Application of standardless analysis in graphite furnace atomic absorption spectrometry: Determination of chromium

Talanta, Vol. 40, No. 3, pp. 347-350, 1993 Printed in Great Britain. All rights reserved

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APPLICATION OF STANDARDLESS ANALYSIS IN GRAPHITE FURNACE ATOMIC ABSORPTION SPECTROMETRY: DETERMINATION OF CHROMIUM YANSHENG ZHENG* and XINGGIJANGSu

Department of Chemistry, Jilin University, Changchun 130023, P.R. China (Received

15 May 1992. Revised 9 July 1992. Accepted

10 July 1992)

Summary-Influences of atomization temperatures on the characteristic mass and the atomic absorption coefhcient of chromium were studied. The experimental results show that the values of characteristic mass have appeared to be. stable to better than 10% when the analyte is atomized in the range of 2500-2800”. The standardless analysis was applied to the determination of chromium in standard sediment and geochemical reference samples and satisfactory results were obtained.

The idea of standardless or absolute analysisid has attracted attention of spectroscopic workers ever since Walsh’s first paper on atomic absorption spectroscopy.’ There are now several approaches that have been proposed for providing graphite furnace conditions approximate to those that are theoretically required for absolute analysis. Typically, these devices have been called “constant temperature furnaces”, a name that seems to have been proposed by WoodriK6 However, the Woodriff furnace was very difficult to use for real analysis. At present, in discussions of the development of graphite furnace atomic absorption spectrometry (GFAAS), it is worth while to speak of the method which is known as the stabilized temperature platform furnace.’ With this system, it has been demonstrated that it is possible to control the experimental conditions so tightly that a unique and constant calibration curve can be assumed as valid. However, any change of sample composition and the formation of stable molecules can be a source of larger errors for application of standardless analysis. Therefore, it is an important condition that a matrix modifier should be used to eliminate interferences for application of standardless analysis in real samples. In atomic absorption spectrometry, the transient atomic absorption signal of a spectral line is the result of the time-dependent density distribution of the analyte atoms in the analysis

volume. The equation which has been derived for GFAA!P is

A(r) =

xexPt-W~h’~!(~~

0)

j+qtj

(1)

m,c’Z(T)Av,s Here m, and e are the mass and the charge of the electron, c is the velocity of light in vacuum, f is the oscillator strength, gi and Ei are the statistical weight and the energy of the lower level for the analytical line, y ’ is a coefficient accounting for hyperfine splitting in the analytical line and the Doppler line width in the light source, 6 is a correction factor for adjacent lines in the light source spectrum, Z(T) is the partition function at temperature T(K), k is the Boltzmann constant, H(a, o) is the Voigt integral for the point of the absorption line centour distant from the line centre by cu = 0.72~ (here a is the damping constant of the Voigt profile), s is the cross section of the tube, N(t) is the total number of free analyte atoms in the analysis volume at time t. The atoms need not be distributed homogeneously over the tube length, but they must maintain a homogeneous distribution in the plane perpendicular to the light beam, because otherwise the A(t) measured will not be linearly related to N(t). 89 The constant K = 0.432JSXS2gi

exp(-

Ei/kT)y ‘dfH(a, CD)

m,c’Z( T) Av,

(2) *Author for correspondence.

will be called the atomic absorption coefficient.

348

YANSIiBNG .zHENG

In graphite furnace atomic absorption spectrometry, the sensitivity is usually expressed as the characteristic mass (mO), i.e., the mass of analyte in picogram corresponding to a peak area of 0.~ A - s. As long as the removal of free analyte atoms from the furnace is a first order process, we get for the peak area absorbance (Ai): Ai = KN,ra/s

(4)

Here m is the analyte mass, NA is the Avogadro number, J4, is the molar mass of the analyte. Substituting equation (4) into equation (3), we come to Ai = K~~?~~/s~~

(5)

For an absolute analysis, the constant 1y in equation (2) should be obtained from universal constants. However, it is possible to obtain rR experimentally if the process of atom generation is fast in comparison with the removal process.**‘**”Therefore, for all practical purposes it is more convenient to measure Kin equation (5) from a known quantity of analyte, as is done for the measurement of the molar absorptivity in W-Vis molecular spectroscopy. The characteristic mass (m& was calculated from the mean peak area from the following equation: m. = (0.0044tAi)m

