manner similar to the procedure normally used in NMR. Thus, together with the resulting line assignments and their area ratios, the struci.ure of many chlorine-containing cornpounds can be determined. Quantitative analys IS appears to be a promising procedure in NQR, as area measurements show relationships similar to those normally encountered in NMR spectra. However, frequency variations are considerably greater in NQR spectra and as a result the sensitivity which varies with frequency poses a greater problem in the comparison of areas. For the example of the 1 -isomer of hexachlorocyclohexane the area ratio of 1 :2:2:1 for the four lines is only approximate. However, the areas are close enough to this ratio to suggest
that some day area measurements in NQR spectra may become as routine as they have been in NMR. ACKNOWLEDGMENT
The author is indebted to W. L. Truett, Wilks Scientific Corp., for use of the Model NQR-1 instrument and for supplying the two samples of hexachlorocyclohexane used in this work. RECEIVED for review March 3, 1967. Accepted March 29, 1967. Delaware Science Symposium, January 21, 1967. Contribution 164.
Theory of Solute Retention in Liquid-Liquid Chromatography David C. Locke Analytical Reseurch Ljiuision, Esso Research and Engineering Co., Linden, N . J . 07036 Daniel E. Martire Department of Chemistry, Georgetown University, Washington, D. C. 20007 Solute retention in I.LC systems based on a gas-liquid chromatographic (GLC) analog is considered theoretically. The distribution coefficient and several chromatographic parameters are defined for LLC. Equations are derived for the specific retention volume, V,, and its temperature-, pressure-, and structure-dependence. v,, is related to the ratio of solute activity coefficients at infinite dilution in the mobile and stationary phases, the molecular weights of the phases, and the eluent density. V,, is slightly pressure-dependent according to thle difference in the solute partial molar volumes in the two phases. The temperature coefficient is proportional to the partial molar enthalpy of transfer of solute from one phase to the other, and is of the same order of magnitude as that in GLC. I n many systems a linear relationship will exist between In V,, and solute carbon number. As in GLC, the potential exists for dletermining a variety of physicochemical informatiam from LLC retention measure. ments.
LIQUID-LIQUID CHROMATOGRAPHY (LLC), which was devised by Martin and Synge in 1940 ( I ) , is a powerful technique for the separation of liquid mixtures and solutions. The LLC systems considered in this paper are based on a so-called GLC analog (2). The analogy represents an experimental sophistication of the more conventional and familiar liquid chromatography arrangements, rather than a new LLC technique. The experimental system includes the use of relatively long, narrow, and thermostated columns packed with longlasting substrates; small ( ~ 1 )samples; chemically homogeneous mobile phases; relatively short analysis times; and continuous and sensi tive detection devices amenable to recorder output. We consider the thermodynamic theory of solute retention in LLC. Chromatographic retention parameters are related to solution properties, and the pressure-, temperature-, and structure-dependences of retention are deduced. As in GLC (3),the recovery of solution thermodynamic information ( 1 ) A. J. P. Martin and R.. L. M. Synge, Biochem. J., 35, 91 (1941). (2) C. Karr, Jr., E. E. Childers, W. C. Warner, and P. E. Estep, ANAL.CHEM., 35, 1290 (1963); 36,2105 (1964).
(3) J. H. Purnell, Endeauor, 23, 142 (1964).
