Determination of platinum by graphite furnace atomic absorption ...

Food Control 17 (2006) 365–369 www.elsevier.com/locate/foodcont

Determination of platinum by graphite furnace atomic absorption spectrometry in foods and beverages using an automated on-line separation-preconcentration system C. Bosch Ojeda, F. Sańchez Rojas *, J.M. Cano Pavoń Department of Analytical Chemistry, Faculty of Sciences, University of Ma´laga, E-29071 Ma´laga, Spain Received 17 October 2004; received in revised form 9 January 2005; accepted 10 January 2005

Abstract The increasing use of automobile catalysts leads to the emission of the platinum-group elements (PGE), mainly platinum, but also rhodium and palladium into the environment. In environmental samples, the low concentration of platinum together with the high concentration of interfering matrix components often requires a preconcentration/enrichment step combined with a matrix-separation. The method employs on-line preconcentration of platinum on a column packed with silica gel functionalised with 1,5-bis(di-2-pyridyl)methylene thiocarbohydrazide (DPTH-gel) placed in the autosampler arm. The method has been applied to the determination of platinum in food and beverage samples with 93–110% recoveries in the range of 0.5–50 mg/kg for a range of spiked foods and 100–102% for spikes of 4 ng/ml for beverages. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Platinum; Food; Beverages; Graphite furnace atomic absorption spectrometry

1. Introduction Increasing Pt concentrations from vehicle catalyst have been reported from a number of countries. The question then arises of whether an increase in Pt concentrations in urban dusts and sediments poses a threat to human health through the inhalation of fine dusts. The health and safety executive lists: watering of the eyes, sneezing, tightness of the chest, wheezing, breathlessness, cough, eczematous and urticarial skin lesions, signs of mucous membrane inflammation as typical of Pt salt sensitisation. Metallic Pt appears to be biologically inert and nonallergenic. Platinum allergy is confined to a small group of charged compounds that contain reactive ligand systems, the most effective of which are chloride ligands. *

Corresponding author. E-mail address: [email protected] (F.S. Rojas).

0956-7135/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodcont.2005.01.002

The allergic response increases with increasing number of chlorine atoms, the most potent compounds being hexachloroplatinic(IV) acid and its ammonium and potassium salts, and potassium and sodium tetrachloroplatinate(II). Non halogenated complexes and neutral complexes including the anticancer drug cisplatin, [Pt(Cl)2(NH3)2] are not allergenic. While occupational exposure to Pt compounds has been monitored intensively over recent years, there is little information on general population exposure and health effects. It has been shown that Pt in road dusts has increased after the introduction of vehicle catalyst. Metallic Pt is considered non allergenic and since the emitted Pt is probably in the metallic or oxide form, the sensitizing potential is probably very low. Platinum from road dusts, however, can be solubilised, and enter waters, sediments, soils and the food chain (Hees et al., 1998). The fact that platinum might have entered the food chain was the reason for the rapid and detailed evaluation of

