Anal. Chem. 1988, 60,638-642
638
Characterization of Porous Carbons for Liquid Chromatography Oscar Chiantore* Istituto di Chimica Macromolecolare, Universitd di Torino, Via Bidone 36, 10125 Torino, Italy
Ivan Novtik and Dusan Berek Polymer Institute, Slovak Academy of Sciences, 842 36 Bratislaua, Czechoslovakia
Porous carbonaceous sorbents for use as hlghperformance llquld chromatography statlonary phases prepared vla a r e p llca technique have been characterized through particle size determlnation, shape analysls, surface area measurements, thermogravlmetry, and dlffuse reflectance Fourier transform Infrared (FTIR) spectroscopy. The carbons were obtalned by pyrolysls of two different organic precursors (a phenolformaldehyde resin and saccharose) at temperatures of 600 O C ; the diffuse reflectance FTIR technique revealed itself a convenient tool for structural group ldentlflcatlon In these intermediate carbons. I t has been shown that the carbons from saccharose are dlfferent from those of phenollc orlgln; both types of carbons, however, have a mlxed character of polar and nonpolar groups, and some of the chemical features of the starting materials are still present In the charred structures. Only by hlgh-temperature (>800 "C) treatments under Inert atmosphere can all the functlonai groups be ellmhated.
The development of new types of stationary phases for high-performance liquid chromatography (HPLC) is continuously and rapidly growing. Among the materials that have been investigated in recent years, porous carbons attracted attention because of their potentiality as completely nonpolar column packings, particularly desirable for reversed-phase chromatography, and also because of their stability over a wide pH range. Up to now many different carbonaceous sorbents for HPLC have been prepared and tested, and critical reviews of the progress, and problems, in this field have been published ( I , 2). One of the methods employed for the preparation of porous carbons is based on the pyrolysis of organic precursors on the surface and within the pores of microparticulate silica gel (3-6). After pyrolysis the silica is removed by alkali digestion, thus leaving a carbonaceous material whose particle size, porosity, and surface area depend upon the silica gel used as a support. The final characteristics of the sorbents are also dependent on the nature and the amount of the organic carbon precursor, on the temperature of primary carbonization, and on eventual postpyrolysis thermal treatments at temperatures higher than 1000 "C (6, 7). The chromatographic behavior of carbonaceous stationary phases can therefore be different as a consequence of many factors. To account for the various behaviors it would be important to investigate the reproducibility of the pyrolysis procedure, the effects of the nature of the carbon precursors, the temperature of pyrolysis and postpyrolysis firing, and so on. The present work reports the investigation on the structural characteristics of some porous carbonaceous sorbents which have been prepared from two different carbon precursors, viz. a phenol-formaldehyde resin, and saccharose. The pyrolysis was made at intermediate temperatures of about 600 "C and attention was focused principally on the surface characteristics
of the packings to obtain evidence both about the reproducibility of the method of preparation and about the differences in the chemical groups present. The physical and chemical characterizations of the carbons were performed through particle size analysis, shape determination via electron microscopy, surface area determination, thermal analysis, and IR spectroscopy. The latter technique has largely been employed throughout this work to monitor the structural chemical groups in the pyrolytic materials. The problems encountered in the IR analysis of carbons have been critically reviewed recently (8). Briefly, the investigation of carbonaceous materials by IR spectroscopy has normally produced scarce and disappointing results, mainly because of the opacity of the samples. In some cases the carbons obtained by the low-temperature pyrolysis at temperatures below 400 "C contain numerous functional groups present in the starting materials and the original skeletal structures are still predominant. These so-called low-temperature chars can be investigated by IR transmission techniques, if the thickness of the sample is thin enough. The carbons obtained a t temperatures higher than 400-450 "C, on the other hand, increase their absorption in the infrared region, and even thin films will be so opaque as to give very poor transmission spectra, especially with traditional dispersive instruments. Improved results can be obtained with Fourier transform infrared (FTIR) spectroscopy, due to the higher sensitivity which is intrinsic to this technique. Under such conditions the detection of functional groups is still feasible by IR spectroscopy for the intermediate chars produced in the range from 450 to 650-700 "C. Further improvement in the data obtainable from FTIR of carbons is gained when, instead of transmission spectra, ancillary techniques for surface probing like diffuse reflectance (DR) are used which overcome the difficulties due to the size and dispersion of the materials, to their scattering characteristics, and to the intrinsic opacity of the samples. In the case of DR-FTIR spectroscopy of carbons, advantage may be taken of the amount of reflected light, which is of higher intensity than transmitted light, and another advantage is the ability of DR-FTIR to measure the surface functional groups of native samples which have not been processed, with possible changes in the sample morphology.
