U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2016–1014

The Effect of Suspended Sediment and Color on Ultraviolet 

Spectrophotometric Nitrate Sensors
Cover photograph. Hach NITRATAX (2-mm), Hach NITRAX (5-mm), and the S::CAN Spectro::lyser (5-mm) nitrate 

sensors in the tank apparatus.
The Effect of Suspended Sediment and Color on Ultraviolet 

Spectrophotometric Nitrate Sensors

By Teri T. Snazelle

Open-File Report 2016–1014

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, Director

U.S. Geological Survey, Reston, Virginia: 2016

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Suggested citation:

Snazelle, T.T., 2016, The effect of suspended sediment and color on ultraviolet spectrophotometric nitrate sensors: 

U.S. Geological Survey Open-File Report, 2016−1014, 10 p., http://dx.doi.org/10.3133/ofr20161014.

ISSN 2331-1258 (online)
iii

Contents

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

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

Description of Spectrophotometric Nitrate Sensors...............................................................................2

Test Procedures .............................................................................................................................................4

Turbidity/Total Suspended-Sediment Effect on Accuracy.............................................................4

Colored Dissolved Organic Matter Effect on Accuracy.................................................................6

Test Results.....................................................................................................................................................6

Turbidity and Total Suspended-Sediment Effect on Accuracy ....................................................6

Colored Dissolved Organic Matter Effect on Accuracy.................................................................7

Summary..........................................................................................................................................................9

Acknowledgments.......................................................................................................................................10

References Cited..........................................................................................................................................10

Figures

1. Photographs of nitrate sensors: Hach NITRATAX 5 mm; Hach NITRATAX 

2 mm S::CAN Spectro::lyser; and Satlantic SUNA V2.............................................................2

2. Schematic Illustration of a spectrophotometer.......................................................................2

3. Photograph taken during the first round of suspended sediment testing 

showing the Hach NITRATAX, Hach NITRATAX, and the S::CAN Spectro::lyser 

nitrate sensors in the tank apparatus .......................................................................................4

4. Photograph taken during the second round of suspended-sediment testing 

showing the Hach NITRATAX, Hach NITRATAX, S::CAN Spectro::lyser, and 

the SUNA V2 nitrate sensors in the tank apparatus ...............................................................5

5. Photograph of the National Institute of Standards and Technology standard 

reference material 1646a-estuarine sediment.........................................................................5

6. Photograph of the prefiltered brewed tea used in the test for colored 

dissolved organic matter.............................................................................................................6

7. Graphs showing difference between the initial nitrate measurement and subsequent 

nitrate measurements of the 5 milligrams-nitrogen-per-liter solution by the 

Hach NITRATAX, Hach NITRATAX, S::CAN, and SUNA V2 nitrate sensors in 

suspended-sediment concentration values from 0–4,500 mg/L ...........................................7

8. Photographs showing the initial clear matrix spike of the turbidity/total suspended 

sediment test and the ending matrix spike of the turbidity/tss test containing the 

addition of 4,500 milligrams per liter of National Institute of Standards and 

Technology standard reference material 1646a-estuarine sediment..................................8

9. Photographs showing the observed color change as the estimated total 

organic carbon increases ...........................................................................................................8

10. Graphs showing percent increase in measured nitrate concentration by the 

Hach NITRATAX, Hach NITRATAX, S::CAN, and SUNA V2 nitrate sensors against 

increasing estimated total organic carbon. Turbidity is plotted as the red squares 

in formazin nephelometric units.................................................................................................9
iv

Tables

1. Nitrate sensor features and manufacturer specifications....................................................3

Note: Tables 2, 3, and 4 are available for download in Excel format at 

http://dx.doi.org/10.3133/ofr20161014.

2. Nitrate measurements by three ultraviolet nitrate sensors in water with a 5-mg-

 NL concentration and with varying concentrations of National Institute of Standards

 and Technology standard reference material 1646a sediment (first test).

3. Nitrate measurements by four ultraviolet nitrate sensors in water with a 5-mg-

 NL concentration and with varying concentrations of National Institute of Standards 

 and Technology standard reference material 1646a sediment (second test).

