Photometric accuracy

Photometric accuracy is the accuracy with which the pen indicates the true transmittance of the sample. It enables the analyst to convert band-depth measurements to accurate quantitative results. 

Five major factors determine photometric accuracy  

Although the inherent accuracy of a spectrophotometric system depends on its design, a knowledge of the factors which contribute to this inherent accuracy may be useful in maintaining an instrument in its best operating condition. In any system the optical alignment interacts with the photometric accuracy in some way. One common cause of this is the nonuniformity of response of detectors over their sensitive areas. As a result, photometric accuracy can be achieved only with a specific alignment. 

In the single-beam instrument, photometric accuracy to a great extent is dependent on the precise linearity of the electronic amplifiers, as well as on the stability of these amplifiers between the time that the 100 % level is established, the time that the zero line is checked, and the time during which the spectrum is obtained. Another factor is the presence of atmospheric absorption bands or solvent bands in the vicinity of the band undergoing a quantitative measurement. This is one of the inherent limitations in the accuracy of single-beam systems which is subject to some control by the analyst. 

In electronic ratio-recording systems as well as single-beam systems, a slide-wire is used in the output recording system. Therefore, the 

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Wavelength accuracy and reproducibility, stray light, resolution, photometric accuracy and reproducibility, noise, baseline flatness, stability, and linearity... 

Photometric accuracy is determined by comparing the difference between the measured absorbance of the reference standard materials and the established standard value. Many solid and liquid standards are commonly used to verify the photometric accuracy of a spectrophotometer. An optically neutral material with little wavelength dependency for its transmittance/absorbance is desirable because it eliminates the spectral bandwidth dependency of measurements. The advantages and disadvantages of various commonly used photometric accuracy standards are summarized in Table 10.6. Even for a relatively stable reference standard, the intrinsic optical properties may change over time. Recertification at regular intervals is required to ensure that the certified values of the standards are meaningful and accurate for the intended use. 

Table 10.6. Comparison of Some Commonly Used Standards for Photometric Accuracy Measurement...

Figure 10.12. Photometric accuracy potassium dichromate solutions.

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... 

Standard reference material (SRM) for wavelength accuracy, stray light, resolution check, and photometric accuracy can be purchased from NIST. Certified reference materials (CRMs) which are traceable to NIST and recertification services can be purchased from instrument manufacturers and commercial vendors [12]. The cost of neutral-density filters and prefabricated standard solutions in sealed cuvettes can be substantial. When purchasing performance verification standards from a secondary supplier other than a national standard organizations such as NIST in the United States and National Physical Laboratory (NPL) in the United Kingdom, make sure that the traceability of the standards are available in the certificates. The traceability establishes the relationship of individual results to the national standard through an unbroken chain of comparisons. 

When using potassium dichromate in a sulfuric acid solution for photometric accuracy measurements, the ambient temperature should be between 21 and 23°C. For measurement at a temperature outside the range, the data should be corrected to the temperature of certification, using instructions provided in the certificate. 

When using optical filters for photometric accuracy measurements, make sure that the filter is placed properly in the sample holder so that the filter is perpendicular to the incident light beam. Problems of reflection from the highly polished surface of the glass filter and interreflection between optical surfaces for metal on quartz filters may occur that affect the readings. 

In case there is a need to perform wavelength accuracy and photometric accuracy measurements for the far-UV region below 240 nm, there are new certified reference standards available from Stama Cell [18]. The wavelength standard is a solution of rare earth oxides solvated in dilute sulfuric acid. The standard exhibits well-characterized absorption bands at 210, 211, 222, 240, and 253 nm (Figure 10.13). The photometric accuracy standard consists of a series... 

Optical Considerations. The objective of this investigation was to determine the amount of water in coexisting phases of the C10PO system. The value of the data is therefore determined by the instrument s photometric accuracy. Several factors which contribute to photometric inaccuracies in infrared microspectroscopy include sample geometry, spherical aberrations caused by the introduction of... 

This method is particularly useful for analysis of the qualitative composition of trace amounts of various substances secured as material evidence in court cases, analysis of the homogeneity of a sample, identification of inclusions and contaminations on a surface, and detection of defects in a structure. Its main drawback is the fact that the physical nature of the microsample can affect the photometric accuracy of measurement and cause distortion of the obtained spectra. 

Different operators, using the same instrument, may obtain different results due to variations in technique which, for example, markedly affects the precision of some automatic pipettes (B19). Manufacturers instructions may give little or no information on how to obtain the best results. The optimal absorbance required to obtain maximum precision varies for different types of spectrophotometer from 0.43-0.88 (H26) the user may not know this if it is not stated in the instructions. Inadequate maintenance is undoubtedly a major source of error, and includes such simple faults as greasy spectrophotometer cuvettes and pipettes and dirty tubing in continuous-flow systems, resulting in excessive sample interaction. Errors of spectrophotometers, arising from poor technique and faults in wavelength accuracy, photometric linearity, and photometric accuracy, are discussed in Section 4.5. 

Photometric accuracy is determined by comparing measured absorbance to a certified absorbance of accepted standards. Solutions of potassium dichromate... 

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