научная статья по теме SIMULTANEOUS DETERMINATION OF PALLADIUM AND RHODIUM USING ON-LINE COLUMN ENRICHMENT AND ELECTROTHERMAL ATOMIC ABSORPTION SPECTROMETRIC DETECTION Химия

Текст научной статьи на тему «SIMULTANEOUS DETERMINATION OF PALLADIUM AND RHODIUM USING ON-LINE COLUMN ENRICHMENT AND ELECTROTHERMAL ATOMIC ABSORPTION SPECTROMETRIC DETECTION»

ЖУРНАЛ АНАЛИТИЧЕСКОЙ ХИМИИ, 2009, том 64, № 3, с. 257-261

^=ОРИГИНАЛЬНЫЕ СТАТЬИ

УДК 543

SIMULTANEOUS DETERMINATION OF PALLADIUM AND RHODIUM USING ON-LINE COLUMN ENRICHMENT AND ELECTROTHERMAL ATOMIC ABSORPTION SPECTROMETRIC DETECTION

© 2009 F. Sánchez Rojas, C. Bosch Ojeda, J. M. Cano Pavón

Department of Analytical Chemistry, Faculty of Sciences, University of Málaga

E-29071, Málaga, Spain Received 11.07.2007; in final form 04.12.2007

Flow injection on-line micro-column pre-concentration was coupled with electrothermal atomic absorption spectrometry (ETAAS) for the simultaneous determination of Rh and Pd in environmental samples. The microcolumn contained 1,5-Bis(2-pyridyl)-3-sulphophenyl methylene thiocarbonohydrazide (PSTH) immobilised on an anion-exchange resin (Dowex 1 x 8-200). The sorbent was tested in a micro-column, placed in the auto-sampler arm, at the flow rate 3.1 mL/min. Elution was performed with 2 M HNO3 and the system was optimised with a 60 s pre-concentration time.

Rhodium and palladium occur naturally with other platinum metals only at very low concentration. Rhodium and palladium are mainly used with platinum in automobile catalysts and in catalysts in the chemical industry.

The treatment of exhaust gases from motor vehicles equipped with catalytic converters has resulted in the removal of about 90% of carbon monoxide, unburned hydrocarbons, and nitrogen oxides from the exhaust [1]. The dust containing these metals is deposited along the roadways, on soil and vegetation, and in bodies of water. A clear link has been established between the increasing use of automobile catalysts and increasing environmental concentrations of precious metals [2-8]. A recent review summarises the literature on platinum group metals in the environment and their health risk [9].

During the first years of automobile catalyst impact research, the focus was on Pt as the main component of Pt/Rh-catalysts. Consequently, much effort was invested in the development of analytical methods for Pt possessing sufficient detection capacity. Pt levels of a variety of environmental matrices are already known. However, corresponding Rh and Pd data are mostly missing.

More often, the choice of the analytical technique depends on the availability and the level of occurrence of the metals and the nature of the sample matrix. For many years, atomic absorption spectrometry (AAS), both flame (FAAS) and electrothermal (ETAAS), has been widely used for the determination of PGEs in various materials. At present, AAS is the most widely used method for the determination of rhodium and palladium as long as the sample can be dissolved in acids. Separation, preconcentration and dissolution of samples are the vital steps in many procedures, due to very low concentration of these metals in many samples and complexity of the matrix. Among the aAs methods developed, the determinations for Pd and Rh are relative-

ly sensitive. Efforts have been made to make methods reliable and practical. Hence, the precision, accuracy and reproducibility of these methods should be carefully checked.

Despite the discontinuous character of the ETAAS technique, many efforts have been made to design procedures which imply on-line pre-concentration of ana-lyte prior to detection [10-18]. Solid phase extraction (SPE) has been demonstrated to be a very effective pre-concentration method. The SPE process can be carried out either in off-line or on-line mode. In latter case the sorption columns are mostly located at the injection loop or mounted on the tip of the auto-sampler arm.

In this work, on-line pre-concentration procedure was realized by the flow injection technique combined with ETAAS detection for the simultaneous determination of rhodium and palladium.

EXPERIMENTAL

Instrumentation. A Perkin-Elmer Zeeman/4100 ZL atomic absorption spectrometer equipped with an AS-70 furnace autosampler was used throughout. Pyrolytic graphite coated tubes with pyrolytic graphite platforms were used in all experiments. The light source was a palladium hollow cathode lamp operated at 25 mA; the selected wavelength was 247.6 nm with a spectral slit width of 0.7 nm and a rhodium hollow cathode lamp operated at 15 mA; the selected wavelength was 343.5 nm with a spectral slit width of 0.2 nm.

The graphite furnace temperature program for the determination of palladium and rhodium is shown in Table 1.

