TECHNICAL NOTES Chromatographic Separation of Arsenic Species with Sodium Bis(trifluoroethy1)dithiocarbamateChelation J y a - J y u n Y u and C. M. Wai*
Department of Chemistry, University of Idaho, Moscow, Idaho 83843
INTRODUCTION Chemical speciation techniques are becoming increasingly important for studying trace elements in natural water systems. The toxicity, bioaccumulation, and transport of many trace elements depend largely on their chemical forms present in the aquatic environment. For example, arsenic can exist in natural waters as arsenite (As3+),aresenate (As5+),monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), and other organoarsenics. The predominant arsenic species in natural waters are usually arsenite and arsenate, with the trivalent state being more toxic to biological systems (I). Selective chelation followed by chromatographic separation and determination is one method of analyzing arsenic species. For instance, methyl thioglycolate (MTG) reacts with MMA, DMA, and inorganic arsenic species to form stable complexes that can be separated by gas chromatography (GC) (2, 3 ) . However, the reducing ability of MTG renders it nonselective for differentiation of As3+and As5+. Daughtrey et al. extracted inorganic As3+and methylarsenics with ammonium diethyldithiocarbamate (DDC) into an organic solvent for GC separation (4). But, the low volatility and thermal instability of As(DDC13 make it difficult for GC quantification, and extensive silylation of the packing materials and column walls must be provided periodically in order to elute the arsenic complexes. Substitution of fluorine for hydrogen in DDC, as in the case of sodium bis(trifluoroethy1)dithiocarbamate(FDDC), can generally enhance the volatility and thermal stability of the resulting metal chelates. Separation of a number of metalFDDC complexes using GC has been reported by Neeb et al. (5-7). However, to our knowledge there is no information available in the literature regarding the complexation of FDDC with As3+ and its simultaneous separation with other metal-FDDC complexes by GC. In this note, we report the results of our recent investigation on the characterization and GC separation of As and other relevant metal-FDDC complexes and the potential applications of this method to arsenic speciation studies.
EXPERIMENTAL SECTION Instrumentation. A Hewlett-Packard (Model 5890) gas chromatograph equipped with a flame ionization detector (FID) and a 63Nielectron capture detector (ECD) was used for this study. Chromatographic signals were recorded and processed by a HP Model 3392A integrator. The splitless injection technique was used in the GC experiments. The columns used were fused-silica capillary columns RSL-150 (polydimethylsiloxane 10 m X 0.53 mm i.d., 1.2-pm film, Alltech Associates), HP-1 (methylsilicone 15 m X 0.53 mm i.d., 2.65-pm film, Hewlett-Packard), RSL-160 (polydimethylsiloxane 10 m X 0.53 mm i.d., 2.65-pm film, Alltech Associates),DB-1 (methylpolysiloxane 30 m X 0.53 mm i.d., 1.5-pm film, J&W Scientific),and DB-5 (5% phenylmethylpolysiloxane 30 m X 0.53 mm i.d., 1.2-pmfilm, J&W Scientific). The flow rate of the carrier gas (helium) was 7-8 mL/min. The following temperature program was used: 5 min at 100 "C, 40 "C/min to 200 "C, 1 min at 200 "C, 20 OC/min to 220 "C, 2 min at 220 "C, 40 "C/min to 260 "C, 10 min at 260 "C. The injection port 0003-2700/91/0363-0842$02.50/0
temperature was 220 "C, and the flame ionization detector was maintained at 280 "C. Thermogravimetric (TG) analysis was performed by using a Perkin-Elmer TGS-1 thermobalance with ca. 1 mg of sample at a scan rate of 10 OC/min from room temperature to 400 "C under a nitrogen atmosphere. Arsenic complexes with FDDC and DDC were also characterized by differential scanning calorimetry (DSC) using a Perkin-Elmer DSC-lb calorimeter. A double-focusing mass spectrometer (VG Model 70/70) coupled to a gas chromatography (HP Model 5790) was also used to characterize the As-FDDC complex. The chemical ionization (CI) method was employed to obtain the mass spectra by using methane as a reagent gas at 150 eV. The mass spectrometer was equipped with a VG-11/250 data system for on-line data acquisition and processing. Reagents. Sodium bis(trifluoroethy1)dithiocarbamate (NaFDDC) waq synthesized according to the procedures given in the literature (8). The starting material bis(trifluoroethy1)aminewas purchased from PCR Research Chemicals (Gainesville,FL). Other chemicals used in the synthesis, including sodium amide, carbon disulfide, and potassium hydroxide, were