Porous Carbon Packings for Liquid Chromatography - Analytical

Report

Κ. Κ. linger Institut fur Anorganische C h e m i e und Analytische C h e m i e Johannes Gutenberg-Universitât 6 5 0 0 Mainz, F.R.G.

Porous Carbon Packings for Liquid Chromatography

The carbon materials developed for HPLC offer certain advantages when compared to reversed-phase silica packings

0003-2700/83/0351 -361 A$01.50/0 © 1983 American Chemical Society

Although gas chromatography is the preferred method for analysis of volatile and thermally stable compounds, column liquid chromatography offers the particular advantage t h a t mixtures of polar compounds, especially those sensitive to decomposition (e.g., biological substances), may be resolved (1). In separating these types of samples by H P L C , reversed-phase packings have been used mostly with organic, hydro-organic, or aqueous solutions as eluents (2-4). Although reversed-phase materials meet the needs of m a n y separation problems, att e m p t s have been made to find other selective packings. Among these, porous carbon has been considered as promising. On introducing such a packing it is of primary interest to know whether it possesses novel properties or perhaps resembles an existing packing material. Its application to special separation problems also should be considered. As a starting point, one might assume t h a t carbon would be similar to reversed-phase silica packings in its chromatographic properties. Colin et al. (5, 6) have discussed this aspect for pyrocarbon-modified silicas and car-

bon blacks, which they synthesized for H P L C purposes (7, 8). Research activities in the field of carbons for H P L C have recently been reviewed, and the conclusion reached t h a t no single carbon combines all the desired features of particle rigidity, a d e q u a t e surface area, and uniform surface chemistry (9). However, it does seem reasonable to reinvestigate the possible benefits carbon may provide as an H P L C packing. Industrial Carbons Carbon is an extremely versatile and widely applied industrial product m a d e by a highly advanced technology (10). So-called activated carbon is manufactured from carbonaceous precursors and is used as an adsorbent, e.g., for water purification (11). Partial combustion and cracking of hy-

A N A L Y T I C A L CHEMISTRY, VOL. 55, NO. 3, MARCH 1983 · 3 6 1 A

(b)

(a) -OH

C

c

II c

c c

-OH

II c

.Cv.

o \

0\ / "•—OH

c II

° \ /OH 0—Si x

^° "-OH

o^

H Ο

H H

?

^sr

Ο O—Si. ^OH 0 /

1 ^

H

Nr

OH

H

H

H

?

Nr H

H

H

H

H

Ή-

-H

H

H

H

-JO

Figure 1. Schematic illustration of the difference in surface character of (a) carbon and (b) reversed-phase silica

drocarbons under controlled condi­ tions yield carbon black, which is used as fillers and pigments (12). Graphite made by calcination above 2700 Κ is widely used as a structural material because of its excellent chemical and thermal stability as well as its electric conductivity (13). Coatings of pyrolytic carbons are deposited on substrates by high-temperature pyrolysis of hy­ drocarbons between 1273 and 2273 Κ (14). Glassy carbon (made from syn­ thetic polymers by pyrolysis of com­ posites) is valued as a structural mate­ rial because of its optimum strengthto-weight ratio. The above-mentioned carbon mate­ rials cover a wide range of bulk struc­ ture from highly ordered crystalline to completely amorphous. A wide diver­ sity is observed in the dispersion of carbons. There exist completely dense and nonporous forms, as well as those that exhibit a well-developed porosity. Carbon blacks are composed of aggre­ gates of particles of colloidal dimen­ sions. The aromatic backbone struc­ ture of carbons ensures that the actual organic chemistry is displayed at the carbon surface (15). Chemisorption of oxygen results in the formation of a variety of polar functional groups, some exhibiting a pronounced acidic or basic character (16,17). Requirements for Carbon Packings

Carbon packings suitable for HPLC should possess the following proper­ ties: • adequate particle strength to with­ stand pressures up to 300-600 bar when packed into columns; • an internal surface area of particles in the range of 50 to 500 m 2 /g to gen­ erate a reasonable retention under chromatographic conditions; • pores within the particles of 5-nm

width and larger to ensure a rapid mass transfer of solutes; • defined surface functional groups that are stable and homogeneously distributed on the surface; and • high chemical resistance, in particu­ lar to buffers over a wide pH range. None of the existing carbon materi­ als meets these requirements. Active carbon may be milled to micropart icles of the desired strength, but ex­ hibits a micropore structure. Agglom­ erates made of carbon black are too soft and must be hardened by an ap­ propriate procedure; a further disad­ vantage is the heterogeneity of the surface. Graphite, which has the most homogeneous surface of all carbons, behaves as a soft nonporous material. Pyrolysis of synthetic polymers yields products also showing micropore structures. It becomes apparent that compro­ mises have to be made and that differ­ ent routes can be pursued in the syn­ thesis of porous substrates. Preparation of Carbons for HPLC

