Quantitative analysis absorption

Table 5.5 shows the main characteristics of UV spectrophotometry as applied to polymer/additive analysis. Growing interest in automatic sample processing looks upon spectrophotometry as a convenient detection technique due to the relatively low cost of the equipment and easy and cheap maintenance. The main advantage of UV/VIS spectroscopy is its extreme sensitivity, which permits typical absorption detection limits in solution of 10-5 M (conventional transmission) to 10 7 M (photoacoustic). The use of low concentrations of substrates gives relatively ideal solutions [20]. As UV/VIS spectra of analytes in solution show little fine structure, the technique is of relatively low diagnostic value on the other hand, it is one of the most widely used for quantitative analysis. Absorption of UV/VIS light is quantitatively highly accurate. The simple linear relationship between... 

In principle, no special Raman instrumentation is needed to perform RRS because RR spectra can be obtained with conventional Raman spectrometers, if only the suitable excitation wavelength is applied. However, resonance Raman scattering is experimentally more difficult to implement than normal spontaneous Raman scattering. The excitation wavelength must be made to match the absorption band of the electronic chromophore of interest. The absorption band makes both the excitation intensity and Raman scattered intensity dependent on sample thickness, complicating quantitative analysis. Absorption of the excitation intensity can damage the sample due to heating and/or photochemistry. 

Quantitative Analysis of Mixtures The analysis of two or more components in the same sample is straightforward if there are regions in the sample s spectrum in which each component is the only absorbing species. In this case each component can be analyzed as if it were the only species in solution. Unfortunately, UV/Vis absorption bands are so broad that it frequently is impossible to find appropriate wavelengths at which each component of a mixture absorbs separately. Earlier we learned that Beer s law is additive (equation 10.6) thus, for a two-component mixture of X and Y, the mixture s absorbance, A, is... 

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). 

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). 

For quantitative analysis, the resolution of the spectral analyzer must be significantly narrower than the absorption lines, which are - 0.002 nm at 400 nm for Af = 50 amu at 2500°C (eq. 4). This is unachievable with most spectrophotometers. Instead, narrow-line sources specific for each element are employed. These are usually hoUow-cathode lamps, in which a cylindrical cathode composed of (or lined with) the element of interest is bombarded with inert gas cations produced in a discharge. Atoms sputtered from the cathode are excited by coUisions in the lamp atmosphere and then decay, emitting very narrow characteristic lines. More recendy semiconductor diode arrays have been used for AAS (168) (see Semiconductors). 

Although the most sensitive line for cadmium in the arc or spark spectmm is at 228.8 nm, the line at 326.1 nm is more convenient to use for spectroscopic detection. The limit of detection at this wavelength amounts to 0.001% cadmium with ordinary techniques and 0.00001% using specialized methods. Determination in concentrations up to 10% is accompHshed by solubilization of the sample followed by atomic absorption measurement. The range can be extended to still higher cadmium levels provided that a relative error of 0.5% is acceptable. Another quantitative analysis method is by titration at pH 10 with a standard solution of ethylenediarninetetraacetic acid (EDTA) and Eriochrome Black T indicator. Zinc interferes and therefore must first be removed. 

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). 

In addition to qualitative identification of the elements present, XRF can be used to determine quantitative elemental compositions and layer thicknesses of thin films. In quantitative analysis the observed intensities must be corrected for various factors, including the spectral intensity distribution of the incident X rays, fluorescent yields, matrix enhancements and absorptions, etc. Two general methods used for making these corrections are the empirical parameters method and the fimdamen-tal parameters methods. 

However, these absorption spectra can be employed as an aid to charaeterization, particularly when authentic reference substanees are ehromatographed on a neighboring track. The use of differential speetrometry yields additional information [64]. Quantitative analysis is usually performed by scanning at the wavelength of greatest absorbance (2m ). However, determinations at other wavelengths can sometimes be advantageous, e.g. when the result is a better baseline. An example is the determination of scopolamine at 2 = 220 nm instead of at =... 

