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The evaluation and comparison of ion chromatography, segmented flow analysis and flow injection analysis for the determination on nitrate in natural surface waters
War. Res. Vol. 23, No. 4, pp. 519-521, 1989 Printed in Great Britain
0043-1354/89 $3.00+ 0.00 Pergamon Press pie
RESEARCH NOTE THE EVALUATION A N D COMPARISON OF ION CHROMATOGRAPHY, SEGMENTED FLOW ANALYSIS A N D FLOW INJECTION ANALYSIS FOR THE DETERMINATION ON NITRATE IN NATURAL SURFACE WATERS EILEEN M. BURKE1, F. XAVmR SUAREZz, DANIEL C. HILLMAN1. and EDWARD M. HEITHMAR2 qnorganic Chemistry Section, Lockheed Engineering and Sciences Company, 1050 E. Flamingo Road, Las Vegas, NV 89119, U.S.A. and 7Quality Assurance Division, Environmental Monitoring Systems Laboratory, U.S. Environmental Protection Agency, Las Vegas, NV 89114, U.S.A. (First received May 1988; accepted in revised form September 1988) Abstract--A study to evaluate the comparability of IC, SFA and FIA for the determination of nitrate in natural surface waters has been completed. Assessmentof method performance was based on the precision, accuracy, detection limit and analysis rate obtained from analyses of synthetic and natural samples by each technique. Precision (as RSD) and accuracy (as % recovery) for each method are typically 3 and 103% for IC, 1 and 101% for SFA and 1 and 100% for FIA, respectively.The method detection limits (expressed as mg/l nitrate ion) were 0.007 mg/l for IC, 0.006 mg/l for SFA and 0.030 mg/l for FlA. IC analysis time is 10 samples/h, however chloride and sulfate can be determined along with nitrate in a sample run. SFA and FIA can analyze 30--60 samples/h, but only nitrate can be determined. The colorimetric method responses were significantlylarger than IC responses. The bias is apparently related to the sample matrix but has not been traced to any single matrix component. No relationship to dissolved organic carbon (DOC) was observed. Key words---comparative evaluation, nitrate, colorimetry, ion chromatography, water analysis
INTRODUCTION
Nitric acid from atmospheric nitrogen oxides can be a prime contributor to acidic deposition. Acidic deposition can lead to a decline in phytoplankton and fish populations in natural surface waters and is suspected to cause an increase in the dissolved heavy metal concentration in some systems. Also, an excessive amount of nitrate in some natural waters will cause eutrophication (National Research Council, 1978; Smith, 1974). For these reasons, nitrate determinations are essential to limnological data bases. The National Surface Water Survey (NSWS) is a project conducted by the U.S. Environmental Protection Agency as part of the National Acidic Precipitation Assessment Program. A primary objective of the NSWS is to quantify the chemistry of low alkalinity lakes and streams in regions of the United States subject to change as a result of acidic deposition (Linthurst et al., 1987). Nitrate is one of the key parameters measured in samples collected during the NSWS. Ion chromatography (IC) was specified for nitrate determinations because of its high sensitivity, specificity and capability of measuring chloride and sulfate in the same sample. Previous methods have included u.v. absorbance spectrophotometry, ion *Author to whom all correspondence should be addressed.
specific electrodes, nitration reactions (e.g. phenodisulfonic acid, brucine, chromotropic acid) and reduction methods utilizing various metals to reduce nitrate to nitrite (nitrite is then coupled with a diazodye and measured colorimetrically; Koupparis et al., 1982; National Research Council, 1978). Nitrate in surface waters has often been determined by cadmium reduction colorimetry utilizing segmented flow analysis (SFA) (Henriksen and Selmer-Olsen, 1970). Similar chemistry has been utilized in flow injection analysis (FIA) (Anderson, 1979; Van Staden, 1982). In order to determine the comparability of historical nitrate data with values obtained in the current survey, a method comparison study among IC, SFA and FIA was required. The study entailed determining the analytical characteristics for each method and comparing the analytical results from the analysis of several dozen lake samples. EXPERIMENTAL
The comparison oflC, SFA and FIA for the determination of nitrate in natural surface waters was completed in two parts. In part one, analytical characteristics of each method (precision, accuracy, detection limit and analysis rate) were determined from replicate analyses of both synthetic and natural water samples. In part two, the methods were compared analyzing 50 lake samples representativeof those in the NSWS.
