Download Lead Speciation in Surface Waters of the Eastern North Pacific

Marine Chemistry, 29 (1990) 221-233
221
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Lead Speciation in Surface Waters of the Eastern
North Pacific
GABRIELE CAPODAGLIO
Department of Environmental Sciences, University of Venice, 30123 Venice (Italy)
KENNETH H. COALE* and KENNETH W. BRULAND
Institute of Marine Sciences, University of Cali/ornia at Santa Cruz, Santa Cruz, CA 95064
(U.S.A.)
(Received November 28, 1988; revision accepted November 20, 1989)
ABSTRACT
Capodaglio, G., Coale, K.H. and Bruland, K.W., 1990. Lead speciation in surface waters of the
eastern North Pacific. Mar. Chem., 29: 221-233.
Titrations using differential pulse anodic stripping voltammetry (DPASV) to detect electroactive lead were carried out on fresh seawater samples and on samples stored acidified to determine
the extent of lead complexation in the eastern North Pacific. Results of these analyses on surface
water samples indicate total dissolved lead concentrations between 17 and 49 pM. The inorganic
or DPASV labile fraction is 30-50%. Titration with lead yielded data consistent with one class of
organic ligand(s), present at low concentration {between 0.2 and 0.5 nM) with a mean value for
a conditional stability constant with respect to inorganic lead of log K'~ond=9.7.The presence of
this ligand, together with the various inorganic ligands in seawater, gives rise to a concentration
of free ionic lead of ~ 0.4 pM.
INTRODUCTION
Chemical properties of heavy metals with respect to their interaction with
suspended particles, sediments and aquatic organisms are strongly affected by
the chemical forms in which dissolved metals exist in natural waters (Salomons and FSrstner, 1984). The inorganic speciation of heavy metals such as
lead has been studied extensively, although the results are at times conflicting
(Sipos et al., 1980; Byrne, 1981; Turner et al., 1981). To understand the toxicity, bioavailability and geochemical reactivity with respect to particle scavenging in the water column, the organic complexation of trace metals also needs
to be known. Although we are beginning to understand more about the organic
*Present address: Moss Landing Marine Laboratories, P.O. Box 450, Moss Landing, CA 95039O45O, U.S.A.
0304-4203/90/$03.50
© 1990 Elsevier Science Publishers B.V.
222
G. CAPODAGLIO ET AL.
complexation of some metals (e.g.copper, Fiirstner and Salomons, 1983; van
den Berg, 1984; Kramer, 1986; Coale and Bruland, 1988), relatively littleis
known about lead.
Lead is a highly toxic element and the predominant source of lead to the
ocean is anthropogenic (Schaule and Patterson, 1981; Flegal and Patterson,
1983; Boyle et al.,1986). Boyle et al. (1986) have shown that different geochemical processes act on lead at differentdepths. A residence time of ~ 2 years
was observed in surface waters where the largest biological activityis concentrated, whereas in deep waters a residence time of a few hundred years was
estimated.
In the last decade the application of differentialpulse anodic stripping voltammetry ( D P A S V ) has been introduced to study speciation of heavy metals
in natural waters (Nurnberg et al.,1976; Plavsic et al.,1982). This technique
provides the possibilityof defining two fractions of dissolved lead: (1) the A S V
electroactive or labilefraction (which includes inorganic or weakly complexed
species) and (2) the inert fraction (strongly complexed metal species that are
not electroactive). Titrations of seawater samples with lead, followed by adequate equilibration and D P A S V analysis, allows estimation of the concentration of ligands which have affinity for lead and the conditional stabilityconstant for the lead-ligand association (Ruzic, 1982; Plavsic et al.,1982).
The abilityto conduct trace metal speciation experiments in the open ocean
poses unique problems that have only recently been overcome. A prerequisite
for speciation studies is the collection and processing of samples uncontaminated for the element of interest as well as other potentially competing metals
and organic contaminants. It is imperative in such studies that the abilityto
obtain the proper total metal value can be demonstrated, especiallyfor metals
such as lead, a ubiquitous and contamination-prone element. M a n y speciation
experiments have been jeopardized or rendered meaningless because of inadequate processing techniques and inadvertent contamination. In this study,
great care was taken to ensure that a high degree of sample integritywas maintained throughout the course of sample collectionand analysis.
