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Data quality in water analysis: validation of ion chromatographic method for the determination of routine ions in potable water
Desalination 213 (2007) 182–188
Data quality in water analysis: validation of ion chromatographic method for the determination of routine ions in potable water Phani Miskaki a*, Efthimios Lytras b, Leonidas Kousouris b, Philippos Tzoumerkas c a
Water Quality Control Department, Athens Water Supply and Sewerage Company (EYDAP SA), Oropou 156, 111 46 Galatsi, Athens, Greece Tel. +30 210-2144263; Fax +30 210-2144261; email: [email protected] b Water Quality Control Department, EYDAP SA, Oropou 156, 111 46 Galatsi, Athens, Greece c Water Quality Control and Protection Division, EYDAP SA, Oropou 156, 111 46 Galatsi, Athens, Greece Received 31 January 2006; revised accepted 5 May 2006
Abstract Ion chromatography provides a simple, fast, small sample volume demanding, and fit-for-purpose methodology for the concurrent determination of 10 ions (F–, Cl–, Br–, NO3– , PO43–, SO42–, Na+, K+, Ca2+ and Mg2+) in surface, ground and potable water samples. Current law legislations, as well as the need to constantly provide accurate and reliable results, enforce public sector laboratories to practice quality management system, according to the EN ISO 17025 standard. In this paper, the work that was undertaken for the accreditation of the above determinations at the chemical laboratory of EYDAP SA, is presented. The laboratory has developed two in-house methods, which are carried out simultaneously by a dual chromatographic system. The method for the analysis of anions is based on the 4110B APHA standard method and the one for the cations based on the ISO14911. Deviations from the standard procedure made a full method validation imperative, therefore, for all 10 ions, method characteristics were determined and assessed towards the fitness-for-purpose scope (range of measurement, calibration, method detection level, level of quantification, repeatability, reproducibility, precision, accuracy, peak resolution). Additionally, measurement uncertainty was determined and procedures for daily measurement quality control, assessment of individual analyst capability, measurement, sample and reagent traceability, and regular participation in proficiency testing schemes were
*Corresponding author. Presented at the International Conference on New Water Culture of South East European Countries-AQUA 2005, 21–23 October 2005, Athens, Greece. 0011-9164/06/$– See front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2006.05.063
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implemented. Methodology and results for method validation, uncertainty quantification and quality control is presented in detail in the full paper. Keywords: Water quality; Ion chromatography; Method validation; Data quality
1. Introduction Ion chromatography (IC) provides a simple, fast, small sample volume demanding, and fitfor-purpose methodology for the determination of routine ions in surface, ground and potable water samples [1–8]. Current law legislations [9], as well as the need to constantly provide accurate and reliable results, enforce public sector laboratories to practice quality management system, according to the EN ISO 17025 standard [10]. In this paper, the work that was undertaken for the accreditation of the concurrent determination of 10 ions (F–, Cl–, Br–, NO3–, PO3– 4 , SO42– , Na+, K+, Ca2+ and Mg2+) by IC, at the chemical laboratory of EYDAP SA, is presented. 2. Experimental 2.1. Apparatus All analysis was carried out at a dual ion chromatographic system of METROHM (two 709 IC Pumps, 733 sample injection systems, precolumns and separator columns, 732 conductivity detectors, including membrane suppressor device assembly for the anion system only). 2.2. Materials Ultra pure water was used for the preparation of all solutions, free from all ions to be determined, filtered through 0.20 μm pore membrane, of conductivity <0.05 μS/cm, produced by the Elga Maxima HPLC system. H2SO4, 50 mmol/L, was the regenerant solution for the anion conductivity suppressor device. Stock standard solutions, of certified concentration 1000 mg/L for each of the ions (F–, Cl–, + + 2+ 2– Br–, NO3– , PO3– and Mg2+), 4 , SO4 , Na , K , Ca
traceable to SRM NIST, were purchased from Merck. All mixed standard solutions of ions for calibration or quality control were prepared by dilution of the stock solutions. 0.45-μm-porediameter membrane syringe filters were used for sample particulate removal. All volumetric glassware used for the preparation of standard solutions was of class A. 2.3. Methodology The laboratory has developed two in-house ion chromatographic methods, one for the anions based on the 4110B APHA standard [1] and one for the cations based on the ISO14911 [6], which are carried out simultaneously by the dual chromatographic system (method details in Table 1, and typical chromatograms in Figs. 1 and 2). Deviations from the standard procedure (presented in Appendix A) made a full method validation imperative, therefore, for all 10 ions, method characteristics were determined and assessed towards the fitness-for-purpose scope. Table 1 Analytical methodology
Separator column Precolumn
Eluent solution
Loop size (μL) Eluent flow rate (mL/min)
Anion system
Cation system
METROSEP Anion Dual 2 IC precolumn cartridge PRP-1 Na2CO3 1.3 mM NaHCO3 2 mM
METROSEP Cation 1-2 METROSEP Cation 1-2 precolumn Dipicolinic acid 0.75 mM Tartaric acid 4 mM 10 1
100 0.8
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3. Results and discussion 3.1. Range of measurement
Fig. 1. Typical anion chromatogram produced by the method presented.
