A Protocol for Minimizing Contamination in the Analysis of Trace Metals in Great Lakes Waters

J. Great Lakes Res. 19(1):175-182 Internal. Assoc. Great Lakes Res., 1993

A Protocol for Minimizing Contamination in the Analysis of Trace Metals in Great Lakes Waters Jerome O. Nriagu, Greg Lawson, Henry K. T. Wong, and Jose M. Azcue National Water Research Institute Box 5050 Burlington, Ontario L7R 4A6

ABSTRACT. The precautions and procedures aimed at minimizing contamination artifacts during the collection, filtration, and analysis of trace metals in fresh water are described. The techniques for proper decontamination of sample containers and labware and for field blanks are also presented. Samples are obtained using a portable clean laboratory and analyzed in a Class 100 pressurized room. Preliminary results obtained using the protocols described cast doubt on most of the published data on trace metal concentrations in the Great Lakes. The profiles of dissolved metal concentrations in the water column of Lake Ontario show very distinctive source-dependent features obfuscated in earlier studies by poor data quality. INDEX WORDS:

Metals, Lake Ontario, sampling, contamination.

INTRODUCTION During the past decade or so, chemists have developed sampling protocols and highly sensitive instrumentation for reliable collection and determination Qf trace metals in natural waters at concentrations below 1.0 ng/L (Patterson et al. 1976; Bruland et al. 1979, 1985; Danielson et al. 1982; Gill and Fitzgerald 1985; Boutron 1979, 1990; Ahlers et al. 1990). These developments show that previously published data were severely compromised by sample contamination and have led to major improvements in our understanding of the biogeochemistry of trace metals in the ocean. Such developments have been eschewed by most analysts in the Great Lakes basin who still tacitly assume that stringent methods are not required for the measurement of trace metals in the "polluted" waters of the Great Lakes. Recent studies using the ultraclean laboratory methodology, however, have shown that the concentrations of many trace elements in Lakes Erie and Ontario are comparable to, or even lower than, the levels commonly reported in the open ocean (Flegal et al. 1988, Coale and Flegal 1989). The work by Flegal and his colleagues clearly show that most of the available data on the concentrations of trace metals in the Great Lakes waters were biased by contamination arti-

facts introduced during the collection, handling, and analysis of samples. Because contamination is a factor during every step of sampling and handling water, rigorous analytical protocols are required since every extraneous object or environment which contacts the sample may positively or negatively affect it (Kosta 1982, Boutron and Patterson 1983, Adeloju and Bond 1985, Gretzinger et al. 1982, Berman and Yeats 1985, Ahlers et al. 1990). Positive contamination involves the release of metals into the sample waters from any surface or atmosphere which results in concentrations that are biased high. Negative contamination entails loss of metals from the sample, generally by adsorption onto surfaces, leading to concentrations that are biased low. These problems necessitate the use of very meticulous measures to reduce the risk of contamination from all sampling equipment, labware, reagents, and even the air in contact with the samples (Mart 1982, Boutron and Patterson 1987, Gorlach and Boutron 1990). Much of our awareness in handling samples at low concentrations in a contamination-free manner is derived from the pioneering work of several authors whose media include seawater, ice and snow, river water, and lake water (Boutron 1979, 1990; Bruland et al. 1979, 1985; Brugmann et al. 1983;

175

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Nriagu et at.

Danielsson et al. 1982; Tramontano et al. 1987; Patterson et al. 1976). Although each medium is different with regards to its matrix or composition, each has been shown to contain very low concentrations of heavy metals and many similarities exist as to how the metal levels are accurately quantified. We have adapted, modified, and sometimes added novel features to the published methods. The focus of this report is on the minimization of sample contamination from such common sources as the laboratory and field environment, the sampling and analytical equipments, the reagents and the analyst. The instrumental analytical procedures are described only briefly.

