Chemically Bonded Stationary Phases In Chrorrnatography In the last several years, chemically bonded stationary phases have greatly augmented the versatility of chroma tographic separations. Although they were first employed as stationary phases rather than column activity moderators in gas chromatography (1), the explosive growth in the utili zation of bonded phases can be linked to the development of high-perfor mance liquid chromatography. The advantages of stationary phases chem ically bonded to the support medium over physically coated phases are evi dent in the areas of column stability and longevity. The most obvious problem over come by bonding the stationary phase to the support is the loss of the sta tionary phase, due to high vapor pres sure in gas chromatography or solubil ity in the mobile phase in liquid chro matography. The proper choice of coated stationary phases and experi mental conditions in gas chromatogra phy often minimizes stationary phase losses to the point where the column is most likely to be ruined by other causes long before significant bleed takes place. Liquid-liquid chromatog raphy presents a much more compli cated case. The mobile phase can dis solve and mechanically strip the sta tionary phase. Often, column bleed is a rapidly progressing problem, causing the chromatographic conditions to change drastically between runs. A partial solution to this problem has been the presaturation of the mobile phase with the stationary phase. This can lead to improved column perfor mance, though not necessarily to the best. Bleed is still encountered due to such factors as temperature changes that affect the stationary phase solu bility. Presaturation techniques can change the nature of the mobile phases and therefore hinder the sepa ration. The stabilization of the experi mental conditions in liquid chroma tography, along with increased column life, can be achieved with bonded sta tionary phases. Column selectivity can also be var ied by changing the nature of the bonded phases. Therefore, the chro1004 A
matographer can choose a bonded phase that is most suitable for his sep aration. At present, most of the practition ers of HPLC have at least one bonded phase column, whereas gas chromatographers still carry out most of their analyses with physically coated phases. This is not to discount the usefulness of bonded phases in gas chromatography. In fact, examples will be shown later to demonstrate that bonded phases can offer excellent separation media. The impression must not be left that bonded phases are immune from routine problems, for they can easily be altered or destroyed if not treated properly. Temperature effects, chemi cal reactions, and even microbial ac tions have been known to render chemically bonded phases ineffective via column degradation. Covalently bonded stationary phases are by far the most often em ployed, and this review will confine its discussion mainly to such phases. Bonded Phase Preparation The first highly publicized ap proach to the bonding of stationary phases is due to Halasz and Sebestian (2) who popularized such phases. They esterified silica gel with alcohols, which resulted in a "brush"-type or monomeric bonded phase of the form Si—Ο—R, R being the bonded moi ety. Such bonded phases, however, are susceptible to hydrolysis; therefore, in liquid chromatography, aqueous solvents cannot be used as mobile phases. At present, most bonded phases are prepared by the reaction of organosilane compounds with silica gel—utilized successfully in the mid1960's by Abel and coworkers (3). A chloro or alkoxysilane is refluxed with silica gel in dry solvents. The chemis try of organosilanes is beyond the scope of this review, and the inter ested reader is directed to the classic text by Noll (4). Bonded phases pre pared from organosilanes are quite immune to hydrolysis due to the for mation of Si—Ο—Si—R bonds. Fig ure 1 shows in a schematic fashion the
Report Eli Grushka Department of Chemistry State University of New York at Buffalo Buffalo, N.Y. 14214
Edward J. Kikta, Jr. FMC Corp. Middleport, N.Y. 14108
reactions that take place between mono-, di-, and trichloro or alkoxy organosilanes and silica gel. Several comments should be made with respect to Figure 1. Currently, it is accepted that the number of accessible OH groups on the silica surface is between 4 and 5 (5). For steric reasons, only two of the three trichloro or trialkoxy groups can react with the surface silanol, as depicted in the figure. The unreacted groups on the organosilane moiety can be easily hydrolyzed to form hydroxyl groups that can react further with excess reagent to form a polymeric coating. Such polymers have been utilized by Aue and coworkers to achieve excellent separations in gas chromatography (6, 7). In liquid chromatography such polymers can lead to deterioration of the efficiency due to stagnant pockets of trapped mobile phase. The free hydroxyl groups can also interfere with the chromatographic separation due to mixed retention mechanisms. To prevent the formation of free hydroxyl groups on the organosilanes and their polymers, the reaction should take place in dry solvents. The water bonded to the silica gel should also be removed, e.g., by heating at 150 °C under vacuum for several hours. The existence of free hydroxyl groups after the bonding of the stationary phase can be determined by methyl red adsorption. In the presence of the slightly acidic hydroxyl groups, a red color is observed. The elimination of the OH groups can be accomplished by silanization after the bonding. Recently, Unger et al. (8) suggested that with bonded hydrocarbons (reversed phase systems), a nonpolar mobile phase such as heptane should be used to monitor the retention times of polar solutes. Such solutes should elute very close to the void volume of the column and exhibit symmetrical peaks. Otherwise, the existence of OH groups is suspected. An excellent discussion on the possible configuration of monoand dichlorosilanes on the surface of the support can be found in the work of Gilpin and Burke (9). There are several other methods
