Shipboard determination of hydrogen peroxide in the western Mediterranean sea using flow injection...

Shipboard determination of hydrogen peroxidein the western Mediterranean sea using ¯ow injection

with chemiluminescence detection

David Price1,a,b, R. Fauzi C. Mantourab, Paul J. Worsfolda,*

a Department of Environmental Sciences, Plymouth Environmental Research Centre, University of Plymouth, Plymouth PL4 8AA, UKb Plymouth Marine Laboratory, Centre for Coastal and Marine Sciences, Prospect Place, West Hoe, Plymouth PL1 3DH, UK

Received 10 March 1998; received in revised form 7 May 1998; accepted 11 May 1998

Abstract

Field studies at a variety of sampling locations in the western Mediterranean have endorsed the use of a novel ¯ow injection-

chemiluminescence method (FI-CL) for the determination of hydrogen peroxide in seawater. Analysis time for a depth pro®le

of 12 samples including standard additions was 45 min, repeatability (relative standard deviation; RSD; n�5) was typically

<5% and reproducibilities at two different sampling stations were 2.5% RSD (mean concentration�83.6 nM H2O2; n�3) and

1.8% RSD (mean concentration�42.5 nM H2O2; n�4). Depth pro®le data, characterised by H2O2 concentrated at the surface

(16.0±154 nM H2O2) and decreasing rapidly below the thermocline, were in agreement with previous studies in similar

oceanographic environments. Samples collected during Lagrangian drift mode indicated that diurnal maxima can occur in the

mid-late afternoon. Batches of sampled and treated seawater were incubated under ambient light conditions whilst onboard

ship to investigate the photogeneration of H2O2. Surface waters had a higher H2O2 photogenerative ef®ciency than deeper

waters and samples collected close to the River RhoÃne plume were particularly effective at generating H2O2, suggesting that

the chromophoric material responsible for the photogeneration of H2O2 had a terrestrial source. Additional incubation

experiments in the laboratory showed that humic material was more ef®cient at photogenerating H2O2 than fulvic material or

hydrophilic macromolecular acids. # 1998 Elsevier Science B.V. All rights reserved.

Keywords: Shipboard ¯ow injection; Hydrogen peroxide; Seawater; Western Mediterranean Sea

1. Introduction

Hydrogen peroxide is thought to play a key role in

redox reactions in marine environments [1,2], parti-

cularly in surface waters (top 10 m) where concentra-

tions in the range 100±1000 nM have been reported

[3]. Hydrogen peroxide is therefore likely to govern

the redox speciation of several trace metals and parti-

cipate in oxidation reactions involving labile and

residual organic compounds. It is predominantly

formed by photochemical generation via free radical

intermediates and dissolved organic chromophores

[4,5] and is found in all natural aquatic environments

Analytica Chimica Acta 371 (1998) 205±215

*Corresponding author. Tel.: +44-1752-233006; fax: +44-1752-

233009; e-mail: [email protected] address: Department of Chemistry and Biochemistry,

Old Dominion University, Norfolk, VA 23529-0126, USA.

0003-2670/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved.

P I I S 0 0 0 3 - 2 6 7 0 ( 9 8 ) 0 0 3 2 2 - 5

[6±10]. Wet and dry deposition of photogenerated

H2O2 in the atmosphere can be a signi®cant transient

source to the oceans [7,11] and biological production

by certain phytoplankton provides a minor input

[12,13]. The role of biota in enzyme-mediated decom-

position of H2O2 [14] however is a more signi®cant

factor than photochemical decomposition [15] in the

breakdown of H2O2 in seawater.

The transient nature of H2O2 in marine waters

necessitates a rapid, ®eld-based analytical method

for its determination. A previous report [16] showed

that ¯ow injection with chemiluminescence (FI-CL)

detection was a robust and reliable technique for the

shipboard determination of H2O2, with a detection

limit (S/N�3) in seawater of 5 nM, a linear range of 5±

500 nM and relative standard deviations (RDSs)

(n�5) typically in the range 1±2%.

