To select a column you first need to decide if you want to do packed or capillary gas chromatography. Packed or Capillary? Packed columns have higher sample capacity than capillary columns, although the difference has been greatly reduced by the large-bore 530 m m capillaries invented by HP. Improvements in detector sensitivity have also reduced the need for large samples. The one area in which packed columns may have an advantage is in analysis of gas samples. For almost all other samples, capillaries provide much better efficiency (narrow peaks) which leads to greatly improved peak separation. In fact, the separating power is so great that many analyses can be done on surprisingly short columns in very brief runs. This time saving translates directly into reduced turnaround time and increased sample throughput. For new or updated methods, we recommend capillary columns unless there is some overwhelming reason for using packed columns. Column material It must be as inert as possible, particularly for trace analysis work or for compounds that tend to tail badly, for example active compounds such as mercaptans or alike. For capillaries, fused silica is the material of choice. There are two basic types of fused silica capillary columns: The Wall Coated Open Tubular or WCOT columns and the Porous Layer Open Tubular or PLOT columns. The stationary phase in WCOT columns is a liquid film coated to the deactivated wall of the column. These are the most widely used columns in gas chromatography. In the PLOT columns the stationary phase is a solid substance that is coated to the column wall. Packed columns may be glass or metal, usually stainless steel. Metal, although inherently active, is durable and suitable for nonpolar materials. But if samples with polar components are to be analyzed, select glass. If even this is too active (peak tailing, sample loss), try a deactivation treatment. Stationary phase When selecting capillary columns the first decision to be made is whether a PLOT column is needed. Here are the typical application areas for the three types of PLOT columns:

 
 Molecular Sieve  Fixed gases, sensitive to water
 Divinylbenzene (DVB) HP-PLOT Q  Complete resolution of C1 to C3 isomers, only partial resolution of isomers of C4 and higher (up to  C14), polar compounds, volatile solvents will tolerate water
 Alumina Al 2 O3  Separation of isomers of C1 to C10, sensitive to water
 

If none of the above mentioned applications is what you are interested in then you will be able to use a WCOT type column. When faced with an unknown sample, first try the column that is presently in your GC. If that does not give satisfactory results, consider what you know about the sample. The basic principle is that analytes like to interact with stationary phases of similar chemical nature. This means that the more you know about your sample the easier it is to find the optimum separation phase. The most important step is to consider the polar character of your analytes:

o Nonpolar Molecules - generally composed of only carbon and hydrogen exhibit no dipole moment.
o Straight-chained hydrocarbons (n-alkanes) are common examples of nonpolar compounds.
o Polar Molecules - primarily composed of carbon and hydrogen but also contain atoms of nitrogen, oxygen, phosphorus, sulfur, or a halogen.     Examples include alcohols, amines, thiols, ketones, nitriles, organo- halides, etc.
o Polarizable Molecules - primarily composed of carbon and hydrogen, but also contain unsaturated bonds. Examples include alkenes, alkynes and    aromatic compounds

Agilent Technologies offers you the right stationary phase for your specific separtion needs: Is your sample a mixture of non polar components of the same chemical type, such as hydrocarbons in most petroleum fractions? Try a nonpolar column such as HP-1 which separates them in (approximate) boiling point order. Perhaps you suspect some aromatic components; try a column such as HP-5 or HP-35 with phenyl groups. Samples with polar or polarizable compounds often resolve well on the more polar and/or polarizable stationary phases that contain phenyl groups and alike. Examples are the HP-210 or HP-225 columns. If even more polar phases are required consider the polyethylene glycol (PEG) phases, also often called the wax phases. Please see the selection charts on the next few pages, which suggest stationary phases based on the application and the polar character of the analytes. Bonding creates chemical bonds between the phase and the column tubing. Crosslinking polymerizes the phase in place to increase its molecular weight. Both processes are happening simultaneously during the manufacturing process of bonded/crosslinked columns and have the desirable effects of increasing temperature stability and reducing column bleed. Bonded/cross-linked columns can be rinsed to remove contamination that might build up over time and allow larger volume injections. Where there is a choice, we recommend the bonded/crosslinked phases over the standard coated version. Film thickness The general rule is that thin films elute components sooner with better peak resolution and at lower temperatures than thick films. This indicates that they are well suited to samples with high-boiling components, closely-spaced components, or temperature sensitive components. The "standard" film thickness is 0.25 to 0.5 m m. These work well for most samples (including waxes, triglycerides, and steroids) eluting up to 300 ° C. For components eluting at higher temperatures, thin films (0.1 m m) are available. While standard or thin films are appropriate for high-boiling components, thicker films are needed to resolve low-boiling materials. Film of 1 to 1.5 m m work well for components eluting between 100 and 200 ° C. Extremely thick films (3 to 5 m m) are needed for gases, solvents, and purgeables to increase their interaction with the stationary phase. Another reason for using a thicker than normal film is to maintain resolution and retention times when changing to a wider bore column. For this reason, wide bore columns tend to be available only with thicker films. Thick films mean more material in the column and therefore more bleed. Temperature limits must be lowered as film thickness rises. Column length As a general practice, 15 m columns are used for fast screening, simple mixtures, or very high molecular weight compounds. The 30 m length has become the most popular one for most analyses. Very long columns (50, 60 and 105 m) are for extremely complex samples. Column length is not a very strong parameter in column performance. For example, doubling column length doubles isothermal analysis time but increases peak resolution by only about 40%. If an analysis is almost but not quite good enough, there are better ways than length to improve it. Consider a thinner film, optimizing the carrier flow through the column, and using temperature programming if you are not already doing so. One special situation is the analysis of samples with extremely active components. These will tail severely if they contact the column material. Relatively short columns with thick films reduce the chance of interaction by having less column material and smothering it with stationary phase to conceal active sites. Inside diameter Increased diameter means more stationary phase, even with the same thickness, for greater sample capacity. It also means reduced resolving power and greater bleed. Narrow columns provide the resolution needed for complex samples, but typically require a split injection because of the low sample capacity. Wider columns avoid this if the loss of resolution can be tolerated. When sample capacity is a major consideration, as with gases, very volatile samples, and purge and trap or headspace sampling, large id or even PLOT columns may be appropriate. Also consider the limitations and needs of your instrumentation. An adapted packed column inlet can use the larger bore capillary column but not the narrow ones. Inlets designed specifically for capillary columns generally handle the entire id range. GC/MS and MSD with direct coupling may require narrow columns because the vacuum pumps cannot handle the high flows used with larger columns. Look at your entire system to discover which parts limit your choice of column diameter.

How do I install a capillary column?
 

