ANALYTICAL CHEMISTRY, VOL, 5 1 , NO. 7, JUNE 1979
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Concentration and Spectrochemical Determination of Trace Metals in Urine with a Poly(dithi0carbamate) Resin and Inductively Coupled Plasma-Atomic Emission Spectrometry Ramon M. Barnes’ and Jeanne Spero Genna Department of Chemistry, GRC Tower I, Unlverslty of Massachusetts, Amherst, Massachusetts
The separatlon and concentrallon capabllltles of a poly(d1thlocarbamate) resln comblned wlth the detectlon power of ICP-AES were examlned for the determlnatlon of 10 trace metals In urlne. The separatlon of analyte metals from the major electrolyte and organlc matrlx constltuents of urlne and thelr concentration by 125 times into a 2-mL flnal volume were demonstrated. The only treatment of the urlne prlor to Its lntroductlon to the resln was collectlon over HCI and flltratlon. Quantltatlve recoverles from the resln were obtalned by uslng a mixed-acld dlgestlon procedure. Slmple acid-matched reference solutlons were adequate for analytlcal callbratlon,
A method of simultaneous multielement analysis that isolates trace elements of interest from biological matrices and provides concentration factors sufficient to allow quantitative determinations a t the nanogram per milliliter level is needed. Among the analytical techniques that provide simultaneous multielement analysis capability, the inductively coupled plasma (ICP) discharge has high potential to satisfy this need. Haas e t al. (1, 2) recently reported the determination of 13 elements (All As, Be, Cd, Co, Cr, Fe, Ni, P b , Se, T i , V, and Zn) and exploratory studies with Ca, Mg, P, B, Cu, and Mn in dilute, normal, and concentrated urine samples using inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Urine samples containing a wide range of total dissolved solids could be analyzed successfully. The approach required: (a) two internal reference elements (Ga and Y) to compensate for variations in nebulizer efficiency that resulted from differences in the amount of total dissolved solids in the sample; (b) a computer-controlled background correction scheme to overcome background shifts and instrumental stray light effects which arose from variable calcium and magnesium concentrations, and (c) an ultrasonic nebulizer with sample aerosol desolvation. This analytical approach lacked sufficient detection power for the quantitative determination of expected intrinsic levels of many trace elements (Al, As, Cd, Co, Cr, Mn, Ni, Pb, V) in urine, and Haas et al. recommended either improved limits of detection or analyte preconcentration for these elements. As an example of the latter, Nixon (3)employed a digestion and bromide distillation procedure for isolating and concentrating As, Ge, Se, and Sn in the ICP-AES analysis of urine. Previously, Dahlquist et al. ( 4 ) reported limits of detection in urine by ICP-AES for As and Hg obtained by conventional gaseous hydride generation and for As, Hg, and P b by thermal atomization from a graphite yarn. Even when state-of-the art limits of detection for ICP-AES with pneumatic nebulization ( 5 )are compared with expected ranges of trace elements in urine (I, 6, 7), Ag, Bi, Hg, Nb, Pb, Sb, Se, and Sn are among the elements that cannot be determined quantitatively without preconcentration. Ion-exchange chelating resins like Chelex 100 have been used for the simultaneous determination of trace elements in urine (81, but alkali and alkaline earth elements are also 0003-2700/79/0351-1065$01 OO/O
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complexed, which would introduce rather than exclude interfering concomitants in ICP-AES. Based upon the earlier developments of Dingman et al. (9, 101, Hackett and Siggia (11) recently synthesized and tested a poly(dithiocarbamate) resin with Cd(II), Co(II), Cu(II), Ni(II), Pb(II), Sb(III), and Zn(I1) in the atomic absorption spectrophotometric determination of P b in snow, Cu in whole milk and tapwater, and Cu, Cd, and P b in river and sea waters, They also demonstrated the selective chelation of certain trace metals and essential exclusion of the alkali and alkaline earth elements as well as Mn and Fe. The elements found to complex quantitatively with the poly(dithi0carbamate) resin were As, Ag, Cd, Cu, Hg, Ni, Pb, Sb, and Zn ( I I , 1 2 ) . Recent studies in our laboratory have shown that All Au, Bi, In, Ir, Pd, Pt, Re, Rh, Ru, Se, Sn, Te, and T1 also complex quantitatively with this resin. Description of these characteristics will be given in a subsequent paper, The ICP-AES method reported in this article for trace elements in urine combines the poly(dithi0carbamate) resin with the simultaneous multielement analysis capability of ICP-AES. Thus, it maintains all of the features of the ICP analysis and, simultaneously, eliminates background shifts in the ICP and instrumental stray light effects arising from the variable concentration of major urine constituents. The approach takes advantage of the selective concentration of specific trace metals provided by this novel resin to separate desired trace metals from interfering concomitants, alkali and alkaline earth elements and the organic matrix of the urine. Thus, the concentration levels of analytes can be increased in the final sample solutions so that pneumatic nebulization can be employed. Correction for variable total dissolved solids content is not required when the simple aqueous reference solutions are made to contain the same acid content as the treated urine samples. The objectives of this investigation include the development of a procedure for the separation of trace metals from the organic and major electrolyte constituents of the urine matrix, concentration of the trace elements employing the poly(dithiocarbamate) resin, and their quantitative multielement determination with ICP-AES. EXPERIMENTAL Apparatus. Metal determinations were performed using two
