U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2015–1065

Results from Laboratory and Field Testing of Nitrate 

Measuring Spectrophotometers

Results from Laboratory and Field Testing 

of Nitrate Measuring Spectrophotometers

By Teri T. Snazelle

Open-File Report 2015–1065

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

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Suggested citation:

Snazelle, T.T., 2015, Results from laboratory and field testing of nitrate measuring spectrophotometers: 

U.S. Geological Survey Open-File Report 2015–1065, 35 p., IUUQdx.doi.org/10.3133/ofr20151065.

ISSN 2331-1258 (online)
iii

Contents

Abstract ..........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Description of Spectrophotometric Nitrate Analyzers............................................................................2

Phase I Laboratory Testing...........................................................................................................................4

Phase I Test Procedures......................................................................................................................4

Phase I Test Results .............................................................................................................................5

Accuracy and Limit of Linearity ................................................................................................5

Drift.................................................................................................................................................5

Operating Temperature...............................................................................................................5

Phase II Field Deployment............................................................................................................................5

Phase II Test Procedures ....................................................................................................................5

Mississippi ....................................................................................................................................6

New Jersey...................................................................................................................................7

Illinois.............................................................................................................................................8

Phase II Test Results ............................................................................................................................9

Mississippi - Low Nitrate Concentration ................................................................................9

New Jersey - Mid-Level Nitrate Concentration ..................................................................10

Passaic River.....................................................................................................................10

Pompton River...................................................................................................................11

Illinois – High Nitrate Concentration ......................................................................................12

Phase II Summary...............................................................................................................................13

Summary........................................................................................................................................................14

Acknowledgments.......................................................................................................................................15

References Cited..........................................................................................................................................15

Figures

[Figures located at end of report]

1. Schematic Illustration of a spectrophotometer

2. Photos of nitrate analyzers: TriOS ProPS, Hach NITRATAX, S::CAN Spectro::lyser 

Satlantic SUNA, , and Satlantic SUNA V2

3. A schematic of a split or double-beam configuration

5. A schematic of a dual-beam configuration

4. A schematic of a single-beam configuration

6. Temperatures applied during temperature testing for nitrate analyzers

7. Accuracy results for TriOS, Hach, SUNA, and S::CAN ultraviolet nitrate analyzers

8. Drift results for TriOS, Hach, SUNA, and S::CAN ultraviolet nitrate analyzers

9. The difference between measured nitrate and standard concentration over an 

extended temperature range, for TriOS, and Hach, SUNA, and S::CAN

10. Top left, the S::CAN, Hach, TriOS, and SUNA analyzers next to an uninstalled deployment pipe. Top right, installed deployment pipes at the test pier. Bottom left, the S::CAN 

and Hach controllers, bottom right, the TriOS Tribox controller and the WaterLog 
iv

H-500XL data logger interfaced with the SUNA

11. Two Bridges, New Jersey. View is looking downstream from the gage house

12. Two Bridges sampling system. Pictured on the left is the open basin that was 

sequentially filled with water from the three intakes

13. Agricultural fields surrounding Kickapoo Creek

14. Site of Kickapoo Creek restoration project including The Grove subdivision seen in 

the background

15. Installation of the nitrate analyzers at USGS site 05579610, Bloomington, Illinois

16. Stacked time series plots for measured nitrate concentration in milligram Nitrogen 

per liter plotted for the East Pearl River gage height in feet, stream velocity in feet per 

second, temperature in degrees Celsius, turbidity in formazin nephelometric units, and 

specific conductance in microseimens per centimeter

17. Stacked time series plots for measured nitrate concentration in milligram nitrogen per 

liter plotted for the Passaic River gage height in feet, temperature in degrees Celsius, 

turbidity in formazin nephelometric units, and dissolved organic carbon in milligrams 

per liter

18. Stacked time series plots for measured nitrate concentration in milligrams nitrogen per 

liter plotted for the Pompton River gage height in feet, temperature in degrees Celsius, 

turbidity in formazin nephelometric units, and dissolved organic carbon in milligrams 

per liter

19. Stacked time series plots for measured nitrate concentration in milligrams nitrogen per 

liter plotted for Kickapoo Creek gage height in feet, and discharge in cubic feet 

per second

20. Difference in milligrams nitrogen per liter between the measured and reference nitrate 

concentrations from eight analyzers at three USGS sites compared to the reference 

nitrate concentration

21. Statistical distribution of the differences in measured and discrete sample nitrate 

concentration from three measurement sites

Tables

1. Analyzer features and specifications........................................................................................3

2. Nitrate plus nitrite results from the East Pearl River............................................................10

3. Nitrate plus nitrite results from the Passaic River................................................................11

4. Nitrate plus nitrite results from the Pompton River..............................................................12

5. Nitrate plus nitrite results from Kickapoo Creek...................................................................13

6. Statistical distribution of the analyzer model’s accuracy in milligrams 

nitrogen per liter .........................................................................................................................14
v

Conversion Factors

Inch/Pound to International System of Units

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4047 square meter (m2)

Flow rate

foot per second (ft/s) 0.3048 meter per second (m/s)

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

International System of Units to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

micrometer (μm) 0.00003937 inch (in.)

nanometer (nm) 0.00000003937 inch (in.)

meter (m) 3.281 foot (ft) 

millimeter (mm) 0.003281 foot (ft)

Volume

liter (L) 33.82 ounce, fluid (fl. oz)

milliliter (mL) 0.0338 ounce, fluid (fl. oz)

Mass

kilogram (kg) 2.205 pound avoirdupois (lb)

milligram (mg) 0.0000022 pound avoirdupois (lb)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as 

°F = (1.8 × °C) + 32.

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as 

°C = (°F – 32) / 1.8.

Results from Laboratory and Field Testing of Nitrate 

Measuring Spectrophotometers 

By Teri T. Snazelle

Abstract 

 Five ultraviolet (UV) spectrophotometer nitrate analyzers were evaluated by the U.S. Geological Survey (USGS) Hydrologic Instrumentation Facility (HIF) during a two-phase evaluation. In Phase I, the TriOS ProPs (10-millimeter (mm) path 

length), Hach NITRATAX plus sc (5-mm path length), Satlantic Submersible UV Nitrate Analyzer (SUNA, 10-mm path length), 

and S::CAN Spectro::lyser (5-mm path length) were evaluated in the HIF Water-Quality Servicing Laboratory to determine the 

validity of the manufacturer’s technical specifications for accuracy, limit of linearity (LOL), drift, and range of operating temperature. Accuracy specifications were met in the TriOS, Hach, and SUNA. The stock calibration of the S::CAN required two 

offset adjustments before the analyzer met the manufacturer’s accuracy specification. Instrument drift was observed only in the 

S::CAN and was the result of leaching from the optical path insert seals. All tested models, except for the Hach, met their specified LOL in the laboratory testing. The Hach’s range was found to be approximately 18 milligrams nitrogen per liter (mg-N/L) 

and not the manufacturer-specified 25 mg-N/L. Measurements by all of the tested analyzers showed signs of hysteresis in the 

operating temperature tests. Only the SUNA measurements demonstrated excessive noise and instability in temperatures above 

20 degrees Celsius (°C). The SUNA analyzer was returned to the manufacturer at the completion of the Phase II field deployment evaluation for repair and recalibration, and the performance of the sensor improved significantly.

In Phase II, the analyzers were deployed in field conditions at three different USGS sites. The measured nitrate concentrations were compared to discrete (reference) samples analyzed by the Direct UV method on a Shimadzu UV1800 bench top 

spectrophotometer, and by the National Environmental Methods Index (NEMI) method I-2548-11 at the USGS National Water 

Quality Laboratory. The first deployment at USGS site 0249620 on the East Pearl River in Hancock County, Mississippi, tested 

the ability of the TriOs ProPs (10-mm path length), Hach NITRATAX (5 mm), Satlantic SUNA (10 mm), and the S::CAN 

Spectro::lyser (5 mm) to accurately measure low-level (less than 2 mg-N/L) nitrate concentrations while observing the effect 

turbidity and colored dissolved organic matter (CDOM) would have on the analyzers’ measurements. The second deployment 

at USGS site 01389005 Passaic River below Pompton River at Two Bridges, New Jersey, tested the analyzer’s accuracy in 

mid-level (2-8 mg-N/L) nitrate concentrations. This site provided the means to test the analyzers’ performance in two distinct 

matrices—the Passaic and the Pompton Rivers. In this deployment, three instruments tested in Phase I (TriOS, Hach, and 

SUNA) were deployed with the S::CAN Spectro::lyser (35 mm) already placed by the New Jersey Water Science Center (WSC). 

The third deployment at USGS site 05579610 Kickapoo Creek at 2100E Road near Bloomington, Illinois, tested the ability of 

the analyzers to measure high nitrate concentrations (greater than 8 mg-N/L) in turbid waters. For Kickapoo Creek, the HIF 

provided the TriOS (10 mm) and S::CAN (5 mm) from Phase I, and a SUNA V2 (5 mm) to be deployed adjacent to the Illinois 

WSC-owned Hach (2 mm). A total of 40 discrete samples were collected from the three deployment sites and analyzed. The 

nitrate concentration of the samples ranged from 0.3–22.2 mg-N/L. The average absolute difference between the TriOS measurements and discrete samples was 0.46 mg-N/L. For the combined data from the Hach 5-mm and 2-mm analyzers, the average 

absolute difference between the Hach samples and the discrete samples was 0.13 mg-N/L. For the SUNA and SUNA V2 combined data, the average absolute difference between the SUNA samples and the discrete samples was 0.66 mg-N/L. The average 

absolute difference between the S::CAN samples and the discrete samples was 0.63 mg-N/L.

Introduction

The U.S. Geological Survey (USGS) Hydrologic Instrumentation Facility (HIF) evaluates the performance of instruments 

and equipment that are used to directly measure hydrologic and biogeochemical data. Instrument and equipment evaluations are 

done primarily to determine if particular devices would be suitable for use by USGS personnel for hydrologic data collection.
2 Results from Laboratory and Field Testing of Nitrate Measuring Spectrophotometers

 This report documents the laboratory and field evaluation of commercially available self-contained ultraviolet (UV) 

spectrophotometer nitrate analyzers. Evaluation reports document the results at the time of testing, and may or may not reflect 

future conditions resulting from improvements to the instrumentation and/or software updates. Five nitrate analyzer models 

were evaluated during a two-phase evaluation. In Phase I, four analyzer models were tested in laboratory conditions at the HIF 

for compliance to the manufacturer’s specifications and for accuracy in National Institute of Standards and Technology (NIST) 

traceable nitrate standards. The SUNA V2 was not available when Phase I testing was conducted. In Phase II, the analyzers were 

field tested at three USGS sites and their measured nitrate concentrations were compared to discrete samples collected at the 

sites. Additional information on the nitrate analyzers and their proper use in environmental waters can be found in the USGS 

Techniques and Methods 1–D5 “Optical Techniques for the Determination of Nitrate in Environmental Waters: Guidelines for 

Instrument Selection, Operation, Deployment, Maintenance, Quality Assurance, and Data Reporting (Pellerin and others, 2013). 

