Uncertainty of nitrate and sulphate measured by ion chromatography in wastewater samples

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1 DOI: /s Research artcle CEJC 5(2) Uncertanty of ntrate and sulphate measured by on chromatography n wastewater samples Brgta Tepuš 1, Marjana Smonč 2 1 Komunalno podjetje Ptuj, 2250 Ptuj, Slovena 2 Faculty of Chemstry and Chemcal Engneerng, 2000 Marbor, Slovena Receved 27 June 2006; accepted 27 November 2006 Abstract: Ths paper presents an evaluaton of measurement uncertanty regardng the results of anon (ntrate and sulphate) concentratons n wastewater. Anons were determned by on chromatography (EN ISO , 1996). The major sources of uncertanty regardng the measurements results were dentfed as contrbutons to lnear least - square or weghted regresson lnes, precson, trueness, storage condtons, and samplng. Determnaton of anons n wastewater s very mportant for the purfcaton procedure, especally the amount of ntrate n waste and potable waters. The determned expanded uncertanty was 6.1 % for ntrate anons and 8.3 % for sulphate anons. The dfference between measurement uncertantes determned by the two methods, the weghted and lnear least - square methods, s neglgble. c Versta Warsaw and Sprnger-Verlag Berln Hedelberg. All rghts reserved. Keywords: Ntrate, sulphate, measurement uncertanty 1 Introducton The determnaton of ntrate and sulphate anons by on chromatography plays a key role n assgnng value to the purfcaton process regardng wastewater. The non-lnearty of the calbraton curves could be an mportant source of error [1]. Ths paper demonstrates, that there s a very small dfference between the measurement uncertantes determned by lnear least - square and weghted regresson, respectvely. The determnaton of ntrate and sulphate anons content plays a key role n assgnng E-mal: cstlna.lab@komunala-ptuj.s E-mal: marjana.smonc@un-mb.s

2 558 B. Tepuš and M. Smonč / Central European Journal of Chemstry 5(2) value to the purfcaton process of wastewater or potable water. The purpose of ths paper s to ntroduce a procedure for evaluatng any uncertanty regardng the results for ntrate and sulphate concentraton obtaned by on chromatography and to dentfy the steps n the analytcal procedure whch could be mproved, the goal of whch s the reducton of the overall measurement uncertanty. The process to evaluate the uncertanty durng the ntrate and sulphate anons determnaton s dvded nto the followng components: descrpton of the method, dentfcaton of uncertanty sources, quantfcaton of uncertanty components, and calculaton of combned and expanded uncertanty. 2 Expermentaton Internatonal Standard Water qualty - Determnaton of dssolved anons by lqud chromatography of ons-part 2 [2] provdes gudance for the determnaton of ntrate and sulphate n wastewater samples. Ths standard apples to those water samples contanng ntrate content rangng from 0.1 to 50 mg/l and sulphate from 0.1 to 100 mg/l, respectvely. In our experments, the lmts of detecton (LOD) for ntrate and sulphate were determned at 1.2 mg/l. LOD was determned by means of standard addton (10 replcates were made for each soluton). The lmts of quantfcaton (LOQ) regardng ntrate and sulphate were determned at 1.7 mg/l. LOQ was calculated from 10 replcates made for a seres of 5 dfferent concentratons. Relatve standard devaton (RSD) was determned to be 3 % for ntrate and 5% for sulphate ons. A range between 1.7 and 50 mg/l was appled for both anons due to the old type of IC and ts poorer performance at lower concentratons. Hgher concentraton of ntrate (or sulphate) can be determned by approprate dluton. Selectvty was determned by the standard addton method. Certan concentratons of standard solutons of ntrate and sulphate were added to the dstlled water. The recovery was between 93 and 105% for the determnaton of ntrate and between 102 and 108% for the sulphate determnaton. Lqud chromatographc separaton of ons was performed by means of a separatng column nstrument for on chromatography (Donex DX-100). Samples were cooled down to 2-5 C and analyzed wthn 48 hours after samplng. Samples were fltered usng black flter papers from Sartorus, Germany (pore sze 0.45 μm) to prevent adsorpton of the anons onto partculate matter or the converson of anons by bacteral growth. Samples were then njected by syrnge wth cellulose acetate flter (pore sze 0.2 μm) nto the guard columns (IonPac AG12A, 4 50 mm, Donex and IonPac AS12A, mm, Donex). A conductvty detector cell was used [3]. A soluton of Na 2 CO 3 (c = mol/l) and NaHCO 3 (c = mol/l) was used as eluent. The ntrate and sulphate certfed reference solutons (C CRM NO3, C CRM SO4 )were purchased from Merck, Germany wth the followng concentratons (Eqs. 1 and 2): C CRM NO3 =(0.997 ± 0.005) g/l (1)

