ANALYTICAL CHEMISTRY, VOL. 50, NO. 1, JANUARY 1978
13
Automated Method for the Determination of Total Dissolved Mercury in Fresh and Saline Waters by Ultraviolet Digestion and Cold Vapor Atomic Absorption Spectrometry Haig Agemian" and A. S. Y. Chau Canada Centre for Inland Waters, Water Quality Branch, 867 Lakeshore Road, P.O. Box 5050, Burlington, Ontario, Canada
A flow-through UV digestor is incorporated into the automated cold vapor atomic absorption spectrometric technique to provide a completely automated method for the determination of total dissolved mercury in fresh and saline natural waters. UV digestion as a means of degradation of organomercurials removes the interference of chloride which is encountered in automated chemical oxidation techniques. The method is shown to degrade seven of the most common organomercurials which may be found in natural waters. The precision of the method at levels of 0.07 pg/L, 0.28 pg/L and 0.55 pg/L Hg was f6.0% , f3.8 % , and fl.O YO,respectively. The detection limit of the system is 0.02 pg/L and the method is capable of analyzing 30 samples per hour.
Several workers ( I , 2) have described methods for the automated determination of total dissolved mercury in natural waters. These methods consisted of digestion of the sample with acid-permanganate followed by reduction of mercury and detection by the cold vapor atomic absorption technique. Bennett et al. (3) later showed that acid-permanganate alone did not recover three methyl mercuric compounds, while the addition of a potassium persulfate oxidation step increased recoveries t o 100%. El-Awady e t al. ( 4 ) confirmed the low recoveries of methylmercury by acid-permanganate. They showed that only about 30% of methylmercury could be recovered by this method, while the use of potassium persulfate produced complete recovery. The automated method for the determination of total dissolved mercury has now been established and is used as the standard method by the U.S. Environmental Protection Agency ( 5 ) and the Water Quality Branch, Canada (6). Although the above automated methods are satisfactory for natural fresh waters, they suffer from Cl- interference, making them nonapplicable t o saline waters. The large amounts of C1- found in saline waters reduce all of the oxidant used in the system, thus interfering with the oxidation of organomercurials. In addition, large amounts of chlorine are produced which unless reduced back to chloride, would absorb a t the 253.7-nm line causing a positive interference. Because of these problems, high chloride samples could not be analyzed in an automated system and require a manual predigestion step. The requirement in our study is to obtain an automated method which could be applicable to fresh waters, brackish waters and seawaters. Goulden and Afghan (7) showed that five organomercurials decomposed readily after irradiation with ultraviolet radiation for 1 h. In the present study it was found that the rate of decomposition of organomercurials increased rapidly in the presence of sulfuric acid and with increased surface area of the UV irradiation. A flow-through UV digestor which had a delay time of 3 min was used to carry out the photooxidation in the automated system. T h e UV radiation has no effect on chloride. T h e method therefore can be applied t o both fresh and saline waters 0003-2700/78/0350-0013$01 .OO/O
