6- vacuum mass generation

After desolvation and subsequent charge concentration, gas-phase ions are produced and propelled toward the high vacuum mass analyzer. ESI is considered to be one of the softest ionization techniques available, i.e., little energy is transferred to the molecule other than that required for ionization. Thus, protonated, deprotonated, or cationized molecules that undergo very little fragmentation are generated, even from highly polar, thermally labile molecules. 

An atmospheric-pressure ion source for electrospray ionization consists of five parts (Figure 2) (1) the pneumatically assisted electrospray needle, used for the introduction of sample solution or LC mobile phase (2) the actual ion source region, where ions are generated from the microdroplets at atmospheric pressure (3) the ion-sampling aperture (4) the atmospheric-pressure to high-vacuum interface and (5) the ion-optical system, where the ions generated in the source are analyte-enriched and transported toward the high-vacuum mass analyzer. 

LC-MS and, in particular, tandem mass spectrometry (LC-MS/MS) techniques, offer a number of experiments for the determination of conjugated materials such as glucuronides from biological fluids.In LC-MS instruments, the typical analyzer used (e.g., quadrupole) analyzes ions that are sampled to the MS vacuum and generates molecular weight information that is, in most cases, limited to the parent ion. In dmg metabolism research, where often... 

This teclnhque can be used both to pennit the spectroscopic detection of molecules, such as H2 and HCl, whose first electronic transition lies in the vacuum ultraviolet spectral region, for which laser excitation is possible but inconvenient [ ], or molecules such as CH that do not fluoresce. With 2-photon excitation, the required wavelengdis are in the ultraviolet, conveniently generated by frequency-doubled dye lasers, rather than 1-photon excitation in the vacuum ultraviolet. Figure B2.3.17 displays 2 + 1 REMPI spectra of the HCl and DCl products, both in their v = 0 vibrational levels, from the Cl + (CHg) CD reaction [ ]. For some electronic states of HCl/DCl, both parent and fragment ions are produced, and the spectrum in figure B2.3.17 for the DCl product was recorded by monitoring mass 2 (D ions. In this case, both isotopomers (D Cl and D Cl) are detected. 

Selenium and precious metals can be removed selectively from the chlorination Hquor by reduction with sulfur dioxide. However, conditions of acidity, temperature, and a rate of reduction must be carefliUy controlled to avoid the formation of selenium monochloride, which reacts with elemental selenium already generated to form a tar-like substance. This tar gradually hardens to form an intractable mass which must be chipped from the reactor. Under proper conditions of precipitation, a selenium/precious metals product substantially free of other impurities can be obtained. Selenium can be recovered in a pure state by vacuum distillation, leaving behind a precious metals residue. 

The main one is the incompatibility of HPLC, utilizing flow rates of ml min of a liquid, and the mass spectrometer, which operates under conditions of high vacuum. Even if this can be overcome, attention must then be focussed on the ionization of the analyte, bearing in mind the limitations of El and Cl discussed earlier in Chapter 3, and the generation of analytically useful mass spectra. 

Mass spectrometers, workhorse instmments described in Chapter 2, require a vacuum to function. A mass spectrometer generates a beam of ions that is sorted according to specifications of the particular instrument. Usually, the sorting depends on differences in speed, trajectory, and mass. For instance, one type of mass spectrometer measures how long it takes ions to travel from one end of a tube to another. Residual gas must be removed from the tube to eliminate collisions between gas molecules and the ions that are being analyzed. As the diagram shows, collisions with unwanted gas molecules deflect the ions from their paths and change the expected mass spectral pattern. 

In a separate set of experiments designed to follow the gas phase reactions of CHj-radicals with NO, CHj- radicals were generated by the thermal decomposition of azomethane, CHjN NCHj, at 980 °C. The CH3- radicals were subsequently allowed to react with themselves and with NO in a Knudsen cell that has been described previously [12]. Analysis of intermediates and products was again done by mass spectrometry, using the VIEMS. Calibration of the mass spectrometer with respect to CH,- radicals was carried out by introducing the products of azomethane decomposition directly into the high vacuum region of the instrument. 

The movement of air in the subsurface during the application of SVE is caused by the pressure gradient that is applied in the extraction wells. The lower pressure inside the well, generated by a vacuum blower or pump, causes the soil air to move toward the well. Three basic equations are required to describe this airflow the mass balance of soil air, the flow equation due to the pressure gradient, and the Ideal Gas Law. 

Most nitrile oxides are unstable, some of them are explosive. This fact hinders the study of their physical properties. Nevertheless, there are a number of publications concerning not only stable but also unstable nitrile oxides. In particular, mass spectral data for nitrile oxides among other unstable compounds containing an N+-X bond are summarized in a review (9). In such studies, the molecular ions must be generated using indirect procedures, including dissociative electron ionization, online flash-vacuum pyrolysis mass spectrometry, or ion-molecular reactions. Their characterization is mainly based on collisional activation and ion-molecular reactions. 

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