Detectors

Once the ions of the compound of interest have been mass analyzed, the ions are directed to the detector of the mass spectrometer. Most detectors are based on the premise that the incoming ion will impact the surface of the detector, thereby forming many secondary electrons. Depending on the geometry of the detector these secondary electrons then impact another part of the detector surface. This process continues resulting in a 105 and higher amplification of the original ion beam. Parameters such as mass and velocity of the impacting ion as well as chemical characteristics of the impacted surface affect the ultimate current amplification in any detector.

The information obtained from the detector is used to generate the mass spectrum. The mass spectrum is a plot of the intensity of the individual mass-analyzed ions plotted as a function of m/z. Usually the most intense ion, termed the base peak, is given a relative abundance of 100% and the rest of the ions in the mass spectrum are normalized to this intensity. Figure 5.1 shows the EI mass spectrum of 2-methoxy-4-vinyl phenol (molecular weight 150 Daltons). The base peak is the molecular ion at m/z 150, a radical cation. The three most intense fragment ions are m/z 135 (loss of CH3), m/z 107 (loss of CH and CO), m/z 77 (C6^+ ; loss of OC^, C2H2, and OH), and m/z 51 (C4H3+ ; loss of C2H2 from m/z 77).

c a a 70 so 90 ioo 110 120 wo 140 150

Figure 5-1. Mass spectrum of 2-methoxy 4-vinylphenol.

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