without the chloride interference. In addition to the above advantage, the method does not require a heating digestion bath or any chemical oxidation; thus lower reagent blanks and better baselines are obtained. EXPERIMENTAL Apparatus. The equipment used :For the analysis consisted of the following: Manifold 1 (Figure 1). (a) An automatic sampler (Technicon AutoAnalyzer I1 sampler with 30-2/1 cam). (b) Proportioning pump (Carlo Erba, Model 08-59-10202). (c) Technicon AutoAnalyzer tubing of specified dimensions and color codes. (d) Ultraviolet digestor as used by Afghan et al. (8). It is made as follows: (UV exposure time, 3 min). A 550-W photochemical lamp (Hanovia quartz mercury lamp Type A catalogue No. 673A-36 Englehard Hanovia, Inc., 100 Chestnut Street, Newark, N.J.) place inside a quartz coil made of Puracil453 quality fused silica tubing approximately 10 m long, 3-mm i.d., 0.6-mm wall thickness and a coil diameter of approximately 12 cm. (NOTE:A proper exhaust must be used in conjunction with the L'V digestor to vent off the ozone produced during irradiation.) (e) Gas separator as reported previously ( I , 3 ) . (f) Detector. Two systems were used for detection of mercury; at the 253.7-nm line: (1) Mercury Monitor (Pharmacia Fine Chemicals). This has a 30-cm long cell and has no background correction. (2) Model 603 Perkin-Elmer atomic absorption spectrophotometer with automatic background correction and equipped with a home-made cell with quartz windows (10-mm diameter and 100 mm long). A mercury hollow cathode lamp was used in the instrument. (NOTE: Both systems produced identical results and were used in the peak height mode of operation.) (g) Strip chart recorder (Hewlett-Packard, Model 7101B). Manifold 2 (Figure 2). As an alternative to Manifold 1. Same as for Manifold 1 (Figure 1) except the pump is replaced with a Technicon AutoAnalyzer I1 proportioning pump with the specified dimensions and color codes for the tubing (Figure 2). (NOTE:All tubing used in manifolds 1 and 2 was clear standard Technicon tubing except for the H2S04line, in which case it was acidflex Technicon tubing.) Reagents. High-purity certified reagents were used for all analyses (Fisher Scientific Company, 184 Railside Itd., Don Mills, Ontario, Canada). (1) Sulfuric acid, 36 N. (2) Stannous sulfate solution, 1070 w/v in 2 N sulfuric acid. (3) Hydroxylamine sulfate (3% w/v)-sodium chloride (3% w/v) solution. (4)Hg stock solutions (mg/L as Hg): (a) Mercuric chloride, 1000 mg/L; (b) Phenylmercuric acetate, 1000 mg/L; (c) Phenylmercuric nitrate, 1000 mg/L; (d) Diphenylmercury, 100 mg/L; (e) Methylmercuric chloride, 500 mg/L; (f) Ethylmercuric chloride, 500 mg/L; (9) Methoxyethylmercuric chloride, 100 rng/L; (h) Ethoxyethylmercuric chloride, 100 mg/L. (NOTE:Because of the low water solubilities of diphenylmercury, methoxyethylmercuric chloride, and ethoxyethylmercuric chloride in water, they were made up in 40%, lo%, and 10% solutions, respectively, of'spectrograde acetonitrile in water. Acetonitrile aids in the dissolution of the above organomercurials and yet it (as well as its UV oxidation products) does not absorb in the 254-nm region of the UV spectrum. The preservative used for the stock solutions was 1% H,S04. At such high levels this preservative is adequate. Procedure. Preseruation. Jenne and Avotins (9) have presented a review of preservatives used for mercury in water. The requirement for a strong oxidizing agent together with a strong acid for the preservation of low levels of mercury has been pointed out. Both potassium permanganate and dichromate have C 1977 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 1, JANUARY 1978
14
PROPORTIONING PJMF
(C
11
Table I. Recoveries of Organomercury Compounds for Different Oxidation Methods in the Automated Systema
Erori
Method 5 PgiL of Hg as organic compound 1. Phenylmercuric acetate 2. Phenylmercuric nitrate 3. Diphenylmercury
4. Methylmercuric chloride 5. Ethylmercuric chloride 6. Methoxyethylmercuric chloride 7 . Ethoxyethylmercuric chloride
I
PROPORTIONING
*
~
a
b
c
74 80 95 7 3 75 95
d
e
61 1 0 2 98 91 46 98 98 9 5 96 93 85 9 5
71 84 1 0 0
65 92 4 1 46 89 8 1 88 88 7 0 75 94 85 91 9 1
a The manifolds in Figures 1 or 2 were used with the appropriate reagent lines given in each method. a, H,SO, ; b, H,SO, + 4% (w/v) K,Cr,O,; c, H,SO, + 0.5% (wiv) KMnO, + 0.5% (w/v) K,S,O,; d, UV oxidation; e, H,SO, + UV oxidation.