To install a column follow the instructions below. Pre Column Installation Check List 1. Replace oxygen, moisture, and hydrocarbon traps as needed. 2. Check gas cylinder pressures to ensure that an adequate supply of carrier, makeup, and fuel gases is available. Minimum carrier gas purity percentages are given in Table A. 3. Clean the injection port, replace critical injection port seals, replaceinjection port liners, and change septa as needed. 4. Check detector seals, and replace as necessary. Clean or replace detector jets as necessary. 5. Carefully inspect the column for damage or breakage. 6. Gather the necessary installation tools: You will need a column cutter, column nuts, ferrules, a magnifying loop, and typewriter correction fluid. Installing the Column: 1.Uncoil approximately 0.5 m (18 in.) of tubing from the column basket at both ends of the column for injector and detector installation. Avoid sharp bends in the tubing. 2.Mount the column in the oven. Use the hanging bracket if available. 3.Install the column nut and Vespel or graphite ferrule at each column end; pull the nut and ferrule down the tubing approximately 5 cm. 4.Score (cut) the column. Score the column about 4 to 5 cm from each end. 5.Make a clean break. Grasp the column between the thumb and forefinger as close to the score point as possible. Gently PULL and bend the column. The column should part easily. If the column doesn't break easily, don't force it! Score the column again in a different place, try for a clean break. 6.Use a magnifying loop to inspect the cut. Make sure the cut is square across the tubing with no polyimide or glass fragments at the end of the tube 7.Install the column in the inlet. Check the GC manufacturer's instrument manual for the correct insertion distance. Mark the correct distance on the column with typewriter correction fluid. Insert the column into the injector. Finger tighten the column nut until correct insertion distance. Mark the it starts to grab the column, and then tighten the nut an additional ¼ to ½ turn so that the column cannot be pulled from the fitting when gentle pressure is applied. 8.Turn on the carrier gas, and establish the proper flow rate. Set head pressure, split flow, and septum purge flow to appropriate levels. See Table B for nominal head pressures. If using a split/splitless inlet, check that the purge (split) valve is ON (open). 9.Confirm carrier gas flow through the column. Immerse the end of the column in a vial of acetone, and check for bubbles. 10.Install the column into the detector. Check the instrument manufacturer's manual for. the proper insertion distance 11.Check for leaks. THIS IS VERY IMPORTANT. Don't even THINK about heating the column without thoroughly checking for leaks. 12.Establish proper injector and detector temperatures 13.Establish proper makeup and detector gas flows. Ignite or turn ON the detector. 14.Purge the column for a minimum of 10 minutes at ambient temperature (approximat1ely x column volumes). 15.Inject a nonretained substance to check for proper injector installation. Examples: butane or methane (FID),headspace vapors from: acetonitrile (NPD), methylene chloride (ECD), air (TCD), argon (mass spectrometer). Proper installation is indicated by a symmetrical peak. If tailing is observed, repeat the injector installation process. Conditioning and Testing the Column: 1. Condition the column at 20?C above the maximum temperature of the analysis or at the maximum temperature of the column (whichever is lower) for 2 hours. IF AFTER 10 MINUTES AT THE UPPER TEMPERATURE THE DETECTOR SIGNAL DOES NOT BEGIN TO FALL, IMMEDIATELY COOL THE COLUMN AND CHECK FOR LEAKS. 2. If you are using Vespel ferrules, recheck tightness after the conditioning process. 3. Confirm final proper average linear velocity by injecting a nonretained substance. See Table Below

 
Table A
Dead Time for Common Capillary Column Lengths
 Column Length, m  Hydrogen (40 cm/sec)  Helium (20 cm/sec)
 10  0 min 25 sec  0 min 50 sec
 15  0 min 38 sec  1 min 15 sec
 25  1 min 02 sec  2 min 05 sec
 30  1 min 15 sec  2 min 30 sec
 50  2 min 05 sec  4 min 10 sec
 60  2 min 30 sec  5 min 00 sec
 
What glass columns are used in the 6890?
 

The glass columns for the 6890 use the same configurations as the 5890 series.

What is a capillary GC column?

n the most basic level, a capillary column is composed of two parts - the tubing and stationary phase (Figure 3). Fused silica and stainless steel are the primary tubing materials. There are numerous stationary phases. Most are high molecular weight, thermally stable polymers that are liquids or gums. The most common stationary phases of this type are the polysiloxanes (sometimes incorrectly called silicones) and polyethylene glycols. The next most common type of stationary phases are small, porous particles composed of polymers or zeolites (e.g., alumina, molecular sieves).

 
 

Tubing
Fused silica is a synthetic quartz of high purity. A protective coating is applied to the outer surface with polyimide being the most common coating material. The polyimide coating is responsible for the brownish color of fused silica capillary columns. The color of the polyimide coating often varies between columns. Column color has no effect on the chromatographic performance of the column. Polyimide coated tubing often darkens after prolonged exposure to higher temperatures. The upper temperature limit of standard polyimide coated fused silica tubing is 360 ? C. High temperature polyimide coated tubing has an upper limit of 400 ? C.

The inner surface of fused silica tubing is chemically treated to minimize interactions of the sample with the tubing. The reagents and process used depend on the type of stationary phase being coated onto the tubing. A silylation process is used for most columns. Silanol groups (Si-OH) on the tubing surface are reacted with a silane type of reagent. Typically, a methyl or phenyl-methyl silyl surface is created for most columns.

Stainless steel capillary columns are used for applications requiring very high column temperatures. Stainless steel tubing is more robust than fused silica tubing, thus it is also used in situations where the possibility of tubing breakage needs to be virtually eliminated. Stainless steel interacts with many compounds, thus it is treated to minimize these undesirable interactions. The inner surface is either chemically treated or it is lined with a thin layer of fused silica. When properly made, the inertness of stainless steel capillary columns rivals those made with fused silica tubing.

Stationary Phases

Polysiloxanes
Polysiloxanes are the most common stationary phases. They are available in the greatest variety and are the most stable, robust and versatile. Standard polysiloxanes are characterized by the repeating siloxane backbone (Figure 4). Each silicon atom contains two functional groups. The type and amount of the groups distinguish each stationary phase and its properties. The four most common groups are listed in Figure 4.

Figure 4. Polysiloxanes

The most basic polysiloxane is the 100% methyl substituted. When other groups are present, the amount is indicated as the percent of the total number of groups. For example, a 5% diphenyl-95% dimethyl polysiloxane contains 5% phenyl groups and 95% methyl groups. The "di-" prefix indicates that each silicon atom contains two of that particular group. Sometimes this prefix is omitted even though two identical groups are present. If the methyl percentage is not stated, it is understood to be present in the amount necessary to make 100% (e.g., 50% phenyl-methyl polysiloxane contains 50% methyl substitution). Cyanopropylphenyl percent values can be misleading. A 14% cyanopropylphenyl-dimethyl polysiloxane contains 7% cyanopropyl and 7% phenyl (along with 86% methyl). The cyanopropyl and phenyl groups are on the same silicon atom, thus their amounts are summed.

For select stationary phases, a low bleed or "ms" version is available. These stationary phases incorporate phenyl or phenyl type groups into the backbone of the siloxane polymer (Figure 5). These types of stationary phases are commonly called arylenes. The phenyl group strengthens and stiffens the polymer backbone which inhibits stationary phase degradation at higher temperatures. This results in lower column bleed and, in most cases, higher temperature limits. The arylene stationary phase substitution can be adjusted to maintain the same separation characteristics as the original, non-arylene version (e.g., DB-5, DB-5MS, HP-5, or HP-5MS). The separations for the two versions are the same or extremely similar. It is rare, but there may be a slight separation difference between the regular and low bleed versions of a stationary phase. There are some unique low bleed stationary phases with no "regular" equivalent.

Polyethylene Glycols
Polyethylene glycols (PEG) are widely used as stationary phases (Figure 6). Stationary phases with "wax" or "FFAP" in their name are some type of polyethylene glycol. Polyethylene glycols stationary phases are not substituted, thus the polymer is 100% of the stated material. They are less stable, less robust and have lower temperature limits than most polysiloxanes. With typical use, they exhibit shorter lifetimes and are more susceptible to damage upon over heating or exposure to oxygen. The unique separation properties of polyethylene glycol makes these liabilities tolerable. Polyethylene glycol stationary phases must be liquids under GC temperature conditions.

Figure 6. Polyethylene Glycol

There are two types of polyethylene glycols in common use as GC stationary phases. One has a higher upper temperature limit ( DB-WAXetr and HP-Innowax ), but exhibits slightly higher activity (i.e., peak tailing for some compounds). The other has a lower upper temperature limit and a lower low temperature limit but exhibits better reproducibility and inertness ( DB-WAX and HP-WAX ). The separation characteristics of the two stationary phases are slightly different. Another variation of polyethylene glycol phases is pH modifi- cations. FFAP columns are terephthalic acid modified polyethylene glycols ( DB-FFAP and HP-FFAP ). These columns are used for the analysis of acidic compounds. Base modified polyethylene glycol stationary phases are also available for the analysis of basic compounds ( CAM and HP-Basic WAX ). Strong acids and bases often exhibit peak tailing for standard columns. pH modified stationary phases may decrease the amount of tailing for strong acids or bases.

Gas-solid
Gas-solid stationary phases are comprised of a thin layer (usually <10 ? m) of small particles adhered to the surface of the tubing. These are porous layer open tubular ( PLOT and HP-PLOT ) columns. The sample compounds undergo a gas-solid adsorption/desorption process with the stationary phase. The particles are porous, thus size exclusion and shape selectivity processes also occur. Various derivatives of styrene, aluminum oxides and molecular sieves are the most common PLOT column stationary phases.

PLOT columns are very retentive. They are used to obtain separations that are impossible with conventional stationary phases. Also, many separations requiring subambient temperatures with polysiloxanes or polyethylene glycols can be easily accomplished above ambient temperatures with PLOT columns. Hydrocarbon and sulfur gases, noble and permanent gases, and low boiling point solvents are some of the more common compounds separated with PLOT columns.

