# Miniaturized Near-Infrared Analyzer for Quantitative Detection of Trace Water in Ethylene Glycol
## 1 Introduction  
Ethylene glycol is a critical, chemical, raw material and a strategic resource for various industries. It is utilized in the production of polyesters, explosives, ethylene glycol aldehyde, as well as antifreeze, plasticizers, hydraulic fluids, and solvents [1]. Controlling water content during ethylene glycol production is essential for ensuring the stability and efficiency of the process. The water content serves as a vital indicator for assessing the quality grade of polyester-grade ethylene glycol. According to relevant standards, first-grade ethylene glycol must contain less than 0.1% (w) water [2], which is thus pivotal for quality control. Regularly measuring the water content of ethylene glycol samples during production enables manufacturers to promptly identify and address issues, optimize production processes, and enhance both product quality and output. Currently, the standard method for measuring water content in industrial liquids is Karl Fischer (K-F) titration. While effective, this traditional technique is relatively complicated and time-consuming [3, 4]. Water molecules demonstrate significant absorption characteristics in the near-infrared (NIR) region, primarily due to O-H stretching and combination vibrations [5, 6, 7]. This property makes NIR spectroscopy a powerful tool for quantitative moisture analysis. TheNIR wavelength range, extending from 780 to 2500 nm, captures overtone and combination vibration absorption information from hydrogen-containing chemical groups such as C-H, O-H, and N-H, encompassing almost all organic compounds and their mixtures [8; 9; 10; 11; 12]. When materials are irradiated with NIR light at wavelengths sensitive to water, different moisture levels produce varying absorption intensities. Consequently, measuring light intensity allows for the indirect determination of moisture content. The strong absorption of O-H groups in the NIR region renders NIR spectroscopy particularly effective for determining trace water content. By analyzing the reflected or transmitted light intensity after NIR irradiation, moisture content can be assessed effectively [13; 14; 15]. Numerous studies highlight the versatility of NIR spectroscopy in moisture analysis. For instance, [PERSON] et al. employed portable NIR spectroscopy combined with machine learning to predict water adulteration in milk [16]. [PERSON] et al. used NIR spectroscopy with nonlinear methods to rapidly and robustly characterize moisture effects in raw organic waste [17]. [PERSON] et al. applied simple mathematical models in conjunction with NIR spectroscopy to calculate moisture content in cellulose [18]. Furthermore, [PERSON] et al. implemented a data fusion strategy that combined micro-NIR spectroscopy and machine vision for the rapid prediction of moisture content during black tea drying [19]. In summary, NIR spectroscopy offers high precision, quick response times, no-contact online measurement, and poses no radiation risk to humans. These advantages make it widely applicable across various production fields, revolutionizing moisture analysis [20; 21]. The above conventional near-infrared water analyzers are typically large and rely on a light path structure that incorporates a halogen lamp and diffraction grating, often with motorized components [22]. This design limits their portability and confines their use to laboratory environments. In contrast, Fourier-transform near-infrared spectrometers (FT-NIR) based on MEMS technology offer significant advantages [23; 24]. For example, a product launched by the Hamamatsu Corporation measures just 8.7 cm \(\cdot\) 4.9 cm and achieves a spectral resolution (FWHM) of 5.7 nm. Although this resolution is slightly lower than that of traditional instruments, it is adequate for routine testing. Successful applications of portable FT-NIR spectrometers include online monitoring of water content and mass transfer rates in fluidized beds [25], as well as water detection in biodiesel using portable infrared spectrometers (FTIR) [26].