1835
Anal. Chem. 1085, 57, 1835-1840 Knox, J. H.; Hartwick, R. A. J . Chromatogr. 1981, 204, 3. barn, M. T. W.; Grego, B.; Hancock, W. S. J . Chromatogr. 1979, 185, 429. Deelder, R. S.;Llnssen, H. A. J.; Konijnendljk, A. P.; van de Venne, J. L. M. J . Chromatogr. 1979, 185, 241. Denkert, M.; Hackzell, L.; Schlll, 0.;Sjogren, E. J . Chromatogr. 1981,
(27) Feibush, 6.;Cohen, M. J.; Karger, B. L. J . Chromatogr. 1983, 282, 3. (28) Hearn, M. T. W. “Advances In Chromatography”; GLddlngs, J. C., Grushka, E., Cazes, J., Brown, P. R., Eds.; Marcel Dekker: New York,
Molnar, I.; Horvath, Cs. J . Chromatogr. 1077. 142, 623. Crommen, J.; Fransson, 6.;Schlll, 0. J . Chromatogr. 1977, 142, 283. Kraak. J. C.; Jonker, K. M.; Huber. J. F. K. J . Chromatogr. 1977, 142,
1980;Vol. 18,p 59. (29) Laub, R. J.; Purnell, J. H. J . Chromatogr. 1978, 161, 59. (30) Bldllngmeyer, B. A.; Demlng, S. N.; Price, W. P., Jr.; Sachok, B.; Petrusek, M. J . Chromatogr. 1079, 186, 419. (31) Zaslavsky, B. Y.; Mestechkina, N. M.; Miheeva, L. M.; Rogozhin, S. V. J . Chromatogr. 1982, 240, 21. (32) Cassidy. R. M.; Fraser, M. Chromatographla 1984, 18, 369.
671. (25) Radjai. M. K.; Hatch, R. T. J . Chromatogr. 1980, 196, 319. (26) Iskandaranl, 2.; Smith, R. L.; Pletrzyk, D. J. J . Liq. Chromatogr. 1984, 7, Ill.
RECEIVED for review January 9,1985. Accepted April 8,1985.
218, 31.
Peak Compression Effects in Ion-Pair Reversed-Phase Liquid Chromatography of Substituted Benzamides Lars B. Nilason* Department of Bioanalytical Chemistry, Research and Development Laboratories, Astra Lakemedel AB, S-151 85 Sodertdlje, Sweden
Douglas Westerlund Department of Analytical Pharmaceutical Chemistry, Uppsala University Biomedical Center, P.O. Box 574,S-751 23 Uppsala, Sweden
Peak compression effects have been studied in Ion-pair reversed-phase iiquld chromatography of substituted benzamldes. The sample Is injected in a soiutlon of an organic anion. A zone wlth a depletion of one of the mobile phase components is created by this organlc anion. The peak for an analyte which coeiutes with the depleted zone wlii be extremely narrow: chromatographic efficlencles corresponding to >lo6 plates/m have been obtalned. A retention model for this effect is proposed.
