Ultrasonic slurry sampling hydride generation atomic fluorescence spectrometry: determination of trace cadmium in soil

Liang Shuxuan1, Lv Tianfeng1, Sun Hanwen1, Qiao Fengxia1, Liu Aichang2
(1College of Chemistry and Environmental Science, Hebei University, Key Laboratory of Analytical Science and Technology of Heibei Province, Baoding 071002; 2China Metallurgy Perambulation Design Academe of limited company, Bao Ding, 071069, China)

Abstract A method has been developed for the determination of cadmium in soil by ultrasonic slurry sampling hydride generation atomic fluorescence spectrometry (USS¨CHG-AFS). Approximately 500 mg, ground to a particle size about 80mm, was mixed with hydrochloric acid solution and agar solution. The samples are agitated for 15 min in an ultrasound water bath. The described procedure permits the use of direct calibration for the determination of cadmium in several soil samples. The influences of instrument operating conditions, slurry preparation and interferences on the signals and the accuracy and precision were determined, with the thiourea and antiscorbutic acid addition as masking reagent that can eliminate the disturbance of partial coexistences ion in the soil£¬with the Co2+ addition that can intensify the Cd hydride generation efficiency. The results for the reference soil sample GBW-07411 agreed satisfactorily with the certified values. The recoveries of the analytes varied in the range from 93 to 104 %. The detection limit was 7.5 ngL-1 of Cd. The relative standard deviations were lower than 5.3 %, demonstrating a good precision for slurry analysis.

    The increasing global emission of Cd compounds mainly related to the Cd industry into the atmosphere, together with aqueous and solid emissions lead to local contamination problems[1]. All these practices produce an accumulation of Cd in soils, allowing it to enter the food chain with the potential to cause serious health problems. Its strong tendency to accumulate in organism makes it one of the most dangerous metals to humans. Occupational exposure to cadmium has long been known to be harmful to the kidney, lung and liver[2]. Such reasons explain the need to monitor Cd in environmental samples.
    Among these most extensively used method is atomic absorption spectrometry by using flame (FAAS) or graphite furnace (ETAAS). Nowadays, hydride generation atomic fluorescence spectrometry (HG-AFS) offers a relatively low-cost and highly sensitive alternative for the determination of Cd.
    The traditional pretreatment of solid samples to dissolve the Cd species is an acid digestion procedure. However, this procedure is time consuming, require adequate laboratory conditions and make use of large quantities of glassware and reagents, increasing the risks of analyte contamination or loss. An alternative for the pretreatment of solid samples is the slurry formation by using the ultrasonic power and direct analysis of the solid suspension. Compared with traditional sample preparation methods, slurry formation combines the benefits of the solid and liquid sampling, avoiding the contamination and the loss of volatile elements and reduced sample amount needed. This mode has been extensively used in AAS[3-5], ICP-MS[6,7] and AFS[8-10].
    To the best of our knowledge, no applications have been reported in the literature concerning the determination of Cd in soil by slurry sampling AFS. The main goal of this work is to develop new analytical methodologies based on the use of ultrasonic slurry sampling for fast and accurate determination of Cd in soil by HG-AFS. In this study, the instrument operating conditions, slurry preparation, particle size, slurry stability, acidity of the medium, accuracy and precision of the method have been optimized. Finally, data for different soil samples were compared with those found after acid digestion, demonstrating that the traditional acid digestion of the samples can be avoided by ultrasonic slurry sampling. This method was applied to the determination of Cd in reference material and some soil samples.
    2.1 Instrumentation
    A hydride generation atomic fluorescence spectrometer AFS-230 (Beijing Haiguang Instrumentals Co. Ltd., Beijing, China) equipped with hollow cathode lamps(Beijing Vacuum Electronics Research Institute, Beijing, China) was employed for determination of Cd. Intermittent flow manifold was in Fig.1. The instrumental parameters were in Table.1.Fig. 1 Intermittent flow manifold

Table 1 Parameters of the HG-AFS instrument

Parameters Conditions
PTM voltage (V) 300
Atomizer temperature (¡æ) 200
Atomizer height (mm) 8
Lamp current (mA) 60
Flow rate of carrier gas (Ar) (mL min-1) 700
Flow rate of shield gas (Ar) (mL min-1) 800
Read mode Peak area
Measure method Std. curve
Read time(s) 10
Delay time(s) 1
Read repeat(times) 3
Injection volume (mL) 0.5

An intermittent flow hydride generation system was employed throughout this work and the schematic diagram of the system is shown in Fig.1. The working of the system can be programmed in several steps for each measurement, at every step the rotation rate and time of the two peristaltic pumps used in this work are listed in Table. 2.

