Li Weili, Ma Yinhai, Peng Yongfang, Zhu Liya, Huang Zhangjie
(Department of Chemistry, Kunming TeacherCollege, Kunming 650031;Department of Chemistry, Yunnan University, Kunming, 650091, China)
AbstractIn this paper, a new method for the simultaneous determination of palladium, platinum and rhodium ions as metal-DHNTR chelates was developed using a rapid column high performance liquid chromatography equipped with on-line enrichment technique. The palladium, platinum and rhodium ions were pre-column derivatizedwith DHNTR to form colored chelates. ThePb-DHNTR, Pt-DHNTR and Rh-DHNTR chelates can be absorbed onto the front of the enrichment column when they were injected into the injector and sent to the enrichment column [ZORBAX Stable Bound, 4.6×10 mm, 1.8m] with a buffer solution of 0.05 mol/L sodium acetate-acetic acid buffer solution (pH 4.0) as mobile phase. After the enrichment had finished, by switching the six ports switching valve, the retained chelates were back-flushed by mobile phase and traveling towards the analytical column. These chelates separation on the analytical column [ZORBAX Stable Bound, 4.6×50 mm, 1.8m] was satisfactory with 63% acetonitrile (containing 0.05 mol/L of pH 4.0 sodium acetate-acetic acid buffer salt and 0.01 mol/L of tritonX-100) as mobile phase. The detection limits (S/N=3) of palladium, platinum and rhodium are 1.4 ng/L, 1.8 ng/L and 2.2 ng/L, respectively. This method was applied to the determination of palladium, platinum and rhodium in water and urine with good results.
Environmental contamination by the platinum group elements (PGEs)has received more and more attentions, and the determination of basal concentrations of those metals has a key role since an increase of their level [1,2] However, the heterogeneous composition of samples and the low concentration levels of palladium, platinum and rhodium involved make the direct measurement of analytes really difficult. Several analytical techniques have been employed with this matrix in recent years and most of the advantages and drawbacks have been reviewed [3-7]. The high performance liquid chromatography method has been proved to be a favorable and reliable technique [8-11]. However, the routine chromatographic methods need a long separation time (more than 10 min is needed).
In this paper, a new reagent, 2,4-dihydroxy-1-naphthalthiorhodanine (DHNTR) was firstly synthesized and used as pre-column derivatization reagents for palladium, platinum and rhodium. A ZORBAX Stable Bound rapid analysis column (4.6×50 mm, 1.8m) was used for the separation of Pd-DHNTR, Pt-DHNTR and Rh-DHNTR chelates on a high performance liquid chromatography equipped with on-line enrichment technique. The palladium, platinum and rhodium were separated completely within 2.0 min in this method. The separation time was greatly shortened compared to the routine chromatographic methods. This method can be applied to the determinationg/L (ppb) level of palladium, platinum and rhodium ions in water and human urine with good results.
On line column enrichment system used is shown in Fig-1. This system includes a Waters quadripump, Waters 515 pump, Waters 996 photodiode array detector, six ports switching valve, large volume injector (containing 10.0 ml samples) and column. The enrichment column is ZORBAX Stable Bound pre-column (4.6×10 mm, 1.8m) and the analytical column is ZORBAX Stable Bound rapid column (4.6×50 mm, 1.8m). The pH value was determined with a Beckman-200 pH meter.