CONCEPTAND
TERMS
If we consider a complexation equilibrium between lead ion and a class of
natural organic ligands, [L n- ],with a stoichiometric ratio of I: 1,we can define
a thermodynamic equilibrium constant K = [PbL 2-n ]/ [Pb 2+ ] [L ~- ]. Thermodynamic constants can be corrected by the use of activity coefficients (7)
of the various species in solutions of high ionic strength such as seawater. W e
can then define K' = K/(~PbL/~Pb~L). Other corrections can be made ifwe consider the complexation side-reactions of lead and organic ligands with inorganic species. Metal cations can be complexed by inorganic anions such as
CO~-, O H - and CI-, and the organic ligand (s) may be coordinated by major
LEAD SPECIATION IN THE EASTERN NORTH PACIFIC
223
cations such as Ca 2+ and Mg 2+. These side-reactions can strongly affect the
free metal ion and free organic ligand concentrations that enter in the thermodynamic stability constant. The side-reaction coefficient (aM and ~L)
(Ringbom and Harju, 1972; Ringbom and Still, 1972) provides an expression
for these side-reactions. The free lead ion and free ligand concentrations are,
respectively, [Pb 2+ ] = [Pb' ]/aPb' and [L"- ] = [L' ]/C~L,, where [Pb'] is the
concentration of dissolved lead present in all the inorganic forms and [L' ] is
the total concentration of the organic ligand present not bound with lead. For
seawater conditions the complexation equilibrium can be expressed as a conditional stability constant (K'cond), related to K' by side-reaction coefficients:
K'cond= [ P b L ] / [ P b ' ] [L'] = [ P b L ] / [ P b 2+ ]o~pb,[Ln- ]O~L, K'cond=g'/oLpb, OLL,.
With DPASV, [Pb'] is the inorganic lead fraction detected, [PbL] is calculated by difference between total Pb and [Pb' ], and K'condis calculated. As ~L'
for natural ligand is not known, the K'condcannot be converted to a thermodynamic stability constant; however, K'condand [L'] for the Pb-ligand interaction in seawater allow an estimation of the extent of lead complexation with
organic ligands.
METHODS
The samples collected for this study were obtained during the August 1987
leg of the VERTEX VII cruise in the eastern North Pacific (see Fig. 1; for
coordinates, see Table 1 }. Seawater was collected in the mixed layer (10 m)
using internally Teflon-coated 30-1 Go-Flo bottles (General Oceanic) deployed
on Kevlar hydrowire (Philadelphia Resins) and tripped with a Teflon messenger. Great care was taken to avoid contamination from the ship. Upon recovery, the Go-Flo bottles were pressurized at 0.5 atm with filtered nitrogen gas
and the samples were collected in 2-1 FEP bottles. Samples for total lead anal!
60°N
• oo
i
°e°°
40ON J
~ Cruz
Pacific Ocean
20°N
i
180°W
I60°W
i
i
140°W
I
i
120°W
I
lO0°W
Fig. 1. Locationof samplingstations in the eastern North Pacific.
224
G. CAPODAGLIOET AL.
TABLE 1
Location,temperatureand chlorophyllfor the stations occupiedduringthe August1987VERTEX
VII cruise
Position
Coordinate
Temperature
(°C)
Chlorophyll
(,ugl -~)
T8
T7.5
T7
TA
TB
TC
TD
TE
TF
TG
TH
55 ° 26' N
52°39'N
50° 19'N
48°03'N
46°52'N
45°25'N
44°11'N
42°44'N
41°24'N
39°51'N
38°31'N
13.4
12.1
12.4
14.1
14.7
15.7
16.1
17.1
17.7
16.9
16.9
0.350
-0.598
0.258
0.173
0.176
0.048
0.068
0.092
0.078
--
147 ° 14'W
146°39'W
144°42'W
140° 19'W
138 ° l l ' W
135°37'W
133°29'W
131°06'W
129°00'W
126°46'W
124°42'W
ysis were collected in 500-ml polyethylene bottles and acidified with 1 ml of
quartz-distilled HC1. The samples collected at stations T7 and T8 (see Fig. 1 )
were filtered by forcing the seawater from the Go-Flo bottle (0.5 atm) through
an acid-cleaned polycarbonate membrane filter (142-mm diameter, 0.3-/lm pore
size, Nuclepore) in a Teflon filter-holder (Millipore). All manipulations of
open samples at sea took place within a positive-pressure clean-air sampling
van or under a Class 100 laminar flow hood within a positive-pressure cleanair analytical van.
