Calibration with an External Standard

This is only valid for areas and concentrations within the limits of the standard curve calibration levels. While in principle one may calibrate against a standard curve that has a substantial y-axis intercept, this will yield valid results only if the background signal level causing the large intercept is the same in both the standards and unknown sample matrices. This is a dangerous and often unveriflable assumption to make, and the use of such calibration curves should be avoided. 

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Analyses based on calibration with an external standard... 

An extract of the sample is separated by reversed phase chromatography. The interfering polymeric matrix is removed by switching columns. A UV detector operating at 235 nm, calibrated with an external standard, is used to determine the level of monomer. 

A pair of elevated anchors with hydrophilic top surface allowed for sample and standard deposition in very close proximity (on the anchors), and thus improved mass calibration with an external standard. The approach actually mimicked an internal standard mass calibration procedure and enabled the analysis of 30 pL sample solutions corresponding to 1.5 amol of angiotensin peptides. 

Because an increase in resolution causes a decrease in sensitivity, it is best to operate at the lowest resolution commensurate with good results. Some instrument data systems will allow calibration with an external reference material such as perfluorokerosene and then use of a secondary reference material for the internal mass reference. Tetraiodothiophene, vaporized using the solids probe inlet, is recommended as the secondary reference. The accurate masses are 79.9721, 127.9045, 162.9045, 206.8765, 253.8090, 293.7950, 333.7810, 460.6855, and 587.5900. For a higher mass standard, use hexaiodobenzene. Because the mass defect for these internal reference ions are so large, a resolution of 2000 is ample to separate these ions from almost any sample ions encountered in GC/MS. 

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. 

The residues R) of famoxadone in all plant samples, expressed in mgkg are calculated using an external standard with one-point calibration according to the following equations ... 

The reaction mixture is sampled after each reaction cycle and analyzed for purity, reaction progress, and the presence and amount of by-products (if any) formed. Analysis is by gas chromatography (HP 5890 Series II Phenomenex Zebron ZB-1 capillary column, Phenomenex Cat. No. 7HK-G001-36) with flame ionization detection in order to quantify the by-product impurities. The quantity of the byproducts is determined using an external standard calibration method. 

Compound 6 contains seven iron-based units [ 12], of which the six peripheral ones are chemically and topologically equivalent, whereas that constituting the core (Fe(Cp)(C6Me6)+) has a different chemical nature. Accordingly, two redox processes are observed, i.e., oxidation of the peripheral ferrocene moieties and reduction of the core, whose cyclic voltammetric waves have current intensities in the 6 1 ratio. Clearly, the one-electron process of the core is a convenient internal standard to calibrate the number of electron exchanged in the multi-electron process. In the absence of an internal standard, the number of exchanged electrons has to be obtained by coulometry measurements, or by comparison with the intensity of the wave of an external standard after correction for the different diffusion coefficients [15]. 

In another investigation, ethylene oxide in polyvinylchloride was determined by dissolving 65 mg of sample in 1 ml of dimethylacetamide [189]. Headspace analysis was conducted on a glass column packed with Porapak T under isothermal conditions. The solvent was removed by back-flushing. An external standard was used for calibration. A vinylchloride monomer was also detected in this analysis (Figure 4.3). 

Difficulties are caused by the need to calibrate the electron beam energy scale using an external standard, often a rare gas. With the commonly used older methods available for processing ionization-efficiency data, it is necessary to assume some relationship between the ionization-efficiency curve for the compound under investigation and the... 

An important feature of modern high-performance liquid chromatography (HPLC) is its excellent quantitation capability. HPLC can be used to quantify the major components in a purified sample, the components of a reaction mixture, and trace impurities in a complex sample matrix. The quantitation is based on the detector response with respect to the concentration or mass of the analyte. In order to perform the quantitation, a standard is usually needed to calibrate the instrument. The calibration techniques include an external standard method, an internal standard method, and a standard addition method. For cases in which a standard is not available, a method using normalized peak area can be used to estimate the relative amounts of small impurities in a purified sample. 

The LAS in soils was quantified through the use of a standard sample of a LAS (C10-C14) as an external standard. Calibration plots were constracted from measurements of peak areas versus known weights of the reference LAS. Both fluorescence and UV detection resulted in linear plots for 0-0.75 ug and 0-2 ug LAS injected respectively and with correlation coefficients (R ) greater than 0.99. The individual LAS homologues were shown to have similar molar responses for fluorescence detection as was found previously for the UV system (Matthijs and De Henau 1987 Kikuchi et al 1986). 

The glass Na-ISE (with 3M KCl Ag/AgCl external reference system where required) is calibrated against standard NaCl solutions in a range appropriate to the determination of the cation in vegetables. In preparing the standard solutions, they should contain 20 mL of 2.5M triethanolamine (TEA)/100 mL of the prepared standard solution. Produce a cahbration curve of Na versus response in mV. Check the cahbration curve daily with an Na standard at the midpoint of the range. 

The calculation of the relative characteristic peak areas on the chromatograms of the volatile pyrolysis products, using an external standard irrespective of the pyrolysis procedure, permits one to take into account the sensitivity of the detector, with easy computation of the ratio between the peak areas of the component of interest and the standard which, under normal conditions (sample size, carrier gas flow-rate, pyrolysis temperatures, etc.) are proportional to the absolute amounts of the pyrolysis products. This method of calculation is essentially a modification of the absolute calibration method in gas chromatography, which had never been used before in Py—GC.To facilitate comparison of the results obtained at different times or on different instruments, the results of individual measurements should preferably be presented in terms of specific yields (or relative characteristic peak areas), i.e., the yield of the volatile pyrolysis products must be calculated per 1 mg (or g or ng) of the pyrolysed sample with respect to 1 mg (or g or Mg) of the external standard. Such a calculation makes sense in the range of sample sizes which affect only insignificantly the specific yield of light pyrolysis products. 

By measuring absorbance changes as a function of time, the cumulative undersize particle distribution can be obtained by mathematical manipulation. However, in converting Equation 34-18 to the size distribution, it is necessary to know how the absorption coefficient K varies with d,. Alternatively, an external standard can be used for calibration. If correction is not made for variations in K. results are valid only for comparison purposes. 

Determine whether an external standard calibration (pure standards) or fortified matrix will be used for the calibration curves in the method. When pure standards are used for calibration, it is highly recommended that an internal standard be included in the method to compensate for loss of analyte during extraction or other phenomena associated with the presence of matrix materials in samples. 

Quantitative determination of the monoisotopic element cesium can only be performed by using a radioactive isotope or a different alkali element as an internal standard or by establishing a calibration curve as an external standard. A plot of such a curve is given in Fig. 13 for [Cs]" . For measurements, a number of standard solutions of CsCl in CCI4 are prepared and analyzed. The resultant values are displayed with an error of 22% corresponding to 2a (a = standard deviation). 

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