Report István Halász Angewandte Physikalische Chemie, Universität d e s S a a r l a n d e s , 6600 Saarbrucken, F e d e r a l R e p u b l i c of G e r m a n y
Columns for Reversed Phase Liquid Chromatography Reversed phases (RP) usually consist of normal hydrocarbons bonded onto the surface of silica, with chain lengths from C1 to C 2 2 . T h e enormously rapid growth in the use of R P in liquid chromatography (LC) is due mainly to their versatility and speed of equilibration. Both polar and nonpolar eluents can be used in R P L C , although polar eluents are more widely used, and hence there are few problems with the solubility of the sample. Consequently both polar and apolar mixtures can be separated. All LC eluents contain more or less water, and retention is a function of this not always well-defined water content of the eluent. T h e equilibration of R P with the eluent, as the water content is changed, is very much more speedy with R P t h a n with silica or alumina. Although in the great majority of cases a completely nonpolar R P is required, occasionally, where mixtures with a broad polarity spectrum are to be separated, an R P with a residue of polar silanol groups can be used. This REPORT is not meant to be an exhaustive review of the subject. Several excellent reviews (1-4) already exist and many monographs (listed in ref. 3) discuss the basic problems of R P . This is a purely personal, and sometimes provocative, review of some selected parts of the subject, and literature citations have been deliberately curtailed. Only the properties of typical routine columns (3-8 m m i.d., 10-30 cm in length) will be discussed. Quite a p a r t from other considerations, there are certain instrumental difficulties to be overcome before microbore (i.d. 1 m m or less) and open tubular columns are suitable for routine use. Even in the triumphal age of the microprocessor, it is worth restating the trivality t h a t samples t h a t are not 0003-2700/80/A351 -1393$01.00/0 (© 1 9 8 0 A m e r i c a n C h e m i c a l S o c i e t v
separated in the column cannot be separated later by calculation. T h e routine analyst is primarily interested in separation, in quantitative analysis, and in baseline separation (R = 1.5) where possible. T h e speed of analysis is usually of secondary importance. In the past few years there have been several excellent theoretical t r e a t m e n t s of the complex mechanism of R P L C (1-4). These have increased understanding of the separation and facilitated the development of techniques such as ion-pair and ligandexchange LC. It is unfortunate t h a t hardly anybody has described his R P exactly enough for others to reproduce it. Unfortunately, although R P is a wonderful tool, it is also one of the most undefined systems in LC. Although virtually any chemist is able to prepare an R P , it is extremely difficult to prepare a good R P more or less reproducibly. Most of the difficulties encountered in the preparation and packing of R P having particle sizes (dp) 3-10 μπι stem from the small dp and from the necessary high specific surface area, which results in a high surface energy. T h e small dp brings us to the fringe of colloid chemistry. Because of the high surface energy the supports are sur face catalysts whose properties can change with passage of time—they "age." Furthermore, two-dimensional chemistry at the surface of a catalyst is unlikely to be the same as chemistry in the bulk phase. Consequently, our knowledge of the reaction mechanism is limited. For example, in our experi ence, some batches of commercial sili ca are unsuitable, when first bought, for the preparation of R P . A year or two later, however, they can be used to prepare good R P . Heat and/or chemi cal treatments sometimes, b u t by no means always, accelerate this aging
C a r b o n N u m b e r of P h e n y l a l k a n e Chain
Figure 1. Relative retentions of phenylalkanes of varying chain length with re spect to ethylbenzene on RP with differ ent chain length Curve a: C 2 2 RP (2.8 μΓηοΙββ/ηη2); Curve b: C i 8 RP (2.9 ^ m o l e s / m 2 ) ; Curve c: C 8 RP (3.2 μΓποΙββ/ηίΐ2); Eluent: methanol, room tempera ture; Silica base: SI 100 (Merck A.G., Darmstadt), specific surface area 350 m 2 /g (120 m 2 /cm 3 ), average pore size 94 Λ
process. Others have probably also ob served t h a t the same person, using the same batch of R P and the identical packing method, is occasionally quite unable to pack a satisfactory column, even though the obvious variables (temperature, humidity, etc.) are con trolled. T h e resolution (R) of two closely neighboring peaks can be approxi mated by:
(1) where tn is the retention time, w the average peak width, a the relative re-
ANALYTICAL CHEMISTRY, VOL. 5 2 , NO. 1 3 , NOVEMBER