In situ quantitation Quantitative analysis (Figs. 2 and 3) could be performed both absorption-photometrically with long-wavelength UV light (2 = 365 nm) or fluorimetrically (2eic = 436 nm Afi = 546 nm [monochromation filter M 546] or 2fi > 560 nm). 

In situ quantitation The quantitative analysis is performed by measuring the absorption of the chromatogram zone in reflectance at 2 = 440 nm (Fig. 1). 

In situ quantitation The absorption-photometric determination in a reflectance mode was performed at A = 330 nm (detection limit ca. 40 ng per chromatogram zone). The fluorimetric analysis was carried out at =313 nm and An > 560 nm (detection limits ca. 10 ng per chromatogram zone) (Fig. 1). 

In situ quantitation The absorption-photometric analysis was carried out at / = 378 nm. 

In situ quantitation The absorption photometric analysis was made at A = 540 nm (ethynylestradiol) and X = 605 nm (diethylstilbestrol). The detection limit for ethynylestradiol was 12ng and that for diethylstilbestrol 3 ng per chromatogram zone. 

Quantitative analysis of the peroxy group of macroinitiators is performed by iodometry [38] and that of the azo group is done by ultraviolet (UV) spectrometry. Recently, type II MAI composed of PU was determined of its azo concentration by UV [20]. When the UV absorption spectral peak of the azo group overlaps other peaks, DSC is available by determining the azo group from the exothermal peak area [1IJ. 

The main techniques employed in quantitative analysis are based upon (a) the quantitative performance of suitable chemical reactions and either measuring the amount of reagent needed to complete the reaction, or ascertaining the amount of reaction product obtained (b) appropriate electrical measurements (e.g. potentiometry) (c) the measurement of certain optical properties (e.g. absorption spectra). In some cases, a combination of optical or electrical measurements and quantitative chemical reaction (e.g. amperometric titration) may be used. 

Beer s Law. We have so far considered the light absorption and the light transmission for monochromatic light as a function of the thickness of the absorbing layer only. In quantitative analysis, however, we are mainly concerned with solutions. Beer studied the effect of concentration of the coloured constituent in solution upon the light transmission or absorption. He found the same relation between transmission and concentration as Lambert had discovered between transmission and thickness of the layer [equation (3)], i.e. the intensity of a beam of monochromatic light decreases exponentially as the concentration of the absorbing substance increases arithmetically. This may be written in the form ... 

Many complexes of metals with organic ligands absorb in the visible part of the spectrum and are important in quantitative analysis. The colours arise from (i) d- d transitions within the metal ion (these usually produce absorptions of low intensity) and (ii) n->n and n n transitions within the ligand. Another type of transition referred to as charge-transfer may also be operative in which an electron is transferred between an orbital in the ligand and an unfilled orbital of the metal or vice versa. These give rise to more intense absorption bands which are of analytical importance. 

It would appear that measurement of the integrated absorption coefficient should furnish an ideal method of quantitative analysis. In practice, however, the absolute measurement of the absorption coefficients of atomic spectral lines is extremely difficult. The natural line width of an atomic spectral line is about 10 5 nm, but owing to the influence of Doppler and pressure effects, the line is broadened to about 0.002 nm at flame temperatures of2000-3000 K. To measure the absorption coefficient of a line thus broadened would require a spectrometer with a resolving power of 500000. This difficulty was overcome by Walsh,41 who used a source of sharp emission lines with a much smaller half width than the absorption line, and the radiation frequency of which is centred on the absorption frequency. In this way, the absorption coefficient at the centre of the line, Kmax, may be measured. If the profile of the absorption line is assumed to be due only to Doppler broadening, then there is a relationship between Kmax and N0. Thus the only requirement of the spectrometer is that it shall be capable of isolating the required resonance line from all other lines emitted by the source. 

In situ quantitation The absorption photometric scan at Xn x = 550 nm had to be carried out immediately, since the colors of the derivatives only remained stable for ca. 10-15 min exact quantitative analysis was not always possible. 

In situ quantitation The absorption photometric analysis in reflectance was carried out either at the absorption maximum of the pyrogallol derivative at = 350 nm (Fig. lA) or at the absorption maximum of the phloroglucinol derivative at = 420 nm (Fig. IB). 

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