Anal. Chem. 1980.
676
The determination limits ( n ~of) the present method are calculated using the GAMANL (7, 8) criterion of the least possible registerable peak area as described by Salbu (9) (Table IV). The determination limits are of the order 10-'-10-3 pg/L for 22 of the 28 elements investigated, which underlines the usefulness of this combined method. Considering the very low concentration of most of these 28 elements in seawater, emphasis should also be put on the fact that contamination problems during electrolysis are avoided as radioactive species only, deposited on the electrode or left in the solution, are being measured. Hence, technical reagents can be utilized. Thus, using the combination of neutron activation analysis and electrolysis as described in this paper, 28 elements in seawater samples may be determined quantitatively. The method has been applied to a study of samples of marine origin from a Norwegian lake, the results of which will be published elsewhere. This method may also be used in multielement analysis of other samples having high content of interfering elements which are not deposited on the electrode, Le., samples of biological and geological origin.
Table IV. Determination Limits of the Present Method Applied to Seawater Samples.
element
determination limit, n ~pg/L ,
background level, counts/ channel
Elements deposited on the mercury electrode Ag As AU
0.30
Cd
0.21 0.04 0.17 20.96 0.04 0.05 0.04 0.42 0.06 0.68 14.16
0.11 0.001
co Cr Fe Ga
Hg La
Mo Sb
Se Zn
2000 1800 2200 2700 3 00 3000 200 2200 3200 100
4300 1900 3200 500
Elements left in solution Ba Br Ca Ce
cs
Eu Na
2.25 0.12
46.92 0.07 0.02 0.03 17.48 0.57
150 150 330 370
Sm Sr
0.03 18.36
Th U
0.03
0.12
Yb
0.14
150
Rb sc
0.002
ACKNOWLEDGMENT The authors are indebted to W. Lund, Department of Chemistry, University of Oslo, for kindly providing the equipment for electrolysis. They express their sincere gratitude to A. C. Pappas for valuable discussions and for having read the manuscript.
40
200 90 25 40 420 150 200 4 20
52. 676-679
LITERATURE CITED
precision varies from 6% (Cr) to 19% (Hg). Because of counting statistics, the reproducibility increases with concentration. The residual solution, after electrolyzing for 1 h, still contains high amounts of Na and Br. After radioactive decay-time of 2 weeks, however, the determination of additional elements, Le., Ba, Br, Ca, Ce, Cs, Eu, Na, Rb, Sc, Sm, Sr, Th, U, and Yb, is carried out by measuring the activities in the residual solution.
(1) Piper, D. Z.; Goles, G. G. Anal. Chim. Acta 1969, 4 7 , 560-563. (2) Bolter, E.; Turekian, K. K.; Schutz, D. F. Geochim. Cosmochim. Acta 1964, 28, 1459-1466. (3) Schutz, D. F.; Turekian, K . K. Geochim. Cosmochim. Acta 1965, 29, 259-313. (4) Florence, T. M. J . Nectroanal. Chem. 1972, 35, 237-245. (5) Lund, W.; Salberg, M. Anal. Chim. Acta 1975, 76, 131-141. (6) Fahland, J.; Herrmann, G. Z.Anorg. Allg. Chem. 1962, 376, 141-153. (7) Gunnink, R.; Levy, H. B.; Niday, E. UCID-15140. 1967. (8) Scheidemann, @, Department of Chemistry, University of Oslo, private communication, 1970. (9) Salbu, B.; Steinnes, E.; Pappas, A. C. Anal. Chem. 1975, 4 7 , 1011-1016.
RECEIVEDfor review October 12, 1979. Accepted December 12, 1979. Based on a thesis by Kari J$rstad.
Simultaneous Determination of Mercury and Cadmium in Biological Materials by Radiochemical Neutron Activation Ana Iys is Robert R. Greenberg Center for Analytical Chemistry, National Bureau o f Standards, Washington, D.C. 20234
A radiochemical procedure has been developed for the simultaneous determination of Hg and Cd in biological matrices. The procedure is based upon bomb dissolution followed by solvent extraction using Ni and Zn diethyldlthlocarbamates. Mercury is separated from Se allowing the use of the '03Hg isotope as well as the '"Hg isotope to quantify Hg.
