Blank contribution

Separate sample blanking requires an additional analytical channel, and is therefore wasteflil of both reagents and hardware. An alternative approach that is used on several automated systems, eg, Du Pont ACA, BM-Hitachi 704, Technicon RA-1000, is that of bichromatic analysis (5) where absorbance measurements are taken at two, rather than one, wavelength. When the spectral curves for the interference material and the chromogen of the species measured differ sufficiently, this can be an effective technique for reducing blank contributions to assay error. Bichromatic analysis is effective for blanks of both the first and second type. 

A plot of log(Mp/p) vs. p yields a from either the slope or the intercept (see Figure 2) the "goodness of fit" is indicated by the relative agreement of a obtained from the slope or the intercept. For CpCo(C0>2 5 the data yield a = 0.81 from the slope and 0.84 frcm the intercept considering the fact that a blank contribution from the n-octane solvent had to be subtracted out of C5- Cio, the agreement is good. Typical values of a are 0.80-0.87 ( ). 

If (a) conjugate formation in the control incubation mixture is unimportant or can be suppressed (Section 3.2), (b) blank contributions are adequately determined, and (c) diazo-coupling of synthetic conjugates is reasonably complete, the difference in extinction of test and control (usually measured at 530-550 nm) allows the amount of pigment synthesized, and thus the value of BCR, to be obtained. 

All the results have been corrected for blank contribution. The XAD-2 cartridge blanks for the evaluation stage and the field cartridge blanks were essentially free of all of the compounds analyzed. However, the field samples, as well as the field blanks, had significant amounts... 

A method blank is a sample containing all components except analyte, and it is taken through all steps of the analytical procedure. We subtract the response of the method blank from the response of a real sample prior to calculating the quantity of analyte in the sample. A reagent blank is similar to a method blank, but it has not been subjected to all sample preparation procedures. The method blank is a more complete estimate of the blank contribution to the analytical response. 

Any real sample can be treated as a mixture of sample material and contaminants of various origins. If the blank contribution is constant in composition and magnitude across analyses, then a simple mass balance can be constructed for the mixture analyzed ... 

Sensitivity. When operated in an ion monitoring mode as a GC detector, a mass spectrometer with standard electron multiplier detection and signal amplification is theoretically capable of producing a response to samples of less than 10 " mol (1 femtomole) [68]. In practice such limits have not been reached due to the combined effects of variable stationary phase and instrument background, sample degradation in gas chromatography or, in isotope dilution, residual blank contributions from... 

Table 1 Comparison of the counting rates R2 in W2 f Y) for six aliquots of the same sample. Blank contribution in W2 estimated at 0.43 CPM. Counting uncertainty on the Net R2 () at k=2 Standard deviation of the set (s) in CPM.

In many cases it is not the background signal from the source or the measurement system but blank contributions that limit the power of detection, the limiting standard deviation is often the standard deviation of the blank measurements and this value must be included in Eq. (139) [44]. From the calibration function the detection limit then is obtained as ... 

When signals include a blank contribution, this will also have fluctuations, which limit the power of detection in practice, the influence of which can easily be shown. If Ix is the lowest measurable signal without blank contribution, Ix the lowest analytical signal including a considerable blank value, Iv the background signal and IBi the blank signal, ffi, (Ix + lu), (Ibi + Iu) and (Ix + IBi + Iu) can be measured and ... 

To summarise, microwave sample preparation reduces blank contribution from environmental exposure, reagent use and losses from evaporation, and in addition, reduces sample preparation times. It also offers a more reproducible method for duplicating metals determination, both within a laboratory and between laboratories (Lamble Hill 1998). But, in the case of contaminated soil sample digestions, due to surface effects of PTFE, a microwave-cleaning step is highly recommended between each bath of samples in order to avoid any memory effects (carry over contamination) from previous digested samples. 

Ideally an analyte-lree matrix material should be used to establish blank levels. In the absence of such a material (e.g. it is not possible to obtain cholesterol free serum), a full reagent blank, subjected to all sample preparation steps, should be used to evaluate possible blank contributions. 

The generation of the analytical signal through a nuclear reaction makes the determinations virtually independent of the chemical form of the elements and their chemical environment. The determinations are, to a certain degree, independent of the sample matrix, and in most cases, can be made free of any blank. The blank-free attribute is aided by the fact that the methods require much less sample manipulation than other techniques and they are not as susceptible to reagent and laboratory contamination. The reduction or elimination of blank contributions includes radiochemical processing performed after irradiation (see Sect. 30.5), where inadvertent chemical contamination would add only nonactivated analyte that would not contribute to the measured signal. 

After irradiation, samples are commonly separated from the rabbit and, if feasible, also from their primary packaging containers to minimize blank contributions from the container materials or external contamination encountered in the irradiation facility. For the measurement of ultrashort-lived nuclides (Ty2 30 s) facilities have been designed that automatically separate the sample from the rabbit by mechanical means. Usually these systems are implemented in a fully automated irradiation and counting process (Bratter et al. 1977 Ismail et al. 2001). 

Measure and record the absorbance of the sample and standard at this Aex. subtracting any solvent blank contribution, using the 5 > 0.25. The absorption spectrophotometer bandpass in these measurements should be nearly the same as the bandpass of ExM, that is typically 4 nm. 

In the absence of a detectable blank, the relationship of the logarithm of peak area to the logarithm of the mass of injected DMS should be linear over at least two orders of magnitude. However, blanks may occur as a consequence of decreasing system performance after prolonged use (e.g., decreased sulphur scrubber efficiency, decreased surface silylation) or because of DMS traces in the seawater samples used for calibration. In this case, significant deviations from linearity are observed at low analyte levels. Therefore, the blank contribution, Wb, to the total amount of injected DMS must be taken into account ... 

Figure 7.8 Uncertainty in Os concentration, determined via ICP-IDMS. The contribution of the spike calibration dominates the uncertainties at a higher concentration level. Blank contributions dominate the uncertainties at concentration levels approaching the determination limit. The standard deviations of the concentrations of replicate digestions were in most cases higher than the standard uncertainty estimated for the analysis of a single sample. It can therefore be assumed that in cases where the standard deviations of replicate measurements are very large, the nugget effect is dominant.

The analyte s concentration ranges are important to identify, in consideration of the sensitivity of the instrument, to establish whether or not a dilution or enrichment step is necessary. In good analytical practice, the calibration range should cover the analyte concentration present in each sample. Additionally further decisions in terms of equipment have to be made. For working in the mg/kg range, conventional glassware for nebuliser and spray chamber can be used, but to go down to the pg/kg or ng/kg level requires the sample introduction system to be made from quartz or PFA (perfluoroalkoxy polymer) to minimise blank contribution, memory effects and cross contamination. 

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