Blank value correction

Figure 1 Influence of the specific mechanical energy on the water-solubility (corrected from the blank values) of some raw materials...

For example, The concentration is derived from a 6 point calibration curve by reading off the concentration, corresponding to the sample absorbance, corrected for the blank value, and multiplying it by the concentration factor . 

There are a few requirements for the application of the standard addition method. The analytical results have to be corrected for blank. Otherwise we would add the blank value to our sample content. Since we are using linear regression we need a linear relationship between signal and concentration. As stated above the homogeneity of variances is also a prerequisite for linear regression. We want to divide our sample into several sub-samples and spike them with known amounts of analyte. This means that we need to divide the sample homogeneously and to precisely add the analyte. 

The first problem is deciding on which of these two common models to use. It has been argued that for spectrophotometric methods where the Beer-Lambert Law is known to hold, Y = bX + e, the force through zero model is the correct model to choose if the absorbance values are corrected for the blank." The correct way to carry out the calibration regression is to include the blank response at assumed zero concentration and use the model Y = bX + a + instead. This may be a nicety from a practical standpoint for many assays but there are instances where a force through zero model could produce erroneous results. Note that the e denotes the random error term. Table 15 contains a set of absorbance concentration data from a UV assay. 

Absorbance of the blank can arise from the color of starting reagents, reactions of impurities, and reactions of interfering species. Blank values can vary from one set of reagents to another, but corrected absorbance should not. 

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... 

Samples which were counted in polyethylene and quartz vials required corrections for the impurity content of the vials. Standard libraries of vial and blank filter paper corrections were added to SPECTRA (see Tables II, III, and IV). We used indicators in the input data to each computer calculation to call out the proper correction library. The code used corrections for polyethylene vials, Suprasil vials, Whatman-41 filters (25.8 cm2), and combinations. The computer also did not print the value for an element in a sample if the microgram quantity was within two times the microgram value of the vial or filter paper. The value output was listed as less than the vial or filter paper value, corrected to proper units. With this restriction, some data were lost, but very small values which were the difference between two larger numbers were eliminated. For example, if a volatile sample plus vial gave a chlorine value of 9.4 fig, the chlorine value output by the computer for the sample would be less than 9.0 fig (referring to chlorine in Table V) rather than the difference of 0.4 fig. If the sample plus vial gave a chlorine value of 20 fig, the value output by the computer would be 11 fig. 

Subtract absorbance of blank, and correct absorbance to original concentration and a 1-cm cuvette path length. Subtract 4 AU to report final value. 

If we take the blank value as our standard, then the 10 fil of H -cyclic AMP added 1781 CPM above background. In Fraction 50, the 10 /a1 of H -cyclic AMP yields 1610 - 9 = 1601 CPM. Thus, only 1601/1781 =89.9% of the added counts were recovered. The corrected count rate of fraction 50 is ... 

Colorimetric tests corrected for blank value (two-slide method) c - Ao+ Afgi(Dji) + + AjC... 

The blank of the acid employed in the digestion step is evaluated by repeating the acid treatment on a 3 + 10 M KCI solution prepared with ultrapure water and KCI. The voltammetric measurement is carried out before and after the acid treatment and the concentration increment observed gives the blank value to be used for correction. Using the Merck Suprapur HCl the Cd and Pb blanks are found to be 17 and 19 pM, respectively (35, 57), while the blank for Cu is under the detection limit (69). Using the NIST ultrapure acid all the blanks are below the detection limit. 

Semm cholesterol exists as a mixture of fatty acid esters and free cholesterol. Quantitation of total cholesterol involves the initial conversion of the esters to free cholesterol, followed by the total conversion of free cholesterol to its oxidation product. This reaction is coupled to the familiar dye-peroxidase indicator reaction. The parameter A50o measurements using stock cholesterol solutions provide a calibration curve. A reagent blank solution is prepared using all components except cholesterol, and this value is subtracted from all measured A50o values, correcting for any background oxidation of the dye. 

When interpreting the BOD curves (Figures 3 and 4), it is essential to know the respiration due to the biomass itself, called the endogenous respiration. In biodegradation tests, it is important to determine endogenous respiration because the measured respiration data from the test assays with the test substance have to be corrected by these blank values. The concept is then to subtract this... 

Values have been corrected for seawater blank values but not for the concentration of the various species that are contributed by the same settling velocity fraction of the natural particles in the seawater used for the experiment. In the case of chlorinated hydrocarbons these natural values should be negligible hence all of the reported concentrations can be taken as effluent-derived. The rapid settling fraction is isolated according to the techniques of the PLOP experiment, which is described in the text, and chemical analysis for the various trace constituents is performed as described in Ref. 10. 

There are some fundamental features that should be part of every good analytical method. The method should require that a blank be prepared and analyzed. A blank is used to ascertain and correct for certain interferences in the analysis. In many cases, more than one type of blank is needed. One type of blank solution may be just the pure solvent used for the sample solutions. This will ensure that no analyte is present in the solvent and allows the analyst to set the baseline or the zero point in many analyses. A reagent blank may be needed this blank contains all of the reagents used to prepare the sample but does not contain the sample itself. Again, this assures the analyst that none of the reagents themselves contribute analyte to the final reported value of analyte in the sample. Sometimes a matrix blank is needed this is a blank that is similar in chemical composition to the sample but without the analyte. It may be necessary to use such a blank to correct for an overlapping spectral line from the matrix in atomic emission spectrometry, for example. 

The EPA Contract Laboratory Program under Superfund (CERCLA) provides another example of how blanks and detection limits are treated.(17)(18) With respect to blanks, the statements of work specify that the laboratory should not blank correct sample responses. In the case of organics analyses, the EPA evaluator and/or data auditor has the authority to blank correct sample responses. In practice this Is never done. For both organics and Inorganics, the absolute blank level Is primarily used as a control to determine If samples need to be reanalyzed. Detection limits are based on replicate analyses of a standard at 3-5 times the required detection limit concentration. The Instrument detection limit Is calculated as being equal to 3 times the standard deviation of the measured value. Since blank correction is not permitted or not done, sample results will all be biased high by an amount equal to the blank response. The absolute blank value (actually usually a multiple of 5 or 10 times the blank value) rather than... 

Correct the sample results for the blank by subtracting the average blank value (in pg) from the analyte weight determined on each filter, correct for DE (if necessary), and calculate airborne concentration in pg/m as described in Section 3.1.8. 

For this reason blank values should be analyzed both before and after a series of analyses. They can be experimentally determined from separately conducted multiple analyses. The mean blank value is subtracted from the information value yi and this corrected information value gives the concentration. 

Blank subtracted, internal standard corrected then is calculated as the blank value subtracted from the sample value 1.0096 x 10 - 3.64356 x 10 = 1.00600 X 10 F The same calculation is performed in Excel for each isotope. Finally, the... 

Blank values. Three sources contribute to the blanks and should be taken into account The vessels, the acids used for digestion, and the filter material. The blanks of the vessels and acids can be obtained by performing the digestion procedure without the filter the blank for the filter material can be derived from digestions of several pre-cleaned filters of each filter stock. Our experience indicates that the blanks for the elements Cr, Ni and Zn, in some cases also for Fe, are mainly caused by the filter material (see also Chapter 2). Obviously, the lower the content of SPM on the filter, the stronger the influence of the blank values for the corrections. 

Equations (8.1) and (8.2) are not corrected for instrumental mass bias (mass discrimination) and blank values. In the literature, one can therefore find much... 

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