from chromatographic retention data is possible. It should be noted that the results developed below in terms of the partition coefficients apply equally to LLC and solvent extraction. Retention Parameters in LLC. The physical basis of LLC is true liquid-liquid partition of the solutes between the two immiscible phases. As such, it is convenient to define an experimental distribution coefficient,
K =
concentration of solute in the stationary phase concentration of solute in the mobile phase
(1)
In order to relate K to chromatographic retention parameters, several assumptions must be met: (a) K must reflect true liquid-liquid partition as the only retention mechanism; (b) K is a constant throughout the column; (c) partition equilibrium is established rapidly relative to the down-stream migration rate of the solute in the mobile phase; (d) axial diffusion is negligible; (e) all possible solute paths through the column are equivalent; and (f) the sample is introduced as a very thin plug. Conditions (b) through (f) are standard chromatographic assumptions for the derivation of Equation 2; (a) is necessary for relating solute retention to fundamental thermodynamic properties of bulk liquid solutions. Constant K values are achieved by operation in the linear portion of the absorption isotherm, and are experimentally realized by admitting very small samples or by extrapolation to zero sample size. Rapid sample injection from a microsyringe or liquid sampling valve satisfies condition (f). Assumptions (c), (d), and (e) are met for those solute molecules in the sample that elute at a time corresponding to the maximum on the recorded elution peak, and declare the independence of peak maximum retention volumes from solute band broadening mechanisms. If we restrict our attention to these “average” molecules, we can consider chromatography to be either an equilibrium process, or one which does not deviate perceptibly from equilibrium, probably the latter (4). Assumption (c) precludes appreciable interfacial mass transfer (4) J. C. Giddings, “Dynamics of Chromatography,” Marcel Dekker, Inc., New York, 1965, p. 4. VOL 39, NO. 8 , JULY 1967
921
resistance. Finally, for our purposes, liquid interface adsorption must also be absent [assumption (a)]. Each individual experimental system will have to be tested to determine whether interfacial adsorption provides a n appreciable contribution to solute retention. Using an analysis similar to that suggested for GLC (9,this effect can be recognized and appropriate corrections made. If we can validly make these assumptions, then it is readily shown that the solute retention volume, VR, the amount of eluent passing through the column required to elute a solute band from the moment of injection to the top of the resulting recorded elution peak, is related to K by VE = Vm
+ KVs
(2)
where Vm is the interstitial volume of the column occupied by the mobile phase, the retention volume of a component of zero K , and V , is the volume of stationary liquid in the column. We may also define a net retention volume, VN = VR Vm = KV,. In order to express retention measurements in a form independent of experimental parameters, and therefore reproducible in any laboratory, the specific retention volume (6) is defined:
vg =
7 2
(T,P) = pzz(T,P)- p22,idea1 (T,P)
+
where ~ L ~ O , ~ (isT the , P ) chemical potential of the pure liquid solute at T and P and xa2is the mole fraction of solute in the solution. Choosing a standard pressure P*, then (T,P) =
(T,P*)
+ ( P - P*)
223-1
pi*'m(T,P*) =
V ~ O
pa**'
(T,P*)
(10)
The activity coefficient at the standard pressure P* is related to that at any pressure P by RT In ya (T,P) = RT In y2 (T,P*)
+
(Oa
-
(P
U ~ O )
- P*)
(11)
where 02 is the solute partial molar volume in the solution. Converting Equation 9 to the standard pressure P*, then in view of Equation 10 we have
RT In (X2'/Xam)
+ RT In -
(O*'
+
(T,P*) (P - P*) =
VaO)
RT In yarn(T,P*)
+ (Darn
- ~ 2 ' ) ( P - P*) (12)
We have defined the experimental distribution coefficient in LLC, where the solutions are very dilute, as
K
ne9 Vrn zn--to nzrnVs = lim
where nat is the number of moles of solute dissolved in chromatographic phase i at equilibrium, and V c is the volume of phase i in the column. For dilute solutions, ,a* = x2'n', where n' is the number of moles of phase i in the column, so that
where vio is the molar volume of pure liquid phase i. From Equations 12 and 14, assuming the solutions are infinitely dilute,
(4)
where pa'(T,P) is the solute chemical potential in the actual solution and (T,P) is the chemical potential the solute would have were the solution ideal: pzt,ideal (T,P) = pzovz(T,P) RT In X a z (5)
+ao"
We choose the standard state of the solute in the solution at any pressure and temperature to be the pure liquid s o l u t e Le., lim ylt = 1. Therefore
(3)
vN/ws = KIPS
where w, is the weight of stationary phase (density p s ) present in the column. In GLC it is necessary to consider the compressibility of the carrier gas. The carrier gas undergoes an acceleration in moving from the high pressure inlet end of the column to the outlet. For liquids, the compressibility is of the order of 10-4 atm-* and is negligible for ordinary pressures. Consequently, the eluent velocity is directly proportional to the pressure drop across the column and no corrections are required. Relationship between Solute Retention and Solution Properties. Consider a binary solution of components 1 (the solvent) and 2 (the solute) at equilibrium at pressure P and temperature T . The solute activity coefficient at T and P , y?(T,P),is defined by RT In
where p;(T,P) and pP(T,P) are the solute chemical potentials in the stationary phase and the mobile phase, respectively. Substitution into Equation 7 and rearrangement yields
(6)
where ~ 2 ' ' " is the solute activity coefficient at infinite dilution in phase i. Finally we must select the standard pressure, P*. Choosing P* = 1 atm, and assuming we validly can set P = B = (1/2)(Pf Po),the mean column pressure, then
+
In terms of the specific retention volume,
where uao is the molar volume of the pure solute and we / ~0.P It ) ~follows that assume ( ~ V ~ O = pz
(T,P) = pa*'' (T,P*)
+
+ RT In xaZya(T,P) (7)
where Mt is the molecular weight of phase i and p,(T) is the eluent density at T .
