The two most common units of measurement for FDOM are Relative Fluorescence Units (RFU) and Quinine Sulfate Units (QSU).

QSU is the common standardized unit, where 1 QSU equals the fluorescence of 1 µg/L of quinine sulfate in 0.05 Molar sulfuric acid. To use QSU, the sensing instrument must be calibrated against QS reference standards.

Several challenges are associated with these standards:

• High-purity quinine sulfate dihydrate is required.
• Precise acid concentration is essential.
• Standards are temperature sensitive.
• Standards degrade with light, so must be stored in the dark.
• Shelf life is limited.
• Standards are easily contaminated.
• Disposal must be handled carefully.

Because of these requirements, and to improve reliability and traceability, most people prefer to purchase commercial QS standards. Standards can be costly and degradation and contamination after opening limits standard shelf life. This leads costs of standards to accumulate quickly. Safe handling and disposal pose an additional challenge for field-based applications.

Relative Fluorescent Units (RFUs) are generally viewed as the more convenient unit of measurement. An RFU measurement is determined relative to an instrument’s range of measurement, usually represented by a scale from 0 to 100, where 0 indicates no fluorescence and 100 represents maximum fluorescence.

The term ‘Relative’ is key to understanding that this measurement is a qualitative indicator of concentration. RFUs will give relative changes in concentration of the fluorescent substance but will not provide an exact concentration. To determine exact concentration, one must perform a site-specific calibration which involves simultaneously comparing water samples for laboratory analysis along with FDOM values. Determining the relationship between FDOM RFUs and laboratory-measured DOC will require correction for environmental factors such as temperature and turbidity.

The range of RFUs can be correlated to a secondary standard–including QS standards–to understand a sensor’s scale of measurement. If one wants to avoid the hassles of QS standards, solid state standards are an excellent option for understanding and calibrating the RFU scale. For more on solid state standards and calibrators, see “How do In-Situ sensors measure FDOM?”

Both QSUs and RFUs enable highly sensitive measurements because monitoring technologies rely on fluorescence, which is a more sensitive tool than light absorbance. For more detail on the process of fluorescence and how it provides this sensitivity, see “What methods are used to measure FDOM?”

QSUs offer one main advantage over RFUs as a standardized unit: they enable the comparison of FDOM readings from different instruments. Measurements in RFUs are comparable between instruments of the same type and manufacturer because they will have the same RFU scale relative to a standard (solid state or QSU). However, reference scales for RFUs will not be the same between manufacturers. So, comparing RFUs between different manufacturers’ instruments is not recommended. If such comparisons are necessary, RFUs can be correlated and normalized against QSUs as a standardized unit.

Taking field measurements always comes with risks of interferences (see “What are common challenges when monitoring FDOM?”). Whether using QSU or RFU, absolute measurements are difficult to capture. Therefore, FDOM monitoring is best approached as an assessment of trends and changes in water quality, where users can establish their own thresholds for how much change might lead to follow-up measurements or actions.

Monitoring FDOM helps water managers understand the dynamics of organic matter transfer in a system and make informed decisions if action is required. DOM in natural waters comes from both natural processes and human sources.

The composition and concentration of DOM—and therefore FDOM—resulting from natural sources can vary based on

• Landscape and vegetation

• Seasonal patterns

• Land use

• Biological activity in and around the water body

The size, hydrology, and structure of the water body itself also influence how organic matter accumulates or moves throughout the system.

Growing populations have increased human influence on the levels and types of organic matter in water bodies. This trend underscores the importance of monitoring FDOM to understand potential impacts on water quality and ecosystem health.

FDOM may be measured in the field or in a laboratory using fluorometers. Fluorometers are instruments designed to detect fluorescent molecules. These molecules absorb light, which “excites” electrons. The energy those electrons hold must either be passed on to another molecule or released as heat or light. Fluorometers detect the energy emitted as light.

The emitted light exists within a narrow range of wavelengths. For each fluorescent molecule, only a specific range of wavelengths can be absorbed; this is called the “excitation light.” The excitation light is a shorter wavelength and higher energy than the emission light, which will have longer wavelengths and lower energy. If you’re interested in understanding this better, you can learn more about the physics of fluorescence and the electromagnetic spectrum here.

These excitation and emission wavelengths facilitate detection of humic-like substances, which are usually the molecules of interest when monitoring FDOM in the environment. For more on the relevance of humics to FDOM monitoring, see "Why measure FDOM?”

FDOM sensors are specialized fluorometers—optical instruments that measure fluorescence in substances, specifically dissolved organic matter in water. Many molecules can absorb light from a specific spectrum and then emit light at a different wavelength, a process known as fluorescence.

These fluorescent molecules are detected using a carefully designed system: an LED emitter generates light at a specific wavelength, and filters isolate the desired excitation spectrum. When this light reaches fluorescent molecules in a water sample, they absorb the energy and emit light at a longer wavelength (lower energy). This emitted light is known as the fluorescence signature.

