Maria Teresa S. R. Gomes

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.

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.

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.

A New Method for Monitoring the Contamination of Glycerol with Diethylene Glycol

A quartz crystal microbalance has been used to check the purity of glycerol.

Critical assessment of the parameters that affect the selection of coating compounds for piezoelectric quartz crystal microbalances

The selection of the compound to be used as a coating for a piezoelectric quartz crystal is of utmost importance in the development of a chemical sensor. The relevant parameters to be evaluated (stability, sensitivity, reversibility, response time, reproducibility, and selectivity), and the main variables affecting the results and influencing the choice of coatings are discussed and illustrated with experiments performed during the evaluation of coatings to detect carbon dioxide.

Analytical advantages of monitoring a particular characteristic frequency in a thickness shear mode acoustic wave sensor

Abstract The oscillator or active method, where just the series resonance frequency ( f s ) can be measured, has been the method of choice for analytical applications of bulk acoustic wave sensors. However, the present work shows that an increase in the analytical signal can be obtained if the frequency at minimum impedance ( f n ) is measured. An example is given, where quantitative analysis is only possible if f n is measured instead of f s . Sensitivity of a particular analysis can be improved if a previous study is performed in order to select the most suitable characteristic frequency ( f s , f n or f p ) to be monitored.

A gas chromatography-quartz crystal microbalance for speciation of sulfur compounds in landfill gas

A new method, based on separation with gas chromatography and detection with a quartz crystal microbalance, was used for the quantification of ethane-1-thiol, propane-1-thiol, ethyl methyl sulfide, propane-2-thiol, butane-2-thiol and butane-1-thiol. The analytical error, the analysis time and the general analytical performance are identical for gas chromatography-mass spectrometry (GC-MS) and gas chromatography-quartz crystal microbalance (GC-QCM). However, the GC-QCM method constitutes a low cost alternative to GC-MS for monitoring individual sulfur compounds in landfill gas. It is an easily assembled apparatus made with an injection device, a furnace, a column, a crystal cell and a quartz crystal.

Iron migration from undamaged and dented juice tinplate cans

BACKGROUND Iron migration from tinplate cans to pineapple drink was studied over time using flame atomic absorption spectroscopy, taking into consideration storage temperature, sell-by date and can condition (dented/undamaged). An organoleptic test, at the sell-by date, was also performed. RESULT Analysis of iron in drinks from tinplate cans, glass and polyethylene terephthalate (PET) bottles was performed up until the sell-by date. For undamaged canned drinks stored at room temperature, iron was found to remain constant at 0.23 ± 0.01 mg L(-1) , from the 24th day until 1 year after production. Iron found in glass and PET bottles also remained constant until the sell-by date, at 0.15 ± 0.04 and 0.12 ± 0.04 mg L(-1) , respectively. However, migration of iron from dented cans was found to be significant. Twelve months after production, iron content in dented cans, stored at room temperature (22 °C) and in a refrigerator chamber (4 °C) was 14.4 ± 1.0 and 5.5 ± 0.4 mg L(-1) , respectively. CONCLUSION Results showed that for a 35 kg child the intake of three damaged canned drinks may contribute to more than 50% of the iron provisional maximum tolerance daily intake.

Starch viscoelastic properties studied with an acoustic wave sensor.

Gelatinization and retrogradation of starch was followed in real time with an acoustic wave sensor. This study relies on the monitorization of the frequency of oscillation of a piezoelectric quartz crystal in contact with a 2.5% emulsion of a commercial maize starch, during heating and cooling. The technique showed to be very powerful and sensitive to most of the changes described in the literature, which have been elucidated by some other techniques. The value for the temperature of gelatinization found using the sensor was confirmed by the analysis of the same starch emulsion by polarized light microscopy. Temperatures of gelatinization were found to vary with the sample heating rate, as follows: 73.5 °C at 2.0 °C/min, 66.0 °C at 1.0 °C/min, and 65.0 °C at 0.5 °C/min. Hysteresis of the studied system was evidenced by the frequency shift before heating and after cooling till the initial temperature. Analysis performed on a 1.5% emulsion of a rice starch heated at 2.0 °C/min and cooled as before, evidenced no hysteresis and showed complete reversibility, in which concerns to the series frequency of the piezoelectric quartz crystal.