Figure 1 . Schematic of reaction of chloro- or alkoxysilanes and silica gel ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977 · 1005 A
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1006 A · ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
used to bond the stationary phase. For example, Locke et al. (10) reported the use of Grignard reagents to attach the stationary phases to the solid support. Gilpin and his coworkers (11) reported an in situ bonding procedure. Unger and his group (8) recently reported the incorporation of the bonded phases in the silica gel matrix, a technique they termed "bulk modification". This procedure is a promising one, since the amount of bonded phase per surface area of the support can be quite high. Most commercially available bonded phases are prepared with either bior trifunctional silanes. Typically, these phases are treated further with chlorotrimethylsilane to remove residual hydroxyl groups. Presently, some manufacturers are beginning to prepare phases from monofunctional silanes. A great variety of phases can be bonded to the support or synthesized on the support once a suitable starting material is bonded to it. The latter approach was described by Novotny (12). However, in practice, the most commonly used bonded phases in liquid chromatography are: amine, —(CH 2 )„NH 2 ; nitrile,—(CH 2 )„CN; phenyl, —(CH 2 ) n C 6 H 5 ; octadecyl, —(CH 2 )i 7 CH 3 and various ion exchangers; more recently, shorter alkane chains were introduced. The most commonly used bonded phases in gas chromatography are various carbowaxes, alkanes, and polymeric organosilicones prepared from the reaction of di- and trichlorosilanes. Calculation of Surface Coverages The amount of the stationary phase bonded to the support is an important parameter frequently needed to compare column performances. This quantity can be reported in several ways. a) From the results of elemental analysis, the weight/weight percent of carbon is reported. If N, CI, etc., are part of the bonded phase, their weight/weight percent should also be given. Typical values for percent carbon on microparticulate silica gel having a surface area of 300-350 m2/g are about 3-4% for a methylsilane and about 20% for octadecylsilane. It is our opinion that reporting the total percent carbon content is the least desirable method of characterizing the bonded phase. b) By use of the elemental analysis results, the surface coverage can be reported in terms of weight of bonded phase per unit surface area of the support. Along the same lines, Unger et al. (8) recommended that surface coverages be reported in terms of the moles of bonded phase per specific surface area. Typical values for
straight chain alkanes bonded to silica gel are 4.5 μπιοΐ/m2 for trimethylsilane and 3.5 μπιοΐ/m2 for octadecyldimethylsilane. Keep in mind that the mea sured surface area can decrease drasti cally by bonding long chain moieties to the support (8). Reporting moles of the bonded phase per meter of sur face area of the support seems like a logical choice. c) The surface coverage can also be reported in terms of the percent sur face silanol groups replaced by the bonded phase. Several assumptions, however, must be made here. The first assumption is that four surface OH groups per 100 A2 (or 1 ran2) are avail able for reaction. The second assump tion is that of the four available OH groups, at maximum only 70-80% will actually react with the organosilane. If a trichloro- or trialkoxysilane is used, it is further assumed that each molecule interacts only with two sur face hydroxyls and that no polymer ization takes place. Similar assump tions might have to be made in the case of bifunctional organosilanes. As an example, if 5% w/w carbon is the result of reacting 10 g of silica gel hav ing a surface area of 400 m 2 /g with trimethoxyoctadecylsilane, then by use of the assumption mentioned above, it can be shown that the surface cover age is 23% of the maximum possible coverage. The detailed calculations
are shown in the insert on this page. This type of calculation gives a good estimate of brush formation, although one must be cognizant of assumptions made. Our own work (13,14) seems to in dicate that when reacting trimethoxyorganosilanes with silica gel, under an hydrous conditions the resultant bonded phases consist of areas of "brushes", i.e., monomeric organosilane, and patches of polymeric materi al. To avoid the formation of polymer ic parts in the bonded phases, it is ad vised that the silica gel be dried, the reaction take place in dry solvents and glasswares, and that the organosilane be monofunctional. Further helpful suggestions can be found in the works of Unger et al. (8) and Karch et al. (15).