Figure 1. Mercury manifold No. 1 PJh'PIA,,
( % recovery i ~ j ) b
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been used as the oxidants. The former is inadequate for samples with high chloride levels since it would be readily consumed by chloride (see Results and Discussion). Carron and Agemian (10) showed that 1%H2S04+ 0.05% K2Cr07make a very effective preservative for sub-ppb levels of mercury in water, for extended periods of time especially when coupled with glass as the container. Therefore in this study water samples (100 mL) were preserved by adding 1 mL concentrated H2S04and 1 mL of 5% K2Cr207 in glass containers a t the start of the dilutions or sampling. Analysis. Prepare dilute mercury standards by serial dilution of the stock solutions given under reagents. The system has a detection limit of 0.02 pg/L and is linear up t o about 5 pg/L. Prepare two sets of standards in the required range; one with mercuric chloride and one with methylmercuric chloride. This procedure checks one standard vs. the other in addition to checking the performance of the system in degrading organomercurials. Manifolds 1 and 2 in Figures 1 and 2, respectively, give equivalent results. They are provided in order to give the method more flexibility with respect to type of pump used. The Carlo Erba pump has a pumping rate one and a half times greater than the Technicon pump and therefore requires accordingly smaller size tubing to obtain the required flow rate. After the system is warmed up and a steady baseline is obtained, standards and samples could be aspirated into the system, automatically from glass sample cups by the Technicon sampler. A rate of 30 samples/h was found to be the practical limit for this system.
RESULTS AND DISCUSSION As stated earlier, automated methods employing wet chemical oxidations for the determination of mercury in water suffer interferences from high chloride levels. The mechanism of chloride interference is as follows: Permanganate and persulfate which are the two oxidants employed in the method have half-cell oxidation-reduction reactions as given below. S , 0 E 2 - + 2e
MnO;
+
z 2S0,2-; E ,
8H'
+
= 2.01 V
5 e C' Mn'+
+
4H,O; E , = 1.51 V
(1) (11)
In the presence of these oxidants, chloride would be readily oxidized to chlorine gas since the C12/C1- couple has a half-cell potential of only 1.358 V. Initially it was thought that the chloride problem could be solved by increasing the number of equivalents of oxidant used in the method to a level in excess of that of chloride in the sample. For an average seawater, the chloride content is about 19ooO mg/L (11). It is easily seen that such a procedure would require excessive amounts of oxidants for the oxidation of chloride and reducing agents for the subsequent reduction of the resulting chlorine. The overall effect of such an attempt is to increase blank values and thus increase detection limits considerably, change kinetics of the reaction due to the increased concentrations and flow rates, and generate problems involving M n 0 2 depositions as a consequence of the use of concentrated K M n 0 4 solutions (3-570 w/v). Dichromate is a widely used oxidizing agent in analytical chemistry and the CrzO?-/C3+ couple has a half-cell oxidation potential of only 1.36 V. This potential is very close to that of the C12/C1- system so that the oxidation reduction reaction would be almost nonexistent between Cr207'- and C1-. Dichromate was tested as a replacement of permanganate and persulfate in the automated system and indeed no reaction occurred between it and C1- under the conditions of analysis. However, complete recoveries of mercury could not be obtained from methyl mercuric chloride. Table I (column b) shows that only about 45% of this compound could be degraded under the analytical conditions. This value agrees well with that reported by El-Awady e t al. ( 4 ) of about 40%. I t can be seen from Table I that of the seven organomercurials tested, CH3HgC1 is the hardest to decompose. In fact, most workers test their recoveries on this compound since it is the most common form of organomercury in the aquatic environment. From the above discussion it is apparent that chemical oxidation of organomercury in the presence of chloride leads to many problems. However, physical methods such as photooxidation would involve a different mechanism, making selective degradation of organomercurials possible. Table I (column e) shows that complete recovery of seven organomercurials is obtained by using the UV digestor described in the Experimental in the presence of sulfuric acid. Figures 1 and 2 show the manifolds used for the automated analysis. T h e recoveries (Table I) are for 5 pg/L Hg solutions which is much higher than mercury levels in most natural waters. The systems provide similar recoveries throughout the working range of the calibration curve. The table shows that recoveries by UV oxidation and H 2 S 0 4 (column e) are complete and comparable to the permanganate-persulfate oxidation method (column c). Under the conditions of analysis, the I J V radiation