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tention, k' the capacity ratio, and η the number of theoretical plates. Since the term (a — 1) appears in equation (1) and since a is close to one in value, the resolution is an extremely sensi tive function of the retention. T h e res olution is only proportional to y/n , so t h a t a change in η has a much smaller effect on R than in a. T h e relative re tention and the k' values of a pair of substances are influenced not only by the composition of the eluent, b u t also by the quality of the stationary phase. Because of this, what follows deals mainly with the properties of the R P and only briefly with the efficiency of the column itself. Properties of the silica b a s e . T h e surface properties of silicas, including those from commercial sources, vary markedly from batch to batch. This, together with the "aging" discussed previously, is one of the main reasons why it is impossible at this time to prepare reproducible R P . T h e permeability of a column packed with spherical particles is about 20-40% better than one packed with irregular particles of similar par ticle size distribution. It is held by some t h a t the quality and reproduc ibility of packing are better with spherical particles. Exactly the oppo site view is held by others, who main tain t h a t spherical particles can be displaced more easily within the bed. It probably depends more on the qual ity of the sieve fraction and on the packer's experience with a given mate rial. One thing is certain: T h e spheri cal material is more expensive than the irregular. T h e quality of commer cial silicas, especially with regard to particle size fractionating, has im proved in the last two years to the ex tent t h a t no further sedimentation is now required and the given average particle size can now be relied upon. T h e capacity ratios (k') attained with a given stationary phase (e.g., Cie) depend upon the specific surface area per unit empty column volume (m 2 /cm 3 ), which in turn depends on the specific surface area per unit weight (m 2 /g) and on the packing den sity (g/cm 3 ). Sometimes smaller spe cific surface areas per gram are com pensated by a higher packing density. T h e great majority of commercial sili cas have specific surface areas, which lie within the most useful range for LC. T h e choice of specific surface area depends upon the problems to be solved, i.e., whether high or low k' val ues are required. Typically, commer cial silicas have surface areas in the range 180-460 m 2 /g (or 90-220 m 2 /cm 3 when packed) (5). Length of the hydrocarbon chain. When the surface of the silica is completely covered with a mono layer of organic bristles, a point t h a t will be discussed later, then, as the
length of the bristles is increased from Ci to C22, the capacity ratio, the rela tive retention, and the maximum sam ple size also increase. T h e above applies to nonpolar samples; for polar samples the contrary can be true. High capacity ratios are desirable when multicomponent systems are to be separated, i.e., when high peak ca pacity is required. For simple mix tures low k' values are desirable in order to increase the speed of separa tion. Figure 1 illustrates the increase of relative retention of apolar samples with increasing chain length of the or ganic coating (6). Here a homologous series of phenylalkanes has been sepa rated using methanol as eluent. T h e retentions are related to ethylbenzene. T h e relative retentions (a) are plotted against the carbon number of the sam ple. T h e highest relative retentions are achieved with C22 bristles (curve a ) , slightly lower relative retentions result when the chain length of the organic coating is C i 8 (curve b) and retention is even lower with Ce bristles (curve c). All the R P were further reacted (capped) with hexamethyldisilazane. T h e average place requirements for an organic bristle were 59 Â 2 for C22, 57 Â 2 for C 1 8 , and 53 Â 2 for C 8 . T h e C 2 2 R P contained 19.6; the Ci 8 , 17.2; and t h e C 8 , 9.8% carbon w/w. As can be seen from Figure 1 the effect of increasing the bristle length of the support (from curve c to curve a) on the relative retention increases with increasing chain length of the nonpolar phenylalkane sample. This is to be expected from a simple picture of the mechanism of the interaction. T h e longer bound alkane chains can interact better with the longer alkane chains of the sample. T h e most usual chain lengths applied in routine R P are C i 8 and C 8 . Shorter chain R P (e.g., C4 or Ci) can be useful when the sample contains polar or polarizable substances. Silanizing technique. There is a great deal of discussion about whether an alkyldimethylchlorosilane, an alkylmethyldichlorosilane, or an alkyltrichlorosilane is the optimum reagent to use. Instead of chlorosilane the corresponding methoxysilane can, of course, be used. T h e trichloro- and sometimes the dichlorosilanes are commercially available and relatively inexpensive. With few exceptions the monochlorosilanes must be specially synthesized. Without question a monochlorosilane can only react with one silanol group of the silica, so t h a t the reaction product is better defined. It is possible for a dichlorosilane to react with two vicinal silanol groups, although this does not necessarily occur. It is, however, extremely improbable t h a t a trichlorosilane will react with three silanol groups for steric reasons.
