The U.S. Geological Survey (USGS) completed a verification study of selected commercially available phosphorus analyzers for their applicability to scientific surface-water applications. In this study, the analyzers were the Hach EZ7800 TOPHO, Hach Phosphax sc, Sea-Bird Scientific HydroCycle-PO4, and the YSI Inc. Alyza IQ PO4. Verification tests included laboratory trials comparing analyzer results to known standards with several known concentrations of dissolved organic matter and waste production estimates. Field trials were completed at the Vermilion River near Danville, Illinois (U.S. Geological Survey station 03339000), where analyzer-measured concentrations were compared against discrete samples across a wide range of environmental conditions from November 2020 to August 2021. Data coverage was closely tracked for analyzer malfunctions and operator errors that caused missing data. Laboratory and field trials indicated that each analyzer is a viable option for scientific surface-water studies depending on environmental conditions. Because of the complexity of the analyzers, a substantial time investiture was required to get maximum data coverage including considerable site infrastructure investments and well-trained technicians. Data coverage was closely related to each analyzer’s ability to handle elevated turbidity levels.
U.S. Geological Survey, 2021, USGS water data for the Nation: U.S. Geological Survey National Water Information System database,
For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment—visit
For an overview of USGS information products, including maps, imagery, and publications, visit
Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.
The Next Generation Water Observing System supported the fieldwork and compilation of the data and writing of this report. The author would like to thank the manufacturers Hach, Sea-Bird Scientific, and YSI Inc. for their cooperation and technical assistance.
The author acknowledges the U.S. Geological Survey Southeast Region for supporting the fieldwork and compilation of the data and writing of this report. The author would also like to acknowledge Aubrey Bunch of the Ohio-Kentucky-Indiana Water Science Center and Paul Reneau of the Upper Midwest Water Science Center for their technical assistance and reviews of the report.
Multiply | By | To obtain |
Volume | ||
---|---|---|
gallon (gal) | 3.785 | liter (L) |
gallon (gal) | 0.003785 | cubic meter (m3) |
gallon (gal) | 3.785 | cubic decimeter (dm3) |
Flow rate | ||
gallon per minute (gal/min) | 0.06309 | liter per second (L/s) |
Multiply | By | To obtain |
Length | ||
---|---|---|
centimeter (cm) | 0.3937 | inch (in.) |
kilometer (km) | 0.6214 | mile (mi) |
meter (m) | 3.281 | foot (ft) |
micrometer (μm) | 0.00003937 | inch (in.) |
nanometer (nm) | 0.00000003937 | inch (in.) |
Area | ||
square kilometer (km2) | 247.1 | acre |
square kilometer (km2) | 0.3861 | square mile (mi2) |
Volume | ||
liter (L) | 0.2642 | gallon (gal) |
Mass | ||
gram (g) | 0.03527 | ounce, avoirdupois (oz) |
kilogram (kg) | 2.205 | pound avoirdupois (lb) |
Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:
°F = (1.8 × °C) + 32.
Phosphorus concentrations are given as milligrams per liter (mg/L) of phosphorus.
A water year is the period from October 1 to September 30 and is designated by the year in which it ends; for example, water year 2021 was from October 1, 2020, to September 30, 2021.
plus or minus
deionized water
dissolved organic matter
formazin nephelometric unit
International Humic Substances Society
National Water Quality Laboratory
orthophosphate
total phosphorus
U.S. Geological Survey
Phosphorus is of interest in surface waters because of its role in eutrophication as a potential limiting nutrient (
Phosphorus cannot be measured via wavelength absorption directly in the solution of interest. Instead, the sample solution must be chemically manipulated. The principles of the first spectrophotometric methods have been largely unchanged over many decades (
Despite the ease at which phosphorus can be determined in a laboratory setting, transforming these methods into an automated field process has been challenging. A fully automated phosphorus analyzer is too complex to match the accuracy of a laboratory analysis. Because of this, the USGS has begun using commercially available analyzers to determine their applicability to scientific goals (
Understanding the design of an automated phosphorus analyzer is important because altering filter size, method, or digestion can all change the amount of phosphorus detected. This report documents the laboratory and field verification of the Hach EZ7800 TOPHO, Hach Phosphax sc, and YSI Inc. Alyza IQ PO4, hereinafter, “EZ7800 TOPHO,” “Phosphax,” and “Alyza,” respectively. Additionally, this report incorporates the Sea-Bird Scientific HydroCycle-PO4 (hereinafter, “HydroCycle”) in field trials; however, it already has been extensively studied in laboratory settings, and therefore, those tests are not repeated (
The purpose of this report is to present results from a phosphorus analyzer verification study of selected commercially available phosphorus analyzers for their applicability to scientific surface-water applications. Analyzers used in this study were the EZ7800 TOPHO, Phosphax, HydroCycle, and Alyza. Verification tests included laboratory trials comparing analyzer results to known standards with and without dissolved organic matter and waste production estimates. Field trials were completed at the Vermilion River near Danville, Illinois (USGS station 03339000), where analyzer results were compared against discrete samples across a wide range of environmental conditions.
