Matrix interference, atomic spectroscopy

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... 

Determination of trace metals in seawater represents one of the most challenging tasks in chemical analysis because the parts per billion (ppb) or sub-ppb levels of analyte are very susceptible to matrix interference from alkali or alkaline-earth metals and their associated counterions. For instance, the alkali metals tend to affect the atomisation and the ionisation equilibrium process in atomic spectroscopy, and the associated counterions such as the chloride ions might be preferentially adsorbed onto the electrode surface to give some undesirable electrochemical side reactions in voltammetric analysis. Thus, most current methods for seawater analysis employ some kind of analyte preconcentration along with matrix rejection techniques. These preconcentration techniques include coprecipitation, solvent extraction, column adsorption, electrodeposition, and Donnan dialysis. 

This book is rooted in an informal discussion with three researchers. Dr Alatzne Carlosena, Dr Monica Felipe and Dr Maria Jesus Cal, after they had some problems measuring antimony in soils and sediments by electrothermal atomic absorption spectrometry. While we reviewed the results and debated possible problems, much like in a brainstorming session, I realized that some of their difficulties were highly similar to those found in molecular spectrometry (mid-IR spectroscopy, where I had some experience), namely a lack of peak reproducibility, noise, uncontrollable amounts of concomitants, possible matrix interferences, etc. 

Spectroscopic analysis can also benefit from a preceding electrochemical preconcentration. In particular, such coupling has been widely used for minimizing matrix interferences in atomic absorption spectroscopy (AAS). For example, lead, nickel, and cobalt have been determined in seawater with no interferences from the high sodium chloride content [80]. By adjusting the deposition potential and the pH, it is possible to obtain information on the oxidation and com-plexation states of the metal ions present [81]. 

Recently, flameless atomization techniques have been developed for atomic spectroscopy, particularly for atomic absorption. Absolute sensitivities of atomic absorption using these flameless atomizers are, for most elements, comparable with or better than those attainable by any other technique. Additionally, unlike most other techniques the atomic absorption method is relatively free from interferences by other components of the sample matrix. Therefore, flameless atomic absorption holds great promise for direct analysis of trace metals in seawater and other environmental samples. This paper reports the successful apphcation of a new design of commercial atomizer to direct analysis of several metals in seawater. 

It is apparent that the spectrophotometric procedures are rather time consuming since several multiple extractions must be performed in order to minimize interferences. Atomic absorption spectroscopy has enjoyed wide popularity in recent years as a trace metal analysis tool. This is due to a number of factors including high sensitivity, selectivity, and ease of sample preparation. With biological fluids, often no sample preparation at all is required, depending on the element analyzed, its concentration and the sample matrix. Because of its advantages, this technique will be treated in some detail. 

Atomic absorption spectroscopy is highly specific and there are very few cases of interference due to the similar emission lines from different elements. General interference effects, such as anionic and matrix effects, are very similar to those described under flame emission photometry and generally result in reduced absorbance values being recorded. Similarly, the use of high temperature flames may result in reduced absorbance values due to ionization effects. However, ionization of a test element can often be minimized by incorporating an excess of an ionizable metal, e.g. potassium or caesium, in both the standards and samples. This will suppress the ionization of the test element and in effect increase the number of test atoms in the flame. 

Analysis by atomic (or optical) emission spectroscopy is based on the study of radiation emitted by atoms in their excited state, ionised by the effect of high temperature. All elements can be measured by this technique, in contrast to conventional flames that only allow the analysis of a limited number of elements. Emission spectra, which are obtained in an electron rich environment, are more complex than in flame emission. Therefore, the optical part of the spectrometer has to be of very high quality to resolve interferences and matrix effects.-... 

ICP offers good detection limits and a wide linear range for most elements. With a direct reading instrument multi-element analysis is extremely fast. Chemical and ionization interferences frequently found in atomic absorption spectroscopy are suppressed in ICP analysis. Since all samples are converted to simple aqueous or organic matrices prior to analysis, the need for standards matched to the matrix of the original sample is eliminated. 

These findings indicate general difficnlties in interpreting the results of flash photolysis experiments with UV visible detection. First, no detailed structural information can be obtained. Second, minnte traces of imparity in the solvent interfere strongly with the spectral behavior. It has now become clear that Cr(CO)s (and other unsaturated carbonyls) are almost always complexed by solvent or matrix molecnles or atoms. Even in a neon matrix there is some interaction to produce the species Ne- Cr(CO)5. Naked Cr(CO)s can be made in the gas phase, and by some rather cunning time-resolved solution experiments. Here, solvated Cr(CO)s is formed by UV photolysis of Cr(CO)6, then the solvating molecule is removed by visible photolysis. Fast time-resolved spectroscopy allows the decay of the naked Cr(CO)s, as it resolvates, to be moiutored. It is clear that this decay is extremely rapid. 

