Anal. Chem. 1989, 6 1 , 544-547
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development of this SRM was provided by the office of the Assistant Secretary for Health Affairs, U.S. Department of Defense. Certain commercial equipment, instruments, or materials are identified in this paper to specify adequately
the experimental procedure. Such identification does not imply recommendation or endorsement by NIST, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.
Shipboard Determination of Aluminum in Seawater at the Nanomolar Level by Electron Capture Detection Gas Chromatography C. I. Measures* and J. M. Edmond Department of Earth, Atmospheric and Planetary Sciences, E34-246, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
A method for the determination of Ai in seawater at levels between 0.6 and 120 nM is described. The technique uses electron capture detection of the l,l,l-trifiuoro-2,4-pentanedione derivative of Ai, whkh is prepared by solvent extraction from a 15-mL sample of seawater. The method, which has a detection iimH of 0.6 nM and a relative precidon of f3.8% at 18.5 nM, has been used at sea on eight oceanographic cruises.
Trace elements with short residence times in the Ocean are, when coupled with strong input functions, nontransient tracers of their injection process. Aluminum is such an element. Despite its being the third most abundant element ( 1 ) in the earth's crust (8.2% by weight), dissolved aluminium concentrations in seawater are reported to fall between 0.2 ( 2 ) and 120 nM (3) in the trace element range. These low concentrations are a result of the low solubility of aluminosilicate phases during continental weathering ( 4 ) and the short residence time of the dissolved form in the ocean. The distribution of dissolved A1 in profiles from the Pacific and Atlantic leads to the conclusion that the dominant process in the delivery of A1 to the marine environment is the partial dissolution of atmospherically derived aluminosilicate dusts in the surface waters of the oceans (5). The short residence time of the element in the dissolved form ensures that background concentrations remain low, allowing the imprint of this input function to remain sharply defined. Shore-based determination of trace elements from samples collected a t sea is the modus operandi for most applications; the ability to acquire precise and accurate data rapidly under the adverse conditions that exist onboard ship is clearly of benefit. Not only does this provide the opportunity to modify sampling strategies as data gathering proceeds, but also provides the ability to identify and solve any unforeseen contamination problems in the field. The technique presented here is an evolution of the method originally presented for the determination of Be in seawater ( 6 ) . While the methods use mutually exclusive handling protocols and different solvents, they are both part of a continuing program that is aimed at adapting and developing analytical techniques for trace elements that exploit the high sensitivity and seagoing capability of electron capture detection gas chromatography. While many techniques for the determination of aluminum in freshwaters have been developed (7-9), only two techniques have been applied successfully to the determination of the element in seawater where the concentration levels are sig0003-2700/89/0361-0544$01 S O / O
nificantly lower. The fluorometric lumogallion method of Hydes and Liss ( I O ) , which has a detection limit of ca. 2 nM and a precision of 5% a t 37 nM, has been used successfully both at sea and in the laboratory on stored samples by a variety of workers to produce high-quality data in the Atlantic and other oceans (11-15). Orians and Bruland ( 2 ) have achieved the best detection limits of ca 0.1 nmol/kg in a shore-based analytical scheme by solvent extraction of 250-g samples of previously frozen Pacific Ocean water with 8hydroxyquinoline in a class 100 clean room and then by using atomic absorption spectroscopy to quantify the concentrated Al.
