ICP slurry introduction for simple and rapid ... - Springer Link

DOI: 10.2478/s11532-007-0037-5 Research article CEJC 5(4) 2007 1148–1157

ICP slurry introduction for simple and rapid determination of Pb, Mg and Ca in plant roots Danuta Baralkiewicz∗, Anetta Kanecka-Hanc, Hanka Gramowska Department of Trace Element Analysis by Spectroscopic Methods, Faculty of Chemistry, Adam Mickiewicz University, 60-780 Pozna´ n, Poland

Received 01 March 2007; accepted 11 June 2007 Abstract: Pb, Mg and Ca were simultaneously determined in plant roots by slurry introduction into inductively coupled plasma optical emission spectrometry (SS-ICP-OES). Slurries were prepared in 0.5% or 5% (v/v) HNO3 with 0.5, or 5% (v/v) Triton X-100. Omission of the Triton X-100 improved results. Compared with wet ashing of the root sample followed by ICP-OES, ICP-MS and FAAS, the method offers: comparable results, simplification of sample preparation, less sample contamination, and reduction in the use of dangerous and corrosive reagents. The precisions varied: 1.7% for Mg, 2.8% for Ca and 4.3% for Pb, and were not significantly different (95% confidence level) from those of conventional analysis. c Versita Warsaw and Springer-Verlag Berlin Heidelberg. All rights reserved. Keywords: Inductively coupled plasma optical emission spectrometry (ICP-OES), slurry sampling, plant root analysis, lead, magnesium, calcium

1

Introduction

Because most atomic spectrometric methods require samples in solution, solids require digestion. Biological and environmental samples consist mainly of organic matter with mineral components, which are normally digested at as high a temperature as possible [1]. Slurry sampling is an attractive alternative to classical digestion methods because dissolution is so time consuming that sample preparation often requires more time than analysis. Combination with a multielement analytical technique, such as inductively coupled plasma optical emission spectrometry (ICP-OES) can make an efficient procedure. Slurry sampling is well-established for direct determination of trace metals in solids (SSICP-OES). It has been widely applied to inorganic materials [2–4], sediments [5, 6], food ∗

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D. Baralkiewicz et al. / Central European Journal of Chemistry 5(4) 2007 1148–1157

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samples [7–9] and plant materials [10–14]. Multielement analysis of plant materials using SS-ICP-OES are: Al, Ba, Ca, Mg, Mn and Zn [10–14]; Ca, Mg and Mn [13]; Ba, Cu, Fe, Pb [11]. The direct introduction of a slurry into ICP-OES reduces sample preparation time by combining matrix destruction, atomization, and excitation into a single step. It also reduces sample contamination, minimizes analyte losses during preparation or due to incomplete release from the matrix, and reduces the use of hazardous or corrosive reagents [9]. Analytical plasmas are generally higher temperature sources than combustion flames, a prerequisite if complex solid matrices are to be efficiently atomized, ionized and excited. However, the analysis of slurries by ICP-OES also presents some difficulties. Slurry concentration is important. Jarvis recommended a total suspended and dissolved solid content of < 2000 μg mL−1 [15]. Krejcowa et al. estimated that at above 1.5% sample inhomogeneity becomes detrimental to precision and accuracy [16]. The slurry particles must be < 20 μm to guarantee efficient nebulization and atomization-excitation in the plasma [17]. Larger particles do not reach the plasma and cause signal loss. Slurry stability is crucial for efficient and reproducible nebulization [18], so stabilizers (e.g. Triton X-100, glycerol) are sometimes added [4, 16, 19]. Precision is highly dependent on sample homogeneity and analyte partitioning between the solid and liquid phases [6]. Hydrochloric acid [4, 14, 16, 19] and various nitric acid concentrations [16, 20] have been used. Good extraction of the analyte is achieved with ultrasonic agitation in nitric acid, ensuring better representativeness of the slurry [21]. Mokgalaka [10] prepared slurries in HNO3 and Triton X-100. The dependence of plant Pb tolerance on Ca and Mg is well known; the harmful effects of Pb on plants are ameliorated as soil Ca concentration increases [22, 23]. In our previous work we prepared the sample as a slurry and applied ETAAS to determine lead in rape leaves. However, the amount of lead in rape roots is too high for direct determination by ETAAS with slurry introduction. Therefore in this work we develop SS-ICP-OES as a simple and fast simultaneous method to determine lead, calcium and magnesium in roots. We investigate the effects of acid concentration, Triton X-100, and slurry concentration. Roots were then analyzed using the SS-ICP-OES method developed.

