cressing either particle diameter or porosity. I n conveni ional paekings, plate height increaser, directly with particle diameter, so t qat column performance deteriorates when coarser 1)ackings are used. The quadratic tlependcnre of pressure drop on particle diameter compared wil h the linear de1)cndence of plate height on particle cliameter wggests that liighcr total plate tlfficicnvies m i l d be achievctl with longer colunins containing largcr diamtriter partirle? hut it is e\trtmely inc.fficic.nt of analysis i iriie to use long rolrimns of poor plate height. T h p swsitivity of pressure tlrop to porositj i i verv appreciable-an incrcasc in e from its normal raluc of 0.4 t o 0.6 increascs Bo nearly 8-fold. Sinre rapillar> columnf, with a porosity of unity, c'an qive good plate heights, thcrP i b no basic reason n hy a n inrrease of Ilorosity of packed colunins should lead to impaired pe .formance. The problem of desirning a packed column N ith higher permeabilii y without sacrifice in plate heiehts th IS requires either
improvemrnt of efficiency with coarser packing particles or use of techniques of packing to give higher porosity TI ithout sacrifice of plate height. The recent work of Halasz and his associates a t the University of Frankfurt (9,10) using loosely packed capillary columns. points toward the fruitfulness of this area of investigation. By using columns of lcss than 0.020-inch inside diameter packed with particles to 1 / 2 the diameter of the column, Halasz and Heine (20) have obtained plate heights comparable to standard columns packed \\ith particles of the same dialmeter, but with porosities and permeabilities 10-fold higher The usefulness and fleyibility of this type of column iiiaT- n ell usher in a nen spiral of advance of column technology, leading to faster and better separations.
(3) DeFord, D. D., Loyd, R. J., Agers, B. O., A x ~ ~ LCHEM. . 35, 426 (1963). ( 4 ) Giddings, J. C., ANAL. CHEM.35, 353 (1963). (5) Giddings, J. C., Ibicl., 34, 72% ( 1 9 6 9 . (6) Giddings, J. C., Seager, S. L., Studii, L. R., Stewart, G. H., Ibid., 32, SGT (1960). ( 7 ) Golay, 51. J. E., ',Gas ChromnD. H. Desty, ed., tography-1958,'' p. 36, Butterworths, London, 1958. (81 Haarhoff. P. C.. Pretorius. 1.. J.. S. Ajrican Chem. Init. 13, 97 (i960). (9) Halasz, I., Papers presented at International Symposium on Gas Chromatography, Houston, and Iiesenrt 11 Conference on Gas Chromstographj-, U.C.L.A., January 1963. (10) Hslasz, I., Heine, E., .Ynture 194, 971 (1962). (11) Littlewood, A . B., "Gas Chroinntography-1958," D. H. Desty, ed., p. 35, Butterworths Scientific Publications, London (1958). (12) Scott, R. P. W., ''Gas Chromatography--1958," D. H. Destp, ed.. p. 189, Rutterworths, London, 1958. \
,
LITERATURE CITED
(1) Bohemen, J., Purnell, J. H., J. Chenz. SOC.1961, 360. ( 2 ) Carman, P. C., "Flow of Gases Through Porous Media," p. 11, Academic Press, New Tork, 1956.
RECEIVEDfor review Julv 18, 1963. Accepted October 17, 1963. Presented a t the 1963 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy.
Inorganic Salts for Gas-Solid Chromatography JOHN A. FAVRE and LYLE R. KALLENBACH' Phillips Petroleum Co
., Research Division,
b Resolutions, as measured b y ability to separate terphenyl isomers, were determined for 44 solid phase inorganic salts including antimonates, borates, carbonates, hydroxides, metal halides, nitrates, oxides, and phosphates. Hiqh resolutisns and low retention times were obtained with some of the salts. Potassium carbonate, borate, antimonate, and phosphate were the best salts tested for separating the terphenyls. 'The addition o f KOH to K2COa packing significantly improved the resolution of the terphenyls. The solid phtrse salt packings can b e operated successfully up to 500" C.
