Quartz balances

The use of resonating quartz crystals for mass-change determinations is very attractive because a sensitivity of about 10 12 g/cmz is feasible. Mass determinations with resonating quartz crystals have been discussed by Plant (48) and Van Empel et al. (49). The principle (49) of the determination is based on the relationship between the variation, A/ of the resonant frequency, f and the mass added, Am, or 

Another approach is to use a thin quartz fiber, 5-20 fj. in diameter and about l cm long, covered with a thin coating of evaporated gold (48). The fiber is held rigidly at one end and rests between two parallel metal plates to which a dc potential is applied. When an audio frequency signal is applied to the fiber, it displays vibrating reed resonances whose frequency depends on the physical parameters of the fiber. If mass is added to the free end of the fiber, the resonant frequencies shift downward, being dependent on the amount of mass. 

Other types of balances using quartz crystals have been described also (48). 

The first application of a quartz crystal for use as a nonisothermal thermobalance was reported by Henderson ei al. (66). This was accomplished through the use of a minicomputer to characterize the temperature-frequency relationship for the crystal and to correct numerically the frequency-temperature-mass relationship to obtain the TG curve of the sample. One advantage of such a system is that very fast heating rates may be employed due to small sample size (t-40 jug typical) and high sensitivity. Thin films of sample exhibit rapid gas diffusion and thus permit thermal equilibrium to be maintained at heating rates of l00°C/min or greater. Applications of this thermobalance include the evaluation of thin films and coatings as well as various polymer studies of pyrolysis, flammability, and so on. 

---

The quartz balance is a tool for detecting the increase (or decrease) of the film mass deposited onto the surface of a quartz resonator, connected to the driving circuit, and registering the shift in a frequency. The dependence is expressed by the Sauerbray equation (Sauerbray 1964) ... 

The principle of operation of a quartz balance can be easily described if an AC potential is placed across a quartz piezoelectric crystal, the crystal will oscillate spatially and the amplitude of this oscillation is greatest at the resonance frequency of the crystal. This resonance frequency, /0, is a function of several parameters, including the mass of the crystal, and the mass of foreign material placed on the crystal. In fact, the change in the resonance frequency, A/0, on placing some foreign material on it is given approximately... 

The quartz balance uses a thin quartz crystal, a few hundred /xm thick, with thin, vapor-deposited gold films on the two sides. Such a crystal has a fundamental mode for shear waves with a frequency in the 1-15 MHz region, which can be excited by application of a corresponding alternating voltage on the two electrodes. The resonance frequency is very sensitive to small mass changes of the system. One... 

Among the large variety of in situ experiments that have been described one can distinguish (1) those whose purpose is investigation of the electrochemical doping process itself cyclic voltammetry, quartz balance [17], mirage effect [18], and ellipsometry [19], and (2) those developed for studies of the properties of the CP UV-near-IR spectroscopy [20], IR [21], ESR [22], conductivity [23], impedence [24], and so on. 

Bis(3,4-dicarboxybenzoyl)phthalocyanines with M = Co(II) or Cu(II) have been covalently bound to linear polystyrene by a Friedel-Crafts reaction [130], The polymers contained 0.13 mmol g (12.4 wt.%) phthalocyanine moieties. The sensitivity of the polymers toward toxic gases were investigated by quartz balance transducers. The sensitivities are 6T0" m mL for NO2 and 2T0 m mL for chloroform and perchloroethylene, respectively. 52 (R = SO2CI, M = Fe) has been covalently bound to the amino groups of a cross-linked, hydrophilic polymer consisting of A/ -acryloyl-P-alanine-(aminoethyl-ene)-amide, A -acryloylpyrrolidine and 7V,A -bis(methacryloyl)-l,2-diamino-ethane as comonomers [131]. The polymer obtained, 52, contains 20 pmol phthalocyanine units per g. The catalytic oxidation of 2,4,6-trichlorophenol or 3,5-di-tert-butylcatechol in the presence of H2O2 or KHSO5 as oxidant has been studied. Around 80% of the trichlorophenol was catalytically oxidized within 2 h at pH 7. 

The preparation chamber, number I in Figs. 1.2 and 1.3, is equipped with two electron beam evaporators and an effusion cell. A four-pocket mini electron beam evaporator serves for deposition of metals from rods and crucibles from each pocket separately, as well as for codeposition of different combinations of the evaporants. A single-pocket e-beam source is used for deposition of the Mossbauer isotope Fe. An effusion cell Is available for evaporation of rare-earth metals. A precise calibration of the deposition rate with a thickness reproducibility of I A is done by a quartz-balance monitor. The deposited structures can be characterized by low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). In this chamber, the samples can be cooled down to about 90 K and heated up to 2300 K by a multifunctional manipulator. 

Another possibility for avoiding the measurement of absolute vapor pressures is provided by sorption methods. In most cases, the polymer is positioned on a quartz balance and the amount of solvent it takes up via the vapor phase is weighted. The so-called flow-through variant [32] works with an open system in contrast to the previous method. 

R.F. Boyer I can answer that question. That work was done at Dow Chemical Company by Prof. Duda [J.L. Duda and J.S. Vrentas, J. Polym. ScL, Part A-2,6, 675-685 (1968)] when he was still at Dow. He simply used a quartz balance and followed the weight pick up of polystyrene from the isopentane and analyzed the data, getting both the solubility and the diffusion rates as a function of temperature. 

---