Analytica Chimica Acta 554 (2005) 17–24

Determination of dissolved inorganic carbon (DIC) anddissolved organic carbon (DOC) in freshwaters by sequential injection

spectrophotometry with on-line UV photo-oxidation

Orawan Tue-Ngeuna,1, Richard C. Sandfordb,∗, Jaroon Jakmuneea, Kate Grudpana,Ian D. McKelviec, Paul J. Worsfoldb

a Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailandb School of Earth, Ocean and Environmental Sciences, University of Plymouth, Drakes Circus, Plymouth PL4 8AA, UK

c Water Studies Centre, School of Chemistry, Monash University, P.O. Box 23, Clayton Campus, Vic. 3800, Australia

Received 3 June 2005; received in revised form 27 July 2005; accepted 17 August 2005

Abstract

on (DOC)lyl red

ed after on-d

L

andre catalytic

ionghly

sses,hodsnicions,tion

herogensub-etry

er,largeplish

An automated sequential injection (SI) method for the determination of dissolved inorganic carbon (DIC) and dissolved organic carbin freshwaters is presented. For DIC measurement on-line sample acidification (sulphuric acid, pH < 2), converted DIC to CO2 which subsequentdiffused through a PTFE membrane into a basic, cresol red acceptor stream. The CO2 increased the concentration of the acidic form of the cresoindicator, with a resultant decrease in absorbance at 570 nm being directly proportional to DIC concentration. DIC + DOC was determinline sample irradiation (15 W low power UV lamp) coupled with acid–peroxydisulfate digestion, with the subsequent detection of CO2 as describeabove. DOC was determined by subtraction of DIC from (DIC + DOC). Analytical figures of merit were linear ranges of 0.05–5.0 mg C−1 forboth DIC and DIC + DOC, with typical R.S.D.s of less than 7% (0.05 mg C L−1–5.3% for DIC and 6.6% for DIC + DOC; 4.0 mg C L−1–2.6% forDIC and 2.4% for DIC + DOC,n = 3) and an LOD (blank + 3S.D.) of 0.05 mg C L−1. Sample throughput for the automated system was 8 h−1 forDIC and DOC with low reagent consumption (acid/peroxydisulfate 200�L per DIC + DOC analysis). A range of model carbon compoundsTamar River (Plymouth, UK) samples were analysed for DIC and DOC and the results showed good agreement with a high temperatuoxidation (HTCO) reference method (t-test,P = 0.05).© 2005 Elsevier B.V. All rights reserved.

Keywords: Dissolved inorganic carbon; Dissolved organic carbon; UV photo-oxidation; Peroxydisulfate; Sequential injection; Freshwater

1. Introduction

The principal forms of carbon in its aquatic cycle[1] aredissolved inorganic carbon (DIC), dissolved organic carbon(DOC), particulate organic carbon (POC) and biotic carbon.The freshwater organic fraction of many elements, includingcarbon, forms an important reservoir with fluvial mechanismsalso transporting 90% of the material moved from land to oceans.Surface water DIC has been used as an indicator of air pollu-tion, water quality and the degree of dissolution of chemicalspecies, e.g. carbonates[2,3]. DOC can be used as an indicator

∗ Corresponding author. Tel.: +44 1752 233127; fax: +44 1752 233035.E-mail address: rsandford@plymouth.ac.uk (R.C. Sandford).

1 Permanent address: Department of Chemistry, Faculty of Science, NaresuanUniversity, Phitsanulok 65000, Thailand.

of aquatic pollution, with high DOC levels resulting in depletof dissolved oxygen and reduced biodiversity leading to hidegraded aquatic ecosystems[4].

The need to elucidate further details of such procecoupled with legislation, has driven the development of metfor DIC, total inorganic carbon (TIC), DOC and total orgacarbon (TOC). DIC, generally present at higher concentratis more easily measured than DOC. Historically, the digesof DOC has used wet oxidation digestion[5] in standardbatch methods[6], often involving high temperatures, taddition of large volumes of hazardous reagents, e.g. hydperoxide, concentrated acids, and peroxydisulfate or withsequent determination by non-dispersive infrared spectrom(NDIR), coulometric titration or conductimetry. Howevthese methods are time consuming, complex, requirevolumes of reagents and samples and are difficult to accom

0003-2670/$ – see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.aca.2005.08.043

18 O. Tue-Ngeun et al. / Analytica Chimica Acta 554 (2005) 17–24

in closed systems and are therefore potentially subject tocontamination.

Many of the flow methods for DIC involve sample acidi-fication to convert inorganic carbon to carbon dioxide (CO2),which then diffuses across an appropriate membrane into areceptor stream where it is determined by spectrophotometry[7–11], potentiometry[12], conductimetry[13–15] or NDIR[16]. Flow injection methods for DOC utilise on-line photo-oxidation and peroxydisulfate digestion (also used for dissolvedorganic phosphorus[17]), to convert DOC to CO2 which isdetected by NDIR spectrometry[18], conductimetry[19,20],spectrophotometry[21], or catalytic conversion to methane fol-lowed by flame ionization detection[22]. Alternative FI systemsuse on-line photocatalytic oxidation by titanium dioxide[23],potassium dichromate in sulphuric acid[24] or microwave diges-tion [25] with conductometric, spectrophotometric and turbiditybased detectors, respectively. FI methods with ICP–AES[26]and high temperature catalytic oxidation (HTCO) with NDIRdetection[27] have also been reported for organic carbon.

