Correction background

Several methods of background correction can be used. One method is to adjust the read-out circuit to read 100% transmission through the background adjacent to the analytical line. This technique basically is a correction based on percentage transmission differences of the clear emulsion and the background. It is useful at low background intensity levels but mathematically is not correct. 

A common method for correcting for background is based on subtraction of the background density from the density of the line plus the background. If the read-out system is adjusted to 100% T on the clear emulsion, then 

The density of the spectral line is calculated by subtracting equation (8-3) from (8-4) to obtain 

Equation (8-5) is almost equivalent to adjusting the galvanometer deflection to read 100 %T through the background. 

Since common practice is to use an internal standard to prepare working curves, another type of approximate correction can be used. This involves a measurement of the percentage transmittances of the clear emulsion, of the background, of the spectral line plus background, and of the internal standard. These data are processed as shown in the following equation  

The atomic absorption lines of each element are very narrow and are easily distinguished from the atomic absorption lines of other elements. They are so narrow in fact that isotopes of the same element absorb at slightly different wavelengths and can be distinguished from each other.  

Each of these methods is based on the fact that broadband absorption is virtually the same at the resonance line as at a wavelength very close to the resonance line. If the background absorption is measured close to the resonance line then a correction can be made for the background absorption at the resonance line. 

In practice, first the absorption of the resonance line is measured. This is a measure of the absorption signal equivalent to the atomic absorption plus molecular background absorption. Secondly, absorption due to the molecular background is measured and the difference between the two is the true atomic absorption measurement. The bases of the techniques for background absorption are as follows. 

The amount of incident radiation absorbed or scattered must be measured and subtracted from the total measured absorbance in order to obtain the net absorbance of the analyte atoms only. 

In all methods of background correction, two measurements are needed. The background correction requires that the spectrometer rapidly alternates between the total absorbance measurement and the background absorbance measurement, especially in electrothermal AAS where the absorption signal lasts only for a few seconds. A beam switching frequency of at least 150 Hz is required. 

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Figure C2.10.1. Potential dependence of the scattering intensity of tire (1,0) reflection measured in situ from Ag (100)/0.05 M NaBr after a background correction (dots). The solid line represents tire fit of tire experimental data witli a two dimensional Ising model witli a critical exponent of 1/8. Model stmctures derived from tire experiments are depicted in tire insets for potentials below (left) and above (right) tire critical potential (from [15]).

Data for the several flame methods assume an acetylene-nitrous oxide flame residing on a 5- or 10-cm slot burner. The sample is nebulized into a spray chamber placed immediately ahead of the burner. Detection limits are quite dependent on instrument and operating variables, particularly the detector, the fuel and oxidant gases, the slit width, and the method used for background correction and data smoothing. 

Other methods of background correction have been developed, including Zee-man effect background correction and Smith-Iiieffje background correction, both of which are included in some commercially available atomic absorption spectrophotometers. Further details about these methods can be found in several of the suggested readings listed at the end of the chapter. 

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. 

The matrix for the standards and the blank should match that of the samples thus, an appropriate matrix is 0.75 M HNO3. Any interferences from other components of the sample matrix are minimized by background correction. 

Background correction is used to compensate for background absorption and scattering due to interferents in the sample. Such interferences are most severe for analytes, such as Zn, that absorb at wavelengths of less than 300 nm. 

Minimizing Spectral Interferences The most important spectral interference is a continuous source of background emission from the flame or plasma and emission bands from molecular species. This background emission is particularly severe for flames in which the temperature is insufficient to break down refractory compounds, such as oxides and hydroxides. Background corrections for flame emission are made by scanning over the emission line and drawing a baseline (Figure 10.51). Because the temperature of a plasma is... 

Method for background correction in flame atomic emission. 

The comparison with experiment can be made at several levels. The first, and most common, is in the comparison of derived quantities that are not directly measurable, for example, a set of average crystal coordinates or a diffusion constant. A comparison at this level is convenient in that the quantities involved describe directly the structure and dynamics of the system. However, the obtainment of these quantities, from experiment and/or simulation, may require approximation and model-dependent data analysis. For example, to obtain experimentally a set of average crystallographic coordinates, a physical model to interpret an electron density map must be imposed. To avoid these problems the comparison can be made at the level of the measured quantities themselves, such as diffraction intensities or dynamic structure factors. A comparison at this level still involves some approximation. For example, background corrections have to made in the experimental data reduction. However, fewer approximations are necessary for the structure and dynamics of the sample itself, and comparison with experiment is normally more direct. This approach requires a little more work on the part of the computer simulation team, because methods for calculating experimental intensities from simulation configurations must be developed. The comparisons made here are of experimentally measurable quantities. 

