HPLC for Pharmaceutical Scientists 2007 (Part 18)

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For much of the early development of liquid chromatography, separations were carried out at ambient temperature and many laboratories did not attempt to regulate or control the temperature of the column. Frequently, the column would be mounted on the side of the pump or detector and thus would be subjected to changes in the room temperature or changes due to external factors, such as sunlight. However, the inﬂuence of temperature on the retention times of analytes was well known and had been studied by a number of groups—in particular, Melander et al. [1]. They demonstrated that for most analytes...

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Nội dung Text: HPLC for Pharmaceutical Scientists 2007 (Part 18)

18 TEMPERATURE AS A VARIABLE IN PHARMACEUTICAL APPLICATIONS Roger M. Smith 18.1 THE INFLUENCE OF TEMPERATURE ON CHROMATOGRAPHY For much of the early development of liquid chromatography, separations were carried out at ambient temperature and many laboratories did not attempt to regulate or control the temperature of the column. Frequently, the column would be mounted on the side of the pump or detector and thus would be subjected to changes in the room temperature or changes due to external factors, such as sunlight. However, the inﬂuence of temperature on the reten- tion times of analytes was well known and had been studied by a number of groups—in particular, Melander et al. [1]. They demonstrated that for most analytes there was a linear relationship between the retention factor of an analyte and the inverse of the absolute column temperature (see Chapter 1). However, for a few samples there has been an increase in retention with increasing temperature usually attributed to entropy effects. In the case of polyethylene glycol oligomers, the optimum separation was achieved with a negative temperature gradient [2]. The retention of leucine-phenylalanine at low pH and high % acetonitrile also increased with increasing tempera- ture [3]. As a result, temperature can play an important role in pharmaceutical analysis. The precise and accurate control of temperature can improve repro- ducibility and method transferability (Section 18.2). In recent years, the use of HPLC for Pharmaceutical Scientists, Edited by Yuri Kazakevich and Rosario LoBrutto Copyright © 2007 by John Wiley & Sons, Inc. 811
812 TEMPERATURE AS A VARIABLE IN PHARMACEUTICAL APPLICATIONS elevated or unconventional temperatures have been examined as methods to alter selectivity and column efﬁciency, either with conventional mobile phases (Section 18.3) or with solvent free systems, such as superheated water (Section 18.4). Although normally the interest has been in elevated temperatures, sub- ambient chromatography has provided a number of interesting separations (Section 18.5). 18.2 EFFECTS ON METHOD TRANSFERABILITY AND REPRODUCIBILITY As pharmaceutical analysis developed and the need for long-term repro- ducibility became more important, instrument manufacturers recognized the need for temperature stability and by the early 1990s started to include column ovens as an integral part of their instruments. In most cases the temperatures were controlled near or just above ambient because the aim was to ensure a reproducible result rather than to employ temperature as a method variable. However, even now, many chromatographers carry out separations at ambient temperature, partly on the assumption that the conditions in a heated or air- conditioned building are constant. The reality is often different and the tem- perature around a column can alter quite markedly. Sunlight can shine on a column, draughts can blow on the column, or the air-conditioning can be pro- grammed to lower the laboratory temperature overnight or at weekends as a cost-saving exercise. The result is that the retention of analyte compounds can move outside predeﬁned retention windows and the system can show daily or long-term variations and poor reproducibility. Of particular concern is that methods that have been developed and tested in one laboratory are often transferred to another laboratory in the same or a different company as the drug product moves from discovery through toxi- cology, stability studies, formulation, scale-up, and eventually to manufactur- ing quality control. Frequently, it is found that at each transfer, a new method optimization and a revalidation are required, each taking time and money. Sur- prisingly, relatively little research had addressed this problem. There are only few reports of interlaboratory collaborative studies where the target has been to assess the transferability of retention or resolution. In contrast, the trans- fer of quantitation has been repeatedly examined, but this is based on relative peaks areas to an internal or external standard measured under the same con- ditions. This usually compensates for differences in retention time. Typically, interlaboratory studies produce retention time reproducibility, which is much worse than intralaboratory measurements. A comparison of the analysis of forensic drugs in different UK labs [4] and in an international study [5] showed wide variation in relative and absolute retention times even through the mobile phases were closely speciﬁed and all the columns were from a single batch of packing material. Within a single laboratory, the situation could be improved markedly by employing temperature control of the column, with an oven or water bath [6].
