Analytical blank

Several terms have been used to define LOD and LOQ. Before we proceed to develop a uniform definition, it would be useful to define each of these terms. The most commonly used terms are limit of detection (LOD) and limit of quantification (LOQ). The 1975 International Union of Pure and Applied Chemistry (lUPAC) definition for LQD can be stated as, A number expressed in units of concentration (or amount) that describes the lowest concentration level (or amount) of the element that an analyst can determine to be statistically different from an analytical blank 1 This term, although appearing to be straightforward, is overly simplified. If leaves several questions unanswered, such as, what does the term statistically different mean, and what factors has the analyst considered in defining the blank Leaving these to the analyst s discretion may result in values varying between analysts to such an extent that the numbers would be meaningless for comparison purposes. 

Blanks may be differentiated into instrumental blank (background and baseline, respectively) and chemical blank (analyte blank). 

Use of immobilised chelating agents for sequestering trace metals from aqueous and saline media presents several significant advantages over chelation-solvent extraction approaches to this problem [193,194], With little sample manipulation, large preconcentration factors can generally be realised in relatively short times with low analytical blanks. 

One litre of unfiltered water was rolled with a carbon sachet and 10g NaCI for 7 days. The carbon was then dried, ashed, digested in aqua regia and analysed by ICP-MS, for Ag, Au, Pd, and Pt. Field and analytical blanks, as well as duplicates were used in all analyses to ensure accuracy (Gray etal. 2001). 

One of the major problems associated with trace level analysis of perfluori-nated acids is background contamination in the analytical blanks. Because of the contamination in blanks, the limits of detection (LOD) of perfluoro-chemicals in water samples are high, in the range of several tens to hundreds of ng/L to a few pg/L (11-15). 

PP Self-test 1.5 In which of the following cases has the limit of detection been reached Signal from sample Sample SD Signal from analytical blank Analytical blank SD... 

Double beam spectrophotometers allow differential measurements to be made between the sample and the analytical blank. They are preferable to single beam instruments for measurements in problematic solutions. For high performance instruments, the bandwidth can be as low as 0.01 nm. 

The detection limit is defined as the concentration of the element that will yield a signal whose intensity is equal to two times the standard deviation of a series of at least 10 measurements of the analytical blank or of a very dilute solution (confidence level 95%). In practice, concentrations should be at least 10 times higher than the detection limit to give reliable measurements (cf. 21.5.3). 

A stock solution of calcium ions was prepared by dissolving 0.1834g of CaCl2.2H20 in 100 ml of distilled water and then further diluting by a factor of 10. From this new solution, three standard solutions were prepared by further dilutions of five, 10 and 20 times, respectively. The unknown sample is itself diluted 25 times. Sufficient strontium chloride was then introduced to eliminate any interference due to phosphate ions. An analytical blank containing the same concentration of strontium was the first solution to be examined by the air/acetylene flame. The results were as follows ... 

Sometimes the analyte is in such low concentration that it is impossible to isolate. It can be noted from equations (17.1) and (17.3) that it is not necessary to know Ax and As individually if their ratio can be determined. To achieve this, a reproducible reaction can be conducted on the labelled standard (analytical blank) and, in an identical fashion, on the sample in order to obtain the same quantity of derivatised compound. Thus the sub-stoichiometric method is similar to the immunochemical method for trace analysis. 

Equations (21.19) and (21.20) are useful for estimating the smallest concentration of an analyte that can be detected, but not quantified, with a given confidence level. Consider one of the means, x2 for example, to be the result from a set of measurements made on the analytical blank. This mean will be noted as xb with a standard deviation of Sb- If i = 1 in equation (21.19), then sp =, vb and, using a value t taken from Table 21.2 for n = nx + nb — 2, we can write ... 

A calibration curve shows the response of a chemical analysis to known quantities (standard solutions) of analyte. When there is a linear response, the corrected analytical signal (= signal from sample — signal from blank) is proportional to the quantity of analyte. Blank solutions are prepared from the same reagents and solvents used to prepare standards and unknowns, but blanks have no intentionally added analyte. The blank tells us the response of the procedure to impurities or interfering species in the reagents. The blank value is subtracted from measured values of standards prior... 

The primary source of data interpretation error in elemental analysis is laboratory contamination affecting the method blank and the samples. Method blank is a volume of analyte-free water prepared and analyzed in the same manner as the samples. Method blank is also called analytical blank or preparation blank. 

Because modern instruments are capable of trace element detection in the part per trillion concentration range, laboratories must take extreme care in preventing contamination during sample preparation and instrumental analysis. To monitor the preparation and analysis processes, laboratories use method blanks that are prepared and analyzed together with the samples. The sources of analytical blank and sample contamination and the measures to eliminate them are as follows ... 

In the authors laboratory many acidic mixtures are used to digest organic matter in food. Containers of various materials are also adopted for the digestion of the samples so as to minimize the analytical blank. The magnitude of the latter, in fact, and the degree of uncertainty associated with it, usually limits the ability to perform reliable quantitative determinations at low concentration levels [6]. 

The improvement of the procedural blank signal using FEP and PFA tubes instead of glass and quartz tubes is shown in Fig. 1.3. The last shows a better decrease of the signal (blank absorbance) and therefore a lower concentration level will be reached. Fluoromaterials possess a series of unique properties such as wide operational temperature range (-200 to +260°C) and high chemical resistance, thus being particularly useful in trace and ultratrace work. Needless to say, a low analytical blank can substantially improve the limits of detection (LoDs) and the accuracy of the method [10]. 

R. K. Skogerboe, The analytical blank sources and effects on lead analyses, J. Assoc. Off. Anal. Chem., 65 (1982), 957-964. 

Teflon beakers or test tubes on a hot plate [25-27], Closed digestion systems produce pressures above atmospheric which provide higher temperatures and facilitate the complete oxidation of the samples digestion time and reagent consumption can be also reduced in this way. In addition, loss of volatile elements is minimized and the rate of digestion is increased, yielding low analytical blanks. 

Table 5.5 summarizes the advantages and disadvantages of the wet digestion techniques discussed in the section Wet decomposition with respect to loss of analyte, blank levels, contamination problems, sample size, digestion time, degree of digestion, and economic aspects. 

Trace element concentration determinations at the part per million and lower levels with IDMS require rigorous attention to contamination of the sample Sources of contamination that comprise the analytical blank have been documented by Murphy (66) These are principally derived from chemical reagents ... 

Finally, the size and of the blank can be used in Eq. (29.9) to correct for the influence of any analytical blank. This approach allows measurement of the of small samples with high precision and accuracy. We estimate that the overall error in our measurement is less than 0.15%o for samples containing more than 0.6 pmol N. 

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