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the platinum content in various environmental samples. Wine serves as an example for the following the path of platinum, beginning with the uptake during the plant growth, continuing with the fermentation process of grape juice and resulting in the final product (Alt, Eschnauer, Mergler, Messerschmidt, & To¨lg, 1997). The analysis of samples of tap water, rose´ wine in tetrabrik package, bottled white wine, canned lemon tea, camomile in filter bags, peach fruit juice UHT semiskimmed milk, wild chicory, beer and lettuce were also reported (Desimoni, Brunetti, & Bacchella, 2002, 2004). Improvements in detection power, reliability, precision, selectivity and economy of analytical methods are still a challenge to the analyst. The determination of trace amounts of essential and toxic elements at very low concentration in various biological and environmental materials are examples of problems whose solution requires improved analytical procedures. Research has been carried out on developing reliable analytical methods for accurate determinations of traces of platinum group metals in environmental samples. Most of the attention has been focused on Pt determination. Accurate determinations of low levels of Pt in environmental samples have been possible using instrumental methods which possess good sensitivities. Electrothermal atomic absorption spectrometry (ETAAS) is a well-established technique with excellent sensitivity, and the equipment is available in many laboratories. Quantitative transformation of the platinum into suitable complexes, isolation from the interfering matrix and preconcentration up to the level detectable are generally required. Among the numerous techniques reported for the preconcentration and separation of platinum the methods using ion exchange resins or sorbent extraction have proved to be especially effective. Recently, new functional resins with chelating properties, prepared by simple immobilization of organic complexing reagents on different solid support, have gained considerable attention. For preconcentration of platinum Separon SGX C18 in the presence of cationic surfactants was proposed (Otruba, Strnadova, & Skalnikova, 1993), Amberlite XAD-7 resin coated with dimethylglyoxal bis(4phenyl-3-thiosemicarbazone) was also studied (Hoshi et al., 1997), platinum was preconcentrated with bis(carboxyl-methyl)dithiocarbamate on Amberlite XAD-4 (Lee, To¨lg, Beinrohr, & Tscho¨pel, 1993) and also using pyrrolidinedithiocarbamate on C18-bonded silica gel (Shah & Wei, 1989). The analytical performance of platinum collection using activated alumina was discussed by Cantarero, Gomez, Camara, and Palacios (1994). A flow-injection technique with on-line sample preconcentration was developed in an attempt not only to enhance sensitivity but also to decrease analysis time. Analytical methodologies for the quantification of platinum in

various biological and environmental samples are critically reviewed in two papers (Balcerzak, 1997; Barefoot, 1997). In the present work, an automatic on-line FI-ETAAS method for the determination of trace amounts is used. A chelating ion-exchange resin was employed for the separation and preconcentration of platinum from different food and beverage samples.

2. Experimental 2.1. Reagents, instrumentation and procedure Reagents, instrumentation and procedure for the determination of platinum has been described previously (Bosch Ojeda, Sańchez Rojas, Cano Pavoń, & Garcia de Torres, 2003): during the 1 min sample loading period, a 2.4 ml/min flow of sample (standard or blank) at pH 5.0, buffered with acetic acid–sodium acetate, is pumped through the microcolumn (located in the sampler arm); the metal ion [Pt(II) or Pt(IV)] is adsorbed on the sorbent microcolumn 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 deionised water and, immediately after, the sampler arm lowers the sample capillary into an autosampler cup (filled with eluent) aspirating 45 ll of 4 M HCl + 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 the eluted Pt(II) or Pt(IV) is 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. 2.2. Sample preparation The general problems in the determination of platinum group elements, irrespective of the analytical method used, are the low concentrations and the lack of certified reference materials (CRM) for quality control. The majority of available reference materials are different types of minerals, ores, etc. (such as SARM-7), and recycled monolith autocatalyst (NISTSRM-2557) or car exhaust catalyst material (IMEP 11), where the concentrations of PGE are considerably higher than in environmental materials and the matrix composition and forms of PGE are very different (Schramel et al., 2000).


3. Results Optimum chemical parameters including sample acidity, and ionic strength, FI variables (loading time, sample flow rate) and ETAAS parameters has been obtained in a previous work (Bosch Ojeda et al., 2003): The optimum pH range was around 3.6–5.6; all subsequent studies were carried out at pH 5.0. The signal value remains constant for buffer concentration equal or greater than 0.1 M; a concentration of 0.2 M buffer was used for subsequent experiments. The effect of sample loading time on the absorption signal of 2 ng/ml Pt was tested; the signal increased almost linearly up to 7 min preconcentration time, after which the slope decreased gradually. Sensitivity enhancements gained by increasing the sample loading time, however, the loading time selected in the experiment was 60 s in order to achieve high sampling frequency with a reasonable degree of sensitivity. Loading time may be higher for samples with low concentrations of platinum. Changes in the flow rate of the sample were studied between 1.6 and 4.7 ml/min, resulting in an optimum sample flow rate of 2.4 ml/ min with the best signal-to-blank ratio. The optimum graphite operating conditions used are summarised in Table 1. It is known that strong acids are effective in dissociating complexes and releasing free metal ions. For this purpose, HNO3, HCl, HNO3 + HCl mixture, at different