1394 A · ANALYTICAL CHEMISTRY, VOL. 52, NO. 13, NOVEMBER 1980
Hence, when di- or trichlorosilanes are used, one must expect t h a t unreacted chlorine will remain in the matrix. Water in the reaction mixture can interact with these bound chlorines and initiate the formation of a bound "silicone" polymer layer. Another possibility is t h a t bound chlorines will remain in the R P to be later hydrolyzed to unwanted silanol groups of undefined concentration. In all R P there are many unreacted silanol groups, and the higher the carbon content of the R P , the better the shielding of these unreacted silanols. In our experience the carbon content of R P prepared using trichlorosilanes is higher t h a n t h a t obtained using monochlorosilanes of identical chain length. If after preparation the R P are further reacted with a trimethylsilanizing agent (e.g., hexamethyldisilazane), then some silanol groups are removed (capped). It can be calculated from the carbon analyses t h a t an R P prepared using octadecyldimethylchlorosilane has had 0.1 μηιοΐβ/ιη 2 silanol capped (6). For a C 1 8 R P prepared using the corresponding trichlorosi lane this value can be as much as 1 ^mole/m 2 (6). Although a good cap ping reaction is not simple and is ex pensive, nevertheless, for the reasons mentioned above, it is always to be recommended as an aid in the prepa ration of good, apolar R P . When chromatographically observ able polymerization occurs at the sur face of the silica, mass transfer in the stationary phase is slowed down. This means t h a t the ascending branch of the h vsu curve becomes steeper, and its minimum is pushed to lower linear velocities (u). Both of these effects are unwanted. However, chromatographi cally unobservable "polymerization" shields unreacted silanol groups on the surface of the silica. Most com mercially available RP exhibit no ob servable "chromatographic polymer ization," so t h a t any interest remain ing in this question is theoretical. T h e concentration of silanol groups available to the sample is one of the factors determining the retention, especially of polar or polarizable sub stances. In our experience, a capped R P correctly prepared from a trichlo rosilane is always more apolar t h a n an uncapped R P prepared from a monochlorosilane. T h a t is, using the more expensive monochlorosilane does not obviate the necessity for capping. T h e extent of the difficulties de scribed above should not be mini mized, as is shown by the following ex ample: Take the same silica and care fully prepare from it two capped Cis R P , one using mono-, the other dichlo rosilane. T h e absolute and sometimes the relative retentions of even con densed aromatic hydrocarbons (not to mention polar compounds) are some-
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bj BURDICK & JACKSON LABORATORIES, INC. 1953 SOUTH HARVEY STREET · MUSKEGON, MICHIGAN 49442 CIRCLE 21 ON READER SERVICE CARD 1396 A · ANALYTICAL CHEMISTRY, VOL. 52, NO. 13, NOVEMBER 1980
Carbon Number of Phenylalkane Chain Figure 2. Logarithm of α for phenylalkanes on RP with different silica bases Curve a: silica base SI 40 (Merck A.G., Darm stadt), average pore size 40 Â, specific surface area 300 m 2 /g, carbon content 10.1 % w/w; Curve b: SI 60, 60 A, 380 m 2 /g, 10.8% w/w; Curve c: SI 100, 100 A, 350 m 2 /g, 10.0% w/w and SI 200, 200 A, 150 m 2 /g, 5.4% w/w. All RP were prepared using octylmethyldichlorosilane and capped. Eluent: acetonitrile, room temperature
