A technique referred to as tunable diode laser absorption spectroscopy (TDLAS) enables fast, accurate and noninvasive measurement of bacteria levels in food, blood supplies and other products derived from living matter. Several detection techniques are currently capable of rapid, accurate measurements of gas compositions, but techniques based on optical spectrometry offer the advantages of being noninvasive, highly sensitive, and able to provide instant responses. “One major advantage TDLAS offers is its ability to achieve very low detection limits, on the order of parts per billion,” said Jie Shao, associate professor at the Institute of Information Optics, Zhejiang Normal University, Jinhua, China. “Apart from concentration, with TDLAS it’s also possible to determine other properties of the gas under observation, such as temperature, pressure, velocity and mass flux.” TDLAS is by far the most common laser-based absorption technique for quantitative assessments of species within a gas phase. It can be used to measure the concentration of specific gaseous species — carbon monoxide, carbon dioxide (CO2), water or methane, to name a handful — within gaseous mixtures by using absorption spectrometry based on tunable diode lasers. Since micro-organism growth is always associated with the production of CO2, it’s possible to assess the microbial growth by assessing the level of CO2 within a closed compartment, such as a bottle or bag. Shao and other researchers from Zhejiang Normal University and Umeå University in Sweden developed an instrument based on TDLAS to assess bacterial growth of various types of samples under a variety of conditions. The researchers’ basic setup involved a tunable diode laser as the light source, beam shaping optics, a sample to be investigated, receiving optics and one or more detectors. The emission wavelength of the laser was tuned over a characteristic absorption line transition of the species within the gas being assessed, causing a reduction of the measured signal intensity, which the researchers used to determine gas concentration. When the wavelength is rapidly tuned across the transition in a specific manner, it can be combined with a modulation technique called wavelength modulation (WM), which enhances the sensitivity of the TDLAS technique. By applying WM-TDLAS to transparent containers of organic substances such as food items or medical samples, bacterial growth can be quickly and accurately evaluated. Micro-organism growth is driven by many factors, which make it difficult to accurately estimate the amount of bacteria within food containers or blood samples at any given time. A better understanding of the growth process of micro-organisms could help reduce food waste and prevent sickness. Within the medical realm, the ability to assess the quality of blood samples quickly and accurately could mean that a larger percentage of blood could be directly tested for bacteria, reducing the risk of bacterial blood contamination. Next, the researchers plan to enhance the technique to allow for assessments of microbial growth in a large variety of samples beyond food items and medical supplies. The research was published in Applied Optics (doi: 10.1364/AO.55.002339).