Atomic spectrometry ionization interference

Eq. (72) also shows that the intensity ratio of the atom and ion lines of an element will change considerably with the electron pressure in the plasma. Elements with a low ionization energy such as Na will thus have a strong influence on the intensity ratios of the atom and ion lines of other elements. This is analytically very important as it is the cause of the so-called ionization interferences, found in classical dc arc emission spectrometry but also in atomic absorption and plasma optical emission as well as in mass spectrometry. 

Interference effects in plasma atomic emission spectrometry comprise (1) nebulization interferences, (2) chemical interferences, (3) ionization interferences, and (4) spectral interferences. The degree of interference varies from one different plasma source and spectrometer used to another. However, the most significant impediment to the effective use of any plasma-AES equipment is spectral interference. 

Interferences are physical or chemical processes that cause the signal from the analyte in the sample to be higher or lower than the signal from an equivalent standard. Interferences can therefore cause positive or negative errors in quantitative analysis. There are two major classes of interferences in AAS, spectral interferences and nonspectral interferences. Nonspectral interferences are those that affect the formation of analyte free atoms. Nonspectral interferences include chemical interference, ionization interference, and solvent effects (or matrix interference). Spectral interferences cause the amount of light absorbed to be erroneously high due to absorption by a species other than the analyte atom. While all techniques suffer from interferences to some extent, AAS is much less prone to spectral interferences and nonspectral interferences than atomic anission spectrometry and X-ray fluorescence (XRF), the other major optical atomic spectroscopic techniques. 

Both the reliable determination of isotope ratios and the quantitation of trace elements generally accompanied by complete destmction of all molecular entities. Opposed to what is normally desired in organic and bio-organic mass spectrometry, ionization methods in inorganic mass spectrometry therefore use conditions which erase any molecular stmcture, i.e., inorganic mass spectrometry particularly employs high temperature plasmas to achieve atomization prior to ionization and mass analysis [27]. Furthermore, ion-neutral reactions need to be suppressed or their products destroyed in order to ensure isotopic patterns free from interferences with isobars [28,29]. 

Flame emission spectrometry is used extensively for the determination of trace metals in solution and in particular the alkali and alkaline earth metals. The most notable applications are the determinations of Na, K, Ca and Mg in body fluids and other biological samples for clinical diagnosis. Simple filter instruments generally provide adequate resolution for this type of analysis. The same elements, together with B, Fe, Cu and Mn, are important constituents of soils and fertilizers and the technique is therefore also useful for the analysis of agricultural materials. Although many other trace metals can be determined in a variety of matrices, there has been a preference for the use of atomic absorption spectrometry because variations in flame temperature are much less critical and spectral interference is negligible. Detection limits for flame emission techniques are comparable to those for atomic absorption, i.e. from < 0.01 to 10 ppm (Table 8.6). Flame emission spectrometry complements atomic absorption spectrometry because it operates most effectively for elements which are easily ionized, whilst atomic absorption methods demand a minimum of ionization (Table 8.7). 

A convenient method is the spectrometric determination of Li in aqueous solution by atomic absorption spectrometry (AAS), using an acetylene flame—the most common technique for this analyte. The instrument has an emission lamp containing Li, and one of the spectral lines of the emission spectrum is chosen, according to the concentration of the sample, as shown in Table 2. The solution is fed by a nebuhzer into the flame and the absorption caused by the Li atoms in the sample is recorded and converted to a concentration aided by a calibration standard. Possible interference can be expected from alkali metal atoms, for example, airborne trace impurities, that ionize in the flame. These effects are canceled by adding 2000 mg of K per hter of sample matrix. The method covers a wide range of concentrations, from trace analysis at about 20 xg L to brines at about 32 g L as summarized in Table 2. Organic samples have to be mineralized and the inorganic residue dissolved in water. The AAS method for determination of Li in biomedical applications has been reviewed . 

Flame atomic absorption spectrometry can be used to determine trace levels of analyte in a wide range of sample types, with the proviso that the sample is first brought into solution. The methods described in Section 1.6 are all applicable to FAAS. Chemical interferences and ionization suppression cause the greatest problems, and steps must be taken to reduce these (e.g. the analysis of sea-water, refractory geological samples or metals). The analysis of oils and organic solvents is relatively easy since these samples actually provide fuel for the flame however, build-up of carbon in the burner slot must be avoided. Most biological samples can be analysed with ease provided that an appropriate digestion method is used which avoids analyte losses. 

Resonant and non-resonant laser post-ionization of sputtered uranium atoms using SIRIS (sputtered initited resonance ionization spectroscopy) and SNMS (secondary neutral mass spectrometry) in one instrument for the characterization of sub-pm sized single microparticles was suggested by Erdmann et al.94 Resonant ionization mass spectrometry allows a selective and sensitive isotope analysis without isobaric interferences as demonstrated for the ultratrace analysis of plutonium from bulk samples.94 Unfortunately, no instrumental equipment combining both techniques is commercially available. 

In addition, quantitative and qualitative elemental analysis of inorganic compounds with high accuracy and high sensitivity can be effected by mass spectrometry. For elemental analysis, atomization of the analysed sample that corresponds to the transformation of solid matter in atomic vapour and ionization of these atoms occur in the source. These atoms are then sorted and counted with the help of mass spectrometry. The complete decomposition of the sample in the ionization source into its constituent atoms is necessary because incomplete decomposition results in complex mass spectra in which isobaric overlap might cause unsuspected spectral interferences. Furthermore, the distribution of any element in different species leads to a decrease in sensitivity for this element. 

The overlap of molecular spectra with atomic lines, which occurs in optical flame spectrometry, has been less commonly encountered with LEI. Native flame species such as OH and CH are not observed because of their high ionization potentials. Resonantly-enhanced multiphoton ionization of molecules such as NO 45,46) may cause interferences in some flames. The LEI spectra of oxides of lanthanum, scandium,... 

The general application of LEI spectrometry to the determination of trace metals has been somewhat limited but progress in this area should continue. Alloy analyses are particularly amenable to LEI spectrometry because of the absence of an ionizable sample matrix. Indium (303.9 nm) has been determined in a nickel-based high temperature alloy31. Atomic absorption spectrometry of this sample requires time-consuming extraction procedures to remove concomitant metals which contribute to spectral interferences. After dissolution of the alloy sample with acids, the concentration of indium was determined to be 35 pg/g by LEI spectrometry. 

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