Quantitative Absorption Spectroscopy

Methods based on the absorption of radiation are powerful and useful tools for the analytical chemist. The ultraviolet region is particularly important for the qualitative and quantitative determination of many organic compounds. In the visible region, spectrophotometric methods are widely used for the quantitative determination of many trace substances, especially inorganic elements. 

The basic principle of quantitative absorption spectroscopy lies in comparing the extent of absorption of a sample solution with that of a set of standards under radiation at a selected wavelength. 

In its simplest form, colorimetry consists of visual matching of the color of the sample with that of a series of standards. A colored compound is first formed by suitably reacting the constituent to be determined, then the colored solutions are racked side-by-side in Nessler tubes for viewing from the top. The approximate concentration of the unknown is estimated by finding which standard most closely matches the unknown in color. Visual colorimetry suffers from poor precision since the eye is not as sensitive to small differences in absorbance as is a photoelectric device. The use of a Duboscq colorimeter constitutes a more refined method of analysis for color comparison. This is equipped with an eyepiece with a split field that permits the ready comparison of beams passing through sample and standard. 

Photometers equipped with filters are suitable for many routine methods that do not involve complex spectra. Spectrophotometers can provide narrow bandwidths of radiation for accurate work and can handle absorption spectra in the ultraviolet region. 

Choice of Wavelength. When filter photometers are employed, a suitable filter is selected in preparing an analytical curve for the unknown substance. With a spectrophotometer, the spectrum of the absorbing substance is determined, and a suitable 

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Measurements of the intensity and wavelength of radiation that is either absorbed or emitted provide the basis for sensitive methods of detection and quantitation. Absorption spectroscopy is most frequently used in the quantitation of molecules but is also an important technique in the quantitation of some atoms. Emission spectroscopy covers several techniques that involve the emission of radiation by either atoms or molecules but vary in the manner in which the emission is induced. Photometry is the measurement of the intensity of radiation and is probably the most commonly used technique in biochemistry. In order to use photometric instruments correctly and to be able to develop and modify spectroscopic techniques it is necessary to understand the principles of the interaction of radiation with matter. 

Quantitative absorption spectroscopy is also perfectly suited for systematic online studies of unimolecular decomposition rates of peroxides and azocom pounds in different solvents as a function of pressure and temperature. This kind of experiment is illustrated in Fig. 4.17, where the decay of dibenzoyl peroxide (BPO) is taken as an example. 

Quantitation can be carried out using absorption or fluorescence spectroscopy. Measurements can be carried out in transmittance or reflectance mode. The basis of quantitative absorption spectroscopy in transmission mode (UVA IS and FUR) is the usual linear relationship of the Beer-Bouguer-Lambert law, which states that the absorbance A of a solute is directly proportional to its concentration c ... 

By changing the configuration of the two cells in the san le compartment of the IR spectrophotometer this enables the determination of either recirculating gas composition or plotting of the baseline spectrum for the catalyst wafer. With the two cells in the double-beam mode, the catalyst baseline and surface spectra are recorded. If the reference cell was placed in the sample beam and an air gap in the reference beam, quantitative absorption spectroscopy was possible. The IR cells thus provide information leading to both reaction rates and mechanistic insights concerning adsorbed species at reaction conditions. 

Zalicki P and Zare R N 1995 Cavity ring-down spectroscopy for quantitative absorption-measurements J. Chem. 

Most potentiometric electrodes are selective for only the free, uncomplexed analyte and do not respond to complexed forms of the analyte. Solution conditions, therefore, must be carefully controlled if the purpose of the analysis is to determine the analyte s total concentration. On the other hand, this selectivity provides a significant advantage over other quantitative methods of analysis when it is necessary to determine the concentration of free ions. For example, calcium is present in urine both as free Ca + ions and as protein-bound Ca + ions. If a urine sample is analyzed by atomic absorption spectroscopy, the signal is proportional to the total concentration of Ca +, since both free and bound calcium are atomized. Analysis with a Ca + ISE, however, gives a signal that is a function of only free Ca + ions since the protein-bound ions cannot interact with the electrode s membrane. 

