Chromatography Forum

I have never used a Combi-PAL but have one that has Liquid and headspace injection options on a 7890 witha an FID... and I have some questions.

The standard Comb-PAL liquid syringe is 10 uL.
In the manual it says that syringe should be used for 1-10 uL... But the overview card lists sample volumes for that syringe down to 0.2uL... Can you get good reproducible injections in the 0.4-0.5uL range with that syringe?

I see there are 5 and 1.2uL syringes available...If I can;t use the 10 uL would the 5 uL be OK or would I need the 1.2 uL?

Does each size need it's own syringe holder plate? If so changing syringe sizes (at least the 1st time) gets more expensive than then just buying another syringe!

Now headspace. I have only done sample loop headspace with the Agilent sampler before not a syringe based unit. On the Agilent HS sampler I use the deactivated 2mm ID straight through liner. It occurs to me as I am injecting gas (a mL on the Agilent HS) into the inlet that a liner with a larger internal volume might be better (I know injection rate matters - some analytes are likely to be migrating at the staring temp so I don't want to have to inject too slowly).

Is that true? Any specific liner recommendations?


Thanks,
- Karen

Hi Karen

I would be wary of trying volumes less than 1 ul with a 10 ul syringe. That said, the main source of variability in how much sample leaves the syringe is not the mechanical movement of the plunger but evaporation and boiling of the solvent in the needle as it goes into the hot inlet - so your pre- and post injection dwells are as important as the syringe volume. For 0.5 ul (and 1 ul) I would use a 5 ul. I believe that 1.2 ul is a plunger in needle design - these have their own problems, samples have to be very clean or the plunger jams in the needle. Below 0.5 ul diluting the sample is well worth considering. Off the top of my head I do not know whether you need a different syringe holder for each.

Since the column flow rate is pretty well fixed the width of the starting band after injection depends only on the injection volume - having a larger liner does not change that. While the larger inlet volume will damp pressure fluctuations a little bit you still need to tailor your injection speed to no more than double the total inlet flow - so to get fast injections you either have to inject smaller volumes or use split injections. A long as the liner does not have glass wool or anything similar in it the geometry makes surprisingly little difference - I have seen indications that baffles and laminar flow cups helped mix the sample gas with the carrier gas but the effect was really small compared with all the other sources of variation.

Peter

Hi Karen,

I have done several tests runs and compared the 10uL, 5uL and 1.2 uL against each other in volumes of 0.1 to 1uL. My finding was that the 10 uL syringe gave the most reproduceable results down to 0.5uL. Below that amount it was difficult to pinpoint a clear winner, but the 5uL most often was closer to the real number and RSD overall. Overall the 1.2 uL -- a needle in needle syringe -- was giving the worst results. But I have to admit, that I used the standard parameters suggested by CTC, which might not be the best parameters with this somewhat special syringe. I also like to point out that carryover was a huge problem with the 1.2uL.

Talking to syringe manufacturers about the result, they suggested that the thickness of the hole in the glass is best for certain flow characteristics, but that is a little beyond my knowledge.

If you want to use the 5uL syringe with the PAL, it might be easiest to turn off the automated syringe detect (through the service menu), upload the 5uL syringe definition and then tell the PAL which syringe size you currently use. The 10uL holder will accept any syringes from 1.2 to 100uL, but the syringe definition (for automated detection) comes with the holder. There are holders for most sizes available if you require it for compliance.

On your headspace injection, I would recommend our getting started guide for headspace (www.autosamplerguys.com -- right on the bottom). Basically Peter is right, try to inject as fast or slightly faster than column flow.

Best Regards,
Ingo Christ
Autosampler Guys LLC
+1-703-780-1500