A sequential injection (SI) method with its inherent advan-tages[28] has also been reported for DIC using a gas diffusioncell and spectrophotometric detection[29], as has the sequen-tial determination of TIC (total inorganic carbon) and TOC (totalorganic carbon) by a gas-diffusion/FI manifold coupled to a bulkacoustic wave impedance sensor[30]. Organic and inorganic car-bon at low�g C L−1 concentrations can also be continuouslyd torw

tiono ighp wita f thelet or Ud sor nl theu rogep

heb ch-n fn s ann tiono– sin-g rouns ho-ti -b[ undi

veda ndD

diffused through a PTFE membrane into a basic cresol red indi-cator stream. The resultant decrease in the absorbance of thecresol red indicator[9] was monitored at 570 nm. DIC + DOCwas subsequently determined after on-line, low power UV irra-diation coupled with acidic peroxydisulfate digestion of a splitsample aliquot, the resultant CO2 being detected as describedabove. DOC was then determined by subtraction of DIC from(DIC + DOC).

2. Experimental

2.1. Reagents

Acidified peroxydisulfate (4% m/v) was prepared by dissolv-ing di-potassium peroxydisulfate (AnalaR, BDH) in sulphuricacid (0.5 mol L−1, Spectrosol, BDH). Cresol red (0.0012% m/v,AnalaR, BDH) was prepared by mixing 6 mL of 0.1% m/v cresolred solution with 1.5 mL of 0.5 mol L−1 sodium hydrogen car-bonate solution (0.0015 mol L−1, Aristar®, BDH) and made upto 500 mL. Ultrapure, low carbon water (18.2 M� cm−1, MQElgastat Maxima, Elga Ltd., UK fitted with low power UV treat-ment) was used throughout.

2.2. Model compounds

A number of commonly reported model organic carbon com-p stiono k),e DH),d cid( nedp 10),n laR,B iuma H),t wasde lution( rdsw

2

tem( a2 lve( ac-t pec-t itha con-v spec-t 0-2-U anOi sed.T d toi tors,

etermined using a UV-shielded, cylindrical thin-film reacith high-sensitivity IR spectrometric detection[31].A clean and efficient alternative to wet oxidative diges

f DOC is UV photo-oxidation using low, medium and hressure mercury lamps which have an emission spectrummaximum at 254 nm, corresponding to the relaxation o

owest excited state (63PO) to the ground state (61SO). A weakermission at 184 nm corresponds to the transition from 61P1 to

he ground state. These wavelengths are the most efficient fegradation of DOC[32–34]. A small number of atoms can aleach excited states higher than 61P1, although their emissioines are weak. The use of UV photo-oxidation minimisesse of hazardous hot reagents e.g. perchloric acid and hyderoxide.

Analytical applications of UV photo-oxidation include treakdown of DOC, an interferent in electro-analytical teiques, prior to trace metal analysis[32] and the oxidation oitrogenous and ammoniacal organic compounds to nitrateitrites prior to molecular spectrophotometry. The absorpf UV light by chromophores in DOC, e.g. –CC–, –C O–, orN N–, results in the formation of highly reactive, transientlet excited species. These subsequently decay to new gtate products (DOC* ) by a series of primary and secondary pophysical and photochemical pathways[35] involving intra- orntermolecular processes[36]. Peroxydisulfate oxidation comined with UV irradiation produces hydroxyl radicals (OH•)

31,37,38]which are amongst the most reactive oxidants fon aqueous systems.

This paper reports an automated SI manifold with impronalytical figures of merit for the determination of DIC aOC using sample acidification to convert DIC to CO2, which

h

V

n

d

d

ounds were used to determine the efficiency of UV digef DOC. These were citric acid (GR for Analysis, Mercthylenediaminetetraacetic acid disodium salt (AnalaR, B-glucose (A.R., Fisons Scientific Equipment), humic aAldrich, 36.7% C, 4.7% H, 1.7% N), composition determireviously on a CHNS analyser (CE instruments EA 11icotinic acid (Sigma), 1,10-phenanthroline hydrate (AnaDH), potassium hydrogen phthalate (AnalaR, BDH), sodcetate tetrahydrate (GPR, BDH), tartaric acid (AnalaR, BD

hiourea (AnalaR, BDH) and urea (AnalaR, BDH). Eachried at 105◦C for 2 h and stock solutions (100 mg C L−1) ofach prepared. A stock sodium hydrogen carbonate so1000 mg C L−1, Aristar®, BDH) was also prepared. Standaere prepared daily by serial dilution.

