Optical Monochromator

The most commonly used light source in photoionization mass spectrometry is a gas-discharge lamp in which the gas and method of excitation can be varied to produce the desired spectrum. The same lamp (e.g., the Hinteregger type or the Tanaka type ) can serve to produce all the gas-discharge spectra to be described here. 

More recently, electron-ion recombination has been used to produce continuum light sources which promise to have much use. With helium gas, such a source produces useful continuum radiation down to wavelengths of about 300 A. 

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Despite the measurement of the emitted radiation by these means it is still possible for scattered or reflected incident radiation to reach the detector. To prevent this, fluorimeters require a second monochromating system between the sample and the detector. Many simple fluorimeters use filters as both primary and secondary monochromators but those instruments that use true optical monochromators for both components are known as spectrofluo-rimeters. Other instruments incorporate a simple cut-off filter system for the emitted radiation while retaining the optical monochromator for the excitation radiation. Because the wavelengths of both excitation and emission are characteristic of the molecule, it is debatable which monochromator is the most important in the design of a fluorimeter. 

Atomic emission from the plasma is focused on to the entrance slit of the monochromator using a combination of convex or plano-convex lenses or a concave mirror. The combination of focusing optics, monochromator and detector is generally referred to as a spectrometer, although the heart of the device is the monochromator. A monochromator is an instrument that... 

The complications just described can be minimized if there is greater selectivity in the ionization process, as is sometime possible when photoionization is used as the excitation mechanism. Because the ionization energy can be more precisely controlled, it is possible in selected cases to produce only the desired reactant-ion species, or at least to minimize production of other ions. As already noted in the earlier section on formation of excited ions, it is also possible to populate specific internal-energy states of some reactant ions by using a photoionization source. One of the earliest photoionization mass spectrometers used to study interaction of internally excited ions with neutrals was that constructed by Chupka et al.91 Such apparatuses typically incorporate a photon source (either a line or a continuum source) and an optical monochromator, which are coupled to the reaction chamber. Various types of mass analyzer, including sector type, time-of-flight (TOF), and quadrupole mass filters, have been used with these apparatuses. Chupka has described the basic instrumental configuration in some detail.854 Photoionization mass spectrometers employed to study interactions of excited ions with neutral species have also been constructed in several other laboratories.80,1144,142,143 The apparatus recently developed by LeBreton et al.80 is illustrated schematically in Fig. 7 and is typical of such instrumentation. 

The basic Raman instrument [20, 21, 39-42] mainly consists of the excitation source, collection optics, monochromator, and detection and recording system. 

Here the source lamp provides a continuous spectral emission over a wide range of wavelengths, and the desired wavelength is isolated by an optical monochromator, usually a grating or prism. The advantage of this type of detector is the ability to select precisely the wavelength of maximum absorption, and hence maximum sensitivity, for the compound of interest. These instruments are naturally more expensive than the fixed-wavelength models. 

Several devices are available for separating ions with different mass-lo-charge ratios. Ideally, the mass analyzer should be capable of distinguishing minute mass differences. In addition, the analyzer should allow passage of a sufficient number of ions to yield readily measurable ion currents. As with an optical monochromator, to which the analyzer is analogous, these two properties are not entirely compatible, and design compromises must be made. 

The construction of a typical monochromator is shown in Figure 10.12. Radiation from the source enters the monochromator through an entrance slit. The radiation is collected by a collimating mirror, which reflects a parallel beam of radiation to a diffraction grating. The diffraction grating is an optically reflecting surface with... 

Infrared instruments using a monochromator for wavelength selection are constructed using double-beam optics similar to that shown in Figure 10.26. Doublebeam optics are preferred over single-beam optics because the sources and detectors for infrared radiation are less stable than that for UV/Vis radiation. In addition, it is easier to correct for the absorption of infrared radiation by atmospheric CO2 and 1420 vapor when using double-beam optics. Resolutions of 1-3 cm are typical for most instruments. 

Endoscopes containing a CCD camera replace the fiber optic bundle with a smaU monochrome CCD chip at the focal point of the fish eye lens (4). 

Quartz also has modest but important uses in optical appHcations, primarily as prisms. Its dispersion makes it useful in monochromators for spectrophotometers in the region of 0.16—3.5 m. Specially prepared optical-quality synthetic quartz is requited because ordinary synthetic quartz is usually not of good enough quality for such uses, mainly owing to scattering and absorption at 2.6 p.m associated with hydroxide in the lattice. 

Because the laser beam is focused on the sample surface the laser power is dissipated in a very smaU area which may cause sample heating if the sample is absorbing and may cause break-down if the sample is susceptible to photodecomposition. This problem sometimes may be avoided simply by using the minimum laser power needed to observe the spectrum. If that fails, the sample can be mounted on a motor shaft and spun so that the power is dissipated over a larger area. Spinners must be adjusted carefully to avoid defocusing the laser or shifting the focal spot off the optic axis of the monochromator system. 

Ultrasensitive Equipment In recent years all components of Raman equipment (laser, sampling optics, filtering, monochromator, and detector) have been clearly improved. This has led to an enormous increase in sensitivity and has enabled direct observation of adsorbed molecules with carefully optimized instruments without the need for further enhancement or resonance effects. 

When recording excitation and fluorescence spectra it must be ensured that monochromatic light falls on the detector This can best be verified in instruments built up on the kit principle or in those equipped with two monochromators (spectrofluonmeters) The majority of scanners commercially available at the moment do not allow of such an optical train, which was realized in the KM3 chromatogram spectrometer (Zeiss) So such units are not able to generate direct absorption or fluorescence spectra for the charactenzation of fluorescent components... 

Fig. 1-4. Simple absorption experiment. Note three important elements of optical system. Two others, the collimator (to give narrow, parallel beam) and the monochromator, have been omitted.

A modern laser Raman spectrometer consists of four fundamental components a laser source, an optical system for focusing the laser beam on to the sample and for directing the Raman scattered light to the monochromator entrance slit, a double or triple monochromator to disperse the scattered light, and a photoelectric detection system to measure the intensity of the light passing through the monochromator exit slit (Fig. 7). 

A detachable monochromator (19) developed by Spex Industries, was another approach in minimizing stray light. It is a modified Czerny-Turner spectrograph which can be coupled to the exit slit of a double monochromator and function as a variable bandpass, variable frequency filter. This accessory, while providing the versatility of a triple monochromator, does not add much mechanical and optical complexity and can be removed when not wanted. 

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