can your integrator do this? Multiple Tailing Peak Analysis
Alkyl-Bonded Phase Applications
Work investigating the retention mechanism on alkyl-bonded phases has been carried out in GC (16-20) and more extensively in HPLC (8, 14, 15, 21-27). The most recent data in gas chromatography seem to show that nonpolar solutes interact mainly with the bonded alkane, and the re tention seems to be due to adsorption on the bonded phase (19). Undoubt edly, unreacted surface OH groups can also affect the chromatographic be havior of the solutes (17). Important
Sample Calculation of Surface Coverage by Bonded Phase 1) Facts: Octadecyltrimethoxysilane reacted with 5 g of silica gel having a surface area of 400 m 2 /g. Elemental analysis indicated 5 wt % carbon. 2) Assumptions: (a) Four accessible OH groups on the silica gel surface, (b) Only 7 0 - 8 0 % of these silanol groups are available for reaction with the organosilane. (c) Only two of the trimethoxy groups react with the surface silanol. (d) No polymerization of the silane takes place. 3) Objective: Find the percent surface coverage of the support by the orga nosilane. The calculations are as follows:
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I) Number of accessible surface OH's: 2 (400 m 2 /g) .ή21 „ . 0 v a (4 OH/nm ) (5 g) —— —„ , = 8 Χ 10 21 OH groups 18 2 2 1.0 X 10~ nm /m II) Number of moles of OH groups in 5 g of silica gel = 0.013. III) Number of moles of the organosilane. Note that because of assumption (c), the number of carbon atoms is 19; 18 from the octadecyl chain plus the remaining methoxy. Five wt % carbon corresponds to 0.25 g C per 5 g of silica gel or 1.1 X 10~ 3 mol C in 5 g silica gel. IV) By use of the two site reactions assumption, the percent of OH involved in the reaction is
(1.1 X 10-3 mol C ) X 2 X 1 0 0 = 1 7 % 0.013 mol OH Thus, 17% of all accessible OH groups have reacted with the organosilane. V) By use of assumption (b) for 75% available OH group, the surface coverage by the octadecylsilane (ODS) is about 23 %. This example shows that large areas of the silica gel surface are bare and that more ODS could have been attached to the support.