Table 2 Program of intermittent flow

Step Time(s) Rotation rate
(r/ min)
1 6 0 No
2 10 100 No
3 6 0 No
4 16 120 Yes

An ultrasound vibrator (KQ3002B, Kunshan, China) was used to stir the samples with agar solution before hydride generation.

2.2 Reagents
All chemicals were of highest available purity, Ultrapure water (18.3W) was used throughout.
The 1.0mgmL-1 of cadmium stock solution(10 % HCl, GSB G62040-90) was purchased from the National Center for Analysis and Testing of Steel Materials. The working standards were obtained from the stock solution after dilution with 1% (v/v) HCl prior to use.
The KBH4 solutions were daily prepared by dissolving proper amounts of KBH4 in 0.5 % (m/v) NaOH solution.
The thiourea and ascorbic acid solution of 50 gL-1 was prepared by dissolving 5.0 g ascorbic acid and 5.0 g thiourea in 100 mL water.
1000 mgL-1of Co stock solution was prepared by dissolving a precisely weighted amount of high pure CoCl2 (Tianjin Chemicals) in 100mL water. The agar solution (2.0gL-1) was prepared by dissolving 1.0 g agar (Haiyang, Guangdong) in 500 mL boiled water.
5% (v/v) HCl used for carrier solution and 10 % (v/v) HCl used in sample treatment was prepared from high purity grade.
NaOH, HNO3 and H2SO4 were of high purity grade.
2.3 Experimental procedure
2.3.1 Soil properties
The soils used in this study were obtained from the main agricultural areas in Baoding (China) varying widely in physicochemical properties. The soil samples, collected from the surface horizon (0-15cm), were air dried and mildly ground to pass through a 80mm stainless steel sieve, homogenized and stored for subsequent analysis.
2.3.2 Slurries preparation
Slurries were prepared by accurately weighing 500 mg soil sample in 50 mL glass flasks and adding 15 mL 10 %(v/v) HCl, 20mL agar solution (2.0gL-1), 10 mL thiourea-ascorbic acid solution of 50gL-1, 0.5mL 1000 mgL-1 of Co2+ solution, then diluted to the mark with ultrapure water. The resulting slurries were dispersed with an ultrasonic vibrator for 15 min in order to obtain homogeneous dispersions. The aqueous standard solutions, in which the concentration of cadmium varied from 0 to 10mgL-1, were prepared as slurries containing 0.8gL-1agar, 3 % (v/v) HCl and 1.0% (m/v) thiourea and ascorbic acid and 1.0 mgL-1 Co2+ by the above mentioned procedure and used for calibration.
2.3.3 Wet digestion
For comparison, determinations were also performed with wet digestion of compost samples. Test portions (500mg) were accurately weighed into borosilicate glass vessels and 1mLconcentrated HNO3, and 3 mL concentrated HCl were added. Then the solution was boiling for 2 hours. 2 mL HClO4 was added in the solution, which was going on boiling until dried. The solution obtained was transferred to a volumetric flask with addition of 30 mL 10%(v/v) HCl, 20 mL 50gL-1 of thiourea and ascorbic acid solution, and 1.0 mL 1000 mgL-1 of Co2+solution, then diluted to 100 mL with water.
2.3.4 Analytical procedure
Total Cd in the sample slurry and the aqueous standard solutions are merged with KBH4 to form the hydrides. The analytes were determined by AFS under the optimized experimental conditions indicated in Table 1 and 2.

