Chromatography Forum

Hello,
I wonder if somebody can tell me, if it is better to use a injection time (into GC from a headspace sampler) higher than 1 minute to increase sensitivity? Does it matter? My results are inconsistent.
Furthermore, I want to know, if the volume of an GC-liner has effect on sensitivity. We used 140µl and 250µl volume liners and observed varying peak areas, although the provider said there can be no variation in peak area due to varying liner volume.

Thanks for comments.

bye, propanal

Propanal,

Do you use spliting of the gas flow flowing into the GC from the HS sampler?
If you do use split ratio, you could reduce it instead of prolonging the inject time from the HS.

Best regards

Hello zokitano,

our split ratio is 5.3/1.

propanal

Ok,

Instead of increasing the original (prescribed in your method) inject time could you try to use smaller split ratio (with original inject time prescribed) like 2:1 or 1:1 and find the appropriate for the desired sensitivity you want to get?

I assume that you're using Perkin Elmer GC...

Hope this helps

hello,

no, we use agilent headspace sampler as well as GC (6890). we were told not to run headspace analysis splitless. the minimum split is 4:1.
do you know, if injection time of 1 or 2 minutes is [b]indeed[/b] responsible for sensitivity?

Depending on whether or not you are running a temperature programme on the GC, increasing the injection time will just make the peaks wider (which compromises resolution) rather than higher (which increases sensitivity). Presuming that you have the 1 ml default loop in your headspacer, at a split of about 5 to 1 and a 320 micron column the loop is swept in 10 s anyway, so increasing injection time will not make the peaks any bigger.

A change in liner volume could influence peak size and repeatability by influencing the mixing of the sample plug from the headspacer with the carrier gas. Also, as the 6-port valve switches there is a pressure and flow pulse in the inlet that causes eratic split ratios.

You will be very lucky to get better than 2 % rsds in peak area from any commercial headspacer.

To increase the size of peaks the easiest way is to increase the sample temperature, presuming that the samples are stable to heat.

Peter

A comment on the effect of liner volume on peak area/sensitivity:

I was told smaller liners give sharper peaks (better sensitivity) although I haven't seen significant difference between a 4 mm liner and a 2 mm liner. But in theory this is true because the dead volume is smaller for a lower volume liner.

Again, in theory liner volume has no effect on peak area. As long as split ratio is kept the same(same colume flow and same split flow), the amount of compounds going into the column is the same, which means that peak area will not change. What the provider said was true. The variation you saw was caused by something else.

Propanal,

I would add several comments. First, the 1 mL loop comment made by Peter is really dead on if you are running split. Even assuming a 0.25 column at 1 mL/min with 5:1 split, the loop is completely swept in 10 seconds so the remaining 50 is essentially dead time.

If you are trying to improve sensitivity, I would offer several suggestions. 1) Move to a single gooseneck liner with restriction down. Put the column right at the beginning of the taper. Or, move to a really narrow bore liner (which would not be my first choice but since you are running gas phase and split it should work. 2) Use # 1 and move to a bigger column if you are not using an MS.

When you talk about inconsistent results, are you talking about your results with changing liners or are you talking about 2, 3, or 4 runs with the same setup?

Best regards.

Short injection times are usually better than long times, simply because once the loop and transfer line are swept there is no reason to have the additional volume in line. The pressure changes when the 6 port valve switches can affect peak shape if the switch comes while a peak is eluting.

For the reproducibility issues, are you sure the samples are actually coming to equilibrium? An easy way to check this is to extend the equilibration time for 5 minutes and see if response goes up and precision improves. Consistent vial sealing is also important for good precision. Saturating the solution with a salt can also improve recovery and precision. Finally, the addition of an appropriate internal standard can significantly improve precision.