Guiochon and co-workers (7, 8) have hardened sized carbon black agglomerates by depositing a thin layer of pyrolytic carbon. Benzene was used as the hydrocarbon source. Load­ ing could be increased above 50% w/w, but then the initial surface area de­ creased and the surface became more heterogeneous. Optimum deposits of 15-20% w/w were found to give suffi­ cient mechanical strength. Surface homogenization was achieved either by high-temperature hydrogénation at 1300 Κ or by thermal treatment at 3000-3300 K. These products (pyrocarbon-modified carbon black, PMCB) gave specif­ ic surfaces on the order of 50 to 100 m 2 /g. Problems arose in synthesizing

362 A · ANALYTICAL CHEMISTRY, VOL. 55, NO. 3, MARCH 1983

5-10-μπι materials, so that most stud­ ies are carried out on batches of d p = 15 μπι. Czechoslovakian workers (18-20) have used polytetrafluoroethylene (PTFE) as a starting material for the synthesis of carbon. The product, called Jado carbon, was made by re­ duction of P T F E with lithium amal­ gam at room temperature and exhib­ ited a defined micropore structure. By a specific treatment, the initially high surface area could be reduced to de­ sired values; at the same time the par­ ticle porosity decreased noticeably and the oxygen content was reduced from 13 to 0% by weight. Analogously, Zwier and Burke (21 ) reduced the fluorocarbon polymer Kel-F 300 with lithium amalgam and removed the polar surface functional groups by treatment with trimethylchlorosilane and thionylchloride, fol­ lowed by a reaction with the Grignard reagent C 8 H 17 MgBr. Ciccioli et al. (22) used commercial graphitized carbon black, Carbopack B, made from saran active carbon. Particle size was d p > 20 μπι. Unger et al. (23, 24) employed suffi­ ciently hard cokes and active carbons as precursors. The material was milled, sized into narrow fractions, and then subjected to various extrac­ tions and heat treatments to achieve carbonization and removal of noncarbonaceous constituents. Annealing to 2073 Κ under inert atmosphere fol­ lowed by treatment with metal salts, partial degassing at 1273 K, removal of salts, and final reduction by hydro­ gen at 673 Κ gave reasonable products with specific surface areas of 50 to 400 m 2 /g and a predominantly nonpolar surface, i.e., with a low concentration of polar functional groups (25). Knox and Gilbert (26) recently in-

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Table I. Comparison of Properties of Carbon and Reversed-Phase Silica Packings Property

Reversed-phase silica

Carbon

Surface functional groups

n-Alkyl, alkylaryl, aryl, residual hydroxyls

Aromatic-type acidic and basic functional groups

Specific surface area

100-200 m 2 /g

5-1000 m 2 /g

Type and extent of particle porosity

Open pores 60-80%

Open and closed pores 30-60%

Mean pore size and pore size distribution

5-25 nm homogeneous mesopores

1-1000 nm heterogeneous micro-, meso-, macropores

troduced a porous glassy carbon made by pyrolysis of a phenol/formaldehyde polymer on a silica template. After carbonization the silica template is dissolved and the residue heated to 2500 Κ and above. The specific sur­ face area ranges from 50 to 500 m 2 /g, and the products offer a high porosity because of the mesopores originating from the porous silica template. All preparations have in common carbonization at elevated tempera­ ture, followed by a second treatment to achieve a homogeneous surface. Al­ though the pore structure parameters and the specific surface area are mea­ sured in most cases, only limited in­ vestigations have been made on the surface chemistry of the products; even if the oxygen content of the ma­ terial is extremely low, polar surface functional groups can still be present in low concentrations. The reversed-phase silicas are made by a two-step procedure (4): 1) The silica is prepared in an aqueous sys­ tem and dried to remove the water from the pore system formed; 2) sur­ face modification (i.e., binding of n-alkyl functional groups) is accom­ plished by a reaction between the sili­ ca and an appropriate silane at tem­ peratures in most cases lower than 473 K. It should be noted that thermal decomposition of bonded organic groups starts at about 600 K. Surface Characteristics