This paper presents further validation of the FI-CL

technique for shipboard deployment in marine envir-

onments and discusses ®eld data (particularly H2O2

depth pro®les) from a four-week cruise in the Med-

iterranean Sea. The photogeneration of H2O2 in dif-

ferent marine waters is also investigated using

shipboard and laboratory based incubation.

2. Experimental

2.1. Sampling

Seawater samples from various depths and loca-

tions in the western Mediterranean were collected

using 10 l PTFE-lined Go-Flo bottles (General Ocea-

nics, Fort Lauderdale, FL) and immediately trans-

ferred to clean amber 50 ml glass bottles (soaked in

2 M HNO3; well rinsed with Milli-Q water). Each

sample bottle was rinsed three times with sample

before collection of the true sample and analysis

was performed within 1 h of sample collection.

The relevant Stations for Cruise D203 onboard

R.R.S. Discovery are shown in Fig. 1(a) (leg 1) and

(b) (leg 2). The cruise embarked at Gibraltar on 2nd

July 1993 and terminated on 2nd August 1993 at Nice

in southern France. The track for leg 1 was west

through the Straits of Gibralter to Station D3 then

east via Stations D5-9 to the Straits of Sicily (arriving

at Station D10 on 9th July) and north-west to Station

MA1, arriving in Monaco on 15th July. The track for

leg 2 was from Monaco (on 17th July) back to Station

MA1, where R.R.S. Discovery remained in Lagran-

gian drift mode for almost three days (Casts MA1/1 to

MA1/26), then to Roustan Bouy (Station MA9), arriv-

ing on 21st July. The RhoÃne plume area was studied in

detail (Stations MA2-8, MF1-3 and ME1-2) before

departing for a second Lagrangian drift Station

at MD1 from 30th July to 1st August and then to

Nice.

2.2. Reagents and standards

All solutions were prepared from ultra-pure deio-

nised water supplied by Milli-Q systems (Millipore,

Milford, MA) in the laboratory and at sea. Luminol,

cobalt(II) nitrate hexahydrate and 30% H2O2 were

analytical grade and supplied by Fluka (Gillingham,

Dorset, UK). All other chemicals, unless otherwise

stated, were reagent grade and supplied by Merck

(Poole, Dorset, UK).

Stock solutions of luminol (1�10ÿ3 M) and cobalt

(II) nitrate (1�10ÿ2 M) were prepared in 0.1 M

sodium carbonate, adjusted to the required pH with

2 M HCl and stored in high density polyethylene

(HDPE) containers. Working solutions were prepared

by serial dilution and stored at 48C to minimise

decomposition. The luminol solution was left for

24 h before use to ensure maximum CL activity.

Three size fractionated seawater samples for ship-

board incubation experiments were kindly donated by

G. Fengler from the University of Hamburg during

Cruise D203. The samples were obtained by collecting

105 l of surface seawater (1.6 m depth) from Station

MA9/10 (4.98E, 43.38N) at 2158 on 27th July 1993.

The seawater was suction ®ltered to remove particu-

late matter (>0.2 mm) and 94.0 l of the ®ltrate was

passed through two modi®ed tangential ®ltration sys-

tems (DT-Modul, Rochem) ®tted with 8000 and

300 Da membrane sizes. Both membranes were com-

posed of modi®ed polyethersulfone and the water

forced through the ®ltration system by a gear pump

(Tuthill). Operating pressures for the 8000 and 300 Da

membranes were 5 and 10 bar respectively. The mem-

brane-retained material was quantitatively transferred

by rinsing with water. Four fractions were obtained

(®ltered seawater (11 l), >8000 Da (redissolved in

1.98 l water), 300±8000 Da (redissolved in 3.92 l

water) and <300 Da (94 l)).

206 D. Price et al. / Analytica Chimica Acta 371 (1998) 205±215

Natural water organic material for incubations in

the laboratory was obtained from three samples col-

lected from the River Dodder, Eire. Filtration, adsorp-

tion chromatography (using Amberlite XAD-4 and

Amberlite XAD-8) and ion exchange chromatography

were used to fractionate and purify H-saturated forms

of humic, fulvic and hydrophilic macromolecular

acids [17]. Each fraction was freeze-dried to allow

quantitative preparation of solutions of known con-

centration. A commercial ionic humic acid salt

Fig. 1. Sampling stations for Cruise D203 onboard R.R.S. Discovery. (a) leg 1 (b) leg 2.

