Gas-Liquid-Solid Chromatography-Theoretical Aspects and Analysis of Polar Compounds Antonio Di Corcia, Arnaldo Liberti, and Roberto Samperi’ lnstituto di Chimica Analitica, Citta Universitaria, 00785 Home. l t a l y
Adsorptive modifications on graphitized carbon blacks, (graphitized Sterling FT and Graphon) carried out by deposition of polymeric macromolecules of polyethylene glycol 1500 (PEG 1500) have been evaluated by increasing the surface concentration of PEG 1500 up to the formation of a few layers on the adsorbing media. The role played by both the liquid and the solid medium in the sorption process of vapors has been investigated by measuring heats, entropies, and capacity ratios for some polar and nonpolar eluates at various liquid/solid ratios. Experimental evidence is given for the occurrence of multilayer adsorption of macromolecules of PEG 1500 on the graphitized carbon surface. A simple, chrornatographic criterion is suggested to evaluate the completion of the first layer of the liquid phase. The great usefulness of liquid-coated solids in the chromatographic analysis has been made evident by evaluating gas-liquid.solid columns in terms of selectivity, efficiency, and thermal stability. Also, the high performance of such columns has been exploited in the elution of some interesting, very dilute aqueous solutions of polar compounds.
Gas-solid (GSC) and gas-liquid (GLC) chromatography are the techniques traditionally used and though the attractive features of the former have been recognized, the latter has been far more extensively applied. The factors limiting the use of GSC have been the lack of adsorption media with geometrical homogeneous surface, the poor reproducibility of adsorbing materials, and the high heats of adsorption which confine the use of GSC to the separation of gases and low-boiling compounds; a n additional limitation is the low column loading. The main drawback is, however, the surface heterogeneity which is responsible to a greater or lesser extent for peak asymmetry and, in some cases, for irreversible adsorption. Though highly homogeneous adsorbing media, such as Porapak and Chromosorbs Century Series have been introduced, some limitations to the extensive use of GSC still remain because of the limited variety of homogeneous materials available. In addition, traces of chemical heterogeneities on the surface of these adsorbing materials prevent the linear elution of tiny amounts of very polar compounds. Several methods have been adopted to linearize adsorption isotherms in GSC, such as the elimination of surface heterogeneities by chemical or thermal treatments and the blocking of surface impurities by coating adsorbents with small amounts of properly selected, high-boiling, polar and nonpolar liquids. The use of these compounds, commonly referred to as “tailing reducers,” is a wellknown practice to obtain symmetrical peaks, which was suggested by Eggertsen e t al. ( I ) . They succeeded in sepa1Fellowship owner
of t h e Fondazione “ G . Donegani,” Rome, I t a l y .
( 1 ) F. T. Eggertsen, H. S. Knight, and S. Groennings, Anal. Chem., 28, 303 (1 956),
1228
A N A L Y T I C A L CHEMISTRY, VOL. 45, NO. 7, JUNE 1973
rating hydrocarbon mixtures by adding squaiane t o a Pelletex Carbon column and. in the submonolayer region of liquid concentration. observed a sharp decrease ( i t the retention time as the liquid/solid ratio was increased i i ’ i . Though this technique has been found to improve noticeably the chromatographic performance of a GS column. no systematic investigation has been made 10 correlate the chromatographic behavior with thermodynamic data upon adsorption on liquid-treated solids. Only a qualitative assessment of the chromatographic behavior of adsorbing media coated with variable amounts of’ liquids has been made (3, 4). Even where liquids are added in tiny amounts just t o block surface heterogeneities, the chromatographic teatures of a GS column may be profoundly altered. This modification in the chromatographic process arises from the combined effects of the chemical and physical properties of both the solid and the liquid phase. The elution of a certain compound through a column where these effec~i are acting together is defined as gas-liquid-soiid chromatography (GLSC),according to Purnell(5). Recently, graphitized carbon black IGCH) cozted w i t h variable amounts of a basic liquid phase. such a i tetraethylenepentamine (TEPA). was successfully used in the linear elution of aliphatic amines (6). Linear elution a t nanogram level of free acids and phenols ‘ 7 , d j ha.been achieved by coating the surface of the G(’H with s suitable amount of a n acid, stationary phase, such as FFAP (Varian Aerograph). More recently, analysis of C Z - C ~acids a t concentrations down t o 0.5 ppm in water solution has been performed on a GLS column packed with Graphon