Some trivial causes for the bending of analytical curves in atomic absorption spectrometry

It is theoretically proven and experimentally demonstrated that atomic absorption analytical curves will be bended if more than one transition falls within the spectral bandpass of the monochromator. If the radiation of all transitions is (unequally) absorbed, as is the case for multiplet transitions, the curvature is only slight. If the radiation of only one transition is absorbed, severe bending may occur and a simple procedure is outlined to obtain linear analytical curves in this case even with low-dispersion instruments.     

Some causes of bending of analytical curves in atomic emission flame spectrometry

Factors affecting the shape of analytical curves in atomic emission flame spectrometry are discussed. Equations are presented by which the effect on the analytical curve of self-absorption, ionization, compound formation, variation in solution flow rate and atomization efficiency, entrance optics, and multiple spectral lines can be considered quantitatively. Theoretical and experimental curves are compared for Na introduced as aqueous NaCI solution into H₂/air, H₂/O₂, and C₂H₂/O₂ flames. The portion dealing with the effect of ionization, compound formation, and variation in solution flow rate and atomization efficiency on the atomic concentration in the flame applies as well to atomic absorption and atomic fluorescence flame spectrometry.     

The influence of hollow-cathode lamp line profiles upon analytical curves in atomic absorption spectroscopy

Analytical curves of Ca, Al, Mn, Ga and In in the acetylene-nitrous oxide flame are calculated using line profiles measured with a pressure-scanned interferometer. The effects of hyperflne structure, flame Une shift and finite hollow-cathode lamp line width are determined and found to decrease sensitivity by 10 to 20% and to bend the curves towards the concentration axis according to a quadratic function that causes deviations from a straight line between 2 and 7% at absorbance one. An increase of the lamp current from 5 to 25 mA d.c. enhances the effects mentioned above. It is shown that cooling the hollow cathode raises the sensitivity by only a few per cent and in some cases even reduces sensitivity. The use of a tunable laser as a primary source can improve sensitivity, especially when the atomic line profile consists of several widely spaced hyperfine components.     

Calculation of non-linearities of the calibration curves in atomic absorption flame spectroscopy

The deviations from the linear relationship between absorbance and concentration in atomic absorption measurements are calculated. Besides the chemical reactions there are four groups of interferences: (a) profiles of emission- and absorption lines (b) hyperfine structure (c) optics around the flame (d) non resonant light detected by the amplifier due to the optical arrangement and/or resolution of the monochromator.

The calculations show that curvature of experimental curves as obtained by apparatus for atomic absorption spectroscopy is caused partly by optical compromises and partly by the emission profile of the source. With a narrow simple emission line profile of the source and a precision monochromator a linear relationship within a few percent is possible up to 95 per cent absorption in a flame at atmospheric pressure.     

Roll-over of analytical curves in atomic absorption spectrometry arising from background correction with pulsed hollow-cathode lamps

A theoretical analysis of background correction systems in atomic absorption spectrometry reveals the interdependence of three phenomena: analytical sensitivity, roll-over of the analytical curve, and wavelength proximity of the background correction. The deuterium lamp system sacrifices wavelength proximity and the Zeeman technique is subject to roll-over. For the newly introduced correction technique using pulsed hollow-cathode lamps roll-over has also been observed, although the effect is reduced by sacrifices of both wavelength proximity and analytical sensitivity.     

Twenty-five years of analytical atomic spectroscopy

Developments in atomic spectrometry are traced over the twenty-five years from 1960–1985. Although a few other methods are briefly mentioned, greatest emphasis is placed on emission, absorption and fluorescence techniques. Two emission approaches are considered in detail and are based on the high-voltage spark and on the inductively-coupled plasma, respectively. Atomic absorption progress is followed from its earliest introduction, through the trial-and-error improvement of instrumentation, to the recent availability of fully automated commercial systems. Finally, the youngest of the methods, fluorescence spectrometry, is described, its strengths and weaknesses reviewed, and its future potential assessed.     

Further observations in atomic absorption spectroscopy

Three observations have been made pertinent to atomic absorption spectroscopy. With a flame atomizer, it is shown that (1) the absorption profile is controlled by metal oxide formation, (2) organo-metallic compounds give rise to greater absorption than metal salts, and (3) absorption by hydroxyl bands affects the apparent metal absorption in certain spectral regions.     

Bridging the gaps in analytical atomic absorption spectrometry

The techniques of analytical, valence electron, atomic spectrometry (absorption, fluorescence and emission), by themselves, are mainly for the determination of the amount of an element in a sample which has been prepared as a . For a large fraction of trace element analysis such an approach is satisfactory. However, there are some other very important requirements in trace element analysis not now adequately being addressed by analytical atomic spectrometry. A selection of these, familiar to the present writer, will be covered in this presentation. Some interesting initiatives to “bridge the gaps” are now being made. The topics to be discussed are; elemental speciation, direct analysis of solids, elemental and natural isotope analysis using plasma sources, micro analysis by laser probe and vapour generation approaches to improved detection limits. This is not intended to be an exhaustive review of these subject areas but will present material of interest together with recently published key papers. Greatest emphasis is given to the very important topic of speciation.     

