Avoidance calibration

In many applications it is not necessary to know the absolute gas partial pressure or concentrations. In some cases, for example, arterial, venous, or expired-to-venous concentrations are sufficient. This avoids calibrations, removes a source of error. 

Samples of urine with variable Na content, and of synovial fluid of limited volume may be analysed for Au using solvent extraction ETA—AAS in order to avoid calibration and matrix problems. Wawschinek and Rainer [98] used dimorpholine thiuramdisulphide/MIBK extraction of Au from... 

The intercomparison data in Figure 8.8 reveals a small number of points that lie within the uncertainty expected by the NPL. There is nothing remarkable about that. In at least two of these cases (one of which I have particular knowledge ), the reason for the close agreement is that the measurements were made comparatively. The sample was compared directly with a calibrated source of Cs measured in the same geometry at the same distance from the detector. In that way, the TCS errors are the same for sample and standard and cancel ont. There is no need for a calibration curve at aU. This would seem to be the most direct way of avoiding calibration errors due to TCS and, unless there are specific reasons to do otherwise, I would recommend direct comparison with... 

Measurement of the conductivity can be carried out to high precision with specially designed cells. In practice, tiiese cells are calibrated by first measuring the conductance of an accurately known standard, and then introducing the sample under study. Conductances are usually measured at about 1 kHz AC rather than with DC voltages in order to avoid complications arismg from electrolysis at anode and cathode [8]. 

As explained in Section 2.13, the use of iz,-plots makes it possible to avoid the involvement of either n or when an alternative adsorptive is being used for evaluating the surface areas of a set of related solids. It is then no longer necessary to exclude the use of isotherms having a low value of c, consequently the method is applicable even if the isotherm of the alternative adsorptive is of Type III (cf. Chapter 5). Calibration of one sample by nitrogen or argon adsorption is still required. 

Second, when filling a pipet or volumetric flask, set the liquid s level exactly at the calibration mark. The liquid s top surface is curved into a meniscus, the bottom of which should be exactly even with the glassware s calibration mark (Figure 2.6). The meniscus should be adjusted with the calibration mark at eye level to avoid parallax errors. If your eye level is above the calibration mark the pipet or volumetric flask will be overfilled. The pipet or volumetric flask will be underfilled if your eye level is below the calibration mark. 

Quantitative mass spectrometry, also used for pharmaceutical appHcations, involves the use of isotopicaHy labeled internal standards for method calibration and the calculation of percent recoveries (9). Maximum sensitivity is obtained when the mass spectrometer is set to monitor only a few ions, which are characteristic of the target compounds to be quantified, a procedure known as the selected ion monitoring mode (sim). When chlorinated species are to be detected, then two ions from the isotopic envelope can be monitored, and confirmation of the target compound can be based not only on the gc retention time and the mass, but on the ratio of the two ion abundances being close to the theoretically expected value. The spectrometer cycles through the ions in the shortest possible time. This avoids compromising the chromatographic resolution of the gc, because even after extraction the sample contains many compounds in addition to the analyte. To increase sensitivity, some methods use sample concentration techniques. 

Figure 14-9 also shows a flowchart for analysis of wet and dry precipitation. The process involves weight determinations, followed by pH and conductivity measurements, and finally chemical analysis for anions and cations. The pH measurements are made with a well-calibrated pH meter, with extreme care taken to avoid contaminating the sample. The metal ions Ca, Mg, Na, and are determined by flame photometry, which involves absorption of radiation by metal ions in a hot flame. Ammorda and the anions Cl, S04 , NO3 , and P04 are measured by automated colorimetric techniques. 

Avoid the use of instrumentation that may have low first cost, but is very expensive to operate or maintain. Blind controllers, for example, are completely unsatisfactory for most applications. The author has seen examples of temperature controllers set at the factory, but with no method of readout or calibration. These almost always require retrofitting of additional instrumentation later. Internal level floats on process vessels that require plant... 

This method avoids errors inherent in handling, sampling, analyzing, storing, introducing into the system, and batch purging with calibration gas. 

Avoid chromatographs requiring mixed carrier gas. Mixed carrier gas can introduce as many inherent accuracy problems as calibration gas, maybe more. There is almost always a way to avoid using mixed carrier gas. Find a way even if it is more expensive. 

The precision rotameter is a secondary calibration device. If it is to be used in place of a primary device such as a bubble meter, care must be taken to ensure that any introduced error will be minimal and noted. The precision rotameter may be used for calibrating the personal sampling pump in lieu of a bubble meter provided it is (a) Calibrated with an electronic bubble meter or a bubble meter, (b) Disassembled, cleaned as necessary, and recalibrated. It should be used with care to avoid dirt and dust contamination which may affect the flow, (c) Not used at substantially different temperature and/or pressure from those conditions present when the rotameter was calibrated against the primary source, (d) Used such that pressure drop across it is minimal. If altitude or temperature at the sampling site are substantially different from the calibration site, it is necessary to calibrate the precision rotameter at the sampling site where the same conditions are present. 

The user must have a good understanding of the individual calibration curves for each individual pore size column in order to avoid mismatching them, which can result in artifacts in polymer distributions. 

The zero-resistance ammeter is seldom employed for routine testing. This instrument requires careful handling to avoid damage, in particular to the galvanometer. Usually two permanent test leads are installed at a set distance apart, and by the initial use of a zero-resistance ammeter a calibration chart of potential between the two leads and current in the structure is drawn up. Thus when routine testing is made, it is only necessary to measure the... 

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