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ANALYTICAL CHEMISTRY, VOL . 49, NO. 12, OCTOBER
1977 ·
1007 A
variations in the retention mecha nisms may be due to whether a brush or polymeric phase is being used and to the magnitude of the surface cover age. Liquid chromatography presents a much more complex case because the mobile phase plays an active part in the chromatographic process and the solute-mobile phase interactions must be considered vis-a-vis the inter actions with the bonded alkyl. The most recent works on reversed phase chromatography seem to indicate that over and above surface deactivation by the alkyl groups, hydrophobic ef fects are responsible for the separa tions and the selectivities. Reversible association of the solutes with the alkane-bonded phases is responsible for the solutes' retention order. This asso ciation is a function of the nature of the mobile phase and its ionic strength. For a more detailed discus sion, the reader is referred to the work of Karger et al. (25) and of Horvath et al. (26, 27). A comprehensive listing of the sig nificant applications of alkyl-bonded phases in GC and HPLC, not to men tion TLC, is far too lengthy. The im portance of alkyl-bonded phases can not be overemphasized since such phases present, unlike silica gel or most other adsorbents, a nonpolar surface. If one were to add up all ap plications carried out with other types of bonded phases, the number would
be considerably fewer than alkyl phase applications. In liquid chromatogra phy the term "reversed phase" chro matography is used for alkyl-bonded phases since aqueous mixtures are used as mobile phases. The most fre quently used alkyl-bonded phase is octadecylsilane (ODS), although shorter hydrocarbon chains are pres ently available commercially. With solutes containing significant alkyl residues such as fatty acids, the retention time can be clearly correlat ed to the length of the residues. This is illustrated in Figure 2 for C12-C20 fatty acid esters (28). Similar exam ples can be shown in GC. Hastings et al. (29) separated (Figure 3) η-hydro carbons using a polyoctadecylsiloxane phase on Chromosorb G. Novel applications of alkyl-bonded phase chromatography are found throughout the literature. A couple of examples will suffice to illustrate their versatility. Kirkland (30) showed that polystyrenes, normally separated by exclusion chromatography, can be sep arated with good resolution using re versed phase chromatography. Figure 4 illustrates this separation. In less than 20 min, Zimmerman et al. (31) separated 20 PTH derivatives of amino acids using gradient elution on a reversed phase chromatographic column. A separation performed on this time scale allows for the rapid de termination of peptide or protein se quences using the Edman degradation method. Figure 5 illustrates the excel lent separation obtained by Zimmer man et al. Alkyl-bonded phases are a very powerful tool for a wide variety of sep aration problems. However, many sep arations are either too difficult or im
possible to achieve using an alkylbonded phase. For these applications other bonded phases have been pre pared. The next section will describe some such phases. Bonded Phases Other Than Alkyl Types Though many bonded phase types have been prepared, only a few have enjoyed widespread acceptance. Next to alkanes, the most commonly used nonpolar phase is bonded phenyl. For certain systems, where χ — π interac tions may he important, significantly different separations can be obtained with the bonded phenyl phase. Many phases of medium polarity have been prepared. In gas chromatography the most popular bonded phases are the bonded carbowaxes (polyethyleneglycols). The most frequently used polar phases in liquid chromatography are the nitrile and the amine phases. The former is less polar than the amine, and it can also be used in a reversed phase mode. The amine phase is capa ble of acting as a weak anion exchang er in aqueous solutions. Bonded ether phases and polyamide phases are used, though not as frequently as other phases of medium polarity. Polar stationary phases such as glycol have been prepared for liquid chroma tographic use. However, when polar surfaces are needed, adsorbents such as silica gel are most often used. Bonded ion exchangers are also avail able and are presently applied rou tinely. Some interesting and varied applications of the above mentioned bonded phases follow. Kirkland (32) illustrated the use of a polyether phase in HPLC for the separation of a variety of mixtures.