The analyzer and its manufacturer-listed specifications are provided in
Table 1. Manufacturer listed specifications for each analyzer.
[mg/L, milligram per liter; μm, micrometer; AC, alternating current; DC, direct current]
Analyzer name | Hach | Hach | Sea-Bird Scientific | YSI Inc. |
EZ7800 TOPHO | Phosphax sc | HydroCycle-PO4 | Alyza IQ PO4 | |
Targeted analyte | Total phosphorus | Orthophosphate | Orthophosphate | Orthophosphate |
Modified method | Blue | Yellow | Blue | Yellow |
Stated range (mg/L of phosphorus) | 0.025–5 | 0.05–15 | 0–0.3, theoretical upper limit of 1.2 | 0.02–15 |
Detection limit (mg/L of phosphorus) | 0.005 | 0.05 | 0.0023 | 0.02 |
Filter size (μm) | 100 | 0.45 | 7.5 (mean) | 0.45 |
Power supply | AC | AC | DC | AC |
When verifying an analyzer, some key parameters warrant consideration, such as targeted analyte, method, and filter size (
The EZ7800 TOPHO is different from all the other analyzers because it is the only analyzer designed to target TP. TP requires an additional digestion step to further break down phosphorus-containing compounds such as organics. It should be noted that the EZ7800 TOPHO does not measure TP directly because it has a 100-μm filter, and TP laboratory methods are unfiltered (
The EZ7800 TOPHO uses the blue method and can measure a range of 0–5 milligrams per liter (mg/L) of phosphorus nearly every 23 minutes. For this study, the EZ7800 TOPHO sampled every 30 minutes. It is a cabinet-based analyzer measuring 68.8 centimeters (cm) tall, 46 cm wide, and 34 cm deep and weighing about 25 kilograms (kg), depending on analyzer options. For surface waters, the analyzer requires the addition of the EZ9020 fast loop filtration system. This filtration system is a continuous flow cell with a stainless steel 100-μm filter mounted inside of it and requires the use of compressed air, which is used to backflush the filter. The sample is drawn from a small, continuously circulating side vessel and sent to the EZ7800 TOPHO’s digestion chamber, where the persulfate digestion takes place. The digestion uses heat and pressure to boil the sample and reagent mixture. To cool the sample back down for absorbance measurements, the analyzer required the use of continuously circulating cold water. Typically, this would be done with continuously flowing tap water at a municipal plant; however, tap water was not available at the testing site. To compensate for this, a water chiller was installed to continuously supply cooling water to the EZ7800 TOPHO. Because of the large amounts of reagent required, the reagent was supplied by the operator and mixed according to manufacturer specifications (
The Phosphax is a cabinet style orthophosphate analyzer originally designed for use at municipal water treatment facilities (
The HydroCycle is different from cabinet-based analyzers. It is the only self-contained analyzer that does not have a separate filtration unit and is specifically designed to be deployed in situ. It is a wet-chemical analyzer that uses the blue method to supply continuous orthophosphate measurements. The instrument range is 0–0.3 mg/L of phosphorus, but the upper range can be increased to 1.2 mg/L if validated with user samples. The analyzer is cylindrical, measuring 56 cm tall with an 18-cm diameter weighing about 7.6 kg with the protective casing. The sample is drawn through the bottom of the analyzer through a coarse copper mesh screen and then through a 5–10-μm filter. This analyzer is the only one without a heated reaction chamber, and therefore, the only one with a variable reaction time with a mean of 15 minutes per sample. Additionally, the analyzer has a calibration standard check that it performs at an operator-specified interval. This standard check is for operator sample verification and does not change the output values of the analyzer. The calibration standard check was run at the manufacturer-recommended interval of every six samples. For this study, the Hydrocycle was run every hour to increase the lifespan of the filter and length between required maintenance. The analyzer was controlled via SDI–12 communications by a Campbell Scientific CR6 for this study.
The Alyza is an online, cabinet-style analyzer originally designed for the continuous monitoring of orthophosphate at municipal water treatment facilities (
The equipment was installed at the Vermilion River near Danville, Illinois (USGS station 03339000;
Table 2. Continuously monitored water properties measured at the Vermilion River near Danville, Illinois, for the 2015–20 water years.
