7.3.1 Carrier gas
With the original designs of photoionization and discharge detectors, only argon and helium were employed as carrier gases, as they simultaneously acted as discharge
FIG. 7.4. The dependence of the PID signal on the discharge tube input for some carrier gases 1351.
gases. In contemporary devices with separated discharge and detection compartments, any carrier gas with an ionization potential higher than the photon energy can be used. Helium, hydrogen and nitrogen have been employed. During a study of the
eKe:t of impurities on the PID signal, air was also used as the carrier gas (Fig. 7.4).
It was found that SPID is about 20 times smaller with air than with the other gases, which can be explained by the capture of the electrons formed by oxygen and water present in the carrier gas and thus by an increase in the recombination rate. It is thus evident that electronegative substances decrease the PID signal and their presence in the carrier gas is undesirable.
7.3.2 Geometric arrangement of the P/D 7.3.2.1 Discharge compartrnent
It has already been mentioned that older PID devices contained directly connected compartments. The eluted substances were then ionized by all the particles formed in the discharge. Responses to permanent gases, when argon was used as the discharge gas, have been reported in the literature. In argon, the maximum photon energy is 11.6 eV. Hence it is clear that the ionization of permanent gases was not caused by interaction with photons but by accelerated electrons and metastable atoms with energies higher than 11.6 eV.
TABLE 7.1
THE OPTICAL TRANSPARENCE LIMITS FOR SOME SUBSTANCES I N THE UV SPECTRAL REGION
Transparence limits
Substance Ref.
[nml [evl
- ~ ~~
LiF I04 11.9 15, 28, 34
MgF, 112 1 1 . 1 10
CaFz 122 10.3 18
Na F I32 9.4 25
BaF, 134 9.2 21
Sapphire 142.5 8.7 40
New PID designs are based on mechanical separation of the discharge and detection parts by an optically transparent substance. Glass and quartz are not transparent for high-energy photons and therefore crystals of alkali and alkaline earth metal fluorides are used as windows in these detectors (Table 7.1). This discharge tube is then an independent part of the PID and, after being filled with a discharge gas, yields a source of photons with a constant spectrum (Table 7.2). As the maximum energy of photons emitted from the discharge tube is determined by the crystal used, it is suitable to employ argon or hydrogen for filling the discharge tube. If a lower photon energy is
130
required, krypton or xenon can be used as discharge gases, or a different window can be placed across the photon flux.
The transparence of lithium fluoride extends t o very short wavelengths and is dependent on the temperature [18] and the humidity [34] and changes on prolonged use [40]. The LiF crystal has a transparency of about 60% for a wavelength of 1215.7 A. MgF, has better mechanical properties than LiF; its chief advantage is its insolubility in water and it is therefore more suitable for practical devices.
TABLE 7.2
PHOTON SOURCES IN THE VACUUM UV SPECTRAL REGION Line spectrum
“41
([eVI)
He 60-110
(20.3- 11.3)
Ne 74- 100
(16.7-12.4)
Ar 106.7- 170
(11.6-7.3)
Kr 123- 180
(10.0- 6.9)
Xe 147-220
(8.45- 5.6)
HZ 180-
(6.9-)
584.3; 537.0 (21.3); (23.1) 735.9; 743.7 (16.8); (16.7)
919.8; 932.0; 1048.2; 1066.7 (13.5); (13.3); (11.8); (11.6) 1164.9; 1235.8
(10.7); (10.0) 1295.6; 1469.6 (9.6); (8.5) 850-1670; 1215.7 (7.4- 14.6); (10.2)
The shape of the discharge tube determines the intensity of the photon flux.
Measurements have shown that the photon flux can be concentrated most effectively by leading it through a capillary (see Fig. 7.1). The distance between the electrodes affects the voltage at which discharge starts. It is therefore generally suitable to keep this distance as small as possible.
