Dispersive spectrometers

The ability to separate ions spatially is called the dispersion of a mass spectrometer. Dispersion is simply the distance between the centers of two ion beams that differ in mass by Am at the collection plate. A simple sector instrument, where the ion beam enters and exist the magnetic field normal to the pole faces and the object and image distances are the same, is known as a symmetrical geometry analyser. Examples are shown in Fig. 8. In this case, the dispersion, D is given by ... 

InfraRed Spectroscopy (IR). Infrared speetroscopy is an efifeetive method for eharaeterization of polymers as to ehemieal structure. IR speetra of the sample examined are obtained by two basie types of IR spectrometers dispersive or Fourier transform (FTIR) ones. Infrared spectra are usually presented as a dependence of absorption (in pereent transmission) on wave length or wave... 

Dukhin, S.A., Installation Handbook and User Manual Model DT-1200 Electroacoustic Spectrometer, Dispersion Technology Inc., Mount Kisco, NY, 2001. 

Sampling techniques for Raman spectroscopy are relatively general, since the only requirement is that the monochromatic laser beam irradiate the sample of interest and the scattered radiation be focused on the detector. The sampling discussion outlined here is applicable to both types of spectrometers (dispersive/ FT). 

Fig. 1. Scale drawing of the dual electron-ion time-of-flight spectrometer. Dispersed synchrotron radiation passes through the chamber containing the N2 reactant gas. The photoelectrons are accelerated to the left detector, and the N ions are accelerated to the right toward the Ar beam. Unreacted N2 ions and Ar+ products are then accelerated through the TOF chamber to the right detector...

There are two types of MIR spectrometers, dispersive and Fourier-transform (FT) spectrometers. Today FT spectrometers are used predominantly. The most significant advantage of FT spectrometers is that radiation from all wavelengths is measured simultaneously, whereas in dispersive spectrometers all wavelengths are measured consecutively. Therefore, a FT spectrometer is much faster and... 

Raman spectroscopy may be performed on either a dispersive instrument or a FT instrument. All of the spectra provided in this chapter were obtained from a FT-Raman instrument (see Fig. 62), featuring a NdiYAG solid-state laser, and a InGaAs (indium-gallium arsenide) detector, combined with a silicon on quartz beam splitter. Note that in the FT-Raman experiment, the sample effectively becomes the source to the FT spectrometer. Dispersive Raman instruments are also popular, and these usually feature a silicon-based array detector (CCD array) in combination with either a visible laser (doubled YAG or HeNe) or a short-wavelength solid-state NIR laser. 

As most commonly used, the technique employs a pulsed laser and a focusing lens to generate a plasma that vaporizes a small amount of a sample. A portion of the plasma light is collected and directed to a spectrometer. The spectrometer disperses the light emitted by excited atoms, ions, and simple molecules in the plasma, a detector records the emission signals, and electronics take over to digitize and display the results. 

Carbohydrate Total carbohydrate Starch Snack foods Oat bran products Neotec 6350 scanning spectrometer, reflectance NIRSystems 6500 scanning spectrometer, dispersive, reflectance Summation of starch and NDF values Enzymatic-colorimetric 8... 

NIRSystems 6500, scanning spectrometer, dispersive, reflectance Nicolet Raman 950 Perten Model DA7000 diode array spectrometer, reflectance... 

Protein and nitrogen Diverse cereal products NIRSystems 6500, scaiming spectrometer, dispersive, reflectance Combustion analysis 28... 

The first requirement is a source of infrared radiation that emits all frequencies of the spectral range being studied. This polychromatic beam is analyzed by a monochromator, formerly a system of prisms, today diffraction gratings. The movement of the monochromator causes the spectrum from the source to scan across an exit slit onto the detector. This kind of spectrometer in which the range of wavelengths is swept as a function of time and monochromator movement is called the dispersive type. 

While a laser beam can be used for traditional absorption spectroscopy by measuring / and 7q, the strength of laser spectroscopy lies in more specialized experiments which often do not lend themselves to such measurements. Other techniques are connnonly used to detect the absorption of light from the laser beam. A coimnon one is to observe fluorescence excited by the laser. The total fluorescence produced is nonnally proportional to the amount of light absorbed. It can be used as a measurement of concentration to detect species present in extremely small amounts. Or a measurement of the fluorescence intensity as the laser frequency is scaimed can give an absorption spectrum. This may allow much higher resolution than is easily obtained with a traditional absorption spectrometer. In other experiments the fluorescence may be dispersed and its spectrum detennined with a traditional spectrometer. In suitable cases this could be the emission from a single electronic-vibrational-rotational level of a molecule and the experimenter can study how the spectrum varies with level. 

Light sources can either be broadband, such as a Globar, a Nemst glower, an incandescent wire or mercury arc lamp or they can be tunable, such as a laser or optical parametric oscillator (OPO). In the fomier case, a monocln-omator is needed to achieve spectral resolution. In the case of a tunable light source, the spectral resolution is detemiined by the linewidth of the source itself In either case, the spectral coverage of the light source imposes limits on the vibrational frequencies that can be measured. Of course, limitations on the dispersing element and detector also affect the overall spectral response of the spectrometer. 

Even while Raman spectrometers today incorporate modem teclmology, the fiindamental components remain unchanged. Connnercially, one still has an excitation source, sample illuminating optics, a scattered light collection system, a dispersive element and a detechon system. Each is now briefly discussed. 

The two essential elements of an electron spectrometer are the electrodes that accelerate electrons and focus them into a beam and the dispersive elements that sort electrons according to their energies. These serve the fimctions of lenses and prisms in an optical spectrometer. The same parameters are used to describe these elements in an electron spectrometer as in an optical spectrometer the teclmology is referred to as electron optics. 

Yuzawa T, Kate C, George M W and Hamaguchi H O 1994 Nanosecond time-resolved infrared spectroscopy with a dispersive scanning spectrometer Appl. Spectrosc. 48 684-90... 

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. 

A mass spectrometer analyzer disperses ions according to their various m/z values. 

In a mass spectrometer, ions can arrive at a multipoint collector as a spatially dispersed beam. This means that all ions of different m/z values arrive simultaneously but separated in space according to each m/z value. Each element of the array, depending on its position in space, detects one particular m/z value (see Chapter 29, Array Collectors ). 

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