Quantitation

In addition to providing molecular weight confirmation and structural information, mass spectrometry can also provide quantitative results (Hoffmann and Stoobant, 2001; Watson, 1997). This is most often done using either a calibration curve (external calibration) or an internal standard, like an isotopically labeled analog or a compound with a closely related structure (internal calibration). Standard addition, the addition of known amounts of the compound of interest to the unknown sample is another frequently used quantitative method. Mass spectrometry is a very universal and sensitive analytical technique. As the methodology has matured over the years it has been used to solve a myriad of far-ranging analytical problems including those involving complex plant phenolics such as anthocyanins.

The data presented in Table 5-1 (next page) were reprinted from Phytochemistry 59: Wood K. V., Bonham, C. C., Miles, D., Rothwell, A. P., Peel, G., Wood, B. C., and Rhodes, D., Characterization of betaines using electrospray MS/MS, p. 759-765, copyright 2002, with permission from Elsevier.

Table 5-1. Electrospray MS/MS spectra of a series of betaine analogs. The product ions in bold face are the base peaks of the MS/MS spectra.

Compound

Structure

Productions (m/z)

glycine-betaine

H3C ch3 o

d9-glycine-betaine

127 68 [M-CH2CO2H] 66 [M-CH2DCO2H]

ß-alanine-betaine h3c

d9-ß-alanine-betaine d3c

Trigonelline d3-trigonelline proline-betaine d6-proline-betaine

CH3 O

CH3 O

OH

144 84 [M-C3H6-H2O] 102 [M-C3H6] 98 [M-HCO2H] 58 [CH2N(CH3)2]+

150 90 [M-C3H6-H2O] 108 [M-C3H6] 104 [M-HCO2H] 64 [CH2N(CD3)2]+

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