Figure 2. Elution orders of phenacyl es ters of some fatty acids
Figure 3. Chromatogram of n-hydrocarbons
Column: nonane bonded to Corasil II, 250 X 4.1 mm. Mobile phase 7 5 % methanol/25% water. Τ = 40 °C; flow rate, 0.63 mL/min. (1) Impurity, (2) lauric, (3) palmitic, (4) stearic, (5) arachidic
Column: ca. 5 wt % polyoctadecylsiloxane on 100-120 mesh chromosorb G, 0.5 m X 4 mm i.d. Pyrex glass column. Initial temperature, 30 °C, programmed at 16 °C/min up to 410 °C. Carrier gas: N 2 at 13 mL/min Reproduced with permission of Journal of Chromatography
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1977
Figure 4. Separation of polystyrenes by reversed phase chro matography
Figure 5. Gradient elution of PTH amino acids from 25 X 0.46 cm Zorbax ODS column
Column: 25 X 0.32 cm i.d., octadecylsilane (ODS) bonded to porous silica microspheres (~7 μπ\). Τ = 50 °C. Mobile phase: acetonitrile; flow rate, 1 mL/min. UV detector at 0.08 AUFS Reproduced with permission of Chromatographia
Solvents: A, 0.01 Ν sodium acetate, pH 4.5; B, acetonitrile. Pressure, 375 psi; initial flow rate, 1.0 mL/min; temperature, 62 °C; sample size, 1-2 nmol of each PTH Reproduced with permission of Analytical Biochemistry
Excellent resolutions and fast analysis times of thiolhydroxamates, phenylenediamine isomers, and substituted phenols were obtained. This paper (32) is significant since it discusses the effect of polymer loading and type on the observed retention characteristics of the chromatographic system. Knox and Vasvari (33) outlined a method for evaluating bonded ion ex changers. Although they considered an anion exchanger, their approach is applicable also to cation exchangers. Hartwick and Brown (34) used bond ed anion exchangers for the separation
of nucleotides, as shown in Figure 6. Schwarzenbach (35) applied an aminopropyl-bonded phase to the separa tion of saccharides as shown in Figure 7. Rabel (36) described a polyamide phase for HPLC. This phase has many sites capable of forming hydrogen bonds. Its maximum potential is with solutes that can form such bonds. Grushka and Scott (37) investigated the use of bonded peptides as phases for HPLC. Kikta and Grushka (38) showed the usefulness of bonded pep tides in amino acids, peptides, and
Figure 6. Separation of mono-, di-, and triphosphate nucleotides of adenine, gua nine, hypoxanthine, xanthine, cytosine, uracil, and thymine Column: Partisil-10 SAX; temperature, ambient; detector sensitivity, 0.08 AUFS. Eluents (low), 0.007 F KH 2 P0 4 , pH 4.0; (high) 0.25 F KH 2 P0 4 , 0.50 F KCI, pH 4.5. Gradient, linear, 0-100% of high concen tration eluent in 45 min; flow rate, 1.5 mL/min. Dashed lines: elution positions of SMP, XDP, and XTP Reproduced with permission of Journal of Chromatography
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ANALYTICAL CHEMISTRY, VOL. 4 9 , NO. 12, OCTOBER
1977
PTH-amino acid separations. Recent work (39) has shown that peptide diastereomers can be separated with such a bonded phase (Figure 8).
Figure 7. Separation of saccharides on propylamine-bonded phase Mobile phase: 2 0 % water-acetonitrile at 0.5 mL/min. Peaks: (1) water, (2) ribose, (3rarabinose, (4) glucose, (5) saccharose, (6) maltose, (7) lactose, (8) maltotriose Reproduced with permission of Journal of Chromatogra phy
One of the most exciting applica tions of bonded phases has been in the separation of optically active enantiomers. Here, we will mention only two recent examples of racemic resolu tions. Dotsevi et al. (40) utilized opti cally active bonded crown ethers for the optical resolution of amino acids and their ester salts. Mikes et al. (41 ) separated racemic helicenes on a bonded chiral charge transfer complexing agent. Applications of bonded phases in gas chromatography are less frequent but not less impressive. For example, Al-Taiar and coworkers (42) prepared thermally stable (up to 350 °C) bond ed phenyl phases on Gas Chrom Q. These phases were characterized by the good peak symmetries obtained on t h e m for solutes covering a wide range of polarities. DeStefano and Kirkland (43) prepared a bonded 3chloropropyl phase for use in gas chro matography. T h e phase was very effi cient, yielding good peak symmetry. Figure 9 illustrates the separation of several chlorinated aromatics, using temperature programming, with this phase.
Summary and Conclusions
Many phases are now available commercially, most notably the alkyl reversed phases for H P L C and the bonded Carbowax columns for GC. T h e new users of bonded phases might be well advised to start with the com mercially prepared column. With some experience, however, bonded phases can be prepared, and a column packed in one's own laboratory. Finally, we would like t o reference several more comprehensive reviews (44-47) and a book (48) on the topic of bonded phases in chromatography.
