INTRODUCTION Platinum, Palladium and Rhodium are members of the Platinum Group Elements (PGEs) and are used in large quantities in automobile catalysts. The PGEs are dispersed on a ceramic material; due to wear and abrasion these elements are co- emitted with the vehicle’s exhaust gases and deposited along roadsides [1]. The natural abundance of these rare elements is very low; traffic is found to greatly influence this background concentration. Besides mechanical transport (rain, wind), additional mobility by entering the food-chain is discussed [2]. In order to investigate PGE’s fate in the biosphere, the uptake potential of plants has to be evaluated. Quantification of PGEs in plant material is generally carried out by digesting the dried plant samples and measuring the resulting solution. One of the shortcomings of this procedure is the inevitable dilution and the W Nischkauer 1 , E Herincs 1,2 , A Limbeck 1 1 Vienna University of Technology, Institute of Chemical Technologies and Analytics, Getreidemarkt 9/164-IAC, 1060 Vienna, Austria 2 University of Natural Resources and Life Sciences, Department of Forest- and Soil Science, Institute of Soil Science, Peter Jordanstr. 82, 1190 Vienna, Austria ETV- ICP- OES PROCEDURE FOR THE DETERMINATION OF PLATINUM GROUP ELEMENTS IN PLANTS ETV- ICP- OES MEASUREMENTS Prior to analysis the graphite boats were cleaned by heating them for 20 seconds to 2250°C in an ETV 4000 graphite furnace (Spectral Systems, Germany). The plant material was then weight into the boats, put into a muffle furnace (Nabertherm LE4) and treated for three hours at 340°C in oxygen atmosphere (2.5 L/min) in order to remove organic carbon [3]. Aqueous standards were directly transferred into the clean boats and the solvent was slowly evaporated by means of an IR- Lamp. Matrix matched calibration was performed by mixing non- contaminated plant material with aqueous standards and subsequent drying. Calibration using “in-lab” reference material was carried out by increasing the tedious sample pre-treatment. Furthermore, rather large sample amounts are required for one digest which makes it difficult to measure small plants or to assess the spatial distribution of PGEs in one plant. In this work, a method for the direct measurement of PGEs in plant material is introduced. Owing to the electro-thermal vaporisation SPECTROMETER All measurements were carried out on a Thermo Scientific iCAP 6500 ICP- OES spectrometer equipped with radial view echelle optic. Three wavelengths were chosen according to their intensity and to the quality of their respective calibration curves (Pt 265.9nm, Pd 340.4nm, Rh 343.4nm). For detailed plasma parameters see table I. CONVENTIONAL ICP- OES MEASUREMENTS were performed by digesting 200mg of the samples with mineral acids Table I conventional measurements ETV measurements plasma power [W] 1400 1300 plasma gas flow rate [L/min] 12 12 radial viewing height [mm] 11 12 nebulizer flow rate [L/min] 0.7 0.52* auxiliary flow rate [L/min] 0.6 0.6 sample flow rate [mL/min] 0.8 - Freon modifier [mL/min] - 10 *sum of carrier gas and cooling gas METHOD DEVEOLOPMENT By analyzing aqueous standard solutions the temperature program was optimized. The application of a gaseous modifier (Freon R12) was found to significantly increase both the signal intensity and the reproducibility. Analysis of increasing sample intakes of contaminated plant material (the “in- lab” reference) showed a linear signal to concentration behaviour for sample weights ranging from 1.5mg to 12mg; vaporisation, sample pre- treatment is reduced to a minimum. The influence of modifiers and sample amount on the signal is investigated. Quantification is carried out and the results are compared with results of conventional ICP- OES measurements requiring sample digestion. reference material was carried out by increasing the sample intake. Prepared samples were analysed using a vaporization temperature of 2250°C. Derived products were transferred to the plasma using an argon stream. Emission signals were recorded in transient signal mode (intensity vs. time). PLANT EXPERIMENTS Greenhouse experiments were performed in hydroponic set-up using four weeks raised plants of Brassica Napus Californium. The plants were subsequently contaminated with PGEs for the time span of four weeks. After harvesting, the plants were washed, dried and ground using a Retsch MM400 mixer mill yielding a fine powder. performed by digesting 200mg of the samples with mineral acids and H 2 O 2 in a microwave (Multiwave 3000, Anton Paar) and diluting the solution with water to a final volume of 25mL. Sample introduction into the plasma was done via an APEX- E nebulizer. METHOD COMPARISON Quantification of the plant material shows that the ETV- method allows for excellent reproducibility which is comparable to the conventional