A history of atomic absorption spectroscopy

Spectrochemical analysis originated with the work of Kirchoff and Bunsen in 1860 but found relatively little application until the 1930s. Arc-spark emission and, to a lesser extent, flame emission methods then became popular. Following World War II flame emission became very popular. In 1955 the modern era of atomic absorption spectroscopy began with the work of Walsh and Alkemade and Milatz. The time since 1955 can be divided into seven year periods. The first was an induction period (1955–1962) when AA received attention from only a very few people. This was followed by a growth period (1962–1969) when most of what we see today was developed, and then by a period of relative stability (1969–1976) when AA contributed greatly to other fields. We are now in a period of great change, which started in about 1976, due to the impact of computer technology on individual laboratory instruments.     

The atomic absorption spectroscopy of lead

Lead can be determined by atomic absorption spectroscopy at 3 wavelengths. The relative sensitivities are 1:1.5:300. No interferences were found from the cations studied. Anionic interferences were numerous and extensive, but were removed by adding EDTA. The use of a “T” -piece increased the sensitivity of atomic absorption when flame atomizers were used. However, extreme care was necessary in controlling flame conditions both with respect to oxygen-fuel ratio and the type of solvent used. The absorption by combustion products in the flame was high, and in many cases, much greater than that of the lead itself.The most sensitive conditions for the determination of lead appeared to be as follows: wavelength, 2170 A; solvent, aqueous or organic; flame, oxy-hydrogen, with the hydrogen atomizing the sample (reversed from normal). Aflame adapter enabled detection limits of 0.013 p.p.m. to be reached.     

The atomic absorption spectroscopy of selenium

Selenium can be determined in aqueous solution by atomic absorption spectroscopy at 1960, 2040,2063 or 2075 Å; the sensitivities for these lines with a Techtron 10-cm air-acetylene burner are in the ratio of 1:9:60:93. When a Beckman tripleburner (air-hydrogen) and a triple-pass optical system are used, the most sensitive1960 Å line provides a sensitivity of 0.5 p.p.m. and a detection limit of 1.0 p.p.m. The performence in air-hydrogen and air-acetylene flames is described,and optimum experimental conditions determined.With the Se 1960 Å line and a Techtron 10-cm air-acetylene burner, selenium extracted into methyl isobutyl ketone as its diethyldithiocarbamate complex gives a sensitivity of 0.30 p.p.m.,which is a 2.4-fold increase over that found in aqueous solution.     

The atomic absorption spectroscopy of Tellurium

Telluiium can be determined by atomic absorption spetroscopy at 2143, 2359, and 2386 A The sensitivities for these lines are in the ratio of 1.9.9 187 With aqueous solutions, a Beckman triple-buiner (air-hydrogen) and a 5-pass optical system, the line 2143 Å has a sensitivity of 0.23 p.p.m , and a detection limit ot 0.076 p.p.m The sensitivities for this line in acqueous and organic solvents, and in air-hydrogen and air-acetylene flames were studied and the optimum conditions determined Where necessary, preconcentration of tellurium by coprecipitation with elemental arsenic, 01 by extraction of K₂Tel₆ or tellurium diethyldithiocarbomate with MIBK can be applied, the latter gives a two-fold enhancement in sensitivity compared with aqueous solutions.     

The atomic absorption spectroscopy of chromium

Conditions were studied for the determination of trace amounts of chromium by atomic absorption spectroscopy. Solution matrix, flame composition, and extraction procedures were the variables studied. A detection limit of 0.006 p.p.m. of chromium was observed with an air-hydrogen flame and methyl isobutyl ketone as the solvent.     

The oscillator strength in atomic absorption spectroscopy

The role of the oscillator strength, f, in the theory of atomic absorption is investigated. For a pure natural broadened absorption line, the peak absorption coefficient α_o is independent of the oscillator strength. The peak absorption coefficient becomes dependent on f only through additional broadening processes such as Doppler or collisional broadening. The peak cross section for resonance absorption, α₀/N₁, for a closed transition with equal statistical weights is given by σ₀ = 2πXXX² = ( )/[cn(ω₀)] (where XXX = and n(ω₀) is the spectral mode density of the radiation field at the resonance frequency ω₀) and physically represents the cross-sectional area per allowed mode of the radiation field per unit time per unit frequency interval, multiplied by a lineshape factor 2/π.A summary is presented of some recent determinations of oscillator strengths of atomic absorption lines, based on lifetime measurements made in this laboratory. The results are used to revise values of the theoretical characteristic mass for Ag, Al, Au, Ca, Cu, Mo, Na, Ti and V used in absolute analysis by graphite furnace atomic absorption spectroscopy.     

Some observations on sulfuric acid reactions in electrothermal atomic absorption spectroscopy with graphite furnaces

Interferences were found when mixtures of sulfuric acid with either nitric or perchloric acid were atomized in the graphite furnace of an atomic absorption spectrometer. The mixtures apparently formed thermally stable compounds with the graphite, which were not removed by pyrolysis at temperatures up to 1100°C. During the subsequent atomization, volatile species were released and their absorption could not be compensated by the conventional deuterium lamp system. The absorption and emission spectra of the species formed in the furnace showed the presence of CS and probably COS, as well as bands appropriate to S₂, SO₂ and NS. The uncorrectable absorptions detected at the 303.8-nm nickel line, the 303.94-nm indium line, and the 304.40-nm cobalt line are thought to be caused by the fine-structure of an unknown molecular compound.