Selenium calibration standard

The precision of the flame measurement step was established using aqueous calibration standards (Table 15.11). The accuracy of the method was evaluated using various petroleum materials spiked with selenium dioxide and dilaurylselenide. The results (Tablel5.III) show an average recovery of 100% and suggest that the method is accurate within the precision of the measurement. 

Prepare a selenium calibration curve by plotting the net selenium response vs. nanograms selenium for each selenium standard. 

In order to bring the photocell to a response that approximates the sensitivity of the human eye for the different parts of the visible spectrum, color-correcting filters are interposed that attenuate the ranges in which the normally used selenium cell is more responsive than the eye, i.e, below 0.53 and above 0.58 wavelength. Standard lamps are used for calibration. 

Due to the great variability in results between different laboratories, there has been a marked lack of agreement on the absolute concentration in blood at which Se deficiency begins. It was found that the interlaboratory coefficient of variation (CV) was unsatisfactory at blood Se levels <0.5 p,mol/L and, at ca. 0.2 p-mol/L, the CV was as high as 57% [83]. In an attempt to improve this situation, Koh [82] determined the effects of using a common set of blood selenium standards to reduce the interlaboratory CV. In a cooperative survey, 43 laboratories performed the assay of Se in 3 blood samples (0.2-2.9 i,mol/L) using either a set of blood Se standards, as supplied, or their own aqueous standards. Using aqueous standards for calibration, the interlaboratory CV s were 82% and 143% at 0.5 and 0.2 xmol/L, respectively. However, CV s were reduced to ca. 29% for blood standards. Thus, the benefit of a common set of standards, preferably of similar matrix and Se concentration is demonstrated. 

Liquid absorbance standards provide a theoretically perfect way of checking the linearity of the photometric scale of an instrument because absorbance is directly proportional to concentration or path length. However, technical imperfections in instruments, cuvets, and filter solutions limit the accuracy of absolute photometric determinations (60). Liquid standards should be prepared for each calibration session. Solid samples should be checked and recertified periodically by an accredited standards testing laboratory. Color standards that change with temperature or light exposure such as the National Bureau of Standards red glass filter No. 2101 that is doped with selenium (50) must be used cautiously as recommended by the standards issuing laboratory. 

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