Molecular absorption spectrometry

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 basic instrumentation used for spectrometric measurements has already been described in Chapter 7 (p. 277). The natures of sources, monochromators, detectors, and sample cells required for molecular absorption techniques are summarized in Table 9.1. The principal difference between instrumentation for atomic emission and molecular absorption spectrometry is in the need for a separate source of radiation for the latter. In the infrared, visible and ultraviolet regions, white sources are used, i.e. the energy or frequency range of the source covers most or all of the relevant portion of the spectrum. In contrast, nuclear magnetic resonance spectrometers employ a narrow waveband radio-frequency transmitter, a tuned detector and no monochromator. 

It is seen by examination of Table 1.11(b) that a wide variety of techniques have been employed including spectrophotometry (four determinants), combustion and wet digestion methods and inductively coupled plasma atomic emission spectrometry (three determinants each), atomic absorption spectrometry, potentiometric methods, molecular absorption spectrometry and gas chromatography (two determinants each), and flow-injection analysis and neutron activation analysis (one determinant each). Between them these techniques are capable of determining boron, halogens, total and particulate carbon, nitrogen, phosphorus, sulphur, silicon, selenium, arsenic antimony and bismuth in soils. 

Since the analytes for atomic spectroscopy are severely limited (elements only), compared to the large number of molecular and complex ion analytes for UV-VIS molecular absorption spectrometry,... 

ISO 78-3 1983 Chemistry - Layouts for standards - Part 3 Standard for molecular absorption spectrometry... 

Other frequently used methods for determining fluoride include ion and gas chromatography [150,204,205] and aluminium monofluoride (AIF) molecular absorption spectrometry [206,207]. Less frequently employed methods include enzymatic [208], catalytic [209], polarographic [210] and voltammetric methods [211], helium microwave-induced [212] or inductively coupled plasma atomic emission spectrometry [213], electrothermal atomic absorption spectrometry [214], inductively coupled plasma-mass spectrometry [215], radioactivation [216], proton-induced gamma emission [217], near-infrared spectroscopy [218] and neutron activation analysis [219]. 

K. Tsunoda, K. Fujiwara, K. Fuwa, Subnanogram fluorine determination by aluminum monofluoride molecular absorption spectrometry. Anal. Chem. 49 (1977) 2035-2039. 

Bellido-Milla, D., Moreno-Perez, J.M., Hemandez-Artiga, M.P. Differentiation and classification of beers with flame atomic spectrometry and molecular absorption spectrometry and sample preparation assisted by microwaves. Spectrochim. Acta B 55, 855-864 (2000)... 

In Chapter 24, we show that quantitative molecular absorption spectrometry is based on Beer s law, which can be written... 

Chapter 26 Molecular Absorption Spectrometry 784 Chapter 27 Molecular Fluorescence Spectroscopy 825 Chapter 28 Atomic Spectroscopy 839... 

The spectrophotometric methods to be discussed (methods of molecular absorption spectrometry) are based on the measurement of absorption of radiation, in the visible and near ultraviolet regions, owing to coloured compounds formed, before the determination, by the elements to be determined. Only seldom is use made of the intrinsic colour of the element itself, in its ionic form. In cases where an element neither forms coloured compounds nor occurs in a coloured form, indirect spectrophotometric methods are applied. 

A paper has been published on the Recommendations of the Analytical Chemistry Section of the International Union of Pure and Applied Chemistry (lUPAC) concerning the nomenclature, symbols and units applied in molecular absorption spectrometry [25]. 

The method relies on the Lambert-Beer relationship, but the molar coefficient of absorption s is not calculated in this case. The instrument yields the absorbance by ratioing the transmitted intensities in the absence and then in the presence of sample. Linearity is only observed for weak concentrations (typically below 3ppm), or for solutions in which the matrix effect is negligible. The methods, comparables to those used in molecular absorption spectrometry, involve classical protocols via a calibration curve. If the matrix is complex, methods using standard additions are used to improve the calibration curve (Figure 13.4). 

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