Mercury and cadmium are two of the more common environmental pollutants. Even at very low levels, these eleThis article not subject to
US. Copyright.
ments have been suspected of causing detrimental health effects. Mercury and cadmium both appear on the Environmental Protection Agency's Priority List of Toxic Substances ( I ) . The Safe Water Drinking Act allows a maximum Hg concentration of 2 pg/L, and marine organisms from water containing more than 0.05 pg/L may be harmful to human consumers (2). The maximum allowable Cd concentration in drinking water is 10 pglL and concentrations above 0.4 pg/L may be harmful to some types of aquatic life (2). Mercury and cadmium are commonly found in biological materials at the ppm or sub ppm level. At these levels,
Published 1980 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 4, APRIL 1980
problems occur in accurately determining the concentrations of these elements for several reasons: losses during sample treatment, contamination, reagent blank, and interferences. This was illustrated by a recent interlaboratory comparison of analyses of oyster homogenate conducted by the International Atomic Energy Agency (IAEA). Ranges of more than a n order of magnitude were reported for both elements ( 3 ) . This illustrates the problems involved in accurately determining the Hg and Cd concentrations in biological materials even when these concentrations are relatively high; these problems become more serious as the Hg and Cd content decreases. An analytical procedure has been developed to determine both Hg and Cd simultaneously by radiochemical neutron activation analysis (RNAA). RNAA offers the advantages of high sensitivity, excellent selectivity, and no reagent blank. Furthermore, the ability to add carriers during the chemical dissolution and separation enables a quantitative recovery of both elements by minimizing volatility losses, and losses of micro amounts into the Teflon and/or glass vessels used. The radiochemical procedure is based upon a bomb dissolution followed by solvent extraction with metal diethyldithiocarbamates in chloroform (4-6). Nickel(I1) diethyldithiocarbamate (Ni(DDC)z) is used to isolate Hg; and Zn(DDC)2 in chloroform, followed by a back extraction into 2 M HC1, is used to isolate Cd. Since Hg is isolated from Se, the 203Hg isotope, as well as the 197Hgisotope, can be used to quantify Hg. Samples containing as little as 1 ng of Hg and/or Cd can be accurately analyzed by this procedure if care is taken to optimize irradiation, decay, and counting parameters. EXPERIMENTAL Preparation of Metal DDC Compounds. The Zn(DDC)2and Ni(DDC)2compounds which were used for analysis were prepared by mixing aqueous solutions of NaDDC, and either Zn(NO3I2or Ni(N0J2 The M(DDC), compound formed was insoluble in water and precipitated. The precipitate was filtered, washed with water, and dissolved in chloroform. An equal volume of ethanol was added to the solution, which was then set aside in a fume hood to allow the chloroform to evaporate at room temperature. After the chloroform evaporated, the M(DDC), compound crystallized in the remaining ethanol. The crystals were filtered and allowed to dry at room temperature ( 4 ) . Dissolution Bombs. The bombs used for sample dissolution were Autoclave 3 digestion bombs obtained from Perkin-Elmer. They consisted of an aluminum shell over an inner Teflon container. The internal volume was approximately 125 mL allowing a rather large sample to be dissolved. Samples could be dissolved at temperatures of up to 160 "C and pressures of up to 50 atm, at which point a safety valve would open releasing excess pressure. The samples used for analysis were dissolved at temperatures of 130-140 "C. Procedure. The samples and primary standards were sealed in cleaned quartz vials and irradiated together in the RT-3 pneumatic tube facility at the NBS Research Reactor. In this position, the thermal neutron flux is 5 X l O I 3 ncm-2-s-1with a Cu/Cd ratio of approximately 70 (7). Irradiation times of 1-4 h were used. The primary standards consisted of a solution prepared from high purity metals dissolved in NBS high purity H N 0 3 (8). After decaying for 3 days, the quartz