all obtained from Aldrich Chemical Co. Stock solutions (loo0 mg/L) of As3+and As5+were prepared according to the procedures given in our previous papers (9, 10). Other stock solutions including Ni, Zn, Hg, Pb and Bi were Baker Analyzed Reagents. Ammonium acetate buffer (2 N) was prepared by mixing 120 g of pure glacial acetic acid (Baker Ultrapure Reagent) and 134 g of concentrated NHIOH (Aldrich ACS Reagent) and diluting to 1L. The pH value was adjusted to 3 by dropwise addition of HN03 and/or NH,OH. Methyl thioglycolate was obtained from Aldrich Chemical Co. Concentrated nitric acid (Baker Ultrapure Reagent) and 30% hydrogen peroxide (Baker Ultrapure Reagent) were used for urine sample digestion. The As(FDDC)~complex was prepared by adding an excess amount of Na(FDDC) solution to an As3+solution at pH 3. The resulting pale yellow precipitate was extracted with chloroform, and the organic phase was washed with deionized water a couple of times after phase separation. Purification of the product was achieved by recrystallizationusing a chloroform/ethanol solution (1:l v/v). All the glassware, plastic vials, and containers used in this study were cleaned with a detergent wash followed by soaking in a 10% nitric acid solution for at least 24 h. After washing, they were rinsed with deionized water several times and stored in a clean hood equipped with a vertical laminar flow filter. Extraction Procedures. Prior to FDDC extraction, water samples were first shaken with chloroform to remove organic materials. A known amount of As3+or As5+was then spiked into the pretreated water in an Erlenmeyer flask. The solution, 20 mL in volume, was adjusted to a desired pH by using an acetate buffer. About 20 mg of Na(FDDC) in 2 mL of deionized water was added to the aqueous phase. A 2-mL aliquot of chloroform was added to the aqueous phase, and the mixture was shaken vigorously for several minutes. After phase separation, the organic phase was transferred into a 1-dram polyethylene vial and was allowed to evaporate to dryness at room temperature in a laminar flow hood. Exactly 100 pL of chloroform was then added into the vial to redissolve the extracted As complex for GC analysis. To determine total As, a second aliquot of the spiked water sample was placed in an Erlenmeyer flask and adjusted to pH 2 with 5 N HN03. Reduction of As6+to As3+was conducted by adding 1 mL each of a 25% sodium thiosulfate solution and a 20% potassium iodide solution. After 15-20 min, 2 mL of a 1% Na0 1991 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 63,NO. 8, APRIL 15, 1991 843 40
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)MA MTG Peak maximum: 138.7.C Onaet: 1 3 5 . 7 * c
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Differential scanning calorimetry (DSC) of As(DDC), and As(FDDC),. Figure 1.
(FDDC) solution was added to the sample. The arsenic complex was extracted by the same procedure described above. The difference in As concentrations between the two aliquots represents the amount of Asb+in the water sample.
RESULTS A N D DISCUSSION Characterization of As(FDDC), Complex. The crystal structure of the synthesized As-FDDC complex was examined by single-crystal X-ray diffraction analysis. The crystal showed an arsenic to ligand ratio of 1:3, consistent with the expected As(FDDC)~structure. The details of the X-ray diffraction data will be reported elsewhere. The result of elemental analysis of the complex also agrees with the theoretical values expected for As(FDDC),. In addition, thermogravimetric (TG) behaviors of AS(FDDC)~ and As(DDC), were investigated. According to our results, As(FDDC), showed complete volatilization under the conditions of the thermal analysis experiments whereas As(DDQ3 was less volatile and decomposed slightly. Some brown residues was left in the sample pan after runs with As(DDC),. The initial inflection of the TGA curves appears around 175 OC for As( F D D 0 3and 230 "C for As(DDC),. Examination by differential scanning calorimetry (DSC) gave endothermic peaks starting a t 135.7 and 142.8 OC, which corresponding to the melting points of As(FDDC), and As(DDC),, respectively (Figure 1). The DSC peak maxima for As(FDDC), and As(DDC), were 138.8 and 145.7 "C, respectively. These results clearly demonstrated the greater thermal stability and volatility displayed by As(FDDC), relative to the analogous As(DDC),. The mass spectrum of pure As(FDDC), was studied for comparison with those obtaned from the chromatographic peaks eluted from the GC columns as discussed in the next section.