MATERIALS AND METHODS

Ultraclean Laboratory Facilities It is now an accepted dogma that the determination of ultratrace concentrations of elements in natural waters must be performed in a controlled laboratory environment (or clean laboratory) so as to alleviate sample contamination by dusts from the laboratory atmosphere and unclean surfaces (Moody 1982). A schematic floor plan of the Class 100 Clean Laboratory in use at the National Water Research Institute (NWRI) is shown in Figure 1. It measures roughly 3 m X 7.2 m and is divided into two sections with a clothing change chamber and sticky mat floor between them. Each room contains a high efficiency particle (HEPA) filter assembly which provides about 100 air changes per hour in the room. Air supply to each room is approximately 3,000 Llsec made up of 30% prefiltered external laboratory air and 70% recirculating air; the high ratio of exhaust to recirculating air is essential in maintaining low fume levels in the laboratory. With a HEPA efficiency of better than 99.9% for 0.5 I..lm particles, the high frequency of air changes in the relatively small rooms serves to maintain the particle count at the Class 100 level. The inner room is maintained at a positive pressure in relation to the outer room which itself has a positive pressure relative to the outside laboratory atmosphere. Where practicable, the fixtures in the laboratory are made of plastic and any unavoidable metal surfaces (such as door knobs and the HEPA filter housing) are coated with epoxy resin. The cabinets are made of wood and the counter tops are covered with Teflon protective overlays. The sealed walls and ceiling are covered with five coats of resistant epoxy resin. The floor consists of seamless, chemi-

••

3.0m

••

Fume Hood Bench ~~

Inner Room

7.2m

1~ 1.2m

1

I

;Bench I

Windo·

Change Area 1. Milli-Q System 2. AA Computer 3. Sticky Mat

Bench:

I

FIG. 1. Schematic floor plan of the Class 100 Clean Laboratory at the National Water Research Institute, Burlington, Ontario.

cally resistant vinyl and the floor drain is capped with a plastic block. All the sample handling, the preparation of reagents, and the final stages in the rinsing of containers and labware are done in the inner room. The outer room is used for instrumental analysis and contains a gaphite furnace atomic absorption AA spectrometer and a Milli-Q water distillation unit. All samples and standards being analyzed on the AA are prepared in the class-l00 section of the NWRI clean lab. At all times in this section the analyst wears full Tyvek coveralls with an attached hood, a Tafetta hair cap, Tyvek booties, and disposable, non-powdered, polyethylene (PE) gloves. In the outer room, regular street clothing or lab coats are worn with the Tafetta cap and Tyvek Booties.

Portable Clean Laboratory All sample handling and field procedures related to sample acquisition are performed inside a

177

Trace Metal Analysis portable clean lab, a schematic of which is shown in Figures 2a and 2b. It is essentially a 2 m X 2.7 m X 3 m wooden box with a small clothing change area at the entrance. It is made of wooden panels fastened onto an aluminum frame structure and sits on a base that can be secured to the deck of a boat or mounted on a trailer. The interior walls are covered with white epoxy paint, and the floor is covered with seamless vinyl. The roof is made of molded plastic with a window for the air intake manifold. The portable laboratory itself is equipped with an arborite work bench covered with Teflon protective overlay and a plastic drain sink. Two HEPA filter modules, with prefilters, are mounted on the ceiling in such a way that they can be lowered into the laboratory during transport or when not in use. The two modules deliver a large volume of filtered air in a vertical stream to the work bench. Four 0.3 m X 0.3 m exhaust openings with flaps, located near the floor, prevent the air

stream from resuspending dust particles from the laboratory floor. Since there is no air recirculation, the cleanliness of the portable lab is believed to be no better than the Class 1000 level. Full Tyvek clean suits with attached hoods and Tyvek booties, as well as disposable, unpowdered, polyethylene gloves, are worn at all times in the lab. All filtering and other sample manipulations are done on the benchtop. All carboys of MQ-water and supplies for filtering and sample manipulation are stored in the cleanroom, under or on top of the bench. Acids and other reagents are kept, single or double bagged, in a PE storage box under the bench and most supplies are secured as much as possible in case of rough weather. Decontamination of Bottles and Labware All the containers and labware used in every stage of sampling and sample handling are made of plas-

Air Intake

I

V

i

Window'

B~slh leraihlith

Bench

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Main Work Area 3.0m

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Exhaust Air ~_,

Floor () Drain

:>Exhaust Air

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Sliding Partition Change Area

Door

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FIG. 2a. Basic outline of the portable clean laboratory used in the study.

