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Determination of fluoride in toothpaste, effluent streams and natural and borehole water using a flow injection system with a fluoride-selective membrane electrode
Pharmaceutica Acta Helvetiae 73 Ž1999. 307–310
Determination of fluoride in toothpaste, effluent streams and natural and borehole water using a flow injection system with a fluoride-selective membrane electrode Raluca-Ioana Stefan a , Jacobus F. van Staden a , Hassan Y. Aboul-Enein b
b,)
a Department of Chemistry, UniÕersity of Pretoria, Pretoria 0002, South Africa Bioanalytical and Drug DeÕelopment Laboratory, Biological and Medical Research Department, MBC-03, King Faisal Specialist Hospital and Research Centre, P.O. Box 3354, Riyadh 11211, Saudi Arabia
Received 17 December 1998; received in revised form 8 March 1999; accepted 14 April 1999
Abstract An automated system for the determination of fluoride in toothpaste, effluent streams, natural and borehole water based on the concept of flow injection with a fluoride-selective membrane electrode as sensor is described. The system is suitable for the on-line monitoring of fluoride at a sampling rate of 60 samples hy1 in the linear range 10y4 –10y1 Žapproximately 1.9–1900 mg ly1 . with an RSD of better than 0.6%. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Fluoride-selective membrane electrode; Flow injection analysis
1. Introduction The fluoride-selective membrane electrode is an ideal detecting device for the determination of fluoride using flow injection systems, especially due to the accuracy of the analytical information obtained. Extensive studies concerning the use of fluoride-selective electrodes in flow injection analysis ŽFIA. have been performed by various research groups ŽSlanina et al., 1980; Trojanowicz and Matuszewski, 1982; Frenzel and Bratter, 1986; Najib and ¨ Othman, 1992; Borzitsky et al., 1993.. It is well known that the limitation of using ion-selective electrodes as detector systems are the limit of detection, influence of pH and selectivity of these devices. Although a very good correlation is obtained between log c and relative peak height in flow injection systems, this is not always the case ) Corresponding author. Tel.: q966-1-442-7859; fax: q966-1-4427858; e-mail: [email protected]
for Nernstian response of the electrode. It was proved that the limit of detection and linear response are given by the nature of the buffer used in the carrier stream ŽCardwell et al., 1987; Hara and Huang, 1997.. By using FIA systems, the selectivity of the electrode is improved due to the very short contact time of the interfering ions with the electrode membrane. The advantages associated with the coupling of the fluoride-selective electrode as sensor to an FIA system, enables this combination to be used in a reliable low cost, robust analyser for the determination of fluoride in toothpaste as well as in effluent streams, natural and borehole water samples. The fluoride content contributes to the quality of toothpaste. The maximum level of the fluoride in water should be 1.500 mg ly1 ŽFifield, 1995.. In order to sustain the quality of life and for healthy reasons, the fluoride content in water should be controlled carefully. This paper reports on the incorporation of a fluorideselective electrode in an FIA system for the determination
0031-6865r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 1 - 6 8 6 5 Ž 9 9 . 0 0 0 0 8 - 4
308
R.-I. Stefan et al.r Pharmaceutica Acta HelÕetiae 73 (1999) 307–310
of fluoride in toothpaste and in effluent streams, natural and borehole water.
Table 2 Selectivity coefficients of the fluoride-selective electrode incorporated into the FIA system Interfering species ŽJ zy . y
Br Cly COy2 3
2. Experimental 2.1. Reagents and solutions
Selectivity coefficients 3.0=10y5 1.4=10y5 1.2=10y5
All measurements are average of seven determinations.
All reagents were prepared from analytical reagent grade chemicals unless specified otherwise. Deionised water from a Modulab system ŽContinental Water Systems, San Antonio, TX, USA. was used for dilution. The solutions were prepared as follows: Total strength adjustment buffer ŽTISAB II. was used as carrier stream. Fifty-seven milliliters of glacial acetic acid and 74 g reagent grade potassium chloride were carefully dissolved in 500 ml distilled water in a 1-l beaker. The pH was adjusted to 5.50 by adding 5 mol ly1 KOH solution. The solution was quantitatively diluted to mark in a 1-l volumetric flask with distilled water. The NaF solutions were prepared by serial dilutions from a standard 0.10 mol ly1 stock solution.
at 10 rev miny1 supplied the carrier streams to the manifold system. Tygon tubing Ž0.51 mm i.d.. was used to construct the manifold; coils were wound round suitable lengths of glass tubing Ž15 mm o.d... A 60-s cycle sampling time was used, giving the system a capacity of about 60 samples hy1 . Data acquisition and device control were achieved using a PC30-B interface board ŽEagle Electric, Cape Town, South Africa.. The FlowTEK ŽMarshall and van Staden, 1992. software package Žobtainable from MINTEK. for computer-aided flow analysis was used throughout for device control and data acquisition.
