Residual Benzene(2 ppm) HS-GC bad repeatability (bad RSD)

Discussions about GC and other "gas phase" separation techniques.

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Hi everyone~
I'm doing the residual benzene method verification, benzene std is 2 ppm, prepared only with water. The problem is the %rsd of the pear area in benzene std, it always over 15%, especially for the bracket std, the peak area always drop a lot. (etc. from 1.3 to 0.6) This situation happened every day.

Here is my method condition:
HS Injection volume: 1mL
Oven Temp: 80°C
Loop Temp:100°C
Transfer Line Temp:110°C
Vial Equilibration Time:40 minutes
Shaker:High

GC-FID:
5.0 mL/min (Constant flow)
Injector Mode: Split1:1
Injector Temperature:280°C

I'm using the split liner without wool, 4 mm x 6.3 mm x 78.5 mm
column: DB-624 30M*053MM, 3.0UM

It's a method verification, so I can't change the std and sample preparation.

Is anyone met the same problem?I'm fresh in GC analysis, please advise me how to resolve this problem.
Thanks so much !
I had an issue with benzene a number of years ago where I also initially went direct to water with that analyte. And I did not have a stable situation with regard to RSD. My recollection (possibly not entirely correct) was that while benzene does have reported adequate solubility in just water at the concentration I was using - getting it properly solubilized in H2O was not so cut and dry. I solved the issue by first dissolving the benzene in methanol prior to adding that to water for my final concentration.
I completely agree with Gizmo but we use DMSO instead of methanol. Also do you inject different vials to calculate RSD? The same vial should not be reinjected via HS.
Best regards,
Dmitriy A. Perlow
I have an Agilent Head Space sampler, and Agilent suggests to use a straight injection liner with 2 mm ID and no glass wool. I have not experience problems with the linearity of a calibration curve, nor the repeatability of peak areas.
Regards,
Gilbert Staepels

Ideas mentioned in this note represent my own and not necesseraly those of the company I work for.
What's your signal/noise ratio like for the Benzene peak? Reproducibility will be harder to reach as the peak height decreases, as it will become indistinguishable from the baseline. Can you post some pictures of your Benzene peak?

I would also use a 2mm ID straight-through liner with no glass wool as mentioned above.

Benzene from water is definitely going to be tricky - are you sure there is no intermediate dilution in DMSO, á la the USP residual solvents method?

Also a good starting point to check when encountering reproducibility problems in headspace is your vial crimping technique - is it uniform for every vial?
If you can tolerate reducing your peak area, you may want to try increasing your split ratio to at least 5:1. Your system may be having a hard time maintaining consistent flows at the ratio of 1:1. I think Agilent recommends 5:1 as a minimum, it may be even higher than that.
To reduce the %RSD you can work with internal standard.
To increment the signal of benzene you can add NaCl to the sample (2g NaCl/10ml water.
Francesc

Sorry for my english
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