JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, DECEMBER 1986, VOL. 1 485

COMMU NlCATlON - ~

Material for publication as a Communication must be on an urgent matter and be of obvious scientific importance. Rapidity of publication is enhanced if diagrams are omitted, but tables and formulae can be included. Communications should not be simple claims for priority: this facility for rapid publication is intended for brief descriptions of work that has progressed to a stage at which it is likely to be valuable to workers faced with similar problems. A fuller paper may be offered subsequently, if justified by later work.

Manuscripts are usually examined by one referee and inclusion of a Communication is at the Editor’s discretion.

Background Atomic Absorption in Graphite Furnace Atomic Absorption Spectrometry

Lindesay R. Macdonald, Thomas C. O’Haver,* Barbara J. Ottaway and (the late) John M. Ottaway Department of Pure and Applied Chemistry, University of Strathclyde, Cathedral Street, Glasgow GI IXL, UK

Keywords: Atomic absorption; graphite furnace; background atomic absorption; matrix modification; magnesium nitrate

In atomic absorption spectrometry, spectral interferences that can occur as a result of a number of different processes have been well characterised in the general literature.1-3 Back- ground absorption in graphite furnace atomisation is by far the most important source of spectral interference, and has led to the essential use of automatic background correction systems for the analysis of most types of sample. In this instance, the observed background absorption can usually be identified as being due to either absorption by molecules with bands overlapping the atomic source line being used, or to scattering of source radiation by incompletely atomised particles in the vapour phase of the atomiser. Modern furnace design has done much to minimise the latter problem.

Spectral interferences due to atomic absorption of the source radiation by concomitant elements in the sample matrix are generally considered to be unusual. A number of important cases of direct spectral line overlap have however been identified,lJ but interference of this type is not considered to be serious unless the line separation is less than 0.01 nm,l although some effects have been noted at line separations greater than 0.04 nm.2 As will be described here however, this effect will depend on the sensitivity and concentration of the concomitant. Other effects have been observed, for example, where the hollow-cathode lamp emits radiation within the band pass of the monochromator from other elements, or from ions of the analyte.1.3 A particularly serious form of interference from such radiation occurs with continuum-source background correction systems, and can give rise to overcompensation or negative background correc- tion errors.4 Examples of this effect have now been well characterised and have led some workers to recommend alternative methods of background correction.4

Chemical matrix interferences that occur in graphite fur- nace atomisation have now also been well documented, and the literature on this problem was the subject of a recent comprehensive review.5 Many workers have described the construction of new atomisation devices, designed to achieve greater atomisation efficiency and reduced chemical interfer- ences, by ensuring that the vaporised material enters a hotter and isothermal furnace atmosphere .6 However , state of the

* On leave from Department of Chemistry, University of Maryland, College Park, MD 20742, USA.

art practical graphite furnace analysis with commercial instru- ment systems most frequently utilises a platform in conjunc- tion with a suitable matrix modifier, to minimise matrix interference effects.7-9 One of the more commonly used matrix modifiers, proposed for many different analytes, is magnesium nitrate.8-1° Combined use of this additive as an ashing aid in dry-ashing procedures and as a modifier for the subsequent determination stage11912 make it an appropriate choice for biological samples. Its effectiveness for many elements also makes it a convenient choice for multi-element analysis procedures. 1 2 ~ 3

A recent investigation of the use of magnesium nitrate as a matrix modifier for the determination of manganese revealed a significant background absorption by the magnesium solu- tion at the manganese wavelength. Initially this was ascribed to manganese contamination of the magnesium nitrate solid used, but a purer form of magnesium nitrate (Aristar grade) supplied by BDH Chemicals Ltd. essentially removed this problem. The possibility of molecular absorption was con- sidered, but a more detailed investigation has now revealed that the background absorption was due to atomic absorption by magnesium atoms. Although the resonance line for magnesium at 285.2 nm is almost 6 nm away from the manganese line used at 279.5 nm, a significant absorption by magnesium atoms at the manganese wavelength was ob- served, and has been characterised by spectral measurements covering a wide range of wavelengths in the UV region. At first, it seemed unlikely that a significant absorption should be observed in the wings of an atom line as much as 6 nm from the line centre , but theoretical calculations using a Lorentzian line profile have confirmed the experimental evaluation. Several other commonly used atomic absorption lines lie sufficiently close to the magnesium line (< 5 nm) to be subject to a similar atomic absorption background absorption interference and results for gallium, lead and tin are also reported.

