MS TIP No. 14 - Reduction of Peak Tailing

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Affiliation: Scientific Glass Engineering

The occurrence of peak tailing and poor reproducibility can occur with standard syringe injection of liquid samples into the GC injection port.

This is a common problem which is consistent with slow volatilization of the sample and/or slow transfer of all of the sample onto the column. It occurs with microvolume syringes when the minute sample being delivered from the syringe forms droplets which wet the syringe needle rather than forming a jet of nebulised liquid from the needle tip. As the needle is rapidly withdrawn from the injection port, it is wiped by the septa causing slow secondary vaporisation near the cool septum cap. Not only poor peak shape is observed but sample size reproducibility will be poor.

Position of quartz wool in the injection liner

Resolution of the problem is achieved by effectively wiping the sample from the needle tip during the injection. High speed autosamplers commonly use quartz wool as the liner packing. The position of the quartz wool should be adjusted to achieve effective wiping of the needle.

Theoretically the quartz wool should be placed in the hottest part of the injector, normally the center (Figure #1). In this position, the quartz wool offers no wiping of the needle tip. Figure #1a shows the resultant chromatogram of a 0.3µl injection from a 0.5BR-HP-0.63 syringe into such an injection port. Unacceptable solvent peak tailing and a poor reproducibility (see Table I) results.

0.2µl injection of 0.02% C10 in Heptane chromatographed on a 25QC3/BP1-0.5 column.

Injector 200 degrees C and oven 120 degrees C.

TABLE I - Reproducibility of 15 injections @0.3 µl

Centrally packed liner Liner packed to wipe syringe needle

15 Injections C10 @0.3µl 15 Injections C10 @0.3µl

RSD = 12.73% RSD = 0.57%

By adjusting the position of the quartz wool so that the needle penetrates approximately 1/3rd the way into the quartz wool on injection (Figure #2), the resultant chromatograms show excellent peak shape (Figure #2a) and reproducibility (see Table I). Linearity of dispensed volume is also improved (Figure #2b). The adjustment of the packing position does not alter the boiling point discrimination characteristics of the injection system.

Recommendation

Adjusting the position of the injection point packing so that the needle tip is wiped during injection, improves peak shape, reproducibility and linearity without adversely affecting boiling point discrimination.

Additional Hint:

Do not over tighten the septum cap when using a high speed autosampler. Deformation of the septum may cause leakage during the rapid injection stroke.