SPME - poor reproducibility, decrease in peak area

[Fee]

Hi,

I am working on a SPME procedure to analyze volatile compounds from wine, including ethyl esters, terpenes, acetates, alcohols, etc. However, looking at the reproducibility of my method (injection of 10 identiques spiked samples, in a row), I observed that some of my internal standards, such as ethyl heptanoate and ethyl nonanoate, were showing decreasing peak area from sample 1 to sample 10.

Of course, compounds quantified using these internal standards also decrease (peak area). However, other compounds, quantified against a pretty stable internal standard also shown decrease in there peak area.

So, can anybody help me to troubleshoot that ? I am using SIM conditions to quantify most compounds (I am using a GC-MS SIM/Scan, with a Gerstell SPME extractor and a Gerstell injector used in pulse splitless mode).

Thanks a lot,

Fée

[AICMM]

Fee,

Very similar application published in Journal of Chromatography A (1143, 2007, pp 8-15) regarding sulfur compounds. Might provide some useful insights into what you are doing. One of the key things they found was that less sample was much better than more.

Best regards.

[Fee]

Thanks for the ref. I'll take a look at that. Actually, looking at my results with someone more experienced than me today, I learned that an error % of 15% was something acceptable. So, it seems that my results are not too scattered for most compounds. I am running the recovery study. We'll see ho it goes...

Thanks again,

Fée

[tolo]

Several points you can check
1. Did you use the appropiate fibre?
2. Did you activate the fibre properly as instructed by the attached leaflet
3. Did you clean the fibre after each run, i.e. dip in the oven after each run
4. Did you check the maximum temp. you can use with fibre?
5. Did you use a 0.75mm liner?

[Fee]

Hi Tolo,

1. Did you use the appropiate fibre?

I experienced other fiber, the best one was a 2 cm 3 phases (CAR, PDMS, DIV) fiber I am currently using.

2. Did you activate the fibre properly as instructed by the attached leaflet

I generally condition my fibers according to the procedure described by Supelco in the booklet.

3. Did you clean the fibre after each run, i.e. dip in the oven after each run

Yes. I am heating the fiber at 240oC during 15 minutes after the injection.

4. Did you check the maximum temp. you can use with fibre?

Yes. It is arounf 270oC if I remember well.

5. Did you use a 0.75mm liner?

I am using a liner made for SPME (according to the lab tech), having a pretty small diameter. I cannot tell you if it has a 0.75 mm diam though.

I hope you have some more tips for me. Do you think that +- 15% error is too wide ? I have to say that most of my compounds are inside 10% error.

Thanks a lot,

Fée