Electrothermal vaporization inductively coupled plasma atomic emission spectrometry determination of gold, palladium, and platinum using chelating resin YPA4 as both extractant and chemical modifier

Talanta 63 (2004) 585–592

Electrothermal vaporization inductively coupled plasma atomic emission spectrometry determination of gold, palladium, and platinum using chelating resin YPA4 as both extractant and chemical modifier Yiwei Wu, Zucheng Jiang, Bin Hu∗ , Jiankun Duan Department of Chemistry, Wuhan University, Wuhan 430072, PR China Received 9 October 2003; received in revised form 23 November 2003; accepted 25 November 2003 Available online 7 February 2004

Abstract A new method for determination of trace gold (Au), palladium (Pd), and platinum (Pt) in environmental and geological samples by electrothermal vaporization (ETV)–inductively coupled plasma atomic emission spectrometry (ICP-AES) with the use of chelating resin YPA4 as both solid phase extractant and chemical modifier has been developed. The resin loaded with analytes was prepared to slurry and directly introduced into the graphite furnace without any pretreatment. The factors affecting the vaporization behaviors of Au, Pd, and Pt were investigated in detail. It was found that, in the presence of YPA4 , Au and Pd could be quantitatively vaporized at lower vaporization temperature of 1900 ◦ C. Compared with the conventional electrothermal vaporization, the vaporization temperature was decreased by 700 ◦ C, and the detection limits for Au and Pd was decreased by a three-fold. However, a little effect of YPA4 on the ETV–ICP-AES determination of Pt was found. Under the optimized conditions, the detection limits (3σ) of Au, Pd, and Pt for this method are 75, 60, and 217 pg, respectively; and their relative standard deviations (R.S.D.) are 4.4, 5.6, and 3.7%, respectively (n = 9, C = 0.2 ␮g ml−1 ). The proposed method has been applied to the determination of trace Pd and Pt in sewage sludge, and the results well agreed with the recommended values. In order to further verify the accuracy of the developed method, a GBW07293 certified geological reference material and an auto catalyst NIST SRM 2557 reference material were analyzed, and the determined values coincided with the certified values very well. © 2004 Elsevier B.V. All rights reserved. Keywords: Chelating resin YPA4 ; Preconcentration and separation; Gold; Palladium; Platinum; ETV–ICP-AES; Chemical modifier

1. Introduction Noble metals have gotten extensively application in fields of petroleum/chemical industry, agriculture, and medicine. Along with the usage of these elements, they inevitably entered the environment by various means (as an example, emitting into the atmosphere with automobile exhaust gases). However, concentrations of noble metals in environmental (also geological) materials are usually too low (even below the detection limit of the instrument) to be determined directly by conventional techniques owning to insufficient sensitivity and matrix interference.Thus, an effective separation and preconcentration procedure is usually necessary prior to determination. ∗ Corresponding author. Tel.: +86-27-87218764; fax: +86-27-87647617. E-mail address: [email protected] (B. Hu).

0039-9140/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2003.11.042

The most widely used techniques for the separation and preconcentration of trace noble metals include fire assay [1,2], coprecipitation [3,4], liquid–liquid extraction [5], big-cycle molecular recognition technology [6,7], ion-exchange and sorption [8–10], HPLC [11,12], and CE [13]. Among these techniques, the methods using ion-exchange resins or sorbent extraction have proved to be especially effective [10,14–16]. Chelating resin possesses excellent adsorption selectivity and high adsorption capacity for noble metals due to its functional group. Therefore, enrichment of noble metals by means of chelating resins have been adopted extensively [17–20]. However, quantitative recovery of these metals is often difficult [18] due to the irreversible adsorption of noble metals on the resins, which results from the strong complexation of noble metals with functional groups of the resins used. The generally approach to this problem is to ignite the resin [21] or mineralize completely the resins loaded the analytes with

