Rapid determination of manganese in sea water by flow-injection analysis with chemiluminescence detection

469

Analytrca Chrmlca Acta, 249 (1991) 469-478 Elsevrer Sctence Pubhshers B.V., Amsterdam

Rapid determination of manganese in sea water by flow-injection analysis with chemiluminescence detection Thomas P. Chapin i, Kenneth S. Johnson * and Kenneth H. Coale Moss Landrng Marme Laboratorres, P. 0 Box 450, Moss Landmg, CA 95039, and Monterey Bay Aquanum 160 Central Avenue, Pacrfc Grove, CA 93950 (USA) (Recetved 6th November

1990; revrsed manuscnpt

Research Instrtute,

recetved 20th February 1991)

Abstract Flow-Injection analysis (FIA) wrth chemiluminescence. detectron was used to determine manganese m sea water. The oxldauon of 7,7,8,8-tetracyanoquinodimethane in an alkalme solutton produces hght. Manganese(I1) catalyzes thrs reactton and its concentration can be determined by measurmg the rate of photon ermsston. A column contammg I-hydroxyqumohne remobilized on a solid support was used m-hne m the FIA system to concentrate manganese The analysts trme was 6 mm per sea-water sample with a correspondmg detection hnut of 0 1 nM (30). Lower detection limits can be obtained by Increasing the amount of manganese loaded onto the column. The relattve standard devtatton of analyses of sea water contatmng l-10 nM manganese was typically 3%. Analyses of North Atlanttc Surface Sea Water (NASS-1) and Coastal Atlantic Surface Sea Water (CASS-1) standards gave manganese concentrations wtthm the accepted ranges, confirming the accuracy of the techmque. Slupboard determmatrons of manganese produced excellent agreement wtth previous rehable data. This technique is wtdely apphcable to a variety of natural water and process-onented studres. Keywork

Chemtlummescence;

Flow

system;

Manganese,

Only recently have reliable sampling and analytical techniques been developed that allow the determination of trace metals in sea water at low nanomolar to picomolar concentrations [l-5]. Most of these techniques have used graphite furnace atomic absorption spectrometry (GFAAS) after preconcentration of the sea-water sample. Analyses utilizing these methods require complex instrumentation, laborious sample preparation and clean laboratory facilities that are usually only available in dedicated shore-based laboratories. As a consequence, field sampling strategies are developed and carried out based on the distribution and concentrations of physical and chemical prop-

’ Present address: School of Oceanography, WB-10, sity of Washmgton, Seattle, WA 98195, USA.

0003-2670/91/%03

50

Umver-

0 1991 - Elsevter Science Publishers

Sea water,

Waters

erties other than (and sometimes unrelated to) trace metal concentrations. Simple, inexpensive methods of trace metal analysis must be developed if shipboard methods of analysis are to become widespread. Some progress has been made through the use of electrochemical [6], gas chromatographic [7] and chemiluminescence techniques [8]. Other methods must be developed to broaden the range of metals that are amenable to near real-time analysis on board ship. The goal of this work was to develop a rapid, sensitive, inexpensive and portable method for the determination of Mn(I1) in sea water. Yamada et al. [9] demonstrated that Mn(I1) can be determined with high sensitivity and selectivity by exploiting the Mn(I1) catalysis of the oxidation of 7,7,8,8-tetracyanoquinodimethane (TCNQ). This chemiluminescent reaction produces photons B V. All nghts reserved

470

at a rate proportional to the concentration of manganese in solution. The technique can be readily adapted to automated analysis using flow-injection analysis (FIA). FIA requires very little sample handling or manipulation and this eliminates many of the stringent “clean” practices often necessary for standard manganese determinations. FIA has proven to be a simple, inexpensive and robust method of analysis that is well suited for use on-board ship [8,10-121. In this paper, the results of work to adapt the manganese-catalyzed oxidation of TCNQ to the determination of manganese in sea water are reported. This method was applied to the shipboard determination of manganese in sea water at stations off the California coast and to the analysis of the Canadian reference seawater standards North Atlantic Surface Sea Water (NASS-1) and Coastal Atlantic Surface Sea Water (CASS-1).

