Calibration curve detection

Treatment of a real, imperfect calibration data set revealed the full complexity and breadth of the calibration curve -detection limit problem, ranging from varying statistical weights to an uncertain model and data containing possible blunders to an artificially imposed response threshold. 

B, A) for the calibration curve. Detection under this circumstance is the principal topic of the next major section treating the detection of the pesticide Fenvalerate by gas chromatography (GC). Some subtle issues will be noted here, however. 

In the text which follows we shall examine in numerical detail the decision levels and detection limits for the Fenval-erate calibration data set ( set-B ) provided by D. Kurtz (17). In order to calculate said detection limits it was necessary to assign and fit models both to the variance as a function of concentration and the response (i.e., calibration curve) as a function of concentration. No simple model (2, 3 parameter) was found that was consistent with the empirical calibration curve and the replication error, so several alternative simple functions were used to illustrate the approach for calibration curve detection limits. A more appropriate treatment would require a new design including real blanks and Fenvalerate standards spanning the region from zero to a few times the detection limit. Detailed calculations are given in the Appendix and summarized in Table V. 

APPENDIX — CALIBRATION CURVE DETECTION LIMITS (Calculation of Table V Results) ... 

In general, quantitative analyses are carried out as usual, using internal or external standards and calibration curves. Detection limits can be achieved in the lower picogram range dependent on the selected derivatization procedure. 

S-shaped calibration curve detected. 2-coeF. equatk... 

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. 

A measurement procedure has been developed that allows to determine the mass of the inclusions as well as their locations with respect to radius, angle, and depth (2). For the depth determination use is made of the approximate 1/R dependence of the magnetic field strength from the distance R to the inclusion When in a first measurement at a small lift off an inclusion is detected, the measurement is repeated at an increased lift off From the signal ratio the depth can be calculated or seen from a diagram like fig. 5a which was generated experimentally. After that, from calibration curves like fig. 5b the absolute value of the signal leads to the mass of the inclusion. 

In a quantitative flow injection analysis a calibration curve is determined by injecting standard samples containing known concentrations of analyte. The format of the caK-bration curve, such as absorbance versus concentration, is determined by the method of detection. CaKbration curves for standard spectroscopic and electrochemical methods were discussed in Chapters 10 and 11 and are not considered further in this chapter. 

A sample is to be analyzed following the protocol shown in Figure 15.2, using a method with a detection limit of 0.05 ppm. The relationship between the analytical signal and the concentration of the analyte, as determined from a calibration curve is... 

The REE values obtained for the U.S.G.S. standards by this method compare well with the literature values obtained by NAA, MS and other methods. Einear calibration curves in the range of about 0 to 200 956 g/g are obtained and detection limits in the range of 0.06 to 0.04 956 /g are achieved. 

It is known that Selenium catalyzes reaction of some dye reduction by Sulphide. On this basis spectrophotometric and test-techniques for Selenium determination are developed. Inefficient reproducibility and low sensitivity are their deficiencies. In the present work, solid-phase reagent on silica gel modified first with quaternary ammonium salt and then by Indigocarmine was proposed for Selenium(IV) test-determination. Optimal conditions for the Selenium determination by method of fixed concentration were found. The detection limit of Se(IV) is 10 ftg/L = 2 ng/sample). Calibration curve is linear in the range 50-400 ftg/L of Se(IV). The proposed method is successfully applied to the Selenium determination in multivitamins and bioadditions. 

This, on the one hand, reduces the detection limit so that less sample has to be applied and, thus, the amounts of interfering substanees are reduced. On the other hand, the linearity of the calibration curves can also be increased and, hence, fewer standards need to be applied and scanned in routine quantitative investigations so that more tracks are made available for sample separations. However, the introduction of a large molecular group can lead to the equalization of the chromatographic properties. 

FIGURE 4.2 Polyethylene oxide, dextran, and protein calibration curves for TSK-GEL SW Columns. Column TSK-GEL SW, two 7.S mm x 60 cm columns in series. Sample , proteins Q, polyethylene oxides O, dextrans. Elution dextrans and polyethylene oxides distilled water proteins 0.3 A1 NaCI in 0.1 M phosphate buffer, ph 7. Flow rate 1.0 ml/min. Detection UV at 220 nm and Rl. 

It is appropriate at this juncture to illustrate the power of chemiluminescence in an analytical assay by comparing the limits of sensitivity of the fluorescence-based and the chemllumlnescence-based detection for analytes in a biological matrix. The quantitation of norepinephrine and dopamine in urine samples will serve as an illustrative example. Dopamine, norepinephrine, and 3,4-dihydroxybenzy-lamine (an internal standard) were derivatized with NDA/CN, and chemiluminescence was used to monitor the chromatography and determine a calibration curve (Figure 15). The limits of detection were determined to be less than 1 fmol injected. A typical chromatogram is shown in Figure 16. 

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