An Adriatic Sea sample was collected about 3 miles offshore of the Lagoon
of Venice for the production of UV-oxidized seawater, and to examine the kinetic lability of metal-organic complexes.
DPASV M E A S U R E M E N T
Measurements on the North Pacific samples were carried out by DPASV
using the apparatus and instrumental modifications described earlier (Mart
et al., 1980; Bruland et al., 1985; Coale and Bruland, 1988). Measurements on
Adriatic Sea samples were carried out with an EG&G Model 264A Polarographic Analyzer, using a rotating glassy carbon disk electrode (EG&G Rotel
2). All potentials reported are relative to Ag/AgC1,KC1 saturated. The sensitivity of the commercial apparatus was ~ 15% lower, primarily because its
maximum electrode rotation speed was 4000 r.p.m.
The procedure for the preparation of the mercury film and blank control was
as described by Coale and Bruland (1988). Measurements were carried out by
deposition at - 0 . 7 5 V and 5000 r.p.m, for 15 min, followed by a quiescent
LEAD SPECIATION IN T H E EASTERN NORTH PACIFIC
225
period of 30 s. The potential scan was then started at 10 mV s- 1 in the positive
direction until a final potential of - 0 . 1 5 V was reached. The electrode was
rotated and held at this potential for 5 min to strip out any residual metals
from the film. All iead additions were allowed to equilibrate for 15 rain before
the next deposition step. For the zero-addition point, DPASV analyses were
carried out using the same operating conditions, except that the deposition
time was increased to 30-60 min. This improved the precision in the Pb' determination. The currents were normalized to 15 min of deposition time in
accordance with the rest of the titration. Upon completion of a titration the
sample volume was measured. The detection limit, expressed as three times
the noise, using 15 rain of deposition time, was 10 pM. Detection limits were
proportionally reduced to 5 and 2.5 pM by increasing the deposition time to 30
and 60 rain, respectively.
Two initial lead additions each increased the lead concentration in the cell
by ~ 40 pM, then larger additions were carried out until a final lead concentration of ~ 3 nM was obtained.
An equilibration experiment was performed using seawater from station T7.5
to examine potential kinetic effects on the complexation between lead and
natural ligands. In this experiment ~ 60-ml aliquots of fresh sample were
transferred to a series of cleaned 60-ml F E P bottles and each aliquot was spiked
with lead to an amount equivalent to the standard additions used in a normal
lead titration. This set of bottles, representing a single titration, was allowed
to equilibrate for 24 h before analysis. These samples were analysed using the
same analytical procedure as for a normal titration except that the electrode
was turned off at the end of each deposition/stripping cycle and the next degassed aliquot transferred to the analytical position.
The total initial concentration of lead was determined by DPASV at the
UCSC Laboratory on samples acidified to pH 2 with hydrochloric acid and
stored in clean polyethylene bottles for at least I week. The method of standard
additions was used and at least four spike additions were made (correlation
coefficients were always > 0.999).
The relative standard deviation (RSD) for the inorganic fraction, for the
zero addition measurement, was 20% ( n = 5 ) for measurements carried out on
an Adriatic Sea sample (lead concentration 40 pM) and with Pacific Ocean
samples replicated. A more extensive statistical evaluation of the repeatability
and significance of parameters available from titrations was carried out on
samples collected in the Antarctic Ocean to study speciation of lead (Capodaglio et al., 1990); the RSD for the total initial concentration was 7% for a
concentration of 78 pM, the RSD for the labile fraction in the initial sample
was 20% for a concentration of 40 pM, for a ligand concentration of 0.37 nM
the RSD was 10%, and for the conditional stability constant, calculated by
linearization of the titration point with the Ruzic and van den Berg method
(Ruzic, 1982; van den Berg, 1982 ), it was 30%.
226
G. C A P O D A G L I O E T AL.
LABORATORY R E S U L T S
The application of ASV techniques to the study of metal-organic ligand interactions has raised a certain amount of controversy and criticism. The principal uncertainties arise from the possible reduction of metal-organic complexes at the potential applied in the deposition step and from the kinetic lability
of metal complexes with organic ligands in the diffusion layer (Tuschall and
Brezonik, 1981 ). Both processes would lead to an underestimation of the conditional stability constant and of the amount of ligands present.