.3. Instrumentation

The instrumentation consisted of a FIAlab 3500 SI sysFIAlab Instruments, Bellevue, WA, USA), comprising500�L syringe pump (SY), an eight port multi-position vaMV), a peristaltic pump (P), a holding coil (HC) and two reion coils (RC1 and RC2). Detection was by diode array srometry (S2000-FI Ocean Optics Inc., Dunedin, FL, USA) whalogen light source (LS-1, Ocean Optics Inc.) and an A/Derter (SAD-500, Ocean Optics Inc.) interface between therometer and computer and fibre optic connectors (16-20V/vis, 200�m, Ocean Optics Inc.). A cell holder (CUV, Oceptics Inc.) and a U-flow flow cell (10 mm path-length, 18�L

nternal volume, Hellma, Southend-on-Sea, UK) were also uhe FI manifold (0.75 mm i.d. PTFE tubing) was configure

nclude a low power UV lamp (15 W, Photochemical Reac

O. Tue-Ngeun et al. / Analytica Chimica Acta 554 (2005) 17–24 19

Fig. 1. SI manifold for the sequential determination of DIC and (DIC + DOC): SY—syringe pump (2500�L), MV—multi-position valve, GD—gas diffusion unit,P—peristaltic pump, UV reactor—PTFE tube (350 cm× 0.75 mm i.d.) coiled around UV lamp (15 W), HC—holding coil PTFE tube (500 cm× 0.75 mm i.d.), RC1and RC2—reaction coils consisting of coiled PTFE tube, RC1 (22 cm× 0.75 mm i.d.) and RC2 (11 cm× 0.75 mm i.d.), carrier—ultrapure water.

Ltd., Reading, UK) with a PTFE photo-reactor coil, an HC, RC1and RC2 of 500, 22 and 11 cm, respectively, and a home-madegas diffusion (GD) unit (Fig. 1). Standard nuts and ferrules(Upchurch, Oak Harbor, USA) were used. The SI instrumenta-tion was controlled by FIAlab 3500 software as was the acquisi-tion of raw data. Peak heights were determined post-acquisitionusing in-house software (Ruthern Instruments, Bodmin,UK).

The on-line UV irradiation system consisted of a low powerUV lamp (15 W, λmax 254 nm, 21 cm long) wound with aphoto-reactor coil (350 cm long, 0.75 mm i.d. PTFE tubing) andmounted in a light-tight, vented box. The gas diffusion unit con-sisted of two rectangular Perspex® blocks, (10.5 cm long, 3.7 cmwide, 1.8 cm thick) with a zigzag flow channel (0.04 mm deep,0.12 cm wide and 28 cm long) machined into each of the blockfaces. A rectangular PTFE gas diffusion membrane (0.17 mmthick, 12 mm wide, RS Ltd., UK) was clamped between thetwo blocks, separating the lower (donor) and upper (acceptor)streams.

2.4. Sample collection, preservation and storage

All samples were filtered immediately after collectionthrough glass fibre filters (0.7�m pore size; GFF, Whatman)to minimise the effects of biological processes which couldresult in a net increase (e.g. release from organisms present)o ere

ashed in a muffle furnace (450◦C for > 4 h) andrinsed withwater before use to minimise leaching of carbon from the fil-ter. Samples were also purged with ultra high purity (UHP) N2(CO2-free) to remove volatile organic carbon (VOC) and dis-solved CO2 and were stored at 4◦C in the dark. The DOCfraction that remained is therefore equivalent to that desig-nated as non-purgeable DOC (NPDOC) for the HTCO referencemethod. All glass and plasticware was rigorously cleaned byimmersion in a nutrient free detergent (3% v/v, NeutraconTM,Decon Laboratories, UK) for 12 h[27], rinsed with water, soakedin HCl (10% v/v, 12 h) and finally rinsed with low carbonwater.

2.5. Procedures

A typical automated SI sequence (Table 1) was firstly theaspiration of alternate aliquots of standard/sample and acidicperoxydisulfate to stack 10 sample and reagent zones in theholding coil (Fig. 1), which were then delivered to the UV photo-reactor to digest DOC. Next, in order to determine DIC, alternatealiquots of standard/sample and sulphuric acid were then aspi-rated into the holding coil to stack 10 sample and reagent zonesin the holding coil. These were then pumped through the gasdiffusion unit, where the CO2 from the breakdown of DIC dif-fused through the PTFE membrane into the cresol red acceptorstream. The reaction zone was then pumped (peristaltic pump)t ion of

r decrease (e.g. microbial consumption) of DOC. Filters w o the diode array detector. For the subsequent determinat

20 O. Tue-Ngeun et al. / Analytica Chimica Acta 554 (2005) 17–24

Table 1Operational procedure for the sequential determination of DIC and (DIC + DOC)

Step Description SYport

MV port SY flow rate(�L s−1)

Directionof SY

Volume (�L) P onor off

Flow rate of pumpP (mL min−1)

Delaytime (s)