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Y. Wu et al. / Talanta 63 (2004) 585–592

mixture of inorganic acid [17,22,23], but it is laborious, time-consuming and easily contaminated. Several analytical techniques, such as atomic absorption spectrometry (AAS) [24], stripping voltammetry (SV) [25], inductively coupled plasma atomic emission spectrometry (ICP-AES) [26], inductively coupled plasma mass spectrometry (ICP-MS) [27–32], and neutron activation analysis (NAA) [33] have been applied to accurately determine trace noble metals in various samples. SV is one of the most sensitive techniques for measuring noble metals, but this analytical technique does not allow multi-element analysis for noble metals. In some cases, ETAAS was applied for noble metals determinations in environmental and geological samples. However, such method suffers from severe matrix effects, especially at the nanogram per gram (ng g−1 ) or lower concentration level of found in environmental samples. Neutron activation analysis is a powerful method for trace noble metals, but it can hardly be considered a routine approach due to the complexity and cost of the required instrumentation. ICP-MS is one of the most suitable techniques for the determination of noble metals [27–32]. The sensitivity of this method can be further increased by use of efficient sample introduction system such as electrothermal vaporization [27], ultrasonic nebulization [31]. However, polyatomic ions originating from the plasma gas, reagents and components of the matrix can affect the measured signal to a large extent. To overcome this problem, Sabastein et al. [31] attempted to determine platinum (Pt), palladium (Pd), and Rh in road dust and river sediment samples using a high mass resolution sector field ICP-MS with ultrasonic nebulization. But for Pd, even the maximum mass resolution attainable did not suffice to resolve all spectral interferences, and the cost of instrument is usually much more expensive than that of the conventional quadrople ICP-MS. ICP-AES does not meet entirely the requirements concerning the determination of the noble metals in environmental and geological samples. However, their limits of detection can approach expected concentration levels by utilizing efficient sample introduction system such as electrothemal vaporization and combing with a preconcentration step. It is well known that the precious metals (especially for platinum group elements) are typical refractory elements. When they were determined by electrothermal vaporization (ETV) plasma spectrometry, an incomplete vaporization, much lower sensitivities and severe memory effects have been encountered, and a very high vaporization temperature of 2700–2800 ◦ C has to be adopted [27,32]. The use of chemical modifier has been proven to be very effective to suppress the formation of refractory carbides, to eliminate the memory effect, and therefore, to improve the analytical performance of the method. In recent years, various chemical modifiers have been reported for ETV plasma spectrometry determination of refractory elements. Of all chemical modifiers, the halogenating reagents (especially, polytetrafluoroethylene (PTFE)) were the most used one

to improve the vaporization behaviors of refractory metal elements due to the facts that majority of metal halides show good chemical stability and excellent volatility. Our group [34–36] has extensively studied the use of PTFE as chemical modifier to improve the vaporization behaviors of refractory metals in ETV–ICP-AES, and the developed method was applied successfully to determination of trace elements in biological, environmental and refractory materials. Vanhaecke et al. [27] developed a method to determine Pt and Rh in environmental matrixes. Solid samples were analyzed directly by ETV–ICP-MS without any pretreatment and a mixture of HCl and HF was used as a modifier to stimulate the vaporization of matrix components during the thermal pretreatment step. However, a very high vaporization temperature (2700 ◦ C) and strong mixture solution of inorganic acid were used, which is unfavorable for the instrument. Byrne et al. [37] also investigated the vaporization mechanisms of the platinum group elements (PGMs) in the graphite furnace by ETV–ICP-MS. The addition of TeCl2 chemical modifier, osmium could be vaporized with a lower vaporization temperature of 1400 ◦ C and the limit of detection was improved obviously. For other five of the platinum group elements, however, no remarkable effect on the vaporization behaviors and analytical sensitivity were observed and a high vaporization temperature of 2600 ◦ C still had to be adopted. It should be noted that the progress in the use of organic chelating regents as chemical modifier to improve the vaporization behaviors of refractory elements at lower temperature have also been made recently. Wu et al. [38] reported a novel method to vaporize the oxinate of palladium at temperature of 900 ◦ C, and the analytical sensitivity was greatly improved. It could be imagined, if one can find a material which could be used as both an effective separation/preconcentration material and a chemical modifier for ETV–ICP-AES determination, this double-function material will provide a new effective way for ETV–ICP-AES determination of trace/ultratrace elements in complicated matrix samples. A chelating resin (YPA4 ) of aminoisopropylmercaptan type with a polythioether backbone, which the total contents of S and N in the resin are 24.89 and 7.82%, respectively, was found to have an excellent adsorption characteristic toward noble metals [39]. The aim of this work is to explore the possibility of using YPA4 resin as both an adsorbent and a chemical modifier for ETV–ICP-AES determination of gold (Au), Pd, and Pt, and to develop a new method, which possesses good selectivity and excellent sensitivity for determination of trace Au, Pd, and Pt. YPA4 resin loaded with the analytes was directly introduced into the ETV device without any pretreatment. The factors affecting the vaporization behaviors of Au, Pd, and Pt were investigated in detail and were optimized. Finally, the proposed method was applied to the determination of trace Au, Pd, and Pt in environmental and geological samples.