Some PLOT columns may occasionally lose particles of the stationary phase. For this reason, using PLOT columns that may lose particles with detectors negatively affected by particulate matter is not recommended. Mass spectrometers are particularly susceptible to this problem due to the presence of a strong vacuum at the exit of the column.

Bonded and Cross-linked Stationary Phases
Cross-linked stationary phases have the individual polymer chains linked via covalent bonds. Bonded stationary phases are covalently bonded to the surface of the tubing. Both techniques impart enhanced thermal and solvent stability to the stationary phase. Also, columns with bonded and cross-linked stationary phases can be solvent rinsed to remove contaminants. Most polysiloxanes and polyethylene glycol stationary phases are bonded and cross-linked. A few stationary phases are available in an nonbonded version; some stationary phases are not available in bonded and cross-linked versions. Use a bonded and cross-linked stationary phase if one is available. All J&W columns with the prefix "DB" are bonded and cross-linked stationary phases.

Column Bleed
Column bleed is the background generated by all columns. It is the continuous elution of the compounds produced from normal degradation of the stationary phase. Column bleed increases at higher temperatures. The appearance of column bleed is best illustrated with a bleed profile or trace, generated by a blank run using a temperature programmed run that reaches the column's upper temperature limit and holds at that temperature for 10-15 minutes. A typical bleed profile is shown in Figure 7.

Figure 7. Bleed Profile

There are several important characteristics to a bleed profile. The baseline is relatively flat during the lower temperature region of the blank run. A sharp rise in the baseline begins at 30-40 ? C below the upper limit of the column and continues until the upper temperature limit is reached. Upon holding at the upper temperature limit, a fairly flat baseline is obtained. Several minutes may elapse before the baseline becomes completely flat. Major deviations from this profile are not due to column bleed. Column bleed is a continuous process - it is not an occasional or momentary event that stops and starts. To obtain a peak a band of volatile material has to enter and travel through the column. Any peaks in a blank run are not from column bleed, but most likely originate from contaminants in the GC system. Even if a mass spectral library search indicates silicon based compounds, the peaks are not from the column stationary phase; they may originate from the septum. On average, polar stationary phases have higher column bleed, and significant bleed occurs at lower temperatures. Column bleed appears to be higher when using a detector that is particularly sensitive to any of the atoms or functional groups in the stationary phase. While the column bleed is not higher, the detector's enhanced sensitivity to the degradation products results in a very large baseline rise. This is usually most prominent for cyanopropyl substituted polysiloxanes used with an NPD or polyethylene glycol columns used with an ECD.

Excessive column bleed appears as a larger rise in the baseline at the higher temperature regions. There is no absolute measurement to indicate when column bleed is excessive. Column bleed is best measured as the difference or change in the background signal at two temperatures. Usually the column's upper temperature limit and a lower value around 100 ? C are used (Figure 7). The absolute background signal is a composite of the background generated by the entire GC system. It is not possible to determine the contribution of column bleed to this total background signal. By measuring the relative amount of column bleed, the other contributors to the background signal are subtracted out. Most columns are tested using FIDs. The output signal for a FID is in picoamps (pA). Bleed levels are usually reported as the difference (DpA) in the FID signal at two temperatures. Because these values are influenced by the detector response they have validity in column comparisons only if both measurements use the same detector under the same conditions, or both detectors are standardized and the bleed reported as pg C per gram of stationary phase under standardized flow conditions.

Column bleed increases as a column is used. Exposing the column to oxygen (air) and/or consistently using the column at or near its upper temperature limit for prolonged periods accelerates the onset of higher column bleed. A sudden or rapid increase in column bleed is usually an indicator of column damage or a problem in the GC system. Prolonged heating of a column above its upper temperature limit, constant exposure of the column to oxygen (usually via a leak), or repeated injection of damaging compounds are the most common causes or problems.

Column Temperature Limits
Columns have lower and upper temperature limits. If a column is used below its lower temperature limit, rounded and wide peaks are obtained (i.e., loss of efficiency). No column damage has occurred; however, the column does not function properly. Using the column at or above its lower limit maintains good peak shapes.

Upper temperature limits are often stated as two numbers. The lower one is the isothermal temperature limit. The column can be used indefinitely at this temperature and reasonable column bleed and lifetime are realized. The upper number is the temperature program limit. A column can be maintained at this temperature for 10-15 minutes without severely shortening column lifetime or experiencing excessively high column bleed. Exposing the column to higher temperatures or for longer time periods results in higher column bleed and shorter column lifetimes. Exceeding the upper temperature limits may damage the stationary phase and the inertness of the fused silica tubing.

Column Capacity
Column capacity is the maximum amount of a solute that can be introduced into a column before significant peak distortion occurs. Overloaded peaks are asymmetric with a leading edge. Overloaded peaks are often described as "shark fin" shaped (Figure 8). Tailing peaks are obtained if a PLOT column is overloaded. No damage occurs if a column is overloaded.

Column capacity is related to stationary phase polarity, film thickness, column diameter and solute retention. Higher column capacities are obtained for solutes that are similar in polarity to the stationary phase. For example, a polar column has a higher capacity for a polar solute than a non-polar solute. Thicker film and wider diameter columns have higher capacities. Column capacity decreases as solute retention increases, thus two similar polarity solutes may overload the column at different amounts with lower capacity for the later eluting solute. Capacity ranges for the most common column diameters and film thicknesses can be found here .

Figure 8.Overloaded Peak

Figure 9. Boiling Point Elution Order for Homologous Series

 
What is the difference between a guard column and retention gap? Why should I use them? How to choose the right column?
 

Guard Columns or Retention Gap

A guard column and retention gap are the same thing, but they serve different purposes. Both are 1-10 meters of deactivated fused silica tubing attached to the front of the column (Figure 27). Deactivated fused silica tubing does not contain any stationary phase; however, the surface is deactivated to minimize solute interactions. A suitable union is used to attach the tubing to the column. In most cases, the diameter of the retention gap or guard column should be the same as the column. If the tubing sizes are different, it is better to have a larger diameter guard column or retention gap than a smaller one.

Figure 27. Retention Gap or Guard Column

Guard columns are used when samples contain non-volatile residues that may contaminate a column. The non-volatile residues deposit in the guard column and not in the column. This greatly reduces the interaction between the residues and the sample since the guard column does not retain the solutes (because it contains no stationary phase). Also, the residues do not coat the stationary phase which often results in poor peak shapes. Periodic cutting or trimming of the guard column is usually required upon a build-up of residues. Guard columns are often 5-10 meters in length to allow substantial trimming before the entire guard column has to replaced. The onset of peak shape problems is the usual indicator that the guard column needs trimming or changing.

Retention gaps are used to improve peak shapes for some types of samples, columns, and GC conditions. Usually a minimum of 3-5 meters of tubing is required to obtain the benefits of a retention gap. The situations that benefit the most from retention gaps are large volume injections (>2 ? L) and solvent-stationary phase polarity mismatches for splitless, Megabore direct and on-column injections. Peak shapes are sometimes distorted when using combinations of these conditions. Polarity mismatches occur when the sample solvent and column stationary phase are very different in polarity. The greatest improvement is seen for the peaks eluting closest to the solvent front or solutes very similar to the solvent in polarity. The benefits of a retention gap are often unintentionally obtained when using a guard column.

Unions
There are a variety of unions that can be used to connect fused silica tubing. Stainless steel, stainless steel-glass combinations, glass press-fit and quick connectors are some of the more common types.

There is a variety of metal unions available. These have higher upfront cost for the fitting and ferrules, but metal unions are reusable. Agilent's Ultimate Union was designed using Agilent's new patented Micro fluidic diffusion-bonded plated technology, Agilent's Ultimate Union let's you easily and quickly make leak free, inert column connections.

Glass press-fit unions are inexpensive but, on occasion, a glass press-fit union will not seal with a particular batch of tubing. The sealing process with Press-Fits is very technique dependent and time consuming, with a recommended half hour curing time. Also, they may develop a leak after numerous temperature program runs.
 

Are there Low Bleed Columns - Fact or Fiction?