EXPERIMENTAL

Reagents

TCNQ and didodecyldimethylammonium bromide (DDAB) were obtained from Kodak Chemicals. 2’,4’,5’,7’-tetrabromofluorescein (Eosin-Y), analytical-reagent grade sodium hydroxide, hydrochloric acid and nitric acid and Ultrex ammonia solution from J.T. Baker and doubly distilled 6 M hydrochloric acid was from G. Frederick Smith Chemical (GFS). Reagent solutions were made up in 18 Ma cm-’ water obtained from a Mill&Q water purification system (Millipore) (MQ water). All plasticware was cleaned by soaking for 24 h in 10% Micro detergent followed by 24 h in a 10% analytical-reagent grade hydrochloric acid bath. After the initial cleaning, the plasticware was stored in 10% hydrochloric acid and rinsed with MQ water prior to use. Reagents were not further purified as any manganese contamination in the reagents would be accounted for in the background chemiluminescence and would not affect the sample signal. No significant difference in sensitivity or background emission was observed when analytical-reagent grade hydrochloric acid was replaced with high-purity GFS 6 M acid, so the less expen-

T P CHAPIN

ET AL

sive product was used. Replacing the sodium hydroxide with Fisher Ultrex ammonia solution actually resulted in a decrease in sensitivity, perhaps owing to an interference effect from ammonium ion. Reagent R,. A 5.0~ml volume of 0.06 M Eosin-Y solution was added to 900 ml of MQ water, 0.05 g of TCNQ and 1.85 g of DDAB were added and the volume was adjusted to 1000 ml with MQ water. This solution was shaken occasionally and sonicated in an ultrasonic bath for at least 30 min. The son&cation helped to solubilize the TCNQ. The final concentrations were 2.5 x 10h4 M TCNQ, 4x 10m3 M DDAB and 3 x 10e4 M Eosin-Y. The reagent solution was usually stable for 24-48 h. Acid and alkali solutions. For 1 M HCl-0.1 M HNO,, 86 ml of HCl (12.1 M, analytical-reagent grade) and 6.5 ml of HNO, (15.8 M, analytical-reagent grade) were diluted to 1000 ml with MQ water. For 0.0025 M HCL, 10 ml of 0.25 M HCL were diluted to 1000 ml with MQ water; and for 0.01 M NaOH, 10 ml of 1.0 M NaOH were diluted to 1000 ml with MQ water. Standards

Manganese stock solutions of 50 and 1 PM were prepared by diluting a 1000 mg 1-l Mn standard (Fisher) with MQ water and acidifying with 25 ~1 of 6 M GFS HCl per 100 ml of standard. Stock solutions were prepared monthly. Working standards were prepared daily by dilution of these stock solutions. Apparatus

FIA was used to mix sample and reagents. The design of the reaction manifold was similar to that utilized by Sakamoto-Arnold and Johnson for the determination of cobalt in sea water [8] (Fig. 1). The sample and reagents were pumped with a Gilson Minipuls 2 peristaltic pump. The flow-rates of the 0.01 M NaOH reagent and the luminescing reagent R, were 1.3 ml min-’ and those of the 0.0025 M HCl and the sample solution were 2.7 ml min-‘. Tyson tubing was used in the pump. Manifold lines consisted of 0.8 mm i.d. Teflon tubing, except for a 0.5 mm i.d. Teflon line from the injection valve to the reaction flow cell. This

FIA OF MANGANESE

IN SEA WATER

471

MULTI-PORT VALVE Vl

SAMPLE LOAD

4.0 mm

MILLI-Q WATER

0.75 mln

1 M HWO.1 M Hi\103

0.5 min

PUMP 2.7

0.0025 M HCI --

2.7 1.3

REAGENT Rl

1.3

0.01 M NaOH

ml mln-l PMT

IOCM 8-HQ

--I

\

3OmmldTFE

1 COMPUTER

8-HYDROXYQUINOLINE

COLUMN CHART &CORDER

Fig. 1 Schematic diagram of the FIA-CL system used for the determmation of manganese m the sample load posltion. The dimensions of the 8-HQ column are shown m the inset.