The essential minimum requirements of the ion chromatographic system applied within the scope of both methods were • Resolution power (R) of the column: In all chromatograms of samples and standard solutions, peak resolution, R, should not fall below R = 1.3 for any pair of peaks [6]. • Method of detection: measurement of the electrical conductivity with suppressor device for the anions, without for the cations. • Applicability of the methods: working ranges according to Table 2. • Calibration: calibration and determination of the quadratic for anions or linear for cations working range. • Guaranteeing the analytical quality: Validity checks of the calibration functions every 10 sample chromatograms.
The range of measurement was dictated by the range of raw and treated water samples that are to be analyzed at the chemical laboratory of EYDAP SA. The dynamic range (DR) for each ion was determined, i.e. the concentration range over which the method had an increasing response (linear or quadratic). Each standard measurement should be within 10% of the true value for acceptance into the DR [1]. Additionally, within the DR, the condition | y′′ – Sx′| ≤ 0.03 Sx′ had to be fulfilled, where y′′ the measured signal and Sx′ = y′ the estimated value for y, as calculated by the calibration function. 3.2. Calibration — linearity When the analytical system was first evaluated, and at intervals afterwards, calibration functions for each ion were established by analyzing at least five multi-ion standard solutions at levels dictated by the range of measurement (Table 2). Calibration curves were constructed by plotting peak area for each ion against concentration. The quality requirement for the acceptance of the calibration function was: correlation coefficient R2 ≥ 0.995. Linear calibration functions were established for the cation and quadratic for the anion methods. 3.3. Method detection level (MDL)
Fig. 2. Typical cation chromatogram produced by the method presented.
MDLs were determined for each ion by analyzing 20 portions of reagent water, over a period of at least 3 days. As Br– and PO43– ions could not be detected in reagent water, standard solutions of those ions in reagent water at a concentration near the estimated MDL were used to determine the MDL. Thus, in the case of Br– and PO43– , standard solutions at concentration levels of 0.010 and 0.025 mg/L respectively, were analyzed 20 times over a period of at least 3 days.
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Table 2
Method characteristics Ion
Range of measurement (mg/L) (range extension by sample dilution)
MDL (mg/L)
LOQ (mg/L)
Calibration levels (mg/L)
F– Cl– Br– NO3– PO43– SO42– Na+ K+ Ca2+ Mg2+
0.020–0.400 (up to 2 mg/L) 1–35 (up to 200 mg/L) 0.002–0.400(up to 2 mg/L) 0.006–30 (up to 150 mg/L) 0.010–0.400 (up to 2 mg/L) 5–80 (up to 400 mg/L) 1–25 (up to 100 mg/L) 0.5–5 (up to 50 mg/L) 10–100 (up to 500 mg/L) 1–25 (up to 100 mg/L)
0.020 0.027 0.002 0.006 0.010 0.009 0.075 0.120 0.260 0.030
0.050 0.068 0.005 0.015 0.025 0.023 0.188 0.300 0.650 0.075
0.025–0.05–0.1–0.2–0.3–0.4 1–2.5–5–10–20–35 0.025–0.05–0.1–0.2–0.3–0.4 0.1–0.5–2.5–10–20–30 0.025–0.05–0.1–0.2–0.3–0.4 5–10–20–40–60–80 1–5–10–15–25 0.25–0.5–1–2–5 10–20–50–80–100 1–5–10–15–25
The standard deviation(s) of the replicate measurements was calculated and MDLs were computed according to MDL = t * s. The t value is selected from a table of the onesided t-student distribution for n–1 degrees of freedom and at 99% level (n = 20, number of measurements). The replicate measurements should be in the range of one to five times the calculated MDL [11]. 3.4. Level of quantitation (LOQ) LOQs were calculated by the equation: LOQ = 2.5 * MDL [11].
