data quality in water analysis toc tn

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Data quality in water analysis:validation of combustion-infraredand combustion-chemiluminescencemethods for the simultaneousdetermination of Total Organic Carbon(TOC) and Total Nitrogen (TN)Vlasoula Bekiaria & Pavlos Avramidisaa Laboratory of Geology for Aquatic Systems, Department ofAquaculture and Fisheries Management, Technological EducationalInstitute of Messolongi, 30200, Messolonghi, GreecePublished online: 21 Feb 2013.

To cite this article: Vlasoula Bekiari & Pavlos Avramidis (2014) Data quality in water analysis:validation of combustion-infrared and combustion-chemiluminescence methods for thesimultaneous determination of Total Organic Carbon (TOC) and Total Nitrogen (TN), InternationalJournal of Environmental Analytical Chemistry, 94:1, 65-76, DOI: 10.1080/03067319.2013.763940

To link to this article: http://dx.doi.org/10.1080/03067319.2013.763940

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Data quality in water analysis: validation of combustion-infrared andcombustion-chemiluminescence methods for the simultaneous

determination of Total Organic Carbon (TOC) and Total Nitrogen (TN)

Vlasoula Bekiari* and Pavlos Avramidis

Laboratory of Geology for Aquatic Systems, Department of Aquaculture and Fisheries Management,Technological Educational Institute of Messolongi, 30200, Messolonghi, Greece

(Received 12 June 2012; final version received 31 December 2012)

Total Organic Carbon (TOC) and Total Nitrogen (TN) were simultaneously determined in watersamples, applying the high-temperature (720C) catalytic combustion (Pt/Al2O3) oxidationmethod. The detection of TOC and TN was performed using an infrared or a chemiluminescencedetector, respectively. Following current law legislations as well as the need to provide accurateand repeatable results, a systematic work has been successfully undertaken for the accreditationaccording to EN ISO 17025/99. Method characteristics (range of measurement, calibration,method detection level, repeatability, reproducibility, precision, accuracy) and measurementuncertainties were determined, while participation in proficiency testing schemes was also imple-mented. The abovemethodswere tested for the study ofwater purification from organic pollutants.To this end, organic dyes have been used as model organic pollutants and anionic hydrogels havebeen used as sorbents. With respect to the colorimetric techniques, TOC-TN characterization isproved to be a reliable tool with significantly lower detection limits of the organic load.

Keywords: water analysis; total organic carbon (TOC); total nitrogen (TN); data quality;water purification; sorbents

1. Introduction

The presence of organic matter in aquatic systems and liquid wastes has attracted an intensiveresearch interest concerning environmental studies [13]. Thus, Total Organic Carbon (TOC) isconsidered as one of the main factors giving quantitative information for the control of waterquality [4,5]. During the last years, owing to the increasing use of TOC analysis in manyapplications related to water samples, numerous international rules and guidelines about theapplied analytical methods have been imposed [6].

One of the most widely used analytical methods is the Combustion-Infrared method,Standard Method (SM) 5310B (Standard Methods for the Examination of Water and WasteWater, American Water Works Association, 21st Edition, 2005) [7]. In this method, applicablefor a variety of samples, a microportion is injected into a heated reaction chamber packed withan oxidative catalyst such as cobalt oxide, platinum group metals or barium chromate. Theproduced CO2 from oxidation of organic and inorganic carbon is measured by means of a non-dispersive infrared analyzer. Because Total Carbon (TC) is measured this way, Inorganic Carbon(IC) has to be measured separately by injecting the sample into a reaction chamber where it isacidified. Then, TOC is obtained by subtraction (TOC = TC-IC).

*Corresponding author. [email protected] article was originally published in this issue with errors. This version has been corrected. Please seeErratum (http://dx.doi.org/10.1080/03067319.2014.892753)

Intern. J. Environ. Anal. Chem., 2014Vol. 94, No. 1, 6576, http://dx.doi.org/10.1080/03067319.2013.763940

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Taking into account the environmental significance of nutrient concentrations in natural waters,the determination of Total Nitrogen (TN) is another subject of interest in the routine laboratoryanalysis of potentially polluted waters [8,9]. During the last years high temperature catalyticoxidation with a chemiluminescence detector has started to be applied for TN analysis [10].Therefore, high-temperature catalytic combustion was investigated for simultaneous determinationof TOC and TN. Using commercially available instruments, a simple, fast and small volumedemanding methodology was provided by coupling a TOC-analyzer to a chemiluminescencedetector for determination of NOx gases (ASTM International, Designation: D5176-08) [11].