4. Nitrate measurements by four ultraviolet nitrate sensors in water with a 5-mg-

 NL concentration and with varying concentrations of filtered tea.

Conversion Factors

SI 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.)

Volume

liter (L)

liter (L)

33.82

1,000

ounce, fluid (fl. oz.)

milliliter (ml)

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

°F=(1.8×°C)+32

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

°C=(°F-32)/1.8
v

Abbreviations

CDOM colored dissolved organic matter

EDOC estimated dissolved organic carbon

ETOC estimated total organic carbon

FNU formazin nephelometric units

HIF Hydrologic Instrumentation Facility (USGS)

NIST National Institute of Standards and Technology

SSC suspended-sediment concentration

TSS total suspended sediment

USGS U.S. Geological Survey

UV ultraviolet

The Effect of Suspended Sediment and Color on Ultraviolet 

Spectrophotometric Nitrate Sensors

By Teri T. Snazelle

Abstract 

Four commercially available ultraviolet nitrate spectrophotometric sensors were evaluated by the U.S. Geological Survey 

Hydrologic Instrumentation Facility (HIF) to determine the effects of suspended sediment concentration (SSC) and colored 

dissolved organic matter (CDOM) on sensor accuracy. The evaluated sensors were: the Hach NITRATAX plus sc (5-millimeters 

(mm) path length), Hach NITRATAX plus sc (2 mm), S::CAN Spectro::lyser (5 mm), and the Satlantic SUNA V2 (5 mm). A 

National Institute of Standards and Technology-traceable nitrate-free sediment standard was purchased and used to create the 

turbid environment, and an easily made filtered tea solution was used for the CDOM test. All four sensors performed well in 

the test that evaluated the effect of suspended sediment on accuracy. The Hach 5 mm, Hach 2 mm, and the SUNA V2 met their 

respective manufacturer accuracy specifications up to concentrations of 4,500 milligrams per liter (mg/L) SSC. The S::CAN 

failed to meet its accuracy specifications when the SSC concentrations exceeded 4,000 mg/L. Test results from the effect of 

CDOM on accuracy indicated a significant skewing of data from all four sensors and showed an artificial elevation of measured 

nitrate to varying amounts. Of the four sensors tested, the Satlantic SUNA V2’s accuracy was affected the least in the CDOM 

test. The nitrate concentration measured by the SUNA V2 was approximately 24 percent higher than the actual concentration 

when estimated total organic carbon values exceeded 44 mg/L. Measured nitrate concentration falsely increased 49 percent 

when measured by the Hach 5 mm, and 75 percent when measured by the Hach 2 mm. The S::CAN’s reported nitrate concentration increased 96 percent. Path length plays an important role in the sensor’s ability to compensate measurements for matrix 

interferences, but does not solely determine how well a sensor can handle all interferences. The sensor’s proprietary algorithms 

also play a key role in matrix interference compensation. The sensors’ ability to compensate for CDOM varied significantly 

during the tests, even among the three with 5-mm path lengths. Results of this evaluation suggest that the proprietary algorithms 

of the nitrate analyzers are more effective compensating for suspended sediment, and less effective compensating for CDOM 

(color) when sensor path length remains constant.

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 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. Evaluation reports 

document the results at the time of testing, and may or may not represent future conditions resulting from updates and improvements to the device. Suspended sediment and colored dissolved organic matter (CDOM) are some of the most problematic 

matrix interferences to optical sensors. They are particularly troublesome with spectrophotometric sensors due to the skewing 

of direct wavelength absorption measurement (a) by scattering of the light (turbidity) or (b) by the absorption of visible and 

ultraviolet (UV) light of CDOM, which biases the measured concentration high (Blough and Del Vecchio, 2002). This report 

describes a laboratory evaluation on the effect of these interferences on the accuracy of UV spectrophotometric nitrate sensors. 