1 Stack 10 AP reagent and S/S zones inHC (DIC + DOC determination)

Out 6 and 5 100 Aspirate 20 (AP), 80 (S/S) Off – –

2 Fill syringe In – 100 Aspirate 1500 Off – –3 Propel AP & S/S zones to UV

reactorOut 7 100 Dispense 1050 Off – –

4 Cleaning HC Out 1 100 Dispense Empty Off – –5 Stack 10 sulfuric acid and S/S zones

in HC (DIC determination)Out 4 and 5 100 Aspirate 20 (Sulfuric acid),

80 (S/S)Off – –

6 Fill syringe In – 100 Aspirate 1500 Off – –7 Transport of CO2 from HC to GD

(DIC determination)Out 3 60 Dispense Empty Off – –

8 Reference scan – – – – – Off – 39 Start absorbance scan (570 nm) – – – – – Off – 15

10 Start pump P – – – – – On 2.9 –11 Pump cresol red indicator (P) and

monitoring of DIC– – – – – On 2.9 65

12 Stop absorbance scan – – – – – On 2.9 –13 Stop pump P – – – – – Off – –14 Propel CO2 from UV reactor to HC

(DIC + DOC determination)Out 7 100 Aspirate 1200 Off – –

15 Fill Syringe In – 100 Aspirate 1300 Off – –16 Propel CO2 (for DIC + DOC) from

HC to GDOut 3 60 Dispense Empty Off – –

17 Reference scan – – – – – Off – 318 Start absorbance scan (570 nm) – – – – – Off – 1519 Start pump P – – – – – On 2.9 –20 Pump cresol red indicator (P) and

determination of DIC + DOC– – – – – On 2.9 65

21 Stop absorbance scan – – – – – On 2.9 –22 Stop pump P – – – – – Off – –23 Fill syringe In – 100 Aspirate 2500 Off – –24 Clean UV reactor Out 7 100 Dispense Empty Off – –

SY—syringe pump, MV—multi-position valve, P—peristaltic pump, AP—acidic peroxydisulfate, S/S—standard/sample, HC—holding coil and GD—gas diffusionunit.

(DIC + DOC) the sample/reagent zone that had been irradiatedin the UV photo-reactor was then pumped back into the holdingcoil and then to the gas diffusion unit and detector as describedabove for DIC. DOC was determined by subtraction of DIC from(DIC + DOC). A reference scan (for 3 s) was stored before eachabsorbance scan. To minimise memory effects and carry overon the tube walls the holding coil and UV reactors were cleanedby acid washing and rinsing with ultra pure water prior to eachanalysis.

3. Results and discussion

The key operational variables for the SI manifold were opti-mised to maximise the efficiency of DOC digestion and sensi-tivity of the method.

3.1. Determination of CO2

Dissolution of CO2 from the donor stream into the basiccresol red indicator/acceptor stream reduced the pH by convert-ing from the basic form to the acidic form, this being monitoredas a decrease in absorbance at 570 nm. This provided a more sen-

sitive method than monitoring the increase in the acidic form ofthe indicator at 430 nm[10].

A key parameter affecting method sensitivity was the perme-ability to CO2 of the hydrophobic PTFE gas diffusion membraneand therefore two commercially available PTFE membranes(0.17 and 0.08 mm thick) were investigated. Under flow con-ditions the 0.08 mm membrane suffered from excessive bowing,was less permeable to CO2 and resulted in a reduced signal.Therefore, the 0.17 mm membrane was selected for all futurework. Although robust and durable, the performance of the mem-brane was evaluated prior to use each day using a referencesodium hydrogen carbonate standard (10.0 mg C L−1), with themembrane routinely replaced every 2 weeks.

The production of CO2 was maximised by effective mixingof standard/sample with the reagent stream, a factor promotedby sequentially aspirating 10 alternate aliquots of each to forma stack of sample/reagent zones in the holding coil (Fig. 1). Thiswas a novel aspect reported for FI methods previously[39] butnot for SI and was used to maximise the sensitivity of the pro-posed method. For the determination of DIC, the volume of eachalternate aliquot was optimised over the range of 40–80�L forstandard/sample and 10–50�L for sulphuric acid. As the volume

O. Tue-Ngeun et al. / Analytica Chimica Acta 554 (2005) 17–24 21

of standard/sample increased, a greater change in absorbancewas observed although for standard/sample volumes in excessof 80�L peak shape deteriorated and no significant differencein signal was seen due to less efficient mixing with the sulphuricacid. Furthermore, no significant difference in absorbance wasobserved for the sulphuric acid optimisation over the 10–50�Lrange used. Therefore, for all subsequent work 10 alternatealiquots of 80�L of standard/sample and 20�L of sulphuricacid were stacked in the holding coil, corresponding to a totalsample/standard volume of 800�L and total sulphuric acid vol-ume of 200�L.

The concentration of cresol red in the acceptor solution wasoptimised over the range 0.0001–0.01% m/v, whilst maintain-ing the sodium hydrogen carbonate at 0.003 mol L−1 (Fig. 2a).The signal reached a plateau at 0.001–0.002% m/v of cresol red,but declined rapidly at higher concentrations when a significantincrease in baseline noise was also observed. Below 0.001%m/v cresol red the decrease in absorbance was also small dueto the stoichiometric limitation of the reaction. A cresol redconcentration of 0.0012% m/v was therefore selected for allsubsequent studies. The effect of varying the sodium hydrogencarbonate concentration, used to prepare the basic cresol redindicator stream, over the range 0.0005–0.01 mol L−1 was alsoinvestigated with a 0.0015 mol L−1 selected for optimum man-ifold sensitivity (Fig. 2b). This resulted in a mean pH of 9.1 forthe cresol red stream.

eamo edap waso( owre s sin forpFt ast,f withg -f

3.2. UV photo-oxidation for DOC determination

The proposed method used UV photo-oxidation with acid-ified peroxydisulfate to digest (DIC + DOC) and therefore thekey operational variables of volume and concentration of acidi-fied peroxydisulfate, sample volume and time of UV irradiationwere optimised to maximise digestion efficiency. Nicotinic acidwas used as a model DOC compound and mixed on-line with theacidified peroxydisulfate, pumped through the UV photo-reactorcoil (Fig. 1), where DIC and DOC were converted to CO2, anddetected as described in Section3.1. The digestion efficiencywas defined as the ratio (%) of the peak height of nicotinic acidrelative to that of sodium hydrogen carbonate.