2.2. Apparatus An Orion fluoride–selective electrode ŽOrion Research Incorporated Laboratory Products Group. Model 94-09 was used for all measurements in combination with a double junction reference Orion electrode Model 90-02. The potentials were measured at room temperature with an Orion Research ŽModel 901. microprocessor Ionalyzer. 2.3. Flow injection system The electrodes were incorporated into the conduits of a flow injection system with basic design similar to that previously described Žvan Staden, 1987.. A Carle microvolume two-position sampling valve ŽCarle No. 2014. containing two identical sample loops was used. Each loop has a volume of 30 ml. A Cenco peristaltic pump operating Table 1 Response characteristics of the fluoride-selective electrode incorporated into the conduits of an FIA system Parameter
Response
Slope Intercept, H8 Linear range ŽLR. Žmol ly1 . Detection limit ŽDL. Žmol ly1 .
0.400"0.002 1.910"0.021 10y4 –10y1 1.8=10y5
All measurements are average of 10 determinations. LR is approximately 1.9–1900 mg ly1 . DL is about 0.34 mg ly1 .
2.4. Preparation of toothpaste samples A gram of Aquafresh toothpaste ŽSmithKline Beecham Consumer, Healthcare, Brentford, UK. was weighted into a 50-ml beaker. The sample was dissolved using 2 ml distilled water and 2 ml concentrated HCl. The solution was diluted to 50 ml in a volumetric flask, using 25 ml TISAB II ŽpH s 5.50., and distilled water.
3. Results and discussion The response characteristics and selectivity coefficients of the fluoride-selective electrode incorporated into the conduits of the flow injection system was evaluated under optimum running conditions. The analytical applicability of the proposed ISE–FIA system was further evaluated on toothpaste, natural and borehole water. 3.1. Electrode response The electrode characteristics of the fluoride-selective membrane electrode incorporated into the FIA system are summarised in Table 1. The equation of the calibration graph is: H s H8 y 0.40 pF
R.-I. Stefan et al.r Pharmaceutica Acta HelÕetiae 73 (1999) 307–310 Table 3 Performance and reproducibility of the proposed ISE–FIA system for fluoride in toothpastea samplesb Sample
Proposed ISE–FIA system. Fluoride concentration in mg ly1
%RSD
1 2 3 4 5 6 7
14.44 14.82 14.55 13.68 13.68 14.82 14.22
0.2 0.4 0.5 0.6 0.6 0.4 0.1
309
COy2 ions do not interfere even when the ratio between 3 the concentration of fluoride and interfering ions Žmol ly1 . is 1:1000. 3.3. Analytical applications
a
Fluoride concentration in tooth pastes14.66 mg ly1 . Mean result of seven tests in each case, with relative standard deviation for the ISE–FIA system.