Because of current law legislations as well as the need to provide accurate and reliableresults many laboratories apply a quality management system according to EN ISO 17025/99standard (ISO/IEC 17025 1999) [12]. Moreover, as a consequence of the law legislationsproviding sensitive state-of-the-art tools and reliable protocols to ascertain the data quality inwater analysis is an important need for scientists involved in the characterization or monitoringof aqueous environmental samples. So, in this paper, we present the methodology for theaccreditation of the simultaneous determination of TOC and TN, using a modern instrumentproviding both features. The parameters selected to be investigated in the present work, TOCand TN, are of special importance, since they are related with the determination of organic loadand nutrients in environmental water samples.

For both TOC and TN investigations, we focused on the reliable determination of a wide variety ofmethods characteristics such as range of measurement, calibration, method detection level, repeat-ability, reproducibility, precision, accuracy and measurements uncertainties. Moreover, these meth-odologies were further tested through participating in proficiency testing schemes, as a part of the workundertaken for the accreditation of the Laboratory of Geology for Aquatic Systems at TechnologicalEducational Institute of Messolonghi for the simultaneous TOC and TN determination.

The above methods are used in our laboratory for monitoring aqueous environmentalsamples as well as on water purification studies from organic pollutants. Among others, forthe water purification studies, anionic poly (sodium acrylate), PANa, hydrogels bearing nega-tively charged COO groups were used as sorbents for the removal of model coloured organicdyes. In our previous studies [13,14], it has been shown that these hydrogels selectively adsorband retain both organic and inorganic cations from aqueous solutions. In these works, theadsorption capacity of the hydrogels against the different types of the studied model pollutantswas determined using UV-Vis absorption spectrophotometry. In the present work, using CrystalViolet as a model organic pollutant, we demonstrate that the determination of TOC-TN is apromising tool for studying organic pollutants at very low concentrations, offering the oppor-tunity for a precise determination of the adsorption capacity of the studied sorbents.

2. Experimental

2.1 Validation of TOC-TN determination

2.1.1 Apparatus

All analyses were carried out using a Shimadzu TOC analyzer (TOC-VCSH) coupled to achemiluminescence detector (TNM-1 TN unit), creating a simultaneous analysis system.

2.1.2 Materials

Ultra pure water of conductivity

Purified air, CO2 free, containing less than 1 ppm hydrocarbon was used as carrier, purgingand reactive gas.

Anhydrous primary-standard-grade potassium biphthalate (C8H5KO4, Merck) was usedfor the preparation of the organic carbon stock solution by dissolving 2.1254 g in ultra purewater and dilution to 1000 mL; 1.00 mL = 1.00 mg carbon. The inorganic carbon stocksolution was prepared by dissolving 4.4122 g of anhydrous sodium carbonate (Na2CO3) and3.497 g of anhydrous sodium bicarbonate (NaHCO3, Merck) in water and diluting to1000 mL; 1.00 mL = 1.00 mg carbon. The nitrogen stock solution was prepared by dissolving7.219 g of anhydrous special reagent grade potassium nitrate (KNO3) in ultra pure water anddilution to 1000 mL; 1.00 mL = 1.00 mg nitrogen. All standard solutions for calibration orquality control of TC, IC and TN were prepared by proper dilution of the above stocksolutions. All volumetric glass-ware used for the preparation of standard solutions was ofClass A.

2.1.3 Methodology

TOC analysis was performed using the Combustion-Infrared method, Standard Method (SM)5310B (Standard Methods for the Examination of Water and Waste Water, American WaterWorks Association). The principle of this method is that a microportion of the sample is injectedinto a heated reaction chamber packed with an oxidative catalyst, which in our case is Pt/Al2O3.The water is vaporized and the organic and inorganic carbon is oxidized to CO2 and water. TheCO2 is transported in the carrier gas streams (purified air) and is measured by means of a non-dispersive infrared analyzer (NDIR analyzer). Because in this way Total Carbon (TC) ismeasured, Inorganic Carbon (IC) is measured separately. The origin of inorganic carbon isCO2 and carbonate salts dissolved in water. Acidifying the sample with HCl acid at pH

Adjustment to optimum combustion temperature (720C) and monitoring to insurestability

Applicability of the methods: working ranges according to the ranges of water samples areto be analyzed

Calibration: calibration and determination of linear response for TC, IC and TN workingrange

Analytical quality: repeat injection until consecutive measurements are obtained that arereproducible to within 10% and validity checks of the calibration functions every threemonths

2.2 Study of the adsorption capacity of PANa hydrogels using TOC-TN measurements

2.2.1 Materials

Acrylic acid (AA), N,N,N,N-tetramethylethylenediamine (TEMED) and methylene bisacry-lamide (BIS) were purchased from Aldrich, while ammonium persulfate (APS) was aServa product. Crystal Violet (CV) was purchased from Aldrich. All reagents were used asreceived.

