Absolute internal reference

Experimentally, Ao j/2 is generally considered equal to the midpeak potential, and is then directly deduced from the voltammograms. This does not generate experimental errors, since an ion of known standard transfer potential (for instance, tetramethyl ammonium, TMA+) must be used as an internal reference to transpose the experimental potential scale (noted E) to the absolute Galvani potential scale, so that is obtained by ... 

As XRF is not an absolute but a comparative method, sensitivity factors are needed, which differ for each spectrometer geometry. For quantification, matrix-matched standards or matrix-correction calculations are necessary. Quantitative XRF makes ample use of calibration standards (now available with the calibrating power of some 200 international reference materials). Table 8.41 shows the quantitative procedures commonly employed in XRF analysis. Quantitation is more difficult for the determination of a single element in an unknown than in a known matrix, and is most complex for all elements in an unknown matrix. In the latter case, full qualitative analysis is required before any attempt is made to quantitate the matrix elements. 

To obtain absolute concentrations of metabolites, calibration techniques are necessary. For this purpose an external calibration compound of known concentration can be measured to which the metabolite signals are referenced. Another possibility is the use of spectral signals from a tissue compound with known concentration serving as internal reference. 

The strategies for determination of absolute coneentration of metabolites using an internal reference are similar to those of external references. Often tissue water or a metabolite which is expected to have a stable concentration in tissue is used. Total creatine and water are metabolites with relatively stable concentration in musculature, although changes in their rotational mobility may change their NMR visibility. Using water as an internal reference, Eq. (5.1) has to by modified by an additional correction factor according to... 

The most accurate measurements of the CMB spectrum to date have come from the Far InfraRed Absolute Spectrophotometer (FIRAS) on the COsmic Background Explorer (COBE) (Boggess et al., 1992). In contradiction to its name, FIRAS was a fully differential spectrograph that only measured the difference between the sky and an internal reference source that was very nearly a blackbody. Figure 9.2 shows the interferograms observed by FIRAS for the sky and for the external calibrator (XC) at three different temperatures, all taken with the internal calibrator (IC) at 2.759 K. Data from the entire FIRAS dataset show that the rms deviation from a blackbody is only 50 parts per million of the peak Iv of the blackbody (Fixsen et al., 1996) and a recalibration of the thermometers on the external calibrator yield a blackbody temperature of... 

To quantitatively determine the absolute amounts of certain esters present in the oxidation products, the mass spectrometer responses to these compounds were compared with the response to the internal reference, octadecane. Using authentic samples of the esters of interest, relative mass spectrometric response factors were determined. From these response factors, listed in Table I, the amounts of esters present in the oxidative degradation products were calculated using an INCOS subroutine. 

Absolute values of (D/H), may be obtained if the NMR signal intensities of the ethanol or other test material are measured relative to that of a working standard (WS), with a known isotope ratio, (D/H)ws. The working standard may be an external reference (in a coaxial tube) or an internal reference added to the test solution. The merits of different referencing procedures have been discussed.334 For measurements on ethanol, tetramethylurea (TMU) is used as an internal reference. (D/H)ws is determined by MS with respect to an internationally agreed water standard, a value of 135 ppm being used currently for TMU.19 The isotope ratio for site i of a compound A may then be calculated from... 

The colloidal state inevitably brings about difficulties for the experimentalist when separation of the disperse phase from the dispersion medium is needed. This is the case when the speciation and concentration of only the free soluble species have to be determined. Separation of the ionic solution from the small colloidal particles for conventional chemical analysis is nontrivial, although separation techniques such as ultracentrifugation, dialysis, and field-flow fractionation have been successfully used. If the soluble species of interest have an active nuclear spin, the liquid NMR technique wiU constitute an alternative and simpler way to characterize and quantify those species without being affected by the disperse phase. An exception is the case where the colloidal species gives a signal that fully overlaps the sharp resonance of the solution entity. As NMR is quantitative, the absolute concentration of the species can be estimated based on an internal reference of known concentration but different chemical shift relative to the sample signals. Alternatively, a calibration curve can be established from a set of external standard solutions (preferably the same substance found in the sample) measured under the same experimental NMR conditions as those applied to the sample. 

Due to the complexity of the samples (e.g., layered composition or inclusions), it is not straightforward to prepare these almost perfectly matrix-matched standards in many situations and, as discussed before, suitable reference materials are often not available. K that is the case, as commonly occurs in archaeometric research, then it is still possible to obtain reliable quantitative information with non-matrix-matched solid standards, under certain conditions. First, the use of an internal reference element, which is recommended to compensate for any variation in the measuring conditions even if matrix-matched standards are used for calibration, becomes absolutely imperative in this case. The signal intensity for one of the isotopes of a selected element should correct for any differences in ablation yield or transport efficiency, for example between the samples and the standard(s). 

FIGURE 55.6. Absolute configuration of chiral Cgo-fullerene cis-3 bisadduct 9 by X-ray internal reference method. 

---