Calibration trace analysis

Throughout this book the use of a number of standard analytical samples is recommended in order that practical experience may be gained on substances of known composition. In addition, standard reference materials of environmental samples for trace analysis are used for calibration standards, and pure organic compounds are employed as standard materials for elemental analysis. 

Rossbach M, Ostapczuk P, Emons H (1998) Microhomogeneity of candidate reference materials Comparison of solid sampling Zeeman-AAS with INAA. Fresenius J Anal Chem 360 380-383. Rossbach M, Stoeppler M (1987) Use of CRMs as mutual calibration materials and control of synthetic multielement standards as used in INAA. J Radioanal Nud Chem Artides 113 217-223. Sargent M (1995) Development and application of a protocol for quality assurance of trace analysis. Anal Proc 32 71-76. 

Applications The application of the isotope dilution technique is especially useful in carrying out precise and accurate micro and trace analyses. The most accurate results in mass spectrometry are obtained if the isotope dilution technique is applied (RSDs better than 1 % in trace analysis). Therefore, application of IDMS is especially recommended for calibration of other analytical data, and for certification of standard reference materials. The technique also finds application in the field of isotope geology, and is used in the nuclear industry for quantitative isotope analysis. 

Especially in the case of biochemical and environmental systems and generally in ultra trace analysis, SAM is frequently applied. By addition of standard solutions to the sample a similar behaviour of the calibration set and the sample is created provided that the analyte is added in form of... 

Although standard addition calibration is an unreliable method if linearity in the range x < x0 is not experimentally verified but only supposed, there is scarcely an alternative in trace and ultra trace analysis when matrix effects are seriously suspected. 

The advantages of CC in ultra trace analysis are shown to be unmistakable. The quantitative reliability of the method was demonstrated by the extension of a calibration graph for phenol to two decades of concentration more when compared with conventional chromatography. A considerable improvement of the signal-to-noise ratio can be achieved in a relatively short time. The method offers excellent prospects for ultra trace analysis in cases where preconcentration of the solute fails. 

A convenient method is the spectrometric determination of Li in aqueous solution by atomic absorption spectrometry (AAS), using an acetylene flame—the most common technique for this analyte. The instrument has an emission lamp containing Li, and one of the spectral lines of the emission spectrum is chosen, according to the concentration of the sample, as shown in Table 2. The solution is fed by a nebuhzer into the flame and the absorption caused by the Li atoms in the sample is recorded and converted to a concentration aided by a calibration standard. Possible interference can be expected from alkali metal atoms, for example, airborne trace impurities, that ionize in the flame. These effects are canceled by adding 2000 mg of K per hter of sample matrix. The method covers a wide range of concentrations, from trace analysis at about 20 xg L to brines at about 32 g L as summarized in Table 2. Organic samples have to be mineralized and the inorganic residue dissolved in water. The AAS method for determination of Li in biomedical applications has been reviewed . 

The method of standard additions can be used with many analytical techniques where interference from the matrix has to be eliminated and is of general use in residue or trace analysis. Essentially the technique involves addition of increasing volumes of a standard solution to a fixed volume of the sample to form a calibration series. An advantage of the technique is that since several aliquots of sample are analysed in order to produce the calibration series the method gives a measure of the... 

Re archers are active in the field of correlation gaschromatography and correlation HPLC " the first application in trace analysis was introduced in 1970 A typical example of the noise reduction property is the determination of a calibration graph of phenol for the higher concentrations with conventional chromatography, and extended to very low concentrations by CC (Fig. 10). The detection limit achieved is about 3 ppt (Laeven et al. ). A correlogram of 10 ng/1 phenol mple is shown in... 

Further details of different strategies in solution calibration are described in the literature.1 29 76 79 Precise and accurate measurements of isotope ratios, which is one of the major advantages of mass spectrometric techniques, are a requirement for the application of isotope dilution techniques in trace analysis, which is also the main goal of the application of isotope dilution in solution based calibration in LA-ICP-MS. 

