in the humidifier was further confused as some neutralization of the humidifier liquid occurred as a result of traces of carbon dioxide volatile acids and halogens in the samples. T o reduce these difficulties it was found advisable to replenish the reservoirs with fresh solutions a t intervals of about two months.
Ronald W. Dickinson of United Kingdom Atomic Energy Authority, Windscale Works. They thank Anthony Hinde of these laboratories for many helpful suggestions, and the Managing Director of the Production Group, United Kingdom Atomic Energy Authority, for permission to publish this account.
ACKNOWLEDGMENT
LITERATURE CITED
The authors acknowledge that the original suggestion for the gas chromatographic use of a Hersch cell was made b y
(1) Berry, R., “Gas Chromatography, 1962,” hf. van Sway, ed., pp. 321-327.
Butterworths, London, 1962.
(2) Cremer, E., Iiraus, T., Bachtold, E., Chem. Ingr.Tech. 33, 632 (1961). (3) Dimbat, Martin, Porter, P. E., StrOBs, F. H., A ~ cHEM, ~ ~28, 2. 9 ~ (1956). (4) Gudzinowicz, B. J., Smith, W. R.,
(5rg;;itJ y: (::i:Lment Practice 195,, pp. 817, 937.
(6) Lovelock, J. E., ANAL. CHEY. 35, 474, (1963). (7) Monkman, J. 4,.,Dubois, L., “Gas Chromatography, Instrument Society of America (1959), pp. 333-337, Academic Press, 1961. RECEIVED for review September 16, 1963. Accepted November 12, 1963.
Liquid Surface Effects in Gas Chromatography ROBERT L. PECSOK, ANGELA d e YLLANA, and AZlZ ABDUL-KARIM Department of Chemisfry, Universify of California, 10s Angeles 24, Calif.
b TO verify the contribution of liquid surface adsorption to the retention time in a gas liquid chromatographic column, four sets of columns were studied. Varying amounts of hexadecane and B,P-thiodipropionitrile (TDPN) were applied on firebrick and Chromosorb-W. The retention behavior of 22 hydrocarbons and oxy compounds of different polarities was investigated. The specific retention volume is essentially independent of liquid load for hexadecane, but varies widely for the nitrile, up to 20% loads. In the latter case, liquid surface adsorption may account for more than half of the retention. Retention volumes can b e computed from a two-term equation involving the volume of liquid and its exposed surface area. Solid-solute effects appear to be insignificant over the regions studied, except for polar solutes with firebrick coated with less than 1 % liquid.
T
identification of solutes and the derivation of thermodynamic data from gas chromatographic data, or conversely the a priori prediction of retention times, have been difficult problems. I n the case of low-loaded columns, and the consequent necessity to use vanishingly small samples, the problem of identification becomes more acute. It is in this region of small percentages of liquid phase that surface effects may predominate over solubility. For the usual liquid loadings of 20 to 30%, i t has been assumed that the specific retention volume, V , (or the partition coefficient, K ) , is a function of only the nature of the solute, liquid phase, and temperature. HE
v, = ~ ~ ~ V N / W= L273K/p~T T 452
ANALYTICAL CHEMISTRY
(1)
where V,,, is the net retention volume corrected for column pressure drop and gas volume in the system, w L is the weight of liquid phase in the column, p~ is the density, and T is the temperature in OK. If Equation 1 is valid, it serves to identify sample components. I n particular, i t should be independent of the amount of liquid on the column. Deviations at low liquid loadings have been often attributed to adsorption effects of the solid support surface. Keller and Stewart (6) have reviewed recent work on the extent of participation of the solid support and the importance of the solid surface area. They conclude that a three-phase model is necessary to account for the distribution of solute at a given cross section of the column, and suggest that even more terms may be necessary in some cases. It has also been reported (6, 22) that the solid effect is noticeable u p to 10 to 20% liquid coating, depending on the polarity of the liquid and solutes. On the other hand, Craig (3) opposes solid effects at high substrate concentrations because the solid surface appears to be sufficiently covered and peaks are symmetrical. He favors an orientation of the liquid molecules due to liquidsolid interaction and a consequent modification of the structure of the liquid. Martin (7) has proposed that adsorption of the solute molecules at the liquidgas interface can be important and may predominate at low liquid loads. His equation is expressed in terms of the net retention volume per gram of packing, V ’ R ~ : V’R~ = k,VL
+
kaAL
(2)
where k. is the usual partition coefficient, k , is an adsorption coefficient for the liquid-gas interface, and V L and
