Instrumentation - The Mass Spectrometer

Isotopes and atomic weights. Nature 105, 617-619 (1920c). 

Mass Spectra and Isotopy, Nobel Lecture, December 12, 1922 available at http //nobelprize.org/nobel prizes/chemistry/laureates/1922/aston-lectures.pdf 

A new spectrograph and the whole number rule. Proc. Roy. Soc. Land. A115,487-514 (1927). 

Babcock, H. D. Some new features of the atmospheric oxygen bands, and the relative abundance of the isotopes 0-16, 0-18. Proc. Nat. Acad. Sci. USA 15, 471 —477 (1929). 

and Menzel, D. H. The relative abundance of the oxygen isotopes, and the basis of the atomic weight system. Phys. Rev. 37, 1669-1671 (1931). 

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There are two basic types of detectors used to measure ion signals, current detectors and ion counters. Each type has different implementations. A third type of detector is an imaging detector. In some SIMS instruments, the mass spectrometer is also an ion microscope, which transmits a stigmatic image of the sample to a detector plane. 

For the most recent LC-MS on the market, an automatic procedure is included in the software package to tune and calibrate in the ESI mode. However, older instruments and/or very specific applications still require manual or semiautomatic procedures to optimize the parameters that affect ion detection. In an LC-MS instrument, the mass spectrometer is tuned and calibrated in three steps (1) ion source and transmission optimization, (2) MS calibration, and (3) fine tuning (detection maximization of one or more particular ions). 

The determination of deviations in isotopic ratios requires very precise measurements. The combustion stage involved is usually carried out immediately before injection into the mass spectrometer. Some instruments have been developed that include a gas chromatograph in line with a tubular combustion oven (containing copper oxide at 800 C) and a low-resolution magnetic sector instrument. The mass spectrometer is equipped with a multicollector that allows recording at each individual mass (see Fig. 16.24). 

MS has long been used for the detection of low molecular weight gases. Hence, the devices are sometimes called residual gas analyzers and are typically magnetic sector or quadrapole instruments. The mass spectrometer is often considered to be the best GC detector since it can usually provide unambiguous identification of eluted components. A variety of mass spectrometers have been used as... 

Despite the speed and accuracy of contemporary analytical techniques, the use of more than one, separately and in sequence, is still very time-consuming. To reduce the analysis time, many techniques are operated concurrently, so that two or more analytical procedures can be carried out simultaneously. The tandem use of two different instruments can increase the analytical efficiency, but due to unpredictable interactions between one technique and the other, the combination can be quite difficult in practice. These difficulties become exacerbated if optimum performance is required from both instruments. The mass spectrometer was a natural choice for the early tandem systems to be developed with the gas chromatograph, as it could easily accept samples present as a vapor in a permanent gas. 

Identifrcation of components in the extracts was conducted by mass spectrometry. The sample was injected onto an HPS890 GC. The chromatographic conditions for the OV-1 column were the same as described for GC analysis. The end of the GC capillary column was inserted directly into the ion source of the mass spectrometer via a heated transfer line maintained at 280°C. The mass spectrometer was a Micromass Prospec high resolution, double-focusing, magnetic sector instrument. The mass spectrometer was operated in the electron ionization mode (El), scanning from ni/z 450 to m/z 33 at 0.3 seconds per decade. 

Like most analytical instrumentation, the mass spectrometer (MS) initially found use as a research system. These complex laboratory-built research instruments were subsequently refined for commercial sale and widespread use, and are now available for routine sample assay. 

Several techniques have been developed for detecting the quantity of ga.s. and in some cases the type of gas, that has permeated through the sample. Perhaps the most imptvrtant of these from the research point of view is the use of a mass spectrometer as the detector [40,41]. In this case a pressure differential may be maintained across the test piece by coupling the low-pressure chamber of the transmission cell directly to the inlet manifold of the instrument. The mass spectrometer offers high sensitivity and the ability to measure the permeation rates of several gases simultaneously. 

Similar to any mass spectrometric experiment, ions that are intended to be converted to neutrals in NR MS should be first generated by appropriate ionization methods. In principle, all ionization methods described in Chapter 2.28 may be used for the generation of ions for NR MS studies. However, only a limited number of ionization techniques have found practical use for this purpose. They are electron impact, chemical ionization, fast atom bombardment, and secondary ion mass spectrometry. One of the reasons for not using other methods is that NR MS experiments are mostly carried out on sector instruments. The mass spectrometers of this type are usually equipped with relatively old methods of ionization. The second reason for using these methods is that they provide high ion fluxes of ions of interest. This condition is crucial for many NR MS experiments because of the overall low total efficiency (<0.1%) of the neutralization-reionization process. 

