Sensors & transducers Optical liquid analysis
prototyping platform lights the way to ubiquitous sensing Scott Hunt, systems applications engineer, Analog Devices
By Sydney Wells, applications engineer, and M
onitoring the environment in real time is critical to improving global sustainability. Having the ability to
quickly analyse a sample and identify a problem is key to a fast resolution with minimal impact to the ecosystem. This drive toward ubiquitous, real-time sensing has shifted requirements for liquid sensors to require smaller size, higher robustness, and lower power while still delivering high quality results. As the industry advances, intelligent platforms for on-the-go sensing are needed. These platforms need to be highly versatile, capable of satisfying unique requirements for a wide range of applications from environmental waters to process control. This article will introduce a portable, real- time sensing solution and prototyping platform for rapid liquid sensing.
A Common LiquiD AnALySiS TeCHnique Various methods exist to test liquids, the purpose being to measure the concentration of an unknown parameter in a sample, such as pH, fluorescence, or turbidity. A popular method is to evaluate liquids optically, as it is noninvasive and provides stable and accurate results. Precision optical liquid measurements require mixed domain knowledge in electronics, optics, and chemistry. In simple terms, the analysis begins with a sample that is exposed to light from a source such as an LED. After interacting with the sample, the resulting light is processed by a photodiode. That
measured response is plotted against the measured responses of a set of standard samples of known concentrations. This is known as a calibration curve. Using the calibration curve, the unknown value can be determined. This describes the general laboratory method for analytical measurements, but to meet the needs of ubiquitous sensing it must be scaled to different analytes and to measurement techniques, as well as fit into a small form factor, all of which increases the complexity of design and evaluation.
moDuLAr ADi SoLuTion for rApiD LiquiD meASuremenT The ADPD4101 from Analog Devices is an optical analogue front end (AFE) capable of driving LEDs and synchronously receiving and processing signals from photodiodes to make high precision optical measurements. The ADPD4101 is highly configurable, featuring a high optical signal-to-noise ratio of up to 100 dB and high ambient light rejection provided by on-chip synchronous detection methods, allowing it to be used without an optically dark enclosure in many cases. The CN0503 reference design was created
to enable rapid prototyping of liquid analysis measurements with the ADPD4101. The CN0503 features the ADPD4101 as its core product, but adds up to four modular optical paths as well as measurement firmware and application software targeted for liquid analysis. The CN0503 interfaces directly with the ADICUP3029 board, which manages the measurement routine and data flow. The ADICUP3029 board can be connected directly to a laptop to view results in the evaluation GUI. The CN0503 can measure fluorescence, turbidity, absorbance, and colorimetry. A sample is prepared in a cuvette and placed in the 3D-printed cuvette holder, which houses the optics, including a lens and beam splitter. The cuvette holder slots into the appropriate optical path for plug-and-play measurement. In addition, the LED and photodiode cards can be switched out for even more customisation. To demonstrate creating calibration curves
Figure 1. An example of a calibration curve for absorbance.
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and measuring unknowns with the CN0503, measurements of pH, turbidity, and
Figure 2. The CN0503 evaluation board.
fluorescence will be shown. The evaluation GUI was used to take measurements to create calibration curves. The noise value and limit of detection (LOD) were calculated to determine the lowest concentration feasible to be detected by the CN0503 for each example.
meASuring pH WiTH ABSorBAnCe Absorbance Background Absorbance involves determining the concentration of a known solute in a solution based on how much light is absorbed at a particular wavelength. The concentration is proportional to the absorbance, per the Beer- Lambert Law. Many colourless analytes can be measured by adding a colour-changing reagent. This example is measuring pH, one of the most common parameters measured across many industries from water quality to wastewater treatment. Absorbance measurements are used
Figure 3. An optical light path for measuring absorbance.
February 2022 Instrumentation Monthly
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