After a brief description of the analyzer models, the laboratory and field procedures and test results are described. 

Description of Spectrophotometric Nitrate Analyzers

Recent advances in technology have led to the development of self-contained, submersible analyzers, which can deliver 

continuous monitoring for nitrate plus nitrite concentration. In contrast to some previous models that relied on reagents and 

cadmium reduction and visible spectrophotometry, these analyzers provide a chemical-free and cost-effective solution through 

the use of UV spectrophotometry (American Public Health Association, 1989). The optical system of a spectrophotometer or 

spectrometer is composed of three main parts (fig. 1): 

• a light source (lamp)—usually deuterium or xenon, 

• a sample optical path length— usually 5–100 millimeters (mm), and 

• the spectrometer detector. 

The matrix to be tested (normally water) flows into the instrument’s optical path length, and is illuminated with a beam of 

light. This same light beam is separated into various wavelengths of the ultraviolet/visible (UV/Vis) spectrum (190–740 micrometers) by means of either a diffraction grating or monochromatic optical filter, and projected on a photoelectric detector. The 

energy measured by the spectrophotometer for each wavelength produces a raw “spectral fingerprint” or absorption spectrum of 

the water sample. The raw absorption spectrum is processed by proprietary signal processing algorithms into a filtered absorption spectrum. The nitrate concentration is calculated from the filtered absorption spectrum using the Beer-Lambert Law, or 

Beer’s Law. 

Eight different UV nitrate analyzers with varying path lengths were evaluated: the TriOS ProPS (10 mm) (TriOS Mess- und 

Datentechnick GmbH [2012]), Hach NITRATAX plus sc (2 mm) (Danaher Business System [2012a]), the Hach NITRATAX 

plus sc (5 mm), the Satlantic SUNA (10 mm), the Satlantic SUNA V2 (5 mm) (Danaher Business System [2012b]), and three 

S::CAN Spectro::lysers (5 and 35 mm) (S::can messtechnik GmbH [2012]) (fig. 2). The path length of the analyzer plays a 

central role in determining how well the instrument compensates for interferences such as turbidity. In general, a shorter path 

length will limit the sensitivity of the analyzer for low concentrations, but will generally increase the nitrate detection range and 

minimize adverse effects from high turbidity and suspended sediment. A longer path length will provide greater sensitivity for 

low-level concentrations, but will reduce the detection range and increase the effect of interferences. The analyzers are available commercially with prices varying from $15,000 to $23,000, depending on the accessories. Analyzers differ in their light 

sources, optical configuration, communication protocols, antifouling measures, and algorithms used to compute nitrate from the 

UV absorbance of the water sample. Table 1 gives a summary of each unit’s available features and technical specifications and 

the path lengths of the tested analyzers. Information on the TriOS range of operation, accuracy, and detection limit were unavailable at the time of this evaluation; this information was included in Pellerin and others (2013). Limit of linearity (LOL) can be 

defined as the point at which the instrument’s performance is no longer linear, and the accuracy of the measurement no longer 

meets the specification (Finite Element Method (FEM), 2010). The operating range of the analyzer should be bracketed at the 

lower end by the limit of detection, which is the smallest concentration that can be reliably measured, and at the upper end by 

the limit of linearity. 

The most common optical configurations in a spectrometer are single beam, double or “split” beam, and dual beam. The 

TriOS and S::CAN have a split or double-beam configuration. A split-beam configuration divides a single light beam into the 

primary or measurement beam and a reference beam prior to the light beam passing through the water sample (HunterLab Inc., 

2008). The reference beam is used to monitor the age of the lamp and to make adjustments for changes in the lamp output, electronic noise, and stray light (fig. 3). 
Description of Spectrophotometric Nitrate Analyzers 3

The SUNA and SUNA V2 have a single-beam configuration. For this optical configuration, there is one beam from the 

light source. The beam passes through the monochromator, which sequentially separates and focuses the wavelengths of light 

onto the detector. The source is not “split” to provide a reference beam. For most single-beam instruments to perform well, the 

light source, detector, and electronics must be reasonably stable (fig. 4). The Hach (2 and 5 mm) is designed with a dual-beam 

fixed wavelength configuration (fig. 5). A dual-beam spectrometer has a mirror that splits a single beam of light into primary and 

secondary beams after passing through the water sample, and directs the beams onto two separate detectors (Comerford, 2011; 

Hach NITRATAX http://www.hach.com., September 2012). The Hach uses a primary beam wavelength of 210 nanometers (nm) 

to measure nitrate plus nitrite, and a secondary reference beam wavelength of 350 nm to correct for interferences such as turbidity or organic matter. This type of configuration accounts for fluctuations in the lamp and electronics as well as interferences 

associated with the water sample matrix. Baseline noise associated with a double or dual beam is normally much lower than the 

other configurations (Comerford, 2011).

Table 1. Analyzer features and specifications.

[UV, ultraviolet; nm, nanometer; mm, millimeter; mg/L, milligrams per liter; %, percent; °C, degree Celsius; dia, diameter; kg, kilogram; max, maximum; m, meter; 

V, volt; mA, milliamp; w.i.g, whichever is greater]

Unit TriOS ProPS HACH NITRATAX 

plus sc Satlantic SUNA Satlantic SUNA V2 S::CAN Spectro::lyser

Detector type UV (200–385 nm)

 spectral analysis

UV (210 nm and 

350 nm) spectral 

analysis

UV (190–370 nm)

 spectral analysis

UV (190–370 nm)

 spectral analysis

UV-Vis (220–720 nm)

 spectral analysis 

Sample path

 length (mm)

1–60 (semifixed) 1, 2, and 5 (fixed) 10, (fixed)

5, 10, (fixed)

0.5–100 (semifixed)

Sample path

 length (mm)

 tested

10 2 and 5 10 5 5 and 35

Lamp Deuterium Xenon Deuterium Deuterium Xenon 

Detector Zeiss Patented 2-beam Zeiss 256 diodes Zeiss Zeiss 256 diodes

Optical

 configuration

Split beam Dual beam Single beam Single beam Split beam

NO3-N

 range (mg/L)

Not available 0.1–25 0.007–28 0.031–56 0.00–70 (typical)

Accuracy Not available +/- 3% of mean

 +/- 0.5 mg/L

0.028 (mg/l) or

 ±10% 0.028 (mg/l) or

 ±10%

2% + 1/(optical path

 length) mg/L

Detection limit

 (mg/L)

Not available 0.10 0.007 0.031 0.005

Operating

 temperature

 range

0–50°C 0–40°C 0–40°C 0–35°C 0–45°C

Dimensions 68 mm dia x 520

 mm

75 mm dia x 323

 mm

50 mm dia x 530

 mm

119 mm x 624

 mm (with wiper)

44 mm dia x 647 mm

Weight (in air) 4.9 kg 3.6 kg 2.5 kg 3.1 kg 2.1 kg

Depth max 1500 m 100 m 500 m 500 m

 100 m (with

 wiper)

Not available

Battery 24 V 12 V or 24 V 12 V 12 V 12 V

Current draw

 (average)

Not available Not available 0.625 amp 0.625 amp 20 W max

Communications RS232 Modbus/RS485,

 Modbus/RS232,

 4–20 mA

RS232, SDI-12,

 4–20 mA

RS232, SDI-12,

 4–20 mA

Modbus/RS232,

 Modbus/RS485,

 4–20 mA, SDI-12
4 Results from Laboratory and Field Testing of Nitrate Measuring Spectrophotometers

Phase I Laboratory Testing

Phase I Test Procedures

From August 2011 through January 2012, the analyzers were evaluated in a controlled laboratory setting for accuracy, limit 

of linearity (LOL), drift, and operating range of temperature. Measured nitrates were compared to standard dilutions with known 

nitrate concentrations that were prepared from two independent nitrate standards (McNaught and Wilkinson, 1997). Manufacturing technical specifications were compared to the laboratory results to determine their validity. An 11.3 ± 1 percent mg-N/L 

Hach-Lange standard (LCW825), and a 1,000 ± 0.5 percent mg-N/L Ricca Chemical standard (5459-16) were used to prepare all 

working standard dilutions (3.7, 5.0, 5.7, 10.0, 11.3, 16.7, 20.0, 25.0, 40.0, and 50.0 mg-N/L). Standard dilutions were prepared 

using the formula:

( )

( )

 * Cn Vn

X

Cs =

, 

where 

X = volume of nitrate stock standard to be diluted in milliliters (mL)

Cn= concentration of the new standard in mg/L

Vn = total volume of the new standard in mL; and

Cs = concentration of the standard to be diluted in mg/L.

The uncertainty of each standard dilution was estimated at 3 percent at 95-percent confidence and was calculated from the 

sum of the squares of the following variables. 

• 1 percent error from the larger impurity of the two certified stock standards (1 and 0.5 percent)

• A single dilution of the stock standard using Class A volumetric glassware with an associated error of ± 0.5 percent

• 1 percent error from the standard deviation from five replicates of a 5.0 mg-N/L solution 

Normal distribution was assumed. Standard accuracy was verified with a calibrated bench top Shimadzu UV1800 spectrophotometer, and standards with deviations greater than ± 3 percent from the target concentration were discarded and reprepared. 

The analyzers were evaluated at room temperature for accuracy, LOL, and drift using the “as received” or manufacturer’s 

stock calibrations. The “stock calibration” is the factory or manufacturer’s calibration that has not been altered. The analyzers 

are shipped calibrated; however, most manufacturers encourage the user to adjust the calibration for local matrix effects. During 

accuracy and LOL tests, approximately 100 standard measurements were made by each analyzer. Ten replicated measurements 

were collected at each concentration. Nitrate difference was calculated as measured minus standard; therefore negative values 

indicated a low bias, and positive values indicated a high bias by the unit. For the drift test, the analyzers were filled with the 

11.3 mg-N/L ± 1 percent standard, sealed with parafilm to prevent external contamination, and programmed to measure nitrate 

every 15 minutes for approximately 20 hours. The drift tests were conducted at room temperature. 

