SPECTROCHIMICA ACTA PART B

ELSEVIER Spectrochimica Acta Part B 50 (1995) 1595-1598

Chemical modification of graphite surfaces for the determination of chromium by electrothermal atomic

absorption spectrometry 1

Krystyna Pyrzyfiska Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland

Received 1 March 1995; accepted 30 June 1995

Abstract

Different kinds of graphite surfaces (electrographite, pyrolytic graphite, zirconium and tungsten car- bide-coated) have been tested for optimization of analytical conditions for the determination of chromium using electrothermal atomic absorption spectrometry. The effect of mineral acids on the peak absorbance signal of chromium has been investigated. Considering pyrolysis temperature and sensitivity, atomization from pyrolytic graphite coated surface showed the best performance.

Keywords: Chromium; Electrothermal atomic absorption spectrometry; Graphite surface; Mineral acid

1. Introduction

Electrothermal atomic absorption spectrometry (ETAAS) is probably the most popular tech- nique today for the determination of chromium in different samples [1-22]. In this method the nature of the atomizer surface is an important factor. The analyte and also the matrix can penetrate into the graphite structure. The slow volatilization of chromium from graphite atomiz- ers was observed due to formation of stable carbides [14,17]. The application of pyrolytic graphite or metal carbide-coated electrographite surfaces could reduce the diffusion of the analyte through the graphite wall and prevent the formation of refractory carbides.

In this paper, different atomization surfaces (electrographite (EG), pyrolitic graphite (PG), zirconium and tungsten carbide-coated electrographite) are investigated, comparing their per- formance for the determination of chromium by ETAAS.

2. Experimental

2.1. Reagents

Mineral acids and other chemicals used were of analytical-reagent grade. De-ionized water from a Millipore purification system was used for the preparation of the solutions.

~This paper has been published in the special issue of the East European Furnace Symposium, Warsaw, 4-7 September 1994.

0584-8547/95/$09.50 1995 Elsevier Science B.V. All rights reserved SSDI 0584-8547(95)01406-3

1596 K. Pyrzyhska/Spectrochimica Acta Part B 50 (1995) 1595-1598

Table 1 Graphite furnace temperature programme for the deter- mination of chromium by ETAAS

Step Temperature/ Ramp/ Hold/ C s s

I 100 10 30 2 900a/1200 b 20 15 3 2500 0 5 ~ 4 20 0 10

"Zr- and W-coated tube. b EG and PG tube. c Read at this step.

A 1000 mg 1-1 standard Cr(III) solution from Merck was used. Working standards were prepared by appropriate dilution of-the stock solution.

2.2. Apparatus

The experiments were carried out with a Beckman 1272 atomic absorption spectrometer equipped with a Pye Unicam GRM 1268 furnace. A Beckman chromium hollow cathode lamp was used at a current of 10 mA. The spectrometer utilized the chromium 357.9 nm line with a spectral bandwidth of 0.2 nm. Sample volumes of 20 Ixl were injected by a micropipette with disposable plastic tips. The deuterium-arc background correction was not used. Argon was used as the purge gas, with an internal flow rate of 300 ml min -1. Table 1 shows the temperature programme. Readings from the spectrometer were taken using the peak height mode.

Unfortunately, we only had available to us very old instruments that forced us to use wall atomization, peak absorbance measurements and no background correction. These conditions may make some of our results difficult to interpret for modern instruments.

Uncoated EG and PG coated tubes and also tungsten or zirconium carbide-coated graphite tubes were used. For coating, the tubes were soaked for 24 h at room temperature in 5% m/v zirconium nitrate or sodium tungstate solutions and then dried for 10 h. Before use, the tubes were conditioned in the ETA atomizer by heating to 1000C (ramp 10C s -l) and held for 30 s and then to 2500C (ramp 10C s -l) for 10 s. This heating procedure was repeated three times under the same conditions.

3. Results and discussion

3.1. Analytical performance

The pyrolysis and the atomization curves obtained for the graphite tube coatings under study are plotted in Fig. 1. When the pyrolysis temperature was varied, the atomization temperature

O.l

0.2

&l

Tempc~atm'~ C

Fig. 1. Pyrolysis and atomization curves for chromium (50 ~g 1 ~) using different graphite surfaces: (o) EG, (o) PG, (D) zirconium and () tungsten carbide-coated graphite.