Table 1 Graphite furnace temperature programme (Vi = 45 ll) Step

Temperature (°C)

Ramp time (s)

Hold time (s)

Argon flow rate (ml/min)

1 2 3 4 5

110 130 1600 2200 2400

1 4 5 0 1

40 30 20 5 2

250 250 250 0 250

Table 2 Working conditions for microwave oven Step

1 2 3

Reagent

Volume (ml)

Power (%)

Time (min)

Aa

Ba

A

B

A

B

A

B

HNO3 H2O2 –

HNO3 HNO3 H2O2

10 5 –

10 10 5

15 15 –

15 30 30

10 14 –

10 22 5

a A: rice, lentil, macaroni, red wine, beer, fish; B: lettuce, chick-pea, milk, orange juice, bovine liver.

concentrations was examined; the results obtained were shown in Fig. 1. In this study a 4 M HCl + HNO3 solution was chosen as eluent. The influence of the volume of eluent used has also been studied. The signal increased as the volume increased up to 40 ll, and then remained constant with further increase in the volume of eluent. An injection volume of eluent of 45 ll was fixed. In this conditions the signal obtained for Pt(II) and Pt(IV) is the same. The characteristic performance data of the FI-ETAAS system for platinum determination are presented in Table 3. The sensitivity was increased by increasing the sample loading time; a loading time of 60 s was selected, in order to fit this time into the cycle of the furnace program, thus good sensitivity was achieved with high sampling frequency. Nevertheless, for samples containing low concentrations of platinum longer loading times can be employed. The resin located in the microcolumn has a high capacity; this enables further reduction of the concentration limit by increasing the sample loading time.

0.1 Nitric acid

Absorbance

Procedures applied to PGE determination can be broadly divided into three stages: the sample decomposition, preconcentration stage and the analyte determination stage. Acid dissolution procedures are extensively used for PGE determination. Mixtures of HCl, HF, HClO4, HNO3, HBr, H2O2, etc., are used in either open or closed vessels. Digestion on hot plates or by the microwave technique is popular. Additional separation and preconcentration steps are needed prior to the determination of the analyte. The acid digestion procedures offer advantages such as low cost and low blanks, and these methods are often preferred for biological and environmental samples. As far as we know, CRM platinum in all the explored matrices are not available. In view of the application of the method to the determination of platinum in foods and beverages, the ability to recover platinum from different samples spiked with platinum was investigated. For this purpose, standard solutions containing platinum were added to 0.2–0.7 g of different food samples or 5 ml of beverage samples and the resulting material was prepared by microwave digestion as are listed in Table 2. Tap water is collected immediately prior to the analysis.

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Hydrochloric acid

0.05

Nitric acid + Hydrochloric acid

0 0

2

4

[eluent] M Fig. 1. Influence eluent concentrations.

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Table 3 Performance of the FI-ETAAS system for platinum determination under the optimum conditions Analytical parameters

Peak-height

Peak-area

Working concentration range (ng/ml) Calibration function (CPt in ng/ml) Correlation coefficient Detection limit (ng/ml) Determination limit (ng/ml) Precision (% RSD, n = 11, CPt = 4 ng/ml) Sampling frequency (1/h) Enrichment factor Concentration efficiency (1/min) Consumptive index (ml)

0–20

0–20

A = 0.018CPt + 0.0263 0.9931 1.0a 2.3a 1.0

A = 0.0292CPt + 0.0277 0.9973 0.8a 1.8a 1.6

29 25.7 12.4 0.09

29 41.7 20.1 0.06

a

With 1 min of loading time.

