Chromatography Forum

Hi,

I just started a job at a beverage processor in their analytical lab and I'm being trained on their GC's. I used one maybe 10 years ago in school but not since - I might as well say I've never used one as I don't even remember it.

When I'm drawing my standards with the 100 microL syringe, I get a tiny bubble at the end of the plunger and am having a difficult time knocking it off. I'd imagine that this bubble is displacing part of my sample size if I can't remove it correct?

Any tips? What I was shown was to turn the sealed standard vial upside down and draw the sample upside down by holding the vial and pulling on the syringe plunger past the 100uL mark and then "flick off any bubbles". Maybe it just takes practice? Thx for any advice.

Are you sure you're using a 100µL syringe? Although it's by no means set in stone(I've used larger and smaller), in my experience 10µL to inject ~1µL is by far and away the most common syringe size for liquid injections into a GC. The bore of a 10µL syringe is fairly small, and bubbles near the plunger will not-in my experience-respond to inverting and flicking(usually 100µL is large enough that that will start to work).

What DOES work reliably for me is vigorously "pumping" the plunger in the sample a few times.

With that said, depending on your detector type and what you're trying to achieve, there's also a certain amount of validity to "sandwich" injections where the sample is a plug between two volumes of air. I find those especially useful when I'm using a TCD and want a measurable air peak(it can be useful for a couple of reasons, including monitoring flow rate drift and calculating the hold-up time) although of course if you're working in a regulated lab you may not be able to deviate from the injection technique in your method. At the same time, I will counter that with the same mantra that I repeat all the time to my students-the exact way you personally inject(barring some really big issues like slow injection speeds or completely pausing in the middle of an injection) is less important than doing it consistently and repeatably every time.

I inject 100uL standards into the vial which already has 10mL of the sample to be tested. The crimped shut vials are all loaded into the GC tray where they are auto tested. I don't have to inject anything directly into the GC itself.

I'm new to GC testing so I may have some terminology incorrect.

I'm wondering if that tiny bubble is critical. Thanks.

I have found when making standards that the bubble doesn't matter if it is always there and the same size. Some 100ul syringes have almost 15ul dead volume in the needle. If you draw a few ul of air, then draw up 30ul of sample, as some say to do, then you pull up air again you will find you actually have 45ul of standard instead of 30ul.

I start with the plunger completely down, draw up the amount needed, measuring by the base of the plunger not the amount of liquid I can see because you will have the dead volume of the needle filled, but the absolute volume will be what is measured at the base of the plunger. Then just inject that and you are getting the amount you measured, since what is in the needle stays in the needle.

It seems to work since I make up calibration curves this way and when I run a blind test sample for certification we are always within a few percent of the known value.

I have found when making standards that the bubble doesn't matter if it is always there and the same size. Some 100ul syringes have almost 15ul dead volume in the needle. If you draw a few ul of air, then draw up 30ul of sample, as some say to do, then you pull up air again you will find you actually have 45ul of standard instead of 30ul.

I start with the plunger completely down, draw up the amount needed, measuring by the base of the plunger not the amount of liquid I can see because you will have the dead volume of the needle filled, but the absolute volume will be what is measured at the base of the plunger. Then just inject that and you are getting the amount you measured, since what is in the needle stays in the needle.

It seems to work since I make up calibration curves this way and when I run a blind test sample for certification we are always within a few percent of the known value.

Thank you for that information!

Another trick is to pull a little air into the syringe while upside down. Push the needle into a septa and then push gently on the plunger. The bubble should move up .