ANALYTICAL CHEMISTRY, VOL. 50, NO. 1 , JANLJARY 1978
has no effect on chloride so t h a t C1- behaves as an inert constituent of t h e sample. Analysis of synthetic mercury solutions of t h e seven compounds in distilled water and synthetic seawater (about 3% w/v NaC1) gave similar recoveries, proving t h a t there was no chloride interference. Furthermore, the absence of any peaks when the UV method was used for seawater without the S n S 0 4 line (Figures 1 and 2) proves t h a t chloride is not oxidized t o chlorine. T h e recovery d a t a in Table I are reported for seven compounds. These compounds include all of t h e forms of mercury which are involved in the proposed mechanism (12, 13) for the hydrochemical behavior of mercury in aquatic ecosystems except that of dimethylmercury. This compound was not tested since it has a very low solubility in water and is very hazardous, making it very hard t o handle. However, t h e rate of UV degradation of this compound was shown t o be faster than t h a t of either t h e phenylmercuric or methylmercuric species ( 7 ) , so t h a t complete recovery of this compound would be expected in the proposed method. In the present automated method, quantitative degradation occurs in a matter of seconds. This result can be explained by the reduced sample cross section and continuous mixing action of the proposed method. The UV reactor utilizes quartz tubings of 4.2-mm cross section (Le., 3-mm i.d. with 0.6-mm wall thickness). T h e low sample cross section allows good penetration of the UV radiation and thus efficient degradation during the 3-min residence time in the digestor. In addition, the continuously moving sample is segmented with air pockets thus further increasing the UV exposed sample surface area as well as generating a slight mixing effect due t o turbulent flow caused by the tube wall friction. This principle is used extensively in Technicon methods (14). T h e acid content of solutions was found to be important for quantitative degradation of the organomercurials. When t h e method was run without the presence of the sulfuric acid line, erratic recoveries were obtained (see Table I, column d). I t is likely t h a t sulfuric acid provides a high boiling acidic medium for effective degradation of organomercurials. After the organomercury oxidation method was established, it was necessary to study the effect of high chloride solutions on the stannous reduction reaction of Hg2+. I t was found that when there was no chloride continuously run in the manifold, chloride in the sample changed the rate of reduction and a shift in baseline was observed. However, when a large amount of chloride was continuously injected into the manifold (Figures 1 and 2 ) , this effect was not seen. Therefore, it is essential to have the hydroxylamine sulfate-sodium chloride line in the manifold as shown in Figures 1 and 2. The reducing agent is included in excess in order to reduce any oxidant (including the Hg preservative in t h e samples, residual chlorine, etc.) which might be present in samples. Agemian and Chau (15)showed that the rates of reduction of solutions containing different amounts of oxidizing agents were different. Therefore, it is essential to normalize all standards and samples with respect t o oxidant content by a mild reduction prior t o the reduction of Hg2+ by stannous ions. Comparison of the proposed method (Figures 1 and 2 ) with the standard Water Quality Method (6) shows t h a t the chemistry and principle of the two systems are identical except for the oxidation step. Therefore, interferences encountered in this method would be identical and they have been reported previously ( 4 , 16). T h e only interference which may be different is t h a t of high sulfide concentrations. Kopp e t al. (16) have indicated t h a t a chemical oxidation step such as permanganate-persulfafate removes this interference. El-Awady et al. ( 4 ) have reported that the automated system utilizing persulfate oxidation removes this interference u p to a level as high as 20 mg/L of S2- (as Na2S). In order to determine
