Calibration curves standardization routines

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. 

Since a standard additions calibration curve is constructed in the sample, it cannot be extended to the analysis of another sample. Each sample, therefore, requires its own standard additions calibration curve. This is a serious drawback to the routine application of the method of standard additions, particularly in laboratories that must handle many samples or that require a quick turnaround time. For example, suppose you need to analyze ten samples using a three-point calibration curve. For a normal calibration curve using external standards, only 13 solutions need to be analyzed (3 standards and 10 samples). Using the method of standard additions, however, requires the analysis of 30 solutions, since each of the 10 samples must be analyzed three times (once before spiking and two times after adding successive spikes). 

Accuracy When spectral and chemical interferences are minimized, accuracies of 0.5-5% are routinely possible. With nonlinear calibration curves, higher accuracy is obtained by using a pair of standards whose absorbances closely bracket the sample s absorbance and assuming that the change in absorbance is linear over the limited concentration range. Determinate errors for electrothermal atomization are frequently greater than that obtained with flame atomization due to more serious matrix interferences. 

This, on the one hand, reduces the detection limit so that less sample has to be applied and, thus, the amounts of interfering substanees are reduced. On the other hand, the linearity of the calibration curves can also be increased and, hence, fewer standards need to be applied and scanned in routine quantitative investigations so that more tracks are made available for sample separations. However, the introduction of a large molecular group can lead to the equalization of the chromatographic properties. 

The "method of standard additions" has been employed as a technique for standardization of atomic absorption analyses of metals In biological fluids (13,21) In this procedure, several concentrations of standard analyte are added to samples of the biological fluid to be analyzed The calibration curve which Is obtained after additions of the standard analyte to the biological fluid should parallel that obtained when aqueous standards are analyzed Extrapolation of the standard additions curve back to a negative Intercept on the abscissa furnishes an estimate of the concentration of the analyte In the original sample (21) This technique Is helpful In assessing the validity of methods of trace metal analysis (11,13,58) However, In the author s opinion, the "method of standard additions" Is neither practical nor reliable as a routine method for standardization... 

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... 

With known analyte concentrations, the processed data provide calibration points, and Immusoft comprises fitting procedures to deduce the corresponding calibration curve that can be stored in the program for further experiments. Different calibration curves can be stored depending for instance on the assay protocol, on the specific features of the chip used or on the medium in which the assay is performed. In most cases, the calibration is performed with six independent chips of eight channels and cumulated in order to get a stable batch calibration. Then the results can be referred to this internal batch calibration. For routine control, one calibration each week is recommended to be sure that the chemistry is still in the specifications (e.g. + 10% of inter-assay standard deviation). 

In routine analysis, one standard may be ran in a batch to check the calibration curve. Within a linear range, the concentration of NH3-N in the sample may be determined from a single standard as follows ... 

The calibration mode selected by the laboratory should also be carefully considered, i.e. standard additions, calibration curve and/or use of bracketing standards. All calibration methods suffer from typical sources of error or drawbacks, e.g. for standard additions non-linearity of the calibration curve, extrapolation difficulties, chemical form of calibrant added, etc. for external calibration (calibration curve) changes of the matrix affecting the linearity of the curve for bracketing standards time-consuming procedures for many routine laboratories, etc. (Quevauviller et al., 1996a Quevauviller, 1998b). 

To place this in a familiar context, in a routine GC determination the result y (usually the concentration of material in a sample) is obtained from a simple calculation based on interpolation from a calibration curve. The curve in turn depends on the values of the concentration standard(s) used to set up the calibration, combined with the values of the mass of sample, dilution volumes etc. xx to xm represent the values of concentration standards used to set up the calibration curve, sample mass, dilution volumes etc. which are used in this calculation of the result. 

Calibration is carried out using standard calibration curves. The simplicity, repeatability, and low cost of the method have allowed its use for routine determination of trihalomethanes in tap water. SOME has also been compared with solid phase microextraction (SPME), purge and trap (P T), and direct aqueous injection (DAI) [10]. This technique offers accuracy comparable with that obtained using P T and DAI. With respect to conventional LEE, the SDME method is more accurate. In contrast to DAI and P T, it requires no special equipment. SDME has been used for extraction of chlorophenols [II], pesticides [12, 13], warfare agents [14], and butanone derivatives [15], and for control of food products [16]. The low costs of the SDME method (typical GC syringe and 2-3 pL of solvent), simplicity, and short extraction time (approximately 15 min) make it particularly suitable for preliminary analyses of organic pollutants in water samples. It can also be an effective alternative to SPME, as it does not require the use of expensive instrumentation. 

