Photometric precision

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... 

Absorption of electromagnetic radiation in the NIR region is caused by overtone and combination vibrations. Polyatomic molecules exhibit many overtone and combination vibrations, their spectral bands overlap and make typical NIR bands look very broad and featureless. Nevertheless, NIR spectra contain molecular information about the sample, and this information can be extracted by means of chemo-metric methods (cf Chapter 13). A prerequisite for chemometric evaluations is high quahty of the collected spectral data. Therefore, wavelength precision, resolution, photometric precision and signal-to-noise ratio are important criteria for the selection of an NIR spectrometer. 

Not many years ago it was believed that the highest photometric precision was always attained at 36.8% T,... 

The second main factor determining photometric accuracy, noise, may generally be controlled by the analyst. The deepest point of the band (maximum absorption), which is of greatest interest in quantitative measurements, can vary by as much as the peak-to-peak noise level hence, photometric precision is limited by this factor. In the more versatile spectrophotometers it is possible to slow down the response of the system, thereby smoothing out and averaging the noise fluctuations and decreasing their peak-to-peak variations. However, if this is done more time must be spent in recording the spectrum to compensate for the slower response of the system. 

Other precautions must be taken when spectra are collected if high photometric precision (repeatability) is required. Photometric precision is not only restricted to quantitative analysis, but is also required for spectral subtraction (Section 9.3), and spectral search systems (Section 10.8). Repeatable sample preparation and placement is critical. If the aperture of the sample cell is small enough to become the limiting aperture in the spectrometer, any shift in the position of the sample from the center of the beam will cause a shift in wavenumber and possibly a change in the ILS function (Section 2.6). A small wavenumber shift will cause a peak maximum to be displaced, and the bandshape and peak absorbance may change due to the change in the ILS function [6] and the discrete sampling of the spectrum (Section 3.1). 

The worked out soi ption-photometric method of NIS determination calls preliminary sorption concentration of NIS microamounts from aqueous solutions on silica L5/40. The concentrate obtained is put in a solution with precise concentration of bromthymol-blue (BTB) anionic dye and BaCl, excess. As a result the ionic associate 1 1 is formed and is kept comparatively strongly on a surface. The BTB excess remains in an aqueous phase and it is easy to determinate it photometrically. The linear dependence of optical density of BTB solutions after soi ption on NIS concentration in an interval ITO - 2,5T0 M is observed. The indirect way of the given method is caused by the fact the calibration plot does not come from a zero point of coordinates, and NIS zero concentration corresponds to initial BTB concentration in a solution. 

Investigated is the influence of the purity degree and concentration of sulfuric acid used for samples dissolution, on the analysis precision. Chosen are optimum conditions of sample preparation for the analysis excluding loss of Ce(IV) due to its interaction with organic impurities-reducers present in sulfuric acid. The photometric technique for Ce(IV) 0.002 - 0.1 % determination in alkaline and rare-earth borates is worked out. The technique based on o-tolidine oxidation by Ce(IV). The relative standard deviation is 0.02-0.1. 

The total cerium content in the single crystal samples on the basis of rare-earth elements is determined by photometry after Ce(III) oxidation by ammonium persulfate. The Ce(III) content is calculated from the difference. Comparison of the determination results of the total cerium content obtained by photometric and atomic emission methods for Li GdlBO ljiCe demonstrated the elaborated procedure precision and systematic error absence. 

In situ quantitation This reagent was not suitable for direct, precise photometric quantitation. 

Visibility Amplitude (III.7) This is the most readily measured observable, the maximum contrast of the interferogram. It contains essential photometric information for images of the source and all information for circularly (or el-liptically) symmetric sources. A high precision measurment requires calibrator sources with known visibilities and / or monitoring of system parameters for calibration. 

Experimental A photometric method was found in the literature which seemed to suit the particular circumstances. Two cyanide stock solutions were prepared, and an electromechanical dispenser was used to precisely prepare solutions of 20, 40,. .., 240 respectively 10, 30, 50,. .., 250 fig CN /100 ml. 10 ml of each calibration solution were added to 90 ml of the color-forming reagent solution and the absorbance was measured using 1-cm cuvettes. (See Table 4.17 (left and middle panels) and data file CYANIDE.dat.)... 

Jagner [28] has also described a semi-automatic titration for high-precision determination of chlorine in seawater, where it has been used for the potentiometric determination of total halides (silver electrode) and alkalinity (glass electrode), and for the photometric titration of total alkaline-earth metals. Several titrations can be effected simultaneously. 

In this development of a flow injection method for the determination of nitrate andnitrite, Anderson [168] chose the Shinn [155] method to reduce nitrate and nitrite because of its high sensitivity and relative freedom from interferences. Anderson [168] used flow injection in the photometric determination of nitrite and nitrate with sulfanilamide and N-( 1-naphthyl) ethylenediamine as reagents, as discussed next. The detection limit is 0.05 xm for nitrite and 0.1 xm for nitrate at a total sample volume of 200 iL. Up to 30 samples can be analysed per hour with relative precision of about 1%. 

Jagner [150] used computerised photometric titeration in a high-precision determination of calcium in seawater. 