Reversed-phase liquid chromatography is widely accepted
as a tool for the separation and quantitation of compounds in complex mixtures. One of the main features is the possibility of optimizing the separation in a simple way by varying the mobile phase composition, e.g., the nature and amount of organic modifying solvent, the pH, the ionic strength, and the nature and concentration of additives such as complexforming or ion-pairing agents. In many cases the injected sample is dissolved in the mobile phase in order to minimize base line disturbances. When a sample with a composition deviating from the mobile phase is injected, the established column equilibria are disturbed and anomalous peaks, called ghost peaks (11,vacant peaks (21,or induced peaks (3),may appear. Such a peak is due to a migrating zone with a deviating concentration of one of the mobile phase components. Peaks of this kind are often regarded as complications or curiosities (1,2).Normally the mobile phase is made up of components not registered by the detector in use and the induced peaks are thus not observed. However, in the so-called UV-visualization liquid chromatography, the mobile phase contains a UV-absorbing component and the UV-trace is monitored (4-10). In such a system the injection of components which interact with the UV-absorbing component in the mobile phase will give rise 0003-2700/85/0357-1835$01.50/0
to two kinds of peaks, The first kind involves one peak for each interacting component in the sample, and the second kind originates from the elution of an excess (positive peak) or a deficiency (negative peak) of the detectable mobile phase component. These peaks are usually called system peaks (4, 5). The UV-visualization technique has been applied to quantitative determinations (8-10). This paper reports on a study, where UV-transparent zones are used to enhance the chromatographic efficiency for UVabsorbing compounds. The analyks, substituted benzamides, were injected dissolved in an acidic buffer containing a high concentration of an organic anion, alkyl sulfate or alkylsulfonate. The acidic mobile phase contained a tertiary amine as a cationic ion-pairing agent. The injection of the organic anion gave rise to a migrating zone with a deficiency of the cationic mobile phase component. A sufficiently high concentration of the injected anion gave a totally depleted zone and an extremely compressed peak was observed for the analyte that coeluted with this depleted zone. With this method it was possible to obtain peak compression for a selected component in the sample by a careful choice of the composition of the mobile phase and of the injected solution. Injections of ion-pairing agents have earlier been used to influence the retention properties of the column. The ionpairing agents have then been injected before or after the injection of the sample and for quite different purposes. Stranahan et al. (11)used pulsed injections of octanesulfonate to influence the separation of substituted anilines and Berry and Shansky (12)used a similar approach to)obtain qualitative information on the charge of the injected components.
EXPERIMENTAL SECTION Chemicals. The sodium salts of pentane-, hexane-, and oc: tanesulfonic acid were obtained from Eastman Kodak Co. (Rochester, NY). The sodium sal& of hexyl and octyl sulfate were 0 1985 Amerlcan Chemlcai Society
1836
ANALYTICAL CHEMISTRY, VOL. 57, NO. 9, AUGUST 1985
obtained from Research Plus Laboratories (Denville, NJ), and Nfl-dimethyl-N-octylamine(DMOA) was obtained from ICN Pharmaceuticals (Plainview, NY). All other chemicals were of analytical or HPLC grade. Model Compounds. Remoxipride ((S)-(-)-3-bromo-N-[(1ethyl-2-pyrrolidinyl)methyl]-2,6-dimethoxybenzamide hydrochloride monohydrate), FLA 908 ((S)-(-)-3-bromo-N-[(l-ethyl2-pyrrolidinyl)methyl]-6-hydroxy-2-methoxybenzamide), FLA 838 ((S)-(+)-3-brorno-2,6-dimethoxy-N-[ (2-pyrrolidinyl)methyl]benzamide hydrochloride), and FLA 965 ((S)-(-)-3-ethyl-N-[(1ethyl-2-pyrrolidinyl)methyl]-2-hydroxy-6-methoxybenzamide hydrochloride) were synthesized at the Department of CNSMedicinal Chemistry,Astra Llikemedel AB (SadeMlje, Sweden). Apparatus. The chromatographicsystem consisted of a Milton Roy Mini-Pump, a Rheodyne 7120 sample injection valve equipped with a 2WpL loop, a Waters 441 W detector (214 nm), and a Laboratory Data Control 3402 recorder or a HewlettPackard 3390 integrator. The refractive index monitor was an Optilab Multiref 901. The chromatographic columns (100 x 4 mm i.d.) were packed with Nucleosil Cle, 5 pm, from Macherey-Nagel (Diiren,F.R.G.) by the upward slurry packing technique with methyl isobutyl ketone as the slurry medium and dichloromethane as the eluent. Chromatographic Conditions. Unless otherwise stated the mobile phase was 23% acetonitrile in phosphate buffer pH 2, ionic strength = 0.025, with the addition of 4 X lom4M DMOA. The flow rate was 1 mL/min. The same column was used for all experimentsexcept in the study of the retention of cetanesulfonate and DMOA by refractive index monitoring, where a later batch of Nucleosil was used. This column performed similarily, but with somewhat different retention times. Comments on Calculation of Plate Count and Retention Times. The plate counts are calculated from the width of the peak at half the height and are used, in this paper, to describe the effects obtained on the chromatographic peaks. This is not correct from a theoretical viewpoint, since the chromatographic conditions vary during the elution of the compounds in this study and the values obtained were in some cases much higher than anticipated from theory. According to our opinion, however, this is the best way to illustrate the dramatic effects on peak shape obtained with this technique. The determination of the volume of the mobile phase within the column was uncertain, not only due to the varying conditions during the run caused by the injection technique but also due to the presence of ion-pairingadditives. Common markers of dead volume, such as nitrate or a sample only slightly different from the mobile phase, did not give reliable and reproducible results. As a consequence, the retention of injected compounds is expressed as retention time rather than the more frequently used capacity ratio (k’).
