Severe syringe degradation during GC analysis

[mikael.lichtert]

Hello,

I'll give a bit of context to answer any potential questions right away.

Our production facility uses commercially bought denatured Ethanol in it's manufacturing process. The Quality Control lab where I work is charged with analyzing the purity of this raw material before use in the process that will, eventually, lead to much needed medicines.

To this end, we use an in-house developed method and a 7820A GC-FID system.
For the last 8 years, no problems were reported, other than the occasional failed run. Failures that were easily remedied by maintenance (changing the liner and/or septum, cleaning the column regularly, preventive 6-monthly maintenance by the supplier).
Our analyte is pure ethanol (around 93%) denatured with Methyl-Ethyl-Ketone (around 2%). Injection by the autosampler was 2µl, on a 10µl Gold Standard syringe. Carrier gas is H2.
The way our internal System Suitability Test is set up, the calibration standard is injected 4 times : 1 time as a "dummy run", 3 times as "actual" runs. The ratio of Analyte/internal standard is calculated; if the RSD is greater than 1%, the run is failed.

Around November 2024, we started getting poor repeatability on the three calibration standard injections. Numerous runs were suddenly "invalid" due to a high RSD. It started relatively benign (around 1.6% RSD) but got to 4.5%.
The invalids popped up across the lab : all the technicians that have performed this analysis for at least 3 years without problems suddenly encountered this drop in repeatability.
Every part that could be changed on this machine was changed (by qualified technicians/engineers from the supplier).
The method did not change.
The technicians follow the Standard Operating Procedure scrupulously.
Still repeatability problems.
It got to the point where we would install a new needle (pre-cleaned with acetone before use), get one or two valid results out of it, and the repeatability of the injections would drop again.

We installed the (slightly adapted) method to a brand new GC (8860 GC-system). First testing and validation of the method went well, but now we use it in routine, we encounter the same problems (RSD of 11.5% sometimes).

Interestingly, as soon as we switch the syringe for a new one, it works, but again, for one or two runs (which typically consist of 7 injections each). When inspecting a used syringe, it is as if the syringe does not aspirate the fluid it is supposed to inject. The latest syringe (still a Gold Standard 10µl syringe) worked perfectly well on Monday 03 MAR 2025, faulted Wednesday 05 MAR 2025.
I tried a manual cleaning of the syringe, and noticed that when I carefully move the piston, liquid does get into the syringe but a multitude of small bubbles are formed. The same result happens at different speeds of moving the piston.

Other, older, syringes, seem to have lost the capacity for aspiration altogether : nothing gets aspirated in the graduated part.

It is clear our problem with repeatability is due to a faulty and unequal injection of the mixture, so logically the fault lies with the syringe. But since we change it regularly (twice a week!) and the problem is recurring, there must be something else going on.

Did anybody encounter similar problems? We (= my colleagues and myself) do not know what the root cause could be. The technicians/engineers of the supplier are equally stumped. Routine analysis has become a headache and if frustration was marketable, I would be a millionaire by now...

Any ideas? Tips? Pointers?

Thank you for reading this long winded post

[fki]

Do you use a "normal" syringe with only a metal plunger or a kind of Teflon tipped plunger? Maybe that there is a difference.

Have you tried to use syringes from another supplier like e. g. SGE? Do you then face the same problems?
I don't know if the kind of syringe is part of your validated process and if you're able to change it.

Do you run other methods with the same GC and the same syringe? Do they run within the validated limits?

[Peter Apps]

Try a syringe finr one of the independent suppliers; Restak, Supelco etc.

[70 eV]

I agree with fki, if you're using a gas tight try a metal plunger or vice versa and double check that the flat part of the syringe body at the top is flush with the back of the ALS.

[Consumer Products Guy]

1. I may have passed over this, but I didn't see what the solvent was that was used for diluting your sample.

2. Are you using a rinse/wash step in-between samples? If so, what solvent.

3. I'm retired, and have never used a GC newer than 6890 units. I seem to remember that one could slow down the speed of the plunger while withdrawing a sample.

