Interferences in FAAS

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... 

Another well-known interference in FAAS is the ionization of alkali metals such as sodium and potassium in the relatively hot air-acetylene flame. Strictly speaking, it is not the ionization itself which causes an interference but the varying degree of ionization depending on the sample matrix and the different degree of ionization in sample and reference solutions. The easiest way to control this interference is by the addition of another easily ionized element, such as cesium, in excess to sample and reference solutions. This way the ionization equilibrium for the analyte element is shifted almost entirely to the atoms. 

The sample is wet digested, if necessary, tin is extracted as Sn(IV)I4 into toluene and back-extracted as stannate with aqueous KOH to reduce interferences in the flame, and is determined by FAAS using a nitrous oxide-acetylene or air—hydrogen flame. 

Flame photometry (see also p. 168) is almost exclusively used for the determination of alkali metals because of their low excitation potential (e.g. sodium 5.14eV and potassium 4.34 eV). This simplifies the instrumentation required and allows a cooler flame (air-propane, air-butane or air-natural gas) to be used in conjunction with a simpler spectrometer (interference filter). The use of an interference filter allows a large excess of light to be viewed by the detector. Thus, the expensive photomultiplier tube is not required and a cheaper detector can be used, e.g. a photodiode or photoemissive detector. The sample is introduced using a pneumatic nebulizer as described for FAAS (p. 172). Flame photometry is therefore a simple, robust and inexpensive technique for the determination of potassium (766.5 nm) or sodium (589.0nm) in clinical or environmental samples. The technique suffers from the same type of interferences as in FAAS. The operation of a flame photometer is described in Box 26.2. 

In the case of step 3, insufficient extraction was found to be responsible in one case for low results and the set of data was withdrawn. It was agreed that the use of nitrous oxide in FAAS was not suitable for eliminating Ca and Fe interferences, which could explain the low results obtained by one laboratory. Air-acetylene (with oxine as releaser) should perhaps be used instead. This last point was confirmed by investigations performed by the University of Barce-... 

ETA-AAS Vs several ml needed for aspiration in FAAS. The sensitivities obtained by ETA-AAS are 100 fold or more compared to FAAS. On the other hand, measurement time is more than FAAS, since the sample has to go through the stages of in situ drying, ashing and atomization, and then cooling of the carbon furnace. When first introduced, the ETA-AAS technique was hailed as extremely sensitive and completely free of interferences. The sensitivity claim is certainly true, but it has now been recommended even by the manufacturers to use ETA-Aas only when FAAS cannot meet the demands of sensitivity. Accordingly, ETA-Aas was widely used for determination of REE in metallurgical and environmental samples, (after preconcentrative separation of matrix in case of seawater) and electronic materials. 

Physical Interferences. In GF-AAS, viscosity and surface tension of the sample solution have a less important part to play than in FAAS. They may affect the reproducibility of micropipetting to some extent, but their main effect is in the degree to which the sample solution spreads inside the cuvette after the injection. This effect can be minimized by the ridged tube design and the use of peak area measurements. 

The determination of chloride involves the precipitation of chloride with silver nitrate, dissolution of the precipitate in ammonia, and determination of silver in the resulting solution by FAAS using air-acetylene or air-hydrogen flames. Standard silver nitrate solutions in ammonia are used for calibration since bromide and iodide interfere in the determination of chloride. 

A well-designed mixing chamber burner with an impact system produces an aerosol with a high percentage of fine droplets which can be completely volatilized and atomized in the laminar flames of low burning velocity used in FAAS. For this reason spectral interferences are rarely observed in FAAS. 

The third type of interference which may be observed in FAAS is the formation of molecules of the analyte element in the gas phase which cannot be dissociated quantitatively—or to the same extent as in the reference solution—into atoms. The most thoroughly investigated interference of this type is the effect of phosphate on the determination of calcium in the air-acetylene flame. This interference is essentially nonexistent in the nitrous oxide-acetylene flame, which is hotter... 