    3.1 Choice of slurry medium
    Slurry preparation in aqueous solution is rarely suitable because most powdered materials undergo rapid sedimentation. This sedimentation of suspended material usually occurs after mixing the slurry. The sedimentation rate depends on the densities of the diluent and solid material, the viscosity of the diluent medium and the radius of the sample particles. The slurry can be stabilized using a highly viscous liquid medium. So far, agar[11,12], glycerol[13], enthanol[14] and Triton-x100[15] have been used as slurry stabilizing agents. In this study, the results showed that the viscosity of agar kept different types of particles in suspension for a sufficient time. Furthermore, it has been stated that the optimum concentration of agar was 0.08 % (m/v) for the homogenization of slurry soil samples. Therefore, in the present work, 2.0 gL-1 agar solution is used as the slurry stabilizing agent.
    3.2 Effect of particle size

    The particle size of the solid material can influence the stabilization and deposition of the slurry, which in turn can influence both accuracy and precision. In the studies, the effect of different particle sizes of soil was investigated. Sample separation in different particle size fractions were carried out by sieving. With the variation in particle size, significant changes in suspended time were observed. The suspended time gradually increased with decreasing particle size, down to the range 76mm-80mm. It was shown the good dispersion and homogeneity of relatively small suspended particles. When the particle size£¼70mm, the suspend time reached a plateau(Fig. 2). So the particle size range 76mm-80mm was selected.Fig.2 Effect of the particle size of soil sample on the suspended time

3.3 Influence of KBH4 concentrations
In the present method, potassium tetrahydroborate(KBH4) solution was used as both reducing agent and hydrogen supplier. The results shown that the signal intensities of analytes would increase when the concentration of KBH4 solution increased from 0.5 to 2 % (m/v), but the signal stability and intensity decreased when the concentration were higher than 2 % (m/v). This is obviously described to the fact that an increase of the reducing reagent concentration corresponds to the formation of more hydride molecules, but too high a concentration tends to produce concomitantly more hydrogen molecules in the gas phase, and the concentration of the hydride was diluted, thus caused a drop in the sensitivity. Hereby, KBH4 solution of 2 % (m/v) was applied to all determinations. The results were presented in Fig. 3.

Fig. 3. Effect of KBH4 concentrations on Cd HG.
Carrier solution, 5 % (v/v) HCl; Cd solution, 2 mgL-1 in 3 % (v/v) HCl,
containing 1.0 mgL-1 of Co2+; 1.0 % (m/v) thiourea and ascorbic acid.

3.4 Acidity effects
Acidity of the solution is a very important parameter that has great influence on the hydride generation, and even critic. In this study, the acidic medium concentration effects of H2SO4, HNO3 and HCl on Cd hydride generation were investigated. The results were presented in Fig. 4. As can be seen in Fig. 4, the Cd signal intensity of HCl system was much higher than HNO3 and H2SO4 systems. So 3 % (v/v) HCl was the most suitable acidity.

Fig. 4. Effect of acidic medium concentration on Cd HG.
Reductant solution, 2 %(m/v) KBH4; Cd solution ,2 mgL-1 in 3 % (v/v) HCl,
containing 1.0 mgL-1 of Co2+; 1.0 % (m/v) thiourea and ascorbic acid.

3.5 Cd hydride generation efficiency enhancement
In previous works, it was widely reported that, in the HCl system, Co2+ acting as a catalyzer that would enhance the Cd hydride generation efficiency to a great extent[16]. In this study, it was necessary to investigate the roles of Co2+ on the Cd hydride generation efficiency enhancement. As can be seen in Fig. 5, the presence of Co2+ significantly enhanced the Cd hydride generation efficiency, however, a maximum enhancement was reached at 1.0 mgL-1of Co2+. The results were presented in Fig. 5.

Fig. 5 Effect of Co2+ concentration on Cd HG.
Reductant solution, 2 %(m/v) KBH4; Carrier solution, 5 %(v/v) HCl ; Cd solution,
2 mgL-1 in 3 % (v/v) HCl, containing 1.0 % (m/v) thiourea and ascorbic acid.

3.6 Effect of thiourea and ascorbic acid
Thiourea and ascorbic acid can influence both accuracy and precision of test, while can eliminate the disturbance of partial coexistences ion in the soil. The concentration of thiourea and ascorbic acid was investigated from 0.0 % (m/v) to 2.5 % (m/v). The signal intensity was stable as the concentration of thiourea and ascorbic acid from 0.2 % (m/v) to 2.5 % (m/v) and thiourea and ascorbic acid have no contribution to the improvement of Cd signal in this work. In fact, thiourea was deduced to act as a masking reagent more than a catalyzer. So 1.0 % (m/v) thiourea and ascorbic acid was chosen. The results were presented in Fig. 6.