519
520
Research Note
Table 1. Summary of method characteristics for IC, SFA and FIA Parameter Precision (as % RSD) Accuracy (as % recovery) IDL (mg/I nitrate) M D L (mg/l nitrate) Analysis rate (samples/h)
IC
SFA
FIA
4+ 1 100 + 6 0.007 0.007 8
3+ 2 102 + 3 0.006 0.006 30
3 +_ 1 99 + 3 0.010 0.030 45
Instrumentation and methods IC. Suppressed IC was performed on a Dionex 2010i with an AG4 pre-column, AS4A separator column, anion micromembrane suppressor, and the Autolon 300 Data System. The NSWS IC protocol for nitrate determination was followed (Hillman et al., 1986). A 0.75 mM NaHCO3/2.2 mM N a 2 C O 3 eluant and 0.025 N H2SO4 regenerant were pumped at a rate of 2.0 ml/min. The sample loop size was 0.30 ml. To improve accuracy separate calibrations were performed to cover the concentration range of interest (0.00-0.20 and 0.20-1.5 mg/l nitrate). SFA. A Technicon GTpc AutoAnalyzerll system was used to determine nitrate by SFA. The standard EPA protocol was followed (EMSL, 1983). A water carrier (ASTM Type II), sulfanilamide/N-(l-Napthyl) ethylenediamine dihydrochloride (SA/NED) color reagent and an ammonium chloride-EDTA buffer reagent system with a copperizedcadmium reduction column was used. As with IC separate calibrations were performed to cover the range of interest (0.00-0.20 and 0.20-1.50 mg/1 nitrate). FlA. A dual channel Lachat QuikChem FIA equipped with an IBM PC/XT Data System was used to determine nitrate by FlA. The FIA protocol developed by Lachat was followed (Steig, 1986; McKnight, 1986). The chemistry of the FIA method is identical to that of the SFA method. Manifold tubing was 0.8 mm i.d. Teflon. Description of experiments Experiment 1. Determination of method eharacteristics. The analytical characteristics for each method were evaluated by analyzing 4 synthetic samples (3 in-house and I EPA reference sample) and 3 natural samples. Each sample was measured 7 times each day on 5 different days. The nitrate concentration in the samples ranged from 0.04 to 1.5 mg/l. The natural samples were spiked as necessary to fall into this range. Experiment 2. Method comparison. The method comparison between IC, SFA and FIA involved a single day of analysis on lake samples obtained during the NSWS. The
same calibration standards were used to calibrate each method to ensure that any difference in response was not due to difference in calibration solutions. Thirteen samples did not contain nitrate and were spiked appropriately.
R E S U L T S AND D I S C U S S I O N
Each method can be described in terms of its analytical characteristics, i.e. its precision, accuracy, detection limit and analysis rate. In this study precision is expressed as percent relative standard deviation (% RSD) calculated by analysis of variance of the synthetic and natural sample results for each concentration level. For synthetic samples, accuracy is expressed as % recovery of the theoretical concentration. For natural samples, accuracy is expressed as % recovery of a spike added to the sample. The instrumental detection limit (IDL) is defined as 3 times the SD of the measured concentration from the analysis of the 0.020 mg/l calibration standard. The method detection limit ( M D L ) is defined as 3 times the SD of the measured concentration from the analysis of a natural sample containing <0.02 mg/l. A summary of analytical characteristics is presented in Table !. Generally, the precision and accuracy estimates for IC, S F A and FIA were comparable. The instrument detection limits for each method were similar, all about 0.01 mg/I. The M D L s for IC and S F A are identical to the IDLs. However, for FIA, the M D L is about 3 times as high as the IDL. This is probably due to a non-optimized flow system leading to a sample matrix-dependent signal. The M D L for IC has the potential to be lowered 1-2 orders of magnitude through the use of a concentrator column (Wetzel et al., 1979). However, in this work, a concentrator column was not tested. S F A and F I A analysis rates are faster than IC analysis rates. Significantly faster rates (60-120 samples/h) for S F A and F I A than those listed have been reported, but analysis conditions are
~0~) 0.25Z 0.2Z
0.15-
I,~ I..Z
0.1-
Z 0 0
0.05-
~u.I
O.
~ -0.05-
S
u. 0
-0.1-
0.2
0.4
0.5
0.8
1.0
1.2
1.4
1.6
CONCENTRATION BY IC (mg/L NO3-)
Fig. 1. A practical view of the differences between the colorimetric and IC method responses for nitrate determination.