EXPERIMENTAL SECTION The carbon sorbents were prepared by the replica technique (3-5) on different spheroidal silica gels soaked with 50-80%, by weight, either phenol formaldehyde resin or saccharose. Pyrolysis was performed at 600 "C for 1 h in an inert atmosphere, and afterward the material was boiled for 30 min in excess of 10% NaOH solution to dissolve the silica. The recovered carbon sorbents were dried a t 110 "C. Some of the samples were pyrolyzed twice: after the first pyrolysis the particles were soaked again with the solution of the carbon precursor and then the pyrolysis was repeated. Surface areas were determined by the BET technique using argon as an adsorbate.
0003-2700/88/0360-0638$01 50/0 C 1988 American Chemical Society
ANALYTICAL CHEMISTRY. VOL. 60. NO. 7, APRIL 1. 1988
839
Table 1. Physical C h c t e r i o t i s of the Pymlitic Carbon8 bulk density, sample
surface area, ma/g
gcmJ
residual silica, W
CF18 CF16 CF32-3
770 859 435 230 735 970
0.26 0.23 0.46
10
CF9 CF19 CF2O
0.56 0.45 0.44
8 22 19 2 2
a,rm
4 rm
26.7 36.6 28.7 31.8 22.4 19.2
32.9
1.23
54.8 35.3
1.50
41.9 32.7 27.2
&lam 1.23 1.32 1.46 1.42
remarks
twice pyrolyzed twice pyrolyzed
e 2. SEM of CF18 carbon sample; magnlficatlon 10000.
Figure 1. EM of CFl8 carbon sampk. magnnicath 1000
m
Thermal analyses were performed on a Du Pont 951 thermobalance driven by a 1090Du Pont control unit under 60 cms/min nitrogen flow. The carbon powders were heated in small cylindrical silica crucibles. Size didbutions of the pyrolytic carlam were determined with a 2600/3600 particle sizer (Malvern Instruments, Spring Lane, Malvern, England) by messuring the weight fractions of particles having dimensions iiicluded in a given number of size bands. The samples for measurement were dispersed in methanol. From the mean average diameter. di,of the size bands and the corresponding weight fractions, w. number and weight average particle diameters were calculated respectively, as follows:
Moreover, the carbon particles possess excellent mechanical stability (5.9). AU these fmdinps indicate that the replication procedure can yield interesting HPLC packings. The alkali treatment of the samples after the pyrolysis was not efficient enough to remove all the silica from the carbon particles made of phenol-formaldehyde resin, and this is shown in the fourth column of Table I where the residual sample weights after buming-off are reported. It is seen that the silica content is particularly high for the carbons which were pyr0ly-d twice. In this case the carbon probably formed a dense layer that prevented effective leaching of the silica matrix. On the other hand almost all of the silica has been removed from the carbons prepared from saccharose. Infrared Analysis. The samples were fmt examined with transmission spectra in KBr pellets; 64 scans were generally sufficient to reach a good signal to no& ratio. The DR spectra obtained on the undiluted powders were corrected with a Kubelka-Munk procedure (10) using the instrument software, in order to compare the results with normal transmission spectra. AU the DR spectra were recorded with 508 scans and submitted to the same medium level smoothing. In Figure 3 the transmiasion spectrum of the sample CF18 (spectrum 1) can be compared with the DR spectrum of the same sample (spectrum 2); it can be seen that the DR spectrum is much better than the one obtained in transmission mode. Furthermore, in the spectra obtained with KBr pellets, abnormally high bands near 3500-3400 cm-' are always present (Figure 3, spectrum I), which are located a t lower frequencies than in the pure carbon and are probably due to the water moled e s adwrbed on the bromide which are d i f i d t to eliminate. Therefore, in the following, only DR spectra will be used for discussion. In Figure 3 the ordinates are absorbances and spectra are vertically displaced in order to avoid overlapping. Carbons from Phenols-Formaldehyde Resin. The porous carbons prepared by pyrolysis of phenol-formaldehyde resins are CF18, CF16. CF9 and CF32-3. The latter two samples were pyrolyzed twice. The DR-FTIR spectra of these