[Temp., temperature; °C, degree Celsius; SC, specific conductance; μS/cm, microsiemens per centimeter; DO, dissolved oxygen; mg/L, milligram per liter; FNU, formazin nephelometric unit; <, less than]
Statistic | Temp. |
SC |
DO |
pH | Turbidity (YSI – EXO2) |
Turbidity (Hach – Solitax sc) |
Nitrate |
Orthophosphate |
Maximum | 32.2 | 918 | 20.9 | 9.1 | 1,300 | 1,818 | 25.1 | 1.368 |
Minimum | 0.8 | 257 | 4.3 | 7.4 | 2.5 | 2.9 | 0.3 | <0.050 |
Median | 17.1 | 598 | 9.6 | 8.3 | 16.3 | 18.1 | 5.7 | 0.189 |
Table 3. Concentrations determined in discrete water samples collected from the Vermilion River near Danville, Illinois, for the 2015–20 water years.
[mg/L, milligram per liter]
Statistic | Orthophosphate |
Total phosphorus |
Nitrate |
Suspended sediment |
Maximum | 1.27 | 1.69 | 10.8 | 2,210 |
Minimum | 0.019 | 0.07 | 0.54 | 1 |
Median | 0.105 | 0.25 | 5.60 | 179 |
Count | 93 | 94 | 90 | 93 |
Map of the Vermilion River Basin showing land use and Vermillion River near Danville, Illinois (U.S. Geological Survey station 03339000).
Figure 1. Map of the Vermilion River Basin showing land use and Vermillion River near Danville, Illinois.
A pumping system was constructed in 2017 to draw water from the river via a submersible pump into various tanks and flowing water chambers. The pumped water has about a 9-m maximum head difference and a 28-m run to the gage house. The pumping system was necessary to meet the flow requirements of all the analyzers while keeping them safely out of the river’s flood zone. The pumped water is distributed from a custom manifold to two tanks and a flow cell. A 30-gallon (gal) induction tank housed the Alyza filter and the Sea-Bird HydroCycle. The EZ7800 TOPHO has the EZ9020 filtration unit that is designed as a flow cell through which water continuously flows. The Phosphax filters were housed in a 12-gal plating tank. All tanks and cells had continuous flow at a minimum of 5 gallons per minute (gal/min), depending on river stage. The building where all the instrumentation was housed was heated during the winter months to prevent freezing.
Field and laboratory trials were designed to verify analyzer results. The verification methods used are as follows:
Laboratory trials were designed to verify the analytical methods of each analyzer across a large selection of known dissolved phosphorus concentrations. Criteria in the verification include the following:
accuracy and precision,
dissolved organic carbon response, and
waste production.
Field testing was devised to verify the analyzers in a surface-water environment that represented conditions typical in many midwestern watersheds. Criteria in the verification include the following:
data quality when compared to discrete samples,
turbidity response and matrix considerations, and
data coverage and maintenance needs.
Laboratory standards and discrete samples were sent to the USGS National Water Quality Laboratory (NWQL) for analysis. Discrete water-quality samples were collected during maintenance visits, during scheduled routine sampling, or with an automated sampler. These samples allowed for quality assurance of laboratory standards and direct comparisons to each analyzer. Analytical method information for the discrete water quality samples is provided in
Table 4. Analytical method information for discrete water-quality samples.
[H2SO4, sulfuric acid; <, less than; mg/L, milligram per liter; µm, micrometer]
Constituent | Preservation | Analysis method | Limit detection method |
Total phosphorus | Chilled, H2SO4 acid to pH<2 | Colorimetry, alkaline persulfate digestion ( |
0.01 mg/L |
Orthophosphate | Filtered (0.45 µm), chilled, dark bottle | Colorimetry, phosphomolybdate reduction ( |
0.004 mg/L |
Laboratory trials were designed to verify each analyzer’s accuracy and precision across a large selection of known phosphorus concentrations. Additionally, standard tests with dissolved organics were performed because they are a known source of interference. Other laboratory tests, such as temperature or turbidity, were not performed. Although temperature affects the rate of the chemical reaction in the blue and yellow methods (
Phosphorus standard concentrations were 0.06, 0.25, 1, 2.5, and 5 mg/L of phosphorus. These standards were chosen based on the manufacturer-stated ranges of the EZ7800 TOPHO, Phosphax, and Alyza. The lowest concentration of 0.06 mg/L was limited by the Phosphax, and the highest concentration of 5 mg/L was limited by the EZ7800 TOPHO. Although the range of each analyzer is slightly different, it was deemed preferable to have comparable results among analyzers rather than the extreme ends of each analyzer’s range. Additionally, the lower range of values can be examined in field trials.
Standards were made on the day of testing using a 16.31 mg/L plus or minus (±) 0.5-percent Hach brand National Institute of Standards and Technology traceable stock phosphorus concentration (catalog number 17149, lot number A0170, expiration June 2025). The stock concentration was diluted to the desired concentration using Type I 18.2 megohm ultrapure deionized water (DIW). Quality control tests on the Type I DIW did not detect any phosphorus; therefore, additional phosphorus from the Type I DIW was considered negligible. Aliquots of each standard were sent to the USGS NWQL for quality control and analysis, and results are included in
All analyzers ran the same standard simultaneously for about 12 hours. Cleaning and (or) calibration cycles were run before and after each standard depending on manufacturer-recommended practices. The operation of these cycles allowed for analyzer stabilization effects, numerous samples, and possible drift while staying within the manufacturers’ recommendations for cleaning regiments.