0.5 mm i.d. line decreased the time required to inject the sample into the luminescing reagents and resulted in higher sensitivity. The injection loop contained a chelator column of immobilized %hydroxyquinoline (8-HQ) which concentrated the manganese from sea water (Fig. 1 and inset). The 8-HQ was immobilized on Fractogel (EM Science) by the technique of Landing et al. (13). The detector flow cell consisted of a 25-cm length of 1.27 mm i.d. clear Tygon tubing coiled into a grey poly(viny1 chloride) (PVC) holder. The PVC holder was bolted to the shutter of a Pacific Instruments Model 3547 photomultiplier tube (PMT) housing. A Hamamatsu R268 PMT was cooled inside the housing to - 15” C with a Pacific Instruments Model 33 Cooler. Cooling of the PMT reduced the detector baseline from about 1000 to 300 counts s-l when monitoring the reagent stream in the flow cell. There was no wavelength discrimination in the system. The photomultiplier output signal was amplified by a Pacific Instruments Model 126 photometer and the analog output was converted to a digital signal using a Metrabyte Dascon-1, 1Zbit analog to digital converter. The analog signal was digitized

m sea water. The uqection valve IS shown

every 0.3 s and stored on disk in a Compaq microcomputer. A strip-chart recorder was also used to provide a backup record. The multi-port stream selection valve (Vi), rotary injection valve (V,) and an Isco ISIS autosampler were all controlled by the microcomputer using the digital output ports of the Dascon-1 board. Tlmrng parameters

The flow manifold for the FIA-CL determination of manganese is shown in Fig. 1. The analysis began with the injection valve, V,, in the load position. Sea water at natural pH (ca. 8) was pumped through the 8-HQ column for 4 min at 2.7 ml mm-‘. Longer sea-water load times decreased the detection limit, but after 8 min of loading the 8-HQ adsorption sites were saturated. After the sea water manganese was loaded onto the 8-HQ column, the stream selection valve, Vi, was then rotated and MQ water was passed through the 8-HQ column for 45 s at 2.7 ml mm’. This step rinsed the 8-HQ column of Ca*+ . and Mg*+ ions, which can potentially interfere. The injection valve, V,, was then switched to the elute position for 45 s and the digital output

T P CHAPIN

472

during this elution-detection period was recorded on disk. In the elute position, 0.0025 M HCl flowed in the reverse direction through the 8-HQ column and eluted the manganese. The chemiluminescence was produced almost instantaneously so the manganese eluted from the column was mixed with the TCNQ-NaOH solution in the flow cell. During the 45-s detection-elution period, the stream selection valve, V,, was switched to the acid wash position. This step filled the line from valve V, to V, with the acid solution (1 M HCl-0.1 M HNO,), which washed any remaining metals off the 8-HQ column when valve V, was switched back to the load position to begin the next analysis. The total analysis time was 6 min per sample. Sample handling Sea-water samples were collected with 10-l General Oceanics Niskin bottles mounted on a General Oceanics rosette. Comparisons of seawater samples collected by the Niskin bottles and 30-l PTFE-lined Go-F10 bottles (General Oceanics) hung on Kevlar wire did not reveal any differences in manganese concentrations greater than 0.15 nM at depths below 1000 m. Unfiltered samples were drawn into acid-cleaned 125 ml polyethylene bottles and placed in a small Class 100 laminar flow hood (EACI) or double bagged in polyethylene ziplock bags. Samples were then either transferred to 30-ml FTFE vials for use with the autosampler or analyzed manually by placing the FTFE intake tubing directly into the 125~ml sample bottle. Samples that were analyzed within 24 h were run at natural pH (ca. 8). No loss of manganese due to wall adsorption was observed for at least 24 h. Samples with a high particulate concentration (frequently encountered near shore or in surface waters) should be filtered to prevent clogging of the 8-HQ column. The analysis time for a twelve-bottle hydrocast with five standards and a blank all measured in triplicate (60 determinations total) was ca. 6 h. Samples that had been stored longer than 24 h were acidified with 6 M GFS HCl (ca. 125 ~1 of HCl per 125 ml of sample) to bring the sample pH to ca. 2. These acidified samples were adjusted to pH 8 with Ultrex ammonia solution just before analysis. The addition of acid and base to the

