Calibration standards levels

The following is an example of GC/MS calibration standard levels. Concentrations other than those shown below may also be prepared and used if necessary. The GC/MS calibration standards may be prepared as set out in Table 3. 

An initial calibration verification standard should be measured after calibration and before measuring any sample. A calibration verification standard is a standard solution or set of solutions used to check calibration standard levels. The concentration of the analyte should be near either the regulatory level of concern or approximately at the midpoint of the calibration range. These standards must be independent of the calibration solutions and be prepared from a stock solution with a different manufacturer or manufacturer lot identification than the calibration standards. An acceptance criterion is set, usually as a maximum allowable percentage variation (e.g., 5%, 10%). The calibration can be continually verified using either a calibration standard or the initial calibration verification standard. Acceptance criteria must be set and action taken when results fall outside the limits (i.e., stop the analysis, investigate, correct the problem and rerun samples run between the verification standards that were not limits). 

Prepare a single multicomponent working standard from the stock standards by making appropriate dilutions with methanol. Concentrations in the working standards should be at such a level that a 20- xL sample added to 100 mL of water gives a calibration standard whose response for each trihalomethane is within 25% of that for the samples to be analyzed. 

Determinarion of MW and MWD by SEC using commercial narrow molecular weight distribution polystyrene as calibration standards is an ASTM-D5296 standard method for polystyrene (11). However, no data on precision are included in the 1997 edition of the ASTM method. In the ASTM-D3536 method for gel-permeation chromatography from seven replicates, the M of a polystyrene is 263,000 30,000 (11.4%) for a single determination within the 95% confidence level (12). A relative standard deviation of 3.9% was reported for a cooperative determination of of polystyrene by SEC (7). In another cooperative study, a 11.3% relative standard deviation in M, of polystyrene by GPC was reported (13). 

Adequate sensitivity should be demonstrated and estimates of the limit of detection (LOD) and the limit of quantitation (LOQ) should be provided. The slope of the calibration line may indicate the ability of the method to distinguish the tme analyte concentration. The LOD of a method is the lowest analyte concentration that produces a reproducible response detectable above the noise level of the system. The LOQ is the lowest level of analyte that can be accurately and precisely measured. For a regulatory method, quantitation is limited by the lowest calibration standard. The techniques for these estimations should be described. 

The above solutions are adequate to fortify 200 mL of control water samples in the range 0.05-20.0 qg of each analyte. Samples fortified at levels above 5.00 qg L are analyzed only with extended calibration standards. 

Detector calibration. A calibration curve is generated for every set of samples with a minimum of five standard levels. The standards are interspersed among the analytical samples of each set. The first and last sample in each analytical sample set must be a standard. 

The calibration curve is generated by plotting the peak area of each analyte in a calibration standard against its concentration. Least-squares estimates of the data points are used to define the calibration curve. Linear, exponential, or quadratic calibration curves may be used, but the analyte levels for all the samples from the same protocol must be analyzed with the same curve fit. In the event that analyte responses exceed the upper range of the standard calibration curve by more than 20%, the samples must be reanalyzed with extended standards or diluted into the existing calibration range. 

Inject a calibration standard solution corresponding to a level at or below the estimated LOQ and obtain a signal-to-noise ratio of at least 9 1. 

Quantitation is performed by the single-point calibration technique. The calibration standard should be at a level similar to the expected residues and should be injected after every 3 samples throughout the sample batch. The mean response of the calibration standards which bracket the sample should be used for the residue calculation. 

Laser ablation ICP-MS (LA-ICP-MS) was established in the early 1990s as a potential routine tool for the measurement of trace and ultra-trace elements in silicate systems for geology. Early studies (Perkins et al. 1993) used sample preparation techniques identical to that used to prepare rock samples for WDXRF, i.e., either a pressed powder disk or a glass bead fusion method (see Appendix VIII). Such studies concluded that LA-ICP-MS had the potential to surpass XRF in terms of the limits of detection achieved and INAA in terms of the speed of analysis (Perkins et al. 1993 481). It has long been recognized that the main limit on the quantitative performance of LA-ICP-MS is the homogeneity at the trace and ultra-trace level of the solid calibration standards available. Subsequent work (e.g., Hollecher and Ruiz 1995, Norman et al. 1996) has demonstrated that some of the international... 

Wegscheider fitted a cubic spline function to the logarithmically transformed sample means of each level. This method obviates any lack of fit, and so it is not possible to calculate a confidence band about the fitted curve. Instead, the variance in response was estimated from the deviations of the calibration standards from their means at an Ot of 0.05. The intersection of this response interval with the fitted calibration line determined the estimated amount interval. 

Slow change to higher or lower values in the T plots, with subsequent leveling off to a constant value characterize this pattern. This behavior usually suggests the slow attainment of an equilibrium value. Inadequately stabilized and equilibrated calibration standards or Q.C. samples will lead to this pattern... 

For a measurement result to be metrologically traceable, the measurement uncertainty at each level of the calibration hierarchy must be known. Therefore, a calibration standard must have a known uncertainty concerning the quantity value. For a CRM this is included in the certificate. The uncertainty is usually in the form of a confidence interval (expanded uncertainty see chapter 6), which is a range about the certified value that contains the value of the measurand witha particular degree of confidence (usually 95%). There should be sufficient information to convert this confidence interval to a standard uncertainty. Usually the coverage factor ( see chapter 6) is 2, corresponding to infinite degrees of freedom in the calculation of measurement uncertainty, and so the confidence interval can be divided by 2 to obtain uc, the combined standard uncertainty. Suppose this CRM is used to calibrate... 