Evaluating The Problem
Checking The Obvious
Ghost Peaks or Carryover
Excessive Baseline Noise
Baseline Instability or Disturbances
Tailing Peaks
Split Peaks
Retention Time
Change in Peak Size
Loss of Resolution
Condensation Test
"Low-Bleed" Columns - Fact or Fiction?

Evaluating The Problem
The first step in any troubleshooting effort is to step back and evaluate the situation. Rushing to solve the problem often results in a critical piece of important information being overlooked or neglected. In addition to the problem, look for any other changes or differences in the chromatogram. Many problems are accompanied by other symptoms. Retention time shifts, altered baseline noise or drift, or peak shape changes are only a few of the other clues that often point to or narrow the list of possible causes. Finally, make note of any changes or differences involving the sample. Solvents, vials, pipettes, storage conditions, sample age, extraction or preparation techniques, or any other factor influencing the sample environment can be responsible.

Checking The Obvious
A surprising number of problems involve fairly simple and often overlooked components of the GC system or analysis. Many of these items are transparent in the daily operation of the GC and are often taken for granted (set it and forget it). The areas and items to check include:

• Gases - pressures, carrier gas average linear velocity, and flow rates (detector, split vent, septum purge).
• Temperatures - column, injector, detector and transfer lines.
• System parameters - purge activation times, detector attenuation and range, mass ranges, etc.
• Gas lines and traps - cleanliness, leaks, expiration.
• Injector consumables - septa, liners, O-rings and ferrules.
• Sample integrity - concentration, degradation, solvent, storage.
• Syringes - handling technique, leaks, needle sharpness, cleanliness.
• Data system - settings and connections.

GC Troubleshooting

Evaluating The Problem
Checking The Obvious
Ghost Peaks or Carryover
Excessive Baseline Noise
Baseline Instability or Disturbances
Tailing Peaks
Split Peaks
Retention Time
Change in Peak Size
Loss of Resolution
Condensation Test
"Low-Bleed" Columns - Fact or Fiction?

Evaluating The Problem
The first step in any troubleshooting effort is to step back and evaluate the situation. Rushing to solve the problem often results in a critical piece of important information being overlooked or neglected. In addition to the problem, look for any other changes or differences in the chromatogram. Many problems are accompanied by other symptoms. Retention time shifts, altered baseline noise or drift, or peak shape changes are only a few of the other clues that often point to or narrow the list of possible causes. Finally, make note of any changes or differences involving the sample. Solvents, vials, pipettes, storage conditions, sample age, extraction or preparation techniques, or any other factor influencing the sample environment can be responsible.

Checking The Obvious
A surprising number of problems involve fairly simple and often overlooked components of the GC system or analysis. Many of these items are transparent in the daily operation of the GC and are often taken for granted (set it and forget it). The areas and items to check include:

• Gases - pressures, carrier gas average linear velocity, and flow rates (detector, split vent, septum purge).
• Temperatures - column, injector, detector and transfer lines.
• System parameters - purge activation times, detector attenuation and range, mass ranges, etc.
• Gas lines and traps - cleanliness, leaks, expiration.
• Injector consumables - septa, liners, O-rings and ferrules.
• Sample integrity - concentration, degradation, solvent, storage.
• Syringes - handling technique, leaks, needle sharpness, cleanliness.
• Data system - settings and connections.

Ghost Peaks or Carryover
System contamination is responsible for most ghost peaks or carryover problems. If the extra ghost peaks are similar in width to the sample peaks (with similar retention times), the contaminants were most likely introduced into the column at the same time as the sample. The extra compounds may be present in the injector (i.e., contamination) or in the sample itself. Impurities in solvents, vials, caps and syringes are only some of the possible sources. Injecting sample and solvent blanks may help to find possible sources of the contaminants. If the ghost peaks are much broader than the sample peaks, the contaminants were most likely already in the column when the injection was made. These compounds were still in the column when a previous GC run was terminated. They elute during a later run and are often very broad. Sometimes numerous ghost peaks from multiple injections overlap and elute as a hump or blob. This often takes on the appearance of baseline drift or wander.

Increasing the final temperature or time in the temperature program is one method to minimize or eliminate a ghost peak problem. Alternatively, a short bake-out after each run or series of runs may remove the highly retained compounds from the column before they cause a problem. Performing a condensation test is a good method to determine whether a contaminated injector is the source of the carryover or ghost peaks

Excessive Baseline Noise

 
What are some possible cause of baseline instability and disturbances?
 

Evaluating The Problem
The first step in any troubleshooting effort is to step back and evaluate the situation. Rushing to solve the problem often results in a critical piece of important information being overlooked or neglected. In addition to the problem, look for any other changes or differences in the chromatogram. Many problems are accompanied by other symptoms. Retention time shifts, altered baseline noise or drift, or peak shape changes are only a few of the other clues that often point to or narrow the list of possible causes. Finally, make note of any changes or differences involving the sample. Solvents, vials, pipettes, storage conditions, sample age, extraction or preparation techniques, or any other factor influencing the sample environment can be responsible.

Checking The Obvious
A surprising number of problems involve fairly simple and often overlooked components of the GC system or analysis. Many of these items are transparent in the daily operation of the GC and are often taken for granted (set it and forget it). The areas and items to check include:

• Gases - pressures, carrier gas average linear velocity, and flow rates (detector, split vent, septum purge).
• Temperatures - column, injector, detector and transfer lines.
• System parameters - purge activation times, detector attenuation and range, mass ranges, etc.
• Gas lines and traps - cleanliness, leaks, expiration.
• Injector consumables - septa, liners, O-rings and ferrules.
• Sample integrity - concentration, degradation, solvent, storage.
• Syringes - handling technique, leaks, needle sharpness, cleanliness.
• Data system - settings and connections.

Ghost Peaks or Carryover
System contamination is responsible for most ghost peaks or carryover problems. If the extra ghost peaks are similar in width to the sample peaks (with similar retention times), the contaminants were most likely introduced into the column at the same time as the sample. The extra compounds may be present in the injector (i.e., contamination) or in the sample itself. Impurities in solvents, vials, caps and syringes are only some of the possible sources. Injecting sample and solvent blanks may help to find possible sources of the contaminants. If the ghost peaks are much broader than the sample peaks, the contaminants were most likely already in the column when the injection was made. These compounds were still in the column when a previous GC run was terminated. They elute during a later run and are often very broad. Sometimes numerous ghost peaks from multiple injections overlap and elute as a hump or blob. This often takes on the appearance of baseline drift or wander.Increasing the final temperature or time in the temperature program is one method to minimize or eliminate a ghost peak problem. Alternatively, a short bake-out after each run or series of runs may remove the highly retained compounds from the column before they cause a problem. Performing a condensation test is a good method to determine whether a contaminated injector is the source of the carryover or ghost peaks.

 
Excessive Baseline Noise
 
Possible Cause Solution Comments
Injector contamination Clean the injector Try a condensation test; gas lines may also need cleaning
Column contamination Bake-out the column Limit the bake-out to 1-2 hours
Column contamination Solvent rinse the column Only for bonded and corss-linked phases
Detector contamination Clean the detector Usually the noise increases over time and not suddenly
Contaminated or low quality gases Use better grade gases; also check for expired gas traps or leaks Usually occurs after changing a gas cylinder
Column inserted too far into detector Reinstall the column Consult GC manual for the proper insertion distance
Incorrect detector gas flow rates Adjust the flow rates to the recommended values Consult GC manual for the proper flow rates
Leak when using an MS, ECD or TCD Find and eliminate the leak Usually at the column fittings or injector
Old detector filament , lamp or electron multiplier Replace appropriate part  
 
Baseline Instability or Disturbances
 
Possible Cause Solution Comments
Injector contamination Clean the injector Try a condensation test; gas lines may also need cleaning
Column contamination Bake-out the column Limit bake-out to 1-2 hours
Unequilibrated detector Allow the detector to stabilized Since detectors may require up to 24 hours to fully stabilize
Incompletely conditioned column Fully condition the column More critical for trace level analysis
Change in carrier gas flow rate during the temperature program Normal in many cases MS, TCD and ECD respond to changes in carrier gas flow rate
 
