Gas-Solid Chromatography of Hydrogen Bonding Compounds Antonio Di Corcia and Fabrizio Bruner lstituto di Chimica Analitica and Laboratorio Inquinamento A tmosferico del Consiglio Nazionale delle Ricerche, Uniuersita dl Roma, 00185 Roma, Italy
The use of gas-solid chromatography is extended to the elution of polar compounds by treating Graphon, MT, and FT with hydrogen at 1000 “C. In comparing the adsorption isotherms before and after treatment, it appears that chemisorbed oxygen, always present in these adsorbents, is removed. Peak tailing is eliminated or strongly reduced and the elution of tiny amounts of alcohols, aliphatic amines, and free carboxylic acids can be performed on thesecolumns. The results given by the adsorbents used are compared with those obtained with GLC and untreated materials; several analytical applications can be realized with this chromatographic material.
THE INTEREST of analytical chemists in gas-solid chromatography has grown rapidly since a number of important results have been achieved with this techniquc The analysis of permanent gases and low boiling liquids is Fossible at room temperature only with gas-solid chromatography and certain problems oT’-Igh selectivity can be solved with it ( I ) . This development is related to a number of achievements concerning the possibility of better controlling the homonogeneity, geometrical structure, and chemical nature of the adsorbents. Moreover, the fact that most of the adsorbents have a high thermal and chemical stability and that the expansion of the working range of the gas chromatographic columns can be now raised up to 500 “C, makes adsorption gas chromatography particularly valuable for use with analytical temperature-programmed columns. Nevertheless, gas- liquid chromatography is more widely used than GSC for the analysis of hydrogen bonding compounds, even though gas-liquid partition presents some serious problems; among these bleeding of the fixed liquids at high temperatures is by far the most important. The chief obstacle to the analysis of such compounds by GSC is the fact that a n otherwise uniform surface is always contaminated to a greater or lesser extent by active functional groups which yield specific interactions. Such chemical inhomogeneity has a profound influence on the adsorption of polar compounds, provoking an initial pronounced “knee-bend’’ in the absorption isotherms, at least yielding the peak asymmetric or, in many cases causing an almost irreversible adsorption, responsible for the so called “ghost peaks.” Several methods have been described to linearize the adsorption isotherms, the most important being the following: Deposition of monomolecular layers of strongly adsorbed substances (especially polymers) on the absorbing surface of a nonporous or macroporous adsorbent (2) ; blocking of the active centers of an inhomogeneous surface with a highly polar liquid (3, 4 ) ; and use of a strongly adsorbed carrier (1) M. Taramasso and A. Timidei, in reprints of the 8th International Symposium on Gas-Chromatography, N. Stock, Ed., Institute of Petroleum, London, 1970, paper 3. (2) C . Vidal-Madjar and G. Guiochon, Nature, 215, 1372 (1967). (3) J. R. Lindsay-Smith and D. J. Waddington, ANAL. CHEM., 40, 523 (1968). (4) A. Di Corcia, D. Fritz, and F. Bruner, ibid., 42, 1500 (1970). 1634
gas or mixing of proper deactivating substances in a common carrier gas (5,6). In some cases, the methods mentioned above attain the aim of eliminating the active centers completely, but each of them has some serious limitations. For example, as already reported in our recent work (4), surface coating of Graphon, a well known example of partially graphitized carbon black, with small amounts of liquids, whose affinity for the active sites is analogous to that of the compounds to be analyzed, yielded good results for the linear elution of highly polar compounds. But, because of the relatively high volatility of the deactivating liquid (TEPA or glycerol), its use is limited to the range of temperature below 100 “C. Our efforts have recently been devoted to the search for adsorbents for gas-solid chromatography which are able to elute polar compounds even at very low concentrations, without the use of any deactivating agent. This would allow working at high temperatures, eliminating bleeding from the column, and this is particularly important for gas chromatography-mass spectrometry coupling. Moreover, the high selective power and versatility of GSC would possibly open new methods for faster and more effective analysis of complex mixtures. On the point of view of the adsorption energy, an ideal adsorbent should be gaussian, i.e., the heats of adsorption on the adsorptive sites should be displayed symmetrically around an