Photometric accuracy test

Since photometric accuracy of the instrument depends on almost all of the performance attributes discussed above, it is better to perform the photometric accuracy test at the final stage of the regular performance verification. Once the factors that may affect the photometric accuracy have been verified, there is a better chance of passing the photometric accuracy test. There is no point in doing the photometric accuracy test if there are known problems in the other performance attributes. 

Check the cell for the performance measurement The quality of the measurement depends strongly on the quality of the cell used. Always use the highest-quality cell available. Make sure that the cell is clean and free of contamination. Contamination can change the absorbance of standard solutions, resulting in failure of the photometric accuracy test. The optical quality of the cell is affected by factors such as closeness to the specified path length, parallelism of the inner and outer faces of the windows, and flatness of the window. Upon... 

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. 

To verify that a spectrophotometer is performing satisfactorily, the device must be shown to be able to operate within the specifications provided for it. Parameters to be tested include (1) wavelength accuracy, (2) spectral bandwidth, (3) stray light, (4) linearity, and (5) photometric accuracy. 

Table 6.1 displays recommended specifications for wavelength accuracy, photometric linearity, and spectrophotometric noise levels for pharmaceutical applications. The first step in validating any NIR method is to test the suitability of these specifications for a given application. Wavelength accuracy tests conducted using appropriate external standards will prevent potential problems that could occur with proprietary internal calibration protocols. The exact nature of any calibration standard must be noted in a validation protocol. Rare earth oxides and glass standards are candidates for such calibration. 

The use of the 96-cells plate and microplate photometers represents a convenient and fast way of quantitative photometric analysis of reactions of the chemical tests and biotests made on the basis of paper materials. The firm microplate photometers are supplied with the necessary software and systems of scanning a plate which carry out not only one-wave, but also the multiwave photometric analysis, that will enable us to increase the accuracy of the analysis. 

The performance verification tests required by major pharmacopoeias for UV-Vis spectrophotometers are listed in Table 10.1 [1-4]. The required performance tests include wavelength accuracy, stray light, resolution, and photometric... 

The wavelength uncertainty test verifies the accuracy and precision of the spectrophotometer x-axis. Typically, the x-axis will be in nanometers for a dispersion instrument and cm for a FT instrument. The use of cm for the spectral axis of an FT instrument is due to the mathematics of the interference term (Atkins 1996). The wavelength standards have stable isolated peaks usually based on a mixture of rare-earth oxides. The center of mass of the peaks is compared to standard values established on master instruments at National Institute of Standards and Technology (NIST). The typical tolerance values for the peak accuracy are 1 nm [19]. The observed precision values are usually much smaller than 1 nm due to the high reproducibility of modern spectrophotometers. The photometric linearity verifies that the y-axis of the spectrophotometer is linear over a typical refiectance range. The linearity is verified by scanning a series of standards of known reflectance (absorbance) values. The measured absorbance is plotted versus the standard values. The USP chapter specifies that the slope of this curve is equal to 1.0 0.05 with an intercept of 0.0 0.05. Photometric standards are available from instrument vendors and third party suppliers. 

The qualification of a Raman spectrometer is described in USP chapter < 1120>. In particular, the tests for the operational and performance qualification of a Raman spectrometer are described x-axis precision, photometric precision, laser power precision and accuracy. The x-axis of the Raman spectrometer is the Raman shift measured in wavenumbers. Before the Raman shift can be determined, both the laser wavelength and spectrophotometer calibration must be determined. The precision of the Raman shift can then be measured using an American Society for Testing and Materials (ASTM) Raman standard material [20]. A commonly used Raman standard material is acetaminophen. The peak position of the known reference peaks can be determined visually, but is better done with a peak location algorithm. The USP chapter on Raman specifies that the peak location should not vary more than... 

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. 

To check the performance and accuracy of the potassium microelectrode serial readings were made in the K calibrating standards. This yielded a mean ratio of potentiometric / photometric K concentration of 1.01 0.02 (S.E.), a ratio which is not significantly different from unity. In addition the performance of the potassium microelectrode was tested in Necturus and rat serum. The concentration was measured in Necturus and rat serum droplets in vitvo... 

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