2.2.2 Synthesis of the hydrogels

The PANa hydrogels were synthesized according to the following procedure: 0.05 mol AA and0.001 mol BIS (crosslinker, 2 mol% over the total monomer concentration) were dissolved in40 mL ultrapure water under stirring at room temperature. The pH of the solution was adjustedto pH = 56 by addition of adequate volume of a NaOH 1M solution. After deoxygenation ofthe solution by N2 bubbling for 30 min, 0.1 g APS (dissolved in 2 mL ultrapure water) and threedrops of TEMED were added. The formation of the gel was almost immediate. The reaction wasleft to proceed for 24 h and, then, the gel was fully neutralized to PANa form by adding anexcess of the NaOH 1M solution (pH = 13). After 24 h, the gel was immersed in ultrapurewater. Water was decanted and renewed daily for 1 week. Finally, the purified swollen gel wascut into ~2 mL pieces and water was removed by freeze-drying.

2.2.3 Adsorption of crystal violet from aqueous solutions

The adsorption of Crystal Violet was studied by the following procedure: ~0.02 g of dryhydrogel was introduced in 100 mL of aqueous solution of Crystal Violet and was left toadsorb for 24 h. Then the gel was removed from the solution and the remaining quantity of thedye was monitored by TOC and TN measurements. The experiments were carried out for fivedifferent initial concentrations of the organic dye, while the studied concentration range wasfrom 20 (6 mg C/L, 0.84 mg N/L) to 1 m (300 mg C/L, 42 mg N/L).

3. Validation of TOC-TN determination-data analysis

3.1 Range of measurements

The range of measurements which are presented in Table 2, were dictated by the range of watersamples that are to be analyzed in the Laboratory of Geology for Aquatic Systems and currentlaw legislations. So, in the case of TC and IC were 0.5100 mg C/L and for TN was 0.5100 mg N/L. These ranges could be extended by sample dilution.

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3.2 Calibration-linearity

When the analytical system was first evaluated, and at three-month intervals afterwards,calibration functions of each parameter were established by analyzing at least five standardsolutions at levels dictated by the range of measurement (Table 2). Calibration curves wereconstructed by plotting peak area for each parameter against concentration. The quality require-ment for the acceptance of the calibration functions was: correlation coefficient R2 0.995[15].Linear calibration functions were established for TC, IC and TN methods. The representativecalibration curves for TC, IC and TN are presented in Figure 1.

3.3 Limit of detection (LOD) level of quantification (LOQ)

Limit of detection (LOD) is the lowest concentration level that can be determined to bestatistically different from a blank (99% confidence). One very important determination thatmust therefore be made is how large a signal needs to be before it can be distinguished from thebackground noise associated with the instrumental measurement. Various criteria have beenapplied for this determination; however, the generally accepted rule in analytical chemistry isthat the signal must be at least three times greater than the background noise [16]. So in ourcase, LODs were determined for each parameter by analyzing 15 portions of ultrapure waterover a period of five days. The standard deviation (s) of the replicate measurements wascalculated and LODs were defined as the concentration of analyte required to give a signal

Table 2. Method characteristics.

ParameterRange of measurement

(mg/L) LOD LOQ (mg/L) Calibration levels (mg/L)

TC 0.5100 0.150 (mg C/L) 0.290 (mg C/L) 0.1-0.2-0.5-1-2-5-10-20-50-100IC 0.5100 0.140 (mg C/L) 0.280 (mg C/L) 0.1-0.2-0.5-1-2-5-10-20-50-100TN 0.5100 0.09 (mg N/L) 0.160 (mg N/L) 0.1-0.2-0.5-1-2-5-10-20-50-100

0 20 40 60 80 100

Concentration (mg/L)

0

400

800

1200

1600

2000

Pea

k A

rea

(a.u

.)

x4A

B

C

Figure 1. Representative calibration curves for TC (A), IC (B) and TN (C). The notation 4 for curve Bmeans that the values for IC peak area are multiplied by a factor of 4, in order to be distinguishable usingthe same axis scale.