same analyst. Reproducibility results were obtained by analysis, over a period of 3 months, of three replicate standard solutions (one each month), by 2 analysts in random use. Forty measurements were carried out during this period, and calibration functions were redetermined in the meanwhile. Method accuracy data were also obtained herewith, and results are presented in Table 3. Method accuracy is also checked periodically by participation in proficiency testing schemes with real samples of raw or treated water. Results are presented in Table 5. 3.6. Measurement uncertainty Combined uncertainty values for each ion are presented in Table 4. The combined uncertainty [12] was calculated according to the following formula:
3.5. Method precision and accuracy Method precision was determined in terms of repeatability and reproducibility, and quantified by the repeatability and reproducibility standard deviation (sr, sR). Repeatability data were obtained by replicate analysis of 20 portions from one standard solution at two concentration levels the same day by the
U = √ Ua2 + Ub2 where Ua is the uncertainty type A, expressed by the standard deviation of the mean value of a number of measurements (at least 10) of a standard solution in one or two concentration
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Table 3
Precision and accuracy data Analyte
F– Cl – Cl – Br – Br – NO3– PO43– SO42– Na+ Na+ K+ K+ Ca2+ Ca2+ Mg2+ Mg2+
Conc. level (mg/L)
0.200 5.00 30.0 0.050 0.200 10.0 0.200 20.0 5.00 10.0 1.00 2.00 50.0 80.0 5.00 10.0
Xmean (n = 20) (mg/L)
0.204 4.58 30.28 0.051 0.204 9.88 0.211 20.29 4.90 9.90 1.01 1.98 50.17 79.8 4.86 9.86
levels and Ub the uncertainty type B, which includes: (1) The uncertainty of the concentration of the standard solution that is used for the system calibration. (2) The uncertainty of the calibration curve. (3) The uncertainty of the conductivity measurement by the instrument’s conductivity detector (according to instrument’s specifications and performance evaluation) (Table 4). 3.7. Quality control Validity checks of the calibration function are carried out every 10 samples, using one multianion and one multi-cation standard solution at concentration levels in the middle of the calibration curve. Results are plotted in Shewhart control charts, individually for each ion. In addition to the ±3 s limits, ±10% accuracy is implemented by the Decision Y2/2600/2001 [9]. Corrective actions are to be taken once out of control conditions are detected through the
Error %
2.2 –8.5 0.9 1.8 1.3 –1.2 5.6 1.4 –2.0 –1.0 0.9 –1.0 0.3 –0.2 –2.8 –1.4
Repeatability (n = 20)
Reproducibility (n = 40)
sr (mg/L)
%RSDr
sR (mg/L)
%RSDR
0.009 0.043 0.25 0.001 0.002 0.075 0.005 0.13 0.05 0.075 0.067 0.03 0.24 0.36 0.042 0.06
4.6 0.9 0.8 2.8 0.8 0.8 2.2 0.6 1.0 0.8 6.7 1.5 0.5 0.5 0.9 0.6
0.010
5.0
0.23
1.2
0.005 0.18 0.028 0.23
2.5 1.2 16.1 1.1
0.17
1.7
0.15 0.48
6.9 0.9
0.11
1.1
control charts. Additional internal quality control is carried out by blank or duplicate analysis. External quality control is also implemented by regular participation in proficiency testing Table 4
Combined uncertainty Analyte
Concentration level (mg/L)
Uncertainty (mg/L)
F– Cl – Cl – Br – NO3– PO43– SO42– Na+ Na+ K+ Ca2+ Mg2+ Mg2+
0.200 5.00 30.0 0.200 10.0 0.200 20.0 5.00 10.0 2.00 50.0 5.00 10.0
±0.022 ±0.24 ±2.8 ±0.010 ±0.6 ±0.02 ±1.1 ±0.26 ±0.5 ±0.06 ±2.4 ±0.26 ±0.5
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Table 5
Proficiency testing results Analyte
Aquacheck Oct. ’03 Lab. value
F – (μg/L) Cl – (mg/L) NO3– (mg/L) PO43– (μg/L P) SO42– (mg/L) Na+ (mg/L) K+ (mg/L) Ca2+ (mg/L) Mg2+ (mg/L)
Aquacheck Jan. ’04
Leap May ’04
Leap Dec. ’04
Leap May ’05
z-score Lab. value z-score Lab. value z-score Lab. value z-score Lab. value z-score
1725 31.4 24.6
0.78 0.06 0.74
775 40.9 18.8
–0.46 –0.62 0.44
260 8.0 1.55
0.1 –0.2 0.0
41.1 21.9 5.7 87.6 6.3
0.27 –0.27 1.15 0.07 –0.43
66.0 40.9 4.9 44.10 9.4
0.00 0.12 0.06 0.40 0.47
34.2 19.7 1.1 6.2 1.5
0.3 0.7 0.7 0.4 –0.1
schemes with certain quality criteria. The laboratory’s performance in such schemes is represented by the derived z-score (considered successful when |z| ≤ 2). 4. Conclusions A fully validated dual ion chromatographic method, complying with current law legislations and the ISO 17025, was developed at the chemical laboratory of EYDAP SA for the concurrent determination of 10 ions (F–, Cl–, Br–, NO3– , PO43–, SO42–, Na+, K+, Ca2+ and Mg2+) in surface, ground and potable water samples. The method was successfully included in the accreditation scopes of the laboratory, granted by ESYD (the national accreditation body), in January 2005.