3 B. Tepuš and M. Smonč / Central European Journal of Chemstry 5(2) C CRM SO4 =(1.000 ± 0.002) g/l (2) Workng reference solutons (C NO3 SO 4 ) 1 were prepared by dluton of the certfed reference soluton to 80 mg/l, 50 mg/l and 36 mg/l, respectvely. By further dluton of the workng reference soluton of γ = 80 mg/l, workng solutons (C NO3 SO 4 ) 2 were prepared wth mass concentratons of 2 mg/l, 4 mg/l, 8 mg/l, 16 mg/l and 24 mg/l (the same for both anons). Adequate concentratons (C ) for workng reference solutons (C NO3 SO 4 ) 1 and workng solutons (C NO3 SO 4 ) 2 were determned by (Eq. 3). C = C CRM f d = C CRMV ppette V 100 (3) C CRM g/l V 100 volume of one mark volumetrc flask, L V ppette volumes of ppettes, L f d dluton factor. Thus, the calbraton curve was establshed by measurng a seres of ntrate and sulphate solutons mxed together, rangng from 2 mg/l to 50 mg/l (see above) of ntrate and sulphate ons, respectvely. The concentratons of each workng reference soluton and workng soluton were measured n three replcates. A new calbraton curve was establshed every tme new batches of reagents or columns were used. The soluton wth the same concentraton of the ntrate and sulphate anons was always poured nto the same lab-calbrated flask. Control charts were created from the results obtaned durng the analyss of laboratory workng reference standards. The laboratory partcpated n profcency tests (Aqua check, UK) and a good performance for determnaton was obtaned. 2.1 Identfcaton of the uncertanty sources Sources of uncertanty are: uncertanty of concentraton of workng reference solutons (C NO3 SO 4 ) 1 uncertanty of concentraton of workng solutons (C NO3 SO 4 ) 2 measurement uncertanty of area counts (A ) from conductvty detector: flow of the effluent volume of the sample conductvty of the moble phase pressure ambent and column temperature uncertanty of weghted or lnear least - square regresson (b w,a w,b 1,B 0 ) effluent preparaton ntercept of the calbraton curve slope of the calbraton curve

4 560 B. Tepuš and M. Smonč / Central European Journal of Chemstry 5(2) measurement uncertanty of area counts (A) n sample dluton of the sample (F d ) repeatablty (F r ) recovery (F re ) storage condtons (F h ) samplng procedure (F s ) The sources of uncertanty n ntrogen and sulphate on determnaton are schematcally presented n Fgure 1. C A C sample dluton 1 volume temperature volume dluton 2 temperature volume weghted regresson or lnear least square regresson CRM flow temperature conductvty pressure area counts dluton temperature volume C NO3-SO4 repeatablty (precson study) storage recovery (trueness study) samplng Fg. 1 Cause and effects dagram for the determnaton of ntrate and sulphate, usng the on chromatography method. 2.2 Quantfcaton of components uncertanty The uncertanty assocated wth the volumetrc flask s volume depends on the uncertanty regardng the volume of the volumetrc flask tself, on the uncertanty assocated wth the use of volumetrc equpment at a temperature dfferent from that of calbraton, and on the repeatablty of volume delvery when preparng the workng solutons (C ).The accuracy lmt of the volumetrc flask s volume was ndcated as type B uncertanty by the manufacturer, wth no dstrbuton data. Trangular dstrbuton was assumed, therefore, the correspondng values were dvded by 6. The uncertanty of the temperature effect was calculated usng an estmate of the temperature range and the volume expanson coeffcent. A temperature varaton of ± 5 C was taken as a reasonable estmate wth 95% confdence. The volumetrc expanson coeffcent of the water was C 1. The repeatablty of volume delvery durng preparaton of the stock reference soluton was determned expermentally by a seres of fll and weght experments on a volumetrc flask. All uncertanty contrbutons were then combned to obtan the volume uncertanty of the volumetrc flask u(v 100 ). The uncertanty sources for delvered volume from the ppettes are: the repeatablty

