Ionic mass

In its simplest form, a mass spectrometer is an instmment that measures the mass-to-charge ratios ml of ions formed when a sample is ionized by one of a number of different ionization methods (1). If some of the sample molecules are singly ionized and reach the ion detector without fragmenting, then the ml ratio of these ions gives a direct measurement of the molecular weight. The first instmment for positive ray analysis was built by Thompson (2) in 1913 to show the existence of isotopic forms of the stable elements. Later, mass spectrometers were used for precision measurements of ionic mass and abundances (3,4). 

Typical decay curves for various ionic masses are shown in Figure 3b, where the effect of differing thermal velocity is clearly evident. Neglect of such decay characteristics can lead to serious discrimination effects. Note that all the experimental curves have the sigmoid shape... 

Use of the Born-Oppenheimer approximation is implicit for any many-body problem involving electrons and nuclei as it allows us to separate electronic and nuclear coordinates in many-body wave function. Because of the large difference between electronic and ionic masses, the nuclei can be treated as an adiabatic background for instantaneous motion of electrons. So with this adiabatic approximation the many-body problem is reduced to the solution of the dynamics of the electrons in some frozen-in configuration of the nuclei. However, the total energy calculations are still impossible without making further simplifications and approximations. 

A more detailed review of this and other early work in ionic mass transfer is given by Tobias et al. (T3). 

Obviously, mass spectrometry is ideally suited for distinguishing between isotopic species, and isotopic labeling is used for mechanistic as well as analytical applications (Chap. 3.2.9). However, the effect of isotopic substitution is not only an effect on ionic mass, but isotopic substitution can have several simultaneous effects, and this complication sometimes produces results which are at first sight curious. [65]... 

Example the molecular ions of nitrogen, N2, carbon monoxide, CO, and ethene, C2H4, have the same nominal mass of 28 u, i.e., they are so-called iso-baric ions. The isotopic masses of the most abundant isotopes of hydrogen, carbon, nitrogen and oxygen are 1.007825 u, 12.000000 u, 14.003070 u and 15.994915 u, respectively. Using these values, the calculated ionic masses are 28.00559 u for Nz"" , 27.99437 u for CQ-", and 28.03075 u for CjH/. This means they differ by some millimass units" (mmu) from each other, and none of these isobaric ions has precisely 28.00000 u (Chap. 3.3.4 and Chap. 6.9.6). 

Although the initial velocity of the desorbed ions is difficult to measure, reported values generally are in the range of 400-1200 m s" The initial velocity is almost independent of the ionic mass but dependent on the matrix. [33,36-38,46,50,51] On the other hand, the initial ion velocity is not independent of the compound class, i.e., peptides show a behavior different from oligosaccharides. [51]... 

The operation of magnetic sector (Chap. 4.3), linear quadrupole (Chap. 4.4), or quadrupole ion trap (Chap. 4.5) mass spectrometers in the repetitive scanning mode is useful for the identification of the components of a mixture. If quantitation is a major issue (below), selected ion monitoring (SIM) is preferably employed the term multiple ion detection (MID) and some others are also in use. [33] In the SIM mode, the mass analyzer is operated in a way that it alternately acquires only the ionic masses of interest, i.e. it jumps from one m/z value to the next. [34-39] The information obtained from a SIM trace is equivalent to that from a RIC, but no mass spectra are recorded. Thus, the scan time spent on a diagnostically useless m/z range is almost reduced to zero, whereas the detector time for the ions of interest is increased by a factor of 10-100. [40] An analogous improvement in sensitivity (Chap. 5.2.3) is also observed. 

Note that molecular or ionic masses cannot be calculated from empirical formulae. 

Carbon dioxide may be readily analyzed by various instrumental techniques, such as IR, GC, and GC/MS. Many portable infrared analyzers are available commercially for rapid, on site monitoring of CO2. Also, it can be analyzed by GC using a TCD or an FID. It readily may be identified by mass spectrometry from its characteristic ionic mass 44. Dissolved CO2 in water... 

Elemental composition C 15.77%, S 84.23% carbon disulfide. It may be analyzed by GC using a sulfur chemiluminescence detector or by GC/MS. A concentration of 1 ppm in the air may be measured by mass spectrometry. The primary characteristic ionic mass for identification of this compound by mass spectrometry is 76. Many GC columns are available commercially. 

Ion mobility spectrometry (IMS), which has the ability to separate ionic species at atmospheric pressure, is another technique that is useful for detect and characterising organic vapours in air [97]. This involves the ionisation of molecules and their subsequent drift through an electric field. Analysis is based on analyte separations resulting from ionic mobilities rather than ionic masses. A major advantage of operation at atmospheric pressure is that it is possible to have smaller analytical units, lower power requirements, lighter weight and easier use. 

Table 3.3 Ionic masses and critical energy deficits of gas ions... 

It is found that with this calibration method ionic masses can be derived with an accuracy of 0.01 to 0.05 u for light to heavy elements. With a proper calibration, the shape of the mass lines also appears much more symmetric and a better mass resolution is achieved. But again, the system cannot be used for an ion energy analysis because of the flight-time-focusing scheme. 

Sulphates (SO42 ), ammonium (NH4+) and nitrates (NO3 ) are the main secondary inorganic aerosol ions as they account for about two thirds of the total ionic mass in PMj and for about 50% in PM10 in Athens [21], These ions represent two different major source categories fuel combustion and vehicular circulation. Theodosi et al. [21] studied the spatial variability of these ions in 2 sites within GAA Lykovrisi (LYK) and Goudi (GOU). LYK is a moderately populated municipality, in the northern part of the GAA, 10 km from the city centre GOU is located... 

Keister, J.C., and G.B. Kasting. 1986. Ionic mass transport through a homogenous membrane in the presence of a uniform electric field. J Membr Sci 29 15. 

Cyclotron Equation Inverse relationship between ionic mass (m)... 

Mass spectrometers (MSs) are classified by the technology they use to separate ionic masses into clusters. Their examples include the quadrupole, magnetic sector, time-of-flight, and ion-trap mass spectrometers. 

The conservation of (ionic) mass in each volume element is given by [1.6.1.61 and reads... 

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