coated with 0.57~H3P04 and 3%‘ Carbowax 20 M (9). The results of the cited works show clearly that hb GLSC the advantages of’ both GSC and GLC can be exploited: linear elution of very polar compounds is made possible, column efficiency is good even a t high linear carrier gas velocities, column “bleeding” is by far attenuated. and mainly that by varying the nature and the surface concentration of the liquid a n extremely wide range of‘ se. lectivity is made available. The present paper deals with the study of adsorptivt modifications obtained from the addition of’ variable amounts of a liquid phase on graphitized carbon black. General information needed to predict the best liquid, solid ratio required to obtain the most satisfactory resuli in a n analytical separation is also given. Some interesting. (2) F. T. Eggertsen and I-. S. Knight. A n a / Chem.. 30, 15 (19581. (3) C. G. Scott, J I n s f . Pefroi.. London. 45, 115 (1959) ( 4 ) 0. Grubner and E. Srnolkova. “Gas Chromatography, ’ 3rd Conf. Anai. Chem.. Prague. Sept. 1959. (5) H . Purneli, “Gas Chromatography,” John Wiley 8 Sons, New York, N . Y . , 1962, p 375. (6) A. Di Corcia. D. Fritz. and F. Bruner, A n a / . Chem.. 42, 1500 (1970). (7) A. Di Corcia, Ana/. Chem., 45, 492 (1973). (8) A . Di Corcia. J. Chromatogr.. in press. (9) A. Di Corcia and R. Samperi, in press.
b
Measurements of isosteric adsorption heats were carried out by injecting small amounts (-40-60 ng) of vapors of the desired compound and by plotting, at five temperatures, values of log V R / T us. T-l, where V R is the volume of the carrier gas (passing through the column during elution of the sample) less V,, which is the volume needed to elute a non-adsorbed gas. The temperature throughout any one set of chromatographic measurements was kept to within f O . l "C. It was possible to keep the carrier gas flow rate to within f 0 . 8 % during the course of a day. Repeated measurements of the retention volumes were in agreement to within 0.3-0.4%. The precision in the measurements of adsorption heats was calculated to be good to within 3%. On the contrary, the precision in measurements of solution heats for alcohols in PEG 1500 was good only to 10% because of tailing in the peaks.
DISCUSSION OF RESULTS In order to obtain information on the modifications to the adsorptive process for eluates on liquid-coated solids, it has been found quite convenient to express the results in terms of isosteric heats of adsorption, q s t , of some, selected compounds as a function of the percentage of the liquid phase (PEG 1500) added on GCB. Values of qst have been calculated for an alkane, pentane, and for a set of alcohols, methanol, ethanol, and propanol adsorbed on Sterling FT-G + PEG 1500 and Graphon + PEG 1500 and are plotted, respectively, in Figure 1, a and b . Dashed lines parallel to abscissa indicate heats of solution for the same compounds obtained with the GL column described in the experimental. Adsorption heat a t zero-coverage for methanol on the C5HIZ _ _ . _.._ _ __ . ._ . _ _ . . _ _ _ _ surface of Sterling FT-G has been measured by Kiselev , , pyq e:3 (10); from this value, limiting adsorption heats for ethanol .2 4 6 .8 to 1.2 1.4 and propanol have been approximately derived, as adsorpof PEG 1500 tion heats can be considered sums of contributions for Figure 1. lsosteric heats of adsorption for pentane, 0 ; methaeach structural group [q(CHa) = 2.1 Kcal/mole and nol, 0 ;ethanol, c1; and propanol, A , on a. Sterling FT-G f q(CH2) = 1.6 Kcal/mole]. PEG 1500; b. Graphon f PEG 1500. Dashed lines indicate Heats of adsorption a t zero-coverage for pentane on heats of solution in PEG 1500 Sterling FT-G and Graphon have been previously calculated (11). The behavior of the adsorbing Sterling FT-G + analytical results concerning elution of very diluted, aquePEG 1500 system is discussed first: At low percentages of ous solutions containing polar compounds are shown. PEG 1500, the adsorption heat for pentane remains unEXPERIMENTAL changed, whereas it increases for alcohols. By increasing the amount of PEG 1500, a rise in qst is observed for all The graphitized carbon blacks (GCB) used in the present work, the eluates and a maximum appears. After the first maxidesignated as Graphon and graphitized Sterling F T (Sterling FT-G) were provided, respectively, by the Cabot Corp., Billerica, mum, adsorption heats show to a greater or lesser extent a Mass., U.S.A. and by Elettrocarbonium, Milano, Italy. The two steep decrease. At relatively higher percentages of PEG carbons listed above possess essentially nonpolar and nonporous 1500, a second and third small but real maxima can be surfaces and gave values for specific surface areas of about 110 observed. and 13 m2 per gram based on nitrogen adsorption (B.E.T.) and The interpretation of this behavior can be made taking by assigning for the cross-sectional