K. Pyrzy6ska/Spectrochimica Acta Part B 50 (1995) 1595-1598

Table 2 Analytical performance data a for chromium using different atomization surfaces

1597

Characteristic mass/ Detection RSD/ pg limit/(la.g 1-~) %b

Electrographite 34.8 0.66 3.4 Pyrocoated graphite 9.7 0.32 1.8 W-coated graphite 16.6 0.45 3.0 Zr-coated graphite 15.9 0.48 2.7

"Values calculated from peak height measurements, b At the 50 p,g 1 -~ level (n = 8).

was set at 2500C. During the measurements with different atomization temperature, the pyrol- ysis temperature was set at 800C. The results obtained show that pyrolysis temperature of 900C and 1200C can be applied to tubes with and without metal salt coatings, respectively, without decrease of the peak absorbance signal of chromium. The slow decrease in pyrolysis curves for zirconium and tungsten carbide-coated surfaces could be attributed to a stepwise decrease of the chemical form of the analyte. The sensitivity (calculated from peak height measurements) was the highest for the pyrolytic graphite coated tube, in agreement with earlier reports [4,6,23]. The 2500C atomization temperature applied was the maximum available temperature with the GRM 1268 furnace we used.

The analytical performance data obtained for the determination of chromium using different surfaces of the graphite tube are presented in Table 2. The sensitivity of chromium determi- nation expressed in terms of characteristic mass (e.g. that mass of analyte which provides a peak absorbance of 0.0044) decreases in the order PG > W-carbide coated = Zr-carbide coated > EG surface (see Fig. 2). The improvement with the use of metal salt coatings relative to the uncoated electrographite tube can be explained by the formation of a zirconium or tungsten carbide layer over the graphite surface, which prevents chromium from coming into physical contact with the reactive surface. The values of the detection limit (Table 2) were calculated as the chromium concentration corresponding to three times the standard deviation of the blank solution (0.1% v/v hydrochloric acid). The highest precision in terms of relative standard deviation (RSD) was also obtained for the PG coated surface.

The lifetime of metal carbide-coated tubes was found to be twice as long in comparison to pyrocoated cuvettes [4,11]. Therefore, these tubes are recommended when the sensitivity is not a critical factor for a particular analysis. According to Hoenig et al. [9] the progressive

0.1

0.0 = .

Uncoated tube

0.5 1 13 2

I Zr-coated tube

1- o . _ __ , , , ,' 0.5 1 1.5 2

Pyrolytic graphite tube

&15

/ I I I I 0.5 1 1.5 2

I W-coated tube I 0.15L / o,r ,

/ , ~ e4 | 0.5 I 1.5 2

Concentration of acids, tool 'l'l

Fig. 2. Peak absorbance signals of chromium obtained for different atomization surfaces as a function of mineral acid concentration: 1, HCI; 2, HCIO4; 3, H2SO4; 4, HNO3.

1598 K. Pyrzytiska/Spectrochimica Acta Part B 50 (1995) 1595-1598

degeneration of the pyrolytic graphite layer appears to be a major factor affecting the determi- nation of chromium in complex matrices.

3.2. Influence of matrix composition

Mineral acids are often used to decompose and/or dissolve chromium samples and the inter- ference effects caused by various acids have been investigated. As seen in Fig. 2, increasing hydrochloric acid concentration results in a considerable signal enhancement in the case of Zr-coated tube. Perchloric and nitric acids decreased the peak absorbance signal of chromium, especially for EG tubes. This may be explained by the destruction of the tube surface in these oxidizing media. Sulphuric acid provides a definite enhancement effect above 1 mol 1-1 concentration when EG and PG tubes are used. This effect could be due to the activation of some carbon sites by H2SO 4. There are contradictory reports concerning the influence of nitric acid on the absorption signal of chromium. According to Ref. [10] this mineral acid causes an increase of chromium signal, while in Ref. [20] signal depressions were observed for this medium in most cases. In a more extensive work, Jackson and West [16] recommended 1% v/v nitric acid as the best medium for the measurement of chromium at low level by ETAAS.

4. Conclusions

Four different atomization surfaces (electrographite, pyrolytic graphite coated, zirconium and tungsten carbide-coated) have been tested and compared for the determination of chro- mium. The best analytical performance was obtained using atomization on a pyrocoated graph- ite tube. In the presence of perchloric and nitric acids, depressing effects on the peak absorbance signal of chromium were observed.

The useful lifetime of pyrolytic graphite coated tubes decreases more rapidly in a strong acidic medium compared to metal carbide-coated surfaces. Continuing studies are in progress to investigate the effects of the metal salt coatings when applied to a pyrolytic graphite surface.

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