Gas phase interferences due to compound formation of the analyte element with a concomitant should not be very significant in ETAAS because a much longer time is available for dissociation compared to FAAS. It was shown by high-temperature equilibrium calculations that gas phase interferences at the temperatures used in ETAAS should actually be rather insignificant [18], The reason why the literature is nevertheless full of reports on such interferences is largely due to an improper use of this technique. Slavin et al. [19], based on the systematic work of L vov [20], introduced a concept which they called stabilized temperature platform furnace (STPF). It is in essence a package of measures which eliminates most nonspectral interferences in ETAAS by atomization under local thermal equilibrium conditions. 

ICP-OES is a destructive technique that provides only elemental composition. However, ICP-OES is relatively insensitive to sample matrix interference effects. Interference effects in ICP-OES are generally less severe than in GFAA, FAA, or ICPMS. Matrix effects are less severe when using the combination of laser ablation and ICP-OES than when a laser microprobe is used for both ablation and excitation. 

Althoi h nonspectral interference effects are generally less severe in ICP-OES than in GFAA, FAA, or ICPMS, they can occur. In most cases the effects produce less than a 20% error when the sample is introduced as a liquid aerosol. High concentrations (500 ppm or greater) of elements that are highly ionized in the... 

Flow injection analysis is based on the injection of a liquid sample into a continuously flowing liquid carrier stream, where it is usually made to react to give reaction products that may be detected. FIA offers the possibility in an on-line manifold of sample handling including separation, preconcentration, masking and color reaction, and even microwave dissolution, all of which can be readily automated. The most common advantages of FIA include reduced manpower cost of laboratory operations, increased sample throughput, improved precision of results, reduced sample volumes, and the elimination of many interferences. Fully automated flow injection analysers are based on spectrophotometric detection but are readily adapted as sample preparation units for atomic spectrometric techniques. Flow injection as a sample introduction technique has been discussed previously, whereas here its full potential is briefly surveyed. In addition to a few books on FIA [168,169], several critical reviews of FIA methods for FAAS, GF AAS, and ICP-AES methods have been published [170,171]. 

The analysis of soils and plant material are common examples used to demonstrate ICP applications. Dahlquist and Knoll(43) compared the preparation and ICP analyses of botanicals (16 elements) and soils (11 elements) with few exceptions the ICP values for the CII botanicals were in excellent agreement with the assigned values, and the soil analyses were in excellent agreement with FAA analyses of soil digests. )ones( 4) reported the analysis of 17 elements in plant material and soils but confirmation of the two analyses was not given. Alder, et. aJ.(75) describe the unique analysis of ammonia-nitrogen in soils by gas evolution into an ICP no interferences were observed from the concomitants evaluated and acceptable recoveries were obtained. Irons et. al.(76) compared the ICP analyses of 13 elements in NBS orchard leaves and bovine liver to the data obtained by FAA and energy dispersive x-ray. 

Flame AAS (often abbreviated FAAS) was until recently the most widely used method for trace metal analysis. However, it has now largely been superseded by inductively coupled plasma atomic emission spectrometry (see Chapter 4). It is particularly applicable where the sample is in solution or readily solubilized. It is very simple to use and, as we shall see, remarkably free from interferences. Its growth in popularity has been so rapid that on two occasions, the mid-1960s and the early 1970s, the growth in sales of atomic absorption instruments has exceeded that necessary to ensure that the whole face of the globe would be covered by atomic absorption instruments before the end of the century. 

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. 

Another type of interference that can arise in the atomiser is called ionisation interferences . Particularly when using hot atomisers, the loss of an electron from the neutral atom in metals with low ionisation energy may occur, thus reducing the free atom population (hence the sensitivity of the analyte determination, for which an atomic line is used, is reduced). These interferences can be suppressed in flames by adding a so-called ionisation suppressor to the sample solution. This consists in adding another element which provides a great excess of electrons in the flame (he. another easily ionisable element). In this way, the ionisation equilibrium is forced to the recombination of the ion with the electron to form the metal atom. Well-known examples of such buffering compounds are salts of Cs and La widely used in the determination of Na, K and Ca by FAAS or flame OES. 

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