(6)

Here m is the mass of analyte in picograms for the particular element. It can be seen from equation (5) that the relation between k: and m, can be expressed as K = 2.30 x 10-‘4iU,t2/morR

(7)

Here t is the tube radius. If we assume that the temperature in the furnace is constant and that the removal proceeds exclusively via diffusion through the graphite tube ends, the residence time can be expressed as: ra = 1*/8D

63)

Here t is the tube length and D is the diffusion coefficient. Substituting equation (8) into equation Q, we come to K = 1.84 x 10-‘3M,Dr2/mo12

(9)

SU

In this paper, the influences of atomization temperatures on the characteristic mass and the atomic absorption coefficient of Cr were studied. The method of standardless analysis was applied to the dete~nation of chromium in standard sediment and geochemical reference samples; satisfactory results were obtained. EXPERIMENTAL

(3)

here No is the total number of analyte atoms deposited in the furnace and rR is the average residence time of atoms in the optical path. We recall that No=m .iV,/M,

andXWGGUANG

Apparat~ A Hitachi 180-50 atomic absorption spectrometer with a GA-3 graphite furnace was used. Pyrolytically coated graphite tubes (made in China) and solid pyrolytic L’vov platforms and pyrolytic V-shaped boatsi (made in our laboratory). The size of the graphite tubes used in our experiments is r = 2.35 mm and I= 30 mm. The slit width was 1.3 nm. A Cr hollow cathode lamp (made in China) was used as a light source and operated at 10 mA. A deuterium arc background system was used throughout. Nz was used as the purge gas, due to its lower cost than Ar at a flow rate of 150 ml/mm and the purge gas was stopped during the atomization step. The 357.9 nm line of Cr was employed for all measurements. The absorption signals were recorded with an XWT164 (made in China) strip chart recorder. Sample solution was injected into graphite tube with a 20 ml Eppendorf micropipette. The temperature of the furnace was corrected using a MT-2 optical pyrometer (made in China). The graphite furnace operating parameters were as follows: dry 80-120”, 30 set; ashing 800”, 30 set; atomization 2700”, 7 set; cleaning 2800”, 3 sec. Reagents Chromium stock solution (1 mg/ml) was prepared by dissolving a suitable amount of K2Cr207 (analytical reagent grade) in quartz sub-boiling distilled water. The working standards were prepared by appropriate dilution with water prepared in a sub-boiling still. A 5%(w/v) solution of the ammonium salt of EDTA was prepared by dissolving 5.00 g of EDTA (analytical reagent grade) in sub-boiling distilled water and adding 15 ml of 25% aqua ammonia (analytical reagent grade) and then diluting to 100 ml with water. Procedure Standard sediment and geochemical reference samples (made in China) were used in this study. Transfer a weighted amount of the sample into

Determination of chromium

349

Table 1. The experimental values of m, and K for Cr at various atomization temperatures 10” x K (cm?

m, 0

V-shaped boat

Temp. CC)

Wall

Platform

V-shaped boat

2300 2200

4.9 4.3

5.9 ::;

:.9

2.23 1.82

1.07 1.36

:z

;z 2600 2700 2800

3.4 3.5 3.2

3.0 2.9 2.7 2.9

2.6 217 2.5 :::

3.22 2.87 3.55 4.26 4.47

2.46 1.81 2.76 3.19 4.10

299 3.25 3.65 3.90 4.19

a polyfluorotetraethylene crucible usually 0.142 g, carefully add 5 ml and 3 ml of concentrated nitric and perchloric acids, respectively. Heat on a sand bath until near dryness, again add 3 ml of perchloric acid, 5 ml of hydrofluoric acid and 0.5 ml of sulphuric acid and continuously heat to dryness. The residue dissolves in sub-boiling distilled water. Add 0.5 ml of 5% (w/v) ammonium salt of EDTA as a matrix modifier and make the final volume to 25 ml for the determination of chromium by GFAAS. RESULTS AND DISCUSSION