The conventional method for quantitative analysis of galHum in aqueous media is atomic absorption spectroscopy (qv). High purity metallic galHum is characteri2ed by trace impurity analysis using spark source (15) or glow discharge mass spectrometry (qv) (16). 

Chemical Analysis. Plasma oxidation and other reactions often are used to prepare samples for analysis by either wet or dry methods. Plasma excitation is commonly used with atomic emission or absorption spectroscopy for quaUtative and quantitative spectrochemical analysis (86—88). 

In addition to the spark emission methods, quantitative analysis directly on soHds can be accompHshed using x-ray fluorescence, or, after sample dissolution, accurate analyses can be made using plasma emission or atomic absorption spectroscopy (37). 

Instrumental Quantitative Analysis. Methods such as x-ray spectroscopy, oaes, and naa do not necessarily require pretreatment of samples to soluble forms. Only reUable and verified standards are needed. Other instmmental methods that can be used to determine a wide range of chromium concentrations are atomic absorption spectroscopy (aas), flame photometry, icap-aes, and direct current plasma—atomic emission spectroscopy (dcp-aes). These methods caimot distinguish the oxidation states of chromium, and speciation at trace levels usually requires a previous wet-chemical separation. However, the instmmental methods are preferred over (3)-diphenylcarbazide for trace chromium concentrations, because of the difficulty of oxidizing very small quantities of Cr(III). 

The capacity factors of SN-SiO, for metal ions were determined under a range of different conditions of pH, metal ions concentrations and time of interaction. Preconcentration of Cd ", Pb ", Zn " and CvS were used for their preliminary determination by flame atomic absorption spectroscopy. The optimum pH values for quantitative soi ption ai e 5.8, 6.2, 6.5, 7.0 for Pb, Cu, Cd and Zn, respectively. The sorption ability of SN-SiO, to metal ions decrease in line Pb>Cu> >Zn>Cd. The soi ption capacity of the sorbent is 2.7,7.19,11.12,28.49 mg-g Hor Cd, Zn, Pb, andCu, respectively. The sorbent distribution coefficient calculated from soi ption isotherms was 10 ml-g for studied cations. All these metal ions can be desorbed with 5 ml of O.lmole-k HCl (sorbent recovery average out 96-100%). 

Metal impurities can be determined qualitatively and quantitatively by atomic absorption spectroscopy and the required purification procedures can be formulated. Metal impurities in organic compounds are usually in the form of ionic salts or complexes with organic compounds and very rarely in the form of free metal. If they are present in the latter form then they can be removed by crystallising the organic compound (whereby the insoluble metal can be removed by filtration), or by distillation in which case the metal remains behind with the residue in the distilling flask. If the impurities are in the ionic or complex forms, then extraction of the organic compound in a suitable organic solvent with aqueous acidic or alkaline solutions will reduce their concentration to acceptable levels. 

As with other diffraction techniques (X-ray and electron), neutron diffraction is a nondestructive technique that can be used to determine the positions of atoms in crystalline materials. Other uses are phase identification and quantitation, residual stress measurements, and average particle-size estimations for crystalline materials. Since neutrons possess a magnetic moment, neutron diffraction is sensitive to the ordering of magnetically active atoms. It differs from many site-specific analyses, such as nuclear magnetic resonance, vibrational, and X-ray absorption spectroscopies, in that neutron diffraction provides detailed structural information averaged over thousands of A. It will be seen that the major differences between neutron diffraction and other diffiaction techniques, namely the extraordinarily... 

Analyses performed may include conventional wet chemistry, coupled with atomic absorption spectroscopy or other metal-scan techniques to provide a quantitative elemental assay, plus X-ray diffraction to determine the major crystalline constituents. 