average value. This can be accomplished only if the surface is geometrically and chemically homogeneous. This is never true even with carbon blacks, which are normally considered homogeneous adsorbents. About fifteen years ago, Millard et al. (7) reported that the surface oxygen complex on Graphon could be removed by heating the adsorbent in a stream of hydrogen at 1000 “C. After the hydrogen treatment, adsorption of water no longer showed the small “knee” at the low-pressure end of the isotherm, thereby indicating the disappearence of the impurity. In this work we repeated the hydrogen treatment on Graphon and extended it to other types of carbon blacks, such as FT and MT. Very satisfactory results have been obtained for the gas chromatographic analysis of mixtures of aliphatic and aromatic amines and mixtures of aliphatic acids and alcohols with very low sample sizes. The shape of the adsorption isotherms before and after the treatment are compared and their effect on the chromatographic properties of the adsorbents is discussed. EXPERIMENTAL
Hydrogen Treatment. The carbon black sample, previously sieved at the desired mesh range, is placed in a piece of iron tubing, which is in turn placed in a cylindrical oven (Bicasa, Milan). The oven is then heated, keeping the ( 5 ) J. Janak, Collection Czech. Chem. Commuri.,19,684 (1953). (6) A. Liberti, G. Nota, and G. Goretti, J . Chronzatogr., 38, 282 (1968). (7) B. Millard, E. G. Caswell, E. E. Leger, and D. R. Mills, J . , Phys. Chem., 59, 876 (1955).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
~
,‘
GRAPHON (hydrogen treated)
---- GRAPHON
..’
/‘
a
Figure 1. (a) Isotherms for methanol at 78 “C on Graphon and on hydrogen-treated Graphon (b) Isosteric heats of adsorption of methanol on Graphon and on hydrogen-treated Graphon I
Qat
b
,
I
0
sample under a pure hydrogen stream up to 1000 “C. This temperature is maintained for about 4 hours. The time is not very critical. In the original paper the treatment was carried on for 12 hours, but in our experience no significant further improvement is observed by extending the treatment beyond 4 hours. A treatment time less than 4 hours can sometimes give some irreproducibility of results. Gas Chromatography. A Carlo Erba model G I gas chromatograph, equipped with a flame ionization detector was used. For the determination of adsorption isotherms, Teflon (Du Pont) tubing (1 m X 3 mm i d . ) was used as column material. The mesh range was 60-80. Methanol of research grade, injected by a 10-pl syringe (S.G.E. Melbourne, Australia), was properly split at the column inlet to obtain the desired sample size (10-6gram). The mesh ranges used for analytical columns were 60-80 and 80-100 for FT and MT, respectively. Glass columns (80 cm X 2.5 mm i d . ) were used. Artificial mixtures were prepared from commercial sources. Graphitized carbon blacks were kindly furnished by Dr. W. R. Smith from Cabot Corp., Billerica, Mass. Pure nitrogen was used as carrier gas. Analytical tests were performed using synthetic mixtures consisting of solutions l/lO,OOO, V/V of each compound, usually in diethyl ether, or in distilled water when low-boiling amines were examined. Sample blends were injected with a 10-pl S.G.E. syringe,
10
20
30
-40
a io6
50
conveniently split to obtain the desired amount. When hydrogen-treated adsorbents were examined, no ghosting phenomena were observed and reproducibility of peak areas for the same amount of any polar compound was about 1-3 %, the range difference being attributed to syringe technique. RESULTS AND DISCUSSION
Isotherm Modification by Hydrogen Treatment. The isotherms and the isosteric heats of adsorption us. coverage of methanol for both untreated and hydrogen-treated Graphon are shown in Figure la and lb. Absorption isotherms have been obtained from the chromatograms, using a method devised by Glueckauf (8) and largely used by Kiselev (9). From the isotherms at various temperatures, the isosteric heats of adsorption at any value of surface coverage have been calculated graphically by plotting the adsorption isosteres as In p us. 1jT and determining the slope, which equals --q,t/R. In these cases, straight lines were obtained and reproducibility of heats of adsorption was good to within 3-4 %. That hydrogen treatment has removed chemisorbed oxygen ~~
(8) E. Glueckauf, Discussiotls Farciduy SOC.,7, 199 (1949). (9) A. V. Kiselev, “Advances in Chromatography,” vol. 4, J. C. Giddings and R . A. Keller, Ed., Dekker, New York, N.Y., 1967.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
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Figure 2. (a) Isotherms for methanol at 33 “C on FT and hydrogen treated FT and on hydrogen-treated FT
v
0
I 1.o
I
I
2 .o
f
I
3.0
I
I
4.0
I
I
5.0
I
I
!
