Non-Dispersive Ultra-Violet Spectroscopic Detection of Formaldehyde Gas for Indoor Environments

Abstract

We describe a simple method for detecting formaldehyde using low resolution non-dispersive UV absorption spectroscopy. A two channel sensor was developed, making use of a strong absorption peak at 339 nm and a neighboring region of negligible absorption at 336 nm as a reference. Using a modulated UV LED as a light source and narrow laser-line filters to select the desired spectral bands, a simple detection system was constructed specifically targeted at formaldehyde. By paying particular attention to sources of noise, a minimum detectable absorbance of 5 × 10 −5 absorbance units (AU) was demonstrated with a 20 s averaging period (as I/I 0). The system was tested with formaldehyde finding a limit of detection of 4.3 ppm for a 195 mm gas cell. As a consequence of the low gas flow rates used in our test system, a time period of over 8 min was used in further tests, which increased the minimum detectable absorbance to 2 × 10 −4 AU, 17 ppm of formaldehyde. The increase was the result of thermal drift caused by unwanted temperature variation of the UV LED and the filters, resulting in a zero uncertainty estimated at −560 ppm°C −1 and 100 ppm°C −1 respectively.

Towards Integrated Mid-Infrared Gas Sensors

“…Various topologies have been implemented to fabricate optical gas sensors (Figure 3), with the most commonly used based on gas cells formed between face-to-face configured emitters and optical detectors [48,53,54] (Figure 3a–c,e). Strategies to miniaturize the gas cell include: the use of enhancement layers, such as photonic crystals [55], optical cavities [56], multi-pass cells [57], or gas enrichment layers [58], to increase the light-gas interaction (Figure 3c); planar configurations of emitters and detectors [44,59] (Figure 3f); or use of waveguides for evanescent-field interaction [60,61,62] (Figure 3g).…”

“…The sensor response signal, ∼I(λ), is typically extracted by means of a lock-in detection technique, from a known frequency used to modulate the emitter [19,65]. A reference detector is often used to compensate for changes in the emitted light [40,48,53,54,58,59,64]. Additional sensors can be used to compensate for environmental parameters such as temperature, pressure or humidity [44,51,59,63,66].…”

“…Gas sensors using optical detectors, to measure I(λ), have been successfully implemented for a variety of gases: acetone (C3H6O) [63], ammonia (NH3) [63], carbon dioxide (CO2) [19,40,43,47,54,58,59,61,64,73], formaldehyde (CH2O) [48], nitric oxide (NO) [53], carbon monoxide (CO) [59,64,74], methane (CH4) [56,59,60,62,64,75,76], or methanol (CH3OH) [77]. Various strategies have been implemented to fabricate gas sensors based on optical detection, Table 1 (Figure 3a–c,e–g).…”

“…These include designs based on tube-like gas cells formed between face-to-face configured emitters and detectors [19,43,48,53,54,58,63,75,76,77] (Figure 3a–c,e), dome-like gas cells with planar configured emitters and detectors [47,59] (Figure 3f), open cells [40,64,73] (Figure 3e), cavity-enhanced cells [56], or waveguides based on evanescent-field interaction [60,61,62] (Figure 3g). Different light sources have been used such as MEMS heaters [19,43,54,63,64,73,77], LEDs [40,47,48,53,76], distributed feedback lasers (DFBs) [62,75], or QCLs [61], and detectors such as photodiodes [47,48,53,62,76], thermopiles [19,43,54,63], pyroelectric detectors [59,64,77], or photoconductive detectors [56,60,61]. Thus far the most popular configuration is based on face-to-face configured tube-like cells for CO2 detection [19,56,60].…”

“…These advancements, in particular the realization of new MIR sources [45], combined with the increasing needs to develop new innovative technologies for healthcare, digital services and other innovation [46], are driving optical gas sensors towards low-cost, mainstream applications [15,19]. Gas sensors based on optical detectors, have been demonstrated for various applications (e.g., breath analysis [19,47], indoor air quality (IAQ) [48], or pollution control [49]). Photoacoustic sensors, using highly-sensitive MEMS microphones, have also emerged as compact, low-cost sensors with high sensitivity and stable operation [44,50,51].…”