Introduction Portable analyzers gain increasing interest for environmental monitoring, food safety or breath analysis. For instance, portable breath analyzers give patients the opportunity to regularly and conveniently monitor their own health at home, decreasing detection time and as a result improving survival rates [1]. For each tracer, many different sensor systems are proposed in the literature. However, too often these systems are only optimized in the lab, without thorough validation with real-world samples [2]. Especially the latter step is crucial to determine if the proposed system is fit-for-purpose.Here, we propose an inexpensive multi-use platform that can be combined with a wide variety of chemo-resistive detector systems (e.g. Al2O3 filters combined flexibly with a Pd:SnO2 sensor [3] or sensor arrays [4]) for their validation in the real application. Furthermore, this pocket-size analyzer can modularly be combined with different sampling adaptors, e.g. for on- and offline breath sampling, or to sample the headspace of liquids. As a proof-of-concept, we applied the developed analyzer to assess the methanol concentration in alcoholic beverages for food safety applications. Methods The analyzer is based on an in-house designed printed circuit board (PCB) featuring a commercial microcontroller, an analog-to-digital converter (ADC) and integrated circuits to heat several metal oxide gas sensors to elevated temperatures, read their film resistances and operate a miniaturized vane pump (Figure 1). The sensors are heated by pulse width modulation at a frequency of 100 kHz. The output of the microcontroller switches thereby between 0 and 3.3 V at a desired duty cycle. A low-pass filter is used to reduce the noise of the output voltage that is subsequently amplified by an operational amplifier using a gain of 1.7. Also, the voltage is automatically adjusted to maintain the desired operating temperature of the sensor. This is achieved by evaluating the resistance of the heating element using a voltage divider and the ADC. The same principle is used to evaluate film resistance. However, an additional unity-gain amplifier and capacitance were used to reduce to influence of the ADC’s internal resistance, enabling to reliably read resistances at >100 MOhm. Wireless communication to a smartphone enables the remote control of this multi-use platform and visualization of results. Results and Conclusions The resulting analyzer is rather compact (2 x 4 x 12 cm3) and weighs less than 100 g. Due to its low power consumption (in total < 1 W), it can easily be powered with a battery, enabling its use, e.g. for diet monitoring at home through breath acetone detection [5]. Depending on application, the platform can be equipped with a wide array of metal-oxide gas sensors and additional filter columns. Thereby, the operating temperature of the sensor can accurately be adjusted between 100 – 450 °C (± 3 °C), as required by the sensors. Also, film resistances can be determined reliably. For instance, such resistances between 1 and 100 MOhm were determined with a high accuracy (99.7%) and low noise (< 1.5%) when using a 10 MOhm reference resistance.As a result, we report a multi-use platform that can easily be utilized by laymen based on its smart-phone connectivity. Due to its modular design, it can be combined with different sensor systems, including selective metal oxide sensors, filter columns or even orthogonal sensor arrays for usage in monitoring of food safety or breath analysis [1], where reliable handheld devices are urgently needed.
Read full abstract
Oct 13, 2023
S C Dutta Roy 
Open Access