Although largely of academic inter­ est, it is worthwhile to consider briefly the surface characteristics of both types of packings. The simplified scheme of Figure 1 indicates the prin­ cipal differences in the structure of the packings. Carbon, of a more or less aromatic type of structure, bears a va­ riety of polar functional groups origi­ nating from the reaction between ac­ tive carbon surface atoms and oxygen. Even when the material is hydrogentreated at 1273 K, the probability of oxygen chemisorption at ambient tem­

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364 A · ANALYTICAL CHEMISTRY, VOL. 55, NO. 3, MARCH 1983

perature still remains. This reactivity will be proportional to the specific surface area of the carbon. These sur­ face functional groups can be princi­ pally identified and their concentra­ tions measured by chemical and phys­ ical analytical methods; a much more sensitive method, however, is to ob­ serve the retention behavior of wellselected test solutes when carbon is examined under chromatographic con­ ditions. The retention of basic, acidic, and hydrophobic test substances re­ flects the net interactions of all sur­ face functional groups with the solute. Porous silica is composed of an oxidic framework of three-dimensionallylinked S1O4 tetrahedra. By coordina­ tion of surface silicon atoms with water, weakly acidic hydroxyl groups are formed, rendering the product hydrophilic. On silanization, about half of these hydroxyls react with the silane. The surface structure of a re­ versed-phase silica—if one can use this term—consists of a highly open framework of solvated and mobile n-alkyl chains with hydroxyl groups at the matrix surface. It behaves more like a microphase, exhibiting swelling and aggregation behavior depending on the type of eluent employed. Table I lists some additional prop­ erties. The surface area of carbon spans a much wider range than that of reversed-phase materials, since micro­ pores in carbon, when present, con­ tribute to a large extent to the specific surface area. On the other hand, se­ vere thermal treatment may drastical­ ly reduce the surface to a few square meters, which gives such materials a low linear sample capacity. The parti­ cle porosity of carbon is often found to be lower than it is for silicas. However, in some cases very fine pores are dis­ tributed within the particles. Typical­ ly, carbons offer a broad pore volume distribution whereas silica's is com­ paratively narrow. Thus, carbon and reversed-phase silica have different structures and

Table II. Comparison of Retention of Solutes on Reversed-Phase Silica and Carbon Columns Capacity factor k Acetonitrlle/ water 40/60 (v/v)

Solute

Benzene Phenol Benzaldehyde Acetophenone Methylbenzoate Nitrobenzene Benzonitrile Anisole Benzoic acid

LiChrosorb RP-8

4.7 3.9 2.2 2.0 3.9 3.6 2.7 4.3 0.3

Tetrahydr of ur an/water 40/60 ( v / v )

carbon

1.3 0.4 2.9 3.25 Large Large 2.6 3.0

-

LiChrosorb RP-8

4.7 4.9 1.9 1.9 2.9 3.4 2.3 3.9 0.4

carbon

1.0 0.3 1.5 1.0 2.6 4.0 0.8 1.5 0.6

Data taken from Reference 37 for LiChrosorb RP-8; data for carbon from Reference 25

even at the same eluent composition one cannot expect with similar reten­ tion and sequence of elution of test so­ lutes. However, this consideration does not predict the extent to which carbon and reversed-phase packings will behave differently in chromato­ graphic use. Chromatography on Carbon Columns

Microparticulate carbons narrowly distributed in the 5-20-μηι range are packed by the usual slurry technique; the slurry liquid composition is adapt­ ed to the wetting behavior of carbon (7, 24, 27). Packing stability studies on carbon columns indicate that pres­ sures above 800 bar at corresponding­ ly high flow rates may be applied without bringing about any change of column bed and particle geometry (28). Depending on the extent of hy­ drophobic surface character, carbons may exhibit a limited wettability toward hydro-organic and buffered solvents. Similar to pyrocarbon-modified carbon blacks (7), carbons made from hard cokes and active carbons (24) cease to be wetted by methanol/ water eluents above 45/55 v/v, while columns packed with Jado carbons (29) are run in neat buffered eluents. Conditioning of carbon columns re­ quires the same amount of eluent as do reversed-phase packings. Excep­ tions have been observed for those packings that contain micropores of less than 0.2-nm mean pore diameter (25). In this latter case, equilibration took a long time, comparable to the time needed for conditioning of dry silica with very nonpolar solvents. Column loadability has been the subject of thorough investigation (5, 24). When normalized to unit surface area of packing, and assuming the col­

