Compound-independent calibration

Beside the qualitative identification of elements the atomic emission detector provides the possibility of determination of elemental composition and empirical formulas with compound independent calibration (Wylie et al., 1990 Quimby et al., 1995). This possibility enables an identification independent from retention times and does not require authentic reference standards for calibration. For structural elucidation the element ratios have been calculated using the method of relative response factors. Triphenylarsine was used as standard. The results have been confirmed by complementary high resolution mass spectrometric investigations. Table 4 summarizes the results of selected compounds. 

A direct result of a compound-independent elemental signal is the ability to measure elemental mole ratios (EMR) in analyzed compounds. For unknown compounds, the AED response in separate elemental channels for the compound is compared with the response to one or several standards. With computer software, first the EMR for known standards and then the empirical formula for an unknown compound are determined. Compound-independent calibration saves much time and cost for analysts, for it is possible to... 

To calculate the concentration, a compound-independent calibration (CIC) is performed using the CLND trace. The relationship between the signal and the concentration is given by the following equation ... 

As mentioned above, AED is a technique particularly well-suited to screen complex samples for multiple compounds containing heteroatoms, such as phosphorus, sulfur or nitrogen, which are especially relevant in the study of chemical warfare agents. Among other GC detectors, AED has unique characteristics, such as compound independent calibration and possible raw formula determination [56]. Albro and Lip-pert have summarized the strengths and weaknesses of the performance of AED relative to other classical detectors for the analysis of chemical warfare agents and their breakdown products. Their conclusions illustrated that AED offers the best combination of sensitivity and selectivity of the detectors investigated for the analysis of sulfur compounds [57]. 

The calibration constant K strongly depends on the instrumental specifications, geometry of the calorimeter (Kd), and the solvent thermoelastic properties (x ) It can be determined through a comparative assay made under the same conditions as the main experiment but using a photoacoustic calibrant instead of the sample compound. The calibrants are substances that have known values of (pm from independent measurements, or more conveniently, substances that dissipate all of the absorbed energy as heat (e/>nr = 1), like ferrocene [286] or ort/w-hydroxybcnzophenone [287]. Note that the computation of K is not really needed, because a direct comparison of the signals obtained with sample and calibration compounds allows it to be eliminated from the calculations. 

The calibration step is critical. In general, the basic principle is always to use two independent calibration solutions. One of these can be made from pure chemicals, for example, Hg° dissolved in concentrated HN03 and diluted to the appropriate volume. For mercury, commercially available standard solutions can be used, but regular checks against a reference standard must be made. Certified reference materials (CRFs) should be used if available, but reference standards can also be prepared from pure mercury compounds. In the absence of aqueous-phase reference standards, solid materials may be used. 

The principal advantage of sensors is therefore their fast response time in comparison to the retention time in chromatography, but the sensor response is partially a sum signal of the favourably enriched analyte and the cross-sensitive compounds, which makes a independent calibration necessary. Furthermore, it is essential for the sensor layer to allow a completely reversible inclusion process, while still maintaining high selectivities and high enrichment factors. 

Detector signal magnitude for an individual element is almost compound independent. The phenomenon of compound independence is used in quantitative analysis and also for identification of unknown compounds. In quantitative analysis, content of a given element in a particular compound is determined using a calibration graph for the same element, but present in various compounds in known amounts. It is then possible to perform quantitative analysis of compounds without standards of those compounds. 

Atomic emission detection shows promise for easing the calibration problem. In practice, AED is not completely Independent of molecular structure, so calibration should be made with a similar compound. However, calibration standards are required for only one or two members of a homologous series (39). 

In case of fast gradient (below 15 min), S could be considered constant for all the investigated molecules and wiU only have a small influence on the retention time of the compounds. Thus, the gradient retention times, of a calibration set of compounds are linearly related to the ( )o values [39]. Moreover, Valko et al. also demonstrated that the faster the gradient was, the better the correlation between t, and < )o [40]. Once the regression model was established for the calibration standards, Eq. 8 allowed the conversion of gradient retention times to CHI values for any compound in the same gradient system. Results are then suitable for interlaboratory comparison and database construction. The CH I scale (between 0 and 100) can be used as an independent measure of lipophilicity or also easily converted to a log P scale. 

For a mixture of enantiomers it is thus possible to determine the ee-value without recourse to complicated calibration. The fact that the method is theoretically valid only if the g factor is independent of concentration and if it is linear with respect to ee has been emphasized repeatedly.84-89 However, it needs to be pointed out that these conditions may not hold if the chiral compounds form dimers or aggregates, because such enantiomeric or diastereomeric species would give rise to their own particular CD effects.88 Although such cases have yet to be reported, it is mandatory that this possibility be checked in each new system under study. 