Y. Wu et al. / Talanta 63 (2004) 585–592

( CH2

2. Experimental

587

C

S )m

CH2 ( NCH2CH2 )4 NH

2.1. Apparatus

CH2CHCH3 CH2CHCH3

The graphite furnace sample introduction device and ICP-AES instrument used in this work were identical with that reported previously [38]. An ICP spectrometric system (Beijing Broadcast Instrument Factory, Beijing, China) with 2 kW plasma generator was used with a conventional quartz torch. A WF-1B type heating device with a matching graphite furnace (Beijing Second Optics, Beijing, China) was used for analyte vaporization. The radiation from the plasma was focused as a 1:1 image on the entrance slit of a WDG 500-1A type monochromator (Beijing Second Optics, Beijing, China) having a reciprocal linear dispersion of 1.6 nm mm−1 . The transient emission signals from plasma were detected with a R456 type photomultiplier tube (Hamamatsu, Japan) fitted with a laboratory-built direct current amplifier, and recorded by a U-135C recorder. The used instrument operating conditions and wavelength are given in Table 1. A HL-2 peristaltic pump (Shanghai Qingpu Instrument Factory, China) was used in separation and preconcentration process. A minimum length of PTFE tubing (0.5 mm i.d.) was used for FI connections. A self-made PTFE microcolumn (20 mm × 3.0 mm i.d.) was used. A DT-40 thermogravimetry instrument (Shimadzu, Kyoto, Japan) was used to investigate the pyrolysis for YPA4 and YPA4 –Au/Pd.

SH

SH

Fig. 1. The structure of YPA4 .

analytical reagent grade. Doubly distilled water was used throughout. 2.3. Preparation of YPA4 The chelating resin YPA4 was purchased from Department of Polymer Chemistry, in Wuhan University, and the preparation method and the detailed structural properties of the resin YPA4 were described in ref. [39]. The chelating resin (YPA4 ) is aminoisopropylmercaptan type with a polythioether backbone, which the total contents of S and N in the resin are 24.89 and 7.82%, respectively, and the structure of YPA4 is shown in Fig. 1. YPA4 with 140 mesh size was immersed in acetone and 1 M HCl for 24 h, respectively, filtered and washed with double distilled water, dried prior to storage for use for the adsorption of metal ions.

3. Separation and preconcentration procedure 3.1. Batch method for separation and preconcentration

2.2. Standard solutions and reagents (1.000 mg ml−1 )

The Au, Pd, and Pt stock solutions were prepared by dissolving of high-purity AuCl3 , Pd(NO3 )2 , and K2 PtCl6 (The first Reagent Factory, Shanghai, China) in 0.1 M nitric acid, respectively. Analytical mixture standard solutions of noble metals were prepared by mixing and diluting the stock solutions. All other reagents used were of

Table 1 ETV–ICP-AES operating conditions Parameters Wavelength (nm) Incident power (kW) Carrier gas (Ar) (l min−1 ) Coolant gas (Ar) (l min−1 ) Plasma gas (Ar) (l min−1 ) Observation height (mm) Entrance slit-width (␮m) Exit slit-width (␮m) Drying temperature (◦ C) Ashing temperature (◦ C) Vaporization temperature (◦ C) Clearing temperature (◦ C) Sample volume (␮l)

Pd: 340.5, Au: 267.6, Pt: 306.5 1.0 0.6 16 0.8 12 25 25 100; ramp 10 s, hold 20 s 800; ramp 10 s, hold 20 s 1900, 3 s (for Au and Pd); 2600, 4 s (for Pt) 2700, 2 s 10