L
OF yuitable organic liquid phaqes for column packings operating aboi e 300" C. has limited vparations a t higher temperatures. I-ze of inorganic molt :n salt mixtures proposed by Phillips ( 4 ) and separation of polyphenyls with snch liquid phabe columns by Hanneman, Spencer, and ,Johnson ( 1 ) led to an investigation of the inorganic salt>. 1:xcellent separations, obtained n i t h the eutectic mixture of L i S 0 3 , S a S 0 3 . and K S 0 3 a t temperatures below t k e melting point (1n.p. 150" C.), suggested possible use of solid pliase inorganic salts as column
ACK
Barflesville, Okla.
packings to improve resolution and decrease tailing of component peaks. This paper reports the results of a survey of 44 inorganic salts, containing a variety of anions and cations, as possible column packings. The two principal criteria used to select the salts were stability up to about 500" C., and a melting point above about 500" C. The salts were compared on the basis of their ability to separate a standard blend of 0-, m-, and p-terphenyl. EXPERIMENTAL
An F&M LIodel 500 linear programmed temperature gas chromatograph was used throughout the study. A 2-foot drying tube packed with molecular sieve was placed in the helium line to dry the carrier gas. The inorganic salts 15ere obtained from Raker Chemical Co., Fisher Scientific Co., Nallinckrodt Chemical Works, and Merck and Co., Inc. The terphenyls and tetrahydrofuran were obtained from Eastman Organic Chemicals. The column packings I\ ere prepared by placing 25 wt. % of the salt dissolved in deionized water on 35- to 50-mesh Johns-llanville Chroniosorb P. The solvent was evaporated on a hot plate a t temperatures up to 600" C., depending on the melting point of the salt. Although pretreating the LiCl packing a t temperatures above the LiCl
melting point caused improved resolution of the polyphenyls ( 3 ) . other salt packings showed decreased resolution after this treatment. As a result, only the LiCl packing was pretreated. The packings were screened to assure 35- to 50-mesh particle size and poured into a &foot tube. A hand vibrator was used to obtain uniform packing. The packed columns 15-ere usually conditioned in the instrument for 1 to 2 hours a t 400" C., depending on the properties of the packing. Ten microliters of a blend of 95% of tetrahydrofuran and 5% of an equimolar mixture of the three terpheiiyls was used for each run. The column temperature was programmed at t h e rate of 15" C. per minute over the range 250' to 400" C. The helium carrier gas was flow-controlled a t 45 cc. per mi nut e. RESULTS
The component peaks in adsorption chromatography frequently are not symmetrical and show raryiiig degrees of tailing. I n programmed temperature chromatography the earlier peaks may be broader than the later peaks. These difficulties are more apparent Present address, Department of Chemistry, University of Oklahoma. Sorman, Okla. VOL. 36, NO. 1, JANUARY 1964
63
2 5 % K z C O 3 ON 3 5 - 5 0 MESH CHROMOSORB HOT PLATE DRIED
with low resolution salt packings than with the examples shown in Figures 1 to 3. For this reason the resolution formula is modifled to include only those portions of each peak that lie between the peak maxima. The resolution was defined as: 25% K28407 ON
W
35-50 MESH CHROMOSORB
r
(0
0
(L
where
T I M E (MINUTES)
Figure 1. Separation of terphenyl blend with K2C03 packing
Table 1. Resolutions and Retention Times of Terphenyl Isomers on Various Inorganic Salt Packings with 25 Wt. Salt on Chromosorb P
yo
Retent ion time, Salt (25 wt. %) K4Sb207a KzC03 KzB407 &Po4 KzHzSbz07 Sa&03 Sic& LiClb NaB02 CSCl ZndP04h SrWO3)2 Sb203 KF Sa2Cr04 ?;aBiO? LiI CaC12 LiLXh Sa1
KBr NaaPO4 RbCl Ba( NOs)* Sa2TVO4
Blank (Chromosorb P ) LiF BaCli K2S04
Blank (activated 600"
Resolution
see. Meta__
Para
para
Orthometa
231 224 119 225 179 276 310 1.59 183 179 416 197 198 174 314 258 232 2i0 298 168 18s 227 210 "Bo 297
1 75 1 65 1 47 1 44 1 43 1 23 1 22 1 15 1 13 1 12 1 07 1 04 1 02 1 00 0 98 0 97 0 96 0 95 0 90 0 89 0 89 0 88 0 85 0 84 0 79
3.02 4.42 5.10 4.37 7.45 4.05 1.84 3.60 5.92 3.50
152
0 0 0 0
70
79 78 76
1.18 1 .RX 0.90 1.90
0 0 0 0 0
76 73 72 69 64
1.26 2.22 1.83 3.04 1.39
2 8'2 300 218
263 173 225 174
238
...
