Photometric repeatability

Photometric repeatability refers to the precision of measured absorbance especially using a standard. 

Photometric repeatability (7 ) is the precision with which a spectrometer can make repeated measurements at the same nominal transmission value over temporal and environmental changes. This specification is calculated as... 

Degradation due to weathering was followed by measuring the absorbance of the dyed film as a function of time of exposure. Spectra were measured using a Shimadzu UV-2550 spectrophotometer (UV-2401 for some replicates) set at 1-nm bandpass and equipped with a holder assembly that ensured repeatable positioning of a specimen each time it was measured. A file specimen that was kept in the dark at room temperature was also measured at each interval. Its spectra revealed that the repeatability of the absorption measurement was very near to the photometric repeatability specified for the spectrophotometer ( 0.001 absorbance units). 

ASTM E-13 discusses the evaluation of photometric repeatability and linearity. They recommend the use of metallic screens with electrolytically perforated holes having conical cross-sections with sharp edges as almost achromatic beam attenuators. Various combinations of Corning filters provide a check on linearity. 

At present, the problems of determining photometric repeatability, linearity, and accuracy in the ultraviolet, visible, and near-infrared regions are far from solved. They are, however, receiving attention from Vandenbelt and others, and the day may yet arrive when cut-and-dried procedures are available and spectroscopists will not need to ingeniously contrive their own systems. 

Micro amounts of sulfur in polymer are usually determined by oxygen flask combustion, sodium peroxide fusion in a metal bomb followed by titration [30], pyroluminescence [36] or ICP-AES. An oxygen flask combustion photometric titration procedure capable of determining total sulfur in polymers in amounts down to 50 ppm was reported. The repeatability of the sulfur determination in polyolefins in the oxygen flask is 40% at 50 ppm level, improving to 2% at the 1 % level [21]. Crompton [31] has also combined Schoniger flask combustion with a colorimetric procedure for the determination of phosphorous in polymers in various concentration ranges (0.01 to 2%, 2 to 13%). 

The usual procedure for measuring the rate of an enzymatic reaction is to mix enzyme with substrate and observe the formation of product or disappearance of substrate as soon as possible after mixing, when the substrate concentration is still close to its initial value and the product concentration is small. The measurements usually are repeated over a range of substrate concentrations to map out how the initial rate depends on concentration. Spectro-photometric techniques are used commonly in such experiments because in many cases they allow the concentration of a substrate or product in the mixture to be measured continuously as a function of time. 

Castro et al. [64] reported a comparison between derivative spectro-photometric and liquid chromatographic methods for the determination of omeprazole in aqueous solutions during stability studies. The first derivative procedure was based on the linear relationship between the omeprazole concentration and the first derivative amplitude at 313 nm. The first derivative spectra were developed between 200 and 400 nm (A/ = 8). This method was validated and compared with the official HPLC method of the USP. It showed good linearity in the range of concentration studied (10—30 /ig/ ml), precision (repeatability and interday reproducibility), recovery, and specificity in stability studies. It also seemed to be 2.59 times more sensitive than the HPLC method. These results allowed to consider this procedure as useful for rapid analysis of omeprazole in stability studies since there was no interference with its decomposition products. 

Using five types of absorption photometric systems commonly employed in clinical laboratories, problems associated with the calibration of such instruments have been depicted. Monoelement CRMs [4] in the range of concentration indicated by the method of measurement applied were used in these situations, and as far as possible, calibration procedures agreed by clinical laboratories have been followed. Each CRM used was repeatedly measured (ten times at least). Whenever possible, a linear calibration curve was fitted. Then a correction factor for calibration and the uncertainty of this factor were determined. The degree of compatibility between the measurement result and the certified value of the CRM was tested in each situation. This algorithm is illustrated in Fig. 4. Accordingly, results on calibration are shown in Table 1. 

In order to resolve the conflicting results of Chapman Thon , and Cremer , a very detailed and thorough set of experiments was performed by Norrish and Ritchie . In this work, analysis was by a photometric technique thus the system was not perturbed and accurate results could be obtained. The experiments of Cremer were repeated and surface effects were found to play some role. However, these effects were minor except at low pressures. Of greater importance was the inhibiting effect of HCl. In general, the results of Norrish and Ritchie completely confirmed those of Chapman , who used a water manometer which dissolved the HCl produced. Thus the inhibition was not pronounced in her system. 

If desired, or required for a particular application, repeat step 4 for a range of integration times, from near the detection limit to near detector saturation. The detector response should be linear with integration time. For an FT-Raman system, a similar test may be performed by varying the laser power to check photometric linearity. 

Photometric linearity is tested using a set of standards with known relative transmittance or reflectance, depending on the application. Repeated measurements on stable standards give information about the long-term stability of an instrument. For analyte systems with absorbance levels below 1.0, at least four reference standards are to be used in the range of 10 to 90%. When analytes provide absorbance values that exceed 1.0, the proposed USP chapter recommends adding a 2 or 5% standard, or both. 

The technical specifications identified and described by most of the manufacturers of absorption photometers for medical use include wavelength accuracy, spectral half-width of spectral radiation flux at the detector, photometric accuracy, percentage of wavelength integrated, false radiation, and photometric short-time repeatability. As discussed previously [2], the Instrumental Performance Validation Procedures, issued by serious manufacturers of analytical instruments, indicate the methods and the reference materials required to test and to maintain optimum spectrometer performance in daily routine analysis. 

Deferoxamine interferes with photometric measurements of iron and UIBC, an important consideration if serum iron and UIBC measurements are used to monitor therapy with deferoxamine. Deferoxamine has a short tm ( lh) therefore, a delay of at least 4h after deferoxamine administration would be appropriate before measurements of serum iron and UIBC are repeated. 

Peak smoothing is also an option, as demonstrated in the spectro-photometric determination of nitrate and nitrite [93]. The signals recorded for the blank and the sample were processed by applying a simple algorithm, allowing the blank signal to be subtracted. Analytical repeatability, however, was impaired due to the error propagation effects involved. 

Becquerel at least knew about Auer s result. Kriiss, one of the founders of photometry (Szabadvary 1966, p. 343), in a paper published together with Nilson, concluded from their absorption photometric studies that the rare earth elements known up to then include at least twenty as yet unknown elements (Kriiss and Nilson 1887). Bettendorf repeated Auer s experiments in 1890 and by subjecting all mother liquors individually to spectrometry he confirmed the existence of praseodidymium and neodidymium (Bettendorf 1890). 

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