D. Price et al. / Analytica Chimica Acta 371 (1998) 205±215 207

(Aldrich, Gillingham, Dorset, UK) was obtained and

used as supplied.

2.3. Instrumentation and procedures

Hydrogen peroxide measurements were made using

an automated FI-CL system described in detail else-

where [16]. The optimised FI manifold is shown in

Fig. 2. The cobalt(II) and luminol streams were

pumped at 1.7 ml minÿ1, merged at a perspex T-piece

and passed into a coiled glass ¯ow cell situated

immediately in front of the photomultiplier tube

(PMT) end window. Seawater samples and standards

(100 ml) were injected into the luminol stream prior to

merging. The chemiluminescence emission

(�max�440 nm) resulting from the oxidation of lumi-

nol was directly proportional to the concentration of

H2O2 and was output to a chart recorder. A standard

additions method (three additions) was used to quan-

tify H2O2 and the order of analysis was kept random to

avoid systematic errors.

2.4. Laboratory and deck based incubations

Acid-cleaned sealed quartz glass tubes and sintered

caps (volume�45±50 ml) were used for all incubation

experiments. During the laboratory studies the tubes

were secured in a thermostated (3±58C) sample tray

supplied with an accelerated exposure unit designed to

simulate global radiation (Suntest CPS, W.C. Heraeus

GmbH, Hanau, Germany). At sea the tubes were held

in a purpose-made rigid PVC tube rack (built in house)

and situated in an unshaded area of the boat deck on

R.R.S. Discovery.

3. Results and discussion

3.1. Analytical performance at sea

Forty-three depth pro®les were obtained during the

course of the cruise from a variety of different ocean-

ographic environments within the western Mediter-

ranean basin. The practical limit of detection (signal-

to-noise�3) while at sea was 10.6 nM H2O2 and the

reproducibility (relative standard deviation; RSD) of

two different surface samples was 2.5% at Station D10

at 11.18E, 37.48N (mean concentration�83.6 nM

H2O2; n�3) and 1.8% at Station MA7 at 4.98E,

43.18N (mean concentration�42.5 nM H2O2; n�4).

Analysis of a depth pro®le of 12 samples and stan-

dard additions required 45 min and within-run repeat-

ability was <5% RSD (each sample or standard addi-

tion was injected 4±6 times). No serious technical

problems were encountered during the course of the

cruise.

3.2. Depth profile data

The weather during the cruise was generally very

settled and warm with minimal rainfall. There was

very little cloud cover but Mistral induced storms were

encountered off the south coast of France. The high

level of incident radiation produced detectable con-

centrations of H2O2 at all of the stations studied. All 43

depth pro®les obtained exhibited characteristic sur-

face maxima (ranging from 16.0±154 nM) followed

by a decrease in concentration with depth until the

practical limit of detection was reached (usually at 40±

60 m). Hydrogen peroxide was not observed in any of

the depth pro®les below 100 m (i.e. <10.6 nM). The

depth pro®le results are generally in agreement with

those reported previously for different water masses,

as shown in Table 1. Typical depth pro®les for meso±

oligotrophic waters at Stations MA1 and MD1 are

shown in Fig. 3 and the corresponding temperature

depth pro®les are shown in Fig. 4. The H2O2 surface

maxima were related to the higher photochemical

activity in surface waters and the decrease in concen-

tration with depth to three additive processes; dilution

Fig. 2. Optimised flow injection manifold with chemilumines-

cence detection for the shipboard determination of hydrogen

peroxide.


of H2O2 enriched surface seawater with lower con-

centration water from below, consumption of H2O2 on

reaction with natural and anthropogenic chemical

species and the increased attenuation of incident

radiation decreasing the rate of H2O2 photo-genera-

tion. These processes resulted in the establishment of

concentration gradients between the photo-active sur-

face waters and deeper waters controlled by the extent

of physical mixing, chemical reactivity and incident

radiation. The close relationship between H2O2 and

temperature depth pro®les at Stations MA1 and MD1

(Figs. 3 and 4), particularly the sharp decrease in

concentration at the thermocline, clearly demonstrates

that H2O2 is affected by the extent of wind induced

mixing.