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equal to the background (blank) plus three times the standard deviation of the blank. Theconcentration of analyte (TOC, TN) which gives a signal equal to the blank sample is defined asmean concentration in blank samples. LOQs were calculated by the following equation:

LOQ mean concentration in blank samples 10 standarddeviation (1)

The LOD and LOQ results for all measurements are presented in Table 2.

3.4 Method precision and accuracy data-proficiency testing results

Precision is a measure of the random error with a series of repeated measurements of the sameparameter within a sample. In our case methods precision was determined in terms of repeat-ability and reproducibility and quantified by the repeatability and reproducibility standarddeviation Sr and SR, respectively.

Repeatability data were obtained by replicate analysis of ten portions from one standardsolution at four concentration levels the same day by the same analyst.

Reproducibility data were collected by analyzing over a period of two months the samestandard solutions (once every five days), by two different analysts. Repeatability and reprodu-cibility data are presented in Table 3. Method accuracy was also checked by participation inproficiency testing schemes and the results are also presented in Table 3.

3.5 Measurement uncertainty

The combined and expanded uncertainties for TOC and TN parameters were evaluated in two steps:(1) identification of the uncertainty sources and their quantification (2) calculation of combined andexpanded uncertainties [17]. In order to identify the uncertainty sources we took into accountthat the concentration C of TOC or TN in the sample is calculated by the following equation:

Table 3. TOC and TN precision and accuracy data-proficiency testing results.

Repeatability Reproducibility

ParameterConcentration

(mg/L)Xmean (n = 20)

(mg/L) Sr%

RSDrXmean

(n = 25) (mg/L) SR%

RSDR

TOC 0.500 0.48 0.02 4.17 0.48 0.02 4.17TOC 5.000 5.27 0.06 1.14TOC 10.00 10.36 0.02 0.23TOC 50.00 49.43 0.10 0.20 50.02 0.56 1.12TOC 90.00 89.15 0.25 0.28TN 0.500 0.54 0.02 3.71 0.52 0.02 3.82TN 5.000 5.12 0.07 1.40TN 10.00 10.18 0.08 0.80TN 50.00 50.56 0.32 0.64 50.78 0.34 0.68TN 90.00 90.43 0.59 0.66Proficiency testing resultsParameter Aquacheck

Round 386Lab Value z score

AquacheckRound 403Lab Value z score

AquacheckRound 406Lab Value z score

TOC 10.84 mg C/L 0.29 6.57 mg C/L 0.48TN 35.2 mg N/L 0.06


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C A B0B1

Fd Fr FR (2)

where:

C is the concentration of TOC or TN in the sample (mg L1)A is the peak area of the sampleB1 is the slope of the calibration curveB0 is the intercept of the calibration curveFd is the dilution factor of the sampleFr is the repeatability factorFR is the recovery factor

Therefore, the sources of uncertainty are: u(A), which is the uncertainty of the measurementof the peak area, u(B0), which is the uncertainty of the intercept of the calibration curve, u(B1),which is the uncertainty of the slope of the calibration curve, u(Fd), which is the dilution of thesample, u(Fr), which is repeatability and u(FR), which is recovery. Following the proceduredescribed in Ref. [17] all the above uncertainties were evaluated for TOC and TN and arepresented in Table 4. The uncertainty of the concentration of TOC and TN in a sample wasdetermined by combining all the above standard uncertainties using the rule for the propagationof errors. To calculate the expanded uncertainty of the result of a measurement at the 95%confidence level, the result of the combined uncertainty (U) was multiplied by a coverage factork of 2.

3.6 Quality control

The calibration functions are tested for their validity every 15 samples using one TOC and oneTN standard solution at concentration 50 mg/L which is in the middle of the calibration curve.Results are plotted in Shewhart control charts separately for each parameter. According to theinstructions given, corrective decisions have to be made when the measurements are out ofthe 3s limits. Moreover, external quality control is also implemented by regular participation inproficiency testing schemes.

4. Results and discussion

4.1 Validation of TOC-TN determination qualification of the methods

TOC is the measuring technique used to monitor the total amount of organic content in watersamples. During the last years this technique is extensively used, especially due to the wide fieldof applications related to water (surface, tap water, waste water). Concerning environmental

Table 4. Sources of uncertainty for TOC and TN measurements-combined uncertainty.