References [1] Standard Methods for the Examination of Water and Wastewater 20th edn., 4110 B, Determination of anions by ion chromatography with chemical suppression of eluent conductivity. [2] EPA method 300.1 Part A, Determination of inorganic anions in drinking water by ion chromatography, Revision 1.0, Cincinnati, OH, 1997.
253 17.5 7.0 21 25.9 12.3 2.1 81.6 4.1
0 –0.5 –0.1 –0.6 0.3 –0.1 0.6 0.6 –0.1
165 11.0 2.6 1566 47.7 8.0 1.0 38 2.1
0 –0.2 –0.1 2.0 0.4 0.5 0.6 0.5 0
[3] EN ISO 10304-1:1995, Water quality — determination of dissolved fluoride, chloride, nitrite, orthophosphate, bromide, nitrate and sulfate ions, using liquid chromatography of ions — Part 1: Method for water with low contamination. [4] METROHM IC Application Note No S-3, Fluoride, chloride, nitrate and sulfate in drinking water. [5] METROHM IC Application Note No S-8, Fluoride, chloride, nitrite, bromide, nitrate and sulfate in surface water. [6] ISO 14911:1998(E), Water quality — determination of dissolved Li+, Na+, NH4+, K+, Mn2+, Ca2+, Mg2+, Sr2+ and Ba2+ using ion chromatography — method for water and wastewater. [7] METROHM IC Application Note No C-1, Sodium, potassium, calcium and magnesium in drinking water. [8] METROHM Application Work AW CH6-0744052002, Cations — to acidify or not? [9] Ministerial Decision Y2/2600 (2001), Quality of water for human consumption, Greek Republic Official Journal 892/11-7-2001. [10] ISO/IEC 17025:1999, General requirements for the competence of testing and calibration laboratories. [11] Standard Methods for the Examination of Water and Wastewater 20th edn., §1030C.1 and §1030C.2. [12] CITAC/Eurachem, Quantifying uncertainty in analytical measurements, 2nd edn., QUAM:2000.P1.
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Appendix 1: Deviations from standard methods
Stationary phase of anion separator column Eluent solution Regenerant solution Sample filtration
APHA 4110 B
In-house method
Styrene divinylbenzene-based low-capacity pellicular anion-exchange resin 0.0018 M Na2CO3 and 0.0017 M NaHCO3 H2SO4 0.025 N 0.2-μm-pore-diam membrane filter
Polymethacrylate with quaternary ammonium groups
Range of measurement
Acidification prior to chromatographic analysis
Linearity criteria of calibration function Validity checks of calibration function
Volumetric flasks used for the preparation of cation standard solutions
0.0013 M Na2CO3 and 0.0020 M NaHCO3 H2SO4 0.100 N 0.45-μm-pore-diam membrane filter
ISO 14911:1998
In-house method
Na 0.1–10 mg/L K 0.1–10 mg/L Ca 0.5–50 mg/L Mg 0.5–50 mg/L Blanks and standards are acidified with HNO3 to a final concentration of 0.001 M. Membrane filtered samples are acidified with HNO3 to pH 3 ± 0.5 Refer to ISO 8466-1
1–25 (up to 100 mg/L) 0.5–5 (up to 50 mg/L) 10–100 (up to 500 mg/L) 1–25 (up to 100 mg/L) None applied Ref. [7]
Minimum 2 calibration solutions (low & high conc. level) after set-up procedure and after each sample series Made of polyethylene
Refer to APHA 3020B, 20th edn., 2000 1 calibration solution (concs. in the middle of calibration range) after set-up procedure and every 10 samples Made of borosilicate glass
Data quality in water analysis: validation of ion chromatographic method for the determination of routine ions in potable water Phani Miskaki a*, Efthimios Lytras b, Leonidas Kousouris b, Philippos Tzoumerkas c a
Water Quality Control Department, Athens Water Supply and Sewerage Company (EYDAP SA), Oropou 156, 111 46 Galatsi, Athens, Greece Tel. +30 210-2144263; Fax +30 210-2144261; email: [email protected] b Water Quality Control Department, EYDAP SA, Oropou 156, 111 46 Galatsi, Athens, Greece c Water Quality Control and Protection Division, EYDAP SA, Oropou 156, 111 46 Galatsi, Athens, Greece Received 31 January 2006; revised accepted 5 May 2006