3.3. Diurnal variations

Fig. 5 shows the results of a Lagrangian drift mode

diurnal study of H2O2 concentration at Station MA1

(6.08E, 41.08N) on 18th July 1993. Light measure-

ments for the day of study were obtained from a

deckbased PAR meter at the bow of the ship and these

data showed that there was very little cloud cover

during the course of the experiment, that sunrise was at

0400 and that maximum light intensity occurred at

1200. The diurnal variations of H2O2 concentration at

selected depth intervals (surface (1.1±1.7 m), 15 m

(14.0±15.3 m) and 40 m (40.0±44.4 m)) are shown

in Fig. 6. These diurnal variations assume that there

was no vertical transport of water between the selected

depths.

The surface water H2O2 concentration was rela-

tively constant (120±130 nM) from 0450 (only 50 min

after sunrise) until 1521. From this time onwards H2O2

at the surface was consumed at a greater rate than its

generation and the concentration decreased to a mini-

mum at 0027 on the next day (84 nM H2O2). The other

depth intervals showed similar generation and decom-

position cycling of H2O2 although the sub-surface

Table 1

Hydrogen peroxide marine field data

Location [H2O2] (nM) Reference

North Atlantic <180 [1]

Gulf of Mexico <150 [2]

Gulf of Mexico <150 [10]

Atlantic 488N to 638S <175 [11]

Sargasso Sea <150 [12]

Peru Upwelling area <50 [18]

Western Mediterranean <150 [19]

Great Barrier Reef <110 [20]

Eastern Caribbean and

Orinoco plume

<420 [21]

Seto Inland Sea <400 [22]

North Atlantic, off coast

of SE USA

<125 [23]

Southern Ocean 498S to 628S <20 [24]

South and Equatorial Atlantic <240 [25]

Western Mediterranean and

RhoÃne plume

<160 This study

Fig. 3. Hydrogen peroxide depth profiles for Station MA1 (68E,

418N) at 12h04 local time on 20th July 1993 and Station MD1

(48E, 438N) at 16h40 local time on 31st July 1993.

Fig. 4. Temperature depth profiles for Station MA1 (68E, 418N) at

12h04 local time on 20th July 1993 and Station MD1 (48E, 438N)

at 16h40 local time on 31st July 1993.


variations were less pronounced than at the surface.

The total H2O2 diurnal pro®le gave a concentration

maximum just after midday and rapidly decreased

thereafter. The sequence of pro®les from 1005 to

0027 demonstrates the potential use of H2O2 measure-

ments as a short-term tracer for vertical transport in the

water column. The increased H2O2 concentration at

ca. 30 m at MA1/9 was probably due to entrainment of

Fig. 5. Diurnal variations in hydrogen peroxide depth profiles at station MA1.


H2O2-rich surface water. The H2O2 anomaly was

slowly assimilated into the sub-surface waters and

traced by pro®les MA1/9 to MA1/14. The potential

of H2O2 measurements at sea to examine vertical

motions near the sea surface was ®rst proposed by

Johnson et al. [19] who observed large anomalies in

H2O2 concentrations at frontal regions of the western

Mediterranean (028W±038E, 36±378N) that were well

correlated with the salinity gradient which was known

to drive the frontal circulation.

