Mass spectrometer, detectors kinds

What kind of information does a mass spectrometer detector give in gas chromatography that is useful for qualitative analysis For quantitative analysis ... 

Time-of-flight mass spectrometers have been used as detectors in a wider variety of experiments tlian any other mass spectrometer. This is especially true of spectroscopic applications, many of which are discussed in this encyclopedia. Unlike the other instruments described in this chapter, the TOP mass spectrometer is usually used for one purpose, to acquire the mass spectrum of a compound. They caimot generally be used for the kinds of ion-molecule chemistry discussed in this chapter, or structural characterization experiments such as collision-induced dissociation. Plowever, they are easily used as detectors for spectroscopic applications such as multi-photoionization (for the spectroscopy of molecular excited states) [38], zero kinetic energy electron spectroscopy [39] (ZEKE, for the precise measurement of ionization energies) and comcidence measurements (such as photoelectron-photoion coincidence spectroscopy [40] for the measurement of ion fragmentation breakdown diagrams). 

Many kinds of detectors have been designed, ranging from the widely used, cheap but robust flame ionization (GC) or ultraviolet absorption type (LC) to the much more exciting and informative, if much more expensive, mass spectrometer. 

More than 20 different kinds of commercial mass spectrometers are available depending on the intended application, but all have three basic parts an ionization source in which sample molecules are given an electrical charge, a tnass analyzer in which ions are separated by their mass-to-charge ratio, and a detector in which the separated ions are observed and counted. 

Mass spectrometers are used primarily as tools for measuring isotopic compositions, although some kinds can also be used to determine elemental abundances. Mass spectrometers have three basic components (1) a means of ionizing the sample (2) a mass analyzer that separates atoms based on their masses and (3) a detector. Most of the time, the mass spectrometer is identified by its source, although the mass analyzer can also be identified. In the next few paragraphs, we will describe the various sources, the different mass analyzers, and the detectors, and then describe the most common configurations used in cosmochem-istry. For more details, see Gross (2008). 

The scope of the use of mass spectrometry in the protein analysis has grown enormously in the past few decades. MS has become an important analytical tool in biological and biochemical research. Its speed, accuracy and sensitivity are unmatched by conventional analytical techniques. The variety of ionization methods permits the analysis of peptide or protein molecules from below 500 Da to as big as 300 Da (Biemann 1990 Lahm and Langen 2000). Basically, a mass spectrometer is an instrument that produces ions and separates them in the gas phase according to their mass-to-charge ratio (m/z). The basic principle of operation is to introduce sample to volatilization and ionization source, and then the molecular fragments from the ionization of the sample are detected by various kinds of detector and the data are analyzed with computer software. 

A cleanup procedure for sample extraction is often required, depending on the type of compound and the kind of sample to be analyzed and on the selectivity of the analytical equipment used in the determination. The use of a selective detector, such as a diode array, or a mass spectrometer can reduce the need for a cleanup procedure in some cases. 

Principal component analysis is most easily explained by showing its application on a familiar type of data. In this chapter we show the application of PCA to chromatographic-spectroscopic data. These data sets are the kind produced by so-called hyphenated methods such as gas chromatography (GC) or high-performance liquid chromatography (HPLC) coupled to a multivariate detector such as a mass spectrometer (MS), Fourier transform infrared spectrometer (FTIR), or UV/visible spectrometer. Examples of some common hyphenated methods include GC-MS, GC-FTIR, HPLC-UV/Vis, and HLPC-MS. In all these types of data sets, a response in one dimension (e.g., chromatographic separation) modulates the response of a detector (e.g., a spectrum) in a second dimension. 

Traditional detectors (i.e., FID electron capture detector, BCD nitrogen-phosphorous detector, NPD) supply only retention data. However, in many cases this is not enough for proper identification of analytes. Application of GC coupled with an MS detector gives much more information (i.e., the mass spectmm of each compound). GC-MS is a well known and frequently used technique that combines the highly effective separation of GC with the high sensitivity and selectivity of MS. Moreover, improvements in analytical instruments based on different types of mass analyzers (ion trap, quadrupole, and TOF) and the development of hybrid Q-TOF has enhanced the analytical capabilities of modem hardware. Different kinds of mass spectrometers are presented in Table 14.2 [119]. 

The mass analyzer. After the process of ionization, the ionized molecules of proteins or peptides enter the section of the mass spectrometer called the mass analyzer, where they are separated based on their mass-to-charge ratio by electric and/or magnetic fields or by measuring the time taken by an ion to reach a fixed distance from the point of ionization to the detector. Different kinds of analyzers are available for the separation of ionized molecules. Among the different kinds of analyzers, two particular kinds, called the quadrupole and the time-of-flight (TOF) analyzers, are the most important from the point of proteomics for their use in mass spectrometers. A particular spectrometer may use one or the other or at times a combination of both quadupole and TOF analyzers. Usually, the machine with the electrospray ionization device carries a quadrople analyzer. A spectrometer with a MALDI device has a TOF analyzer or a combination of quadrupole and TOF analyzers in succession to each other. Certain spectrometers called tandem spectrometers (MS/MS) contain two or three quadruples and a TOF analyzer. 

A time of flight (TOP) mass spectrometer measures the mass-dependent time it takes ions of different masses to move from the ion source to the detector. This requires that the starting time (the time at which the ions leave the ion source) is well-defined. Therefore, ions are either formed by a pulsed ionisation method (usually matrix-assisted laser desorption ionisation, or MALDI), or various kinds of rapid electric field switching are used as a gate to release the ions from the ion source in a very short time. 

Whenever you plan to combine a mass spectrometer with a second detector, it is this additional detection principle that tells you how to technically realize the hyphenation. Do not forget - the mass spectrometer always ehminates your analyte while measuring and detecting it, so it must be the last detector in your instrument arrangement. If you want to add a nondestructive detector to your system - all spectroscopic detectors are of that kind - you can simply connect this in line with your LC column upfront and the mass spectrometer behind. One thing to take care of is the additional volume of the detector flow cell, which in most cases also adds a measurable contribution to band broadening. Using UV... 

As for any physical quantity (Section 1.1), measurement of mJz values of ions is ultimately a comparison of the unknown with a standard of some kind. In the case of a mass spectrometer, the raw data transferred to the computer do not involve tnlz values at all but are specifications of the times at which electrical signals (arising from arrival of ions at the detector) are transferred to the computer during a scan of the electric and/or magnetic fields that the particular analyzer employs to separate the ions. (In the case of time of flight analyzers the relevant quantity is the time elapsed since the ions were pulsed into the flight mhe, see Section 6.4.7.) To transform these times of detection into mlz values, it is necessary to obtain a cahhration of time vs mlz obtained using mass cahbration standards that yield ions of known mlz calculated from their molecular formulae and values of atomic masses (Table 1.4). It can be mentioned that... 

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