FIG. 2b. Schematic floor plan of the portable clean laboratory.

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Nriagu et al.

tic material which can stand up to strong acid treatment for extended periods. The sample bottles are made of high density linear polyethylene (HDLPE) primarily although some low density (LDLPE) variety is employed occasionally. The AA vials for the autosampler are made of LDLPE. Teflon is used for storing high purity acids, purified freon-1l3, AA and reference standards, and other high purity reagents. Beakers, separatory funnels, washbottles, watchglasses, stir bars and rods, tweezers, and the filtration system are all made of Teflon also. Volumetric flasks, measuring cylinders, pipets, and pipet tips used in the laboratory are made of polypropylene. The choice of material for the containers and labware was based partly on the metal contents of the various plastics and glass (Table 1). The cleaning of all the containers, the filtration device, and the labware follows a nine-step procedure (below) which was adapted from the method described by Tramontano et ai. (1987). All the times given are the minimum normally employed.

off acid and rinse with RO-water, shake off excess water. 6. Soak in warm (40-50°C) 2 M reagent grade nitric acid bath for 72 h. 7. Rinse inside with 0.5% reagent grade nitric acid then rinse entire piece with quartz distilled water. 8. Place piece(s) in heavy polyethylene bag, rinsed outside and inside with quartz distilled water for transfer to the clean lab. 9a. For bottles, volumetric flasks, jars, separatory funnels, and other containers rinse inside with MQwater and then fill with 0.2% ultrapure nitric acid (Seastar) until use. 9b. For beakers, AA vials, pipet tips, watchglasses, volumetric cylinders, and other small items either rinse with MQ-water and store in PE bags until use or place in a small tub containing 0.2% ultrapure nitric acid (Seastar) until use.

1. Degrease in Versa Clean (Fisher) soap bath for 24 h, rinse well with reverse osmosis (RO) "distilled" water, shake off excess water. 2. Fill bottle with or soak piece in bath of reagent grade acetone for 1 h, drain off acetone and allow to air dry and then rinse with RO-water, shake off excess water. 3. Fill bottle with or soak piece in plastic bath of reagent grade concentrated hydrochloric acid for 1 h, rinse with RO-water, shake off excess water. 4. Fill bottle with or soak piece in plastic bath of reagent grade concentrated nitric acid for 1 h, rinse with RO-water, shake off excess water. 5. Fill bottle with or soak piece in small plastic bath of reagent grade 6 M nitric acid for 72 h, drain

The treatment above is given to all new and unknown bottles and materials. Once they have gone through the treatment they are only subjected to steps 5-9 during subsequent cleanings. If the piece is exposed to solutions with high metal concentrations it is put through all the acid wash treatments of steps 3-9. The large HDLPE carboys used as water reservoirs for the Milli-Q system are cleaned first with dilute soap solution, filled for at least a week with 6 M and then 2 M HN0 3 , after which they are rinsed thoroughly with MQ-water. The GO-Flo bottles receive a 2 M HN0 3 leaching and DD-water rinse before they are taken to the field. The heavy polyethylene bags in which the sample bottles are taken into the field are soaked in ei-

TABLE 1.

Trace metal contents of common laboratory ware material. Concentration range (Ilg/g)

0.1-10

10-100

Material

0.001-0.01

0.01-0.1

Polyethylene and polypropylene Polyvinyl chloride Teflon

Hg, Cu, Sb, Co

Ni, Cr, Mn, AI, Se, As

Cd, Pb, Sr, Fe

Zn (Ti, AI)*

Zn, Pb, Sn, Cd, Ni, Cr, Mn

(So, AI)*

Pb, Co, Cs, As

Cu, Co, As, Sb Cu, Cd, Mn, Cr, Fe, Ni, Zn, Co Cd, Cu, Cr, Pb, Ni, Zn, Co, As La, Au, Rb, As, Co, Se

