Cold-trap preconcentration method for the determination of mercury in

Cold-Trap Preconcentration Method for the Determination of Mercury in Sea Water and in Other Natural Materials William F. Fitzgerald, W. Berry Lyons, and Carlton D. Hunt Marine Sciences Institute and Department of Geology, University of Connecticut, Groton, Conn. 06340

A cold-trap preconcentration procedure has been developed and incorporated into a standard flameless atomic absorption analysis of Hg in sea water and in other environmental samples. The cold-trap is created by the immersion In liquld N2 of a glass U-tube packed with glass beads (BO/ 100 mesh). After reduction, purging, and trapplng, the Hg is removed from the glass column by controlled heating, and the gas phase absorption of eluting Hg is measured. This procedure has been employed for both shipboard and laboratory analyses of Hg in sea water and for the determination of Hg in mixed zooplankton (seston) and in oceanic manganese crusts. In sea water, a detection limit of 2 ng Hg/l. was attained.

photometry has been developed and employed for the analysis of Hg in sea water, and in various biological and geological materials. The entire procedure takes approximately nine minutes and no extraction or metal amalgams are used. The cold-trap consists of a glass U-tube packed with small glass beads and immersed in a liquid N2 bath. The advantages of this technique are speed and the ability to make precise determinations of Hg at levels as low as 0.2 nanograms in various aqueous samples. Knudson and Christian (20) have described a similar collection device consisting of a simple U-tube that was used for the determination of volatile hydrides of As, Sb, Bi, and Se by flameless atomic absorption.

EXPERIMENTAL Atomic absorption and atomic fluorescence techniques using closed system reduction-aeration have been applied widely to determine Hg concentrations in natural samples (1-15). In many applications such as the analysis of Hg in open ocean sea waters where the Hg concentrations can be as small as 10 ng/l. (11, 15-18), a preconcentration stage is generally necessary. A preliminary concentration step may separate Hg from interfering substances, and the lowered detection limits attained are most desirable when samples are rare or the quantity of sample material is limited. Concentration of Hg prior to measurement has been commonly achieved either by amalgamation on a noble metal (3, 10, 12, 1 5 ) or by dithizone extraction (5, 14, 17). Preconcentration and separation of Hg has also been accomplished using a cold-trap a t thetemperature of liquid N2 for the determination of Hg in geological materials (9) and in biological samples (19). This paper describes a rapid and precise cold-trap preconcentration technique for the determination of trace amounts of Hg in solution. A simple but effective noncontaminating cold-trap method for concentrating Hg prior to its measurement by gas phase atomic absorption spectro(1) W. R. Hatch and W. L. Ott,Anal. Chem.,40, 2085 (1968). (2) M. E. Hinkle and R. E. Learned, U.S.Geol. Surv., Prof. Pap., 650-D, 251 (1969). (3) M. J. Fishrnan, Anal. Chem., 42, 1462 (1970). (4) G. Lindsteat, Analyst, (London)95, 264 (1970). (5) Y-K Chau and H. Saitoh, Envlron. Scl. Techno/., 4, 839 (1970). (6) S.H. Ornang, Anal. Chim. Acta, 53, 415 (1971). (7) S. H. Omang and P. E. Paus. Anal. Chim. Acta. 56, 393 (1971). ( 8 ) J. H. Hwang, P. A. Ullucci. and A. L. Malenfant, Can. Spectrosc., 16, 1 (1971). (9) S.R. Aston and J. P. Riley, Anal. Chim. Acta, 59, 349 (1972). (10) V. I. Muscat, T. J. Vickers, and A. Andren, Anal. Chem., 44, 218 (1972). (11) G. Topping and J. M. Pirie, Anal. Chim. Acta, 62, 200 (1972). (12) R. A . Carr, J. B. Hoover, and P. E. Wilkniss, Deep-sea Res., 19, 747 (1972). (13) S. H. Ornang, Anal. Chim. Acta, 63, 247 (1973). (14) D. Gardner and J. P. Riley, Nature (London),241, 526 (1973). (15) J. Olafsson,Anal. Chim. Acta., 68, 207 (1974). (16) T. M. Leatherland, J. D. Burton, M. J. McCartney, and F. Culkin, Nature (London),232, 112(1971). (17) R. Chester, D. Gardner, J. P. Riley, and J. Stoner, Mar. Pollut., Bull., 4, 28 (1973). (18) W. F. Fitzgerald and C. D. Hunt, Abstr., Bull. /'Union Oceanogr. h.,A-5, August, 1973. (19) H. L. Rook, T. E. Gills, and P. D. LaFleur. Anal. Chem., 44, 1114 (1972).