The MA1 diurnal cast data was used to calculate net

H2O2 generation rates (when the concentration

between casts increased) and consumption rates (when

the concentration between casts decreased). The

ranges of generation and consumption rates were

0.0201±0.0315 nM H2O2 minÿ1 and (ÿ)0.0111±

(ÿ)0.0975 nM H2O2 minÿ1, respectively, as shown

in Table 2. These rates are in good agreement with

those reported previously, e.g. Moffett and Za®riou

[26] calculated generation rates in un®ltered coastal

waters of 0.045±0.057 nM minÿ1 when exposed to

¯uorescent light and 0.083±0.100 nM minÿ1 under

ambient sunlight and Johnson et al. [19] reported

consumption rates in the western Mediterranean of

(ÿ)0.050±(ÿ)0.120 nM minÿ1. The mean generation

and consumption rates at depth (40.0±44.4 m) were

less than at the surface (1.1±1.7 m) due to enhanced

biological and photochemical activity in the shallow

waters for both generation [12,27] and consumption

[14,15] processes. Diurnal variations in the surface

waters at Station MA1 in the western Mediterranean

(84±123 nM H2O2) were in excellent agreement with a

similar study in the Gulf of Mexico that reported 100±

130 nM H2O2 [10]. Both waters were relatively oli-

gotrophic but larger diurnal trends were reported for

more productive waters, e.g. the H2O2 concentration

in Jack's Lake, Ontario, peaked at 200±400 nM H2O2

in the late afternoon and decreased to <10 nM

during the night [8] while in more marine in¯uenced

water (Patuxent River Estuary, Maryland) H2O2 varied

from 180 to 350 nM H2O2 [28]. The variation in

H2O2 concentration ranges is likely to be due to

matrix differences between the fresh, estuarine and

marine waters which affect the light quality and

penetration characteristics (e.g. the extent to which

light attenuation is dependent on the concentration of

suspended solids, which tends to be higher in estuarine

waters), physical mixing, biological activity (e.g. with

respect to catalase activity) and the type and con-

centration of H2O2-generating dissolved organic

chromophores.

3.4. Geographical distribution of H2O2

As described above, the time of day greatly affected

the concentration of H2O2 in the sampled waters.

However, a ®rst order comparison of the geographical

variations of H2O2 was made by classi®cation of the

depth pro®le data into ®ve geographical areas. The

geographical distribution of H2O2 in the western

Mediterranean is shown in Table 3. Depth intervals

of 0±20 m, 21±40 m, 41±80 m and >80 m were chosen

to assess the depth pro®le characteristics from the

different areas. The population sizes for each geogra-

phical area were not consistent, e.g. the Straits of

Gibraltar area comprised only 24 data points, of which

Fig. 6. Diurnal variations of hydrogen peroxide at different depth

intervals at Station MA1 on 18th July 1993.

Table 2

Net generation or consumption of H2O2 at Station MA1

Time period

(local time)

Generation (�) or consumption (ÿ) of H2O2

(nM minÿ1)

1.1±1.7 m 14.0±15.3 m 40.0±44.4 m

0450±1252 ÿ0.021 n/a n/a

0450±1005 n/a ÿ0.029 ÿ0.011

1005±1252 n/a ÿ0.070 �0.030

1252±1521 �0.020 �0.032 �0.031

1521±1950 ÿ0.043 ÿ0.028 ÿ0.024

1950±0027 ÿ0.098 ÿ0.064 ÿ0.023


15 were samples from below 80 m depth. However, a

number of ®ndings were evident from this limited data

set.

1. The highest H2O2 concentrations were determined

in the Straits of Sicily ([H2O2]max�143 nM) and

the inner RhoÃne ([H2O2]max�154 nM) areas. An

interesting result was that the highest H2O2

concentration (154 nM) determined throughout

the entire period of the cruise (Cast MA8/1) was

collected at the shallowest recorded sampling

depth (0.2 m) implicating a possible micro-layer

enhancement of H2O2.

2. The oligotrophic classified stations had higher

mean H2O2 concentrations than the other four

geographical areas for the 0±80 m depth ranges.

The Straits of Sicily casts also had high H2O2

concentrations, particularly in the top 20 m of

the water column. The decrease in concentration

with depth is well demonstrated and was in agree-

ment with the typical depth profiles shown earlier

(Section 3.2). At depths >80 m very low H2O2

concentrations were typical at all five areas (mean

concentration �13 nM).