The sensor’s detector, equipped with its own set of filters, captures this emitted light and measures its intensity. Because fluorescence intensity is proportional to the concentration of fluorescent molecules, the sensor can estimate the concentration of target substances in the water. As concentration increases, more molecules absorb the excitation light, typically resulting in stronger fluorescence.

Measurements are typically expressed in Relative Fluorescence Units (RFUs). RFUs indicate relative changes in concentration of the fluorescent substance, but they do not provide an exact concentration without calibration.

FDOM specifically refers to the subset of dissolved organic matter that fluoresces when excited by light in the 360–390 nm range. The detector in an FDOM sensor captures the emissions in the 460–530 nm range, providing valuable data about organic content in water systems.

In-Situ fluorometers also include a reference detector for Integrated Optical Compensation. This reference detector measures changes in LED brightness and automatically adjusts each reading to compensate for variations caused by temperature or age of the sensor.

The most notorious challenge when monitoring FDOM in the field is fouling. When buildup blocks the sensor window, the sensor cannot transmit or receive light. Because light delivery and detection are essential to the sensor's functionality, any obstruction will interfere with data accuracy. To ensure reliable FDOM measurements, regular maintenance coupled with passive and mechanical antifouling tools is essential.

For all fluorometers, there is an inverse relationship between fluorescence and temperature. This presents the greatest challenge at very low ranges of sensor detection. At these ranges, a change from 0.1 to 0.2 RFU, for instance, may look like an increase in FDOM, when it may actually represent a decrease in temperature while measuring in deeper waters or a temperature drop due to seasonal change. On-board temperature sensors can help with understanding the influence of temperature on FDOM measurements and enable temperature corrections. The importance and level of temperature’s effect on fluorescence is project specific and any corrections should be developed empirically on a project-by-project basis.

All optical sensors experience interference from turbidity. Storm events, high sediment loads and different types of water bodies can all increase turbidity and its effects on FDOM monitoring. High turbidity increases the chances that the wavelengths of light will not have a clear path to and from the sensor. Particulates in water can block the light that reaches fluorescent molecules and light emitted by the fluorescent molecules can similarly be blocked from reaching the sensor. Additionally, suspended particles provide surfaces where DOM can adsorb. This removes fluorescent compounds from the dissolved fraction and can change the fluorescent characteristics of the target molecules. Thus, the relationship between turbidity and FDOM is complex. It is not always linear and can be influenced by different particle types.

In all cases, however, turbidity is most likely to lead to an underestimation of FDOM. Best practice is to always measure turbidity alongside FDOM. At the very least, this enables an awareness of possible turbidity interference. In some cases, turbidity measurements can help develop a mathematical correction for turbidity interference. The threshold at which turbidity interference occurs, in either nephelometric turbidity units (NTUs) or formazin nephelometric units (FNU), depends on both the monitoring site and project objectives. Any turbidity corrections must be developed empirically from site-specific measurements.

The inner filter effect is a type of interference gaining interest in environmental monitoring. “Inner” refers to a light-filtering effect created by compounds contained within a sample. For example, if there is a very high concentration of DOM, molecules may absorb some of the excitation light, reducing the amount of light that reaches molecules deeper in the sample. Emitted fluorescence might also be absorbed by other fluorescent molecules in the water.

Both of these processes lead to a non-linear response and an underestimation of FDOM. The effect seems most severe in highly colored samples; having both a CDOM and an FDOM sensor together may be useful for these environments.

Another response to this challenge is to collect samples and dilute them, to see if there is a linear response with FDOM and concentration of the sample. As with temperature and turbidity, the objectives of the monitoring program must drive the decision of whether and how to address these interferences.

Like all components, LED brightness in optical sensors will decrease over time with extended use. In-Situ FDOM sensors include a primary detector and secondary reference detector to measure LED drift and automatically compensate each reading. This feature enhances stability at very low detection limits and significantly reduces sensor drift caused by changes in temperature or time.

In most water quality platforms, installing more than one optical sensor causes interference among sensors. Each In-Situ optical sensor runs at a unique frequency that creates a digital signature for each light source. Detectors for FDOM sensors use this signature to isolate FDOM wavelengths from other light sources and measure only the targeted parameter. This allows multiple optical sensors to run simultaneously in one instrument.

Ambient light can interfere with optical measurements when sensors cannot distinguish between internal and external light sources. In-Situ FDOM sensors separate light from the sensor from other ambient light sources, enabling the same degree of accuracy regardless of lighting conditions or deployment location.

Interferences between sensors and inaccuracies in algae monitoring data are more likely to occur when fluorometers excite a wider range of wavelengths. In-Situ fluorometers include one parameter per sensor. Each sensor excites a small range of the light spectrum to increase accuracy while minimizing interference from other fluorescence sources.