Fig.1 On-line enrichment system using the valve-switching technique
Pump A, Waters 515 Pump. Pump B, Waters 2690 Alliance quadripump. Injector can contain 10 ml of sample. Six ports switching valve (Waters Corporation). Enrichment Column, ZORBAX (4.6×10 mm, 1.8 m m). Analytical column, ZORBAX (4.6×50 mm, 1.8 m m). Detector, Waters 996 photodiode array detector. MP A, 0.05 mol/L of pH 4.0 sodium acetate-acetic acid buffer solution. MP B, 68% acetonitrile (containing 0.05 mol/L of pH=4.0 sodium acetate-acetic acid buffer salt and 0.01 mol/L of tritonX-100)
2.2 Synthesis of DHNTR
The DHNTR was synthesized as following procedure: 50 mL of acetic acid was added to the sample of 1.5 g of thiorhodanine and 1.7 g of 2,4-dihydroxy-1-naphthalaldehyde, and the mixture was heated gently to dissolve the thiorhodanine and 2,4-dihydroxy-1-naphthalaldehyde completely. The solution was refluxed for about 1.0 h, and 0.5 mL of concentrated sulfuric acid was added dropwise during refluxing. After the color of the solution turned red, the refluxing was stopped and the sample was poured into 150 mL of distilled water. To this solution, a small amount of aqueous ammonia was added. Thereafter, the precipitants were separated by filtration, and were recrystallized twice with absolute alcohol. The yield is 46%. The structure of DHNTR was verified by elemental analysis, IR, HNMR and MS. Elemental analysis: C14NO, calculated (found), 52.64 (52.47)% C, 2.84 (2.76)% H, 4.39 (4.46)% N, 30.12 (30.05)% S. IR (KBr) (cm-1): 3600 (-OH), 3315 (-N-H；3060, 3020 (=C-H；1660 (N-H；1566, 1548, 1515, 1450 (C=C；1292 (C-N；1171, 1215 (C=S；825 (Ar-H；806 (C=C-H). HNMR (solvent: DMSO-d) (,ppm): 4.82 (1H, s, C-OH, H 1); 6.24 (1H, s, Ar-H, H 2); 4.76 (1H, s, C-OH, H 3);7.54-7.88 (4 H, m, Ar-H, H 4-7); 6.56 (1H, s, -C=C-H, s, H 8). MS (EI) (m/z): 319 (M+·). All of those experimental results show that the DHNTR has the following structure, as Scheme 1.
All of the solutions were prepared with ultra-pure water obtained from a Milli-Q50 SP Reagent Water System (Millipore Corporation, USA). Palladium, platinum and rhodium standard solution: 1.0 mg/ml (Obtained from Chinese Standards Center), a working solution of 0.2 m g/ml was prepared by diluting this standard solution. HPLC grade acetonitrile (Fisher Corporation, USA). A sodium acetate-acetic acid buffer solution (0.5 mol/L, pH 4.0) was used. DHNTR solution (2.0×10-4 mol/L) was prepared by dissolving DHNTR with 95% ethanol. Mobile phase A: 0.05 mol/L pH 4.0 sodium acetate-acetic acid buffer solution. Mobile phase B: 63% acetonitrile (containing 0.05 mol/L of pH 4.0 sodium acetate-acetic acid buffer salt and 0.01 mol/L of tritonX-100). All other reagents used were of analytical reagent-grade. The glass and Teflon ware used were soaked in 5% of nitric acid for at least 2 h, and then thoroughly wash with pure water.
2.4 Standard Procedure
A 0-15 ml of 0.2 m g/ml standard or sample solution was transfered into a 25 ml of volumetric flask. To which, 4.0 ml of 1.0×10-4 mol/L DHNTR solution, 3 ml of 0.5 mol/L sodium acetate-acetic acid buffer solution (pH 4.0) and 1.0 ml of 1 % TritonX-100 solution were added. The solution was diluted to volume with water and mix well. After 10 min, a 10.0 ml of solution was introduced into injector and sent to enrichment column with mobile phase A at flow rate of 2.0 ml/min. When the enrichment had finished, by switching the valve of six ports switching valve, the metal-DHNTR chelates, which absorbed onto the foreside of enrichment column, were eluted by mobile phase B at the flow rate of 2.0 ml/min in reverse direction and traveled towards the analytical column. The chelates were separated on the analytical column. A tridimensional (X axis: retention time, Y axis: wavelength, Z axis: absorbance) chromatogram was recorded from 400-650 nm with photodiode array detector and the chromatogram of 545 nm is shown in Fig.2.
