Principles and Applications of Mass Spectrometry

These two techniques are characterized by high ionization efficiency for a wide range of biological molecules while preserving the analyte and enabling quantification. 

Electrospray ionization (ESI) enables the transfer of ions from solution to the gas phase 

In addition, since ions are analyzed in packets, TOF MS has a natural compatibility with the pulsed ion release of MALDI interfaces. If there is interest in a given m/z value, the pulsing frequencies can be adjusted to increase the duty cycle and the sensitivity by about one order of magnitude (lower micromolar range). 

The suitability of MS detectors for quantitative analyses is debated. For example, ESI is a competitive process and, occasionally, matrix (background) material interferes with the ionization of the analyte [40]. These effects of ion suppression are especially aggravated when several species coelute, such as in the case of biological extracts or with direct infusion (without previous separation). Generally, hydrophilic species are more sensitive to ion suppression than hydrophobic ones, which tend to concentrate on the droplet surface during ESI [41]. In MALDI, the sample dispersion is often inhomogeneous or the matrix crystals unevenly distributed on the surface. A truthful representation of the sample composition is obtained exclusively upon thorough laser desorption of the entire spot. In addition, with some instruments the transmission of ions in the mass 

The contribution of MS to identification of compounds and quantification of their concentration is complementary to other detection techniques and, despite being very practical and versatile, it remains fundamentally replaceable. However, knowledge of molecular weight is a prerequisite for techniques that rely on the synergies with stable isotopic tracers. In fact, powerful analytical methods exist to obtain important insights on cell dynamics from the ratiometric measurement of marked and not-marked species (or atoms). We cite, for example, (1) relative abundances of virtually all metabolites or proteins in two separate cultures are quantified based on the isotope dilution theory [43 5] (2) information on the mechanisms and kinetics of nonlinear chemical processes can be extracted from response tracer experiments [46 7] and (3) the labeling patterns in metabolic intermediates are used to resolve the relative rate in convergent reactions in vivo [48,49]. 

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