In this study, an effort was made to minimize these potential sources of error.
To determine whether there was an effect due to kinetic lability of metalorganic complexes, titration experiments were carried out at three different
rotation rates on an Adriatic Sea water sample. No significant differences were
observed in the conditional stability constant using rotation speeds of 4000,
3000 and 2000 r.p.m. (see Table 2 ). This indicates that the Pb-organic ligand
complexes were kinetically inert with respect to the electrochemical techniques. However, to minimize the potential for a kinetic contribution and to
maximize sensitivity, the highest rotation speed was used. To minimize any
possible reduction of organically complexed lead, a relatively weak deposition
over-potential (for high-chloride media) was applied ( - 0 . 7 5 V). To determine the optimum deposition potential for this study, a pseudo-polarogram
was made (curve obtained by plotting the oxidation current vs. the deposition
potential). For deposition potentials more negative than -0.72 V, no increase
in peak current was observed. A potential of -0.75 V was selected because it
is the weakest possible potential in the reproducible portion of the pseudopolarogram plateau.
Analogous to the work of other authors for copper (Plavsic et al., 1982; Coale
and Bruland, 1988 ), an attempt was made to verify better the DPASV technique by testing it against well-characterized model ligands. EDTA {28.1 nM)
was added to UVSW and allowed to equilibrate with lead additions for at least
1 h before analysis. These titrations were modelled according to the linear
transformation method used by Ruzic (Ruzic, 1982; van den Berg, 1982 ). Lead
TABLE2
Ligand concentration and conditional stability constant measured by titration with lead at three
different rotation speeds
Rotation speed
(r.p.m.)
Concentration
(nM)
K'
(1 tool -1)
4000
3000
2000
7.2
8.1
7.9
3.8 × lOs
3.7 X 10 s
4.4 × 10 s
LEAD SPECIATION IN THE EASTERN NORTH PACIFIC
227
was added until at least four sequential additions gave a linear response. The
sensitivity, obtained from the slope of the response to the last additions, was
used to calculate the inorganic fraction observed for each addition. If lead and
the organic ligand form a complex whose binding stoichiometry is 1:1, plotting
the ratio of inorganic lead to organically complexed lead [Pb' ]/[PbL] vs. inorganic lead [Pb' ] should yield a straight line. The slope and the intercept of
this line allow calculation of the concentration of the ligand and the conditional stability constant (Ruzic, 1982; Coale and Bruland, 1988).
The experimental results yielded values very close to those expected: the
EDTA concentration measured was 27.3 nM against 28.1 nM added and the
measured log K'condwas 8.6, which is in good agreement with the result calculated for seawater at our experimental conditions (at pH =7.8 and low CO2
content, a log apb, = 0.9 was calculated) of log K'cond= 8.5. These results further
support the observations of others using DPASV with model ligand systems
(Raspor et al., 1980).
To observe the DPASV response to lead additions in the absence of complexing organic ligands, UV-oxidized seawater (UVSW) (Coale and Bruland,
1988) was titrated according to the procedure outlined above. Results show a
linear relation between anodic current and lead concentration; the final concentration was 4.3 nM (correlation coefficient 0.9996 ).
To investigate the specificity of the natural ligands which show affinity for
lead, some tests were performed using seawater samples. Iron and copper were
added in large excess (20 nM) with respect to the lead complexing ligand. After
30 min equilibration and subsequent deposition, the lead signal during the
stripping cycle showed no change, indicating no significant competition between lead and the added metals. In a second experiment, two aliquots of the
same sample were prepared. To one aliquot 0.83 nM of Fe (III) was added and
equilibrated. Both aliquots were then titrated and analysed. Results of the two
samples yielded ligand concentrations and a conditional stability constant of
[ L ] = 0.49 nM, K'cond= 2.4 × 109 1 mol- 1 and [ L ] = 0.56 nM, K'~ond= 3.3 × 109 l
mol-1, respectively, indicating little, if any, competition by iron for the ligands
which show affinity for lead.