A L are the volume and surface area of the liquid per gram of packing. H e supports this theory with a limited amount of data for hydrocarbons with two liquid phases down to 1.5% loads. Recently he has studied the excess concentration at the liquid surface by surface tension measurements ( 8 ) . These results seem to confirm his earlier interpretation of the effects of liquid surface area on the gas chromatographjc retention volume. Severtheless, the importance of this effect has not been completely accepted (9, IO). I n view of the importance and controversial nature of the concept of liquid surface adsorption in a gas chromatographic column, we have extended and amplified the types of measurements made by Martin to cover a wider variety of solutes and even lower loaded columns. Four sets of columns using n-hexadecane and O,p’-thiodipropionitrile (TDPN) on firebrick and Chromosorb-IFr were used with liquid loads varying from 0 to 15%. Txentytwo representative solutes including oxygenated compounds of diverse polarities were used. Solid adsorption effects are important only a t very low loadings. Retention data can be explained by Equation 2 over a considerable range of conditions. EXPERIMENTAL
Gas Chromatographs. Commercial instruments (Aerograph A-90-P and A-9O-ACS) were modified b y relocating t h e column in a n external water bath maintained a t 25.0’ + 0.1’ or 50’ + 0.1’ C. Connections t o t h e instrument were made with short lengths of insulated capillary tubing and Swagelok fittings. T h e pressure was regulated with a two-stage reducing gauge on t h e t a n k and another
pressure - control valve on the instrument. The inlet Fressure was measured with a mercury manometer. The helium carrier gas was passed through a molecular sieve precolumn to remove impurities. Flow rale was adjusted t o give convenient elution times (30 t o 200 ml. per minute:. Flow rate was measured by a soap film flowmeter, with the measured rate corrected t o column temperature and for water vapor pressure in the usual way. Columns. Firebrivk columns were prepared from 42- t o 60-mesh GC-22 Super Support ( C o m t Engineering Laboratories). Chromosorb-TV columns were prepared from 60- to SO-mesh material (TVilkens Instrument and Research). Both matirials were used without washing or any chemical pretreatment. n-Hexadecane (Wilkens) was used as received. @,P’-Thiodipropionitrile (Eastman Kodak, yellow label) was recrystallized seven times to obtain a colorless iquid melting at
25’ C. To coat the support, a weighed amount of dried solid was added t o a solution of a known :tmount of liquid phase in about 100 ml. of solvent. The flask was connected t o a rotating evaporator, the temperature was increased and the pressure was cecreased until no solvent odor remaine1-i. It was then weighed and packed into a column. All columns were 1/4-inch 0.d. copper tubing approximately 6 feet lcng, filled straight with gentle tapping and then coiled to fit the thermostat. The weight of material packed mas determined by difference and was known to the nearest 0.1 mg. All columrs were preconditioned at room temperature, to avoid loss of liquid, until a stable base line was achieved. Operating Technique. Individual samples were injccted from a Hamilton 1-p1. syringe, using 0.1 pl. or less. For columns with less than 1% liquid, the sample was drawn into the syringe, which was then pumped with air several times bef x e the residual vapor was injected. Reproducibility of sample size was not inportant to this study. Retention times were measured from the air peak. Each reported time is the average of three or more separate determinations reproducible to better than 1%. Samdes. The solutes. each in the highesc purity available, 6ere obtained from a variety of commercial sources and were used as received.
Surface Area Measurements.
An
all-glass constant-volume apparatus was used in which t h e pressure was measured with :t sensitive Pirani gauge in t h e range cf 1 to 20 mm. Because of the presence of the liquid coating on the sample, it was not possible t o degas in the usual manner. A 3- to 5-gram portion of t h e sample was carefully weighed i n the sample tube attached t o the manifold. T h e sample compartment was evacuated to 50 microns, and a liquid nitrogen bath put around it, while pumping for exactly 2 more hours. After this degassing procedure, a certain pressure of krypton was let into the calibrated
manifold and the pressure measured. Then the stopcock between the manifold and sample chamber was opened and the krypton was partially adsorbed on the sample surface. h number of measurements were made on each sample by adding successive increments of krypton to the system. Temperatures mere measured by a thermocouple in a well in the sample chamber. Specific surface area of the sample, S,, was calculated from: 8, = UV,/W = u / W ( a P ) (3) where is the cross-sectional area of one atom of K r = 19.5 i: 0.4 sq. A.
