Ion source, of mass spectrometer

Glucuronides are often thermally unstable in the ion source of mass spectrometers and can break down to the aglycone via thermally induced... 

In the 1980s, attempts were made to enable continuous introduction of liquid samples (especially aqueous buffer solutions) to ion sources of mass spectrometers. An early con-tiuous flow interface was based on the fast atom bombardment (FAB) ion source [70]. However, it was the ESI interface that greatly facilitated temporal profiling of dynamic... 

Polyphenols are thermally labile compounds and their evaporation without decomposition is impossible even in the ion source of mass spectrometer, where high vacuum exists. In this situation, soft ionization methods have to be applied for... 

Ion sources of mass spectrometers, which were dis-cus.sed in some detail in the previous section and in Chapter 11. convert the components of a sample into ions. In many cases the inlet sv stem and the ion source are combined into a single component. In either case. 

Figure 1. Low pressure apparatus. (1) inlet, (2) vacuum pump, (3) manometer, (4) ion source of mass spectrometer.

Tandem mass spectrometry has been used to demonstrate that M+ as well as MH+ of substituted A-(ort/zo-cyclopropylphenyl)benzamides isomerizes before the fragmentation, with formation of 3-aryl-1-ethyl-lH-benzoxazines and 5-ethyl-2-oxodi-benzoazepines (Scheme 5.14). The methyl group in /V-[ortho-( 1 -methylcvclopropyl )-phenyl]benzamides quenches the latter process, leaving the formation of benzoxazines as the only cyclization reaction. A subsequent chemical experiment in solution confirmed the mass spectral predictions [24]. A similar study confirmed the analogy of cyclization of substituted A-(ort/zo-cyclopropylphenyl)-A -aryl ureas and N- ortho-cyclopropylphenyl)-A -aiyl thioureas in the ion source of mass a spectrometer and in solution [25]. 

Allelochemicals found in extracts of such botanical materials as plant leaves can often be well separated by liquid chromatography (LC). Identification of the separated components on-line by mass spectrometry (MS) is of great value because LC has the ability to deliver samples into the ion source of the spectrometer with low or no thermal decomposition. 

Given the variety of forms and the nature of possible samples, there are numerous methods of introducing a sample to a mass spectrometer. The sample can be ionised either before or in the ion source of the spectrometer. 

The mass spectra of the gases evolved from the deuterated SWNT sample heated in vacuum were measured with the MI 1201V mass spectrometer. Gas ionization in the ion source of the spectrometer was produced with a 70-eV electron beam. To obtain the gas phase, the sample was placed in a quartz ampoule of a pyrolyzer that was connected to the injection system of the mass spectrometer through a fine control valve. Then the ampoule was evacuated to a pressure of about 2-x 10-5 Pa in order to remove the surface and weakly bound impurities from the sample. After the evacuation, the ampoule was isolated from the vacuum system and the sample was heated to 550°C in five steps. At each step, the sample was kept at a fixed temperature for 3 h then the fine control valve was open and the mass-spectrometric analysis of the gas collected in the ampoule was performed. After the analysis, the quartz ampoule was again evacuated, the valve was closed, and the sample was heated to the next temperature. The measurements were carried out over the range 1 < m/z < 90, where m is the atomic mass and z is the ion charge. The spectrometer resolution of about 0.08% ensured a reliable determination of the gas-phase components. 

A suitable method for recording the atoms or molecules released is mass spectroscopy. Mass spectroscopy is extremely sensitive to the release of ions, less sensitive to neutrals because of the ionization efficiency (typically 0.01%) of the ion source of the spectrometer. 

The first reliable spectroscopic analysis of saturated sulfur vapor was published by Berkowitz and Marquart [28] who used a combination of a Knud-sen effusion cell with a mass spectrometer and generated the sulfur vapor by evaporating either elemental sulfur (low temperature region) or certain metal sulfides such as HgS which decompose at high temperatures to sulfur and metal vapor. These authors observed ions for all molecules from S2 to Ss and even weak signals for Sg and Sio. From the temperature dependence of the ion intensities the reaction enthalpies for the various equilibria (1) were derived (see Table 1). Berkowitz and Marquart careMly analyzed their data to minimize the influence of fragmentation processes in the ion source of the spectrometer. They also calculated the total pressure of sulfur vapor from their data and compared the results with the vapor pressure measurements by Braune et al. [26]. The agreement is quite satisfactory but it probably... 

T. O. Tiernan, Development of a High-Efl5ciency Negative-Ion Source for Mass Spectrometers, OAR Progress 1969, p. 49, Office of Aerospace Research Report OAR 69-0017, AD 699300 (1969). 