3.80 4.56 2.48 2.27 1.85 0.78 3.07 1.69 2 . ,54 1.36 3.07 1.6s 3.56 2.32
Dissolved in IiOH solution. Packing pretreated by heating above melting point before filling column. a
b
64
rn
ANALYTICAL CHEMISTRY
YI Y LT
Rlz = resolution of peaks 1and 2 T 1 and Tt = retention times of peaks 1 and 2 TI+ = time from T1 to intersection of tangent and base line. The tangent is drawn to the point of inflection of the trailing side T 2 - = time from intersection of tangent and base line to T2. The tangent is d r a m to the point of inflection of the front side of peak 2
I n Table I the salts are arranged according to their abilities to separate the rn- and p-terphenyl isomers. These isomers are difficult to separate and their resolution is a good meawre of the relative effectiveness of the various salts. A resolution of 1.00 or above indicates a satisfactory separation. I n all cases the elution order mas 0-, rn-, p-; therefore the retention times for p-terphenyl indicate the total time required for the separation. Since K4SbzOi, which gave the best resolution, was solubilized in water with KOH, the column also contained KOH. The good resolution suggested that KOH alone might give good resolution or that it might act as a modifier on other salts. Table I1 gives results on the effectiveness of KOH alone and as a modifier on KsCOS,the second-best salt in Table I. The table also shows the effect of varying the concentration of KiC03. DISCUSSION
The data in Table I qhow a wide range of effectiveness for the salts studied. Correlation between the cation or anion and the effectiveness is not apparent: LiCl and KzC03 gave much better results than KC1 and Li2C03. S o correlation was found between the total time for elution and the resolution. These observations are not unexpected, since adsorption, which is dependent on the nature of the surface presented to the organic molecules rather than directly on the composition of the salt, is the process involved. Koberitein ( 2 ) has shown that the retention time in gas adsorption chromatography is proportional to the energy of the adsorption site. Resolution, however, is determined not only by the average adsorption energies for the components to be resolved, but also on the diqtribu-
X
W
X a
0
Y U
LT
+ W z 3
5:
0
1 2 3 TIME (MINUTES)
4
Figure 2. Separation of terphenyl blend with K2B407packing
tioii of energies, since broad diGtribution. will lead to broad and unresolved peaks. The selection of a column for analytical use is not dependent solely on resolution. il KnC03column, for example. is more than adequate for separating the terphenyls (Figure 1) and the superfluous resolution might be traded for time with proper optimization of conditions. However we have found that the retention times on K&03 are considerably longer for polyphenyls hea\ ier than the quaterphenyls. On the other hand, LiC1, n-hich hac a barely adequate resolution, has been used satisfactorily in our laboratory to separate polyphenyls as high as the liexaphenyls ( 3 ) . The present study Table 11. Resolutions and Retention Times of Terphenyl Isomers on Various Inorganic Salt Packings
Resolution illetapara
Orthometa
2 09 2.08 2.0;
4.64 -1.90 5.20
1.9s
5.34
1.85
4,s;
Ii*co,'( 25
224
1.75 I 62
3.02 4.42
IiOH (2) I