2

Experimental

2.1 Instrumentation All ICP measurements were carried out using a Varian (Australia) Vista-MPX, CCD Simultaneous ICP-OES spectrometer. The sample introduction system consists of a slurry nebulizer and cyclonic spray chamber. An ultrasonic processor (Sanopuls, Germany) with a 3 mm titanium probe provided automatic agitation of the slurry. Operating parameters and selected analytical lines are listed in Table 1. The results were compared with those obtained by microwave digestion followed by ICP-OES and FAAS in our laboratory, as well as ICP-MS by an independent laboratory. A Varian Spectra AA Plus flame atomic

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absorption spectrometer and an Agilent (UK) 7500ce with Octopole, Reaction System ICP-MS spectrometer were used. Table 1 Instrumental operating conditions. Spectrometer

Varian, Vista-MPX CCD Simultaneous ICP-OES

Power (W) Nebulizer Plasma argon gas flow rate (l min−1 ) Auxiliary argon gas flow rate (l min−1 ) Nebulizer argon gas flow rate (l min−1 ) Nebulizer pump (rps) Analytical wavelength (nm)

1200 Slurry Nebulizer (Varian) 15 1.5 0.9 0.10 Pb 220,353 Mg 285,213 Ca 317,933

2.2 Plant root preparation Pisum Sativum L. seeds were soaked in water for 4 h and germinated in the dark at 24 ◦ C for 3 days. Seedlings were cultivated hydroponically in Hoagland medium supplemented with 1 mM Pb(NO3 )2 . Roots were sampled 96 h after the application of lead ions, rinsed and then frozen in liquid nitrogen. Samples were dried to constant weight at 100 ◦ C in a conventional drying oven, thoroughly ground in a mechanical mortar (0.5 h) and passed through a 30 μm sieve. The resulting powder was kept in tightly closed plastic containers until analysis.

2.3 Reagents and standards All reagents were prepared from analytical grade chemicals. HNO3 and H2 O2 were supplied by Merck. Triton X-100 (Merck) was of suprapure grade. All solutions were prepared in distilled water passed through a Milli-Q (Millipore, USA) purification system. Calibration curves were established by multielement aqueous standards (1.0; 5.0; 10.0; 20.0; 35.0 and 50.0 mg L−1 for Pb, Ca and Mg), prepared by dilution of 1000 mg L−1 (Merck, Germany) aqueous standards. The blank and standards contained the same amount of dispersant.

2.4 Slurry preparation and measurements In a clean bench, weighed homogenized plant sample (0.01 g) was quantitatively transferred into polypropylene autosampler cups and 15 mL solution added (0.5% or 5.0% HNO3 with 0.5 or 5.0% Triton X-100). Three slurries were prepared for each sample and each sample measurement was replicated at least five times. Just before nebulization, the slurry was ultrasonically agitated for 5 min at 50 W. The sample injection tube was


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immersed about 20 mm above the bottom of the flask and signal was acquired at intervals of 2 min under static conditions [14]. After aspiration of each suspension, a 10% v/v nitric acid wash was flushed through the nebulizer-torch system to remove any residual sample, minimizing memory effects. Two calibration techniques were compared: aqueous calibration and standard additions. Internal standards were used to compensate for plasma fluctuation.

2.5 Microwave digestion For comparison, microwave digestion was also used to prepare roots. Approximately 0.5 g of root powder was accurately weighed into a 100 ml Teflon PTFE vessel. Five millilitres of concentrated nitric acid (60% w/v) was added, the vessel was capped and irradiated (CEM model MDS 2000) according to the program in Table 2. After cooling to room temperature the vessels were vented, 1 ml of H2 O2 was added, and the heating repeated. The digested samples were quantitatively transferred to 10 mL volumetric flasks and filled to the mark with deionized water. Blanks were prepared in a similar manner. Table 2 Closed vessel microwave digestion program. Step 1 2

3

Power [W]

Time [min]

Temp. [◦ C]

Max .pressure [PSI]

Hold [min]

600 600

8 7

120 180

350 350

2 4

Results and discussion

3.1 Stability and concentration of slurries Under the conditions of analysis, signal intensity remained constant for at least 20 min for Pb, Ca and Mg (Fig. 1). Signal stability for 20 min is satisfactory, as most ICPOES analyses require less than 2 minute acquisition times. After a wash with 10% HNO3 , remeasuring the sample yielded similar results, eliminating the possibility that the stability was due to memory effects. Table 3 shows that the percent of suspension dissolved varies with the liquid. When analyte extraction was improved by ultrasonic agitation and acid, slurry stability became less critical [24]. Root dissolution in different nitric acid concentrations (0.5 and 5.0%) is generally similar (54.9% and 55.8% respectively). However, adding Triton X-100 reduced dissolution by about 20% and the organic Triton X-100 lowered the plasma temperature, decreasing the signal. The Mann -Whitney test (Table 3) showed a significant difference in dissolution (p ≤ 0.05) with and without Triton X-100. Based on these results, 0.5% nitric acid was chosen as medium.