+
Table
I.
per atom (2), V , is the volume of adsorbed gas when the entire adsorbent surface is covered with a complete monolayer, ti‘ is the weight of the sample, (Y and R are the slope and y-intercept of the BET plot, p / N ( p o - p ) us. pip,,, and p , was assumed t o be 2.800 microns at liquid nitrogen temperature (11). All samples, both dry and liquidcoated, were treated exactly alike, so that any errors would be consistent and the relative areas would be significant. The continuous flow method using a gas chromatograph (4)was tried and found to be unsatisfactory. The results by this method showed a gradual in-
Effect of Amount of Liquid Phase on Retention Volume V
(ml./gram)
R ~ O
Calculated values of V R ~ from ’ Equation 2, given in parentheses below corresponding observed values Per cent of liquid phase Compounds 0.30 0.60 1.05 3.02 5.98 8.75 13.98 A. TDPS-Chromosorb-W columns 25’ C. 0.47 0.66 n-Pentane ... ... ... ... 0.38 2.02 1.40 1.03 Cyclopentane 0.26 0.38 0.62 (0.32) (0.34) (0.38) (0.61) (1 .00) (1.36) (2.08) 0.92 1.21 ‘0.79‘ 2,3-Dimethylbutane 0.86 0.55 0.55 0.62 (0.64) (0.60) (0.59) (0.61) (0.75) (0.90) (1.23) 0.90 1.14 0.66 0.56 0.62 0.68 0.79 2-Methylpentane (0.67) (0.63) (0.61) (0.62) (0.74) (0.88) (1.17) 0.92 1.04 1.39 0.86 0.61 0.63 0.73 3-Methylpentane (0.71) (0.67) (0.66) (0.69) (0.86) (1.04) (1.41) 2.63 1.51 ‘1.88’ 1-Hexene 0.93 0.86 0.91 1.14 (0.96) (0.93) (0.94) (1.09) (1.48) (1.87) (2.67) 1.52 1.02 1.18 n-Hexane 0.79 0.73 0.74 0.83 (1.56) (0.82) (0.78) (0.76) (0.78) (0.96) (1.1;) 3.00 2.03 0.75 0.63 0.79 1.09 1.58 hlethylcyclopentane (0.72) (0.72) (0.76) (1.02) (1.54) (2.04) (3.03) 34.06’ 57.08 2.64 3.72 5.65 12.99 Benzene (2.48) (3.55) (5.31) (12.99) (24 81) (35.64) (56,;s) 3 .00 4.05 2.31 Cyclohexane 0.88 0.76 0.91 1.43 (0.80) (0.83) (0.91) (1.39) (2.24) (3.02) (4.57) 12.01 7.75 1.21 1.33 1.73 3.27 5.55 Cy clohexene (1.19) 11.36) (1.67) (3.15) (5.54’) (7.76) 2.36 ’1.99‘ ‘2.19’ 2.66 3.00 3.62 ‘2.39‘ n-Heptane (3.77) (2.41) (2.27) (2.18) (2.13) (2.48) (2.88) 3.31 2.66 2.93 2.91 2.78 2,2,4-Trimethyl2.56 2.74 pentane (3.00) (2.80) (2.64) (2.40) (2.56) (2.82) (3.45) 2.61 2.01 Diethyl ether 1.24 0.65 0.75 0.66 (0.73) (0.76) (0.83) (1.26) (2.01) (2.73) 4.18 4.03 5.74 7.78 9.95 n-Butyl ethyl ether 4.32 .~ (4.84) (4.71) (4.77) (5.68) (7.87) (10.04) 15.01 22.77 Propionaldehyde 2.66 8.15 I .68 3.63 (1.89) (2.53) (3.60) (8.32) (15.65) (22.39) 24.82 17.43 Isobutyraldehyde 9.78 3.09 4.76 4.09 13.71) (4.27) (5.34) (10.17) (18.02) (25.29) 38.88 7.61 Ethyl acetate ‘5.41’ 6.38 ‘16.99’ 27.88 (9.42) (16.59) (28.48) (39.53) (7.21) (7.94) 25.61 36.33 Acetone 4.09 3.28 13.56 5.96 (4.91) (6.56) (14.03) (25.79) (36.62) ( 3 .”) 25.75 46.52 65.88 8.97 11.27 Methyl ethyl 6.30 ketone (7.32) (9.08) (12.14) (25.94) (47.66) (67.66)
:
~I
B. TDPS-Chromosorb-W columns a t 50’ C. n -46 1.26 0.66 0.81 0.34 0.34 (0.33) (0.43) (0.64) (0.83) (1.23) (0.32) 0.43 0.75 0.33 0.50 n-Hexane 0.30 0.27 (0.51) (0.73) (0.26) (0.30) (0.41) (0.27) 0.72 1.49 0.90 0.29 Methylcyclopentane 0.26 ’0.46’ (1.44) (0.43) (0.70) (0.95) (0.27) (0.23) 9.48 12.41 n Rn 21.83 4.97 1.97 Benzene 0.90 (4.69) (9.18) (13.28) (21.18) (1.73) (0.61) 2.20 1.03 ’ 1.35’ 0.63 0.36 0.30 Cyclohexane (1.00) (1.45) (2.13) (0.34) (0.58) (0.26) 3.08 5.18 2.33 1.32 0.45 0.59 Cy clohexene (0.57) (1.24) (2.29) (3.25) (5.12) (0.34) 1.04 0.96 1.50 0.81 0.66 n-HeDtane 0.56 (1.48) (0.65) (0.70) (0.88) (1.07) (0.70) 0.98 I .37 0.91 2,2,4-Trimethyl0.90 0.76 (1.35) (0.77) (0.90) (0.80) pentane (0.89) I-Hexene