area of Nz a value of 16.2 A2. into consideration that the general shape of these curves The adsorbing materials were sieved and the 60-80 mesh fraction was used. The materials were then coated with appropriate is analogous to the ones obtained for multimolecular adamounts of polyethylene glycol 1500 (PEG 1500), previously dissorption of a single component on a homogeneous surface. solved in methanol. The minima amounts of PEG 1500 needed to The thermodynamic evaluation has been also made in block surface heterogeneities and obtain perfectly symmetrical terms of surface coverage ( e ) , which is the ratio between peaks for alcohols were, respectively, about 0.2% and 0.6% w/w for the number of adsorbed macromolecules and the ones Sterling FT-G and Graphon. These amounts may undergo slight needed to form one complete monolayer. The completion variations depending on the particular batches the adsorbents come from. of one monolayer is assumed to be formed just after the A GL column was prepared by coating 60-80 mesh Chromosorb first maximum observed. W with 30% PEG 1500. Assuming that the adsorbing surface .is very honiogeThe apparatus used was a Carlo Erba gas chromatograph neous, a t low surface coverage of PEG 1500 there is hardly model GI (Milan, Italy) equipped with a flame ionization detector a possibility for a single molecule of eluate being adsorbed and connected to a Leeds & Northrup Speedomax model G reon the top of preadsorbed macromolecules of PEG 1500: corder operating with a 1-mV, full-scale response. At the maximum sensitivity of the amplifier system (1 X 1) about 1.5 pA give the adsorption heat remains thus unchanged up to a cera full-scale response of the recorder. The chromatographic appatain surface coverage. The progressive rise in heat with ratus is constructed so as to allow the sample to be injected dithe increase of the surface coverage is due to lateral interrectly into the column. All chromatographic columns used were
't~-:-,-, ~~~
~~
~
,
,
I
,
I
made from 2-mm i.d. glass tubing 1.4m in length. Nitrogen was used as carrier gas.
(10) A. V . Kiselev. Discussions F a r a d a y SOC . 40, 2 0 5 ( 1 9 6 5 ) . (1 1 ) A. Di Corcia and R. Samperi. in press. A N A L Y T I C A L C H E M I S T R Y , V O L . 45, N O . 7, J U N E 1973
1229
actions which can be established on the adsorbing surface between molecules of PEG 1500 and those of the eluate. As soon as macromolecules of PEG 1500 of the first layer are more or less closely packed, a two-dimensional condensation can take place. Adsorption heats for eluate molecules on the top of this monomolecular layer are generally smaller than the heats for adsorption on the last portions of the GCB surface being occupied, as a consequence, a steep drop in the heat-coverage curve is observed near the completion of the first layer. On the top of this layer, other macromolecules of PEG 1500 can be adsorbed and a second layer starts. The completion of the second layer is again indicated by the appearance of a more or less pronounced maximum in the adsorption heats. The subsequent maxima and minima are less pronounced as the adsorption into the second and succeeding layers is not so well defined as in the first. As the deposition of layers is continued, heat of adsorption will approach condensation heat. It is quite important to evaluate differences in the adsorption process between polar and nonpolar eluates in terms of the surface coverage of liquid phase. At low surface concentration, 0.2% PEG 1500, adsorption heats for alcohols show a rise in heat, which is steeper for the more polar compounds, whereas adsorption for pentane does not show any change in heat. The OH groups of adsorbed alcohol molecules can establish long-range, specific, attractive forces with the ether oxygen atom of -CHzOCHzlinks of polyethylene glycol macromolecules, while for pentane only weak, nonspecific, Van der Waals forces are operative. The completion of a more or less closely packed monolayer of PEG 1500 can be evaluated at about 0.6590w/w. At the completion of this monolayer, a new adsorbing surface containing functional groups alternating regularly with hydrocarbon groups is achieved. This layer is thus able to adsorb polar molecules more strongly than nonpolar ones. Pentane shows, therefore, a much sharper drop in qst than alcohols. Only about one-half of the amount required to fill the first monolayer is needed to fill the second layer. This decrease can be presumably accounted for by the large macromolecules bridging depressions on the surface of the solid. It follows that the surface area of the first layer is lower than that of the adsorbing medium. In the filling up of the second layer, lateral interaction contributions once more cause an increase in the adsorption heat for alcohols and a second maximum is observed near the completion of the second layer of