Total evaporation headspace GC- standards vs. samples

[osp001]

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?

[osp001]

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.)

[MSCHemist]

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.

[osp001]

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.

[MSCHemist]

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.

[mckrause]

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!