Low-temperature, photoaggregation techniques employing ultraviolet-visible absorption spectroscopy have also been used to evaluate extinction coefficients relative to silver atoms for diatomic and triatomic silver in Ar and Kr matrices at 10-12 K 149). Such data are of fundamental importance in quantitative studies of the chemistry and photochemistry of metal-atom clusters and in the analysis of metal-atom recombination-kinetics. In essence, simple, mass-balance considerations in a photoaggregation experiment lead to the following expression, which relates the decrease in an atomic absorption to increases in diatomic and triatomic absorptions in terms of the appropriate extinction coefficients. 

Csilibrsition. Quantitative binding analysis requires knowing the concentration of FLPEP, which can be determined for a stock solution of FLPEP by absorption spectroscopy. The quenching by the antibody is essentially quantitative, and the relative amounts of free and bound ligand are calculated from the relative fluorescence intensity. 

Once the FBA has been identified, ultraviolet absorption spectroscopy affords a rapid and accurate method of quantitative analysis. Care must be taken when interpreting the spectra of stilbene-type compounds, since turns to cis isomerisation is promoted by ultraviolet radiation. Usually, however, a control spectrum of the turns isomer can be obtained before the compound undergoes any analytically significant isomerisation. FBAs are often marketed on the basis of strength comparisons determined by ultraviolet spectroscopy. 

Scheinost AC, Kretzschmar R, Pflster S, Roberts DR. Combining selective sequential extractions, X-ray absorption spectroscopy, and principal component analysis for quantitative zinc speciation in soil. Environ. Sci. Technol. 2002 36 5021-5028. 

Quantitative analysis in flame atomic absorption spectroscopy utilizes Beer s law. The standard curve is a Beer s law plot, a plot of absorbance vs. concentration. The usual procedure, as with other quantitative instrumental methods, is to prepare a series of standard solutions over a concentration range suitable for the samples being analyzed, i.e., such that the expected sample concentrations are within the range established by the standards. The standards and the samples are then aspirated into the flame and the absorbances read from the instrument The Beer s law plot will reveal the useful linear range and the concentrations of the sample solutions. In addition, information on useful linear ranges is often available for individual elements and instrument conditions from manufacturers and other literature. 

Allan Walsh, in 1955, was the pioneer for the introduction of atomic absorption spectroscopy (AAS), which eventually proved to be one of the best-known-instrumental-techniques in the analytical armamentarium, that has since been exploited both intensively and extensively in carrying out the quantitative determination of trace metals in liquids of completely diversified nature, for instance blood serum-for Ca2+, Mg2+, Na+ and K+ edible oils-Ni2+ beer samples-Cu+ gasoline (petrol)-Pb2+ urine-Se4+ tap-water-Mg2+ Ca2+ lubricating oil-Vanadium (V). 

In atomic absorption spectroscopy (AAS) the technique using calibration curves and the standard addition method are both equally suitable for the quantitative determinations of elements. 

The elements present in a host of pharmaceutical substances are determined quantitatively by atomic absorption spectroscopy, for example Pd in carbenicillin sodium Cu, Pb and Zn in activated charcoal Fe in ascorbic acid Ag in cisplatin Ph and Zn in copper sulphate Zn in glucogen Zn in insulin Pb in oxprenolol hydrochloride Ni in prazosin hydrochloride Zn in sodium sulphite heptahydrate, and Cd and Pb in zinc oxide. 

The lead content of biological samples is usually very small, rendering gravimetric methods impracticable, and methods have often relied upon the formation of coloured complexes with a variety of dyes. More recently, the development of absorption spectroscopy using vaporized samples has provided a sensitive quantitative method. Oxygen measurements using specific electrodes offer a level of sensitivity which is unobtainable using volumetric gas analysis. 

The most convenient methods are those that permit simultaneous identification and quantitation of the test substance. Unfortunately these are relatively few in number but probably the best examples are in the area of atomic emission and absorption spectroscopy, where the wavelength of the radiation may be used to identify the element and the intensity of the radiation used for its quantitation. 

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