5 B. Tepuš and M. Smonč / Central European Journal of Chemstry 5(2) of the delvered volume, the uncertanty of the calbraton of that volume and the uncertanty resultng from the dfference between the temperature n the laboratory and that of the calbraton of the ppettes. Glass ppettes, class A were used. The slope of the weghted regresson lne b w and ntercept a w was calculated usng the equatons from 4 to 8. Data for ntrate and sulphate anons were determned to be heteroscedastc. The weghted regresson lne had to be calculated to gve addtonal weght to those ponts where the error bars were the smallest: t s more mportant for the calculated lne to approxmate such ponts than to approxmate the ponts representng hgher concentratons wth larger errors. Ths result s acheved by gvng each pont a weghtng nversely proportonal to the correspondng varance (s 2 ). Thus, f the ndvdual ponts are denoted by (x 1,y 1 ), (x 2,y 2 )etc., then the ndvdual weghts,w 1,w 2, etc., are gven by Eq. 4 [4]. w = s 2 s 2 /n w x y n x w ȳ w b w = w x 2 n x 2 w (4) (5) w weghts s 2 varance b w weghted slope of calbraton curve a w weghted ntercept of calbraton curve a w =ȳ w b w x w (6) n number of calbraton ponts x w and ȳ w represent the co-ordnates of the weghted centrod through whch the weghted regresson lne must pass. These coordnates are gven by Eqs. 7 and 8. The weghted centrod s much closer to the orgn of the graph than the unweghted centrod, and the weghtng gven to the ponts nearer the orgn ensures that the weghted regresson lne has an ntercept very close to ths pont. Because of ths, the weghted regresson method provdes more realstc results for error estmaton, n theory. So we decded to determne the practcal results. w x x w = (7) n w y ȳ w = (8) n x w, ȳ w co-ordnates of the weghted centrod n number of calbraton pont The slope of the lnear least - square calbraton curve B 1 and calculated blank B 0

6 562 B. Tepuš and M. Smonč / Central European Journal of Chemstry 5(2) were calculated usng Eqs. 9 to 12. B 1 = C concentraton of ndvdual ponts C concentraton of sample A area counts of ndvdual ponts A area counts of the sample (C C ) (A A ) (C C ) 2 (9) B 0 = A B 1 C (10) A = 1 A (11) n C = 1 C (12) n n number of calbraton pont The concentraton of ntrate or sulphate (C NO3 SO 4 ) was calculated from Eq. 13 for weghted regresson and from Eq. 14 for lnear least - square regresson. B 1 slope of calbraton curve B 0 ntercept of calbraton curve F d dluton factor of sample C NO3 SO 4 = A a w b w F d F r F re F h F s (13) C NO3 SO 4 = A B 0 B 1 F d F r F re F h F s (14) F r repeatablty factor (precson study) F h storage factor F re recovery factor (trueness study) F s factor for samplng procedure 3 Results and dscusson 3.1 Quantfcaton of the components uncertanty Concentraton uncertanty of workng reference solutons (C NO3 SO 4 ) 1 The concentratons of ntrate and sulphate ons for workng reference solutons were calculated usng Eqs. 1 to 3. The values were as follows: g/l, g/l and g/l for ntrate and g/l, g/l and g/l for sulphate, respectvely. The concentraton uncertanty of the workng reference soluton s assocated wth

7 B. Tepuš and M. Smonč / Central European Journal of Chemstry 5(2) the uncertanty of the certfed reference soluton, the volume of the volumetrc flask for the preparaton of the workng reference soluton, and the volumes of ppettes (3.6 ml, 5 ml and 8 ml). The rectangular dstrbuton was assumed for the certfed reference soluton. For calculatng the concentraton uncertanty of the workng reference solutons, the data presented n Table 1 for ntrate were used, plus those presented n Table 2 for sulphate. Table 1 Uncertanty components and ther relatve standard uncertanty of ntrate workng reference solutons. Unt Value Uncertanty u Uncertanty (RSD=u/Value) C NO3 g/l V 8 L V 5 L V 3.6 L V 100 L Table 2 Uncertanty components and ther relatve standard uncertanty of sulphate workng reference solutons. Unt Value Uncertanty u Uncertanty (RSD=u/Value) C SO4 g/l V 8 L V 5 L V 3.6 L V 100 L The concentraton uncertanty of workng reference solutons (C NO3 SO 4 ) 1 were calculated usng Eq. 15 and were determned to be g/l for C NO3 80, g/l for C NO3 50, g/l for C NO3 36, g/l for C SO4 80, g/l for C SO4 50 and g/l for C SO4 36. u(c NO3 SO 4 ) 1 =(C NO3 SO 4 ) 1 u(c CRM NO 3,CRM SO 4 ) 2 + u(v ppette) 2 CCRM NO 2 3,CRM SO 4 Vppette 2 + u(v 100) 2 V (15) Concentraton of workng solutons(c NO3 SO 4 ) 2 The calculated values obtaned from Eq. 3 were 1.99 mg/l, 3.99 mg/l, 7.98 mg/l, mg/l and mg/l for ntrate, and 2 mg/l, 4 mg/l, 8 mg/l, 16 mg/l and 24 mg/l for sulphate, respectvely. The uncertantes of the concentratons of the workng calbraton solutons for curves were assocated wth the uncertanty of the concentraton of workng reference solutons of ntrate or sulphate (80 mg/l), the volume of ppette