Calibration is necessary for in-situ spectrometry in TLC. Either the peak height or the peak area data are measured, and used for calculation. Although the nonlinear calibration curve with an external standard method is used, however, it shows only a small deviation from linearity at small concentrations [94.95 and fulfils the requirement of routine pharmaceutical analysis 96,97J. One problem may be the saturation function of the calibration curve. Several linearisation equations have been constructed, which serve to calculate the point of determination on the basis of the calibration line and these linearisation equations are used in the software of some scanners. A more general problem is the saturation function of the calibration curve. It is a characteristic of a wide variety of adsorption-type phenomena, such as the Langmuir and the Michaelis-Menten law for enzyme kinetics as detailed in the literature [98. Saturation is also evident for the hyperbolic shape of the Kubelka-Munk equation that has to be taken into consideration when a large load is applied and has to be determined. 

If the system has a spectral detector, spectral evaluation such as peak purity and compound identity should be part of the test. Additional tests should be developed if there are other software functions used in routine analysis, but they are not part of the sample or standard analysis test. If the system is used over a wide concentration range with multiple calibration points, the tests should be run over many concentrations to verify correct function of the calibration curve. 

The dilution methods of sample preparation of petroleum fractions for metal analysis are limited to the concentration of metals in each fraction. The dilution method is applicable for routine monitoring crude and lubricating oils, providing the concentration of metals is quantifiable using standard calibration curve, internal standard or standard addition method. The method is unsuitable for low concentration of metals especially as those can build up accumulatively, causing poisoning in all catalytic fractionation plants. The low concentration of toxic metals may be undetected by dilution methods and may escape monitoring if a more sensitive method is not used. 

The input parameters define the compound quantities and positions, solvents, temperatures, and equilibration time. The first step of the work flow is to dispense the compound with an automated powder dispenser. The parameters from Library Studio are used to drive a powder dispenser from Autodose (Geneva, Switzerland). The next step of the work flow requires the liquid handler to take the parameters outlined in Library Studio and to dispense the solvents across the plate. Besides the routines to aspirate and dispense, the liquid-handler platform has the capability to stir and control temperature. Concurrently, while this part of the work flow is executed, the compound standards, LC methods, and calibration curves are generated for... 

Quantitative polarographic analysis is based on the linear linkage between the diffusion current and bulk concentration of the electroactive species. In general, the most precise measurements of concentration are carried out by constructing a calibration curve with a set of standard solutions. In routine work, 1% precision can be obtained (3, 6, 18,... 

Calibration is carried either by means of internal standard addition or against a calibration curve from lead-spiked blood. For routine purposes the much simpler and faster method of calibration with lead-spiked blood can be applied. To minimise matrix effects the use of Zeeman GF-AAS and pyrolytically coated graphite tubes with a L vov platform is recommended, but this is instrument-dependent. 

The instrument components required for reflectance measurements are similar to those for transmission. There are two standard geometric conflgurations for light source and detector, an incidence angle of 0° with the detection instrumentation at 45° or vice versa. Reflectance measurements are not so sensitive to variation in thickness and uniformity of surface layer, and give more precise and accurate measurements. A modified form of the Rubella Munk function [64] relates intensity of reflectance beam to concentration, however, for routine work calibration curves would be used. 

The second aspect of method validation is the experimental work. This typically involves initial experiments with analytical standards to confirm the reliability and repeatability of calibration of the system using only standards. The next step usually involves a series of analytical runs, conducted over several days or weeks, in which one or more analysts prepare calibration curves and analyze replicates of the typical analyte/matrix combinations and concentrations that are to be routinely analyzed using the method. The final phase of validation typically includes several runs in which fortified or incurred materials, again representing typical analyte/matrix combinations and concentrations, are provided blind to the analyst(s). The results are then summarized in a validation report, which again should receive appropriate peer review within the laboratory prior... 

This is called the Ilkovic equation. For a particular capillary and pressure head of mercury, is a constant. Also, the value of n and that of the diffusion coefficient for a particular species and solvent conditions are constants. Thus, (l is proportional to the concentration C of the electroactive species, and this is the basis for quantitative analysis. The Ilkovic equation is accurate in practice to within several percent, and routinely 1% precision is possible. It is commonplace to use standard additions to obtain a calibration curve or an internal standard. Internal standards are useful when chemical sampling and preparation procedures involve the possibility of losses. The principle is that the ratio of the diffusion currents due to the sample and the added standard should be a constant for a particular electrolyte. 

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