Ammonia Detection in the UV Region Using an Optical Fiber Sensor Hadi Manap, Gerald Dooly, Sinead O'Keeffe, Elfed Lewis Department of Electronic & Computer Engineering University of Limerick Ireland e-mail: [email protected] Abstract—This paper describes an optical fiber sensor for the 1.1 Current ammonia sensors and their working principle monitoring of ammonia gas. An open path optical technique is used to analyze the absorption lines of ammonia within the There are many types of NH3 sensors that have been ultra violet region. Experimental results describing the previously reported. They can be categorized into five main operation of the sensor are presented and are compared with types and each of them has their own advantages and theory. A comparison between a commercial sensor has been disadvantages. carried out and cross sensitivity testing with regard to oxygen and carbon dioxide gas is reported. 1.1.1. Metal oxide semiconductor The first type of NH3 gas sensor is metal oxides 1. INTRODUCTION semiconductor. Usually this type of sensor is based from tin dioxide (SnO ) [3]. This gas sensor operates on the principle Increased ammonia (NH ) emissions are one of the 2 3 of conductance change due to chemisorption of gas concerns of the European Union, especially NH that is 3 molecules to the sensing layer. Much research has been done naturally produced by livestock farming. According to the on this type of sensor demonstrating promising results using Official Journal of the European Communities, a directive a sensor that is rugged and inexpensive [4-8]. However this setting national emission ceilings (NECs) has been set in type of sensor can have some drawbacks, the primary one 2001 to combat NH emission. The aim of the directive is to 3 being that it has a low level of selectivity. There are different gradually improve, through a stepwise reduction of the approaches to cater to this selectivity problem such as neural pollutants, the protection both of human health and the network method [9-11], mathematical modelling [12] or by environment throughout the EU countries. By 2010, EU using metals or additives that enhance the chemisorption of member states must have limited annual national emissions so specific gasses [13,14]. that they do not exceed the emission ceilings [1]. They must also ensure that these emission levels do not exceed the 1.1.2. Catalytic ammonia sensor emission ceilings in any year after 2010. According to European Environmental Agency, EEA report 2003, 93% of The working principle of catalytic NH3 sensor is based on the charge particle in the catalytic metal. When the EU15 NH3 emission in 2001 was caused by agriculture sector as shown in Fig. 1. Agriculture sector is the main contributor concentration of the test gas introduced to the sensing layer changes, it affects the concentration of the charge carrier in to NH3 emission since 1990. Hence, a new NH3 sensor that suits the agriculture environment need to be developed as an the catalytic metal. This change in charge carriers can be alternative to current ammonia sensor. quantified using a field effect device, like a capacitor or a transistor [15,16]. The main disadvantage of this sensor system is that they are not immune to electrical interference since the reading is based on the charge carrier, which is sensitive to electrical field. 1.1.3. Conducting polymer gas sensors This is another type of NH3 gas sensor, usually based on polyaniline [17,18] as the conducting polymer. In other research [19,20], polypyrrole is used as polymer film which causes a change in the conductivity of the material, making it a suitable material for measurement in resistance [21] or electrical current for NH3 detection [19]. This kind of sensor has a good detection level down to 1 ppm as reported in this paper [22]. However it suffers from irreversible reaction Figure 1. Sector split of NH emissions in 2001 (%), EEA report 2003. 3 978-1-4244-5335-1/09/$26.00 ©2009 IEEE 140 IEEE SENSORS 2009 Conference which makes this sensor less sensitive over time when gas, σ (cm2/Molecule) is the absorption cross section and N 3 exposed to NH3 [20]. (Molecules/cm ) is the gas concentration. 