Mass/charge ratio

This technique relies on the formation of ions hy various means in a high-vacuum chamber, their acceleration hy an electrical field and subsequent separation hy mass/charge ratio in a magnetic field and the detection of each species. It can he used for both inorganic and organic substances, be very sensitive, and be of value in examining mixtures of compounds especially if linked to glc. Usually this is a laboratory technique but portable or transportable models are now available. ... 

A detailed description of sources used in atmospheric pressure ionization by electrospray or chemical ionization has been compiled.2 Atmospheric pressure has been used in a wide array of applications with electron impact, chemical ionization, pressure spray ionization (ionization when the electrode is below the threshold for corona discharge), electrospray ionization, and sonic spray ionization.3 Interferences potentially include overlap of ions of about the same mass-charge ratio, mobile-phase components, formation of adducts such as alkali metal ions, and suppression of ionization by substances more easily ionized than the analyte.4 A number of applications of mass spectroscopy are given in subsequent chapters. However, this section will serve as a brief synopsis, focusing on key techniques. 

It is possible to determine the masses of individual ions in the gas phase. Strictly speaking, it is only possible to measure their mass/charge ratio (m/e), but as multi charged ions are very much less abundant than those with a single electronic charge (e= 1), m/e is for all practical purposes equal to the mass of the ion, m. The principal experimental problems in mass spectrometry are firstly to volatilise the substrate (which implies high vacuum) and secondly to ionise the neutral molecules to charged species. 

The mass analyzer separates the ion beams emerging from the ion source according to their m/e (mass/charge) ratios. As the ion beam passes through the magnetic field, the ions are deflected into circular paths, the radii of which are proportional to the square root of mje. Thus, the ions are separated into beams, each characterized by a particular value of mje. 

A mass analyzer converts components of a mixture into ions based on their mass/charge ratio (m/z ratio)... 

The mass spectrum of the analyte represents the statistical abundance of each type of ion formed, as a function of its mass/charge ratio in increasing order (Fig. 16.1). Under the same operating conditions, the spectrum is reproducible and characteristic of the sample. 

There are several other chemometric approaches to calibration transfer that will only be mentioned in passing here. An approach based on finite impulse response (FIR) filters, which does not require the analysis of standardization samples on any of the analyzers, has been shown to provide good results in several different applications.81 Furthermore, the effectiveness of three-way chemometric modeling methods for calibration transfer has been recently discussed.82 Three-way methods refer to those methods that apply to A -data that must be expressed as a third-order data array, rather than a matrix. Such data include excitation/emission fluorescence data (where the three orders are excitation wavelength, emission wavelength, and fluorescence intensity) and GC/MS data (where the three orders are retention time, mass/charge ratio, and mass spectrum intensity). It is important to note, however, that a series of spectral data that are continuously obtained on a process can be constructed as a third-order array, where the three orders are wavelength, intensity, and time. 

In mass spectrometry the sample is vaporized, and bombarded with electrons so that the molecules are ionized. The detector measures the mass/charge ratio, from which the molecular weight is determined and the molecule identified. Radicals often give the same fragment ions as the parent molecules, but they can be distinguished because lower energies are needed for the radical. 

Actually, it depends on the mass-to-charge ratio of the ion. However, it is unusual for the ionization sector to remove more than one electron. Therefore, the charge on the molecular ion is almost certain to be +1. Therefore, the mass/charge ratio is equal to the mass/1, which is numerically equal to the mass of the particle. 

Electrophoresis in the CZE mode takes place in an open tube and in a free solution without any separation matrix in the capillary. The separation is based on the mass/charge ratio of the analytes. It is appropriate for the separation of nucleosides and nucleotides. It is not well suited for medium to large oligonucleotides, because their mass/charge ratio tend to be smaller. The use of a separation matrix becomes necessary for these species. Various capillary systems, including bare fused silica capillaries and surface-coated capillaries, have been used in CZE. 

Interference between scattered electron beam caused by electrostatic field Interference between scattered neutron beam caused by atomic nuclei Determination of the mass/charge ratio and relative abundance of the ions formed upon electron bombardment... 

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