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Apparatus. The mercury analyses were conducted using a Coleman Instruments Hg analyzer (MAS-50) equipped with a Leeds & Northrup Speedomax Recorder (Model XL 601). The aqueous sample solution was contained in a 250-ml Pyrex (Corning Glass Works) glass bubbler placed a t one end of a sampling train employing Nz as the purging and carrier gas. The gas-flow system also included a flow regulator, two by-pass valves, a water absorber, a Hg cold-trap, a gas cell, and a gas washing bottle containing a 10% solution of KMn04. A schematic diagram of the entire system is shown in Figure 1. The sparging vessel was connected directly through amber latex rubber tubing (i.d. 3 mm; length 60 cm) to a Pyrex glass drying tube (i.d. 18 mm; length 18 cm) containing colorless silica gel (6/20 mesh) as a water absorbent. The Hg cold-trap followed this absorbing stage, and connection was made using a 75-cm length of rubber tubing. The Hg cold-trap consisted of an 8-in. Pyrex glass (i.d. 4 mm) U-tube (width 1.5 in.), which was packed with glass beads (80/lOO mesh) to form a 6-cm column at the bend of the tube. The U-tube was wrapped with Nichrome wire (diameter 0.06 mm) yielding 5 windingdin. over the entire lower 6 in. of the tube. The wire-wound U-tube was placed inside an insulating Pyrex glass covering (0.d. 10 mm; i.d. 8 mm) such that only the non-wire wrapped sections were exposed. This complete U-tube apparatus was designed to fit a 1-liter Dewar flask containing liquid Na t o provide the trap and concentration step for Hg vapor. The leads from the heating wire were connected to a Powerstat variable transformer which allowed the column to be heated electrically upon removal from the liquid N2 bath. Three-way Teflon (Du Pont) stopcocks (4-mm bore) were placed before the bubbler and after the drying tube to permit the sparging vessel and the drying column to be bypassed during the heating and elution step. Connections were made with 3-mm i.d. rubber latex tubing. The Hg, which is rapidly vaporized and eluted from the column during the heating step, was fed directly by polyethylene tubing (i.d. 3 mm; length 70 cm) to the gas cell of the Coleman Hg Analyzer. The absorption of elemental Hg in arbitrary units was displayed on a recorder using a 25X scale expansion. After the carrier gas had passed through the gas cell, it was directed into a 300-ml gas washing bottle containing 100 ml of 10% KMn04. At this stage, the elemental Hg was oxidized and removed from the gas flow. Reagents. Except where noted, all chemicals used were ACS certified reagent grade quality supplied by the J. T. Baker Chemical Company. The N2 carrier gas was 99.8% pure (Chemetron Corp.). Deionized water for reagent preparation and experimental studies was produced by passing Pyrex glass distilled water through an activated charcoal absorbent and two mixed bed ion exchangers (Continental Deionization Service). A primary standard mercury solution was made by dissolving 1.354 grams of freeze-dried spectographic grade HgC12 (Johnson, Matthey Chemi(20) E. J. Knudson and G. D. Christian, Anal. Lett., 6 , 1039 (1973).

ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974

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3. Composite calibration curve for Hg in sea water samples measured over a three-week period. The Hg spike quantities are in ng and the Hg absorption in arbitrary units

cal Ltd.) in 1000 ml of distilled deionized water. From this solution, 10-ppm and 100-ppb spiking solutions were made daily. A 20% stannous chloride solution was prepared by dissolving 72 grams SnC12 in 20 ml concentrated HCI and bringing to volume with 3N H2S04. The standard Hg solution, Hg spiking solutions, and the stannous chloride reagent were stored in high density polyethylene bottles. The 10% KMn04 oxidizing solution was prepared by dissolving 100 grams KMn04 crystals in 1 liter of distilled deionized water. The column packing material was Anaport Glass Beads (80/100 mesh). In earlier studies, the column-packing material employed was Analabs Chromosorb W-HP (SO/lOO mesh 1.5% OV-17 and 1.96% QF-1). Although the latter column was equally efficient, it must be conditioned before use a t 150 "C for approximately two hours. No column conditioning, other than normal blank measurements, was necessary with the glass bead column. Colorless silica gel (Fisher Scientific Co., 6/20 mesh) was used as the drying agent. Procedure. With the U-tube column immersed in the liquid nitrogen bath, a 100-ml sample solution is placed into the gas bubbler. The sparger is inserted and the valves switched to the flow through position. The purging rate for the N:! aeration is 0.5 I./min and 7 psi. After purging is completed (7 minutes), the valves are returned to the by-pass position, and the flow rate of the carrier gas increased to 0.7 l./min (7 psi). The column is removed from the liquid nitrogen bath and the U-tube heated through the wire windings using the variable transformer. The transformer can be simply switched on by prior calibration of the voltage setting to give a col-

umn temperature of 225 "C measured on the outside wall of the U-tube after 60 seconds of heating. The elemental Hg is vaporized and eluted from the column in 1-2 sec and the entire operation required less than 10 sec from the time the U-tube is removed from the cold trap. During the heating and elution cycle (60 sec), the flow rate of the carrier gas decreases from 0.7 l./min to 0.15 I./min. The absorption a t the mercury wavelength (2537 A) occurs in the gas cell 1 2 sec after the heating cycle is initiated. The absorption is recorded in arbitrary units and the maximum height noted. This represents the sample and system blank. After the response has returned to the initial base line (60 sec), the heating is stopped and the column cooled in air for 30 sec. The U-tube is then returned to the liquid N2 bath. The carrier gas flow is set to 0.5 l./min and the system is ready for the SnClz reduction and N? aeration step. Total time for this operation is 9 minutes. A 0.5-ml addition of the SnC12 reagent js added to the 100-ml sample to reduce Hg to its elemental state. The sample solution is mixed by hand shaking for 5 sec and then the Teflon stopcocks are switched to the flowthrough position. After the latter operation, the manipulations are identical to the steps outlined in the procedure for establishing the system blank. The absorption peak (2537 A) of mercury is recorded and its maximum height noted. Appropriate spikes of HgC12 standards were added to the sample matrix and the procedure was repeated. Three spike additions were usually made. A system blank is repeated and the mercury concentration determined from an individual calibration curve for each sample.

Figure 2. Representative calibration curve for the measurement of

Figure

ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974

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Table I. Mercury Concentrations Measured in a Variety of Natural Samples Using the Cold-Trap Preconcentration Gas Phase Detection of H g Hg concentrationa Sample type

Coastal waters of Long Island Sound Continental slope waters Open ocean w a t e r s , northwest Atlantic Ocean Mixed zooplanktonb (seston) northwest Atlantic Ocean NBS Standard Ref e r e n c e Material 1571: Orchard Leaves NBS Certified Value Manganese c r u s t s from mid-Atlantic Ridge between 26"31"N and 25"W

Raw acidified

Photooxidized

21-33 ng/l.

45-78 ng/l.

10 ngA.

41 ng/l.

2-11 ng/l.

5- 10 ng/l.

Acid digestion

References

90- 510 ng/g

150 i 17 ng/g

155 i 15 ng/g 12-130 ng/g

a The range of Hg concentrations indicated for open ocean sea water, seston, and manganese crusts represents the variations associated with geographical location within a large oceanic region. * Dry weight.