Al,W Fe, AI, Mn Cd, Pb, Cu, Ni, Zn, Cr, Ti, Fe

Mn,Al

Ni, Cu, Cr

Pb,Zn,Fe

Polycarbonate Glass Silica

Tl, Y, U, Sc, Hg, Ag, Se Hg, As, Mn, Cd, Mo, Co, Sb, Se

#Based on compilation by Kosta (1982) *Some types only

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Trace Metal Analysis ther the 2 M or the 6 M nitric acid baths for 24 h and then are rinsed with quartz distilled water. They are transferred in cleaned bags to the inner room of the clean lab where they are further rinsed with MQ-water and stored until use. The storage tubs are thoroughly rinsed with RO-water before use, packed with bottles in the clean lab, the lids are then secured before they are stored outside the clean room until they are taken to the field. It should be noted that the sample bottles are taken to the field still containing the final cleaning solution. They are packed individually in acid washed PE bags and then packed five bottles per larger bag. Five sets of five bottles are placed in each PE storage tub for transport to the field. The tubs are only opened in the portable clean lab. Ultrapure Water Water is widely recognized as a major source of contamination in reagents and samples. In fact, the levels of many trace elements in the Great Lakes are lower than those obtainable in many laboratory water purification systems. The need for minimizing the metal contents of the analytical water samples cannot be over-emphasized. The ultrapure water used in our laboratory is derived from a three-stage demineralization process. The first stage is the general purpose in-house reverse osmosis (RO) "distilled" water delivered in plastic pipes and faucet. The second stage is the redistillation of the RO water in a quartz still (Coming AG-3 system). In the final stage, the quartz (or doubly) distilled water is fed into a Milli-Q system (Millipore Corp., Bedford, Mass.) to produce water with the following blank values: Cd < 1.0 ng/L, Cu < 0.4 ng/L, Ni < 0.2 ng/L, Pb < 0.4 ng/L, and Zn < 0.05 ng/L. The blank values can be further reduced by repeated recirculation of the water through the Milli-Q cartridges. Acids and Other Reagents Doubly quartz distilled (sub-boiled) nitric and hydrochloric acids (Seastar, Sidney, British Columbia) are used to acidify water samples, reference standards, and for the digestion of biological and sediment samples. The final rinse solutions for containers and labware are also made with these high purity acids. High purity ammonia solution (Seastar) and doubly Teflon distilled (sub-boiled) glacial acetic acid are employed for buffers. The solutions of diethylammonium diethyldithiocarbamate

(DDDC) and ammonium pyrolidinedithiocarbamate (APDC) are purified by repeated extraction with freon-Il3. The freon itself is purified by sub-boiling distillation.

WATER SAMPLING METHODS Subsurface water samples are obtained by means of 5-L Go-Flo bottles attached to Kevlar rope and tripped using a Teflon messenger. The Go-Flo bottle (General Oceanics, Inc., Miami, Florida) is cocked inside the clean lab, taken to the sampling depot in its PE bag and quickly attached to the Kevlar line. After obtaining the sample, the filled Go-Flo bottle is put back in its bag and quickly transported to the portable clean lab where the sample can be processed. It should be noted that because of the pollution plume around the research vessel, the GoFlo is not recommended for obtaining samples at depths of less than 10m from the airwater interface. Filtration of Water Samples Sample filtration often is a major source of contamination. All fittings and tubing used as part of the filtration apparatus are made of Teflon. Water samples are processed through an in-line filter device connected to the Go-Flo's or to a HDLPE reservoir into which the surface samples have been transferred. The custom made in-line filter assembly consists of membrane supports between circular plates held together by wing-nut clamps. Holes are drilled in the plates to allow water to be suctioned through the filter membrane directly into sample bottles housed inside an evacuatable chamber. The chamber, made of clear acrylic, is sealed by means of viton O-ring to a flat base plate. A Teflon tube from the lower side of the membrane support to the sample bottle passes through a hole with Viton 0ring at the sealed top of the chamber. An outlet at the side of the chamber is connected to a vacuum pump to evaculate the chamber and induce flow of water through the filter membrane into the sample bottle. The vacuum pump is oil-free (Aircadet model, Gast, Fisher) and is located outside the portable clean lab. Exhausts from the pump are directed away from the clean lab by PE tubing. The filter membranes in use are made of polycarbonate (Nuclepore), with 0.4 J.lm pore size and 95 mm in diameter. Each membrane is acid leached in 20% ultrapure nitric acid (Seastar) at least I week before a cruise and remains soaking in a MQ-water

