Calibration check compounds

The next step is to show that the response for the analysis of any target compound is linear. This step is known as the initial calibration and is achieved by the analysis of standards for a series of specified concentrations to produce a five-point calibration curve (Figure 41.2a, b). On subsequent days, a continuing calibration must be performed on calibration check compounds to evaluate the calibration precision of the GC/MS system. 

Continuing calibration for a Series Method is performed using calibration check compounds. Surrogate compounds are added to the matrix before sample preparation to evaluate recovery levels. To check GC retention times, internal standards are added to a sample after its preparation for analysis. 

Acceptable calibrations for indicator compounds, such as the calibration check compounds (CCCs) in EPA Methods 8260 and 8270, shown in Example 4.14, do not eliminate the need for acceptable calibrations of the target analytes. 

Example 4.14 Calibration check compounds for EPA Methods 8260B and 8270C... 

Chemical Abstract Service calibration check compounds continuing calibration verification California Code of Regulations... 

Continuing calibration for a Series Method is performed using calibration check compounds. 

Quantitative analysis is performed by the calibration technique. A new calibration curve with a standard solution of each diphenyl ether herbicides is constructed, plotting the peak area against the amount of standard solution injected. Each diphenyl ether herbicide in the sample is measured by using the peak area for each standard. Before each set of measurements, the GC and HPLC system is checked by injecting more than one standard solution containing ca 0.01-2 mg L of each compound. 

Conventional XRF analysis uses calibration by regression, which is quite feasible for known matrices. Both single and multi-element standards are in use, prepared for example by vacuum evaporation of elements or compounds on a thin Mylar film. Comparing the X-ray intensities of the sample with those of a standard, allows quantitative analysis. Depending on the degree of similarity between sample and standard, a small or large correction for matrix effects is required. Calibration standards and samples must be carefully prepared standards must be checked frequently because of polymer degradation from continued exposure to X-rays. For trace-element determination, a standard very close in composition to the sample is required. This may be a certified reference material or a sample analysed by a primary technique (e.g. NAA). Standard reference material for rubber samples is not commercially available. Use can also be made of an internal standard,... 

For a mixture of enantiomers it is thus possible to determine the ee-value without recourse to complicated calibration. The fact that the method is theoretically valid only if the g factor is independent of concentration and if it is linear with respect to ee has been emphasized repeatedly.84-89 However, it needs to be pointed out that these conditions may not hold if the chiral compounds form dimers or aggregates, because such enantiomeric or diastereomeric species would give rise to their own particular CD effects.88 Although such cases have yet to be reported, it is mandatory that this possibility be checked in each new system under study. 

Operation of detectors with their associated alarm panels should be checked and calibrated after installation. Detector performance can be impaired in a hostile environment by blockages to the detector (i.e., ice, salt crystals, wind blown particles, water or even fire fighting foam, or by inhibition of the catalysts by airborne contaminants such as compounds of silicon, phosphorus, chlorine or lead. It is essential that detectors and alarm panels be checked and re-calibrated on a routine basis. 

System suitability allows the determination of system performance by analysis of a defined solution prior to running the analytical batch. System suitability should test the entire analytical system, chromatographic performance as well as the sensitivity of the mass spectrometer for the compounds of interest. Some LC-MS SOPs reference analytical methods as the source of operating details for a given analysis. This works particularly well for quantitative analysis, where analytical methods include critical details on instrument parameters and special calibrations that might be required for a particular analyte. Thus, system suitability testing provides the daily [3] checking of the system. 

It is important that there is a low level of doubt about the identity of the compound being measured and a high level of certainty that the quantity determined is a true reflection of the amount actually present. To achieve this, confirmatory techniques usually employ complex separation procedures to isolate the compound of interest and require calibration procedures that involve adding known amounts of the compound to uncontaminated specimens of the material under analysis. As a final check on the validity of the method it is recommended that the method should be evaluated in a collaborative trial in a minimum of three independent laboratories against a standard reference material. 

As individual sample QC checks, internal standards are important in compound quantitation. They should be monitored with the same care as other QC checks. The deterioration of internal standard area counts (the area under a chromatographic peak) reflects the changes in the analytical system that may eventually degrade the quality of analysis to an unacceptable level. EPA Methods 8260 and 8270 require that the area for each internal standard be within the range of —50 to +100 percent of the area of the internal standards in the most recent CCV (EPA, 1996a). This requirement may be used as a general rule for all other methods that use internal standard calibration. 

The calibration step is critical. In general, the basic principle is always to use two independent calibration solutions. One of these can be made from pure chemicals, for example, Hg° dissolved in concentrated HN03 and diluted to the appropriate volume. For mercury, commercially available standard solutions can be used, but regular checks against a reference standard must be made. Certified reference materials (CRFs) should be used if available, but reference standards can also be prepared from pure mercury compounds. In the absence of aqueous-phase reference standards, solid materials may be used. 

The performance of the system is tested by injecting 2 xl of OPCW check mixture running the Performance Check Injection method. The composition of the mixture is given in Annex 2. The retention times of the series of nine hydrocarbons in the check mixture are used by AMDIS for calibration of the RI the other seven compounds are used for assessing the performance of the GC and MS part of the system. Two components of the check mixture, chloromethylaniline and dibenzothiophene, are used for evaluation of isotopic ratios for chlorine and sulfur measured by the mass spectrometer. The example chromatogram of the check mixture is presented in Figure 5. 

The refractometer is essentially an analytical instrument, used to determine the composition of binary mixtures (as in Exp. 14) or to check the purity of compounds. Its most common industrial application is in the food and confectionery industries, where it is used in saccharimetry —the determination of the concentration of sugar in syrup. Many commercially available refractometers have two scales one calibrated directly in refractive index, the other in percent sucrose at 20°C. 

In general, new columns should be calibrated in the laboratory in which they will be used, and a standard mobile phase and a standard series of compounds should be available for this purpose. The resolution obtained under the standard conditions of the laboratory at the start of the useful life of the column should be recorded, together with the date of the analysis. All should be part of the record for that column. At the first sign of problems with this column, its performance should be checked against these records, using the same mobile phase and standards. 

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