[Nescafe]

Hi,

Using our LRMS machine and  MeOH as the solvent to run the sample, I always see either M+H peak or M + 23 (Na) peak. I never see the M+. I was told by my senior graduate students that I must use a protic solvent to run my sample like MeOH or H2O. Why not an aprotic solvent like Acetonitrile? Using an aprotic solvent would I not just see the M+ and the fragmented peaks? I don't see why I must use a protic solvent....

Additionally, I also don't understand how you get the M + H peak. I read in the following article that M + hnu --> M+ + e- followed by the reaction M+ + S --> MH+ + S(-H).

http://www.ncbi.nlm.nih.gov/pubmed/15519219

How does a M+ which is a radical cation abstract a proton, mechanistically speaking. I just don't see it :S

Nescafe.

[orgopete]

A great advance in mass spectrometry came with using liquid chromatography interfaces, especially when the liquid chromatography was reverse phase. Reverse phase virtually allows everything injected to elute. There isn't a polar hold up as occurs with normal chromatography. As a consequence, water is a co-eluent.

What is also not being appreciated here is the value of chemical ionization rather than electron impact mass spectroscopy. The amount of fragmentation that occurs with e.i. ms is much greater. The decompositions are chemical reactions. If compounds are known, that may enable a direct suggestion of a compound's identity, but for unknown compounds, predicting fragmentation of an unknown compound is just an additional level of difficulty to its identification.

Practically speaking, LRMS is making it really easy for you to get a mass spectrum or to get a lot of spectra with an autosampler. It doesn't replace GC-MS, but GC requires volatile samples. LRMS is a good compliment. Samples that are not volatile are good candidates. LRMS usually gives an M+H, that is, you can determine the MW of your sample. That is usually the first good piece of information you need to identify a sample. (I agree the M+23 or other adducts are a nuisance.)

[fledarmus]

You got good information from Orgopete. I wanted to highlight this sentence, however, because I think this directly answers your question.

What is also not being appreciated here is the value of chemical ionization rather than electron impact mass spectroscopy.

Mass spectrometry requires measures the mass of charged particles by shooting them close to a magnet and seeing how much their path changes. Lighter molecules get pulled closer to the magnet, heavier particles are less affected. The difference between chemical ionization and electron impact (or electrospray) is that in chemical ionization, a proton is transferred to generate the charged particle. In electron impact, and electron is knocked off of the molecule to generate the charged particle. That means that in chemical ionization mass spec, the "molecular ion" will be the molecule plus a proton - M+H⁺. In electron impact mass spec, the molecular ion will be the molecule minus an electron - M⁺. You can get more details here http://en.wikipedia.org/wiki/Chemical_ionization and here http://en.wikipedia.org/wiki/Electron_ionization

As Orgopete indicates, the radical cation formed by knocking an electron out of a molecule in EI is less stable than the protonated species formed by adding a proton in CI - the radical cations tend to fragment more and you see smaller molecular ion peaks and larger fragment peaks. On the other hand, chemical ionization techniques tend to be sensitive to the ability of the molecule to attach to a proton, and the sensitivity of the technique to different molecules can differ widely based on the structure of the molecule.