Monochromators and Detectors

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. 

Radiations outside the ultraviolet, visible and infrared regions cannot be detected by conventional photoelectric devices. X-rays and y-rays are detected by gas ionization, solid-state ionization, or scintillation effects in crystals. Non-dispersive scintillation or solid-state detectors combine the functions of monochromator and detector by generating signals which are proportional in size to the energy of the incident radiation. These signals are converted into electrical pulses of directly proportional sizes and thence processed to produce a spectrum. For radiowaves and microwaves, the radiation is essentially monochromatic, and detection is by a radio receiver tuned to the source frequency or by a crystal detector. 

The basic instrumentation used for spectrometric measurements has already been described in the previous chapter (p. 277). Methods of excitation, monochromators and detectors used in atomic emission and absorption techniques are included in Table 8.1. Sources of radiation physically separated from the sample are required for atomic absorption, atomic fluorescence and X-ray fluorescence spectrometry (cf. molecular absorption spectrometry), whereas in flame photometry, arc/spark and plasma emission techniques, the sample is excited directly by thermal means. Diffraction gratings or prism monochromators are used for dispersion in all the techniques including X-ray fluorescence where a single crystal of appropriate lattice dimensions acts as a grating. Atomic fluorescence spectra are sufficiently simple to allow the use of an interference filter in many instances. Photomultiplier detectors are used in every technique except X-ray fluorescence where proportional counting or scintillation devices are employed. Photographic recording of a complete spectrum facilitates qualitative analysis by optical emission spectrometry, but is now rarely used. 

The optical path for flame AA is arranged in this order light source, flame (sample container), monochromator, and detector. Compared to UV-VIS molecular spectrometry, the sample container and monochromator are switched. The reason for this is that the flame is, of necessity, positioned in an open area of the instrument surrounded by room light. Hence, the light from the room can leak to the detector and therefore must be eliminated. In addition, flame emissions must be eliminated. Placing the monochromator between the flame and the detector accomplishes both. However, flame emissions that are the... 

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... 

Atomic emission spectroscopy can be employed, generally with an inductively coupled plasma for thermal excitation. The sample is introduced into the plasma as a mist of ultrafine droplets, and the monochromator and detector are set to measure the intensity of an atomic emission line characteristic of the element. This technique is powerful, general, sensitive, linear, and able to measure over 70 elements, and, as a result, is widely used. Response is typically linear over four orders of magnitude in concentration with relative standard deviations of 1 to 3%. In low-salt aqueous solutions, detection limits range from 10 to 1000 nanomolar without preconcentration. Significant problems with saline samples remain, but use of Babington nebulizers alleviates these problems somewhat. 

Atomic absorption sample is vaporized and atomized in high temperature flame. Atoms of the analyte element absorb light of a specific wavelength from a hollow cathode lamp, passing through the flame. Amount of energy absorbed by these atoms is measured, which is proportional to the number of atoms in the light path. Components lamp, flame, monochromator, and detector. 

In UV-vis-NIR spectrometers, the monochromator and detector are switched simultaneously. Step-like artifacts can be generated at this switch, and it is then questionable which part of the spectrum represents the correct absolute intensity. By nature, NIR detectors are susceptible to thermal radiation, and the step at the change-over to or from the NIR range and also the noise in the NIR range increase with temperature (Melsheimer et al., 2003). Sometimes authors present the UV-vis and NIR sections of the spectrum separately, disguising step-like artifacts at the transition. 

Raman scattering was first observed in 1928 and was used to investigate the vibrational states of many molecules in the 1930s. Initially, spectroscopic methods based on the phenomenon were used in research on the structure of relatively simple molecules. Over the past 20 years, however, the development of laser sources and new generations of monochromators and detectors has made possible the application of Raman spectroscopy to the solution of many problems of technological interest. 

Figure 13 represents an instrument having dual-channel optics. This means that there are facilities for two line sources and one continuum source, each line source having its own monochromator and detector. Many workers have advocated the usefulness of this type of instrument. Certainly the recently introduced microcomputerised dual channel models are easier to use than their earlier counterparts, so it is worthwhile to summarise the possible attractions of simultaneous dual element analysis. One obvious possibility is the analysis of two elements in a sample at the same time, thus halving analysis time. A less obvious attraction is the possibility of analysing via an internal standard. This is where a second element, either already present or added to the sample, is measured and ratioed to the analytical element. The... 

A flame emission spectrometer therefore consists of an atom source, a monochromator and detector and is therefore simpler instrumentally than the corresponding atomic absorption system. Particular developments engendered by atomic absorption have restimulated interest in flame emission spectrometry after a dormant period. Chief of these is the use of the nitrous oxide—acetylene flame which is sufficiently hot to stimulate thermal atomic-emission from a wide range of metal elements. 

The basic instrumentation used for spectrometric measurements has already been described in the previous chapter (p. 274). Methods of excitation, monochromators and detectors used in atomic emission and absorption techniques are included in table 8.1. Sources of radiation physically separated... 

Measuring the portion of the diffracted intensity that passes through receiving slits, monochromator and detector windows. 

The spectra of atomic absorption are obtained with instruments called atomic absorption spectrometers. These instruments, as already described for other types of spectrometers, consist of the light source, monochromator and detector. However, the atomic absorption spectrometers and atomic emission spectrometers differ from all other spectral spectrometers by the absence of the sample chamber. Instead of the sample chamber, they contain a burner. A schematic of the atomic absorption spectrometer is shown in Figure 2.54. 

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