ET AL

stored samples introduced a blank of 0.2 nM. The elution characteristics of the manganese peak were also changed, which can affect peak-area integration if the injection spike coelutes with the manganese signal (see below). Therefore, as often as possible, the samples were analyzed within 24 h at natural pH (ca. 8) to avoid the inconvenience and manganese blank introduced by acidification and sample storage.

RESULTS

AND

DISCUSSION

Initially attempts were made to determine manganese in sea water using the single-valve manifold described by Yamada et al. [9]. However, this system lacked sufficient sensitivity for the direct determination of manganese at concentrations less than 40 nM. Efforts were therefore focused on a system that incorporated a column of an immobilized ion exchanger, 8-HQ, to concentrate the manganese. This resulted in a loo-fold increase in peak height. The following results were derived using the two-valve system incorporating the 8-HQ ion-exchange column. Optimzatlon of reactlon conditions The concentrations of TCNQ, Eosin-Y, DDAB, sodium hydroxide and hydrochloric acid were all optimized with the FIA-CL system with the in-line 8-HQ column for standards in MQ water and sea water. The concentration of one reagent was varied while the other reagent concentrations remained constant. In general, the concentrations of the reagents were kept to a minimum in order to reduce the background chemiluminescence and reagent blanks while still maintaining adequate sensitivity and stability. A system consisting of two reagent solutions, one containing sodium hydroxide and the other the organic reagents, was selected for this work, as suggested by Yamada et al. [9]. A mixed reagent (R,) containing 0.5 X lop4 M TCNQ, 4 x 1O-3 M DDAB and 3 X lop4 M Eosin-Y appeared to give optimum results (Figs. 2-4). However, it was found that this solution was stable for only a few hours before sensitivity dropped and the baseline began to drift. Increasing the TCNQ concentration to

FIA OF MANGANESE

473

IN SEA WATER

:I 0

2

4

a

8

,o

0 12

TCNQ CONCENTRATION

14

(10’

16

16

BASEUNE

,

20

0

01

M)

Rg 2. Effect of TCNQ concentranon on the signal and background of a 5 nM manganese sea-water sample.

2.5 x low4 M produced a reagent solution that was stable for 24-48 h. This TCNQ concentration is ca. ten times greater than that suggested by Yamada et al. [9]. The chemiluminescence efficiency was dependent on the reaction pH. Control of the final reaction pH was achieved by adjusting the sodium hydroxide concentration in the second reagent and the hydrochloric acid concentration in the carrier stream that was used to elute the 8-HQ column. A pH greater than 11 was required in the flow cell to produce chemiluminescence (Fig. 5). Changes in the pH of the solution flowing through the detector cell could also produce shifts in the background light detected by the PMT. Variations in pH can result from the large difference in proton concentration that occurs when the valve V, switches to the elute position and the MQ rinse

1

04

03

CONCENTRATION

05

06

(mM)