umn porosity to be equal, the linear sample capacity 0n.i [following Sny­ der's definition (30)] is found to be on the same order for carbon and re­ versed-phase silicas (9). Values com­ parable to 00.1 are obtained (5, 24) when 00.5 is employed, where #o.5 is the mass of solute per gram of packing that produces a 50% reduction in col­ umn efficiency. When deviations be­ tween 0o.i and 0o.s arise, they are de­ pendent on the type of eluent and sol­ utes. The kinetic performance of any HPLC column is characterized by its flow resistance factor φ' and the shape of the plot of the reduced plate height (h) against the reduced linear velocity (v) (31,32). The plot h vs. ν follows the equation

pared to silica and reversed-phase sili­ ca. The steeper the isotherm and the higher the capacity factor, the smaller the linear sample capacity 0o.i will be. One cause for the peak tailing of strongly retained solutes may be an overload effect. Apart from the iso­ therm effect, carbon may contain mi­ cropores that dramatically retard the kinetics of mass transfer. Again this effect is expected to be proportional to the retention of solutes. The poor peak shape can also arise from surface heterogeneities of carbon packings. This observation demands additional investigation to test the above predic­ tions and to overcome these problems by synthesizing carbons with an ap­ propriate pore and surface structure. The lack of good chromatographic efficiency for retained solutes com­ pared to reversed-phase silica columns is one major reason for the limited practical application of carbon col­ umns. As is shown by a series of chromato­ graphic separations, carbons possess a particular advantage: Eluents over the whole pH range (20, 24, 29) can be used without any chemical decomposi­ tion. Chemical stability, however, is a direct function of the extent of the surface area; a carbon packing with a high specific surface area (>500 m 2 /g, with micropores) is much more sensi-

100F

10 : (a)

h = B/v + A;>1/3 + Cv with a minimum h of 2-5 at ν in the range of 1-10 with A 8.0, but this is not the case for carbon. Unger et al. sepa­ rated mixtures of alkaloids on carbon in methanol/buffered eluents with pH up to 12 (24). Jado carbons were em­ ployed for amino acid separations in

370 A · ANALYTICAL CHEMISTRY, VOL. 55, NO. 3, MARCH 1983

neat buffered eluents between pH 0 and 8 (29). Peak tailing, frequently observed in carbon chromatography, is eliminated by adding small amounts of modifier to the eluent, silmilar to silica chroma­ tography (9). The modifier molecules are probably adsorbed at the most ac­ tive surface sites of carbon, thus pro­ viding a better surface homogeneity. The merits of reversed-phase ion pair chromatography (RPIPC) in the resolution of ionic species are widely recognized (38-40). Carbons are also suitable as supports in ion pair chro­ matography, provided that the counterion chosen is not strongly retarded at the given range of eluent composi­ tion (41). For carbon made from hard coke and with specific surface area ~ 5 m 2 /g, the counterion tetrabutylammonium (TBA) meets these demands in methanol/water mixtures. As for RPIPC, retention on carbon in the ion pair mode is also dependent on the counterion concentration and on the pH of the eluent. Table III demon­ strates the effect of ion pair reagent on the retention of carboxylic acids at otherwise constant conditions at pH 9.0. Further examinations have indi­ cated that differences arise in the se­ lectivity between reversed-phase packings and carbons in the ion pair mode (25). Selectivity of Carbon vs. Reversed-Phase Packings The difference in surface structure of both packings indicates that chro­ matographic discrimination will be different. Retention of solutes at the carbon surface resembles an adsorp­ tion process more closely than that at the surface of a reversed-phase pack­ ing; the latter exhibits a rather ex­ tended interfacial layer. The retention mechanisms on reversed-phase silicas and pyrocarbon-coated silicas have been treated by Colin and Guiochon (6). Both packings offer a hydropho­ bic as well as a hydrophilic selectivity,

each in a q u i t e different way, however. F r o m t h e linear plot of log k vs. n, a' values can be calculated from:

a r a t i o n of alkaloids, catecholamines, m e t h y l a t e d b e n z e n e s (see Figure 3) a n d carboxylic acids (24, 25,41).

