Detection of toluene diisocyanate in air with a coated piezoelectric crystal: Part 2. Development of an Instrumental Method For Personal Monitoring

Gases in the atmosphere are monitored with chemically-coated quartz piezoelectric crystals; the method is demonstrated in the use of polyethylene glycol for toluene diisocyanate determination. A microprocessor is used to control the gas sample flow through the detector head as well as the data acquisition. A computer-based procedure for data treatment permits signal integration and background drift correction, resulting in a theoretical detection limit of about 0.006 ppm. The design of a basic portable instrument for piezoelectric crystal monitoring of toxic gas is described with a view to future modification and microprocessor control.     

Detection of toluene diisocyanate with a coated quartz piezoelectric crystal: Part 4. A portable automatic detector with humidity correction

Microcomputer-controlled measurement of the frequency of two differently-coated quartz piezoelectric crystals and subsequent data processing permits the immediate correction of instrumental response for fluctuations in water vapour concentration and the display of the corrected toluene diisocyanate concentration in air. Crystal pairs with coatings of polyethylene glycol 400 or 1540 or tri-n-octylphosphine oxide and cobalt(II) chloride were used to illustrate the performance of the instrument. Toluene diisocyanate can be detected over the range 0.1—15 ppm in atmospheres with relative humidities ranging from 30 to 60% without significant interference from changes in water concentration.     

Detection of toluene diisocyanate with a coated quartz piezoelectric crystal: Part 3. Practical coatings for a humidity-corrected detector

Dithizone and tri-n-octylphosphine oxide (TOPO) were tested as coatings for quartz piezoelectric crystals to be used in a detector for touene diisocyanate (TDI). Cobalt(II) chloride and polyethylene glycol (PEG) with molecular weights of 400 and 1540 were tested for suitability as coatings for detecting humidity variations so that correction for humidity could be obtained from a two-crystal system. Sensitivities for TDI of 12 and 75 Hz ppm^?1 were found with dithizone and TOPO, respectively, between 0 and 1 ppm TDI. Cobalt chloride-coated crystals showed a sensitivity for water of 0.053 Hz ppm^?1 at 8000 ppm water and about 40 Hz ppm^?1 for TDI at the 2 ppm level. PEG-400 and PEG-1540 provided sensitivities to water of 0.038 and 0.051 Hz ppm^?1 respectively, and the response was linear over the range 9000—13 000 ppm water. All coatings showed irreversible behaviour towards TDI.     

Determination of lead in solution with a piezoelectric quartz crystal coated with copper oleate

Copper(II) oleate was coated on a piezoelectric quartz crystal, and the copper removed by passing EDTA solution. The remaining coating reacted with aluminium, copper(II), iron(III) and lead ions in a flowing acidic solution, to form absorbed compounds which changed the frequency of the crystal. Lead (3–40 μM) could be determined at pH 5.5–5.8 with good reproducibility. Interfering metal ions (Al³⁺, Cu²⁺, Fe³⁺) were masked with acetylacetone.     

A piezoelectric crystal detector for determination of acetoin in air

A method is presented for the determination of acetoin (3-hydroxy-2-butanone) in air with a piezoelectric crystal detector coated with semicarbazide. The response time is about 5 min, is fully reversible, and is selective for acetoin in the presence of the interferences normally found in air. The detector has a sensitivity of 12.4 Hz 1 μl^?1 for acetoin and the response varies linearly with concentration in the 50– μl l^?1 range.     

Determination of hydrogen cyanide in air using mass amplification by heavy ligand replacement on a coated quartz piezoelectric crystal

Piezoelectric crystals coated with bis(pentan-2,4-dionato)nickel are used to detect the presence of hydrogen cyanide in the range 13–93 ppm (10^?4 mol mol^?1). The displacement of the heavy ligand pentan-2,4-dione by a light molecule (HCN) yields an increase in sensitivity, compared to simple absorption of the analyte (i.e., mass amplification). A continuous flow system is used at relative humidities of 40–92%. The lifetime of such a device is limited by hydrolysis. Reproducibility from coated-crystal to coated-crystal is poor, probably owing to inconsistencies in coating techniques.     