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Nriagu et aI.

bath until used in the field. Both sides of the membrane holder are first rinsed with slightly acidified Milli-Q water and then with Milli-Q water. The membrane is taken from the MQ-water bath using Teflon coated tweezers and carefully set onto the lower plate of the filter holder. The membrane is rinsed with MQ-water when it is on the plate and the plates are clamped together. A sample bottle is drained of its final cleaning solution and placed inside the vacuum chamber. Vacuum is applied and the sample is filtered, the first 100 mL being discarded. When the bottle is full, 2 mL of the ultrapure nitric acid (Seastar) are added per litre of sample to lower the pH to about 1.7. The bottle is put back in its individual acid washed PE bag and labelled. The sample bottles are again packed five per bag, then five sets of bottles per box, and the lid (of the box) secured by tape. The boxes containing the bottles are stored in a cold room until the samples are analyzed.

Procedural Blanks Procedural blanks are used to estimate the amount of each element that is added to the samples from the reagents, walls of sample containers, the filtration apparatus, the air in the clean laboratories, and during instrumental measurements. Such blanks thus represent a key feature in the analysis of trace metals in natural waters (Rossmann and Barres 1988, Boutron 1990, Ahlers et ai. 1990). Procedural blanks are prepared in triplicate in the field, usually at every other sampling station. They consist of aliquots of the Milli-Q water which have been filtered, processed, and exposed to the portable clean laboratory environment in a manner similar to the actual samples. The blanks are acidified, packed and stored with the samples, and analyzed exactly as the samples. SAMPLE ANALYSIS The protocols described above have been used to measure the concentrations of Cd, Cu, Cr, Pb, Mn, Fe, and Zn in the waters of Lake Ontario. The samples were collected between 22 and 25 July 1991 on board the CSS Limnos, research vessel of Environment Canada. Subsequently, the water samples were analyzed by means of a graphite furnace atomic absorption spectrometry (GFAAS) equipped with a Zeeman background corrector (Varian Spectra 400 model). Unlike seawater with a high salt content which can interfere with the analysis of

metals in the graphite furnace by direct injection, the waters of the Great Lakes seem to be relatively free of such interfering matrix. Some of the metals (Cu, Zn, Fe, Mn, and Cr for instance) can thus be analyzed by direct injection into the graphite furnace, the major limitation being the instrumental detection limit for the particular element. For the other trace elements where a preconcentration step is necessary, the samples are treated with ammonium pyrrolidinedithiocarbamate (APDC) and diethylammonium diethyldithiocarbamate (DDDC) and the complexed metals extracted using freon-1l3 (Daniels son et ai. 1982, Brugmann et ai. 1983, Statham 1985, Lo et ai. 1982). The metals taken up by the organic solvent are back-extracted into dilute nitric acid to achieve a 1()(): 1 concentrated solution which is then analyzed by the GFAAS. The lead contents were quantified by direct injection of the water samples into a laser-excited atomic fluorescence spectrometer (LEAFS); this instrument has a detection limit of 0.4 nglL Pb (Cheam et ai. 1992).

PRELIMINARY RESULTS AND DISCUSSION At low concentrations of the trace metals, the precision of a measurement is very dependent on the quality of the blank determinations (Rossmann and Barres 1988, Boutron 1990). Table 2 shows the concentrations of trace metals in field blanks run at two stations during the Lake Ontario cruise. These concentrations obtained using our protocols are much lower than those reported by Rossmann and Barres (1987) in their field blanks. They are comparable to the laboratory extraction blanks (LEB) for Cu and Pb reported by Coale and Flegal (1989). The high Zn levels in our field blanks may be related to the use of polyethylene sample bottles which may be rich in zinc (see Table 1). Teflon contains less Zn but a trade off has to be made between the high costs of Teflon bottles and those made of linear polyethylene.

