IEEE SENSORS JOURNAL, VOL. 18, NO. 4, FEBRUARY 15, 2018 1723 An On-Line Fourier Transform Infrared Spectrometer Used for Biological Process Measurement Sha Huang, Bangsheng Yin, Libo Zeng, and Qiongshui Wu Abstract—In order to real-time online monitoring the reaction substrates for biological processes, we designed an on-line Fourier transform infrared spectrometer measurement system, which has the advantages of low cost, simple and compact structure, good reliability, and strong shock resistance compared with the traditional Fourier transform infrared spectrometer. This system consists of two core components, i.e., a high-performance interferometer and an attenuated total reflectance (ATR) probe. In the interferometer, we 1) combined corner cube mirrors and flat mirrors to avoid tilting of the moving mirror; 2) set 60° incident angle to the ZnSe beam splitter to improve the luminous flux; 3) shared folded optical path with both the infrared light and the reference laser to improve compactness in structure; and 4) drive the moving mirror by a parallelogram swing flexible support module to make it move smoothly. In the ATR probe, we utilized two large diameter and inner coating light pipes to transmit the incident and outgoing light to enhance the luminous flux further. We also simulated the optical system and did some off-line and online measurement experiments. The results show the spectrometer has satisfactory reliability, large luminous flux, and online measurement capability. It can be real- time online monitoring the biological process through measuring the concentration of the related bioreactor reactant. The online analysis has wide application prospects in biological, chemical testing, material analysis, and many other fields. Index Terms—ATR probe, biological process, Fourier trans- form infrared spectrometer, online monitoring. I. I NTRODUCTION T HE measurement of substrate in microbial fermenta- tion process is of great significance for the parameter optimization of biological process. To obtain the related para- meters of the biological process, indirect measurement method and off-line measurement method are often used in scientific researches and industrial production. However, the off-line measurement can’t meet our demand of real-time measurement and requires the bioreactor tank to be opened, which may Manuscript received October 31, 2017; revised December 10, 2017; accepted December 13, 2017. Date of publication December 21, 2017; date of current version January 18, 2018. This work was supported by the National Science and Technology Support Plan Projects of China under Grant 2011BAF02B02. The associate editor coordinating the review of this paper and approving it for publication was Prof. Aime Lay-Ekuakille. (Sha Huang and Bangsheng Yin contributed equally to this work.) (Corresponding author: Qiongshui Wu.) S. Huang, L. Zeng, and Q. Wu are with the College of Elec- tronic Information, Wuhan University, Wuhan 430072, China (e-mail: 2016202120056@whu.edu.cn; lbzeng@whu.edu.cn; qswu@whu.edu.cn). B. Yin is with the Atmospheric Sciences Research Center, State University of New York at Albany, Albany, NY 12203 USA (e-mail: byin@albany.edu). Digital Object Identifier 10.1109/JSEN.2017.2785806 cause some contamination of the substrate. Indirect measure- ment method is often costly and with unsatisfactory accuracy. Moreover, the demand of on-line measurement of the substrate concentration for biological process is growing. There is an urgent need to develop a kind of on-line measurement system which can detect the substrate concentration fast and accurately. In recent years, the off-line measurement techniques for the substrate concentration have been advanced in sev- eral directions, such as spectroscopy, mass spectrometry, chromatography, etc. At the same time, the related on-line measurement techniques are also in ceaseless development. Currently, the best detection methodology is the mid-infrared spectroscopy [1]. At present, the application of infrared spec- troscopy technology has been maturely developed and widely used by many manufactures, including the Nicolet, Brook, Mettler, etc. However, most of these manufacturers’ detection equipment only works for off-line detection and the devices that can be applied to on-line detection of biological process are very limited and expensive. To address this issue, we devel- oped a low-cost on-line Fourier transform infrared (FTIR) spectrometer that can be applied to measure the biological process. With this on-line FTIR spectrometer, quantitative biological component information of nutrient solution in bio- logical process can be obtained real-timely, non-destructively and continuously. II. DEVELOPMENT OF THE FOURIER TRANSFORM I NFRARED SPECTROMETER The overall structural design of FTIR spectrometer is shown in Fig.1. It consists of two light sources (i.e., infrared light source and laser light source), a paraboloidal mirror, a Michel- son interferometer, an Attenuated Total Reflectance (ATR) probe, and some auxiliary subsystems. In this design, the core components are the Michael interferometer and ATR probe [2]. Overall, the infrared light becomes a parallel beam after reflected by the paraboloidal mirror, and then enters the interferometer, and finally rips into the ATR probe with the formation of the interference signal. The infrared light from ATR probe enters into an HgCdTe (MCT) detector, which turns a light signal into an electrical signal. Through a series of subsequent processes, such as signal amplification and de-noise filtering, the electrical signal is recorded by the 1558-1748 © 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.