3. Although productivity data is unavailable, it was

assumed that the most productive waters were

those closest to the RhoÃne plume. However, the

less productive waters (oligotrophic and Straits of

Sicily) had the highest H2O2 concentrations. A

contributing factor was the fact that the oligo-

trophic and Straits of Sicily Stations were further

south than the RhoÃne Stations and therefore

Table 3

Geographical distribution of hydrogen peroxide in the western Mediterranean

Depth (m) Number of data points [H2O2]Mean (nM) [H2O2]Min (nM) [H2O2]Max(nM) S.D. (nM)

Straits of Gibraltar (D3 and D5)

0±20 2 33 20 46 13

20±40 2 21 17 25 4

40±80 5 18 13 23 3

>80 15 13 11 16 3

Oligotrophic (D8, D9, D16, MA1, MA2 and MA3)

0±20 34 91 51 136 21

20±40 17 61 33 88 15

40±80 40 28 11 65 12

>80 63 13 11 27 4

Straits of Sicily (D10, D11, D12, D12A, D14)

0±20 13 87 34 143 29

20±40 14 44 17 82 15

40±80 23 21 11 35 6

>80 44 13 11 46 6

RhoÃne, outer (MA4, MA5, MA6, MD1 and MF1)

0±20 49 72 19 123 23

20±40 32 28 11 76 14

40±80 30 13 11 23 4

>80 21 11 11 11 0

RhoÃne, inner (MA7, MA8, MA9, MF3 and ME2)

0±20 24 57 16 154 35

20±40 15 20 11 33 6

40±80 15 13 11 21 4

>80 7 11 11 11 0


received a greater light flux for the photochemical

generation of H2O2.

3.5. Photogeneration of H2O2

The photogeneration of H2O2, the most dominant

generation process of H2O2 in the marine environ-

ment, was investigated at sea and in the laboratory.

The effect of water origin on the photogeneration of

H2O2 was examined using shipboard incubations and

ambient sunlight (Fig. 7). The eutrophic/mesotrophic

RhoÃne plume surface sample (MA9) generated by far

the greatest concentration of H2O2 (401 nM after

360 min), probably due to River RhoÃne derived com-

ponents increasing the photochemical generation

potential of the sample. These photochemically active

components were assumed to be terrestrial in origin,

with humic and/or fulvic acids being the most likely

chromophores as these chemical species play an

important role in many natural aquatic photochemical

processes as light absorbing compounds [29]. The

deeper MA9 sample (39.4 m) generated signi®cantly

less H2O2 during the incubation (80 nM in 360 min).

This large difference over only 40 m of depth corre-

lated with differences in other water properties (e.g.

the salinity of the surface sample was 35.1 while at

39.4 m depth the salinity was 38.1) and was due to a

lack of vertical mixing between the plume in¯uenced

surface water and the deeper, denser Mediterranean

seawater at MA9. The surface D15 sample generated

slightly more H2O2 than the deep (2800 m) sample but

much less than the corresponding surface sample at

MA9. Lower concentrations of terrestrially derived

humic and fulvic compounds is the most likely reason

for the decreased photoreactivity of the surface D15

sample. This is corroborated by the more geographi-

cally remote location of D15 than MA9 (see Fig. 1).

3.6. Effect of molecular size on photogeneration

The effect of molecular size of the dissolved organic

matter in seawater on its potential to generate H2O2

was investigated by incubating ultra-®ltered samples

(see Section 2.2) as described in Section 2.4. The

dissolved organic carbon (DOC) content of each

fraction, as determined by two independent methods,

was 4.9% for the >8000 Da ®ltrate, 24.5% for the 300±

8000 Da fraction and 62.1% for the <300 Da ®ltrate

(total recovery 91.5%). The results for a 660 min

incubation are shown in Fig. 8. The data for the

300±8000 Da fraction have been corrected for the

420, 540, 600 and 660 min measurements because

the CL emission was off scale (>12 V) and assumes

that there was 100% recovery for the three fractions

compared with ®ltered seawater.

The 300±8000 Da fraction generated 62.3% of

the total H2O2 produced at a rate of 0.64

nM (mg C)ÿ1 hÿ1 while only accounting for 24.5%

of the total DOC content. It can therefore be deduced

that the majority of the most ef®cient H2O2 generating

chromophoric molecules were 300±8000 Da in size

Fig. 7. Effect of water origin on the photogeneration of H2O2.

MA9 samples were collected in the RhoÃne plume (58E, 438N) and

D15 samples in oligotrophic waters (68E, 408N).

