Reference Techniques

Impedance tests on materials are often performed over a wider frequency range than those on electrochemical cells. In particular, measurements at the high frequency end of the spectrum ( lMHz) are more difficult to perform with high accuracy, and measurement errors due to cables and instrumentation become more pronounced in this frequency range, requiring speciahzed reference and normalization techniques to reduce or eliminate these effects and obtain accurate and repeatable results. 

Usually external reference capacitors can be connected to the interface allowing an ultra-stable reference to be used if one is available. This also provides best matching of cable errors since the same type of connection cables can be used on the sample and on the reference. The reference technique is especially usefiil when using external amplifiers for high voltage tests, though the reference capacitor must be chosen with care to withstand the applied voltage level. 

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Major advantages of LVI methods are higher sensitivity (compare the 100-1000 iL volume in LVI to the maximum injection volume of about 1 iL in conventional splitless or on-column injection), elimination of sample preparation steps (such as solvent evaporation) and use in hyphenated techniques (e.g. SPE-GC, LC-GC, GC-MS), which gives opportunities for greater automation, faster sample throughput, better data quality, improved quantitation, lower cost per analysis and fewer samples re-analysed. At-column is a very good reference technique for rapid LVI. Characteristics of LVI methods are summarised in Tables 4.19 and 4.20. Han-kemeier [100] has discussed automated sample preparation and LVI for GC with spectrometric detection. 

Activation analysis is based on a principle different from that of other analytical techniques, and is subject to other types of systematic error. Although other analytical techniques can compete with NAA in terms of sensitivity, selectivity, and multi-element capability, its potential for blank-free, matrix-independent multielement determination makes it an excellent reference technique. NAA has been used for validation of XRF and TXRF. 

Nevertheless, the introduction of time-of-flight (ToF) analysers for SIMS analyses at the beginning of the 1980s, as well as the recent development of liquid ion sources delivering cluster projectiles now permit the analysis of organic materials with high sensitivity and selectivity. Moreover, thanks to its excellent lateral resolution (in the order of micrometres), and its minimal sample preparation, ToF-SIMS has become the reference technique for chemical imaging by mass spectrometry. 

For reference methods, HPLC with various detectors has become the standard reference technique for analysis of food additives, but new developments in this area are mainly linked to detector technology. Diode array detectors have not totally met the expectations of food analysts in terms of their specificity and LC-MS is likely to fill the gap. Specific detection with biosensor chips may also have a future for certain analyses. The use of combined LC-MS/DAD systems is... 

The easing of the requirement for high-quality optics by using this two-reference technique, and employing those characteristics which are essentially inherent to the... 

The reasons why solvent extraction has become the reference technique for the reprocessing of spent nuclear fuels at industrial scale (and will probably also be chosen in the future for the recovery of long-lived radionuclides) are the following (32, 33) ... 

Strindberg L, Lonnroth P. Validation of an endogenous reference technique for the calibration of microdialysis catheters. Scandinavian Journal of Clinical and Laboratory Investigation 2000, 60, 205-212. 

In EMEP, ICP-MS is dehned as the reference technique. The exception is mercury, where cold vapor atomic fluorescence spectroscopy (CV-AFS) is chosen. Other techniques may be used, if they are shown to yield results of a quality equivalent to that obtainable with the recommended method. These other methods include graphite furnace atomic absorption spectroscopy (GF-AAS), flame-atomic absorption spectroscopy (F-AAS), and CV-AFS. The choice of technique depends on the detection limits desired. ICP-MS has the lowest detection limit for most elements and is therefore suitable for remote areas. The techniques described in this manual are presented with minimum detection limits. Table 17.2 lists the detection limits for the different methods. 

K. Robards and P. Towers, Chromatography as a reference technique for the determination of clinically important steroids, Biomed. Chromatogr., 4 1 (1990). 

The earliest detectors of S02 in the atmosphere were automated calorimetric wet chemical devices. The reference technique for determining the SOz content of atmospheric air is based on the absorption of S02 from an air sample by a solution of sodium tetrachloromercurate, which, upon the addition of formaldehyde and a pararosaniline dye, forms a strong purple dye complex. Several manufacturers have automated this procedure. Properly maintained, these units provide an excellent record of S02 concentration in the air, because they operate on the basis of a chemical reaction that is specific for S02. 

The quality of a calibration also depends on the quality of the reference samples used. Glucuronic acid is a particular example the enzymatic method for this analysis is not reliable in that it lacks robustness. This problem has not been identified in the literature and is probably linked to non-identified matrix effects. The first FTIR calibrations were carried out using the enzymatic method as a reference and, even though gluconic acid has highly qualitative absorption characteristics, the results obtained by FTIR were unreliable. Subsequent calibrations carried out using more reliable methods such as capillary electrophoresis as a reference technique have resulted in FTIR analyses of high quality in terms of precision and accuracy. 

