Background correction techniques

The influence of matrix concomitants often cannot be recognised or quantified. Progress in background correction techniques (e.g. direct Zeeman-AAS [218]), furnace techniques, microweighing, and electronic signal processing have gradually made possible the elimination... 

CONTENTS Preface, Joseph Sneddon. Analyte Excitation Mechanisms in the Inductively Coupled Plasma, Kuang-Pang Li and J.D. Winefordner. Laser-Induced Ionization Spectrometry, Robert B. Green and Michael D. Seltzer. Sample Introduction in Atomic Spectroscopy, Joseph Sneddon. Background Correction Techniques in Atomic Absorption Spectrometry, G. Delude. Flow Injection Techniques for Atomic Spectrometry, Julian F. Tyson. 

Another type of background correction system that has found some use is that developed by Smith and Hieftje. The Smith-Hieftje background correction technique is of especial use when there is strong molecular interference, such as that observed by phosphate on selenium or arsenic determinations. If the hollow-cathode lamp is run at its normal operating... 

Explain how the following background correction techniques work (a) beam chopping (b) deuterium lamp (c) Zeeman. 

Sneddon, J., Background correction techniques in atomic spectroscopy, Spectroscopy, 2, 38, 1987. 

Molecular absorbance and scatter are overcome by use of background correction techniques. In its simplest form, this may be achieved by measuring absorbance immediately adjacent to the determinant atomic absorption line. If light from a... 

S. Smith, R.G. Schleicher, and G.M. Hieftje, New Atomic Absorption Background Correction Technique , Paper 422, 33rd Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Atlantic City, 1982. 

The topic of interferences in AAS analyses is dealt with in Chapter 3 of this book. In general it is wise to take the precaution of checking for sample matrix interferences using the method of standard additions, to make general use of background correction techniques unless proven unnecessary, and to match closely the matrices of samples and standards. These precautions will limit the likelihood of errors due to a variety of potential interferences. 

However, the use of high resolution spectrometers and advanced background correction techniques coupled with the flexibility of choice provided by the numerous possible emission lines means that it is possible to conduct analyses on the majority of samples without spectral interference. 

Spectral interferences are uncommon in AAS owing to the selectivity of the technique. However, some interferences may occur, e.g. the resonance line of Cu occurs at 324.754 nm and has a line coincidence from Eu at 324.753 nm. Unless the Eu is 1000 times in excess, however, it is unlikely to cause any problems for Cu determination. In addition to atomic spectral overlap, molecular band absorption can cause problems, e.g. calcium hydroxide has an absorption band on the Ba wavelength of 553.55 nm while Pb at 217.0nm has molecular absorption from NaCl. Molecular band absorption can be corrected for using background correction techniques (see p. 174). The operation of a flame atomic absorption spectrometer is described in Box 27.6. 

Inductively coupled plasma-atomic emission spectrometry was investigated for simultaneous multielement determinations in human urine. Emission intensities of constant, added amounts of internal reference elements were used to compensate for variations in nebulization efficiency. Spectral background and stray-light contributions were measured, and their effects were eliminated with a minicomputer-con-trolled background correction scheme. Analyte concentrations were determined by the method of additions and by reference to analytical calibration curves. Internal reference and background correction techniques provided significant improvements in accuracy. However, with the simple sample preparation procedure that was used, lack of sufficient detecting power prevented quantitative determination of normal levels of many trace elements in urine. 

A major difficulty encountered with atomic absorption techniques is the presence of incompletely absorbed background emission from the source and scattered light from the optical system. As the background becomes more intense relative to the absorption of the analyte, the precision of the measurement decreases dramatically. For this reason, several background correction techniques have been implemented. A commonly used method is the method of proximity, which was discussed in relation to inductively coupled plasma spectroscopy. 

Figure 7.41 Direct spectral overlap of Pt and Cr emission lines. No background correction technique can solve this problem. A line with no interference must be found, an interelement correction factor must be applied or the elements must be separated chemically. [From Boss and Fredeen, courtesy of PerkinElmer Inc. (www.perkinelmer.com).]...

Use the same type of background correction technique The metallic analyte is converted to atomic vapor in four steps ... 

Dulude G (1992) Background correction techniques in atomic absorption spectrometry. In Joseph S (ed.) Advances in Atomic Spectroscopy, vol. 1. Amsterdam Elsevier. 

The disadvantage for SS-AAS is that the method is a single element technique, and for both SS-AAS and SS-ICP that special apparatus and a homogeneous solid are required. On the other hand, it can be stated that this technique is useful to determine the homogeneity of a solid. For SS-AAS an excellent background correction technique, e.g., use of the Zeeman effect, is needed to correct for the high nonanalyte peak due to the high amount of mass which may be analyzed. 

Irgafos 168 concentrations. Different techniques for normalisation and calibration were compared. Another 30 samples (with replicates totalling 80 samples) of three different HDPE products were used for testing the background correction techniques. Also Leardi et al. [174] have applied multivariate calibration (PLS) for the prediction of additive concentrations in PE films from FTIR data (cfr. also Chp. 7.2.3). 

Maia et al. [332] eliminated interferences in the direct determination of Hg in powdered coal samples by means of analyte transfer during the pyrolysis step from the platform to a graphite tube wall. This graphite tube was permanently modified with Pd, and detection limits in the range of 0.025-0.05 pg/g were obtained. In simultaneous multi-element determinations of Cu, Cr, Al, and Mn in urine, Pd was also very successfully used as a matrix modifier [333]. The use of Pd as a modifier, and also in combination with dynamic background correction techniques such as the Smith-Hieftje technique (see Section 4.6.3), enables a considerable enhancement of the analytical accuracy of AAS, as shown in the case of As determinations [334]. 

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