 Four nitrate sensors were tested during this two-phase evaluation: Hach NITRATAX plus sc 5-millimeter (mm) path 

length, Hach NITRATAX plus sc 2-mm path length, S::CAN Spectro::lyser 5-mm path length, and Satlantic SUNA V2 5-mm 

path length (fig. 1). In the first phase, sensor accuracy was tested against the effect of highly turbid water with heavy suspendedsediment concentration (SSC). The second phase evaluated the effect of CDOM on sensor accuracy. 
2 The Effect of Suspended Sediment and Color on Ultraviolet Spectrophotometric Nitrate Sensors

Description of Spectrophotometric Nitrate Sensors

Recent advances in technology have led to the development of self-contained, submersible sensors that can deliver continuous monitoring for nitrate-plus-nitrite concentration. In contrast to some previous models that relied on reagents, cadmium 

reduction, and visible spectrophotometry, these sensors provide a chemical-free and cost-effective alternative through the use of 

UV spectrophotometry (American Public Health Association, 1989). The optical system of a spectrophotometer comprises three 

main parts (fig. 2): 

• A light source (lamp) - usually deuterium or xenon.

• A sample optical path length - usually 2 to 100 millimeters (mm).

• A photoelectric detector. 

The matrix to be tested (normally water) flows into the instrument’s measurement window and is illuminated with a beam 

of light. This light beam is separated into various wavelengths in the UV-visible (UV-Vis) spectrum (190–740 nanometers (nm)) 

by means of either a diffraction grating or monochromatic optical filter and is subsequently 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. 

The Hach NITRATAX (2 mm and 5 mm) uses a primary light-beam wavelength of 210 nm to measure nitrate plus 

nitrite, and a secondary reference light-beam wavelength of 350 nm to correct for matrix interferences. The SUNA V2 measures nitrate plus nitrite using a 217–240-nm wavelength light beam. The spectrum is processed into approximately 35 divisions or “channels” from which the absorbance is calculated and the contributing absorbing constituents are decomposed using 

the method developed at the Monterey Bay Aquarium Research Institute (MBARI) (Johnson and Coletti, 2002). The S::CAN 

Figure 1. Photographs of nitrate sensors: Hach NITRATAX 5 mm (top left); Hach 

NITRATAX 2 mm (top right); S::CAN Spectro::lyser (bottom left); and Satlantic SUNA 

V2 (bottom right).

UV light source Spectrometer

Sample

volume

UV light

Figure 2. Schematic Illustration of a spectrophotometer.
Description of Spectrophotometric Nitrate Sensors 3

Spectro::lyser UV-Vis measures the complete absorbance spectrum between 190 and 720 nm. This spectrum is resolved into a 

256-wavelength “fingerprint” that makes it possible to monitor multiple parameters simultaneously and compensate for uncharacterized substances that interfere with these parameters. The analyzers are available commercially with prices varying from 

$15,000 to $25,000, depending on optional accessories. The instruments differ in their light sources, optical configuration, 

measurement path length, communication protocols, antifouling measures, and algorithms used to compute nitrate from the 

UV absorbance of the water sample.

Natural waters contain a mixture of substances that can create challenges for optical sensors. These substances can reduce 

the amount of transmitted light (turbidity) or absorb light in the same UV spectral range as nitrate and cause a bias in the calculated concentration. Two factors that influence how well a sensor can compensate for these interfering substances are the path 

length and data-processing algorithms. In general, a shorter path length will generally expand the nitrate detection range and 

minimize adverse effects from matrix interferences, such as turbidity, but will limit the sensitivity of the sensor at low concentrations. A longer path length will provide greater sensitivity at low concentrations, but will reduce the detection range and increase 

the effect of interferences. The effect of CDOM on UV absorption is not as readily managed. The S::CAN Spectro::lyser 

UV-Vis measures and reports turbidity, estimated total organic carbon (ETOC), and estimated dissolved organic carbon 

(EDOC), independently. To estimate total organic carbon and dissolved organic carbon, the Spectro::lyser scans wavelengths 

from 200–735 nm and, using a proprietary algorithm, translates this information into an estimated value. The Spectro::lyser 

calculates the amount of interference and subtracts it from the reported nitrate measurement. The user manuals for the Hach 

NITRATAX (Danaher Business System [2012a]) and SUNA V2 (Danaher Business System [2012b]) state that CDOM (color) in 

natural waters does not affect absorption, and, therefore, does not interfere with nitrate measurement (Danaher Business System, 

2014 [SUNA]; 2015 [Hach]).