a,, = l/X(wi/di) a. = Xwidi and the polydispemit of particle dimensions was expressed through the ratio a,/% Surface functional groups on the carbon sorbents were moni t o d by FIT3 speroseopy on a Perkin-Elmer 1500 instrument. by weight The speetra were taken either on KBr pellets ('lam sample concentration) in transmianion mode or on the undiluted samples with a Perkin-Elmer diffuse reflectance accessory. All the spectra were recorded at 4-em-' resolution. RESULTS AND DISCUSSION
Phy~icnlCharacterization. The sorbents that were investigated are listed in Table I. The first four samples were prepared by pyrolysis of a phenol formaldehyde resin, while the last two of the table are from saccharose. From Table I i t is evident that the surface area of carbon sorbents can be easily adjusted even when starting from the name silica gel. Scanning electron micrographs of these carbons show that the particles have spheroidal shapea and a spongelike porous texture (Figures 1and 2) similar to parent silica gel. It was found that also the diameter and the external shape of carbon particles prepared by the replication procedure can be simply controlled by the size and shape of silica gel particles (4.9) while the pore diameter of carbon particles reflects to some extent the porous structure of the silica gel matrix (4.5,s).
640
ANALYTICAL CHEMISTRY, VOL. 60, NO. 7, APRIL 1, 1988
Scheme I OH
OH I
I
OH I
A
Ib
4000
kk-iko
B
OH
OH
OH
OH
I
1 OH
,boo
OH
&cH*-cH2& do0
cm-' Figure 3. FTIR spectra of porous carbons: (1) transmission spectrum of CF18, KBr pellet; (2) DR spectrum of CF18; (3) DR spectrum of CFl8 oxidized at 400 O C ; (4) DR spectrum of CF18 heated under vacuum at 900 O C ; (5) DR spectrum of CF20 carbon from saccharose. samples are basically identical with minor differences in relative intensities which might be related to slight differences in the pyrolysis conditions. All these carbons present a number of absorptions which are due to the presence of several functional groups; Le., the charring process did not reach the stage of a complete destruction of the starting structure. For the assignment of the bands, reference can be made to a recent IR study performed with photothermal beam deflection spectroscopy on the chars produced by pyrolysis of a phenol-formaldehyde resin of the Novolak type (11). (1)The presence of phenolic structures is indicated by the broad OH stretching band near 3540 cm-' and by the narrower and relatively strong bands a t 1320 cm-' (OH deformation) and 1220 cm-' (C-0 stretching). ( 2 ) The CH stretching region in the range 3100-2800 cm-' shows the presence of both aromatic and aliphatic components. The former component is the origin of the band above 3000 cm-l; aromatic out-of-plane deformations and vibrations are also responsible for the two bands near 820 and 760 cm-'. Aliphatic structures are still relatively abundant in these carbons, as indicated by the CH stretching near 2900 cm-' but also by the band near 1470 cm-' which might be attributed, at least partially, to deformation of CH, structures. It has been shown (11) that the intensities of these two CH bands vary in the same way with the aliphatic content. Therefore, in addition to methylene bridges already present in the original resin (structure A), the groups indicated in B and C formed during the pyrolysis through branching and fragmentation reactions, according to Scheme I, would also be responsible for these absorptions. (3) The presence of ketonic structures is indicated by the two absorptions near 1700 and 1660 cm-', respectively. The
C
latter band is generally associated with conjugated carbonyl groups, as in benzophenone units (D), and it has been proposed that they are formed by reaction of the methylene bridges with the water evolved during degradation (11,12).