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(a) GC chromatogram of (5 ng) and As(FDDC), (40 ng of As3+) in CHCI,. Conditions: RSL-150 column, 1-pL injection, temperature program described in the Experimental Section. (b) GC chromatogram of As(FDDCb extracted from a well water sample collected from Putai, Tainan, Taiwan. Operation conditions: same as (a). (c) GC chromatogram of Zn (15 ng), Ni (10 ng), Hg (18 ng), Pb (20 ng), A s (45 ng), and Bi (30 ng) FDDC complexes. Operation conditions: same as (a). (d) GC chromatogram of MTG complexes of MMA, DMA, and As3+. The peak labeled with an asterisk is MTG Figure 2.
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Figure 3. Mass spectrum of As(FDDC), obtained on a VG 70170 spectrometer with CI. The spectrum of the chromatographic peak is identical with that of pure As(FDDC),.
GC Separation of As(FDDC),. Five FSOT capillary columns with different lengths and film thicknesses were used to evaluate the resolution and reproducibility for the separation of As and other relevant metal-FDDC complexes. The RSL-150 column (10 m X 0.53 mm and 1.2 pm film thickness) was found to give the most satisfactory results. A representative FID chromatogram obtained for the As(FDDC)~ complex together with C&42 as an internal standard is shown in Figure 2a. The mass spectrum (Figure 3) of the chro-
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 8, APRIL 15, 1991
matographic peak is identical with that of pure As(FDDC)~, indicating that it is not a decomposition product. The As(FDDC& chromatograms obtained from the RSL-160 and the HP-1 columns showed both band broadening and tailing of the As(FDDC), peak. The broadening is probably caused by an enhanced interaction of As(FDDC)~with the thicker liquid film in the RSL-160 and HP-1 columns. Increase in column length, as in the case of DB-5 and DB-1, also caused As(FDDQ3 peak broadening. Therefore, a shorter column with a thin film as in the case of RSL-150 is expected to provide better separations for As(FDDQ3 and other metal-FDDC complexes. Using this RSL-150 column, a good separation of Zn2+-, Ni2+-, Hg2+-, Pb2+-, Bi3+-, and As3+-FDDC complexes can be achieved in 20 min with excellent reproducibility, as shown in Figure 2c. Arsenite in aqueous solution can be quantitatively extracted with Na-FDDC into chloroform in the pH range 2-6 according to our experiments. pH 3 was chosen as the standard condition for extracting As(FDDC), in all our experiments. At this pH, other metal-FDDC complexes, including those of Zn, Ni, Hg, Pb, and Bi, can also be quantitatively extracted. The pH dependence for the extraction of some metal-FDDC complexes (Ni, Pb, and Bi) has been reported by Neeb and Schneider (11). Our experiments indicate that the pH ranges for quantitative extraction of Zn2+and Hg2+are 2.8-6.2 and 2.0-6.5, respectively. The stability of As(FDDC)~in chloroform during storage was evaluated by GC. No noticeable decomposition was observed when an As(FDDC), solution in chloroform was stored in a refrigerator a t 4 OC overnight. However, the concentration of As(FDDC)~diminished and a prominant chromatographic decomposition peak was observed if the solution was stored over 24 h. Therefore, we recommend that after extraction, the chloroform solution should be stored in a refrigerator and analyzed within 1 day. The extraction procedure actually also serves as a preconcentration step for As3+. The organic to aqueous phase ratio in the extraction experiments in this study was set at 1:10, providing a 10-fold increase in concentration. If the ratio is greater than 1:20, incomplete extraction of As3+may result according to our experiments. A larger preconcentration factor can be obtained by evaporation of the organic phase to dryness followed by reconstitution of the solution with a small amount of CHC13. For example, if the initial sample aliquot is 10 mL and the final volume of CHC13 is 100 pL, a preconcentration factor of 100 can be obtained. Applications. The detection limit of As(FDDC)~in chloroform using the given instrument settings with the RSL-150 column and FID has been estimated to be around 10 ng of As (1.33 X mol)/l-pL injection on the basis of a signal to noise ratio of 3. The calibration curve is linear in the concentration