The concentrations of sulphuric acid and peroxydisulfatewere varied over the range of 0.0–2.0 mol L−1 and 1–5% m/v,respectively. The optimum sulphuric acid concentration was0.5 mol L−1, with the digestion efficiency reduced at higheracid concentrations (Fig. 2c). For peroxydisulfate, the digestionof (DIC + DOC) was more efficient at higher concentrations,although it was difficult to ensure complete dissolution at per-oxydisulfate concentrations in excess of 4% m/v. Therefore, 4%m/v was chosen as the best compromise between method sensi-tivity and ease of reagent preparation.

The effect of varying the volumes of standard/sample andacidified peroxydisulfate was investigated over the ranges of40–80 and 10–50�L, respectively, as described for the deter-m erw xi-m

tedo rbonc rveda suret s inf fora

oto-o l car-b undst oldc cid,

F nd sb NaHi %) os

The flow rates of the sample/sulphuric acid donor strver the range 10–100�L s−1 (syringe pump) and the cresol rcceptor stream over the range 0.8–4.2 mL min−1 (peristalticump) were investigated. No significant difference in signalbserved for either the low (0.5 mg C L−1 NaHCO3) or high10.0 mg C L−1 NaHCO3) concentration standards over a flate range of 10–100�L s−1 (syringe pump). Below 40�L s−1

levated blanks were observed and sample throughput waificantly constrained due to the extended time requiredroduction of the signal peak (5 min per peak at 10�L s−1).urthermore, the best precision was at 60�L s−1 which was

herefore the flow rate chosen for all future work. In contror the cresol red acceptor stream a sharp signal optimumood precision was observed at 2.9 mL min−1

, which was thereore selected for all future studies.

ig. 2. Optimisation of cresol red indicator, sodium hydrogen carbonate alank (ultrapure water) (�); (b) NaHCO3 concentration in indicator stream.

n acid–peroxydisulfate stream for DOC (�). Recovery is defined as ratio (tandard. Error bars— 1S.D. (n = 3). AU—absorbance units.

g-

ination of DIC. An 80�L standard/sample aliquot togethith 20�L of acidified peroxydisulfate resulted in the maum (DIC + DOC) digestion efficiency.The optimum time for UV digestion of DOC was investiga

ver the range 15–240 s. Using nicotinic acid as a model caompound, no significant improvement in signal was obset photo-oxidation times greater than 60 s. However, to en

he complete digestion of more refractory DOC compoundreshwaters[6], a photo-oxidation time of 240 s was selectedll subsequent work.

In order to evaluate the digestion efficiency of the UV phxidation/acid–peroxydisulfate digestion process 12-modeon compounds were chosen to represent DOC compo

ypically found in freshwaters. Using the optimum manifonditions, recoveries for KHP, EDTA, citric acid, tartaric a

ulfuric acid in acid–peroxydisulfate stream for DOC: (a) cresol red concentration (�),CO3 standard (10.0 mg C L−1) (�); blank (ultrapure water) (�); (c) sulfuric acidf the peak height of nicotinic acid (10.0 mg C L−1) to NaHCO3 (10.0 mg C L−1)

22 O. Tue-Ngeun et al. / Analytica Chimica Acta 554 (2005) 17–24

Fig. 3. Digestion efficiency of 11-model carbon compounds. Recovery is defined as ratio (%) of the peak height of each model carbon compound (10.0 mg C L−1)to that of NaHCO3 (10.0 mg C L−1). Error bars—1S.D. (n = 3).

Fig. 4. Typical SI peaks for the sequential determination of DIC and (DIC + DOC).

thiourea, urea,d-glucose and sodium acetate were 94–107%,and for nicotinic acid, 1,10-phenanthroline and humic acid 88,82 and 81%, respectively, with a mean R.S.D. of 3.3% (Fig. 3,n = 3). The reduced recoveries for these three model DOC com-pounds was consistent with the high chemical stability of theC N bonds in nicotinic acid and 1,10-phenanthroline, and thehigh molecular weight range and refractory nature of humic acid.

3.3. Analytical figures of merit

Calibrations were performed over the range 0.05–5.0mg C L−1 for sodium hydrogen carbonate (R2 = 0.9596, Y =0.0278X + 0.0342) and the more refractory potassium hydrogenphthalate (R2 = 0.9875,Y = 0.0310X + 0.0239) which were usedas model DIC and DOC compounds, respectively. The closesimilarity of the two gradients indicated that sodium hydrogencarbonate could be used as a standard for both DIC and DOCdeterminations. Typical SI peaks for DIC and (DIC + DOC) areshown inFig. 4.