Tailing Peaks
 
Possible Cause Solution Comments
Column Contamination Trim the column Remove 1/2-1 meter from the front of the column
Column Contamination Solvent rinse the column Only for bonded and cross-linked phases
Column activity Irreversible Only affects active compounds
Solvent-phase polarity mismatch Change sample solvent More tailing for the early eluting peaks or those closest to solvent front
Solvent-phase polarity mismatch Install a retention gap 3-5 meter retention gap is sufficient
Solvent effect violation for splitless or on-column injections Decrease the initial column tempterature Peak tailing decreases with retention
Too low of a split ratio Increase the split ratio Flow from split vent should be 20 mL/min or higher
Poor column installation Reinstall the column More tailing for the early eluting peaks
Some active compounds always tail None Most common for amines and carboxylic acids
 
Split Peaks
 
Possible Cause Solution Comments
Injection technique Change technique Usually related to erratic plunger depression or  having sample in the syringe needle
Mixed sample solvent Change the sample solvent to a single  solvent Worse for solvents with large differences in polarity or boiling points
Poor column installation Reinstall the column in the injector Usually a large error in the insertion distance
Sample degradation in the injector Reduce the injector temperature Peak broadening or tailing may occur if the temperature is too low
Sample degradation in the injector Change to an on-column injector Requires an on-column injector
 
Retention Time
 
Possible Cause Solution Comments
Change in carrier gas velocity Check the carrier gas velocity All peaks will shift in the same direction by approximately the same amount
Change in column temperature Check the column temperature Not all peaks will shift by the same amount
Change in column dimension Verify column identity  
Large change in compound concentration Try a different sample concentration May also affect adjacent peaks
Leak in the injector Leak check the injector A change in peak size also usually occurs.
Blockage in a gas line Clean or replace the plugged line More common for the split line; also check flow controllers and solenoids
 
Change in Peak Size
 
Possible Cause Solution Comments
Change in detector response Check gas flows, temperatures and settings All peaks may not be equally affected
Change in detector response Check background level or noise May be caused by system contamination and not the detector
Change in the split ratio Check split ratio All peaks will not by equally affected
Change in the purge activation time Check the purge activation time For splitless injectors
Change in injector volume Check the injection technique Injection volumes are not linear
Change in sample concentration Check and verify sample concentration Changes may also be caused by degradation,evaporation, or variances in sample temperature or pH
Leak in the syringe Use a different syringe Sample leaks passed the plunger or around the needle; leaks are often not readily visible
Column contamination Trim the column Remove 1/2-1 meter from the front of the column
Column contamination Solvent rinse the column Only for bonded and cross-linked phases
Column activity Irreversible Only affects active compounds
 
Loss of Resolution
 
Possible Cause Solution Comments
Decrease in Separation
Different column temperature Check column temperature Differences in other peaks will be visible
Different column dimensions or phase Verify column identity Differences in other peaks will be visible
Coelution with other peak Change the column temperature Decrease column temperature and check for the appearance of a peak shoulder or tail
Increase in peak width
Change in carrier gas velocity Check carrier gas velocity A change in retention time also occurs
Column contamination Trim the column Remove 1/2 to 1 meter from the front of the column
Column contamination Solvent rinse the column Only for bonded and cross-linked phases
Column contamination Trim the column Remove 1/2-1 meter from the front of the column
Column contamination Solvent rinse the column Only for bonded and cross-linked phases
Change in the injector Check the injector settings Typical areas: split ratio, liner, temperature, injection volume
Change in sample concentration or solvent Try a different sample concentration Peak widths increase at higher concentrations
 

Condensation Test
Use this test whenever injector or carrier gas contamination problems are suspected (e.g., ghost peaks or erratic baselines).

• Leave the GC at 40-50�C for 8 or more hours.
• Run a blank analysis (i.e., start the GC, but with no injection) using the normal temperature conditions and instrument settings.
• Collect the chromatogram for this blank run.
• Immediately repeat the blank run as soon as the first one is completed. Do not allow more than 5 minutes to elapse before starting the second    blank run.
• Collect the chromatogram for the second blank run and compare it to the first chromatogram.
• If the FIRST chromatogram contains a substantially larger amount of peaks and baseline instability, then that is an indication that there is    contamination upstream of the capillary column (ie. contaminated inlet, dirty carrier gas, etc.).
• If BOTH chromatograms contain few peaks or very little baseline drift, it can be assumed that the carrier gas and/or inlet are relatively clean.
• If BOTH chromatograms contain a significant amount of noise and/or baseline drift, then that usually is an indication that the detector or detector   gases are contaminated.

"Low-Bleed" Columns - Fact or Fiction?
Prof. Walt Jennings
Cofounder, J&W Scientific Incorporated

Several manufacturers offer "low bleed" columns. In some cases, these are merely selected from the standard production process, but in other cases the columns are actually "synthesized" for low bleed. In recent years, it has been established that where functional groups (i.e. phenyl) are inserted into the polysiloxane chain as aryl inclusions, as opposed to being attached to the chain as pendant groups, the resultant phase possesses increased thermal and oxidative resistance. Columns coated with such phases emit lower levels of bleed signal and are capable of going to higher temperatures. The increased thermal resistance is apparent only at temperatures above ca. 300 degrees. While some users can reap the benefits of these developments, others find little or no improvement.. their bleed signals are still too high.

True column bleed, of course, comes only from the column. What the user perceives as bleed is usually the total signal reaching the detector, which is the summation of signal from the septum (this gives a typical silicone mass spectrum), the injector, and the detector, all of which is usually blamed on the column.

It is good procedure to first check the detector. Disconnect and remove the column, and place an undrilled cap on the column attachment fitting. Activate the detector, and note the signal at 50 degrees. Increase the oven temperature to 320 degrees, and again note the signal. On a pristine detector, the FID signal will increase by one to two picoamps. If the increase exceeds this level, attention should be directed to cleaning the detector, make-up gas and hydrogen lines. Once the detector signal falls to an acceptable level at 320 degrees, attention should be directed to the injector. If the injector liner is visibly soiled, the injector should be cooled, dissembled and interior cavities scrubbed with solvent and natural bristle brushes or cotton swabs. After assembling the injector, a "jumper tube" (one to three meters of uncoated fused silica or steel tubing) is then used to connect the injector directly to the detector. The injector heater should be energized, and the oven set at 320 degrees. Any increase in "bleed" signal over that observed with the detector alone must come from the front end of the instrument, and may originate with the septum, the carrier gas line, in-line regulators, valves, or flow controllers.

Wrap a new septum in aluminum foil, ensuring that one face is smooth, and install this, smooth side down. If the signal emanating from the jumper tube is decreased, it indicates a need for better quality septa. If the signal is still high, materials entrained in the carrier gas may have deposited in lines, valves, or regulators, which should be dissembled and cleaned or replaced.

When the combined signal from the injector and detector falls to an acceptable level (one to two picoamps @ 320 degrees on an FID), the user is ready to install and reap the benefits of a true low-bleed column. The bleed rate of conventional columns is normally high enough to mask signal from the injector and detector unless these latter are heavily contaminated. With low bleed columns, the signal from the injector and detector assumes increased importance. This spurious signal is not infrequently limiting, and is usually (and incorrectly) perceived as "column bleed".

 
Is it necessary to condition new columns?
 

PID/FID

Question:
I've noticed an interesting phenomenon when running a diesel standard on my tandem PID/FID; hydrocarbons eluting after C16 start to significantly decrease and also tail very badly. What causes this?

Answer:
This is a symptom of condensation in the PID. As the relative volatility of solutes decreases, the effect becomes more pronounced. Normally, this phenomenon can be minimized by increasing the temperature of the detector, up to the upper temperature limit (about 250�C). The life of the PID lamp is greatly reduced with higher temperatures, so compounds should be restricted to the volatile range.

Calibration Curve

Question:
EPA Method 8270 (GC/MS of semivolatile organics via capillary column techniques) requires a five-point calibration curve, typically 20, 40, 80, 120 and 160 ppm. Because the EPA recommends a 30 meter, 0.25 mm I.D. capillary column with a 1.0 �m film, my chromatograms exhibit all of the symptoms of overload for the 120 and 160 ppm standards. What can I do?