ELEVATED TEMPERATURE AND PHARMACEUTICAL SEPARATIONS 813 The values were then quite usable for quality control and identiﬁcation espe- cially when the system was calibrated with standards at frequent intervals. The main residual source of variation was then batch-to-batch differences between columns, although these differences have been reduced in recent years, and uncertainties in the preparation of the mobile phase, which can be reduced by close control of the protocols used. As part of a project to develop a certiﬁed reference material for high- performance liquid chromatography [7], it was necessary to demonstrate that the proposed method would yield identical results in different laboratories and on different equipment. However, initial results using a speciﬁed temperature and columns from a single batch of packing material gave poor interlabora- tory results, and temperature variations were suspected as a cause [8]. It was found that although the ovens in each laboratory were set to the same nominal temperature, different oven types, air, fan air, convection/conduction, and water bath gave signiﬁcantly different results, the worst results coming from heaters where the column rested against a heated block. The effective tem- perature could be up to 6°C lower than the set value, and this could be mon- itored by using the changes in shape selectivity and hydrophobicity of a test mixture [9]. Similar observations of oven variability were made by Paesen and Hoogmartens [10]. The protocol for the CRM was then tightened so all the laboratories used a water bath or circulating water jacket and speciﬁed lengths of eluent preheating tubing. This gave interlaboratory and intralaboratory variations that were comparable and within the acceptable range [11]. Thus to achieve good transferability of a method, not only the obvious factors, such as column make and mobile phase, need to be deﬁned, but also the method of maintaining a constant temperature needs to be speciﬁed. Part of the cause of the problem is attributable to differences in the dissi- pation of the ﬁctional heating generated by the movement of the mobile phase through the stationary phase. In a liquid bath, this heat is readily lost to the bath as the external temperature of the column is constant along its length, whereas in a noncontrolled or static air system the mobile phase elutes from the column at a higher temperature (2–3°C) than the inlet [12]. There is also an axial temperature gradient in each case. The effect of different tempera- ture control was also examined by Welsch et al. [13], who found differences between air oven and water baths on normal-phase separations and also studied the effect of inlet temperatures. These effects were later studied for reversed-phase separations by Wolcott et al. [14], who suggested a number of temperature-related reasons for poor method transferability and suggested how different effects changed the temperature proﬁle within the column. 18.3 ELEVATED TEMPERATURE AND PHARMACEUTICAL SEPARATIONS Although temperature has been used for many years to alter separation prop- erties, especially selectivity and efﬁciency, the operating range has usually been
814 TEMPERATURE AS A VARIABLE IN PHARMACEUTICAL APPLICATIONS modest, typically up to 40–60°C. The principal aim has been to establish a con- trolled system sufﬁciently above ambient temperature so that day-to-day changes in the laboratory conditions have no effect on the separation. Although the use of non-ambient temperatures might offer advantages in bio- analytical methods, it has been noted that the selection often seems arbitrary and without speciﬁc justiﬁcation [15]. However, Brinkman et al. [16] com- mented that temperature was a variable that should be considered in method development. This comment was echoed in a recent review of the use of mod- erate temperature changes for drug assays by Zhu et al. [17], who noted that “temperature should be considered as a useful variable to control resolution only when components in a mixture are of different types.” For many years, analysts have been deterred from applying signiﬁcantly ele- vated temperatures because of concern about the volatility of mobile