Operating temperature specifications were examined at the HIF using an ESPEC environmental chamber over a temperature range of 5–40 degrees Celsius (°C). Except for the S::CAN, the analyzers were tested using the manufacturers’ stock calibrations. A high bias was observed in the S::CAN’s measurements during the initial room temperature testing. To correct for this 

bias, an offset was programmed into the unit by the manufacturer prior to the temperature specification testing. Each analyzer 

was subjected to the same environmental chamber, temperature cycling program, and 10 mg-N/L standard. Chamber temperatures were set at 20, 5, 20, 40, and 20 °C (fig. 6). The analyzers and the nitrate standard were allowed to acclimate overnight 

at 20 °C in the chamber prior to the start of the temperature test. The chamber temperature was slowly ramped to the next test 

temperature and held for 2 hours to stabilize the sample temperature of the analyzers prior to initiating measurements. 
Phase II Field Deployment 5

Phase I Test Results

Accuracy and Limit of Linearity 

The TriOS specifications list a spectral accuracy of 0.2 nm and no measurement accuracy in concentration. The detection range is listed as 0–100 mg/L “depending on water matrix and components.” Because of the ambiguous specifications, the 

data were used to describe the accuracy limits achieved by the TriOS in testing. As shown in figure 7, TriOS met a ± 10 percent 

accuracy specification with few exceptions between zero and 16.7 mg/L, but accuracy limits achieved above 20 mg/L were up 

to 25 percent, with sensor readings biased high at high concentrations. Manufacturing specifications for the Hach 5-mm path 

length claim a detection range of 0.1–25 mg-N/L. Test results indicated that the analyzer was linear to 18 mg-N/L, but failed to 

meet the manufacturing specification at higher concentrations. The SUNA’s performance was based upon its freshwater calibration, and met the manufacturer’s accuracy specifications with few exceptions. SUNA’s test results indicated a much higher LOL 

than the specified detection range of 0.007–28 mg-N/L. The SCAN stock calibration resulted in a curvilinear response, with a 

14–17-percent bias between 5.6 and 16.7 mg/L. Like the SUNA, the S::CAN demonstrated a higher limit of linearity.

Drift

The results of the drift test at a nitrate concentration of 10 mg-N/L for the analyzers are shown in figure 8. The TriOS data 

set had a standard deviation (SD) of 0.17 mg-N/L, and showed a slight downward trend. There was no indication of trending 

or obvious drift in the Hach data. The Hach showed good stability during the drift test, and resulted in a 0.04 mg-N/L SD. The 

SD for the SUNA’s data set was 0.41 mg-N/L. There was little indication of drift, but there was evidence of noise or instability. The S::CAN drift test was repeated three times due to a large persistent upward trend in the data. Leaching of the 5-mm 

path length insert seals was proposed by the manufacturer as an explanation of the upward drift. To test this theory, the test was 

repeated over a longer period of time. During the retest, the increase in measured concentration plateaued after 3 days, as would 

be expected if the seals were leaching. The same test was performed on another S::CAN Spectro::lyser equipped with a 35-mm 

sample window without insert seals. Data from this S::CAN showed no indication of trending or of drift. The 5-mm window 

data set resulted in an SD of 1.1 mg-N/L. In contrast, the 35-mm window data set showed a standard deviation of 0.04 mg-N/L, 

or roughly 29 times less than the 5-mm window. This finding supported the theory that the seals were leaching and changing the 

concentration in the previous test. The effect of seal leachate on instrument drift should be minimal in many applications with 

flowing water such as rivers.

Operating Temperature

The effect of temperature on measurement accuracy was also evaluated. TriOS and Hach met their respective accuracy 

limits (fig. 9), and demonstrated good stability during the temperature cycling. However, a hysteresis effect, where measured 

concentrations at the same temperature are higher for rising temperatures than for falling temperatures, was evident in the data. 

The average difference in TriOS nitrate measurements below 20 °C was 0.3 mg/L, and above 20 °C was 0.4 mg/L. For the Hach, 

the average difference in concentration below 20 °C was 0.2 mg/L, and above 20 °C the difference increased to an average of 

0.3 mg/L. The SUNA met the accuracy criteria at temperatures less than 25 °C, and showed a negligible hysteresis effect in data 

below 20 °C. For temperatures above 25 °C, the measurements became noisier and failed to meet the manufacturer’s accuracy 

specifications. A retest at temperatures above 25 °C confirmed the results of the first test; however, testing was limited to a single 

unit. As stated earlier, an offset was applied to the S::CAN’s stock calibration prior to the temperature test to correct for the 

high bias observed during the room temperature testing. After the application of the offset, a low bias became apparent in the 

S::CAN’s data. The S::CAN measurements in the 10 mg-N/L standard were an average of 1.9 mg-N/L lower than the standard 

value, but there was no apparent drift over the temperature cycle. The S::CAN results also showed evidence of hysteresis similar 

to the TriOS. The average difference in concentration below 20 °C was 0.3 mg-N/L, and 0.4 mg-N/L above 20 °C. 

Phase II Field Deployment

Phase II Test Procedures

Phase II was an extensive evaluation that used three deployment sites to evaluate the analyzers performance in field 

conditions. The sites were located in Mississippi, New Jersey, and Illinois. Nitrate concentrations were low at the Mississippi 

site (<1 mg-N/L), mid-level at the New Jersey site (0.6–4.1 mg-N/L), and high at the Illinois site (16.6–22.2 mg-N/L). The 
6 Results from Laboratory and Field Testing of Nitrate Measuring Spectrophotometers

analyzers’ measured values were collected raw, and were not corrected for fouling or calibration drift as per “USGS Techniques 

and Methods 1–D3” (Wagner, 2006), nor matrix bias as per “USGS Techniques and Methods 1–D5” (Pellerin and others, 2013). 

Mississippi

The first deployment from February to May 2012 was at USGS site 02492620 located on the East Pearl River in Hancock 

County, Mississippi. The Pearl River begins in Neshoba County, Mississippi, and splits into the East Pearl and the West Pearl 

Rivers, 50 miles above the mouth of the Pearl River. The East Pearl empties into Lake Borgne where the Pearl River Channel 

meets the Gulf Intracoastal Waterway. USGS site 02492620 is tidally affected. Site conditions, including gage height, temperature, and water quality, are monitored routinely as part of the continuous monitoring program. The analyzers were deployed 

in separate 4-inch (in.) schedule 40 polyvinyl chloride (PVC) pipes. The TriOS controller, the Tribox, and the Water Log© 

H-500XL data logger used to interface the SUNA were mounted in the adjacent gage house (fig. 10; bottom right). The S::CAN 

and the Hach controllers were mounted on the test pier. Each analyzer was secured so that the optical window would hang at a 

depth of 11 feet (ft) below the pipe cap and would remain submerged in the river over the anticipated range of stages. The analyzers sampled every half hour. 

Three of the four analyzers were deployed with their stock calibrations. The S::CAN was initially deployed with the manufacturer’s “global river calibration,” but as the deployment progressed two adjustments to the analyzer’s calibration offset were 

required on March 6 and April 3, 2012. Following the advice of the SUNA manufacturer, the measurement integration window 

was reduced from 750 to 250 milliseconds on March 20 to improve the unit’s performance. The integration window setting of 

a spectrometer determines how much light the detector is exposed to, and is analogous to the shutter speed of a camera. The 

higher the integration time, the longer the detector is exposed to light. Too much light can cause a “blinding” effect and will 

negatively affect the measurements. The SUNA was not recalibrated, and data points collected before the reduction of the integration window were not used to assess accuracy. 

Site visits were conducted weekly, during which the optical windows of the analyzers were cleaned with a soap solution 

and weak (1 percent) hydrochloric acid (HCL), and data were exported from the analyzers manually using a laptop and flash 

drive. A site visit log was kept to record site conditions, calibration verification checks, and the analyzers’ precleaning and 

postcleaning nitrate measurements. During the site visits, discrete samples were collected at the deployment depth (11 ft) using 

a Van Dorn water sampler, and were analyzed by the Direct UV method on a Shimadzu UV1800 bench top spectrophotometer. 

Splits of the reference samples were also sent to the USGS National Water Quality Laboratory (NWQL) in Denver, Colorado, 

for comparison testing and were analyzed by NEMI method I-2548-11 with a reporting limit of 0.01 mg-N/L. A comparison of 

data by the two methods revealed a positive bias of approximately 0.25 mg-N/L in the Direct UV method. The NWQL enzymereduction colorimetric method is designed to remove interferences, such as colored dissolved organic matter (CDOM) or metals 

during the preparation of the samples. There is no sample preparation for the Direct UV method. The Direct UV method compensates for most interfering compounds by using wavelength subtraction or through the second derivative method; therefore, 

the 0.25 mg-N/L difference between the two methods was most likely due to matrix interference. 

To compensate for matrix interferences affecting the Direct UV method, a matrix spike was employed to serve as the 

evaluation reference sample. The U.S Environmental Protection Agency (EPA) Forum on Environmental Measurements (FEM) 

defines a matrix spike as an “environmental sample to which known quantities of the target analytes are added in the laboratory 

(Forum on Environmental Measurements, 2010).” The volume of spiking solution should not increase the sample volume by 

more than 5 percent. If less than 5 percent, the volume of the spiking solution is considered insignificant, and does not need to 

be taken into account (Thermo Scientific, 2011). For most laboratory purposes, a matrix spike is used to demonstrate that the 

sample preparatory steps of the method have not adversely affected the analyte concentration (U.S Environmental Protection 

Agency, 2004). In this case, the matrix spike was used to adjust for the unpredictable effect that site matrix interferences have 

on the Direct UV method. To prepare the matrix spike, the grab sample was filtered through a Pall 0.45-micron (μm) filter to 

remove particulates. This filtered water was used to fill to the meniscus of a Class A 100 milliliter (mL) volumetric flask containing 0.5 mL of a 1,000 mg-N/L nitrate standard. The spike or “known quantity of target analyte” concentration used in this evaluation was 5 mg-N/L, and was calculated using the following formula:

X = (Cn *Vn )/(Cs ),

where

X = volume of nitrate stock standard to be diluted in mL

Cn = desired concentration of the matrix spike in mg/L

Vn = desired volume of matrix spike in mL, and

Cs = concentration of the spiking solution in mg/L.
Phase II Field Deployment 7

To prepare 100 mL of a 5 mg/L matrix spike from a 1,000 mg/L nitrate stock solution, the volume of the stock nitrate solution to 

be diluted in the filtered river water is

X = (5 mg/L * 100 mL)/(1000 mg/L) = 0.5 mL.

