Calibration problems proposed solutions

One final advantage not yet mentioned is that coulometry is an absolute technique needing no calibration with standard solutions—the electron is the standard. This minimizes error and eliminates the preparation and storage—and problems— of standard solutions in fact, the electron has been proposed as a permanent, non-destructible standard for the analysis of materials. 

This GLS estimator is akin to inverse variance-weighted regression discussed in Section 8.2.3. Again there is a limitation V can be inverted only when the number of calibration samples is larger than the number of predictor variables, i.e. spectral wavelengths. Thus, one either has to work with a limited set of selected wavelengths or one must apply other solutions which have been proposed for tackling this problem [5]. 

Despite all these developments, some problems remain. One of them concerns the comparison of results obtained from different instruments. Solutions involving standards are proposed. But most of them suffer from drawbacks thermal or photochemical instability, multistep preparation, wavelength inadequacy between standard and actual fluorophore emission. Many solutions have been advocated to calibrate luminometers [8],... 

Evaluation of the performance of a method or an instrument represents the largest use of CRMs. Examples are widely reported in the literature. One could even say that the development of a new method or instrument without evaluation of the performances with (a) CRM(s) is an incomplete task. Besides these research tasks, CRMs for calibration or validation are also used to assess the performance of instruments by the manufacturer himself to demonstrate the possibilities of his instrument or by the customer who wishes to evaluate the proposed instrument before purchasing it. CRMs produced by independent official or regulatory bodies to validate instrument performance or calibration sets have been under development for several years. They have in particular allowed the solution of inaccuracy problems in the biomedical sector where calibration test kits of automatic instrument manufacturers were not comparable and even led to different results between countries such arguments supported many BCR projects for... 

Sensitivity of the instrument, which refers to the system response to small changes in concentration, is determined by the signal to noise ratio (SNR) of the measurement, as well as, the concentration slope. The SNR of each measurement may be considerably smaller than variation between measurements made on different occasions. Measurements of concentration from absolute fluorescent intensity (O) are possible provided that the calibration of the instrument remains unchanged. Reliable fluorescent intensity measurements across different instruments or from repeated measurements with the same instrument are possible only if the above factors can either be eradicated from consideration or be controlled with an experiment independent calibration coefficient. One effective solution proposed the development of a set of standard reference materials (SRMs) for use with identical instruments [9]. Thus, for a particular instrument, the concentration of a test solution can be expressed as a ratio of the fluorescent counts for the test and SRM. Subsequently, it was recommended that the concentration of the test solution could be expressed in molecular equivalent soluble fluorophore (MESF) units. This protocol was developed with respect to the use of flow c) tometers, which can be operated close to the ideal experimental conditions. Despite this, measurements of fluorescent intensity in terms of MESF units stiU have problems, as most fluorescence-based instruments cannot be guaranteed to operate under the same conditions from day to day. 

Although the analyst may be unaware of its existence in the sample, signal suppression serves to decrease system sensitivity and raises the working detection limit. Spectral enhancement, on the other hand, increases the system sensitivity and lowers the working detection limit. "Enhancement is a well known problem in atomic absorption spectroscopy ( 12). A variety of approaches, such as Zeeman effect correction have been proposed for its elimination (11). To avoid artificially high results, calibration standards must contain concentrations of the enhancing species equivalent to those in the sample. Ordinary standard solutions are not representative of the analytical situation."... 

Despite the problems discussed above, LIPS has been successfully applied in a number of process analysis applications. In some cases, the LIPS system has been used off-line as in the application described by Ottesen in which air cooled metallic substrates were used to collect fly ash deposits from a pulverised coal combustion for subsequent analysis off-line [70]. Calibration standards were prepared by spraying aqueous solutions onto heated substrates using an air brush and the method was found to work well provided that the deposits were sufficiently thin to permit complete ablation. Other workers have proposed on-line LIPS systems for process control. An example is the apparatus proposed by Sabsabi [71] for in situ analysis of pre-selected components of homogeneous solid compositions. In particular, the author proposed that the system could be used for measurement of the concentration of active ingredients (e.g. drugs) in pharmaceutical products such as tablets, by monitoring an element present in the active component (e.g. 

Polymers with differing chemical structures will usually have a different coil volume in solution and produce a displaced calibration curve. Various procedures have been proposed to overcome this problem and to produce a Universal calibration curve. These include the g factor method [16], the use of hydrodynamic volume [17], and unperturbed dimensions [18]. The use of broad-molecular-mass distribution polymers as calibration standards has also been employed. These procedures are discussed more fully in Chapter 3. They are only important if accurate relative molecular masses are required if the technique is being used simply for qualitative purposes, these procedures are not important. 

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