Spectrochimica Acta, Vol. 24B, pp. 13 to 18. Pergamon Press 1969. Printed in Northern Ireland

A spectral interference in atomic absorption spectroscopy

J. E. AI&AN Ruakura Soil Research Station, Hamilton, New Zealand

(Received 9 April 1968)

Abstract-A mutual interference between gallium and manganese which occurs in atomic absorption spectroscopy when absorption measurements are made at Ga 4032.982 and Mn 4033.073 A here reported has been shown to be due to the overlapping of these two absorption lines. The mutual interference has been studied in four different flames: air-hydrogen, air- propane, air-acetylene and nitrous oxide-acetylene, and the results have been discussed in terms of line broadening theory, flame temperature and flame composition.

ONE of the advantages of using atomic absorption spectroscopy rather than emission spectroscopy for analytical purposes is that the former provides far greater selec- tivity [l]. In emission spectroscopy the resolution depends on the optical perform- ance of the spectrograph, monochromator or filter whichis used. In atomic absorption spectroscopy on the other hand, provided that the light source emits the line spec- trum of the element to be determined, the resolution of the system is far greater as it depends, not on the optical system, but only on the breadth of the absorption lines in the flame. As these can be expected to have breadths, at half intensity, between 0.01 A and O-1 A the occurrence of spectral interference, which can be defined as the situation where the absorption spectrum of one element is overlapped by that of another, is greatly reduced. As a result little has been published on this subject.

Two types of spectral interference may be distinguished; molecular, where the interfering spectrum is a molecular band, and atomic where the interference is caused by an atomic line.

KOIRTYOHANN and PICKETT [2, 31 have shown that molecular absorption bands occur in some flames, particularly when solutions having a high salt content are analysed, and that these bands can cause analytical interference if they extend over the absorption line of the element being determined.

The present paper is concerned with atomic spectral interference and describes in detail the mutual interference caused by the overlapping of the two lines Ga 4032.982 A and Mn 4033.073 A, which was briefly reported earlier [4]. Since this paper was first submitted for publication it has been brought to the author’s notice that other pairs of interfering absorption lines have been recently reported [5-71.

111 121 131 141 161

161 171

J. E. ALLAN, Analyst 83, 466 (1958). S. R. KOIRTYOHANN and E. E. PICKETT, Anal. C%em. 37, 601 (1965). S. R.KOIRTYOHANN ~~~E.E.PIc~TT, AnaLChem. 38,585 (1966). Annual Report of Research Division, New Zealand Department of Agriculture (196667). T.G. COWLEY,J. A.FIORINO,J.O.RASMUSON andV. A.FASSEL, 18th Ann. Mid-Ante&an Symp. Spectrg (1967). D. C. MANNING and F. FERNANDEZ, At. A&or&on Newsletter 7, 24 (1968). V. A. FASSEL,J. O.RASMUSON and T. G.COWLEY, Spectrochim. Acta 23B, 579 (1968).

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14 J. E. ALLAN

EXPERIMENTAL

During the course of analytical work it was found that when solutions containing gallium were analysed for manganese, high results were obtained when the absorption of the group of manganese lines at 4030.8, 4033.1 and 4034.5 A was measured.

Preliminary investigations showed that when gallium solutions were sprayed into the flame, absorption at the wavelength of the manganese line at 4033.073 A could be observed, but no absorption was detectable at the wavelengths of the other, more sensitive, manganese lines at 4030.8 and 2794.8 A. Similarly when manganese solutions were sprayed, absorption could be observed at the wavelength of the gallium line at 4032.982 A but not at the wavelengths of the other more sensitive gallium lines at 2874.2 and 4172.1 A.

Spectrographic examination of the spectrum of the gallium lamp showed that it was free from the strongest manganese lines at 4030.8, 4034.5 and 2794.8 A and similarly the spectrum of the manganese lamp was free from the strongest gallium lines at 4172-l and 2874.2 A.

The above observations eliminate the possibility that either the solutions used or the hollow cathode lamps were contaminated with the other element and estab- lished that the observed interaction between gallium and manganese was in fact due to the overlapping of the manganese line at 4033.073 A and the gallium line at 4032.982 A.

The phenomenon was investigated quantitatively by spraying a series of solutions containing from 10 to 5000 ppm of manganese, and another series containing from 10 to 5000 ppm of gallium into each of four different flames and measuring the absorption at 4033.073 A and at 4032.982 A.

The measurements were made with a Techtron AA-4 atomic absorption spectro- photometer, the monochromator of which had a reciprocal linear dispersion of 33 A/mm. For the absorption measurements at 4033.073 A, a slit width of 10 ,um was used, while the source was a manganese hollow cathode lamp (A.S.L.) operating at 10 mA. For the measurement at 4032.982 A the slit was 50 pm and the source a gallium hollow cathode lamp (A.S.L.) operating at 5 mA.