0.5 M HNO3 was selected as adsorption media. A portion of sample solution containing Au, Pd, and Pt (0.1–10.0 ␮g) was transferred to a 1 ml centrifuge tube, the acidity was adjusted to the desired value with 1 M HNO3 , and the final volume was diluted to 1.0 ml. Then, 20 mg of chelating resin YPA4 was added, and the mixed solution was stirred mechanically for 5 min to facilitate adsorption of the metal ions onto the chelating resin YPA4 . After centrifugation, the supernatant was removed. The resin loaded with the analytes was evaporated carefully to near dryness, and then prepared to 1.0 ml slurry by adding 0.1% (m/v) agar. 3.2. Micro-column separation and preconcentration method A portion of sample solution containing Au, Pd, and Pt (0.1–10.0 ␮g) were passed through the micro-column (20 mm × 2.0 mm i.d.) at flow rate of 1.0 ml min−1 by using a peristaltic pump. Then the resin retained the analytes was washed down from the column by double distilled water. After centrifugation, the supernatant was removed. The resin loaded with the analytes was evaporated carefully to near dryness, and then prepared to 1.0 ml slurry by adding 0.1% (m/v) agar.

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Y. Wu et al. / Talanta 63 (2004) 585–592

3.3. Recommended procedure for ETV–ICP-AES 20

c'

15

Signal intensity (a.u)

After selecting analytical wavelength by pneumatic nebulization system, it was disconnected from the plasma torch, and replaced by the graphite furnace device. After plasma stabilizing, 10 ␮l prepared sample was pipetted into the graphite furnace with microsyringe, and the sample inlet hole was then sealed with a graphite cone. Under the optimized conditions, the analytes loaded on chelating resin YPA4 were quickly vaporized from the graphite furnace, and were introduced into the ICP by carrier gas (Ar). The emission signal was recorded and the peak height was measured for quantification.

b' 10

b

a' 5

c

a 0

(A)

Time/s

e 20

4. Results and discussion

15

Signal intensity (a.u)

As above described, the chelating resin YPA4 showed good adsorption selectivity, high adsorption capacity, excellent stability and quick kinetic characteristics for noble metals. However, these metals had to be stripped from the resin by using strong complexing agent thiourea, which is tedious and time consuming. Thus, in this work, the resin loaded analytes was attempted to introduce directly into the electrothermal vaporizer in the form of slurry without desorption. Fig. 2 is the typical signal profiles of the analyte without the addition of YPA4 and the analyte after adsorbed by YPA4 . As can be seen, in the presence of YPA4 , it greatly changed the vaporization behaviors of Au and Pd. Intense and sharp emission signal profiles were obtained for Au and Pd after adsorbed by YPA4 , and the Au and Pd could be quantitative vaporized at lower temperature of 1900 ◦ C (shown by g and h in Fig. 2). What is more, no obvious memory effect was observed at temperature of 2600 ◦ C (shown by g and h in Fig. 2), indicating that the analytes had been vaporized completely. On the contrary, at the same conditions, the emission signal profiles for Au and Pd without YPA4 (shown by a and b in Fig. 2) were weaker and the peak height was one third and one tenth of those obtained by Pd and Au loaded on YPA4 , respectively, and a severe memory effect could be seen at temperature of 2600 ◦ C (shown by a and b in Fig. 2). It should be noted that Au, Pd, and Pt without YPA4 could be quantitatively vaporized at 2600 ◦ C and almost had no memory effects (Fig. 2(B)). However, Pt after adsorbed by YPA4 showed a quite different vaporization behavior from the Au and Pd after adsorbed by YPA4 . The presence of YPA4 has a little effect on the vaporization of Pt (shown by f and j in Fig. 2), and the signal intensity of Pt without YPA4 at 2600 ◦ C (shown by f in Fig. 2) is the 90% of that with YPA4 (shown by j in Fig. 2) at the same temperature. Although Pt with or without YPA4 has no obvious memory at the vaporization temperature of 2600 ◦ C (shown by f and j in Fig. 2), severe memory could be observed at 2600 ◦ C( shown by i in Fig. 2) after Pt with YPA4

10

d

5

f'

e'

d' 0

(B)

Time/s

j

h

20

i' Signal intensity (a.u)

4.1. Vaporization behaviors of Au, Pd, and Pt

f

15

g 10

5

g'

i h'

j'

