3.1 Physical principles of the detection
3.1.2 Effect of the electric field intensity
Charged particles are attracted towards poles in an external electric field. During the resultant movement, the particles are accelerated and their energy increases. This acceleration depends on the electric field intensity, E (Vlcm), and the particle free path, which is proportional to the pressure p (torr) and the particle mass.
As electrons have a low mass, they are substantially accelerated in an electric field, even at low E / p values. On the other hand, ions with large masses are only slightly accelerated in the E / p range 1 to 200 Vcm-' . torr-'. Ion energies vary around 1 eV.
In addition to the pressure of the gas through which the particle moves, the quali- tative properties of this gas must also be considered. The average particle energy will be higher in monatomic than in diatomic gases, because of the lower number of possible energy states in the former (see Fig. 3.3). Thus, under gas chromatographic conditions when helium is used as the carrier gas, there is a greater probability of accelerating the secondary electrons to an energy sufficient for direct ionization of the target substance; when nitrogen is used as the carrier gas, the probability of successive ionization by an accelerated secondary electron is negligible.
FIG. 3.3. The characteristic energy of elcctrons in He, N, and 0, as a function of E / p 191.
FIG. 3.4. Calculated energy distributions for electrons in helium for E / p values of 3, 6 and 10 V/cm. torr 191.
The energy distribution also changes with a change in the electric field intensity.
With increasing E / p values, a greater number of particles of higher energy is obtained, as shown in Fig. 3.4.
Hence it is clear that, when a d.c. electric field is used for collecting charged particles formed by ionization, changes i n particle energies also occur and result in further reactions involved in the detection reaction mechanism.
Accelerated electrons lose their energy by colliding with atoms of molecules of the medium through which they are moving and also by collision with solid parts of the detector. These interactions lead to
(1) heating of the anode;
(2) heating of the carrier gas;
(3) ionization by electron capture;
(4) excitation of atoms or molecules of the carrier gas;
( 5 ) direct ionization of atoms or molecules.
If the probabilities of the individual interactions are expressed qualitatively, it is found that, with increasing electric field intensity, the probability of the loss of the electron energy by process (1) increases, by processes (2), (3) and (4) decreases and by process (5) again increases. For the probabilities of processes (4) and (5), it is
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found that (4) 2 ( 5 ) i.e., excitation to various states is always more probable than direct ionization [lo].
Acceleration of electrons under normal pressure and low E / p values (about 1 to 10) does not generally yield sufficient energy for direct ionization. This acceleration leads to a pronounced increase in the production of electrons with an energy of about 3 eV and the electron capture ionization probability thus significantly increases. This process is common to all ionization detectors and is discussed below.
Excitation of atoms or molecules may sometimes lead to the formation of so-called super-excitated states or, with monatomic gases such as argon and xenon, which are present as impurities in carrier gas containers, to the formation of metastable states.
While most super-excited states disappear by emission of energy, metastable states participate in the ionization of substances in the detection space. Process (I), leading to ionization of substances through so-called surface ionization, also contributes to the overall increase in the ionization current. It generally holds that the cross-sections of individual ionization mechanisms are additive [4].
It follows from the above discussion that the electric field intensity significantly affects the electron history, especially that of secondary electrons.
Various designs of collecting electrodes and various electric field intensities .are employed for gas chromatographic measurements. The applied d.c. voltages lie in the range 100 to 2000 V, and have a pronounced effect on the E / p value for electrode distances of 1 to 10 nim. All detector designs employ E / p values from 0.2 to 4,000 V/cm . torr.
Lower electric field intensities ( E / p < 10) are chiefly applied under the conditions corresponding to the flame ionization detector, the thermionic detector using an alkali metal, the photoionization detector, etc. With these types of detector, there are no changes in the secondary electron distribution and ionization by secondary electrons does not contribute t o the measured ionization current.
The second group contains detectors employing high E / p values, usually above 500. The helium and argon detectors and some versions of the photoionization de- tector belong in this group. Electrons are accelerated and their energy is increased above 10 eV, so that the probability of collision leading to ionization of the target substance increases considerably. The measured current consists of several contribu- tions, among which ionization with accelerated secondary electrons is also important.
The marked dependence of the experimental results on the electric field intensity must be borne in mind. However, it is also true that commercial instruments d o not always specify the electrode distance and the collecting voltage, so that it is often very difficult to compare results obtained with different instruments.
In some experiments, the dependence of the polarity of the electric field on the experimental arrangement of the detector was studied. In general, the collection of ions and electrons is almost instantaneous at suitable electric field intensities and is not affected by the geometric arrangement of the poles. It must be borne in mind that ions formed by the ionization of molecules move much more slowly than electrons
and that the velocity of ionic movement does not change with their free path, in contrast to the situation with electrons, An increase in the electron velocity leads to the formation of a space charge, which causes a decrease in the ionization detector signal and an increase in the noise. The formation of a space charge is probable with currents above 10-9A, i.e., in JlDA systems and in some helium detectors. I t occurs only very rarely in FID and E CD systems and not at all in a PID with separated spaces. Whether the measured current is negative or positive is determined by the design of the electronic part of the instrument and is decided by the manufacturer.
In recent years, some ionization detectors have employed a pulse electric field for collecting ion pairs. There are devices with variable pulse width and with different pulse frequencies. It is evident that the greater the pulse frequency, the more the pulse arrangement will resemble that employing a constant d. c. field. With a lower pulse frequency, the effect of the electric field on the fate of secondary electrons is decreased.