External standard absorption

M HNO3. The concentration of Cu and Zn in the diluted supernatant is determined by atomic absorption spectroscopy using an air-acetylene flame and external standards. Copper is analyzed at a wavelength of 324.8 nm with a slit width of 0.5 nm, and zinc is analyzed at 213.9 nm with a slit width of 1.0 nm. Background correction is used for zinc. Results are reported as micrograms of Cu or Zn per gram of FFDT. 

Standardizing the Method Equations 10.32 and 10.33 show that the intensity of fluorescent or phosphorescent emission is proportional to the concentration of the photoluminescent species, provided that the absorbance of radiation from the excitation source (A = ebC) is less than approximately 0.01. Quantitative methods are usually standardized using a set of external standards. Calibration curves are linear over as much as four to six orders of magnitude for fluorescence and two to four orders of magnitude for phosphorescence. Calibration curves become nonlinear for high concentrations of the photoluminescent species at which the intensity of emission is given by equation 10.31. Nonlinearity also may be observed at low concentrations due to the presence of fluorescent or phosphorescent contaminants. As discussed earlier, the quantum efficiency for emission is sensitive to temperature and sample matrix, both of which must be controlled if external standards are to be used. In addition, emission intensity depends on the molar absorptivity of the photoluminescent species, which is sensitive to the sample matrix. 

The %w/w lead in a lead-based paint Standard Reference Material and in unknown paint chips is determined by atomic absorption using external standards. 

In the analysis of seawater, isotope dilution mass spectrometry offers a more accurate and precise determination than is potentially available with other conventional techniques such as flameless AAS or ASV. Instead of using external standards measured in separate experiments, an internal standard, which is an isotopically enriched form of the same element, is added to the sample. Hence, only a ratio of the spike to the common element need be measured. The quantitative recovery necessary for the flameless atomic absorption and ASV techniques is not critical to the isotope dilution approach. This factor can become quite variable in the extraction of trace metals from the salt-laden matrix of seawater. Yield may be isotopically determined by the same experiment or by the addition of a second isotopic spike after the extraction has been completed. 

Finally, although temperature had a large effect on both the position (wavelength) and the intensity of the water absorption bands in the emulsion NIR spectra, careful experimentation demonstrated that the 1618 nm vinyl C—H band used in the calibration model did not shift in either position or intensity with temperature, in the temperature range used in these studies (25-75° C). Therefore, it was not necessary to correct the calibration model for temperature effects, either by the use of internal or external standards, or by including temperature variations in the calibration set. 

The standard addition method is commonly used in quantitative analysis with ion-sensitive electrodes and in atomic absorption spectroscopy. In TLC this method was used by Klaus 92). Linear calibration with R(m=o)=o must also apply for this method. However, there is no advantage compared with the external standard method even worse there is a loss in precision by error propagation. The attainable precision is not satisfactory and only in the order of 3-5 %, compared to 0.3-0.5 % using the internal standard method 93). 

Frequently, quantitative analyses are based on external-standard calibration (see Section 8C-2). In atomic absorption, departures from linearity are encountered more often than in molecular absorption. Thus, analyses should never be based on the... 

External Standard Calibration The relationship between the analytical line intensity and the concentration mu.st be determined empirically with a set of standards that closely approximate the samples in overall composition. We then assume that absorption and enhancement effects arc identical for samples and standards, and the empirical data are used... 

Determining the Relationship between Absorbance and Concentration 1 he method of external standards (sec Section ID-2) is most often used to establish the absorbance versus concentration relationship. After deciding on the conditions for the analysis, the calibration curve is prepared from a series of standard solutions that bracket the concentration range expected for the samples. Seldom, if ever, is it safe to assume adherence to Beer s law and use only a single standard to determine the molar ab,soq)tivity. It is never a good idea to base the results of an analysis on a literature value for the molar absorptivity. 

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. 

Determination of Lead in Drinking Water Using Graphite Furnace Atomic Absorption Spectroscopy (GFAA) External Standard Versus Internal Standard Caubration Mode... 

The spectra of the radicals and magnetic nanoparticles were recorded at room temperature using Bruker EMX 8/2.7 X-band spectrometer at a microwave power of 5 mW, modulation frequency 100 kHz and amplitude 1 G. The first derivative of the resonance absorption curve was detected. The samples were placed into the cavity of the spectrometer in a quartz flat cell. Magnesium oxide powder containing Mn ions was used as an external standard in ESR experiments. Average amount of spin labels on protein macromolecules reached 1 per 4-5 albumin macromolecules and 1 per 2-3 thrombin macromolecules. Rotational correlation times of labels were evaluated as well as a fraction of labels with slow motion (x > 1 ns). 

With external standardization, an equivalent analyte concentration in the standard and sample should yield the same analyte fluorescence signal. The accuracy of the determinations is dependent on interferences, chemical equilibrium involving the analyte, scattering, and quenching. Analyte interference due to absorption, scattering, or quenching may be... 

The frequency modifications, known as chemical shifts are usually measured with respect to the single sharp absorption in 85% orthophosphoric acid, which is used as an external standard. The same nucleus, if in a similar chemical environment, either within the same molecule, or in different kinds of molecules, will in general show the same kind of chemical shift. The chemical shift is given by the expression... 

Matrix effects frequently lead to varying extraction yields. The headspace and purge and trap techniques in particular are affected by this type of problem. If measures for standardizing the matrix are unsuccessful, the standard addition procedure can be used, as in, for example, atomic absorption (AAS) for the same reason. Here the calibration is analogous to the external standardization described earher and involves addition of known quantities of the analyte to be determined (Miller, 1992). The calibration samples are prepared with constant quantities of the sample material by addition of corresponding volumes of the standard solution. One sample is left as it is, that is, no standard is added. 

In order to standardize absorption frequencies to account for different magnetic field strengths in different laboratories, a chemical shift scale has been developed to remove the dependence on the external magnetic field strength. This is accomphshed by dividing the observed frequency of a resonance, v, by the frequency of a reference compound, (Equation 2.4). The units of chemical shift are in ppm, or... 

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