Talanta 69 (2006) 631–636 Highly sensitive optical humidity probe H. Dacres, R. Narayanaswamy ∗ School of Chemical Engineering and Analytical Science, The University of Manchester, Faraday Tower, P.O. Box 88, Sackville Street, Manchester M60 1QD, UK Received 11 July 2005; received in revised form 28 September 2005; accepted 25 October 2005 Available online 2 December 2005 Abstract A highly sensitive optical humidity probe based on reflectance measurements has been developed using Nafion ® -crystal violet (CV) films. This sensor can be used to calibrate relative humidity (RH) in the range 0–0.25% with a detection limit (blank signal + 3σ b , where σ b = the standard deviation (S.D.) of the blank signal) of 0.018% RH (∼4.37 ppm) and exhibited low hysteresis. The sensor films were fully reversible in dry nitrogen and reversal times were shown to be dependent on exposure time and % RH. The response to 1% RH was highly reproducible (S.D. = 1.67%, number of samples (n) = 5). Hydrogen chloride gas did not interfere with the response of the sensor to RH but did reduce sensor reversal times. This sensor displayed sufficient sensitivity that it could be used to detect ppm levels of moisture in process gases such as nitrogen and HCl. © 2005 Elsevier B.V. All rights reserved. Keywords: Relative humidity sensor; Moisture detection; Reflectance probe; Optical fibre sensor 1. Introduction The ability to monitor low levels of moisture has become even more critical in the semiconductor-manufacturing process since the demand for process gases with more stringent moisture specifications has continued to grow. Commonly used process gases include both nitrogen (N 2 ) and hydrogen chloride (HCl) [1]. Moisture is a particularly problematic contaminant in gas supplied from a cylinder as it can adsorb strongly onto metallic surfaces and cannot be easily removed by purging with dry inert gases. Humidity is often referred to as relative humidity (RH) and is defined as the ratio (%) of the concentration of water vapour in the gas to the saturated concentration at the same tem- perature and pressure. Humidity can also be expressed as volume Abbreviations: ANN, artificial neural networks; a.u., arbitrary units; CO 2 , carbon dioxide; CoCl 2 , cobalt (II) chloride; CV, crystal violet; H, hydrogen; HCl, hydrogen chloride; HPC, hydroxypropylcellulose; kHz, kilohertz; LiCl, lithium chloride; N 2 , nitrogen; NH 3 , ammonia; NIR, near-infrared; nm, nanometres; NO 2 , nitrogen dioxide; PCS, plastic clad silica; PMMA, polymethylmethacry- late; ppm, parts per million (v/v); PVA, poly(vinyl alcohol); r, correlation coefficient; RH, relative humidity; S.D., standard deviation; s/n, signal/noise ratio; wt, weight ∗ Corresponding author. Tel.: +44 161 306 4891; fax: +44 161 306 4911. E-mail address: Ramaier.Narayanaswamy@manchester.ac.uk (R. Narayanaswamy). concentration (ppm). In this paper, both units of concentration will be referred to intermittently. Recent research in optical humidity sensing has been described in the literature [2–5]. Cobalt (II) chloride (CoCl 2 ) was one of the earlier reagent schemes investigated for this pur- pose [2]. Immobilised in gelatine it could be used to determine the RH of air between 40% and 80% [2] and, when immo- bilised on acetylated cellulose substrates, it could determine RH down to 4% [3]. Recently an evanescent wave sensor has been fabricated using PVA doped with CoCl 2 coated on the surface of a U-bend core of a plastic clad silica (PCS) fibre and this sensor responded to RH in the range 10–90% with a detection limit of 15% (∼3660 ppm) and it displayed fast response and reversal times <1 s [4]. Using the same U-bend sensing principle phenol red was incorporated into a polymethyl- methacrylate (PMMA) film and the resulting sensor was shown to respond to RH in the range 20–80% with a response time of 5s [5]. In the identification of a suitable reagent scheme for the deter- mination of moisture in process gases crystal violet in Nafion ® is of considerable interest. It is well known that the colour of CV is very sensitive to the acidity of protons [6] and this property has been exploited in the development of an optical humidity sen- sor. Nafion ® is a perfluorosulfonate resin in which hydrophilic perfluorinated ether side chains terminate with sulfate groups, which are periodically attached to hydrophobic perfluoroethy- 0039-9140/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2005.10.037