phases and the stability of stationary phases and analytes. More recently as a spin-off from work with supercritical ﬂuid chromatography, many laboratories learned how to handle separations in pressurized columns (up to 300 bar), and hard- ware with pressure-resistant detector ﬂow cells became available. As a con- sequence, the expertise and equipment were commercially available, which could control mobile phases above their boiling point. This has enabled the examination of separations under conditions up to 250°C based on either con- ventional mobile phases or less common solvents, such as superheated water [18]. Temperatures above 80°C, where pressure has to be applied to prevent the mobile phase from boiling, are usually termed either pressurized, super- heated, or subcritical conditions, the latter two terms being more frequently applied to separations with just water as the mobile phase. Either the separa- tions can be isothermal or a temperature gradient can be employed, which generates an effect similar to gradient elution, speeding up the later compo- nents [19, 20]. However, concern is often expressed that the mass of a packed HPLC column might cause the internal temperature to lag behind the oven setting but as long as the internal temperature is reproducible, a valid method can be developed. A number of early studies employed packed capillary columns with a low thermal mass [19]. Three main aims have driven these studies: the use of temperature as a variable to optimize separations, an interest in improved efﬁciency, and the potential for “green” separations methods, such as superheated water chromatography, which can eliminate the organic solvent from the mobile phase. 18.3.1 Effect of Temperature on Selectivity Although temperature has been proposed as a variable in altering selectivity, it has not been widely used, because the majority of analytes show very similar changes on changing temperature (especially over the limited conventional temperature range). Signiﬁcant differences may be observed if temperature can cause ionization changes or if analytes with very different functional
ELEVATED TEMPERATURE AND PHARMACEUTICAL SEPARATIONS 815 groups are present. However, care must be taken in these situations because relative retention changes with column temperature could result in a lack of method robustness, especially if caused by ionization changes. A few application and studies have examined temperature effects, such as the selectivity dependence of the carotenoids [21] on different columns from 25–45°C. Studies of the prediction of the inﬂuence of temperature and solvent strength on the separation of 47 basic acidic and neutral drugs compounds were reported by Zhu et al. [22] in an interlaboratory collaborative study. More recently the inﬂuence on temperature on selectivity has been reviewed by Dolan [23, 24]. The changes in retention and selectivity can also be exploited in the thermally tuned tandem column concept by Mao and Carr [25], in which the temperatures of two sequentially linked columns containing different sta- tionary phases can be altered to provide the optimum separation. The tech- nique was applied to the separation of barbiturates, phenylthiohydantoin amino acids [26], and selected basic pharmaceuticals, such as antihistamines (Figure 18-1) [27]. Berthod et al. [28] examined the effect of temperature on chiral separations between 5°C and 45°C using four macrocyclic glycopeptides phases; and although the efﬁciencies increased with temperature, in 83% of cases the chiral selectivity decreased. 18.3.2 Effect of Temperature on Separation Efﬁciency One of the reported advantages of raising the temperature of a chromato- graphic separation is an increase in peak efﬁciency. This is usually attributed Figure 18-1. Separation of antihistamines on linked columns with different tempera- tures. ODS at 40°C and PBD-ZrO2 at 35°C. Mobile phase 40 : 60 methanol/pH 7 buffer. Solutes: 1, pheniramine; 2, chlorpheniramine; 3, thenyldiamine; 4, bromopheniramine; 5, cyclizane; 6, pyrrobutamine; 7, chlorcyclizane; 8, thonylamine; 9, meclizane. (Reprinted from reference 28, with permission. Copyright 2001, American Chemical Society.)