Answer:
0.25 mm I.D. capillary columns with a 1.0 �m film can handle approximately 125 to 175 ng1 of each individual analyte in a matrix; this means that if a 1.0 �L sample mix has 100 ppm each of two components, for a total of 200 ng, the column will not overload. Faced with column overload, the analyst may select a column with greater capacity (e.g., a 0.32 mm I.D. column with comparable b). Unfortunately, not all benchtop GC/MS systems can handle the greater flow rates of 0.32 mm I.D. columns. Another approach is to position the top of the column within the injector so that a smaller amount of sample is introduced to the column. The calibration curve remains linear because the injector discrimination is constant for all concentrations. This might be the simplest solution for most analysts. Alternatively, split injection could reduce the amount of analyte on column (e.g., 25:1). Many of the late eluting and trace concentration compounds will become difficult to detect.

Water Injections

Question:
Our lab routinely injects samples with an aqueous matrix, and we commonly have problems getting reproducible results. Can we improve this?

Answer:
Water has one of the largest vapor volumes of the common laboratory solvents. For water injections, the injector liner may be too small to accommodate the vaporized mixture, so the excess vapor will "backflash" outside of the injection port. This vapor mixture condenses on cooler surfaces resulting in a loss of sample. In the case of water, losses can be minimized by setting the injection port temperature to between 150 and 220�C (lowering the expansion volume) or using a smaller injection volume.

Guard Columns

Question:
How long should a guard column be?

Answer:
Guard columns are typically from 0.5 to 10 meters long. Although there are no definitive lengths that are good for all samples, the following guidelines can be used.

If the sample matrix is relatively "clean" (a small concentration of non-volatile compounds) and the solutes are active, the guard column should be 0.5 meter to 1 meter in length. If the sample matrix is dirty, the guard column should be longer (to collect the nonvolatile compounds). Five to ten meters help simplify system maintenance. With use a guard columns saturates and it becomes necessary to replace it. The longer guard column allows the user to simply cut off the first meter or so and reinstall it into the injector instead of replacing the entire guard column.

Flow on dual column assemblies

Question:
I'm having a problem matching flows on my homemade dual-column assemblies and resolving some of my anlaytes. Help!

Answer:
Even though a column manufacturer specifies particular dimensions, the dimensions are not exact. This can cause problems when coupling columns in a single injection, dual-column analysis. J&W offers a column connection service, but if you want to do it yourself, try the following.

Connect the columns with the Y splitter and guard column. Verify the integrity of the connection. Heat the columns to a temperature at which you can inject a detectable, unretained compound.1 I like 150�C. Note the elution time of the compound. If it elutes more than 0.1 minute apart, cut 10-15 cm from the column with the later time. Repeat the process until the nonretained compound elutes within 0.1 minute on both columns, making the column's flow rates nearly the same.

Run a standard under typical run conditions until resolution criteria are met. Pick a member of a pair of compounds that is difficult to resolve on one or both columns. Raise the column oven temperature high enough so that it will elute the compound between 5 and 10 minutes, inject it and note the elution times on both columns.

When installing new dual columns for the same analysis, repeat the steps in paragraph two, inject the chosen compound and adjust the head pressure until the retention time for both columns is within a percentage or two of the previously recorded times.

For a list of compounds for different detectors, refer to: Rood, D. A Practical Guide to the Care, Maintenance, and Trouble Shooting of Capillary Gas Chromatography Systems; Huthig, Heidelberg, 1991.

Conditioning New Columns

Question:
I've heard conflicting opinions about conditioning new columns. Some of my coworkers say it isn't necessary, some say you should bake the thing overnight, and others say you should ramp the column slowly. So what's the deal? Is it necessary to condition a new column? If so, how?

Answer:
Condition a new capillary column at approximately twenty degrees higher than the final temperature of your oven program without exceeding the upper temperature limit of the column. If a temperature higher than the isothermal temperature limit of the column is needed for your analysis, recondition the column at that higher temperature, but, again, don't exceed the upper program limit.

When you install your column, purge it with at least three volumes of carrier gas prior to ramping it to the conditioning temperature. The total column conditioning time will depend on the type of application you're running and how much bleed is acceptable. The lower the detection limit that's needed, the longer the column will need to be conditioned. (Column bleed is closely related to the polarity and the film thickness of the stationary phase.) Polar and thick film columns bleed more and require more conditioning. For most applications, 30-60 minutes of conditioning is usually sufficient.

But how can you really determine when a column is sufficiently conditioned?

A flame ionization detector (FID) works best for monitoring the baseline during conditioning. Toward the end of the temperature ramp (i.e., 30-40�C below the isothermal upper temperature limit), the baseline will rise, then come down and level off, at which time you may consider the column conditioned. There are those that report detector fouling during conditioning when using other types of detectors (e.g., ECD, MS), but it's generally considered a safe practice to condition the column while connected to these detectors.

One more thing: don't condition a column overnight. Column life expectancy is greatly reduced when the column is stored at high temperatures. If you're experiencing an excessive amount of bleed for more than two hours, bring the oven down to room temperature and locate the source of the problem (usually oxygen entering the column from loose fittings or a leaky septum). Baseline signals that mimic column bleed can also originate from residues present in the GC itself.

One more note: if the column has not been in use for a while, a mild conditioning step may be needed to drive off contamination which may have condensed inside the column during storage. Also, there is nothing to suggest a limit to the ramp rate of the oven when conditioning a column.

Inlet activity problems

Question:
How do you have chemically deactivate injection liners? How do I know when I have an activity problem in my inlet?

Answer:
Most inlet liners are made of borosilicate glass (e.g., Pyrex). Borosilicate glass exhibits characteristics advantageous to gas chromatography. It has a low coefficient of thermal expansion and is resistant to thermal shock. Most glasses contain Lewis acid sites, and in the borosilicates, these are in the form of boron, metal oxides, and surface silanols. These sites can interact with solutes in the sample, resulting in tailing peaks. These sites may also contribute to solute degradation. Thus, when you experience tailing peaks or loss in sensitivity for chromatographically active solutes, you may be experiencing an activity problem in the inlet liner.

The deactivation process entails two basic steps: a leaching step to remove metal oxides at the glass surface and a derivatization step to deactivate surface silanols. Leaching involves soaking the inlet liner in a 25% mineral acid solution (e.g., hydrochloric, nitric, and sulfuric acids, but not chromic acid), usually overnight at room temperature. This portion of the deactivation process can be shortened to several hours if the acid solution is mildly heated (65 ? C).

The derivatization step is more involved. After leaching, the liner is heated to remove free and bound water from the surface of the glass2), and then it is derivatized with a chemical agent to deactivate the surface silanol groups The choices for derivatizing agents are numerous, and methods are just as varied.

Although simple deactivation procedures exist, and are fairly effective (40-50%), the procedure is a very thorough deactivation procedure, which produces a more chemically inert liner than is commonly commercially available. This procedure is especially effective for very active compounds.

For more information on the properties of glass and chemical deactivation, we recommend two books by Walt Jennings: Analytical Gas Chromatography, Academic Press, and Comparisons of Fused Silica and Other Glass Columns in Gas Chromatography , H ? thig. Also, Dean Rood's book, A Practical Guide to the Care, Maintenance, and Troubleshooting of Capillary Gas Chromatographic Systems , offers discussions on poor peak shape and activity phenomena.

Flowmeter readings

Question:
I am getting a different reading on my flowmeter than I get if I inject an unretained compound and calculate the flow. The unretained compound elutes in 1.04 min on my 30 meter, 0.32 mm I.D. DB -5, which gives me a calculated 2.31 mL/min flow rate. My flowmeter says the flow rate is 4.59 mL/min. Am I doing the calculation wrong, or is my flowmeter wrong?

Answer:
Both answers are correct, but they answer different questions. The flowmeter is measuring the flow rate at the exit end of the column, whereas the calculated flow rate is a measure of the average flow rate through the column. The calculation should look something like this.
 
What causes column degradation?
 

Column Breakage
Fused silica columns break wherever there is a weak point in the polyimide coating. The polyimide coating protects the fragile fused silica tubing. The continuous heating and cooling of the oven, vibrations caused by the oven fan and being wound on a circular cage all place stress on the tubing. Eventually breakage occurs at a weak point. Weak spots are created when the polyimide coating is scratched or abraded. This usually occurs when a sharp point or edge is dragged over the tubing. Column hangers and tags, metal edges in the GC oven, column cutters and miscellaneous items on the lab bench are just some of the common sources of sharp edges or points.