Table 1 gives a summary of each sensor’s available features, technical specifications, path lengths, and firmware of the 

sensors at the time of testing. Additional information on the analyzers can be found in USGS Open-File Report 2015–1065, 

“Results from laboratory and field testing of nitrate measuring spectrophotometers” (Snazelle, 2015) and in 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) .

Table 1. Nitrate sensor features and manufacturer specifications.

[UV, ultraviolet; nm, nanometer; mm, millimeter; NO3-N, nitrate-nitrogen; mg/L, milligrams per liter; %, percent; °C, degree Celsius; dia, diameter; kg, 

kilogram; Max, maximum; m, meter; VAC, volts alternating current; VDC, volts direct current; amp, ampere; mA, milliamp]

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

Detection

 protocol

UV absorption at

 210 nm & 350 nm

UV spectral analysis 

(190–370 nm)

UV-Visible Spectral analysis 

(220–720 nm)

Available Measurement

 path lengths (mm)

1, 2, and 5 (fixed) 5, 10, 100 (fixed) 0.5–100 (semifixed)

Sample path length (mm) tested 5 and 2 5 5 

Software version of tested

 analyzer

5 mm - version 3.06

2 mm - version 3.13

Version 2.5.0 Information unavailable

Lamp Xenon Deuterium Xenon 

Detector Patented 2 beam Zeiss Zeiss 

Optical configuration Dual beam Single beam Split beam

NO3-N range (mg/L) 5 mm - 0.1–25

2 mm - 0.1–50

0.031–56 0.005–70 (typical)

Accuracy ±3% of mean ±0.5 mg/L 0.028 (mg/L) or ±10% 2% + 1/(optical path

 Length in mm) (mg/L)

Detection limit (mg/L) 0.10 0.031 0.005

Operating temperature range 0 ± 40°C 0–35°C 0–45°C

Dimensions 75 mm dia x 323 mm 119 mm x 624 mm (with wiper) 44 mm dia x 647 mm

Weight (in air) 3.6 kg 3.1 kg 2.1 kg

Depth max 100 m 500 m

100 m (with wiper)

Not available

Power 120 VAC or 24 VDC 12 VDC 12 VDC

Current draw (average) Not available 0.625 amp 20 W max

Communications Modbus/RS485, Modbus/RS232, 

4–20 mA

RS232, SDI-12,

 4–20 mA 

Modbus/RS232, Modbus/RS485,

 4–20 mA, SDI-12
4 The Effect of Suspended Sediment and Color on Ultraviolet Spectrophotometric Nitrate Sensors

Test Procedures

The tests were conducted in two phases. Phase one testing, turbidity/SSC effect on accuracy, evaluated the effect of suspended sediment on the nitrate measurements made by the analyzers. Phase two testing, CDOM effect on accuracy, evaluated 

the effect of color on the nitrate measurements made by the sensors.

To create the testing environment for the analyzers, an open-top Chem-tainer10-gallon tank was purchased along with a 

stand (part number [pn] TC1528AP, The Tank Depot). The tank was designed with a cylindrical cone bottom, which was retrofitted with a drain tube and stopcock. Also purchased was an electric variable-speed 1/15th horsepower mixer (pn 34745K71, 

McMaster-Carr), a stand with swivel connector (pn 35265K11, McMaster-Carr), a 4-inch stainless steel propeller (pn 8364T5, 

McMaster-Carr), a 12-inch stainless steel drive shaft (pn 8004K421, McMaster-Carr), and two pairs of mounting brackets 

(pn 1307T221, McMaster-Carr). 

The measurement interval for all four sensors was set at once per minute. Measurements were made based upon the 

manufacturer’s default calibrations and no offset values were applied. Signal averaging was initiated to obtain the numerical 

mean of three spectra or “counts” per reported value. The SUNA V2 was programmed to operate in continuous mode and the 

“deployed in salt water” option was disabled. 