Frg. 4. Effect of Eosm-Y concentratton on the srgnal and bacground of a 5 nM manganese sea-water sample

solution is injected into the HCl stream. This pH change creates an injection spike that can be difficult to resolve from the signal due to manganese (Fig. 6). The HCl and NaOH concentrations in the reagent solutions were selected to minimize the injection spike, maintain sufficient acidity in the HCl solution to elute the manganese from the column and sufficient alkalinity in the NaOH solution to optimize the chemiluminescence signal. This required the weakest possible acid concentration to elute the column and just enough NaOH to reach a pH greater than 11 within the flow cell. These conditions produced two different effects: the pH gradient between the acidic eluent solution and the MQ solution was minimized, reducing the injection spike, and a weak HCl solution caused

20 15,

02

EOSIN-Y

6’

0

110 c

113

1

114

EFFLUENT 117

2

pH 123

3

4

5

NaOH HCI RATIO DDAS CONCENTRATION

(mM)

Ftg. 3. Effect of DDAB concentratton on the srgnal and background of a 10 nM manganese sea-water sample.

Fig. 5. Effect of NaOH to HCl ratto and effluent pH on the signal, background and mjechon spike of a 5 nM manganese sea-water sample.

414

T P CHAPIN

ET AL

sufficient manganese onto the column. The 3 mm i.d. column minimized back-pressure, permitting the peristaltic pump to move the sample solution through the column at flow-rates as high as 4 ml mm-‘. Interferences

160’

0

I 30 20 10 Time After Injection (seconds)

I 40

Fig. 6. Resolution of the lqectlon spike from a 0.88 nM sea-water sample as recorded by the d@al data.

the manganese to elute slowly from the column, which separated the manganese signal from the injection spike (Fig. 6). The best results were obtained with an NaOH to HCl molar concentration ratio in the reaction mixture between 1.5 : 1 and 2 : 1 (Fig. 5). For the given TCNQ, DDAB and Eosin-Y concentrations and flow-rates, 0.01 M NaOH and 0.0025 M HCl were chosen. The flow-rate of the HCl elution stream was twice that of the NaOH reagent stream (2.7 vs. 1.3 ml mm-‘), so these reagent solutions still maintained the optimum 2 : 1 NaOH to HCL molar ratio in the flow cell. &Hydroxyqumoline columns The 8-HQ resin was immobilized

onto Fractogel utilizing the method of Landing et al. [13]. An 8-HQ slurry was pipetted into a column that was l-l.5 cm long and 3 mm i.d. (Fig. 1). The 8-HQ was retained in the column by plugs of glass-wool at each end. These columns were jam-fitted over the 0.8 mm i.d. injection loop tubing and this created a leakproof seal. The columns proved to be extremely stable and could be used for several months. Manganese was retained on the column at pH values greater than 7 (Fig. 7). Sea water could therefore be loaded directly from the sample bottle at natural pH (ca. 8). The apparent low retention of manganese at high pH probably results from the rapid oxidation of Mn(I1) to Mn(IV) in alkaline solutions. Relatively high flow-rates were used to minimize the time needed to load

The determination of Mn(I1) by the chemiluminescent oxidation of TCNQ was fairly selective. Yamada et al. [9] reported that only Fe(II), Fe(II1) and Mg2+ interfered at 0.1 mM concentrations but did not interfere below 0.01 mM. Unfortunately, sea water contains 53 mM of Mg2+, and this would interfere significantly unless the Mg2+ was removed. The interference effects of magnesium in sea water were determined by analyzing a 53 mM Mg2+ solution in MQ water. The Mg2+ standard produced a signal equivalent to ca. 1 nM manganese. This signal disappeared after rinsing the 8-HQ column with MQ water for 45 s, which strips Mg 2+ from the column [8]. In addition to Mg 2+, the following m etals were determined to assess their interference on manganese analyses: Co(II), Ni(II), Cu(II), Zn(II), Cr (VI), Pb(II), Cd(II), Ag(I), Mo(VI), Al(III), Fe(I1) and Fe(II1). Metal concentrations were tested at levels that were five times greater than the maximum concentrations found in open-ocean sea water [14]. There were no significant interferences except from Mo(VI), Cu(I1) and Fe(I1) (Table 1). When Mo(V1) was determined at natural sea-water levels, the interference was negligible. Molybdate