Dissolved organic matter (DOM) was verified using the 0.25 mg/L phosphorus standard and various amounts of the Suwannee River natural organic matter (catalog number 2R101N) from the International Humic Substances Society (IHSS). The 0.25 mg/L phosphorus standard was chosen based on median environmental conditions of the site while being within each analyzer’s stated range. An elemental compositional analysis of the organic matter indicates phosphorus was not detected. Three DOM concentrations were made using 10, 50, and 250 mg/L of Suwannee River natural organic matter. Each DOM standard was passed through a 0.45-μm capsule filter as a precaution to avoid damage to the analyzers. An aliquot of each standard was collected in triplicate and sent to the NWQL for verification of the phosphorus concentration. The mean OP for each DOM concentration was compared against analyzer results. Results are provided in appendix 1 (
The measurement of phosphorus requires chemical manipulation using toxic heavy metals and strong acids. Waste in this study was disposed of properly according to local regulations; however, it is important to note that regulatory requirements vary from State to State. Reagent chemical concentrations for each analyzer are not available because they are typically considered proprietary information by manufacturers. However, volumes of total waste and reagent usage are available. Total waste production is defined as the total amount of liquid passed through the analyzer and includes reagents, samples, and rinse water. Reagent volumes are defined as the volumes that contain heavy metals and strong acids. Total waste volumes were not directly measured because manufacturers frequently adjusted reagent, rinse, and sample volumes to improve analyzer performances. For this reason, it is advised that volumes in this report only be used as approximations.
Field verifications took place at the Vermilion River near Danville, Illinois (USGS station 03339000) from November 20, 2020, to August 12, 2021. During this time, the analyzers were verified for accuracy, matrix considerations, and maintenance needs. Analyzers were compared to NWQL samples under a wide variety of site conditions to assess accuracy. The river matrix was verified based on turbidity effects on the analyzers. Maintenance needs were performed according to manufacturer recommendations, and data gaps were tracked.
Typical USGS practices for optical sensors involve regular fouling and calibration checks to verify proper sensor operation and data quality (
Analyzer malfunctions and operator errors are costly and cause data gaps that are detrimental to the study’s scientific goals. Data gaps can be directly related to environmental conditions, the complexity of operating the analyzer, and the extent of maintenance requirements. Documenting these issues is an important step in understanding what causes data gaps and what can be done to mitigate them.
Analyzer malfunctions are defined as problems that can be attributed to the analyzer itself. Typically, these malfunctions are the result of a component failure, such as hardware or software malfunctions. These complications are usually more costly because they require a larger time investment by the user to diagnose the issue and repair it or send the analyzer to the manufacturer for repair. Such repairs involve constant communication with the manufacturer for guidance and support. The magnitude of the failure and the ability of the manufacturer to assist in a timely manner usually determine the amount of data lost during these periods.
Operator errors are equally as disruptive as analyzer errors but typically not as severe because they can often be quickly rectified. Examples of operator errors include reagents running out; not performing recommended part replacements in a timely manner; and more commonly, human errors. Operator errors also include site-specific errors that equally affect all analyzers, such as the failure of a water pump that supplied the analyzers with river water. Operator errors are subjective but give a sense of the difficulty in effectively maintaining and operating an analyzer.
Laboratory testing of each phosphorus analyzer was completed in February 2021, which was near the beginning of the study to minimize the effects of analyzer maintenance needs regarding laboratory results. Any error caused by analyzer maintenance requirements are included in the field trials. Laboratory verification was performed using known standards with and without IHSS natural organic materials. Additionally, waste production was estimated based on manufacturer-supplied information.
Laboratory testing results of various phosphorus standard concentrations analyzed using the EZ7800 TOPHO, Phosphax, and Alyza are provided in
Table 5. Laboratory trial statistics of phosphorus concentrations for each analyzer by standard.