0

(C)

Time/s

Fig. 2. Signal profiles for Pd, Au, and Pt (A) without YPA4 at 1900 ◦ C: a = 5 ng Au, b = 5 ng Pd, and c = 10 ng Pt; a , b , and c are their residual signals of empty firing at 2600 ◦ C, (B) without YPA4 at 2600 ◦ C: d = 5 ng Au, e = 5 ng Pd, and f = 10 ng Pt; d , e , and f are their residual signals of empty firing at 2600 ◦ C, and (C) with YPA4 at 1900 ◦ C: g = 5 ng Au, h = 5 ng Pd, and i = 10 ng Pt; g , h , and i are their residual signals of empty firing at 2600 ◦ C; with YPA4 at 2600 ◦ C: j = 10 ng Pt; j is its residual signal of empty firing at 2600 ◦ C.

vaporized at 1900 ◦ C (shown by i in Fig. 2). These results indicated that: (1) the chelating resin YPA4 is an effective chemical modifier for ETV–ICP-AES determination of Au and Pd at lower temperature. (2) Compared with no YPA4 , the presence of YPA4 not only remarkably enhanced the sensitivity of Au and Pd, but also greatly lowered the vaporization temperature of Au and Pd (from the conventional

100

100

80

80

60

Q-TG,mass loss (%)

Signal intensity(a.u)

Y. Wu et al. / Talanta 63 (2004) 585–592

Au Pd Pt

40

YPA4 YPA4-Au YPA4-Pd

60

40

20

20

400

589

500

600

700

800

900

1000

1100

1200

0

1300

Ashing temperature(¡æ)

Fig. 3. Effect of ashing temperature on signal intensity. Conditions: Au, Pd: 5 ng, and Pt: 10 ng adsorbed by YPA4 ; drying, 100 ◦ C, ramp 10 s, hold 20 s; ashing time 20 s; vaporization, 1900 ◦ C, 3 s (for Au and Pd); vaporization, 2600 ◦ C, 4 s (for Pt).

2600–1900 ◦ C). The possible reasons are that the complexes formed between Au/Pd and function group of YPA4 was stable and easily volatile, and the analytes were vaporized from the graphite furnace into ICP in the form of complexes. Thus, the transport efficiency of Au/Pd from ETV to the plasma may be greatly increased. (3) Without YPA4 , a very high vaporization temperature is required for Au and Pd. This may be due to the poor thermochemical stability of the inorganic compounds of Au and Pd, which is easily decomposed to the metal Au and Pd in the graphite furnace and then vaporized from the vaporizer in the form of atom Au (bp 2700 ◦ C) and Pd (bp 2960 ◦ C) at very high temperature. (4) Unlike Au and Pd, a very high vaporization temperature is still required for Pt with YPA4 . The reason that YPA4 has different effect on vaporization behaviors of Au/Pd and Pt is still unknown, some further research work on this is undergoing.

0

200

400

600

Temperature (¡æ)

Fig. 4. Variation of Mass-loss Q-TG with pyrolysis temperature for YPA4 , YPA4 –Au, and YPA4 –Pd in an argon atmosphere (40 ml min−1 ), heating rate 20 ◦ C min−1 , 20 mg (resin): 0.5 ␮g (Au/Pd).

was shown in Fig. 4. As can be seen, the majority of the resin, whatever adsorbing or not adsorbing Au and Pd, could be decomposed within the temperature of 600–700 ◦ C. The results supported powerfully the reliability of selecting 800 ◦ C as the ashing temperature. Under the selected ashing temperature of 800 ◦ C, the effect of ashing time on the signal intensity of Au, Pd, and Pt was investigated. The results showed that there were no obvious differences in singal intensity of Au, Pd, and Pt when ashing time was changing from 5 to 40 s. Therefore, an ashing time of 20 s was chosen. 4.3. Effect of vaporization temperature and time Using an ashing temperature of 800 ◦ C and ashing time of 20 s, the influence of the vaporization temperature on signal intensity of Au, Pd, and Pt adsorbed by YPA4 was studied and the results are shown in Fig. 5. As can be seen, Au and

4.2. Effect of ashing temperature and time 100

80

Signal intensity (a.u)