Sensor accuracy was verified prior to testing with a Hach-Lange 50-milligrams-per-liter (mg/L) (11.3 mg-N/L) nitrate 

standard (lot number 13196). An acceptance range of plus or minus (±) 10 percent was established for the calibration 

verification. All sensors met the criteria: the Hach (2 mm) measured 11.5 milligrams nitrogen per liter (mg-N/L), the Hach 

(5 mm) measured 10.8 mg-N/L, the S:CAN measured 11.3 mg-N/L, and the SUNA V2 measured 10.8 mg-N/L. 

Turbidity/Total Suspended-Sediment Effect on Accuracy

Phase I testing was originally conducted with only three sensors—two versions of the Hach NITRATAX plus sc (5 mm 

and 2 mm) and the S::CAN Spectro::lyser. The test was later repeated and included the Satlantic SUNA V2. For the first round 

of testing, the tank was filled with 28 liters (L) of deionized water, and the mixer was set to produce a flow of approximately 

1.5 feet per second. A 5-mg-N/L matrix spike was created by adding 140 milliliters (mL) of a 1,000-mg/L nitrate standard (Ricca 

Chemical, pn 5307-16) to the 28 L of agitated deionized water (fig. 3).

Figure 3. Photograph taken during the first round of suspended 

sediment testing showing the Hach NITRATAX (2-mm), Hach 

NITRATAX (5-mm), and the S::CAN Spectro::lyser (5-mm) nitrate 

sensors in the tank apparatus.
Test Procedures 5

The formula used to calculate the proper volumes for the matrix spike is shown below.

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 the matrix spike was estimated at 1.22 percent at 95-percent confidence and was calculated from the sum 

of the squares of the following variables (standard uncertainties) (Kirkup, 2006): 

• 0.5-percent error from the certified stock standard.

• Fourteen volumes of deionized water from a 2-L volumetric flask with a 0.025-percent uncertainty (0.35-percent 

error).

Normal distribution was assumed for the variables. 

When the Phase I test was repeated, the tank was filled with 32 L of deionized water to accommodate the optical path 

length in the SUNA V2, which is located higher in the body compared to the other sensors. The 5-mg-N/L matrix spike was created by adding 160 mL of the previously mentioned 1,000 mg/L nitrate standard to the 32 L of deionized water. The uncertainty 

of the matrix spike for this test was estimated at 1.28 percent at 95-percent confidence and was derived from the previously 

mentioned variables with 16 volumes of deionized water from a 2-L volumetric flask with an uncertainty of 0.028 percent. The 

sensors were submerged in the spiked solution and allowed to acclimate for several minutes prior to starting the test (fig. 4). 

Initial measurements from the analyzers in the clear spiked solution were monitored to ensure they were working properly, and 

the turbidity of the clear solution was recorded as the baseline for the test.

The turbid solution was created by the addition of National Institute of Standards and Technology (NIST) Standard 

Reference Material 1646a-estuarine sediment. This grey-colored sediment was characterized and certified to be free of nitrogencontributing substances (fig. 5). Aliquots of this sediment were weighed gravimetrically on a Mettler AE200 digital balance and 

systematically added to the agitated spike solution in which the nitrate sensors were submerged. 

Figure 4. Photograph taken during the second round of 

suspended-sediment testing showing the Hach NITRATAX 

(2-mm), Hach NITRATAX (5-mm), S::CAN Spectro::lyser (5-mm), 

and the SUNA V2 (5-mm) nitrate sensors in the tank apparatus.

Figure 5. Photograph of the National Institute of Standards and 

Technology standard reference material 1646a-estuarine sediment.
6 The Effect of Suspended Sediment and Color on Ultraviolet Spectrophotometric Nitrate Sensors

Nitrate and turbidity concentrations were recorded for SSC values that ranged from 10 to 2,000 mg/L during the first round 

of testing, and from 10 to 4,500 mg/L during the second test. Turbidity was measured using a Hydrolab Datasonde 5x (Ds5X) 

equipped with a calibrated Hydrolab 5 series self-cleaning turbidity probe. The manufacturer specifications list the accuracy of 

the Hydrolab 5 series as ±1 percent (0.1–100 formazin nephelometric units [FNU]), ±3 percent (101–400 FNU), and ±5 percent 

(401–3000 FNU) (Hach Hydromet Application Notes, [2014]). 