TABLE 2. Concentrations of trace metals in field blanks generated during a cruise on Lake Ontario. Element Cu Cr Pb Mn Fe Zn

Concentration (ng/L)

No. of Analysis

4.4 1.8 3.2 5.0 34 28

15 19 19 19 15 15

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Trace Metal Analysis

TABLE 3. Trace metal concentrations (ng/L) in water samples from Lake Ontario collected during 22-25 July 1991. Cd

Cu

Pb

Mn

Fe

Cr

357

10 8.0

626 807 765

9.4 8.4 8.7

98 159 678

337 523 1,037

162 215 174

224 139 256 208 226 231 331

31 15 13 4 5 34

969 984 941 814 869 885 881

16 12 15 6.3 5.6 5.1 21

242 244 127 69

1,649 2,307 796 438 289 407 574

328 349 469 362 408 383 373

Depth(m)

Zn

Station 79 2 10 19

137

Station 33 1.0 10 25 50 75 100 130

The concentrations of trace metals at two stations in Lake Ontario are shown in Table 3. Station 79 (Lat. 44°4"29'N; Long. 76°31 "42'W; 21 m depth) is a shallow, nearshore location just west of Wolfe Island in the far eastern part of the lake. Station 33 (Lat. 43°35"49'N; Long. 78°48"17'; 134 m depth), is a deep and offshore location near the center of the western basin of the lake. The Pb and Cu concentrations in surface waters obtained using the present protocols are comparable to those reported by Coale and Flegal (1989). Our Zn values are, however, higher than the <3-115 nglL (average 12 ng/L) reported by Coale and Flegal. The Pb, Cd, and Zn data which Rossmann and Barres (1988) reported for the near-surface waters of Lake Ontario are not directly comparable to our results. The authors noted that blanks accounted for over 50% of the measured concentrations of the three metals and furthermore that the Pb, Cd, and Zn concentrations were below the limit of detection for a large number of their 1985 samples. The concentrations they reported for the three elements in their 1981 samples are generally much higher than our results. It should be noted, however, that when the concentrations in the samples are relatively high, the effects of sample contamination tend to be minimized which may account for the fact that our preliminary data for Cu, Fe, and Mn are generally similar to those of Rossmann and Barres (1988). Table 3 gives the first glimpse of realistic variations of trace metal concentrations with depth in Lake Ontario. At Station 33, the concentrations of Cd, Pb, Mn, and Fe decrease markedly with depth

19 99

below the air-water interface. Such a profile can be generated by high atmospheric fluxes of these metals to the eplimnion and a higher population of particulate scavengers in the hypolimnion (Nriagu 1986, Flegal et al. 1988). Regeneration from sediments due to particle resuspension and release of metal-rich porewaters from the sediments (Rosa et al. 1983) presumably accounts for the observed increase in concentrations near the sediment-water interface. The profiles of Cu, Zn, and Cr remain fairly constant in the water column suggesting that their concentrations in the lake water are being dominated by different biogeochemical processes andlor sources. Factors responsible for the differences in the observed trace metal profiles, especially the depletion of some metals below the thermocline, are now being actively investigated. With the exception of increases near the sediment-water interface, the trace metal concentrations at Station 79 show no distinctive trend with depth, as to be expected from a shallow, well-mixed body of water. REFERENCES Adeloju, S. B., and Bond, A. M. 1985. Influence of laboratory environment on the precision and accuracy of trace element analysis. Anal. Chern. 57:1728-1733. Ahlers, W. W., Reid, M. R., Kim, J. P., and Hunter, K. A. 1990. Contamination-free sample collection and handling protocols for trace elements in natural fresh waters. Aust. J. Mar. Freshwater Res. 41:713-720. Berman, S. S., and Yeats, P. A. 1985. Sampling of seawater for trace elements. CRC Critical Reviews in Anal. Chern. 16(1):1-14.

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Submitted: 22 September 1992 Accepted: 8 December 1992