vials were cleaned and opened, and the samples were weighed and transferred to the Teflon-lined bombs. Sample sizes varied from 200-500 mg. Mercury and Cd carriers (1mg and 0.1 mg), along with Cu and Se holdback carriers (0.1 mg each), were added, and the samples were dissolved in 6 mL of H N 0 3 and 2 mL of H2S04(3:l) at temperatures of 13&140 "C for about 2 h. If silica was present, a few drops of HF were also added. After dissolution, the bombs were cooled to -15 "C and opened. The samples were allowed to warm to room temperature in a fume hood and about 2 mL of deionized water were added dropwise to drive off NO2. Two mL of 30% H202were then added dropwise to destroy any remaining reduced nitrogen compounds. At this point the solutions
677
became clear with no visible particulate material. The volume of each solution was adjusted to about 50 mL with deionized water, 100 mg of Zn2+holdback carrier was added (as the nitrate), and the pH was adjusted to 1.5 with ammonia. The solutions were then transferred to 125-mL separatory funnels and shaken for 2 min with 25 mL of 0.005 M Ni(DDC)*in CHC13using a shaking machine. The organic fractions containing Hg were drained into 125-mL linear polyethylene bottles, and the aqueous fractions were washed with 10 mL of CHC1,. The washes were combined with the appropriate Ni(DDC)2solutions, which were retained for counting. Linear polyethylene bottles were used because CHC1, was found to evaporate through the walls at a much slower rate than with conventional polyethylene bottles. The aqueous fractions, still in the separatory funnels, were then shaken for 2 min with 25 mL of 0.005 M Zn(DDC)2in CHC1,. The Zn(DDC)2solutions were transferred to another set of separatory funnels and each aqueous fraction was washed with 10 mL of CHC13,which was added to the appropriate Zn(DDCI2solution. These solutions were then shaken for 15 s with 20 mL of 2 M HC1 and the organic fractions were then discarded. The HC1 solutions containing Cd were drained into 125-mL polyethylene bottles which were retained for counting. Two different procedures were followed for the standards. One set of standards was prepared by pipetting a known amount of irradiated solution into each of the bombs with carriers and some unirradiated sample material. This material was dissolved in the same manner as were the samples used for analysis. Another standard was pipetted directly into 1 M HN03 along with carriers. Both types of standards were then subjected to the same separation scheme used for the samples. Counting. The samples and standards were counted on Ge(Li) detectors with active volumes of 60 to 75 om3 coupled to 4096channel pulse height analyzer systems. The resolution of these detectors (FWHM) varied from 0.7 to 1.0 KeV at 122 keV, and from 1.9 to 2.1 keV at 1332 keV. Vertically-oriented (uplooker) detectors were used so the samples could be counted directly on top of the detectors for a good counting geometry. The Ni(DDC)* solutions containing Hg could be counted immediately after separation for the 67.0-keV Au X-ray, and the 77.5-keV combination y-ray and Au X-ray produced by the decay of l9Hg, and/or after a decay of several weeks for the 279-keV y-ray from '03Hg. The HC1 solutions containing Cd were allowed to decay for at least 24 h to establish the equilibrium between lI5Cd and its daughter 115mIn.The 336-keV line from 115mIn,and the 528-keV line from '16Cd were both used for analysis. RESULTS AND DISCUSSION Possible Interferences. The Hg X-rays produced by the decay of lg8Au can interfere with the Au X-rays emitted by Ig7Hg. T h e Hg K a 2X-ray (69.0 keV) and the Au K,, X-ray (68.8 keV) overlap, as do the Hg KB1,3,5 (80.1 keV) and the Au KB2,4,6 (80.2 keV) X-rays. T h e Hg X-rays, however, d o not interfere with the Au Ka2X-ray (67.0 keV) or with the combination Au K81,3,5 X-ray-lg7Hg y-ray (observed a t 77.5 keV), so these lines should be used to quantify Hg when using the lg7Hgisotope. Many detectors currently available can fully resolve lines 2 keV apart in this energy region (i.e., F W H M 0.7 keV a t 122 keV) including large volume Ge(Li) and Ge(HP) detectors, as well as low energy photon detectors. Although a small quantity (-10%) of the Au present in a sample does accompany Hg through the separation procedure, the interference for the 68.8- and 80.1-keV lines is usually small in biological materials. 