FEF sensors measure the material of interest from a single side. They are formed by a coplanar distribution of electrodes. Figure 2 shows the cross-sectional view of an FEF sensor. The spatial wavelength of the sensor, which is defined as the distance between centers of electrodes with the same polarity, can be varied depending on the excitation of the electrodes. The penetration depth of the sensor is mainly dependant on the spatial wavelength and is approximated as one-third the spatial wavelength. Conceptually, the penetration depth is an effective measure of the rate at which the electric field decays away from the sensor surface. Variations in penetration depth also can be obtained from the various electrode excitation patterns to provide the sensor with access to different layers of the test specimen. In Figure 2, the parameters l ₁, l ₂, and l ₃ represent three different ways of connecting the electrodes. D represents the driving electrodes; an AC voltage signal is applied to these. S represents the sensing electrodes. At these electrodes, current–voltage measurements are made. G represents the guard electrodes. In the table below the figure, the cross-sectional view of the sensor in Figure 2, each row corresponds to one of the three connection schemes; each column corresponds to the types of connections for the electrode directly above the column used in the different connection schemes. For l ₁, every other electrode is driven and the undriven electrodes are treated as sense electrodes. For l ₂ and l ₃ several undriven fingers are chosen as guard electrodes and only the current–voltage at the sensing electrodes is measured. The guard electrodes are either connected to ground or kept at the same voltage potential as the sensing electrodes by using unity-gain buffer amplifiers. The spatial wavelength of the sensor is increased (λ₃ > λ₂ > λ₁) by using various connection schemes. A ground plane below the electrode surface eliminates the effects of underlying material and stray noise in the medium. Major advantages of this technology include one-sided access to materials under test, profiling and imaging capabilities, and model-based signal analysis.

Direct sensing or preconcentrators? There are many cases in which gas or liquid analytes must be sensed in small concentrations close to, or below, the detection threshold of a dielectric spectroscopy measurement device. Direct sensing is simpler than preconcentration; however, direct sensing is not always possible. Preconcentration should be performed in the following cases:

• When the analyte is at a concentration at or below the measurement threshold
• To select an analyte of interest from a mixture of gases or liquids.

The advantage of direct sensing is that it is does not require the modification of the measurement setup for a particular analyte. However, because the preconcentration process acts as an amplifier, the signal-to-noise ratio may better than the direct-sensing case.

Dielectric spectroscopy has been used with gaseous analytes as well. With gases, the preconcentrator material will typically form a compound with the gas and may require heating of the substrate to increase the analyte absorption. Metal compounds are used as preconcentrators for gaseous analytes (27, 28). For example, use of Ni-PC (tetra-t-butyl phthalocyaninatonickelCII) coating as a preconcentrator improves the sensitivity and signal-to-noise ratio of interdigital sensors toward ethanol fumes (27). Preconcentrators improve the selectivity of the sensor for a particular analyte because it is possible to select a preconcentrator with an affinity for only the analyte of interest. Hagen et al., provided examples of highly specific preconcentrators in which the addition of a chromium layer between interdigitated gold electrodes and a zeolite Pt-Na-Y membrane resulted in highly selective sensors for hydrocarbon vapors such as butane (29).