VOL. 36, NO. 3, MARCH 1964
453
crease in area as the amount of liquid phase was increased. It was assumed t h a t this anomalous behavior is caused Of the liquid by the temperature "m the constant-vollowered to -196' C. With ume technique the cooling process was rather slow. The range of the area measurements is i 10%. RESULTS AND DISCUSSION
A plot of V , us. per cent liquid phase should give a straight line parallel t o the abscissa. In the present study, a series of these plots was drawn for the Table
Compounds n-Pentane Cyclopentane 2,3-Dimethylbutane 2-Methy lpentane 3-Methy lpentane
1-Hexene n-Hexane Methylcyclopentane Benzene Cyclohexane Cyclohexene n-Heptane 2,2,4-Trimethylpentane Diethyl ether n-Butyl ethyl ether Propionaldehyde Isobutyraldehyde Ethyl acetate Acetone Methyl ethyl ketone
Cyclopentane 2,3-Dimethylbutane 2-Methylpentane 3-Methylpentane 1-Hexene n-Hexane Methylcy clopentane Benzene Cyclohexane Cy clohexene n- Heptane 2,2,4-Trimethylpentane
454
0
II.
different sets of columns. Typical plots are shown in Figure 1. Hexadecane Columns. Plots similar t o those in Figure 1 were obtained for all other solutes on hexadecane columns, regardless of whether the solid support was firebrick or Chromosorb-W. I n the latter case, there is no deviation from a horizontal line until the liquid load is decreased to less than 1.0 t o 1.5% rather than the 3% level observed with firebrick support. With either solid the hydrocarbons elute in order of boiling point, except for inversion of n-heptane and
-
2,2,4-trimethylpentane. Increasing the temperature from 25' to 50" C. decreases the retention volumes b y 50 t o 70% without changing the order of elution. Polar solutes on hexadecane columns gave skewed peaks and nonreproducible retention times. This effect is presumably esplained by solidsolute interactions. P,P'-Thiodipropionitile Columns.
In
contrast to the regular behavior observed on hexadecane, the plots in Figure 1 show t h a t V gvalues for TDPN vary with per cent liquid to well beyond 10% loads and do not level off even at
Effect of Amount of Liquid Phase on Retention Volume, VQO (ml./gram)
0.31 0.78 0.81 (0.86) 1.73 (1.82) 1.90 (1.93) 1.88 (2.01) 2.67 (2.72) 2.30 (2.34) 1.91 (1.98) 5.88 (4.94) 2.17 (2.16) 3.02 (3.01) 6.50 (6.92) 8.00 (8.67) 2.50 (1.98) 16.24 (13.62) 7.44 (4.09) 13.10 (9.26) 26.70 (18.66) 16.24 (9.23) 31.20 (17 .lo) 0.33 0.62 0.67 0.69 0.91 (0.88) 0.80
(0.78) 0.71 (0.62) 1.89 (0.92) 0.81 (0.68) 1.09 (0.78) 2.08 (2.00) 2.44 (2.54)
ANALYTICAL CHEMISTRY
0.60
1.oo
Per cent of liquid phase 3.11 6.00
A. TDPN-firebrick columns 25' C. 0.71 0.76 0.74 0.72 1.01 1.37 0.82 0.79 (1.38) (0.86) (1.07) (0.87) 1.62 1.62 1.53 1.57 (1.61) (1.75) (1.68) (1.59) 1.70 1.77 1.69 1.66 (1.84) (1.66) (1.65) (1.77) 1.80 1.81 1.75 1.70 (1.80) (1.93) (1.77) (1.86) 2.83 2.59 2.59 2.49 (2.75) (2.55) (2.62) (2.55) 2.10 2.08 2.07 2.20 (2.07) (2.04) (2.25) (2.16) 2.49 2.07 1.88 1.84 (2.45) (1.91) (2.08) (1.93) 26.64 15.27 7.75 6.08 (26.43) (7.32) (5.88) (15.47) 2.14 3.17 2.08 2.50 (3.20) (2.15) (2.14) (2.53) 6.82 4.62 3.15 3.05 (6.82) (4.73) (3.29) (3.10) 6 .OO 5.88 5.96 6.39 (5.75) (6.62) (5.86) (6.34) 6.92 7.20 7.28 7.67 (6.68) (7.88) (8.27) (7.07) 3.03 1.96 2.06 (2.91) (2.31) (i:96) (1.97) 14.45 12.77 13.41 (14.21) (12.99) (i3:i7) (12.84) 11.27 16.98 6.19 (17.14) (10.41) (4:62) (5.47) 21.84 15.06 10.67 (21.94) (15.04) (9:60) (10.27) 35.79 25.91 19.42 (36.61) (26.47) (19.74) (18194) 29.27 17.96 12.02 (29.46) (18.83) (11.18) (9 :96) 54.42 34.80 23.97 (54.44) (34.82) (20.69) (18:43) B. TDPK-firebrick columns a t 50' C. 0.66 0.41 0.35 0.66 0.60 0.60 0.66 0.62 0.62 0.73 0.66 0.65 1.10 0.94 0.89 (1.04) (0.91) (0.85) 0.80 0.74 0.74 (0.77) (0.72) (0.73) 0.81 0.69 (0:98) (0.76) (0.63) 9.72 2.63 5.45 (9.34) (5.12) (1.92) 1.36 1 .oo 0.82 (1.30) (0.95) (0.72) 2.75 1.77 1.11 (2.59) (1.64) (0.95) 1.99 1.94 1.85 (1.82) (1.77) (1.85) 2.20 2.20 2.35 (2.10) (2.14) (2.33)
_ _ _ _ ~ 12.00 8.99
14.90
0.79 1.74 (1.76) 1.66 (1.71) 1. I 2 (1.43) 1.86 (1.92) 3.08 (3.08) 2.19 (2.19) 2.87 (2.93) 38.27 (38.43) 3.94 (4.00) 8.93 (9.20) 6.03 (5.94) 6.84 (6.63) 3.57 (3.63) 16.77 (16.12) 27.00 (24.56) 28.72 (29.76) 48.09 (48.34) 40.51 (41.28) 76.41 (76.29)
0.94 2.27 (2.12) 1.81 (1.83) 1.78 (1.83) 2.02 (2.02) 3.46 (3.44) 2.31 (2.34) 3.55 (3.43) 49.52 (50.01) 4.92 (4.81) 11.78 (11.53) 6.56 (6.20) 7.16 (6.70) 4.42 (4.36) 18.18 (18.13) 31.55 (31.14) 38.20 (37.39) 57.11 (59.84) 52.20 (52.77) 94.08 (97.51)
0.93 2.50 (2.43) 1.87 (1.88) 1.91 (1.86) 2.16 (2.13) 3.65 (3.67) 2.46 (2.41) 3.74 (3.79) 60.36 (59.73) 5.26 (5.43) 13.27 (13.42) 5.73 (6.24) 6.03 (6.53) 5.34 (4.92) 19.16 (19.47) 36.64 ( 37.73) 45.24 (43.63) 69.84 (69.12) 63.74 (62.29) 118.28 (115.08)
0.84 0.69 0.65 0.76
0.97 0.70 0.68 0.79 1.40 (1.43) 0.87 (0.95) 1.35 (I .49) 16.61 (18.33) 2.00 (2.10) 4.60 (4.67) 1.96 (2.12) 2.03 (2.25)
1.187 0.74 0.73 0.83 1.47 (1.57) 0.90 (1.01) 1.59 (1.70) 21.95 (22.06) 2.36 (2.42) 5.28 (5.52) 2.10 (2.20) 1.96 (2.26)
1.21
(1.23) 0.84
(0.86) 1.23 (1.24) 13.64 (13.92) 1.67 (1.70) 3.62 (3.65) 1.95 (1.96) 2.14 (2.16)
'Oo0k
% Liquid on F l r i b r i c k
Figure 1. Effect of amount of liquid phase on specific retention volume, Vu (ml. per gram of liquid), at 2 5 ' C., using firebrick Nitrile columns. 0 1-hexere, cyclopentane Hexadecane columns. 0 n-hexane, 0 cyclopentane
A n-hexane, 0 1-hexene,
A
Typical retention data are given in Tables I and 11. The slope of a plot of Vauo us. % T D P N varies with nature of the hydrocarbon from a very steep one for benzene, to nearly zero for n-heptane, then becoming negative for 2,2,4-trimet hylpentane. These differences determine several changes in order elution as the per cent of T D P N is increased. I n general, as the per cent of T D P N is increased, branched, paraffins move forward and cyclo compounds move backward with respect to the corresponding normal paraffins. When the temperature is raised to 50' C., the order of elution changes again, and the olefins move forward with respect to the paraffins. The retention times of hydrocarbons on Chromosorb-W columns are much shorter than on firebrick a t comparable per cent liquid loads. Only in the case of benzene and cyclohexane do the
,O0I
I
/
300
20% loads. On T D P Y columns nonpolar compounds d e v a t e more from the predicted behavior than polar compounds-e.g., benzens, which fell between the two sets of curves in Figure 1. This anomaly is not consistent with solid-solute interactions because stronger adfiorption effects would be espected a i t h a nonpolar liquid (hexadecane) t h i n with a polar liquid (TDPN) which can more effectively cover active sites, the solid being the same in each (case.