For the four flames used, the support gas was maintained at 15 Ib/in.2; the same atomiser and spray chamber was used throughout, and the light beam was limited in height to about 5 mm by lens masks. For the air-acetylene flame (Techtron burner AB-41), the air-propane flame (Techtron burner AB-42) and nitrous oxide- acetylene flame (Techtron burner AB-40) the fuel gas was adjusted to give rich, but non-luminous flames. For the first two flames the height of the burner was set so that the light beam passed through the flame immediately above the blue cone, while for the latter the light beam passed through the centre of the red zone. The burner used for the air-hydrogen flame was that designed in this laboratory [S] modified to fit a Techtron spray chamber. The hydrogen flow was adjusted to give maximum absorption and the light beam passed immediately above the burner top.

RESULTS AND DISCUSSION

The data in Table 1 have been derived from the initial linear portions of the curves relating absorbance to concentration. These will be examined from two points

[S] 0. E. CLINTON, Spectrochim. Acta 16, 985 (1960).

A spectral interference in atomic absorption spectroscopy 15

of view. Firstly, the factors affecting the “absorbance ratio” will be discussed and, secondly, consideration will be given to the analytical interference between the two elements.

Table 1. Absorbance, absorbance ratio and analytical interference for gallium and manganese in four flames

Blame Nitrous

Air- Air- Air- oxide- hydrogen propane acetylene acetylene

Gallium

Absorbance for 300 ppm Ga at 4032.982 A 0.508

Absorbance for 300 ppm Ga at 4033.073 A 0.062

Ratio: Abs. at 4033.073/abs. at 4032.982 o-12

Manganese

Absorbance for 50 ppm Mn at 4033-073 A 0.380

Absorbance for 50 ppm Mn at 4032.982 A O-016

Ratio: Abs. at 4032_982/abs. at 4033.073 0.042

Apparent CIa concentration produced by 100 ppm Mn

19 mm

Apparent Mn concentration produced by 100 ppm Ga 2.7 ppm 0.38 ppm 0.63 ppm 5.0 ppm

-

O-123 0.260 0.398

0.011 0.019 O-024

0.089 0.073 0.060

0.475 0.500 O-080

0.016 O-016 0.0024

0.034 0.032 o-030

79 ppm 37 mm 3-6 ppm

Absorbance ratio

For the purpose of this paper the term absorbance ratio means the ratio of the absorbance in the wing of the line to that at the peak, as will be clear from Table 1

where the ratios are given for both the manganese and the gallium line in the different flames used. The value of this ratio will obviously be some measure of the extent to which the line will suffer interference and it will be a function of the breadth and shape of the absorption line.

In flames, the breadth and shape of an absorption line depends almost entirely on the Doppler and Lorentz broadening. Doppler broadening is due to the random movement of the absorbing atoms and the breadth at half intensity, An, is propor- tional to (T/N)1/2 while Lorentz broadening, due to collisions of the absorbing atom

with foreign particles, results in a breadth at half intensity AL, which is proportional to

02nT1/2

where T = absolute temperature, M = atomic weight of the absorbing atom, m = molecular weight of the perturbing species, n = number of perturbing particles per unit volume and o 2 = the effective cross section for collisional broadening.

When both types of broadening are present, the shape or profile of the absorption

16 J. E. ALLAN

line is given by a Voigt distribution and can be calculated from tables provided that AD and AL are known [9, lo].

However, both Mn 4033.073 and Ga 4032.982 exhibit hyperfine structure which must be taken into account in the determination of the overall shape of the absorp- tion lines. WHITE and RITSCHL [l l] partially resolved the Mn line into five main components having wavelength separations from the strongest component of 0, +0.0142, +0.0269, $0.0359 and +0*0419 A. Relative intensities were not given. However, from the knowledge that I = 512 and J = 512 these can be assigned from Tables [12] and are as follows: 100, 82, 63.5, 45.5 and 36.3. The hyperfine structure of Ga 4032.982 has been shown by JACKSON [13] to consist of six components with the following wavelength separations : 0, +0*0036, +0*0147, +0*0179, +0*0292 and +0*0327 A and the following relative intensities: 4, 6, 10, 2, 6 and 4.

The shape of each hyperfine component will be determined by the Voigt distribu- tion so that the overall absorption curve, obtained by summing the various overlap- ping Voigt profiles, is substantially wider than would be calculated for a simple line. Although the calculation of the exact relationship between the overall absorption curves and the measured absorbance ratios is complicated by the presence of hyperfine structure in the emitted radiation from the hollow cathode lamps and will not be attempted here, it is obvious the hyperfine structure of the two lines is a major factor contributing to their overlap.

