Total ion current of fast atom bombardment mass spectrometry fitted by non-linear curve

Some alkaloids and alkali metal salts were analyzed by fast atom bombardment mass spectrometry (FABMS) with glycerol as matrix, the intensities of their protonated molecules varying with time were investigated. It was found that the curve was decreased exponentially and both alkaloids and salt sample can be fitted by equation I = a + b e−kt. a, b were constants with a characteristic distribution depending on the ionizability of the sample in the matrix, t was time which was expressed by scan number in this experiment. It was observed that parameter a would be zero when the sample was salt of alkali; when sample was alkaloid, the ratio of initial intensities of protonated molecules was only related to the concentration of the sample, which can be used to analyze sample quantitatively.