/ AL
Figure 3. Determination of retention coefficients, k, and k,, at 25OC. 1 -Hexene. n-Hexane.
A Firebrick, 0 Chromororb-W A Firebrick, 0 Chromasorb-W
VR~O'S reach a common value a t high loadings.
VL "106 -
LL
Figure 4. Determination of retention coefficients, k, and k,, at 25OC.
A
Firebrick, 0 Chromosorb-W A Firebrick, 0 Chromororb-W
Acetone. Benzene.
first suggested by Martin (7). Typical plots are shown in Figures 3 and 4. I n each case the best line was drawn from a least squares treatment. The slope and intercept, which give values for k, and k,, respectively, in Equation 2, are recorded in Table 111. The values for both k , and k, are the same for firebrick and Chromosorb-W. With the values of k , and k, from Table 111, VRso was calculated for each solute a t the various liquid loadings used as noted by the numbers in parentheses in Tables I and 11. The agreement with the observed values is remarkably good. The average deviation for all compounds in a given column was of the order of 1 to 5y0 relative for liquid loads greater than 1%; and 10 to 15% relative for columns as low as 0.3% liquid load. However, deviations as high as 50% were encountered for oxy compounds on firebrick columns with less than 1% T D P N . The effect of liquid surface adsorption is not negligible, as shown for typical situations in Figures 5 to 7 . Adsorption
3.0~
Surface Adsorption on TDPN Columns. T o explain the preceding
5
0
10
15
% Liquid I'hase
Figure 2. Effect of amount of liquid phase on surface area, AL (sq. meters per gram) 0 0
Chromosarb-W Firebrick
observations, t h e specific surface areas per gram of packing of all column packings were determined. T h e results are shown in Figure 2. T h e surface decreases, as expected, with increasing amounts of liquid load, and within experimental error, the same value was found with both hexadecane and T D P N for the same solid. The areas of the bare supports agree well with values reported previously: 3.9 sq. meters per gram for firebrick [earlier values 4.14 (4) and 4.8 ( I ) ] , and 1.4 sq. meters per gram for Chromosorb-W [earlier value 1.2 ( I ) ] . Plots of VRo0/AL us. VL/AL were made for all compounds, in a manner
5 10 % T O P N on F i r e b r i c k
0
I5
Figure 5. Contributions to retention volume for n-hexane on firebrick at 2 5 " C.
--0
Calculated from Equation 2 Observed values
VOL. 36, NO. 3, MARCH 1964
455
irreversible adsorption on the solid surface causes bad tailing and nonreproducibility. I n the case of solid adsorption, the retention volume extrapolates to the retention time observed on the bare solid. With liquid surface adsorption, the extrapolated value differs from the observed value on the bare support-for example, VE," for benzene on bare firebrick is 14.9 ml. and the extrapolated value is 5.8 ml. Some preliminary, though not as extensive, measurements comparing firebrick with firebrick treated with hexamethyldisilazane, have shown that retention volumes are indeed independent of the nature of the solid surface for the cases studied. Typical results are shown in Table IV. Here, the areas of the solid and liquid surfaces in the two columns were identical. Of course, as the per cent liquid is decreased below 1%, differences due to solid adsorption can be expected.
3.0 r
5
10
15
T O P N on C n r o m o r o r b W
Figure 6. Contributions to retention volume for 1 -hexene on Chromosorb-W a t 25°C.'