PEG 1500. For pentane, weak, lateral interactions with macromolecules of the second layer result in a quite distinct step in qst curve which rapidly tends to approach the heat of solution. The highly homogeneous nature of Sterling FT-G which allows the deposition process of the liquid phase to occur as multimolecular adsorption is well-evidenced by no initial drop in qst a t low coverages, by a well-defined maximum a t 8 1, by an almost vertical fall in qst a t 8 = 1, and by the appearance of the second and third maxima near the completion of the second and third layers of macromolecules, respectively. Surface heterogeneities can hinder to a greater or lesser extent multilayer deposition of the liquid phase. Graphon possesses a higher degree of surface heterogeneity than Sterling FT-G. Then, the behavior of heats of adsorption on the Graphon PEG 1500 system presents some differences, though the general shape of q s t curves is analogous to the one already discussed. The initial drop in qst for pentane and propanol is due to a preferential adsorption
-
+
1230
A N A L Y T I C A L C H E M I S T R Y , VOL. 45, NO. 7, J U N E 1973
of PEG 1500 on high-energy sites of the Graphon surface, which then become unavailable for adsorption of eluates. The appearance of maxima and minima in the heat-coverage curves is not so well defined as for adsorption on Sterling FT-G + PEG 1500. The smoothness in the heat curves indicates that multimolecular deposition of the liquid phase does not occur in a regular way and adsorption of macromolecules of PEG 1500 can take place on the top of other macromolecules before the filling of the layer. Nevertheless, the formation of one layer of PEG 1500 can be approximately established a t about 4.5% w/w. In conclusion, the action of a liquid phase added on homogeneous adsorbing media causes a deactivation of the surface at very low percentages. An increase in the surface concentration results in an increase of adsorption energies for eluates by means of lateral interactions. At the surface concentration needed to form a dense monolayer, adsorption of eluates on the top of macromolecules occurs; thereafter a combined effect is observed due to solution into the liquid film and adsorption on its outer layer. It should be pointed out that partial orientation of the liquid layers nearest to the solid can, however, affect partitioning properties of the liquid phase to a greater or lesser extent, depending upon the surface activity of the adsorbing medium, the nature, and the amount of the liquid phase. Plots of differential standard entropy changes on adsorption for pentane and ethanol a t 89 "C as a function of the surface concentration of PEG 1500 are shown in Figure 2. Such calculations are made to supply information on the modifications of the degrees of freedom of individual, adsorbed molecules as the surface concentration is varied. Additional criteria to indicate the completion of the monomolecular layer can also be obtained. Standard adsorption entropies have been calculated according to a relation previously reported ( I I). Adsorption entropy loss values at zero-coverage for pentane on Graphon and Sterling FT-G shown here have been calculated previously (12).
In all cases, an increase in the entropy loss on adsorption is observed as the surface concentration of PEG 1500 is increased and before a macromolecular monolayer is formed. The addition of the liquid phase progressively reduces thermal motions of adsorbates and decreases the number of possible configurations of the adsorbate-adsorbent system. As far as the highly homogeneous surface of Sterling FT-G is concerned, the pronounced minimum in the entropy loss on adsorption near the completion of the first layer of PEG 1500 and the subsequeut sharp increase for pentane indicate that the first layer is virtually filled before the second one gets appreciably under way. The adsorption of molecules with functional groups is greatly localized on a polar liquid phase surface. This behavior is made evident by the fact that, a t the completion of the monolayer, the rise in adsorption entropy for ethanol is not high if compared with that for pentane. As far as PEG 1500-coated Graphon is concerned, the initial increase in the adsorption entropy for pentane confirms the presence on the surface of this adsorbing material of some irregularities. where the adsorption for eluates is localized to some extent. Adding small amounts of a polar-liquid phase is sufficient to block topographical irregularities and neutralize chemical heterogeneities. This initial local concentration of deactivating macromolecules on active sites provokes some irregularities in the deposition of the liquid phase, as it is shown by the smoothness of entropy curves and particularly by the fact that the first minimum is not very pronounced both for pentane and ethanol.