Since no background correction can be made, dot maps of minor and trace constituents are subject to possible artifacts caused by the dependence of the bremsstrahlung on composition, particularly with EDS X-ray measurement. 

Ix is the background-corrected net intensity of the principal peak of analyte X, Kx a proportionality factor for the absolute sensitivity of the standard reference, e. g. an Ni plate, and c the concentration of X. Multielement analyses are based on known relative sensitivities S ... 

P. B. Eamswoith, M. Wu, M. Tacquai d and M. L. Lee, Background correction device for enhanced element-selective gas cltromatograpltic detection by atomic emission spec-ti oscopy , Appl. Spectr. 48 742-746 (1994). 

Multichannel instruments are capable of measuring the intensities of the emission lines of up to 60 elements simultaneously. To overcome the effects of possible non-specific background radiation, one or more additional wavelengths may be measured and background correction (see Section 21.12) can be achieved. 

To produce an analytical method, the operator must select the power level of the plasma, the wavelength for each element (preferably free from spectral interferences), and the vewing height at which the plasma is to be seen for each element. Further, it may be necessary to apply background correction intervals are set using the graphics capability. 

It should be stressed that background correction methods should always be used in furnace AAS. The background effect in this case may be as high as 85 per cent of the total absorption signal. 

In the previous section it has been shown that the measured sample absorbance may be higher than the true absorbance signal of the analyte to be determined. This elevated absorbance value can occur by molecular absorption or by light scattering. There are three techniques that can be used for background correction the deuterium arc the Zeeman effect and the Smith-Hieftje system. 

Deuterium arc background correction. This system uses two lamps, a high-intensity deuterium arc lamp producing an emission continuum over a wide wavelength range and the hollow cathode lamp of the element to be determined. 

All instruments should be equipped with a background correction facility. Virtually all instruments now have a deuterium arc background correction. The Zeeman system is also available in instruments marketed by the Perkin-Elmer Corporation and the Smith-Hieftje system by Thermo Electron Ltd. 

It is advisable to employ background correction (see Section 21.11), especially when the 217.0 nm line is used. 

Fick s law 592 Filter funnel 102 Filter papers 115 folding of, 116 incineration of, 120, 121 macerated, 450 quantitative, (T) 116 Filter pulp 450 Filtering crucibles 102 Filters, optical 661 Filtration 102, 106, 115 accelerated, 450 technique of, 116, 117 with filter papers, 116 with filtering crucibles, 117 Flame emission spectroscopy 779, 797 background correction, 795 elementary theory of, 780 D. of alkali metals by, 812... 

It is gratifying that no empirical calibrating factor was needed with the Fe-55 source, which means that the results were predictable from Equation 5-6 by insertion of accepted values for the mass absorption coefficients. The deviation corrected by the introduction of this empirical factor (Equation 5-7) was of the kind produced by the filtering of polychromatic beams. About all that can be said about such empirical factors and about background corrections is this Always unwelcome, not to be introduced unless necessary, the need for them does not in itself make a method less desirable, but it does usually indicate that something is incompletely understood. 

Tungsten ores often contain traces of molybdenum that need to be determined before the ore is processed. When the tungsten content is known, as it usually is, that element can serve as a built-in standard for the determination of molybdenum. In the work to be described, the intensity ratio was measured for molybdenum Ka and tungsten Lyl. The general approach thus resembles that of Eddy and Laby to the analysis of brass (7.10), but conditions are less favorable in the present instance. The background corrections necessary were somewhat involved, and they will be discussed in Chapter 8. See Figure 8-1 c. 

Group I, the simplest, usually requires only that the analytical line be counted, that a background correction be applied if necessary, and that the content of the element sought be obtained by simple proportion from counts made on a suitable standard, which can usually be prepared by adding the element to a synthetic matrix. As the content of the element increases, absorption and enhancement effects may come into... 

A working curve was constructed for each element from counting data obtained on a number of chemically analyzed standards apparently no. background correction was necessary. By use of these simple curves, and without allowing for absorption or enhancement effects, satisfactory approximate results were obtained for both iron and manganese, as is shown by the data in Table 7-10, which are representative of those for a series of 40 samples. 

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