Injection volume vs split ratio

[ricard]

Hello,

I want to reduce the contamination in my GC/MS system. Then, i will simply reduce my injection volume and proportionaly reduce the split ratio. Ex: inject 0,5 ul instead of 1 ul but reducing split ratio from 1:50 to:1:25. Is it exactly the same or will be perfomance ( chromatographic quality, sensitivity, etc) affected any way?

Thanks in advance,

Ricard

[Peter Apps]

No, it will not be exactly the same, but unless you have some really critical separations or unusually tight expectations for repeatability I doubt that the difference will actually make any difference - there might even be an improvement in some parameters.

For such a simple change why not just try it and see what happens ? Don't think, measure !

Peter

[Don_Hilton]

If the issue is to keep junk from slowly fouling the GC column or mass spec source, a change of inlet liner may help.

If you are not using a liner with a deactivated wool plug, it may be worth a try. This can help by catching nonvolatiles as condensate on the wool, keeping them out of the GC colulmn. If you keep the inlet temperature of the inlet blow the maximum temperature used in the GC run, you are less likely to trap high boiling gunk from the inlet at the head of your column.

This does require sufficiently frequent changes of the liner to prevend the accumulated gunk from affecting your sensitivity and the chromatography.

[AICMM]

ricard,

As Peter said, no they won't be the same. People who think they are doing dilutions with split ratio's are sadly mistaken.

Best regards,

AICMM

[ricard]

Hi again,

well, as it's said, when one reduce the injection volume and proportionally the split ratio, the effective difference is a minimum (some minimum issue to account for?). Logically, it's impossible to dilute in this way.

By the other hand, if you hold the injection volume constant and change the split ratio, which are the fundamental problems to consider this a proper dilution (if one has calibrated with a constant split ratio as a reference factor of dilution).

Thanks everybody.

[Peter Apps]

By coincidence I have just compared a 10:1 split injection of a 10 ng/ul solution with a splitless injection of a 1 ng/ul solution under two sets of conditions. With classical hot vaporizing split injections the split injection peaks are nearly half the predicted size. With PTV from just below the solvent boiling point the split peak sizes are close to predicted. I put the discrepancy down to the pressure pulse from flash vaporization causing a temporary disruption of flows and split ratio.

The pressure pulse and its impact on split ratio will be different for different injection volumes. If you want accurate, robust results you have to run all your samples and standards under the same conditions.

Peter

[mckrause]

Your initial statement was that you wanted to reduce the contamination in your GC-MS system. Please note that changing the injection volume but keeping the split ratio constant results in carrying the equivalent mass to the MS.

Peter's assertion regarding calibration is correct. Unless you can verify laminar flow in the injector, you will not be able to extrapolate split ratio/area responses.

[ricard]

Hi mckrause,

I don't understand the fitness of your assertion "Please note that changing the injection volume but keeping the split ratio constant results in carrying the equivalent mass to the MS" since, as i understand it, the mass transferred to MS doing that is effectivily less if the injection volume is less. The other way around.

Otherwise, if you reduce proportionaly the injection volume with the split ratio, the mass transferred to MS (column and detector) is the same, but you avoid some contamination in the injection (liner, injection chamber, syringe) port. Any way, as you has inform to me, any change in split ratio or volume injection need a new calibration with the same conditions (standards,samples,)

Ricard

[carls]

If you get the desired signal (S/N at the low end of your calibration) when you reduce the injection volume and adjust the split flow then you will reduce the rate of inlet fouling since you are injecting half the matrix into the inlet. Just run the samples and standards with the same conditions.