The selection of an appropriate ashing temperature is very important for removing the organic matrix of the resin and preventing the ashing loss of the analytes. Fig. 3 shows the influence of ashing temperature on signal intensity of the analyte adsorbed by YPA4 . It could be seen that there was no ashing loss of the analytes within the temperature of 800 ◦ C for Au, 900 ◦ C for Pd, and 1000 ◦ C for Pt. However, when the ashing temperature was over 800 ◦ C for Au, 900 ◦ C for Pd, and 1000 ◦ C for Pt, the signal of the analytes decreased rapidly with the increase of the ashing temperature. Based on the experimental results, an ashing temperature of 800 ◦ C was selected for the simultaneous determination of Au, Pd, and Pt. The pyrolysis of YPA4 and YPA4 –Au/Pd was investigated by thermogravimetry in an argon atmosphere (40 ml min−1 ) with the heating rate of 20 ◦ C min−1 . The variation of mass loss with pyrolysis temperature for YPA4 and YPA4 –Au/Pd

Au Pd Pt

60

40

20

0 1200

1400

1600

1800

2000

2200

2400

2600

2800

Vaporization temperature(¡æ)

Fig. 5. Effect of vaporization temperature on signal intensity. Conditions: Au, Pd: 5 ng, and Pt: 10 ng adsorbed by YPA4 ; drying, 100 ◦ C, ramp 10 s, hold 20 s; ashing, 800 ◦ C, ramp 10 s, hold 20 s; vaporization time, 4 s.

Y. Wu et al. / Talanta 63 (2004) 585–592

Pd show similar vaporization behaviors in ETV–ICP-AES and the vaporization behaviors of Pt are quite different from those of Au and Pd. For Au and Pd, the emission signal was observable over the temperature of ∼1300 ◦ C and enhanced with the increase of temperature. The maximum signal intensity could be obtained at about 1900 ◦ C, and kept unchanged with the temperature further increasing from 1900 to 2700 ◦ C. While for Pt, the signal intensity of Pt vaporized at 1900 ◦ C is about 5% of that at 2700 ◦ C and enhanced rapidly with the increase of temperature. The maximum signal intensity could be obtained at about 2600–2700 ◦ C. Compared with the reported vaporization temperature of 2700 [27], 2800 [32], and 2600 ◦ C [37], the vaporization temperature of Au and Pd is obviously lowered with the use of YPA4 as chemical modifier. This suggests that the mechanism of vaporization for Au and Pd in the presence of YPA4 is significantly changed. With the use of YPA4 as chemical modifier, complexes may be formed between Au/Pd and the function groups in YPA4 , which could be more efficiently vaporized from the furnace. Thus, the ETV–ICP-AES signals for Au and Pd may be significantly enhanced and 1900 ◦ C was chosen to vaporization of Au and Pd. Unlike Au and Pd, YPA4 have a little effect on the vaporization of Pt and 2600 ◦ C was selected for Pt. Under the selected vaporization temperature of 1900 ◦ C (for Au and Pd) and 2600 ◦ C (for Pt), the effect of vaporization time on the signal intensity of Au, Pd, and Pt was studied. The experimental results show that vaporization time has little effect on signal intensity of Au, Pd, and Pt. A vaporization temperature of 1900 ◦ C (for Au and Pd) and 2600 ◦ C (for Pt) and a vaporization time of 3 s (for Au and Pd) and 4 s (for Pt), therefore, were selected as vaporization conditions for Au, Pd, and Pt. 4.4. Effect of amount of YPA4 Previous studies [39] demonstrated that YPA4 resin was an excellent absorbent for Au, Pd, and Pt and the adsorption capacity of the resin for Au, Pd, and Pt is 67.2, 64.8, and 27.6 mg g−1 , respectively. In this work, YPA4 was tried to be used not only as absorbent, but also as a chemical modifier for ETV–ICP-AES determination of Au, Pd, and Pt. Thus, the effect of the amount of YPA4 in the slurry on the emission signal intensity of Au, Pd, and Pt in ETV–ICP-AES were examined. For this purpose, the experiment was designed: the slurry of 6% YPA4 loaded with analytes was prepared by mixing YPA4 loaded analytes with 0.1% (m/v) agar as stabilizer. The slurry series (4, 2, and 0.5%) were obtained by diluting 6% YPA4 loaded analytes slurry with 0.1% agar solution. By this treatment, the analyte in resin slurry is diluted by 1.5-, 3-, and 12-folds, respectively. Fig. 6 showed the results obtained by the above experiment. It can be seen that the signal intensity of Au and Pd increases linearly with the increasing of YPA4 amount from 0.5 to 6% (corresponding to increasing analyte concentration linearly) with the correlation of 0.9999 and 0.9998, respectively (Fig. 6). However, the signal intensity of Pt does not increase linearly with the