For DIC and (DIC + DOC) the proposed method had anLOD of 0.05 mg C L−1 and a linear range of 0.05–5.0 mg C L−1

(for DIC Y = 0.0537X + 0.0169,R2 = 0.9868; for (DIC + DOC)Y = 0.0395X + 0.0023,R2 = 0.9959,Fig. 5) and was thereforesuitable for the determination of DIC and DOC in natural waters.The analytical figures of merit compared well with those ofO tersu -t rr oda h theme al

(alkaline peroxydisulfate) FIA method with spectrophotomet-ric detection. Sample throughput for the proposed automatedsystem was 8 h−1 for DIC and DOC.

The proposed method also incorporated the inherent advan-tages of SI which included a robust, easily modifiable manifoldwith low sample and reagent consumption i.e. 800 and 200�Lacid/peroxydisulfate, respectively, per DIC or (DIC + DOC)

F -d

shima et al., who reported a method for DIC in natural wasing sulphuric acid to convert DIC to CO2, FIA and spec

rophotometric detection (an LOD of 0.14 mg L−1 and a lineaange of 0.12–12 mg C L−1 [9]). For DOC the proposed methlso had an improved LOD and linear range compared witethod of Edwards et al.[21] (LOD of 0.1 mg C L−1 and lin-ar from 0.1–2.0 mg C L−1) for an on-line UV photo-chemic

ig. 5. Typical calibrations for DIC and (DIC + DOC). NaHCO3 and KHP stanards (0.05–5 mg C L−1), respectively, were used. Error bars—1S.D. (n = 3).

O. Tue-Ngeun et al. / Analytica Chimica Acta 554 (2005) 17–24 23

Tabl

e2

Rec

over

yof

DIC

and

DO

Cad

ded

tom

odel

carb

onst

anda

rds.

NaH

CO

3—

sodi

umhy

drog

enca

rbon

ate,

KH

P—

pota

ssiu

mhy

drog

enph

thal

ate,

ED

TA—

ethy

lene

diam

inet

etra

acet

icac

id.

Sam

ple

Mod

elca

rbon

com

poun

d(m

gC

L−1

)D

IC(m

gC

L−1

DIC

+D

OC

(mg

cL−

1)

Doc

(mg

CL−

1

NaH

CO 3

KH

PE

DTA

Ure

aG

luco

seD

ICad

ded

X̄±

S.D

.

(n=

3)R

ecov

ery

(%)

DIC

and

DO

Cad

ded

X̄±

S.D

.(n

=3)

Rec

over

y(%

)D

ocad

ded

X̄±

S.D

.

(n=

3R

ecov

ery

(%)

12.

0–

––

–2.

02.

3±0.

311

5±15

2.0

2.4±

0.1

120±

50.

00.

1±0.

4–

22.

01.

0–

––

2.0

2.0±

0.3

100±

153.

02.

6±0.

187

±3

1.0

0.6±

0.4

60±

403

2.0

–1.

0–

–2.

02.

1±0.

110

5±5

3.0

3.0±

0.2

100±

71.

00.

8±0.

280

±20

42.

0–

–1.

0–

2.0

2.2±

0.2

110±

103.

02.

9±0.

297

±7

1.0

0.7±

0.4

70±

405

2.0

––

–1.

02.

02.

2±0.

111

0±5

3.0

3.1±

0.2

103±

71.

00.

9±0.

190

±10

62.

01.

01.

01.

0–

2.0

2.3±

0.1

115±

55.

04.

5±0.

190

±2

3.0

2.2±

0.1

73±

37

2.0

1.0

1.0

–1.

02.

02.

2±0.

211

0±10

5.0

4.5±

0.1

90±

23.

02.

3±0.

277

±7

82.

0–

1.0

1.0

1.0

2.0

2.2±

0.1

110±

55.

04.

6±0.

392

±6

3.0

2.4±

0.3

80±

109

2.0

1.0

–1.

01.

02.

02.

2±0.

111

0±5

5.0

5.4±

0.4

108±

83.

03.

2±0.

510

7±

17

analysis, which compared well with typical FI methods (4 mLof acid/peroxydisulfate[21]).

3.4. Interference study

Water samples contain ions that could potentially reducethe efficiency of the UV digestion step. Therefore the effectof commonly occurring anions (nitrite, nitrate, phosphate fluo-ride, chloride, bromide, iodide) and cations (iron(II), iron(III),zinc(II) and copper(II)) was investigated at concentrationsin excess of those found in natural waters. KHP standards(5.0 mg C L−1) were spiked with fluoride, iodide and bro-mide (100 and 500 mg L−1), chloride (50, 100, 500 mg L−1),nitrite, nitrate, phosphate, iron(II), iron(III), zinc and copperat 50 mg L−1. All potential interferents except for 100 and500 mg L−1 chloride, resulted in less than±10% relative error.At concentrations of 100 and 500 mg L−1 chloride the photo-chemical digestion gave an increased acidity in the donor streamwhich resulted in an increased signal at the detector (570 nm).Therefore, the proposed method is not suitable for the determi-nation of DOC in seawater samples.

