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.
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.