Self-evidently, collisions between the ions and gas molecules disturb the ion beam and have to be avoided to the lar st possible extent. As a result, in all MS instrumentation, the mass spectrometer and detection system are brought under... 

The charged particle movement described above permitted the development of a powerful instrument - the mass spectrometer a device for sorting ions on their specific charge q/m. Such an opportunity is extremely tempting for modem chemistry for the analysis and synthesis of new substances and for many other problems. 

The mass spectrometer tends to be a passive instrument in these applications, used to record mass spectra. In chemical physics and physical chemistry, however, the mass spectrometer takes on a dynamic function as a... 

The Z-spray inlet causes ions and neutrals to follow different paths after they have been formed from the electrically charged spray produced from a narrow inlet tube. The ions can be drawn into a mass analyzer after most of the solvent has evaporated away. The inlet derives its name from the Z-shaped trajectory taken by the ions, which ensures that there is little buildup of products on the narrow skimmer entrance into the mass spectrometer analyzer region. Consequently, in contrast to a conventional electrospray source, the skimmer does not need to be cleaned frequently and the sensitivity and performance of the instrument remain constant for long periods of time. 

In one instrument, ions produced from an atmospheric-pressure ion source can be measured. If these are molecular ions, their relative molecular mass is obtained and often their elemental compositions. Fragment ions can be produced by suitable operation of an APCI inlet to obtain a full mass spectrum for each eluting substrate. The system can be used with the effluent from an LC column or with a solution from a static solution supply. When used with an LC column, any detectors generally used with the LC instrument itself can still be included, as with a UV/visible diode array detector sited in front of the mass spectrometer inlet. 

Before sample preparation, the laboratory must demonstrate that the mass spectrometer is operating satisfactorily. First, the instrument must be tuned by calibration using one of two compounds. 

In many applications of mass spectrometry, it is necessary to obtain a mass spectrum from a sample dissolved in a solvent. The solution cannot be passed directly into the mass spectrometer because, in the high vacuum, the rapidly vaporizing solvent would entail a large pressure increase, causing the instrument to shut down. 

Mass spectrometer configuration. Multianalyzer instruments should be named for the analyzers in the sequence in which they are traversed by the ion beam, where B is a magnetic analyzer, E is an electrostatic analyzer, Q is a quadrupole analyzer, TOP is a time-of-flight analyzer, and ICR is an ion cyclotron resonance analyzer. For example BE mass spectrometer (reversed-geometry double-focusing instrument), BQ mass spectrometer (hybrid sector and quadrupole instrument), EBQ (double-focusing instrument followed by a quadrupole). 

The various SNMS instruments using electron impact postionization differ both in the way that the sample surface is sputtered for analysis and in the way the ionizing electrons are generated (Figure 2). In all instruments, an ionizer of the electron-gun or electron-gas types is inserted between the sample surface and the mass spectrometer. In the case of an electron-gun ionizer, the sputtered neutrals are bombarded by electrons from a heated filament that have been accelerated to 80—... 

The mass spectrometer usually found on ICPMS instruments is a quadrupole mass spectrometer. This gives high throughput of ions and resolutions of 1 amu. Only a... 

Detection limits in ICPMS depend on several factors. Dilution of the sample has a lai e effect. The amount of sample that may be in solution is governed by suppression effects and tolerable levels of dissolved solids. The response curve of the mass spectrometer has a large effect. A typical response curve for an ICPMS instrument shows much greater sensitivity for elements in the middle of the mass range (around 120 amu). Isotopic distribution is an important factor. Elements with more abundant isotopes at useful masses for analysis show lower detection limits. Other factors that affect detection limits include interference (i.e., ambiguity in identification that arises because an elemental isotope has the same mass as a compound molecules that may be present in the system) and ionization potentials. Elements that are not efficiently ionized, such as arsenic, suffer from poorer detection limits. 

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. 

Gas chromatography/mass spectrometry (GC/MS) is the synergistic combination of two powerful analytic techniques. The gas chromatograph separates the components of a mixture in time, and the mass spectrometer provides information that aids in the structural identification of each component. The gas chromatograph, the mass spectrometer, and the interface linking these two instruments are described in this chapter. 

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