Microchemical Journal 159 (2020) 105555 Available online 21 September 2020 0026-265X/© 2020 Published by Elsevier B.V. Environmentally friendly liquid medium for a cost-effective long-path absorption liquid core waveguide with a gas diffusion fow analysis system Wasin Somboot a, d , Jaroon Jakmunee a, b, c , Tinakorn Kanyanee a, b, c, * a Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand b Center of Excellence for Innovation in Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand c Research Center on Chemistry for Development of Health Promoting Products from Northern Resources, Chiang Mai University, Chiang Mai 50200, Thailand d The Graduate School, Chiang Mai University, Chiang Mai 50200, Thailand A R T I C L E INFO Keywords: Liquid core waveguide Long-path absorption spectrophotometry Gas diffusion fow injection Green chemical analysis Ammonium determination Environmentally friendly ABSTRACT An environmentally friendly modifed fow analysis carrier with a high refractive index liquid to perform a total internal refrection phenomenon in a typical Tefon tube was achieved as a cost-effective long-path liquid core waveguide (LCW) for absorption spectrophotometry. A home-built LED-photodiode based long-path colorimeter was demonstrated to enhance the sensitivity of chemical analysis without using a preconcentration unit. A low toxicity carrier such as syrup or ethanol was selected to improve the sensitivity for NH 4 + ion determination via butterfy pea fower extract reagent in a gas-diffusion fow injection (GD-FI) system. A linear calibration range of NH 4 + ion determination in the GD-FI based on syrup modifed LCW (mLCW) system of 10–500 µmol L 1 can be obtained with a detection limit of 9.8 µmol L 1 , repeatability of <2% RSD, and 12 injections h 1 sample throughput. The proposed cost-effective and environmentally friendly mLCW shows the potential to apply with other natural/synthetic chromogenic reagents for the green chemical analysis approach. 1. Introduction The world of modern and green analytical chemistry is looking for less toxicity and locally available reagent for chemical analysis [1]. The alternative materials with lower toxic solvent or reagent medium should be sought out for the new method. The use of natural reagents to replace conventional synthetic chemicals has been mentioned as an alternative way for green analytical chemistry [2–5]. However, the high limit of detection would be a drawback of using some natural reagents [3]. A liquid core waveguide (LCW) is one of the interesting optical tools to enhance the limit of detection (LOD) for various spectrophotometric techniques, including UV–Vis absorption, fuorescence, chem- iluminescence, and Raman measurement. A typical LCW employed a commercially available low refractive index (RI) Tefon AF [6]. Since the RI of the Tefon AF tubing (1.29) is lower than that of a core liquid such as water (1.33), the total internal refection (TIR) in the liquid can be performed, and the light is guided through the core liquid towards the end of tube. As a result, the long-path optical measurement can be achieved, and the LCW has been widely used for the sensitivity enhancement of an absorption spectrophotometric detection. The Tefon AF coating on fused silica capillary was applied in many analytical felds of environmental, biochemical, and food chemistries [7]. The fow analysis with an LCW detection approach was mentioned as one of the green analytical methodologies in terms of green signal acquisition and automation systems [8]. However, the commercially available low RI material such as Tefon AF is very expensive. The LCW approach can be performed previously without using the expensive Tefon AF by applying the higher RI liquid medium, instead of using the lower RI cladding tube with ordinary aqueous solution [9]. Most of the high RI liquid for modifying the core medium for LCW is the highly toxic solvent such as tetrachloroethylene, carbon tetrachloride [10–11], carbon disulfde for determination of phosphorus [12], iodine [13], and Cu 2+ [14] in water samples, and NaOH for acetone measurement in exhaled breath [15]. Obviously, many natural chromogenic reagents can be well extracted in ethanol, which was reported as a high RI but low toxicity organic solvent. Again, the syrup, which can be found in a kitchen, would promote the TIR phenomenon for a cost-effective LCW. Ammonium is a key parameter for investigating the nitrogen cycle and indicating water quality [16]. The ammonium determination pro- vides an essential information for the environmental scientist. Various analytical techniques, including spectrophotometry, fuorometry, and * Corresponding author at: Department of Chemistry, Faculty of Science, Chiang Mai University, Huay Keaw Rd., Chiang Mai, Thailand. E-mail address: tinakorn.kanyanee@cmu.ac.th (T. Kanyanee). Contents lists available at ScienceDirect Microchemical Journal journal homepage: www.elsevier.com/locate/microc https://doi.org/10.1016/j.microc.2020.105555 Received 15 August 2020; Received in revised form 16 September 2020; Accepted 16 September 2020