The value of the measured absorbance ratio is also affected by the asymmetry and wavelength shift of the absorption line, both of which result from the collisional broadening process, but which have received little attention in flames. BEHMENBUR~ [14] hasreportedontheNa5890line inanacetylene-oxygen-nitrogenflameat 2500°K. No detectable asymmetry was found but the line was shifted towards the red by about 0.02 A. In the present work a red shift of this magnitude could appreciably increase the absorbance ratio, particularly for gallium where the absorbance measured in the wing of the line is on the red side of the peak, and would therefore increase as the absorbance at the peak decreased.

The different absorbance ratios obtained in the four different flames should be explicable in terms of the temperature and the composition of the burnt flame gases. With the narrow unshielded flames of the type used for analytical work, as were used here, the entrainment of atmospheric air makes it difficult to know precisely the temperature and composition of the inner zone traversed by the light beam, hence the discussion that follows will be qualitative only.

As noted above, AD varies as T112 while AL, because a2 is expected on theoretical grounds to vary as T-115 [15], varies as T-7’10. From these relationships and the tables of Voigt distributions it can be seen that the absorbance ratio will decrease with increasing temperature. This is in accord with the results obtained for the three

[9] A. C. G. MITCHELL and M. W. ZEMANSKY, Resonance Radiation and Excited Atoms. Cam-

bridge University Press ( 1934). [lo] D. W. POSENER, Awtralian J. Php. 12, 184 (1959). [ll] H. E. WHITE and R. RITSCHL, Phy~. Rev. 35, 1146 (1930). [12] H. E. WHITE, Inltroduction to Atomic Spectra. McGraw-Hill (1934). [13] D. A. JACKSON, Whys. Rev. 103, 1738 (1956). [14] W. BEHMENBURG, J. Quant. Spectq Radiative Transfer 4, 177 (1964). [15] F. W. HOFMANN and H. KOHN, J. Opt. Sot. Am. 51, 512 (1961).

A spectral interference in atomic absorption spectroscopy 17

hydrocarbon flames, where the absorbance ratio decreases in the order air-propane, air-acetylene and nitrous oxide-acetylene. In these flames the composition of the burnt gases will be rather similar, while the temperature increases in the above order.

This temperat~e effect will not, however, account for the higher absorbance ratios found in the air-hydrogen flame, the temperature of which is between that of the air-propane flame and the air-acetylene flame. The higher ratios must, therefore, be attributed to the difference in composition, In the four flames studied, nitrogen is the predominant constituent and accounts for 6575% of the burnt gases. In the air-hydrogen flame the remainder is mainly H,O, while in the three hydrocarbon flames, the presence of oxides of carbon substantially reduces the proportion of H,O. It seems reasonable, therefore, to attribute the higher absorbance ratio in the hydrogen flame to the higher proportion of H,O in this flame. This implies that 0.2 for the interaction of the metal atoms and H,O molecules is greater than that for the interaction with oxides of carbon. Eelated to this increase in a2 there could well be an increase in the red shift of the absorption lines, leading to a greater increase in the absorbance ratio for gallium for the reason mentioned earlier.

Analytical interference

The figures for analytical interference in Table 1 show a large variation among the four flames. The magnitude of the analytical interference is a function of the absorbance ratio and the relative sensitivity of the two elements in the particular flame. It can be seen that in this particular case the latter factor varies to a much greater extent than the former and is therefore largely responsible for the variation in the interference.

If an acceptable value for analytical interference cannot be obtained by choice of flame, the only alternative is to use another absorption line, although this may result in a different concentration range being available.

It should be noted that for the purpose of this investigation a slit sufficiently narrow to isolate the Mn 4033.073 line has been used. The normal analytical procedure is to use a slit sufficiently wide to pass three manganese resonance lines at 4030.8, 4033-l and 4034.5 A, whereby the magnitude of the gallium interference is reduced by a factor of about 2.6.

CONCLUSION

It has been found that a mutual interference between gallium and manganese occurs when absorption measurements are made at 4032.982 and 4033.073 A respectively, and it has been shown that this is due to the overlapping of these two absorption lines.

It has been noted that the extent of this overlap is substantially influenced by the hyperfine structure of these lines and that the absorption by gallium atoms of light emitted by manganese atoms could be increased by the theoretically expected red shift of the gallium absorption line.

The differences found among the four flames is explicable, at least qualit,atively, in terms of their temperature and composition and infer that the collision cross section for the interaction between the metal atom and H,O is greater than that between the metal atom and oxides of carbon.

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18 J. E. ALJAN

The magnitude of the analytical interference has been found to vary widely among the four flames and it has been shown that while this is partly due to the variation in the absorbance ratio it is predominantly due to the variation in the relative sensitivity of the two elements.

Acknowledgements-I am indebted to Mrs V. 0. M. BEECH for the observation that high manganese results were obtained in the presence of gallium and for her assistance with the experimental work, and to Dr R. REEVES, Massey University, for the loan of a gallium hollow cathode lamp.