[All concentrations are in milligrams per liter of phosphorus; NWQL, National Water Quality Laboratory]
Phosphorus standard | NWQL | Statistic | Hach EZ7800 TOPHO | Hach Phosphax sc | YSI Inc. Alyza IQ PO4 |
0.06 | 0.057 | Mean | 0.177 | 0.065 | 0.051 |
Median | 0.175 | 0.065 | 0.054 | ||
Standard deviation | 0.022 | 0.004 | 0.005 | ||
Minimum | 0.140 | 0.054 | 0.040 | ||
Maximum | 0.22 | 0.073 | 0.056 | ||
Count | 24 | 68 | 71 | ||
0.25 | 0.247 | Mean | 0.25 | 0.25 | 0.23 |
Median | 0.24 | 0.25 | 0.22 | ||
Standard deviation | 0.027 | 0.006 | 0.007 | ||
Minimum | 0.22 | 0.23 | 0.22 | ||
Maximum | 0.31 | 0.26 | 0.24 | ||
Count | 46 | 73 | 109 | ||
1.0 | 0.986 | Mean | 0.77 | 0.99 | 1.01 |
Median | 0.74 | 0.99 | 1.01 | ||
Standard deviation | 0.074 | 0.010 | 0.005 | ||
Minimum | 0.66 | 0.97 | 1.00 | ||
Maximum | 0.95 | 1.03 | 1.02 | ||
Count | 49 | 68 | 81 | ||
2.5 | 2.506 | Mean | 1.69 | 2.50 | 2.60 |
Median | 1.59 | 2.50 | 2.60 | ||
Standard deviation | 0.234 | 0.019 | 0.009 | ||
Minimum | 1.44 | 2.46 | 2.56 | ||
Maximum | 2.10 | 2.56 | 2.65 | ||
Count | 48 | 73 | 101 | ||
5.0 | 5.077 | Mean | 3.89 | 5.07 | 5.17 |
Median | 3.87 | 5.06 | 5.17 | ||
Standard deviation | 0.198 | 0.047 | 0.009 | ||
Minimum | 3.53 | 5.01 | 5.15 | ||
Maximum | 4.40 | 5.22 | 5.19 | ||
Count | 32 | 83 | 114 |
Residual phosphorus concentrations for each analyzer and standard. Residual concentrations were determined by subtracting the phosphorus concentration determined by the National Water Quality Laboratory (NWQL) from the phosphorus concentration measured by the analyzer.
Figure 2. Boxplots showing residual phosphorus concentrations for each analyzer and standard.
As the EZ7800 TOPHO does not have a stated accuracy by the manufacturer, discrete samples are recommended to determine data quality. The EZ7800 TOPHO had substantial issues performing calibrations and deviated substantially from standard values. The range of absolute differences between the mean analyzer concentration and the National Water Quality Laboratory concentration were 0.003–1.187 mg/L of phosphorus. The problem was determined to be sample contamination from the previous run, meaning the analyzer was not draining or rinsing effectively. After working with the manufacturer, it was determined to be a design flaw. The sample contamination was most problematic with large, consecutive changes in concentration, such as the recommended 0 and 5 mg/L concentrations for calibration. The EZ7800 TOPHO uses a two-point calibration where the mean of three measurements of each standard is used to determine the linear regression coefficients. It is possible to manually change the EZ7800 TOPHO’s linear regression coefficients, and several attempts were made using the standard results to develop a linear regression as shown in
Residual phosphorus concentrations for tested calibration options for the Hach EZ7800 TOPHO. Residual concentrations were determined by subtracting the phosphorus concentration determined by the National Water Quality Laboratory (NWQL) from the phosphorus concentration measured by the analyzer. [Factory, factory default calibration; 0/5, linear regression using only 0 and 5 milligrams per liter of phosphorus standards; All, linear regression using all standards; Quadratic, quadratic regression using all standards]
Figure 3. Boxplots showing residual phosphorus concentrations for tested calibration options for the Hach EZ7800 TOPHO.
The manufacturer-stated accuracy for phosphorus concentrations measured by the Phosphax is 2 percent ± 0.05 mg/L. For all standards tested, the Phosphax measurements were within the manufacturer-stated accuracy. The range of absolute differences between the mean analyzer concentration and the National Water Quality Laboratory concentration were 0.003–0.008 mg/L of phosphorus. The analyzer had a downward pattern in median values as the concentration of the standard increased (
The mean concentrations (
The effects of three concentrations of IHSS Suwannee River natural organic matter on analyzer-measured phosphorus concentrations are shown in
Table 6. Laboratory trial statistics for phosphorus concentrations measured by each analyzer for three standard concentrations of dissolved organic matter.