b
x where H is the relative peak height and pF s ylog w cy F ; with a correlation coefficient of 0.9994. It is clear from the results in Table 1 that with the linear response range between 10y4 and 10y1 mol ly1 Žapproximately 1.9–1900 mg ly1 ., the very high precision obtained and the stability of the ISE–FIA system with time, the proposed ISE–FIA system seems to be ideally suitable as a reliable low cost analyser especially in the manufacturing of toothpaste and in the on-line monitoring of water samples. 3.2. SelectiÕity of the electrode The effect of some of possible interfering species on the response of the electrode was studied by using the mixed solutions method. The selectivity coefficients presented in Table 2, show that the response of the fluoride-selective membrane electrode is not affected by the presence of Bry, Cly and COy2 ions and this fact demonstrates the speci3 ficity of the electrode for fluoride assay. Bry, Cly and
The response characteristics as well as the selectivity of the fluoride-selective membrane electrode incorporated into the conduits of the FIA system assure a reliable assay of fluoride in toothpaste and in effluent streams, natural and borehole water. The accuracy of the proposed ISE–FIA system was tested by comparing the results of a number of toothpaste samples with the value of 14.66 mg ly1 given by the manufacturer. Results compared favourably as can be seen from Table 3. Fluoride can be determined in toothpaste with an average recovery of 99.73% and an RSD of 0.6% on seven tests of each sample. The accuracy of the proposed ISE–FIA system was also tested for water samples by comparing the results of a number effluent streams, natural water and borehole samples with those obtained by a standard automated segmented method. For the determination of fluoride in natural and borehole water samples where the values were below the detection limit of 0.34 mg ly1 of the proposed system, it was necessary to test the accuracy of proposed system by the standard additions method due to the low fluoride concentrations in these water samples. Known amounts of fluoride were added to these water samples which were processed after addition. The results compared very well with those obtained by a standard automated segmented method as seen from Table 4. The average recovery of fluoride in water samples is 100.35%. Analysis were also performed in a random order to test carry-over effects. Carry-over from one sample to another is negligible at a sample throughput of 60 samples hy1 .
Table 4 Performance and reproducibility of the proposed ISE–FIA system for the determination of fluoride in effluent streams, natural and borehole water a Sample
Automated segmented method. Fluoride concentration in mg ly1
Proposed ISE–FIA system with standard addition. Fluoride concentration in mg ly1
Ž%RSD.
1 2 3 4 5 6 7 8
0.269 0.311 0.328 0.796 0.514 1.220 0.905 0.516
0.285 b 0.304 b 0.323 b 0.794 0.514 1.219 0.905 0.514
0.2 0.3 0.3 0.25 0.5 0.2 0.25 0.4
a b
Mean result of 10 tests in each case, with relative standard deviation for the proposed ISE–FIA system. Done by using the standard additions method.
310
R.-I. Stefan et al.r Pharmaceutica Acta HelÕetiae 73 (1999) 307–310
4. Conclusion The proposed ISE–FIA system is suitable for on-line fluoride monitoring at a rate of approximately 60 actual samplesrhy1 . As a low cost, low maintenance Žespecially with the low cost fluoride-selective electrode as detector. reliable analyser, the system should be particularly attractive for the on-line determination of the fluoride in the manufacturing of toothpaste and in the on-line monitoring of fluoride in effluent streams, natural and borehole water. The only disadvantage of the proposed fluoride-selective electroderflow injection system is the detection limit of 0.34 mg ly1 .
References Borzitsky, J.A., Dvinin, A., Petrukhin, O.M., Urosov, Y.I., 1993. Flow cell with double slope factor for potentiometric determination of fluoride at low concentrations. Analyst 118, 859–861. Cardwell, T.J., Cattrall, R.W., Hausen, P.C., Hamilton, I.C., 1987. Buffer
systems for use with the fluoride electrode in flow injection analysis. Anal. Chem. 59, 206–208. Fifield, F.W., 1995. The Analysis of water in environmental analytical chemistry. In: Fifield, F.W., Haines, P.J. ŽEds.., P-367. Blackie Academic and Professional, Chapman & Hall, London, UK. Frenzel, W., Bratter, P., 1986. The fluoride ion-selective electrode in flow ¨ injection analysis: Part I. General methodology. Anal. Chim. Acta 185, 127–136. Hara, H., Huang, C.C., 1997. Buffer composition suitable for determining very low fluoride concentrations using a fluoride ion-selective electrode and its application to the continuous analysis of rain water. Anal. Chim. Acta. 338, 141–147. Marshall, G.D., van Staden, J.F., 1992. Computer-aided flow-analysis for laboratory use and process analysis. Anal. Instrum. 20, 79–100. Najib, F.M., Othman, S., 1992. Simultaneous determination of chloride, bromide, iodide and fluoride with flow injectionrion-selective electrode systems. Talanta 39, 1259–1267. Slanina, J., Lingerak, W.A., Bakker, F., 1980. The use of ion-selective electrodes in manual and computer-controlled flow injection systems. Anal. Chim. Acta 117, 91–98. Trojanowicz, M., Matuszewski, W., 1982. Limitation of linear response in flow-injection systems with ion-selective electrodes. Anal. Chim. Acta 138, 71–79. van Staden, J.F., 1987. Flow injection determination of inorganic bromide in soils with a coated tubular solid-state bromide-selective electrode. Analyst 112, 595–599.
