A Low-cost, User-friendly, Smartphone-based Measurement Technique for Dissolved Nutrients J. Feng, B. Khakipoor, K. E. Siman, A. W. Smith, H. T. King The University of Akron Akron, OH, U.S.A. April 2018

Outline Need for Accessible Water-quality Measurement Techniques Design of an Accessible Water-quality Measurement System Prototype of a Low-cost and User-friendly Spectrometer Standard Operation Procedure and Calibration Results Directions for Future Modification and Development

California Drought Global Fresh-water Shortage Source: UNESCO World Water Assessment Programme California Drought Source: USGS "Earth's water distribution" Fresh water is limited: 2.5% of water on earth is fresh water, 1.2 % of which is surface water Demand for water is rapidly growing Water shortage problem manifests in various ways all around the world

The Need for Water-quality Data USD $ 300k + Source: Woods Hole Oceanographic Institution Regional water issue: Lake Erie HABs Lack of water quality data (e.g., in the great lake area) Current technology are expensive and inaccessible to ordinary citizens Public concern towards environment provides opportunity for innovations USD $ 3,000 +

Accessible by Design: the Open-source Way SpecPhone e.g., d = 1.0 μm, m = 1 color λ θ Blue 445 nm 26.4o Green 532 nm 32.1o Red 650 nm 40.5o Grating Equation: d∙sinθm = m∙λ (m = 0, ±1, ±2 …)

A Low-cost Spectrometer small strip of diffraction grating light-blocking cover battery pack solder-less connection 3D-printed sample holder mirror current-limiting resistor installed inside battery pack bolt and spacer Laser-cut Acrylic enclosure

A Low-cost Spectrometer Designed to be easy-to-make and easy-to-use Custom parts are made with 3D printer and laser cutter

Standard Operation Procedure Add 2mL of nitrate sample to test tube Add 4 drops of reagent_1 Shake rigorously for 60 s Add 4 drops of reagent_2 Shake rigorously for 60 s spectra wait 10 m put cuvette into spectrometer water sample

Commercially-available Test Kits Nitrates Phosphates Nitrites Ammonia pH High-range pH

Calibration for Dissolved Nitrate Color difference between solutions with difference nitrate concentrations is detectable by eye Mixing of red and green is perceived as yellow / orange Color change from light yellow to dark red corresponds to decrease of green It is observed that concentration of nitrate is correlated with absorption of green light 0.0 ppm 1.0 ppm 2.0 ppm 5.0 ppm 10.0 ppm water 1.0 ppm 2.0 ppm 5.0 ppm 10.0 ppm

Misalignment of Diffraction Grating Data Extraction from Smartphone Photos sample reference reference spectrum sample spectrum diffraction grating slit spectrum Misalignment of Diffraction Grating Edge of spectrum (along width) is defined by slit Color bands in spectrum always align with grating Spectrum images may be rotated to correct for grating misalignment

Quantification of Spectra in RGB Channels Pixel Intensity Averaged across Width of each Spectrum Pixel Value Pixel Number sample spectrum reference spectrum Random noise can be filtered out by averaging Signal processing in each RGB channel should be performed separately

Correction for Exposure and White Balance 1.0 ppm 2.0 ppm 5.0 ppm 10.0 ppm

Correlation between Absorbance and Concentration 535 ± 5 nm A: measured absorbance L: path length c: analyte concentration −log10(I / I0) = A = ε∙L∙c Beer-Lambert Law: Sample spectrum are rescaled the same way that allows reference spectrum to be aligned Light intensity are calculated as the integrals under the normalized intensity curves A narrow band of 10 nm (in wavelength) is used to account for smearing of spectrum Beer-Lambert law is observed from test data log10IG = a∙c + b a = −ε∙L b = log10IG,0

Correlation between Absorbance and Concentration 535 ± 5 nm

Correlation between Absorbance and Concentration 500 nm – 570 nm Bands of 10 nm (in wavelength) from 500 nm to 570 nm is checked for log-linear relationship: 505 ± 5 nm = 500 nm – 510 nm 515 ± 5 nm = 510 nm – 520 nm 525 ± 5 nm = 520 nm – 530 nm 535 ± 5 nm = 530 nm – 540 nm 545 ± 5 nm = 540 nm – 550 nm 555 ± 5 nm = 550 nm – 560 nm 565 ± 5 nm = 560 nm – 570 nm

Phosphate Tests All three channels change intensity

Phosphate Tests R-channel Result

Future Modifications Mobile App & Website Light Source: replace LED light plate with stronger LED to improve signal-to-noise ratio Mobile App & Website Cover: a snap-on cover will be implemented for pedagogical purpose Recyclability: replace Acrylic with plywood as housing material to allow for recycling Modularity: modular design to allow for easy repair/replace and upgrade Extended Functionalities: measuring heavy metals and chlorophyll; soil measurement

Grating, light source and battery Error Estimation Grating, light source and battery Mixing and operation User and phone manufacturing