[DOM, dissolved organic matter; mg/L, milligram per liter; NWQL, National Water Quality Laboratory]
DOM |
NWQL phosphorus |
Statistic | Hach EZ7800 TOPHO |
Hach Phosphax sc |
YSI Inc. Alyza IQ PO4 |
10 | 0.244 | Mean | 0.164 | 0.250 | 0.201 |
Median | 0.166 | 0.250 | 0.202 | ||
Standard deviation | 0.021 | 0.009 | 0.006 | ||
Minimum | 0.131 | 0.223 | 0.187 | ||
Maximum | 0.204 | 0.270 | 0.212 | ||
Count | 12 | 23 | 23 | ||
50 | 0.248 | Mean | 0.235 | 0.265 | 0.19 |
Median | 0.239 | 0.263 | 0.189 | ||
Standard deviation | 0.014 | 0.01 | 0.014 | ||
Minimum | 0.201 | 0.244 | 0.141 | ||
Maximum | 0.263 | 0.309 | 0.247 | ||
Count | 32 | 68 | 77 | ||
250 | 0.288 | Mean | 0.244 | 0.308 | 0.451 |
Median | 0.266 | 0.301 | 0.447 | ||
Standard deviation | 0.075 | 0.03 | 0.006 | ||
Minimum | 0.055 | 0.275 | 0.347 | ||
Maximum | 0.318 | 0.443 | 0.563 | ||
Count | 28 | 72 | 75 |
Residual phosphorus concentrations for each analyzer for three dissolved organic matter concentrations made using a 0.25 milligram per liter phosphorus standard mixed with several concentrations of Suwannee River natural organic matter. Residual concentrations were determined by subtracting the phosphorus concentration determined by the National Water Quality Laboratory (NWQL) from the phosphorus concentration measured by the analyzer.
Figure 4. Boxplots showing residual phosphorus concentrations for each analyzer for three dissolved organic matter concentrations made using a 0.25 milligram per liter phosphorus standard mixed with several concentrations of Suwannee River natural organic matter.
Manufacturer-specified total volume and reagent volume is listed in
Table 7. Approximate total waste and reagent volumes per sample.
[mL, milliliter]
Analyzer characteristic | Hach EZ7800 TOPHO | Hach Phosphax sc | Sea-Bird Scientific |
YSI Inc. Alyza IQ PO4 |
Total waste (mL) | 100 | 20 | 30.7 | 0.9 |
Reagent usage (mL) | 1.5 | 0.06 | 0.2 | 0.001 |
Field data were gathered from November 20, 2020, through August 12, 2021. Time series data are shown in
Table 8. Summary of concentrations determined in discrete samples collected from the Vermilion River near Danville, Illinois, during the study period.
[mg/L, milligram per liter]
Statistic | Orthophosphate |
Total phosphorus |
Maximum | 0.744 | 1.208 |
Minimum | 0.01 | 0.05 |
Median | 0.078 | 0.25 |
Count | 67 | 67 |
Timeseries of continuously measured phosphorus concentrations by each analyzer at the Vermilion River near Danville, Illinois (U.S. Geological Survey station 03339000) and orthophosphate and total phosphorus concentrations determined by the National Water Quality Laboratory (NWQL) in discrete samples collected from the Vermillion River at the same site. (Data from
Figure 5. Graph showing timeseries of continuously measured phosphorus concentrations by each analyzer at the Vermilion River near Danville, Illinois, and orthophosphate and total phosphorus concentrations determined by the National Water Quality Laboratory in discrete samples collected from the Vermillion River at the same site.
Linear regressions between OP discrete samples and each analyzer are shown in
All analyzers in the study used some combination of automated cleaning or calibration. Because of the complexity and importance of these processes, discrete samples become paramount to verifying accuracy and proper operation of the analyzer. This can be problematic because most operators send verification samples to laboratories for analysis, and it may take weeks to receive results. Additionally, multiple check samples are usually required to conclude whether or not there is a problematic pattern. Combined, these issues indicate that long periods of poor data quality are likely before a problem is diagnosed.
Linear regressions for each analyzer showing relation between orthophosphate concentrations measured by the analyzer and orthophosphate concentrations determined by the National Water Quality Laboratory in discrete samples collected from the Vermilion River near Danville, Illinois (U.S. Geological Survey station 03339000).
Figure 6. Graphs showing linear regressions for each analyzer showing relation between orthophosphate concentrations measured by the analyzer and orthophosphate concentrations determined by the National Water Quality Laboratory in discrete samples collected from the Vermilion River near Danville, Illinois.
Concentrations of orthophosphate in discrete samples determined by the National Water Quality Laboratory regressed against the difference between analyzer-measured concentrations and discrete sample concentrations in samples collected from the Vermilion River near Danville, Illinois (U.S. Geological Survey station 03339000).
Figure 7. Graphs showing concentrations of orthophosphate in discrete samples determined by the National Water Quality Laboratory regressed against the difference between analyzer-measured concentrations and discrete sample concentrations in samples collected from the Vermilion River near Danville, Illinois.