EXPERIMENTAL SECTION Apparatus. A Hewlett-Packard 5792 gas chromatograph (ECD-GC) equipped with a 10-15-mCi 63Ni electron capture detector was used for all determinations. The ECD-GC was run in the split mode (split ratio approx. 401) with a 15 m X 0.3 mm 0.d. 0.2-pm film DB 210 capillary column (J&W Scientific). The detector temperature was maintained at 350 OC;the injection port and column were heated to 250 and 130 "C, respectively. The pressure at the top of the column was set at 15 psi, equivalent to a linear velocity of about 45 cm/s. The column gas was hydrogen (Matheson zero grade), which had been passed through a 13X molecular sieve trap (HP no. 5060-9084). Detector makeup gas (95% argon, 5% methane) was also cleaned by using a 13X molecular sieve and an oxygen trap (Matheson no. 6406) and supplied at 45 mL/min. Distillation of the l,l,l-trifluoro-2,4pentanedione (Htfa, Eastman Kodak) has been described earlier (6). Small-volume separatory funnels are not commercially available; the use of the smallest ones available (125 mL) results in a considerablefraction of the solvent being left behind on the walls of the vessel. A simple low-volume (ca. 20 mL) Teflon separatory funnel can be constructed from an approximately 18 cm X 1.5 cm piece of heat shrink Teflon tubing shrunk onto a standard Teflon seperatory funnel stopcock. Sample shaking was performed with a Burrel Model 75 wrist action shaker. Reagents and Standards. The buffer, 1M sodium acetate, is prepared from twice recrystallized material; all of the commercially available grades were found to contain unacceptably high levels of Al. The recrystallization procedure is as follows: 87 g of NaAe3H20is dissolved in 67 mL of sub-boiled distilled water in a 250-mL Teflon bottle. The solution is filtered through an 0.45-pM Nuclepore filter held in a previously acid leached filtration unit (Millipore). To this solution is added 100 mL of absolute ethanol, which has been redistilled in the Teflon microstill used for the Htfa purification. After the precipitate has formed (overnight in a refrigerator), the material is filtered in the same manner as above. The once recrystallized material is dried under a filtered stream of air at room temperature, and a second recrystallization performed, as above. The yield at each step is approximately 50%. 1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 6, MARCH 15, 1989
Toluene (Package Chemical Co. Toluol grade) is purified by redistillation through a 5 ft X 1.5 in. glass still. We have found that a single distillation of this rather impure starting material produces solvent that is free of extraneous peaks and superior to the best pesticide grades available. The redistilled toluene is spiked with the internal standard, 2,6-dichlorobiphenyl(Analabs), at about 50 ng/mL. A working standard of ca 1 nM Al/mL is prepared by serial dilution of a commercially available standard (Alfa Products). The acidity of the dilution is maintained by the inclusion of 100 p L of 6 N HC1 in each 100 mL of standard. Precleaned plastic containers were used for all standard dilutions and the storage of all standards. The backwashing solution is prepared by adding 1%by weight sodium fluoride to a 1 M sodium hydroxide solution. Recommended Procedure. To 15 mL of sample in a 35-mL Teflon bottle is added 200 pL of 1 M sodium acetate solution, 1 mL of toluene, and 20 pL of Htfa. The bottle is then closed and shaken for 1 h. At the end of this time, and when the phases have separated, the contents are poured into a small Teflon funnel. The aqueous phase is run to waste immediately, and the organic phase is run into a Teflon bottle containing 1 mL of the 1 M sodium hydroxide backwashing solution. This bottle is then shaken immediately for a period of 30 s, after which the contents are poured into a small glass vial with a Teflon-lined cap. The sodium hydroxide phase is siphoned out with a Pasteur pipet, and the remaining toluene phase is rinsed three times with approximately 2 mL of distilled water, with care being taken to ensure good aqueous-phase removal at each stage. The extracts, thus treated, can be stored for at least several weeks at -15 "C. A standard curve is prepared by extracting A1 standards from samples of sub-boiled water in the same manner as above. An alternate aqueous matrix that we have used is deep Pacific Ocean water that had been stored unacidified for several years. The advantage of the latter is that its naturally low level of Al, if anything, decreases with time due to adsorption of A1 on the walls of the