Chemically Reactive Solids as Column Packings for Gas Chromatography JACK J. DUFFIELD' and L. B. ROGERS2 Department of Chemistry and laboratory for Nuclear Science, Massachusetts Institute of Technology, Cambridge 39, Mass.
b Successful chromatographic use of a chemically reactive solid, silver nitrate, has been demonstrated using very small samples of unsaturated hydrocarbons. Although such a column has io be operated at relatively higher temperatures to make the unsaturates mobile, its use should simplify many analyses because saturates are not retained.
S
OLIDS such
as alumina, silica, charcoal, and molecular sieves, which depend upon physical sorption as the primary separative force, have been used as column packings in gas chromatography. Chemisorption, however, is frequently more selective than physical sorption and should permit some separations to be made on shorter columns. The studies most similar to the present one are two by Janak. In one study, samples of hydrogen were chromatographed using palladium on Celite (8); in the other study, retention of olefins was reported to be greater on a zeolite in which sodium in the lattice had been replaced by silver (9). More frequently, chemical reactivity has been superimposed upon gas liquid partitioning behavior. Bradford, Harvey, and Chalkley (6) were the first to report that by dissolving silver nitrate in polyethylene glycol, unsaturates were retained more strongly than saturates, which appear to be salted out. Unfortunately, these columns tend t o decompose a t 85' C. and above (4). Phillips, Bayer, and coworkers (1-3, 11, 12) have used melted heavy-metal salts of long-chain fatty acids as column liquids to separate amines. I n a special case, oxygen retention by hemoglobin has been used to characterize blood type (6). The chief reason for examining the use of chemically reactive solids was the hope of demonstrating a much higher selectivity. At the same time, an indication of the feasibility of l Present address: A plied Physics Corporation, 2724 S o d Peck Road, Monrovia, Calif. Present address: Chemistry Department, Purdue University, Lafayette, Ind.
obtaining heats of reaction, free from solvent effects, was desired. EXPERIMENTAL
Columns. Although crystals of silver nitrate were used to fill one column, most columns contained Johns-Manville 80- to 100-mesh Chromosorb W or P coated with a silver salt. Impregnation of 10 grams of Chromosorb W (or 15 grams of Chromosorb PI firebrick) was done by drying the support at 110' C., pouring i t slowly, with stirring, into 40 ml. of an aqueous solution of the silver salt, removing most of the water on a rotary evaporator protected from strong light. The evaporator was stopped frequently to break up large aggregates of packing which formed when nearly dry. Further drying was done at 80' to 90' C. under a vacuum of 0.01 to 0.03 mm. of mercury. After the packing had been loaded into 60 cm. of 8-mm. borosilicate glass tubing, it was attached to the apparatus and argon was passed through it slowly for 1 to 2 hours a t 250" to 300" C. Approximately 150 ml. per minute flow rate was used for all measurements of distribution ratios. Apparatus. The injection system, column, and associated heaters were constructed along standard lines from glass or stainless steel. The p r a y ionization detector was obtained from Research Specialties Corp. A Varian G-11 recorder, modified to give 5 m v . full-scale response, was used. A prepurifier for the argon carrier gas was necessary to maintain stable activity of the column over long periods of time. The argon passed first through a 15-cm. length of 8-mm. borosilicate tubing filled with 60- to 80-mesh crystalline silver nitrate. The center portion was heated to about 130' C. and protected from light. The argon next passed through a 60-cm. U-tube of 1 : l O w./w. silver nitrate on Chromosorb W. Most of the U-tube was immersed in ice water. In time, the contents of the heated tube turned black just before the hot zone, which remained colorless. The iced portion of the second tube became yellowbrown. Samples. The method of Pitkethly (19) waa used to obtain very small amounts of solute. When samples of 10" to lO-"J mole were used, the eluted peaks were quite symmetrical whereas large samples gave skewed elution peaks.