In liquid-liquid chromatography, the condition of equilibrium is that
Temperature-Dependence of Retention. From Equation 4, it follows that
(P
pn3
- P*>
UZO
(T,P) = pzm (T,f')
(8)
( 5 ) R. L. Martin, ANAL.CHEM., 33, 347 (1961); 35, 116 (1963). (6) A. B. Littlewood, C . S. G . Phillips, and D. T. Price, J. Chem. SOC.,1955, 1480.
922
ANALYTICAL CHEMISTRY
RT In yam( T J ) = p Z e(T,1)
(18)
where the superscript e emphasizes that the activity coefficient is an excess property, an excess arising because of differences between real and ideal solutions. We also know that
Thus
where hae and Sae are respectively the partial molar excess enthalpy and excess entropy of mixing of solute with solvent. Then
Martin (9) arrived at a result similar to Equation 24 by expressing the free energy of distribution as the sum of molecular group contributions. This is basically the same approach as that of Pierotti et al. (8). However, because we have related Vu to molecular structure through the activity coefficient ratio, Equation 17 (or 25) in conjunction with the relevant contributions of Equation 23 enables prediction of those systems where a linear retention volume-structure correlation will not be found. DISCUSSION
or
where hae,* is the partial molar excess enthalpy of mixing of solute with phase i, dL(hlB)is the partial molar enthalpy of transfer of solute froin mobile phase to stationary phase, and - 0); = Aiia is the difference between the solute partial molar volumes in the two phases. a m is the coefficient of thermal expansion of the mobile phase. The term in A& is negligible compared with the other two terms. Structure-Dependence of Retention. The variation of solute retention in LLC with the molecular structure of the solute molecules depends upon the relationship between the activity coefficients and structure. The correlations of Pierotti et al. (7) should be very useful in this aspect of LLC, having previously beein applied to GLC (8). In Pierotti's treatment, the activity coefficients are correlated with the structure of solute and solvent by a general equation, In yam = Ai,a
+ Blna/nl + Ca/n2 + D (n2 - n d a
+ K/nl
(23)
where 2 refers to solute and 1 to solvent, A, B, C, D,and F a r e empirical coefficients dependent upon the nature of the solute and/or solvent, and n is roughly the number of carbon atoms in the hydrocarbon portions of the molecules involved. In LLC, the activity coefficients for members of an homologous series of solutes in both the stationary and mobile phases can be represented in the simplest case by the first two terms on the right hand side of Equation 23. Although A&, (Equation 17) increases slightly with increasing carbon number, to a first approximation we can assume that A89 remains constant withiin an homologous series, and we then have In MmIMspm = ki
+ kga
(24)
where kl and ka are differences between the empirical constants for each phase. In these systems a linear relationship between In Vu and na can be anticipated, and in principle structural information is available from retention measurements. This behavior is frequently observed in GLC systems (3). In other solutionii a more complicated activity coefficient-structure correlation is required, and In V, may not be linear in solute carbon number. (7)G. J. Pierotti, C. H. Deal, and E. L. Den, Znd. Eng. Chem., 51, 95 (1959) and subsequent papers. ( 8 ) G. J. Pierotti, C. H. Deal, E. L. Derr, and P. E. Porter, J. Am. Chem. SOC.,78,2989 (1956).