FIELD RESULTS AND DISCUSSION
Samples for Pb complexation analysis were collected at 11 locations in the
eastern North Pacific (see Fig. 1 ). The titrations were carried out on the ship
within 3 days of collection. Results including total lead, ASV labile lead, ligand
concentrations and conditional stability constants are presented in Table 3.
The ligand concentrations and the relative conditional stability constant were
calculated by the graphical linearization illustrated in the laboratory results
section. Figure 2 shows a typical titration curve and the corresponding linearization plot according to the Ruzic and van den Berg method.
228
G. CAPODAGLIO E T AL.
TABLE 3
Concentrations of lead species, ligand and conditional stability constants for the samples collected
in the North Pacific Ocean
Position
[ Pb ] t
(pM)
[ Pb' ]
(pM)
[ Pb e + ]
(pM)
[L ]
(nM)
log (K'cond)
T8
T7.5
T7
TA
TB
TC
TD
17
31
30
49
27
25
25
4
15
8
11
.
10
22
0.1
0.5
0.2
0.3
0.22
0.24
0.31
0.51
10.0
9.8
9.4
9.6
TE
30
10
TF
TG
TH
30
41
37
13
24
29
0.24
0.24
0.54
0.56
0.41
0.47
9.8
9.9
9.6
9.6
9.8
9.7
.
.
0.3
0.7
0.3
0.4
0.7
0.9
.
1.5
o
<
::L
4
1.0
C
5
-Q
\
"U
2
o.s
g
g
cL
0
/yl
p
/
//
0D
I
1
Pb
concentration,
2
nM
Fig. 2. Lead titration of station TE; ( O ) current vs. total concentration, ( O ) inorganic lead/
organically complexed lead ratio vs. inorganic lead.
Graphical linearizations of inorganic/organically complexed lead vs. inorganic lead showed a kinetic effect (Ruzic and Nicolic, 1982). To examine this
phenomenon, the sample collected at T7.5 was titrated using a 10-min equilibration time (5 rain less than the normal time of 15 min) between standard
additions, and the results were compared with a titration equilibrated for 24 h.
The kinetic effect was not apparent in the sample equilibrated for 24 h (see
Fig. 3 ). The ligand concentrations and conditional stability constants obtained
229
LEAD SPECIATION IN THE EASTERN NORTH PACIFIC
15
I
0 10 min.
• 24 hr.
i
~
0
o
o
0
I
1
I
2
3
Pbinorg '(nM)
Fig. 3. Linear t r a n s f o r m a t i o n of lead t i t r a t i o n of t h e T7.5 sample using ( O ) 10 rain a n d ( Q ) 24
h of equilibration time.
by linear transformation of the experimental data were [L']=0.23 nM,
Kcond=5.5X 109 1 tool-1 and [L'] =0.24 nM, K'cond=6.0X !09 1 tool-I with 10
rain and 24 h of equilibrationtime, respectively.For the titrationequilibrated
for only 10 rain,the zero-addition point and the last six points of the titration
were used to estimate [L' ] and K'co,d.Thus, even though an apparent kinetic
effect was observed, the results obtained by the linear transformation of the
data and the interpretationof Ruzic and Nicolic (1982) were not significantly
different.Using the 10-rain equilibratedtitrationand the kinetictreatment of
titrationdata presented in Ruzic and Nicolic (1982), a complexation rate constant of kf= 5 × 104 was estimated.
The bulk of the samples were equilibratedfor 15 rain and exhibited a slight
kinetic effect in the initialadditions,as a resultof lack of complete equilibration of the added lead with the organic ligands. However, estimates of the ligand concentrations and conditional stabilityconstants could stillbe readily
made by linearization of the zero-addition and the last six titration points.
Good precision was obtained in the measurement of ligand concentrations; the
R S D s were ~ 15%, calculated from the slope of the straightline obtained from
the graphical linearization.Lower precision was observed in the measurement
of the conditional stabilityconstant obtained from the intercept ( R S D was
35%).