7«

Conclusion

a' = log ( k ' „ + 1 / k ' „ ) T h e s e are a m e a s u r e of t h e discrimi­ n a t i o n of solutes differing by one CH2 or CH3 group, or by one ring carbon a t o m , a' values on carbons (9) fall in t h e range between 0.25 to 0.45 for a m e t h y l e n e or a m e t h y l group, a' val­ ues for ring carbon a t o m s in P N A mol­ ecules are a p p r o x i m a t e l y 0.25-0.28. T h e highest values of a' are o b t a i n e d for m e t h y l derivatives of a r o m a t i c c o m p o u n d s . T h e value a' of c in Fig­ u r e 2 for carbon is 0.28, c o m p a r e d t o a' = 0.12 for LiChrosorb R P - 1 8 a t t h e s a m e e l u e n t composition. T h u s , car­ b o n offers a m u c h more p r o n o u n c e d h y d r o p h o b i c selectivity for P N A . T h e higher r e t e n t i o n a n d b e t t e r selectivity of carbons with h y d r o p h o b i c com­ p o u n d s a p p e a r to arise from stronger dispersion interactions, also resulting in b e t t e r steric recognition. P r e l i m i n a r y results also indicate t h a t carbon packings show a h y d r o philic selectivity toward polar solutes, as is shown in T a b l e II. T h i s p r o p e r t y results from t h e polar functional groups a t t h e carbon surface. How­ ever, t h e limited d a t a d o n o t p e r m i t a simple correlation t o be d r a w n be­ tween t h e elution sequence of polar solutes a n d their molecular dipole m o ­ m e n t , molecular cross-sectional a r e a etc. It is also worth noting t h e variation in t h e elution order of some polar sol­ utes on carbon upon s u b s t i t u t i n g acetonitrile by t e t r a h y d r o f u r a n u n d e r otherwise c o n s t a n t conditions (see T a b l e II). T h e s e solvents differ mainly in their proton donor a n d acceptor properties, t h e strong dipole contribu­ tion to t h e solvent polarity p a r a m e t e r P ' being equivalent (42). T h e more basic character of t e t r a h y d r o f u r a n a p ­ pears t o cause t h e change in r e t e n t i o n of a c e t o p h e n o n e a n d benzaldehyde, b o t h bearing basic carbonyl groups. S u c h a discrimination is only observed on carbon and does n o t occur on Li­ Chrosorb R P - 8 , as shown by inspec­ tion of T a b l e II. T h i s is strong evi­ d e n c e for a stationary-phase effect. Significant differences are seen on c o m p a r i n g t h e elution sequence of polar solutes on t h e reversed-phase a n d t h e carbon column. Control of se­ lectivity of polar solutes seems to be affected by t h e t y p e of polar surface functional groups on t h e packings a n d t h e i r h y d r o p h o b i c e n v i r o n m e n t at t h e surface (see Figure 1). Silica bears a single t y p e of weakly acidic hydroxyl, s u r r o u n d e d by long-chain n-alkyl groups. T h e carbon, however, develops a variety of polar groups, some being acidic and some basic, a n d all b o n d e d t o an a r o m a t i c matrix.

2

3

4

51

J 1

8

'

1

1L

6

4 2 time(min)

0

Figure 3. Separation of benzene deriva­ tives on carbon made from hard coke (28). Sequence of solutes: 1 = metha­ nol; 2 = benzene; 3 = toluene; 4 = ethylbenzene; 5 = isopropylbenzene; 6 = tertbutylbenzene; 7 = 1-methylstyrene. Column dimensions, 250 X 4 mm; packing, carbon made from hard coke (dp = 8.9 Mm); eluent, methanol/ water (55/45 v/v); detector, UV 254 nm; flow rate, 0.26 cm/s; pressure, 242 bar; injection volume, 5 μΙ_

