Parts of a spectrometer

Of course, the most precious part of a spectrometer is the magnet Both field stability and field homogeneity are of prime importance and have a huge effect on the quality of spectra. A tittle calculation will show how difficult it is to achieve the required field stabil-... 

J The plural is used here since different theoretical model chemistries give rise to different spectrometers . In rough terms, the methods and approximations which constitute different theoretical model chemistries can be liken to the different component parts of a spectrometer. 

Computer-Assisted Infrared. The impact of computers on spectroscopy has been high. In vibrational spectroscopy, it was felt strongest in the 1970s when the revolution in computer technology brought prices down into the range where it was practical to have a dedicated computer as an integral part of a spectrometer. 

Figure 16.3. Typical electron-impact source. The source is mounted on a frame and inserted into the flight tube of the mass spectrometer. The voltages in parentheses are typical values for the component parts of a spectrometer operating at 5000 V accelerating potential. Note that the target is at +100 V with respect to the filament, and the filament is at —70 F with respect to the ionization chamber.

There are alternative means of source frequency stabilisation " one is to couple an oscillator to a resonant cavity machined from a temperature insensitive alloy as part of a spectrometer, e.g. Zhu et alP Oscillator modules are available commercially, e.g. Farran," Elva-l, " that have low phase noise and can be employed as local oscillators in narrow band spectrometer designs. Thirup et aV... 

The low MW power levels conuuonly employed in TREPR spectroscopy do not require any precautions to avoid detector overload and, therefore, the fiill time development of the transient magnetization is obtained undiminished by any MW detection deadtime. (3) Standard CW EPR equipment can be used for TREPR requiring only moderate efforts to adapt the MW detection part of the spectrometer for the observation of the transient response to a pulsed light excitation with high time resolution. (4) TREPR spectroscopy proved to be a suitable teclmique for observing a variety of spin coherence phenomena, such as transient nutations [16], quantum beats [17] and nuclear modulations [18], that have been usefi.il to interpret EPR data on light-mduced spm-correlated radical pairs. 

A multipoint ion collector (also called the detector) consists of a large number of miniature electron multiplier elements assembled, or constructed, side by side over a plane. A multipoint collector can be an array, which detects a dispersed beam of ions simultaneously over a range of m/z values and is frequently used with a sector-type mass spectrometer. Alternatively, a microchannel plate collector detects all ions of one m/z value. When combined with a TOP analyzer, the microchannel plate affords an almost instantaneous mass spectrum. Because of their construction and operation, microchannel plate detectors are cheaper to fit and maintain. Multipoint detectors are particularly useful for situations in which ionization occurs within a very short space of time, as with some ionization sources, or in which only trace quantities of any substance are available. For such fleeting availability of ions, only multipoint collectors can measure a whole spectrum or part of a spectrum satisfactorily in the short time available. 

Separation of ions according to their m/z values can be effected by magnetic and/or electric fields used as mass analyzers, which are described in Chapters 24 through 27. However, apart from measurement of m/z values, there is often a need to be able to transmit ions as efficiently as possible from one part of a mass spectrometer to another without any mass separation. 

For efficient transmission of ions, multipolar guides provide a means of reducing ion losses during their transit from one part of a mass spectrometer to another. It is useful to understand some of the reasons for ion losses. 

One very important group of infrared instruments consists of spectrometers used for quantitative measurements either as part of a continuous industrial monitoring process or for environmental studies. These instruments are normally purpose-made, dedicated machines designed to run virtually automatically, and are normally intended only to measure a single compound or family of compounds. 

Tn a double mass spectrometer several types of ion-molecule reactions - can be observed (a) charge exchange, A++B- A + B+, often followed by dissociation of B+ (b) transfer of part of A+ or B (e.g., proton transfer or hydride ion transfer) during the collisions (c) reactions at increased pressure in the collision chamber. 