Colored Dissolved Organic Matter Effect on Accuracy

The difficulty in the phase 2 testing (CDOM effect on accuracy) was in finding a suitable additive that would provide the 

brownish hue found in many surface waters and that would not interfere with the sensors at the wavelengths of interest. Food 

coloring was purchased and initially tried. A drop of food coloring was added to 1L of deionized water and an aliquot was tested; 

every color produced a false-positive nitrate concentration of 5 to10 mg-N/L when analyzed on a calibrated UV-Vis benchtop 

spectrophotometer. Tea was also tested as a surrogate. A family-size tea bag of cold brew tea was added to 1L of deionized water 

and allowed to diffuse for approximately 15 minutes (fig. 6). Initial tests on the benchtop spectrophotometer indicated a false 

nitrate concentration of 23 mg-N/L. 

After the tea was filtered through a Pall 0.45-micron Versapor membrane filter, the nitrate concentration read less than 

detection when analyzed on the benchtop spectrophotometer. The filtered tea was selected as the CDOM source for the 

evaluation.

For the CDOM test, the tank was filled with 32 L of deionized water, and as with the suspended sediment test, spiked to a 

nitrate concentration of 5 mg-N/L, using the same spiking solution as in the phase 1 test. The four sensors were submerged in 

the tank and the mixer was adjusted to maintain a flow of approximately 1.5 feet per second. Measured aliquots of the filtered 

tea were added to the solution, and turbidity, ETOC, and nitrate concentrations were recorded to determine the effect of color on 

the measured nitrate concentrations. The S::CAN’s ETOC values were not verified for accuracy prior to testing; therefore, these 

values were noted only as an increase in concentration from the baseline reading. 

Test Results

Turbidity and Total Suspended-Sediment Effect on Accuracy 

SSC concentrations of 10 mg/L to 4,500 mg/L were added to the water bath and the effect on nitrate accuracy was recorded. 

Tables 2 and 3 summarize the results from the two tests, and figure 7 shows the difference between the initial nitrate measurements and subsequent nitrate measurements during the two tests as the SSC increased.

Figure 6. Photograph of the prefiltered brewed tea used in the 

test for colored dissolved organic matter.
Test Results 7

Little or no change was observed in any of the sensors’ nitrate measurements during either of the phase 1 tests, although a 

noticeable change in water clarity was observed as the SSC was increased (fig. 8). Only at 4,500 mg/L SSC did the S::CAN’s 

accuracy fail to meet the acceptance criteria (± 0.31 mg-N/L), with a difference of 0.50 mg-N/L between the initial and measured concentration. The SUNA V2 and both NITRATAX sensors (2 mm and 5 mm) consistently met their respective manufacturer specifications for accuracy.

Colored Dissolved Organic Matter Effect on Accuracy

Turbidity remained low during the color test and averaged only 5.5 FNU as measured by the Hydrolab 5 series turbidity 

sensor. Estimated total organic carbon, as measured by the S::CAN Spectro::lyser, increased from 13.7 mg/L in deionized water 

to 44.6 mg/L at the completion of the test, and a noticeable change in water color was observed as the ETOC increased (fig. 9). 

Figure 10 shows the percent increase of measured nitrate concentration for each sensor compared to the increasing ETOC 

values. It is important to emphasize that the increase in nitrate concentration was not legitimate. The color additive, the filtered 

tea, was tested prior to the test for nitrate concentration using a calibrated benchtop spectrophotometer, and nitrate concentration 

registered at less than detection. All four sensors were adversely affected to varying degrees. The SUNA V2 performed the best 

in the evaluation. The SUNA’s measured nitrate stayed within 5 percent of the true value in ETOC concentrations up to 20 mg/L. 

