HS-GC RSD problem with residual solvents

[tinsang]

Hello guy’s.
I‘ doing HS- GC method development of residual solvents, ethylmethylketon(MEK) and 1- propanol.
The RSDs for peak area of the two analytes from 5 injections of the standard solution are not bad (about 2%). But the customer will have to raise the sample volume (1000 µl instead 100 µl). Unfortunately, the RSD by 1000µl was terrible (8%). Should I raise the agitator temperature or extend the incubation time?
Method parameters:
Diluent: Water
Standard Solution: MEK :0,03 mg/ml
1-Propanol: 0,1 mg/ml
FID Detector.
Inlet temp.: 250°C; Detector temp.:300°C
Oven temp.: 40°C for 6 min., 15°C/min to 300°C
Column: DB 1 (30*0,32mm*1µm)
Carrier gas: Helium, Flow rate: 1,9 ml/min, Split ratio: 10 to 1
Injection volume: 1 ml
HS: Agitator: 80°C, Incubation time: 30 min. Syringe temp. 90°C
HS-Vial size: 10 ml and I’m putting 100 µl into the HS vial.
Thanks in advance.
Tin sang

[chromatographer1]

Neither

The solution is to reduce the sample volume or just accept the poor RSD values.

The sample should be in equilibration after 30 minutes !

Any longer and the septa tend to leak or it may absorb the solvents, both will hurt your RSD values. Hotter temperatures will do the same thing.

You have provided another example of my preaching about headspace.

Large samples do NOT improve reproducibility or accuracy. They DECREASE IT.

best wishes,

Rod

With 100µL you should be equilibrated in 10 to 12 minutes.

[krickos]

Hi

tinsang when you state a RSD of 8% does this go for both solvents or one of them? Reason for askings is that for polar solvents in water it is generally good to keep the HS oven a bit under the bp of the lowest bp the ploar solvents of intrest, think bp of MEK is 80 so would be intresting to know if RSD improves if you drop HS oven temp to 70 for the 1000ul samples.

[chromatographer1]

With a 10mL vial and 1mL sample it is easy to oversaturate the headspace with solvent, and in agreement with the previous post, when you stop shaking and heating and the vial is moved to be sampled when near the boiling point of the solvent, you can lower the concentration in the headspace, thus the high variability. This also increases the absorption of the septum material of the solvents, thus the high variability.

Reduce the sample size. You have shown it can be reproducible. Make your customer change, not the physical laws of science. Or they can keep their 8% RSD. The choice is theirs.

best wishes,

Rod

[tinsang]

@ Krickos: I’ve got the RSD of MEK about 8% and even 13% of 1-propanol for the 1000 µl sample. Do you still think that I should drop HS oven temp to 70°C?

@ Rod: Thank you for your support. I’ll try next week to change the customer’s opinion. His argument was that the solution limit of detection (0, 5 to 1ppm) would be too low for the 100µl sample. I haven’t tried this concentration until now, but I’ve injected several times (5x) the solution with the concentration of 10 ppm of 1-propanol and 3 ppm of MEK for the 100µl and the RSD’s of both worsened (MEK: 10,9% and 1-propanol:8,7%), even though the specification of the limit of detection can be up to 15 %. Should I reduce in this case the incubation time to 15 min.?

Tinsang

[DSP007]

You have to remember about the existence of the equilibrium vapor pressure equation and its application to miscible and immiscible liquids.In Russia it is the first year of medical school, hated physical and colloid chemistry (fizkolda) . Propanol is unlimited mixed with water and has a vapor pressure equilibrium / peak area responsed by concentation of solution/ , etylmethylketone miniscrible mixted in water , full evaporated with water boil / peak area responded by "full amount etilmetylketone in probe " /.

[Peter Apps]

Taking 1 ml out of the headspace of a 10 ml vial lowers the pressure by 10% in round numbers. I wonder if the poor repeatability with the larger sample volume is due to a slower re-equilibration with the changed headspace conditions as the sample is removed ? If it is it would help if you could reduce the volume of headspace taken into the syringe - but this would make the peaks correspondingly smaller and so could only work if you have sensitivity to spare.

Do you know why the customer wants to use 1 ml samples ? Is it cast in stone ?

What is your injection speed ?

Peter

[tinsang]

Hello Peter.
Sorry for the late reply.
1.The reason that the customer wants to use 1000 µl solution: It’s not able to see 1 ppm or less than 1 ppm of solvent for a lower concentration of sample only using 100 µl of solution.
2.Injection speed: 30 ml/min.
Regards,
Tinsang

[chromatographer1]

You are putting 0.1mL of a 0.030mg/mL solution in your 10mL vial. That is 3000ng each of MEK and IPA.