The phosphorus concentrations measured by the EZ7800 TOPHO had a moderate relation with OP discrete samples (
The OP concentrations measured by the Phosphax had a consistent high bias compared to OP concentrations determined in discrete samples and a significant negative linear correlation with increasing concentration (
The OP concentrations measured using the Sea-Bird Scientific Hydrocycle-PO4 had a strong regression with OP concentrations determined in discrete samples (
The OP concentrations measured by the Alyza have a strong linear regression with orthophosphate concentrations in discrete samples and has a consistent low bias that increases with concentration (
A large portion of phosphorus is associated with particulates in the Vermilion River. Discrete sample data indicate that the mean OP concentration is about 63 percent of TP concentration at turbidities less than 100 formazin nephelometric units (FNUs) but only about 17 percent at turbidities greater than 100 FNUs. Additionally, TP concentrations have a moderately strong relation with turbidity (
Regression of turbidity and total phosphorus concentrations determined by the National Water Quality Laboratory in discrete samples collected from the Vermilion River near Danville, Illinois (U.S. Geological Survey station 03339000). (Data from
Figure 8. Graph showing regression of turbidity and total phosphorus concentrations determined by the National Water Quality Laboratory in discrete samples collected from the Vermilion River near Danville, Illinois.
The EZ7800 TOPHO has a filter with a mean pore size of 100 μm and an added persulfate digestion. Although the EZ7800 TOPHO does not measure a true TP concentration, this analyzer is designed to analyze more particulates and break down phosphorus-containing compounds to make its analysis more comparable to TP. The EZ7800 TOPHO had no relation with any other properties except OP. This includes turbidity, which did have a moderately strong coefficient of determination with TP (
The filter for the Sea-Bird Scientific Hydrocycle-PO4 has a mean pore size of 7.5 μm, which is more than 10 times larger than the 0.45-μm filter used to determine OP. The larger filter size was hypothesized to have caused this analyzer to have a high bias when compared to discrete OP samples because it allowed more particulate-associated phosphorus into its analysis. That relation had been observed previously with Cycle-PO4, the predecessor to this analyzer (
The Sea-Bird Scientific Hydrocycle-PO4 has extensive quality control software that allows the user to analyze sample runs. One of those tools has the ability to analyze light absorption during the chemical reaction of each sample. Using this tool, it is hypothesized that the full color development may not always be present during the correct part of its analysis. An example of the problem with an onboard calibration standard check is shown in
Example of reaction curve issue from Sea-Bird Scientific Hydrocycle-PO4 software.
Figure 9. Graph showing example of reaction curve issue from Sea-Bird Scientific Hydrocycle-PO4 software.
Data coverage is listed in
Table 9. Missing data point summary.
Analyzer | Total possible data points | Missing data points | Percent data coverage |
Hach EZ7800 TOPHO | 12,149 | 2,892 | 76 |
Hach Phosphax sc | 24,704 | 1,314 | 95 |
Sea-Bird HydroCycle-PO4 | 6,467 | 4,310 | 33 |
YSI Inc. Alyza IQ PO4 | 24,733 | 5,482 | 78 |
The EZ7800 TOPHO had 76-percent data coverage. Despite quarterly servicing, the tubing was subject to clogging, meaning the daily cleanings were not effective. The operator error was much higher largely because of the amount of reagent consumption. From
The Phosphax had 95-percent data coverage. Most of the missing data were from a period where internal leaking took place and needed repair. Additionally, the communications hardware malfunctioned, but the analyzer was able to continue sampling during this period, which allowed the data to be recoverable. If the data had not been recoverable, the Phosphax would have had about 86-percent data coverage during the study. Many internal components were replaced quarterly, which likely resulted in the high percentage of data coverage.
The Sea-Bird Scientific Hydrocycle-PO4 had 33-percent data coverage. The Hydrocycle initially operated above the recommended range of 0.3 mg/L of phosphorus, but within the hypothetical manufacturer extended range of 1.2 mg/L of phosphorus (
The Alyza had 78-percent data coverage. Hardware malfunctions were mainly related to clogged parts and halted further analysis until the issue was rectified. It is possible the daily cleaning should have been run more frequently to help with hardware issues. Annual maintenance, which includes hardware replacement, was not done because the study period was less than a year. Many of the hardware solutions were part of the annual maintenance routine, indicating the Vermilion River may be harsher than the environment intended for the analyzer. The analyzer also had several software malfunctions related to communication errors between different parts of the analyzer. These were often fixed by resetting the analyzer or firmware upgrades.
Phosphorus is an important aspect of nutrient research because of its association with impaired waters. More frequent phosphorus data would be beneficial to meet surface-water scientific goals, and several analyzers can be used to continuously measure phosphorus concentrations. Phosphorus analyzers use complex automated wet-chemistry reactions to determine phosphorus concentrations, with most being designed for use in municipal water treatment facilities. This report presents the results of an analyzer verification study of selected commercially available phosphorus analyzers for their applicability to scientific surface-water applications. Analyzers in this study were the Hach EZ7800 TOPHO (hereafter referred to as “EZ7800 TOPHO”), Hach Phosphax sc (hereafter referred to as “Phosphax”), Sea-Bird Scientific HydroCycle-PO4 (hereafter referred to as “Hydrocycle”), and the YSI Inc. Alyza IQ PO4 (hereafter referred to as “Alyza”).