Parameter u(A) No unit u(B0) No unit u(B1) L/mg u(Fd) No unit u(Fr) No unit u(FR) No unit

TOC 0.64 0.818 0.084 0.000089 0.0317 0.015TN 0.52 0.213 0.017 0.000089 0.0121 0.011

Combined uncertaintyConcentration level (mg/L) Uncertainty (mg/L)

TOC 50.00 3.12TN 50.00 2.23


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samples TOC was developed as a new technique to replace the time-consuming COD (ChemicalOxygen Demand) and BOD (Biochemical Oxygen Demand) measurements, conventionallyused in environmental monitoring. Moreover, as COD results in the production of hazardouswastes such as mercury and hexavalent chromium [18], the success of this replacement has beenexamined in literature [1820] and the results show that TOC correlates very well with BODand COD, while the fastness of determination enables prompt action when pollution is detected.Concerning TN analysis it is well known that TN is a typical nutrient monitoring index giving afast and quantitative analysis of the temporal and spatial distribution characteristics of nutrientsubstances enabling to adequately estimate the state of environment and describe environmentalchange processes conditioned by anthropogenic forces [21]. These findings indicate thatachieving accurate TOC and TN detection with ensured analyte recovery is necessary. So, ourstudy demonstrates how to develop and validate both methods for simultaneous environmentalmonitoring applications.

To our knowledge no reports are available for validation of TOC and TN for such applica-tions. Concerning only TOC analysis previous validation studies come from pharmaceuticalindustry research where this technique is used for cleaning validation in pharmaceuticalmanufacturing [22]. The comparison of our results with these literature data results, first,confirms the good linear relationship between the measured compound concentration and theTOC response of the analyzer. Moreover, the LOD and LOQ of our results are comparable to therespective literature data [22], indicating the high sensitivity of this method. Finally, in this study wefound that precision and accuracy was less than 5% in all studied concentration areas, while theextended uncertainty of the measurement was less than 7%, which is in full agreement with therespective literature data. TN validation study gave also the same satisfactory results concerningsensitivity, precision and accuracy, demonstrating that both analysis are suitable for measuringwith low detection limit organic load and nitrogen nutrients in environmental samples.

4.2 Study of the adsorption capacity of PANa hydrogels towards Crystal Violet using TOC-TN measurements

Cross-linked poly(sodium acrylate), PANa (Figure 2), is a synthetic hydrophilic polymerbearing carboxylate anions along the macromolecular chain [23,24]. This material probablyrepresents the simplest case of an anionic hydrogel, able to adsorb cationic species, likepositively charged pollutants or organic dyes [25] and higher valence metal ion [26]. Thecombination of electrostatic attractions with hydrophobic interactions is usually the drivingforce in the case of cationic organic species, while polymer/metal ion coordination complexesare often formed when higher valence metal ions are involved.

The aqueous Crystal Violet (Figure 2) solution is strongly coloured blue-violet as a result ofa strong absorption band with high extinction coefficient centered at ~600 nm. As discussed inprevious studies [13,14], after treatment with PANa hydrogel, absorbance of the solution washardly detectable indicating that most of the organic dye molecules has been removed fromwater under these experimental conditions. Instead, the initially transparent PANa hydrogel wasthen strongly-coloured, providing a direct visual evidence for the strong adsorbing capacity ofthe hydrogel towards Crystal Violet. In the present work, instead of UV-Vis absorption spectro-photometry, we employ TOC-TN measurements to determine the quantity of the organic dyeremaining in solution and the quantity extracted by the hydrogel.

The Langmuir isotherms for the adsorption of Crystal Violet in mg C/L and in mg N/L arepresented in Figures 3 and 4 respectively. In these representations, the quantity Qe expresses themg of the adsorbed substance per gram of the adsorbent and it is calculated by the followingequation [27]:


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Qe C0 CeVm (3)

where C0 is the initial concentration of the studied substance (in mg/L), V is the volume of thesolution used for the adsorption (in L) and m is the mass of the adsorbent (in g). Figure 5 showsthe variation Ce/Qe versus Ce. It represents an analysis of the Langmuir isotherm and it producesthe binding constant KL by the following equation:

CeQe

1KL

1Qe;max

Ce (4)

where KL is the binding constant expressed in L/g and Qe,max is the maximum quantity of theadsorbed substance per gram of the adsorbent. From this study it is verified that Crystal Violet isstrongly adsorbed from this gel with adsorption percentage almost 100% at the lower studiedconcentrations. The values of the adsorption parameters are calculated to Qe,max = 275 mg C/gor 38.6 mg N/g and KL = 833 L/g, which can be compared with previously published results forstrong adsorption [13,14,27]. If the results in Figures 3, 4 and 5 are expressed in mmol of dyeinstead of mg C or mg N the two curves coincide, as expected.