The final concentration of the raw discrete (reference) samples from the matrix spike was calculated using the following 

formula:

C_reference = C_(matrix spike) - C_spike,

where 

 C_reference = reference sample concentration in mg-N/L

 C_(matrix spike) = measured concentration of the spiked sample in mg-N/L (discrete sample + 5 mg-N/L); and

 C_spike = known concentration of the added spike in mg-N/L (5 mg-N/L).

This value was compared to the raw (unspiked) filtered river water measurements, and on average was 0.24 mg-N/L lower 

in concentration. The matrix spike compensated for stubborn interferences like color and eliminated the high bias caused from 

the interference. The difference in concentration was similar to the concentration difference seen between the Direct UV measurement and the reduction method and validated the theory of measurement interference contribution. Analyzer accuracy was 

determined by the differences between the measured values to the C_reference standards.

New Jersey

In October 2012, the HIF partnered with the USGS New Jersey (NJ) Water Science Center (WSC) to deploy the analyzers 

at USGS site 01389005 Passaic River below Pompton River at Two Bridges, N.J. (fig. 11). The station is located 400 ft downstream from the mouth of the Pompton River, on the left bank of the Passaic River. 

 The upper Passaic River travels through suburban New Jersey and empties the Great Piece Meadow Swampland. The 

upper Passaic River has a highly circuitous route and normally has higher nitrate concentrations than the Pompton River. The 

Pompton River drains several reservoirs upstream from the USGS site, and tends to have a green hue from algal growth during 

the warm season. The water at the convergence of these two rivers is poorly mixed and creates two distinct water matrices. A 

system of three pumps collects sample water from the Passaic River (right intake), the Pompton River (left intake), and either 

the Passaic or Pompton Rivers, depending on the relative amounts of stream flow (middle intake). Only measurements made of 

water from the right and left intakes were used in this evaluation. At Two Bridges, the Hach NITRATAX (5 mm), TriOS ProPs 

(10 mm), and Satlantic SUNA (10 mm) from Phase I were operated in conjunction with an existing S::CAN Spectro::lyser 

(35 mm) operated by the NJWSC as part of its continuous monitoring program (fig. 12). 

The four analyzers and a YSI model 6920 multiparameter sonde were installed in an open basin in the gage house. At 

scheduled intervals, the basin was automatically and sequentially filled with water from each of the three intakes. The waterquality sensor and the nitrate analyzers measured the characteristics of water pumped sequentially from the three separate 

intakes. Site conditions, including gage height, temperature, specific conductance, pH, dissolved oxygen, turbidity, and dissolved 

organic carbon (DOC), were also monitored for each intake. A deionized (DI) water wash was used to automatically clean the 

analyzer windows of the TriOS, S::CAN, and SUNA, and the Hach’s integrated wiper was used to clean its optical window. 

Three of the analyzers—the TriOS, Hach, and SUNA—measured nitrate using their stock calibrations. The S::CAN at the beginning of the deployment was on loan from the manufacturer while the NJWSC-owned unit was being repaired. The S::CAN also 

had a stock calibration. This loaner unit was replaced on October 22, 2012, with one equipped with a manufacturer-developed, 

custom calibration for the local site conditions. 

The S::CAN and water-quality monitor data were collected and transmitted by the NJWSC-owned data logger. A WaterLOG® H-522+ data collection platform (DCP) was installed and used to collect and transmit data from the TriOS, Hach, and 

SUNA over the Geostationary Operational Environmental Satellite (GOES). The SUNA is SDI-12 compliant and was directly 

connected to the DCP. A computer with Windows XP was used to interface the DCP with the TriOS and Hach analyzers. The 

TriOS controller was connected to the computer using Modbus over TCP/IP through an Ethernet cable and the Hach’s sc controller was connected to the computer using an RS-232/RS-485, Modbus to serial converter. Data from the TriOS and the Hach 

were written by the computer into unused memory locations in the DCP using Labview, and the DCP was set up to transmit the 

memory locations as two of its measurements. The S::CAN and the water-quality sonde were programmed to sample each intake 

hourly. The DCP retrieved the data from the TriOS, Hach, and SUNA analyzers every 6 minutes, and transmitted the previous 10 

measurements from each analyzer hourly along with the battery voltage. The DCP and the SUNA were powered by a 12-volt (V) 
8 Results from Laboratory and Field Testing of Nitrate Measuring Spectrophotometers

battery in the gage house, and the TriOS, Hach, and S::CAN were powered by alternating current (AC). Data from the NJWSC 

S::CAN are usually corrected prior to being disseminated based on a regression analysis of collected water samples, which relate 

a NWQL-determined concentration to the in situ S::CAN-determined concentration. For the evaluation analysis, the S::CAN’s 

data were used “raw” without this correction. 

Between October and December 2012, 22 discrete samples were collected—11 each from the Passaic and the Pompton 

Rivers—and were analyzed by NEMI method I-2548-11 at the NWQL. These samples were collected during biweekly site visits 

by the NJWSC staff and on alternating weeks by cooperators from the Passaic Water Valley Commission. During site visits 

conducted by the NJWSC staff, the analyzers’ optical windows were cleaned with a soap solution and 1-percent HCL. Fouling 

checks were conducted by plugging and filling the basin with water from the left intake (Pompton River), and a site visit log was 

kept to record the results and the site conditions. 

Illinois

In April 2013, the HIF partnered with the USGS Illinois (IL) WSC to test the performance of the analyzers at USGS site 

05579610 Kickapoo Creek at 2100E Road near Bloomington, Ill., a site with high nitrate concentrations. For this deployment, 

the HIF supplied the TriOS ProPs (10 mm) and S::CAN Spectro::lyser (5 mm) from Phase I, and a Satlantic SUNA V2 (5 mm), 

that were operated in conjunction with the ILWSC-owned Hach NITRATAX plus sc with a 2-mm optical path length. Gage 

height and discharge were recorded at the site as part of the ILWSC continuous monitoring program. No other water parameters 

were monitored at the site during the deployment. 

USGS site 05579610 is located approximately 6 miles southeast of the city of Bloomington, and receives waters from two 

tributaries. The northern tributary runs along the east side of the city of Bloomington and drains subdivisions and a portion of 

the Central Illinois Regional Airport. The eastern tributary consists primarily of runoff from the surrounding agricultural fields 

(fig. 13).

In 2007, the city of Bloomington was awarded a $950,000 EPA grant to implement several strategies to control nonpoint 

source pollution, including sediment and nutrients, along Kickapoo Creek in McLean County, Ill. A residential development in 

Bloomington, The Grove on Kickapoo Creek, was constructed in conjunction with the restoration project (fig. 14). The project, 

which consisted of a large-scale restoration of approximately 2,800 linear ft of Kickapoo Creek and approximately 40 acres of 

the adjacent floodplain, provided an opportunity to identify the impacts that a residential development might have on an agricultural watershed.

The four analyzers were deployed at the Kickapoo Creek restoration project in separate PVC pipes that were anchored to 

the creek bank and further secured with rip-rap rock (fig. 15). Three 4-in.-diameter schedule 40 PVC pipes were used for the 

Hach, TriOS, and the S::CAN. The SUNA V2 was too large for a standard 4-in. schedule 40 PVC pipe, and required an 8-in.-

diameter pipe. The controllers for the Hach, TriOS, and S::CAN were mounted in the adjacent gage house. The SUNA is SDI-12 

compliant and does not come equipped with a separate controller. 

 Fouling of the SUNA V2 was controlled by the integrated Hydro-Wiper, which cleaned the SUNA V2 5-mm path length 

prior to every measurement. The Hach was programmed to engage its wiper twice an hour to combat fouling, and the TriOS and 

the S::CAN used an hourly air blast supplied by an external air compressor to clean the optical windows. 

The Hach was programmed to sample hourly. Data were collected and transmitted by the ILWSC-owned data logger, and 

was retrieved through the USGS National Water Information System (NWIS) Web site at http://waterdata.usgs.gov/il/nwis/

uv/?site_no=05579610. Similar to the New Jersey deployment, a WaterLOG® H-522+ DCP was installed and used to collect 

and transmit data from the SUNA, TriOS, and S::CAN over GOES. A computer with Windows XP was used to interface the 

HIF-supplied DCP with the TriOS and S::CAN analyzers. The SUNA V2 was directly connected to the DCP. The TriOS controller was interfaced to the computer using Modbus over TCP/IP through an Ethernet cable, and the S::CAN’s controller was 

connected to the computer through a serial port using an Instrumental Solutions Inc. (ISI) MS1-2150 Modbus/SDI-12 Protocol 

Converter. The converter operates as a Modbus “master”/SDI-12 “slave” to make Modbus data available as SDI-12 (Instrumental Solutions Incorporated, 2011). More information on the MS1-2150 is available at the Instrumental Solutions Inc. Web site: 

http://isidev.net/products. Data from the TriOS and the S::CAN were written into unused memory locations in the DCP using 

Labview, and the DCP was set up to transmit the memory locations as two of its measurements. The DCP read all three analyzers every 6 minutes, and transmitted the data once an hour. Both DCPs and the SUNA were powered by a 12-V battery in the 

gage house, and the TriOS, Hach, and S::CAN ran on AC power.

Reference samples were collected between April and July 2013, and analyzed by NEMI method I-2548-11 at the NWQL. 

During site visits, the analyzers’ optical windows were cleaned with a soap solution and site conditions with preclean and postclean nitrate measurements were recorded. 
Phase II Field Deployment 9

Phase II Test Results

Mississippi - Low Nitrate Concentration 

Continuous nitrate data for the four analyzers were evaluated for the following site conditions—gage height, stream 

velocity, temperature, turbidity, and specific conductance (fig. 16). Initially, the Hach and the S::CAN’s controllers were erroneously programmed to record data every 60 seconds, which resulted in a data file too large to be preserved and deletion of 2 

weeks of the initial data before the measurement interval was corrected. A site log recorded the analyzers’ weekly nitrate results 

at the time that discrete samples were collected. The performances of the Hach and the S::CAN were evaluated by the difference between the analyzers’ log values and the discrete sample concentration on the days the continuous data were missing. 

The 3-week gap in the TriOS data was due to a faulty installation. At 4.9 kilograms (kg), the TriOS was the heaviest of the four 

analyzers. The TriOS was deployed in such a way that the body of the TriOS was not sufficiently supported, which put a strain 

on the communication cable. The cable failed on March 30, 2012, and a replacement cable was not obtained for approximately 

3 weeks. As mentioned earlier, the SUNA’s integration period was reduced (March 20, 2012) and the S::CAN’s calibration was 

adjusted twice (March 6, 2012, and April 3, 2012). These changes to the instrument settings are shown as blue arrows in figure 

16. The values of the discrete samples that were collected and analyzed independently are plotted as red squares on the time 

series for the four analyzers.

