Analytical procedures calibration curve

In most analytical procedures, calibration is carried out by means of a calibration curve using com-pound(s) prepared with chemicals of an appropriate purity and verified stoichiometry. Matrix effects must often be taken into account and, consequently, the calibration solutions should be matrix-matched. CRMs of pure compounds may be used for calibration. However, matrix CRMs should in principle not be used for the purpose of calibration unless no other suitable calibrants are available, with the exception of those methods (e.g., spark source mass spectrometry, wavelength-dispersive XRF, etc.) that require calibration with CRMs of a similar, fully characterized matrix (e.g., metal alloys, cements). For such methods, accuracy can only be achieved when certified RMs are used for the calibration. 

The method of least squares finds the best straight line through experimental data points. We will apply this procedure to analytical chemistry calibration curves in the next section. 

The choice between X-ray fluorescence and the two other methods will be guided by the concentration levels and by the duration of the analytical procedure X-ray fluorescence is usually less sensitive than atomic absorption, but, at least for petroleum products, it requires less preparation after obtaining the calibration curve. Table 2.4 shows the detectable limits and accuracies of the three methods given above for the most commonly analyzed metals in petroleum products. For atomic absorption and plasma, the figures are given for analysis in an organic medium without mineralization. 

The "method of standard additions" has been employed as a technique for standardization of atomic absorption analyses of metals In biological fluids (13,21) In this procedure, several concentrations of standard analyte are added to samples of the biological fluid to be analyzed The calibration curve which Is obtained after additions of the standard analyte to the biological fluid should parallel that obtained when aqueous standards are analyzed Extrapolation of the standard additions curve back to a negative Intercept on the abscissa furnishes an estimate of the concentration of the analyte In the original sample (21) This technique Is helpful In assessing the validity of methods of trace metal analysis (11,13,58) However, In the author s opinion, the "method of standard additions" Is neither practical nor reliable as a routine method for standardization... 

Using a newly developed, transversely heated graphite atomizer and D2-back-ground correction (for details see Sections 2.2 and 4.3), Cd, Pb and Cr were determined in cement and river sediment samples. Of the various calibration approaches applied the best results, also in comparison with wet chemical procedures, were achieved with calibration curves constructed by means of different BCR CRMs with different analyte concentrations and usually n = to individual intakes (Nowka and Muller 1997). 

This analytical method, based on TXRF, enables a large number of trace elements to be determined simultaneously. The range is suitable for different areas of the sea. The motivation to use TXRF resulted mainly from the characteristic features of the method its high detection power, its universal calibration curve, which eliminates the need for matrix-dependent standard samples or standard-addition procedures, the simple preparation of the sample films, and of course the possibility of multielement determination. 

Quantitative analysis demands that an analytical measurement can be accurately and reliably related to the composition of the sample in a strict proportionality (p. 2). The complexity of relationships, especially for instrumental techniques, means that the proportionalities need to be practically established in calibration procedures. For a typical simple calibration, a range of standards is prepared containing varying amounts of the analyte. These are then analysed by the standard method and a calibration curve of signal us amount of analyte is plotted. Results for unknowns are then interpolated from this graph (Figure 2.7). An important convention is... 

A procedure which enables the response of an instrument to be related to the mass, volume or concentration of an analyte in a sample by first measuring the response from a sample of known composition or from a known amount of the analyte, i.e. standard. Often, a series of standards is used to prepare a calibration curve in which instrument response is plotted as a function of mass, volume or concentration of the analyte over a given range. If the plot is linear, a calibration factor... 

Procedure A standard calibration curve is plotted by thoroughly mixing together about 10% (w/w) of the analyte with the KBr-KSCN mixture and then grinding the same intimately. Now, the ratio of the thiocyanate absorption at 2125 cm-1 to a selected band absorption of the analyte is plotted against the percent concentration of the sample. Likewise, an identical disc is prepared with the unknown sample and the same KBr-KSCN mixture. Finally, its absorbance ratio is determined and the concentration (of unknown sample) is read off directly from the standard calibration curve. 

Implications For B ethod Development. The effects of the calibration process on precision suggest the need for an additional step in the development of an analytical method. A suggested flow chart is shown in Figure 9. The analyst should first develop a method of adequate accuracy and precision without using calibration curves. The calibration step is then added, and the precision is rechecked. If precision has been excessively degraded, the analyst can choose among alternative calibration strategies, such as use of more standard measurements and use of the multiple-curve procedure. 

In Section 3, calibration was defined as a procedure by which an instrument or measuring device is tested in order to determine what its response is for an analyte in a test sample or samples for which the true response is either already known or needs to be established. If the true response is already known, one then makes an adjustment, if possible, so that the known response is, in fact, produced. If one cannot adjust to give the known response, the device is defective and is taken out of service and repaired. If the true response needs to be established, one establishes it via a single standard, or perhaps via a calibration curve or standard curve created using a series of standards, and then correlates the response of unknowns with that of the known quantity or quantities. 

Two synthetic internal standards are used (nor-DCA and nor-CA) to perform an external standardisation method. Calibration curves for each compound are built, starting from known amounts of standards ranging from 0.02 pg to 0.2 pg. BA standards are prepared and analysed as described in below in section 5.4.5.3 Analytical, Procedure . 

As a calibration procedure in ICP-MS via calibration curves, external calibration is usually applied whereby the blank solution is measured followed by a set of standard solutions with different analyte concentrations (at least three, and it is better to analyze more standard solutions in the same concentration range compared to the sample). After the mass spectrometric measurements of standard solutions, the calibration curve is created as a plot of ion intensities of analyte measured as a function of its concentration, and the linear regression line and the regression coefficient are calculated. As an example of an external calibration, the calibration curve of 239 Pu+ measured by ICP-SFMS with a shielded torch in the pgC1 range is illustrated in Figure 6.15. A regression... 

For most chemical analyses, the response of the procedure must be evaluated for known quantities of analyte (called standards) so that the response to an unknown quantity can be interpreted. For this purpose, we commonly prepare a calibration curve, such as the one for caffeine in Figure 0-7. Most often, we work in a region where the calibration curve is a straight line. 

The procedure we use assumes that the errors in the y values are substantially greater than the errors in the x values.7 This condition is usually true in a calibration curve in which the experimental response (y values) is less certain than the quantity of analyte (x values). A second assumption is that uncertainties (standard deviations) in all the y values are similar. 

A calibration curve shows the response of an analytical method to known quantities of analyte.8 Table 4-7 gives real data from a protein analysis that produces a colored product. A spectrophotometer measures the absorbance of light, which is proportional to the quantity of protein analyzed. Solutions containing known concentrations of analyte are called standard solutions. Solutions containing all the reagents and solvents used in the analysis, but no deliberately added analyte, are called blank solutions. Blanks measure the response of the analytical procedure to impurities or interfering species in the reagents. 

---