The SUNA’s data showed a 24-percent error for the maximum ETOC concentration. The NITRATAX 5-mm path length had the 

next best performance with measured nitrate remaining within 5 percent of the true value in ETOC concentrations up to 17 mg/L. 

NITRATAX data resulted in a 49-percent error at the maximum ETOC concentration recorded during the test. 

0

200

400

600

800

1,000

1,200

–1

–0.8

–0.6

–0.4

–0.2

0

0.2

0.4

0.6

0.8

1

0 1,000 2,000 3,000 4,000 5,000

Turbidity, in FNUs

Total suspended solids, in milligrams per liter

NITRATAX 2 mm NITRATAX 5 mm

S::CAN 5 mm SUNA V2 5 mm

Difference between initial measurement and subsequent nitrate measurements (mg-N/L) 

Difference between initial measurement and subsequent nitrate measurements (mg-N/L) 

0

200

400

600

800

1,000

1,200

–1

–0.8

–0.6

–0.4

–0.2

0.2

0.4

0.6

0.8

1

0 1,000 2,000 3,000 4,000 5,000

Turbidity, in FNUs

Total suspended solids, in milligrams per liter

0 1,000 2,000 3,000 4,000 5,000

Total suspended solids, in milligrams per liter

0 1,000 2,000 3,000 4,000 5,000

Total suspended solids, in milligrams per liter

0

200

400

600

800

1,000

1,200

–1

–0.8

–0.6

–0.4

–0.2

0

0.2

0.4

0.6

0.8

1

Turbidity, in FNUs

0

200

400

600

800

1,000

1,200

–1

–0.8

–0.6

–0.4

–0.2

0.2

0.4

0.6

0.8

1

Turbidity, in FNUs

EXPLANATION

Measured nitrate

Accuracy specifications

Turbidity, in formazin

nephelometric units (FNUs)

Figure 7. Difference between the initial nitrate measurement and subsequent nitrate measurements of the 5 milligrams-nitrogenper-liter (mg-N/L) solution by the Hach NITRATAX (2-mm), Hach NITRATAX (5-mm), S::CAN (5-mm), and SUNA V2 (5-mm) nitrate 

sensors in suspended-sediment concentration values from 0–4,500 mg/L.
8 The Effect of Suspended Sediment and Color on Ultraviolet Spectrophotometric Nitrate Sensors

Figure 8. Photographs showing the initial clear matrix spike of the turbidity/total suspended sediment (tss) test 

(left) and the ending matrix spike of the turbidity/tss test containing the addition of 4,500 milligrams per liter of 

National Institute of Standards and Technology standard reference material 1646a-estuarine sediment (right).

17.5 mg/L 19.1 mg/L 21.6 mg/L

27.5 mg/L 35.5 mg/L 44.6 mg/L

Figure 9. Photographs showing the observed color change as the estimated total organic carbon (ETOC) 

increases (left to right, top to bottom). Measured aliquots (in milligrams per liter, mg/L) of filtered “nitrate-free” tea 

were added to increase the ETOC values.
Summary 9

Data from the NITRATAX 2-mm remained within 5 percent of the true value in ETOC concentrations up to 17 mg/L, and 

showed a 75-percent error at the maximum ETOC concentration. The S::CAN’s measured values remained within 5 percent of 

the true value in ETOC concentrations less than 15 mgL, and showed a 96-percent error at the maximum ETOC concentration.

Theoretically, the NITRATAX 2 mm should have performed better than the NITRATAX 5 mm due to the shorter path length, 

and initial test results verified this theory. The nitrate concentration reported by the 2-mm sensor was relatively unaffected up to 

an ETOC concentration of 15.5 mg/L, while the concentration reported by the 5-mm sensor was adversely affected at an ETOC 

concentration of 14.6 mg/L. At an ETOC concentration of 16.5 mg/L, both the 2-mm and the 5-mm sensors showed an approximate 6-percent increase in nitrate concentration. This result suggests that path length alone does not determine a sensor’s ability 

to compensate for all interferences. Table 4 summarizes the volumes of tea solution added and the resulting increases in ETOC 

and “false” nitrate concentration for each tested nitrate sensor.

