Chromatography Forum

When doing normal liquid injections on the GC, anyone notice if gas-tight syringes give significantly better results than, let's say, microliter syringes...?

We usually have gas-tight syringes, but we were looking into buying a smaller volume (Hamilton 1.2 uL) syringe, which is not gas-tight (difference in the plunger design).



Thanks,

Roxanne.

I've only used gastight syringes for sample dilution and gas sample injection ( but I prefer to use a valve sample loop for gases ).

I would expect a gastight syringe to be inferior to a syringe designed for injection of most solutions over expected syringe lifetimes. I'd also expect a syringe from one of the guality manufacturers, eg SGE, Hamilton to be more durable and precise than some other brands.

My preference for manual liquid injection is a guided plunger-in-barrel model, and sometimes for small volumes, a needle-in-plunger model.

Maybe you should purchase a couple of syringes and perform a simple comparison, as I suspect operater filling and inejction techniques will have more significance on precision.

Please keep having fun,

Bruce Hamilton

You probably aren't going to be able to find a gas tight syringe (that is one with a PTFE plunger tip) smaller than 10 or maybe 5uL. The plungers become too small to keep a PTFE tip securely in place.

The main advantage of gas tight syringes when used with liquid samples is that they are less prone to plunger binding. This can be useful if you are using an autosampler and have samples that that leave a buildup on the inside of the glass syringe barrel.

Regards

Anthony

There are actually three issues here - does a teflon tip on the syringe plunger give better performance than an all metal plunger ?, the relationship between syringe volume and design, and the performance of syringes of different volume.

NB that not all gas tight syringes have the gas seal (only) at the plunger tip, some also have a seal around the plunger at the top of the barrel.

"Gas tight" and "microliter" are not exclusive categories of syringe; many gas tight syringes have volumes of a few microliters.

When both are new there is no difference between a teflon-tipped and an all metal plunger, but as the plungers wear with time the teflon will take up some of the slack while the metal will not, so teflon tipped plungers tend to last a bit longer.

In theory you might get some carryover by absorption into the teflon, but with reasonable syringe washing this should not be a problem.

Syringes smaller than 5 ul have the plunger in the needle, and the plunger is all metal. The repeatability of injection volume (as a percentage of the volume) is worse for plunger in needle than for plunger in barrel when the volume is a given fraction of the syringe capacity - if you inject 1 ul from a 5 ul plunger in barrel you get better repeatability than injecting 0.2 ul from a 1 ul plunger in needle.

With plunger in needle syringes doing hot vaporizing injections the plunger and the liquid film around it get hot during injection and this can cause eratic volumes of the film around theplunger (as opposed to the measured sample volume in front of the plunger) to boil out of the needle tip.

So if you see a difference between the 1.2 ul syringe and the "microliter" or gas tight syringes that you have been using it is most likely to be because it has the plunger in the needle, not because it is not "gas tight".

Peter

I remember that there are other issues between plunger in needle/plunger in barrel syringes.

1) Grob claimed that plunger in needle syringes were more subject to discrimination, as the sample tended to boil out of the needle rather than being physically expelled, which was more likely to happen in the plunger in barrel design. Discrimination means high b.p components might remain in the syringe Another point against plunger in needle design.

2) In plunger in barrel designs, in a totally cold injection, the syringe is designed to deliver the stated volume while leaving the needle full of liquid. In a hot vaporizing injector, this remaining sample in the needle partially or totally boils , increasing the volume injected-sometimes by a variable amount depending on how long the needle resides in the hot injector. This problem is mostly overcome by autosamplers, which either inject so quickly that the needle remains relatively cool, or inject so reproducibly that the extra amount introduced is reproducible.

Plunger in barrel syringes can improve their delivery variability by user techniques such as using a Chaney adapter and by using solvent plugs behind the sample.

For example, when injecting liquid samples I have been able to inject with RSD values of 1% when using a direct injection system. Splitters tend to increase RSD values by at least 1%.

Gas sampling valves also tend to have a variability of 1% or so unless very precise gas filling techniques are used.

best wishes,

Rod