Quaternary ammonium and tertiary sulfonium compounds can be measured directly by fast atom bombardment mass spectrometry (FAB-MS) using a glycerol matrix. The compounds are typically detected as their protonated molecular ions (M+H⁺). Alternatively, betaines can be esterified with alcohols to give betaine esters that are particularly sensitive to detection because the derivatives are precharged (M⁺) and have increased surface activity in the glycerol matrix. Quantification is accomplished using a suitably labeled internal standard (e.g. deuterium-9 labeled glycine betaine).

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Rhodes D, Rich PJ, Brunk DG, Ju GC, Rhodes JC, Pauly MH, Hansen LA 1989 Development of two isogenic sweet corn hybrids differing for glycinebetaine content. Plant Physiol. 91: 1112-1121.

Rhodes D, Rich PJ, Myers AC, Reuter CC, Jamieson GC 1987 Determination of betaines by fast atom bombardment mass spectrometry: identification of glycine betaine deficient genotypes of Zea mays. Plant Physiol. 84: 781-788.

Wood KV, Strinham KJ, Smith DL, Volenec JJ, Hendershot KL, Jackson KA, Rich PJ, Yang W-J, Rhodes D 1991 Betaines of alfalfa: characterization by fast atom bombardment and desorption chemical ionization mass spectrometry. Plant Physiol. 96: 892-897.