Assessment of ability to detect low concentration analyte with near-infrared spectroscopy based on pre-concentration technique

• An equation was obtained to evaluate the detection ability depending on Langmuir model.
• PLS models were built for three batch solutions to validate the ability of DR-NIR.
• Carbaryl as low as 0.8 mg/g adsorbed on the resin can be detected by DR-NIR.
• DR-NIR based on enrichment technique has potential ability for ppm-level analysis.

This paper aimed to assess the ability of detecting low concentration analyte in water by near-infrared diffuse reflectance spectroscopy (DR-NIRS) based on pre-concentration technique. HZ818 resin was employed as the adsorbent, on which aqueous carbaryl was adsorbed, and when adsorption equilibrium was achieved, concentration of carbaryl in solution was detected by HPLC for adsorption isotherm fitting, meanwhile DR-NIR spectrum of the adsorbed resin was measured for quantitative analysis by partial least squares (PLS) regression. Langmuir model was chosen for fitting the experimental data of adsorption isotherm with R2 of 0.9991 and root mean squared error (RMSE) of 0.3472 mg/g, which was used to develop an equation declaring the relationship between initial concentration of carbaryl in solution and the equilibrium adsorption mass. With the developed equation, the lowest detection concentration was estimated and the effect of solution volume and mass of resin on the lowest detection concentration were investigated also. Furthermore, ‘NIR spectrum-carbaryl concentration’ calibration models were built for a serial of solutions in different concentration ranges to validate the detection ability of NIR. The results showed that with an acceptable prediction error as low as about 0.8 mg/g carbaryl adsorbed on the resin can be detected by DR-NIR, so that the lowest detection concentration was exhibited by the developed equation to be 1.7 mg/L under fixed solution volume of 1 L and resin mass of 2 g. It revealed that it was possible to quantitatively detect analyte in solution at ppm-level by the combination of DR-NIRS and pre-concentration technique.