--0
Calculated from Equation 2 Observed values
on the liquid surface is the main factor contributing to retention of 1-hexene at low loadings; even at 15% i t amounts to one third of the observed retention time on firebrick and one fifth on Chromosorb-W. For less polar compounds, like n-hesane, i t accounts for one half of the retention a t 15% T D P N on firebrick and one quarter on Chromosorb-W. Oxy compounds behaved in a similar manner, but with a larger contribution from solution a t intermediate and high loadings. Solid-solute forces interfered with the measurements only in the case of methanol and ethanol. Both behave normally on TDPN-Chromosorb-W columns; methanol tails only below 0.6% liquid; but with firebrick, tailing was observed a t 3% for ethanol and 6% for methanol. The characteristics of solid adsorption are completely different from liquid adsorption; besides an increase in retention time, the
CONCLUSIONS
25O
456
E,
The surface forces of the liquid phase in a gas chromatographic column determine whether the solute will adsorb on the liquid-gas interface. When the nature of the solute and substrate is such that the surface tension of the liquid-layer decreases with adsorption of the solute, this phenomenon will occur. Adsorption on the solid can occur simultaneously, although in most cases it is less significant. The solid support does influence separation with polar phases, but mainly because of differences in the estent rather than the nature of its surface. Reducing the amount of liquid phase will retard the component that is least soluble, relative to more soluble components. A similar effect can be obtained by using a solid with a larger
7.4 . .6.8
8.6 9.3 20.7
c.
5oo
E , X 106, cm. 51.5 54.5 56.9 66.3
196.2
15.7 15.6 21.8 34.0 17.8 93.6 451.6 30.7
246.7 23.3
306.5
240.2 495.7 227.4
97.6 281.3
469.1
454.1 838.8
ANALYTICAL CHEMISTRY
54.8
58.9 76.2 78.4 105.0 54.0 381.2 94.3
421.3
k.
3.2 2.7 3.7
0
5
10
15
% T D P N on F i r e b r i c k
Figure 7. Contributions to retention volume for 1-hexene on firebrick a t
25" C.
---
Calculated from Equation 2
0 Observed values
Table 111. Values of k , and k, for TDPN Columns (Same values apply to both firebrick and Chromosorb-W)
Compounds n-Pentane 2 3-Dimethylbutane 2-Methy lpentane 3-Methylpentane n-Hexane n-Heptane 2,2,4-Trimethylpentane Cyclopentane Methy lcyclopentane Cyclohexane 1-Hexene Cy clohexene Benzene Diethyl ether n-Butyl ethyl ether Propionaldehyde Isobut yraldehyde Ethyl acetate Acetone hiethyl ethyl ketone
40r
c.
surface area, keeping the amount of liquid phase constant. For polar substrates, the calculation of partition coefficients and other thermodynamic data must take into account the contribution of liquid surface adsorption. Comparing results reported here with those of Martin ( 7 ) , we note a general agreement of all values of k,, usually well within +loyo of each other. However, most of our values fork, are about 75 to 80% of his values at 25" C., and 70 to 75% at 50" C. Thus we show a smaller effect due to liquid surface adsorption relative to bulk solubility, but with a slightly greater temperature effect with an apparent heat of adsorption of about -8 kcal. per mole. We presume that the differences in the two sets of k , values are caused by differing purities of the T D P N . It is clear t h a t traces of impurities will have a large effect on the surface tension of the solvent.
E , X lo6, cm. 18.6 19.1
20.4
4.8 9.1
21.8 56.3
7.3 7.8 10.8 16.3 8.7 40.4 170.5
72.1 8.9 17.0 18.3 24.6
18.9
12.7
Table IV.
Comparison of Treated and Untreated Support
Both columns contained firebrick coated with 9.37, TDPN and were operated under identical conditions a t 25°C. Specific retention volume, ml./gram of pack1ng UnHMDS treated treated Compound 1 38 1.33 Cyclopentane 0... 7 7 0.77 3-Methylpentane 1.GO 1.61 1-Hexene 1.89 1.90 Methylcy clopentane 41.2 38.8 Benzene 3.02 2.88 Cyclohexane 8.34 8.07 Cyclohexene
It has been tacitly ztssumed t h a t the solid surface is completely covered at all liquid loads. With 0.3y0 liquid phase on firebrick, it can be shown that the mean thickness of i,he liquid layer is about 8 A, a reason,ible value for a monolayer. Therefor?, even at this load, the exposed surface of the packing is due entirely to the liquid phase. T h a t a liquid layer clf 8-A. thickness exhibits a normal surface tension has been proved only by implication. Further, i t is not posiible to tell from these data whether a phase separation has occurred, giving a surface layer of pure solute. At 25” C. both hexadecane and TDPN are near their freezing points. If the liquid had a seinirigid structure, one would espect surface effects to be
exaggerated. However, the results at the two temperatures are consistent with each other, thus giving credibility t o a liquid surface at 25’ C. LITERATURE CITED
W. J., Lee, E. H .,,,Wall, R. F., “Gas Chromatography, H. J. Noebels, R. F. Wall, N. Brenner, eds., p. 21, Academic Press, Xew York, 1961. (2) Beebe, R. A., Beckwith, J. B., Honic, J. M.. J . Am. Chem. SOC. 67. 1554 i 1945): ( 3 j Craig, B. >I., “Gas Chromatography,” N. Brenner, J. E. Callen, M. D. Weiss, eds., p. 37, Academic Press, Kew York, 1962. (4) Ettre, L. S., “Comparative Surface Measurements of Sumort Materials of Gas Chromatograph?,’’ 11th Pittsburgh Conference on -4nalytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 1, 1960. (1) Baker,
(5) Fukuda, T., Japan Analyst 8 , 627 (19591. ( 6 j Keller, R. A., Stewart, G. H., ANAL. CHEM.34, 1834 (1962). ( 7 ) Martin, R. L., Ibid., 33,. 347 (1961). (8) Ibid., 35, 116 (1963). (9) Ottenstein. D. M.. J . Gas Chromatoo. ’I (4), 11 (1963). ’ 0 ) Purnell, J. H., “Gas Chromatography,” p. 419, Wiley, Xew York, 1962. 1) Rosenberg, A. J., J . Am. Chem. SOC. 78.2929 f 14%) 2) .Scholz, .R. G., Ph.D. dissertation, Purdue University, June 1961.
