Rtx015-16-628

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GC COLUMNS

Guard/Retention Gap Columns

Guard Columns and Retention Gaps

Guard columns and retention gaps are widely used in gas chromatography. The con-

cept of the guard column is to trap nonvolatile material at the head of the column, not

allowing the material to reach the analytical column. The concept of the retention gap

is to help focus the compounds transferred from the inlet to a small band at the head of

the analytical column in order to reduce chromatographic peak broadening. Both con-

cepts (trapping nonvolatile material and refocusing the target analytes) may take place

when a piece of deactivated tubing is connected to an analytical column as in Figure 1.

Analyte Focusing

There are two injection techniques where the retention gap is used to help focus target

analytes at the beginning of the analytical column: cool on-column injection and split-

less injection.

For cool on-column injection, the purpose of a retention gap is to help focus the

sample components when introducing a liquid sample directly into the retention gap.

The cool on-column injection is performed by inserting the syringe needle into the

retention gap (this can be accomplished with a 0.53 mm ID retention gap and a 26s

gauge syringe) and transferring the liquid sample directly into the retention gap. The

injection is made with the injector and column oven set below the boiling point of the

solvent. As the solvent is evaporated, the volatile target analytes migrate in the solvent

towards the analytical column, and the heavier analytes will be distributed over the re-

tention gap. As the oven temperature increases, the target analytes vaporize and move

unretained down the retention gap column until the compounds reach the liquid

stationary phase of the analytical column. At this juncture, the target analytes are

trapped/focused by the liquid phase forming a narrow injection band.

The retention gap may also be useful in hot vaporization injections when the transfer

of the compounds from the inlet to the column does not form a focused band. Typical

applications include water injections or injections using small ID columns, where split

or tailing peaks would indicate an unfocused band. In these applications, the target

analytes are trapped in a nonuniform or longitudinally diffuse band at the head of the

retention gap (Figure 2a). As the oven temperature is increased, the solvent and target

compounds are vaporized and move unretained through the retention gap (Figure

2b). When the target compounds come in contact with the stationary phase, they are

refocused in a narrow band (Figure 2c), improving the chromatography.

Figure 1:

A guard/retention gap column connected to an analytical column

Injector

Guard/Retention Gap

Column

Detector

Analytical Column

Press-Tight

Connector

did you know?

We test our guard columns/transfer lines

with a comprehensive test mix to ensure

high inertness.

please note

Having trouble making a leak-free

connection? Try our “built in”

Integra-Guard® columns!

See

page 23

for details.

Figure 2:

Retention gaps are

used to focus components in a

tight band at the beginning of

the analytical column.

Sample introduction: a liquid film of

solvent and sample is deposited in the

first length of capillary.

As oven temperature increases, the

solvent evaporates and the target

compounds elute unretained through

the retention gap until they contact the

analytical column.

When target compounds come in

contact with the stationary phase, they

are refocused on the analytical column,

resulting in a narrow initial band width.