Photometry reflectance

In reflectance photometry, diffuse reflected fight is measured. The reflected light results from illumination, with diffused light, of a reaction mixture in a carrier or from the diffusion 

TABLE 3-4 Recommended Absorbance Values for Acidic Potassium Dichromate Solutions 

---

Ng R.H., Sparks K.M., Statland B.E., Colorimetric determination of potassium in plasma and serum by reflectance photometry with a dry-chemistry reagent, Clin. Chem. 1992 38 1371. 

Figure 6.4 Cross-section through a dry reagent slide for use in the Vitros Chemistry System, previously known as the Kodak Ektachem analyser. A range of slides, which vary in the nature, number and composition of the layers, is available for a variety of analytes in blood serum. The sample (approximately 10 /d) is applied to the spreading layer and reactions take place as it permeates through the various layers. Detection is by reflectance photometry.

Hardy, J. and Fanni, J. 1981. Application of reflection photometry to the measurement of milk coagulation. J. Food Sci. 46, 1956-1957. 

Over conventional reflectance photometry, IRES retains advantages 1) and 2), with 3) being partially lost (the substrate contribution to A only is measurable) however, sensitivities are a factor of 10 or more lower. Polarization modulation techniques that determine only Ae, and not the corresponding dispersion property (6,J), retain advantage 1) over wavelength modulation photometry,but not advantage 2),and retain advantage 3) only in part, as above. 

To avoid the difficulties of translucency and the irregularities in the structure of the paper, reflection photometry has been proposed (K17, L4, R6, R7, S14, T3). Originally a linear protein/dye relationship was claimed for reflectance measurements (L4) but soon the same error as for translucency readings was found (Fig. 37). A theoretical account... 

Owen, J. A., Determination of serum-protein fractions by zone electrophoresis on paper and direct reflection photometry. Analyst 81, 26 (1956). 

Fig. 3. (a) The principle of reflectance photometry, (b) The relation of reflectance measurements to concentration. 

S105 Lott, J.A. and Khabbaza, E. (1985). Haemoglobin analysis on whole blood by reflectance photometry. J. Automat. Chem. 7, 197-200. 

Automated chemistry analyzers have traditionally relied on photometers and spectrophotometers for measurement of absorbance. Alternative approaches now being incorporated into analyzers include reflectance photometry and fluorom-etry. Immunoassay systems have used fluorescence (IMX), chemiluminescence (Centaur and Immulite), and electrochemiluminescence (ELECSYS) to enhance sensitivity. Ion-selective electrodes and other electrochemical teclmiques are also widely used. Principles of these measurement techniques have been discussed previously (see Chapter 4). This section reviews the special features and application of the various approaches to automated analysis. 

A formazan dye (l,5-bis[3,5 dichloro-2-hydroxyphenyl]-3-formazan carbonitrile) forms a complex with magnesium at alkaline pH, which has been measured at 630 nm by thin-film reflectance photometry. N,hf -[l,2-ethanediyibis[oxy-2,l-phenylene]bis(N-carboxymethyl)] glycine is used to chelate calcium. This thin-film reflectance method shows relatively little interference from icteric, Upemic, and hemolyzed specimens. Elevated calcium concentrations cause a measurable but small overestimation. 

Besides the above mentioned gravimetric methods, there are many, more sophisticated commercial instruments for measuring moisture, some suitable for on-line applications and monitoring. Those are based on backscatter radiation, electrical conductance, near-infrared absorption or reflection photometry and most require prior calibration. 

W. Werner and W. Rittersdorf, Reflectance Photometry, in Methods ofEnzymatic Analysis, Vol. 1 (eds. J. Bergmeyer and M. Grassi), Verlag-Chemie, Weinheim, 1983, p. 305. 

Dry chemistry analyzers use no liquid reagents. The reagents and other sample preparation methods are layered onto a slide. The liquid sample is placed on the slide, and after a period of time the color developed is read by reflectance photometry. Ion-selective electrodes have been incorporated into the same slide format. 

---