Chromatography Forum

Greetings.

What is the effect of injection volume of a gas on retention time and detection limits using a GCMS?

Thanks.

Injecting (manually) much above 0.1 mL gas on typical capillary column causes significant peak spreading. With respect to increasing sensitivity it's diminishing returns at larger injection volumes. You can work out for yourself what length of your column would be occupied by 0.1 mL if it was evenly spread out. It could be more than 1 metre, although cold-trapping may minimise the true initial spread of analyte.

The injection time ("width" of injection) should ideally be no greater than 20% of typical peak width at half height. However fast you tried to inject a large gas volume eg. 1 mL this is not going to happen in reality. For a start half the gas sample might blow back out of the septum.

The peak spreading effect is improved, but not by much, if you use a powered gas sample valve in place of a syringe. 0.1 mL is still my maximum recommendation.

The effect of gas volume on retention time is hard to predict. It may be very small or it may be confounded by the analyte load on the column.

LCMS Student,

Assuming you are shooting the standard and the sample the same way, retention time should be un-affected by the injection volume.

If you are using an inlet (not a valve) the larger the volume injected the more you alter the pressure inside the inlet and the uglier the chromatography gets. I have seen vast improvement with a valve versus a syringe but we were shooting 2 mL's at a time. Plus you run the risk of exceeding the liner capacity and pushing sample into places it really should not go.

If you use packed columns or even micro-packed columns, you should be able to shoot higher volumes because the flows are higher and the sample gets swept out of the inlet much quicker. The only way to do that, though, based on you post is with a jet separator since you probably cannot support packed column flows on your MS.

If you are using a valve, then you must ensure that the valve is in the inject position long enough that the contents are swept at the flow rate going through the valve (split or splitless.) The peak width cannot be better than the time it takes to move the material out of the loop and on to the column.

Couple of things to consider. If you are running fixed gases, I would not suggest an MS since it is probably leaking things you are looking for. If you are running volatiles in a gas, you could think about cryo-focusing to retain sharp band of analyte while shooting more gas.

Best regards.