2
8
6
4
1
2
4
6
8
10
12
14
2
4
6
8
10
1
,
12
14
PEG 1500
' l a 01
Figure 2. Differential adsorption entropies at 89 "C for pentane and ethanol vs the amount of PEG 1500 deposited on Graphon (upper scale abscissa) and Sterling FT-G (lower scale abscissa). Dashed lines refer to adsorption on Sterling FT-G 4- PEG 1500 and full lines to adsorption on Graphon PEG 1500
+
i
8ol t t
9.1
Q=2
I
I
a
0.3
1 b
K
t
A -
'fo of
Figure 3. Capacity ratios vs. the relative amount of PEG 1500 on a. Sterling FT-G ratios on the gas solution column of PEG 1500 are indicated as short lines In Figure 3, a and b , are shown plots of a chromatographic parameter, that is the capacity ratio ( k ) , defined as the ratio between the corrected retention volume and the dead volume, us. the surface coverage of liquid phase a t 89 "C on Sterling FT-G and Graphon, respectively. At the same temperature, k values have been measured on a gas-liquid column of PEG 1500. As the chromatographic k parameter is related to the free enthalpy changes of the chromatographic process, plotting k values yields direct, meaningful information on the modification to the adsorption process caused by the addition of molecules of a liquid phase. The shape of the k curve for butanol adsorption on modified Graphon can be used to illustrate variations in the adsorption free enthalpy due to preadsorption of macromolecules. Filling high-energy sites by the liquid phase results in an initial decrease in adsorption free enthalpy: butanol is thus less retained. As the surface coverage of PEG 1500 increases, so does the retention time of butanol, since the adsorption heat increases by lateral in-
+ PEG
PEG 1500
1500; b. Graphon 4- PEG 1500. Capacity
teractions displayed between the adsorbed eluate molecules and -CH20CH2- links of PEG 1500. At higher surface concentrations, a sharp drop in the retention of butanol is observed. This is explained by the progressive, sharp decrease of the configurational entropy upon adsorption and by the reduction of the available surface for adsorption; these effects are not counterbalanced by the increase in the adsorption heat. In the region where multimolecular adsorption takes place, the k curve is smoothed and its slight increase is likely due t o the contribution of partial solution into the liquid film which starts to form. Capacity ratio-coverage curves for other compounds and for adsorption on Sterling FT-G have a similar trend, the differences depending upon the degree of homonogeneity of the solid medium and upon the strength of the interactions between the liquid phase and eluate molecules. It is worth noting that from the completion of one monomolecular layer of PEG 1500, the capacity ratio of pentane remains almost unaLered. This suggests a very simple, chromatographic method for an approximate estiANALYTICAL CHEMISTRY, VOL. 45, NO. 7, JUNE 1973
1231
mation of the monolayer capacity of adsorbing materials, providing the eluate is properly chosen. k values calculated on GLS columns are constantly higher than on PEG 1500 GL columns. However, a comparison in terms of analysis time has, in this case, no practical meaning, because this low-polar, liquid phase cannot be employed for linear elution of alcohols in GL chromatography.