6

,

I

5-

c g4 =

3-

$

2_

2

1

Fig. 7. Detector

3

5 7 Sample pH

response

9

11

as a function of the pH of the

FIA OF MANGANESE

415

IN SEA WATER

TABLE 1 Effect of trace metal interference relative to a 5 nM manganese Signal a Interferent

Signal relative to 5 nM Mn (W

Interferent

Signal relative to 5 nM Mn (W

25 nM 500 nM 100 nM 1 nM 45 nM 5 nM 500 pM 5 nM 200 nM

1 15 1 0 3 0 0 0 0

1 nM 60 nM 15 nM 3 nM 15 nM 30 nM 6 nM 3 nM

1 1 50 5 1 30 5 5

Cr(IV) b MO (VI) b MO (VI) Pb(I1) b Zn(I1) b Zn(I1) Ag(I1) b Cd(I1) b Al(II1) b

Co(I1) b N1(11)b Fe(I1) b Fe(I1) Fe(II1) b Cu(I1) b Cu(I1) Cu(I1)

a 100% signal IS eqnvalent to that produced by a 5 nM manganese standard m sea water. b Concentration five times greater than maxmmm oceamc values [14].

should not be a significant interferent as it shows little variability in sea water [14]. Copper(I1) gave a slight emission at 6 nM but at a concentration of 3 nM it did not produce detectable chemiluminescence. If high levels of Cu(II), relative to manganese, are likely to be encountered, the copper can be complexed with

tetraethylenepentamine (TEPA). This ligand has an extremely high binding constant for copper, ca. seventeen orders of magnitude greater than 8-HQ [15]. TEPA may be added directly to the sample or to the MQ rinse to suppress the copper interference (Table 2). Iron(I1) interfered at a 3 nM concentration. Fortunately, the kinetics of the oxidation of Fe(I1) to Fe(II1) are rapid at pH 8 and interfering amounts of Fe(I1) should not accumulate under most sea-water conditions. High Fe(I1) concentrations, such as those found in hydrothermal fluids or anoxic waters, might interfere with manganese determinations. Iron interference could be reduced by the addition of a siderophore, such as deferoxamine mesylate. This reagent has been used to reduce iron interference in the spectrophotometric determination of manganese in hydrothermal fluids (16). The reagent could be added to the sample or MQ wash in a manner similar to that for TEPA for Cu(I1) interference. Calibration The system response to manganese was determined by making standard additions of Mn(I1) to deep sea-water samples that contain low levels of manganese. The resulting calibration graphs

TABLE 2 Effect of addttron of TEPA to sample and to MQ column wash a Species

5 nM 5 nM 30 nM 30 nM 5 nM 5 nM 5 nM 5 nM 5 nM 5 nM 5 nM 5 nM 5 nM 5 nM

Mn(II) Mn(II) Cu(I1) Cu(I1) Mn(I1) Mn(I1) Mn(I1) Mn(I1) Mn(I1) Mn(I1) Mn(I1) Mn(I1) Mn(I1) Mn(I1)

+ 30 nM Cu(I1) + 30 nM Cu(I1) + 30 nM Cu(I1)

+ + + +

30 nM 30 nM 30 nM 30 nM

TEPA added to sample

TEPA added to MQ wash

Relative

w

(Pll-‘)

c@

0 100 200 0 100 200 300

100 99 40 1 120 106 101 100 100 98 120 112 102 96

1-l)

0 100 0 100 0 100 200

Cu(I1) Cu(I1) Cu(I1) Cu(I1)

a 100% signal ts eqnvalent

to 5 nM Mn. Concentration

of TEPA solutton = 0.4 mM.

