Standardizations, and Blank Corrections

Signals are measured using equipment or instruments that must be properly calibrated if Sjneas is to be free of determinate errors. Calibration is accomplished against a standard, adjusting S eas until it agrees with the standard s known signal. Several common examples of calibration are discussed here. 

A 10-mL volumetric pipet was calibrated following the procedure just outlined, using a balance calibrated with brass weights having a density of 8.40 g/cm. At 25 °C the pipet was found to dispense 9.9736 g of water. What is the actual volume dispensed by the pipet  

If the buoyancy correction is ignored, the pipet s volume is reported as 

The American Chemical Society s Committee on Environmental Improvement defines standardization as the process of determining the relationship between the measured signal and the amount of analyte. A method is considered standardized when the value of k in equation 5.1 or 5.2 is known. 

A reagent of known purity that can be used to make a solution of known concentration. 

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Example Standardization and Blank Correction in Karl Fischer Titration... 

An alternative to MSA in ICP-MS analysis is the internal standard technique. One or more elements not present in the samples and verified not to cause an interelement spectral interference are added to the digested samples, standards, and blanks. Yttrium, scandium, and other rarely occurring elements or isotopes are used for this purpose. Their response serves as an internal standard for correcting the target analyte response in the calibration standards and for target analyte quantitation in the samples. This technique is very useful in overcoming matrix interferences, especially in high solids matrices. 

Method. Sample, standards and blank solutions are individually spiked with a known and constant concentration of an internal standard. The internal standard corrects for most sample effects e.g. viscosity, change in nebulisation efficiency, etc., and the results are reported as if the sample behaves exactly similar to the standards. This will be expanded in Chapter 7 using an automated system. 

The classic way to compensate for a physical interference is to use internal standardization (IS). With this method of correction, a small group of elements (usually at the ppb level) are spiked into the samples, calibration standards, and blank to correct for any variations in the response of the elanents caused by the matrix. As the intensities of the internal standards change, the elanent responses are updated every time a sample is analyzed. The following criteria are typically used for selecting an internal standard ... 

The matrix for the standards and the blank should match that of the samples thus, an appropriate matrix is 0.75 M HNO3. Any interferences from other components of the sample matrix are minimized by background correction. 

With a prescriptive approach to quality assessment, duplicate samples, blanks, standards, and spike recoveries are measured following a specific protocol. The result for each analysis is then compared with a single predetermined limit. If this limit is exceeded, an appropriate corrective action is taken. Prescriptive approaches to quality assurance are common for programs and laboratories subject to federal regulation. For example, the Food and Drug Administration (FDA) specifies quality assurance practices that must be followed by laboratories analyzing products regulated by the FDA. 

In a performance-based approach to quality assurance, a laboratory is free to use its experience to determine the best way to gather and monitor quality assessment data. The quality assessment methods remain the same (duplicate samples, blanks, standards, and spike recoveries) since they provide the necessary information about precision and bias. What the laboratory can control, however, is the frequency with which quality assessment samples are analyzed, and the conditions indicating when an analytical system is no longer in a state of statistical control. Furthermore, a performance-based approach to quality assessment allows a laboratory to determine if an analytical system is in danger of drifting out of statistical control. Corrective measures are then taken before further problems develop. 

Running a blank determination. This consists in carrying out a separate determination, the sample being omitted, under exactly the same experimental conditions as are employed in the actual analysis of the sample. The object is to find out the effect of the impurities introduced through the reagents and vessels, or to determine the excess of standard solution necessary to establish the end-point under the conditions met with in the titration of the unknown sample. A large blank correction is undesirable, because the exact value then becomes uncertain and the precision of the analysis is reduced. 

Procedure. To determine the purity of a sample of boric acid, weigh accurately about 0.8 g of the acid, transfer quantitatively to a 250 mL graduated flask and make up to the mark. Pipette 25 mL of the solution into a 250 mL conical flask, add an equal volume of distilled water, 2.5-3 g of mannitol or sorbitol, and titrate with standard 0.1 M sodium hydroxide solution using phenolphthalein as indicator. It is advisable to check whether any blank correction must be made dissolve a similar weight of mannitol (sorbitol) in 50 mL of distilled water, add phenolphthalein, and ascertain how much sodium hydroxide solution must be added to produce the characteristic end point colour. 

A comparison of the blank corrected values before and after conjugation should give an indication of the percent of peptide coupled. To be more quantitative, a standard curve must be run to focus in on the linear response range of the peptide-Ellman s reaction. Using cysteine as a representative sulfhydryl compound (similar in Ellman s response to a peptide having one free sulfhydryl), it is possible to obtain very accurate determinations of the amount which coupled to the activated carrier. Figure 19.20, discussed previously in this section, shows the results of this type of assay. 

Determinate errors may be constant or proportional. The former have a fixed value and the latter increase with the magnitude of the measurement. Thus their overall effects on the results will differ. These effects are summarized in Figure 2.1. The errors usually originate from one of three major sources operator error instrument error method error. They may be detected by blank determinations, the analysis of standard samples, and independent analyses by alternative and dissimilar methods. Proportional variation in error will be revealed by the analysis of samples of varying sizes. Proper training should ensure that operator errors are eliminated. However, it may not always be possible to eliminate instrument and method errors entirely and in these circumstances the error must be assessed and a correction applied. 

Method Performance. A blank sample, prepared using the same procedure as for the samples, was included with every five samples. PCB 28 and y-HCH were the only compounds detected in the blanks. Detection limits, calculated as mean blank +3 SD, were typically 2.3-13.3 pg/pF = 0.02-0.12 ng/g soil. Results were not blank corrected. Replicate analysis (the same soil sample extracted three times) was done for several samples. The relative standard deviation (RSD) for replicate analysis was always less than 20% (n = 3). Analytical recoveries were monitored with the aid of two recovery standards mirex for FI and 5-HCH for F2. The mean recovery for mirex was 100 ... 

The following blank-corrected readings were obtained for the determination of nickel in steel, using nickel standards dissolved in iron solution (10 g k ). The determination was performed by atomic absorption spectrometry using an air-acetylene flame and the 232 nm nickel line. 

Make a blank detn on the reagents and apply correction if necessary. Standardize the cerate by titrating 40.0 ml of soln with std Na oxalate obtained from the NBS (National Bureau of Standards)... 

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. 

B 7. A whole, peeled orange weighed 100 g. Three sections (30 g) were extracted with metaphosphoric acid and the total extract filtered and diluted to 100 mL. A sample of 10 mL of the filtered extract required 4.10 mL of DCIP after blank correction. Also, 1 mg of standard ascorbic acid required 7.2 mL of DCIP (blank corrected). How many milligrams of ascorbic acid are present in the whole, peeled orange ... 

The protein-to-protein variation in color response was measured at 1000 pg/ml for each protein in duplicate using the standard tube protocol. Within each assay, the average net or blank corrected absorbance was determined for each protein. The average net absorbance for each protein was divided by the average net absorbance obtained with BSA and expressed as a ratio. The standard deviation (SD) and the coefficient of variation (CV) is presented for the fourteen proteins assayed on the three methods. By comparing the CV s. the relative degree of protein-to-protein variation to be expected with the three methods can be assessed. 

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