Chromatography Forum

Does shaking the vials on low in the headspace sampler have an effect on the chromatography if I am analyzing solvents (methanol, ethanol, acetonitrile, TEA, toluene) in an aqueous soln, pH 12?

I have been running a method for a while with the shaker off, but I recently changed the shaker setting to low. Now I am randomly seeing broad/tailing peaks in my chromatograms. These peaks are not in every injection or in every run, but are randomly seen every now and then.

I find it hard to believe that changing this setting is causing the peaks to change shape.

Thanks.

CK:
That is a very interesting observation and your observations seem to fit in with my preliminary findings on the effect of gentle mixing of ALS vials on the chrom.

If shaking the vials really does change the shape (as opposed to the size) of peaks then several fundamentals of chromatography are going to have to be reconsidered.

Is it the analyte peaks that are distorted, or are you seeing extra distorted peaks in addition ot the normal analyte peaks ?.

What make and model instruments do you have, and what are the operating conditions ?

Peter

CK,

Random, broad, tailing peaks suggests carryover to me. Now that you are shaking, components that used to not get on column in high concentrations are getting on but they elute later than your run time. Try extending your run time substantially for a run or two and see what else might be out there.

Best regards.

Agitating the vials during incubation enhances the recovery of some compounds. By changing your method you may be recovering substances not found before, increasing the amount of some compounds present or even causing breakdown of compounds that now spend more time in the gaseous phase than they used to because they reach equilibrium quicker.
You should really be running a series of runs to check the recoveries and if you are incubating too long

Thanks for the replies.

It is only the analyte peaks that are distorted and I am not seeing any extra peaks, even when I extend the run time.

I am using an Agilent 6890 with an Agilent 7694 headspace sampler.

Hi CE,

Is it possible that agitating the vials would only affect 2 or 3 out of 20 injections and not all injections a during a run?

I would suggest you run several samples and then run blanks for several hours. See if the broad tailing peaks come out long after the samples are injected. If they are eluting in a delayed manner that would explain your evidence for all details.

Rodney George
consultant USA

Are the distorted peaks bigger in area than the usual ones, and by how much ?

Peter

yes, the distorted peaks are bigger. around ranging from 10-50% bigger.

Please post details of your set up and operating conditions, and an example of a chromatogram (instructions in a sticky on the LC page)

Peter

Below are the operating parameters for my method.

I attached an overlay of two of chromatograms. The second chromatogram is the same as the first, but zoomed in on the baseline.

Another set of samples (~20 injections) were run using the same method and the shaker off...all of the peaks look good. Could the poor peak shape be a result of the shaker being turned on low? The experiments seem to point in that direction, but does that make sense?

GC parameters:
Injection port temp 180C
FID temp 260C
Split flow 25 mL/min

HS parameters:
Vial pressure 10psi
Oven temp 85C
Loop temp 95C
Transfer line 110C
Vial equil time 10 min
Pressurization time 0.2 min
Loop fill time 0.15 min
Loop equil time 0.05 min
Inj time 1.0 min
Shaker on Low

Thanks again.

The chromatograms are very informative.

There are no new peaks with the shaking of the vial.

But I suspect depending upon how much volume is in the vial and I will assume there is more than 2ml and probably 5-18 mL, that with the shaking the recovery of the ethanol is increased, increased enough in fact to carry an amount of water with it onto the sample loop and the 'refocusing' section of the GC column.

The tailing shown demonstrates that the column phase is not capable of focusing the vapor plug with its present content and volume.

Diagnostic experiment is needed.

Reduce the sample loop by 10 to 100 times and see if the tailing goes away even with the vials being shaken. I believe the tailing will go away as the vapor load on the column is reduced.

If this proves true then the best solution IMHO would be to reduce the sample size by 5 to 100 times and review the results.

Good luck.

Rodney George
consultant

What do you mean, "Reduce the sample loop by 10 to 100 times"? What parameter should I change?

Thanks.

I was not suggesting that you change a parameter of the software. I was advising you to replace a piece of hardware in the sampling system.

It is a hardware change: the internal volume of the sampling loop attached to your injection valve.

Are you using a 0.25mL or a 1mL loop? Change the loop (a simple 5/16" wrench should be sufficient) from its present size to a smaller one, 0.05mL or 0.10mL.

If you are not able to change the loop then increase the split ratio 100X in your split injector of the GC from its present setting.

I hope I am not confusing or ambiguous.

best wishes,

Rodney George