Dielectric spectrometer

In quadrupole-based SIMS instruments, mass separation is achieved by passing the secondary ions down a path surrounded by four rods excited with various AC and DC voltages. Different sets of AC and DC conditions are used to direct the flight path of the selected secondary ions into the detector. The primary advantage of this kind of spectrometer is the high speed at which they can switch from peak to peak and their ability to perform analysis of dielectric thin films and bulk insulators. The ability of the quadrupole to switch rapidly between mass peaks enables acquisition of depth profiles with more data points per depth, which improves depth resolution. Additionally, most quadrupole-based SIMS instruments are equipped with enhanced vacuum systems, reducing the detrimental contribution of residual atmospheric species to the mass spectrum. 

The choice of mass spectrometer for a particular analysis depends on the namre of the sample and the desired results. For low detection limits, high mass resolution, or stigmatic imaging, a magnetic sector-based instrument should be used. The analysis of dielectric materials (in many cases) or a need for ultrahigh depth resolution requires the use of a quadrupole instrument. 

Setting the block of frequency parameters can sometimes be omitted for subsequent similar samples if the spectrometer is sufficiently stable. Similar samples means that the sample tubes come from a single batch of quartz tubing (i.e., they have identical inner and outer diameter within a small tolerance) and the samples are physically similar (e.g., frozen buffered protein solutions). Such samples are invariant as dielectric, and they all will cause the same shift in resonator frequency within circa 1 MHz. 

We have seen in Chapter 2 that the frequency of an EPR spectrum is not a choice for the operator (once the spectrometer has been built or bought) as it is determined by the combined fixed dimensions of the resonator, the dewar cooling system, and the sample. Even if standardized sample tubes are used and all the samples have the same dielectric constant (e.g., frozen dilute aqueous solutions of metalloproteins), the frequency will still slightly vary over time over a series of consecutive measurements, due to thermal instabilities of the setup. By consequence, two spectra generally do not have the same frequency value, which means that we have to renormalize before we can compare them. This also applies to difference spectra and to spectra... 

The most commonly used LC/MS interfaces in pharmaceutical analysis are ESI and APCI. An ESI interface on the majority of commercial mass spectrometers utilizes both heat and nebulization to achieve conditions in favor of solvent evaporation over analyte decomposition. While ionization in APCI occurs in the gas phase, ionization using ESI occurs in solution. Attributes of a mobile phase such as surface tension, conductivity, viscosity, dielectric constant, flow rate and pFi, all determine the ionization efficiency. They therefore need to be taken into consideration and controlled. 

In the crossed-coil spectrometer two flux paddles (I) are frequently used to adjust the phase of the flux intersecting the receiver coil relative to the phase of H. These paddles are metallic and dielectric disks or rings which modify the direction and phase of the rf transmitter voltage near the receiver coil. Lumped parameter circuits are often used 68) to accomplish the same end. 

For our purpose, it is convenient to classify the measurements according to the format of the data produced. Sensors provide scalar valued quantities of the bulk fluid i. e. density p(t), refractive index n(t), viscosity dielectric constant e(t) and speed of sound Vj(t). Spectrometers provide vector valued quantities of the bulk fluid. Good examples include absorption spectra A t) associated with (1) far-, mid- and near-infrared FIR, MIR, NIR, (2) ultraviolet and visible UV-VIS, (3) nuclear magnetic resonance NMR, (4) electron paramagnetic resonance EPR, (5) vibrational circular dichroism VCD and (6) electronic circular dichroism ECD. Vector valued quantities are also obtained from fluorescence I t) and the Raman effect /(t). Some spectrometers produce matrix valued quantities M(t) of the bulk fluid. Here 2D-NMR spectra, 2D-EPR and 2D-flourescence spectra are noteworthy. A schematic representation of a very general experimental configuration is shown in Figure 4.1 where r is the recycle time for the system. 

Finally in this section, we refer to classic studies on gas phase interactions carried out with a pulsed electron beam high ion source mass spectrometer, which have yielded details of hydrogen bonding of substituted pyridinium ions to water in the gas phase (79JA1675). These measurements afford thermodynamic data for the stepwise hydration of pyridinium ions XC6H4NH(OH2)n for values of n varying between 0 and 4. The attenuation of substituent effects is much less than for aqueous solution, because although the water molecules cluster round NH in the gas phase, they cannot provide an overall solvation network, the dielectric constant of which in the liquid phase serves to reduce the influence of the substituent dipole. 

A laser-induced ToF mass spectrometer (LIMA-2A) was manufactured by Cambridge Mass Spectrometry Ltd., Cambridge, UK, for micro local analysis and was used to analyze thin sections of biological samples in the transmission mode or bulk material in the reflection mode.150,151 Typical LIMA applications in microelectronics include identification of impurities in dielectrics, microlocal analysis, depth profiling, thick film analysis and investigations on hybrid circuits. 

A second use of microwave spectroscopy is the measurement of dipole moments. These are obtained by measuring the frequency shifts of lines in the applied electric field of a Stark-modulated spectrometer. This method of dipole-moment determination is superior to the older method of measuring dielectric constants. For example, impurities in the sample will not affect the dipole moment as measured by microwave spectroscopy. The dipole moment of a substance present to the extent of a few percent can be accurately measured if its microwave spectral lines can be assigned. The components of d can be determined, thus giving its orientation in the molecule, in addition to its magnitude. 

An ESR spectrometer (Varian model E-3) was used to observe and quantify Mn2+ species at a field strength of 3155 50 G and a frequency of 9.5 GHz. A flat fused silica ribbon cell (Wilmad Glass No. WG-812) was used at very low concentrations to optimize the signal-to-noise ratio by minimizing dielectric losses. Microwave power was set routinely to 4 mW, but was occasionally raised to optimize sensitivity at very low concentrations. Quantitation was based on the height of the lowest-field peak in the first derivative of the absorption spectrum. As reported by others (63), this technique is characterized by precision and accuracy of about 1% relative standard deviation over a linear range from CIO"6 to If)"4 M (<0.05-5 mg/L). 

One of the fluids was pure Mazola corn oil and the other was the same oil colored with oil-based Teal dye and doped with oil-miscible antistatic Stadis 450 to increase the conductivity and permittivity [91]. The latter values were measured with a broadband dielectric spectrometer in a spatially uniform low electric field for frequencies of 0.5-1 kHz. 

A nonlinear dielectric spectrometer has been designed around a standard IBM PC and realised almost completely in software, with a minimum of extraneous hardware [129]. 

Fig. 7. Dielectric spectrometer schematic Two standard four-terminal electrode chambers are connected to A/D converters and on into a PC. Fourier analysis is done by the PC to produce the nonlinear dielectric spectra...

Mopsik, F. I. Precision time-domain dielectric spectrometer, Rev. Sci. Instr. 55, 79 (1984)... 

Figure 14. Imaginary part of the dielectric permittivity e,(co) of (a) fluoroaniline (7 — 173 K) and (b) toluene (Tg = 117 K), both type B glass formers showing in addition to the main relaxation (a-process) a secondary relaxation peak (p-process) numbers indicate temperature in K. Unfilled symbols represent data obtained from a broad-band spectrometer [6,153]. Filled symbols represent data from a high-precision bridge [137] interpolations for fluoroaniline (solid lines) were done by applying the GGE distribution (a-process) and a Gaussian distribution (p-process) of relaxation times [142], and these for toluene (dashed lines) were done by the gamma distribution (a-process) and a Gaussian distribution (p-process) [6] (cf. Section IV.C.2).

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