RESULTS AND DISCUSSION Effects of an Organic Anion Present in the Injected Solution. A mixture of four substituted benzamides was injected dissolved in seven different media and the chromatographic efficiencies were measured; the results are shown in Figure 1. If the sample was dissolved in mobile phase (medium I), the efficiencies were low due to the large injected volume (200pL). Normal efficiencies for a column of this type, about 40000 plates/m, were obtained if the sample was dissolved in pure pH 2 buffer (medium 2); the compounds are then concentrated on the top of the column during the injection (13). It was possible to influence the efficiencyfor each of all four benzamides by injecting them together with organic anions with different alkyl chain length. This is illustrated in Figure 1 by the efficiencies obtained with media 3-7. An increase of the efficiency was, for example, observed for the least retained compounds, FLA 838 and remoxipride, when the sample was injected together with hexyl sulfate, an anion with a rather short alkyl chain. In contrast, the efficiencies were decreased far below the normal level for these compounds when the sample was injected together with the most lipophilic anions.
20000
t b
20001 1
I
!
I
I
2
3
4
5
dlssolutlon medlum
I
~
6
7
No
(
)
Figure 1. Effects of an organic anion present in the injected solution on chromatographic efflclencies: the mobile phase was 23 % acetonitrile in phosphate buffer pH 2 with 0.6 mM DMOA; analytes FLA 838 (O), remoxlpride (0),FLA 908 (A), and FLA 965 (M), 200 ng was injacted of each analyte. Dissolutlon medium: 1, mobile phase; 2,pH 2 buffer; 3, 5 mM pentanesulfonate In pH 2 buffer; 4, 5 mM hexyl sulfate in pH 2 buffer; 5, 5 mM heptanesulfonate in pH 2 buffer; 6, 5 mM octanesulfonatein pH 2 buffer; 7 , 5 mM octyf sulfate in pH 2 buffer.
i
i
154
0
1
2
3
4
5
dissolution medium
i
6 (
No
7
>
Flgure 2. Effects of an organic anion present in the injected solution on retention times; chromatographic conditions and symbols as in Figure 1. The number of theoretical plates was unaffected for the more retained analytes, FLA 908 and 965,when media 3-5 were used. However, a 3-fold increase of the plate count was observed for FLA 908 when the sample was injected together with octanesulfonate and for FLA 965 when octyl sulfate was used. It was thus possible to significantly improve the chromatographic efficiency for each benzamide by the inclusion of the proper anion in the injected solution; a more lipophilic analyte needed a more lipophilic anion for this effect to occur. The corresponding retention times, which were also affected by the character of the injected anions, are shown in Figure 2. Effects of a Variation of the Organic Anion Concentration i n the Injected Solution. It was demonstrated above (Figure 1)that hexyl sulfate seemed to be the optimal anion regarding peak compression effects for remoxipride. This effect, however, was strongly dependent on the concentration
ANALYTICAL CHEMISTRY, VOL. 57, NO. 9, AUGUST 1985
1837
1OOOO[
40001 N/m
I1
il
10001
0
2
4
6
hexylsulphote
8 (
1
0
mM )
Figure 3. Effects of the hexyl sulfate concentration In the injected solution on chromatographic efficiency. The mobile phase was 23% acetonitrile In phosphate buffer pH 2 with 0.4 mM DMOA. Symbols are glven In Figure 1.