100

Signal intensity (a.u)

590

Au Pd Pt

80

60

40

20

0 0

1

2

3

4

5

6

Concentration of YPA4(%)

Fig. 6. Effect of amount of YPA4 . Conditions: Au, Pd: 5 ng and Pt: 10 ng adsorbed by YPA4 ; drying, 100 ◦ C, ramp 10 s, hold 20 s; ashing, 800 ◦ C, ramp 10 s, hold 20 s; vaporization, 1900 ◦ C, 3 s.

increasing of YPA4 amount from 0.5 to 6%, and just increases linearly with the increasing of YPA4 amount from 0.5 to 4% with the correlation of 0.9996. This shows that the amount of YPA4 ranging from 0.5 to 4% has no obvious effect on the signal intensity of Au, Pd, and Pt. In this work, a 2% YPA4 was employed. 4.5. Enrichment factor In order to explore the possibility of enriching low concentrations of analytes from large volumes, the dynamic absorption preconcentration was performed to examine the effect of sample volume on the retention of Au, Pd, and Pt. The volume of 10, 25, 50, and 100 ml of sample solutions were prepared with the content of Au and Pd fixing at 5 ␮g and Pt at 10 ␮g, and then operated as described previously. The results in Table 2 shows that the quantitative recoveries (>90%) were obtained for the sample volumes of less than 100 ml for Au, Pd, and Pt. In this work, 100 ml of sample solution was adopted for the preconcentration of Au, Pd, and Pt, the resin loaded with analytes was prepared to 1 ml slurry, thus, an enrichment factor of 100 is achieved by this method. 4.6. Detection limits and precision With the use of established experimental parameters as shown in Table 1, the analytical performance detection of the Table 2 Effect of sample volume on the recovery of Au, Pd, and Pt Sample volume (ml)

Concentration (␮g ml−1 )

Recovery (%)

Au

Pt

Pd

Au

Pt

Pd

100 50 25 10

0.05 0.1 0.2 0.5

0.1 0.2 0.4 1

0.05 0.1 0.2 0.5

90.5 96.0 98.0 99.5

97.1 98.5 96.7 101.4

98.7 99.2 98.7 99.1

Y. Wu et al. / Talanta 63 (2004) 585–592 Table 3 Limits of detection (LODs) and precision (R.S.D.) Element

Au(III) Pd(II) Pt(IV) a b

R.S.D.a (%)

4.4 5.6 3.7

Detection limit (ng ml−1 ) YPA4 chemical modifier

No chemical modifer

7.5 (0.075)b 6.0 (0.060)b 21.7 (0.22)b

19.7 20.6 25.8

Pd and Au 0.2 g ml−1 , Pt 0.5 ␮g ml−1 ; 20 ␮l injection, n = 9. After the preconcentration.

Table 5 Analytical results of Au, Pd, and Pt in GBW07293 geological reference material and Pd and Pt in auto catalyst NIST SRM 2557 reference material Sample

Element

Founda (ng g−1 )

Certified value (ng g−1 )

GBW07293c

Au(III) Pd(II) Pt(IV)

42.34 ± 1.67 532 ± 31.7 411 ± 27.8

45 ± 2.1 568 ± 29.3 440 ± 26.2

NIST SRM 2557

Pd(II) Pt(IV)

