Flame emission spectral analyses

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). 

In flame emission spectroscopy, light emission is caused by a thermal effect and not by a photon, as it is in atomic fluorescence. Flame emission, which is used solely for quantification, is distinguished from atomic emission, used for qualitative and quantitative analyses. This latter, more general term is reserved for a spectral method of analysis that uses high temperature thermal sources and a higher performance optical arrangement. 

At a high enough temperature, any element can be characterised and quantified because it will begin to emit. Elemental analysis from atomic emission spectra is thus a versatile analytical method when high temperatures can be obtained by sparks, electrical arcs or inert-gas plasmas. The optical emission obtained from samples (solute plus matrix) is very complex. It contains spectral lines often accompanied by a continuum spectrum. Optical emission spectrophotometers contain three principal components the device responsible for bringing the sample to a sufficient temperature the optics including a mono- or polychromator that constitute the heart of these instruments and a microcomputer that controls the instrument. The most striking feature of these instruments is their optical bench, which differentiates them from flame emission spectrophotometers which are more limited in performance. Because of their price, these instruments constitute a major investment for any analytical laboratory. 

Janssen (Jl) suggested that spectral analysis, until then used only for qualitative observations, was suitable also for quantitative work. He felt that such a development would be particularly advantageous in the case of elements like sodium which were difficult to determine by classic procedures. His suggestions bore fruit 3 years later when Champion et al. (Cl) constructed an instrument for the determination of sodium in plant ash. A solution of plant ash was introduced into the flame by means of a platinum wire and the emission intensity measured by comparing it by means of a visual photometric attachment with light from a reference constant-intensity sodium flame. This spectronatrometre was the first flame photometer and when one considers that it was capable of an accuracy of between 2 and 5 %, it is interesting that it was not for more than 70 years that the method was applied to clinical problems. 

For most elements, flame emission analysis requires the elimination or recognition of spectral interferences and excitation problems, which can usually be ignored in atomic absorption. 

Simple, low-dispersion monochromators or even interference filters are used for most flame emission applications since few atomic line spectral interferences are expected as a result of the limited population of the higher-lying excited states. For high-temperature sources such as ICPs, higher-dispersion spectrometers are typically used. Instruments set up to do simultaneous multielemental analysis can use direct readers with PMT detection. However, most modern detections systems for this type of source for simultaneous multielemental analysis employ a high-dispersion eschelle grating spectrometer and an array detector such as a CCD or CID. 

Flame AAS may be used to determine the concentration of most metals by using an acetylene-based flame. An instrumental arrangement for a flame atomic absorption spectrometer is presented in Fig. 11. There are virtually no spectral interferences that affect AAS. Nonetheless, the main limit to analysis by AAS is due to physico-chemical interferences. The proliferation of ICP instruments has significantly reduced the overall application of flame AAS in most industrial applications. More than likely, the use of flame atomic absorption will dwindle to only a few specialized applications (similar to flame emission instruments). 

It would appear that measurement of the integrated absorption coefficient should furnish an ideal method of quantitative analysis. In practice, however, the absolute measurement of the absorption coefficients of atomic spectral lines is extremely difficult. The natural line width of an atomic spectral line is about 10 5 nm, but owing to the influence of Doppler and pressure effects, the line is broadened to about 0.002 nm at flame temperatures of2000-3000 K. To measure the absorption coefficient of a line thus broadened would require a spectrometer with a resolving power of 500000. This difficulty was overcome by Walsh,41 who used a source of sharp emission lines with a much smaller half width than the absorption line, and the radiation frequency of which is centred on the absorption frequency. In this way, the absorption coefficient at the centre of the line, Kmax, may be measured. If the profile of the absorption line is assumed to be due only to Doppler broadening, then there is a relationship between Kmax and N0. Thus the only requirement of the spectrometer is that it shall be capable of isolating the required resonance line from all other lines emitted by the source. 

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 . 

D. The measurement of Li in brine (salt water) is used by geochemists to help determine the origin of this fluid in oil fields. Flame atomic emission and absorption of Li are subject to interference by scattering, ionization, and overlapping spectral emission from other elements. Atomic absorption analysis of replicate samples of a marine sediment gave the results in the table below. 

