Signal intensities, absolute

The number of published reports detailing the APPI technique is still relatively small. Some preliminary accounts indicate lower absolute signal intensities when compared to APCI, but this is accompanied by a significant reduction in the intensity of background signals for overall improvement in the signal-to-noise (S/N) ratio. The net outcome is comparable or better detection limits for a limited set of test compounds significantly promise for future applications. 

Increasing resolution does not affect the relative intensities of the peaks, i.e., the intensity ratios for m/z 28 32 40 44 in the spectrum of air basically remain constant (Fig. 3.18). However, an increase of resolution is usually obtained at the cost of transmission of the analyzer, thereby reducing the absolute signal intensity (Chap. 4.3.4). Accordingly, isotopic patterns are not affected by increasing resolution up to / = 10,000 beyond, there can be changes in isotopic patterns due to the separation of different isotopic species of the same nominal mass (Chap. 3.4). 

Absolute" Signal Intensities (Long Data Collection Times)... 

In the first and obvious approach, "absolute" signal intensities are measured. Since very long data collection times are required, this method is only useful in studying equilibrated, i.e. nonchanging, systems. 

Figure 4. Variation of the absolute signal intensities of signals in the substrate...

Both signal -to-noise and absolute signal intensity were the criteria used to select the mode of ionization for LC-MS (Table IV). Except for the sulfates and 4-nitrophenol in PCI, there is a decrease in S/N when the solvent is changed from acetonitrile to acetonitrile-buffer. The opposite is observed for the sulfates and 4-nitrophenol. In most cases, there is a parallel loss of signal intensity when going from acetonitrile to acetonitrile-buffer. The deleterious effects of the buffer on S/N is least pronounced in PCI. With acetonitrile-ammonium formate mobile phase, Isobutane PCI provides optimum sensitivity when compared to methane PCI or El. In both methane and isobutane NCI, peak broadening was unacceptably large, and NCI spectra were difficult to interpret. Based on these considerations, isobutane PCI was chosen for LC-MS studies. 

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