Fig. 6 Effect of thiourea and ascorbic acid concentration on Cd HG.
Reductant solution, 2 %(m/v) KBH4; Carrier solution, 5 %(v/v) HCl ;
Cd solution ,2 mgL-1 in 3 % (v/v) HCl, containing 1.0 mgL-1 of Co2+.

3.7 Interference studies
2 mgL-1 of Cd with a mixture solution containing 1 mgL-1 of Ca2+, Mg2+, Na+, K+, Fe2+, Al3+or single ion solutions containing 0.1 mgL-1each of Ti4+, Fe3+, Pb2+, Cu2+, Ni2+, Sn4+, Mn2+ and Cr3+, 1 mgL-1 Zn2+, V5+, Mo6+ and Cr6+ were determined respectively under the presence of thiourea and antiscorbutic acid. The results of these experiments suggested no interference were found The presence of thiourea and antiscorbutic acid reduced the interference of coexistences ion in soil samples on Cd.
3.8 Analytical performance
A successive measurement of a 0.2 mgL-1of Cd solution gave a RSD of 3.1 % for eleven runs, which indicated good reproducibility of the proposed method.
The analysis of a series solutions containing 0.0£¬2.0£¬4.0£¬6.0£¬8.0£¬10.0 mgL-1 of Cd, respectively, gave a calibration function of If = 2.8+15.84X (If, the signal intensity and X, the concentration of Cd, expressed in mgL-1), of which the correlation coefficient is 0.9997.
The detection limits (DL) for Cd species was evaluated on the basis of the standard deviation (S.D.) of the measured signals (11 times) for the blank solution, and was calculated by the formulate DL=3S.D.K-1 (here K is the slopes of the calibration graph). As results, the detection limits was 7.5 ngL-1.
The feasibility of the method was evaluated by analyzing the standard reference material of GBW07411. The results listed in Table 3 reveal that this method is of high accuracy and precision. The results were presented in Table 3.

Table 3 Analysis of standard reference materials

Sample Certified (mgg-1)a Found (mgg-1)a
Soil(GBW07411) 28.2¡À1.3 28.6¡À0.75

a means and standard deviations from 7 replicate measurements.

3.9 Soil sample analysis
The proposed USS-HG-AFS method was applied to the determination of trace amounts of Cd in soil samples collected from suburb cropland. The Cd content of soil samples was shown in Table 4. Results from USS-HG-AFS are in good agreement with those obtained after acid digestion. The recovery of Cd from soil samples was in the range of 93-104 %. The RSD was from 3.8-5.3 %. This is indicative of excellent recovery of copper. The results were presented in Table 4.

Table 4 Results from determination of cadmium in soil samplesa

Sample Found
RSD(%) Added
Acid digestion
1 0.52¡À0.020 3.8 0.30 0.81¡À0.025 99 0.51¡À0.016
2 0.63¡À0.025 4.0 0.30 0.95¡À0.015 102 0.66¡À0.013
3 0.25¡À0.010 4.0 0.30 0.52¡À0.025 95 0.25¡À0.018
4 0.31¡À0.012 3.9 0.30 0.64¡À0.012 104 0.32¡À0.008
5 0.15¡À0.008 5.3 0.30 0.42¡À0.013 93 0.18¡À0.014

a means and standard deviations from 5 replicate measurements

    The developed procedure offers a simple and convenient alternative to those based on a previous complete digestion of samples for the determination of Cd in soil. This method, based on the excellent sensitivity attainable by AFS and on a soft room-temperature treatment of the samples, is a safe and comfortable methodology for operators, which reduces the reagent consumption and time of analysis and offers a possibility to explore the presence of Cd in soil. The developed method is simply and accurate, so is promising for routine analysis of trace of Cd in other more complex materials.

Acknowledgement  This work was supported by Nature Science Foundation of HeBei province, China, for much support to the studied subject(230110).