Investigation of atomic spectra yields atomic energy levels. An important chemical application of atomic spectroscopy is in elemental analysis. Atomic absorption spectroscopy and emission spectroscopy are used for rapid, accurate quantitative analysis of most metals and some nonmetals, and have replaced the older, wet methods of analysis in many applications. One compares the intensity of a spectral line of the element being analyzed with a standard line of known intensity. In atomic absorption spectroscopy, a flame is used to vaporize the sample in emission spectroscopy, one passes a powerful electric discharge through the sample or uses a flame to produce the spectrum. Atomic spectroscopy is used clinically in the determination of Ca, Mg, K, Na, and Pb in blood samples. For details, see Robinson. 

In environmental analysis, flame photometry is most widely used for the determination of potassium, which emits at 766.5 nm. It is also often used for the determination of sodium at 589.0 nm, although spectral interference problems (see Chapter 3) then may be encountered in the presence of excess calcium because of emission from calcium-containing polyatomic species. Molecular species are more likely to be found in cooler flames than in hotter flames. Some instruments use single, interchangeable filters, while others have three or more filters, for example for the determinations of potassium, sodium and lithium,... 

If the flame background around the wavelength to be used is variable, as a consequence of variable amounts of potentially interfering matrix elements, it is advisable to scan the emission spectrum in the vicinity of each wavelength of interest for each sample and standard in routine analysis. Spectral scan rate, the wavelength interval studied for each determination, and electronic damping must all then be carefully optimized. 

Silicate, nickel, and cobalt tend to interfere in the air-acetylene flame, although nickel and cobalt are rarely present in sufficient excess to cause a problem. Silicate interference may be eliminated at modest excesses by the use of lanthanum as a releasing agent or by using a nitrous oxide-acetylene flame. Very careful optimization is sometimes necessary, for example in the analysis of freshwaters, when concentrations are very low. It is important to use a narrow spectral bandpass and to make sure that the correct line is being used, because the hollow cathode lamp emission spectrum of iron is extremely complex. If you have any doubts about monochromator calibration, check the sensitivity at adjacent lines ... 

The flame is a chemical reaction which takes place in the gas phase. The ideal flame for atomic absorption would generate the correct amount of thermal energy to dissociate the atoms from their chemical bonds. The most commonly used flames are aii -acetylene and nitrous oxide—acetylene. The choice of oxidant depends upon the flame temperature and composition required for the production of free atoms. These temperatures vary the molecular or chemical form of the element. Air and acetylene produce flame temperatures of about 2300°C and permit the analysis by atomic absorption of some thirty or so elements. The nitrous oxide—acetylene flame is some 650°C hotter and extends the atomic absorption technique to around 66 elements. It also permits the successful analysis of most elements by flame atomic emission, in many cases at fractional parts per million levels, providing adequate spectral resolution is available. 

Since the hollow-cathode lamp spectra used in AAS are relatively simple, spectral bandwidths narrower than 0.1 nm are seldom if ever used. In atomic emission analysis, however, higher resolving power is often essential, particularly when the excitation source (e.g. the nitrous oxide—acetylene flame) is producing a complex spectrum. The instrument should, therefore, provide a wide range of slit settings and a convenient digital display of the wavelength in use for the operator. 

The oldest of the spectroscopic radiation sources, a flame, has a low temperature (see Section 4.3.1) but therefore good spatial and temporal stability. It easily takes up wet aerosols produced by pneumatic nebulization. Flame atomic emission spectrometry [265] is still a most sensitive technique for the determination of the alkali elements, as eg. is applied for serum analysis. With the aid of hot flames such as the nitrous oxide-acetylene flame, a number of elements can be determined, however, not down to low concentrations [349]. Moreover, interferences arising from the formation of stable compounds are high. Further spectral interferences can also occur. They are due to the emission of intense rotation-vibration band spectra, including the OH (310-330 nm), NH (around 340 nm), N2 bands (around 390 nm), C2 bands (Swan bands around 450 nm, etc.) [20], Also analyte bands may occur. The S2 bands and the CS bands around 390 nm [350] can even be used for the determination of these elements while performing element-specific detection in gas chromatography. However, SiO and other bands may hamper analyses considerably. 

Atomic spectroscopy is the oldest instrumental elemental analysis principle, the origins of which go back to the work of Bunsen and Kirchhoff in the mid-19th century [1], Their work showed how the optical radiation emitted from flames is characteristic of the elements present in the flame gases or introduced into the burning flame by various means. It had also already been observed that the intensities of the element-specific features in the spectra, namely the atomic spectral lines, changed with the amount of elemental species present. Thus the basis for both qualitative and quantitative analysis with atomic emission spectrometry was discovered. These discoveries were made possible by the availability of dispersing media such as prisms, which allowed the radiation to be spectrally resolved and the line spectra of the elements to be produced. 

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. 

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