64Coppercan interfere indirectly with the lS7Hganalysis by producing a high background level of radiation. A small, but variable, fraction of the Cu present in the sample is usually found in the Ni(DDC)2fractions. Since the half-life of ls7Hg is much greater than %u (64 h vs. 13 h), a delay of a few days before counting could be used t o improve the peak to background ratio. 82Brominecan also interfere with the ls7Hg determination by producing a high background level of radiation. Some elemental Br is often formed during the sample dissolution
678
ANALYTICAL CHEMISTRY, VOL. 5 2 , NO. 4, APRIL 1980
Table I. Comparison of Hg and Cd Concentrations Observed in NBS Orchard Leaves (SRM 157 1)with NBS Certified Values
sample 1 2
3
4 5 6
50 0
0 50
100
1.50
2 00
PH
mean i 2s certified
Figure 1. Cadmium extracted a s a function of pH
a n d a large fraction (on the order of 40%) extracts in the CHC13solution of Ni(DDC)2. The s2Br levels can be reduced by washing the aqueous fractions several times with CHC13 prio: to extraction with Ni(DDC)2. Since a small fraction of the Hg present extracts into CHCl, (-0.5%) the samples and standards must be subjected to the same number of washes. Alternatively, Hg can be quantified using the longer lived "3Hg isotope after t h e 82Br decays away. T h e 279-keV y-ray from 75Sedirectly interferes with the 279-keV 203Hgy-ray. Since 75Sealso emits -,-rays a t 136 and 265 keV which are approximately 6 and 3 times more intense than the 279 line, any Se interference could easily be detected. T h e addition of the 100 pg of Se holdback carrier in the sepwation procedure results in a 75Sedecontamination factor of lo4 or more. No 75Sewas observed in the Ni(DDCI2fractions of any of the biological materials analyzed. T h e HCl fraction containing Cd also contains some of the 65Zn and 69mZnoriginally present in the irradiated sample. This is due to exchange between the inorganic Zn of the dissolved sample and the Zn of the Zn(DDC), used to extract Cd. Both Zn isotopes can interfere with the Cd determination by elevating the background level of radiation; however, this effect is usually small for biological materials. The use of 100 mg of Zn holdback carrier results in a Zn decontamination of about 20. Since this procedure depends upon the quantitative recovery of Hg from the bomb, this was checked a t various temperatures using 203Hgtracers. T h e two bombs used to dissolve the samples were both checked for Hg loss a t 130, 140, and 150 "C. Two hundred fifty mg of unirradiated oyster material was dissolved in each bomb with 1 mg of Hg carrier a n d a known amount of tracer a t each of the three temperatures. These temperatures were maintained for at least 5 h. T h e solutions were removed from the bombs and compared to a similar amount of tracer solution. In each case, Hg was quantitatively recovered (100 f 1%).Since the samples analyzed were dissolved a t temperatures of 13C-140 O C , Hg loss was not a problem. An additional test of Hg loss was carried out by comparing the different types of standards used for each set of samples analyzed. Since one standard was dissolved in each of the bombs with unirradiated sample material, any loss would be visible upon comparison with the third standard which was pipetted directly into 1 M "0,. No differences were observed. Since extractions with metal DDC compounds are often dependent on p H , tracer experiments were carried out to determine the relative amounts of Hg and Cd extracted as a function of pH. Solutions were prepared to simulate conditions after sample dissolution. Each solution contained 6 m L of H N 0 3 , 2 mL of H2S04, 2 mL of H 2 0 z ,Hg and Cd tracers, Hg, Cd, Se, Cu, and Zn carriers, and deionized water to bring the volume to approximately 50 mL. The solutions were adjusted to various pHs ranging from 0 to 2 with am-
a
concentration, ng/$ Cd ("'Cd/ l l s m I n ) b Hg (197Hg)b Hg (ZG3Hg)b 117 145 153 127 166 161 118 154 159 111 148 146 108 152 148 115 160 161 116 k 1 3 110
i
Dry weight basis.