8 564 B. Tepuš and M. Smonč / Central European Journal of Chemstry 5(2) (2.5 ml, 5 ml, 10 ml, 20 ml or 30 ml) and volume of the 100 ml volumetrc flask for preparaton of workng solutons (see Tables 3, 4). Table 3 Uncertanty components and ther relatve standard uncertanty of ntrate workng soluton. Unt Value Uncertanty u Uncertanty (RSD=u/Value) V 2 L V 4 L V 8 L V 16 L V 24 L C NO3 80 g/l V flask100 L The concentraton uncertantes of the workng reference solutons were calculated usng Eq. 16. They were g/l forc NO3 2, g/l forc NO3 4, g/l forc NO3 8, g/l for C NO3 16 and g/l forc NO3 24. u(c NO3 SO 4 ) 2 =(C NO3 SO 4 ) 2 u(c NO 3 80, SO 4 80) 2 C 2 NO 3 80, SO u(v ppette) 2 V 2 ppette + u(v 100) 2 V (16) Table 4 Uncertanty components and ther relatve standard uncertanty of sulphate workng soluton. Unt Value Uncertanty u Uncertanty (RSD=u/Value) V 2 L V 4 L V 8 L V 16 L V 24 L C SO4 80 g/l V flask100 L The concentraton uncertantes of the workng reference solutons were calculated usng Eq. 16 and were g/l for C SO4 2, g/l for C SO4 4, g/l for C SO4 8, g/l for C SO4 16and g/l for C SO4 24. The measurement uncertanty of area counts (A,A) from conductvty detector: The standard measurement uncertanty of area count from the conductvty detector of solutons (A ) and of the sample (A) were estmated as the standard devatons of the mean of four replcate determnatons of each conductvty measurement, and contrbuted to the combned and expanded uncertantes n Tables 5 and 6.

9 B. Tepuš and M. Smonč / Central European Journal of Chemstry 5(2) The measurement uncertanty due to the dluton of the sample The standard uncertanty of the sample dluton factor (F d ) was estmated from the uncertanty of the volumetrc equpment and the uncertanty assocated wth the temperatures dfferent from those for calbraton, smlar to those descrbed above, and contrbuted to the combned and expanded uncertantes n Tables 5 and Precson study The precson was nvestgated at concentratons coverng the full range specfed n the method range [5]. Four concentratons were chosen for nvestgaton (0.90 mg/l, 3.06 mg/l, mg/l and mg/l for NO 3, but 1.05 mg/l, 2.68 mg/l, mg/l and mg/l for SO 2 4 ) wth 6 replcates at each concentraton level (relatve standard devatons (RSD) were 0.023, 0.013, 0.016, for ntrate and 0.030, 0.037, 0.012, for sulphate, respectvely). It was found that there s no sgnfcant dfference between the relatve standard devatons for each sample. Ths ndcates that the precson s proportonal to the analyte concentraton. The relatve standard devatons were pooled to gve a sngle estmate whch can be appled to the concentraton range covered by the precson study (Eq. 17). RSD pool NO3 SO 4 = (n 1 1).RSD1 2 +(n 2 1).RSD (17) (n 1 1) + (n 2 1) +... It was dscovered, that the RSD pool for ntrate s 1.73% and for sulphate 2.52%. 1, 2...RSD are relatvely standard devatons calculated for a sample at concentraton level 1, 2,..., and 1, 2...n are the number of replcates for these samples. The results calculated wth the lnear least - square and wth weghted regresson were combned nto one calculaton because four determnatons n the precson study were not enough to calculate the uncertanty measurement of each anon, separately Trueness study Trueness s estmated n terms of overall recovery [6, 7],.e., the rato of the observed value to the expected value. Recovery can be evaluated n a number of ways, n our case from the results of nterlaboratory tests (Eqs. 18, 19 and 20). RMS bas = (bas ) 2 n (18) u (bas )= u(c ref )= S R n (19) RMS 2 bas + u(c ref) 2 (20) bas the sum of the squares for all bases between the referenced and analyzed samples durng nterlaboratory tests,