T h e specific polar group selectivity of carbon suggests t h a t this should be fully exploited by applying t e r n a r y a n d q u a t e r n a r y solvent m i x t u r e s ; fur­ t h e r work in this direction is p l a n n e d . Applications M o s t of t h e r e p o r t e d s e p a r a t i o n s on c a r b o n s have been carried o u t by t h o s e workers who synthesized t h e p a r t i c u l a r packings. Commercial car­ b o n s are n o t yet on t h e m a r k e t . Mix­ t u r e s of a r o m a t i c h y d r o c a r b o n s , m e t h ­ yl b e n z e n e s , a n d phenols as well as p o l y c h l o r i n a t e d biphenyls, steroids, a n d sulfur- a n d nitrogen-containing c o m p o u n d s (5, 6) have been resolved on pyrocarbon-modified carbon blacks. Isomer selectivity was d e m o n ­ s t r a t e d with m i x t u r e s of u n d e c e n e iso­ m e r s , isomeric aromatics, a n d a d a m a n t a n e s (33, 43). J a d o carbon has been e m p l o y e d m a i n l y for t h e s e p a r a t i o n of a m i n o acids (20). C a r b o p a c k Β was found t o be highly suitable for t h e res­ olution of a l k y l - s u b s t i t u t e d benzenes, n a p h t h a l e n e s , triazines, p h t h a l a t e es­ ters, analgesics a n d a m i n o acids. Car­ bons m a d e from h a r d cokes a n d active c a r b o n s have b e e n a p p l i e d for t h e sep­

372 A · ANALYTICAL CHEMISTRY, VOL. 55, NO. 3, MARCH 1983

T h e carbon m a t e r i a l s developed for H P L C to d a t e d o n o t m e e t all t h e re­ q u i r e m e n t s necessary for selective a n d effective packings, such as rigidity of microparticles, a d e q u a t e surface area, a n d uniformity of surface. T h e m o s t serious d i s a d v a n t a g e is t h e poor efficiency for strongly r e t a i n e d com­ p o u n d s , which also limits t h e peak ca­ p a c i t y . C u r r e n t a n d future a t t e m p t s in t h e synthesis of carbon m a y eliminate these drawbacks. On t h e o t h e r h a n d , it is certain t h a t c a r b o n offers a h y d r o p h o b i c a n d hyd r o p h i l i c selectivity t h a t is different from t h a t of r e v e r s e d - p h a s e silica. T h e selectivity of carbon t o w a r d polar sol­ u t e s h a s n o t y e t been fully exploited; in t h i s respect, one further a d v a n t a g e is t h e p H stability of carbon packings. References (1) Karger, B. L.; Giese, R. W. Anal. Chem. 1978,50, 1048 A. (2) Horvath, C ; Melander, W.; Molnar, I. Anal. Chem. 1977, 49, 142. (3) Molnar, I.; Horvath, C. J. Chromatogr. 1977,142,623. (4) Halâsz, I. Anal. Chem. 1980, 52, 1393 A. (5) Colin, H.; Ward, N.; Guiochon, G. J. Chromatogr. 1978,149,169. (6) Colin, H.; Guiochon, G. J. Chromatogr. 1978,158, 183. (7) Colin, H.; Eon, C ; Guiochon, G. J. Chromatogr. 1976,7 79,41. (8) Colin, H.; Eon, C ; Guiochon, G. J. Chromatogr. 1976, 722, 223. (9) Knox, J. H.; Unger, K. K. J. Liquid Chromatogr., in press. (10) Shreve, R. N.; Brink, J. Α., Jr. "Chem­ ical Process Industries"; McGraw Hill: London, 1977; p. 120. (11) Cheremisinoff, P. N.; Ellerbusch; F., Eds.; "Carbon Adsorption Handbook"; Ann Arbor Science: Ann Arbor, Mich., 1978. (12) Medalia, A. I.; Rivin, D. In "Charac­ terization of Powder Surfaces"; Parfitt, G. D.; Sing, K. S. W., Eds.; Academic Press: London, 1976. (13) Fischbach, D. B. "Chemistry and Physics of Carbon"; Walker, Ph. L., Jr., Ed.; Dekker: New York, 1971; Vol. 7, p. 1. (14) Bokros, J. C. "Chemistry and Physics of Carbon"; Walker, Ph. L., Jr., Ed.; Dekker: New York, 1969; Vol. 5, p. 1. (15) Puri, B. R. "Chemistry and Physics of Carbon"; Walker, Ph. L., Jr., Ed.; Dek­ ker: New York, 1971; Vol. 6, p. 191. (16) Boehm, H. P.; Diehl, E.; Heck, W. Ind. Carbons Graphites 1966, 2, 369. (17) Boehm, H. P.; Diehl, E.; Heck, W.; Sappok, R. Angew. Chem. Int. Ed. 1964, 3, 669. (18) Plzâk, Z.; Dousek, F. P.; Jansta, J. J. Chromatogr. 1978, 747, 137. (19) Patzelova, V.; Jansta, J.; Dousek, F. P. J. Chromatogr. 1978, 748, 53. (20) Smolkova, E.; Zima, J.; Dousek, F. P.; Jansta, J.; Plzâk, Ζ. J. Chromatogr. 1980, 797,61. (21) Zwier, T. H.; Burke, M. F. Anal. Chem. 1981,53,812. (22) Ciccioli, P.; Tappa, R.; di Corcia, Α.; Liberti, A. J. Chromatogr. 1981,206, 35. (continued on p. 375 A)

