0.2 microlitre injection volume for GC - too small?

Discussions about GC and other "gas phase" separation techniques.

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Hi All, I'm having trouble obtaining good RSD's with a 0.2microlitre injection using a 1microlitre syringe.

I'm using GC7890B with 7693 autosampler and Agilent syringe 5188-5246. That’s a 1microlitre syringe gauge 23.

Fairly standard conditions and I'm confident there are no chemistry issues.

My fundamental question is should it be possible to achieve good RSD’s with this small injection from the one microlitre syringe or is this outside the performance of this syringe/below the minimum injection volume for this syringe. I believe it should be possible to inject down to 10% of a syringe’s volume, e.g 1microlitre from a 10microlitre syringe, 0.5microlitre from a 5microlitre syringe and so on. Does this rule not hold for the one microliter syringe?

Am I trying to achieve the impossible with a 0.2microlitre injection from a 1microlitre syringe??

Any advice you can give would be greatly appreciated.

Kind Regards
Steve
I would say no, it's not impossible.

In the past, I've gone as small as .5µL with a standard 10µL syringe in an Agilent autosampler and seen under 5% RSD.

Just today, we were doing a teaching lab where students were making .4µL injections from a 10µL syringe by hand, and they had to keep at it until they could get 10%.

One issue I have run into with newer Agilent GCs is that there's a charcoal filter on the split vent, and if it starts getting saturated it can mess with the split ratios and give poor repeatability on any injection volume. It's not really something I run into terribly often as a lot of my injections are splitless, but I have to remember to do a few split injections every month or two to check for repeatability and make sure it's not causing problems.
Since that size syringe normally has the plunger contained within the needle and the entire volume is contained in the needle also, the accuracy could be affected some by the type of injection being performed.

Are you doing the fast injection method or a slower injection method?

If slower, then you are heating the solvent and evaporating it before the plunger dispenses it, which is worse the lower the boiling point of the solvent is.
The past is there to guide us into the future, not to dwell in.
For a volume that small, I initially wonder if you should have glass wool high in the liner to "wipe" the needle. With the volume being positively forced out of the needle as James implies, it seems to me that it is not necessary for wiping; but maybe necessary to make volatilization more uniformly repeatable.
benhutcherson wrote:
I would say no, it's not impossible.

In the past, I've gone as small as .5µL with a standard 10µL syringe in an Agilent autosampler and seen under 5% RSD.

Just today, we were doing a teaching lab where students were making .4µL injections from a 10µL syringe by hand, and they had to keep at it until they could get 10%.

One issue I have run into with newer Agilent GCs is that there's a charcoal filter on the split vent, and if it starts getting saturated it can mess with the split ratios and give poor repeatability on any injection volume. It's not really something I run into terribly often as a lot of my injections are splitless, but I have to remember to do a few split injections every month or two to check for repeatability and make sure it's not causing problems.


That's a tough class! :)
I've recently cleaned out the split vent lines with solvent and changed the filter but that didn't solve the issue.

Do you think in general one microlitre syringes are available & sold in order to allow small injection volumes or just to improve repeatability of a 1 microlitre injection?

Are you aware of any/many methods which use a small injection volume? I've seen a few in literature but most use a 1 microlitre injection volume, so I'm concerned I'm trying something which is just not realistic. I'm also using a high-ish split ratio (67:1). The whole aim of this is to remove the final dilution step in the lab but now I'm starting to think it's not worth it!

Thanks for your help so far
LALman wrote:
For a volume that small, I initially wonder if you should have glass wool high in the liner to "wipe" the needle. With the volume being positively forced out of the needle as James implies, it seems to me that it is not necessary for wiping; but maybe necessary to make volatilization more uniformly repeatable.


I'm using the focus liner from agilent which is deactivated and has two wads of wool.
James_Ball wrote:
Since that size syringe normally has the plunger contained within the needle and the entire volume is contained in the needle also, the accuracy could be affected some by the type of injection being performed.

Are you doing the fast injection method or a slower injection method?

If slower, then you are heating the solvent and evaporating it before the plunger dispenses it, which is worse the lower the boiling point of the solvent is.


I'm doing fast injection and have experimented with different speeds, pre & post dwell times, air gaps but none have had an effect. I'm also using the Agilent focus liner which is deactivated and has two 'chambers' with glass wool in.
Thanks for your help so far.
We use either a 0.5ul or 0.2ul (depending on the capability of the autosampler) when doing aqueous injections for organic acids. The less water you inject the better. For organic solvents though we inject at least 1ul.
The past is there to guide us into the future, not to dwell in.
James_Ball wrote:
We use either a 0.5ul or 0.2ul (depending on the capability of the autosampler) when doing aqueous injections for organic acids. The less water you inject the better. For organic solvents though we inject at least 1ul.


Hi, I'm using hexane as solvent, in your experience do you think that is okay as a 0.2microlitre injection with autosampler?
saw5813 wrote:
James_Ball wrote:
We use either a 0.5ul or 0.2ul (depending on the capability of the autosampler) when doing aqueous injections for organic acids. The less water you inject the better. For organic solvents though we inject at least 1ul.


Hi, I'm using hexane as solvent, in your experience do you think that is okay as a 0.2microlitre injection with autosampler?


I think the boiling point should be high enough with hexane, but you have to remember you only have 0.2ul in the tip of the needle, and it has to remain there from the time it removes the needle from the vial until it depresses the plunger inside the inlet. Any evaporation of the solvent before the plunger is depressed will cause variations in the amount delivered and how narrow the band of solvent delivered to the column will be.

Unless you have very limited sample volume, you will see better reproducibility using 1ul injections with a higher split ratio. You can set the split ratio to 300:1 during injection then drop back to lower flow rate a minute later and have approximately the same amount of sample delivered to the column with a 1ul injection(170:1 would work the same for a 0.5ul injection, which is probably the maximum you can inject from a 1ul syringe).
The past is there to guide us into the future, not to dwell in.
saw5813,

What kind of inlet are you using? If SSL why are you not using a higher split ratio instead?

Have you tried a different line, say one with much less glass wool or no glass wool?

Best regards,
AICMM wrote:
saw5813,

What kind of inlet are you using? If SSL why are you not using a higher split ratio instead?

Have you tried a different line, say one with much less glass wool or no glass wool?

Best regards,


Sorry for the later reply. I have solved this problem now.

In case it is of use to anyone reading this thread in the future;

The inlet is SSL used in split mode. I'm currently using 67:1 split ratio (2.7ml constant flow + 3ml purge flow)

My analyte has over 50 components of wide boiling point range so inlet discrimination is a real problem. This is why I cannot simply use a bigger split ratio.

Also my hydrogen generator has 450mlmin capacity so running this method dual channel is near the limit until the gas saver comes in.

The whole reason to use a smaller injection volume was to eliminate a dilution step in the lab to save time and solvent.

The poor repeatability in the injection using the one micolitre syringe was due to the syringe type; the one microlitre syringe is plunger in needle and even when using the packed, deactivated Agilent Focus liner the sample was sitting on the end of the needle rather than being expelled from it like the plunger in barrel type syringe would do. I found using a long (30s) post injection hold time with the one microlitre syringe gave a repeatible injection but some discrimination was still evident due to the high total flow rate. I have now moved to a 5microlitre plunger in barrel syringe but have fould this gives good enough repeatablity even with a 0.2ul injection volume with no need for any post injection hold time.

Hope this makes some sense for anyone following the thread in the future
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