Signal

476

T P CHAPIN

were used to determine the concentrations of manganese in other samples. Calibration graphs were calculated with both peak height and peak area. Peak-height calibrations were linear from ca. 1 to 100 nM, but were not linear below 1 nM. The peak-area calibration was linear from 0.1 to 100 nM. Peak area should be used for calibration, especially for samples with low manganese concentrations. Care should be taken to ensure that the injection spikes are not included in the integration. The detector response to manganese standard additions was reproducible over the course of many days of analysis (Fig. 8). The signal increase was ca. 325 counts s-l per 1 nM of manganese. This slope depends on many parameters, such as the flow cell geometry, PMT efficiency and flowrates, and therefore it will vary from system to system. The precision of replicate analyses was also good, with a typical relative standard deviation of 3% for concentrations in the range l-10 nM. Fig. 9 shows a chart recorder output for replicate analyses of a series of manganese additions to sea water. The detection limit (estimated as three times the standard deviation of the blank) was 0.1 nM with a 4-min load time. There was a significant difference between the elution times of manganese standards made up in MQ water and those made up in sea water. MQ standards eluted much faster than the sea-water standards. The small injection spike, which was equivalent to about 0.2-0.4 nM of manganese,

ET AL

i40nM I +30nh4 +POnM

h

+lOnM

M

llLJJ +orM

Fig. 9. Chart recorder output for standard additions of manganese to a 1.09 nM sea-water sample analyzed by FIA-CL at sea.

was difficult to resolve from the manganese signal for standards prepared in MQ water. This made blank determinations with MQ more complicated than those with sea water as the small blank signal is combined with the injection spike. System blanks were therefore determined using manganese-free sea water. K. Bruland and L. Miller (University of California, Santa Cruz) kindly provided us with a sample of trace metalclean sea water prepared in their laboratory [17]. The blank determined with this manganese-free sea water was equivalent to 0.1 nM manganese for a 4-min sample load. This value equalled the detection limit for the 4-min load and indicates that the technique and sample handling procedures were sufficient to avoid contamination. Owing to the limited supply of trace metal-clean sea water, the system blank was only determined for a 4-min load time. MQ blanks were run in every series of analyses and were used only to detect analytical problems. All manganese concentrations in sea water were calculated by assuming that the blank was negligible. This assumption appears valid, as shown below, if care is taken to ensure that the manganese peak and injection spike are not coincident. Accuracy

0 0

1

STANDARD

2

ADDITIONS

3

OF MANGANESE

4

5

(nM)

Rg. 8. Peak height vs. manganese added m standard addltlons curves used dunng typlcal manganese determmatlons performed at sea.

Boyle et al. [3] stated that results for determinations of trace metals in seawater should not be accepted unless they meet two criteria: interlaboratory agreement and oceanographic con-

FIA OF MANGANESE

477

IN SEA WATER

sistency. The Canadian sea-water trace metal standards NASS-1 and CASS-1 were analyzed in the laboratory to verify that results obtained with the FIA-CL method could meet the first of these criteria. These standards have been rigorously analyzed using a number of analytical techniques for many of the trace metals including manganese [18]. The acidified CASS-1 and NASS-1 standards were adjusted to pH 8 with ammonia solution prior to analysis. The results are presented in Table 3. The excellent agreement with the NASS-1 and CASS-1 trace metal standards verifies the accuracy of the FIA-CL technique at both low and high manganese concentrations. It is incumbent upon environmental chemists to demonstrate that new analytical techniques can satisfy the second criterion stated by Boyle et al. [3] as well. Oceanographic consistency of a data set is demonstrated when detailed profiles “show smooth variations related to the hydrographic and chemical features displayed by conventionally measured properties” [3]. A range of natural seawater samples were analyzed to verify that the FIA-CL method produced results that are oceanographically consistent. Sea-water samples were collected from the surface to a depth of 2000 m at a station off the California coastline during June 1989 and analyzed for manganese at sea aboard the R/V Point Sur. The manganese concentrations obtained for these samples (Table 4) are shown in Fig. 10 together with the manganese profile determined by Landing and Bruland [2] at a nearby station. The latter values were measured in a shore-based laboratory by GFAAS after concentrating the samples with Chelex-100 resin. Both data sets show manganese maxima in the surface layer, a sharp decrease through the thermocline and a mid-depth maximum that is associated with

TABLE FIA-CL

3 analysts

of CASS-1

and NASS-1

standard

WI Value

Mn concentratton

(nM)

CASS-1

NASS-1

Certtficate FIA-CL

41.3*3 1 42 9+2.8 (n = 4)

0.40*0 13 048fO.O7(n=3)

sea waters

0

04

MANGANESE (nM) 2 3

1

4

5

.o .