100000c
1'0 min.
5
N/m
Figure 5. R I trace after In]ection of 2 mM octanesulfonate In phosphate buffer pH 2: I, DMOA deficiency peak; 11, DMOA-octanesulfonate Ion pair. Chromatographic conditions are glven In Figure 3.
1OOOOC
Table I. RI Peaks after DMOA Injections"
injected DMOA concn in
concn of
mobile phase, mM
DMOA, mM
10000 0
2
4
6
octanesulphonota
8
1 (
mM
0
)
Figure 4. Effects of the octanesulfonate concentration In the in]ected solutions on chromatographlc eff Iclency; chromatographic conditions as In Figure 3, symbols as In Flgure 1.
of the organic anion; a sudden peak sharpening effect occurred when the hexyl sulfate level exceeded 1.5 mM (Figure 3). The earliest eluting compound, FLA 838, was also influenced but a t a higher concentration (4 mM). The chromatographic performance of the latest eluting compound, FLA 908, was not influenced at all by this short anion. However, a dramatic peak-sharpening effect was observed for FLA 908 with the more lipophilic octanesulfonate (Figure 4). The effect started at an injected concentration of about 1.5 mM of the anion and reached a maximal efficiency of >lo6 theoretical plates per meter. The earlier eluting compounds, remoxipride and FLA 838, were also influenced by octanesulfonate, but the effect started a t higher levels of the anion, a t about 4 and 8 mM, respectively. The effeds were not as dramatic as for FLA 908; the maximal efficienciesfor these two compounds were about 10 times lower and corresponding to those shown in Figure 3. Retention of Octanesulfonate. The retention of one of the UV-transparent organic anions was studied by utilizing a refractive index monitor. Octanesulfonate (OS) was chosen as the model compound since the most dramatic peak-
0.4
0.5 1 0.5 1
0.4 0.8
2 0.5
0.8 0.8
1 2
0 0
0.4
retention, min 6.7 6.2 4.3 4.1 3.8 3.4 3.3 3.2
re1 peak height 16 26 41 54 79
64 83 91
'Mobile phase: 23% acetonitrile in phosphate buffer pH 2 with 0-0.8 m M DMOA. sharpening effects had been obtained with this anion. The peak height measuremenb from the RI chromatograms were unfortunately rather imprecise because of irregular noise. The RI trace after an injection of octanesulfonate is shown in Figure 5. Two major peaks with opposite directions were apparent in the chromatogram, disregarding the front disturbances. In order to study the origin of these two peaks, DMOA and octanesulfonate were injected and, in addition, the DMOA concentration in the eluent was varied. The injection of DMOA gave rise to one positive peak (Table I) and increasing amounts of the compound resulted in a nonlinear increase of the peak heights accompanied by peak deformation and decreasing retention times. This behavior indicated that the adsorption capacity of the column was exceeded, giving a convex adsorption isotherm similar to the adsorption isotherm found for DMOA on a bonded phase column with pentanol-containing acidic mobile phases (14).
1838
ANALYTICAL CHEMISTRY, VOL. 57, NO. 9, AUGUST 1985
Table 11. RL Peaks after Octanesulfonate Injections' DMOA concn in mobile phase, mM
injected concn of OS, mM
0 0 0.4 0.4 0.4 0.8
a
2 5 2 5 10 2
0.8
5
0.8
10
negative peak retention, min re1 peak height
5.8 5.3 5.2 4.6 4.8 4.7
positive peak retention, min re1 peak height 5.2 4.2 11.3 11.4 11.4 12.8 12.9 12.9
47 42 46 53
111 91
37 76 61 61 60 104 100 99
Mobile phase as in Table I.