vessel. Whatever the medium used for the blank determination, it is important that the aqueous-phase contribution to the blank be determined separately, since this is not part of the sample blank and as such must not be subtracted from the analyte signal. This can be accomplished fairly simply by running three blanks with aqueous-phase volumes of 15, 10, and 5 mL, but constant reagent additions. A least-squaresfit of the signal against the aqueous-phasevolume gives an intercept at zero volume which is the equivalent of the reagent plus handling blank. Gas Chromatography. Procedures for the packing and deactivation of the injection port liner and deactivation of the capillary column are the same as those described previously (6). The injection port is held at 250 "C and the detector is maintained at 350 "C for consistency with the Be technique. It is interesting to note however that we have seen no change in response to the Al(tfaI3chelate with detector temperatures ranging from 250 "C to 350 " C , unlike the factor of 2 increase in sensitivity seen in the case Be(tfa)2over the same range of detector temperatures. The oven is maintained at 130 "C. Three-microliter aliquots of the extracts can be injected into the ECD-GC immediately after the chelate extraction procedure has been completed. The extracts may however be stored in a freezer at -15 "C for at least several weeks for later determination or reinjection, provided they have been carefully rinsed free of the NaOH backwash solution. The stability of the extracts negates the requirement to prepare fresh standards on a daily basis. A typical chromatogram replotted from the digitized output of an HP 3392 integrator is shown in Figure 1. A profile of A1 concentration with depth, obtained by using the technique at sea, is shown in Figure 2. The elevated concentrations observed in the surface waters are the result of the dissolution of atmospheric dust, while the elevated deep-water values arise from the recently descended North Atlantic Deep Water.
RESULTS A N D DISCUSSION Contamination Control. Sampling. Water samples collected on large-scale oceanographic cruises are subject t o the measurement of many different parameters. This can lead to problems in devising a sampling protocol that ensures the sampler material requirements for one parameter do not
2 32
545
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1500m
u
5 10000
0
500-
L Figure 1. Chromatogram of an AI determination on a seawater sample from a depth of 2750 m in the Atlantic Ocean (station 44) shown in Figure 2. The Al(tfa), peak at 2.32 min corresponds to 222 pmol of AVmL of toluene, equivalent to 14.8 nM in the original seawater sample. The internal standard, 2,6-dichlorobiphenyl,has a retention time of 2.93 min. 0-
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Figure 2. AI data from the Atlantic Ocean 36'18' N 72'21' W. All determinations were performed at sea aboard the R.V. Knorr . preclude the accurate determination of another. In cases where such conflicts do arise, priority is given to measurements that are part of the core project. This can mean that it is not always possible to use the ideal sampling system to collect trace element samples, which are rarely part of the core program. It is then important to know which components of commonly used sampling systems are compatible with the collection of uncontaminated samples for A1 determinations. The cost of ship time precludes classic control experiments in sampling, and therefore the recommendations consist of a synthesis of our experiences to date. Five-liter Niskin bottles (General Oceanics) are fitted with silicone rubber rather than Viton O-rings, and also the black rubber tubing used to close the bottles is replaced with an epoxy-coated stainless steel spring. Before they are sent to sea, the bottles, minus springs, are leached in ca. 0.1 N HC1 overnight; in cases where there has been no prior access to the bottles, we have found that soaking the bottles onboard ship in an ethylenediaminetetraacetic acid (EDTA) solution is also an effective cleaning technique. Bottles thus prepared have provided uncontaminated samples when either deployed on normal hydrographic wire or mounted on 2501 Girard bottles. We have routinely exchanged the black O-rings in our Niskin bottles for red silicone O-rings due to the contamination effects seen for other trace elements. Recent work was done in the South Atlantic, where the simultaneous sampling for Freons, which have the reverse O-ring requirement, ne-
546
ANALYTICAL CHEMISTRY, VOL. 61, NO. 6, MARCH 15, 1989