A 1-liter flask fitted with a groundglass joint was evacuated, filled with argon, and sealed with a no-air rubber stopper. Using a syringe and needle, a 10-pl. portion of liquid isomeric hexene or hexane was injected; it immediately volatilized. A 10-pl. sample of the resulting vapor could be withdrawn easily for column study. RESULTS
Factors Affecting Distribution Coefficients. Logarithms of the dis-
tribution ratios, log K, for seven isomeric hexenes are listed in Table I. Two isomeric hexanes which were injected were not retained to a measurable extent a t the operating temperature of the impregnated columns. Most of the values in Table I have been calculated on the basis of 1 meq. of silver at 170' C., but, for comparative purposes, one column of data for the cis-2-hexene is shown before conversion to the milliequivalent basis. The olefins are listed from left to right in the normal order of elution for silver nitrate-impregnated Chromosorb W. The very small values for crystals were extrapolated from observations a t lower temperature. The elution order of cis-4-methyl-2-pentene and 1-hexene interchanged with Chromosorb P support or silver sulfate impregnation. In both cases of cistrans isomers, the trans isomer always eluted before the cis. As expected, Table I shows that, because of a smaller surface area, crystals of silver nitrate did not permit the silver to be used nearly so effectively as when the salt was spread on a support. Table I also shows that, although larger amounts of silver nitrate on Chromosorb W increased the distribution ratio, the effectiveness per milliequivalent of silver decreased. This is presumably due to the formation of microscopic crystals or blocking of pores. Note that the values for 0.680:lO of silver nitrate on Chromosorb W appear to be anomalously high. We did find, however, that significant amounts of silver could not be recovered after long periods of contact with dilute nitric acid. Only complete dissolution of the Chromosorb in hydroVOL 34, NO. 10, SEPTEMBER 1962
* 1193
fluoric acid permitted all of the silver to be titrated. It is possible that a t a level of 0.680:lO such losses (by reaction with the support or diffusion into its pores?) became insignificant while, a t the same time, loss of specific activity through the formation of large crystals had not become important. The greater effectiveness of firebrick as a support is so large as to be beyond doubt even when the increased surface area compared to diatomaceous earth is taken into account. Although, near room temperature, the unimpregnated firebrick exhibits a larger retention for Cc hydrocarbons than the diatomaceous earth, retention was insignificant a t 180' C. (and above) where the distribution ratios were measured. The only rationalization we can suggest for the greater effectiveness of silver sulfate compared to the nitrate is the likelihood that the lower water solubility of the former may promote formation of smaller crystalline aggregates having a larger specific surface area. The column-conditioning temperature, although sufficiently high to melt silver nitrate, would not fuse the sulfate. Once again, as in the silver nitrate-impregnated supports, the effectiveness per milliequivalent of the silver ion decreased as the amount on the support increased. Other than the greater activity of silver sulfate, the chief difference was the splitting of %methyl-1-pentene into two peaks. The first peak, which was broader and smaller than the second, was eluted slightly before a normal sample of 4methyl-1-pentene but did not coincide with any samples run. It may have been due to 2-methyl-2-
Table 1.
pentene, a compound not investigated, but which might have been produced by rearrangement. Conversion of silver nitrate to silver chloride by passage of moist hydrogen chloride through a column decreased the absolute values for the distribution ratios. This was indicated by lower column temperatures (about 40' C.) a t which comparable values were obtained. When relative distribution ratios for the olefins were compared, the activity of each olefin had been decreased by a different amount. Only a small part of this difference can be attributed to the fact that the extrapolations of silver nitrate data involved lines having different shapes of log K us. 1/T. The reproducibility of column packing preparations seemed quite good considering the usual difficulties of working with gas-solid chromatography, Four columns containing different preparations of silver nitrate-impregnated Chromosorb W gave a range of distribution ratios, per milliequivalent of silver, within 10% of the average value. Two preparations of silver nitrate on Chromosorb P gave virtually identical results. A packing which had not been conditioned a t 250' C. and another which had been impregnated using dilute nitric acid (0.01N) instead of water showed very nearly the same results for the seven olefins. When twice as much water was used in preparing one packing, the distribution ratios seemed somewhat higher than normal. In this case, however, the longer periods on the rotary evaporator resulted in the production of more fines which may account for the greater activity.