COO-

Na+COO-

Na+

N

N

NH2+

CI-

COO-

Na+

COO-

Na+

COO-

Na+

COO-

Na+COO-

Na+

Na+

COO-

COO-

Na+

-OOCNa+

-OOCNa+

COO-

Na+

Figure 2. Chemical structure of Crystal Violet and schematic depiction of the PANa hydrogel used in thepresent work.


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In the respective photometric study the variation of Ce/Qe with Ce gave straight lines,indicating that adsorption obeys the Langmuir model [14]. Moreover, the intercept of the linewas close to 0. This suggested that the binding constant KL was very large for the adsorbed dye,in agreement with previously reported results where cross-linked copolymers of sodium acrylatewith N,N-dimethylacrylamide were investigated [13]. However, it was not possible to reliablydetermine the accurate value of KL from those experimental data, as a reliable determination ofCe in very dilute solutions was needed [14]. So, the most important result of this study is thereliable determination of the accurate value for KL from the present experimental data, since areliable determination of Ce in very dilute solutions was possible.

0 100 200

Ce (mg C/L)

230

240

250

260

270

280

Qe

(mg

C/g

)

Figure 3. Plot of the quantity Qe of the adsorbed Crystal Violet (in mg C/L) as a function of theconcentration Ce (in mg C/L) of the dye remaining in solution.

0 10 20 30 40

Ce (mg N/L)

30

32

34

36

38

40

Qe

(mg

N/g

)

Figure 4. Plot of the quantity Qe of the adsorbed Crystal Violet (in mg N/L) as a function of theconcentration Ce (in mg N/L) of the dye remaining in solution.


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5. Conclusions

Two fully validated methods for the simultaneous determination of TOC and TN in watersamples were developed. The procedure was undertaken according to ISO 17025. This studydemonstrates that TOC and TN analysis are suitable for measuring organic load and nitrogennutrients in environmental samples giving reliable results with low limits of detection, excellentlinearity, precision, and accuracy. Moreover, it was also shown that TOC-TN measurements canbe used in water purification studies from organic pollutants giving a reliable determination ofthe organic load in very dilute solutions. This is of special interest when the organic pollutantscannot be readily detected otherwise.

AcknowledgementsThis research has been co-financed by the European Union (European Social Fund ESF) and Greeknational funds through the Operational Programme Education and Lifelong Learning of the NationalStrategic Reference Framework (NSRF) Research Funding Program: Thales. Investing in knowledgesociety through the European Social Fund; research project Thales: Design and Development of NovelFunctional Copolymers with a Block Architecture Self-organization Abilities and Controlled Binding/Release of Biocidal Species.

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0 50 100 150 200 250

Ce (mg C/L)

0.0

0.2

0.4

0.6

0.8

1.0

Ce

/Qe

(g/L

)

A

B

Figure 5. Plot of the quantity Ce/Qe of the adsorbed Crystal Violet as a function of the concentration Ceof the dye remaining in solution (A) in mg C/L and (B) in mg N/L.


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Abstract1. Introduction2. Experimental2.1. Validation of TOC-TN determination2.1.1. Apparatus2.1.2. Materials2.1.3. Methodology

2.2. Study of the adsorption capacity of PANa hydrogels using TOC-TN measurements2.2.1. Materials2.2.2. Synthesis of the hydrogels2.2.3. Adsorption of crystal violet from aqueous solutions

3. Validation of TOC-TN determination-data analysis3.1. Range of measurements3.2. Calibration-linearity3.3. Limit of detection (LOD) level of quantification (LOQ)3.4. Method precision and accuracy data-proficiency testing results3.5. Measurement uncertainty3.6. Quality control

4. Results and discussion4.1. Validation of TOC-TN determination qualification of the methods4.2. Study of the adsorption capacity of PANa hydrogels towards Crystal Violet using TOC-TN measurements

5. ConclusionsAcknowledgementsReferences

data quality in water analysis toc tn

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