Microwave Resonance Spectroscopy Research Articles

Non-contact monitoring of the freeze-drying process of microparticles using microwave resonance spectroscopy

Spray freeze-drying has emerged as a promising alternative to conventional vial freeze-drying. In this study, we have developed a non-contact monitoring technique for the freeze-drying of microparticles utilizing microwave resonance spectroscopy. Spectral changes, indicative of the degree of drying, were successfully captured during the freeze-drying process of particulate samples ranging in size from approximately 0.6–3.5 mm. The observed spectral patterns demonstrated significant dependence on both the mean particle size and the formulation type (mannitol or sucrose). The partial least squares method was employed to extract data series strongly correlated with the drying progress. Multiple regression analysis was then utilized to derive regression equations, yielding values representing the drying progress based on the intensity values at selected frequencies within the spectra. The resulting regression equations accurately replicated the experimentally estimated drying kinetics. Notably, a robust regression equation was obtained, demonstrating applicability to various formulations and particle sizes, with coefficient of determination values ranging from 0.95 to 0.99. Furthermore, it was suggested that a correlation between the obtained spectra and the change in moisture content during secondary drying. Microwave resonance spectroscopy proves to be a versatile technique for monitoring freeze-drying processes, offering insights that can enhance the efficiency and adaptability of this critical pharmaceutical manufacturing step.

Development of a Nondestructive Monitoring Technique for Vial Freeze-Drying Process using Microwave Resonance Spectroscopy

In this study, we developed a nondestructive monitoring technique for the drying progress of products in vials during the freeze-drying process using microwave resonance spectroscopy. An aqueous mannitol solution, commonly used as an excipient for injectable drugs, was used as the model material. The drying progress was determined using a mathematical model (i.e., Rp−Kv model), in which the parameters of mass and heat transfer were experimentally determined while considering the variation of the drying progress and position of the product on the drying shelf. A strong correlation was found between the extent of drying and changes in the microwave resonance spectrum. The simulation results were regressed against the changes in the resonance peak frequency and resonance intensity. Using this regression equation, the extent of drying could be predicted from the acquired spectral data. In this way, we were able to estimate the design space (the relationship between the operating conditions, internal temperature of the product, and sublimation rate), which changed with the extent of drying from the spectrum obtained inline, showing that it is possible to set the appropriate operating conditions based on the spectra.

Non-destructive monitoring of food freezing process by microwave resonance spectroscopy with an open-ended coaxial resonator

Abstract In this study, microwave resonance spectroscopy was applied to monitor the freezing process of food product using an open-ended coaxial resonator. The resonator was designed to detect a signal corresponding to the dielectric property of a product placed in space over a probe. Frozen sucrose solutions with various concentrations and temperatures were employed, and the spectra patterns obtained and their variations upon freezing were investigated. The peak intensities of the spectra were significantly dependent on the concentrations and temperatures of the sucrose solutions, whereas the intensities obtained from pure water (i.e., ice) did not significantly depend on temperature. The dependence of the peak intensities on temperature was found to comprehensively represent the temperature of the core part of the products. The proposed monitoring technique can be employed to detect the status of frozen products in terms of temperature, as demonstrated in the freezing process of a sucrose-agar solution.

Monitoring of primary drying in the freeze-drying process using an open-ended coaxial microwave resonator

An open-ended coaxial microwave resonator was designed and applied for monitoring the primary drying stage in the freeze-drying process by microwave resonance spectroscopy. A stainless steel cylindrical resonator with an inner cavity and cylindrical core was designed to sense the electrical permittivity and permeability (i.e., dielectric property) in the space over the resonator. The obtained spectrum patterns and peak intensities in the frequency range of 2500–3000 MHz responded sensitively to different volumes of ice over the probe. Monitoring of freeze-drying of a dextrin solution and sliced apple was successfully achieved by tracking the peak shift and/or the intensity of selected peak frequencies. The end-point of drying estimated from the microwave resonator perfectly corresponded to the point estimated from the product temperature during drying. This sensing device can potentially be applied for in-line monitoring of the freeze-drying process without contact to product, especially in a freeze-drying process.

Field driven recovery of the collective spin dynamics of the chiral soliton lattice

We investigate the magnetic field dependence of the spin excitation spectra of the chiral soliton lattice (CSL) in the helimagnet CrNb3S6, by means of microwave resonance spectroscopy. The CSL is a prototype of a noncollinear spin system that forms periodically over a macroscopic length scale. Following the field initialization of the CSL, we found three collective resonance modes over an exceptionally wide frequency range. Upon further reducing the magnetic field toward 0 T, the spectral weight of these collective modes was disrupted by the emergence of additional resonances whose Kittel-like field dependence was linked to coexisting field polarized magnetic domains. The collective behavior at a macroscopic level was only recovered upon reaching the helical magnetic state at 0 T. The magnetic history of this noncollinear spin system can be utilized to control microwave absorption, with potential use in magnon-driven devices.

Generalized Subset Designs in Analytical Chemistry.

Design of experiments (DOE) is an established methodology in research, development, manufacturing, and production for screening, optimization, and robustness testing. Two-level fractional factorial designs remain the preferred approach due to high information content while keeping the number of experiments low. These types of designs, however, have never been extended to a generalized multilevel reduced design type that would be capable to include both qualitative and quantitative factors. In this Article we describe a novel generalized fractional factorial design. In addition, it also provides complementary and balanced subdesigns analogous to a fold-over in two-level reduced factorial designs. We demonstrate how this design type can be applied with good results in three different applications in analytical chemistry including (a) multivariate calibration using microwave resonance spectroscopy for the determination of water in tablets, (b) stability study in drug product development, and (c) representative sample selection in clinical studies. This demonstrates the potential of generalized fractional factorial designs to be applied in many other areas of analytical chemistry where representative, balanced, and complementary subsets are required, especially when a combination of quantitative and qualitative factors at multiple levels exists.