Furthermore, the retention time decreased rapidly with increasing DMOA concentration in the eluent. Injections of odanesulfonate gave a positive peak only slightly retained in a mobile phase without DMOA (Table 11). The addition of increasing concentrations of the counter ion DMOA to the mobile phase gave, as expected, increasing retention times of the positive peak, since OS is retained as an ion pair with DMOA. The injection of OS into a mobile phase containing DMOA will result in a distribution of the anion as an ion pair with DMOA into the stationary phase. This zone will then contain an excess of DMOA, which will elute with a retention time characteristic of the ion pair. However, at the same moment as this zone of excess DMOA is created, a zone with a corresponding depletion of the cation will also be formed within the column. This zone will elute with a, for DMOA, characteristic retention time. This is in accordance with the findings of other authors (15,16). Thus the positive peak in Table I and the negative peak in Table I1 both reflect concentration changes of the same compound, DMOA, and they should therefore have the same retention time. However, this was not the case; the retention times of the negative DMOA peaks were significantly longer. The reason behind this phenomenon was given by Helfferich (I7) and was also discussed later by other authors (18-20). According to Helfferich, the rate of travel for individual molecules of a mobile phase component is governed by the distribution ratio, while the rate of travel of a concentration pulse of the same component is inversely proportional to the slope of the sorption isotherm. With a linear adsorption isotherm the retention times of the two DMOA peaks would have agreed. On the other hand a convex isotherm, as in this case, will give a steeper slope for the decrease in DMOA concentration seen as the negative peak and thus result in a longer retention time. A more detailed study of the negative peak heights (Table 11) reveals that at the low DMOA concentration (0.4mM) in the eluent the heights were constant and independent of the octanesulfonate concentrations injected. This indicated that already 2 mM of OS resulted in a complete depletion of DMOA from the mobile phase due to the enrichment effect on the top of the column. At the high DMOA concentration (0.8 mM) a complete depletion seemed to be obtained when the OS concentration was 5 mM and the maximal peak height was about twice the size of that obtained at the lower DMOA concentration. The amounts of depleted DMOA were calculated from the peak areas (Table 111). The results demonstrated a good agreement between the amounts of injected octanesulfonate and depleted DMOA. As a summary of these studies: Injections of octanesulfonate in a system containing the counter ion DMOA in an acidic mobile phase gave rise to two major peaks. The first peak was due to a deficiency of DMOA and the second contained the corresponding excess of DMOA eluting as an ionpair with the anion, octanesulfonate.
Table 111. Amount DMOA Depleted from Mobile Phase injected octanesulfonate
a
concn, mM
amt, nmol
2 5
400 1000
DMOA, depleted amt, nmol DMOA 0.4 mMin DMOA 0.8 mMin eluent
eluent
360
320 960
a
Deformed peak.
RI-trace
UV-trace
1'0 mi?
5
0
Figure 6. Slmultaneous recording of UV and RI traces. The sample was dissolved in 4 mM octanesulfonate. The first UV peak Is FLA 838, the second Is remoxipride, and the last peak is FLA 908, 40 ng injected of each.
