Mass spectrometer features

Note The triple-quadrupole picture is a hybrid mass spectrometer featuring an LQIT. Both q2 and Q3 have been used as LQIT (MDS Sciex Corp.) The LQIT scan modes are not covered in this table. Figure reprinted with permission hum J. Am. Soc. Mass Spectrom. 2003, 14, 1130—1147. 

A connnon feature of all mass spectrometers is the need to generate ions. Over the years a variety of ion sources have been developed. The physical chemistry and chemical physics communities have generally worked on gaseous and/or relatively volatile samples and thus have relied extensively on the two traditional ionization methods, electron ionization (El) and photoionization (PI). Other ionization sources, developed principally for analytical work, have recently started to be used in physical chemistry research. These include fast-atom bombardment (FAB), matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ES). 

Magnetic sectors can be used on their own, or in conjunction with energy analysers to fomi a tandem mass spectrometer. The unique features of the reverse geometry instrument are presented from this point. 

Instrumental Interfaces. The basic objective for any coupling between a gas chromatograph (gc) and a mass spectrometer (ms) is to reduce the atmospheric operating pressure of the gc effluent to the operating pressure in the ms which is about 10 kPa (10 torr). Essential interface features include the capability to transmit the maximum amount of sample from the gc without losses from condensation or active sites promoting decomposition no restrictions or compromises placed on either the ms or the gc with regard to resolution of the components and reliability. The interface should also be mechanically simple and as low in cost as possible. 

All commercially available SIMS systems have in common some type of computer automation, an ion source, a high-vacuum environment, and some type of mass spectrometer. While the specifics may vary from system to system, the basic requirements are the same. The hardware feature that tends to distii uish the various systems is the type of mass spectrometer used. These fall into three basic catego-... 

Laser ionization mass spectrometry or laser microprobing (LIMS) is a microanalyt-ical technique used to rapidly characterize the elemental and, sometimes, molecular composition of materials. It is based on the ability of short high-power laser pulses (-10 ns) to produce ions from solids. The ions formed in these brief pulses are analyzed using a time-of-flight mass spectrometer. The quasi-simultaneous collection of all ion masses allows the survey analysis of unknown materials. The main applications of LIMS are in failure analysis, where chemical differences between a contaminated sample and a control need to be rapidly assessed. The ability to focus the laser beam to a diameter of approximately 1 mm permits the application of this technique to the characterization of small features, for example, in integrated circuits. The LIMS detection limits for many elements are close to 10 at/cm, which makes this technique considerably more sensitive than other survey microan-alytical techniques, such as Auger Electron Spectroscopy (AES) or Electron Probe Microanalysis (EPMA). Additionally, LIMS can be used to analyze insulating sam-... 

The great advantage of the mass spectrometer is its abihty to use mass, more accurately the mass-to-charge ratio, as a discriminating feature. In contrast to, for example, the UV detector, which gives rise to broad signals with little selectivity, the ions in the mass spectrum of a particular analyte are often characteristic of that analyte. Under these conditions, discrete signals, which may be measured accurately and precisely, may be obtained from each analyte when they are only partially resolved or even completely umesolved from the other compounds present. 

The advantage of using a mass spectrometer as the detector is associated with cases (ii) and (iii) above. In particular, because mass may be used as a discriminating feature, it is possible to use an isotopically labelled analyte as an internal standard. These have virtnally identical physical and chemical properties to the unlabelled analogue, and are therefore likely to experience similar losses during... 

There are a number of features worthy of note in this figure. For example, there is a difference in retention times, determined by the two detectors, of ca. 0.32 min, and this reflects the fact that they are used in series, i.e. the column effluent passes through the UV detector on its way to the mass spectrometer. 

As stated previously, the advantage of the mass spectrometer is that mass can be used as a discriminating feature and this may allow quantitative measurements to be made on unresolved components. 

There are two notable features of the quantitative performance of this type of interface. It has been found that non-linear responses are often obtained at low analyte concentrations. This has been attributed to the formation of smaller particles than at higher concentrations and their more easy removal by the jet separator. Signal enhancement has been observed due to the presence of (a) coeluting compounds (including any isotopically labelled internal standard that may be used), and (b) mobile-phase additives such as ammonium acetate. It has been suggested that ion-molecule aggregates are formed and these cause larger particles to be produced in the desolvation chamber. Such particles are transferred to the mass spectrometer more efficiently. It was found, however, that the particle size distribution after addition of ammonium acetate, when enhancement was observed, was little different to that in the absence of ammonium acetate when no enhancement was observed. 

The main features of f.a.b.-m.s. are shown schematically in Fig. 1. The hardware consists of (i) an atom gun (or ion gun, see later) which is either mounted on the source housing of the mass spectrometer or, if small enough, inside the housing on the source itself, (it) a sample probe to the end of which is attached a small metal target onto which the sample is loaded, and (Hi) suitable source-optics for the efficient extraction of ions into the analyzer of the mass spectrometer. 

Such techniques imply analysis of chemical products of photolysis. Application of mass-spectrometers of various types is often hampered by a number of circumstances. These difficulties will be discussed later on. The EPR method, which is currently the most extensively employed technique, features low sensitivity and is usually used for analysis of primary fragments of photolysis. For this purpose, the radicals produced are frozen on the walls of a quartz pin and are thus accumulated inside the device. On one hand, this approach allows one to overcome the sensitivity threshold of the device. However, on the other hand, this excludes the possibility of direct kinetic measurements. The SS technique permits the use of weak light sources for detecting active particles under... 

Mass spectrometry has a number of features and advantages that can make it a very valuable tool for the identification of organic additives in polymers (Table 6.2). The range of products that can be studied is limited by the ionisation method used and the performance of the mass spectrometer. Mass spectrometry... 

The main features of DCI-MS and DCI-MS/MS are given in Table 6.14. DCI has gained rapid popularity because it is relatively simple to adapt to almost any mass spectrometer and gives results similar to FD, but in a more simple manner. It is not a substitute for FD, but it is less expensive and generally produces more fragmentation information than FD. For many compounds, molecular ions will be obtained where conventional solids probes would not do the job. DCI is known for the specificity provided by choosing reagent gas with different proton affinities. The major... 

The particle-beam, interface has been used for direct introduction of extracts into the mass spectrometer without chromatographic separation [55]. In fact, chromatographic separation is not always essential, especially if structural information is available about the analytes of interest. The main features of this particular approach are ... 

Smith and Udseth [154] first described SFE-MS in 1983. Direct fluid injection (DFT) mass spectrometry (DFT-MS, DFI-MS/MS) utilises supercritical fluids for solvation and transfer of materials to a mass-spectrometer chemical ionisation (Cl) source. Extraction with scC02 is compatible with a variety of Cl reagents, which allow a sensitive and selective means for ionising the solute classes of interest. If the interfering effects of the sample matrix cannot be overcome by selective ionisation, techniques based on tandem mass spectrometry can be used [7]. In these cases, a cheaper and more attractive alternative is often to perform some form of chromatography between extraction and detection. In SFE-MS, on-line fractionation using pressure can be used to control SCF solubility to a limited extent. The main features of on-line SFE-MS are summarised in Table 7.20. It appears that the direct introduction into a mass spectrometer of analytes dissolved in supercritical fluids without on-line chromatography has not actively been pursued. 

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