Experimental TWO commercially available instrument systems were used for the investigations, a Varian Spectra AA 40 with a GTA-95 graphite tube atomiser and a Perkin-Elmer 3030 atomic absorption spectrometer with an HGA-500 heated graphite atomiser. Both were equipped with data stations for peak display and with D2-arc background correction systems.

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486 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, DECEMBER 1986, VOL. 1

1.2

1.0

0.8 C

2 2 0.6 0.4

0.2

Magnesium absorption measurements were made with appropriate hollow-cathode lamps at the following wavelengths: Mn 279.5, Pb 283.3, Sn 286.4 and Ga 287.4 nm. In addition, the magnesium sensitivity of 0.006 pg 1-1 (0.0044 A) was established with a magnesium lamp at 285.2 nm using the Varian instrument system. Wide range spectral measure- ments were made point by point using discrete atomisations in each instance with the D2-arc continuum-light sources.

Solutions of magnesium nitrate were prepared from a sample of Aristar salt supplied by Mr. T. F. McCombie previously of BDH Chemicals Ltd. of Poole, Dorset. Appro- priate concentrations were prepared as indicated in the text. In all instances concentrations refer to YO m/V of Mg(N03)2.

- - - -

- -

Theory To confirm the observed experimental measurements, calcu- lations of atomic absorption line profiles were made on the following basis. It is known that the width of an atomic line is determined primarily by two broadening mechanisms: Doppler broadening, which is a consequence of the thermal motions of the analyte atoms, and pressure or collisional broadening, which is a consequence of the collisions of atoms with other atoms and molecules. Doppler broadening pro- duces a line of Gaussian shape, whereas pressure broadening produces a line of Lorentzian shape. The final shape of an atomic line subject to both of the broadening mechanisms is described by the Voigt profile, which is a convolution of the Gaussian temperature component and the Lorentzian com- ponent. At the pressures and temperatures typical of practical analytical atomisers such as the electrothermal atomisation systems used here, both of these sources of broadening are significant. However, in the wings of the line, only the Lorentzian collisional component is significant,l4 because the Gaussian temperature component drops off very much more rapidly than the Lorentzian component at wavelengths far removed from the line centre. As wavelengths of interest in this study were no closer than 1 .O nm from the magnesium line centre, it was assumed that line profile calculations could be calculated using a Lorentzian shape. Thus the absorbance at any wavelength A(h) can be calculated from15

. . . . (1) A0

1 + [(h-ho)/0.56h]2 A(h) =

where A. is the absorbance at the line centre ho and 6h is the collisional line width. The value of A. cannot be measured for high concentrations of magnesium, but can be estimated14 from

A0 = O.O044C/S . . . . . .

lo' I

I "

200 210 220 230 240 250 260 270 280 290 300 310 Wavelengthlnm

Fig. 1. Spectrum obtained by repeated atomisation of 20 p1 of 1% mlV magnesium nitrate solution usin the background mode with Varian Spectra AA 40 system. Spectral s i t width 0.1 nm, atomisation temperature 2300 "C. Gas stop at atomisation. Crosses represent the peak-height absorbance measured during each atomisation

where C is the analytical concentration in pg ml-1 and S is the sensitivity or characteristic concentration, i. e., the concentra- tion in pg ml-1 which gives an absorbance of 0.0044.

