Chromatography Forum

I'm having trouble reconciling some issues between running standards and samples in headspace GC.

Specifically, when using Shimadzu's HS-20 headspace sampler and their LabSolutions software, I'm getting all mixed up when it comes to running standards and samples.

For example, if I run a standard with 10 uL 1000 ppm ethanol as a total evaporation (TE) sample, do I put in sample quantity as "10", or should I do it by mass because my samples are going to be run on a weight basis?

Replying to my own message because I have yet another question.

Ran into a problem where acetone and isopropanol peaks on HS-GC seemed to overlap, so I ran them individually (Data 1 is 10 uL of 1000 ppm chromatographic grade acetone in a 10 mL headspace vial, Data 2 is 10 uL of 1000 ppm isopropanol).

Then I ran them in combinations: Data 3 is 2 uL of 100 ppm acetone and 2 uL of 1000 ppm isopropanol.

Data 4 is 2 uL of 1000 ppm acetone and 2 uL of 100 ppm isopropanol.

Data 5 is 2 uL of 1000 ppm of each solvent.



Why are these peaks now co-eluting? (And my apologies if I posted this in the mass spec section- I had thought I originally posted it in GC, but perhaps someone moved it.)

I do ethanol via total evaporative headspace. I use acetonitrile as an ITSD. If acetone is yours I find it tends to be very ubiquitous so there is a high risk of either contamination or the samples having endogenous acetone.

I make my samples and standards as a liquid with the acetonitrile spiked in and then syringe it into a headspace vial (sometimes I need to centrifuge the solid samples). I run calibrations at 10, 50, 250 and 1000ppm etOH (solution) and the only dilution factor is result * (soln vol/sample mass).

What coln are you using? I typically use a wax but I find IPA and ETOH are no resolved but my samples have no IPA.

I make my samples and standards as a liquid with the acetonitrile spiked in and then syringe it into a headspace vial (sometimes I need to centrifuge the solid samples). I run calibrations at 10, 50, 250 and 1000ppm etOH (solution) and the only dilution factor is result * (soln vol/sample mass).

With these vials being pressurized to 15-20 psi, are you ever concerned that piercing the septa in order to add samples to a headspace vial might cause a leak?

What coln are you using? I typically use a wax but I find IPA and ETOH are no resolved but my samples have no IPA.

A 30-meter x 0.25 mm x 1.4 uM Rxi-624 Sil MS, which Restek seems to favor for this sort of application.

I syringe the sample in before crimp-capping them. The vials are not pressurized until just before loop fill though the static head pressure is probably significant during equilibration. I use a Tekmar 7000 and GC/MS SIM.
loop/valve sampler.

The 624 should be good. I usually use a wax because that is what I have available. I use .32mm .5um 30m wax for many of my headspace flavor apps.

Bad idea to add sample through the septa. Add the sample to the vial and then cap it. We pipet into the vials; micropipettors are a lot more precise than syringes. I agree with the assessment of acetone as an internal standard; I don't like it, either.


MSChemist: "I usually use a wax because that is what I have available"

LOL!! Isn't that the truth! Can't count the number of times I've resorted to convoluted chromatography because I happen to already own the column!