3
6.0
P (torr)
qat
(Kcal/mole,)
from Graphon is well evidencec. ,:cause at any value of partial pressure of methanol in the gas phase, the surface of Graphon treated shows much less affinity for the hydrogen bonding adsorbate than the untreated material, since at every pressure the amount of methanol adsorbed is lower; after the treatment, the initial curvature of the adsorption isotherm is appeased, showing a more gaussian distribution of the adsorptive sites; as a consequence, the isosteric heat curve for treated Graphon, Figure 16, lies quite markedly lower than that of the untreated material. However, because of a residual specific activity of the surface, the heat curve is not completely flat at low surface coverage. Better results are obtained when FT and MT, two other well-known examples of the G.C.B. series with surface areas of about 15 and 8 m2/g, respectively, are purified by hydrogen at 1000 “C. Adsorption isotherms and isosteric heats, relative to the adsorption of methanol on FT, are shown in Figures 2a and 2b. The behavior of methanol on MT is quite similar. In this case, the great influence which a small amount of residual oxygen has on the adsorption of the hydrogenbonding molecules is emphasized by the fact that the contaminated surface yields a type I1 isotherm, whereas the clean surface gives a type I11 isotherm. This demonstrates that, owing to high surface homogeneity, adsorbate-adsorbate interactions come into play during adsorption of methanol on purified FT also at low surface coverage. This results in an increase of the heat of adsorption with increasing surface coverage (Figure 26). Analytical Applications. Analytical applications of gas chromatography offer a striking confirmation of the very important improvement which can be obtained in the analysis 1636
4
b
of “polar” mixtures, by using graphitized carbon blacks previously decontaminated by hydrogen at 1000 “C. Before discussing the results of this work, it is useful to make a survey on the present situation in gas-solid chromatography of polar compounds. To date, the best examples of adsorbents available for the analysis of polar compounds are the Porapak series, the Chromosorb “Century Series,” and the graphitized carbon blacks. The first two types of adsorbents, which are porous cross-linked polymers, allow the analysis of any polar compound. Nevertheless, there are three serious limitations to the extensive use of these adsorbents. First, these polymers cannot be used at temperatures above 230-250 “C, because at higher temperature thermal degradation occurs. Second, because of the high surface development of these polymers, the compounds injected exhibit a large retention. For this reason, coupled with their thermal instability, analyses of polar compounds must be confined to the first terms of each homologous series. Last, but not least, relatively large samples of strong hydrogen-bonding compounds must be injected, in order to avoid tailing of the peaks. This is probably due to a certain residual specific activity of these adsorbents. As far as graphitized carbon blacks are concerned, their surface density of hydrophilic sites is somewhat larger than the preceding adsorbents, but these supports have no problems of chemical or thermal stability and surface development of FT or MT does not hinder the elution of high-boiling compounds. The forthcoming results show that the potentiality of GSC can be greatly enhanced by the use of purified graphitized carbon blacks.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
:16
3
n
4
0
Figure 3. Chromatogram showing the elution of C4-Cs n-alcohols. Column, 0.8 in X 2.5 m; packing material, FT (hydrogen treated); sarnple size, 3 X gram; temperature, 165 “C; flow rate, 30 m l / d n
