Determination of carbon monoxide using a coated quartz crystal sensor

Abstract

IntroductionThe principle of detection by piezoelectric crystal is based on the fact that the vibration frequency of an oscillating sensor decreases in the presence of an amount of material added to its surface. As shown by Sauerbrey [1], the vibrating frequency of a quartz crystal, to a first approximation, changes proportionally to the mass deposited onto or removed from one or both their faces. Because the TSMR (thickness shear mode resonator) fundamental frequency is rather low (10 MHz), it is assumed that viscosity effects contribute to the frequency changes due to gas sorption to a negligible extent only [2]. Gaseous pollutants can be selectively sorbed by the detector if an ideal coating is deposited on the metallic electrodes. The observed frequency change is a measure of the amount of sorbed gas. The type of interaction taking place between the analyte molecules and the active coating determines the sorption and desorption characteristics. Lowenergy, perfectly reversible interactions such as physisorption generally lack a high degree of selectivity. On the other hand, the formation of chemical bonds or the chemisorption process tend to be less reversible. One approach to overcoming these problems includes the application of sensor arrays to compensate the low selectivity [3,4]. Another solution to this problem includes searching for "intermediate interactions", i.e., interactions that are weaker than chemisorption (≈300 kJ mol -1 ) but stronger than physisorption (0-40 kJ mol -1 ), such as coordination [5]. A central metal atom surrounded by neutral or charged (often organic) ligands is responsible for these interactions, where one or more donor atoms on these ligands interact with the metal ion. So, selectivity can be influenced by the choice of both the metal ion and the ligand, from both an eletronic and steric point of view [5]. When gaseous molecules interact with metal complexes in this way, they themselves become coligands either by occupying free coordination sites or by displacing other ligands. Earlier work in the field of coordination chemistry on mass-sensitive Abstract: Carbon monoxide was detected and determined by a piezoelectric quartz crystal sensor coated with nickel(II)-phthalocyanine 50 % (v/v) solution in glycerine. Studies on the effect of temperature, flow rate, and some possible interferents were carried out. Calibration curves, sensor stability (lifetime) and the precision of measurements were also verified. The resulting selectivity is probably due to the coordinative binding between the electronically unsatured metal complexes and the analyte. The analytical curve is linear in the concentration range 0.10 to 1.0 % (v/v).

Review of Dissolved CO and H2 Measurement Methods for Syngas Fermentation

“…QCM sensors fabricated with HgO or Ag 2 O coatings utilize their reactions with CO at an elevated temperature to deposit a layer of mercury or silver metal on the surface of the microbalances [ 150 , 151 ]. The metal–organic complex coatings can directly absorb CO from ambient air and change the mass of the microbalances [ 152 , 155 , 156 ].…”

“…QCM sensors using metal–organic complex coatings are highly sensitive and were reported with a detection limit at the ppm level [ 152 , 155 ]. The response time ranged in seconds [ 156 ] to minutes [ 155 ]. However, QCM sensors fabricated with noble metals or volatile metal oxides also respond to other combustible or reducing gases [ 150 , 154 ].…”

“…However, QCM sensors fabricated with noble metals or volatile metal oxides also respond to other combustible or reducing gases [ 150 , 154 ]. QCM CO sensors fabricated with the zinc crptand22 ligand complex and nickel phthalocyanine complex were reported as sensitive to NO 2 [ 152 ] and SO 2 /NO 2 [ 156 ], respectively. QCM sensor with a palladium acetamide complex coating was reported free of interference from H 2 , SO 2 , and H 2 S; however, the CO absorption on the coating is irreversible [ 155 ].…”

“…When the piezoelectric material has an oscillation frequency at the MHz level, QCM sensors are highly sensitive in CO measurements with high mass sensitivity [151]. The reported CO sensing coatings in QCM sensors include noble metals (platinum, palladium, and platinum-iridium alloy) [154], volatile metal oxides (HgO and Ag 2 O) [150,151], and various metal-organic complexes, such as the zinc crptand22 ligand complex [152], palladium acetamide complex [155], and nickel phthalocyanine complex [156]. Sensors using noble metals coatings exploit the heat generated from CO oxidation to increase the resonance frequency of the piezoelectric materials [154].…”