7.3.2.2 Detection compartment
Eluted substances pass through the detection compartment of the PID at atmospheric pressure. This part is mechanically connected to the discharge tube. The electrodes in the detector are outside the photon flux, thus giving a low background current and a low detector noise. The electric field intensity is very small in order to prevent electron acceleration. The effective volume of the detector requires the length of the absorption layer t o be as large as possible. This requirement was taken into conside- ration [35] in the construction of a detector with an internal volume of 0.14 cm3.
The ionization currents in the PID are the lowest in the group of ionization detec- tors. Therefore, it is very important to employ perfectly shielded electric leads and to ground the whole measuring system property. If these requirements are not met, an increase in the detector noise occurs.
1.4 PID APPLICATIONS
In early work, a PID without separated compartments was frequently used for the analysis of permanent gases. Modern devices do not permit determinations of permanent gases and limit PID applications t o substances with I P < 11.2 eV. This energy is sufficient to ionize most organic substances and some inorganic gases. The advantage of the PID lies in the fact that the carrier gas does not contribute to the ionization current, leading to a very small background current. When the PID is
TABLE 7.3
PRINCIPAL PARAMETERS OF SOME IONIZATION DETECTORS
Detector parameters
Background current [A]
Noise [A]
Linear dynamic range The lowest detectable amount [mole/sec]
Substances detected
Ionization efficiency Carrier gas
FID
[241 5 1 0 - 1 2
3 10-14 7 \ 1 0 - l ~
~
6 propane organic
ArD PID
1 x 1 0 - 8 1 x 10-12 i x 1 0 - l ~ 5.5
propane organic and some inorganic Ar 5 x 1 0 - 2
6 x 4 oxygen miscellan- eous 4 x 10-11 4 x lo-"
I x 1 0 - 3
He
9 x 10-11 3 x 1 0 - l ~ I x 1 0 - j ~ 4
benzene organic and some inorganic I x H,, N,, He
compared with the FID, it is found that the FID exhibits a broader linear dynamic range, while the other parameters are comparable. Several ionization detectors are compared with the photoionization detector with unseparated and separated com- partments in Table 7.3. The PID ranks among the most sensitive gas chromatographic detectors. Its simplicity, small volume and the possibility of employing very low carrier gas flow-rates make this detector suitable for use in very exacting gas chromato- graphic analyses.
132
7.5 LITERATURE
1. Arnikar H. J., Rao T. S., Karmarkar K. H.: Indian J. Chem. 5, 480 (1967) 2. Arnikar H. J., Rao T. S., Karmarkar K. H.: J. Chromatogr. 38, 126 (1968) 3. Bache C. A., Lisk D. J.: Biomed. Appl. Gas Chromatogr. 2, 165 (1968)
4. Braun W., Peterson N. C., Bass A. M., Kurylo M. J.: J. Chromatogr. 55, 237 (1971) 5. Dagnall R. M., Smith D. J., West T. S.: Anal. Lett. 3, 475 (1970)
6. Dagnall R. M., Deans D. R., Pratt S. J., West T. S . : Talanta 17, 1009 (1970) 7. Dagnall R. M., Deans D. R., Fleet B., Risby T. H.: Talanta 18, 155 (1971) 8. Dagnall R. M., West T. S . , Whitehead P.: Anal. Chem. 44, 2074 (1972) 9. Driscoll J. N.: Ger. Offn. 2,211,720 (Nov. 2, 1972)
10. Duncanson A., Stevenson R. W. H.: Proc. Phys. SOC. 72, 1001 (1958)
1 I . Ehrhardt H., Linder F., Tckkat T.: Advances in Mass Spectrometry (Kendricke, ed.), Vol. 4, 12. Freeman R. R., Wentworth W. E.: Anal. Chenz. 43, 1987 (1971)
13. Goldbaum L. R., Domanski T. J., Schloegel E. L.: J. Gas Chromatogr. 6, 394 (1968) 14. Goloskokov V. V., Kuzimina V. T., Levina L. E., Panyushkin V. V., Pimenov V. V.: Prib.