Simultaneous Recording of UV and RI Response. The RI monitor was coupled in series with the UV detector in order to study the relation between the peaks obtained by the two detection principles. The injection of FLA 838, remoxipride, and FLA 908 dissolved in a 4 mM octanesulfonate solution demonstrated an extremely narrow peak for FLA 908, while
ANALYTICAL CHEMISTRY, VOL. 57, NO. 9, AUGUST 1985
1839
Flow
/ FLA DMOAIOS965
ion-pair
A 0
0
2
4
6
octanaeulphanata
10
8 (
mM
ion-pair
)
Figure 7. Comparison of the retention of the DMOA deficiency peak and UV peaks when Injecting different concentrations of octanesulfonate; chromatographic conditions as in Figure 3, symbols as in Figure 1, except DMOA deficiency peak ( 0 ) . the other two peaks had a normal appearance (Figure 6). A comparison with the RI response demonstrated that FLA 908 coeluted with the back of the DMOA depleted zone when the concentration of the injected octanesulfonate solution exceeded 2 mM (Figure 7). The extreme peak compression effect started a t the same concentration (Figure 4). The retention times for remoxipride and FLA 838 reached a plateau a t 4 and 8 mM of octanesulfonate, respectively (Figure 7). The moderate peak sharpening for these compounds started at these concentrations(Figure 4). The retention times for the DMOA depleted zone were corrected for the hold-up volume between the detectors. Retention Model. Chromatographicseparations of amines on alkyl-bonded silica-based supports often result in tailing peaks, an effect which is generally considered to be a consequence of residual silanol groups (e.g., ref 21 and 22). The use of aliphatic amines as additives to the mobile phase has been found to improve the peak shapes (23,24).The presence of an amine in the mobile phase will furthermore often decrease the retention of analytes of related chemical structures, which has been interpreted as a consequenceof a competition for available sites on the support (25). The injection of a sample will disturb the established equilibria on the column. A solute which is more retained than the organic modifier component of the mobile phase will, for example, displace a portion of the organic solvent which is adsorbed to the support. This interference will then create a zone within the column that deviates in composition from the mobile phase. This zone will travel through the column with a rate which is characteristic for the modifier (15). The large negative peak in the front in Figure 5 is the result of this organic modifier displacement, since acetonitrile is not retained in this system. In the present study the injected solution contained a high concentration of an organic anion. This means that the distribution of the retained mobile phase component, DMOA, will also be influenced. When the concentrationof the injected organic anion is adequately high, a zone with a complete depletion of DMOA will be created. Analytes that are injected in such a solution will experience the depleted zone at the start of the chromatography, and depending on the properties of the analyte, three different situations are possible. It is to be noted that the concentration of injected anion is very high compared to the concentration of the analytes, therefore their influence on column equilibria is negligible.
DMOA Remoxi- FLA depletedzone pride 838
depleted DMOA zone
v
Figure 8. Schematic representation of the peak compression effect.
I h’benzamide > k’,,,,. In the starting zone the analyte will move very slowly until it reaches the normal mobile phase behind the zone. In the normal mobile phase the analyte moves more slowly than the zone. The consequence is that the peak shape and the retention are hardly influenced at all (e.g., see FLA 908 in Figure 3). 