Fig. 8. Shipboard photogeneration of H2O2 in filtered seawater and

in size fractionated seawater. Samples were collected from 1.64 m

depth at Station MA9 (58E, 438N) and incubated for 540 min from

09h30 on 29th July 1993.


and that the concentration of DOC did not directly

correlate with the amount of H2O2 generated. This

300±8000 Da fraction of Rhone plume origin con-

tained terrestrially derived fulvic and low-to-medium

molecular weight humic acids and a variety of sub-

stituted and unsubstituted aromatic and medium-to-

long chain aliphatic compounds. The <300 Da frac-

tion accounted for 62.1% of the total DOC and 30.4%

of the total H2O2 generated but gave the lowest H2O2

generation rate of 0.12 nM (mg C)ÿ1 hÿ1 . This frac-

tion comprised a variety of low molecular weight

inorganic and organic species, e.g. nitrates, carbohy-

drates, hydrogen carbonates and short chain aliphatic

compounds. The >8000 Da fraction accounted for

4.9% of the total DOC and 7.2% of the total H2O2

generated but the generation rate of 0.38 nM

(mg C)ÿ1 hÿ1 was higher than for the <300 Da frac-

tion. This suggests that the large molecular weight

humic acids in this fraction were moderately photo-

reactive.

The link between H2O2 generation ef®ciency and

humic/fulvic acid concentration was further investi-

gated in the laboratory by irradiating aqueous solu-

tions of river Dodder humic acid (DHA), fulvic acid

(DFA) and hydrophilic macromolecular acid (HMA)

and a commercial ionic humic acid salt (AHA) with a

solar simulator (1.8 kW Xe-arc lamp) for 180 min at

full power (194.0 W mÿ2). The results (Fig. 9)

showed that each organic fraction generated consider-

able quantities of H2O2 during the course of the

incubation with the fulvic acid fraction being 50%

less ef®cient than the other three. Additional incuba-

tions resulted in a linear relationship between humic/

fulvic acid concentration and H2O2 generation over

the range 0.05±2.0 mg lÿ1 (r�0.9411±0.9915; n�5).

There was also a small but measurable increase in

H2O2 from incubation of a Milli-Q water blank

(0.92 nM minÿ1). These results were in broad agree-

ment with the shipboard incubation data reported

above and suggest that ®eld based H2O2 measure-

ments may be useful as a surrogate for the photode-

gradation rate of DOC.

4. Conclusions

Shipboard results con®rm the feasibility of using a

¯ow injection-chemiluminescence method for H2O2

in seawater. The instrumentation is well suited to the

harsh conditions of deployment at sea and the analy-

tical ®gures of merit show excellent repeatability

(<5%) and reproducibility (1.8±2.5%). Hydrogen per-

oxide depth pro®les exhibited characteristic surface

maxima followed by a rapid decrease with depth to

below the practical limit of detection beyond the

thermocline. The pro®les were affected by the amount

of incident radiation, physical mixing processes and

seawater composition. Hydrogen peroxide depth pro-

®les varied diurnally, with surface water concentra-

tions showing the most signi®cant variation. At

Station MA1 in the central western Mediterranean

the concentration of H2O2 ranged from 84±123 nM.

Hydrogen peroxide generation and consumption rates

were in good agreement with previous studies. Ter-

restrially in¯uenced waters were more ef®cient at

photogenerating H2O2 than less productive waters

offshore in deck based incubations and laboratory

studies indicated that humic compounds are important

precursors in the formation of H2O2.

Acknowledgements

The authors wish to acknowledge the Natural Envir-

onment Research Council and Plymouth Marine

Laboratory for the award of a NERC CASE student-

ship to D.P., Lulwa Ali (UoP), Tim Fileman (PML)

and Jun Zhou (PML), and the crew, of®cers and staff

Fig. 9. Effect of River Dodder humic and fulvic acid fractions on

the photogeneration of H2O2. DHA�river Dodder humic acid;

DHA�river Dodder fulvic acid; HMA�river Dodder hydrophillic

macromolecular acid; AHA�commercial ionic humic acid salt.


of Research Vessel Services while onboard the R.R.S.

Discovery during cruise D203. We would also like to

thank Guenther Fengler and Alessandro Spitzy (Uni-

versity of Hamburg) for the fractionated seawater

samples.

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Shipboard determination of hydrogen peroxide in the western Mediterranean sea using flow injection...

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