3.5. Freshwater sample analysis

The proposed method was validated by determining therecovery of various combinations of the 5-model DIC and DOCc withr

risonw ncem ctedu un-nw rbid-i

TC ic oxi-d C inf

S

123456789

m-p((( 9R

ompounds when good recoveries were observed for DICeduced recovery for some combinations of DOC (Table 2).

The proposed method was also validated by compaith a high temperature catalytic oxidation (HTCO) refereethod (Shimadzu TOC-5000) for freshwater samples collesing clean sampling protocols from the Tamar River at Gislake (Plymouth, UK) in March 2004 (Table 3). The river wasell oxygenated (dissolved oxygen 95%), with moderate tu

ty (10–20 mg L−1), pH 7.2± 0.2, conductivity of 190�S, with

able 3omparison of the proposed SI and reference high temperature catalytation (HTCO) methods for the sequential determination of DIC and DO

reshwatersa

ample Carbon (mg L−1)

SI HTCO

DIC DIC + DOC DOC DIC DOC

1.2± 0.04 1.2± 0.05 0.03± 0.05 1.2± 0.01 0.04± 0.032.3± 0.06 2.7± 0.07 0.39± 0.07 2.2± 0.03 0.47± 0.032.8± 0.23 3.6± 0.11 0.84± 0.11 2.6± 0.09 1.0± 0.033.2± 0.06 4.6± 0.11 1.4± 0.11 3.2± 0.04 1.1± 0.036.5± 1.02 11.5± 0.67 5.0± 0.67 7.0± 0.55 5.1± 0.156.0± 0.66 11.9± 0.86 5.9± 0.86 6.2± 0.20 6.1± 0.166.2± 0.42 13.0± 0.31 6.8± 0.31 6.3± 0.05 7.4± 0.047.1± 0.68 11.2± 0.95 4.1± 0.95 6.6± 0.15 5.1± 0.035.8± 0.28 12.1± 0.63 6.3± 0.63 6.7± 0.05 6.5± 0.09

a All results are the mean of three analysis± one standard deviation. Sales 1–4 model carbon standards: 1—NaHCO3 (1.0 mg C L−1); 2—NaHCO3

2.0 mg C L−1) + KHP (0.5 mg C L−1); 3—NaHCO3 (2.5 mg C L−1) + KHP0.5 mg C L−1) + EDTA (0.5 mg C L−1); 4—NaHCO3 (3.0 mg C L−1) + KHP0.5 mg C L−1) + EDTA (0.5 mg C L−1) + glucose (0.5 mg C L−1); samples 5–iver Tamar (Gunnislake, Plymouth, UK) collected at 10 min intervals.

24 O. Tue-Ngeun et al. / Analytica Chimica Acta 554 (2005) 17–24

typical PO43−-P concentrations of 40–100�g L−1 and NO3

−-N concentrations of 2–3 mg L−1. The HTCO method utilisedoxidation at 680◦C with a 0.5% Pt on Al2O3 catalyst, cou-pled with a non-dispersive infrared spectrometer (NDIR) todetect the CO2 [27]. Results for DIC and DOC determinedby the proposed SI method were in good agreement withthose of the HTCO reference method (t-test at 95% confidenceinterval, t = 0.76 for DIC andt = 1.9 for (DIC + DOC), criticalt 2.3).

4. Conclusions

This paper reports a robust, automated SI method incorpo-rating acid digestion of DIC, UV photo-chemical digestion of(DIC + DOC) and a gas diffusion unit for the spectrophotomet-ric determination of DIC and DOC in freshwaters. The methodwas linear from 0.05 to 5.0 mg C L−1 for DIC and (DIC + DOC),with typical R.S.D.s of less than 7% (0.05 mg C L−1–5.3% forDIC and 6.6% for (DIC + DOC); 4.0 mg C L−1–2.6% for DICand 2.4% for (DIC + DOC),n = 3) and an LOD 0.05 mg C L−1.The proposed method was therefore suitable for DIC and DOCin freshwaters. Sample and reagent consumption were low (800and 200�L, respectively, per DIC or (DIC + DOC) analysis) andthe method had a good sample throughput of eight DIC and DOCdeterminations h−1.

Compared with batch and high temperature catalytic oxida-t pler od),p oto-c OCw ltern samp rtedf f lowp withh tract rnsa nedw atede on-t withg ersa forD OC[

A

om-m ni-v ramC entsf

References

[1] S.S. Butcher, R.S. Charlson, G.H. Orians, G.V. Wolfe, Global Biogeo-chemical Cycles, Academic Press, 1992.

[2] S. Manahan, Environmental Chemistry, 5th ed., Lewis, London, 1991.[3] N. Maniasso, S. Sato, M.F. Gine, A.O. Jacintho, Analysis 121 (1996)

1617.[4] K. Robards, I.D. McKelvie, R.L. Benson, P.J. Worsfold, N.J. Blundell,

H. Casey, Anal. Chim. Acta 287 (1994) 147.[5] T.T. Gorsuch, The Destruction of Organic Matter, Pergamon Press,

Oxford, 1970.[6] L.S. Clesceri, A.E. Greenberg, A.D. Eaton, Standard Methods for the

Examination of Water and Wastewater, 20th ed., American Public HealthAssociation, 1998.