D
The two carbonyl bands are not intense; in the CF9 carbon (not shown in Figure 3) they appear just as shoulders on the side of the strong peak near 1600 cm-'. This indicates that the oxidative reactions are not likely to occur frequently during pyrolysis. (4) The strong band near 1600 cm-', always present in the spectra of non-oxygen-free carbonaceous materials, has been shown to be caused by a C=C stretching in a polyaromatic t active in the presence of oxygenated system, which becomes E structures in the carbon (13). (5) The sharp band near 1260 cm-' indicates the presence of diphenyl ether structures, which might be formed through condensation reactions Ar-OH
+ HO-Ar
-
Ar-0-Ar
+ H20
(6) The absorption near 880 cm-' might be due to an aromatic out-of-plane deformation typical of some tetrasubstitutions. The polysubstituted aromatic structures could also be responsible, in part, for the complex shape of the spectra in the 1500-1400 cm-I range, in addition to the contribution of aliphatic deformations which has already been suggested above. (7) The broad band which can be seen in the range 1100-1000 cm-I is due to the silica whose presence in these pyrolitic carbons has already been demonstrated. The band
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 7, APRIL 1, 1988
intensities can be compared with the silica contents reported in Table I: samples CF9 and CF32-3, having higher silica content, present proportionally more intense bands in this region. Further spectroscopical information has been obtained after thermal and chemical treatment of the carbons. Some of the samples were degassed under vacuum at 400 "C and then oxidized a t the same temperature by exposing them to 160 Torr of pure 02. Spectrum 3 in Figure 3 was obtained with the CF18 after oxidation and it can be compared with the same carbon prior oxidation. The major changes induced by the oxidation can be summarized as follows: A fair decrease in the phenolic group concentration is indicated by the decrease of the OH band in the region 3700-3200 cm-'. Both aliphatic and aromatic CH bands in the range 3100-2800 cm-' are reduced; the decrease of the complex band between 1500 and 1400 cm-' also seems to be connected with the destruction of the CH, structures. On the other hand, the carbonyl bands already present in the original sample appear enhanced by the oxidation procedure, and the possibility of new contributions cannot be ruled out after looking a t the complex shape of the bands in the region 1760-1660 cm-'. In the oxidation of phenol-formaldehyde chars, lactone and carboxylic acid structures were proposed as responsible for bands between 1760 and 1725 cm-' (14). These findings suggest that the oxidation occurs mainly a t the aliphatic groups which have been seen to be abundantly present in these carbons; at the same time branching and cross-linking reactions take place resulting in a substantial decrease in the amount of phenolic residues. Upon prolonged heating under vacuum at T = 900 "C, the carbonization of the material proceeds so extensively that the IR spectrum loses any residual structure and shows only a strong continuum absorption, as can be seen for the CF18 sample in spectrum 4 of Figure 3. All carbons heated to high temperature are then apparently equal from the chemical point of view. Carbons from Saccharose. The carbon samples CF19 and CF20 were prepared by pyrolysis of saccharose. Their DR-FTIR spectra are similar, and in Figure 3 the sample CF20 (number 5) is shown. It can be seen that carbons from saccharose are substantially different from those obtained via phenol-formaldehyde resin. The spectra of the CF19 and CF20 carbons show the same features as those of the chars obtained in the vacuum pyrolysis of cellulose at temperatures of 500-600 "C, recently reported (15). The presence of OH groups is indicated mainly by the absorption near 3400 cm-' but contribution of hydroxyls to the complex pattern of the 1400-1200-cm-' region cannot be ruled out. The char has a low aliphatic character, as indicated by the weak bands near 2900 cm-'; the aromatic character prevails and is shown by the band near 3100 cm-l and the triplet in the range 900-700 cm-'. Oxygenated structures of the ether type might be responsible for absorptions near 1300-1200 cm-', whereas the band near 1700 cm-' should be due to carbonyl groups, most likely of a ketonic nature, formed by oxidative dehydration of the saccharidic molecules (15). The 1600-cm-' band is the characteristic absorption of all oxidized carbonaceous materials, due to the activated C=C modes. The silica content of these samples is fairly low and consequently does not contribute very much to the signals in the range 1100-1000 cm-'. From the IR spectroscopic analysis it can be deduced that the charring of saccharose has proceeded with extensive condensation of the starting molecules and aromatization of the resulting network; a t this stage, however, oxygen-containing groups are still largely present either as residuals OH or as ether links or as newly formed C=O groups.