In Figure 3, elution of C4-Cs n-alcohols is shown. This chromatogram is particularly interesting as far as sample size, column efficiency, and elution time are concerned. Sample size, referred t o each component, is, in fact, only about 3 nanograms. Nevertheless, all peaks are still perfectly symmetrical. Moreover, it is noteworthy that the number of effective plates, referred to n-octanol, is 1800 per meter of column. That GSC yields, especially at high linear gas velocities, a higher column efficiency than GLC, has been explained (IO) in terms of a mass-transfer coefficient that is more favorable in the case of gas adsorption than gas-liquid partition. This difference becomes more pronounced when polar mixtures have to be eluted. In this case, in order to avoid negative adsorption effects by the supports, the liquid phase film thickness must be rather high, and this affects the mass-transfer coefficient negatively and causes peak broadening. Finally, another interesting feature shown by the chromatogram reported above is the short retention time exhibited by the compounds eluted. In this connection it is interesting to note that a t 75 “C the retention time of n-butanol on a Hz-treated FT column is only 3.5 minutes. We have found that the same compound eluted at the same temperature, flow rate, and column length has a retention time of 8.5 minutes using Chromosorb W coated with 20% Carbowax 600. Using other stationary phases, such as FFAP or Trimer Acid, nbutanol requires even higher times to be eluted. (10) A. V. Kiselev and Y. I. Yashin, in “Gas-Adsorption Chromatography,” Plenum Press, New York, N.Y., 1969, p 2.
1
3
5
7
9 time (niin)
11
Figure 4. Chromatogram showing the elution of C,-C, free fatty acids Column, 0.8 m X 2.5 mm; packing material, I T (hydrogen treated); sample size, 1 X lo-’ gram; temperature, 190 “C; flow rate, 30 ml/min
That analysis of polar mixtures can be performed by GSC within a shorter time than by G L C is further confirmed by the chromatogram of Figure 4, where the elution of C4-Cs free fatty acids at 185 “C is reported. To emphasize what is said above, a comparative test has been made. At 150 “C, 0.1 pg of n-butyric acid was eluted within 50 sec, using Hdreated FT. On the other hand, to elute the same compound, at the same operating conditions, using FFAP, Ethofat, and Trimer Acid as liquid phases ( I I ) , elution time of 8 min, 6.4 min, and 5.3 min, respectively, was needed. Retention times are used instead of specific retention volumes because the operating conditions of G L C and GSC are so different and so critical that the only way to make a comparison of the results obtained with the two techniques is to look at the analysis time required to have the most effective separation of certain compounds in the two cases. Furthermore, the differences in the physical dimensions to be used for expressing specific retention volumes in G L C and GSC make a numerical comparison on these completely unsignificant. As can be seen, at a level of about 0.1 pg, free fatty acids give some tailing of peaks. These broadened trailing edges can be due to different reasons: adsorption by the injection system, self-dimerization of the acidic molecules (12), or residual basic activity not eliminated by the hydrogen treatment. In spite of this, from the point of view of the amount (11) W. K. Lee and R. M. Bethea, .I. Gas Cliromatogr., 6 , 582 (1968). (12) A. T. James and A. P. S. Martin, Biochenz. J . , 50,679 (1952).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
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C
8
I
1
0
1
2 I me
,
3 ,m n,
,
I
0
2
1
ttme / m l n )
L
l
0
1
3
5
7
I me m n l
