I need to identify four compounds (VOCs) from a MS spectrum including propionic acid, valeric acid, etc.
Should I utilize standards before my run?
Or there is a database we can utilize to recognize VOCs, based on the complete spectum of my samples?
Claudia , since you mention identify it means you want to perform qualitative analysis. If that is the case YOU DO NOT need to run standards as the similarity search from the MS library (database) would give you the names of these components if present. But if you intend to quantify then you will need to run the standards.
Claudia , if you need qualitative analysis, YOU DO NOT need to run standards as the similarity search from the MS library (like NIST Library) would give you the names of these components if present. But if you intend to quantify then you will need to run the standards and should calibrate.
You did not mention which compounds are the "etc." compounds. Some carbonic acids have very similar mass specs, in this case you need to run standards as the similarity search from the MS library (database) is not reliable.
Thank you for your very valuable comments! VOCs that we need to identify are: propionic, valeric, acetic, isovaleric, butyric, and isobutyric acids.
I'm wondering if these MS libraries are free?
The company that makes a GC column to separate C2 to C6 organic acids should be able to give you all you need. It should give the order of elution for your simple compounds for you to confirm on your column. There are lots of books on Modern Organic Analysis.
"I'm wondering if these MS libraries are free?" Not free.
Identification of compounds is not based on EI spectra alone (that is only one dimension of analysis). Qualitative ID should be based on Method Retention time plus other orthogonal methods too. That is why the exact method used may affect the results obtained and we can not rely on just the EI spectra alone.
The spectra of the compounds that you are looking for contain ions that are very common to a wide range of compounds. There is nothing inherently "unique" about their spectra, nor is there anything about the chromatography that would allow you to uniquely identify them. Technically (read it in a book) the answers you received are correct. Practically (actually do it) the answers that you got are garbage for your compound set. You'll need to establish chromatographic conditions such that you know where these compounds elute. You need standards.
Hi Claudia,
as Mark Krause already mentioned, the MS spectra of the lower aliphatic acids do not have unique characteristics.
As in most cases during identification, it is a good idea to run standards prior real samples to get precise information about the retention time with your individual machine.
Do NOT always believe everything a library search is suggesting.
regards, pb
The old timey way was to make a derivative of the whole acid fraction to get good chromatography on your column (ethyl esters are good), then do methane chemical ionization GCMS, so you see each whole molecule intact plus a little bit of methane adduct ion. Because an electron impact MS library search is not believable for acids or hydrocarbons. (and some other compounds as stated by P. Behrend above).
I also would use standards in order to identify the compounds; this is also a good practice for publications, not just a library matching, but also confirmation with the reference standards.
I would just run the standards after my run, not before, to avoid possible contaminations.
Hi Claudia,
free access to mass spectra (EI) is available on the following website:
https://webbook.nist.gov/chemistry/name-ser/
Of course, this library cannot be compared to commercial libraries in terms of quality and quantity, but for simple compounds or on a small budget you can work with it.
Regards
Markus
P.S.: Your compounds are only correctly identified if the mass spectrum and retention times are identical to your standards under the given chromatographic/mass spectrometric conditions.
There are a database we can utilize to recognize VOCs, based on the complete mass-spectrum of samples?
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