SPME Results Interpretation

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

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I have a couple of questions. We do SPME analysis for Geosmin & MIB, and we use IBMP as our Internal Standard, and IPMP as our Surrogate compound. We have a Shimadzu GCMS-QP2010 SE. When it comes to our Standards, it finds all four compounds correctly when we do a Similarity Search in our Library. When it comes to our unknown (Reservoir and Finished Water) samples, most of the time, we get messages like: "Ratio of reference ions does not match" or "No peak found in Window/Band range" or "No peak found".

Sometimes, the software/method parameters identifies a peak as our compound (usually Geosmin) at the right retention time, and yields a ppt result, however, most of the time when we then look at the Similarity Search results in our Library, it doesn't list the correct compound as a result option in the top 25 similarity results.

If the software automatically identifies a peak at the right retention time, should we assume that it is for the correct compound (Geosmin or MIB) even if when we look at the Similarity Search results in our Library it doesn't list the correct compound as an option?

Or if the Library Similarity Search results do not list the correct compound as a result option, should we treat the peak that it did identify and the ppt results that it gave us as if it didn't find a peak and the results should be recorded as not applicable or a 0ppt?

We have the option of manually integrating peaks that are not automatically identified by our software/method parameters, but usually when I do this, and then look at the Similarity Search results, it usually does not list the correct compound as a result option.

So, should I do a manual peak integration if a peak is found at the right retention time, even if it means that when I do a Similarity Search using my Library it does not give me the correct compound as a result option in the top 25 results?

Our software does not give us the ability to increase our Similarity Search result options to more than 25 results.

The representative we "learned" from only gave us like 5-6 days worth of "training", but those 5-6 days were spread out over the span of close to 2 years, and when I email him questions like these, the answers I get are, "use your best judgment."

I want to make sure that I am interpreting the results correctly. If the software/method parameters finds a peak and gives me a ppt result for my target compound because it was found at the right retention time with the correct primary and secondary masses and the right ratio of my reference ions, then why does it not list my target compound as one of the result options on my similarity search?

And if the software/method parameters don't identify a peak because the ratio of the reference ions isn't correct, or the peak is very small or not a sharp peak, but a peak is at the right retention time, should I manually integrate it?

If I do integrate a peak manually, and then do a similarity search, but it doesn't list my target compound as a search result option, should I just act as if it is my compound? Or should I not save the changes I made by manually integrating the peak, and just act as if nothing was found?

Help please.
I assume that you are running SIM to get the detection limit(s) you need. Does your "Similarity Search" only include the ions you're tracking in your method? If it is looking for masses across the entire spectrum and you're operating in SIM mode, that could be some of your problem. In SIM mode, all other masses in the spectrum are ignored and won't be collected by the software.

When I do this sort of thing, I like to choose at least 3 ions - one for quantitation (hopefully the base peak in the spectrum, gives best sensitivity if it works out that way) and 2 qualifiers. I run a known standard (easily detectable but not a ridiculously high concentration) on my GCMS and measure the ratios of the qualifiers relative to the quantitation ion. Unless your spectral library was collected on your instrument, the library spectra will differ slightly from what your instrument provides for the same compound.

As an example, consider styrene in water. For styrene you might monitor 51, 78, and 104 with 104 as the quantitation ion. These are the 3 largest masses in the spectrum. I would sort the chromatogram by these masses accordingly and integrate the styrene peak each time. The ratio of the 51/104 and 78/104 peak areas from my standard are my "theoretical" values. I allow for +/- 20% of those values to allow me to say "yes, it's styrene". The 20% comes from reading a number of papers where researchers are carrying out analyses like this. It seems like a good tolerance to me (compensating for experiemental error). For a sample, if even one of the ratios is outside of that +/- 20% of the theoretical value, it allows me to disqualify the material detected at the target retention time as "not styrene". If it fits inside that range, I have to say it's styrene and then I can use the 104 response to calculate the concentration (from my calibration data generated previously, using only m/z = 104 from my standards).

I hope this helps. Others might have some suggestions as to what they do for these types of situations.
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