Nitrate concentrations in the river did not vary dramatically during the deployment period. Concentrations ranging from 

less than detection (0 reading) to 1.87 mg-N/L were recorded by the analyzers, and the discrete sample concentration ranged 

from 0.33–0.84 mg-N/L. Gage height varied from 8.84 to 14.46 ft during the deployment, and the stream velocity ranged from a 

slightly negative value due to tidal influence to 3.73 feet per second (ft/s). Specific conductance was recorded with a maximum 

value of 102 microseimens per centimeter (μS/cm). Water temperature varied from 14.0 to 27.2 °C . Although low for the majority of the deployment time, turbidity increased temporarily to a maximum of 667 formazin nephelometric units (FNU) after a 

local storm; and although unmonitored, the highly colored water of the East Pearl River indicated the presence of DOC both of 

which can increase baseline noise at low wavelengths and cause a positive bias in measured nitrate concentration. 

The Hach analyzer displayed the most accurate and consistent performance throughout the deployment. Measured nitrate 

concentrations from the TriOS and SUNA analyzers were consistently lower than the discrete sample concentrations. The 

S::CAN’s measured nitrate concentration was initially higher; however, the application of the second calibration adjustment 

improved the S::CAN’s performance. Data from the SUNA showed several zero values during the deployment, which most 

likely resulted from the absence of an active anti-fouling mechanism.

In late March a round of local storms produced nearly 10 in. of rain in a 48-hour period. River conditions at the site 

changed in response to this event with increases in nitrate, turbidity, and stream velocity, and a decrease in specific conductance. The change in nitrate concentration was verified by the reference samples collected before and after the event, and the 

change was registered to varying degrees by all four analyzers. The dynamic nature of nitrate in the river during this event, even 

though it was over a relatively small concentration range, demonstrated the value that continuous monitoring made possible by 

these analyzers. Table 2 summarizes the standard and measured nitrate concentrations observed during the deployment. Nitrate 

concentrations of the reference samples never exceeded 1 mg-N/L—a concentration range much smaller than what is found in 

water samples from many surface-water and groundwater sites. Turbidity increased substantially from an average of less than 

100 FNU to more than 600 FNU after the storm, and could have contributed to the rise in nitrate concentration observed in the 

data from the analyzers.
10 Results from Laboratory and Field Testing of Nitrate Measuring Spectrophotometers

New Jersey - Mid-Level Nitrate Concentration 

Continuous nitrate data for the four analyzers were evaluated for the following site conditions—gage height, temperature, 

turbidity, and DOC (figs. 17 and 18). The gap in the continuous data was the result of the power loss that occurred after Hurricane Sandy made landfall on October 29, 2012. 

Passaic River

The nitrate concentrations of the Passaic River varied substantially during the deployment period. Measured concentrations 

of 0.56 to 5.2 mg-N/L were recorded by the analyzers, while the reference samples ranged from 1.2 to 4.1 mg-N/L. Gage height 

was the same for both rivers and varied from 3.3 to 5.5 ft during the deployment. The Passaic River’s average turbidity was 

4.0 FNU, and average DOC was 5.2 mg/L. Water temperature varied from 4.5 to 16.5 °C. 

The Hach and the SUNA provided the most accurate and consistent data for water from the Passaic River. The Hach’s 

reported range was 0.85 to 4.7 mg-N/L with 0.15 mg-N/L average absolute difference from the discrete sample concentration. 

The SUNA had an average absolute difference of 0.16 mg-N/L and a 0.88 to 4.68 mg-N/L measured nitrate range. Average absolute difference between the TriOS measurements and the discrete samples was 0.24 mg-N/L in a 0.72 to 4.31 mg-N/L range. The 

S::CAN differed from the discrete samples an average of 0.34 mg-N/L in a 0.56 to 5.16 mg-N/L nitrate range. A summary of the 

results from the Passaic River is listed in table 3. 

Table 2. Nitrate plus nitrite results from the East Pearl River. 

 [FNU, formazin nephelometric unit; mg-N/L, milligrams nitrogen per liter]

Date and time Site turbidity

(FNU)

Discrete sample 

concentration1 (in 

mg-N/L) 

TriOS ProPs 10 millimeters

(in mg-N/L)

Hach NITRATAX

5 millimeters

(in mg-N/L)

Satlantic SUNA 

10 millimeters

(in mg-N/L)

S::CAN 

Spectro::lyser 5 

millimeters

(in mg-N/L)

2/24/2012 16:00 41 0.45 0.21 0.61 No data3 1.414

3/1/2012 18:30 48 0.33 0.26 0.70 No data3 1.554

3/6/2012 14:03 38 0.38 0.24 0.62 No data3 0.53

3/14/2012 15:46 66 0.46 0.28 0.69 No data3 0.91

3/20/2012 13:26 355 0.52 0.06 0.72 0.23 0.51

3/23/2012 14:00 537 0.84 0.37 0.73 0.14 1.3

3/29/2012 12:45 100 0.59 No data2 0.74 0.0 1.4

4/3/2012 15:30 30 0.62 No data2 0.63 0.21 0.955

4/10/2012 19:00 38 0.45 No data2 0.69 0.24 0.81

4/17/2012 13:00 22 0.53 No data2 0.49 0.22 0.32

4/25/2012 13:00 29 0.50 0.28 0.6 0.41 0.41

5/2/2012 14:00 23 0.37 0.11 0.42 0.0 0.0

5/8/2012 15:00 16 0.54 0.18 0.5 0.18 0.44

1

Discrete sample concentration analyzed by the Direct UV method on a Shimadzu UV1800 and at the U.S. Geological Survey National Water Quality 

Laboratory NEMI method I-2548-11 (0.01 mg-N/L limit of detection). 

2

Data collected by the TriOS after the failure of the communication cable. 

3

Data collected before the adjustment of the Satlantic Submersible UV Nitrate Analyzer’s integration window. 

4

Data collected by S::CAN with stock calibration. 

5

Data collected by S::CAN after second calibration adjustment.
Phase II Field Deployment 11

Pompton River

For the Pompton River, the Hach, and SUNA again provided the most accurate results when compared to the discrete 

sample concentrations analyzed at the NWQL. During the testing period, the four analyzers measured a cumulative range of 

0.38 to 3.2 mg-N/L. The discrete samples varied in concentration from 0.62 to 2.09 mg-N/L during the testing period. Although 

the overall nitrate concentration of the Pompton River was lower than the Passaic River, the analyzers consistently tracked the 

fluctuations in concentration. Turbidity averaged 2.6 FNU in the Pompton River, and the DOC averaged 3.4 mg/L. Neither interference appeared to adversely affect the analyzers’ performance. The temperature of the Pompton River ranged from 4.5 to 15.1 

°C, which was 1.4 °C cooler than the highest temperature observed for the Passaic River. 

 The concentration range measured by the Hach was 0.72 to 3.21 mg-N/L, and the average absolute difference between the 

Hach measured values and the discrete samples was 0.08 mg-N/L. Early in the deployment, the SUNA’s continuous data falsely 

showed a zero concentration that was caused from an insufficient water depth in the sample basin during the fill from the left 

intake. Because the SUNA’s optical path length is located higher in the unit’s body than the other three analyzers, a greater water 

depth was needed. The pump rate was adjusted to increase the depth in the basin, and the data recorded prior to the pump adjustment were disregarded. The removal of these data points resulted in a measured range of 0.71 to 3.2 mg-N/L, and an average 

absolute difference of 0.12 mg-N/L from the discrete samples. 

The TriOS had an average absolute difference of 0.18 mg-N/L between the measured values and the discrete samples. The 

measured range during the testing period was 0.52 to 2.8 mg-N/L. The S::CAN had a 0.20 mg-N/L average absolute difference 

between the measured values and the discrete samples, and the measured range in the Pompton River was 0.38 to 2.8 mg-N/L. A 

summary of the results from the Pompton River is shown in table 4.

Table 3. Nitrate plus nitrite results from the Passaic River. 

 [FNU, formazin nephelometric unit; mg-N/L, milligrams nitrogen per liter]

Date and time Site turbidity 

(FNU)

Discrete sample 

concentration1

(in mg-N/L)

TriOS Props 

10 millimeters

(in mg-N/L)

Hach NITRATAX 

5 millimeters

(in mg-N/L)

Satlantic SUNA 

10 millimeters

(in mg-N/L)

S::CAN 

S::pectrolyser 

35 millimeters

(in mg-N/L)

10/18/12 15:00 1.9 3.62 3.55 3.67 3.66 4.302

11/2/12 14:00 5.5 1.17 1.12 1.37 1.23 1.4

11/6/12 11:00 3.8 1.16 1.08 1.37 1.38 1.42

11/13/12 14:00 2.8 1.92 1.88 2.17 2.09 1.94

11/15/12 11:00 3.3 1.93 1.84 2.15 2.19 1.91

11/20/12 10:00 2.3 2.43 2.14 2.49 2.58 2.33

11/27/12 14:00 2.7 3.81 3.34 3.83 3.86 3.68

11/29/12 14:00 2.6 4.12 3.75 4.26 3.94 4.06

12/4/12 14:00 2.7 3.82 3.41 3.89 3.93 3.7

12/11/12 10:00 4.7 2.33 1.74 2.02 2.04 1.79

12/14/12 11:00 4.4 2.1 1.94 2.26 2.32 2.03

1

Discrete sample concentration analyzed at the U.S. Geological Survey National Water Quality Laboratory NEMI method I-2548-11 (0.01 mg-N/L limit 

of detection).

2

Data collected by loaner S::CAN with stock calibration.
12 Results from Laboratory and Field Testing of Nitrate Measuring Spectrophotometers

Illinois – High Nitrate Concentration

Several problems with DCP and analyzer data transfer occurred during the Kickapoo Creek deployment, which resulted in 

transmittal of several weeks of erroneous values of 15.7 mg-N/L for the S::CAN, and zero values for the TriOS. The correct values were recorded by the S::CAN and TriOS controllers, and were used for the data analysis for the period of the communication 

issue. It is theorized that issues with data transfer occurred due to an interference between the S::CAN Modbus and the TriOS 

controller. Continuous data from the analyzers are compared to the reference sample concentrations and two site conditions—

gage height and discharge (fig. 19). 

