Multipoint calibration

Corrective action Do multipoint calibration invalidate data collection since last zero/ span check within control limits... 

Documentation Data volume includes all quality control forms, e.g., zero/span control charts and multipoint calibration results... 

Since many ion exchange columns exhibit mixed-mode interactions with analytes, factor analysis has been found to be useful in optimization.84 A 3-year, comprehensive review of inter-laboratory errors in determinations of the anions chloride, nitrate, and sulfate and the cations sodium, potassium, magnesium, and calcium suggested that multipoint calibration is essential and nonlinear calibration desirable.102 The need for nonlinear calibration was confirmed by an extended quality assurance study of chloride, sulfate, and nitrate in rainwater.103... 

Where the calibration of the method is very stable a one-point calibration to verify a previous multipoint calibration maybe sufficient... 

Rhoderick GC, Miller WR. 1990. Multipoint calibration of a gas chromatograph using cryogenic preconcentration of a single gas standard containing volatile organic compounds. Anal Chem 62 810-815. 

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. 

Electrode. Your pH meter user s guide is a valuable source of information on how to calibrate the pH electrode, since specific instructions or steps may be required to obtain reliable pH data. A two-point or multipoint calibration, using buffers is required, before the pH is measured. Daily calibration before use is necessary to determine the slope of the electrode. This serves the dual purpose of determining if the electrode is working properly and storing the slope value in the instrument s memory. A calibration of at least two points is needed to determine the slope of the electrode. 

Creation of a new multipoint calibration followed by sample reanalysis Procedure errors... 

Typically, laboratories choose the analyte PQL value at 2-10 times its MDL. The selection, however, is not entirely arbitrary because the laboratories must use the selected PQL concentration value as the lowest standard in the multipoint calibration curve. This enables the laboratory to assure that even at a low concentration level, the analyte is detected, identified, and quantified correctly. Therefore, the PQL may be also defined as a concentration that is 2-10 times greater than the MDL and that represents the lowest point on the calibration curve during routine laboratory operations. 

Initial calibration is a multipoint correlation between several analyte concentrations in standard solutions and the instrument s response. This multipoint correlation, referred to as the calibration curve, may be mathematically expressed as a linear or non-linear regression. ICP-AES instruments are excluded from this requirement since they have the software that does not allow multipoint calibrations to be performed. 

Figure 4.6 Linear and non-linear multipoint calibration curves.

Similar to target analytes, surrogate standards must be calibrated with multipoint calibration curves. 

Some instruments maintain initial multipoint calibration curves current longer than others. For example, for organic methods, an initial calibration may serve from one month to half a year. However, even under the most careful laboratory conditions, sooner or later instruments will require a new multipoint calibration. 

Surrogate standard recovery measures analytical accuracy for each individual sample. Approved methods for organic compound analysis usually recommend the surrogate standard selection. Similar to target analytes, multipoint calibration curves are prepared for surrogate standards. 

The RSDs of the multipoint calibrations for the poor performers exceed the method... 

Csample is compound concentration in the sample (pg/kg or pg/ ) Asample is peak area of a compound in the sample RFaverage is the average multipoint calibration RF (jig/ml)... 

RF average or CFaverage is the average multipoint calibration RF (pg) or CF (pg ) W(ample is weight (kg) or the volume (liter) of the analyzed sample... 

Asamole Cis -sample - Als x RF age x Wstmpk Csample is compound concentration in the sample (/rg/kg or /rg/1) Asample is peak area of a compound in the sample Cis is quantity of the internal standard (/rg) RFaverage is the average multipoint calibration RF (unitless) Wsample is weight (kg) or volume (liter) of extracted sample... 

Two calibration methods were recommended multipoint calibration of at least four levels over the range of masses injected (each compound being quantified within the calibration range), and response factors used where the mass of the determinand was within 10-15% of the mass of the calibrant. 

The transition should be unaffected by the presence of other standards so that several samples can be run simultaneously to obtain a multipoint calibration in a single experiment. 

It is very important and the most important task for the analyst who is responsible for operation and maintenanee of analytical instrumentation. Calibration is followed by a verifieation proeess in which specifications can be established and the analyst ean evaluate whether or not the calibration is verified or refuted. A calibration that has been verified can be used in acquiring data from samples for quantitative analysis. A calibration that has been refuted must be repeated until verification is achieved, e.g., if, after establishing a multipoint calibration for benzene via a gas chromatographie determinative method, an analyst then measures the concentration of benzene in a certified reference standard. The analyst expects no greater than a 5% relative error and discovers to his surprise a 200% relative error In this case, the analyst must reconstruct the calibration and measure the certified reference standard again. Close attention must be paid to those sourees of... 

The external standard is appropriate when there is little to no matrix effect between standards and samples unless the same matrix is used for preparation of standards. To illustrate this elimination of a matrix effect, consider the situation whereby an aqueous sample is extracted using a nonpolar solvent. The reference standard used to construct the ES calibration is usually in an organic solvent such as methanol, hexane, or iso-octane. The analytes of interest are now also in a similar organic solvent. ES is also appropriate when the instrument is stable and the volume of injection of a liquid sample can be reproduced with good precision. A single or multipoint calibration curve is usually established when using this mode. 

The SA mode is used primarily when there exists a significant matrix interference and where the concentration of the analyte in the unknown sample is appreciable. SA becomes a calibration mode of choice when the analyte-free matrix cannot be obtained for the preparation of standards for ES. However, for each sample that is to be analyzed, a second, so-called standard added or spiked sample must also be analyzed. This mode is preferred when trace metals are to be determined in complex sample matrices such as wastewater, sediments, and soils. If the analyte response is linear within the range of concentration levels anticipated for samples, it is not necessary to construct a multipoint calibration. Only two samples need to be measured, the unspiked and spiked sample. Assume that v represents the amount of analyte originally present in an unknown sample and that y represents the amount of standard analyte added. The instrument response is directly related to the concentration of analyte in both the unknown sample and in the spiked sample. We also know that the total concentration Cs is directly proportional to the total response R - These relationships are... 

Precision can be evaluated for replicate measurement of the ICV following establishment of the multipoint calibration by calculating the confidence interval for the interpolated value for the ICV concentration. Triplicate... 

Multipoint calibration covering one or more orders of magnitude above the 0.1-ppb instrument detection limit (IDE) for C-GC-ECD based on a 1-pL injection volume with a 10 1 split ratio should satisfy a minimum correlation coefficient of 0.9500. Correlation coefficients are calculated within the Turbochrom (PE-Nelson) software itself or by using a computer program. 

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