Research Note different (Buccafuri et al., 1987; Koupparis et al., 1982). IC analysis rates are slower because later eluting peaks (e.g. sulfate) must elute prior to analyzing the next nitrate sample. By the same token, multi-element analyses can be performed in the same analysis. The results from the analyses of 34 lake samples by IC, SFA and FIA showed a definite difference between IC and the colorimetric analyses. The colorimetric analysis (SFA and FIA) results are statistically larger than IC results (alpha = 0.05, Wilcoxon signed rank test). From a practical standpoint, the differences are generally < 10%, as seen in Fig. 1. Initially, it was theorized that color from dissolved organic carbon (DOC) in the sample could lead to the positive bias (with respect to the IC results). However, a regression of the difference between methods vs DOC reveals no obvious trends (correlation coefficients < 0.1 and slopes < 0.003). The positive bias is most likely due to the overall sample matrix characteristics which effect the colorimetric analysis differently than the IC analysis. In conclusion, the analytical characteristics of the colorimetric methods (SFA and FIA) are similar to those for the chromatographic method (IC). If only nitrate is of interest, SFA or FIA are most likely the method of choice due to the higher analysis rate. If other anions are of interest (such as chloride and sulfate), IC may be the preferred method. In the analysis of low ionic-strength natural surface waters, SFA and FIA results are statistically larger than IC results. Generally the relative difference is < 10%. This bias must be considered when comparing nitrate data obtained from colorimetric analyses to nitrate data obtained by IC analyses. DOC was investigated as the potential cause for the difference but no relationship was found. The bias is apparently related to the sample matrix but has not been traced to any single matrix component. Further investigations are necessary in order to determine the cause and possible elimination of the bias. Acknowledgements--This study has been funded by the United States Environmental Protection Agency under contract number 68-03-3249 to Lockheed Engineering and Sciences Company. This work has not been subjected to Agency reviewand does not necessarilyreflectthe viewof the
521
Agency.The mention of trade names or commercialproducts does not constitute endorsement or recommendation for use. We thank Rick Buell, Forest Garner and Annalisa Hall for their contribution to the preparation of this paper.
REFERENCES
Anderson L. (1979) Simultaneous determination of nitrite and nitrate by flow injection analysis. Analyt. ehim. Acta 110, 123-128. Buccafuri A., Conetta A. and Burns D. A. (1987) Analysis of low level nutrients in the gg/I range using a high speed random access continuous-flow analyzer. 29th Rocky Mountain Conference, Denver, Colo., Paper No. 73. Environmental Monitoring and Support Laboratory (1983) Methods for chemical analysis of water and wastes. EPA-600/4-79-020, Environmental Protection Agency, Cincinnati, Ohio, Method 353.2. Henriksen A. and Selmer-Olsen A. R. (1970) Automatic methods for determining nitrate and nitrite in water and soil extracts. Analyst 95, 514-518. Hillman D. C., Potter J. F. and Simon S. J. (1986) National surface water survey eastern lakes survey (phase I--synoptic chemistry) analytical methods manual. EPA-600/486-009, Section 6. Koupparis M. A., Walczak K. M. and Malmstadt H. V. (1982) Automated determination of nitrate in waters with a reduction column in a microcomputer-based stoppedflow sample processing system. Analyt. chim. Acta 142, 119-127. Linthurst R., Landers D. H., Eilers J. M., Brakke D. F., Overton W. S., Meier E. P. and Crowe R. E. (1987) Characteristics of Lakes in the Eastern United States, Vol. 1. Population Descriptions and Physico-Chemical Relationships. EPA 600/4-86/007, Environmental Protection Agency, Washington, D.C. McKnight R. (1986) Quikchem method No. 10-107-04-!-c. Lachat Chemicals Inc., Mequon, Wis. National Research Council (1978) Nitrates: An Environmental Assessment. National Academy of Sciences, Washington, D.C. Smith R. L. (1974) Ecology and Field Biology, 2nd edition. Harper & Row, New York. Steig S. (1986) Quikchem method No. 10-107-04-1-b.Lachat Chemicals Inc., Mequon, Wis. Van Staden J. F. (1982) Automated simultaneous determination of nitrate and nitrite by pre-valve reduction of nitrate in a flow-injectionsystem. Analyt. chim. Acta 138, 403-408. Wetzel R. A., Anderson C. L., Schleicher H. and Crook G. D. (1979) Determination of trace level ions by ion chromatography with concentrator columns. Analyt. Chem. 51, 1532-1535.