The microcolumn containing PSTH-Dowex was a glass tube (3 cm x 3 mm id) packed to a height of 0.5 cm; at both ends of the microcolumn, polyethylene frits were

Table 1. Graphite furnace temperature programs

Step Temperature (°C) Ramp time (s) Hold time (s) Argon flow rate (mL/min)

Pd Rh Pd Rh Pd Rh Pd and Rh

l 11C 11C l l 20 ЗС 250

2 1З0 14C 5 5 ЗС ЗС 250

З 9CC 17CC 1С 1С 20 20 250

4 22CC 24CC С С 5 5 0

5 24CC 24CC l l 2 2 250

fixed to prevent material loss. On the end of this column a piece of sample capillary of the sampler arm, in imitation of the sample tip of the sampler arm was placed. Thus the sample tip of the sampler arm was replaced with this mi-crocolumn, permitting normal working of the sampler.

A peristaltic pump, P (Gilson Minipuls 3), fitted with a vinyl pump tube (1.65 mm id), was used for loading the sample. A Rheodyne Type 50 six-port rotary valve was used as a switching valve. Transport lines were made using 0.8 mm id Teflon tubing. The peristaltic pump and the selection valve were readily controlled electronically via two switches on the autosam-pler that were actuated when the autosampler arm was down. The process was thus fully automated without altering the software of the AA spectrometer. For sample digestion, a microwave oven, Microdigest 301 controlled by Prolabo TX-32, was used. All glassware used was washed with 10 % nitric acid for 24 h and rinsed with de-ionised water immediately before use.

Reagents and standard solutions. Analytical reagent grade chemicals were used throughout. PSTH-

Table 2. Working conditions for microwave oven

Sample Step Reagent Volume (mL) Power (%) Time (min)

A* l HNO;, 1С l5 1С

2 H2O2 5 15 14

B* l HNO;i 1С l5 1С

2 HNO;! 1С ЗС 22

З H2O2 5 ЗС 5

C* l HCl l5 5С 5

HNO;i 5

2 - - ЗС 1С

* A: lentil, fish, rice, lettuce, chick-pea, apple; B: liver, vegetation; C: catalyst, SRM 2557.

Dowex was synthesised as described elsewhere [19]. A standard 1000 Mg/mL Pd(II) and rhodium(III) solutions (CertiPUR, Merck) were used. A pH 8.0 buffer was prepared by mixing 63.0 mL of 0.1 M boric acid with 6.2 mL of 0.1 M borax and diluting to 250 mL with deio-nised water. HNO3 (Merck), 2 M was used as eluent.

FI-enrichment/separation procedure. The FI

manifold is shown in Figure. It operated as follows: during the 1 min sample loading period, a 3.1 mL/min flow of sample (standard or blank) at pH 8.0, buffered with boric acid-borax, is pumped through the microcol-umn (located in the sampler arm); the metal ions, palladium and rhodium ,were adsorbed on the sorbent mi-crocolumn and the sample matrix is sent to waste; then, the switching valve is actuated and the pumps of the AS-70 furnace autosampler are connected, permitting the operation of the autosampler in the normal mode; a wash step takes place with de-ionised water and, immediately after, the sampler arm lowers the sample capillary into an autosampler cup (filled with eluent) aspirating 55 ^L of 2 M HNO3; then, the sampler arm swings over to the graphite furnace and the tip of the sampler capillary is inserted into the dosing hole of the graphite tube where eluted Rh(III) and Pd(II) were deposited as a drop; the sampler arm then returns to its initial position and the cycle of the furnace operation commences; while the temperature programme is running, the switching valve is again turned to start a new loading of the sample (standard or blank); thus, when the spectrometer gives the measurement, the microcolumn is ready for a new injection of eluent.

Preparation of samples. The certified reference material (CRM) analysed to determine the accuracy of the proposed procedure was National Institute of Standard and Technology (NIST), Standard Reference Material (SRM) 2557 catalyst. The sample was first prepared in accordance with the instruction of the analysis certificate, after which an accurately weighed amount of 0.1 g was subjected to microwave digestion. The working conditions of the microwave oven are listed in Table 2. After digestion, the sample was diluted to 100 mL with de-ionised water in a calibrated flask.

SIMULTANEOUS DETERMINATION OF PALLADIUM AND RHODIUM USINQ 259

ET-AAS

Figure. Schematic diagram of FI-ETAAS system for the preconcentration, separation and determination of palladium and rhodium. W1 and W2, waste; Vs, selection valve. For further detail see text.

As far as we know, CRM for palladium and rhodium in all the explored matrices are not available. In view of the application of the method to the determination of palladium and rhodium in vegetation and food samples, the ability to recover palladium and rhodium from different samples spiked with palladium and rhodium was investigated. For this purpose, standard solutions containing palladium and rhodium were added to 0.2-1.0 g of different solid samples and the resulting material was prepared by microwave digestion as is listed in Table 2. After digestion, the solutions obtained were evaporated to eliminate acid excess, neutralised and, finally, the samples were diluted to 25 ml with deionised water in a calibrated flask.

Tap water was collected immediately prior to the analysis.

RESULTS AND DISCUSSION

Experimental parameters. Optimum chemical parameters including sample acidity, ionic strength, FI variables (loading time, sample flow rate) and ET-AAS parameters have been separately obtained in previous works [20, 21] for palladium and rhodium, respectively. In this

study, with the aim of investigating the possibility of simultaneously determining Pd(II) and Rh(III) in mixtures,

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