It is rare for a column to spontaneously break. Column manufacturing practices tend to expose any weak tubing and eliminate it from use in finished columns. Larger diameter columns are more prone to breakage. This means that greater care and prevention against breakage must be taken with 0.45-0.53 mm I.D. tubing than with 0.18-0.32 mm I.D. tubing.

A broken column is not always fatal. If a broken column was maintained at a high temperature either continuously or with multiple temperature program runs, damage to the column is very likely. The back half of the broken column has been exposed to oxygen at elevated temperatures which rapidly damages the stationary phase  The front half is fine since carrier gas flowed through this length of column. If a broken column has not been heated or only exposed to high temperatures or oxygen for a very short time, the back half has probably not suffered any significant damage.

A union can be installed to repair a broken column. Any suitable union will work to rejoin the column. No more than 2-3 unions should be installed on any one column. Problems with dead volume (peak tailing) may occur with multiple unions.

Thermal Damage
Exceeding a column's upper temperature limit results in accelerated degradation of the stationary phase and tubing surface. This results in the premature onset of excessive column bleed , peak tailing for active compounds and/or loss of efficiency (resolution). Fortunately, thermal damage is a slower process, thus prolonged times above the temperature limit are required before significant damage occurs. Thermal damage is greatly accelerated in the presence of oxygen. Overheating a column with a leak or high oxygen levels in the carrier gas results in rapid and permanent column damage.

Setting the GC's maximum oven temperature at or a few degrees above the column's temperature limit is the best method to prevent thermal damage. This prevents the accidental overheating of the column. If a column is thermally damaged, it may still be functional. Remove the column from the detector. Heat the column for 8-16 hours at its isothermal temperature limit. Remove 10-15 cm from the detector end of the column. Reinstall the column and condition as usual. The column usually does not return to its original performance; however, it is often still functional. The life of the column will be reduced after thermal damage.

Oxygen Damage
Oxygen is an enemy to most capillary GC columns. While no column damage occurs at or near ambient temperatures, severe damage occurs as the column temperature increases. In general, the temperature and oxygen concentration at which significant damages occurs is lower for polar stationary phases. It is constant exposure to oxygen that is the problem. Momentary exposure such as an injection of air or a very short duration septum nut removal is not a problem.

A leak in the carrier gas flow path (e.g., gas lines, fittings, injector) is the most common source of oxygen exposure. As the column is heated, very rapid degradation of the stationary phase occurs. This results in the premature onset of excessive column bleed, peak tailing for active compounds and/or loss of efficiency (resolution). These are the same symptoms as for thermal damage. Unfortunately, by the time oxygen damage is discovered, significant column damage has already occurred. In less severe cases, the column may still be functional but at a reduced performance level. In more severe cases, the column is irreversibly damaged.

Maintaining an oxygen and leak free system is the best prevention against oxygen damage. Good GC system maintenance includes periodic leak checks of the gas lines and regulators, regular septa changes, using high quality carrier gases, installing and changing oxygen traps, and changing gas cylinders before they are completely empty.

Chemical Damage
There are relatively few compounds that damage stationary phases. Introducing non-volatile compounds (high molecular weight or high boiling point) in a column often degrades performance, but damage to the stationary phase does not occur. These residues can often be removed and performance returned by solvent rinsing the column (see next section for more information on column contamination and solvent rinsing).

Inorganic or mineral bases and acids are the primary compounds to avoid introducing in a column. The acids include hydrochloric (HCl), sulfuric (H 2 SO 4 ), nitric (HNO 3 ), phosphoric (H 3 PO 4 ) and chromic (CrO 3 ). The bases include potassium hydroxide (KOH), sodium hydroxide (NaOH) and ammonium hydroxide (NH 4 OH). Most of these acids and bases are not very volatile and accumulate at the front of the column. If allowed to remain, the acids or bases damage the stationary phase. This results in the premature onset of excessive column bleed, peak tailing for active compounds and/or loss of efficiency (resolution). The symptoms are very similar to thermal and oxygen damage.

Hydrochloric acid and ammonium hydroxide are the least harmful of the group. Both tend to follow any water that is present in the sample. If the water is not or only poorly retained by the column, the residence time of HCl and NH 4 OH in the column is short. This tends to eliminate or minimize any damage by these compounds. Thus, if HCl or NH 4 OH are present in a sample, using conditions or a column with no water retention will render these compounds relatively harmless to the column.

The only organic compounds that have been reported to damage stationary phases are perfluoroacids. Examples include trifluoroacetic, pentafluoropropanoic and heptafluorobutyric acid. They need to be present at high levels (e.g., 1% or higher). Most of the problems are experienced with splitless or Megabore direct injections where large volumes of the sample are deposited at the front of the column.

Since chemical damage is usually limited to the front of the column, trimming or cutting 1/2-1 meter from the front of the column often eliminates any chromatographic problems. In more severe cases, 5 or more meters may need to be removed. The use of a guard column or retention gap will minimize the amount of column damage; however, frequent trimming of the guard column may be necessary. The acid or base often damages the surface of the deactivated fused silica tubing which leads to peak shape problems for active compounds.

Column Contamination
Column contamination is one of the most common problems encountered in capillary GC. Unfortunately, it mimics a very wide variety of problems and is often misdiagnosed as another problem. A contaminated column is usually not damaged, but it may be rendered unusable.

There are two basic types of contaminants: nonvolatile and semi-volatile. Nonvolatile contaminants or residues do not elute and accumulate in the column. The column becomes coated with these residues which interfere with the proper partitioning of solutes in and out of the stationary phase. Also, the residues may interact with active solutes resulting in peak adsorption problems (evident as peak tailing or loss of peak size). Active solutes are those containing a hydroxyl (-OH) or amine (-NH) group, and some thiols (-SH) and aldehydes. Semivolatile contaminants or residues accumulate in the column, but eventually elute. Hours to days may elapse before they completely leave the column. Like nonvolatile residues, they may cause peak shape and size problems and, in addition, are usually responsible for many baseline problems (instability, wander, drift, ghost peaks, etc.).

Contaminants originate from a number of sources with injected samples being the most common. Extracted samples are among the worse types. Biological fluids and tissues, soils, waste and ground water, and similar types of matrices contain high amounts of semivolatile and nonvolatile materials. Even with careful and thorough extraction procedures, small amounts of these materials are present in the injected sample. Several to hundreds of injections may be necessary before the accumulated residues cause problems. Injection techniques such as on-column, splitless and Megabore direct place a large amount of sample into the column, thus column contamination is more common wirh these injection techniques.

Occasionally contaminants originate from materials in gas lines and traps, ferrule and septa particles, or anything coming in contact with the sample (vials, solvents, syringes, pipettes, etc.). These types of contaminants are probably responsible when a contamination problem suddenly develops and similar samples in previous months or years did not cause any problems.

Minimizing the amount of semivolatiles and nonvolatile sample residues is the best method to reduce contamination problems. Unfortunately, the presence and identity of potential contaminants are often unknown. Rigorous and thorough sample cleanup is the best protection against contamination problems. The use of a guard column or retention gap often reduces the severity or delays the onset of column contamination induced problems. If a column becomes contaminated, it is best to solvent rinse the column to remove the contaminants.

Maintaining a contaminated column at high temperatures for long periods of time (often called baking out a column) is not recommended. Baking out a column may convert some of the contaminating residues into insoluble materials that cannot be solvent rinsed from the column. If this occurs, the column cannot be salvaged in most cases. Sometimes the column can be cut in half and the back half may still be useable. Baking out a column should be limited to 1-2 hours at the isothermal temperature limit of the column.

Solvent Rinsing Columns
Solvent rinsing columns involves removing the column from the GC and passing milliliters of solvent through the column. Any residues soluble in the rinse solvents are washed from the column. Injecting large volumes of solvent while the column is still installed is not rinsing a column nor does it remove any contaminants from the column. A capillary GC column must have a bonded and cross-linked stationary phase before it can be solvent rinsed. Solvent rinsing a non-bonded stationary phase results in severe damage to the column.