EVALUATION OF GLS COLUMN In order to evaluate the working mechanism and feasibility of GLS columns in separating eluates, differences in (AG’ - A G ) / T a t 89 “C for some, selected, isomeric pairs have been calculated and plotted us. the surface coverage of PEG 1500 on Sterling FT-G (Figure 4). This term, which is equal to R In a , where a is the separation factor, is graphically represented as resultants from the plots of the A S - A S term and AH’ - A H / T . An interesting feature to be outlined is the initial decrease in heat differences for the isomeric pairs except the 2-pentanone + 3-pentanone pair. Starting from a surface coverage of about 0 = 0.5, differences in adsorption heats turn to increase, as would be expected considering the contributions of lateral interactions by the liquid phase to be added on to adsorption on the GCB surface. An explanation for this initial decrease can be attempted considering that the energy of adsorption depends upon the distance between the force centers of the individual, adsorbed molecules and the solid surface. Differences, in adsorption heats for an isomeric pair, then, arise mainly by differences in geometric structure and orientation of molecules relative to the surface of GCB. Adding macromolecules results in introduction of force centers other than surface carbon atoms. Perturbations caused by lateral interactions modify the arrangement in the orientation of individual, adsorbed molecules. Thus, differences in energy of adsorption on the carbon surface for isomeric pairs may be remarkably altered. The behavior shown by the 2-pentanone + 3-pentanone pair can be presumably due to the fact that interaction potentials for the two isomers with the carbon surface are perturbed to the same extent, so that differences in adsorption energy remain unchanged. After the drop in the heat differences at the completion of the first monolayer and before a second layer of PEG 1500 is completed, the heat difference for the 1-propanol 2-propanol pair tends to reach the value attainable in the pure solution process. This leads to the conclusion that even at relatively low liquid/solid ratios the adsorption process tends to change toward a solution process. As can be seen, modifications in the orientation of individual, adsorbed molecules by means of lateral forces emanating from macromolecules of PEG 1500 sharply affect differences in adsorption entropies for isomeric pairs. The steeper decrease in entropy loss differences with respect to the decrease in heat differences accounts for the initial increase in separation factors for 1-propanol + 2propanol and 3-methyl-butanal 2-methyl-butanal. In all cases, for polar, isomeric pairs, maxima in the separation factors are attained before the completion of the first monolayer. Because of the limited number of systems considered, it is difficult to indicate the “ideal” liquid/solid ratio to be employed for any analytical separation in order to realize the best separation factors. Moreover, each analytical separation involves many factors which have to be carefully considered in order to obtain a “tailor made” chromatographic column able to yield the best analytical result. Nevertheless, an adsorbing medium almost com-
16)
-
2
10-
1 5 ‘vl a
8-
g
1
i
v)
4
6-
4-
2-
f
-24
-
_ _ _ _ _ _ _ _ _ _ - _ - - - - - - - ----- 1
,
e=i ,
I
,
e=z ,
,
,
8-3 ,
,
,
,
,
+
+
1232
A N A L Y T I C A L C H E M I S T R Y , VOL. 45, N O . 7, J U N E 1973
pletely shielded by one monolayer of a suitable liquid phase appears to be best in order to achieve good separation factors in the chromatographic elution. The separating power of a chromatographic column depends both upon the selectivity of the packing material and upon the spreading of the chromatographic band. In this connection, measurements of column efficiency were carried out as a function of the linear carrier gas velocity on some GLS columns packed with Graphon PEG 1500 (Figure 5 ) . Van Deemter curves have been calculated a t 100 “C for two well-retained eluates, butanol and pentanone. Van Deemter curves relative to the elution on the Sterling FT-G + PEG 1500 system showed a behavior similar to that cited above and for this reason are not reported. As can be seen, all GLS columns investigated exhibit a very good efficiency which, especially for elution on the
+
1.3
1
*'
H (mmi ' 2-
/'C'
t
1.1 -
10-
.9 -
.8.7 -
.6-
.5.4 -
5.
-
10
20
15
U ~cmlsecl
Figure 5. Column efficiency vs. the linear carrier concerning the elution of 2-pentanone, 0 . and butanol, e , on Graphon 1 YO PEG 1500 (-), Graphon 4 % PEG 1500 (- - ) , and Graphon 9% PEG 1500 ( - -)
+
+
+
m
1 IpAl
t
6o
50
40
30
20
10
70
90
110
130
150
170
190
210
t('C1
Figure 6. Dependence of ionic current, I , of a flame ionization detector on the column temperature, t, for: Chromosorb W PEG 1500, 0 ;Graphon 5% PEG 1500, .; Graphon'+ 9%, A ;Graphon 4- 0.6% PEG 1500, 0 ; and Graphon 4% PEG 1500, 0
+
+
low-coated Graphon surface, is more than 2000 plates per meter of column a t optimal carrier gas velocity. The effi1%PEG 1500 ciency of the column filled with Graphon is only slightly dependent upon the carrier gas velocity. It has been shown previously (7) that a t high linear carrier gas velocities, the performance of a GLS column with a low liquid/solid ratio is by far higher than that of a GL column. In this respect, a GLS column behaves as a GS column. Such a column, provided the solid medium is nonporous, has a large mass-transfer coefficient since the kinetic process upon adsorption is governed mainly by diffusion in the gas phase to the solid surface. By increasing the liquid/solid ratio, GLS columns show a progressive increase in the slope of the right-hand branch of the Van Deemter curve. Also, the minimum is shifted toward low values of carrier gas velocities. The experimental results show that solids coated with the minimum amount of liquid phase needed t o achieve symmetrical peaks, are the best packing materials in order to obtain high column efficiencies.