times as different as if stationary phases of different types had been prepared. It is worth mentioning again and again that the optimal apolarity of an R P depends on both the eluent and the mixture to be separated. Some commercial RP are prepared not by exhaustively silanizing the silica, but by silanizing to a given predetermined carbon content, irrespective of any batch to batch variation in the surface properties of the silica. The main advantage of this approach is that the absolute retentions of apolar compounds become more reproducible. Another advantage is that the preparation itself is simpler. The disadvantages of this approach are the nonreproducibility of the absolute and relative retentions of polar and polarizable substances and the possibility of tailing with such substances. The difficulties of adequate description can be further illustrated using the next example, where capped RP were carefully prepared using the same silanizing agent (octylmethyldichlorosilane) and the same technique, but with silicas from the same producer (Merck A.G., Darmstadt) having different pore size distributions. Relative retentions as a function of the side chain carbon number of the phenylalkanes (apolar) are shown in Figure 2 (6) for C 8 RP. Although the specific surface areas of these silicas range from 150 m 2 /g to 380 m 2 /g, the surface coverage (the average space requirement per bristle) is similar. The average pore sizes are 40, 60,100, and 200 Â. Even though the logarithm of the relative retention is plotted, it is quite evident that the RP appear to be of three different types. Only the relative
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retentions of t h e two with average pore sizes 100 Â and 200 Â (350 m 2 /g and 150 m 2 /g) are indistinguishable. If the samples had been more polar, t h e differences would have been even more striking. Pore size distribution. When silica is reacted to form R P , t h e pore size distribution and t h e specific surface area are shifted to smaller values. This effect is illustrated in Figure 3. T h e silica base, whose pore size distribution is indicated by t h e solid line, had an average pore diameter of 82 Â and a specific surface area of 400 m 2 /g. T h e Ci8 R P prepared from it, whose distribution is represented by the broken line, had an average pore diameter of 51 A and a specific surface area of 310 m 2 /g. T h e pore size distributions were measured by elution chromatograph using methylene chloride as eluent (7,8). T h a t is, t h e mean pore diameter is reduced by 38%. This displacement increases with t h e increasing chain length of t h e organic bristle and with t h e decreasing pore size of the silica. T h e chromatographic pore size distribution of R P is also a function of the eluent, especially for water-containing eluents when they are buffered (6, 9). This effect is well known in t h e field of ion exchange (10), b u t has been rarely emphasized for R P . One of the consequences of this is t h a t t h e retention volume of an inert peak is dependent upon t h e eluent. When t h e eluent is made more hydrophilic (or t h e buffer is changed), t h e approach to equilibrium can be slow and before equilibrium is reached t h e holdup volume can change with time. If the pore volume available to t h e " i n e r t " sample is changing, then t h e linear velocity will change, even if the flow rate is kept constant. Of course, t h e retentions of retarded peaks can also be affected. T o eliminate unwanted and sometimes inexplicable exclusion effects it is recommended t h a t silica with an initial average pore diameter of 80 Â or more be used as R P . Column packing. In general it is more difficult to pack R P than to pack silica into good columns, and R P columns are also generally less stable. This can be pictured as being due to the organic surface of t h e particle being better "lubricated." T h u s it slips more easily against its neighbors. Whether t h e balanced density or t h e viscosity method is used to pack R P columns, t h e composition of the dispersing medium has t o be adjusted by trial and error and changed from batch to batch of R P , sometimes from day to day. This unexpected sensitivity is probably caused by t h e ill-defined wettability of t h e R P by t h e organic dispersing agent of uncertain water content, or by changes in t h e packing temperature.