Determination of fluoride in toothpaste, effluent streams and natural and borehole water using a flow injection system with a fluoride-selective membrane electrode Raluca-Ioana Stefan a , Jacobus F. van Staden a , Hassan Y. Aboul-Enein b
b,)
a Department of Chemistry, UniÕersity of Pretoria, Pretoria 0002, South Africa Bioanalytical and Drug DeÕelopment Laboratory, Biological and Medical Research Department, MBC-03, King Faisal Specialist Hospital and Research Centre, P.O. Box 3354, Riyadh 11211, Saudi Arabia
Received 17 December 1998; received in revised form 8 March 1999; accepted 14 April 1999
Abstract An automated system for the determination of fluoride in toothpaste, effluent streams, natural and borehole water based on the concept of flow injection with a fluoride-selective membrane electrode as sensor is described. The system is suitable for the on-line monitoring of fluoride at a sampling rate of 60 samples hy1 in the linear range 10y4 –10y1 Žapproximately 1.9–1900 mg ly1 . with an RSD of better than 0.6%. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Fluoride-selective membrane electrode; Flow injection analysis
1. Introduction The fluoride-selective membrane electrode is an ideal detecting device for the determination of fluoride using flow injection systems, especially due to the accuracy of the analytical information obtained. Extensive studies concerning the use of fluoride-selective electrodes in flow injection analysis ŽFIA. have been performed by various research groups ŽSlanina et al., 1980; Trojanowicz and Matuszewski, 1982; Frenzel and Bratter, 1986; Najib and ¨ Othman, 1992; Borzitsky et al., 1993.. It is well known that the limitation of using ion-selective electrodes as detector systems are the limit of detection, influence of pH and selectivity of these devices. Although a very good correlation is obtained between log c and relative peak height in flow injection systems, this is not always the case ) Corresponding author. Tel.: q966-1-442-7859; fax: q966-1-4427858; e-mail: [email protected]
for Nernstian response of the electrode. It was proved that the limit of detection and linear response are given by the nature of the buffer used in the carrier stream ŽCardwell et al., 1987; Hara and Huang, 1997.. By using FIA systems, the selectivity of the electrode is improved due to the very short contact time of the interfering ions with the electrode membrane. The advantages associated with the coupling of the fluoride-selective electrode as sensor to an FIA system, enables this combination to be used in a reliable low cost, robust analyser for the determination of fluoride in toothpaste as well as in effluent streams, natural and borehole water samples. The fluoride content contributes to the quality of toothpaste. The maximum level of the fluoride in water should be 1.500 mg ly1 ŽFifield, 1995.. In order to sustain the quality of life and for healthy reasons, the fluoride content in water should be controlled carefully. This paper reports on the incorporation of a fluorideselective electrode in an FIA system for the determination
0031-6865r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 1 - 6 8 6 5 Ž 9 9 . 0 0 0 0 8 - 4
308
R.-I. Stefan et al.r Pharmaceutica Acta HelÕetiae 73 (1999) 307–310
of fluoride in toothpaste and in effluent streams, natural and borehole water.
Table 2 Selectivity coefficients of the fluoride-selective electrode incorporated into the FIA system Interfering species ŽJ zy . y
Br Cly COy2 3
2. Experimental 2.1. Reagents and solutions
Selectivity coefficients 3.0=10y5 1.4=10y5 1.2=10y5
All measurements are average of seven determinations.
All reagents were prepared from analytical reagent grade chemicals unless specified otherwise. Deionised water from a Modulab system ŽContinental Water Systems, San Antonio, TX, USA. was used for dilution. The solutions were prepared as follows: Total strength adjustment buffer ŽTISAB II. was used as carrier stream. Fifty-seven milliliters of glacial acetic acid and 74 g reagent grade potassium chloride were carefully dissolved in 500 ml distilled water in a 1-l beaker. The pH was adjusted to 5.50 by adding 5 mol ly1 KOH solution. The solution was quantitatively diluted to mark in a 1-l volumetric flask with distilled water. The NaF solutions were prepared by serial dilutions from a standard 0.10 mol ly1 stock solution.
at 10 rev miny1 supplied the carrier streams to the manifold system. Tygon tubing Ž0.51 mm i.d.. was used to construct the manifold; coils were wound round suitable lengths of glass tubing Ž15 mm o.d... A 60-s cycle sampling time was used, giving the system a capacity of about 60 samples hy1 . Data acquisition and device control were achieved using a PC30-B interface board ŽEagle Electric, Cape Town, South Africa.. The FlowTEK ŽMarshall and van Staden, 1992. software package Žobtainable from MINTEK. for computer-aided flow analysis was used throughout for device control and data acquisition.