range 25-250 ng/pL (3.34 X lo4 to 3.34 X M) with a correlation coefficient of 0.98 for a typical injection of 1 pL of sample. With this detection limit and a preconcentration factor of lo3, this method should be able to detect As in original water samples at the 10 ppb level (1.33 X lo-' M). In real environmental samples, the presence of other extractable organic and inorganic species can raise the background and hence the detection limit of As. The detection limit of this method may be further lowered by increasing the preconcentration factor during extraction. The detection limit may also be lowered by using a more sensitive detector, e.g. ECD instead of FID. However, we found that the As(FDDC), peak was broadened when the ECD was used under identical chromatographic conditions and the reproducibility of the peak was not as good as in the case of FID. This irregular behavior of metal-FDDC complexes in GC/ECD analysis has been reported in the literature previously (6). Careful optimization of the ECD operation parameters is required in order
Table I. Recovery of Spiked AsJ+ and As6+ in Natural Waters and Urine Samples
amt added, Mg As3+ As5+
river watera
urine
recovery As5+
total As
98.2 A 3.2 100.7 f 2.8
3.00 5.00
1.00 2.50 3.00 5.00
%
As3+
3.00 5.00 2.00 2.50
98.6 f 3.0 101.4 f 3.5 97.5 f 3.5 98.5 f 3.5
97.0 99.1 96.4 98.1
f 3.5
f 2.7 f 2.0 f 1.9
Collected from Spring Valley Reservior, Troy, Idaho. to continue research along this direction. The aresenate (As5+)concentration in a water sample can be determined with a second aliquot by using sodium thiosulfate and potassium iodide reduction followed by FDDC extraction. The conditions for the reduction of As5+to As3+ have been described in our previous papers (9,10). Therefore, from two aliquots, one with thiosulfate reduction and the other without reduction, the concentrations of both As5+ and As3+ can be determined by using FDDC extraction and GC analysis. According to the literature, total inorganic arsenic species (AsN + As3+)can be extracted by MTG and determined by GC (3). A typical chromatogram obtained after MTG extraction of MMA, DMA, and inorganic arsenic species in a water sample using the RSL-150 column is shown in Figure 2d. However, our experiments indicated that MTG extraction often resulted in incomplete recovery of inorganic As species, usually around 75-85%. The MTG extraction method is less reliable than the FDDC extraction described in this note for inorganic arsenic species due to lower D (distribution coefficient) values. Recovery of As3+and As5+using this extraction method and GC with a FID was studied with natural river water spiked with 3-5 pg of the inorganic arsenic species. The results are given in Table I. The percentages of recovery of As3+ and AsSf were usually around 97-102% with a standard deviation of about 3%. Recovery of As3+from a urine sample was also found in the range 96-98% with a standard deviation of about 3 9'0. Prior to FDDC extraction, the urine sample was digested with HN03 and H202according to the procedures described in the literature followed by the reduction procedure described above (12). Because of oxidation during digestion, only total inorganic As can be determined in urine samples. This method has been applied to arsenic speciation in a deep well water sample collected from the "black-foot disease" area in southwest Taiwan. Arsenic has been suspected as a possible cause of this endemic peripheral vascular disease (13). The well water was found to contain 331 f 10 ppb As3+and 906 f 14 ppb total inorganic As. The results agree with those determined by the solvent extraction and neutron activation analysis (NAA) method described in our previous papers (9, 10). The GC chromatogram of the extracted As3+species from this water sample is shown in Figure 2b. The concentrations of arsenic species in this well water are much higher than the U S . Public Health Service's recommended limit of 50 ppb total As (1). The high level of As3+found in this well water is alarming because the toxicity of arsenite has been estimated to be 50 times greater than arsenate ( I ) . In another environmental water sample collected from the Travonia mine in Butte, MT, the total inorganic As was found to be 218 12 ppb and the As5+ content was