0043-1354/89 $3.00+ 0.00 Pergamon Press pie
RESEARCH NOTE THE EVALUATION A N D COMPARISON OF ION CHROMATOGRAPHY, SEGMENTED FLOW ANALYSIS A N D FLOW INJECTION ANALYSIS FOR THE DETERMINATION ON NITRATE IN NATURAL SURFACE WATERS EILEEN M. BURKE1, F. XAVmR SUAREZz, DANIEL C. HILLMAN1. and EDWARD M. HEITHMAR2 qnorganic Chemistry Section, Lockheed Engineering and Sciences Company, 1050 E. Flamingo Road, Las Vegas, NV 89119, U.S.A. and 7Quality Assurance Division, Environmental Monitoring Systems Laboratory, U.S. Environmental Protection Agency, Las Vegas, NV 89114, U.S.A. (First received May 1988; accepted in revised form September 1988) Abstract--A study to evaluate the comparability of IC, SFA and FIA for the determination of nitrate in natural surface waters has been completed. Assessmentof method performance was based on the precision, accuracy, detection limit and analysis rate obtained from analyses of synthetic and natural samples by each technique. Precision (as RSD) and accuracy (as % recovery) for each method are typically 3 and 103% for IC, 1 and 101% for SFA and 1 and 100% for FIA, respectively.The method detection limits (expressed as mg/l nitrate ion) were 0.007 mg/l for IC, 0.006 mg/l for SFA and 0.030 mg/l for FlA. IC analysis time is 10 samples/h, however chloride and sulfate can be determined along with nitrate in a sample run. SFA and FIA can analyze 30--60 samples/h, but only nitrate can be determined. The colorimetric method responses were significantlylarger than IC responses. The bias is apparently related to the sample matrix but has not been traced to any single matrix component. No relationship to dissolved organic carbon (DOC) was observed. Key words---comparative evaluation, nitrate, colorimetry, ion chromatography, water analysis
INTRODUCTION
Nitric acid from atmospheric nitrogen oxides can be a prime contributor to acidic deposition. Acidic deposition can lead to a decline in phytoplankton and fish populations in natural surface waters and is suspected to cause an increase in the dissolved heavy metal concentration in some systems. Also, an excessive amount of nitrate in some natural waters will cause eutrophication (National Research Council, 1978; Smith, 1974). For these reasons, nitrate determinations are essential to limnological data bases. The National Surface Water Survey (NSWS) is a project conducted by the U.S. Environmental Protection Agency as part of the National Acidic Precipitation Assessment Program. A primary objective of the NSWS is to quantify the chemistry of low alkalinity lakes and streams in regions of the United States subject to change as a result of acidic deposition (Linthurst et al., 1987). Nitrate is one of the key parameters measured in samples collected during the NSWS. Ion chromatography (IC) was specified for nitrate determinations because of its high sensitivity, specificity and capability of measuring chloride and sulfate in the same sample. Previous methods have included u.v. absorbance spectrophotometry, ion *Author to whom all correspondence should be addressed.
specific electrodes, nitration reactions (e.g. phenodisulfonic acid, brucine, chromotropic acid) and reduction methods utilizing various metals to reduce nitrate to nitrite (nitrite is then coupled with a diazodye and measured colorimetrically; Koupparis et al., 1982; National Research Council, 1978). Nitrate in surface waters has often been determined by cadmium reduction colorimetry utilizing segmented flow analysis (SFA) (Henriksen and Selmer-Olsen, 1970). Similar chemistry has been utilized in flow injection analysis (FIA) (Anderson, 1979; Van Staden, 1982). In order to determine the comparability of historical nitrate data with values obtained in the current survey, a method comparison study among IC, SFA and FIA was required. The study entailed determining the analytical characteristics for each method and comparing the analytical results from the analysis of several dozen lake samples. EXPERIMENTAL
The comparison oflC, SFA and FIA for the determination of nitrate in natural surface waters was completed in two parts. In part one, analytical characteristics of each method (precision, accuracy, detection limit and analysis rate) were determined from replicate analyses of both synthetic and natural water samples. In part two, the methods were compared analyzing 50 lake samples representativeof those in the NSWS.