Abstract Ion chromatography provides a simple, fast, small sample volume demanding, and fit-for-purpose methodology for the concurrent determination of 10 ions (F–, Cl–, Br–, NO3– , PO43–, SO42–, Na+, K+, Ca2+ and Mg2+) in surface, ground and potable water samples. Current law legislations, as well as the need to constantly provide accurate and reliable results, enforce public sector laboratories to practice quality management system, according to the EN ISO 17025 standard. In this paper, the work that was undertaken for the accreditation of the above determinations at the chemical laboratory of EYDAP SA, is presented. The laboratory has developed two in-house methods, which are carried out simultaneously by a dual chromatographic system. The method for the analysis of anions is based on the 4110B APHA standard method and the one for the cations based on the ISO14911. Deviations from the standard procedure made a full method validation imperative, therefore, for all 10 ions, method characteristics were determined and assessed towards the fitness-for-purpose scope (range of measurement, calibration, method detection level, level of quantification, repeatability, reproducibility, precision, accuracy, peak resolution). Additionally, measurement uncertainty was determined and procedures for daily measurement quality control, assessment of individual analyst capability, measurement, sample and reagent traceability, and regular participation in proficiency testing schemes were
*Corresponding author. Presented at the International Conference on New Water Culture of South East European Countries-AQUA 2005, 21–23 October 2005, Athens, Greece. 0011-9164/06/$– See front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2006.05.063
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implemented. Methodology and results for method validation, uncertainty quantification and quality control is presented in detail in the full paper. Keywords: Water quality; Ion chromatography; Method validation; Data quality
1. Introduction Ion chromatography (IC) provides a simple, fast, small sample volume demanding, and fitfor-purpose methodology for the determination of routine ions in surface, ground and potable water samples [1–8]. Current law legislations [9], as well as the need to constantly provide accurate and reliable results, enforce public sector laboratories to practice quality management system, according to the EN ISO 17025 standard [10]. In this paper, the work that was undertaken for the accreditation of the concurrent determination of 10 ions (F–, Cl–, Br–, NO3–, PO3– 4 , SO42– , Na+, K+, Ca2+ and Mg2+) by IC, at the chemical laboratory of EYDAP SA, is presented. 2. Experimental 2.1. Apparatus All analysis was carried out at a dual ion chromatographic system of METROHM (two 709 IC Pumps, 733 sample injection systems, precolumns and separator columns, 732 conductivity detectors, including membrane suppressor device assembly for the anion system only). 2.2. Materials Ultra pure water was used for the preparation of all solutions, free from all ions to be determined, filtered through 0.20 μm pore membrane, of conductivity <0.05 μS/cm, produced by the Elga Maxima HPLC system. H2SO4, 50 mmol/L, was the regenerant solution for the anion conductivity suppressor device. Stock standard solutions, of certified concentration 1000 mg/L for each of the ions (F–, Cl–, + + 2+ 2– Br–, NO3– , PO3– and Mg2+), 4 , SO4 , Na , K , Ca
traceable to SRM NIST, were purchased from Merck. All mixed standard solutions of ions for calibration or quality control were prepared by dilution of the stock solutions. 0.45-μm-porediameter membrane syringe filters were used for sample particulate removal. All volumetric glassware used for the preparation of standard solutions was of class A. 2.3. Methodology The laboratory has developed two in-house ion chromatographic methods, one for the anions based on the 4110B APHA standard [1] and one for the cations based on the ISO14911 [6], which are carried out simultaneously by the dual chromatographic system (method details in Table 1, and typical chromatograms in Figs. 1 and 2). Deviations from the standard procedure (presented in Appendix A) made a full method validation imperative, therefore, for all 10 ions, method characteristics were determined and assessed towards the fitness-for-purpose scope. Table 1 Analytical methodology
Separator column Precolumn
Eluent solution
Loop size (μL) Eluent flow rate (mL/min)
Anion system
Cation system
METROSEP Anion Dual 2 IC precolumn cartridge PRP-1 Na2CO3 1.3 mM NaHCO3 2 mM
METROSEP Cation 1-2 METROSEP Cation 1-2 precolumn Dipicolinic acid 0.75 mM Tartaric acid 4 mM 10 1
100 0.8
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3. Results and discussion 3.1. Range of measurement
Fig. 1. Typical anion chromatogram produced by the method presented.