-

m

.

06

. 08

.o

-

.

DEPTH (km)

lo _

’

IL?-

-

14 18

18 -

20

0.

o LANDING 8 ERIJLAND (1987

-

.

. FIA-CL STATION 5

*=

Fig. 10. Vertical profile of manganese analyzed by FIA-CL at Statton 5 (36O25’N, 123”OS’W) m June 1989 compared wrth data reported by Landing and Bruland [2] for a station (36O30’N, 124OO’W) occupted m September 1980

the oxygen minimum layer (Table 4). This profile is consistent with our understanding of the geochemical behavior of manganese [1,2,5]. The excellent agreement of the FIA-CL manganese values with the reliable GFAAS data of Landing and Bruland [2], together with the correlations of manganese with the supportive physical and chemical data in Table 4, confirm the oceanographic consistency of the concentrations determined with this analytical technique. Conclusions FIA coupled with CL detection is utilized to permit rapid, inexpensive, portable, accurate and precise determinations of manganese in sea water at natural concentrations with very little sample handling or processing. This technique has many applications in process-oriented studies. It has been shown to operate well on-board ship, where it can be used to provide near-real-time feedback to experimental design. The results obtained for samples analyzed with the FIA-CL method are in excellent agreement with previous measurements of manganese in sea water.

478 TABLE

T P CHAPIN

4

Hydrograpmc and manganese (36O25’N, 123”08’W) a Depth b (m)

Potential temperature

sahmty (9~) ’

data

for

0, d

PS89

Station

Oxygen

Mn

(PM)

(nM)

5

(“C) 1 5 10 20 30 40 50 60 80 100 150 200 300 400 500 600 700 800 900 1000 1200 1500 1750 2000

14.13 14 14 14.12 13.57 12.43 11.32 10.74 9.90 9.40 9.04 8.24 7.79 6.71 6.34 5.76 5.03 4.77 4.45 4.09 3.80 3.28 2.65 2.26 192

33.28 33.28 33.28 33 32 33.34 33.37 33.39 33.54 33.67 33.74 33.97 34.06 34.08 34.20 34.25 34.28 34.35 34.38 34.42 34.45 34.49 34.54 34.57 34.60

24 83 24.83 24.83 24.98 25.22 25.45 25.57 25.83 26.01 26.13 26.43 26.56 26.74 26.87 26.99 27.10 27.19 27.25 27.32 27 37 27.45 27.55 27.61 27.66

274 275 275 275 266 245 229 193 156 125 123 95 57 32 18 11 13 16 19 23 36 51 63 82

3.17 3.15 3.07 3.25 2.66 2.34 1.92 1.14 0 87 0.77 0.64 0.71 0.76 1.21 1.18 1.06 121 1.04 1.09 0.95 0 75 0 91 0.61 0.64

a Samples collected and analyzed at sea by FIA-CL on 4th June, 1989. b Bottom depth = 3200 m. ’ Sahmty IS reported m Practical Salmity Umts. d e, is (density - 1) x 1000.

This research was supported by ONR grant NO001489-J-1070 and NSF grant OCE8609437. The authors are indebted to the crew and officers of the R/V Point Sur for their assistance with the shipboard operations. They also thank Virgina Elrod and Carol Chin for their analytical and

ET AL

intellectual support. K. Bruland and L. Miller kindly provided trace metal-free sea water.

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