11k’hnvunide ktzone. The analyte will move very slowly in the starting zone, but when it meets the normal mobile phase at the back of the zone it will move with the same rate as the zone. The result will be that the analyte is enriched between the back of the zone and the mobile phase. The most efficient enrichment would occur if the analyte moved slowly in the zone and somewhat faster than the zone in the normal mobile phase. This ideal situation seemed to be achieved for FLA 908 when it was injected together with moderately high concentrations of octanesulfonate (Figure 4). I11 klbenzamide< htmm.In this case the analyte will also within the zone move faster than the zone. As soon as the analyte reaches the front of the zone, the first arrived molecules will start to travel considerably faster than within the zone. The consequence will be a peak broadening. Consider, as an example, the low efficiencies (10000 to 20000 plates/m) for remoxipride and FLA 838 when they were injected together with comparatively low amounts of octanesulfonate (Figure 4). The mechanism behind the increasing efficiencies for the same compounds together with higher octanesulfonate concentrations (>6mM) is unclear. Figure 8 is an attempt to visualize the peak compression effect. The acetonitrile concentration of the mobile phase was found to have a marked influence on the degree of peak compression. In Figure 4, where 23% acetonitrile was used, the plate count for FLA 908 was about lo6. A slightly higher acetonitrile concentration, 23.5%, gave 2.5 X lo6 plates/m while lower or much higher concentrations gave drastically reduced plate counts. These results indicate that the retention of the analytes and the depleted zone is not altered to the same extent; however the behavior of the different zones when altering the acetonitrile concentration has not yet been systematically studied. The consequence of this is that the concentration range giving peak compression for a given combination of analyte and ion-pairing agents is very narrow. A careful control of the mobile phase composition is thus necessary when peak compression is used for quantitative applications. The general applicability of the peak compression effects remains to be elucidated but it should theoretically be possible N
1840
Anal. Chem. l W 5 , 57, 1840-1846
to cover a wide range of analytes with different lipophilicities by using a series of alkyl sulfates or alkylsulfonates together with amines with different chain lengths. Preliminary experiments have indicated that organic anions other than octanesulfonate can, in combination with DMOA or other aliphatic amines, result in peak compression effects on compounds other than FLA 908. In this study UV-transparent zones were used to sharpen UV-absorbing peaks. Alternative possibilities include the use of nonfluorescent zones for the sharpening of fluorescent peaks or the use of electrochemical detection in combination with electrochemically nonactive zones. It must also be emphasized that in contrast to gradient elution, where similar effects may be achieved, no reequilibration of the column is necessary between the runs, since an immediate equilibration occurs after the elution of the zones. The peak compression effect has been applied to the quantitative determination of a selected compound in a complex matrix (manuscript in preparation). CONCLUSIONS This study has demonstrated the possibility of obtaining extremely narrow peaks for selected compounds in ion-pair reversed-phase liquid chromatography. The phenomenon is suggested to be a consequence of an enrichment of the analyte in a narrow band between the bulk mobile phase and a zone with a complete deficiency of a competing mobile phase component. This zone is created at the start of the chromatography by injecting the analyks in a solution with a high concentration of a UV-transparent organic anion. The anions are distributed to the stationary phase as ion pairs with a cationic mobile phase component, thus also depleting a zone of the mobile phase of this component. The prerequisite conditions for this peak sharpening to occur include a total depletion of the cationic mobile phase component within the zone and the coelution of the analyte and the depleted zone. It is anticipated that the peak compression effect in principle is applicable to any analyte by the proper choice of conditions and that it may prove to be of advantage in ap-
plications where an improvement in detection limit and/or selectivity is needed. Registry NO.FLA 908,96393-00-9; FLA 838,82935-35-1;FLA 965, 96393-01-0; Remoxipride, 80125-14-0; sodium pentanesulfonate,22767-49-3;sodium hexanesulfonate,2832-45-3; sodium octanesulfonate,5324-84-5;sodium hexylsulfate, 2207-98-9;sodium octylsulfate, 142-31-4.
