Multiple point calibration

BCR Analytical Approach for the Certification of PAHs in Natural Matrix CRMs Prior to the certification analyses for the CRM, each participating laboratory has to prepare standard solutions of the analytes to be determined from certified reference compounds (purity >99.0 %) to calibrate their instruments for response and response linearity (multiple point calibration), detection limit, and reproducibility. In the case of PAH measurements, reference compounds of certified purity are used as internal standards, which are not present at a detectable concentration in the matrix to be analyzed (e.g. indeno[i,2,3-cd]fluoranthene (CRM 267), 5-methylchrysene (CRM 081R), benzo[f ]chry-sene (CRM 046), picene (CRM 168), and/or phenanthrene-dio). 

The aids to chromatography include a) resolution calculations on chromatograms of standard mixtures to monitor column performance, b) calculation of Kovats retention index for help in identifying peaks, and (c) multiple point calibration curves for improved quantitation. The file searching routines access two sets of data. Information (such as molecular formula, molecular weight) is stored on 3100 compounds from the Arctander data( ). This allows a quick computer search through the data which is difficult... 

Prerequisites for the Calibration Types. It depends on the design of the analytical procedure as to which regression parameters are meaningful and which results are acceptable. In other words, the model to be used for quantitation must be justified. For a singlepoint calibration (external standardization), a linear function, zero intercept, and the homogeneity of variances are required. The prerequisites for a linear multiple-point calibration are a linear function and in case of an unweighted calibration also the homogeneity of variances. A non-linear calibration requires only a continuous function. With respect to the 100%... 

The external standard method is the simplest one and should therefore only be used for simple analytical problems. The injection must be performed with good reproducibility thus it is recommended that the complete loop filling method be used (see Section 4.6). With multiple-point calibration it is not recommended to inject different volumes from a reference stock solution (e.g. 10, 20, 30, 40 and 50 (xl) because it is well possible that neither accuracy nor precision of these injections are high enough. It is better to prepare a number of calibration solutions with different concentrations and to inject equal volumes of them (with complete loop filling or with exactly the same procedure as is used also for the sample). 

Measurement of the available microwave power is assessed by measuring the temperature rise in 1 kg of water exposed to microwave radiation for a fixed period of time. By producing a multiple-point calibration, the analyst... 

Charrier et al. accurately measured the equilibrium temperature of the solid-solid phase transition of high purity caesium nitrate by stepwise heating and by the method of extrapolation to zero heating rate. The authors reported a mean value of 154.3 0.1 C, that was obtained by using two different heat-flux DSC instruments. Because the transition temperature of caesium nitrate is close to that of the fusion of indium, indium can be used as a single-point calibrant. This approach decreases the error that may arise when the calibration value for the material of interest is determined by interpolation between two-point or multiple-point calibrations [66],... 

A fifth spectrophotometric method for the quantitative determination of the concentration of Pb + in blood uses a multiple-point standard addition based on equation 5.6. The original blood sample has a volume of 1.00 mb, and the standard used for spiking the sample has a concentration of 1560 ppb Pb +. All samples were diluted to 5.00 mb before measuring the signal. A calibration curve of Sjpike versus Vj is described by... 

How do we find the best estimate for the relationship between the measured signal and the concentration of analyte in a multiple-point standardization Figure 5.8 shows the data in Table 5.1 plotted as a normal calibration curve. Although the data appear to fall along a straight line, the actual calibration curve is not intuitively obvious. The process of mathematically determining the best equation for the calibration curve is called regression. 

Standardizations using a single standard are common, but also are subject to greater uncertainty. Whenever possible, a multiple-point standardization is preferred. The results of a multiple-point standardization are graphed as a calibration curve. A linear regression analysis can provide an equation for the 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. 

Two-point calibration 0 and 10 FTU One point within every turbidity range One point within every turbidity range Multiple points within every turbidity range... 

The chart of this instrument is 10 in. wide and has rectangular coordinates. The scale is uniform if graduated in millivolts and departs from uniformity according to the temperature electromotive force relation of the couple, if calibrated in degrees of temperature. In multiple-point recorders a print wheel carrying four or more numbers is substituted for the pen. This wheel is turned in synchronism with the commutator so that the proper number is printed. A small inked roller turns in contact with the print wheel. The latter is depressed once a minute, a length of time sufficient to allow the balancing action to be completed after each couple is connected. 

Demonstration of linear response enables the use of single point calibration techniques, which can offer considerable cost savings regarding the number of calibration materials that must be maintained and the time it takes to calibrate the on-line analyzer. A 2000 ppmw standard was made with 7.88 g of pure dibutyl sulfide diluted in 1 L of toluene. This standard was further diluted sequentially 1 1 nine times until the expected sulfur concentration in the sample was down to 3.91 mg/kg. Each of these 10 samples were injected multiple times in a 6000 series analyzer that was set for a 2500 ppmw full scale range and calibrated with the 2000 ppmw sample (see Figs. 8 and 9). Data from the results of the 16.13 ppmw sample were lost due to a data acquisition problem. The data, shown in Table 3, indicate a % RSD of better than 1 % in the sulfur range 62-2000 mg/kg. At lower sulfur levels of 4-31 mg/kg, the %RSD is 11-2 %. 

A traceable standard carbon black supplied by Micromeritics Corporation (Sydney, Australia) with a recommended multiple-point BET-SSA of 113 5 m /g was used to calibrate and commission a newly designed, all stainless steel, high-pressure manometric adsorption apparatus. 

It is important to note that simulated distillation does not always separate hydrocarbons in the order of their boiling point. For example, high-boihng multiple-ring-type compounds may be eluted earher than normal paraffins (used as the calibration standard) of the same boiling point. Gas chromatography is also used in the ASTM D 2427 test method to determine quantitatively ethane through pentane hydrocarbons. 

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