Primary particles emitted by a suitable source collide with the target substance, S, which is the carrier gas, gradually losing their energy. This process of particle slow- down or thermalization proceeds differently depending on the kind of particle and carrier gas. Simultaneously, it is also necessary to consider whether the carrier gas is monatomic or diatomic.
In monatomic gases, of which particularly helium and sometimes argon are used for gas chromatographic purposes, ionization (equation (3.2)), the formation of helium metastable states (equation (3.3)) or the formation of exited helium atom states (equation (3.4)) takes place during primary electron slow-down:
ionization
> B' + He+ f e
metastable
state p' + Hem
B + He
excited state
+ /I' + He*
B + He
excited state
h v + He . He*
(3.3) (3.4) (3.5) If photons are used as a source of ionization energy, only the formation of an excited state (equation (3.5)) can be expected, as photon energies are not sufficient for excitation to higher energy levels to occur.
The secondary electron energy exhibits a Maxwell distribution. I n the ionization of neon (ZP = 21.3 eV) by 1 keV electrons, the following energy distribution for the secondary electrons was found: 65% had energies higher than 13.6 eV, 40% higher than 27.2 eV and 25% higher than 54.4 eV. The secondary electron distribution is always of the Maxwell type, even if. for example, 75 eV primary electrons are used, only the maximum shifts to lower values. Hence the thermalization of primary electrons proceeds through several collisions, in which the ionization or excitation processes can always be repeated.
Diatomic gases are much more efficient than monatomic gases in slowing down primary particles. It generally holds that the more complex the molecule, the lower is its ionization potential and the greater the choice of possible energy states. Thus the probability of non-elastic collision is increased. Of diatomic gases, nitrogen is the most frequently used in gas chromatography. In this type of carrier gas, ionization (equation (3.6)) or the formation of excited states (equation (3.7)) takes place. The formation of long-lived super-excited states of diatomic molecules is negligible
compared with the probability of ionization. The energies of these long-lived mole- cular states are lower than the energies of metastable states, e.g., N: 11.88 eV [ll]
and He"' 20.6 eV, but are sufficient for the direct ionization of a number of substances.
However, the chief difference lies in the de-exitation process. While the cross-section for ionization by a metastable state is the largest compared with other particles, the probability of ionization by an excited long-lived particle is small:
When using photons as the source of the ionization energy, the formation of excited states usually occurs (equation (3.8)).
Polyatomic substances, occurring as impurities in the carrier gas or added to it deliberately, e.g., hydrocarbons or water vapour, lead to more efficient slowing down of primary particles owing to the large number of excitable electronic, vibrational and rotational states.
3.3.2 Recombination
The ion pairs formed by ionization fill the effective detector space and collide with the medium and with other ion pairs as they move under the influence of the electric field. Collisions with the medium decrease their velocity, but their number remains constant. Hence this type of collision does not cause a decrease in the measured ionization current. However, collisions among ion pairs lead to a decrease in the number of ions and thus also to a decrease in the measured ionization current.
A secondary electron can recombine with a positive ion directly (equation (3.9) and (3.10)), or in the form of a negative ion after being captured by an electronegative molecule in the medium (equations (3.11) to (3.14)):
X,+ 3- e + (Xz)* -+ X* + X 4- energy (3.9)
X + -t Y - -4- Z + XY -1- Z (3.11)
X Y + i - z- -+ x - Y + Z (3.12)
X f 1 Y' --f X" -1 y" (3.13)
X + 4- Y - --+ XY 4- Irv (3.14)
70
Electron - ion recombination without radiative transition (equation (3.9)) is more probable, the recombination coefficient having a value of while the radiative transition reaction (equation (3.10)) has a recombination coefficient of lo-’’. The values of the recombination coefficients for reactions that take place under gas chromatographic conditions are given in Table 3.4.
TABLE 3.4
RECOMBINATION COEFFICIENTS OF SOME INTERACTIONS [ 9 ]
Reaction a [cm3/sec]
O + + e 3 O* + h v (2000 K)
H + -t e --f H* + hv 4.8 x (250 K); 1.3 X (2000 K) NT + e --f N* -+ N*
0; + e + O* + O*
3.4 x l o - ” (250 K); 0.8 X
2.8 x (300 K)
1.7 x (300 K)
Ion - ion recombination is far more probable than electron - ion recombination and attains a maximum value in a pressure region around 760 torr (see Fig. 3.5).
Three-body recombination (equation (3.1 1)) or dissociative recombination (equation (3.12)) is most probable.
I 0
t
0
- 2432
[to..]
FIG. 3.5. Ion-ion recombination in air; a is the recombination coefficient [ 9 ] .
All recombination processes lead to a decrease in the measured ionization current and should be suppressed under the experimental conditions as much as possible by decreasing the dimensions of the measuring cell and by hastening the collection of charged particles.
The occurrence of a space charge is also often encountered in connection with the collection of charged particles. A cloud of heavy ions, which have a low mobility compared with that of electrons (less than l/lOOO), moves very slowly towards the electrode and recombines with electrons moving in the opposite direction. However, this phenomenon starts to be operative at higher ion concentrations (above 10' ions/cm3) and need not be considered under gas chromatographic conditions except for the TlDA and some helium detectors.
3.3.3 Background current of the ionization detector
By gradual slowing down of the primary particles in the carrier gas, a certain number of ion pairs is formed and a measurable current occurs after the application of an electric field. This ionization current, passing in the absence of an eluted substance, is known as the background current.
The background current is caused by direct ionization of impurities present in the carrier gas (icIMp), by ionization of the carrier gas itself (icHe) and by de-excitation of metastable states of monatomic gases (icdeeex):
bci = icHe + i~,,, + icde-ex (3.15)
The magnitude of the background ionization current is determined by the properties of the carrier gas, i.e., by the collision cross-sections. From the above expression, it therefore follows that, as the nitrogen ionization cross-section is greater than that of helium, i.e., icN, > icHe, the background current will be higher when nitrogen is used as a carrier gas than when helium is used. The background ionization current increases with the amount of impurities present and is highest for artificially prepared mixtures enriched with hydrocarbons.
3.4 LITERATURE
1. cermak V., Ozenne J. B.: Znt. J . Mass Spectrom. ton Phys. 7 , 399 (1971) 2. Girenko D. B.: Ref. Zh. Khim. 1972, 12 N 419
3. Hartmann C. H.: Anal. Chem. 45, 733 (1973)
4. Karavaeva V. G., Revelskii I. A,, Zhukhovitskii A. A.: Zaood Lab. 39, 275 (1973) 5. Lovelock J. E., Maggs R. J., Adlard E. R.: Anal. Chem. 43, 1962 (1971)
6. Lubkowitz J. A,, Parker W. C.: J. Chromatogr. 62, 53 (1971)
7. Lubkowitz J. A., Montoloy D., Parker W. C.: J. Chromatogr. 76, 21 (1973) 8. LukaE S . , SevEik J.: Chromatoyraphia 5, 258 (1972)
9. McDaniel E. W.: Collision Phenomena in Ionized Gases, J. Wiley & Sons, Inc., New York, London, Sydney 1964
10. Penning F. M.: Electric Discharges in Gases, Russian transl., Moscow 1960 11. Simpson T. H.: J. Chromatogr. 38, 24 (1968)
12. Wisniewski J. V., Mikkelsen L.: Facls Methods 7, 3 (1966)
4. The Electron Capture Detector (ECD)
The ionization of eluted substances by electron capture was first utilized in gas chro- matography in 1960 [37]. The pronounced dependence of the response for some atoms and functional groups on the measuring conditions has led to extensive use of this principle.