1.1.4. Optical gas sensing 2.1 Unit Conversion NH3 optical gas sensing can be divided into two types. The concentration of test gas, N in the Beer Lambert Law 3 The first is based on optical absorption when NH3 is exposed in equation (1) is given in unit Molecules/cm , so we need to to the light source such as lasers. It has very low detection change the unit to ppm as the pollution gas concentration is limit, 1 ppb and a response time of 1 s has been reported normally read in this unit. [23]. It also shows a good selectivity from other interfering The ideal gas law (PV = nRT ) is also used in this unit gases such as CO2 and water vapor [24]. However this type of sensor can cost thousands of dollars and is not suitable to conversion, where P = pressure (atm), V = volume (L), n = detect NH at small volumes. Another type of optical gas mass of substance (moles), R = ideal gas constant (0.082 atm 3 L mol-1K-1), T = absolute temperature (ºC + 273) in degrees sensing is based on a change in colour when NH3 reacts with a reagent. The best example is using a pH paper. The Kelvin. spectrum of the detected gas will change based on the atm ⋅ L coloration reaction. There are different coloration reactions (0.082 ) (273 K) V RT mol ⋅ K in use for dissolved NH3 such as the Nessler reaction [25]. = = (2) This NH3 detection method is commercially available and n P 1 atm can be used for determining the total NH3 concentration in L water. However this reagent is toxic and therefore is not = 22.4 suitable for frequent use. mol Standard temperature and pressure are defined as 1 atm 1.1.5. Indirect gas analyzers and 0 ºC (273 K). Under these conditions, a mole of an ideal Most of the available gas sensors suffer from poor gas occupies a volume of 22.4 L. selectivity towards NH3 [2]. Hence, the gas sensor system should comprise a selection mechanism that allows only the At times, gaseous concentrations are expressed using mixed units of mass per unit volume such as mg/m3. The gas of interest to be introduced to the detector of the system. 3 A few techniques such as gas diffusion separation with gas relationship between ppm and mg/m depends on the density permeable membranes have been done [26–28]. This sensor of the gas which depends on its pressure, temperature and molecular weight as shown in equation (3). is sensitive enough to detect NH3 below 1 ppm [28]. Since this sensor is using membranes for gas separation, it will mg ppm × ω 273 P(atm) suffer from a low lifespan. ρ( ) = × × (3) 3 L T (K) 1 atm m 22.4 All of NH3 sensors discussed above have their own mol drawbacks. Hence a new NH3 sensor needs to be developed as an alternative for the current detection method. where ρ(mg/m3) is the density of the gas, ω is the 2. THEORY molecular weight, P(atm) is the measured pressure and T(K) Different gas species absorb light at different is the measured temperature of the test gas. characteristic wavelengths and for NH3 gas, it has its own Equation (3) is commonly used in many text books [44- specific gas absorption spectrum. A comprehensive 47] and can be simplified as shown in (4) with assumption collection of absorption cross sections for gaseous molecules the test gas temperature is 298 K and at 1 atm pressure. can be accessed from the Max Planck Institute, MPI Mainz database including NH gas [29]. The data vary from source mg ppm × ω 3 ρ( ) = (4) to source and they depend on temperature and wavelength m3 L range. Only one that suits the experimental condition 24.4 parameter is selected and compared with the measured mol results. Before we can use (4) and read the concentration in ppm For NH absorption comparison with the theory, the unit, we need to find the relation between the density of the 3 gas, ρ(mg/m3) and the concentration of test gas, Beer-Lambert Law has been utilised. The Beer-Lambert law 3 described the linear relationship between absorbance and N(Molecules/cm ). concentration of an absorbing species and its general form is Since N(Molecules/cm3) * (1000 mg/1g) * (1 cm3/10-6m3) shown in (1). * (1 mol/6.022x1023 Molecules) * (ω g/mol) = ρ(mg/m3) 3 I (−σ .N.l) Hence the relation between N(Molecules/cm ) and = e (1) ρ(mg/m3) is given by, I o ρ -9 Where I is the transmitted intensity, Io is the incident N = N A x 10 (5) intensity, l(cm) is the distance that light travels through the ω 141 where N A is Avogadro’s constant. In order to calculate the gas concentration, N in parts per million, ppm, (4) and (5) are plugged in the Beer Lambert Law equation (1) above; Hence I − [ln ][24.4] I ppm = o (6) σ × × × −9 N A l 10 Figure 2. Infrared spectrum for ammonia and water vapour [43]. Using (6), we can accurately calculate the gas concentration with σ known. This σ value varies depend on the wavelength measured. This method of gas concentration Based on UV spectrum shown in Fig. 3, the water vapour calculation was similarly described in [30-32].