Mass spectrometer, detectors soft ionization

As shown in Figure 15.1, there are three main components of every mass spectrometer. The ion source is used to produce gas-phase ions by capture or loss of electrons or protons. In the mass analyzer, the ions are separated according to their mlz ratios ions of a particular mlz value reach the detector, and a current signal is produced. This section describes the soft ionization sources, mass analyzers, and detectors that are used in experiments involving biological macromolecules. 

Mass spectrometry is an analytical technique that can determine precisely the atomic or the molecular weight of atoms or molecules once they have been ionized. There are four key elements in mass spectrometer the sample introduction, the source where ionization occurs, the mass analyzer, and the detector. Mass spectrometry can analyze many different types of samples that range from solid, liquid, or gases. First, the molecules have to be ionized either under vacuum or at atmospheric pressure. Depending on the ionization technique, either molecular ions (M ) with an odd electron number or protonated ions ([M + H] with an even electron number are formed in the positive mode and M , M or ([M - H] in the negative mode. Ionization techniques are often classified into soft ionization, where little or no fragmentation occurs, and hard ionization, where fragmentation is extensive. Electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI) are... 

The mass spectrometer has been touted as the universal chromatographic detector. This claim is not true Like any other detector, the mass spectrometer exercises some degree of preference for materials being presented to it. Consistent with this thesis, we have accumulated a growing list of molecules that are mass spectrometrically silent. Small molecules— in particular small aromatic molecules—exhibit a combination of volatility comparable with the mobile-phase solvents and a reduced ionizability under the ESP soft ionizing conditions to show minimal or no response in the mass spectrometer. Not every compound has a significant UV-vis chromophore, but many UV-silent compounds respond very well in the mass spectrometer. In particular, a number of commonly used excipient molecules do not have useful chromophores. Even... 

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