cessitated the admixture of bottles containing either black or red O-rings in the same profile. The results indicated that after their first use, where considerably elevated values were seen, the bottles with the vacuum-baked black O-rings yielded A1 concentrations that were similar to those found in the red O-ring bottles above and below them. It should be stressed, however, that as there are no control samples from red O-ring bottles at the same depth, these results cannot be interpreted unambiguously. Less successful has been the deployment of Niskin bottles mounted on a rosette system. Initial work with these systems produced erratic profiles with A1 concentrations higher than those observed in samples collected by the Niskin/Girard method. Recent experience in the Arctic however with 5-L Niskin samplers mounted on a painted, steel rosette frame yielded samples whose A1 concentration was analytically indistinguishable from zero in the upper water column. This means that the anodized A1 materials that are always used to constuct the central pylon part of the rosette system do not themselves pose a serious contamination problem, and suggests that the even the use of A1 framing material for the rosette body may be acceptable if the material is treated in some manner such as anodizing or painting. Processing. The Teflon reaction bottles and reagent containers are leached in boiling 0.1 M HC1 for approximately 1 h before their first use; between uses, three rinses with approximately 5 mL of acetone are sufficient. The Teflon funnel is soaked in HC1 before its first use and is rinsed with acetone twice and water once between uses. All reagents are stored in Teflon bottles that have been acid-leached. All procedures up to the rinsing of the extract in the glass vial are carried out in a laminar flow bench. The 1 M sodium hydroxide used for removing the excess reagent from the toluene phase is extremely contaminated with Al, and we have found no suitable technique for purifying it. Although it is not used as a reagent in the reaction, we have found that a t the stage a t which the organic phase is added to it, for the backwashing step, appreciable “blank” levels of Al(tfa)3can form no matter how rapidly the shaking step is commenced after the additions. The obvious expedient of using ammonia as a backwashing solution is not a practical alternative due to the formation of what is assumed to be NH,tfa which is extracted into the solvent phase and partially decomposes on the column. We have found that the addition of 170by weight sodium fluoride to the NaOH solution suppresses the formation of Al(tfa)3during the backwash stage. Presumably it is the production of fluoride complexes of A1 that are kinetically stable on the few-second time scale of the backwash procedure that is responsible for this effect. The addition of EDTA, while not tested, might have a similar effect. Internal Standard. In this work 2,6-dichlorobiphenylwas used. The choice was largely determined by the compound’s retention time (2.9 min under the conditions used here) and its suitability for other tfa separations used in this laboratory. The internal standard normalizes the injection volume and hence reduces the imprecision inherent in the injection process. Care should be exercised when the 2,6-dichlorobiphenyl is used for normalizing. As a column deteriorates, or if it is improperly deactivated, the absolute sensitivity of the internal standard is adversely affected, due to poor peak shape, and becomes unreliable for normalizing. Precision, Accuracy, a n d Recovery. The precision of the technique (Table I) was determined by the replicate analysis of a freshly collected seawater sample under shipboard conditions; the relative standard deviation, 3.8% a t 18.5 nM, is therefore a realistic assessment of all the errors, except for sampling, of the technique under field conditions. Generally
Table I. Reproducibility of AI Determination on a Seawater Samplen
sample no.
concn. of Al, nM
10
15
17.6 18.6 18.2 18.0 19.6 18.9
mean std dev % re1 std dev
18.5 0.71 3.8
11 12
13 14
“Samples were 15-mL aliquots of seawater collected from a 3000-m depth at 3 5 O 20’ N 71’ 32’ W in the Atlantic Ocean. Determinations were performed at sea in real time within 8 h of sample collection. Table 11. Recovery of Aluminum Spikes (nM) from Seawater Samplesn
initial A1
spike
recovdb
70 recovery
8.8 & 0.1 8.8 f 0.1
12.35 24.70
21.1 f 0.2 34.3 f 0.3
98 103
a Samples were aliquots of Atlantic surface water processed onboard ship. * Mean of two replicates.