ICP- OES method. See table II for results of analyzing one sample with both methods. Obviously, the ETV- method overestimates both the Pt and the Rh content. Comparison of signal vs. concentration plots show pronounced matrix effects (see graphs). The slope of the matrix matched calibration curve is significantly steeper than the slope received with the aqueous calibration. When increasing the sample weight of a contaminated plant sample whose PGE content is known via the conventional ICP- OES method (“in-lab” reference material), the slope is again higher. These observations lead to the conclusion that in the presence of sample matrix the vaporisation and/or the transport efficiency of PGEs into the plasma is enhanced. Table II conventional measurements (n = 6) ETV measurements (n = 3) [ng/mg] [ng/mg] Palladium 0.61 ± 0.14 (23.8%) 1.34 ± 0.12 (8.8%) Platinum 27.41 ± 0.64 (2.3%) 40.36 ± 1.30 (3.1%) Rhodium 7.68 ± 0.31 (4.1%) 11.52 ± 0.72 (6.2%) OUTLOOK In order to enable accurate determination , aqueous calibrations will be corrected by means of in-lab reference materials. As the matrix effect is probably due to organic carbon, the ashing process (duration and temperature) is going to be revisited. ACKNOWLEDGEMENT The authors thankfully acknowledge the financial support from the Austrian Science Fund - FWF (Project Nr.: P20838-N17) REFERENCES [1] F. Zereini, C. Wiseman and W. Püttmann, Changes in Pd, Pt, and Rh Concentrations, and Their Spatial Distribution in Soils Along a Major Highway in Germany from 1994 to 2004, Environ. Sci. Technol. 41 (2007) 451-456 [2] F. Vanhaecke, M. Resano, M. Pruneda-Lopez and L.Moens, Determination of Platinum and Rhodium in Environmental Matrixes by Solid Sampling-ETV-ICP-MS, Anal. Chem. 74 (2002) 6040-6048 [3] A. Detcheva, P. Barth, J.Hassler, Calibration possibilities and modifier use in ETV ICP OES determination of trace and minor elements in plant materials, Anal. Bioanal. Chem. 394 (2009) 1485–149 The authors thankfully acknowledge the graphical feed-back from A. Nischkauer and C. Puls. weights ranging from 1.5mg to 12mg; hence the standard sample amount for one ETV- measurement was set to 5mg.
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INTRODUCTION Platinum, Palladium and Rhodium aremembers of the Platinum Group Elements (PGEs) and are used inlarge quantities in automobile catalysts. The PGEs are dispersed ona ceramic material; due to wear and abrasion these elements are co-emitted with the vehicle’s exhaust gases and deposited alongroadsides[1]. The natural abundance of these rare elements is verylow; traffic is found to greatly influence this backgroundconcentration. Besides mechanical transport (rain, wind), additionalmobility by entering the food-chain is discussed[2].
In order to investigate PGE’s fate in the biosphere, the uptakepotential of plants has to be evaluated. Quantification of PGEs inplant material is generally carried out by digesting the dried plantsamples and measuring the resulting solution. One of theshortcomings of this procedure is the inevitable dilution and the
W Nischkauer1, E Herincs1,2, A Limbeck1
1 Vienna University of Technology, Institute of Chemical Technologies and Analytics, Getreidemarkt 9/164-IAC, 1060 Vienna, Austria2 University of Natural Resources and Life Sciences, Department of Forest- and Soil Science, Institute of Soil Science, Peter Jordanstr. 82, 1190 Vienna, Austria
ETV- ICP- OES PROCEDURE FOR THE DETERMINATION OF
PLATINUM GROUP ELEMENTS IN PLANTS
ETV- ICP- OES MEASUREMENTS Prior toanalysis the graphite boats were cleaned by heatingthem for 20 seconds to 2250°C in an ETV 4000graphite furnace (Spectral Systems, Germany). Theplant material was then weight into the boats, putinto a muffle furnace (Nabertherm LE4) and treatedfor three hours at 340°C in oxygen atmosphere (2.5L/min) in order to remove organic carbon[3].Aqueous standards were directly transferred into theclean boats and the solvent was slowly evaporatedby means of an IR- Lamp. Matrix matchedcalibration was performed by mixing non-contaminated plant material with aqueous standardsand subsequent drying. Calibration using “in-lab”referencematerialwascarriedout by increasingthetedious sample pre-treatment. Furthermore, rather large sample
amounts are required for one digest which makes it difficult tomeasure small plants or to assess the spatial distribution of PGEs inone plant.
In this work, a method for the direct measurement of PGEs inplant material is introduced. Owing to the electro-thermalvaporisation
SPECTROMETER All measurements were carried out on aThermo Scientific iCAP 6500 ICP- OES spectrometer equippedwith radial view echelle optic. Three wavelengths were chosenaccording to their intensity and to the quality of their respectivecalibration curves (Pt 265.9nm, Pd 340.4nm, Rh 343.4nm). Fordetailed plasma parameters seetable I.
