Electron Capture Detectors

The invention of the electron capture detector (ECD) is attributed to Lovelock in 1957 (1). The detector measures the electrical conductivity of an effluent gas stream resulting from exposure to ionizing radiation from a radionuclide.

Figure 1: Schematic of a typical ECD detector. Red species are β particles; green species are nitrogen make-up gas molecules; blue species are analyte molecules.

The ECD (see Figure 1 for a schematic) is a selective detector that responds to electronegative analyte molecules capable of "capturing electrons"-more specifically, halogenated, organometallic and nitro-containing compounds. The radionuclide is ⁶³Ni and around 10 millicuries, and is typically embedded in a foil within the detector cell that emits beta particles:

These negatively charged particles collide with make-up gas molecules and ionize them, and the subsequently produced electrons go on to ionize additional make-up gas molecules. Nitrogen is typically used as make-up gas in modern ECD detectors, due to its low ionization potential (low excitation energy).

The free electrons produced are accelerated toward a cathode within the detector, and a standing current is obtained:

When an electronegative analyte is eluted, the analyte molecules capture some of the "background" electrons, and this results in a reduction of the standing (background) current:

One might wonder why the negatively charged analyte ions do not also migrate towards the cathode and maintain the same level of standing current. The answer lies in the fact that the process of neutralization of the charged analyte with nitrogen cations happens very rapidly, at a much shorter time scale than the recombination of the initially liberated electrons with nitrogen cations, and, therefore, the number of charged species reaching the anode or cathode is reduced. The size of the reduction in the standing current is proportional to the analyte concentration.

The carrier gases used for ECD operation should be pure and dry. Oxygen and water are both electronegative, and as such contribute to a noisy baseline if they are present in the carrier or makeup gases, even in trace amounts.

When optimized, the detector can achieve sensitivity in the picogram (10^–12 g) range, but because the change in standing current is relatively low, the detector linear range is limited to around three or four orders of magnitude, depending upon the electronegativity of the analyte.

Improved performance and linearity can be obtained by operating the detector in a "pulsed" mode. A square wave pulse is applied at a frequency (typically a width of 1 ms at intervals of 20–50 µs) that maintains a constant current in the detector cell; to maintain the current in the presence of an analyte, the pulse frequency has to be increased. The signal is generated in proportion to the frequency of the applied pulse.

The cleanliness of the detector needs to be maintained at all times, which often means care with sample preparation. Chromatographic peaks obtained from a dirty ECD detector show a distinctive negative dip at the start or end of the peak, and counterintuitively the response of the detector can often increase as detector performance deteriorates (although signal to noise will deteriorate).

Collision or reaction cells can be added to the instrument to avoid the interference of polyatomic isobaric species with target analytes, by dissociating potentially interfering molecular species, or creating reactions products with the target analytes that have a unique mass. Ammonia is often used to generate these reaction products. Alternatively, high resolution (accurate mass) analyzers can be used in order to discriminate between target analyte elements and their nominally isobaric interferents, due to their ability to spectrometrically discriminate between very small mass differences.

References

(1) J.E. Lovelock, J. Chromatogr. 1, 35 (1958).