Marquet, P., Saint-Marcoux, F., Gamble, T.N., Leblanc, J.C. (2003). Comparison of a preliminary procedure for the general unknown screening of drugs and toxic compounds using a quadrupole-linear ion-trap mass spectrometer with a liquid chromatography-mass spectrometry reference technique. J. Chromatogr. B Anal. Technol. Biomed. Life Sci. 789(1) 9-18. 

Hand sieving is the reference technique by which other sieving techniques should be judged. For instance, in the French standard NFX 11-57, it states ... 

NIR has been used to determine the mass median diameter of a micronized active compound contained in a lactose monohydrate at a concentration of 4% by weight and a size between 8 and 20 pm [223]. Multivariate statistical analysis was applied to zero order NIR spectra using particle size distributions by low angle laser light scattering as a reference technique. Due to its speed, simplicity and low operating costs it was demonstrated that this is a viable alternative to other methods used to carry out this type of analysis. 

The multichannel nature of the SPD could be used to reduce the effect of flicker noise by utilizing the classic internal reference technique. The intensities of a few carefully selected lines are simultaneously integrated with the analyte lines and used as introduction-evaporization-atomization-excitation indicators. This approach can significantly reduce flicker noise and thus improve the S/N performance of the system as shown in Table VII. As expected, at low concentration levels, where the system becomes RFI noise limited, this method offers a very limited S/N improvement. 

Interest in the roles of both essential and non-essential trace metals in human health and disease has undergone an enormous expansion in the last thirty years. This has come about partly due to major advances in our knowledge of inorganic biochemistry (Frausto da Silva and Williams. 1991), as well as the wider introduction into clinical laboratories of powerful analytical techniques such as graphite furnace atomic absorption spectrometry (Delves, 1987 Slavin, 1988). Developments in instrumentation and chemical matrix modification techniques have also brought about dramatic improvements in analytical performance (Delves. 1987 Baruthio et al.. 1988 Slavin, 1988 Christensen et al., 1988 Savory and Wills, 1991). Other analytical techniques, such as inductively-coupled plasma emission spectrometry (ICP) and ICP-mass spectrometry are also finding wide application in the clinical analysis of trace elements (Kimberly and Paschal, 1985 Delves and Campbell, 1988 Melton et al., 1990). Although the cost of such Instruments tends to restrict their use only to specialist centres, they have very important roles as reference techniques in the characterisation of reference materials (Delves and Campbell, 1988). 

Quantitative Applications - The use of stable-isotope-labeled compounds as internal standards for the quantitation of drugs and metabolites in biological fluids offers a unique combination of sensitivity and selectivity of detection for pharmacokinetic studies. The principles of the technique have been outlined, and applications up to 1981 have been compiled. Specific aspects of the isotope dilution method, e.g. its utility as a reference technique and associated procedures for handling the data generated from the use of multiple isotope tracers simultaneously, have been discussed. Examples of isotope dilution methods used in clinical psychopharmacology have also been reviewed. ... 

Accuracy of a method of analysis generally is determined by analyzing standards with matrices the same or similar to the samples of interest. These standards have known isotopic abundances and/or concentrations. The limit to this approach often is the accuracy of the reference technique that was used to characterize the standard. 

Table 2.3 Determination of bromate in Evian water and in three types of ozonated matrices comparison between the reference technique (IC/CD) and the methylene blue method (after Ingrand et at, 2002). Reprinted from Trends in Analytical Chemistry, Vol. 21, No. 1, Ingrand et at, Determination of bromate... , pp. 1-12, 2002, with permission from Elsevier...

The KBr spectrum is employed in the internal-reference technique. In this case, a weak absorption band of the adsorbent (e.g., the overtones of the silica or zeolite lattice vibrations, 2100-1600 cm [119]) acts as the internal reference, and the ratio of adsorbate absorption to the reference absorption is proportional to the quantity of molecules adsorbed. The advantage of this is that it does not require to control precisely the sample weights [120]. 

Although vitamins can be determined, both qualitatively and quantitatively, by MS, routine analysis is usually best conducted by other means (e.g., HPLC with ultraviolet (UV) or fluorescence detection, immunoassay methods, or microbiological methods). Analytically, MS does have an important role as a reference technique, especially when used in isotope... 

The accuracy or validity of FP results can first be estimated by comparison with a reference technique, i.e. a conventional profile such as QDA. Note that when conducting an FP with consumers from a market research perspective, this comparison criterion may not hold and following discussion on validity takes on a different meaning. 

Today, HPLC is used as a reference technique to analyze any type of vitamin [25]. This technique possesses simultaneous detection and determination of vitamins in one sample [17]. HPLC is often used for the simultaneous qualitative and quantitative analysis of water-soluble vitamin and fat-soluble vitamin in biological matrices such as plasma and urine [26-28]. 

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