 The Hach 2-mm path length provided the best performance for Kickapoo Creek. The Hach’s average absolute difference 

between the measured values and the discrete sample concentrations was 0.16 mg-N/L. The range of nitrates measured by the 

Hach was 5.5–24.3 mg-N/L. The S::CAN had the second lowest deviation with a 2.83 mg-N/L average absolute difference 

between the measurements and the discrete samples. The S::CAN’s measured range was 4.3–23.7 mg-N/L. Average absolute 

difference for the TriOS was 2.91 mg-N/L with a measured range of 0-32.4 mg-N/L. The SUNA’s average difference was 

3.50 mg-N/L in a 0-19.1 mg-N/L measured range. 

The TriOS, Hach, and SUNA consistently tracked the fluctuating nitrate concentrations of Kickapoo Creek. The S::CAN 

data indicate a definite downward trend, possibly due to optical degradation from fouling. A local calibration adjustment to the 

S::CAN in early June improved the agreement of the measured values to the discrete samples. Notably, the first three analyzers 

all recorded a decrease in nitrate concentration on April 17, 2012. This decrease was caused by a local storm and the resulting dilution of the creek with rainwater. Gage height and discharge both increased as a result of the precipitation. Conversely, 

an increase in nitrate concentration was recorded on May 30, 2012, following preciptation. Gage height and discharge also 

increased during this precipitation event, but the resulting precipitation fell more slowly than during the April 17th event. The 

increase in the nitrate concentration of Kickapoo Creek was most likely from runoff from the surrounding agricultural fields. 

Table 5 summarizes the results from the Kickapoo Creek deployment. TriOs and S::CAN data listed in the table are the 

measured data retrieved from the analyzers’ software. The June 17, 2012, TriOS data point occurred after the catastrophic failure 

of the analyzer, and was not considered in the accuracy assessment.

Table 4. Nitrate plus nitrite results from the Pompton River. 

 [FNU, formazin nephelometric unit; mg-N/L, milligrams nitrogen per liter]

Date and time Site turbidity 

(FNU)

Discrete sample 

concentration1

(in mg-N/L)

TriOS ProPs 

10 millimeters

 (in mg-N/L)

Hach NITRATAX 

5 millimeters

(in mg-N/L)

Satlantic SUNA 

10 millimeters

(in mg-N/L)

S::CAN 

Spectro::lyser 

35 millimeters

 (in mg-N/L)

10/18/12 15:00 1.1 1.87 1.94 1.98 No data2 2.43

11/2/12 14:00 3.1 0.62 0.56 0.73 0.71 0.66

11/6/12 11:00 1.2 0.72 0.68 0.8 0.87 0.68

11/13/12 14:00 1.4 1.69 1.57 1.7 1.86 1.61

11/15/12 11:00 1.4 1.52 1.37 1.63 1.66 1.46

11/20/12 10:00 1.2 1.62 1.43 1.76 1.76 1.55

11/27/12 14:00 2.0 2.02 1.78 2.08 2.12 1.86

11/29/12 14:00 2.7 2.12 1.85 2.28 2.16 1.94

12/4/12 14:00 2.2 2.1 1.71 2.08 2.03 1.79

12/11/12 10:00 3.5 1.52 1.28 1.5 1.66 1.4

12/14/12 11:00 2.2 1.69 1.46 1.72 1.87 1.61

1

Discrete sample concentration analyzed at the U.S. Geological Survey National Water Quality Laboratory NEMI method I-2548-11 (0.01 mg-N/L limit of 

detection).

2

Data collected from SUNA before the adjustment of the basin water level.

3

Data collected by loaner S::CAN with stock calibration.
Phase II Field Deployment 13

Phase II Summary

Eight analyzers with various optical path lengths from four manufacturers were evaluated in phase II: a Hach NITRATAX 

plus sc (5-mm path length), a Hach NITRATAX plus sc (2-mm path length), three S::CAN Spectro::lysers (5- and 35-mm path 

lengths), one TriOS ProPs (10-mm path length), one Satlantic SUNA (10-mm path length), and one Satlantic SUNA V2 (5-mm 

path length). These analyzers were placed in three different locations with low, mid-level, and high nitrate concentrations. Differences between the measured concentrations and the reference samples for all eight analyzers are compared to the discrete 

sample concentrations from the three deployment sites (fig. 20). Outliers, such as the data collected after the failure of the TriOS 

and before the fill level of the sampling tank was adjusted for the SUNA, were not included in the analysis. 

The nitrate concentration of the discrete samples collected from the three sites ranged from 0.33 to 22.2 mg-N/L. The 

TriOS data (fig. 20) have an overall absolute measured difference of 0.46 mg-N/L over the three-site cumulative nitrate range. 

The Hach results (fig. 20), which included data from two analyzers, the 5-mm and the 2-mm path lengths, showed good agreement with the discrete sample concentration. The combined Hach data resulted in a 0.13-mg-N/L average absolute difference 

from the discrete samples. The SUNA results (fig. 20) consist of data from the SUNA (10-mm path length) and the SUNA 

V2 (5-mm path length). The S::CAN results (fig. 20) include data from the HIF-owned S::CAN (diamonds), the New Jersey 

“loaner” unit (squares), and the NJWSC-owned S::CAN (triangles). The New Jersey loaner unit measured data based upon its 

stock “global” calibration only. The HIF-owned S::CAN measured nitrate concentrations with the stock calibration only, and 

with a new calibration offset that was applied by the manufacturer. The NJWSC S::CAN measured data based upon a sitespecific custom calibration. The SUNA and SUNA V2 combined data produced a 0.66-mg-N/L average absolute difference from 

the discrete samples, and data from the three different S::CAN analyzers resulted in a 0.63- mg-N/L average difference. The data 

collected with the Hach analyzers illustrated the effect of path length on measurement range of nitrate. The Hach with the shortest optical path length of the tested analyzers (2 mm) had the smallest differences at the highest nitrate concentrations in the field 

deployments, and the longer path length (5 mm) had a poor response for concentrations greater than 20 mg/L in the laboratory 

accuracy tests (fig. 7). In the field deployments, the two analyzers with the next shortest path length of 5mm—the S::CAN and 

the SUNA—did not perform as well as the Hach at the higher nitrate concentrations. 

Measured turbidities for the low- and mid-nitrate level sites were low except during peak flows (approximately 600 FNU 

for the low-level site and 100 FNU for the mid-level site). Based on the previous time series results (figs. 16, 17, and 18), data 

from these sites did not indicate any obvious effect on any unit measurements based on the changing turbidity levels. It is 

suspected that the high-nitrate-level site had higher suspended sediment and turbidity levels because of the agriculture land use 

within the basin, but no measurements were made to confirm this hypothesis. The field measurements (fig. 20) and the laboratory 

measurements (fig. 7) without turbidity effects had dissimilar trends over the same range of nitrate concentrations. Differences 

in path length may account for some of the variances in response, but optical path length alone does not determine accuracy. The 

largest differences between the field measurements and the laboratory measurements occurred at the higher concentrations, and 

may have been the result of turbidity, suspended sediment, and dissolved organic matter effects, in addition to the likely existence of a decreasing response to higher absorbance associated with high nitrate concentration (that is, a nonlinear response). 

The statistical distribution of the measured-to-reference differences from figure 20 is shown in figure 21 and table 6. 

Table 5. Nitrate plus nitrite results from Kickapoo Creek. 

[mg-N/L, milligrams nitrogen per liter]

Date and time

Discrete sample 

concentration1

(in mg-N/L)

TriOS ProPs

10 millimeters

(in mg-N/L)

Hach NITRATAX 

2 millimeters

(in mg-N/L)

Satlantic SUNA V2

5 millimeters

(in mg-N/L)

S::CAN Spectro::lyser

 5 millimeters

(in mg-N/L)

4/11/13 16:00 22.2 17.62 22.1 16.4 20.1

5/7/13 14:20 19.1 17.68 18.9 16.3 15.2

5/21/13 14:10 16.6 14.60 16.4 14.9 9.18

6/3/13 9:45 21.5 17.89 21.7 16.7 22.1

6/17/13 10:40 17.9 No data2 18.0 15.5 17.8

1

Discrete sample concentration analyzed at the U.S. Geological Survey National Water Quality Laboratory NEMI method I-2548-11 (0.01 mg-N/L limit of 

detection).

2

Data collected from TriOS after failure.
14 Results from Laboratory and Field Testing of Nitrate Measuring Spectrophotometers

Summary

The recent commercial availability of UV spectrophotometers provides the opportunity to monitor nitrate concentration in 

situ, and thereby increase the frequency of sample analysis by eliminating the logistical constraints of discrete sampling. There 

are several choices available in these analyzers, with variations in detector, light source, and optical path length. Interferences 

like turbidity and DOC present important challenges to the analyzer design. Generally, a longer path length will increase the 

sensitivity of the unit and improve its ability to measure lower concentrations, but the analyzer normally will be limited more by 

interferences such as turbidity rather than by a shorter path length. 

In this report, eight examples of five models of UV nitrate analyzers were evaluated in two phases—a controlled laboratory 

setting and in the field at three USGS monitoring sites. The accuracy of the analyzers was compared to NIST traceable standards 

and to discrete samples collected at the sites and analyzed locally using the Direct UV method, and at the USGS National Water 

Quality Laboratory. Measured analyzer values were collected raw, and were not corrected for fouling or calibration drift or for 

matrix bias. The findings of this report show how robust some sensors are against fouling, drift, and matrix bias, but in no way 

do these findings diminish the importance of data correction as noted in USGS protocols.

Accuracy, drift, limit of linearity (LOL), and operating temperature were tested in phase I. Accuracy requirements were met 

by the manufacturer (stock)-calibrated TriOS ProPs, Hach NITRATAX, and the Satlantic SUNA. The S::CAN Spectro::lyser’s 

calibration required several adjustments before meeting the manufacturer’s accuracy specifications. Instrument drift was 

observed in the S::CAN and was the result of insert seals leaching into the water sample. Except for the Hach, all laboratory test 

models met their specified LOL. The SUNA was excessively noisy and unstable in temperatures of about 20 degrees Celsius or 

above. Phase II tested the accuracy of the analyzers at USGS monitoring sites with low-, mid-level, and high-nitrate concentrations. In addition to varying nitrate concentrations, there were also differences in the environmental conditions and interferences, 

such as turbidity and DOC, at the three sites that potentially affected the performance of the analyzers. 