CIRCLE 185 ON READER SERVICE CARD 1398 A · ANALYTICAL CHEMISTRY, VOL. 5 2 , NO. 13, NOVEMBER 1980
Pore Diameter
Figure 3. Chromatographic pore size distribution of a silica base and a capped C i 8 RP produced from it Solid line: silica base; Broken line: C 18 RP produced from it; Average pore diameters: 82 Λ (sili ca) and 51 A (RP); Specific surface areas: 400 m 2 /g (silica) and 310 m 2 /g (RP); Binding reagent: octadecyl trichlorosilane; Eluent: methylene chlo ride
Column efficiency can be described either in terms of t h e number of theo retical plates (n) for a column with given length or in terms of the height equivalent to a theoretical plate (h). In routine analysis it is usually of less importance than t h e relative reten tion, as was shown in equation (1) and discussed above. T h e h value is a function of t h e linear velocity, of t h e capacity ratio, of t h e viscosity of t h e eluent (which is inversely proportional to t h e interdiffusion coefficient), and sometimes, surprisingly, of t h e column length. For this last reason it pays t o be cautious when quoting numbers of plates per meter by extrapolation from d a t a obtained using a 20- or 30-cm column. T h e plate number of an unretained peak is not a good mea sure of t h e efficiency of a column since the plate number changes markedly for even slightly retained compounds. For several reasons it is sensible to work a t linear velocities corresponding to umin (i.e., a t the minimum of t h e h vs u curve), because here t h e efficien cy changes least with changes in ca pacity ratio (k'), and many commonly used solvents are rather viscous, etc. When choosing t h e optimum linear velocity, it must be remembered t h a t Umin decreases with increasing capaci ty ratio. T h e value of hmin is, for parti cles in t h e size range 3-15 μτη, roughly proportional to t h e particle diameter (dp), b u t t h e permeability of t h e col u m n is proportional to d%. Conse quently, a large price in increased pressure drop has t o be paid for in creasing column efficiency by decreas ing t h e particle size. For a large pro portion of routine analyses, short col u m n s (10-30 cm in length) having ef ficiencies of 5000 to 10 000 plates, where the pressure drop (50 bar t o 150
bar) is not a limiting factor, are suit able. T h e separation time can be re duced when both the particle size and the column length are reduced. In rou tine practice only two length of col umns—for example, 10 cm and 25 cm—are required. In order to mini mize the influence of extra column broadening effects as the column is shortened, its internal diameter should be increased. For example, if a 10-cm-long, 4-mm i.d. column is packed with 5-μπι particles, and then used with many of the commercially available instruments, a great deal of the efficiency of the column will be de stroyed by extra column broadening effects. A column is often too efficient for a given separation in t h a t better than baseline separation is achieved. Instead of taking a shorter column to increase the speed of the separation, the usual practice is to disturb t h e ef ficiency of the column by increasing the linear velocity. T h e consequence is a high pressure drop over the column. There are, of course, complicated sep arations, where extremely high effi ciency and u p to 500 bar (or more) inlet pressures are required. However, p u m p s of such pressure capability are sometimes ordered merely for prestige purposes, or to ameliorate the effect of using columns t h a t are too efficient, even though the manufacturers offer p u m p s t h a t are suitable and cheaper. It is always true t h a t the speed of analysis can be increased by decreas ing the particle size. However, difficul ties encountered in column packing and stability using particles smaller t h a n 5 Mm have yet to be satisfactorily solved. At the moment 7 μηι-10 μτη particles are to be recommended for routine use. For methanol (viscosity 0.6 centipoise), hmin values of around 3 (i p to 4 dp can be claimed for capaci ty ratios in the range of 1 to 2. Peak Asymmetry. Symmetrical Gaussian peaks are assumed in the theory of chromatography, including equation (1). Unfortunately, to our knowledge, such a peak has never been observed. Actual peaks have tail ing and/or leading edges and some times even shoulders. T h e efficiency (h or n) is always calculated from the "mother peak." T h e peak asymmetry (As) is defined as the ratio of the peak half widths at a given peak height. T h e lower down the peak the asymme try is measured, the larger the As is. T h e smallest distance between two peaks is along the baseline, conse quently, As should be defined here. Because of detector noise, among other factors, an acceptable compro mise is to measure As at 10% of peak height (11). T o extract any meaning from the basic equation (1), in a first approach an As2h or n/As2 value is re quired if the efficiency is calculated