100,
641
I
h
E a0 l-
I
: W
L
t
60
l
200
~
400
~
600
TEMPERATURE
(
a00 "C)
~
1000
*
Figure 4. Thermogravimetric curves of CF 18 (-) and CF20 (- - -) carbons: heating rate, 10 "C/min; atmosphere, nitrogen 60 cm3/min.
In comparison with the carbons obtained from phenolic resins, it might be said that those from saccharose appear to have a fairly higher aromatic character but also, at the same time, higher content of polar groups. Thermogravimetry. Evidence that the pyrolytic porous carbons so far described did not reach a very high charring level is also obtained from the thermogravimetric analysis performed in an inert atmosphere. At a high carbonization stage the structure of the carbon would be a stable polyaromatic network which can be heated at temperatures of 800-900 "C, and higher, without any important weight loss, the only products eventually evolved being light gaseous molecules like hydrogen and short hydrocarbons. Thermal analysis performed on both types of our pyrolytic porous carbons shows that by heating them up to 1100 "C the samples undergo substantial weight losses, in the range of 2535% of the original carbon mass. Typical results are shown in Figure 4 for the carbons CF18 and CF20. Because of the incomplete aromatization of the char and the presence of many oxidized structures, these "intermediate" carbons, under heating, not only continue developing more and more crosslinked structures but also generate fair amounts of volatile products mainly through fragmentation and elimination reactions. CONCLUSIONS At the temperatures employed in the preparation of our samples, a carbonaceous polymer network is formed which still maintains some of the chemical features of the starting material, together with new functional groups generated in the degradative process. The materials obtained at this stage can be conveniently analyzed by FTIR, especially in the diffuse reflectance mode. The carbons obtained by pyrolysis of saccharose are different from those of phenolic origin; both types of carbons, however, have a mixed character of polar and nonpolar groups, and this will certainly affect the chromatographic behavior of such phases, as it has already been demonstrated for one of these sorbents (16) and for others of a similar nature (17). T o obtain carbons where polar functional groups have been completely eliminated, high temperature (>800 "C) treatments under inert atmosphere are necessary. As upon controlled oxidation of carbons, a selective modification of their structures is obtained (14, 18), it may be argued that strictly controlled structures having different degrees of polar groups could be prepared by oxidation, under different conditions, of high-temperature deactivated carbons. The influence of such treatments on the surface area and pore size of the particles, however, should be carefully investigated in the case of sorbents for liquid chromatography.
*
Anal. Chem. 1988, 60,642-648
642
ACKNOWLEDGMENT The authors thank C. Morterra for helpful discussion and suggestions and P. Hrdlovic for his assistance with the manuscript. They also acknowledge the Chemistry Department of New York University for providing the electron micrographs. Registry No. Carbon, 7440-44-0. LITERATURE CITED Unger, K. K. Anal. Chem. 1983, 55. 361A-375A. Knox, J. H.; Unger, K. K.; Mueller, H. J. Liq. Chromatogr. 1983, 6(S1). 1-36. Czechoslovak Patent CS. A 0 221 197. Novlk. I.: Berek. D. 6th Discussion Conference “ChromatoaraDhv of Polymers and Poiymers in Chromatography”, Prague, 1978,-C 27: Berek, D.; Novik, I . 2nd Danube Symposium, Progress in Chromatography Carlsbad, 1979, C1.l. Knox. J. H.; Gilbert, M. T. German Patent 2 946 688-4 U.K. Patent 2035282 B US Patent No. 4 263 268. Fehl, L.: Srnolkovi. E.;Hronkovl, J. Collect. Czech. Chem. Commun. 1982,47, 582-587.
(8) Morterra, C.; Low, M. J. D. Mater. Lett. 1984. 2. 289-293. Low, M. J. D.; Morterra, C. Carbon 1983,21, 275-281. (9) Gilbert, M. T.; Knox, J. H.; Kaur, B. Cbromatographia 1982, 76, 138-143. Kubelka. P.; Munk, F. Z . Tech. Phys. 1931, 12, 593-599. Morterra, C.; Low, M. J. D. Carbon 1985,23, 525-530. (12) Jackson, W. M.; Conley, R. T. J . Appl. Polym. Sci. 1984, 8 , 2 163-2193. (13) Morterra, C.; Low, M. J. D. Spectrosc. Lett. 1982, 15, 689-697. (14) Morterra, C.; Low, M. J. D. Langmuir 1985. 7, 320-326. 115) Morterra, C.; Low, M. J.D. Carbon 1983,27, 283-288. i16j Skuthanovi. E.; Feltl, L.; Smolkovi-Keulemansovi, E.; Skuthan, J. J , Cromatogr. 1984,292, 233-239. (17) Elketova, N. A. J. Chromatogr. 1986, 364, 425-430. (18) Morterra, C.; Low, M. J. D.; Severdia, A. G. Carbon 1984,22, 5-12.