Iniorference effects in atomic mass spectroscopy fall into two broad categories spectroscopic interferences and nonspecirosCi)pic interferences. The latter effects are similar to the matrix effects encountered in optical emi.ssion and absorption methods and generally can be managed by means of the various methods described in the two previous chapters. 

Because many elements have several strong emission Hnes, AES can be regarded as a multivariate technique per se. Traditionally, for quantitative analysis in atomic emission spectroscopy, a single strong spectral line is chosen, based upon the criteria of Hne sensitivity and freedom of spectral interferences. Many univariate attempts have been made to compensate spectral interferences by standard addition, matrix matching, or interelement correction factors. However, all univariate methods suffer from serious limitations in a complex and Hne-rich matrix. 

In atomic emission spectroscopy flames, sparks, and MIPs will have their niche for dedicated apphcations, however the ICP stays the most versatile plasma for multi-element determination. The advances in instrumentation and the analytical methodology make quantitative analysis with ICP-AES rather straightforward once the matrix is understood and background correction and spectral overlap correction protocols are implemented. Modern spectrometer software automatically provides aids to overcome spectral and chemical interference as well as multivariate calibration methods. In this way, ICP-AES has matured in robustness and automation to the point where high throughput analysis can be performed on a routine basis. 

Within the density-matrix formalism (Vol. 1, Sect. 2.9) the coherent techniques measure the off-diagonal elements pab of the density matrix, called the coherences, while incoherent spectroscopy only yields information about the diagonal elements, representing the time-dependent population densities. The off-diagonal elements describe the atomic dipoles induced by the radiation field, which oscillate at the field frequency radiation sources with the field amplitude Ak(r, t). Under coherent excitation the dipoles oscillate with definite phase relations, and the phase-sensitive superposition of the radiation amplitudes Ak results in measurable interference phenomena (quantum beats, photon echoes, free induction decay, etc.). 

ETV may also serve as sample introduction for inductively coupled plasma (ICP)-atomic emission spectroscopy (AES)/MS providing the possibility of in situ sample preparation by selective vaporization of different sample components, using appropriate heating programs. By the reduction/elimination of matrix components, spectral interferences can be minimized and matrix effects in the plasma decreased. 

Depending on the detection technique employed and the purpose of the analysis, it is occasionally sufficient to conduct a relatively simple group separation to isolate the rare earths from the matrix. Neutron activation analysis (NAA), inductively coupled plasma/atomic emission spectroscopy (ICP/AES), and mass spectrometry (ICP/MS) are examples of techniques that have been applied for simultaneous detection/quantitation of individual lanthanides in a mixture of lanthanides. Chemical separation techniques are often required prior to application of these methods because of the susceptibility of element-specific techniques to interferences that may compromise the analysis. 

Several methods are available for the determination of total aluminum in biological and other materials. Chemical and physicochemical methods are in most practical situations insensitive and inaccurate X-ray fluorescence is specific but lacks sensitivity neutron activation analysis is complex and subject to interferences, although it is a very sensitive technique. Nuclear magnetic resonance spectroscopy is not very sensitive but useful to get information on speciation [33]. Graphite furnace atomic absorption spectrometry (GFAAS) is the most widely used technique and can produce reliable results, provided that the matrix effects are recognized and corrected. Savory and Wills [19] reviewed chemical and physicochemical methods for the determination of aluminum in biological materials, e.g. X-ray fluorescence, neutron activation analysis, atomic emission spectrometry, flame emission, inductively coupled plasma emission spectroscopy, and AAS. 

Several different methods have been utilized for measuring iron in these biological samples. However, spectrophotometry is the most widely used because it does not require unusual equipment and is readily amenable to automation. Atomic absorption spectrometry is effectively used for tissue and urine analyses [33-35], but unreliable results are obtained with serum due to sensitivity limitations as well as matrix and hemoglobin interferences [35]. Other methods utilizing inductively coupled plasma emission spectroscopy [36], coulometry [37], proton induced X-ray emission [38], neutron activation analysis [39], radiative energy attenuation [40], and radiometry with Fe [41] have been described but, with the exception of coulometry, have not become standard procedures in the clinical chemistry laboratory, inasmuch as sophisticated and expensive instrumentation is required in some instances. However, some of them, e.g., neutron activation, may be the method of choice for definitive accurate analysis. 

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