References (1) C. Rossi, S. Munari, C. Cengari, and G. F. Tealdo, Chim. Ind. (Milano), 42, 724 (1960). (2) I. Halasz and I. Sebestian, Angew. Chem. Int. Ed., 8, 453 (1969). (3) Ε. W. Abel, F. W. Pollard, P. C. Uden, and G. D. Nickless, J. Chromatogr., 22, 23 (1966). (4) W. Noll, "Chemistry and Technology of Silicones", Academic Press, New York N.Y., 1968. (5) Κ. K. Unger, Angew Chem. Int. Ed., 11,267(1972). (6) W. A. Aue and C. R. Hastings, J. Chro matogr., 42, 319 (1969). (7) W. A. Aue and S. Kapila, "Bonded Sta tionary Phases in Chromatography", E. Grushka, Ed., p 13, Ann Arbor Science, Ann Arbor, Mich., 1974.
(8) K. K. Unger, N. Becker, and P. Roumeliotis, J. Chromatogr., 125,115 (1976). (9) R. K. Gilpin and M. R. Burke, Anal. Chem., 45,1383 (1973). (10) D. C. Locke, J. T. Schmermund, and B. Banner, ibid., 44,90 (1972). (11) R. K. Gilpin, P. J. Camillo, and C. A. Janicki, J. Chromatogr., 121,13 (1976). (12) M. Novotny, "Bonded Stationary Phases in Chromatography", E. Grush ka, Ed., p 199, Ann Arbor Science, Ann Arbor, Mich., 1974. (13) E. Grushka and E. J. Kikta, Jr., Anal. Chem., 46,1370 (1974). (14) E. J. Kikta, Jr., and E. Grushka, ibid., 48,1098 (1976). (15) K. Karch, I. Sebestian, and I. Halasz, J. Chromatogr., 122,3 (1976). (16) J. N. Little, W. A. Dark, P. J. Farlinger, and K. J. Bombaugh, J. Chromatogr. ScL· 8,647 (1970). (17) B. L. Karger and G. Sibley, Anal. Chem., 45, 740 (1973). (18) J. J. Pesek and J. E. Daniels, J. Chro matogr. Sci., 14, 258 (1976). (19) J. J. Pesek and J. A. Graham, Anal. Chem., 49,133 (1977). (20) S. Mori, J. Chromatogr., 135, 261 (1977). (21) R. E. Majors and M. J. Hopper, J. Chromatogr. Sci., 12,767 (1974). (22) D. C. Locke, ibid., ρ 433. (23) H. Hemetsberger, W. Maasfeld, and H. Ricken, Chromatographia, 7, 303 (1976). (24) J. H. Knox and A. Pryde, J. Chroma togr., 112,171(1975). (25) B. L. Karger, R. Gant, A. Hartkopf,
This brief review of the bonded phase applications shows t h a t many varied problems can be efficiently solved using bonded phases. T h e po tential of new phases is not always rec ognized by the average user. T h e next few years should show an increase in the use and production of new and novel phases designed to tackle many difficult separation problems in both GC and LC.