a b

method was evaluated. The limits of detection, defined as the analyte concentration that gives a signal that was three times the standard deviation of the blank. The limits of detection (LODs) and the relative standard deviation (R.S.D.) of the proposed method were listed in Table 3. Compared with vaporization of Au, Pd, and Pt without the use of YPA4 as chemical modifier, the detection limits could be decreased by about 2.62-fold for Au, 3.43-fold for Pd, and 1.19-fold for Pt. Considering the theoretical preconcentration factor of 100 after preconcentration by YPA4 , the detection limits of the proposed method for Au, Pd, and Pt were 75, 60, and 217 pg ml−1 . 4.7. Sample analysis For analysis of trace amount of Pd and Pt in sewage sludge (provided by Duisburg University, German), 1.500 g of sewage Sludge was weighed and dissolved in 25 ml of HNO3 –HClO4 –HF (4:2:1 v/v/v) under mild heating and vaporized to near dryness. Then 10 ml aqua regia was added to the residue and the solution was evaporated carefully again to dryness, and finally dissolved in 10 ml of 0.5 mol l−1 nitric acid. 20 mg YPA4 was added to the above solution digested and then operated according to the procedure developed. The average values of five replicate determinations and the recommended values are also given in Table 4. A good agreement between the determined values and the recommended values was obtained. In order to verify the validity of the procedure, the method has been applied to the determination of the content of Au, Pd, and Pt in GBW07293 geological standard reference material, meager platinpalladium ore (provided by Table 4 Analytical results of Pd and Pt in sewage sludge Sample

Element

Founda (ng g−1 )

Recommendedb (ng g−1 )

Sewage sludge

Au(III) Pd(II) Pt(IV)

– 38.76 ± 1.23 72.80 ± 2.71

– 40 70

–: Not detected. a Mean ± average deviation, n = 5. b Results provided by the University of Duisburg, German.

591

c

230.9 ± 24b 1122 ± 18b

233.2 ± 19b 1131 ± 11b

Mean ± average deviation, n = 5. ␮g g−1 . The certified value was shown in ref. [40].

the Institute of Geophysical and Geochemical Prospecting, Langfang, China), and Pd and Pt in auto catalyst NIST SRM 2557 reference material (National Institute of Standards and Technology, Gaithersburg, USA). 1.200 g of GBW07293 geological rock reference material and 6.000 g sodium peroxide were weighed in nickel crucibles and placed in a muffle furnace for 15 min at 700 ◦ C. The residues were dissolved in hot water in a beaker. Then the obtained solutions were heated to near dryness, and the residues were dissolved in dilute nitric acid. After centrifugation, the supernatant was heated to near dryness and the residues were dissolved in 10 ml of 0.5 mol l−1 nitric acid. 0.050 g of auto catalyst NIST SRM 2557 reference material was weighed and dissolved in 5 ml of HNO3 –HClO4 –HF (4:2:1 v/v/v) under mild heating and vaporized to near dryness. Then 5 ml aqua regia was added to the residue and the solution was evaporated carefully again to dryness, and finally dissolved in 5 ml of 0.5 mol l−1 nitric acid. 20 mg YPA4 was added to the above digested sample solution and then operated according to the procedure developed. The average results of five replicate determinations and the certified results are given in Table 5. As could be seen, the analytical results obtained were in good agreement with the certified values.

5. Conclusion A chelating resin YPA4 contains S and N donor atoms used as both an adsorbent and a chemical modifier for ETV–ICP-AES determination of Au, Pd, and Pt has been reported in this paper. The double effects of YPA4 resin provide a new effective strategy for separation/preconcentration and ETV–ICP-AES determination of trace/ultra-trace elements. In the presence of YPA4 , Au and Pd could be quantitatively vaporized at lower vaporization temperature of 1900 ◦ C, but for Pt, a much higher vaporization temperature of 2600 ◦ C is still required. Compared with the conventional vaporization temperature of 2600 ◦ C, a decrease of 700 ◦ C for Au and Pd in vaporization temperature is beneficial to prolong the lifetime of evaporator. What is more, the detection limits could be decreased by about 2.62-fold for Au,

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3.43-fold for Pd with the use of YPA4 as chemical modifier, and just 1.1 for Pt and even no increase for Rh (so Rh was omitted in this paper). In addition, no elution used makes the whole analytical operation be simplified. Therefore, it is possible to concentrate analytes from a small sample volume with a higher enrichment factor. The proposed method is applicable to the determination of trace/ultrace noble metals in complicated matrix sample, such as biological, environmental, and geological samples.

Acknowledgements Financial supports from National Nature Science Foundation of China and Wuhan Municipal Science and Technology Committee are greatly acknowledged.

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