The total lead concentrations vary between 17 and 49 pM and show little
spatial structure (see Fig. 4 ). The horizontal distribution of ligand concentration and associated conditional stability constants are plotted in Fig. 5A and
B. The ligand concentration varies between 0.2 and 0.5 nM. Although there
appears to be a possible increase in ligand concentration within the California
Current, no apparent trends in the stability constants for this association are
evident. The ASV labile fraction that we consider as the inorganic fraction,
230
G.CAPODAGLIOETAL
80
50
O
0~0
Total Pb
@--@
I n o r g a n i c Pb
"
o
/-o
/
30
0.._..0__0/0-0
o.
so
. / @\
/
lo
0
~o~o
~
@/0
/
_
~O
I
I
t
I
I
i
I
I
t
I
I
T-8
T-7.6
T-7
T-A
T-B
T-C
T-D
T-E
T-F
T-G
T-H
Station
Fig. 4. Geographical d i s t r i b u t i o n o f total lead ( O ) a n d inorganic fraction ( • ).
11
"a
co %O
ko
0
.j
9
A
0.6
t
I
I
I
t
I
I
I
I
I
I
I
I
I
I
J
I
I
I
I
I
I
7.5 7
A
B
C
D
E
F
G
H
~'
¢- 0.4
¢C:~ 0.2
.J
B
0.0
8
T-Station
Fig. 5. Geographical distribution of (A) log K'condand (B) lead-complexing ligand concentration.
[Pb' ], represents an average of 48% of the total. The free lead fraction was
calculated by the application of side-reaction coefficients, as previously described. A value for the inorganic side-reaction coefficient of log ~Pb' = 1.51
(for p H = 8 . 1 ) was used (Turner et al., 1981). Free lead concentrations obtained are shown in Table 3 and vary between 0.1 and 0.5 p M in the Subarctic
Gyre, and from 0.3 to 0.9 p M in the California Current. The free lead fraction
LEAD SPECIATION IN THE EASTERN NORTH PACIFIC
231
represents ~ 1.4% of the total lead concentration, about half the value expected if only inorganic ligands were considered (Turner et al., 1981 ).
No appreciable differences were noticed in lead speciation between filtered
samples (T7 and T8) and an unfiltered sample collected between those positions (T7.5) (see Table 3). This is consistent with results observed for copper
(Coale and Bruland, 1988). Thus, the bulk of the organic complexation exists
in the dissolved ( < 0.3 nM) phase. However, possible complexation due to
colloids cannot be ruled out.
For elements naturally present in biological systems as co-factors, e.g. copper and iron, ligands with high selectivity might be expected in natural systems. However, there is no apparent explanation for the high specificity observed for the lead complexing ligands. The high value of the inorganic sidereaction coefficient of Fe(III) in surface seawater (log C~Fe=12.0; Turner et
al., 1981 ) might be the reason for the low competition of this element for the
lead complexing ligands; however, this does not explain the low competition
from copper. More extensive studies will be needed to address the apparent
selectivity of this class of ligands for lead.
A prerequisite for accurate speciation studies is the ability to collect, process
and store samples without contamination. Even picomolar levels of inadvertent lead contamination could significantly alter the results obtained for lead
speciation. Our total dissolved lead concentrations are consistent with those
previously determined in the California Current waters (Bruland et al., 1985)
and in eastern North Pacific samples (Schaule and Patterson, 1981; Flegal et
al., 1986). We can therefore be confident that underestimations of the complexing ligands have not occurred as a result of trace metal contamination.
CONCLUSIONS
According to this study, the organically complexed lead fraction contributes
a significant portion of the total (50%) and must be considered in speciation
models for lead in surface seawater. Comparison between samples equilibrated
with lead for 10 rain and 24 h shows that the kinetics of the association of lead
with ligands is not as fast as observed for copper (Plavsic et al., 1982; Coale
and Bruland, 1988).
We are unable to define the exact nature and origin of this class of complexing ligands. However, the results emphasize the importance of complexation
with organic ligands for elements such as lead which are not involved in naturally occurring biological processes. Such complexation can affect the entry
of toxic metals in marine food webs and the marine sedimentation cycle.
ACKNOWLEDGEMENTS
We wish to extend our sincere thanks and appreciation to J. Martin, M.
Gordon and S. Fitzwater for the use of their sampling equipment and clean-
232
G. CAPODAGLIOET AL.
air sampling van used in this study; to R. Flegal for helpful comments during
the development of this manuscript; and to the crew and officers of the R/V
"Wecoma" for their assistance at sea. G.C. thanks the National Research
Council of Italy (CNR) for financial support for his stay at UCSC laboratories.
This work was supported by ONR contract N00014-83-K-06383-P0003.
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