Figure 8. Separation of peptide diastereomers on L-valyl-L-alanyl-L-proline bonded to silica gel (10 μπι) Column: 25 cm X 3.1 mm i.d. Mobile phase: dis tilled deionized water at 1.5 mL/min. Detector: UV at 254 nm, 0.05 AUFS. Peaks: (1) D-leucyl-Ltyrosine, (2) L-leucyl-L-tyrosine, (3) L-tyrosyl-Lleucine
Figure 9. Separation of chlorinated aromatic hydrocarbons Column: 0.45% 3-chloropropyl-bonded phase on 115-170 mesh Zipax, 2 ft X 0.13 in. i.d., glass; car rier flow: He at 50 mL/min; sample: 0.5 μΙ_ of about 5 mg/mL of each component in benzene; flame ionization detector sensitivity, 1 Χ 1 0 - 9 amps, full scale Reproduced with permission of Journal of Chromatographic Science
1012 A · ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
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and P. H. Weiner, ibid., 128, 65 (1976). (26) C. Horvath, W. Melander, and I. Molnar, ibid., 125,129 (1976). (27) C. Horvath, W. Melander, and I. Molnar, Anal. Chem., 49, 142 (1977). (28) H. D. Durst, M. Milano, E. J. Kikta, Jr., S. A. Connelly, and E. Grushka, ibid. 47,1797 (1975). (29) C. R. Hastings, W. A. Aue, and F. N. Larsen, J. Chromatogr., 60, 329 (1971). (30) J. J. Kirkland, Chromatographia, 8, 661 (1975). (31) C. L. Zimmerman, E. Appella, and J. J. Piseno, Anal. Biochem., 77, 569 (1977). (32) J. J. Kirkland, J. Chromatogr. Sci., 9,206(1971). (33) J. H. Knox and G. Vasvari, ibid., 12, 449 (1974). (34) R. H. Hartwick and P. R. Brown, J. Chromatogr., 112,651(1975). (35) R. Schwarzenbach, ibid., 117,206 (1976). (36) F. M. Rabel, Anal. Chem., 45,957 (1973). (37) E. Grushka and R.P.W. Scott, ibid., ρ 1626. (38) Ε. J. Kikta, Jr., and E. Grushka, J. Chromatogr., 135,367(1977). (39) E. Grushka and G.W.K. Fong, u n p u b lished results, SUNY at Buffalo, N.Y., 1977. (40) G. Dotsevi, Y. Sogah, and D. J. Cram, J. Am. Chem. Soc, 98, 3038 (1976). (41) F. Mikes, G. Boshart, and E. Gil-Av, J. Chromatogr., 122, 205 (1976). (42) A. H. Al-Taiar, J.R.L. Smith, and D. J. Waddington, Anal. Chem., 46,1135 (1974). (43) J. J. DeStefano and J. J. Kirkland, J. Chromatogr. Sci., 12, 337 (1974). (44) D. C. Locke, ibid., 11,120(1973). (45) A. Pryde, ibid., 12, 486 (1974). (46) V. Rehak and E. Smolkova, Chroma tographia, 9,219(1976). (47) I. Sebestian and I. Halasz, "Advances in Chromatography", Vol 14, J. C. Giddings, E. Grushka, J. Cazes, and P. R. Brown, Eds., ρ 75, Marcel Dekker, New York, N.Y., 1976. (48) "Bonded Stationary Phases in Chro matography", E. Grushka, Ed., Ann Arbor Science, Ann Arbor, Mich., 1974.
SPECTRASPAN IV is an up-to-date analytical system that offers substantial capability at a modest price. For multichannel analysis capability ask for information on Spectraspan III.
E l i G r u s h k a ( r i g h t ) is a n a s s o c i a t e p r o f e s s o r of c h e m i s t r y a t S U N Y , Buf falo. H i s r e s e a r c h i n t e r e s t s a r e in t h e a r e a s of c h r o m a t o g r a p h i c t h e o r i e s , e n h a n c e m e n t of c h r o m a t o g r a p h i c r e s o l u t i o n , a n d d e s i g n i n g b o n d e d p h a s e s for selective separations. A low cost integrated system for sequential analysis
SPECTRAMETRICS INCORPORATED 204 ANDOVER STREET, ANDOVER, MA 0 1 8 1 0 (617)475-7015 CIRCLE 195 ON READER SERVICE CARD
1014 A · ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
E d w a r d J . K i k t a , J r . (left), is a r e s e a r c h c h e m i s t for t h e F M C C o r p . , A g r i c u l t u r a l C h e m i c a l G r o u p in M i d dleport, N.Y., where he supervises t h e a n a l y t i c a l c h e m i s t r y s e c t i o n of t h e for mulations group. His current research centers around analytical methods de v e l o p m e n t for n e w p e s t i c i d e p r o d u c t s .
