Optimal spectrophotometric range

For many measurements, we have to obtain the most accurate and precise value that we can, given the performance of the instrument. In order to do so, we need to operate in the optimal spectrophotometric range for both accuracy and precision. The majority of instruments actually measure the apparent transmittance, T, of the sample, which is converted to the more useful absorbance, A, by 

Ideally, the transmittance scale on a linear detector is fixed by a 0% T measurement (dark current measurement on the detector) and a 100% T measurement (total illumination of the detector by Iq). A sample attenuates the Iq intensity signal, and the sample transmittance and hence the absorbance are obtained. All these individual measurements are subject to noise and drift errors and combine to give an overall measurement standard deviation, yj. This standard deviation is related to the relative error of measurement, rc/C, and may be obtained by rearranging Equation (1.7) and obtaining the partial derivative. Note that the molarity, M, in Equation (1.5) has been replaced by C the concentration in g L-1. The relative error function [9] is given by Equation (1.8)  

the calculation of the relative error would be straightforward if it were not for the fact that the value of overall measurement standard deviation, aj, is not independent 

Standard deviation of a measurement aT Source of variability Relative error function  

If we assign typical values for k and k2 of 0.3% T and 1.3% T for then we are able to draw graphs of the three functions. This is shown in Fig. 15. 

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It was shown that Zn + adsorbed onto SG-PVSA composite film as Zn(Phen) complex. It can be detected spectrophotometrically after treatment with anionic dye Bengal Rose (BR). Ternary complex Zn + - Phen-BR formed on the surface under optimal conditions. SG-PVSA film was used for determination of Zn + by spectrophotometric method. The calibration graph was linear in the concentration range 2,5T0 - STO mol/l. 

It is known that Selenium catalyzes reaction of some dye reduction by Sulphide. On this basis spectrophotometric and test-techniques for Selenium determination are developed. Inefficient reproducibility and low sensitivity are their deficiencies. In the present work, solid-phase reagent on silica gel modified first with quaternary ammonium salt and then by Indigocarmine was proposed for Selenium(IV) test-determination. Optimal conditions for the Selenium determination by method of fixed concentration were found. The detection limit of Se(IV) is 10 ftg/L = 2 ng/sample). Calibration curve is linear in the range 50-400 ftg/L of Se(IV). The proposed method is successfully applied to the Selenium determination in multivitamins and bioadditions. 

Use of 10 pm LiChrosorb RP18 column and binary eluent of methanol and aqueous 0.1 M phosphate buffer (pH 4.0) according to suitable gradient elution program in less than 20-min run time with satisfactory precision sensitivity of spectrophotometric detection optimized, achieving for all additives considered detection limits ranging from 0.1 to 3.0 mg/1, below maximum permitted levels Simultaneous separation (20 min) of 14 synthetic colors using uncoated fused silica capillary column operated at 25 kV and elution with 18% acetonitrile and 82% 0.05 M sodium deoxycholate in borate-phosphate buffer (pH 7.8), recovery of all colors better than 82%... 

A method for determination of sodium isoascorbate (see 2) in boiler feed water, where it is used for deoxygenation, consists of following the reaction kinetics of Rhodamine B (13) in the presence of KBrOs, measuring at 555 nm. A linear correlation exists between the catalytic effect of the analyte on the reaction rate and its concentration Fe(III), Ca(II) and Mg(II) in the 5-200 ppm range interfere with the analysis . The effects of solvents, pH, surfactants, metal ions and other food additives on the absorbance were studied for the micelle-enhanced UV spectrophotometric determination of the food preservative sodium D-isoascorbate. The optimal conditions were using water at pH 7-8 as solvent and polyvinyl alcohol as surfactant, which causes an up to 3-fold increase of the UV absorbance. ... 

Figure 16.27 illustrates the dependence of the relative error on the transmittance, calculated for a constant error of 0.01 T in reading the scale. It is evident from the figure that, while the minimum occurs at 36.8% T, a nearly constant minimum error occurs over the range of 20 to 65% T (0.7 to 0.2 A). The percent transmittance should fall within 10 to 80% T (A = 1 to 0.1) in order to prevent large errors in spectrophotometric readings. Hence, samples should be diluted (or concentrated), and standard solutions prepared, so that the absorbance falls within the optimal range. 

A low-cost assay of iodine in foods and urine can be performed using catalytic spectrophotometric methods, namely those based on the Sandell—Kolthoff reaction. These methods, however, are not optimal for determination of the lower range of iodine levels occurring in foodstuffs, because of possible interference. 

The same FIA system is employed by Arruda and coworkers to determine MSG in food samples [21]. As a difference, the GIAD enzyme is obtained from pumpkin [Cucurbita maxima), and the enzymatic reactor is prepared in a completely different way. As the preparation of enzymatic reactors based on enzyme extracts is known to be a laborious procedure, the authors propose to prepare the reactor with a naturally immobilized enzyme. For this purpose, a polyethylene column is filled with 200 mg of square pieces of the outer layer of Cm. maxima. A piece of the fruit is first cut and rinsed with distilled water. Then the outer layer is cut in square pieces with 2-mm sides and 1-mm thick and washed with phosphate buffer solution. This column is incorporated into the FIA system. The obtained linear range with the optimized FIA system is 10-100 mmol L and the analytical frequency is 40 h h The method is applied to the analysis of real samples (meat softener, meat soup, and spices), and the results are compared with those obtained applying the conventional spectrophotometric method to the same samples. The implemented procedure has a comparable precision to the conventional method (deviation values lower than 5%). This enzymatic reactor can be used for 21 days (200 determinations) without significant changes. 

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