A mobile lab-on-a-chip device for on-site soil nutrient analysis

In this paper, a mobile sensor for on-site analysis of soil sample extracts is presented. As a versatile tool for scanning ion concentrations in liquid samples, it especially allows the analysis of NO3, NH4, K and PO4. The sensor mainly consists of a microfluidic chip in which the sample ions are separated in an electric field (capillary electrophoresis) and the individual ion concentrations are detected by a conductivity measurement. For the adaption of the device to field conditions, two major concerns were addressed. Firstly, nano-porous material was used as a barrier between the sample container and the analysis channel of the microfluidic chip. This prevents pressure driven leakage of the sample into the chip due to non-horizontal orientation of the device. Secondly, a new method for the injection of the sample into the chip was used. It reduces the number of fluidic connections between chip and operation device to three instead of the commonly used four connections. The sensor performance was tested on multi-ion solutions with calibration series for NO3, NH4, K and PO4. For the first on-site test, a quick soil nutrient extraction procedure with water was used. The sensor data was compared to standard laboratory results. The potential of the sensor for soil nutrient analysis is discussed together with required improvements of the sensor performance and of the nutrient extraction procedure.