Jagner and Kerstein [654,655] used computer-controlled high-precision complexiometric titration for the determination of the total alkaline earth metal concentration in seawater. Total alkaline earths were determined by photometric titration using EDTA with eriochrome Black as indicator. The method yielded 63.32 (xmolekg-1 for the total alkaline earth concentration in standard seawater of 3.5% salinity. The precision was about 0.01%. 

Connell and Smithies (C2) have proposed a photometric peroxidase method with guaiacol as substrate and H202 instead of ethyl hydroperoxide. According to Nyman (N7), the method is less specific than Jayle s method. In the clinically interesting low range of HbBC (0-30 mg/100 ml) the method is unreliable. A simplification has been presented by Owen et al. (02). No report exists on its specificity, accuracy, or precision. 

If one of the stars in the binary is not a neutron star, then the tests become less precise. Suppose that one observes the optical light from the companion to a neutron star. In addition to the spectral information that allows measurement of P and i i, one also has photometric information (e.g., the total optical flux from the companion). The companion is distorted into a pear shape by the gravity of the neutron star, with the point towards the neutron star. Therefore, from the side there is more projected area and hence greater flux than from either end. If the orbit is edge-on (i = 90°) then the flux varies maximally if the orbit is face-on (i = 0°) then there is no variation. Therefore, by modeling the system one can estimate the inclination from the flux variations. This is called the method of ellipsoidal light curves (Avni Bahcall 1975). 

In a method described by Bates and Carpenter [8] for the characterization of organosulphur compounds in the lipophilic extracts of marine sediments these workers showed that the main interference is elemental sulphur (S8). Techniques for its elimination are discussed. Saponification of the initial extract is shown to create organosulphur compounds. Activated copper removes S8 from an extract and appears neither to create nor to alter organosulphur compounds. However, mercaptans and most disulphides are removed by the copper column. The extraction efficiency of several other classes of sulphur compounds is 80-90%. Extracts are analyzed with a glass capillary gas chromatograph equipped with a flame photometric detector. Detection limit is lg S and precision 10%. 

The results obtained with ISEs have been compared several times with those of other methods. When the determination of calcium using the Orion SS-20 analyser was tested, it was found that the results in heparinized whole blood and serum were sufficiently precise and subject to negligible interference from K and Mg ([82]), but that it is necessary to correct for the sodium error, as the ionic strength is adjusted with a sodium salt [82], and that a systematic error appears in the presence of colloids and cells due to complexa-tion and variations in the liquid-junction potential [76]. Determination of sodium and potassium with ISEs is comparable with flame photometric estimation [39, 113, 116] or is even more precise [165], but the values obtained with ISEs in serum are somewhat higher than those from flame photometry and most others methods [3, 25, 27, 113, 116]. This phenomenon is called pseudohyponatremia. It is caused by the fact that the samples are not diluted in ISE measurement, whereas in other methods dilution occurs before and during the measurement. On dilution, part of the water in serum is replaced by lipids and partially soluble serum proteins in samples with abnormally increased level of lipids and/or proteins. 

As with any process analytical application, instrument selection is based on the required analytical merits (sensitivity, dynamic range, precision and accuracy, etc.), process and enviromnental conditions, integration complexities (mechanical and controls automation) and operational and maintenance requirements. Because of the wide disparity in analytical performance and functionalities among photometric and spectroscopic LIE process instruments, selection should be carefully weighed on the basis of the technical problem, instrumental cost, implementation complexities, ease of use, conunercial and legacy maturity, level of vendor support and cost of ownership. 

Among the analytical methods presently used for the characterization of natural and synthetic peptides and proteins, the primary value of amino acid analysis is the determination of absolute peptide and protein content in solids and solutions and the quantitation of their amino acid composition and stoichiometry. It involves two steps, i.e. complete hydrolysis of peptides and proteins, followed by photometric determination of the released amino adds. The steps are laborious and time-consuming, and there is a continuous need for improvement of the techniques to increase precision and sensitivity. 

Noise and Drift. Electronic, pump, and photometric noise poor lamp intensity, a dirty flow cell, and thermal instability contribute to the overall noise and drift in the detector. Excessive noise can reduce the sensitivity of the detector and hence affect the quantitation of low-level analytes [13,14]. The precision of the... 

For still greater precision photometric measurement of the distribution of blackening on the film may be made each reflection appears as a hump on the blackness-distance curve, and the position of the peak can be taken as the position of the reflection. In work of such precision as this, some account must be taken of several sources of error arising from the particular experimental circumstances. 

At present few hospital laboratories use gas chromatographic methods because colorimetric, fluorometric, and other spectro-photometric methods can give data more rapidly, are less expensive, and are often of sufficient accuracy and precision for clinical evaluation of the patient. Radioimmunoassay procedures are available, but these are limited because of availability of the necessary antibodies and a sufficient demand to justify the expense of setting up the laboratory. For versatility and adaptability, many different analyses can be done on a single gas chromatograph. The only requirement is the proper column and choice of operating conditions. Furthermore, the techniques, in many cases, are now automated. The utility of GC coupled with... 

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. 

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