519
520
Research Note
Table 1. Summary of method characteristics for IC, SFA and FIA Parameter Precision (as % RSD) Accuracy (as % recovery) IDL (mg/I nitrate) M D L (mg/l nitrate) Analysis rate (samples/h)
IC
SFA
FIA
4+ 1 100 + 6 0.007 0.007 8
3+ 2 102 + 3 0.006 0.006 30
3 +_ 1 99 + 3 0.010 0.030 45
Instrumentation and methods IC. Suppressed IC was performed on a Dionex 2010i with an AG4 pre-column, AS4A separator column, anion micromembrane suppressor, and the Autolon 300 Data System. The NSWS IC protocol for nitrate determination was followed (Hillman et al., 1986). A 0.75 mM NaHCO3/2.2 mM N a 2 C O 3 eluant and 0.025 N H2SO4 regenerant were pumped at a rate of 2.0 ml/min. The sample loop size was 0.30 ml. To improve accuracy separate calibrations were performed to cover the concentration range of interest (0.00-0.20 and 0.20-1.5 mg/l nitrate). SFA. A Technicon GTpc AutoAnalyzerll system was used to determine nitrate by SFA. The standard EPA protocol was followed (EMSL, 1983). A water carrier (ASTM Type II), sulfanilamide/N-(l-Napthyl) ethylenediamine dihydrochloride (SA/NED) color reagent and an ammonium chloride-EDTA buffer reagent system with a copperizedcadmium reduction column was used. As with IC separate calibrations were performed to cover the range of interest (0.00-0.20 and 0.20-1.50 mg/1 nitrate). FlA. A dual channel Lachat QuikChem FIA equipped with an IBM PC/XT Data System was used to determine nitrate by FlA. The FIA protocol developed by Lachat was followed (Steig, 1986; McKnight, 1986). The chemistry of the FIA method is identical to that of the SFA method. Manifold tubing was 0.8 mm i.d. Teflon. Description of experiments Experiment 1. Determination of method eharacteristics. The analytical characteristics for each method were evaluated by analyzing 4 synthetic samples (3 in-house and I EPA reference sample) and 3 natural samples. Each sample was measured 7 times each day on 5 different days. The nitrate concentration in the samples ranged from 0.04 to 1.5 mg/l. The natural samples were spiked as necessary to fall into this range. Experiment 2. Method comparison. The method comparison between IC, SFA and FIA involved a single day of analysis on lake samples obtained during the NSWS. The
same calibration standards were used to calibrate each method to ensure that any difference in response was not due to difference in calibration solutions. Thirteen samples did not contain nitrate and were spiked appropriately.
R E S U L T S AND D I S C U S S I O N
Each method can be described in terms of its analytical characteristics, i.e. its precision, accuracy, detection limit and analysis rate. In this study precision is expressed as percent relative standard deviation (% RSD) calculated by analysis of variance of the synthetic and natural sample results for each concentration level. For synthetic samples, accuracy is expressed as % recovery of the theoretical concentration. For natural samples, accuracy is expressed as % recovery of a spike added to the sample. The instrumental detection limit (IDL) is defined as 3 times the SD of the measured concentration from the analysis of the 0.020 mg/l calibration standard. The method detection limit ( M D L ) is defined as 3 times the SD of the measured concentration from the analysis of a natural sample containing <0.02 mg/l. A summary of analytical characteristics is presented in Table !. Generally, the precision and accuracy estimates for IC, S F A and FIA were comparable. The instrument detection limits for each method were similar, all about 0.01 mg/I. The M D L s for IC and S F A are identical to the IDLs. However, for FIA, the M D L is about 3 times as high as the IDL. This is probably due to a non-optimized flow system leading to a sample matrix-dependent signal. The M D L for IC has the potential to be lowered 1-2 orders of magnitude through the use of a concentrator column (Wetzel et al., 1979). However, in this work, a concentrator column was not tested. S F A and F I A analysis rates are faster than IC analysis rates. Significantly faster rates (60-120 samples/h) for S F A and F I A than those listed have been reported, but analysis conditions are
~0~) 0.25Z 0.2Z
0.15-
I,~ I..Z
0.1-
Z 0 0
0.05-
~u.I
O.
~ -0.05-
S
u. 0
-0.1-
0.2
0.4
0.5
0.8
1.0
1.2
1.4
1.6
CONCENTRATION BY IC (mg/L NO3-)
Fig. 1. A practical view of the differences between the colorimetric and IC method responses for nitrate determination.