The essential minimum requirements of the ion chromatographic system applied within the scope of both methods were • Resolution power (R) of the column: In all chromatograms of samples and standard solutions, peak resolution, R, should not fall below R = 1.3 for any pair of peaks [6]. • Method of detection: measurement of the electrical conductivity with suppressor device for the anions, without for the cations. • Applicability of the methods: working ranges according to Table 2. • Calibration: calibration and determination of the quadratic for anions or linear for cations working range. • Guaranteeing the analytical quality: Validity checks of the calibration functions every 10 sample chromatograms.
The range of measurement was dictated by the range of raw and treated water samples that are to be analyzed at the chemical laboratory of EYDAP SA. The dynamic range (DR) for each ion was determined, i.e. the concentration range over which the method had an increasing response (linear or quadratic). Each standard measurement should be within 10% of the true value for acceptance into the DR [1]. Additionally, within the DR, the condition | y′′ – Sx′| ≤ 0.03 Sx′ had to be fulfilled, where y′′ the measured signal and Sx′ = y′ the estimated value for y, as calculated by the calibration function. 3.2. Calibration — linearity When the analytical system was first evaluated, and at intervals afterwards, calibration functions for each ion were established by analyzing at least five multi-ion standard solutions at levels dictated by the range of measurement (Table 2). Calibration curves were constructed by plotting peak area for each ion against concentration. The quality requirement for the acceptance of the calibration function was: correlation coefficient R2 ≥ 0.995. Linear calibration functions were established for the cation and quadratic for the anion methods. 3.3. Method detection level (MDL)
Fig. 2. Typical cation chromatogram produced by the method presented.
MDLs were determined for each ion by analyzing 20 portions of reagent water, over a period of at least 3 days. As Br– and PO43– ions could not be detected in reagent water, standard solutions of those ions in reagent water at a concentration near the estimated MDL were used to determine the MDL. Thus, in the case of Br– and PO43– , standard solutions at concentration levels of 0.010 and 0.025 mg/L respectively, were analyzed 20 times over a period of at least 3 days.
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Table 2
Method characteristics Ion
Range of measurement (mg/L) (range extension by sample dilution)
MDL (mg/L)
LOQ (mg/L)
Calibration levels (mg/L)
F– Cl– Br– NO3– PO43– SO42– Na+ K+ Ca2+ Mg2+
0.020–0.400 (up to 2 mg/L) 1–35 (up to 200 mg/L) 0.002–0.400(up to 2 mg/L) 0.006–30 (up to 150 mg/L) 0.010–0.400 (up to 2 mg/L) 5–80 (up to 400 mg/L) 1–25 (up to 100 mg/L) 0.5–5 (up to 50 mg/L) 10–100 (up to 500 mg/L) 1–25 (up to 100 mg/L)
0.020 0.027 0.002 0.006 0.010 0.009 0.075 0.120 0.260 0.030
0.050 0.068 0.005 0.015 0.025 0.023 0.188 0.300 0.650 0.075
0.025–0.05–0.1–0.2–0.3–0.4 1–2.5–5–10–20–35 0.025–0.05–0.1–0.2–0.3–0.4 0.1–0.5–2.5–10–20–30 0.025–0.05–0.1–0.2–0.3–0.4 5–10–20–40–60–80 1–5–10–15–25 0.25–0.5–1–2–5 10–20–50–80–100 1–5–10–15–25
The standard deviation(s) of the replicate measurements was calculated and MDLs were computed according to MDL = t * s. The t value is selected from a table of the onesided t-student distribution for n–1 degrees of freedom and at 99% level (n = 20, number of measurements). The replicate measurements should be in the range of one to five times the calculated MDL [11]. 3.4. Level of quantitation (LOQ) LOQs were calculated by the equation: LOQ = 2.5 * MDL [11].