Fig. 2 Chromatogram of standard sample (a) and Occupationally exposed human urine samples (b). The concentration of palladium and platinum is 1.0 m g/L in standard sample.
3 RESULT AND DISCUSSION
3.1 Precolumn Derivation
The optimal pH for the DHNTR reacts with metal ions is 2.5 – 5.8 for palladium, 2.8 – 5.2 for platinum, and 2.2 – 4.8 for rhodium, so a 0.5 mol/L of pH 4.0 sodium acetate-acetic acid buffer solution was recommended to control pH.
It was found that 0.5 ml of 1.0×10-4 mol/L DHNTR solution was sufficient to complex 5.0g of palladium, platinum and rhodium, respectively. But in real samples, the foreign ions, such as Hg2+, Pb2+, Cu2+, Ag and the like, form complex with DHNTR and consume reagents. Therefore, the amount ofDHNTR must be in excess. In this experiment, 4.0 ml of 1.0×10-4 mol/L DHNTR solution was recommended.
The experiments show that in the nonionic surfactants or cationic surfactants medium, the sensitivity of the metal-DHNTR chelates was increased markedly. Various nonionic surfactants and cationic surfactants enhance the absorbance in the following sequence: TritonX-100 > Tween-80 > Tween-20 > CTMAB > CPB. Therefore, TritonX-100 was selected as additive in this experiment. The use of 0.6-1.4 ml of TritonX-100 solution give a constant and maximum absorbance in this experiment. Accordingly, 1.0 ml TritonX-100 solution was recommended.
The DHNTR can react with Pd(II), Pt(II) and Rh(III) rapidly. The reaction was complete for 5 min at room temperature, and the complex was stable for at least 6 h.
3.2 On-Line Enrichment
Because the Pd-DHNTR, Pt-DHNTR and Rh-DHNTR chelates are stable in weak acid medium. To avoid the chelates decomposing during the enrichment, a 0.05 mol/L of sodium acetate-acetic acid buffer solution of pH 4.0 (mobile phase A) was selected as mobile phase to send the chelates to the enrichment column and a ZORBAX Stable Bound pre-column (4.6×10 mm, 1.8 m m) with pH range 1-11.5 was selected as enrichment column. Experiments showed that the volume of 10 ml sample injected was sensitive enough to determine Pt(II), Pd(II) and Rh(III) in water, urine and soil samples, so a 10 ml sample injection was recommended.
3.3 Spectrophotometric Properties
The absorption spectrum of metal-DHNTR chelates was obtained by measured with a Shimidzu UV-2401 spectrophotometer. Results show that the maximum absorption is 545 nm for Pd-DHNTR chelate, 552 nm for Pt-DHNTR and 542 nm for Rh-DHNTR chelate. Therefore, the 545 nm was selected as detecting wavelength.
3.4 Chromatographic Separation
The experiments showed that the Pd-DHNTR, Pt-DHNTR and Rh- Pd-DHNTR chelates have a good stability in the presence of weak acid buffer solution and TritonX-100 medium. The pH of mobile phase within 2.5-3.8 and containing a 0.008-0.12 mol/L of TritonX-100 in the mobile phase can avoid the metal-chelate decomposing in the course of separation and get a good peak shape. So acetonitrile/water (63/37) (containing 0.05 mol/L of pH 4.0 sodium acetate-acetic acid buffer salt and 0.01 mol/L of tritonX-100) was selected as mobile phase. To shorten the chromatographic separation time, A ZORBAX Stable Bound rapid analysis column (4.6×50 mm, 1.8 m m) was selected in this experiment. With rapid analysis column, the palladium, platinum and rhodium chelates were separated completely within 2 min. Compared to the routine chromatographic method, more than 85% of separation time was shortened.