Retention Volume. The fundamental equation of LLC, Equation 17, can be derived by a different route. If we define a thermodynamic distribution coefficient, K', as the ratio of solute activities in the two phases, then because we have selected the solute standard state to be the pure liquid solute, K' = 1 (Equation 10). Relating K' to the experimental distribution coefficient, K, the ratio of solute concentrations in the two phases, then for infinitely dilute LLC solutions,
Vo
= Yam*"Mm/^/aS'rnMsPm
(25)
which is identical to Equation 17 for A62 = 0 or P = 1. Equation 17 or its simplified version, Equation 25, not only allows us to deduce the temperature- and structure-dependence of solute retention in LLC, but in addition enables us to predict distribution coefficients and retention volumes a priori for any system in which the activity coefficients are known or can be calculated from solution theory. It is interesting to note that in GLC, Vu0: (yzmM s pao)-'. Here, solute retention is related to a solute property (vapor pressure, pao), to a solvent property (molecular weight, Ms), and to a solute-solvent interaction parameter (solute activity coefficient, yam). In LLC, the solute property has cancelled out. Consequently, the relative retention, which is the important parameter in the resolution of two solutes, will depend only upon differences in the solution behavior of the solutes in the two phases. Quantitatively, in GLC, QI = (V,o)a/(Vuo)b = ( P a oYam)o/(PaoYam)a,whereas in LLC,
In a sense we have lost a degree of freedom with the disappearance of the solute property, but we have gained one by introducing the second solvent and its associated interactions with the solutes. This should ultimately work to our benefit. The solute property (vapor pressure) ratio can be varied only through a temperature change. In LLC, the activity coefficient ratio in the second phase can be altered in two ways: by varying the temperature and by changing the nature of the second phase. On the balance, then, we have gained selectivity not attainable in GLC. The mobile phase extracts solutes from the stationary phase, and can be used to enhance or suppress solute retention. In GLC, the carrier gas acts only to sweep those solute molecules in the vapor phase down the column, ordinarily being an essentially inert, neutral fluid. In chromatography we generally want K values in the range from 1 to 100. For K < 1, separation of solutes becomes difficult because they elute very rapidly. For K > 100, analysis times become excessively long, and under ordinary ~
(9) A. J. P. Martin, Biochem. SOC.Symposium (Cambridge, Eng.), 3, 4 (1950). VOL 39, NO. 0, JULY 1967
923
conditions with packed columns, solute peaks become low and diffuse. These conditions imply that the stationary phase should be selected to be a better solvent for the sample than the mobile phase4.e.) the solutes should spend proportionately more time in the stationary phase while traversing the column. Nonetheless, the differential solute solubility between the two phases cannot be too large or the solutes will have excessive retention times. The fact that immiscible solvent systems are required in LLC generally implies that the dilute solutions formed by dissolving a solute in the two chromatographic phases will be distinguished by distinct differences in their respective deviations from ideal behavior. The measure of this solution nonideality, the activity coefficients, will therefore be different in each phase. Consequently the ratio of solute activity coefficients will differ from unity, and this is why LLC works. These considerations should be familiar to practicing chromatographers; we have expressed this in concise, quantitative form in Equations 17 and 25. In general, in solutions in which there are only weak interactions (dispersion forces) between all constituents, the activity coefficients will tend to be within an order of magnitude of unity. Where strong, specific solutesolvent interactions exist, 7 2 " is small, perhaps as small as 0.001 when chemical complex formation occurs. If the solvent-solvent and/or solute-solute interactions are stronger than solutesolvent interactions, large positive deviations from ideality and thus large activity coefficients are encountered because the solution components cannot effectively compete for each other. Effect of Solvent Molecular Weight. The ratio of phase molecular weights also appears in Equation 17 (and Equation 25). For a given system, the effect of a molecular weight difference is constant and only affects the activity coefficient ratio required to produce K values between 1 and 100. In general, the mobile phase may be a relatively simple organic liquid of fairly low molecular weight. The stationary phase may also be of low molecular weight, but may also be a typical GLC stationary phase, or a polymer. Thus in general we will find M,/M8 5 1. The influence of molecular weight is made apparent when retention volumes are compared on columns made with two solvents, say x and y, which are chemically similar, but of different molecular weight, M z > My. If we use the same mobile phase and operating conditions in both columns, the ratio of retention volumes on the two columns will be
where 72'9" and yzUvmare, respectively, the solute infinite dilution activity coefficients in the two solvents x and y of molecular weights M z and Mu. The effect of the stationary solvent molecular weight can be visualized if we take logarithms of Equation 28 and use a Flory-Huggins expression (1U-13) for the activity coeficients,
where we take r to be the ratio of molar volumes of solvent (10) D. E. Martire, "Gas Chromatography 1966" (Proceedings of Sixth Intern. Symposium on GC, Rome), A. B. Littlewood, ed., Elsevier, Amsterdam, 1967. (11) M. L. Huggins, Ann. N . Y.Acad. Sci., 43,1(1942). (12) P. J. Flory, J. Chem. Phys., 10, 51 (1942). (13) D. E. Martire and J. H. Purnell, Trans. Faraday SOC.,62,610 (1966).