The analyzers were assessed using multiple laboratory and field tests. Laboratory trials were designed to assess each analyzer’s accuracy and precision with and without dissolved organic matter (DOM), as well as an estimation of waste production. Standard values of 0.06, 0.25, 1.0, 2.5, and 5.0 milligrams per liter (mg/L) of phosphorus were used for accuracy and precision trials. The DOM trials used a 0.25-mg/L phosphorus standard with 10, 50, and 250 mg/L of natural organic matter added. Field trials were designed to investigate each analyzer’s response to a typical midwestern river by analyzing discrete samples, turbidity response, and data coverage.
Laboratory trials indicate that accuracy and precision typically decreased as concentration increased across all analyzers. Ranges of absolute differences between the mean analyzer concentration and the National Water Quality Laboratory concentration were 0.006–0.094, 0.003–0.008, and 0.003–1.187 mg/L of phosphorus for the Alyza, Phosphax, and EZ7800 TOPHO, respectively. Standard deviations ranged from 0.005 to 0.009, 0.004 to 0.047, and 0.022 to 0.234 mg/L of phosphorus for the Alyza, Phosphax, and EZ7800 TOPHO, respectively. Additionally, the automated cleaning and calibration procedures likely affect analyzer accuracy positively and negatively. Laboratory standard tests revealed issues with the calibration procedure of the EZ7800 TOPHO. Responses to increased DOM concentrations were observed at the 250-mg/L concentration, where the standard deviation was about three times higher for the Hach Phosphax and EZ7800 TOPHO compared to the 10- and 50-mg/L standards. The measured mean phosphorus concentration of the Alyza more than doubled when comparing the 250-mg/L standard to the 50- and 10-mg/L DOM concentrations. Waste production ranged from about 100 milliliters to less than 1 milliliter per analyzer sample run.
Field verification was completed at Vermilion River near Danville, Illinois (U.S. Geological Survey station 03339000). The analyzers were evaluated for accuracy, turbidity response, and data coverage. Strong correlations (coefficient of determination equal to or greater than 0.9) were observed from the Alyza, Phosphax, and Hydrocycle. In addition, a moderate correlation (coefficient of determination of 0.47) was observed for the EZ7800 TOPHO when phosphorus concentrations measured by the analyzer were regressed against orthophosphate (OP) concentrations determined in discrete samples. Discrete concentration data revealed potential systematic errors in the EZ7800 TOPHO that affected accuracy. Regression results between the analyzers and turbidity did not indicate any relation, and neither did the difference between the analyzer-measured OP concentration and the discrete sample OP concentration that was regressed against turbidity despite the larger filter pore size of the HydroCycle and EZ7800 TOPHO. Maintenance frequency and analyzer malfunctions were most affected by elevated turbidity levels across all analyzers. High turbidity levels were a challenge because of tubes clogging and the impairment of optical chambers. How each analyzer handled turbidity was the largest single indicator of analyzer performance. Discrete samples ensure proper analyzer performance but require additional processing. Several discrete samples were required to diagnose problematic patterns resulting in poor data quality.
Laboratory and field tests indicate that each phosphorus analyzer could be a viable tool for scientific goals depending on the surface water of interest. Because of the complexity of the analyzers, they require a substantial time investiture to operate effectively and would likely require an experienced and well-trained technician. Additionally, three of the four analyzers were designed for usage in municipal water treatment facilities and required substantial infrastructure upgrades to adapt them to surface-water applications.
This appendix lists all laboratory testing standards submitted to U.S. Geological Survey’s National Water Quality Laboratory for verification. All other data for discrete samples are available on the U.S., Geological Survey’s National Water Information System (
[mg/L, milligram per liter, DIW, deionized water]
Standard concentration |
Orthophosphate concentration |
Total phosphorus concentration |
0 (DIW) | Not detected | Not detected |
0.06 | 0.061 | 0.057 |
0.25 | 0.247 | 0.247 |
1.0 | 1.01 | 0.986 |
2.5 | 2.76 | 2.51 |
5.0 | 5.39 | 5.08 |
[DOM, dissolved organic matter; mg/L, milligram per liter]
DOM concentration |
Orthophosphate concentration |
Total phosphorus concentration |
10 | 0.244 | 0.240 |
0.245 | 0.244 | |
0.242 | 0.230 | |
50 | 0.246 | 0.254 |
0.249 | 0.247 | |
0.249 | 0.240 | |
250 | 0.286 | 0.305 |
0.291 | 0.278 | |
0.287 | 0.303 |
For more information about this publication, contact:
Director, USGS Central Midwest Water Science Center
405 North Goodwin
Urbana, IL 61801
217–328–8747
For additional information, visit:
Publishing support provided by the
Rolla and Lafayette Publishing Service Centers