ICP systems. The experimental facilities and the operating conditions employed are given in Table I. The analysis wavelengths used with both ICP systems are listed in Table 11. Resin columns were made with disposable Pasteur pipets (Fisher No. 13-678-6B),and sample reservoirs were 1-L, 500-mL, and 250-mL separatory funnels that had been modified by sealing a 1-inch piece of 7-mm 0.d. tubing t o the funnel outlets. Reagents. Reagents for the synthesis of the poly(dithi0carbamate) resin included polyethyleneimine, 1800 molecular weight, (PEI-18) (Dow Chemical Co., Midland, Mich., or Polysciences Inc., Warrington, Pa.), polymethylenepolyphenyl isocyanate (PAPI) (Upjohn Polymer Chemicals, Kalamazoo, Mich.),ACS reagent grade ammonium hydroxide,carbon disulfide (Fisher Scientific Co.), and pyridine (Eastman Kodak Co., C 1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 7, JUNE 1979
Table I. Experimental Facilities and Operating Conditions inductively coupled plasma system I Plasma-Therm model HFS-5000 radiofrequency generator D generator, 40 MHz, 1000 W. power load coil 3 turns, 1/5-in.diameter. aerosol generator cross flow nebulizer, PlasmaTherm model TN-l. nebulizer chamber barrel spray chamber, PlasmaTherm model SC-2. plasma torch assembly quartz torch, Plasma-Therm model T1.5.
(20).
gas flows plasma argon flow rate auxiliary argon aerosol argon average sample uptake rate observation height of plasma imaging optics scanning monochromator grating slit width slit height photometer photomultiplier digital readout
1 5 L/min. none flow rate, 1.0 L/min; pressure, 1 2 psi. 1 . 6 mL/min 1 0 m m above the t o p of the load coil. quartz lens, 2-in. diameter, 200mm focal length (Oriel. iiA-11-661-37), 1:l image. Heath model EU-700-56 (Benton Harbor, Mich). 1180 lines/mm 30 u m 5 rim Heath model EU-703-31 Heath model EU-701-30 Hewlett-Packard 3420A digital voltmeter,
Table 11. ICP-AES Element and Wavelength List element As I Bi I Ca I1 Cd I1 cu I Hg 1 Mg I1
wavelength, nm 193.76 289.80 393.37 226.50 324.75 253.65 279.55
system I1 Forrest Electronics ICP generator, 26.2 MHz, (18) 850 W. 1.5 turns, '/a-in. diameter. cross flow nebulizer as described by Kniseley e t al. ( I 9 ) Scott-type chamber as described by Scott e t al. ( 2 0 ) . quartz torch. Outer tube 18-mm i.d. Aerosol tube 1.5-mm i.d.
element Ni I Pb I Sb I Se I Sn I Te I UI
wavelength, mm 341.48 405.78 217.58 196.09 303.41 238.58 385.96
Rochester, N.Y.). ACS reagent grade metals, salts, acids, and hases were used throughout. Poly(dithi0carbamate) Resin Synthesis. The poly(dithiocarbamate) chelating resin was synthesized by reacting polyethyleneimine (PEI-18) cross-linked with polymethylenepolyphenyl isocyanate (PAPI) followed by treatment with carbon disulfide and isopropyl alcohol. The PEI-l8/PAPI (8.0/1.0 molar equivalent ratio) resin was prepared according to Hackett and Siggia (11) using 72.12 g PEI-18, 27.88 g PAPI, dioxane, 200 mL ammonium hydroxide, 300 mL carbon disulfide, and 800 mL isopropyl alcohol. Resin Column Preparation. The 8.5 cm long by 0.5 cm o.d. disposable pipet columns were silanized following the procedure described by Hackett (12) with a 15% solution of methylchlorosilane in toluene to which 25 mL of pyridine was added. Without silanization of all of the column and container glass surfaces, some metals were lost from the samples (12). A small plug of glass wool was packed into each column to hold the resin in place, and the empty columns and glass wool plug were soaked in concentrated nitric acid until ready for use. The columns were drained and rinsed several times with distilled, deionized water prior to filling with resin. After sieving, 100 mg of the 60/80 mesh size resin was slurried with 2 mL distilled, deionized water. The rinsed columns were filled with the resin slurry, and half of the water was drained. The columns were attached to the outlet of the sample reservoir with a Tygon tubing sleeve. The sleeve and column were then filled with distilled, deionized water. Metal Recovery from Resin. Although destructive elution with hot 8 M nitric acid removed many metals complexed by the resin (11, 12),the elution technique was time consuming, left an incompletely digested yellow residue in the column, and yielded
1 0 . 4 L/min. none flow rate, 0.65 L/min; pressure, 21 psi. 1.5 mL/min. 1 5 m m above the t o p of the load coil. quartz lens, 2-in. diameter, 200m m focal length (Oriel. #A-11661-37), 1:l image. model EU-700-56 (Heath, Benton Harbor, Mich). 1180 lines/mm 75 u m 5 rim Heath model EU-703-31. Heath model EU-701-30. Hewlett-Packard 3430A digital voltmeter.