For measurements of isotope ratios or isotope abundances, any of the mass spectrometers discussed in the previous chapters, such as SSMS, LIMS, GDMS56 and LA-ICP-MS,6 are of benefit for the direct isotope analysis of solid samples. SSMS and LIMS are rarely applied in isotope analysis due to their relatively low precision. Several applications of the isotope dilution technique as a calibration strategy in SSMS, mostly on geological samples, are known.57-59 GDMS has been mostly applied in multi-element trace analysis and depth profiling and plays only a minor role... 

Attempts to optimize polymer-based electrodes for trace analysis have started very recently. Ceresa et al. have reported an ion-selective electrode optimized for the determination of Pb2+ in drinking water. The detection limit was 0.7 ppb (3 x 10 9M) which is somewhat poorer than the best LOD reported so far for Pb2+-selective electrodes [10] but the former was optimized for ruggedness and response time rather than LOD. Nevertheless, the obtained LOD was still adequate for the targeted application since it was about 20-fold lower than the 15 ppb action limit for Pb2+ in drinking water imposed by the USA EPA [79]. The authors used ICPMS as a reference method and obtained excellent correlation for samples of concentration 3nM. It was shown that the calibration procedure required ca. 10 min for stable readings in micromolar to nanomolar concentration levels. Moreover, the authors... 

In contrast with usual calibration (case B), modeling in trace analysis, and also of environmental relationships (case A), will probably fail to fulfil condition (2) because x is usually also subject to errors. Alternative linear models must then be considered. 

In principle, all performance measures of an analytical procedure mentioned in the title of this section can be derived from a certain critical signal value, ycrit. These performance measures are of special interest in trace analysis. The approaches to estimation of these measures may be subdivided into methods of blank statistics , which use only blank measurement statistics, and methods of calibration statistics , which in addition take into account calibration confidence band statistics. 

Remember the basic rules organic methods typically require a five-point calibration trace element AA methods use a calibration blank and three standards the ICP-AES instruments are calibrated with one standard and a calibration blank inorganic analysis methods use three- to five-point calibration curves. 

In analytical chemistry, several calibration methods have been developed and introduced into laboratory practice [1], Most of them can be recommended for use in trace analysis. The methods differ from each other in terms of (a) preparation of the calibration solutions, (b) interpretation of the measurement results and construction of calibration graphs, and (c) calculation of the final analytical results. We present the calibration methods exploited most often in chemical analysis and point out their advantages and drawbacks, which are especially significant from the point of view of the specific conditions characterizing trace analysis. 

The evident advantages of the set of standards method is its simplicity and great efficiency, as the calibration graph, once constructed, can serve for analyte determination in many samples. This is the main reason for its popularity among analysts. However, as described above, the method has to be applied with special caution, especially in trace analysis. Therefore, it must be carried out in strict accordance with developed and well-verified analytical procedures. Otherwise, it can be used only after diagnostic tests confirm the absence of interferents in the sample analyzed or indicate how the interference effect can be eliminated. 

The most important advantage of this calibration approach is that each calibration solution contains the analyte in the environment of all sample components, including potential interferents. Thus, if the calibration dependence is distorted as a result of interferents, there is still a chance to reconstruct it accurately by means of the calibration graph. From this point of view, the standard addition method is one way to eliminate (or rather compensate for) the interference effect. Furthermore, it gives a chance to compensate for the effects of all interferents, independently of their kind, number, and concentration in the sample. This feature supports application of the standard addition method to trace analysis. 

Unfortunately, the method suffers several significant drawbacks, which should also be taken into account in trace analysis [2]. First, in principle, the method leads to greater random analytical errors than the set of standards method when both calibration approaches are performed under the same experimental conditions. Second, because the method is based on extrapolation, in some circumstances it can be also a source of serious systematic errors. 

Because of the problems described above, the standard addition method can only be recommended in trace analysis with serious reservation. However, the possibility of compensating for the interference effect, even when unexpected or caused by unknown sample components, is so great an advantage that this calibration approach deserves greater interest in analytical practice than it is given at present. 

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