InfIuences of atomization and K

temperature on m,

There are parameters influencing the stability of standardless analysis by graphite furnace atomic absorption spectrometry in which the temperature of the atomization step is a main factor.i3 Frech and Baxter14 have studied the temperature dependence of atomization efficiencies of some elements in the graphite furnace. They pointed out that the atomization efficiency for most elements reaches a plateau, often in the higher temperatures region. We observed the influences of atomization temperatures on the experimental values of m, and K when

Wall

Platform

the sample was evaporated from the wall, using the platform and the V-shaped boat techniques. The results are listed in Table 1. As shown in Table 1, the influences of atomization temperatures on the experimental values of m. and K showed basically the same tendency when the samples were evaporated in three different ways. The value of m. decreased with an increase in the atomization temperature, and then reached a stable value after 2500”. The stability of m, values obtained by vaporizing of the three different ways appeared to be better than 10% in the range of 2500-2800”. However, the m, values are the smallest in the V-shaped boat furnace and the m, values in the platform furnace are less than that for the wall of the furnace at temperatures higher than 2600”. On the other hand, the value of K increases with increasing atomization temperature in the range of 2200-2800”. Standardless analysis of Cr It can be seen from Table 1 that the values of m, for Cr appeared to be stable to better than 10% when the analyte was atomized in the range of 2500-2800”. Therefore, we investigated the determination of Cr without standards. The determination of Cr in the standard sediment and geochemical reference samples by the three

Table 2. Standardless determination of Cr in samoles Calculating method of m, Sample* GSS-4 GSR3 GSD-2 GSD-8

Calculating method of K

Calibration curve method

Content,t &rig

RSD, %

Content,? &r/g

RSD, %

Content,? &rlg

RSD, %

Expected value I@lg

368.4 135.0 12.2 7.5

1.2 3.1 1.8 4.4

358.7 131.0 12.1 7.4

5.7 3.7 1.9 1.5

358.4 131.0 12.1 7.4

3.3 5.0 2.2 2.7

370 134 12.2 7.6

rns was 2.9 pgjO.0044 A.s K was 4.26 x 10-l’ cm’. *GSS-4 and GSR3 are Geochemical reference samples (China). GSD-2 and GSD-8 are drainage sediment samples (China). tValues are the mean of five measurements,

YANSHENG ZHENGand XINOGIJANG Su

350

methods with 2%(v/v) ammonium salt of EDTA as matrix modifier was carried out. The analytical results are listed in Table 2. They indicate that standardless analysis applied to determination of Cr in practical samples provides satisfactory results. But it must be emphasized that a frequent check of characteristic mass will be a key link which assures accurate analytical results and that use of a matrix modifier to eliminate the interferences will be a necessary condition for application of standardless analyses in a practical sample. REFERENCES 1. B. V. L’vov, Spectrochim. Acta, 1990, 4SB, 633.

2. B. V. L’vov, V. G. Nikolaev, E. A. Norman, L. K. Polzik and M. Mojica, ibid, 1986,4lB, 1043. 3. B. V. L’vov, ibid., 1978, 33B, 153. 4. C. S. Rann, ibid., 1968, 23B, 827. 5. A. Walsh, ibid,, 1955, 7, 108. 6. R. Wood%, Appl. Spectrosc., 1974, 28, 413. 7. W. Slavin, Graphite Furnace AAS. A Source Book. Perkin-Elmer Corp., Norwalk, 2nd Ed., 1991. 8. W. M. G. T. van den Broek and L. de Galan, Anal. Ckem., 1977, 49, 2176. 9. S. L. Paveri-Fontana, G. Tessari and G. Torsi, ibid., 1974, 46, 1032. 10. Y. S. Zheng, R. Woodtiand J. A. Nichols, ibid., 1984, 56, 1388. 11. Y. S. Zheng and Y. S. Liu, Fe& Ifuaxue 1987,15,162. 12. Y. S. Zheng and F. Zhu, Jilin Daxue Ziran K&we, 1987, 1, 103. 13. W. Slavin and G. R. Camrick, S’crrochim. Acta, 1984, 39B, 271. 14. W. Frech and D. C. Baxter, ibid., 1990, 4SB, 867.