Analysis, design and use of a Fourier-transform spectrometer for near infrared glucose absorption measurement

Abstract

Noninvasive blood glucose monitoring is a long pursued goal in clinical therapy as an invaluable tool that would aid in the treatment of diabetes. The importance of such device is marked by the market value of glucose testing devices, which was estimated to be more than $2.5 billion worldwide in 1997, and growing at 10-15% a year. In this project, a modular Fourier transform spectrometer for the near infrared wavelength region was designed and built. The work was motivated by the need for a versatile and dedicated instrument for research in the area of blood glucose noninvasive measurement. Selection and design of each element of the spectrometer is discussed, with the aim of optimizing the signal-to-noise ratio in the near infrared region. Careful analysis of sources of error aids greatly to comprehension of the limiting source of inaccuracy, which enables both instrumental and procedural optimization without the need for exhaustive experimentation. Currently, absorption of aqueous glucose solution at 5930cm-1 is being investigated. The ability to resolve milli-molar levels of aqueous glucose concentration is found to be greatly dependent on the instrumental as well as procedural factors such as the optical path length of the solutions. Two instrumental errors found to be significant are photodetector noise and digitization noise, the later being the current instrumental noise limitation for simple, glucose-water transmission study. Digitization noise comes from the quantization error due to the limited resolution of our 12-bit analog-to-digital converter. With the present arrangement, the achievable signal-to-noise ratio is 0.67 per mmoVL for 10-mm pathlengths and 0.18 per mmoVL for 2-mm pathlengths of aqueous glucose solutions. A few methods to improve the signal-to-noise ratio are discussed.