Combining equations (1) and (2):

0.0044C A(h) = S(1 + [(h-ho)/0.56hI2} * * * * (3)

Only absorbances up to about 2.0 can be expected to be directly measurable without large errors from stray light and other sources. However, absorbances in the wings of the line profile should increase with concentration as long as the atomisation efficiency remains constant, and the atomic vapour remains completely contained within the atomiser volume. If either of these conditions fails, as will be likely at very high concentrations, experimentally observed absor- bances will be smaller than those predicted by equation (3).

Calculations were performed on an IBM PC-AT with an 80287 math chip using a program written in TURBO PASCAL.

Results and Discussion The absorption spectrum between 200 and 310 nm for 2 0 4 injections of 1% m/V magnesium nitrate obtained using the Varian Spectra AA 40 at a spectral slit width of 0.1 nm is shown in Fig. 1. Measurements were obtained in the back- ground mode using the D2-arc lamp, and each point represents a separate atomisation. The effect of the logarithmic scale for absorbance should be noted. Measurements close to 285.2 nm were clearly either not possible or subject to large errors. The shape of the spectral profile in the 280-290 nm region clearly resembles a broadened atomic line, although much broader than normally observed at lower concentrations. As the magnesium sensitivity of this instrument using a 2 0 4 aliquot of solution was found to be 0.006 pg 1-1, the concentration of 1% m/V magnesium nitrate is approximately l o 9 times greater. Small shoulders close to 283 and 279 nm may be due to impurities of Pb and Mn, respectively, although there are a number of weak magnesium lines in the 279-nm region. The peaks in the 253-254 nm region have not so far been identified but were observed on both instruments used here and are the subject of further study.

To confirm the atomic absorption character of the spectral feature between 280 and 290 nm, measurements were repeated with the D2-arc for magnesium nitrate concentra- tions of 0.01, 0.1, 0.5 and 1.0% mlV, and compared with calculations based on equation (3). The measured value of the characteristic concentration of 0.006 pg 1-1 was used and a value of 0.002 nm for the collisional width.14 Figs. 2-5 show the comparison of calculated absorbances (solid line) and experimental values (crosses) measured with the Varian

1.4 1

X

1

280 281 282 283 284 285 286 287 288 289 290 Wavelengthh rn

Fig. 2. Spectrum obtained by repeated atomisation of 20 p1 of O.0lo/~ mlV magnesium nitrate solution using the same conditions as those for the spectrum in Fig. 1. Continuous line indicates absorbance calculations based on the Lorentzian profile given in equation (3). Crosses represent the peak-height absorbance measured during each atomisation

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JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, DECEMBER 1986, VOL. 1 487

I Z ox- I ' *%*-4-

280 281 282 283 284 285 286 287 288 289 290 Wavelengthhm

Fig. 3. Spectrum obtained by repeated atomisation of 20 p1 of 0.1% m/V magnesium nitrate solution using the same conditions as those for the spectrum in Fig. 1. Continuous line indicates absorbance calculations based on the Lorentzian profile given in equation (3). Crosses represent the peak-height absorbance measured during each atomisation

280 281 282 283 284 285 286 287 288 289 290 Wavelengthhm

Fig. 4. Spectrum obtained by repeated atomisation of 20 p1 of 0.5% mlV magnesium nitrate solution using the same conditions as those for the spectrum in Fig. 1, Continuous line indicates absorbance calculations based on the Lorentzian profile given in equation (3). Crosses represent the peak-height absorbance measured during each atomisation

2.0 1 I 2.5

X I X

I X

I 1 I I 1 I I I I

280 281 282 283 284 285 286 287 288 289 290 Wavelengthhm

Fig. 5. Spectrum obtained by repeated atomisation of 20 p1 of 1.0% m/V magnesium nitrate solution using the same conditions as those for the spectrum in Fig. 1. Continuous line indicates absorbance calculations based on the Lorentzian profile given in equation (3). Crosses represent the peak-height absorbance measured during each atomisation system. The agreement is excellent at 0.01 and 0.1% magnesium nitrate, but there is evidence of concentration dependent broadening at higher concentrations, where the measured absorbance values are greater than those calcu- lated. Equation (3) assumes that the atomisation efficiency remains constant and that all the atomic vapour remains within the atomic absorption volume. If either of these conditions should fail, it would be expected that the experimental atomic absorption measurement should be less than theoretical, rather than more as observed at 0.5 and 1.0%

