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]. 

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... 

Figure 15.11 Analysis of methanol and water in tetrahydrofuran [reproduced with permission from R.K. Bade, L.V. Benningfield, R.A. Mowery and E.N. Fuller, Int. Lab., 11(8), 40 (1981)]. Conditions sample, 40pl of THF with 1% each of methanol and water column, 25cm x 7.7 mm i.d. stationary phase, OR-PVA-500 mobile phase, Imlmin THF temperature, 5O C dielectric constant detector.

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. 

It was not until 1979 that the first dielectric constant detector for LC became commerically available (19). Beninngfield and Mowrey (20) described this detector in detail including several applications and Bade et al. (21) also reported a number of applications for this particular device. A diagram of the detecting cell is shown in Figure 12. 

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.

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