A Handheld Colorimeter for Photometric Detection

Introduction

As we all know, water is an important natural resource for the survival and development of human beings. Therefore, water quality monitoring is attracting increasing attention. Currently, the monitoring instruments are bulky, expensive, and time-consuming. A handheld colorimeter can improve the flexibility and automation of water quality monitoring. In order to test the performance of the handheld color meter, the standard solution with different phosphorus concentrations was prepared, while ascorbic acid was used as the reducing agent and ammonium molybdate solution was used as the chromogenic reagent. Then phosphorus was reduced by ascorbic acid, and a blue complex was formed by the color reaction of ammonium molybdate solution, which absorbed the maximum intensity at a wavelength of 880 nm. Assays were implemented by employing a digital source meter as a data acquisition system; software was developed as the visual display of data. A solenoid mini-pump was used for fluid propelling. In this work, the effect of residual measurement was considered to extend the linear response range, which is superior to other methods in phosphorus detection. The effect of interference is determined in order to demonstrate the selectivity of our device. Compared with commercial colorimeters, the volume of our device can be miniaturized because of microfluidic technology. Furthermore, the low cost, less consumption of samples, wider range of applicability, and high precision may extend this device for fieldwork water monitoring.

Materials and Methods

Theoretical Principle

The basic principle of our sensor is the Beer-Lambert law, which is a traditional attenuation theory for light transfer and can be applied to detect the ion concentrations in liquid. It exhibits a linear relationship between the analysis and light attenuation. The Beer-Lambert law is given by: A= -log10 It/I0=log10(1/T)=klc

where A is the absorbance of the sample solution, I0 is the initial light intensity before passing through solution, It is the intensity of transmitted light, T is the transmittance, k is the attenuation coefficient, l is the optical path length crossing over the sample solution and c is the concentration of the substance to be measured.

In our design, we applied MEMS (Micro-Electro-Mechanical System) technology to fabricate the device. The MEMS technology enabled the reduction of the length of l, which is beneficial to the volume reduction of the sensor. To ensure high precision in the preparation process, a laser source with a specific wavelength was adopted to enhance the value of k.

The designed structure is shown in the below picture:

MEMS designed structure

Results and Discussion

The application of a caliber 4mm handheld color meter provides a method for further research in miniaturization and integration. The compact size allows the device to be used in the portable measurement. The feature of miniaturized flow cells makes it possible to use a low reagent consumption and to produce less waste liquid. The detection limit for phosphorus in water is 0.02 mg/L, with a high resolution of 0.01 mg/L in a large concentration range from 0.05 mg/L to 7.60 mg/L. Compared with other equipment in photometric detection, the effect of residual measurement is considered to extend the linear measurable range, which can be in a range of 0–10.16 mg/L for phosphorus detection, indicating it can be applied in more cases. The advantage of low consumption and small waste liquid makes it easier to be used in situ detection. The proposed method can also be used to detect other elements, just by changing the light source to the corresponding absorption wavelength. All the above features make it a novel and feasible method for photometric detection in practical applications.

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