The quantity c nj(Nj is the concentration of analyte emerging from the last theoretical plate into the detector, so Equation [3.11] corresponds to the desired theoretical expression for Rd(V) for cases in which the chromatographic detector has a concentration dependent response (Section 4.4.8 and Appendix 4.1) UV-visible absorption detectors are an important example since their response is described by the Beer-Lambert Law, but elecirospray ion sources for mass spectrometers can also behave in this fashion in some circumstances (Section 5.3.6b). Electron ionization ion sources provide a response that is mass flux dependent (Section 4.4.8) however, for a fixed mobile phase flow rate U (volume per unit time), the conversion from c ni(N) to the mass flow rate is trivial and this distinction is not important in the discussion of the present Section although the practical imphcations are discussed in Section 5.3.6b. 

Another recent approach has been to insert an ion mobility device between the ion source and mass spectrometer. The additional separation power added by the differences in ion transit times through a buffer gas can often distinguish between analyte ions and the background ions that originate in the ion source (and thus can not be chromatographically separated). This multidimensional approach is feasible because of the differences in timescales among the HPLC peaks (seconds), ion mobility (tens of milliseconds) and SIR or MRM dwell times (a few... 

In preparing samples for GC/MS, one simple fact must be kept in mind, namely, that everything injected onto the gas chromatographic column will be deposited into the mass spectrometer with the exception of those sample components which remain in the injection port or on the column. For volatile components this is not a concern as they are pumped away by the spectrometer vacuum system without consequence, but semivolatile materials may deposit in the ion source of the spectrometer with resultant loss of sensitivity, increased maintenance, and other unfavorable results. It is not nncommon for normal column bleed to eventually degrade system performance. For particularly valuable samples, such as metabolite extracts, biological samples, or other samples obtained through extensive effort, the contamination threat must be tolerated as the cost of analysis. However, if sample cleanup is possible without significant sample alteration, then a reasonable effort should be made to prevent contamination of the spectrometer. 

Several advanced PyMS configurations have been described. Boon et al. [712] have presented a multi-purpose external ion source FTICR mass spectrometer for rapid microscale analysis of complex mixtures. External source DT-FTlCR-MS allows obtaining nominal mass spectra, temperature windows, HRMS data and exact elemental composition and MS/MS data on selected ions. For more detailed structural analysis of the more volatile part of the pyrolysate PyGC-MS and PyGC-HRMS are frequently applied. Laser pyrolysis experiments benefit... 

Ionization is a crucial process occurring in the ionization source of mass spectrometers There are several requirements about the ionization process (i) The ionization process and ion extraction from the ionization source should be reasonably efficient to maintain low detection limits (high sensitivity) and (ii) the ionization efficiency, desirably, should not be sample dependent and the generated ion current should stay steady for rehable quantitation. The current state-of-the-art mass spectrometers are equipped with efficient ionization sources however, for quantitation the use of internal standards is strongly recommended. 

In essence, a guided-ion beam is a double mass spectrometer. Figure A3.5.9 shows a schematic diagram of a griided-ion beam apparatus [104]. Ions are created and extracted from an ion source. Many types of source have been used and the choice depends upon the application. Combining a flow tube such as that described in this chapter has proven to be versatile and it ensures the ions are thennalized [105]. After extraction, the ions are mass selected. Many types of mass spectrometer can be used a Wien ExB filter is shown. The ions are then injected into an octopole ion trap. The octopole consists of eight parallel rods arranged on a circle. An RF... 

It is possible to detemiine the equilibrium constant, K, for the bimolecular reaction involving gas-phase ions and neutral molecules in the ion source of a mass spectrometer [18]. These measurements have generally focused on tln-ee properties, proton affinity (or gas-phase basicity) [19, 20], gas-phase acidity [H] and solvation enthalpies (and free energies) [22, 23] ... 

These thin wires are supported on a special carrier that can be inserted into the ion source of the mass spectrometer after first growing the whiskers in a separate apparatus. Although the wires are very fragile, they last for some time and are easily renewed. They are often referred to as emitter electrodes (ion emitters). 

As described above, the mobile phase carrying mixture components along a gas chromatographic column is a gas, usually nitrogen or helium. This gas flows at or near atmospheric pressure at a rate generally about 0,5 to 3.0 ml/min and evenmally flows out of the end of the capillary column into the ion source of the mass spectrometer. The ion sources in GC/MS systems normally operate at about 10 mbar for electron ionization to about 10 mbar for chemical ionization. This large pressure... 

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