4. We assayed for ethanol in finished consumer products such as hand sanitizers (which are about 65% ethanol v/v) by GC routinely without issues.

[MichaelVW]

I've had trouble with repeated injections on one instrument. The fan that cools the injector tower was not working, plus some of the insulation around the S/SL inlet was cracked. The sample turret and rinse vials was getting quite hot, evaporating rinses prematurely, and also keeping the syringe hot. Sample would literally drip out, which I only noticed by using a black light flashlight and watching closely (I was running 8270 so my internal standard was highly fluorescent).

But since you've got the problem on two different instruments that wouldn't seem to be the case.

Can you use more sample pumps?

Are the syringes getting clogged with septa from the vial caps? You could try a new syringe and prepare a standard with low volatility (alcohol in water for example) and do a couple dozen runs overnight with the septa punched out of the caps. Yes, with no cap the RSD should be bad... if it gets *better* than what you've been seeing, that might be your answer.

Has water content in samples changed enough to cause a different expansion ratio when you inject?

Also... what if the syringe isn't the problem? You said the ratio of internal standard to sample is what is varying between runs. So even if a slightly different amount is being injected each time, that shouldn't matter too much.

What internal standard are you using?

Compare raw counts across runs for both the analytes and the IS. See whether the RSD of one is low and the RSD of the other is high, or if counts of either one trend in a particular direction.

Have you compared full method files to make sure nothing accidentally changed? It happens.

[mikael.lichtert]

Do you use a "normal" syringe with only a metal plunger or a kind of Teflon tipped plunger? Maybe that there is a difference.

Have you tried to use syringes from another supplier like e. g. SGE? Do you then face the same problems?
I don't know if the kind of syringe is part of your validated process and if you're able to change it.

Do you run other methods with the same GC and the same syringe? Do they run within the validated limits?

Hello,

Yes, it is a normal, standard syringe of 10µl, a fixed needle, with a conical needle point style. The plunger tip material is PTFE.

So far we haven't tried needles from different suppliers, it is not allowed by our SOP. The investigation is handled by another team that does not really share information...but doesn't know WHAT the problem is either.

We run another method on the GC, but that test has different System Suitability Testing (if the peak's Area Under the curve is below a certain calculated level, the test is invalid). It uses the same syringe. This test is a bit more robust, as there are occasionally some problems (AUC is too low due to a faulty injection) but on the whole it performs well.

[mikael.lichtert]

Try a syringe finr one of the independent suppliers; Restak, Supelco etc.

Thank you, I will suggest that to the investigative team.

[mikael.lichtert]

I agree with fki, if you're using a gas tight try a metal plunger or vice versa and double check that the flat part of the syringe body at the top is flush with the back of the ALS.

Thank you for your suggestions.

We noticed at some point that the flat part of the syringe was indeed not flush with the back of the ALS. The needle was turning on its axis while the injector was working. Cause was that the brackets were a bit too bent downwards.

When we corrected the situation, the needle was perfectly fixed in its housing but that didn't solve the issue.

[Jake]

To add, we run 1000s of samples a week by GC/FID and have found over the years since COVID, in general, the quality and reliability of consumables from all vendors has really gone down (yes, we consider syringes a consumable). We get clearly bad batches of liner seals, septa, vial caps, etc. It could be, as Michael suggests, you're have an issue with coring of either low quality septa or vial caps.

We were also experiencing poor injection precision (with PAL-2's), and for one issue, as others suggested, we were able to resolve by doing 4 sample fill/purges before injecting the sample. Seems excessive, but it was really worked well for us. We now do this as standard injection on all nine of our GCs.