924
ANALYTICAL CHEMISTRY
to solute, r = v10/u20 = Ml/p1u20,p1 is the solvent density, and W is an interaction energy parameter. Then
where x and y refer to the two solvents and 2 refers to the solute. If x and y are both larger molecules than the solutei.e., u ~ ~ >> / u1, and ~ ~ if x and y are chemically similar, e.g. members of an homologous series such as hexane and eicosane or polypropylene glycol 1200 and 2000-then at a given temperature WzyN WZ, and pz N pu = p8. Thus
All else being equal, once we have decided on a chemical type for the stationary phase, shorter retention times will result if we use a higher molecular weight compound of that type. The solute-solvent interactions are comparable within an homologous series of solvents ( W2, 'v Wz2),but the molar volumes will differ considerably. Thus, the effect of molecular weight on V, enters through the entropic (size) contribution to the activity coefficient, as expressed by the FloryHuggins theory (Equation 28). As shown by the form of Equation 30, the effect on V, will be most pronounced for MzIMu< 20. It is also interesting to consider the consequence of changing stationary phase molecular weight on the relative retention, a = (V,)u/( v g ) b , Using Equations 26 and 28, there results
where az and a, are, respectively, the relative retentions of compounds a and b on the two substrates x and y , and unosa and U Z are ~ the ~ molar ~ volumes of the pure solutes a and b, respectively. In terms of relative retention, then, if the two solute molar volumes are equal, changing the stationary phase molecular weight will cause a reduction in the retention volumes of the two components, but it will have no effect on the relative retentions. If > u ~ ~ relative p ~ , retention and consequently resolution will suffer by increasing the molecular weight. Finally, if uZotb > u20*a,aZ will increase with M z and the larger the difference in the molar volumes, the more beneficial the solvent molecular weight increase becomes. In this case, lower retention volumes and better resolution result using higher molecular weight stationary phases than those of lower molecular weight. If the above considerations are applied to the mobile phase, the results comparable to Equations 30 and 31 are identical in form, but the roles of M z and Mu are reversed. The conclusions are precisely the reverse of those for the stationary phase. For shortest analysis times, then, we should use a high molecular weight stationary phase and a low molecular weight mobile phase. It may be possible to utilize this effect to determine solvent molecular weights under controlled conditions, as has been done in GLC (13). Effect of Column Pressure. The mean column pressure also has a minor effect on the specific retention volume. This has been observed in liquid-solid systems by Piel (14), who used an ingeniously simple centrifugal device to generate micro-column inlet pressures greater than 1000 atm. Because the difference in solute partial molar volumes in the two phases is generally quite small, the numerical value of the ~~~g~
(14) E. V. Piel, ANAL.CHEM., 38, 670 (1966); also private com-
munication.
ensure meaningful and accurate yz" data, we must have pressure term in Equation 17 may commonly be I0-O to of the gas phase nonideality (solute-carrier gas knowledge 10-2. Thus the effect will be observable only when very interaction second virial coefficient) (15) in order to correct accurate measurements are made at relatively high pressures. retention measurements for gas phase interactions. Accurate At very high pressures (>lo3 atm), the ordinarily negligible virial coefficient data are available or can be reliably estimated mobile phase compressibility must be considered, and therefor nonpolar solute vapors. These data are neither readily fore also the effect of pressure on the molar volumes (densiavailable nor accurately calculable for polar solute vapors. ties) of the two phases. Thus Equation 17 will be useful for The resulting inability to correct for gas phase imperfections pressures up to about LO"lO3 atm and Equation 25 should can lead to significant errors in 72". However, more exadequately describe retention under ordinary operating tensive and reliable liquid phase activity coefficient data are conditions-Le., mean clAumn pressures close to 1 atm. available or can be more easily measured for polar solutes Effect of Temperature, Equation 22 predicts a linear variathan second virial coefficient data. Consequently the data tion of In V , with re1:iprocal column temperature. It is available from LLC should be of more direct, practical value interesting to compare temperature effects in LLC with GLC. and interest to physical chemistry and chemical engineering In GLC, the temperature dependence of Vg0is largely determined by the heat of vaporization of the solute from soluthan the solution data obtainable by GLC. tion, which is of the order of 5 kcal/mole. In LLC, V , From the temperature dependence of the specific retention volume (Equation 22), at low column pressures it is possible depends instead upon the difference in the partiaI molar to evaluate enthalpies of transfer of solute from mobile