Ask a simple question about derivatization of gc-ms

[fds206303]

Hi all,
I am a rookie,how to analyze primary alcohol,second alcohol,tertiary alcohol derivatized with BSTFA，eg:CH2-CH2-CH2-CH2CH2-O-SI(CH3)3, wo want to have a good knowlege of the clevage of bond in detail.
thanks

[Consumer Products Guy]

Typically use solvent such as DMF or pyridine.

GCMS EI spectrum will typically show MW of the derivatized alcohol less 15 units. The GCMS spectra of derivatized alcohols are more distinctive and easier to search using a GCMS search library. Smaller alcohols will be derivatizesd as well but can be lost in the solvent and BSTFA peaks.

A single trimethylsilyl group will add 72 (adds 73 less 1 for the hydrogen atom). You'll see a 73 ion, and obviously must have excess of the derivatization agent.

[fds206303]

primary alcohol,second alcohol,tertiary alcohol derivatized with BSTFA,there are fragment ions like 89,103 117 .I want to know where they come from？ Can you explain it and exhibit its fragment ion?

[Consumer Products Guy]

primary alcohol,second alcohol,tertiary alcohol derivatized with BSTFA,there are fragment ions like 89,103 117 .I want to know where they come from？ Can you explain it and exhibit its fragment ion?

No, haven't needed to look into fragments to explain or find identity of such materials not on our custom search library for a long time. Not since tracking down identity of what turned out to be diethylene glycol -2TMS.

[fds206303]

yeah,I know what is the meanings you want to convey,eg: we find a material have a match of 87 with the standard in NIST,when we have this single material stabdard injected into gcms,the retention time is different with 1.5minute delayed.so I want to have a konwledge of the laws of the cleavge to reanlylyse the material to know the metebolism pathway.

[Consumer Products Guy]

The NIST search library serves a purpose, but we've developed our own propietary search libraries for components found in our type of products and for fragrance components.

[Don_Hilton]

I would suggest starting with a text on mass spectrometry. It sounds like you are trying to solve structures by examining spectra. There is an approach to doing this - and I would suggest the text book.

Fragmentation is in some ways a very simple process that leads to a result that can be difficult to sort out.

You will find that a molecule is ionized, with the site of ionization being more likely to be a non-bonded electron pair (such as on oxygen or silicon) than on a pi bond and that more likely than on a sigma bond. Once the molecule is ionized, it will commonly rearrange to separate the free radical and the charge. The charge will go the location that is energetically favorable. So, in many cases, when a molecule fragments, the charge tends to stay with a particular portion of the molecule, and there little or no information available about the portion that leaves as the free radical.

I will point out that match scores can be misleading. They can help to select likely canidates for further examination. You need to see if the molecuar ion, if present is seen and if major ions are present in both spectra. TMS derivatives can be particulalry difficult because they generate many of the same ions, regardless of the parent compund and a prime site for ionization is the TMS group.

I will add that I suspect that for most of us, we try to us library searches and retention time matches where we can. Solving structures from sepectra can be a time consuming task - and not always sucessful.

[fds206303]

Don_Hilton
Thank you for your explanation and advice,Can you recommend a good text book about bond cleavage and rearrangement? this book can best show its principle of bond cleavge.

[LiVD]

I'd take a look at Fred McLafferty's Interpetation of Mass Spectra.

[Don_Hilton]

McLafferty is the great old classic. I like it as well as Watson and Sparkman - which is newer (2007) and covers a wider range of ionization techniques.

But, for understanding TMS ethers, you may want to look in the literature as well. I would post a couple of references, but my literature file is at work and I"m not there this week.

[fds206303]

Dear Don_Hilton
Thank you for your reference book. I prefer to understand TMS esters and want to know your literature. If you will be at leisure next week, please post a couple references to my email:fdsasdzqs@163.com

thank you

[Don_Hilton]

Let's see if the US Government shutdown is over by next week. If so, I'll be back to work...