A column rinse kit is used to force solvent through the column (Figure 36). The rinse kit is attached to a pressurized gas source (N 2 or He), and the column is inserted into the rinse kit. Solvent is added to the vial, and the vial is pressurized using the gas source. The pressure forces solvent to flow through the column. Residues dissolve into the solvent and are backflushed out of the column with the solvent. The solvent is then purged from the column, and the column is properly conditioned.

Figure 36. Solvent Rinse Kit

Before rinsing a column, cut about 1/2 meter from the front (i.e., injector end) of the column. Insert the detector end of the column into the rinse kit. Multiple solvents are normally used to rinse columns. Each successive solvent must be miscible with the previous one. High boiling point solvents should be avoided especially as the last solvent. The sample solvent(s) is often a good choice. Methanol, methylene chloride and hexane are recommended and work very well for the majority of cases. Acetone can be substituted for methylene chloride to avoid using halogenated solvents; however, methylene chloride is one of the best rinsing solvents. If aqueous based samples (e.g., biological fluids and tissues) were injected, use water before the methanol. Some residues originating from aqueous based samples are only soluble in water and not organic solvents. Water and alcohols (e.g., methanol, ethanol, isopropanol) should be used to rinse bonded polyethylene glycol based stationary phases (e.g., DB-WAX , DB-WAXetr , HP-INNOWax , DB-FFAP , HP-FFAP ) only as a last resort .

Table 13 lists the suggested solvent volumes for different diameter columns. Using larger solvent volumes is not harmful, but rarely better and merely wasteful. After adding the first solvent, pressurize the rinse kit, but stay below 20 psi. Use the highest pressure that keeps the solvent flow rate below 1 mL/min. Except for most 0.53 mm I.D. columns, the rinse kit pressure will reach 20 psi before the flow rate reaches 1 mL/min. Longer rinse times are required when using heavy or viscous solvents, and for longer or smaller diameter columns. When all or most of the first solvent has entered the column, add the next solvent. The previous solvent does not have to vacate the column before the next solvent is started through the column.

Table 13. Solvent Volumes for Rinsing Columns

 
Column I.D. (mm) Solvent Volume (mL)
0.18-0.2 3-4
0.25 4-5
0.32 6-7
0.45 7-8
0.53 10-12
Using larger volumes will not damage the column.
 
 

After the last solvent has left the column, allow the pressurizing gas to flow through the column for 5-10 minutes. Install the column in the injector, and turn on the carrier gas. Allow the carrier gas to flow through the column for 5-10 minutes. Attach the column to the detector (or leave it unattached if preferred). Using a temperature program starting at 40-50 ? C, heat the column at 2-3 ? /min until the upper temperature limit of the column is reached. Maintain this temperature for 1-4 hours until the column is fully conditioned.

Column Storage
Capillary columns should be stored in their original box when removed from the GC. Place GC septa over the ends to prevent debris from entering the tubing. Upon reinstallation of the column, the column ends need to be trimmed by 2-4 cm to ensure that a small piece of septa is not lodged in the column.

If a column is left in a heated GC, there should always be carrier gas flow. The carrier gas flow can be turned off only if the oven, injector, detector and transfer lines are turned off (i.e., not heated). Without carrier gas flow, damage to the heated portion of the column occurs.
 
 
What is the best way to condition a capillary column?
 
You should condition a new capillary column at approximately twenty degrees higher than the final temperature of your oven program without exceeding the upper temperature limit of the column. If a temperature higher than the isothermal temperature limit of the column is needed for your analysis, recondition the column at that higher temperature, but, again, don't exceed the upper program limit.
When you install your column, purge it with at least three volumes of carrier gas prior to ramping it to the conditioning temperature. The total column conditioning time will depend on the type of application you're running and how much bleed is acceptable. The lower the detection limit that's needed, the longer the column will need to be conditioned. (Column bleed is closely related to the polarity and the film thickness of the stationary phase.) Polar and thick film columns bleed more and require more conditioning. For most applications, 30-60 minutes of conditioning is usually sufficient.
But how can you really determine when a column is sufficiently conditioned?
A flame ionization detector (FID) works best for monitoring the baseline during conditioning. Toward the end of the temperature ramp (i.e., 30-40?C below the isothermal upper temperature limit), the baseline will rise, then come down and level off, at which time you may consider the column conditioned. There are those that report detector fouling during conditioning when using other types of detectors (e.g., ECD , MS ), but it's generally considered a safe practice to condition the column while connected to these detectors.
Conditioning a column overnight is not recommended. Column life expectancy is greatly reduced when the column is stored at high temperatures. If you're experiencing an excessive amount of bleed for more than two hours, bring the oven down to room temperature and locate the source of the problem (usually oxygen entering the column from loose fittings or a leaky septum). Baseline signals that mimic column bleed can also originate from residues present in the GC itself.
Additional note: if the column has not been in use for a while, a mild conditioning step may be needed to drive off contamination which may have condensed inside the column during storage. Also, there is nothing to suggest a limit to the ramp rate of the oven when conditioning a column.
 
 
 
 
 
What are Duraguard columns?
 
DuraGuard Columns are GC Capillary Columns with Built-In Projection.
 
What type of the PLOT columns are available with Agilent Technologies?
 
How to condition Capillary or Packed GC Columns?
 
What are the outer diameters of common capillary columns?
 

The Outer Diameter mentioned bellow inludes thickness of polyimide coating of about 20 um
For 100 um id the od is 363 +/- 12 um
200 um id the od is 360 +/- 10 (old 335 +/- 10) um
250 um id the od is 360 +/- 10 (old 355 +/- 10)um
320 um id the od is 435 +/- 15 um
530 um id the od is 673 +/- 25 um
1/8" SS packed columns have an internal diameter 2.3 mm

 
How to recover the GC column catalog inventory while relaoding the ChemStation software?
 

When reinstalling the ChemStation software, it is often desirable to recover the 6890 GC column catalog inventory from the previous installation. The following is a procedure for performing this recovery.
The 6890 GC column catalog inventory is stored in four (4) files in the d:\HPCHEM\DRIVERS subdirectory.
To recover the column catalog inventory copy the following files from the previous installation to the new installation
d:\HPCHEM\DRIVERS subdirectory.
CATALOG.DB
CATALOG.PX
6890COL.DB
6890COL.PX
Where: d = drive designation where the ChemStation software is installed.

 
How to choose right column for my GC analysis?
 

The heart of Gas chromatography is the column. There are many varieties and deciding between packed and capillary, polar and non-polar, and other variables including film thickness and stationary phase, is often confusing. Agilents technical Support Team is always available to answer any question related to columns and most applications. Please refer the attached document for some of the details.

 
How to troubleshoot peak shape problems?
 
How does Agilent Technologies compare the performance of its columns relative to competitors, and what type of tests are carried out?
 
How can I increase the sample throughput and analysis speed using megabore columns?
 
What is the significance of N, the number of theoretical plates for a given column, and why is it important?
 
The theoretical plate count, N, is also called column efficiency, and can be calculated for a given column and analyte under a defined set of conditions. The higher N value a column has, the greater resolving power the column has. Theoretical plate count is one of the key measures that are monitored during column production as part of Agilent's rigorous quality control system. During use a column's efficiency tends to drop with age and number of samples analyzed, which can indicate that a column needs to be conditioned or replaced. Please refer to the attached document for more information about theoretical plate count and other performance measures.why is it important?
 
What are the Quality Control Specifications that are available for GC Capillary Columns?
 

Agilent Technologies has the tightest QC specs in the GC column business. Some of the QC criteria for GC capillary columns are:
Column efficiency (Plates per meter).
Retention Index window.
Column Bleed.
The column plate count efficiency, given by plates per meter, is a measure of the resolving power of the GC column. The higher the plates-per-meter value, the greater the column's resolving power.
The specifications for retention index involve narrow retention index windows. The smaller the retention index window for a given GC column product, the higher the level of column-to-column retention reproducibility.
Column bleed specifications represent the difference, in picoamperes of flame-ionization (FID) response, measured at the upper temperature limit of the column and the isothermal test temperature (generally 110°C to 125°C).Lower bleed performance results are desirable and preferred for a GC column.because they:
Afford better signal-to-noise ratios for improved detection.
Allow higher upper temperatures for shorter run times.
Require less detector maintenance.
Improve cleanliness of mass spectra.
Offer potential for longer column life.

 
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