The Hach NITRATAX 2 mm showed good accuracy at the Kickapoo Creek site when subjected to high sediment loads. The 

SUNA V2 with its 5-mm path length, although promoted to be accurate in highly turbid waters, showed a low bias relative to 

the discrete samples taken at Kickapoo Creek, indicating that path length alone does not ensure accuracy. Although most of the 

measured-to-discrete sample comparisons showed good correlation during the evaluation, deviations to the testing criteria and 

to the manufacturing technical specifications were found. A total of 40 discrete samples were collected from the three deployment sites and analyzed. The nitrate concentration of the samples ranged from 0.33–22.2 mg-N/L. The average absolute difference between the TriOS measurements and discrete samples was 0.46 mg-N/L. For the combined data from the Hach 5-mm and 

2-mm analyzers, the average absolute difference between the Hach and the discrete samples was 0.13 mg-N/L. The SUNA and 

SUNA V2 combined data resulted in an average absolute difference of 0.66 mg-N/L, and the combined data from the three different S::CANs resulted in a 0.63-mg-N/L average absolute difference from the discrete samples. The results of this study show 

the value of in situ testing for nitrates. The results also can offer insight to the user when selecting the appropriate analyzer for 

an application, and can assist manufacturers in improving the performance of the analyzers tested—not only in a laboratory setting, but in the environment as well. 

Table 6. Statistical distribution of the analyzer model’s accuracy in milligrams nitrogen per liter.

Statistic TriOS ProPs 

(10 millimeters)

Hach 

NITRATAX 

(5 millimeters)

Hach 

NITRATAX 

(2 millimeters)

SATLANTIC 

SUNA 

(10 millimeters)

SATLANTIC 

SUNA V2 

(5 millimeters)

S::CAN 

SPECTRO::LYSER 

(5 milllimeters)

S::CAN 

SPECTRO::LYSER 

(35 millimeters)

Median –0.21 0.11 0.00 0.04 –2.89 –0.04 –0.09

Minimum –3.52 –0.31 –0.21 –0.69 –6.10 –7.41 –1.18

Maximum 0.07 0.37 0.11 0.26 –1.69 1.22 0.68

1st quartile –0.38 0.03 –0.19 –0.33 –5.39 –0.23 –0.25

3d quartile –0.07 0.18 0.11 0.14 –2.21 0.32 0.05
References Cited 15

Acknowledgments

The author thanks Lisa Carper and Jacob Gibs (retired) of the USGS New Jersey Water Science Center, and Paul Terrio,

Timothy Straub, and Nick Siudyla of the USGS Geological Illinois Water Science Center for their help and support in 

this project. 

References Cited

American Public Health Association, 1989, Standard methods for the examination of water and wastewater, 17th ed.: Washington, D.C., American Public Health Association.

Comerford, Jeffrey, 2011, Application Note: Measuring baseline corrected spectra on a Carey 50 Dual Beam Spectrophotometer, 

SI-A-1192, Mulgrave, accessed April 11, 2012, at http://www.chem.agilent.com/Library/applications/uv77.pdf.

Danaher Business System [2012a], Hach products and ordering: Hach Company Web page, accessed September 3, 2012, at 

http://www.hach.com.

Danaher Business System [2012b], SUNA V2 UV nitrate sensor: Satlantic Web page, accessed September 3, 2012, at http://

www.satlantic.com/suna.

Forum on Environmental Measurements Method Validation Team, Environmental Measurement Glossary of Terms, January 

2010, accessed April 10, 2012, at http://epa.gov/fem/pdfs/Env_Measurement_Glossary_Final_Jan_2010.pdf.

HunterLab Inc., 2008, Application Note: Double vs. Single Beam Spectrophotometers, v. 9, no. 12, Reston, accessed April 2, 2012,

at http://www.hunterlab.se/wp-content/uploads/2012/11/Double-Versus-Single-Beam-Spectrophotometers.pdf.

Instrumental Solutions Inc. Modbus/SDI-12 Protocol Converter Brochure, 2011, accessed September 4, 2013, at http://isidev.

net/yahoo_site_admin/assets/docs/MS1.310131826.pdf.

McNaught, A.D., and Wilkinson, A., 1997, International Union of Pure and Applied Chemistry (IUPAC) Compendium of 

Chemical Terminology – the Gold Book (2d ed.): Oxford, Blackwell Scientific Publications, 48 p. [Also available at http://

goldbook.iupac.org.]

Patton, C.J., and Kryskalla, J.R., 2011, Colorimetric determination of nitrate plus nitrite in water by enzymatic reduction, automated discrete analyzer methods: U.S. Geological Survey Techniques and Methods, book 5, chap. B8, at http://pubs.usgs.gov/

tm/05b08/.

Pellerin, B.A., Bergamaschi, B.A., Downing, B.D., Saraceno, J.F., Garrett, J.A., and Olsen, L.D., 2013, Optical techniques 

for the determination of nitrate in environmental waters; Guidelines for instrument selection, operation, deployment, 

maintenance, quality assurance, and data reporting: U.S. Geological Survey Techniques and Methods, book 1, chap. D5, 37 p., 

at http://pubs.usgs.gov/tm/01/d5/.

S::can messtechnik GmbH, S::can home page [2012]: S::can messtechnik GmbH Web page, accessed September 2, 2012, at 

http://www.s-can.us/.

Thermo Scientific, 2011, Application Note: Matrix Spiking–Why Spike and How to Do it, Log 112: Thermo Scientific, Environmental & Process Instruments Division, accessed January 6, 2013, at www.thermoscientific.com/waterlibrary.

TriOS Mess- und Datentechnik GmbH [2012], Welcome to TriOS Optical Sensors: TriOS Mess- und Datentechnik GmbH Web 

page, accessed October 10, 2012, at http://trios-science.com.

U.S. Environmental Protection Agency (EPA), 2004, EPA memorandum, Assuring the Competency of Environmental Protection 

Agency Laboratories, accessed September 23, 2012, at http://www.epa.gov/fem/pdfs/labdirective.pdf.

Wagner, R.J., Boulger, R.W., Jr., Oblinger, C.J., and Smith, B.A., 2006, Guidelines and standard procedures for continuous 

water-quality monitors—Station operation, record computation, and data reporting: U.S. Geological Survey Techniques and 

Methods, book 1, chap. D3, 51 p. + 8 attachments, at http://pubs.usgs.gov/tm/2006/tm1D3/.
Figure 1. Schematic illustration of a spectrophotometer.

UV light source Spectrometer

Sample

volume

UV light

Figure 2. Photos of nitrate analyzers: TriOS ProPS, (top left); Hach NITRATAX, (top center) S::CAN Spectro::lyser (top right) 

Satlantic SUNA, (bottom left), and Satlantic SUNA V2 (bottom right).
Figure 3. A schematic of a split or double-beam configuration.

 Lamp

Primary

Reference

Detector

Monochromator

Beam splitter

Sample

Figure 4. A schematic of a single-beam configuration.

 Lamp

Figure 4. A schematic of a single-beam configuration.

Monochromator Detector

Sample

 Lamp

Figure 5. A schematic of a dual-beam configuration.

Monochromator

Primary detector

Reference detector

Sample

Beam

splitter 

Figure 5. A schematic of a dual-beam configuration.
0

5

10

15

20

25

30

35

40

45

0 200 400 600 800 1,000 1,200 1,400 1,600 1,800

Chamber temperature in, degrees Celsius 

Elapsed time in minutes

Figure 6. Temperatures applied during temperature testing for nitrate analyzers.

Figure 6. Temperatures applied during temperature testing for nitrate analyzers.

Figure 7. Accuracy results for TriOS (top left), Hach (top right), SUNA (bottom left), and S::CAN (bottom right) ultraviolet nitrate analyzers. 

Error bars represent the estimated uncertainty, ± 3 percent, of the standard concentration.

.

 

Accuracy results for TriOS (topleft), Hach (top right), SUNA (bottom left), and S::CAN (bottom right) ultraviolet nitrate analyzers . Error bars represent the estimated uncertainty, ± 3 percent, of the standard concentration.

–10

–8

–6

–4

–2

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10

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–8

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–2

0

2

4

6

8

10

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–8

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–2

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4

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8

10

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–8

–6

–4

–2

0

2

4

6

8

10

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Difference between measured and standard, in milligrams per liter 

Trios assigned accuracy limits

Trios (10 mm)

Suggested trios limits

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Hach accuracy limits

Hach (5 mm)

0 5 10 15 20 25 30 35 40 45 50 55

Suna accuracy limits

Suna (10 mm)

0 5 10 15 20 25 30 35 40 45 50 55

Standard concentration of nitrate nitrogen in milligrams per liter

S::can accuracy limits

S::can (5 mm)

EXPLANATION

EXPLANATION

EXPLANATION EXPLANATION
Figure 8. Drift results for TriOS (top left), Hach (top right), SUNA (bottom left), and S::CAN (bottom right) ultraviolet nitrate analyzers. Plotted is 

the difference in successive measurements from the initial measurement over time. 

Figure 9. The difference between measured nitrate and standard concentration over an extended temperature range, for TriOS (top left), 

and Hach (top right), SUNA (bottom left), and S::CAN (bottom right).

–2.0

–1.5

–1.0

–0.5

0

0.5

1.0

1.5

2.0

–2.0

–1.5

–1.0

–0.5

0

0.5

1.0

1.5

2.0

–2.0

–1.5

–1.0

–0.5

0

0.5

1.0

1.5

2.0

–2.0

–1.5

–1.0

–0.5

0

0.5

1.0

1.5

2.0

Difference of successive measurement from initial, in milligrams nitrogen per liter 

Trios (10 mm) Hach (5 mm)

0 200 400 600 800 1,000 1,200

Elapsed time in minutes

Suna (10 mm)

0 200 400 600 800 1,000 1,200

0 200 400 600 800 1,000 1,200 0 200 400 600 800 1,000 1,200

Elapsed time in minutes

Scan (5 mm)

Figure 8. Drift results for TriOS (top left), Hach (top right), SUNA (bottom left), and S::CAN (bottom right) ultraviolet nitrate analyzers. Plotted is the difference in successive measurements from the initial measuremen

EXPLANATION EXPLANATION

EXPLANATION EXPLANATION

Figure 9. The difference between measured nitrate and standard concentration over an extended temperature range, for TriOS (top left), Hach (top right), SUNA (bottom left), and S::CAN (bottom right).