A 2.0

1.8 t I

0 0.2 0.4 0.6 0.8 1.0 1

Magnesium nitratel concentration, %

Fig. 6. Background absorption measurements at different wavelengths as a function of magnesium nitrate concentration. Conditions as for Fig. 1. A, 284; B, 282; C, 280; D, 275; and E, 270 nm. Peak-height absorbance measured during each atomisation

1 .a

0.8

0.6

0.4

0.2 a, C

5 0 v) 2 1.c

0.8

0.6

0.4

0.2

0 0.2 0.4 0.6 0.8 1 .o 1.2 Magnesium nitrate concentration, %

Fig. 7. (a) Background absorption and (b) corrected atomic absorption signals at the resonance lines for (A) manganese 279.5 nm, (B ) lead 283.3 nm, (C) tin 286.3 nm and (D) gallium 287.4 nm for different concentrations of magnesium nitrate. Measurements made with the Perkin-Elmer 3030MGA-500 stem. Spectral slit width 0.1 nm, atomisation temperature 2300 (!. Peak-height absorbance measured during each atomisation

magnesium nitate. The relatively increased absorbance is also illustrated by the upward curvature of the absorbance plots at several wavelengths shown in Fig. 6, It seems likely that the collisional width is increased at these higher concentrations and further evaluation of this effect is in progress. There may

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488 JOURNAL O F ANALYTICAL ATOMIC SPECTROMETRY, DECEMBER 1986, VOL. 1

Table 1. Comparison of background absorbance values obtained with the Perkin-Elmer 3030 and Varian Spectra AA 40 systems

Peak height, absorbance Magnesium

nitrate concentration,

Yo

0.01

0.1

0.5

1.0

Instrument* Varian PE Varian PE Varian PE Varian PE

Manganese, 279.5 nm

0.009 0.021 0.036 0.034 0.223 0.167 0.578 0.323

Lead, 283.3 nm

0.015 0.021 0.109 0.084 0.806 0.640 2.409

>2.0

Tin, 286.3 nm

0.027 0.032 0.269 0.187 2.348 1 SO6 3.166

>2.0

Gallium, 287.4 nm

0.027

0.074

0.454

1.152

-

-

-

-

* SDectral slit width: Varian 0.1 nm, PE 0.2 nm. Atomisation temperature: Varian 2300 “C, PE 2400 “C.

indeed be an atomisation rate effect but the effect of collisional half width appears to be greater.

It is clear from Fig. 1 that background absorption measure- ments in excess of 0.5 A are observed in a 12-nm range from 279 to 291 nm for 1% magnesium nitrate. Over the small wavelength range the absorption in the wings of such a line profile will be continuum, albeit sloping, in nature. If the absorption is not too great the background correction system should take account of it. For elements commonly determined by atomic absorption only four have their resonance lines within this range, manganese at 279.5 nm, lead at 283.3 nm, tin at 286.3 nm and gallium at 287.4 nm. The background absorption levels measured at these wavelengths for magne- sium nitrate concentrations between 0 and 1.0% using the Perkin-Elmer 3030/HGA-500 system are shown in Fig. 7(a) and a comparison with values from the Varian instrument is given in Table 1. The magnitude of the signals is clearly dependent on the closeness of the analyte line to the magnesium line centre. Failure of typical D2-arc background correction systems can obviously be expected at 0.3% magnesium nitrate at the tin wavelength, and at 0.75% for lead and gallium. For manganese there should be no problem up to at least 1% magnesium nitrate [see Fig. 7(b)]. Not all the measurements in Fig. 7(a) show the same continuous concave curvature illustrated in Fig. 6, although this is a feature of many of the measurements. In some instances there is a convex curvature, which may be a reflection of an effect caused by a decrease in atomisation efficiency. This also is the subject of further study. The comparison between the two instruments shown in Table 1 shows very similar background absorbance behaviour. The slightly higher values on the Varian instrument may be a function of tube dimensions and volume, vapour diffusion effects or even slit width.