poor recovery of some metals. Hackett (22) observed quantitative uptake of mercury and silver by the resin but low recovery by destructive elution. Recent studies in our laboratory demonstrated that Pt, Pd, Se, Sn, Re, and T1 behave similarly. On the other hand, total digestion of the resin with nitric acid improved recoveries of Pd, Pt, and Re. With total nitric acid digestion, however, subsequent dilution with nitric acid was required to prevent hydrolysis. In the present procedure, a mixture of nitric and sulfuric acids, widely used for the digestion of organic materials (13), was employed. The resin, containing the sequestered metals, was transferred from the glass column into a 10-mL beaker by forcing air from a pipet bulb placed at the constricted end of the column. One milliliter of 1-to-1 (v/v) nitric acid-sulfuric acid was added to the resin, and with the beaker covered with a watchglass, the resin was gently heated on a hot plate, usually 2 to 3 min, until a clear solution was obtained. The solution was transferred to a volumetric flask and diluted to volume with distilled, deionized water. When 2-mL final volumes were used, the reference solutions were prepared to contain 50% (v/v) 1-to-1 nitric acid-sulfuric acid mixture. Single-element tests of this procedure gave recoveries of 94% for Cu a t the 20 pg/mL level and 99% for Hg a t the 5 pg/mL level in the digested resin solution after 50-fold concentration from a 500-mL original solution. Studies were also conducted for the simultaneous recovery of copper, cadmium, and lead from aqueous solutions adjusted to pH 5 prior to their introduction to the resin columns. Other recovery tests were preformed for copper and nickel, copper and mercury, and selenium and tin. Solutions (100 mL) were adjusted to pH 5 except for the selenium and tin which were made pH 1. Results for these investigations are given in Table I11 as determined by ICP-AES using the conditions in Table I. Urine Sample Treatment. Urine samples from local volunteers were collected over concentrated HC1 (50 mL/L urine) ( 2 4 ) ,filtered through membrane filters (Gelman Instrument Co., Ann Arbor, Mich., Fisher No. 09-735 and 09-730-261, and refrigerated. For recovery studies and analysis, 250-mL aliquots of urine from either individual 2-L samples or from a composite urine sample were placed in silanized glass reservoirs, attached to prepared columns, and allowed to flow through the resin a t approximately 2.5 mL/min. Uranium Recovery, Uranium complexation with the poly(dithi0carbamate) resin had not been studied previously, and
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Table 111. Recovery of Elements from Poly(dithi0carbamate) Resin Columns with Laboratory Prepared Samples in Water final concentration, kg/mL amount no. of metal (s) added, pg replicates expected measured recovery, 510 copper cadmium lead
50 50 50
4 4
copper nickel
50 50
2 2
copper mercury
50 50
2 2
selenium tin
50 50
2 2
uranium
100 200 500
2 2 2 2
1000
4
Copper, cadmium, and lead (pH 5 ) 5.0 4.9, 5.0, 4.9, 4.9 5.0, 5.4, 5.0, 5.4 5.0 4.7, 4.8, 4.7, 5.0 5.0 Copper and nickel (pH 5 ) 5.0 4.8, 4.9 5.0 4.8, 4.5 Copper and mercury (pH 5 ) 5.0 4.9, 4.9 5.0 4.9, 4.8 Selenium and tin (pH 1) 5.0 4.8, 4.8 5.0 4.9, 4.9 Uranium (pH 6) 20.0 19.6, 20.0 39.8, 39.6 40.0 100.0 95.0, 97.0 192.0, 196.0 200.0
Table IV. Concentrations of Elements in Composite Urine Sample, pg/L element untreated digested extracted Cu 65.9 i 6.6a 67.0 t 6.0 63.1 z 5.7 65.5 r 6.4 66.6 z 5.3 63.4 t 5.7 Cd 22.9 t 3.4 26.8 I4.4 25.4 i 3 . 5 24.3 = 3.6 27.8 t 4.6 25.3 t 3.1 Pb 33.8 = 5 . 1 30.4 i 3.9 30.0 i 2.7 30.8 I 4.6 33.6 I4.7 32.0 i. 2.9 Ni 105.9 z 9.5 112.0 c 8.9 111.2 10.0 107.0 f 9.6 112.0 t 8.9 109.8 z 9.8 Hg 11.2 I1.7 13.6 * 1.6 C 12.8 t 1.9 12.4 I1 . 3 u
