Background spectral correction

Another common treatment consists of background spectral correction by subtracting the baseline spectrum before the peak appearance. Drifts in the baseline of electropherograms can be circumvented by absorbance subtraction. Beyond these simple corrections, more sophisticated treatments for detrending and noise hltering using wavelets, artihcial neural networks, and so on can be used. 

Interferences in atomic absorption measurements can arise from spectral, chemical and physical sources. Spectral interference resulting from the overlap of absorption lines is rare because of the simplicity of the absorption spectrum and the sharpness of the lines. However, broad band absorption by molecular species can lead to significant background interference. Correction for this may be made by matrix matching of samples and standards, or by use of a standard addition method (p. 30 et seq.). 

With this technique, problems may arise with interference, such as background absorption—the nonspecific attenuation of radiation at the analyte wavelength caused by matrix components. To compensate for background absorption, correction techniques such as a continuous light source (D2-lamp) or the Zeeman or Smith-Hieftje method should be used. Enhanced matrix removal due to matrix modification may reduce background absorption. Nonspectral interference occurs when components of the sample matrix alter the vaporization behavior of the particles that contain the analyte. To compensate for this kind of interference, the method of standard addition can be used. Enhanced matrix removal by matrix modification or the use of a L vov platform can also reduce nonspectral interferences. Hollow cathode lamps are used for As, Cu, Cr, Ni, Pb, and Zn single-element lamps are preferred, but multielement lamps may be used if no spectral interference occurs. 

Reported analysis of soil standard reference materials (IAEA Soil 5, IAEA SL-1 and BCR-141) indicate that about 16-20 elements may be determined with very good analytical characteristics in sediments and soils after introduction of background and spectral corrections (Liese, 1985a,b). Severson et al. (1992) reported the determination of 26 elements in soils. The accuracy and precision have been proven by analysis of NIST-SRMs. The concentration range varies between 0.1 (Se) p.g/g to %. Seventeen elements were reliably determined in soil and sediment standard reference materials without introducing any corrections (Kanda and Taira, 1988). 

Spectral correction involves taking the ratio of the data to a background reference in order to remove the instrument response component. Because the filter-based NIR-CI instruments operate in a true staring or DC mode, it is also necessary to subtract the dark camera response from the image cube. The correction to reflectance (R) is therefore ... 

Few spectral regions were selected (figure not shown), arotmd 12000-8800 cm range, probably selected for background/baseline correction, 8700-7890 and 7600-6260 cm , where CH and OH overtones are present, respectively. 

In the following section, we discuss the fluorescence properties of AOM in terms of peaks observable in the spectrally corrected EEM data according to the scheme of Coble (1996), while at the same time attempting to reconcile the myriad of peak tables published in the past 20 years, with some speculation as to commonality among results. For the purpose of this chapter, we use the term peak in the context of spectroscopy practice as anything that exceeds the signal to noise of the background of the spectrum. The term... 

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... 

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. 

Practically all classical methods of atomic spectroscopy are strongly influenced by interferences and matrix effects. Actually, very few analytical techniques are completely free of interferences. However, with atomic spectroscopy techniques, most of the common interferences have been studied and documented. Interferences are classified conveniently into four categories chemical, physical, background (scattering, absorption) and spectral. There are virtually no spectral interferences in FAAS some form of background correction is required. Matrix effects are more serious. Also GFAAS shows virtually no spectral interferences, but... 

International or in-house standards in combination with fundamental parameters software, lead to the same accuracy as conventional analysis using regression analysis of standards. Provided that accurate standards are available, the main factors that determine the accuracy of XRF are the matrix absorption correction and (in the case of EDXRF) the spectrum evaluation programme, i.e. correction for spectral overlap and background. 

The need for improved background correction performance has generated considerable interest in applying the Zeeman effect, where the atomic spectral line is split into several polarised components by the application of a magnetic field. With a Zeeman effect instrument background correction is performed at, or very close to, the analyte wavelength without the need for auxiliary light sources. An additional benefit is that double-beam operation is achieved with a very simple optical system. 

Spectral interferences may arise from the close proximity of other emission lines or bands to the analyte line or by overlap with it. They can often be eliminated or minimized by increasing the resolution of the instrumentation, e.g. changing from a filter photometer to a grating spectrophotometer. Alternatively, another analyte line can be selected for measurements. Correction for background emission is also important and is made by monitoring the emission from a blank solution at the wavelength of the analyte line or by averaging measurements made close to the line and on either side of it. 

Instrumental correction for background absorption using a double beam instrument or a continuum source has already been discussed (p. 325). An alternative is to assess the background absorption on a non-resonance line two or three band-passes away from the analytical line and to correct the sample absorption accordingly. This method assumes the molecular absorption to be constant over several band passes. The elimination of spectral interference from the emission of radiation by the heated sample and matrix has been discussed on page 324 et seq. 

In the method described by Willie et al. [167] atomic absorption measurements were made with a Perkin-Elmer 5000 spectrometer fitted with a Model HGA 500 graphite furnace and Zeeman effect background correction system. Peak absorbance signals were recorded with a Perkin-Elmer PRS-10 printer-sequencer. A selenium electrodeless lamp (Perkin-Elmer Corp.) operated at 6W was used as the source. Absorption was measured at the 196.0nm line. The spectral band-pass was 0.7nm. Standard Perkin-Elmer pyrolytic graphite-coated tubes were used in all studies. 

The method of standard additions is a useful procedure for checking the accuracy of a determination and overcoming interferences when the composition of the sample is unknown. It should be noted that the method cannot be used to correct for spectral interferences and background changes. At least three aliquots of the sample are taken. One is left untreated to the others known additions of the analyte are made. The additions should preferably be about 0.5x, x and 2x, where x is the concentration of the unknown. It should also be noted that the volume of the addition should be negligible in comparison with the sample solution. This is to prevent dilution effects... 

Other types of background correction have also been developed. The Zeeman effect background correction system started gaining popularity in the early 1980s. An atomic spectral line when generated in the presence of a strong magnetic field can be split into a number of components... 

Spectral interferences from the overlap of molecular bands and lines (e g. the calcium hydroxide absorption band on barium at 553.55 nm) cannot be so easily dismissed. Lead seems to be particularly prone to such non-specific absorption problems at the 217.0 nm line (e g. sodium chloride appears to give strong molecular absorption at this wavelength). This type of problem is encountered in practical situations, but can sometimes be removed by the technique of background correction (see Section 2.2.5.2). 

We have shown that the radiant flux spectrum, as recorded by the spectrometer, is given by the convolution of the true radiant flux spectrum (as it would be recorded by a perfect instrument) with the spectrometer response function. In absorption spectroscopy, absorption lines typically appear superimposed upon a spectral background that is determined by the emission spectrum of the source, the spectral response of the detector, and other effects. Because we are interested in the properties of the absorbing molecules, it is necessary to correct for this background, or baseline as it is sometimes called. Furthermore, we shall see that the valuable physical-realizability constraints presented in Chapter 4 are easiest to apply when the data have this form. 

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