2.2. Apparatus An Orion fluoride–selective electrode ŽOrion Research Incorporated Laboratory Products Group. Model 94-09 was used for all measurements in combination with a double junction reference Orion electrode Model 90-02. The potentials were measured at room temperature with an Orion Research ŽModel 901. microprocessor Ionalyzer. 2.3. Flow injection system The electrodes were incorporated into the conduits of a flow injection system with basic design similar to that previously described Žvan Staden, 1987.. A Carle microvolume two-position sampling valve ŽCarle No. 2014. containing two identical sample loops was used. Each loop has a volume of 30 ml. A Cenco peristaltic pump operating Table 1 Response characteristics of the fluoride-selective electrode incorporated into the conduits of an FIA system Parameter
Response
Slope Intercept, H8 Linear range ŽLR. Žmol ly1 . Detection limit ŽDL. Žmol ly1 .
0.400"0.002 1.910"0.021 10y4 –10y1 1.8=10y5
All measurements are average of 10 determinations. LR is approximately 1.9–1900 mg ly1 . DL is about 0.34 mg ly1 .
2.4. Preparation of toothpaste samples A gram of Aquafresh toothpaste ŽSmithKline Beecham Consumer, Healthcare, Brentford, UK. was weighted into a 50-ml beaker. The sample was dissolved using 2 ml distilled water and 2 ml concentrated HCl. The solution was diluted to 50 ml in a volumetric flask, using 25 ml TISAB II ŽpH s 5.50., and distilled water.
3. Results and discussion The response characteristics and selectivity coefficients of the fluoride-selective electrode incorporated into the conduits of the flow injection system was evaluated under optimum running conditions. The analytical applicability of the proposed ISE–FIA system was further evaluated on toothpaste, natural and borehole water. 3.1. Electrode response The electrode characteristics of the fluoride-selective membrane electrode incorporated into the FIA system are summarised in Table 1. The equation of the calibration graph is: H s H8 y 0.40 pF
R.-I. Stefan et al.r Pharmaceutica Acta HelÕetiae 73 (1999) 307–310 Table 3 Performance and reproducibility of the proposed ISE–FIA system for fluoride in toothpastea samplesb Sample
Proposed ISE–FIA system. Fluoride concentration in mg ly1
%RSD
1 2 3 4 5 6 7
14.44 14.82 14.55 13.68 13.68 14.82 14.22
0.2 0.4 0.5 0.6 0.6 0.4 0.1
309
COy2 ions do not interfere even when the ratio between 3 the concentration of fluoride and interfering ions Žmol ly1 . is 1:1000. 3.3. Analytical applications
a
Fluoride concentration in tooth pastes14.66 mg ly1 . Mean result of seven tests in each case, with relative standard deviation for the ISE–FIA system.
b
x where H is the relative peak height and pF s ylog w cy F ; with a correlation coefficient of 0.9994. It is clear from the results in Table 1 that with the linear response range between 10y4 and 10y1 mol ly1 Žapproximately 1.9–1900 mg ly1 ., the very high precision obtained and the stability of the ISE–FIA system with time, the proposed ISE–FIA system seems to be ideally suitable as a reliable low cost analyser especially in the manufacturing of toothpaste and in the on-line monitoring of water samples. 3.2. SelectiÕity of the electrode The effect of some of possible interfering species on the response of the electrode was studied by using the mixed solutions method. The selectivity coefficients presented in Table 2, show that the response of the fluoride-selective membrane electrode is not affected by the presence of Bry, Cly and COy2 ions and this fact demonstrates the speci3 ficity of the electrode for fluoride assay. Bry, Cly and
The response characteristics as well as the selectivity of the fluoride-selective membrane electrode incorporated into the conduits of the FIA system assure a reliable assay of fluoride in toothpaste and in effluent streams, natural and borehole water. The accuracy of the proposed ISE–FIA system was tested by comparing the results of a number of toothpaste samples with the value of 14.66 mg ly1 given by the manufacturer. Results compared favourably as can be seen from Table 3. Fluoride can be determined in toothpaste with an average recovery of 99.73% and an RSD of 0.6% on seven tests of each sample. The accuracy of the proposed ISE–FIA system was also tested for water samples by comparing the results of a number effluent streams, natural water and borehole samples with those obtained by a standard automated segmented method. For the determination of fluoride in natural and borehole water samples where the values were below the detection limit of 0.34 mg ly1 of the proposed system, it was necessary to test the accuracy of proposed system by the standard additions method due to the low fluoride concentrations in these water samples. Known amounts of fluoride were added to these water samples which were processed after addition. The results compared very well with those obtained by a standard automated segmented method as seen from Table 4. The average recovery of fluoride in water samples is 100.35%. Analysis were also performed in a random order to test carry-over effects. Carry-over from one sample to another is negligible at a sample throughput of 60 samples hy1 .
