Mass Spectrometer Instrument Designs

There are two principal components of mass spectrometers the ionization chamber, where ionization of the sample occurs, and the mass analyzer, where ion sorting and detection occur. Mass spectrometer instruments vary in design with regard to both of these components. Thus far we have mentioned only one ionization technique, electron impact (El). In Section 9.18A we discuss El ionization in more detail, as well as discuss two other important ionization methods electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI). 

A classic and highly useful reference on MS, NMR, and IR methods is Silverstein, R. M. and Webster, F. X. Spectrometric Identification of Organic Compounds, 6th ed. Wiley New York, 1998. 

Electrospray Ionization—A Technique Especially Useful for Biomolecules 

Electrospray ionization works especially well for mass spectrometry of proteins, carbohydrates, and nucleic acids. ESI mass spectrometry has been used to study protein molecular weights and sequence, enzyme-substrate complexes, antibody-antigen binding, drug-receptor interactions, and DNA oligonucleotide sequence, as well as simply for small molecules that cannot be ionized by electron impact. 

MALDI— A Technique Useful for Both Biomolecules and Synthetic Polymers 

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High-precision isotope ratio mass spectrometers are designed with combinations of multiple Faraday cup detectors and multiple miniature electron multipliers (used as ion counters) for simultaneous isotope measurement. For example, the TRITON and NEPTUNE multicollector mass spectrometers from Thermo Electron Corporation can be configured with up to nine Earaday cups and eight ion counters to detect 17 ion beams simultaneously. Details of these instruments can be found at www.thermo.com. The use of multicollector instruments improves precision by two to three orders of... 

Isotope mass spectrometers are designed to measure the abundance of an isotope in an unknown gas sample compared with that in a known reference gas. The layout of a typical instrument analyser is shown in Figure 1.3. There are important differences between the isotope mass spectrometer and the high resolution analytical mass spectrometer. In both types of instnunent the sample is ionised... 

Each type of mass spectrometer has its associated advantages and disadvantages. Quadrupole-based systems offer a fairly simple ion optics design that provides a certain degree of flexibility with respect to instrument configuration. For example, quadrupole mass filters are often found in hybrid systems, that is, coupled with another surface analytical method, such as electron spectroscopy for chemical analysis or scanning Auger spectroscopy. 

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. 

GC-C-IRMS was first demonstrated by Matthews and Hayes (1978). However, it was somewhat later that Barrie and others (Barrie et al., 1984) coupled a GC, via a combustion interface, to a dual collector mass spectrometer to produce the forerunner of today s GC-C-IRMS instruments. Even so, true determinations of 815N values of individual compounds by GC-C-IRMS remained elusive until finally demonstrated by Hayes and co-workers (Merritt and Hayes, 1994). More recently the precision of GC-C-IRMS instruments has been improved further still with uncertainties in 813C values as small as 0.5 %o for samples containing 5 pmol C and 0.1 %o for 100 pmol samples having been demonstrated (Merritt and Hayes 1994). Instruments available commercially today, from several manufacturers, all conform to the same general principles of design. 

Many older isotope ratio mass spectrometers employed symmetrical geometry. Most new instruments employ asymmetric design known as extended geometry, providing greater dispersion for the same radius ( 2x that of a symmetrical geometry). 

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