CONVENTIONAL ICP- OES MEASUREMENTS wereperformedby digesting200mg of the sampleswith mineral acids
Table I conventionalmeasurements
ETV measurements
plasma power [W] 1400 1300
plasma gas flow rate
[L/min] 12 12
radial viewingheight
[mm] 11 12
nebulizer flowrate
[L/min] 0.7 0.52*
auxiliary flowrate
[L/min] 0.6 0.6
sample flowrate
[mL/min] 0.8 -
Freon modifier
[mL/min] - 10
*sum of carrier gas and cooling gas
METHOD DEVEOLOPMENT Byanalyzing aqueous standard solutionsthe temperature program was optimized.The application of a gaseous modifier(Freon R12) was found to significantlyincrease both the signal intensity and thereproducibility.Analysis of increasing sample intakes ofcontaminated plant material (the “in-lab” reference) showed a linear signal toconcentration behaviour for sampleweights ranging from 1.5mg to 12mg;
vaporisation, sample pre-treatment is reduced to aminimum. The influence ofmodifiers and sample amounton the signal is investigated.Quantification is carried outand the results are comparedwith results of conventionalICP- OES measurementsrequiring sample digestion.
referencematerialwascarriedout by increasingthesample intake. Prepared samples were analysedusing a vaporization temperature of 2250°C. Derivedproducts were transferred to the plasma using anargon stream. Emission signals were recorded intransient signal mode (intensity vs. time).
PLANT EXPERIMENTS
Greenhouse experiments wereperformed in hydroponic set-up using four weeks raised plants ofBrassica Napus Californium. The plants were subsequentlycontaminated with PGEs for the time span of four weeks. Afterharvesting, the plants were washed, dried and ground using aRetsch MM400 mixer mill yielding a fine powder.
performedby digesting200mg of the sampleswith mineral acidsand H2O2 in a microwave (Multiwave 3000, Anton Paar) anddiluting the solution with water to a final volume of 25mL. Sampleintroduction into the plasma was done via an APEX- E nebulizer.
METHOD COMPARISON Quantification of the plant materialshows that the ETV- method allows for excellent reproducibilitywhich is comparable to the conventional ICP- OES method. Seetable II for results of analyzing one sample with both methods.
Obviously, the ETV- method overestimates both the Pt and the Rhcontent. Comparison of signal vs. concentration plots showpronounced matrix effects (see graphs). The slope of the matrixmatched calibration curve is significantly steeper than the slopereceived with the aqueous calibration. When increasing the sampleweight of a contaminated plant sample whose PGE content isknown via the conventional ICP- OES method (“in-lab” referencematerial), the slope is again higher. These observations lead to theconclusion that in the presence of sample matrix the vaporisationand/or the transport efficiency of PGEs into the plasma is enhanced.
Table IIconventionalmeasurements
(n = 6)
ETV measurements
(n = 3)
[ng/mg] [ng/mg]
Palladium 0.61 ± 0.14 (23.8%) 1.34 ± 0.12 (8.8%)
Platinum 27.41 ± 0.64 (2.3%) 40.36 ± 1.30 (3.1%)
Rhodium 7.68 ± 0.31 (4.1%) 11.52 ± 0.72 (6.2%)
OUTLOOK
� In order to enable accurate determination , aqueous calibrations will be corrected by means of in-lab reference materials.
�As the matrix effect is probably due to organic carbon, the ashing process (duration and temperature) is going to be revisited.
ACKNOWLEDGEMENT The authorsthankfully acknowledge the financialsupport from the Austrian Science Fund -FWF (Project Nr.: P20838-N17)
REFERENCES [1] F. Zereini, C. Wiseman and W. Püttmann, Changes in Pd, Pt, and Rh Concentrations, and Their Spatial Distribution in Soils Along a Major Highway in Germany from 1994 to 2004, Environ. Sci. Technol. 41 (2007) 451-456[2] F. Vanhaecke, M. Resano, M. Pruneda-Lopez and L.Moens, Determination of Platinum and Rhodium in Environmental Matrixes by Solid Sampling-ETV-ICP-MS, Anal. Chem. 74 (2002) 6040-6048[3] A. Detcheva, P. Barth, J.Hassler, Calibration possibilities and modifier use in ETV ICP OES determination of trace and minor elements in plant materials, Anal. Bioanal. Chem. 394 (2009) 1485–149
The authors thankfully acknowledge the graphical feed-back from A. Nischkauer and C. Puls.
weights ranging from 1.5mg to 12mg;hence the standard sample amount forone ETV- measurement was set to 5mg.
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