Figure 5. Chromatograms showing the elution of aliphatic and aromatic amines a t several temperatures on MT (hydrogen treated) (a) Column, 0.8 m X 2.5 mm; sample size, 3 X lo-' gram; temperature, 94 "C; flow rate, 25 ml/min; 1, methylamine; 2, ethylamine; 3, trimethylamine; 4, ierlbutylamine; 5, isobutylamine; 6, n-butylamine (b) Column, 0.8 m X 2.5 mm; sample size, 1 X lo-' gram; temperature, 133 "C; flow rate, 20 ml/min; 1, dimethylamine; 2, n-propylamine; 3, pyrrolidine; 4, diethylamine; 5, piperidine; 6, di-n-propylamine; (c) Column, 0.8 m X 2.5 mm; sample size, 1 X lo-' gram; temperature, 243 "C; flow rate, 15 ml/min; 1, Aniline; 2, n-heptylamine; 3, di-n-butylamine; 4, N-methylaniline; 5, n-octylamine; 6, Iv,N-dimethylaniline; 7, n-nonylamine; 8, N,N-diethylaniline
injectable, our column is more than competitive with respect to GLC columns. Besides, no "ghosting" phenomena were observed during our experiments. Finally, in Figure 5 chromatograms of amines at several temperatures on hydrogen-treated M T are reported. In the case of amines, adequate applications of gas chromatography have not been completely achieved. This fact is largely due to the ease of hydrogen-bond formation with amines. Only two recent papers (4, 13) deal with quantitative measurements of amines at very low concentration by gas chromatography. However, they have a limitation in that, because of the relative volatility of the stationary phases used, only the first terms of the amines can be eluted. It has already been shown (14) that amines can be eluted by gas-solid chromatography using a nonspecific adsorbent, like MT. But if the sample size is less than gram, badly skewed peaks are obtained. Moreover, we found that if the amount of very active amine, e.g., n-propylamine, is about gram, almost no signal is observed on the recorder paper. On the contrary, after hydrogen treatment at 1000 "C,the minimum sample size injectable can be lowered to about 0.1 pg. As can be seen from Figure 5c, peaks relative to less active amines, like aromatic and long-chain aliphatic amines, (13) G. R. Umbreit, R. E. Nygren and A. J. Testa, J . Chromatogr., 43, 25 (1969). (14) A. V. Kiselev and Y . I. Yashin, Zh. Fiz. Khim., 40, 603 (1966),
1638
are perfectly symmetrical, thus indicating that, in this case, the sample size can be further decreased. Another interesting feature shown by the chromatograms relative to amines, is that, using appropriate temperature programming, a large number of amines at low concentration can be analyzed. Low boiling amines must, in fact, be eluted on very polar liquid phases, which cannot usually be used at the high temperatures needed for the analysis of the high boiling terms. Different columns must also be employed. In eluting amines, M T yielded more symmetrical peaks than FT. The reasons for this behavior have not been clarified, and it seems to depend on the fact that the impurities of the starting materials relative to the two graphitized carbon blacks are quite different. On the other hand, MT shows the same degree of deactivation as FT with regard to alcohols and fatty acids. But, because of the very low mechanical resistance of MT, which causes difficulties in its handling and strongly affects the performance of the GC columns, FT was preferred. Furthermore also in the elution of amines, no "ghost peaks" have been observed. As has been pointed out in the Experimental Section, consecutive elutions of the same amine gave reproducibility of the peak areas of about 3 %. Finally, in Table I a comparison between the treated and untreated graphitized carbon blacks is reported. The quantities listed refer to the minimum sample size for which reproducible retention times are obtained. The results described above show that GSC of hydrogen
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
~~
Table I.
Sample n-Amyl-alcohol
Data Elution time, min
T, "C
0.7
160
Minimum sample size, go 1 X lo-'
0.7
1.3
160 190
3 X 10-9 1 X lW4
1.3 1.4
190 243
1 X lo-' 1 X lo-'
Adsorbent Sterling FT Sterling FT (Hz treated) n-Butyric acid Sterling FT Sterling FT (He-treated) N-Methylaniline Sterling MT Sterling MT (Hdreated) This is the minimum amount to times.