[7] W.E. van der Linden, Anal. Chim. Acta 151 (1983) 359.[8] S. Motomizu, K. Toei, T. Kuwaki, M. Oshima, Anal. Chem. 59 (1987)

2930.[9] Y. Wei, M. Oshima, T. Takayanagi, S. Motomizu, J. Flow Injection Anal.

18 (2001) 156.[10] V. Kuban, P.K. Dasgupta, Talanta 40 (1993) 831.[11] M.H.C. Stoll, K. Bakker, G.H. Nobbe, R.R. Haese, Anal. Chem. 73

(2001) 4111.[12] P. Linares, M.D. Lugue de Castro, M. Valcarcel, Anal. Chim. Acta 225

(1989) 443.[13] R.M. Carlson, Anal. Chem. 50 (1978) 1528.[14] J.J.R. Rohwedder, C. Pasquini, Analysis 116 (1991) 841.[15] T. Aoki, Y. Fujimaru, K. Fujie, Anal. Chim. Acta 284 (1993) 167.[16] H. Kimoto, K. Nozaki, S. Kudo, K. Kato, A. Negishi, H. Kayanne,

Anal. Sci. 18 (2002) 247.[17] I.D. McKelvie, B.T. Hart, T.J. Cardwell, R.W. Cattrall, Analyst 114

[[[[ , K.

[[ 90)

[[ im.

[ 87.[ iller,

[[[[[[ 342

[ nal.

[ 994)

[ nical

[[ rk,

[ vie,

ion (HTCO) methods the proposed method provides a simapid (8 h−1), accurate (good agreement to HTCO methrecise (<7% R.S.D.) low cost and low power (15 W) phhemical approach to sequentially determine DIC and Dith one sample injection. The sequential aspiration of 10 aate aliquots of sample and reagent to form a stack ofle/reagent zones in the holding coil, not previously repo

or SI, enhanced the sensitivity of the method. The use oower UV photo-oxidation minimises the ozone producedigher power UV systems and therefore eliminates the ex

ion typically required, minimising health and safety concend making it suitable for field deployment. When combiith the inherent advantages of SI e.g. a robust, automasily modified multiparametric manifold that minimises c

amination, has low reagent and sample consumptionood sample throughput[29], the proposed method also offn improved LOD over previously reported FI methodsIC [9] and an improved LOD and linear range for D

21].

cknowledgements

The authors would like to acknowledge the Thailand Cission on Higher Education (CHE) for funding, Naresurn U

ersity, The Post Graduate Education and Research Proghemistry (PERCH) for their support and Ruthern Instrum

or supplying the integration software.

,

--

-

,

in

(1989) 1459.18] K.J. Collins, P.J.L.B. Williams, Mar. Chem. 5 (1977) 123.19] W. Schreurs, Hydrol. Bull. 12 (1978) 137.20] K. Koshy, M. Mataki, Lab. Robot. Automat. 12 (2000) 157.21] R.T. Edwards, I.D. McKelvie, P.C. Ferrett, B.T. Hart, J.B. Bapat

Koshy, Anal. Chim. Acta 261 (1992) 287.22] G. Cauwet, Mar. Chem. 14 (1984) 297.23] R.W. Matthews, M. Abdullah, G.K.C. Low, Anal. Chim. Acta 233 (19

171.24] E. Ballesteros, A. Rios, M. Valcarcel, Analyst 122 (1997) 309.25] J.N.G. Paniz, E.M.M. Flores, V.L. Dressler, A.F. Martins, Anal. Ch

Acta 445 (2001) 139.26] O. Emteryd, B. Andersson, H. Wallmark, J. Microchem. 43 (1991)27] G. Spyres, M. Nimmo, P.J. Worsfold, E.P. Achterberg, A.E.J. M

Trend. Anal. Chem. 19 (2000) 498.28] J. Ruzicka, G.D. Marshall, Anal. Chim. Acta 237 (1990) 329.29] M. Miro, J.M. Estela, V. Cerda, Talanta 60 (2003) 867.30] X.L. Su, L.H. Nie, S.Z. Yao, Anal. Chim. Acta 349 (1997) 143.31] S.A. Huber, F.H. Frimmel, Anal. Chem. 63 (1991) 2122.32] J. Golimowski, K. Golimowska, Anal. Chim. Acta 325 (1996) 111.33] M. Kolb, P. Rach, J. Schafer, A. Wild, Fresen. J. Anal. Chem.

(1992) 341.34] K. Yokoi, T. Tomisaki, T. Koide, C.M.G. van den Berg, Fresen. J. A

Chem. 352 (1995) 547.35] E.P. Achterberg, C.M.G. van den Berg, Anal. Chim. Acta 291 (1

213.36] N.V. Blough, R.G. Zepp, Woods Hole Oceanographic Institute Tech

Report Woods Hole Oceanographic Institute, WHO 90–99, 1990.37] L. Dogliotti, E. Hayon, J. Phys. Chem. 71 (1967) 2511.38] L.M.L. Nollet, Handbook of Water Analysis, Marcel Dekker, New Yo

1999.39] A.J. Lyddy-Meaney, P.S. Ellis, P.J. Worsfold, C.V. Butler, I.D. McKel

Talanta 58 (2002) 1043.