Ionization

In order to obtain a mass spectrum, a sample must be ionized and put into the gas phase.  The CIC MS Facility has four main ways to do this:

1) ESI (electrospray ionization) [J. B. Fenn, Journal of Biomolecular Techniques 13 (3): 101-118 (2002)]

2) MALDI (matrix-assisted laser desorption/ionization) [K. Tanaka, H. Waki, Y. Ido, S. Akita, Y. Yoshida, T. Yoshida, Rapid Communications in Mass Spectrometry 2: 151-153 (1988) and M. Karas, D. Bachmann, f. Hillenkamp, Anal. Chem. 57: 2935-2939 (1985)]

3) ASAP-MS (atmospheric solids analysis probe - mass spectrometry) [C. N. McEwen, R. G. McKay, B. S. Larsen, Analytical Chemistry 77: 7826-7831 (2005)]

4) EI (electron impact ionization) [F. W. McLafferty, F. Turecek, "Interpretation of Mass Spectra," 4th ed. University Science Books, Mill Valley, CA 1993]

The chemistry of mass spectrometry ion formation:

protonation/deprotonation:  M + [A + H]+ --->    [M+H]+  +  A      or      M + [B - H]-   --->   [M-H]-  +  B

adduction (forming complexes with ions):  M  +  Na+  --->   [M+Na]+

electron transfer (oxidation/reduction (redox)):  M  +  e-  --->   [M]+   +  2e-     or       M  + e-  --->  [M]-

chemical ionzation (charge exchange):  M  +  [X]+   --->  [M]+   +   X       or         M  +  [Y]-   --->  [M]-  +  Y

separation of preformed ions:  [R4N]+    [R4P]+    [Na]+    [K]+      [Br]-     [(RO)2PO2]-

Ways to convert solids and liquids to gases:

1) Evaporate into a flow of gas.  2)  Carry into the gas phase with a matrix that desorbs easily.   3)  Desorb thermally.

Generally speaking, heating a sample with a low boiling point will cause it to become a gas.  But just heating most samples, for instance, sucrose, causes them to decompose.  So the idea behind electrospray ionization is to take advantage of solutions where molecules are separated by solvent and quickly remove the solvent before the molecules can condense.  Evaporation is a cooling process, so ESI sources must be heated.  In gas chromatography, the injection port heats the sample solution and mixes it with inert gas, so that the sample molecules are left suspended in the gas phase.  In MALDI, the dry sample, dispersed in a matrix, is carried into the gas phase when the matrix is heated by a laser, so that it explodes into the vacuum.  ESI, MALDI, and GC are considered gentle ways to put solids and liquids into the gas phase and are thus the mainstays of mass spectrometry.

When trying to obtain a mass spectrum of a new compound, it is useful to think about how to ionize it, and how to put it into the gas phase.