[mchem]

Our lab has been seeing the same thing with the 10uL gas tight Agilent gold syringes that match up with your timeline. Used to get 4 ish months of use and now it’s barely 1 month. It’s the PTFE plunger tip that is shrinking, eventually it shrinks so much and it won’t pull up any liquid at all. But I’ve also seen it twice where right before it fails, it seems to be drawing up too much liquid and the responses are triple than what is normal - which causes carry over because it’s overloading the liner capacity.

Sometimes (not often) removing the plunger and tapping the tip on a hard surface can get it to form a tighter seal. It’s gotten an extra week or two out of a syringe, but it’ll shrink again and lose suction. We’ve also been using replacement plungers to reduce costs. For identification purposes this works, but if you’re quanting and require low RSD’s I can see it being an issue. A tech recommended to try tapping it even before the syringe’s first use, but I haven’t seen much success, still seems to fail a month later. I was going to try heating it slightly to regain the seal, but I imagine once it cools the PTFE may just shrink excessively again.

I haven’t seen this issue with either the 5 uL metal syringes or the 50 uL gas tight syringes (yet), seems to be just the 10’s and only recently. Have you had any success yet with fixing it or finding an alternative vendor?

[CharapitsaS]

Hello,

from your letter it follows that "Our analyte is pure ethanol (around 93%) denatured with Methyl-Ethyl-Ketone (around 2%). Injection by the autosampler was 2µl, on a 10µl Gold Standard syringe. Carrier gas is H2".
Perhaps the following modernized internal standard method will be useful for your lab, very close to the problem you are solving

ILIADe 453:2024 | CLEN Method

Determination of Isopropyl Alcohol and Methyl Ethyl Ketone in Alcoholic Products by GC-FID

Version 13 February 2024

https://taxation-customs.ec.europa.eu/s ... method.pdf

In the section "1. Scope" it is indicated that
“This method is suitable for the determination of isopropyl alcohol (IPA) and methyl ethyl ketone (MEK) in denatured alcohol and alcohol containing solutions or drinks with analyte content ranging from 0.1 to 5 L per hL absolute ethanol using gas chromatography-flame ionization detection.
The same analytical procedure can be used for other formulations of volatile denaturants, i.e. methanol, acetone, tert-butyl alcohol, ethyl acetate, methyl isopropyl ketone, methyl isobutyl ketone, toluene or ethyl sec-amyl ketone."

In the section «2. Principle» it is indicated, that
"The concentration of the denaturants is determined by capillary gas chromatography with FID detection. Ethyl alcohol itself is used as internal standard and all data for the concentration of denaturants are calculated in relation to the content of ethanol. There is no need for any further internal standard compound.”

49 laboratories (on 4 samples) took part in the Interlaboratory Study (ILS) of the method in 2021.

For Completely Denatured Alcohol (CDA) with a MEK (robust mean) content of 0.97-1.00 L/hL EtOH, according to the results of the ILS, the values of Repeatability 0.01 L/hL EtOH and Reproducibility 0.07 L/hL EtOH were obtained.

I hope the proposed method will be useful for your laboratory.

Best regards,
Sairhei Charapitsa

[tlahren]

Long time no post for me...

I agree with many of the posts relating to the PTFE plunger being the issue. In my experience, I only use PTFE or non-metal plungers when doing gas phase injections (eg., headspace). If you are doing liquid injections of your ethanol samples I would use/advocate standard liquid sampler syringes; The ones with simple metal plunger. Also, use adequate rinses before and after samples. If there is an issue switching to a different syringe due to SOP variance etc., please advocate to modify the SOP. SOPs can be changed in nearly every industry with appropriate documentation. I have had PTFE plungers and the glue around the needle (glue holding the needle into the glass body) by using chlorinated solvents. It takes some time but it usually results in what you described: bubbles pulling up instead of just liquid.

As someone else stated, I too have seen lower quality in consumables lately.

I know I'm late to the response, but I just got back into the forum today and I figured I'd try to help if you haven't solved the issue yet.