;:1;
RECEIVEDfor review A D d 14. 1987. AcceDted November ~ ~4. 1987. This work has been carried out under the auspices of the official cooperation on “Preparation and Characterization of New Polymeric and Chromatographic Materials” between the C.N.R*,Italy, and the Academy Of Czechoslovakia. I
~
~~~
A-
~~~
~
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Characterization and Automation of Sample Introduction Methods for Capillary Zone Electrophoresis Donald J. Rose, Jr., and James W. Jorgenson*
Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290
An autosampler Is described that allows the comparlson of two sample lntroductlon methods for capillary zone electrophoresls (CZE), electromlgratlon, and hydrodynamic flow. The reproduclblllty Is 4.1 % and 2.9 % relative standard devlatlon for the two methods, respectively, whereas no difference Is seen In terms of zone broadenlng. The hydrodynamic flow method Is capable of maklng lntroductlonsfrom sample volumes as small as 250 nL. In addition, because the autosampler Is computer controlled, complete automation of the sample introductlon process for CZE Is posslble.
Capillary zone electrophoresis (CZE) is a high-resolution technique that separates species in open-tubular capillaries based on their differential rate of migration in the presence of an electric field. The CZE apparatus (Figure 1) consists of an open-tubular fused silica capillary, filled with operating buffer and dipping into two buffer reservoirs, a direct current high-voltage power supply connected to the buffer reservoirs by electrodes such that an electric field is applied between the ends of the capillary, and a detector, to monitor zones migrating through the capillary. To begin a separation in CZE, sample is usually introduced into the end of the capillary at the high-voltage electrode. This sample introduction process can affect the sample quantity introduced, the run-to-run reproducibility of the quantity introduced, and the electrophoretic separation efficiency. Sample introduction in CZE can be accomplished in a number of ways. An electric sample splitter has been built to introduce sample from a large sample capillary into a separation capillary (1). Also, a rohq-type injector habeen used for sample introduction in CZE (2). These types of instrumentation for samde introduction have been designed for capillaries with an internal diameter (i.d.) of 200-300 pm. I
0003-2700/88/0360-0642$01.50/0
For capillaries with a 25-75 pm id., low dead-volume coupling of sample introduction instrumentation to the separation capillary is difficult to achieve. Therefore, sample introduction into smaller capillaries is accomplished by using electromigration or hydrodynamic flow to move sample into the end of the capillary. In electromigration sample introduction, the buffer reservoir at the high-voltage electrode (see Figure 1) is replaced with the sample vial such that the capillary and electrode dip into the sample solution. The high voltage is applied for an interval of time causing sample to enter the end of the capillary due to both electrophoretic migration of charged sample ions and electroosmotic flow of the sample solution. In hydrodynamic flow sample introduction, the buffer reservoir is replaced with the sample vial such that the capillary dips into the sample solution. The sample vial is raised vertically to a specified height for an interval of time, creating a height difference between the liquid levels of the sample vial and the buffer reservoir at the grounded electrode. This height difference results in hydrostatic pressure which flows sample into the end of the capillary. (Alternatively, a vacuum can be applied to the end of capillary at the grounded electrode to pull sample into the capillary.) This paper will present the details and performance of an autosampler capable of performing both electromigration and hydrodynamic flow sample introduction in CZE. The autosampler allows the direct comparison of the two sample introduction methods in terms of reproducibility, zone dispersion, and preferential introduction of sample components.
THEORY In general, the quantity of sample, Q, introduced in CZE can be described as the product of the sample volume intraduced and the sample concentration, C, such that
62 1988 American Chemical Society
= 1m-T
(1)
~
~
-