A coated piezoelectric crystal detector for the determination of hydrogen sulfide

A method is presented for the detection and determination of hydrogen sulfide. It is based on the selective and reversible adsorption of hydrogen sulfide on the surface of a piezoelectric crystal coated with a cadmium iodide/urea/-glycerol solution. The proposed detector has a response time of 30 s with linear response over the concentration range 2–25 μl l^?1 (ppm).     

Measurement of sulfur dioxide in automobile exhausts and industrial stack gases with a coated piezoelectric crystal detector.

A portable instrument operating on a car battery and based on a quadrol-coated piezoelectric crystal detector has been successfully used for monitoring sulfur dioxide in auto exhausts and refinery stack gases. The concentrations of sulfur dioxide in the auto exhausts lie in the range 20–50 p.p.m. Up to 300 p.p.m. of sulfur dioxide occurs in the refinery stack gases.     

A new type of piezoelectric detector in liquid: Part 2. Computation of the equivalent circuit parameters of a piezoelectric crystal with a separated electrode and of series piezoelectric sensors in a non-electrolyte solution

An improved equivalent circuit model is proposed for a piezoelectric crystal with a separated electrode. Equations are derived for the equivalent circuit parameters in a non-electrolyte solution and are verified experimentally. The resonance frequency f_s is given by f_s = f₀[1 + C₁/2(C′₀ + C_s)], where f₀, C₁ and C′₀ are the resonance frequency, motional capacitance and the shunt capacitance of the crystal respectively and C_s is the solution capacitance. The mechanical quality factor is the same as that of the crystal. The motional resistance, motional inductance, motional capacitance and shunt capacitance are respectively K², K², K⁻² and K⁻¹ times those of the crystal, where K = 1 + C′₀/C_s. The influence of the permittivity, density and viscosity of the solution and the configuration of the sensors on the equivalent parameters are investigated. The equivalent circuit parameters of a series piezoelectric crystal are also calculated and measured.     

Detection of Trace concentrations of gases with coated piezoelectric quartz crystals

Coated piezoelectric crystal sensors for determination of trace constituents in gases are very sensitive. Detection limits down to the sub-ppb range have been reported, using injection techniques. From previous and presented data, this high sensitivity is found to be in disagreement with Sauerbrey's eqn., which allows a maximum sensitivity of about 1 ppm. By considering the adsorption of the gases on the coated surface and the calibration procedures used, the validity of Sauerbrey's eqn. is discussed.     

Determination of ammonia in the air using piezoelectric resonance sensors coated with humic acids

Natural macromolecular compounds, humic acids (HA), have been used to form gas-sensitive layers of piezoelectric resonance gravimetric sensors. HA have been extracted from humus using alkaline extraction and purified by electrodialysis through ion-exchange membranes. Species have been deposited onto the electrode surface of piezoelectric quartz resonators by air-spraying their aqueous solutions, the layer thickness being 100–200 nm. In determining ammonia with the sensor obtained on the basis of purified HA species, the detection limit is ∼10 mg/m³. Gas-sensitive layers on the basis of humic acids are more stable as compared to the conventionally used gas-sensitive layers based on pyridoxine hydrochloride or ascorbic acid.     

Determination of iron(III) and aluminum in solution with a piezoelectric quartz crystal coated with silicone oil

The frequency of the crystal connected to an integrated circuit oscillator changed on absorption of iron(III) or aluminum ar pH 3.8–5.8 and 5.0–6.0, respectively. Absorption of iron(III) at pH ≥ 5.0 was prevented by addition of excess of sulfate. Iron(III) was determined at pH 4.7; 5–100 μM solutions of both species may be determined with a r.s.d, of ≤3%.     

Detection of complement C1-inhibitor with a piezoelectric immunosensor

A novel piezoelectric immunosensor has been developed for the detection of human complement C1-inhibitor. Anti-C1-inhibitor antibody was immobilized onto the gold electrodes of a 9 MHz AT-cut piezoelectric crystal. Coating the crystal with polyethyleneimine adhesion, followed by a glutaraldehyde cross-linking method to immobilize antibody showed better results than the physical adsorption method with respect to sensitivity and reproducibility. Under the optimized experimental conditions, the sensor showed good response to the C1-inhibitor in the range from 2.0 × 10^–8 to 1.2 × 10^–6 g. Other proteins in human serum did not remarkably interfere with the detection. The crystals could be regenerated 5 times, when bound materials on the crystal surface were eluted by strong acid and strong alkali solution and subsequently cleaned in an ultrasonic cleaner.     