Relative Heats of Reaction. Table I1 lists the slopes, -A€€/2.3R, of plota of log K vs. 1/T for most of the columns in Table I as well as the temperature range over which measurements were made. Although the scatter in absolute values is quite high, relative comparisons can be made. If the only quantity measured were the heat of reaction between the olefins and silver ions, these values would be the same for a particular olefin. The fact that there is scatter and that they are not identical is an indication that other factors, such as an interaction with the support or an anion effect, are present. Although, for a given c o l u m , the spread in values between olefins is small, it appears in every case that the heat of reaction per mole of olefin is somewhat greater for the cis isomer than the trans, a result expected from steric effects. In obtaining the data for Table 11, no significant change in slope was found on going through the melting point for pure silver nitrate a t 212' C. (10). No change in activity was observed on cooling or reheating because the columns had been preheated at 250' C. before use. Similarly, the distribution ratios a t 150' C. were virtually unaffected by a period of heating to 250' C. between two sets of runs. In comparing the heats of reaction for silver nitrateimpregnated supports with silver nitrate solutions in polyethylene glycol, one finds that impregnated supports yield much higher values. For example, using 4methylI-pentene on column packings of 2.5:lO w./w. Carbowax 400 on firebrick, the slope, -AH/R, of In K us. 1/T,
Values for Isomeric Hexenes of Log K per Milliequivalent of Silver Ion on Various Columns at 170' C.
Column (w./w.) 30- t o 40-mesh AgNOa Crystal8 AgNO: on Chromosorb W 0.034:lO 0,170:10 0.340:lO 0.680:lO 2.550:10 7.650: 10
trans-4Methyl-% pentene
trans-%
Hexene
PMethyl1-pentene
%Methyl1-pentene
Methyl-% pentene
1-Hexene
Hexene
CM-2-
cis'-% Hexene
-2.84
-2.74
-2.64
-2.49
-2.53
-2.48
-2.37
-0.02
0.24 0.22 0.27 0.47 0.03 -0.32
0.35 0.30 0.32 0.54 0.11 -0.32
0.46 0.47 0.46 0.68 0.22 +0.22
0.55 0.56 0.56 0.81 0.37 -0.06
0.57 0.68 0.60 0.81 0.36 -0.08
0.61 0.60 0.60 0.82 0.38 -0.05
0.68 0.65 0.70 0.91 0.46 0.05
-0.37 +0.32 0.70 1.24 1.27 1.35
1.18
1.20
1.34
1.47
1.51
1.48
1.57
1.23
1.01 0.87 0.59
1.04 0.91 0.62
1.19 1.05 0.77
1.28 1.15 0.88
1.33 1.22 0.92
1.32 1.19 0.90
1.40 1.28 0.99
0.19 0.d6
cis-4-
AgNOs OD Chromosorb P 0.170:15 Ag2S04on
Chromosorb W 0.078:lO 0.156:10 0.312:lO a
Log K not log K/meq. Ag+
1194
0
ANALYTICAL CHEMISTRY
0.93
Table II. Slopes of Log
K
vs. 1 /J Plots for Isomeric
CSUnsaturates on Various
Columns Containing Silver Nitrate or Sulfate
Slope X l o w 3
Column 30- to 40-mesh AgNOa crystah W./w. AgNOs on Chromosorb W
0.034:lO 0.170:lO 0.680:lO 2.550:10 7.650:10
W./w. AgNOp on Chromosorb P 0.170:15
W./w. Ag,S04 on Chromosorb W 0.078:10 0.156:10 0.312:10
trans-4
cis-4trans-2- 4-Methyl- 2-Methyl- Methyl-2Hexene 1-pentene 1-pentene pentene 1-Hexene
Temperature Range ( O C.)
No. of points
Methyl-2pentene
101 to 140
(5)
3.49
3.56
3.67
3.63
3.62
3.49
3.71
82 - - t,n - - -I31 -140 to 170 170 to200 164 to224 170 t o 235
(5)
(4j
3 - 11 --
3 02
3.14
(4) (6) (6)
4.18 4.80 3.88 3.66
4.21 4 82 4.01 3.79
4.68 3.92 3.66
3.14 4.16 4.96 4.08 3.73
3.20 4.29 4.73 4.02 3.76
3.11 4.18 4.80 3.88 3.66
3.24 4.46 4.91 4 02 3.85
178 to 220
(6)
4.38
4.34
4.47
4.43
4.45
4.38
4.53
145 to 210 157 to 199 150 to215
(6) (4) (6)
3.88 4.23 4.20
3.87 4.17 4.15
3.90 4.12 4.22
4.03 4.49 4.42
4.07 4.36 4.30
3.88 4.23 4.20
4.18 4.43 4.38
changed from 2160 to 2810 upon addition of 0.60 gram of silver nitrate. For silver nitrate used directly on firebrick and the same sample, the slope was 10,300. Thus, the heat of complex formation between the gas and solid may be determined, independent of solvent (partitioning liquid) effects. Column Efficiencies. The number of the theoretical plates reported in Table I11 for d r y columns, although not exceedingly high, demonstrates the effectiveness of the columns with small samples. The number of plates per milliequivalent of silver gives a rough measure of the relative efficiency of the packing. Although the temperatures and distribution ratios are not the same for each packing, a few conclusions can be drawn. First, a decrease in temperature and increase in distribution ratio should make the columns more efficient, but the opposite is observed in the first three packings listed. Clearly, the more silver nitrate on the Chromosorb W, the more nearly the packing approaches the behavior of crystals, which is highly inefficient. Second, greater effectiveness of Chromosorb P compared to Chromosorb W is indicated. Third, the value for silver sulfate appears t o be somewhat lower than expected but it may be due to a difference in crystalline nature, an anion effect, or, most probably, the somewhat lower operating temperature. Other Studies. Solutes other than paraffinic isomeric hexenes and hexanes were examined briefly. Cyclohexane, like paraffinic saturates, was not retained while benzene and cyclohexene were. The paraffinic unsaturates investigated had retention
4.ii
mixtures because saturated hydrocarbons are not retained. I n working with halides, severe decompositions may occur as judged by reports in which selective reactions have been effected to remove one organic halide from a mixture ( 7 ) .