+
+ 30%
Heats of adsorption are generally higher than heats of condensation and therefore, volatility of molecules adsorbed as monolayers on solid media is much lower than that of molecules in the corresponding bulk liquid. For this reason, Kiselev (12) predicted that adsorbed monolayers of stationary phases can be employed a t temperatures higher than the upper limit of temperature a t which the corresponding GL columns can operate. To obtain experimental evidence, thermal stabilities of some GLS column have been tested and compared with that of a GL column (30% PEG 1500). In Figure 6, the background current of the FID is plotted as a function of the column temperature. From the graph, it is evident that column "bleeding" is considerably reduced when the liquid phase is deposited on an adsorbing surface a t the rate of one or a few monolayers. This behavior suggests that GLS columns can be usefully employed a t programmed temperature (12) A. V . Kiselev, "Advances in Chromatography," 4, J. C. Giddings and R . A. Keller, Ed., Dekker, New York, N . Y . , 1967.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 7, JUNE 1973
1233
N
m r x
31
N
m x
-
i 1 1
1
0
1
1
1
2
1
4
1
6
1
i ,
-. ,
8
1
10
time lminl
Figure 7. Chromatogram of the elution of a water solution of c 2 - C ~ aliphatic aldehydes. Column, 1.4 m X 2 m m ; packing 0.2% PEG 1500; column temperature, material, Sterling FT-G
+
101 "C; linear carrier gas velocity, 11 cm/sec; sample size of each aldehyde, -30 ng 1, Ethanol; 2, propanal; 3, 2-methylpropanal; 4, butanal; 5, 2-methyl-butanal;6, 3-methyl-butanal;7, pentanal and for the determination of traces. In addition, operating a t higher temperatures decreases retention times. The most satisfactory results, as far as the thermal stability is concerned, are obtained with a GLS column (Graphon + 4% PEG 1500) where a monomolecular layer is almost completed. In these conditions, a maximum contribution to the adsorption heat for PEG 1500 is given by adsorbate-adsorbate interactions and a strong decrease of volatility of PEG 1500 is observed.
ANALYTICAL APPLICATIONS To give experimental evidence of the usefulness of GLS column in chromatographic analysis, some interesting separations concerning the elution of polar solutes in aqueous solution were performed. Water solutions containing about 20-40 ppm of each component can be chromatographed quite easily by direct injection into the chromatographic columns. I t is required, however, that each column be preconditioned, once and for all. This is performed by injecting 2 ~1 of water a t 130-140 "C several times. After this treatment, a weak signal for water is observed which, in practice, does not affect the chromatographic piofile. In Figure 7, a chromatogram showing the elution of a water solution containing about 30 ppm of each c 2 - C ~aldehyde on Sterling FT-G 0.2% PEG 1500 is reported.
+
1234
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 7, J U N E 1973
is,l" I
1
0
2
I 4
-
1
6 time
rmin)
Figure 8. Chromatogram of the elution of a water solution of some volatile, polar compounds. Column, 1 . 4 m X 2 m m ; packing material, Graphon 1 % PEG 1500; column temperature, 85 "C; linear carrier gas velocity, 9 cm/sec; sample size of each component, 20 f 30 ng. 1 , methanol; 2, ethanal; 3, methyl formate: 4 , acetonitrile; 5 , ethanol; 6, dichoromethane; 7, propanal; 8, 2-propanone; 9, 2-propanol: 10, ethyl formate; 1 1 , methyl acetate: 12, 1-propanol
+
The elution of small amounts of such compounds on a GL column of 30% PEG 1500 resulted in badly skewed peaks, presumably due to some kind of chemical reaction occurring between the eluates and the liquid phase. By adding small amounts of PEG 1500 on GCB, a perfectly linear elution of aldehydes is obtained and only at surface cov0.7 does a slight tailerages of PEG 1500 higher than e ing in the peaks occur, which increases further as the surface coverage is increased. It is worth noting the separation of 2-methyl butanal and 3-methyl butanal. The difficulty in separating this isomeric pair by GLC arises from the slight difference in the boiling point (-0.2 "C) of the two components. The nonpolar, flat surface of the graphitized carbon black is, however, particularly suitable in separating molecules according to differences in their geometric structure, disregarding the existence of functional groups in the molecule and this property is indeed exhibited. The determination of volatile components of a water solution is usually made through the separation of the compounds from the sample and identification by GLC. However, the sample processing can involve some losses. If the water solution is directly injected into a GL column, the analysis of the components in many cases can be affected. Also, rarely does a single stationary phase suffice to separate all components. The determination of aqueous, volatile, polar mixtures can be made easier by direct injection into a single GLS column by properly adjusting the liquid/solid ratio. As an example, in Figure 8 is shown a chromatogram concerning the rapid analysis of a water mixture containing polar compounds (aldehydes, alcohols, ketones, esters,
-
4
Figure 9. Chromatogram of the elut!on of a water solution of c1-c5
E
-
h
.
