Chromatography Forum

I am elicting experience and opinions concerning maximal injection volumes for gaseous samples.

I am analyzing air samples containing vapor phase hydrocarbons on a 0.53um capillary column, direct on-column injection. I use a gastight syringe for sample injection

First, is there a limit based on the column flow rate? How could I estimate this?

Second, could I experience compression/condensation issues, especially for the higher boiling components?

Evan Cooper

<not a GC expert but... as long as you aren't 1) injecting gases that expand a lot 2) injecting too quickly (at most, stay a bit under the system's flow rate) - and you're cold trapping the stuff of interest on a retention gap (or on the column) - I would guess that the limit would be based on the size of your syringe. I could see where reproducibility could be an issue if you are trying to inject really huge volumes... an autosampler or programmable syringe pump will be necessary to prevent numerous problems.

Ravenwork,

You need to watch the amount of gas injected based on the flow rate and the head pressure involved. Both parameters are critical. Also important is the temperature of the column and the phase thickness of the capillary. These are related to the sample plug focusing alluded to by DR.

First, assuming the vapor phase hydrocarbon may be in the worse case, methane, let us assume that almost no focusing will be experienced in your sample injection. thus the peak width of methane will be the time in which you inject the sample if you do not exceed the volume per sec of the carrier flow. Thus if you inject 50 µL and the carrier flow rate is 50 µL/sec the minimum peak width initially is one second. IF it takes you 2 seconds to inject the 50 µL the peak width minimum will be 2 seconds.

If you inject 100µL in one sec the best possible minimum peak width is still 2 seconds.

This is assuming your injector temperature is the same temperature of your gas sample. If your temperature is not the same then you have a volume increase of the sample due to an increase in temperature, as DR warned quite correctly in his post.

Thicker films will tend to focus this hydrocarbon plug but you will find that the methane through butane peaks will not focus much especially if you are using low headpressure or a higher than ambient oven temperature.

This is one case where a packed column with a heavy loading of methylsilicome on Chrom P or G will work better than a capillary column. Why? It is due to the higher head pressure and usable flow rates that are several times higher than a capillary will allow. These permit much higher sample injection volumes before the sample plug 'smears'.

You will also find that you might get better results with a porous polymer packed column starting at a low temperature and ramping the oven to over 200°C.

Another alternative is to place a restrictor at the end of the 0.53mm capillary to increase the head pressure needed to attain the required flow rate.

The answer to your second question is YES.

best wishes,

Rod