1.4 243 1 X lo-' obtain reproducible retention
lowered, so that treated FT and MT give considerable scope for rapid analysis with columns of high performance. However, the most interesting point arising from this work is the possibility of eliminating bleeding of the stationary phase, which causes serious trouble for gas chromatographymass spectrometry coupling. The use of our columns coupled with mass spectrometry can lead to cleaner and more reliable mass spectra, and, consequently, to easier identification of an extended range of compounds, including the very polar, high-boiling and multifunctional ones, which are contained in very complex mixtures of interest in the fields of biological and biomedical chemistry. ACKNOWLEDGMENT
bonding compounds with hydrogen-treated graphitized carbon blacks is not only competitive with, but in many cases definitely more convenient than, GLC on porous polymers. The range of linearity of GSC is extended so that small amounts of any polar compound can be detected. Column efficiency is enhanced and retention times are considerably
The authors are indebted to A. Liberti for helpful discussions and R. Samperi for technical assistance. Received for review March 30, 1971. Accepted June 22, 1971. This work is supported by the Consiglio Nazionale delle Ricerche.
Electrooxidation of Iodine on Smooth Platinum in Acetic Acid Medium Influence of Adsorption of Electrode Products upon Voltammetric Limiting Currents Roland0 Guidelli and Giovanni Piccardi Institute of Analytical Chemistry, University of Florence, Florence, Italy
Iodine in acetic acid gives a single anodic voltammetric curve on smooth platinum. The mean limiting current decreases progressively with the rest time of the electrode at a given constant potential tending to an asymptotic value, owing to the adsorption of electrode products. The use of a normal pulse-polarographic technique applied to a solid platinum microelectrode permitted us to eliminate the inconvenience due to adsorption. I t was thus possible to attribute the anodic wave of iodine to its electrooxidation to I". A plausible rate-determining stage for the overall electrode process l2+ I' is the following: l2+ I 1 lsdsorbed e. Some general considerations about the influence of the adsorption of electrode products upon voltammetric limiting currents are presented.
IN A PRECEDING paper (I) the authors reported that in acetic acid, iodine gives two cathodic waves on a smooth platinum microelectrode, due to its reduction to Ia- and I-, respectively. Tne aim of the present work is to study the behavior of iodine toward electrooxidation on platinum in acetic acid medium. To this end two different polarographic techniques were employed, herein referred to as techniques I and 11, both making use of a platinum microelectrode with periodical renewal of the diffusion layer (DLPRE) described by Cozzi
and coworkers (2). In the cell of the DLPRE a Teflonfinned piston containing an iron nucleus is moved by a magnetic coil, activated by short current pulses, producing a rapid laminar flow of the solution around a hemispherical platinum microelectrode. This flow is interrupted abruptly by a valve about 30 msec (washing period, t,) from its start. With technique I, the cell of the DLPRE is directly connected to a conventional polarograph. In this case the piston is moved at regular intervals of time t l , of the order of 3 to 5 sec. The washing period, t,, is therefore followed by a much longer period of time, ( t l - t,) t l , during which the solution is perfectly still. If the potential applied to the DLPRE is such as to allow the Occurrence of a charge-transfer process, then, during the washing period t, the depolarizer reaches the electrode surface both by diffusion and convection. At the end of period t,, the depolarizer moves only by diffusion, so that the instantaneous current decreases with time. At the end of period tl, a new washing sweeps away to a large extent the effect of prior electrolysis ; hence, the instantaneous current increases sharply, reaching a maximum value, ,.,i at the end of the new washing period. The geometry of the cell and the energy of the washing are so adjusted that ,i
(1) R. Guidelli and G. Piccardi, A n d . Lert., 1, 779 (1968).
(2) D. Cozzi, G. Raspi, and L. Nucci, J . Elecrroanal. Chem., 12, 36 (1966).
+
+
-
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