Species measurement methods

Essential features of an automated method are the specificity, ie, the assay should be free from interference by other semm or urine constituents, and the sensitivity, ie, the detector response for typical sample concentration of the species measured should be large enough compared to the noise level to ensure assay precision. Also important are the speed, ie, the reaction should occur within a convenient time interval (for fast analysis rates), and adequate range, the result for most samples should fall within the allowable range of the assay. 

The precipitation method of separation involves the addition of salts such as ammonium sulfate or solvents such as polyethylene glycol to the reagent mixture to cause precipitation of the large molecular weight bound species. These methods of precipitation lack specificity and work well only when there is a large difference between the molecular weight of the material being measured and that of the bound complex of it. 

In the laboratory, soil water content is measured by drying in the oven and with a pressure plate apparatus. Drying soil can change the form and species of components present, and for this reason, most soils are air dried carefully or at temperatures only slightly above room temperature before analysis. A number of different field measuring methods are used mostly to determine the amount of water available for plant use. 

Many probes are now known that display changes in fluorescence lifetime on complexation of the analyte, photophysical properties some of them are summarized in Table 10.2. While we have listed the lifetimes of the free and the bound forms of the probes, there is no straightforward equation to calculate the analyte concentration using the mean lifetime as was in the case of the absorbance and intensity (Eqs. (10.14) and (10.15)). The mean lifetime depends not only on relative concentration of the probe species (free and complexed) but also on their decay times, quantum yields, and to some extent on the measurement (method or conditions). While the mean lifetime is independent of total probe concentration, this value generally depends not only on analyte concentration but also on excitation and observation wavelengths.03 ... 

A number of significant oxygenated organic particulate compounds and gas-phase free radicals are formed by the reactions of gas-phase hydrocarbons (see Table 6-1 and (Chapter 2). The measurement methods for these substances are complicated and in the research stage. Their description is beyond the scope of this chapter. It is of major importance to develop methods for measuring hydroxyl and peroxyhydroxyl radicals, as well as the various oxygen species formed with ozone (see Chapter 12). 

Fluorescence is the basis of a number of measurement methods for atmospheric gases. In the case of SOz, for example, a Zn (213.8 nm) or Cd (228.8 nm) lamp is used to excite the SOz and the fluorescence in the 200- to 400-nm region is monitored (Okabe et al., 1973 Schwarz et al., 1974). Again, this technique works for those species whose excited states are sufficiently... 

Lawson, D. R., H. W. Biermann, E. C. Tuazon, A. M. Winer, G. I. Mackay, H. I. Schiff, G. L. Kok, P. K. Dasgupta, and K. Fung, Formaldehyde Measurement Methods Evaluation and Ambient Concentrations during the Carbonaceous Species Methods Comparison Study, Aerosol Sci. Technol., 12, 6A-16 (1990). 

There are two reasons for considering the mechanisms in this detail. Firstly, the potential at which preparative experiments are carried out is determined by which is the basic species. Secondly, methods for determining the rate of protonation (k ) depend on a knowledge of the mechanism. The rate of protonation of an EGB by an acid of known pK is the most convenient measure of basicity (kinetic basicity). 

Rotational Raman spectroscopy is a powerful tool to determine the structures of molecules. In particular, besides electron diffraction, it is the only method that can probe molecules that exhibit no electric dipole moment for which microwave or infrared data do not exist. Although rotational constants can be extracted from vibrational spectra via combination differences or by known correction factors of deuterated species the method is the only one that yields directly the rotational constant B0. However for cyclopropane, the rotational microwave spectrum, recording the weak AK=3 transitions could be measured by Brupacher [20],... 

The acetyl bromide procedure was developed by Johnson et al. (1961) to determine lignin content in small samples from woody species. This method uses acetyl bromide (4.10) to acetylate unbound hydroxyl groups in the lignin (4.11), while the hydroxyl group on the a-carbon is substituted with a bromine group. The substituted lignin derivative (4.12) is soluble under acidic conditions, and its concentration can be measured with a spectrophotometer at 280 nm. 

Needs for improved measurement methods differ depending on whether one is considering low or high transmission rate materials. In the former case one needs very sensitive detectors. Selectivity is also desirable so that interferences from extraneous species can be avoided. In the case of high transmission rate materials instrumental time constants and saturation effects need to be better understood. In all cases there is a need for more convenient instruments and a better knowledge of their operating principles. 