10 566 B. Tepuš and M. Smonč / Central European Journal of Chemstry 5(2) n average number of partcpated laboratores, and S R nterlaboratory standard devaton. An uncertanty of 1.97% was determned for ntrate and 3.23% for sulphate over nne nterlaboratory tests (nne dfferent samples, each wth three replcates - Eq. 20) Uncertanty assocated wth storage condtons The standard uncertanty assocated wth the effect of storage condtons on the results was determned expermentally. The samples whch were analyzed wthn a two-week tme perod, were fltered and stored n the dark at temperatures between 2 Cand5 C. It was dscovered, that the uncertanty for ntrate was 1.24% and for sulphate, 0.55% (samples were analyzed n ten replcates each week) Uncertanty due to samplng The varance [8, 9] (replcate analyses of the same sample prepared n laboratory), sub - samplng transport varance (analyses of replcate samples taken n the feld from the bulk sample) and total samplng varance (analyss of bulk samples obtaned by separate applcaton of the samplng procedure) was determned. The comparson between the dfferent estmates of varance descrbed above can be used to dentfy the most mportant sources of measurement uncertanty. After combnng all contrbutons, the determned uncertantes were 0.61% for ntrate anons and 0.19% for sulphate anons Calculaton of combned and expanded uncertanty The standard uncertanty of nput parameters for dfferent anons s presented n Tables 5 and 6. The uncertanty of ntrate and sulphate were determned by combnng the standard uncertantes ofa,c,f d,f r,f re,f h,f s. Uncertantes were combned by usng the rule of error propagaton. To calculate the expanded uncertanty of measurement results at the 95% confdence level, the result for the combned uncertanty (U) was multpled by a coverage factor k of 2. Generally, the measurement result s determned from other quanttes, and the relatonshp between result y and the values of the nput parameters can be expressed by a model of Eq. 21, wherex represents model nput parameters (A,C,F d,f r,f re,f h,f s ). The uncertanty of the result u(y) depends on the uncertanty of the nput parameters (x ) and s descrbed by Eq. 22 (varables are ndependent). y/ x s a senstvty coeffcent; the partal dfferental of y wth respect to x and u(x ) denotes the uncertanty n y arsng from the uncertanty n x. These senstvty coeffcents descrbe how the value of y vares wth changes of the parameter x. y = f(x 1,...x n ) (21) u(y) = ( ) 2 y u(x ) x (22)

11 B. Tepuš and M. Smonč / Central European Journal of Chemstry 5(2) Table 5 Uncertanty components and ther relatve standard uncertanty regardng ntrate anon. Unt Value Uncertanty u Uncertanty (RSD=u/Value) A no unt 4.99E E E-03 A 2 no unt 3.77E E E-02 A 4 no unt 7.24E E E-02 A 8 no unt 1.46E E E-02 A 16 no unt 3.04E E E-03 A 24 no unt 4.68E E E-03 A 36 no unt 7.22E E E-03 A 50 no unt 1.04E E E-03 C 2 g/l 1.99E E E-03 C 4 g/l 3.99E E E-02 C 8 g/l 7.98E E E-03 C 16 g/l 1.60E E E-03 C 24 g/l 2.39E E E-03 C 36 g/l 3.59E E E-03 C 50 g/l 4.99E E E-03 F d no unt E E-05 F r no unt E E-02 F re no unt E E-02 F h no unt E E-02 F s no unt E E Expanded measurement uncertanty of ntrate The evaluaton of measurement uncertanty s one of the requrements of the standard EN ISO/IEC [10] whch must be completed by laboratores to obtan accredtaton. The result of the measurement was 25.1 mg/l for the ntrate and the evaluated combned uncertanty was 0.75 mg/l. To obtan an expanded uncertanty at the 95% confdence level, the combned uncertanty was multpled by the coverage factor k of 2. Therefore, the expanded measurement uncertanty of ntrate ons was 25.1 ± 1.5 mg/l (5.95%) when applyng weghted regresson, and 24.7 ± 1.5 mg/l (6.09%) when applyng the lnear least - square regresson. 3.4 Expanded measurement uncertanty of sulphate The result of the measurement was 25.1 mg/l for sulphate and the evaluated combned uncertanty was mg/l. To obtan an expanded uncertanty at the 95% confdence level, the combned uncertanty was multpled by the coverage factor k of 2. Therefore,

