Area measurement, precision

The fact that only 2000 channels are available is irrelevant as the spectra can be regarded as a 2000 channel slice of a 4096-channel spectrum - or 8192 or 16384 channel spectra, for that matter. What matters is the number of channels within a peak. These spectra were recorded at about 0.5 keV/channel and I deduced in Chapter 5, Section 5.5.2 that this is, in many respects, optimal for best peak area measurement precision. 

Orifice. Orifice viscometers, also called efflux or cup viscometers, are commonly used to measure and control flow properties in the manufacture, processing, and appHcation of inks, paints, adhesives, and lubricating oils. Their design answered the need for simple, easy-to-operate viscometers in areas where precision and accuracy are not particularly important. In these situations knowledge of a tme viscosity is uimecessary, and the efflux time of a fixed volume of Hquid is a sufficient indication of the fluidity of the material. Examples of orifice viscometers include the Ford, Zahn, and Shell cups used for paints and inks and the Saybolt Universal and Furol instmments used for oils (Table 5). 

Diffusivity and tortuosity affect resistance to diffusion caused by collision with other molecules (bulk diffusion) or by collision with the walls of the pore (Knudsen diffusion). Actual diffusivity in common porous catalysts is intermediate between the two types. Measurements and correlations of diffusivities of both types are Known. Diffusion is expressed per unit cross section and unit thickness of the pellet. Diffusion rate through the pellet then depends on the porosity d and a tortuosity faclor 1 that accounts for increased resistance of crooked and varied-diameter pores. Effective diffusion coefficient is D ff = Empirical porosities range from 0.3 to 0.7, tortuosities from 2 to 7. In the absence of other information, Satterfield Heterogeneous Catalysis in Practice, McGraw-HiU, 1991) recommends taking d = 0.5 and T = 4. In this area, clearly, precision is not a feature. 

To measure the conductivity of a solution it is placed in a cell carrying a pair of platinum electrodes which are firmly fixed in position. It is usually very difficult to measure precisely the area of the electrodes and their distance apart, and so if accurate conductivity values are to be determined, the cell constant must be evaluated by calibration with a solution of accurately known conductivity,... 

The Relative Precision of Peak Height and Peak Area Measurements... 

Results of Figures 4 and 5 suggest that the pore size of these materials can be precisely and nearly continuously modified by the Cs content, and accordingly the catalytic function is controlled. The pore size of Cs2.1 was smaller than that of Cs2.2, as was estimated to be less than 5.9 A finm the adsorption of benzene. In accordance with this, Cs2.1 had an activity for the dehydration of 2-hexanol, but was inactive for other reactions, irrespective of its considerably high sxuface area measure by N2 (55 m g" ). 

Table 4.12 Precision of methods of peak area measurement Method of measurement Relative precision, %...

Changing the attenuation is not required, and (Hi) Offers highest precision in peak-area measurement. 

The relationship between the concentration of the solute and the peak produced in the chromatogram is, strictly speaking, only valid for peak area measurements, but in most instances it is more convenient to measure peak height. Such peak height measurements should only be used when all the peaks are very narrow or have similar widths. The tedium and lack of precision associated with non-automated methods of peak area measurements may be overcome using electronic integrators, which are features of most modern instruments. 

Quantitative measurements in NMR are based on the area of the signals present in the spectrum. Signal areas can be produced as numerical values proportional to the area or, on less modern instruments, from the integration plots that are superimposed on the spectrum (Fig. 9.1). For the proton, the precision obtained in area measurements does not exceed l % even if continuous wave instruments are used at slow scanning speeds. In l3C NMR, it is preferable to add a relaxation reagent in order to avoid saturation related to relaxation times that alter the intensity of the signal. Using the molar ratios that are easily accessible from the spectrum, it is possible to deduce concentrations. 

Together, raw data and results from calibration checks, spike recoveries, quality control samples, and blanks are used to gauge accuracy. Analytical performance on replicate samples and replicate portions of the same sample measures precision. Fortification also helps ensure that qualitative identification of analyte is correct. If you spike the unknown in Figure 0-5 with extra caffeine and the area of a chromatographic peak not thought to be caffeine increases, then you have misidentified the caffeine peak. 

McNair and Bonelli (16) report on a study made comparing a number of techniques for area measurement wherein the entire chromatographic system was analyzed. An eight-component sample was used. The relative standard deviation of 10 replicate analyses by the different techniques is recorded in Table 4.4. In general the data in Tables 4.3 and 4.4 are consistent if one remembers Table 4.3 is measurement technique precision only and Table 4.4 is entire system precision. 

Besides determination of stereosequences, high-field carbon-13 NMR can be used to trace out chain branches and end groups by precise signal area measurements with an accuracy of 0.01 to 0.02%, as verified for polyethylene samples [528]. 

There are several conclusions that can be drawn from these results. First, the peak areas have a pooled RSD of about 5%, a value which is much better than that expected for densitometric scanning of electrophoresis gels. Second, note that the standard deviation of the peak area measurements was essentially independent of the volume injected. Thus, the relative standard deviation of the peak area dramatically decreases as the injection volume increases. An injection volume of at least 10 nL is required to obtain good precision (2-3%, excluding a single poor replicate... 

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