from the standard deviation of the symmetrical part of the peak. T h e most meaningful way to define the ef ficiency of a column, for routine sepa rations, is to quote As2h together with the height As was measured. Asymme try factors are always larger for ex tremely polar substances, which inter act with the unshielded residual silanol groups. This effect, while extreme ly important for the separation of such substances, is very difficult to quantitate and hence such substances are not used for the quantitative charac terization of peak symmetry. Extra column effects are included in the measured value of As; consequently, As is greatest for the inert peak. Tol erable values for As are 0.9-1.3 for an inert peak, and around 0.95-1.15 for a peak of h' = 2. (Note: an As of 1.3 re duces the efficiency by 69% and the resolution by 30%.) T h e measured As is a function of the quality of the R P , the way the column is packed, the sample size, the temperature, and also of the quality of the instrument to which it is attached. Concluding remarks. At present all commercially available R P are sili ca based. Silica itself is a more or less undefined system, which cannot be absolutely reproducibly prepared. It is a pleasant surprise t h a t manufactur ers are able, at the moment, to pro duce silica whose specific surface areas and pore size distributions are con stant within 5-10%. Unfortunately, their chemical activity is not nearly so constant. T h e preparation of R P in volves chemistry a t the surface of a high surface energy catalyst. Our prac tical and theoretical understanding of such reactions is very incomplete. Last but not least, the particle sizes of 5 μπι or less are not very much bigger t h a n colloidal particles. It is hopeless there fore to expect an absolutely reproduc ible R P . T h e relatively high reproduc ibility of presently available commer cial R P is a tribute to the a m o u n t of effort t h a t has been p u t into their de velopment. Roughly the following re producibility can be claimed for a good (capped) R P column in a suit able chromatographic instrument: (1) capacity ratios constant within 10%; (2) relative retentions constant within 5%; (3) asymmetry factor (k' = 2) for nonpolar samples between 0.95-1.15; (4) plate height at k' - 2, for nonpolar samples with methanol as eluent, around 3 to 4 particle diameters, at the minimum of the h v s u curve. To obtain such a performance the R P will almost invariably be capped, and the average place requirement for a Cis bristle will be around 50 Â 2 . Generally when selecting R P a very apolar packing is required. However, depending on the problem, it is possible t h a t R P containing residual polar
(silanol) groups or having exclusion chromatographic properties could produce optimal separations. T h e advent of reliable, extremely nonpolar R P has made a large proportion of separation problems routine. However, to find a suitable column for the remaining separations still requires trial and error.
Literature Cited (1) H. F. Walton, Anal. Chem., 50,36 R(5) (1978). (2) B. L. Karger and R. W. Griese, Anal. Chem., 50,1048 A (1978). (3) H. F. Walton, Anal. Chem., 52,15 R(5) (1980). (4) J. J. DeStefano, A. P. Goldberg, J. P. Larmann, and N. A. Parris, Industrial Research & Development, 99-103 (April 1980). (5) R. Ohmacht and I. Halâsz, to be published. (6) H. Schmidt, H. Engelhardt, lecture given at the III International Symposium on Liquid Chromatography, Salzburg 1978. (7) I. Halâsz and K. Martin, Angew. Chem. Int. Ed. Engl., 17,901 (1978). (8) W. Werner and I. Halâsz, J. Chromatogr. Sci, 18, 277 (1980). (9) W. Werner, Ph.D. Dissertation, Universitàt des Saarlandes, Saarbrûcken, 1976. (10) F. Helfferich, "Ionenaustauscher Vol. I," Verlag Chemie, Weinheim, 1959. (11) J. Asshauer and I. Halâsz, J. Chromatogr. Sci., 12,139 (1974).
István Halász received his PhD in chemistry at the University of Szeged, Hungary, in 1949. He served after that in two positions simultaneously—as docent at the Technical University of Budapest, and as head of the Department of Adsorption and Catalysis, Central Research Institute of Chemistry, at the Hungarian Academy of Sciences, Budapest—until 1956. Halász held positions in German chemical industry and as lecturer and associate professor at J. W. Goethe University, Frankfurt/Main, from 1957 to 1970. After a sabbatical year at Northeastern University in Boston and at the University of Nice in France, he was appointed in 1971 to the Chair for Applied Physical Chemistry at the University des Saarlandes, Saarbrücken, a post he has held since that time. Halász is a winner of the 1980 M.S. Tswett Chromatography Medal (see ρ 1418 A, this issue. )
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