3.5 Calibration Graphs
Under optimum conditions, regression equations of metal-DHNTR chelates were established based on the standard sample injected and its peak areas. The limits of detection are calculated by the ratio of signal to noise (S/N=3). The results were shown in Table 1. The reproducibility of this method was also examined for 10 m g/L of Pd(II), Pt(II) and Rh(III). The relative standard deviations (n=10) were also shown in Table 1.
Table 1Regression Equation, Coefficient and Detect limit
|Components||Regression Equation||Linearity Range
|Coefficient||Detect limit (ng/L)||RSD%
|Pd-DHNTR||A = 2.02×10 C -1589||8 – 7500||r=0.9992||1.4||2.8|
|Pt-DHNTR||A = 1.96×10 C -1422||5 – 4600||r=0.9987||1.8||3.2|
|Rh-DHNTR||A = 1.73×10 C + 1927||12 – 6400||r=0.9989||2.2||3.4|
Under the pre-column derivatization conditions, the foreign ions of Cu(II), Hg(II), Pb(II), Tl(III), Bi(III), Ag(I), Au(III) can react with DHNTR to form color chelates. To examine the selectivity of this method, the interference of these foreign ions was investigated.But no peak was observed or were separated completely with Pd(II), Pt(II) and Rh(III) chelates under the chosen elution conditions. However, they would consume the precolumn reagent. The tolerance limit for the HPLC analysis was expressed as the maximum amount to be determined within an error of ± 5%. When 4.0 ml of 1.0×10-4 mol/L DHNTR was used, with 10g/L of Pd(II), Pt(II) and Rh(III) respectively, the tolerance amount with an error of ± 5% was 2500g/L for Cu(II), Hg(II), Pb(II), Ag(II) and 600g/L for Tl(III), Bi(III), Au(III). If the amount of DHNTR increased, the metal ion tolerances would also increase. Resultsshow that this method isofhigh selectivity.
Taking an appropriate volume (planting effluents 20 ml, river water 200 ml, human urine 50 mL) of the sample in a 500 mL flask. The samples were concentrated to about 5 mL by heating on a hot plate, and was transferred into a 25 mL teflon high-pressure microwave acid-digestion bomb (Fei Yue Analytical Instrument Factory, Shanghai, China). To which, 2.0 ml of concentrated nitric acid and 3.0 ml of 30% hydrogen peroxide was added. The bombs were sealed tightly and then positioned in the carousel of the microwave oven (Model WL 5001, 1000 W, Fei Yue Analytical Instrument Factory, Shanghai, China). The system was operated at full power for 6.0 min. The digest was evaporated to near dryness. The residue was dissolved with 5 ml of 5% of hydrochloric acid and transferred into a 25 ml of calibrated flask quantitatively, then diluted the solution to the volume with 5% hydrochloric acid. The palladium, platinum and rhodium contents were analyzed by using a proper volume of this solution according to general procedure. The results (deducted the reagents blank) were shown in Table 2.
Table 2Determination results (ng/g) of the samples
|Samples||Found (ng/g)||ICP-MS Method (ng/g)||RSD% (n＝5)||Recovery％ (n=5)|
The proposed method has the following advantages: (1)2,4-dihydroxy-1-naphthalthiorhodaninewas firstly synthesized and used as pre-column derivatization reagent for Pd, Pt and Rh ions, and the ZORBAX rapid analysis column was used for the separation of Pt-DHNTR, Pd-DHNTR and Rh-Pd-DHNTR chelates. The DHNTR can reacts with palladium, platinum and rhodium rapidly at room temperature. The Pt-DHNTR, Pd-DHNTR and Rh-DHNTR chelates were separated completely within 2 min. Compared to the routine chromatographic method, more then 85% of separation time was shortened. (2) By on-line enrichment system, a large volume of sample (10 ml) can be injected, and the sensitivity of the method was greatly improved. In a word, for the determination of palladium, platinum and rhodium, this method is of high sensitivity and high selectivity.