–7.0

–6.0

–5.0

–4.0

–3.0

–2.0

–1.0

0

1.0

2.0

–7.0

–6.0

–5.0

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–3.0

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–1.0

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1.0

2.0

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–6.0

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–4.0

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–1.0

0

1.0

2.0

TriOS accuracy limits

TRIOS (10 mm)

Hach accuracy limits

HACH (5 mm)

0 5 10 15 20 25 30 35 40 45

Temperature, in degrees Celsius

SUNA accuracy limits

SUNA (10 mm)

0 5 10 15 20 25 30 35 40 45

0 5 10 15 20 25 30 35 40 45 0 5 10 15 20 25 30 35 40 45

Temperature, in degrees Celsius

S::CAN accuracy limits

S::CAN (5 mm)

Difference of successive measurement from initial,

in milligrams nitrogen per liter 

EXPLANATION EXPLANATION

EXPLANATION EXPLANATION
Figure 10. Top left, (left to right) the S::CAN, Hach, TriOS, and SUNA analyzers next to an uninstalled 

deployment pipe. Top right, installed deployment pipes at the test pier. Bottom left, the S::CAN and Hach 

controllers, bottom right, the TriOS Tribox controller and the WaterLog H-500XL data logger interfaced with 

the SUNA. 

Figure 10. Top left (left to right) the S::CAN, Hach, TriOS, and SUNA analyzers next to an uninstalled deployment pipe. Top right, installed de

Bottom left, the S::CAN and Hach controllers. bottom right, the TriOS Tribox controller and theWaterLog H-500XL data logger inte

Figure 11. Two Bridges, New Jersey. View is looking downstream from the gage house. Figure 11. Two Bridges, New Jersey. View is looking downstream from the gage house. 
Figure 12. Two Bridges sampling system. Pictured on the left is the open basin 

that was sequentially filled with water from the three intakes. A fluorometer is 

located right of the basin.

Figure 13. Agricultural fields surrounding Kickapoo Creek.

Figure 12. Two Bridges sampling system. Pictured on the left is the open basin that

was sequentially filled with water from the three intakes. A

fluorometer is located right of the basin.
Figure 14. Site of Kickapoo Creek restoration project including The Grove 

subdivision seen in the background.

Figure 15. Installation of the nitrate analyzers at USGS site 05579610, 

Bloomington, Illinois.
Figure 16. Stacked time series plots for measured nitrate concentration in milligrams nitrogen per liter 

(mg-N/L) plotted for the East Pearl River gage height in feet, stream velocity in feet per second (ft/sec), 

temperature in degrees Celsius, turbidity in formazin nephelometric units (FNU), and specific conductance 

in microseimens per centimeter (μs/cm). Red boxes represent the reference samples collected from the East 

Pearl River during the testing period and the blue arrows denote the reduction in the Satlantic Submersible UV 

Nitrate Analyzer’s integration window and the two local adjustments of the S::CAN’s calibration.

Figure 16. Stacked time series plots for measured nitrate concentration in milligram Nitrogen per liter plotted for 

the East Pearl River gage height in feet, stream velocity in feet per second, temperature in degrees Celsius,

turbidity in formazin nephelometric units, and specific conductance in microseimens per centimeter. Red boxes

represent the reference samples collected from the East Pearl River during the testing period and the blue arrows 

denote the reduction in the Satlantic Submersible UV Nitrate Analyzer’s integration window and the two local adjustments of the S::CAN’s calibration.

0

0.5

1.0

1.5

2.0

TriOS 

(10 millimeter)

Nitrate

(mg-N/L) 

Hach 

(5 millimeter)

Nitrate

(mg-N/L) 

Suna 

(10 millimeter)

Nitrate

(mg-N/L) 

S::can 

(5 millimeter)

Nitrate

(mg-N/L) 

Temperature,

in degrees

Celsius Gage height, in feet Stream velocity, in ft/sec

Specific

conductance

(μS/cm)

0

0.5

1.0

1.5

2.0

0

0.5

1.0

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2.0

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30

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4

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1,000

Turbidity, in FNU 

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100

150

02/21/12 02/28/12 03/06/12 03/13/12 03/20/12 03/27/12 04/03/12 04/10/12 04/17/12 04/24/12 05/01/12

Date of measurement
Figure 17. Stacked time series plots for measured nitrate concentration in milligrams nitrogen per liter 

(mg-N/L) plotted for the Passaic River gage height in feet, temperature in degrees Celsius, turbidity in formazin 

nephelometric units (FNU), and dissolved organic carbon in milligrams per liter (mg/L). Red boxes represent the 

reference samples collected from the Passaic River during the testing period.

Figure 17. Stacked time series plots for measured nitrate concentration in milligram nitrogen per liter plotted for the Passaic River gage height in feet,

temperature in degrees Celsius, turbidity in formazin nephelometric units, and dissolved organic carbon in milligrams per liter.

Red boxes represent the reference samples collected from the Passaic River during the testing period.

0

2

4

6

0

2

4

6

0

2

4

6

0

2

4

6

0

5

10

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40

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50

100

0

10

20

10/17/2012 10/24/2012 10/31/2012 11/7/2012 11/14/2012 11/21/2012 11/28/2012 12/5/2012 12/12/2012

Date of measurement

TriOS 

(10 millimeter)

Nitrate

(mg-N/L) 

Hach 

(5 millimeter)

Nitrate

(mg-N/L) 

Suna 

(10 millimeter)

Nitrate

(mg-N/L) 

S::can 

(35 millimeter)

Nitrate

(mg-N/L) 

Temperature,

in degrees

Celsius

Gage height,

in feet 

Dissolved

organic

carbon

(mg/L) Turbidity, in FNU 
Figure 18. Stacked time series plots for measured nitrate concentration in milligrams nitrogen per liter 

(mg-N/L) plotted for the Pompton River gage height in feet, temperature in degrees Celsius, turbidity in 

formazin nephelometric units (FNU), and dissolved organic carbon in milligrams per liter. Red squares 

represent the reference samples collected from the Pompton River during the testing period. The 

October 18, 2012, reference point (black square) was not included in the Satlantic Submersible UV 

Nitrate Analyzer’s accuracy evaluation due to the insufficient filling of the sample basin.

Figure 18. Stacked time series plots for measured nitrate concentration in milligrams nitrogen per liter plotted for the Pompton River gage height in feet,

temperature in degrees Celsius, turbidity in formazin nephelometric units, and dissolved organic carbon in milligrams per liter.

Red squares represent the reference samples collected from the Pompton River during the testing period. The October 18, 2012,

reference point (black square) was not included in the Satlantic Submersible UV Nitrate Analyzer’s accuracy evaluation due to the insufficient filling of the sample basin.

0

2

4

0

2

4

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2

4

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2

4

0

5

10

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40

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50

100

0

10

20

10/17/2012 10/24/2012 10/31/2012 11/7/2012 11/14/2012 11/21/2012 11/28/2012 12/5/2012 12/12/2012

Date of measurement

TriOS 

(10 millimeter)

Nitrate

(mg-N/L) 

Hach 

(5millimeter)

Nitrate

(mg-N/L) 

Suna 

(10 millimeter)

Nitrate

(mg-N/L) 

S::can 

(35 millimeter)

Nitrate

(mg-N/L) 

Temperature,

in degrees

Celsius

Gage height,

in feet 

Dissolved

organic

carbon

(mg/L) Turbidity, in FNU 
Figure 19. Stacked time series plots for measured nitrate concentration in milligrams nitrogen per liter (mg-N/L) plotted for 

Kickapoo Creek gage height in feet, and discharge in cubic feet per second (ft3/s). Red squares represent the reference samples 

collected during the testing period. 

Date of measurement

TriOS 

(10 millimeter)

Nitrate

(mg-N/L) 

Hach 

(2 millimeter)

Nitrate

(mg-N/L) 

Suna V2 

(5 millimeter) 

Nitrate

(mg-N/L) 

S::can 

(5 millimeter)

Nitrate

(mg–N/L) 

Gage height,

in feet 

Discharge

(ft3/s)

Figure 19. Stacked time series plots for measured nitrate concentration in milligrams nitrogen per liter plotted for Kickapoo Creek gage height in feet, 

and discharge in cubic feet per second. Red squares represent the reference samples collected during the testing period.

0

10

20

30

0

10

20

30

0

10

20

30

0

10

20

30

0

5

10

15

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250

500

4/3/13 4/10/13 4/17/13 4/24/13 5/1/13 5/8/13 5/15/13 5/22/13 5/29/13 6/5/13 6/12/13 6/19/13
Figure 20. Difference in milligrams nitrogen per liter (mg-N/L) between the measured and reference nitrate concentrations from 

eight analyzers at three USGS sites compared to the reference nitrate concentration. The difference, calculated as measured minus 

reference concentration, shows any bias in the data. 

Figure 20. Difference in milligrams nitrogen per liter between the measured and reference nitrate concentrations from eight analyzers at three USGS sites

compared to the reference nitrate concentration. The difference, calculated as measured minus reference concentration, shows any bias in the data.

–10

–5

0

5

0

5

10

Difference between measured and discrete sample

concentration (mg-N/L ) 

TRIOS

TRIOS 10 mm

HACH

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10

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–5

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–5

10

0 5 10 15 20 25

SUNA

0 5 10 15 20 25

0 5 10 15 20 25 0 5 10 15 20 25

Discrete sample concentration (mg-N/L) Discrete sample concentration (mg-N/L)

Discrete sample concentration (mg-N/L) Discrete sample concentration (mg-N/L)

SCAN

EXPLANATION HACH 5 mm HACH 2 mm

EXPLANATION

SUNA 10 mm SUNA V2 5 mm

EXPLANATION HIF S::CAN 5 mm 

WSC S::CAN 35 mm

LOANER S::CAN 35 mm

EXPLANATION
Figure 21. Statistical distribution of the differences in measured and discrete sample nitrate 

concentration from three measurement sites (mg-N/L, milligrams nitrogen per liter; 

mm, millimeters).

Difference between measured and discrete

sample concentration (mg-N/L ) 

Figure 21. Statistical distribution of the differences in measured and discrete sample nitrate

concentration from three measurement sites.

–8

–6

–4

–2

0

2

4

6

8

TRIOS PROPS

(10 mm)

HACH NITRATAX

(2 and 5 mm)

SATLANTIC SUNA

(10 mm) and

SUNA V2 (5 mm)

S::CAN

SPECTRO::LYSER

(5 mm)

median

Q1

minimum

maximum

Q3

EXPLANATION
For additonal information, visit http://water.usgs.gov/hif/

USGS Publishing Service Center staff:

Marilyn A. Billone, Editor 

James E. Banton, layout

Snazelle—Results from Laboratory and Field Testing of Nitrate Measuring Spectrophotometers—Open-File Report 2015–1065

ISSN 2331-1258 (online)

http://dx.doi.org/10.3133/ofr20151065