Summary

The recent availability of commercial ultraviolet (UV) spectrophotometers provides the opportunity to monitor nitrate 

concentration in situ, and thereby increases the potential frequency of sample analysis by eliminating the logistical constraints 

of discrete-only water sampling. However, interferences like suspended sediment and colored dissolved organic matter (CDOM) 

present major challenges to the sensor abilities. In this report, four of the most popular models of UV nitrate sensors were tested 

to determine what effect suspended-sediment concentration (SSC), from a certified nitrate-free sediment standard and CDOM 

produced from filtered tea, would have on sensor accuracy. 

In this evaluation, there was little effect on the sensors’ accuracy from the National Institute of Standards and Technology 

standard-reference-material-induced SSC. Accuracy specifications were met by the Hach NITRATAX (5 mm), Hach NITRATAX 

(2 mm), and the Satlantic SUNA V2 (5 mm) in the SSC test with concentrations from 10 to 4,500 mg/L. The accuracy 

specifications were not met in the phase I test by the S::CAN Spectro::lyser (5 mm) for SSC values above 4,000 mg/L. The 

effect of CDOM significantly affected measurement accuracy of all four sensors, with errors ranging from 20 to 96 percent Turbidity, in FNUs

Total organic carbon, in milligrams per liter

NITRATAX 2 mm NITRATAX 5 mm

S::CAN 5 mm SUNA V2 5 mm

Percentage of increase in nitrate concentrationPercentage of increase in nitrate concentration

Turbidity, in FNUs

Total organic carbon, in milligrams per liter

Total organic carbon, in milligrams per liter Total organic carbon, in milligrams per liter Turbidity, in FNUs

Turbidity, in FNUs

EXPLANATION

Measured nitrate

Turbidity

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Figure 10. Percent (%) increase in measured nitrate concentration by the Hach NITRATAX (2-mm), Hach NITRATAX 

(5-mm), S::CAN (5-mm), and SUNA V2 (5-mm) nitrate sensors against increasing estimated total organic carbon. Turbidity is 

plotted as the red squares in formazin nephelometric units (FNUs).
10 The Effect of Suspended Sediment and Color on Ultraviolet Spectrophotometric Nitrate Sensors

of measurement values for the maximum-tested ETOC concentration of 45 mg/L. The SUNA V2 performed the best in the 

CDOM test with a 24-percent false increase in measured nitrate concentration at an ETOC concentration of 45 mg/L. Nitrate 

concentration was falsely elevated at the maximum ETOC concentration by 49 percent for the NITRATAX 5-mm sensor, and 

75 percent for the NITRATAX 2-mm sensor. The S::CAN’s measured nitrate concentration artificially doubled from 5 mg-N/L to 10 

mg-N/L by the end of the test (that is, at the 45-mg/L ETOC concentration). Results of this evaluation indicate that although 

suspended sediment and CDOM interfere with all of the UV spectrophotometric nitrate sensors, the sensors are more effective 

compensating for suspended sediment and less effective compensating for CDOM. These results suggest that the end user of 

these sensors should be aware of the potential skewing of the data due to these interferences, especially if the site is one that has 

highly colored (chromophoric) water.

Acknowledgments

The author thanks Jacob Gibs (retired) of the U.S. Geological Survey (USGS) New Jersey Water Science Center; Paul 

Terrio of the USGS Illinois Water Science Center; and Janice Fulford of the USGS Hydrologic Instrumentation Facility (HIF) 

for providing valuable technical information throughout the study. Finally, many thanks go to the staff of the HIF Water Quality 

Servicing Laboratory for their help and support in this project. 

References Cited

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

USGS Publishing Service Center staff:

Marilyn A. Billone, Editor 

James E. Banton, layout
Snazelle—The Effect of Suspended Sediment and Color on Ultraviolet Spectrophotometric Nitrate Sensors—Open-File Report 2016–1014

ISSN 2331-1258 (online)

http://dx.doi.org/10.3133/ofr20161014