Mass spectrometer determination

Aqueous samples extracted by purge and trap method the sorbent trap heated and back flushed with He to transfer the analyte onto a capillary GC column interfaced to a mass spectrometer determined by GC/MS. 

Mass spectrometers determine atomic and molecular isotope ratios. Table 2 lists the relative isotopic abundance of elements commonly encountered in pharmaceutical analysis [3,4]. The values in Table 2 have been empirically determined and refinements in the values are necessary as atomic mass measurements improve, but for this discussion any inaccuracies in the table are insignificant. For some elements there are only two naturally occurring isotopes. For example, if you were to randomly sample carbon atoms in nature, 99% of the time you would find 12C, and roughly 1% of the time a 13C would turn up. Other elements, such as chlorine and bromine, have elemental isotope ratios that are not as heavily... 

Ion cyclotron resonance mass spectrometers determine the ratio of an ion from the rotational frequency of the ion within the magnetic field [36—38]. The relationship between an ion s ratio and it rotational frequency is given by the following ... 

The MALDI-TOF mass spectrometer determines the MW of the protein. Because it also analyzes protein mixtures (separates protein mixtures substantially better than SDS gel electrophoresis), it can perform many functions of SDS gel electrophoresis. For example, the MALDI-TOF provides convincing data about the purity of proteins. As far as the sensitivity is concerned, you need 10 to 100 fM less protein for the MALDI-TOF spectnun than for the silver staining of an SDS gel. However, the MALDI-TOF mass spectrometer offers more than the SDS gel electrophoresis. 

Bel] Measurements of the activity of Cu in Fe rich C-Cu-Fe alloys using the Knudsen cell-mass spectrometer. Determination of copper and carbon solubility in liquid iron at 1600°C... 

High-resolution mass spectrometers determine the exact molecular mass, which allows a compound s molecular formula to be determined. 

Modem mass spectrometers are used in a very wide variety of situations, so it is almost impossible to have a simple set of criteria that would determine whether a quadrupole or magnetic sector instmment would be best for any particular application. Nevertheless, some attempt is made here to address major considerations, mostly relating to cost. 

There is a more important use. Suppose a mass spectrometer has accurately measured the molecular mass of an unknown substance as 58.04189. Reference to tables of molecular mass vs. elemental composition will reveal that the molecular formula is CjH O (see Table 38.2). The molecular formula for an unknown substance can be determined which is enormously helpful in identifying it. 

One method for measuring the temperature of the sea is to measure this ratio. Of course, if you were to do it now, you would take a thermometer and not a mass spectrometer. But how do you determine the temperature of the sea as it was 10,000 years ago The answer lies with tiny sea creatures called diatoms. These have shells made from calcium carbonate, itself derived from carbon dioxide in sea water. As the diatoms die, they fall to the sea floor and build a sediment of calcium carbonate. If a sample is taken from a layer of sediment 10,000 years old, the carbon dioxide can be released by addition of acid. If this carbon dioxide is put into a suitable mass spectrometer, the ratio of carbon isotopes can be measured accurately. From this value and the graph of solubilities of isotopic forms of carbon dioxide with temperature (Figure 46.5), a temperature can be extrapolated. This is the temperature of the sea during the time the diatoms were alive. To conduct such experiments in a significant manner, it is essential that the isotope abundance ratios be measured very accurately. 

Special instruments (isotope ratio mass spectrometers) are used to determine isotope ratios, when needed, to better than about 3%. Such special instruments are described in Chapters 6, 7, and 48. The methods of ionization and analysis for such precise measurements are not described here. 

A double-focusing mass spectrometer can mea.sure mass accurately to several decimal places, thus enabling the determination of molecular formulae and elemental compositions of fragment ions. 

Mass spectrometer. An instrument in which ions are analyzed according to their mass-to-charge ratio (m/z) and in which the number of ions is determined electrically (or via scintillator, vidicon, etc.). 

Effusion separator (or effusion enricher). An interface in which carrier gas is preferentially removed from the gas entering the mass spectrometer by effusive flow (e.g., through a porous tube or through a slit). This flow is usually molecular flow, such that the mean free path is much greater than the largest dimension of a traverse section of the channel. The flow characteristics are determined by collisions of the gas molecules with surfaces flow effects from molecular collisions are insignificant. 

In Surface Analysis by Laser Ionization (SALI), a probe beam such as an ion beam, electron beam, or laser is directed onto a surfiice to remove a sample of material. An untuned, high-intensity laser beam passes parallel and close to but above the sur-fiice. The laser has sufficient intensity to induce a high degree of nonresonant, and hence nonselective, photoionization of the vaporized sample of material within the laser beam. The nonselectively ionized sample is then subjected to mass spectral analysis to determine the nature of the unknown species. SALI spectra accurately reflect the surface composition, and the use of time-of-flight mass spectrometers provides fast, efficient and extremely sensitive analysis. 

One of the important advantages of ICPMS in problem solving is the ability to obtain a semiquantitative analysis of most elements in the periodic table in a few minutes. In addition, sub-ppb detection limits may be achieved using only a small amount of sample. This is possible because the response curve of the mass spectrometer over the relatively small mass range required for elemental analysis may be determined easily under a given set of matrix and instrument conditions. This curve can be used in conjunction with an internal or external standard to quantily within the sample. A recent study has found accuracies of 5—20% for this type of analysis. The shape of the response curve is affected by several factors. These include matrix (particularly organic components), voltages within the ion optics, and the temperature of the interffice. 

It is possible to measure equilibrium constants and heats of reaction in the gas phase by using mass spectrometers of special configuration. With proton-transfer reactions, for example, the equilibrium constant can be determined by measuring the ratio of two reactant species competing for protons. Table 4.13 compares of phenol ionizations. 

Mass spectrometers exploit the difference in the mass-to-charge (m/z) ratio of ionized atoms or molecules to separate them from each other. The m/z ratio of a molecule is also a highly characteristic property that can be used for determining chemical and structural information. Further, molecules can be fragmented in distinctive ways in mass spectrometers, and the fragments that arise also provide quite specific structural information about the molecule. The basic... 

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