same analyst. Reproducibility results were obtained by analysis, over a period of 3 months, of three replicate standard solutions (one each month), by 2 analysts in random use. Forty measurements were carried out during this period, and calibration functions were redetermined in the meanwhile. Method accuracy data were also obtained herewith, and results are presented in Table 3. Method accuracy is also checked periodically by participation in proficiency testing schemes with real samples of raw or treated water. Results are presented in Table 5. 3.6. Measurement uncertainty Combined uncertainty values for each ion are presented in Table 4. The combined uncertainty [12] was calculated according to the following formula:
3.5. Method precision and accuracy Method precision was determined in terms of repeatability and reproducibility, and quantified by the repeatability and reproducibility standard deviation (sr, sR). Repeatability data were obtained by replicate analysis of 20 portions from one standard solution at two concentration levels the same day by the
U = √ Ua2 + Ub2 where Ua is the uncertainty type A, expressed by the standard deviation of the mean value of a number of measurements (at least 10) of a standard solution in one or two concentration
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Table 3
Precision and accuracy data Analyte
F– Cl – Cl – Br – Br – NO3– PO43– SO42– Na+ Na+ K+ K+ Ca2+ Ca2+ Mg2+ Mg2+
Conc. level (mg/L)
0.200 5.00 30.0 0.050 0.200 10.0 0.200 20.0 5.00 10.0 1.00 2.00 50.0 80.0 5.00 10.0
Xmean (n = 20) (mg/L)
0.204 4.58 30.28 0.051 0.204 9.88 0.211 20.29 4.90 9.90 1.01 1.98 50.17 79.8 4.86 9.86
levels and Ub the uncertainty type B, which includes: (1) The uncertainty of the concentration of the standard solution that is used for the system calibration. (2) The uncertainty of the calibration curve. (3) The uncertainty of the conductivity measurement by the instrument’s conductivity detector (according to instrument’s specifications and performance evaluation) (Table 4). 3.7. Quality control Validity checks of the calibration function are carried out every 10 samples, using one multianion and one multi-cation standard solution at concentration levels in the middle of the calibration curve. Results are plotted in Shewhart control charts, individually for each ion. In addition to the ±3 s limits, ±10% accuracy is implemented by the Decision Y2/2600/2001 [9]. Corrective actions are to be taken once out of control conditions are detected through the
Error %
2.2 –8.5 0.9 1.8 1.3 –1.2 5.6 1.4 –2.0 –1.0 0.9 –1.0 0.3 –0.2 –2.8 –1.4
Repeatability (n = 20)
Reproducibility (n = 40)
sr (mg/L)
%RSDr
sR (mg/L)
%RSDR
0.009 0.043 0.25 0.001 0.002 0.075 0.005 0.13 0.05 0.075 0.067 0.03 0.24 0.36 0.042 0.06
4.6 0.9 0.8 2.8 0.8 0.8 2.2 0.6 1.0 0.8 6.7 1.5 0.5 0.5 0.9 0.6
0.010
5.0
0.23
1.2
0.005 0.18 0.028 0.23
2.5 1.2 16.1 1.1
0.17
1.7
0.15 0.48
6.9 0.9
0.11
1.1
control charts. Additional internal quality control is carried out by blank or duplicate analysis. External quality control is also implemented by regular participation in proficiency testing Table 4
Combined uncertainty Analyte
Concentration level (mg/L)
Uncertainty (mg/L)
F– Cl – Cl – Br – NO3– PO43– SO42– Na+ Na+ K+ Ca2+ Mg2+ Mg2+
0.200 5.00 30.0 0.200 10.0 0.200 20.0 5.00 10.0 2.00 50.0 5.00 10.0
±0.022 ±0.24 ±2.8 ±0.010 ±0.6 ±0.02 ±1.1 ±0.26 ±0.5 ±0.06 ±2.4 ±0.26 ±0.5
P. Miskaki et al. / Desalination 213 (2007) 182–188
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Table 5
Proficiency testing results Analyte
Aquacheck Oct. ’03 Lab. value
F – (μg/L) Cl – (mg/L) NO3– (mg/L) PO43– (μg/L P) SO42– (mg/L) Na+ (mg/L) K+ (mg/L) Ca2+ (mg/L) Mg2+ (mg/L)
Aquacheck Jan. ’04
Leap May ’04
Leap Dec. ’04
Leap May ’05
z-score Lab. value z-score Lab. value z-score Lab. value z-score Lab. value z-score
1725 31.4 24.6
0.78 0.06 0.74
775 40.9 18.8
–0.46 –0.62 0.44
260 8.0 1.55
0.1 –0.2 0.0
41.1 21.9 5.7 87.6 6.3
0.27 –0.27 1.15 0.07 –0.43
66.0 40.9 4.9 44.10 9.4
0.00 0.12 0.06 0.40 0.47
34.2 19.7 1.1 6.2 1.5
0.3 0.7 0.7 0.4 –0.1
schemes with certain quality criteria. The laboratory’s performance in such schemes is represented by the derived z-score (considered successful when |z| ≤ 2). 4. Conclusions A fully validated dual ion chromatographic method, complying with current law legislations and the ISO 17025, was developed at the chemical laboratory of EYDAP SA for the concurrent determination of 10 ions (F–, Cl–, Br–, NO3– , PO43–, SO42–, Na+, K+, Ca2+ and Mg2+) in surface, ground and potable water samples. The method was successfully included in the accreditation scopes of the laboratory, granted by ESYD (the national accreditation body), in January 2005.