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Figure 3. Recovery of Ai as a function of time. The experiment was performed at sea by using 15-mL aliquots of a surface seawater sample; ail other conditions are as in the recommended procedure. the l u precision for data interpretation purposes is quoted as i 5 % or f0.2 nM, whichever is greater. It should be pointed out that although the within-the-day precision is ca. f4%, the accuracy of reproducing a data point on a day-to-day basis is ca. f 6 % , due to the extra error associated in fitting a standard curve. While the accuracy of the technique has not been assessed by reference to any independently certified standard, the comparability of the values obtained by this technique and those of the lumogallion method (IO)on the same samples of seawater has been demonstrated previously (5). Figure 3 shows that the recovery of A1 is essentially quantitative in about 20 min a t pH 5.2. In general use, samples are shaken for a period of 1 h. This allows a safety margin precluding the necessity t o make the ligand addition very accurately and obviating any temperature control requirements. Recovery of A1 spiked into samples of seawater (Table 11) is quantitative within the precision of the technique. Blank. Table I11 shows the 1 u variation of the blank to be ca. f0.19 nM. The limit of detection, defined as 3u, is therefore ca. 0.57 nM.
ANALYTICAL CHEMISTRY, VOL. 61, NO. 6, MARCH 15, 1989
of this small-volume shipboard technique.
TabGIII. Reproducibility of the Blank" amt of Al, nM
meanb std dev 70 re1 std dev limit of detection
547
1.70 1.38 1.67 1.33 1.73 1.56 0.19 12.2
0.57
"Samples were 15 mL of unacidified deep Pacific water (see text). bMean is not adjusted for known value of aqueous phase (1.02 n M detection limit is defined however as 3 times the standard deviation of the blank and is therefore independent of its absolute value. All sample processing and determinations were performed at sea.
Data Processing. An HP 3392A integrator was used to process the gas chromatograph signal. When the system was used at sea, the data report from the integrator was transmitted to an HP 85B computer, which performed intermediate concentration calculations and displayed a real-time running plot of the A1 data vs depth. This procedure was extremely useful, enabling rapid identification of samples with suspect values, and allowing immediate reinjection of the extracts. Full listings of both the integrator and computer programs are available from the authors.
ACKNOWLEDGMENT We thank the Chief Scientist, Bill Jenkins, and the entire complement of the R.V. Knorr for both the opportunity to participate in, and their help throughout, the Western Boundary Experiment. Registry No. Al, 7429-90-5; HzO, 7732-18-5; F3CCOCH2COCHB, 367-57-7.
LITERATURE CITED (1) Ernst, W. G. Earth Materials; Prentlce-Hall: Englewocd Cliffs, NJ, 1969. (2) Orians, K. J.; Bruland, K. B. Earth Planet. Sci. Left. lS88, 78, 397-410. (3) Measures, C. I.; Edmond, J. M. J . Geophys. Res. 1987, 9 3 , 591-595. (4) Marlng, H. B.; Duce, R. A. Earth. Planet. Sci. Lett. 1987, 8 4 , 381-392. (5) Measures, C. I.. Edmond, J. M.; Jickells, T. J. Geochim. Cosmochim. Acta 1988, 5 0 , 1423-1429. (6) Measures, C. I.; Edmond. J. M. Anal. Chem. 1988, 58. 2065-2069. (7) May, H. M.; Helmke, P. A.; Jackson, M. L. Chem. Geol. lS79, 24, 259-269. (8) Royset, 0. Anal. Chem. 1987, 5 9 , 899-903. (9) MacCarthy, P.; Klusman, R.; Rice, J. A. Anal. Chem. 1987, 5 9 , 306R-337R. (10) Hydes, D. J.; Liss, P. S. Analyst 1978, 701, 922-931. (11) Hydes, D. J. Geochim. Cosmochim. Acta. 1983. 4 7 , 967-973. (12) Moore, R. M.; Millward, 0.E. Geochim. Cosmochim. Acta 1984. 48. 235-241. (13) Caschetto, S.; Wollast, R. Mar. Chem. 1979, 7 , 141-155. (14) Stoffyn. M.; Mackenzie, F. T. M a r . Chem. 1982, 1 1 . 105-127. (15) Kremllng, K. Deep-sea Res., PartA 1985, 32, 531-555.
CONCLUSION The method described above has been used successfully by us a t sea on eight occasions in the Atlantic Ocean, and the Mediterranean and Greenland seas where concentrations varied from