Detection of complement C1-inhibitor with a piezoelectric immunosensor

A novel piezoelectric immunosensor has been developed for the detection of human complement C1-inhibitor. Anti-C1-inhibitor antibody was immobilized onto the gold electrodes of a 9 MHz AT-cut piezoelectric crystal. Coating the crystal with polyethyleneimine adhesion, followed by a glutaraldehyde cross-linking method to immobilize antibody showed better results than the physical adsorption method with respect to sensitivity and reproducibility. Under the optimized experimental conditions, the sensor showed good response to the C1-inhibitor in the range from 2.0 x 10(-8) to 1.2 x 10(-6) g. Other proteins in human serum did not remarkably interfere with the detection. The crystals could be regenerated 5 times, when bound materials on the crystal surface were eluted by strong acid and strong alkali solution and subsequently cleaned in an ultrasonic cleaner.     

A coated piezoelectric crystal detector for the selective detection and determination of hydrogen sulfide in the atmosphere

A method for the selective detection and determination of hydrogen sulfide in the atmosphere is presented. This method utilizes the reversible adsorption of H₂S on a piezoelectric quartz crystal coated with an acetone extract of soots resulting from the burning of organochlorine compounds. The extract of a soot prepared from chlorobenzoic acid provided the best substrate. The method is useful in the concentration range 1–60 ppm.     

Microdetermination of sulfite with a piezoelectric crystal detector

A piezoelectric method is proposed for the determination of sulfite in concentration range 1 × 10^–7–1 × 10^–5 mol/1. The method is based on the redox reaction of sulfite with iodine followed by measuring frequency change of the piezoelectric crystal caused by the unreacted iodine. The method is applied to the determination of sulfite in liquor.     

Determination of carbon monoxide in ambient air using piezoelectric crystal sorption detection

A new piezoelectric quartz crystal (PQC) sorption detector was developed to monitor carbon monoxide (CO) at sub-ppm level in ambient air. Out of the 28 coating materials studied, the palladium(II) acetamide complex with a 1:10 mole ratio of Pd(II) to acetamide was found to be the best. The detection is based on a non-reversible gas/coating interaction with sensitivity depending on gas flowrate. For 5-15 min exposure at a flowrate of 50 ml/min, the working ranges were found to vary from 0.7 to 40 ppm (total exposure from 8 to 160 μg CO) and detection limits (S/N=2) from 0.7 to 2 ppm CO (total exposure to 8 μg CO). The repeatability at 10 ppm CO was 11.8% (R.S.D. for n=3). The sensor lifetime was found dependent on exposure up to 160 μg CO or not exceeding 1000 Hz accumulative shift of frequency to avoid saturation of active sites at the crystal surface. No interference to CO detection was found for H₂, H₂S, SO₂, NO₂, CO₂, HCHO, gasoline and water vapors at concentrations much higher than ambient air. Compared to existing CO monitor, the PQC detector developed has advantages of adequate selectivity, high sensitivity, fast response and a much lower detection limit for detecting CO at sub-ppm levels. However, it is limited by the total exposure to a maximum of 160 μg CO that restricts its application to intermittent monitoring of low CO concentration. The present work has demonstrated the advantages of using strong non-reversible interaction to enhance PQC sensitivity, as the total exposure can be adjusted easily by a suitable control of the gas flowrate.     

Titrations with piezoelectric monitoring : Part 1. Fundamental theory and titration of acids and bases

The use of a piezoelectric quartz crystal detector is proposed for monitoring the course of a titration reaction by observation of the frequency shift of the oscillating crystal. Satisfactory titrations can often be obtained using the acid-base technique, which is much more sensitive than the classical conductimetric titration and oscillometric titrration, with a lowest titratable concentration that is much lower but which can still be applied in the presence of a large amount of unreacting electrolyte.