Further Reading
5.3 Instrumentation
5.3.1 Basic Components for Spectrometric Instrumentation .1 Some Important Optical Units
5.3.1.4 Detectors
atomic fluorescence spectrometry. Probably, the most frequent use of lasers in atomic spectrometry is laser ablationtechnique; this is now highly commercialized.
In this technique, the laser falling on solid sample surface ablates the sample, an aerosol consisting of free atoms, volatilized species and solid particles with a size of less than 5µm is formed. This mixture can be transported by using a carrier gas such as Ar, into an Ar plasma where the subsequently formed species can then be deter- mined by ICP-OES or ICP-MS. Some typical lasers are given in Table 5.2.
many devices, is now being replaced by array detectors in some instrument designs.
The operation principle of a PMT detector is shown in Figure 5.26. A PMT detector contains a cathode, an anode and typically 9–11 dynodes in vacuum; the system has a transparent quartz envelope. The cathode is made from a material having a photoe- missive surface, photons striking the cathode cause the ejection of photoelectrons that travel to the first dynode, which is held at a potential of approximately 90 V dc more positive than the cathode. The other dynodes in series and finally the anode are all held at successively higher positive potentials. Electrons arriving at the first dynode cause the ejection of 3-5 other electrons that will travel to the second dynode that is held at a more positive potential; consequently after passing through some number of dynodes, a large number of electrons will reach the anode at the end. The current between the anode and the cathode is measured and above a threshold value for the applied voltage, this current is linearly correlated to the number of photons or the power or radiation.
PMTs are presently not only the most sensitive light detectors, but they also have the largest dynamic range as high as six orders of magnitude. This property is very important for their use with analytical techniques having inherently large dynamic ranges. Photocathode materials are photoemissive surfaces usually consisting of alkali metals in various combinations and mixtures to give relatively low values for work function,W. According to the principles of photoelectric effect, the energy of the oncoming photon,hν, should be at least as high as the work function W; any excess in the light energy is transferred to the ejected electron as its kinetic energy.
The work function of photoemissive surfaces is never as low as the energy of the IR photons; therefore photoelectric detectors cannot be used in this spectral region.
The cathodes can work on transmission or reflection modes. The most employed photoemissive surfaces are Ag–O–Cs and Sb–Cs for UV and VIS, bialkali (Sb–Rb–Cs and Sb–K–Cs) for UV and VIS, high-temperature bialkali (Na2.K.Sb), multialkali (Na–K–Sb–Cs) with UV–VIS response extended to near IR, wide range Ga–As–Cs with a flat UV–VIS response and Cs–Te or Cs–I for solar blind (only UV) type of responses. Some examples for photoemissive surface responses are given in Figure 5.27.
Advances in electronics technology and science have resulted in relatively novel products as strong alternatives to PMTs; these are array detectors.Small light detectors with sizes in micrometre scale are now available. A classical, scan- ning monochromator such as the one shown in Figure 5.18 has an exit slit and a single detector to receive the selected portion of wavelength from the focal plane.
In alternative monochromator designs, there is no exit slit; grating and thus the focal plane is stationary. A large number of individual, small detectors are located in an array on the focal plane; each detector receives an allocated portion of the spectrum. Such systems are called photo diode array (PDA) detectors. Since light signals of all the wavelengths are measured simultaneously, the whole spectrum can be obtained in periods in the order of a second. The individual detector size limits the spectral resolution. The sample has to be placed before the entrance slit, after the source. Since the period of exposure for the sample is very short, pho- todecomposition of molecules in measurement zone is not likely for UV–VIS molecular spectrometry. In addition, any stray light induced in sample cuvette
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will be eliminated by the wavelength selector, resulting in improved linearity.
PDA systems are very popular for molecular spectrometry, especially with spec- trometric detection used as coupled with chromatographic separation. Using a computer for data acquisition, three-dimensional plots can be easily stored; the information thus obtained contains the change of signal with both time and wavelength. Therefore chromatograms at any wavelength and spectra at any elu- tion time can be obtained by using the selected data. The information obtained is very rich.
Photodiodes are usually silicon diodes; radiation falling on the detector surface alters the charge characteristics, finally a recharging current is measured and corre- lated with the quantity of radiation received. The individual photodiode detector size is typically 20–25µm, this is analogous to an exit slit size. The working range is typ- ically 190–1100 nm. The use of PDA systems is very common in molecular UV–VIS spectrometry.
Array detectors are also used widely in atomic spectrometers. In many atomic emission spectrometers two-dimensional arrays of charge transfer devices (CTD) are employed with Echelle polychromators and the two-dimensional focal plane formed therein, such as shown in Figure 5.23. The operating principle of CTDs is somewhat similar to that of photodiodes; however these devices can be used in two- dimensional arrays. There are different kinds, such as charge coupled device (CCD) and charge injection device (CID). Working range is from UV to near IR. The sur- face area of a single CTD is commonly 15–30µm squared.
Regarding performance characteristics, PDAs have the lowest sensitivity, PMTs are best and CTDs can now compete with PMTs but require usually longer integra- tion times.
Figure 5.27 Response curves for several photoemissive surfaces (Reproduced with permis- sion from Hamamatsu Photonics K. K., Japan)
Further Reading
1. J.D. Ingle Jr. and S.R. Crouch,Spectrochemical Analysis, Prentice-Hall, New Jersey, 1988.
2. D.A. Skoog, F.J. Holler and T.A. Nieman,Principles of Instrumental Analysis, 5th edn, Saunders College Publishing, Philadelphia, 1998.
3. G. Currell, Analytical Instrumentation, Performance Characteristics and Quality, Wiley, Chicester, 2000.
4. K.A. Rubinson and J.F. Rubinson, Contemporary Instrumental Analysis, Prentice-Hall, Upper Saddle River, NJ, 2000.
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