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Figure 3. Calibration curves for Hg spiked into different matrix waters: -0-as HgCI, and -.-as CH3HgCI in distilled water; -0-as HgCI, and -a.s CH3HgCI, in Hamilton Harbour water, Hamilton, Canada (25 pg/L Cl-); and -A-as HgCI, and -&as CH3HgCI in seawater from New Brunswick, Canada (18 000 mg/L CI-)
Table 11. Recovery of Hg as CH,HgCl from Waters of Varying Chloride Content Hg
Sample 1 2 3 4 5 6
7 8 9 10 11 12 13
Chloride, mg/L 7 700 4 400 410 150 13 000 9 700 10 000 7 600 2 000 5 600 1 0 000 9 800 1 8 000
concn + 0.3 N E I L Hg as Hg Reconcen- CH,HgCl covery, tration, spike, % iglL PglL 0.14 0.11 0.09 0.10 0.26 0.16 0.13 0.17 0.08 0.14 0.09 0.17 0.19
0.43 0.42 0.39 0.39 0.58 0.49 0.46 0.49 0.39 0.43 0.40 0.45 0.49
97 103 100 97 106 110 110 106 103 97 103 93 100
the extent of this interference in the present method, solutions of 1 pg/L of Hg2+were prepared with varying sulfide concentrations up t o a level of 100 mg/L of S2-(as Na2S). T h e solutions were analyzed with the system shown in Figures 1 and 2 with and without the UV digestor on. It was found that with UV digestion the mercury signal was unaltered for sulfide levels as high as 100 mg/L; while without UV digestion, a level of 1 mg/L of S2- reduced the recovery of Hg clown to 50%. Figure 3 shows t h a t satisfactory calibration curves and standard additions curves are obtained for different waters using Hg Clz and CH,HgCl in the range of normal analysis in this laboratory. In addition Table I1 provides recovery data for 0.3 pg/L CH3HgC1 from samples of varying chloride
16
ANALYTICAL CHEMISTRY, VOL. 50, NO. 1, JANUARY 1978
content. These samples are typical high chloride samples analyzed in this laboratory. Samples 1-12 (Table 11) are estuarine waters obtained from the Moosonee area, Ontario, Canada, where t h e Hudson Bay salt waters mix with local creek waters. Sample 13 is a seawater from t h e New Brunswick coast, Canada. T h e accuracy of t h e method was further checked by analyzing t h e N B S (National Bureau of Standards) Standard Reference materials for Mercury in Water No. 1641 and No. 1642. T h e No. 1642 was run direct and a value of 1.15 ng/mL was obtained compared t o the certified value of 1.18 0.04 ng/mL. T h e No. 1641 which is certified a t 1.49 pg/mL was diluted 1000-fold a n d analyzed. A value of 1.53 pg/mL was obtained.
*
ACKNOWLEDGMENT T h e authors thank Y. K. Chau for his comments on t h e original manuscript a n d C. Pacenza for her secretarial help.
LITERATURE CITED (1) P. D. Goulden and B. K. Afghan, "An Automated Method for Determining Mercury in Water", in "Advances in Automated Analysis, 1970, Technicon International Congress", Vol. 2, Mediad, Inc., Tarrytown, N.Y. (2) B. W. Bailey and F. C. Lo, Anal. Chem.. 43, 1525 (1971).
(3) T. B. Bennett, Jr., W. H. McDaniel, and R. N. Hemphill, "Advances in Automated Analysis, 1972 Technical International Congress", Vol. 8, Mediad, Inc., Tarrytown, N.Y. (4) A. A. El-Awady, R. B. Miller, and M. J. Carter, Anal. Chem., 48, 110 (1976). (5) "Method fw Chemical Analysis of Water and Wastewater", EPA Publication No. EPA-625/6-74-003, U.S. Environmental Protection Agency, Office of Technology Transfer, Washington, D.C. 20460. (6) "Analytical Methods Manual", Inland Waters Directorate, Water Quality Branch, Ottawa, Canada, 1974. (7) P. D. Goulden and B. K. Afghan, "An Automated Method for Determining Mercury in Water", Technical Bulletin No. 27, Inland Waters Branch, Department of Energy, Mines, and Resources, Ottawa, Canada, 1970. (8) B. K. Afghan, P. D. Goulden, and J. F. Ryan, Water Res., 6, 1475 (1972). (9) E. A. Jenne and P. Avotins, J . Environ. Qualify, 4 , 427 (1975). (IO) John Carron and Haig Agemian, Anal. Chim. Acta, 9 2 , 61 (1977). (11) "The Sea", M. N. Hill (Genl. Ed.), Volume 2, Interscience Publishers, New York, N.Y., 1963. (12) G. L. Baughman, J. A. Gordon, N. L. Wolfe, and R. G. Zepp, "Chemistry of Organomercurials in Aquatic Systems", EPA-660/3-73-012, National Environmental Research Center, Office of Research and Development, U.S. E.P.A., Corvallis, Ore. 97330. (13) I. R. Janasson, "Mercuy in the Environment", Gedogical S w e y of Canada, Papet 70-57, Geological Survey of Canada, 601 Booth St., Ottawa, Ontario, Canada. (14) "The Industrkl Auto Analyzer", Manual TNO-0210-00, Technicon Industrial Systems, Tarrytown, N.Y. 10591 (15) Haig Agemian and A. S.Y. Chau, Anal. Chim. Acta, 7 5 , 297 (1975). (16) J. F. Kopp, M. C. Longbottom, and L. B. Lobring, J . A m . Water Works ASSOC.,64, 20 (1972).