Table 4 Performance and reproducibility of the proposed ISE–FIA system for the determination of fluoride in effluent streams, natural and borehole water a Sample
Automated segmented method. Fluoride concentration in mg ly1
Proposed ISE–FIA system with standard addition. Fluoride concentration in mg ly1
Ž%RSD.
1 2 3 4 5 6 7 8
0.269 0.311 0.328 0.796 0.514 1.220 0.905 0.516
0.285 b 0.304 b 0.323 b 0.794 0.514 1.219 0.905 0.514
0.2 0.3 0.3 0.25 0.5 0.2 0.25 0.4
a b
Mean result of 10 tests in each case, with relative standard deviation for the proposed ISE–FIA system. Done by using the standard additions method.
310
R.-I. Stefan et al.r Pharmaceutica Acta HelÕetiae 73 (1999) 307–310
4. Conclusion The proposed ISE–FIA system is suitable for on-line fluoride monitoring at a rate of approximately 60 actual samplesrhy1 . As a low cost, low maintenance Žespecially with the low cost fluoride-selective electrode as detector. reliable analyser, the system should be particularly attractive for the on-line determination of the fluoride in the manufacturing of toothpaste and in the on-line monitoring of fluoride in effluent streams, natural and borehole water. The only disadvantage of the proposed fluoride-selective electroderflow injection system is the detection limit of 0.34 mg ly1 .
References Borzitsky, J.A., Dvinin, A., Petrukhin, O.M., Urosov, Y.I., 1993. Flow cell with double slope factor for potentiometric determination of fluoride at low concentrations. Analyst 118, 859–861. Cardwell, T.J., Cattrall, R.W., Hausen, P.C., Hamilton, I.C., 1987. Buffer
systems for use with the fluoride electrode in flow injection analysis. Anal. Chem. 59, 206–208. Fifield, F.W., 1995. The Analysis of water in environmental analytical chemistry. In: Fifield, F.W., Haines, P.J. ŽEds.., P-367. Blackie Academic and Professional, Chapman & Hall, London, UK. Frenzel, W., Bratter, P., 1986. The fluoride ion-selective electrode in flow ¨ injection analysis: Part I. General methodology. Anal. Chim. Acta 185, 127–136. Hara, H., Huang, C.C., 1997. Buffer composition suitable for determining very low fluoride concentrations using a fluoride ion-selective electrode and its application to the continuous analysis of rain water. Anal. Chim. Acta. 338, 141–147. Marshall, G.D., van Staden, J.F., 1992. Computer-aided flow-analysis for laboratory use and process analysis. Anal. Instrum. 20, 79–100. Najib, F.M., Othman, S., 1992. Simultaneous determination of chloride, bromide, iodide and fluoride with flow injectionrion-selective electrode systems. Talanta 39, 1259–1267. Slanina, J., Lingerak, W.A., Bakker, F., 1980. The use of ion-selective electrodes in manual and computer-controlled flow injection systems. Anal. Chim. Acta 117, 91–98. Trojanowicz, M., Matuszewski, W., 1982. Limitation of linear response in flow-injection systems with ion-selective electrodes. Anal. Chim. Acta 138, 71–79. van Staden, J.F., 1987. Flow injection determination of inorganic bromide in soils with a coated tubular solid-state bromide-selective electrode. Analyst 112, 595–599.