References [1] Standard Methods for the Examination of Water and Wastewater 20th edn., 4110 B, Determination of anions by ion chromatography with chemical suppression of eluent conductivity. [2] EPA method 300.1 Part A, Determination of inorganic anions in drinking water by ion chromatography, Revision 1.0, Cincinnati, OH, 1997.
253 17.5 7.0 21 25.9 12.3 2.1 81.6 4.1
0 –0.5 –0.1 –0.6 0.3 –0.1 0.6 0.6 –0.1
165 11.0 2.6 1566 47.7 8.0 1.0 38 2.1
0 –0.2 –0.1 2.0 0.4 0.5 0.6 0.5 0
[3] EN ISO 10304-1:1995, Water quality — determination of dissolved fluoride, chloride, nitrite, orthophosphate, bromide, nitrate and sulfate ions, using liquid chromatography of ions — Part 1: Method for water with low contamination. [4] METROHM IC Application Note No S-3, Fluoride, chloride, nitrate and sulfate in drinking water. [5] METROHM IC Application Note No S-8, Fluoride, chloride, nitrite, bromide, nitrate and sulfate in surface water. [6] ISO 14911:1998(E), Water quality — determination of dissolved Li+, Na+, NH4+, K+, Mn2+, Ca2+, Mg2+, Sr2+ and Ba2+ using ion chromatography — method for water and wastewater. [7] METROHM IC Application Note No C-1, Sodium, potassium, calcium and magnesium in drinking water. [8] METROHM Application Work AW CH6-0744052002, Cations — to acidify or not? [9] Ministerial Decision Y2/2600 (2001), Quality of water for human consumption, Greek Republic Official Journal 892/11-7-2001. [10] ISO/IEC 17025:1999, General requirements for the competence of testing and calibration laboratories. [11] Standard Methods for the Examination of Water and Wastewater 20th edn., §1030C.1 and §1030C.2. [12] CITAC/Eurachem, Quantifying uncertainty in analytical measurements, 2nd edn., QUAM:2000.P1.
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Appendix 1: Deviations from standard methods
Stationary phase of anion separator column Eluent solution Regenerant solution Sample filtration
APHA 4110 B
In-house method
Styrene divinylbenzene-based low-capacity pellicular anion-exchange resin 0.0018 M Na2CO3 and 0.0017 M NaHCO3 H2SO4 0.025 N 0.2-μm-pore-diam membrane filter
Polymethacrylate with quaternary ammonium groups
Range of measurement
Acidification prior to chromatographic analysis
Linearity criteria of calibration function Validity checks of calibration function
Volumetric flasks used for the preparation of cation standard solutions
0.0013 M Na2CO3 and 0.0020 M NaHCO3 H2SO4 0.100 N 0.45-μm-pore-diam membrane filter
ISO 14911:1998
In-house method
Na 0.1–10 mg/L K 0.1–10 mg/L Ca 0.5–50 mg/L Mg 0.5–50 mg/L Blanks and standards are acidified with HNO3 to a final concentration of 0.001 M. Membrane filtered samples are acidified with HNO3 to pH 3 ± 0.5 Refer to ISO 8466-1
1–25 (up to 100 mg/L) 0.5–5 (up to 50 mg/L) 10–100 (up to 500 mg/L) 1–25 (up to 100 mg/L) None applied Ref. [7]
Minimum 2 calibration solutions (low & high conc. level) after set-up procedure and after each sample series Made of polyethylene
Refer to APHA 3020B, 20th edn., 2000 1 calibration solution (concs. in the middle of calibration range) after set-up procedure and every 10 samples Made of borosilicate glass