Given you are splitting 10:1, you should be able to see less than that level, much less, at least 10 if not 30 times less.

I would heat the vial no more than 15 minutes. Equilibrium is reached around 10-12 minutes.

I would also make sure that my vials are sealing, I suspect they are not.

In my AC paper I detected samples of IPA and MEK at 0.5 ng of solvent. It was a Varian 3500, and I was injecting 1mL unsplit, from 3.35mL vials, still given the 5X sensitivity improvement of the GC and the 10:1 improvement of the non-split injection,and the 3x increase of the smaller vials I used, you should be able to see 100-200 ng of solvent. That is 15-30 times lower than the level of your present sample.

You are using teflon lined silicone septa, I hope.

One parameter to change, change the split to 5:1.

Increase sample size to 200µL from 100µL.

Make certain you are not leaking HS from your vials.

Good luck,

Rod

ps I hope you are not using a syringe to sample from the vials. You are using a fixed loop HS analyzer, right? If you are using a syringe, then you have found the source of your problem. You must allow air to enter the vial as you pull out the 1mL sample from the vial SLOWLY. A second needle inserted into the septum will facilitate this. Keep the tip of the second needle near the top of the vial and your fill needle tip at the bottom of the vial. Keep your syringe HOT before you sample and inject quickly. I would also be happy with less than 10% RSD values with a syringe.

[chromatographer1]

And if you want to improve your analysis try SPME fibers and a syringe.

best wishes,

Rod

[Peter Apps]

Hello Peter.
Sorry for the late reply.
1.The reason that the customer wants to use 1000 µl solution: It’s not able to see 1 ppm or less than 1 ppm of solvent for a lower concentration of sample only using 100 µl of solution.
2.Injection speed: 30 ml/min.
Regards,
Tinsang

Internet gremlins - possible double post

Hi Tinsang

Unless the analyte partitions so strongly to the headspace that its concentration in the liquid sample drops significantly during equilibration, detection limit is not dependent on sample volume, so it might be worth trying to demonstrate a 1 ppm (whatever that means in real units) detection limit with a 100 ul sample.

However - your injection speed is too high and you are very likely to be suffering eratic disruptions to the split ratio that will degrade repeatability. How reliable was the 2% repeatability - did you achieve it with more than one set of replicates ? Try using a 0.1 ml sample of headspace injected at 3 ml/min.

Peter

[chromatographer1]

Of course, with an injection speed you must be using a syringe, automated, not manual as I inferred.

Peter is right. At 1.9mL per min with 19mL/min total flow your injection rate of 30mL/min is too fast.

3mL/min might be a bit slow (peaks starting out at 20 sec wide) I would increase it to 10 mL/min.

Good luck,

Rod

[Peter Apps]

3mL/min might be a bit slow (peaks starting out at 20 sec wide) I would increase it to 10 mL/min.

Good luck,

Rod

Hi Rod

The 3 ml/min will be good only with the 0.1 ml injection volume - then the starting band is 2 s wide. I am hoping also that taking a smaller sample in the first place will be less disruptive to the headspace equilibrium in the vial.

Peter

[chromatographer1]

Good morning, Peter.

An oversight on my part. You are quite right.

My apologies.

Rod

[tinsang]

Hello Rod,
Hello Peter,
Thank you both for your advice.
@Rod: you have misunderstood my previous post about the concentration of solution:
There are two solutions:
1. Standard solution: 100 ppm or 10000 ng of 1-propanol and 30 ppm or 3000 ng MEK.
RSD % (5 Injections) of 1-propanol: 1,3
RSD % (5 injections) of MEK: 1,6
2. Limit of detection solution: 10 ppm or 1000 ng of 1-propanol and 3 ppm or 300 ng MEK.
RSD % (5 Injections) of 1-propanol: 8,7
RSD % (5 injections) of MEK: 10,9.
-The septa are Teflon silicon.
-HS vials didn’t leak.
@Peter: you are right that the detection limit is not depended on sample volume but the customer is not satisfied with the RSD of the limit of detection solution.
I achieved it with one set of replicates.

Next week I will start again my analysis as below:
Standard solution (6 Inj.): 100 ppm or 10000 ng of 1- propanol and 35 ppm or 3500 ng of MEK
Limit of detection solution (6 Inj.): 0,5 ppm or 50 ng of 1-propanol and MEK.
Do you both agree to change the parameter?
Injection speed: 3 ml/ml instead 30 ml/min.
Incubation time: 12 min. instead 30 min and of course with 100 µl sample volume
I’ll report you the result of my work. Have a nice week end.
Regards,
Dao