phase excess enthalpies of mixing of solute with solvent in the two to stationary phase. If the partial molar enthalpy of mixing phases. From Equation 20, we can readily calculate that is known in one phase, h: in the second phase can be calcuhae increases by about +1.4 kcal/mole for each order of lated. magnitude increase in y2". For Yaw = 1, hae = 0. To By studying the effect of column pressure on retention, it determine a typical numerical value for Equation 22, assume V , = 100 cc/gm, Mm/Msp m = 1, so that y ~ ~ , ~= /100. y ~ ~ may , ~ be possible to evaluate differences in the solute partial molar volumes in the two phases. Although the pressure If the solute-stationary phase solution is roughly ideal, yplsm = 1, y2m,m= 100, and (hpesm - htm8) = 2.8 kcal/mole. effect is of the same order of magnitude as the compressibility, Commonly am = 10-3 deg-l. Thus RT2am'v 0.2 kcal/mole the measurement may be worth pursuing for solutes difficult to handle by conventional techniques, such as very slightly at 300°K. Finally, in an extreme case the solute partial molar volume difference between the two phases could be 10 soluble materials. By combining this information, differences in partial molar cc. For a mean column pressure of 10 atm, this contribution to the temperature dependence becomes 0.002 kcal/mole, excess entropies and free energies of mixing between the two which is negligible for normal column pressures. Thus in phases can be deduced, because LLC, R (d In V,)/[d(l/T)] = 3 kcal/mole, which is less than RT In y2mtm/y2t*m= Aple = Ahae TAS2' (32) the GLC temperature dependence but of the same order of Again, the potential exists for determining the individual magnitude. This will pieobably be true in general. quantities as well. The accuracy of these measurements The significant temperature coefficient implies the necessity remains to be experimentally tested. Presumably, retention for good column temperature control for reproducible volumes can be determined to within =k1 %, which implies retention volume measurements. It is anticipated that the an accuracy of a few in the activity coefficients and corrange of temperatures required is somewhat shorter in LLC responding accuracies in the related thermodynamic quantithan in GLC, because the solutes need only be kept in solution ties. rather than in the vapor phase. Higher column operating LLC has several advantages over conventional techniques. temperatures may prove a problem in many LLC systems Data is obtained rapidly; the method is technically simple. because of the increasi:d phase miscibility with increasing Because only very small samples are required, and because temperature. In LLC, however, we have the option of LLC is a separation technique, measurements can be made on changing the mobile phase rather than increasing the temrare and/or impure materials, and in addition, several inperature to reduce retention times. In addition, because dividual measurements can be made on a single chromatosamples can be introduced at the column temperature, a hot gram. In the elution mode of operation, distribution coeffiinjection block is not required. Thermal decomposition cients and activity coefficients are calculated at effectively should consequently never have to be a problem in LLC. infinite dilution. If the frontal analysis technique is employed, The possibility of cryogenic LLC is also obvious, using as in which a continuous solution of sample in eluent is introeluents Freons or liquified gases and vapors at very low duced into the column, it should be possible to measure districolumn temperatures. bution coefficients and activity coefficients at any concentraPhysical Measurements. A variety of physicochemical intion. Work is proceeding on a theoretical treatment of formation is apparently available from LLC retention data, frontal analysis LLC similar to the one presented here for From retention measurements at pressures near atmospheric, elution LLC. we can directly calculatl: distribution coefficients and activity Because of a dearth of LLC data in the literature appropricoefficient ratios; if ya'-" is known to be unity in one phase, ate for the testing of the theory presented here, experimental if it can be calculated, or if it has been independently meastudies are in progress to this end (16). sured, we can obtain :'a" in the second phase. Although RECEIVED for review December 29, 1966. Accepted April 19, GLC also offers reliable 7 2 " data, LLC has two important 1967. Presented in part Division of Analytical Chemistry, advantages. In GLC, we are largely restricted to volatile 152 Meeting, ACS, New York, September 1966. solutes in nonvolatile solvents. However, both phases in LLC can be relatively simple molecules while the solutes can (15) D. H.Desty, A. Goldup, G. R.Luckhurst, and W. T. Swanton, be of either low or high molecular weight, and of either "Gas Chromatography 1962," M. van Swaay, ed., Butterworths, simple or complex structure. LLC is also probably better London, 1962,p. 67. suited for polar solute systems than is GLC. In GLC, to (16) D.C. Locke, J. Gas Chromorog., 5, 202 (1967).
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925
