João A.B.P. Oliveira

Detection of volatile amines using a quartz crystal with gold electrodes

Abstract A quartz crystal with gold electrodes is used to detect and quantify volatile amines. The magnitude of the observed frequency decrease is highly dependent on the roughness of the surface of the gold electrodes and sensitivity can be increased by rubbing them with alumina. Water increases the observed signal, leading to important errors in the amine quantification. However, the presence of minor amounts of water is readily detected, as the plotted frequency signal shows a marked distortion. The effect of water on the interaction between amine and gold was studied by infrared spectroscopy.

Quantification of CO2, SO2, NH3, and H2S with a single coated piezoelectric quartz crystal

Abstract Previous experiments showed that a quartz crystal coated with tetramethylammonium fluoride tetrahydrate (TMAF) could be used to quantify CO2. It was also known that TMAF absorbed SO2 and H2S, although there were no studies involving quartz crystal microbalances (QCMs) with the latter gas. The remarkable stability of the frequency of the TMAF-coated crystals was reason enough to test them for a series of gases of environmental interest: CO2, SO2, NH3, H2S, and CO. Studies of sensitivity of TMAF to those gases are reported, along with a detailed discussion of the signal shape. Traps of specially prepared molecular sieves were used in an attempt to separate some of the analytes.

Comparison of two methods for coating piezoelectric crystals

Abstract Quartz piezoelectric crystals were coated with triethanolamine using two procedures: spray and syringe methods. The reproducibility of both coating methods was evaluated comparing the relative standard deviation of batches of four crystals coated by each of the procedures. The relative standard deviation obtained with the spray method is a hyperbolic function of the coating frequency and is, at least, three times lower than the one calculated from results of the syringe method, which is not frequency dependent. An explanation for the poorer reproducibility of the syringe method is given, based on the radial sensitivity theory and the non-uniformity of the coating distribution over the crystal, as evidenced by microscopic photographs.

Determination of Sulfur Dioxide in Wine Using a Quartz Crystal Microbalance

A new method for the analysis of both total and bound SO(2) in wine is proposed, based on a quartz crystal microbalance (QCM), and it is compared with the widely used Ripper method. The proposed method is faster than the Rippers, and the instrumentation is either home-made or widely available. When both methods are applied to the same sample, the results obtained using the QCM method are bracketed in an interval less than one-tenth the size of that obtained using the Ripper method. Although the SO(2) concentrations found using the QCM method correlate well with the ones obtained with the Ripper method, the results are systematically higher, which can be explained as due to the absence of interferences known to affect the Ripper method.

Trends in alkanes and PAHs in airborne particulate matter from Oporto and Vienna: identification and comparison.

A total of 56 weekly samples from Oporto and 40 from Vienna were collected and analysed, for 23 n-alkanes and 16 polycyclic aromatic hydrocarbons (PAH), by GC-MS after extraction with a toluene/methanol mixture. Total and elemental carbon were in the same range of values for both sampling sites. Although parts of the spectrum of species in both sampling sites were constant over the sampling period there is no evidence for suggesting a universal tracer for alkanes. For both Oporto and Vienna, Principal Component Analysis (PCA) identified two PCs and in both cases the first PC contained only PAHs while the second contained only alkanes. This separation between alkanes and PAHs and the observation of an alternating pattern with higher concentrations of odd carbon numbered from C27 to C30 (natural emissions from biological origin) is believed to result from a separation between anthropogenic and biological contributions associated with the first and second PC, respectively.

An electronic nose based on coated piezoelectric quartz crystals to certify ewes' cheese and to discriminate between cheese varieties.

An electronic nose based on coated piezoelectric quartz crystals was used to distinguish cheese made from ewes’ milk, and to distinguish cheese varieties. Two sensors coated with Nafion and Carbowax could certify half the ewes’ cheese samples, exclude 32 cheeses made from cow’s milk and to classify half of the ewes’ cheese samples as possibly authentic. Two other sensors, coated with polyvinylpyrrolidone and triethanolamine clearly distinguished between Flamengo, Brie, Gruyère and Mozzarella cheeses. Brie cheeses were further separated according to their origin, and Mozzarella grated cheese also appeared clearly separated from non-grated Mozzarella.

Performance of a tetramethylammonium fluoride tetrahydrate coated piezoelectric crystal for carbon dioxide detection

A coated quartz crystal microbalance is used for the determination of carbon dioxide. Coating of crystals with tetramethylammonium fluoride tetrahydrate is reported for the first time, and its behaviour, in terms of CO2 interaction at different temperatures, is discussed. This new coating gives greater stability to the calibration curve than N,N,N′ ,N′-tetrakis(2-hydroxyethyl)ethylenediamine but less sensitivity for carbon dioxide, especially at higher CO2 concentrations.

Detection of CO2 using a qaurtz crystal microbalance

Abstract Several amines have been evaluated with respect to stability, sensitivity and reversibility, regarding the analysis of carbon dioxide. The sensitivity of the amines has been related to their molecular structure and reactivity, and THEED was shown to be the best choice. However, 1,2-diaminoethane, 1,3-diaminopropane and 1,2-diaminopropane can be used, at least at 1 °C, with a smaller sensitivity and for shorter periods.

Anti-fungal activity of SiO2/Ag2S nanocomposites against Aspergillus niger.

Submicron particles of amorphous SiO(2) have been used to grow Ag(2)S nanophases at their surfaces. SEM and TEM analysis showed morphological well-defined nanocomposite particles consisting of Ag(2)S nanocrystals dispersed over the silica surfaces. These SiO(2)/Ag(2)S nanocomposites were investigated as anti-fungal agents against Aspergillus niger in different experimental conditions, including as nanofillers in cellulosic fibres. The anti-fungal activity in these composite systems is suggested to result from a synergistic effect due to Ag(2)S anti-fungal centres and the SiO(2) surfaces in promoting the adsorption of the fungus.

Suitability of PZT ceramics for mass sensors versus widespread used quartz crystals

Abstract Two piezoelectric materials, quartz and ceramic (PZT), were evaluated as mass sensors. Quartz crystals (polished and unpolished) and one ceramic (Nb–Mn–PZT) were used to detect and quantify fructose in aqueous solutions and s -butylamine in vapour. The frequencies at minimum impedance (fn), maximum impedance (fm), series resonance frequency (fs) and parallel resonance frequency (fp) were monitored. Sensitivities of the devices to the chemical compounds are compared and related to the observed changes of the electrical parameters of the equivalent circuit. Roughness of the surface of the discs (ceramic and quartz crystals) and electrodes were measured and their influence in the observed frequency responses discussed.