Gad Frishman

Kinetic study of the thermal inactivation of cholinesterase enzymes immobilized in solid matrices.

The thermal inactivation of immobilized cholinesterase enzymes (ChE) in solid matrices where the protein unfolding is blocked was studied, thus enabling investigation of the kinetics of the inactivation process directly from the native structure to the inactivated state. The thermal inactivation of butyrylcholinesterase (BChE), recombinant human acetylcholinesterase (rHuAChE), and eel acetylcholinesterase (AChE) enzymes was studied in dry films composed of poly(vinyl pyrollidone) (PVP), bovine serum albumin (BSA) and trehalose at 60 degrees -120 degrees C. The kinetics follows a bi-exponential decay equation representing a combination of fast and slow processes. The activation enthalpy DeltaH(#) and the activation entropy DeltaS(#) for each of the three enzymes have been evaluated. The values of DeltaH(#) for the fast process and for the slow process of BChE are 33+/-3, and 28+/-2 kcal/mol, respectively, and the values of DeltaS(#) are 0.84+/-0.04, and -18.2+/-0.5 cal/deg, respectively. The appropriate value of DeltaH(#) for rHuAChE is 26+/-2 Kcal/mol, for both processes and the values of DeltaS(#) are -17.6+/-0.9, and -23.0+/-0.9 cal/deg, respectively. Similarly, the values of DeltaH(#) for eelAChE are 30+/-3, 31+/-1 kcal/mol, and the values of DeltaS(#) are -6.7+/-0.5, -9.1+/-0.2 cal/deg respectively.

Optical-fibre sensors for blood gases and pH, based on porous glass tips

Abstract Optical-fibre sensors have been developed for the determination of blood gases and pH. Chemical binding of pyrenebutyric acid and 7-hydroxy-4-methylcoumarine-3-acetic acid to a porous tip of a fused silica fibre produces a sensitive measuring device for molecular oxygen and pH. Insertion of the pH sensor into a solution of bicarbonate, entrapped within a polypropylene membrane, yields a pCO2 sensor. Excitation of the oxygen-sensor tip within the range 340–360 nm yields emission with a maximum intensity at 460 nm. An intensity ratio ≈ 10 between signals measured in pure gas-phase nitrogen and oxygen is obtained. Excitation of the sensitive tip within the range 360–380 nm yields emission with a maximum intensity at 457 ± 3 nm. The excitation spectrum is red shifted about 19–30 nm relative to that obtained in solution. The maximum intensity of the excitation spectrum for the acidic form of the bound coumarine derivative is at 356 ± 3 nm, whereas for the basic form it is at 380 ± 3 nm. The apparent pK of the bound coumarine derivative is 7.45 ± 0.30. The bias and the precision for pO2 determinations in blood samples are about −1 and ± 1.5 torr, respectively. For pH determinations, the respective values are about 0.02 and 0.04 pH units and for pCO2 about 1 and 2.5 torr. The sensors described are suited for production on an industrial scale.

Explosive Detection by Microthermal Analysis

Differential scanning microcalorimetry at high heating rates of ∼ 300°C/s was performed on 30- to 100-µm-size explosive particles using two MEMS-based thermal conductivity gauges in air and under N2. The gauges consist of a thin-film Si3Nx membrane with a centrally located Al thin-film heater, which is surrounded by six thin-film Si/Al junctions, creating a temperature-sensitive thermopile (∼ 1.3 mV/K) with an effective sensitive area of ca. 200 × 200 µm. Heating was carried out by applying a linear voltage ramp during 1.6 s. The measurements were performed in a specially designed exposure chamber having a transparent glass lid that enabled optical observation of the thermal process. Besides explosives (TNT, RDX, picric acid, urea nitrate, and TATP) we have studied nonexplosive materials, organic and inorganic, in order to see whether the explosives have a unique response. The materials we studied were oxygen-poor and -rich organic compounds (polyethylene and sugars, respectively), sea sand, and iron flakes. Clear, well-resolved exotherms were obtained at moderated temperatures (∼ 250°C) for all types of explosive materials tested by us. In addition, all explosives exhibited a melting endotherm preceding the exotherm. Sea sand and iron showed no peaks at the heating temperature range. Polyethylene showed an endotherm representing its melting. The sugars showed an endotherm but also an exotherm when heated to elevated temperatures (> 370°C). The thermogram of each material depends on its properties and is characterized by a unique pattern. This pattern may enable the detection and identification of explosive particles using this technology.

Temperature effect and chemical response of surface acoustic wave (SAW) single-delay-line chemosensors

Abstract The effects of temperature and chemical vapour absorption on the frequency shift of coated and uncoated SAW single-delay-line oscillators are reported. It is found that the temperature-induced frequency-shift function depends on the nature of the coating of the delay-line surface. It changes from the flat parabolic temperature dependence of an uncoated ST-quartz delay line near room temperature, to a steep, almost linear, curve for polymer-coated delay lines in the same temperature range. The slope of the curves depends on the nature of the polymers. The dual-delay-line concept, where an uncoated SAW delay line is used as a reference element to compensate for the ambient temperature variations, is therefore not universal and should be implemented with great caution. The response of an array of four different SAW delay-line sensors to various chemical vapours is represented by pie charts, visualizing chemical pattern variability that can be used for pattern recognition. The frequency shifts are correlated with calculated theoretical partition-coefficient values, based on thermodynamic parameters and regular solution theory. It is shown that this approach may be used to predict the interaction between the polymer and a specific target chemical vapour. A simple method for a preliminary design of a SAW chemosensor is thus suggested.

Flow cell for fluorescence studies of solid particle surfaces

Abstract A flow cell designed for studying the fluorescence properties of molecules adsorbed or chemically bound to solid particle supports is described. Gases or liquids are directed through a chamber containing a sample and the exciting light and emitted fluorescence signals are guided to the sample surface and then to a photomultiplier by means of a fibre-optic bundle. The applicability of the flow cell is demonstrated by fluorescence measurements on some fluorophores adsorbed or chemically bound to solid particle supports.

Pressure and gas composition effects on the operation of the pulsed flame photometric detector

The effect of pressure and hydrogen/oxygen ratio of a burning gas mixture on pulsed flame emission time-dependence was investigated in the range of 0.1–5 atm using a specially designed pulsed flame photometric detector (PFPD). We studied the pressure and gas composition effect on the pulsed flame delayed light emission of sulfur, phosphorus, and nitrogen-containing organic compounds. The optimal pressure conditions for nitrogen detection, intensity, and emission time delay was found to be 0.4 bar, at which the detection sensitivity could be improved by a factor of 2. For phosphorus, the optimal pressure obtained was 1.3 bar with 40% sensitivity improvement (compared with 1 bar). In the case of sulfur detection, two emission maxima were obtained, at 1.1 and 0.6 bar, at H/O ratio of 5. Increasing the H/O ratio resulted in the appearance of only one peak at 1 bar, and enhancement of the sensitivity by a factor of 2.4 at H/O ratio of 10.3. From the analytical point of view, we found that emission intensity is practically unchanged by the pressure and the H/O ratio for all three elements investigated in the range of 0.8–1.1 bar and H/O of 5–6. Thus, in addition to excellent sensitivity and improved selectivity, the PFPD can be applied under a variety of atmospheric pressure conditions in field environmental applications.