Are there 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.

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

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".

 
 
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