Pyknometric volume measurement of a quasispherical resonator

We have measured the internal volume of a 1 litre, diamond-turned copper quasispherical resonator with a fractional uncertainty of approximately 1 part in 106 using two independent techniques. This is in response to the need for a uniquely accurate measurement of resonator volume, for the purpose of measuring the Boltzmann constant in pursuit of the redefinition of the kelvin. The first technique is a pyknometric measurement using water as a liquid of known density. We describe the development of a procedure that results in stable, reproducible volume measurements. We provide a detailed discussion of the factors that affect the water density, such as dissolved gases. The second technique is microwave resonance spectroscopy. Here, we measure the resonant frequencies of the TM1n modes and relate them to the dimensions of the resonator. We evaluate the frequency perturbations that arise from the coupling waveguides and the electrical resistivity of the copper surface. The results of the microwave measurements show evidence of a dielectric coating on the surface. We propose that this is an oxide layer and estimate its thickness from the microwave data. Finally, we compare the volume estimates from the two methods, and find that the difference is within the combined uncertainty.

Plasmon-cyclotron resonance in two-dimensional electron gas confined at theGaN∕AlxGa1−xNinterface

Edge magnetoplasma modes are studied in the two-dimensional electron gas confined at the $\mathrm{Ga}\mathrm{N}∕{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ interface using standard microwave resonance spectroscopy. The position and shape of the resonance line are described by the theory for the dimension-dependent plasmon-cyclotron coupling and the Drude model of momentum relaxation. The analysis of the resonance line shape provides a contactless method for the determination of the sheet electron concentration and the mobility of the two-dimensional electron gas. In addition, we observe Shubnikov--de Haas oscillations using the same microwave resonance spectrometry. We compare values for the cyclotron and quantum mobilities obtained from the plasmon-cyclotron linewidth and the magnetic field dependence of the Shubnikov--de Haas signal, respectively.

Optically Detected Microwave Resonance and Carrier Dynamics in InAs/GaAs Quantum Dots

The properties of small-sized high density InAs/GaAs quantum dots (emitting at 1.25 eV) are studied by means of optically detected microwave resonance spectroscopy and time resolved photoluminescence techniques. The results are discussed in terms of trapping and thermal escape of the carriers as well as their relaxation and recombination in quantum dots. The data are compared with those recently obtained on shallowly formed InAs quantum dot structures.

Observation of cyclotron resonance in an InAs/GaAs wetting layer with shallowly formed quantum dots

A thin InAs/GaAs wetting layer with shallowly formed InAs quantum dots (QD's) is investigated by means of optically detected microwave resonance spectroscopy. The absorption of W-band (95 GHz) microwaves is observed via the detection of changes in the total photoluminescence intensity of the InAs QD's. A strong and anisotropic signal at low fields is ascribed to cyclotron resonance of the electron in the two-dimensional wetting layer, corresponding to an effective mass of ${0.053m}_{0}.$ Further microwave-induced signals at higher fields are tentatively attributed to magnetic resonance transitions between spin states of the holes confined in the shallow dots.

Negative ions and particle formation in low-pressure halocarbon discharges

The growth of particles in a radiofrequency (RF) (13.56 MHz) plasma at pressures from 25 to 200 mTorr in mixtures of CF4, CF2Cl2 and argon has been studied experimentally. A planar configuration was used, with a silicon wafer on the powered electrode. The electron density has been measured with microwave resonance spectroscopy using a cylindrical cavity surrounding the plasma. The same geometry has been used to measure the density of various species of negative ions by detecting the extra electrons created by laser-induced photo detachment. It appears that the negative ion density is much larger in the case of CF2Cl2 than in the case of CF4. There seems to be hardly any dependence of the negative ion concentration on the CF2Cl2/Ar partial pressure ratio in the range where powder growth occurs. However, the attachment rate to chlorine is found to be much higher than to fluorine. Furthermore the gas phase discharge chemistry has been studied using infrared absorption spectroscopy. Both a tunable diode laser system, and a Fourier transform spectrometer have been applied. The CF2 concentration appears to decrease strongly when powder growth occurs. The SiF4 concentration then has a maximum. The results indicate that the presence of chlorine in the plasma feed gas is essential. In CF4 no particle formation is detected. The wafer surface is blackened during powder formation. SEM inspection indicates that this is caused by micromasking. Considering all the information, we arrive at the conclusion that particle growth is initiated by micromasking at the Si surface combined with a highly directional etching process. Due to residual isotropic etching the particles are released from the surface and enter the plasma, where they start coalescing and growing under the influence of CF2 polymerization.

On the structure of β-Ga 2O 3

The deviation from the symmetry 2 m to 1 of β-Ga 2O 3 as reported by Wolten and Chase must be very small. It is apparently not measurable by X-ray intensity differences, by EPR of both Fe 3+-and Cr 3+-doped β-Ga 2O 3, by Mössbauer effect spectroscopy of Fe 3+-substituted β-Ga 2O 3, or by optical and microwave resonance spectroscopy. Second harmonic generation, as measured by the Kurtz-Perry technique, and pyroelectricity are absent. It is pointed out also that single crystals of high-pressure phase Ga 2− x In x O 3 have been reported to be isostructural with the monoclinic β-Ga 2O 3.