RECEED for review May 16,1977. Accepted October 11,1977.
Sputter-Atomization Studies with a Glow Discharge C. G. Bruhn' and W. W. Harrison* Department of Chemistry, University of Virginia, Charloftesville, Virginia 2290 1
Cathodic sputtering in a glow discharge is studied as a means of atomization for atomic absorption spectrometry. A scanning electron microscope Is used to examine the microstructure arising on the cathode surface during ion bombardment. The effects of discharge gas, sputter time, current, pressure, and cathode material are examined. The analysis of Ca, Mg, Zn, Au, Ni, and Sn in solution residues shows detection limits ranging to a few ng with precisions of 3-8 %
.
Although the glow discharge is one of the oldest spectroscopic sources, both for photons and ions, it has never become a major tool for t h e analytical chemist, other than in its role as a hollow cathode line source for atomic absorption. T h e cathodic sputtering phenomenon, central to the glow discharge action, can be analytically useful, however, as an atomization ( 1 , 2 ) ,excitation ( 3 , 4 ) ,and ionization (5,6) source. A cursory consideration of t h e discharge reveals a simple gas diode operation, but this apparent simplicity can be deceptive. No unified theory has evolved t o explain fully the complex processes (e.g., excitation, ionization) involved. Indeed, many theoretical treatments ignore completely the aspect of perhaps most interest t o t h e analytical chemist, t h a t of cathodic sputtering. T h e glow discharge was early recognized as a possible solids atomization source for atomic absorption. Metals and alloys
' Op leave from Departamento de Anilisis Instrumental, Escuela
de Quimica y
Farmicia, Universidad de Concepcibn,Concepcih, Chile. 0003-2700/78/0350-0016$0 1.OO/O
(2, 7-9) have -een sputtered and anL-jzed directly against metal standards. The analysis of elements in solution residues (10,11) has also been reported. Other workers (12-14) have used atomic absorption t o study ground state atomic populations as part of plasma diagnostics or sputter investigations. Interest in our laboratory has centered around t h e use of the glow discharge as a source for optical emission (15, 16), more recently for solids mass spectrometry (6, 17, 18), and, in this report, for atomic absorption. Our goal is to learn more about the basic phenomena occurring within the discharge and subsequently t o apply this information to analytical problems. The cathodic sputter surface studies described here are followed by preliminary d a t a demonstrating some analytical possibilities of t h e glow discharge as an atomization source.
EXPERIMENTAL Glow Discharge Source. A low pressure glow discharge source operating in the abnormal mode (19,20) was used. The design of the source (Figure 1)shows a glass envelope consisting of two 3-cm i.d. glass joints (Kontes Glass Co.) with a Viton O-ring vacuum seal. The source has two vacuum ports. The lateral one (1)acts as the main exhaust port for the glass chamber, both for initial evacuation and for flow operation. The upper port (2) has a brass tubing (3) of 0.15-cm i.d. inserted concentrically through a 0.63-cm Cajon Ultra Torr fitting which is coupled to the Pyrex body of the glass envelope. Through this tubing, the discharge gas can be continuously bled into the source chamber for flow operation. This tubing also serves as the anode, centrally positioned at 2.3 cm above the bottom of the cathode. The cathode (4) is a replaceable copper or graphite hemicylinder of 1.80-cm length, mounted on a holder ( 5 ) of machinable glass ceramic
D 1977 American Chemical Society
