Spectrophotometry precision

A number of poorly defined terms have been used for distinguishing various types of colorant that may occur in sugar materials. It appears probable that precise spectrophotometry may eventually help to show whether the distinctions which have been made between the various colorants (such as caramels, humin, humic acids, melanoidins, etc.) are justified from this point of view. 

Precision spectrophotometry (differential spectrophotometry) is a technique that involves comparing an unknown solution with a reference. The reference scale is set at zero using a solution of a highly colored (radiation-adsorbing) species in place of a reagent blank. Concentrations of the unknown higher than the reference are then measured against this zero in the usual way. 

BeryUium aUoys ate usuaUy analyzed by optical emission or atomic absorption spectrophotometry. Low voltage spark emission spectrometry is used for the analysis of most copper-beryUium aUoys. Spectral interferences, other inter-element effects, metaUurgical effects, and sample inhomogeneity can degrade accuracy and precision and must be considered when constmcting a method (17). 

It may happen that AH is not available for the buffer substance used in the kinetic studies moreover the thermodynamic quantity A//° is not precisely the correct quantity to use in Eq. (6-37) because it does not apply to the experimental solvent composition. Then the experimentalist can determine AH. The most direct method is to measure AH calorimetrically however, few laboratories Eire equipped for this measurement. An alternative approach is to measure K, under the kinetic conditions of temperature and solvent this can be done potentiometrically or by potentiometry combined with spectrophotometry. Then, from the slope of the plot of log K a against l/T, AH is calculated. Although this value is not thermodynamically defined (since it is based on the assumption that AH is temperature independent), it will be valid for the present purpose over the temperature range studied. 

Sutin et al have made a detailed study of the chloride-catalysed paths in deuterated water media, the exchange occurring more slowly in this solvent than in water. Accurate values of, in both water and heavy water, were obtained by spectrophotometry and enabled more precise rate parameters to be calculated. For aqueous media, values are 22.8 l.mole sec fork (n = 0.50 Af and 20 °C),... 

Because of peak overlappings in the first- and second-derivative spectra, conventional spectrophotometry cannot be applied satisfactorily for quantitative analysis, and the interpretation cannot be resolved by the zero-crossing technique. A chemometric approach improves precision and predictability, e.g., by the application of classical least sqnares (CLS), principal component regression (PCR), partial least squares (PLS), and iterative target transformation factor analysis (ITTFA), appropriate interpretations were found from the direct and first- and second-derivative absorption spectra. When five colorant combinations of sixteen mixtures of colorants from commercial food products were evaluated, the results were compared by the application of different chemometric approaches. The ITTFA analysis offered better precision than CLS, PCR, and PLS, and calibrations based on first-derivative data provided some advantages for all four methods. ... 

Precision of analytical techniques should only be described verbally or comparatively (e.g., the precision of coulometry is high or the precision of spectrophotometry is better then that of OES ). 

Klinkhammer [432] has described a method for determining manganese in a seawater matrix at concentrations ranging from about 30 to 5500 ng/1. The samples are extracted with 4 nmol/1 8-hydroxyquinoline in chloroform, and the manganese in the organic phase is then back-extracted into 3 M nitric acid. The manganese concentrations are determined by graphite furnace atomic absorption spectrophotometry. The blank of the method is about 3.0 ng/1, and the precision from duplicate analyses is 9% (1 SD). 

When the samples were returned to the laboratory the pH was adjusted to approximately pH 8 using concentrated ammonia (Ultrapure, G. Frederick Smith). Chelating cation exchange resin in the ammonia form (20 ml Chelex 100,100 - 200 mesh, Bio-Rad) was added to the samples and they were batch extracted on a shaker table for 36 hours. The resin was decanted into columns, and the manganese eluted using 2N nitric acid [129]. The eluant was then analysed by graphite furnace atomic absorption spectrophotometry. Replicate analyses of samples indicate a precision of about 5%. 

It has been reported that the differential determination of arsenic [36-41] and also antimony [42,43] is possible by hydride generation-atomic absorption spectrophotometry. The HGA-AS is a simple and sensitive method for the determination of elements which form gaseous hydrides [35,44-47] and mg/1 levels of these elements can be determined with high precision by this method. This technique has also been applied to analyses of various samples, utilising automated methods [48-50] and combining various kinds of detection methods, such as gas chromatography [51], atomic fluorescence spectrometry [52,53], and inductively coupled plasma emission spectrometry [47]. 

For example, X-ray fluorescence spectrometry may provide rapid but rather imprecise quantitative results in a trace element problem. Atomic absorption spectrophotometry, on the other hand, will supply more precise data, but at the expense of more time-consuming chemical manipulations. 

It therefore became more convenient to monitor the reaction progress with UV/Visible spectrophotometry, because all the pyridine N-oxides have strong absorption bands near 330 nm, with e 103Lmol 1cm 1. Two approaches for the analysis of the kinetic data were used. In the first but much less precise method, the initial reaction rates were calculated from the objective method of fitting the experimental values of [PyOL to this function (30) ... 

The use of spectrophotometry to monitor enzyme-catalysed reactions (Table 8.6) is a very convenient and popular method owing to the simplicity of the technique and the precision that is possible. The technique lends itself readily not only to temperature control using water-jacketed or electrically heated cell holders but also to the measurement of initial velocities by continuous monitoring and recording techniques or by automated analysis systems. 

The enzyme horseradish peroxidase is a hemoprotein and the region of the Soret band exhibits large differences between the position and extinction coefficients of the uncombined and combined forms. Both forms were first studied by spectrophotometry, but the E—S complexes were 0 labile that they could not be examined extensively by any other spectroscopic method. Using rapid-scanning spectrophotometry and rapid mixing, Chance was able to distinguish the spectra of compound I and II and determine the various rate constants of the multistep reaction with rather poor precision. 

Diclofenac sodium, famotidine and ketorolac were analysed utilising their formation of a coloured charge transfer complex with 2,4 dichloro-6-nitrophenol. The complexes were detected by UV/visible spectrophotometry at 450 nm. The method was not affected by the presence of common excipients in the formulations analysed. The precision and accuracy of the method was comparable to that of HPLC methods used to analyse the same samples. ... 

Although its precise structure has not yet been settled, the hydrated electron may be visualized as an excess electron surrounded by a small number of oriented water molecules and behaving in some ways like a singly charged anion of about the same size as the iodide ion. Its intense absorption band in the visible region of the spectrum makes it a simple matter to measure its reaction rate constants using pulse radiolysis combined with kinetic spectrophotometry. Rate constants for several hundred different reactions have been obtained in this way, making kinetically one of the most studied chemical entities. 

AA—atomic absorption spectrophotometry precision and accuracy estimates in relative %. 

Slight mismatch between sample and reference cuvets, over which you have little control, leads to systematic errors in spectrophotometry. For best precision, you should place cuvets in the spectrophotometer as reproducibly as possible. Random variation in absorbance arises from slight misplacement of the cuvet in its holder, or turning a flat cuvet around by 180°, or rotation of a circular cuvet. 

Figure 18-6 Errors in spectrophotomefric measurements due to dark current noise and cell positioning imprecision in a research-quality instrument. [Data from L D. Rothman. S. R. Crouch, and J. D. Ingle. Jr.."theoretical and Experimental Investigation of Factors Affecting Precision in Molecular Absorption Spectrophotometry." Anal. Chem. 1975,47, 1226.]...

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