Coloured, or chromophoric dissolved organic matter (CDOM), is a naturally occurring dissolved matter that absorbs UV light in water. It’s usually made up of tannins that are released from the breakdown of plant material.
A fraction of CDOM fluoresces when it absorbs light of a certain spectrum, and is called fluorescent dissolved organic matter, or FDOM.
For accurate CDOM/FDOM monitoring, try using our CDOM sensor with the Aquaprobe. This sensor is fixed response fluorometer, which provides excitation at 365nm (UV) and detects any resultant fluorescence between 450nm and 520nm.
Our CDOM sensors can be installed into many of our probes – below are some examples. To see all of the probes that can house the CDOM sensor please visit the Products section.
The Aquasonde-5000 has 4 spare ports. They can house any combination of ISE and Optical sensors.
Installing the CDOM sensor into an Aquaprobe is a very simple process. Simply unscrew the blanking plug from an appropriate aux socket, apply some silicon grease to the thread of the sensor (grease provided), and screw in the sensor. Full calibration is required after installation.
A core aim at Aquaread is to make our products easy to use, which is why we have simplified and automated the calibration process as much as possible. The CDOM sensor is calibrated at 2 points. A zero point and a 100µg/l point using Aquaread’s CDOM-CAL solution.
When taking measurements at a particular site there can occasionally be a difference between measured values with the sensor and laboratory data. These differences are caused by a variety of factors.
You can apply a multiplier to the data obtained from our sensor in order to make it more closely correlate with lab data for that particular site. To do this we use a grab sample factor. This factor can be calculated by dividing the average grab sample value (measured in the lab) by the average value measured by our CDOM sensor.
This is a great addition to the software when trying to correlate new data with historic readings from the same site.
Determination of CDOM in the field using fluorescence measurement techniques will never be as accurate as measurements made in a lab using traditional techniques.
Factors adversely affecting accuracy include:
Fluorescence measurement techniques are ideal for researchers who are interested in detecting the presence or absence of a specific substance and measuring relative fluorescence changes that can be used as an indication of increasing or decreasing concentrations.
Fluorescence measurement techniques are not ideal for quantitative measurement. In order to obtain more accurate results, data obtained with the fluorometer in the field should be post-calibrated with data from standard laboratory analysis of grab samples acquired during the study.
If grab sample data is available, a Grab Sample Factor (GS Factor) should be calculated and input on the calibration screen in order to improve the accuracy of future readings.
CDOM/FDOM is used to measure the relative amount of dissolved organic material (DOM) in the water. Although it is naturally occurring, human influence through aspects such as logging, agriculture, effluent discharge, and wetland drainage can affect the levels of CDOM in freshwater and estuarine systems.
A CDOM sensor works by sending a light beam into the water to be tested, at a fixed wavelength. This wavelength of light excites the matter which subsequently fluoresces at a known wavelength band. The level of excitation measured is proportional to the amount of CDOM present in the water.
To use Aquaread’s CDOM sensor, first install it into your probe of choice, then perform a full calibration using CDOM-CAL solution from Aquaread. Once calibrated it’s good to go. Simply put the probe into the body of water, allow the reading to stabilise, and take your measurement.
If you record a difference in chlorophyll levels vs a lab test at the same site, you can use the lab data to apply an offset, fine tuning the probe to detect correctly at that particular site.
It is vital to measure the levels of CDOM/FDOM and understand their trends because they can have a significant effect on aquatic ecosystems. Raised levels of CDOM/FDOM can inhibit the growth of phytoplankton and limit photosynthesis, damaging the food chain and limiting the production of oxygen in water bodies.