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Fig. 1 Pb emission intensity from root slurry in four liquid media. Table 3 Suspension dissolved in the liquid phase. Liquid phase

% suspension dissolved

p∗

54.9 36.3 55.9 38.6

0.0086

0.5% HNO3 0.5% HNO3 + Triton X - 10 5% HNO3 5% HNO3 + Triton X - 100 ∗

0.0090

- statistical significance of the difference using the Mann-Whitney test

3.2 Effect of slurry concentration We observed the effect of slurry concentration on signal intensity from 0.017% to 0.13% (w/v). Optimizations are shown in Fig 2 and Fig. 3a, 3b. A linear response is obtained for slurry concentrations in the range of 0.017 - 0.083% (w/v) for Pb; however, for Mg and Ca the plot is linear from 0.017 to 0.01%. For concentrations greater than 0.083% for Pb and 0.1% for Ca and Mg, the signal intensity became nonlinear, probably because more mass drained from the chamber. Routine analysis used 0.067% (w/v) which permitted determinations with RSD less than 3%. A further result is that aqueous calibration standards can be used to quantify these elements in this type of suspended material. This avoids the need for certified reference materials, which are not commercially available at this level of lead in plants.

3.3 Analytical characteristics Results obtained from the two calibration methods did not differ significantly (t-test, 95% confidence level). Calibration with aqueous standards can be used for determination of Pb, Ca, and Mg in slurry samples with good linearity.


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Fig. 2 Pb signal and precision during slurry nebulization ICP-OES.

Analytical figures of merit are summarized in Table 4. The detection limit (lowest concentration level statistically different from blank) is defined as three times the within batch standard deviation of a blank determination, corresponding to the 99% confidence level. The limits of detection (LOD) were calculated from the standard deviation (3σ) of the blank solution and the slope. A precision of 2 − 4% RSD was obtained based on six replicates of same samples. This is comparable with the 5% RSD in published work on SS-ICP-OES [7, 9]. Carrion et al. obtained better precision for slurry sampling (∼ 1%) than for wet ashing (∼ 2%) [13]. Thus, the precision of the slurry method is similar that of conventional acid digestion. In general, all three independent methods were comparable for each of the three elements.

Table 4 Analytical figures of merit. Element

ICP OES LOD RSD, n = 6 a solution a solution slurry slurry (ng mL−1 ) (ng g−1 ) (%) (%)

Pb Ca Mg a

closed microwave

1.5 0.8 0.09

2.1 1.1 1.2

1.7 1.3 0.9

3.9 2.8 2.3

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Fig. 3 (a) Ca signal and precision during slurry nebulization ICP-OES. (b) Mg signal and precision during slurry nebulization ICP-OES.

3.4 Determination of lead, calcium and magnesium and method validation Pb, Ca and Mg were determined in the roots (Table 5) by the proposed method. The absence of certified reference material to test the accuracy of the analytical procedure is a major difficulty in the determination of Pb, Ca and Mg in roots. To validate our results they were compared with other methods carried out in our laboratory (microwave digestion followed by ICP-OES and FAAS) and by an independent laboratory (microwave digestion and ICP-MS). For a confidence level of 95%, the t-test showed no statistically significant difference between the means. The results of slurry analysis agree well with the conventional acid dissolution method.


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Table 5 Comparison of techniques (mg kg−1 ). Element Pb Ca Mg

ICP OES Slurry sampling

ICP OES∗ Microwave digestion

ICP MS∗∗ Microwave digestion

FAAS∗ Microwave digestion

41.40 ± 0.67 2.11 ± 0.15 1.06 ± 0.08

39.61 ± 1.21 1.98 ± 0.17 1.03 ± 0.06

41.98 ± 0.52 2.07 ± 0.05 1.22 ± 0.02

39.62 ± 1.23 1.90 ± 0.11 1.08 ± 0.27

∗ results reported by our laboratory ∗∗ results reported by independent laboratory

4

Conclusion

In summary, the optimized procedure presents good detection limits and precision for the determination of lead, calcium and magnesium in plants. It minimizes problems due to contamination, evaporative losses of the more volatile elements, and incomplete dissolution. It also saves much time and sample throughput in root QC analysis is significantly improved. Slurry sampling offers an interesting alternative for similar materials.

Acknowledgements This work was financially supported by the State Committee for Scientific Research (KBN), Poland, Grant No. 1 T09D 057 30.

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