times longer than benzene but shorter than cyclohexene. n-Propyl chloride, n-propyl bromide, and ethyl iodide were held slightly with a loss in sensitivity (decomposition?), compared to a plain Chromosorb W column, which was worst for the iodide. Methylene chloride, chloroform, and carbon tetrachloride were not retained. Amines gave very small peake, even at 250’ C., which may have been due to impurities rather than the amines. Ether, acetone, and methanol were held slightly at 100’ C. on all Chromosorb W packings, possibly because of sorption by the support itself. Silver nitrate packings should prove useful for aromatics and olefins but relatively useless for amines, except at much higher temperatures. Such packings should greatly simplify the analyses for unsaturates in complex
LITERATURE CITED
,
Table 111. Typical Numbers of Theoretical Plates for 60-cm. Columns Using 1 0-lo Mole of 2-Methyl-1 -pentene
Nl
Column 30- to 40-mesh AgNOs crystals 2.550: 10 w./w. AgN08 on Chromosorb W 0.17O:lO w./w. AgNOa on Chromosorb W 0.170:15 w./w.
N 127
meq. Ag+
T O
6.
K
0.55 125 8 . 5 6
(1) Barber, D. W.,. Phillips, C. S. G., Tusa, G. F., Verdin, A., J. Chem. SOC. 1959,18. (2) Bayer, E., Reuter, K. H., Born, F., Angew. Chem. 69, 640 1957. (3) Bayer, E., “Gas dhromatography1958,” Desty, ed., p. 169, Butterworths, London, 1958. (4) Bednas, M. E., Russell, D. S., Can. J. Chem. 36, 1272 (1958). (5) Bradford, B. W., Harvey, D., Chalkley, D. E., J. Znst. Petroleum 41, 80 (1955). (6) Gil-Av, E., Herzberg-Mindy, Y., J. Am. Chem. SOC. 81,4749 (1959). (7) Harris, W. E., McFadden, W. H., ANAL.CHEM.31, 114 (1959). (\ 8- ,) Janak. J.. Ann. N . Y . Acad. Sci. 72. 606 (1959).‘ (9) Janak, J. “Vapour Phase Chromatography,’” Desty, ed., p. 247, Butterworths, London, 1957. (10) Lange, N. A,, “Handbook of Chem-
istry,” 9th ed., McGraw-Hill, New York, 1956. ( 1 1 ) Phillips, C. S. G., “Gas Chromatography,” Coates Noebels, and Fagerson. eds.. D. 51. Academic Press. New (1 \ -
370 54.5
195 5.19
190
409
150 4.37
Chromosorb P 424 0.312:lO w./w. Ag2S0, on Chromosorb W 226
951
190 5.37
254
180 4.97
AgN03 on
cis-2-
Hexene
matography Symposium in Edinburgh,” June, 1960. (13) Pitkethly, R. C., ANAL.CHEM.30, 1309 (1958).
RECEIVED for review April 9, 1962. Accepted July 8, 1962. Division of Analytical Chemistry, 138th Meeting, ACS, New York, September 1960. Sup orted in part by the United States Itomic Energy Commission under Contract AT(30-1) 905.
VOL 34, NO. 10, SEPTEMBER 1962
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