LITERATURE CITED
(11) (12) (13) (14) (15) (16)
(17) (18) (19) (20) (21) (22) (23) (24) (26)
Berek, D.; Bleha, T.; Pevna, 2 . J. Chromatogr. Scl. 1978, 14, 560-563. Slals, K.; Krejci, M. J. Chromatogr. 1974, 91. 161-166. Stranahan, J. J.; Demlng, S. N. Anal. Chem. 1982, 54, 1540-1546. Denkert, M.; Hackzell, L.; Schlll, G.; Sjogrefl, E. J. Chromatbgr. 1981, 218, 31-43. Hackzell, L.; Schlll, G. Chromatographla 1982, 15, 437-444. Bldllngmeyer, B. A.; Warren, F. V., Jr. Anal. Chem. 1982, 54, 2351-2356. Barber, W. E.; Carr, P. W. J. Chromatogr. 1984, 301, 25-38. SachQk, B.; Deming, S. N.; Bldllngmeyer, B. A. J. Llq. Chromatogr. IW2, 5 , 389-402. Barber, W. E.; Carr, P. W. J. Chromatogr. 1983, 260. 89-96. Warren, F. V., Jr.; Bldlingmeyer, B. A. Anal. Chem. 1984, 56, 482-491 _ _ ._.. Stranahan, J. J.; Demlng, S. N.; Sachok, B. J. Chromatogr. 1980, 202, 233-237. Berry, V. V.; Shansky, R. E. J. Chromatogr. 1984, 284, 303-316. Westerlund, D.; Carlqvlst, J.; Theodorsen, A. Acta Pharm. Suqc. 1979, 16, 187-214. Jansson, S. 0.;Andersson, I.; Persson, B. A. J. Chromatogr. 1981, 203, 93-105. Mchrmlck, R. M.; Karger, B. L. J . Chromatogr. 1980, 199, 259-273. Bldlingmeyer, B. A.; Deming, S. N.; Price, W. P., Jr.; Sachok, B.; Petrusek, M. J. Chromatogr. 1979, 186, 419-434. Helfferlch, F. J. Chem. Educ. 1984, 41, 410-413. McCormlck, R. M.; Karger, 8. L. Anal. Chem. 1980, 52, 2249-2257. Meknder, W. R.; Erard, J. F.; Horvath, Cs. J. Chromatogr. 1983, 282. 2 11-228. Buffham. B. A. J. Chromatogr. Scl. 1984, 22, 249-251. Jansson, S. 0. J. Llq. Chromatogr. 1982, 5 , 677-691. Karger, B. L.; LePage, J. N.; Tanaka, N. I n "Hlgh Performance Liquld Chromatography"; Horvath, Cs., Ed.; Academic Press: New York, 1980. Sokolcwskl, A.; Wahlund, K.-G. J. Chromatogr. 1986, 189, 299-316. Westerlund, D.; Erbson, E. J . Chromatogr. 1979, 185, 593-603. Tllly Melln, A.; Llungcrantz, M.; Schlll, G. J. Chromatogr. 1979, 185, 225-239.
RECEIVED for review December 5,1984. Accepted March 27, 1985.
Ligand-Exchange High-Performance Liquid Chromatography of Dialkyl Sulfides Hiroaki Takayanagi,* Osamu Hgtano, Kazumi Fujimura, and Teiichi Ando Department of Industrial Chemistry, Faculty of Engineering, Kyoto University, Sakyo-ku, Kyoto 606, Japan The technique !A llgand-exchange chromatography has been found to be satisfactorily applkable to the separatlon of dlalkyl sulfides. Copper( I I ) 2-amino-1-cyciopentene-1-dtthlocarboxylate (ACDA) bonded silicas showed the best s e k tlvlty when hexane contalnlng methanol or acetonttrlle was used as the moblie phase. The retentlon was found to be affected by the lengths and the degree of branching of the alkyl chains of sample sulfldes, and a h e a r relationship was observed between the capaclty factors and the concentratlon of methanol or acetonltrile. The use of Ag(1)-ACDA stationary phase wlth methanol as the moblle phase also gave an efficient separation of sulfides, where the elution order of dl-n -alkyl sulfides was reversed.
Organic sulfur compounds have now become of importance 0003-2700/85/0357-1840$01.50/0
in synthetic, environmental,and biological chemistry. Dialkyl sulfides are typical of them and now are often used as soft ligands for various metal ions. Although basicities of sulfides or complex formation constants of some sulfide-derivedcomplexes have been reported as an index of their ligating ability (1-4), these data are limited to those measured in strongly polar media. Since relatively apolar solvents are preferred when sulfides are used in synthetic or analytical applications ( 5 , 6 ) ,these data in such solvents would be of more interest. The metal ion species of the complex is also to be taken into account in evaluating the ligating ability of sulfides. Although much has been reported on liquid chromatography (LC) separation or determination of some sulfides (7-9), the total LC characterization of these compounds is still lacking. Since attempts to resolve them by common LC techniques such as adsorption or reversed-phase LC were unsuccessful (IO), ligand-exchange chromatography (LEC) 0 1985 American Chemical Soclety