Other Iodine Compounds. By studying the isomer shift and the quad-rupole hfs for a series of compounds, Perlow and Perlow (29) have calibrated the isomer shift scale which is independent of Equation 7. The number of p holes, hp, for the compounds of I2, KICI4 H2O, KIC12 H2O, and ICl has been inferred from the measured value of the quadrupole coupling constant, eqQ, and from the symmetry of the iodine lattice site. Since they found that the isomer shifts and hp are linearly related for these compounds, it appears that 5 electrons do not participate directly in the bonding, and the sp hybridization is negligible. Because of this linearity, they have been able to determine dh/dhp, which leads to a calibration of the isomer shift scale. 

Use of Kq Instead of Vg in the calibration of columns produces a calibration curve that is independent of column dimensions and pore volume. To obtain Kq for any species requires the determination of Vg and Vj in addition to Vg. Vg is usually taken as the elution volume of an excluded polymer while Vj is equal to V-j - Vg. The volume V-j is the total permeation volume of the column and is measured with a low molecular weight compound that totally permeates particle matrices. 

Final adjustment with HC1 may be needed if the pH is more than one unit away from a pfCa value. When two buffering materials are present, the composition should be calculated independently for each. The measurement of pH should always be done with great care because it is easy to make errors. Everything depends upon the reliability of the standard buffers used to calibrate the pH meter.7 Often, especially during isolation of small compounds, it is desirable to work in the neutral pH region with volatile buffers, e.g., trimethylamine and C02 or ammonium bicarbonate,... 

Because SPME extracts compounds selectively, the response to each compound must be calibrated for quantification. A specific compound can be quantified by using three GC peak area values from solvent injection, static headspace (gas-tight syringe), and SPME. The solvent injection is used to quantify the GC peak area response of a compound. This is used to quantify the amount of the compound in the headspace. The SPME response is then compared to the quantified static headspace extraction. These three stages are necessary because a known gas-phase concentration of most aroma compounds at low levels is not readily produced. A headspace of unknown concentration is thus produced and quantified with the solvent injection. Calibration must be conducted independently for each fiber and must include each compound to be quantified. 

The TMS absorption in the 60 MHz proton spectrum of toluene (Fig. 3.42) is indicated and this is the reference point (0 Hz) for all absorptions in the spectrum. The spectrum shows two absorptions which appear at 128 Hz and 419 Hz downfield from TMS. If the spectrum of toluene is recorded on an instrument operating at 100 MHz their absorptions occur at 213 Hz and 698 Hz respectively. In order to make direct and rapid comparisons between spectra recorded on instruments operating at different frequencies, the positions of absorptions are normally quoted on the <5 scale which is independent of the instrument operating frequency. The <5 value is obtained by dividing the position in Hz by the instrument frequency (in MHz) and is expressed in parts per million (p.p.m.). Thus for toluene the two absorptions appear at 5 2.13 (128/60 or 213/100) and 8 6.98 (419/60 or 698/100). The chart paper normally used for recording spectra is calibrated in 8 values and therefore this calculation is not usually necessary, but it is often needed to determine the position of absorptions which have been offset (see Fig. 3.47). The 8 value may relate to any reference compound and therefore the particular reference used (e.g. TMS) must be quoted. Earlier literature used the similar r scale this related solely to TMS which was given a value of 10. The two scales can be readily interconverted since r = 10 — 8. 

AQ/Qo = (Q - Qo)/Qo, where Q0 is charge consumption without analyte and Q is that at certain thrombin, concentration. An example of calibration curve for two independently prepared electrodes is shown in Fig. 47.3. It is seen, that results are well reproducible. Statistical analysis, performed earlier [4] revealed that standard error is approximately 11%. Interferences of this aptasensor with other compounds, human serum albumine (HSA) and human IgG are relatively low. An example is shown in Fig. 47.4, where calibration curve for thrombin is compared with those for HSA and IgG. Please note, that concentrations of HSA and IgG are much higher in comparison with that of thrombin. 

It is important that there is a low level of doubt about the identity of the compound being measured and a high level of certainty that the quantity determined is a true reflection of the amount actually present. To achieve this, confirmatory techniques usually employ complex separation procedures to isolate the compound of interest and require calibration procedures that involve adding known amounts of the compound to uncontaminated specimens of the material under analysis. As a final check on the validity of the method it is recommended that the method should be evaluated in a collaborative trial in a minimum of three independent laboratories against a standard reference material. 

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