RESULTS A N D DISCUSSION Analytical Curves. Working calibration curves were established in distilled deionized, fresh and sea water as well as in acid solutions of marine zooplankton, orchard leaves, and manganese crusts. In Figure 2, a calibration curve is reproduced for the determination of Hg in sea water. The mercury concentration in ngb. has been plotted against gas phase Hg absorption in arbitrary units. Using log-log coordinates, the Hg response is linear over a wide concentration range, which brackets conveniently the expected variability for Hg (10 < ng/l < 200) in marine waters. A slight curvature takes place a t concentrations >500 ngb. Recently, a large number of sea water samples were analyzed for their Hg content (18).Under these circumstances, calibration curves were established at the beginning and end of a sample series (7-10 samples). Only one spike was added to each sample to ascertain the proper behavior of the system. In Figure 3, a composite calibration curve for sea water has been reproduced. This curve is based on the response obtained for spike additions of 1.0, 2.5, 5.0, and 10 ng Hg to 100-ml sea water samples over a period of 3 weeks. The number of spikes making up this composite graph are 3, 19, 20, and 9, for the 1.0 ng, 2.5 ng, 5.0 ng and 10 ng Hg additions, respectively. The average value for each addition has been plotted and the brackets indicate the standard deviation. These curves demonstrate the applicability of the coldtrap preconcentration technique to low concentration ranges of Hg. Approximately 0.20 ng of Hg can be determined with a 25X scale expansion. Since the response depends on the vaporization and elution of trapped Hg from the column, the calibration curves were similar for other aqueous media including acidified ("03) distilled deionized water. Therefore, this cold-trap procedure appears to separate effectively reducible Hg species from interfering substances that might be associated with differing solution matrices. 1884

The flow rate of the carrier gas is the most important parameter governing sensitivity and reproducibility. The proper elution rate must be determined experimentally and maintained accurately. A flow rate of 0.7 l./min at 7 psi was optimal for the column used in this present investigation. Cleaning Procedure. Much care must be given to cleaning sample containers, reagent bottles, pipets, and other laboratory materials that will come in contact with the sample. A nondetectable blank response was obtained for the sample handling steps by employing the following cleaning procedure. All glass and polyethylene ware were first soaked in cold concentrated HCl for 24 hours and rinsed with distilled deionized water. Then the containers bath. The final cleaning were left overnight in a 10% "03 step was the careful washing (with 6 final rinses) in distilled deionized water. Teflon (FEP) sample bottles were soaked in concentrated "03 for 24 hours and then rinsed a t least 6 times with distilled deionized water. Polyethylene pipet tips (Eppendorf) were washed in hot concentrated "03 acid until the color additives were removed. Applications. The cold-trap preconcentration gas phase method has been used for the measurement of Hg in a variety of natural substances. In Table I, Hg determinations in natural fresh and sea water (18,21),oceanic crustal material (22),marine zooplankton (23),and orchard leaves, NBS standard reference material 1571, ( 2 3 ) have been summarized. Since the purpose of this communication is to demonstrate the sensitivity, precision, and versatility of the cold-trap method, the above studies will be briefly discussed. Details regarding the significance of these various data can be found in the appropriate references. (21) W. F. Fitzgerald and W. E. Lyons, Nafure(London),242,452 (1973). (22)R. E. Scott, P. A. Rona, L. W. Butler, A. J. Nalwalk, and M. R. Scott, Na-

ture(London),239,77 (1972). (23)W. F. Fitzgerald, C. D. Hunt and W. E. Lyons, in "Base-line Studies of Pollutants in t h e Marine Environment (Heavy Metals. Halogenated Hydrocarbons and Petroleum)," background papers for a workshop sponsored by the N.S.F. Office for the I.D.O.E., Brookhaven Natl Lab., 24-26 May, 1972.

ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974

Sample Storage. To preserve natural water samples for Hg analysis, much care must be exercised to prevent loss of Hg during storage (24-26). Coyne and Collins (24) recommended preacidification of the sample bottle with concentrated H N 0 3 to yield a final pH of 1 in the sample solution. However, when this procedure was used for storage of sea water, fresh water, and distilled deionized water in commonly employed low density polyethylene storage containers, abnormally high absorption was observed. This absorption a t the Hg wavelength (2537 A) is due to the presence of volatile organic plasticizer material and any polyethylene residue leached by the concentrated H N 0 3 and by the acid solution at pH 1. It would appear that those procedures employing acidified sample storage in polyethylene bottles and gas phase Hg detection, may be subject to artificial Hg absorption due to the presence of organic material. In sea water investigations, samples were stored after collection in Pyrex glass containers and in Teflon (FEP) bottles. The containers were preacidified with concentrated “03 (J. T. Baker Chemical Co., Ultrex) to yield a final pH between 1 and 2. Time studies to test the efficiency of the sample preservation showed that sea water solutions containing between 5 to 50 ng HgA. could be stored for 1 month (length of study) without significant loss of Hg. However, Pyrex glass containers occasionally showed unusually high system blanks and, therefore, Teflon ware would appear to be the preferred storage vehicle. No studies have been conducted concerning the efficiency of various sample containers for other types of samples (e.g., river water). Natural Waters. A survey for the presence of inorganic and organically associated Hg in the surface waters of the Long Island Sound environs and in the surface microlayer and subsurface waters at eight stations between Bermuda and Narragansett, R.I., has been conducted. In this work, the total Hg present in these waters has been divided into two fractions, inorganic and organically associated Hg. The inorganic measurement represents the amount of Hg available for reduction using the cold-trap procedure in pre-acidified raw unfiltered sea water samples, with a pH range between 1.6-2.2. This measurement can include many organo-Hg species present a t the in situ pH of sea water that release Hg when the pH is lowered to about 2.0. The total Hg measurement is carried out in aliquots of the pre-acidified sea water in which the organic matter (measured as organic carbon) has been destroyed by ultraviolet photo-oxidation (27, 28). Therefore, the amount of Hg determined as the difference between the “inorganic” and total Hg measurement represents a very stable Hgorganic associaion. The measurements summarized in Table I indicate that the most significant quantities of organically associated Hg occur in the waters of the continental shelf and slope. This strongly associated organic Hg fraction was not observed in the open ocean samples of either the surface microlayer or the subsurface waters of the northwest Atlantic Ocean. (24) (25) (26) (27)

R. V. Coyne and J. A. Collins, Anal. Chem.. 44, 1093 (1972). R . A. Carr and P. E. Wilkniss, Environ. Sci. Techno/., 7, 62 (1973)

C. Feldman. Anal. Chem.. 46. 99 11974). F. A. J. Armstrong, P. M. Wiliiams, and’J. 13. H. Strickland, Nature (London), 211, 481 (1966) (28) W. F. Fitzgerald. Ph.D. Thesis, Dept. of Earth and Planet. Sci., M.I.T., and Dept. of Chemistry, W.H.O.I., (1970).

Therefore, the study of Hg in the coastal and estuarine environment must consider seriously the probem of Hg speciation. Marine Zooplankton. Samples of mixed zooplankton from the northwest Atlantic Ocean were collected on a transect from Bermuda to the New York Bight region. The zooplankton were stored frozen in polyethylene petrie dishes. The samples were later melted, homogenized, dried, and weighed. One-gram samples were digested in concentrated re-distilled H N 0 3 acid and analyzed using the same procedure outlined for the natural waters. There appears to be no correlation between location and Hg concentration of the mixed zooplankton. However, the limited sampling precludes statements concerning the amounts and distribution of Hg in zooplankton us. geographical location. Manganese Crusts. Determinations of Hg in manganese crusts from the floor of the north Atlantic Ocean were also conducted. One gram of manganese crust from three different locations in the north Atlantic Ocean were weighed and digested in 40 ml of aqua regia. The insoluble residue was dissolved in 15 ml of hot hydrofluoric acid. The acid solutions were combined and made up to 1000 ml with distilled deionized water. One-hundred milliliter aliquots were analyzed using the cold-trap method. These data were used as supporting evidence for magmatic injection of Hg along the mid-Atlantic Ridge. Orchard Leaves. To test the accuracy of this method for biological material, NBS standard reference material 1571, orchard leaves, was lyophilized, digested, and analyzed following the identical procedure employed for the zooplankton studies. The Hg concentrations obtained for the orchard leaves compare quite favorably with the amounts of Hg certified by the National Bureau of Standards (Table I). Unfortunately, sea water Hg standards at concentrations of