The Dielectric Constant Detector

For semipolar substances or mixtures of semipolar substances and non-polar substances the above equation has to be modified to the following form 

For polar substances or mixtures of polar substances and semi polar substances, however, the relationship breaks down and there is no simple function that describes refractive index in terms of 

In normal chromatography the mobile phase is usually Jess polar than the solutes being eluted as they need to be retained on the column to achieve a separation. Thus, the presence of a solute in the mobile phase will, in general, increase the dielectric constant of the mobile phase. In reversed-phase chromatography the converse often applies. The solute is usually less polar than the mobile phase and, consequently, the dielectric constant of the mobile phase is decreased by the presence of a 

In practice the sensory element often takes the form of a cylindrical or parallel plate condenser. To maintain column efficiency, the volume of the condenser has to be very small and, as the sensitivity of the device is directly related to capacity of the condenser, the plates have to be very close together. 

A is the eurea of each plate in cm2, d is the distance between the plates in cm. 

---

Bulk property detectors such as the refractive index detector or the dielectric constant detector are often particularly pressure sensitive and for that reason wide diameter exit tubes are strongly recommended, however, such detectors are not useful for multidimensional analyses. It follows that the pressure sensitivity of the detector should be specified by the manufacturer. The pressure response Dp should be given as the output in mV for unit pressure change in ihe detector cell. The pressure response should be given in both mV/psi and mV/kg/m2, It is also recommended that the pressure noise is given in terms of that pressure change which would provide a signal equivalent to the noise, i.e. 

One of the first on-line liquid chromatography detectors to be developed in the early forties was, in fact, a bulk property detector, the refractive index detector (1). Bulk property detectors continuously monitor some physical property of the column eluent and by the use of a suitable transducer provide a voltage - time output that is either proportional to the physical property being measured, or made proportional to the concentrations of the solute eluted. The properties of the mobile pheuse that are most commonly monitored in commercially available bulk property detectors are refractive index, electrical conductivity, and dielectric constant, the dielectric constant detector being the least popular of the three. 

The dielectric constant detector is a differential-type detector with both a reference and a sensor cell. Figure 12 is a cut-away diagram of the detector cell. Each cell consists of a concentric cylinder (inner electrode) inside a larger diameter cylinder (outer electrode) which forms the outer wall of the cell. The two cylinders are electrically isolated with a cylindrical flow path. In operation, the mobile phase flows through the wall of the outer cylinder, divides around the small inner cylinder, and exits 180 degrees from the entrance port. The inner cylindrical electrodes are 1.26 cm in diameter and 0.625 cm in length. The outer cylinder of each cell has a dual diameter which provides each cell with two different spacings ("d" spaces)... 

Figure 13. The separation of triglycerides employing the dielectric constant detector.

Different approaches utilizing multidimensional EC or SEC systems have been reported for the analysis of middle distillates in diesel fuel. A method, based on the EC separation of paraffins and naphthenes by means of a micro-particulate, organic gel column has been described (23, 24). The complete system contained up to four different EC columns, a number of column-switching valves and a dielectric constant detector. However, the EC column for the separation of paraffins and naphthenes, which is an essential part of the system, is no longer commercially available. 

Nonspectroscopic detection schemes are generally based on ionisation (e.g. FID, PID, ECD, MS) or thermal, chemical and (electro)chemical effects (e.g. CL, FPD, ECD, coulometry, colorimetry). Thermal detectors generally exhibit a poor selectivity. Electrochemical detectors are based on the principles of capacitance (dielectric constant detector), resistance (conductivity detector), voltage (potentiometric detector) and current (coulometric, polarographic and amperometric detectors) [35]. 

Johansson (46) designed a microwave circuit that accurately measured the dielectric constant of pure gases as well as detected components in a mixture. The detector lacked sensitivity but had the advantage of good thermal stability. 

It follows that as the capacity (C) is directly proportional to the dielectric constant of the material between its plates, then the electrical impedance of the cell will vary inversely as the dielectric constant. The most appropriate circuit to use in dielectric constant measurement is an appropriate electrical "bridge", the detector cell being situated in one arm of the bridge. If the cell can be designed to have a capacity greater than 100 pF, then a Wein bridge can be used however such a... 

One of the early dielectric constant detectors was that designed by Grant [10] but the detector cell had a volume of 2-3 ml. Poppe and Kunysten (11) described a dielectric constant detector which included a reference cell for temperature compensation. The cell consisted of two stainless steel plates 2 cm x i cm X 1 mm separated by a gasket 50 pm thick. The two cells were identical and clamped back to back, sharing a common electrode. 

The Separation of Some Triglycerides Monitored by a Dielectric Constant Detector... 

In capillary electrophoresis (CE), several criteria can be applied to classify solvents [e.g., for practical purposes based on the solution ability for analytes, on ultraviolet (UV) absorbance (for suitability to the UV detector), toxicity, etc.]. Another parameter could be the viscosity of the solvent, a property that influences the mobilities of analytes and that of the electro-osmotic flow (EOF) and restricts handling of the background electrolyte (BGE). For more fundamental reasons, the dielectric constant (the relative permittivity) is a well-recognized parameter for classification. It was initially considered to interpret the change of ionization constants of acids and bases according to Born s approach. This approach has lost importance in this respect because it is based on too simple assumptions limited to electrostatic interactions. Indeed, a more appropriate concept reflects solvation effects, the ability for H-bonding, or the acido-base property of the solvent. 

The early applications of the piezoelectric crystal detectors were limited to the measurement in the gas phase, because of the common impression that stable oscillation cannot be obtained in the liquid phase. However, recent advances in PZ research have shown that quartz crystals can oscillate in contact with solution, and several studies have been reported addressing the theoretical aspects of the oscillating frequency of piezoelectric crystals in solution. Nomura and Okuhara (92) demonstrated that the frequency change of a crystal immersed in an organic solvent depends on the density and viscosity of the solvent, and was not affected by the dielectric constant ... 

Supercritical water, on the other hand, with a critical temperature of 374 °C and a critical pressure of 227 bar, cannot be used for chromatography above the critical point, since almost nothing can withstand this temperature. However, since the dielectric constant of water decreases with increasing temperature, subcritical water has been attempted as mobile phase, at temperatures of 100-200°C, with the flame ionization detector (FID), but only with limited success. 

It follows that as the capacity C is directly proportional to the dielectric constant between the electrodes of the cell, then the electrical impedance of the cell will also vary inversely with the dielectric constant. Furthermore, the change in impedance with dielectric constant will increase with the frequency of the applied potential. Consequently, if the cell impedance is measured by a suitable detecting system, the sensitivity of the detector will be improved by operating at a higher frequency. 

---