10
154 * 1 6 155 I1 5
155 i 1 3 156 i 1 5
Isotope used.
Table 11. Comparison of Hg and Cd Concentrations Observed in NBS Bovine Liver (SRM 1577) with NBS Certified Values
sample 1 2 3 4
5 6 mean i 2s certified
concentration, ng/p" Cd ('I5Cd/ 5m1nlb Hg (197Hg)b Hg (*GsHg)b 307 264 295 28 5 293 28 2 288
16.0 15.9 19.7
17.2 17.4 17.7
*
29
2 7 0 t 40
Dry weight basis.
17.3 * 2.8 16i 2
16.3 18.4 16.4 17.0 16.7 15.8 16.8 I 1.8
16 * 2
Isotope used.
monia and extracted with Ni(DDC)2and Zn(DDC)% Mercury was found to extract quantitatively over this range, while the Cd extraction was depressed a t the lower pHs as shown in Figure 1. At p H 0.50 and above at least 99%, the Cd was extracted. Bajo and Wyttenbach (5) observed a similar depression of the Cd fraction extracted by Zn(DDQ2, at low pHs, from aqueous solutions of H2S04,FINO3, and HC10,. Two NBS Standard Reference Materials (SRMs) with certified Hg and Cd concentrations, Orchard Leaves (SRM 1571) and Bovine Liver (SRM 1577), were analyzed by this procedure. The results obtained are compared with the NBS certified concentrations in Tables I and 11. The NBS values were determined using two or more independent analytical techniques ( 9 , I O ) . T h e Hg concentrations determined from both isotopes agree with each other and with the certified values. The Cd concentrations also compare favorably with the certified values. The observed sample-to-sample variations are consistent with the counting statistics. The IAEA Oyster Homogenate (MA-M-l), discussed earlier, was also analyzed and the results obtained are compared, in Table 111,with the IAEA overall average values, and with the averages of results meeting Chauvenet's test. T h e Hg concentrations listed for the present work are t h e mean values determined from the two isotopes. It should be noted t h a t the uncertainties listed for the IAEA results are standard errors, which tend to minimize variations among laboratories. In view of the different meanings of the uncertainties, a direct comparison cannot be made between the IAEA results and those determined in this work. The IAEA value for Cd of 2.3 pg/g (all values met Chauvenet's test) is relatively close to the value of 2.49 pg/g in this work. Five of t h e IAEA Hg values were rejected by Chauvenet's test decreasing the overall mean from 0.32 pg/g to 0.20 pg/g, compared to a value of 0.150 pg/g determined in this work.
Anal. Chem. 1980, 52, 679-682
Table 111. Comparison of Hg and Cd Concentrations Observed in IAEA Oyster Homogenate MA-M-1 with IAEA Results concentration, p g / $ sample 1
2 3
4 5
6 mean
i
2s
Cd 2.44 2.56 2.49 2.54 2.36 2.53 2.49
IAEA, overall meanb 2.3 IAEA, Chauvenet's testb 2.3
i f
Hg
0.150 0.152 0.140 0.156 0.148 0.155 0.15 0.150 i 0.012 0.2 ( 3 0 ) 0.32 t 0.06 ( 2 9 ) 0.2 (30) 0.20 i 0.02 (24)
i
a Dry weight basis. Reference 3 ; numbers in parentheses are number of values used; errors are one standard
-__--.
P1-v-n).
In summary, this technique allows the accurate determination of Hg and Cd in biological materials at the sub-ppm level. The procedure is relatively simple and can be used to analyze both elements in a single sample.