15. Karmen A., Giuffrida L., Bowman R. L.: Nature 191, 906 (1961) 16. Karmen A., Bowman R. L.: Nature 196, 62 (1962)
17. Karpov L. Ya., Kazakevich V. E., Dorbatenko V. D.: USSR Pat. 193,142 (March 2, 1967) 18. Knudson A. R., Kupperian J. E.: J . Opt. SOC. Am. 47, 440 (1957)
19. Krico-Electronic. K. G.: Ger. Offen. 1,958,751 (May 27, 1971)
20. Lakshtanov V. Z., Markevich A. V., Dobychin S. L.: Zh. Prikl. Khini. 40, 2492 (1967) 21. Laufer A. H., Pirog J. A., McNesby J. R . : J . Opt. SOC. Am. 55, 64 (1965)
22. Locke D. C.. Meloan C . E.: Anal. Cheni. 37, 389 (1965) 23. Lovelock J. E.: Nature 188, 401 (1960)
24. Lovelock J. E.: A I ~ . Chem. 33, 162 (1961) 25. Melvin E. H.: Phys. Rev. 37, 1230 (1931)
26. Opregaarg M.: Norw. Pat. 120,095 (Apr. 26, 1969) 27. OstojiE N., Sternberg Z.: Chromatographia 7, 3 (1974)
28. Patterson D. A,, Vaughan W. H.: J. Opt. SOC. Am. 53, 851 (1963)
29. Price J. G. W., Fenimore D. C., Simmonds P. G., Zlatkis A.: Anal. Chem. 40, 541 (1968) 30. Robinson C. F., Brubaker W. M.: US Pat. 2,959,677 (Nov. 8, 1960)
31. Roesler J. F.: Anal. Chem. 36, 1900 (1964) 32. Schnell E., Platz T.: Microchim. Acta 1969, 1285 33. Schnell E., Fuchs H.: Microchim. Actn 1972, 97 34. Schneider E. G . : Phys. Rev. 49, 341 (1936) 35. Sevtik J., Kr);sl S.: Chromatographia 6 , 375 (1973)
36. Sharpe J.: Nuclear Radiation Deteclors, Methum, London 1955, p. 130 37. Vree P. H., Fontion A,: US Pat. 3,540,851 (Nov. 17, 1970)
38. Watanabe K.: J. Chem. Phys. 26, 542 (1957)
39. Williams H. P., Winefordner J. D.: J. Gas Chromatogr. 6, 11 (1968) 40. Yakovlev S. A.: Pribory i Tekhn. Eksperim. 7, 175 (1962)
41. Yamane M.: J. Chromatogr. 9, 162 (1962) 42. Yamane M.: J. Chromatogr. 11, 158 (1963) 43. Yamane M.: J. Chromatogr. 14, 355 (1964)
p. 705, The Institute of Petroleum, London 1968
Tekh. Eksp. 1973, 175
The helium and argon detectors, utilizing metastable atomic states for the ionization of eluted substances, are also included among ionization detectors. Although only the helium detector is discussed below in more detail, all the conclusions drawn are
TABLE 8.1
THE EXCITATION POTENTIALS OF THE METASTABLE STATES OF THE RARE GAS ATOMS
Potential [eV]
ionization excitation
Gas - -~ - Configuration Ref.
He 24.5 70.6 21s
19.8 23s
He2 -1 5
16.6 2 s 4
16.55 7 s 5
Pl PO p2
Ne 21.5 17.6
1 3 3
Ar 15.76 11.83
11.72 11.55
N; 11.88
40 40 17 12 34 34 21 40 40 7
equally valid for the argon detector, as the two detectors differ only in the energy of the metastable atomic states formed (see Table 8.1). This type of detector is the only ionization detector operating in the multiplication region of the volt-ampere char- acteristic.