Table 4 Results for platinum determination in food and beverage samples Sample

Added (mg/kg)

Found (mg/kg)a

Recovery (%)

Rice

50 5 0.5 50 5 0.5 50 5 0.5 20 1 10 1 20 1 20 1

50.0 ± 0.7 4.9 ± 0.2 0.47 ± 0.10 48.6 ± 3.3 4.8 ± 0.1 0.47 ± 0.12 50.4 ± 0.7 5.1 ± 0.8 0.52 ± 0.09 19.6 ± 1.0 0.93 ± 0.20 10.3 ± 0.3 0.98 ± 0.15 21.6 ± 0.3 1.07 ± 0.10 21.9 ± 0.9 0.97 ± 0.08

100.0 98.0 94.0 97.2 96.0 94.0 100.8 102.0 104.0 98.0 93.0 103.0 98.0 108.0 107.0 109.5 97.0

Added (ng/ml)

Found (ng/ml)a

Lentil

Lettuce

Chick-pea Macaroni Fish Bovine liver

3.1. Sample analysis As far as we know, no certified reference material exists containing platinum at concentration levels of the order of the samples under study. This explains why the same matrices were used as control materials. In view of the application of the method to the determination of platinum in food and beverage samples, the ability to recover platinum from different samples spiked with platinum was investigated. All samples were arbitrarily selected and acquired from a local superstore. In a previous study, the tolerance limits found show that platinum can be determined in the presence of a variety of species. For this purpose, standard solutions containing different quantities of platinum were added to samples and the resulting material was prepared as described under Experimental. Standard additions method was used in all instances and the results were obtained by extrapolation. The results of these analyses are summarised in Table 4, and indicated excellent recoveries in all instances.

4. Conclusions The low concentration of Pt in environmental and biological samples demands appropriate methods for their separation and preconcentration, prior to the analytical determination. FI-on-line column preconcentration-ETAAS has revolutionised trace element analysis in samples with complicated matrices. The system proposed in this paper has the advantage of being simpler than other FI-ETAAS because the process is fully automated without complicated hardware and software; in fact modification of the software of the spectrometer was not necessary. The use of expensive and sophisticated instruments is also avoided. High speed, ease of use

Tap water Milk Orange juice Red wine Beer

4 4 4 4 4

4.0 ± 0.2 4.0 ± 0.1 4.1 ± 0.8 4.1 ± 0.2 4.0 ± 0.3

100.0 100.0 102.5 102.5 100.0

a Means ± standard deviation for three replicate measurements of four individual preparations.

and automation, selectivity and relative freedom from interference make this method suitable for platinum determination in different food and beverage samples. Acknowledgements The authors thank the Ministerio de Ciencia y Tecnologıá for supporting this study (Project BQU200302112) and also the Junta de Andalucia. References Alt, F., Eschnauer, H. R., Mergler, B., Messerschmidt, J., & To¨lg, G. (1997). A contribution to the ecology and enology of platinum. Fresenius Journal of Analytical Chemistry, 357, 1013–1019. Balcerzak, M. (1997). Analytical methods for the determination of platinum in biological and environmental materials. Analyst, 122, 67R–74R. Barefoot, R. R. (1997). Determination of platinum at trace levels in environmental and biological materials. Environmental Science and Technology, 31, 309–314. Bosch Ojeda, C., Sańchez Rojas, F., Cano Pavoń, J. M., & Garcia de Torres, A. (2003). Automated on-line separation-preconcentration system for platinum determination by electrothermal atomic absorption spectrometry. Analytica Chimica Acta, 494, 97–103. Cantarero, A., Gomez, M. M., Camara, C., & Palacios, M. A. (1994). Online preconcentration and determination of trace platinum by flow-injection atomic-absorption spectrometry. Analytica Chimica Acta, 296, 205–211. Desimoni, E., Brunetti, B., & Bacchella, R. (2002). Cathodic stripping voltammetric determination of platinum in some foods and

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