The power of mass spectrometry lies in the fact that the mass spectra of many compounds are sufficiently specific to allow their identification with a high degree of confidence, if not with complete certainty. If the analyte of interest is encountered as part of a mixture, however, the mass spectrum obtained will contain ions from all of the compounds present and, particularly if the analyte of interest is a minor component of that mixture, identification with any degree of certainty is made much more difficult, if not impossible. The combination of the separation capability of chromatography to allow pure compounds to be introduced into the mass spectrometer with the identification capability of the mass spectrometer is clearly therefore advantageous, particularly as many compounds with similar or identical retention characteristics have quite different mass spectra and can therefore be differentiated. This extra specificity allows quantitation to be carried out which, with chromatography alone, would not be possible. 

The devices described are mostly used in portable instruments. For a deeper understanding, extended studies are recommended. After a general review of the most important parts of optical spectrometers, we proceed now to colorimetric particularities. 

Generally we are used to dealing with molecules. However, MS studies ions. Consequently, the first event that must occur inside the mass spectrometer is the transformation of molecules into ions. This process is called ionization and it occurs in the first part of a mass spectrometer the ion source. 

Once ions have been produced and analysed they must be detected. Indeed, the detector is the final part of a mass spectrometer. At the start of MS, detectors were composed of a fluorescent screen or a photographic plate the modem instruments are equipped with detectors able to transform the signal produced by the ion beam into an electric current that is transmitted to the data system. 

One of the best tools for metabolite profiling is the hybrid QTRAP MS/MS system (Applied Biosystems).119-121 While the hybrid QTRAP MS/MS was initially considered a premier tool for metabolite identification, it has more recently been seen as a tool for quantitation and metabolite profiling. Li et al.122 described the use of a hybrid QTRAP MS/MS system for discovery PK assays plus metabolite profiling in the same analytical procedure. Because QTRAP MS/MS may be used as a triple quadrupole MS system, it can be used as part of a quantitative HPLC/MS/MS system. Because QTRAP MS/MS also has linear ion trap capabilities, it can be used for metabolite screening and characterization—essentially it combines the capabilities of a triple quadrupole mass spectrometer and a linear ion trap mass spectrometer. 

Most GC-MS instruments do not detect small compounds and are restricted to fragments with masses in the range of 60-600 amu.2 However, mass spectrometers that can measure small and large masses are available, although usually not in the same instrument, and can be used as part of a GC-MS system. 

Modem gratings may be etched on concave surfaces so that they will serve a dual purpose of diffracting light and also focusing the radiation. This decreases the number of parts in a spectrometer and also decreases losses in intensity by having fewer optical parts. 

Note Equation 4.3 describes the velocity of any ion after acceleration in an electric field, and therefore it is valid not only for TOE-MS, but for any part of a mass spectrometer handling beams of ions. 

As an essential part of a mass spectrometer, the ion separation system has the task of separating the fast-flying ions (with different masses m and charges z (with z = n-e) formed in an ion source and extracted from this source using an ion optic system) with respect to their different mass-to-charge (m/z) ratios. The separated ion beams are than supplied to the ion detection system for spatial or time resolved ion detection and registration. The mass spectrum is then the 2D representation of ion intensity as a function of the m/z ratio. 

The introductory chapter is brief but provides an ample introduction to mass spectrometry and leaves one comfortable as he/she moves on to the historical and instrumentation chapters that follow. A few of the basic equations are given as part of the review of basic concepts. In these few pages Dr Becker clearly introduces the concepts of atomic mass units relative to carbon, isotopes and isotope abundance. Figures 1.1 and 1.2 go hand in hand in providing the reader with the three major parts of a mass spectrometer (source, ion separation, detection) and show various alternatives for each of these. The subtle use of color in these and subsequent figures adds an attractive benefit for the reader. 

Analysis by atomic (or optical) emission spectroscopy is based on the study of radiation emitted by atoms in their excited state, ionised by the effect of high temperature. All elements can be measured by this technique, in contrast to conventional flames that only allow the analysis of a limited number of elements. Emission spectra, which are obtained in an electron rich environment, are more complex than in flame emission. Therefore, the optical part of the spectrometer has to be of very high quality to resolve interferences and matrix effects.-... 

In most applications the low signal levels necessitate the use of signalaveraging techniques. A wide variety of multichannel analyzers, mini computers, and microprocessors suitable for this application are now commercially available. Frequently these devices must issue commands and/or receive information from parts of the spectrometer that are at elevated voltage levels, and this can best be achieved by means of optical coupling. 

---