RECEIVEDfor review June 28, 1963. Accepted November 7 , 1963. Contribution No. 1547 from the Department of Chemistry, University of California, Los Angeles. Taken in part from a dissertation presented by Angela de Yllana to the Graduate Division, University of California, Los Angeles, in partial fulfillment of the requirements for the degree of master of science, June 1963.
Through- t Limitations in Preparative Gas Chromatography Flu
DONALD T. SAWYER Department of Chemistry, University of California, Riverside, Calif.,
HOWARD PURNELL Department of Physical Chemistry, Universify of Cambridge, Cambridge, England
b The limitations on sample size imposed by feed-volume and solution concentration are discussed from the standpoint of established theory. Numerical examples are used to illustrate that optimum sample sizes for maximum column efficiency are considerably smaller than those which are commonly used in preparative gas chromatographic separations The same limitations to sample size apply also for analytical separations. The theory is extended to establish the conditions for maximizing the rate of sample separation.
S
has been achieved in applying gas chromatography t o preparative separations, and broadly, three approaches have shown promise. First, the counter-current method (not strictly chromatograpliic) initiated by Freund, Benedek, arid Szepesy ( 5 ); second, frontal analj& as used by Skarstrom ( I d ) and more recently in the elegant 1%-orkof Reilley, Hildebrand, and Ashley (11); and, fins lly, conventional elution chromatogra Jhy employing either large columns or multicolumn arrays. The first two methods are essentially based on an ability to recover two fractions per column and, thus, while offering large throughput, are likely to offer procedural difficLlties when many OME SUCCESS
component mixtures are t o be dealt with. I n the treatment of complex mixtures i t is highly likely t h a t gas chromatography may compete with other methods, in particular, distillation. Thus, in the immediate future, elution chromatography probably represents the major point of attack. This aspect of preparative chromatography is to be considered here although the other techniques mentioned may well ultimately be developed considerably and competitively. Preparative scale elution chromatography has u p t o now been exclusively a laboratory process. The quantities processed are thus in the scale of milliliters. Little has been done to make the method economic, mainly because i t is commonly accepted that increased throughput demands increased column cross-section and, in a single column system, that this inevitably leads to considerable loss of inherent column efficiency. However, there is good reason to believe that the reported loss of efficiency with increased diameter is not entirely due to reduced column performance but is to some extent caused by excessive sample scale-up and badly designed and operated injection systems. Only when the deleterious effects of excessive sample size and feed volume have been eliminated can a
proper estimate of wide column capabilities be made. So t h a t the maximum efficiency of separation can be realized for a given column, it is necessary t o ensure that, for the chosen experimental conditions, the efficiency should not be adversely affected b y the size of the injected sample, or b y the manner of its iniection. This is true for analytical as well as for preparative separations. Column performance may be adversely affected either by excessive feed volume or b y operation at excessively high concentrations of solute (or adsorbate) within the solvent (or adsorbent) in the early part of the column. For the latter case the relevant partition isotherm is no longer linear; both of these factors may lead to considerable band broadening and, possibly, distortion. This paper evaluates limiting sample sizes and feed volumes on the basis of well established principles and theory. Sumerical examples which relate to situations that are realistic in preparative gas chromatographic practice are given. Although a single column is used in these examples the conclusions apply equally to multicolumn systems. Finally, some proposals are given to enable greater .ample throughput per unit time. The latter is of outstanding importance for large VOL. 36, NO. 3, MARCH 1 9 6 4
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