Conclusions It is clear that high concentrations of magnesium nitrate give rise to a spectral interference on analyte resonance lines in the 270-300 nm region. The interference appears as a background absorption effect and is unusual in that it is caused by atomic absorption by magnesium atoms in the wings of the 285.2-nm resonance line. The largest background effects are caused at the resonance lines of gallium, lead, manganese and tin. The most serious problem arises when magnesium nitrate is used as a matrix modifier and it has been recommended for lead,8>9J6 manganese8-10.16 and tin.8,9,16 Most workers recommend magnesium nitrate concentrations of the order of 0.01% , but levels up to 0.25% are commonly recommended. In their paper on the determination of manganese, Slavin et a l l0 suggested that the only problem of using magnesium nitrate concentrations as high as 1.5% would be obtaining the salt efficiently free from contamination. Indeed Frech et al. 16 used a concentration of 1.7% in their investigations of the interference effects of copper chloride and sodium chloride on

a number of elements including manganese. In the light of the results reported in this paper we would suggest that care should be taken to check the performance of the background correction system when higher concentrations of magnesium nitrate are used, particularly for the four elements investi- gated here. Magnesium is one of the most sensitive elements by atomic absorption, and will give more significant back- ground absorption than other matrix modifiers of somewhat lower sensitivity such as nickel and palladium. Background absorption contributions from these materials are however possible and will be the subject of detailed investigation. Equation (3) illustrates that the surprisingly broad-band structure of the magnesium atom is a function of the high sensitivity at 285.2 nm and the high concentrations sometimes used during matrix modification procedures in graphite furnace atomisation.

The authors thank Dr. C. B. Mullins of Varian Associates Ltd., Walton-on-Thames, for the loan of the Spectra AA 40 system used in this study. The provision of a sample of Aristar magnesium nitrate by Mr. T. F. McCombie is also gratefully acknowledged.

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4. 5.

6. 7.

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References Kirkbright, G. F., and Sargent, M., “Atomic Absorption and Fluorescence Spectroscopy,” Academic Press, London, 1974. Price, W. J., “Spectrochemical Analysis by Atomic Absorp- tion,” Heyden, London, 1979. Welz, B., “Atomic Absorption Spectrometry,” VCH Verlags- gesellschaft mbH, Weinheim, 1985. Slavin, W., and Carnrick, G. R., At. Spectrosc., 1986, 7 , 9. Slavin, W., and Manning, D. C., Prog. Anal. At. Spectrosc., 1982, 5 , 243. Ottaway, J. M., Anal. Proc., 1984, 21, 5 5 . Slavin, W., Manning, D. C., and Carnrick, G. R., At. Spectrosc., 1981, 2 , 137. Slavin, W., Carnrick, G. R., Manning, D. C., and Pruszkow- ska, E., At. Spectrosc., 1983,4, 69. Slavin, W . , and Carnrick, G . R., At. Spectrosc., 1985, 6 , 157. Slavin, W., Carnrick, G. R., and Manning, D. C., Anal. Chem., 1982,54, 621. Veillon, C., Patterson, K. Y., and Bryden, N. A., Anal. Chim. Acta, 1984, 164, 67. Lewis, S. A., O’Haver, T. C., and Harnly, J. M., Anal. Chem., 1985, 57, 2. Schlemmer, G., and Welz, B., Paper E077 presented at the XXIV Colloquium Spectroscopicum Internationale, Garmisch- Partenkirchen, September 1985. Parsons, M. L., Smith, B. W., and Bentley, G. E., “Handbook of Flame Spectroscopy,” Plenum, New York, 1975. Kindervater, J., and O’Haver, T. C., J . Anal. At. Spectrom., 1986, 1, 89. Frech, W., Lundberg, E., and Cedergren, A. , Can. 1. Spectrosc., 1985, 30, 123.

Paper J6l80 Received August 28th, 1986

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