Absolute and Relative Sensitivity

In the analysis of simple solutions with known composition, for ICP-AES short term precision between 0.2 3 % relative standard deviation for analytes with concentrations ten times the detection Hmits may be achieved, while over several hours a 5 % RSD can be expected. This precision in combination with its robustness make ICP-AES superior to ICP-MS for the determination of minor and major components. With ICP-AES most elements (about 73) can be determined routinely at the 10 pg L level or better in solution with radial viewing. In favourable cases and/or axial viewing, detection Hmits 1 pg can be achieved. Eor a single emission line, a Hnear range of four orders of magnitude may be easily attained and as many as six orders of magnitude can be observed in favourable cases. 

The linear range can be extended by multivariate caUbration, as different emission lines with different sensitivities can be utilised for trace, minor, and major concentrations. While the speed of ICP-AES analysis will depend on whether simultaneous or sequential instruments are used for detection, generally this can vary between 2 and 6 samples per minute. Today, ICP-AES systems can be operated unattended overnight, due to the modem automated designs and safety inherent in the use of inert argon gas. 

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Detectors. A detector capable of continuously monitoring effluent from the column is essential for efficient HPLC analyses. Considerations in connection with detector performance include absolute and relative sensitivity, drift characteristics, noise, linearity, specificity, and band spreading resulting from detector design. The selection of a proper detector is essential for successful analysis, both from the standpoint of sensitivity and elimination of effects of interfering compounds (specificity). 

Fig. 13.9 The best absolute and relative sensitivity, response rate and recovery efficiency reached for different binary copolymers. ANI aniline, 4ABA 4-aminobenzoic acid, AA anthramiic acid, 3ABA 3-aminobenzoic acid, 3ABSA 3-aminobenzenosulfonic acid. Reprinted from30 with permission (copyright 2008 American Chemical Society)...

Conpa ison of the methods to cjuantitate 0 -EtT with respect to absolute and relative sensitivity... 

Several kinds of detection systems have been applied to CE [1,2,43]. Based on their specificity, they can be divided into bulk property and specific property detectors [43]. Bulk-property detectors measure the difference in a physical property of a solute relative to the background. Examples of such detectors are conductivity, refractive index, indirect methods, etc. The specific-property detectors measure a physico-chemical property, which is inherent to the solutes, e.g. UV absorption, fluorescence emission, mass spectrum, electrochemical, etc. These detectors usually minimize background signals, have wider linear ranges and are more sensitive. In Table 17.3, a general overview is given of the detection methods that are employed in CE with their detection limits (absolute and relative). 

Ordinarily water content may be determined by a Karl Fischer titration, but for the low concentrations in aliphatic hydrocarbons this method is filled with difficulties. To obtain even modest accuracy such as 5 ppm at HtO levels about or below 30 ppm as specified by the German Standard DIN S1777 relatively large eluent volumes (>2001 ) are required. On the other hand, the absolute and relative retentions are sensitive to 1 to 2 ppm of water consequently, two different batches of eluent with identical water content according to the above mentioned Karl Fischer titration can yield different retention values. Therefore, it is impossible to use titration as the sole method for eluent standardization. The reproducibility of adsorption chromatography, however, can be increased by continuously recycling a sufficiently lapge volume in a closed system to maintain equilibrium. 

In all these applications, sensitive instruments for the determination of isotope abundances are required. In this connection, the conventional ionic-type mass spectrometer is used almost exclusively for the determination of absolute and relative isotope abundances with the exception of the wide use of infrared methods for the determination of hydrogen to deuterium ratios. 

Freezing the gas to a low temperature increases the population of state m with respect to n both in absolute and relative terms. This is of particular consequence as levels nearer to the ground state are measured. Significant sensitivity enhancement can be obtained by fi eezing the gas down to a few K rotational temperature as the absorption measurement is carried out in, for example, a supersonic jet. There is a trade-off, however, between population increase of the lower states by depopulation of the higher states, because the intermediate states populations too will increase as the rotational temperature falls. This is reached in OCS, for example, at y = 5 and y = 6 at 4 K. 

The absolute and relative yields of the two products are sensitive to the solvent used. In the more polar solvents, the ratio trans-.cis is increased (5.8 in diethyl ether and 1 in CCl ). 

The first catalyst used by Ziegler et al. [5,82] for the polymerization of ethene was a mixture of TiCU and A1(C2H5)3, each of which is soluble in hydrocarbons. In combination they form an olive-colored insoluble complex that is very unstable. Its behavior is very sensitive to a number of experimental parameters, such as Al/Ti ratio, temperature and time of mixing of all components, and absolute and relative concentrations of reactants [83]. After complexation, TiC is reduced by a very specific reduction process. This reduction involves alkylation of TiC with aluminum alkyl molecules followed by a dealkylation reduction to a trivalent state ... 

The sensitivity of IS S is a strong function of mass and of the experimental parameters used. So many factors are involved in determining the. sensitivity of an element in a particular ca.se that it is not appropriate to state specific numbers unless all the parameters involved are completely specified. Nevertheless, guidelines have been provided in the literature. Leys [152] has suggested that absolute sensitivities range from about 0.3 to lO" monolayers for Li to Au, and relative sensitivity factors for 2 keV and 2 keV - Ne are shown in Fig. 67 [137]. The curve for He+ indicates that the relative sensitivity varies by a factor of more than 500 over the periodic table and that elements with Z < 10 have low relative sensitivities. The relative sensitivity is proportional to the scattering cross section and inversely proportional to the neutralization probability. The detection of higher atomic number elements is favored because the cross section increases, and the neutralization probability decrea.ses with Z. 

The absolute pressure may have a significant effect on the vapor—Hquid equiHbrium. Generally, the lower the absolute pressure the more favorable the equiHbrium. This effect has been discussed for the styrene—ethylbenzene system (30). In a given column, increasing the pressure can increase the column capacity by increasing the capacity parameter (see eqs. 42 and 43). Selection of the economic pressure can be faciHtated by guidelines (89) that take into consideration the pressure effects on capacity and relative volatiHty. Low pressures are required for distillation involving heat-sensitive material. 

Ix is the background-corrected net intensity of the principal peak of analyte X, Kx a proportionality factor for the absolute sensitivity of the standard reference, e. g. an Ni plate, and c the concentration of X. Multielement analyses are based on known relative sensitivities S ... 

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