Research Note different (Buccafuri et al., 1987; Koupparis et al., 1982). IC analysis rates are slower because later eluting peaks (e.g. sulfate) must elute prior to analyzing the next nitrate sample. By the same token, multi-element analyses can be performed in the same analysis. The results from the analyses of 34 lake samples by IC, SFA and FIA showed a definite difference between IC and the colorimetric analyses. The colorimetric analysis (SFA and FIA) results are statistically larger than IC results (alpha = 0.05, Wilcoxon signed rank test). From a practical standpoint, the differences are generally < 10%, as seen in Fig. 1. Initially, it was theorized that color from dissolved organic carbon (DOC) in the sample could lead to the positive bias (with respect to the IC results). However, a regression of the difference between methods vs DOC reveals no obvious trends (correlation coefficients < 0.1 and slopes < 0.003). The positive bias is most likely due to the overall sample matrix characteristics which effect the colorimetric analysis differently than the IC analysis. In conclusion, the analytical characteristics of the colorimetric methods (SFA and FIA) are similar to those for the chromatographic method (IC). If only nitrate is of interest, SFA or FIA are most likely the method of choice due to the higher analysis rate. If other anions are of interest (such as chloride and sulfate), IC may be the preferred method. In the analysis of low ionic-strength natural surface waters, SFA and FIA results are statistically larger than IC results. Generally the relative difference is < 10%. This bias must be considered when comparing nitrate data obtained from colorimetric analyses to nitrate data obtained by IC analyses. DOC was investigated as the potential cause for the difference but no relationship was found. The bias is apparently related to the sample matrix but has not been traced to any single matrix component. Further investigations are necessary in order to determine the cause and possible elimination of the bias. Acknowledgements--This study has been funded by the United States Environmental Protection Agency under contract number 68-03-3249 to Lockheed Engineering and Sciences Company. This work has not been subjected to Agency reviewand does not necessarilyreflectthe viewof the
521
Agency.The mention of trade names or commercialproducts does not constitute endorsement or recommendation for use. We thank Rick Buell, Forest Garner and Annalisa Hall for their contribution to the preparation of this paper.
REFERENCES
Anderson L. (1979) Simultaneous determination of nitrite and nitrate by flow injection analysis. Analyt. ehim. Acta 110, 123-128. Buccafuri A., Conetta A. and Burns D. A. (1987) Analysis of low level nutrients in the gg/I range using a high speed random access continuous-flow analyzer. 29th Rocky Mountain Conference, Denver, Colo., Paper No. 73. Environmental Monitoring and Support Laboratory (1983) Methods for chemical analysis of water and wastes. EPA-600/4-79-020, Environmental Protection Agency, Cincinnati, Ohio, Method 353.2. Henriksen A. and Selmer-Olsen A. R. (1970) Automatic methods for determining nitrate and nitrite in water and soil extracts. Analyst 95, 514-518. Hillman D. C., Potter J. F. and Simon S. J. (1986) National surface water survey eastern lakes survey (phase I--synoptic chemistry) analytical methods manual. EPA-600/486-009, Section 6. Koupparis M. A., Walczak K. M. and Malmstadt H. V. (1982) Automated determination of nitrate in waters with a reduction column in a microcomputer-based stoppedflow sample processing system. Analyt. chim. Acta 142, 119-127. Linthurst R., Landers D. H., Eilers J. M., Brakke D. F., Overton W. S., Meier E. P. and Crowe R. E. (1987) Characteristics of Lakes in the Eastern United States, Vol. 1. Population Descriptions and Physico-Chemical Relationships. EPA 600/4-86/007, Environmental Protection Agency, Washington, D.C. McKnight R. (1986) Quikchem method No. 10-107-04-!-c. Lachat Chemicals Inc., Mequon, Wis. National Research Council (1978) Nitrates: An Environmental Assessment. National Academy of Sciences, Washington, D.C. Smith R. L. (1974) Ecology and Field Biology, 2nd edition. Harper & Row, New York. Steig S. (1986) Quikchem method No. 10-107-04-1-b.Lachat Chemicals Inc., Mequon, Wis. Van Staden J. F. (1982) Automated simultaneous determination of nitrate and nitrite by pre-valve reduction of nitrate in a flow-injectionsystem. Analyt. chim. Acta 138, 403-408. Wetzel R. A., Anderson C. L., Schleicher H. and Crook G. D. (1979) Determination of trace level ions by ion chromatography with concentrator columns. Analyt. Chem. 51, 1532-1535.