The amount of internal standard added should be similar to the amount of analyte contained in the sample. In fact, errors are minimized when the relative responses for analyte and internal standard are comparable. If a wide concentration range is to be measured, the amount of internal standard should be selected to maximize precision where it is critically important. As a general rule, with a six-level calibration curve, the internal standard concentration, should be between the second and third calibration standard concentration depending on the relative instrument response for analyte and internal standard and precision at the LLOQ level (i.e., internal standard should be closer to LLOQ if the precision of the LLOQ is low). Many bioanalytical laboratories use a written policy for rejecting sample with low internal standard even though it is not suggested by the 2000 Washington conference final report. 

Calibration standards Prepare serial dilutions of stock standards in 1 mmol/1 HC1 to give concentrations of 0, 25, 50, 100, 250 and 500 nmol/1, and in the case of samples with very high levels also 1000 nmol/1. 

Using the slope and intercept of the linear regression equation generated for the calibration standards, calculate the trans level (as percent of total fat) for each test sample by substituting the trans band integrated area into the equation ... 

Mix milk samples or calibration standards, containing known progesterone levels, with the AP-prog solution in a 3 5 ratio. 

Monodispersed polystyrene sols are used as calibration standards for electron microscopes, light scattering photometers, Coulter counters, particle sieves, etc. Monodispersed silica is used for antireflection lens coatings. Monodispersity (even at a modest level) can usefully be exploited in photographic film, magnetic devices, pharmaceutical preparations and catalysis. 

The laboratory quality control program has several components documentation of standard operating procedures for all analytical methods, periodic determination of method detection levels for the analytes, preparation of standard calibration curves and daily check of calibration standards, analysis of reagent blank, instrument performance check, determination of precision and accuracy of analysis, and preparation of control charts. Determination of precision and accuracy of analysis and method detection limits are described under separate subheadings in the following sections. The other components of the quality control plan are briefly discussed below. 

The column and conditions for HPLC analysis are outlined earlier in this chapter. If isomeric aldehydes or ketones coelute, alternate HPLC columns or mobile phase composition should be used. The DNPH reagent solutions should be always freshly prepared. Calibration standards should be made in methanol from solid DNPH derivatives. Intermediate standards should be prepared according to the anticipated levels for each component. 

PCBs in soils and wastewaters can be rapidly screened on site or in the laboratory by immunoassay technique (Chapter 1.13). Immunoassay test kits are now commercially available from many suppliers. The samples can be tested at the calibration levels of 1 to 50 ppm. The kit primarily contains antibody-coated test tubes or magnetic particles, assay diluent, PCB-enzyme conjugate, a color-forming substance, and a solution to quench the reaction. The method does not distinguish accurately one Aroclor from another. PCBs can be measured semiquantitatively by comparing the optical density of the color formed in the sample against a set of calibration standards using a spectrophotometer. 

While the establishment and implementation of national traceability systems is left to the countries and their legislation, the mutual recognition of these national systems calls for an international mechanism, at least if worldwide mutual recognition is aimed at. The MRA of the Meter Convention for national measurement standards and calibration and measurement certificates issued by NMIs [2] can be expected to at least provide for one important aspect of the task, namely the establishment of international equivalence at the national standards level. Together with the success of the work currently carried out by the International Laboratory Accreditation Cooperation (ILAC) for the harmonization of national laboratory accreditation schemes, the MRA will be the key to comparability and hence to ac-... 

Abstract Current developments in Germany for establishing a traceability system for chemical measurements are reported. The focus is on a dissemination mechanism which employs chemical calibration laboratories accredited within the framework of the German Calibration Service (DKD) and acting as multipliers between the national standards level and the user level by providing the user with calibration means which are traceable to the SI via national standards. At the national standards level, a network of high-level chemistry institutes coordinated by the national metrology institute, PTB, provides the primary references for chemical measurements. 

It consists of three levels. At the top of the structure a network of national laboratories provides the primary chemical measurement standards and ensures that these are linked up with the international reference framework for chemical measurements. Via primary reference materials and reference measurements, a secondary level consisting of accredited chemical calibration laboratories, including verification authorities in the regulated area, is connected to the national standards level. 

The calibration laboratory is connected to the national standards level via comparison measurements on laboratory samples taken from the calibration laboratory, which are analysed by the national standards laboratory (PTB or DGKC) and the calibration laboratory to be accredited or re-evaluated. Agreement within predefined limits is required as a proof of the competence of the calibration laboratory. The sample with the known value (the national laboratory s value) is then used by the calibration laboratory as measurement standard for its work. It is the advantage of this kind of transferring standards over the transfer of reference materials from the shelf that these standards perfectly match the matrices occurring in the calibration laboratory. 

A tested example of a practical traceability structure has been described which has already proved useful in several fields of chemical analysis. It makes use of a national calibration service as a successful and efficient dissemination mechanism. An essential part of the structure is a network of high-level chemistry institutes at the national standards level providing the end points of traceability and coordinated by the national metrology institute. The need for a network of competence seems to be typical for metrology in chemistry for which in most countries the resources are largely to be found outside of the national metrology institute. 

Method validation was carried out in six (monkey) or seven (human) validation batches. Each batch included eight levels of standard calibrators, four levels of VS at concentrations at the LLOQ, low, mid, and high concentrations within the calibrator range, and two additional concentrations higher than the ULOQ. The >ULOQ VS were run at dilution factors of 20 and 400 to mimic the expected high concentration samples. The accuracy and precision data of the VS in human plasma are listed in Table 6.3. 

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