’-1‘
aliphatic alcohols. Column, 1.4 m
X
2 mm: sample size
of each component, -30 ng; linear carrier gas velocity, -10 cm/sec. Columns: a. Graphon 5% PEG 1500; column temperature, 98 “C; b. Graphon -!- 0.6% PEG 1500; column temper-
+
+
ature, 126 “C; c. Graphon 2% PEG 1500; column temperature, 131 “C. 1, methanol: 2, ethanol: 3, 2-propanol; 4, l-propanol; 5, 2-methyl-2-propanol;6, 2-butanol; 7, 2-methyl-1-propanol; 8 , 1-butanol; 9, 2-methyl-2-butanol:10, 3-methyl-2-butanol; 11, 3-pentanol; 12, 2-pentanol; 13; 2-methyl-1-butanol; 14, 3-methyl-1-butanol;15, 1 -pentanol
+
and other compounds), on a GLS column (Graphon 1% PEG 1500). GLSC allows chemical and physical properties of an adsorbing medium and of a stationary phase to be exploited in full for analytical purposes. It should be pointed out, however, that in exploiting this technique, neither can the liquid phase be regarded merely as a “tailing reducer” nor the adsorbing medium merely as an almost “ideal” support for stationary phases. On the contrary, by continuously adjusting the liquid/solid ratio, a “tailor made” column able to yield the best fractionation of a given mixture can be prepared. In Figure 9 are shown chromatograms concerning the elution of a water mixture containing small amounts (-30 ppm of each component) of all aliphatic alcohols up to Cg except neopentanol, which is, however, of minor interest in practical analysis. As can be seen, a near base-line separation for all components is achieved in 20 min by using Graphon 2% PEG 1500 as packing material (Figure 9c). With the same analysis 0.6% time, neither the column filled with Graphon PEG 1500 (Figure 9b), where the liquid acts mainly as a “tailing reducer” nor the column packed with Graphon 5% PEG 1500, where the solid medium acts mainly as support (Figure 9a), are able to separate all components of the mixture. The great usefulness of GLSC as analytical tool is once more emphasized by considering that a difficult fractionation such as the complete separation of isomers of pentanol, has been realized so far only by the use of capillary columns coated with blended mixtures (13, 14).
+
+
+
CONCLUSION Measuring heats and entropies of adsorption for polar and nonpolar eluates a t various liquid/solid ratios appears to be a very useful means to investigate the combined effects on the adsorption process of the specific properties of both the liquid and the solid media. Experimental results clearly show that GLSC is a powerful tool, able to solve analytical problems which are difficult to solve by either gas-liquid or gas-adsorption chromatography. The selectivity of GLS columns can be continuously varied by changing the liquid/solid ratio as well as by taking advantage of the large variety of stationary phases. The range of selections is even larger than the one available for GLC, if allowance is made for the fact that a number of stationary phases are not of satisfactory use in the analysis of aqueous polar mixtures. From our results, even if definitive conclusions cannot be drawn, a graphitized carbon surface partially shielded by nonvolatile molecules of a suitable liquid phase appears to be the best system to exploit the advantages of GLSC.
N
.?.
P9XL
Received for review November 30, 1972. Accepted January 18, 1973. / - a
I 1-
(13) R . D. Schwartz and R. G. Mathews, J. Chromatogr Sci (1969). (14) V . Palo and J. Hrivnak, J Chromatogr 59, 154 (1971)
7 , 593
7, JUNE 1973
1235
A N A L Y T I C A L C H E M I S T R Y , V O L . 45, N O .