679
LITERATURE CITED (1) Behar. J. V.; Schuck. E. A,; Stanley, R . E.; Morgan, G. B. Environ. Scj. Techno/. 1979. 13. 34-39. (2) Quinby-Hung, M . S. Am. Lab. 1978, 70(13). 17-37. (3) "Intercalibration of Analytical Methods on Marine Environmental Samples", Progress Report 13, 1976, International Laboratory of Marine Radioactivity (Monaco). (4) Wyttenbach, A.; Bajo, S . Anal. Chem. 1975, 45. 1813-1817. (5) Bajo, S.;Wyttenbach, A. Anal. Chem. 1977, 49, 158-161. (6) Gallorini, M.; Greenberg. R. R.; Gills, T. E. Anal. Chem. 1978, 50, 1479-1481. (7) Becker, D. A.; LaFleur, P. D. J . Radioanal. Chem. 1974, 79, 149-157. (6) Kuehner, E. C.; Alvarez, R.; Pauisen, P. J.; Murphy, T. J. Anal. Chem. 1972, 44, 2050-2056. (9) Office Of Standard Reference Materials, National Bureau of Standards, Aua. 1977. Certificate of Analvsis-Standard Reference Material 1571 (OGhard Leaves). (10) Office of Standard Reference Materials, National Bureau of Standards, June 1977. Certificate of Analysis-Standard Reference Material 1577 (Bovine Liver).
RECEIVED for review September 20,1979. Accepted December 17, 1979. Certain commercial equipment, instruments, or materials are identified in this paper in order to adequately specify the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply t h a t the material or equipment identified is necessarily the best available for the purpose.
Angle Resolved Mass Spectrometry with a Reversed Geometry Spectrometer D. M. Fedor and
R. G. Cooks"
Depadment of Chemistty, Purdue University, West La fayette, Indiana 47907
Resolution of the angle into which the products of kilovolt energy Ion-molecule reactions are scattered has been achieved with a reversed geometry double focusing mass spectrometer. Angle selection is in the z-direction normal to the plane of analysis. This is achieved by a simple modification of the Instrument involving addition of two angle-selecting slits. Performance data are given and compared with scattering in the plane of analysis using a double focusing mass spectrometer of conventional geometry. Three types of angle resolved experiments are described: (i) energy loss spectra for inelastic Ion-atom collisions, (ii) mass spectra, showing differential survival of individual mass ions as a function of scattering angle, and (hi) collision-induced dissociation or collisional activation spectra of ions taken using angular resolution as an extra resolution element. The polyatomic ion data are presented In terms of relative Ion abundance as a function of scattering angle and as doubly differential (angle and fragment mass) spectra.
Angular resolution in mass spectrometry provides a n example of how differential measurements increase the power of a technique and of what they cost in terms of signal-tonoise-to-analysis time. Selection of the scattering angle through which the products of kilovolt energy ionic collisions are directed has previously been largely confined to atomic systems ( I ) . Recently this type of resolution has been extended to molecular systems using a modified double focusing mass spectrometer of conventional Nier-Johnson geometry ( 2 , 3 ) . The instrument modifications are straightforward and
the resulting spectra have potential value in analysis (3). An important recent observation is that ions scattered to a given angle without charge alteration are energized by approximately the same amount and that the energy can be varied by varying the angle ( 4 , 5). This feature, when applied to selection of the products of collision-induced dissociation, amounts in fact to preselection of reacting ions on the basis of their internal energies. Control or determination of ion internal energies has always been a weak point in conventional mass spectrometry (6), hence the potential value of this discovery. T h e present paper seeks to combine the advantages of angular resolution with those of prior mass-selection in studies using collision-induced dissociation. Thus, the task is to modify a reversed geometry mass-analyzed ion kinetic energy spectrometer for angular studies. The result is access to a doubly differential experiment in which the products arising from the selected ion, ml+, are distinguished in terms of kinetic energy (proportional to fragment ion mass in collision-induced dissociations) and in terms of scattering angle. Soft (glancing) collisions between kilovolt energy ions and neutral targets are of interest here; they result in excitation of either collision partner or in simple charge transfer or stripping (7). Typical of these processes are collision-induced dissociation,
+ + N
m2+ + m3 charge exchange of doubly charged ions, mI2+ N ml+ N+
t 2)
and charge stripping, ml+ N
(3)
ml+
0003-2700/80/0352-0679$01.00/0C 1980 American Chemical Society
+
+ + e-
mI2+ N
(1)
