Single-point external standardization

For a single-point external standard (omitting the internal standard) the relationship between peak area, A2, and the concentration, C2, ofp-xylene is... 

A multiple-point standardization presents a more difficult problem. Consider the data in Table 5.1 for a multiple-point external standardization. What is the best estimate of the relationship between Smeas and Cs It is tempting to treat this data as five separate single-point standardizations, determining k for each standard and reporting the mean value. Despite its simplicity, this is not an appropriate way to treat a multiple-point standardization. 

Case 3. Linearity demonstrated from 50% of the ICH reporting limit to 150% of the shelf life specification of a related substance, and a significant y-intercept is observed (Figure 3.8). Due to the significant -intercept, a single-point calibration (e.g., high-low or one-point external standard calibration) is not suitable. In this case, multiple-point external standard calibration is the most appropriate. See Section 3.3.3 for more discussion of the significant y-intercept. 

External Standard. In this approach, related substance levels are determined by calculation using a standard curve. The concentration of related substance is determined by the response (i.e., peak area of individual related substance) and the calibration curve. A reference standard of the drug substance is typically used in the calibration. Therefore, a response factor correction may be required if the response of related substance is very different from that of the drug substance. A single-point standard curve (Figure 3.4) is appropriate when there is no significant v-intercept. Otherwise, a multipoint calibration curve (Figure 3.5) has to be used. Different types of calibration are discussed in Section 3.2.3. 

A single point calibration may be used instead of a working calibration curve for quantitation by either external or internal standard method, if the response from the single point standards produces a response that deviates from the sample extract response by no more than 20%. The solvent for preparing calibration standards should preferably be the same one used to make the final sample extract. Hexane, isooctane, or methyl-feri-butylether is an appropriate solvent for the analysis of chlorinated pesticides by GC-ECD. 

The standard addition method must be adopted when the sample matrix is very complex and the instrumental fluctuations are difficult to control. In this method, the unknown sample is first analyzed. A known concentration of the standard solution of the analyte is added to this unknown sample, and the mass spectrometry response is measured again to provide the response factor (i.e. the response per unit concentration). The concentration of the unknown is calculated by multiplying the signal intensity of the unknown with the response factor. This single-point calibration is, however, less precise. To enhance the precision of this method, several increments of the standard solution of the analyte are added to a fixed amount of the unknown sample. After each addition, the mass spectrometry response is also measured, and the calibration curve is obtained as in the external standard method (Figure 14.2). The jc-axis intercept of the calibration curve provides the concentration of the analyte in the unknown sample. 

When quantification is required (Tier II), an external standards method (ESTD) is used. The quantitation technique typically requires a comparison of five peaks (minimum of three) between the chromatograms of an unknown sample and that of standard Aroclor obtained under identical conditions. Quantitation of either Aroclors 1016 or 1260 is performed using a five-point calibration of a mixed Aroclor standard containing Aroclors 1016 and 1260. All remaining Aroclors are quantitated from single point calibrations. Calibration is verified daily by comparison of results obtained for analysis of... 

It has become fairly common to adopt the manufacture of combinations of internal reference electrode and its inner electrolyte such that the (inner) potential at the glass electrode lead matches the (outer) potential at the external reference electrode if the glass electrode has been placed in an aqueous solution of pH 7. In fact, each pH glass electrode (single or combined) has its own iso-pH value or isotherm intersection point ideally it equals 0 mV at pH 7 0.5 according to a DIN standard, as is shown in Fig. 2.11 the asymmetry potential can be easily eliminated by calibration with a pH 7.00 0.02 (at 25° C) buffer solution. 

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