Figure 8.1 Schematic classification of complexation measurement methods as a function of the perturbations that they can create at the discriminator (sensitive part of the analytical system that enables differentiation of the chemical species of interest from the other components present) and in solution. The compound reacting with the discriminator and the nature of the discriminator are shown in parentheses, a Constant cell volume methods are less perturbing than variable volumes, b Possibility of ligand release by organisms, c Possibility of interactions with the indicator (ligand with suitable absorbance or fluorescence properties added into the test solution in spectro-metric methods), d Possibility of contamination of very dilute media by ISE membranes (redrawn from Buffle, 1988).

Hydride generation AAS is a well-established technique for the determination of selenium because of its selectivity and sensitivity. The detection limits are improved by concentrating the hydride prior to the transfer to the spectrometer. Cryogenic condensation in a U-tube at liquid nitrogen temperature is a preferred method of pre-concentration. However, selenium hydride is formed essentially only from Se. The selectivity for Se has been combined with various chemical preparation steps to determine the sum of SeIV and Se and the total selenium content of samples. Se concentrations are measured by the difference between the (Se + Se ) and Se contents. The difference between total selenium and (Se + Se ) contents represents the concentrations of Se° and Se2- species. These methods have been used for determinations of selenium species in natural waters. Soluble organic matter in some groundwater samples interferes with the hydride generation determination of selenium. Isolation of humic substances by their adsorption on resins has overcome this problem. 

Some chemists feel that the mole is an unnecessary SI unit as they make measurements in mass/mass or mass/ volume units, using ratio methods. The definition and the importance of the mole has been discussed elsewhere [8], and the distinction has been made between its importance as a concept, the importance of the related atomic mass values, and the lesser role of the mole as a unit for actually reporting results. A distinctive feature of the mole is the need to define the entity . This is an extra dimension compared with other SI units. For example, it is not necessary to ask, is this a mass when measuring the mass of an object, in the way that it is critical to ask, is this lead before attempting to measure the amount of lead. A mole measurement thus requires two issues to be addressed, namely identity and amount. It follows therefore that traceability claims must show unbroken chains covering both of these issues. It is because of the existence of a vast number of chemical species that it is necessary to clearly specify and separate the specified chemical entities from all other possible chemical entities prior to measurement. This leads to complex chemical measurement processes, with considerable attention to validation of the measurement method being required. 

Chloramphenicol acetyl transferase (CAT). This bacterial enzyme was the first reporter protein used for studying the transcriptional activity of eukaryotic regulatory sequences (Gorman et al., 1982). CAT inactivates chloramphenicol, an inhibitor of prokaryotic protein synthesis, by converting it to the mono- or di-acetylated species. Measurement of CAT activity requires a 14C-radiolabeled chloramphenicol or acetyl-CoA and, therefore, an additional step is neccessary to separate the radio-labeled reactant from the product. Novel detection methods based on fluorescent substrates or ELISA assays, which do not use radiolabeled reagents, have been described more recently (Bullock and Gorman, 2000). 

R. Comelis, J. DeKimpe, X. Zhang, Trace elements in clinical samples revisited -speciation is knocking at the door. Sample preparation, separation of species and measurement methods, Spectrochim. Acta, 53B (1998), 187-196. 

Several integral measurement methods were evaluated in the early stages of this program. The wet-chemistry bubbler system deployed in Europe for remote measurement purposes cannot distinguish different chemical species. Since the deposition velocity is very species dependent, clear distinction among different chemical species is required to derive dry deposition rates. Filterpack methods have limitations as well. At the 1982 Technical Committee meeting of the National Atmospheric Deposition Program, conducted in St. Louis, a... 

This Waterloo was unfortunate for the author considers the boron method to be basically sound and potentially extremely powerful. The answer to the benthic enigma may lie in species-to-species differences in the boron isotope vital effect for benthic foraminifera. The measurement method use by Sanyal et al. (1996) required a large number of benthic shells in order to get enough boron to analyze. This created a problem because, as benthics are rare among foraminifera shells, mixed benthics rather than a single species were analyzed. If the boron isotope pH proxy is to... 

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