12 568 B. Tepuš and M. Smonč / Central European Journal of Chemstry 5(2) Table 6 Uncertanty components and ther relatve standard uncertanty regardng sulphate anon. Unt Value Uncertanty u Uncertanty (RSD=u/Value) A no unt 6.69E E E-03 A 2 no unt 5.33E E E-02 A 4 no unt 9.87E E E-02 A 8 no unt 1.96E E E-03 A 16 no unt 4.03E E E-03 A 24 no unt 6.21E E E-03 A 36 no unt 9.76E E E-03 A 50 no unt 1.38E E E-03 C 2 g/l 2.00E E E-03 C 4 g/l 4.00E E E-03 C 8 g/l 8.00E E E-03 C 16 g/l 1.60E E E-03 C 24 g/l 2.40E E E-03 C 36 g/l 3.60E E E-03 C 50 g/l 5.00E E E-03 F d no unt E E-05 F r no unt E E-02 F re no unt E E-02 F h no unt E E-03 F s no unt E E-03 the expanded measurement uncertanty of sulphate ons was 25.1 ± 2.1 mg/l (8.29%) when applyng the weghted regresson method, and 24.9 ± 2.1 mg/l (8.34%) when applyng lnear least - square regresson method. The followng data contrbute to the uncertanty: precson, trueness, storage condtons, samplng data, concentraton data, and area counts. The equatons for calculatng curves were dfferent (weghted and lnear least - square). The results for measurement uncertantes dffer very lttle n both cases, so we can conclude, that the measurement uncertantes of ntrate and sulphate by on chromatography can be very well determned by applyng the lnear least - square method, whch s less complcated compared to weghted regresson. The largest contrbuton to measurement uncertanty was precson and trueness for both anons. 4 Conclusons Measurement uncertantes for ntrate and sulphate anons were estmated. Two dfferent statstcal methods, the weghted and the lnear least - square method, were appled

13 B. Tepuš and M. Smonč / Central European Journal of Chemstry 5(2) to calculate the measurement uncertantes. Combned and expanded uncertantes were determned. The largest contrbuton regardng the measurement uncertanty was precson and trueness regardng both anons. The expanded uncertanty for ntrate anon measurements was determned to be approxmately 6.1% and apprxomately 8.3% for sulphate anons. It was establshed that the dfference between measurement uncertantes determned by two appled statstcal methods (the weghted and lnear least - square method), s neglgble. Acknowledgment Ths research was supported by the Mnstry of Hgher Educaton, Scence and Technology of the Republc of Slovena (Project number ). References [1] R. Mosello, G.A. Tartar, A. Marchetto, S. Polesello, M. Banch and H. Muntau: Ion chromatography performances evaluated from the thrd AQUACON freshwater analyss nterlaboratory exercse, Accred. Qual. Assur., (2004), Vol. 9(4-5), pp [2] European Standard EN ISO : 1996, Water qualty- Determnaton of dssolved anons by lqud chromatography of ons- Part 2: Determnaton of bromde, chlorde, ntrate, ntrte, orthophosphate and sulphate n waste water (ISO : 1995), Internatonal Organzaton for Standardzaton, Geneve. [3] Donex: DX-100 Ion Chromatograph wth SRS Control Operators Manual, [4] J.N. Mller and J.C. Mller: Statstcs and Chemometrcs for Analytcal Chemstry, Pearson, England, [5] G.A. Tartar, A. Marchetto and R. Mosello: Precson and lnearrty of norganc analyses by on chromatography, J. Chromatogr. A, Vol. 706, (1995), pp [6] V.J. Barwck, S.L.R. Ellson: VAM project 3.2.1, Development and Harmonsaton of Measurement Uncertanty Prncples, Part (d): Protocol for uncertanty evaluaton from valdaton data, [7] S.L.R. Ellson, M. Rosslen and A. Wllams: Quantfyng Uncertanty n Analytcal Measurement; EURACHEM/CITAC, [8] EUROLAB Techncal Secretarat, Measurement Uncertanty n Testng, Techncal Report No. 1/2002, Germany, [9] Internatonal Standard ISO :1999, Water qualty-samplng-part 14: Gudance on qualty assurance of envronmental water samplng and handlng, Internatonal Organzaton for Standardzaton, Geneve. [10] European Standard EN ISO/IEC 17025: 2005, General requrements for the competence of testng and calbraton laboratores, European Commttee for Standardzaton, Brussels.