(23) Unger. Κ.; GoeU. Η. Ger. Of/en IS7». 2.802Ϊ (24) Unger. K.: Roumeliotis. P.; Mueller. H.; Goett. H. J Chromatogr. 1980. 21)2. 3. (25) Mueller. H.; Roumeliotis. P.: Unger. Κ. Κ.. in pre··. (26) Knox. J. H.; Gilbert, M.T.DOS 2\946.688. (CI COI B31/08) June 12. 1980. Brit. Appl. 78/45397. Nov. 21,1978. U.S. Patent. (27) Gilbert. M. T.: Knot. J. H.; Singh. B.. unpublished work. (28) Goetz. H. PhD. Thesis. Technische Hochschule. 6100 l)arm»tedt. K.R.G..

197a (29) Zima. J.; Smolkova. E. J. Chromatogr. 1981.207.79.

(.10) Snyder I.. K. "Principle· of Adsorp. lion (hromaWncraphv"; IVkker: New York. 1968; p. 168. (31) Knoi. J. H. "High Performance Liq­ uid Chromatography"; Kdinburgh Uni­ versity Press: Edinburgh. 1978: p. 5. (32) Brutow. P. Α.; Kim». J. H. Chromatographta 1977.10. 279. (33) Guiochon. G ; Colin. H. "Chromatog raphy Review"; Spectra Physics. Santa Clara, 1978.4,8. (»4) Snyder. L. R.; Kirkland. J. J. "Intro­ duction to Modern Liquid Chromatogra­ phy"; Wiley: New York. 1979; p. 257. (35) Tanaka. N.;Tokuda. Y ; Iwaguchi. K.; Araki. M. J. Chromatogr 1982,239.761. (36) Roumeliotis. P. Ph.D. Thesis. Technische Hochschule. 6100 Darmstadt. P.R.G.. 1977. (37) Bakalyar. St. Κ . Mcllwrick. H. Roggendorf. K. J Chromatogr 1977. /42T353. (38) WMund.K. G.J Chromatogr. 1975. 7/5.411. (39) Johansson. 1. M : Wahlund. K. G.; Schill. G. J. Chromatogr. 1978.149. 281. (40) Gloor. R.: Johnson. E. L J. Chroma­ togr. Sri. 1977, IS. 413. (41) l'nger. K.. presented at the 13th In­ ternational Symposium on Chromatogra­ phy. Cannes. June 30 July 4.1980. (42) Glajch. J. L: Kirkland; J. J.: Squire. K. M.; Minor. J. M. J Chromatogr. 1980. 199.57. (43) Colin. H.: Guiochon. G.; Prusova. D. J Chromatogr. 1982.234.1.

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HIGH PURITY ACIDS • ACS REAGENT ACIDS • REDISTILLED ACIDS Produced by taking ACS Reagent Grade acids and redistilling them in glass. This distaiiatlon yields high purity acids with very low or absent trace elements.

DOUBLE-DISTILLED ACIDS Κ. Κ. Unger is professor of chemistry at Johannes Gutenberg-Universi tat, Maint, F.R.G. He received his PhD in chemistry at the Technische Hochschule of Darmstadt, F.R.G.. in 1965. He was visiting professor at Northeastern University in 1973 and at National University of Singapore in 1982. His research focuses on the synthesis and characterization of tailor-made porous adsorbents and supports and their application in var­ ious fields, particularly HPLC.

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ORCLE 102 ON REAOER SERVICE CARD ANALYTICAL CHEMISTRY. VOL. 55. NO. 3. MARCH 1983 · 37» A