816 TEMPERATURE AS A VARIABLE IN PHARMACEUTICAL APPLICATIONS to a reduction in the viscosity of the eluent and an increase in the diffusion rate of the analyte as the temperature is increased. A higher diffusion rate should reduces the mass transfer term effect (the C term) in the van Deemter equation but can also worsen the inﬂuence of longitudinal molecular diffusion in the column (the B term) [29]. The improvement in efﬁciency is normally regarded as most signiﬁcant for larger analytes, such as biological and synthetic macromolecules, whose size reduces their mobility [30]. For smaller molecules the effects are relatively small, and often an increase in efﬁciency can be attributed to a reduction in the retention factor on raising the temperature. A second factor, which inﬂuences peak shape and apparent efﬁciency, is the temperature of the incoming mobile phase relative to the column tempera- ture. The presence of this underestimated factor may have obscured or con- fused previous studies of efﬁciency. Frequently, it has been claimed that the mobile phase in a high-temperature separation must be heated to the same temperature as the column; otherwise, peaks distortion and broadening are observed [18, 31]. This was demonstrated by Thompson and coworkers [32, 33], who reported the band-broadening effect of a thermal mismatch and advocated the use of narrow bore columns to reduce the effects. Guillarme et al. [34] also demonstrated the need for some preheating of the mobile phase and considered the length of tubing required for effective preheating (within 5°C of the oven) particularly with high ﬂow rates. However, even early studies including those by Cooke et al. [35] and by Poppe and Kraak [36] demonstrated that using a mobile phase slightly cooler than the column temperature can improve column efﬁciency. Usually, differ- ences of 10–20°C gave the highest efﬁciency. For example, Mayr and Welsch [12], who found the highest efﬁciency for the separation of ﬁve hormone steroids was obtained with the incoming mobile phase at 10°C and a column at 30°C (Figure 18-2). Spearman et al. [9] reported that in one case reducing the inlet to 37°C below the column temperature optimized the results. The effect is thought to have two origins. First, a cooler mobile phase and by implication cooler sample causes an initial sample focusing at the head of the column. Second, the cooler eluent ﬂow reduces the analyte mobility at the center of the column, thus balancing the enhanced temperature and hence mobility in the center of the column caused by friction heating (Figure 18-3) [14]. At its most serious, not only efﬁciency but also peak distortion has been observed caused apparently by a temperature in-balance. The selection of the optimum column inlet temperature is not totally clear, and this is an area of ongoing research. In such a situation the internal diameter of the column might also effect the equilibration process but Molander et al. [37] found that even using a tem- perature gradient, the differences were minimal for columns narrower than 4.6 mm internal diameter. A recent study has found that elevated tempera- tures, up to 70°C, markedly improved the efﬁciency and peak shapes of bases with intermediate pH eluents [38].
ELEVATED TEMPERATURE AND PHARMACEUTICAL SEPARATIONS 817 Figure 18-2. Separation of hormones on ChromSpher UOP column at 30°C at differ- ent eluent inlet temperatures: a, 5°C; b, 10°C; c, 22°C; d, 30°C. Compounds: 1, thiourea; 2, hydrocortisone; 3, nortestosterone; 4, dehydro-17a-methyltestosterone; 5, testos- terone; 6, 17a-methyltestosterone. (Reprinted from reference 12, with permission from Elsevier.) 18.3.3 Other Temperature Effects A side effect of a lack of temperature control is that changes can alter the refractive index of the mobile phase, causing baseline disturbances and reduc- ing sensitivity The problem is principally with refractive index detection [39], but it can also inﬂuence spectroscopic detectors and their light path can be distorted. Temperature has also been reported to alter the nature of some sta- tionary phases. For example, it caused a change in the chiral selectivity of the resolution of dihydropyrimidone acid and its methyl ester on amylose and cel- lulose stationary phases [40]. 18.3.4 Applications of Elevated Temperatures Almost all the high-temperature work on pharmaceutical compounds has employed reversed-phase separations. In a series of studies since the 1990s, Greibrokk and co-workers [41, 42] have examined the role of elevated tem- perature and temperature gradients. Many were devoted to polymer separa- tions where the use of a temperature gradient speeded up the larger oligomers and provided clear advantages because of the complexity of the sample. As part of the optimization of the conditions for a separation on a packed
818 TEMPERATURE AS A VARIABLE IN PHARMACEUTICAL APPLICATIONS Figure 18-3. Band-broadening due to thermal effects. (a) Ideal case, no thermal effects; (b) effect of incoming mobile phase that is at a lower temperature than the column; (c) effect of frictional heating; (d) combined effect of cold incoming mobile phase and frictional heating. An oven temperature of 70°C is assumed. (Reproduced from refer- ence 14, by permission from Elsevier.) capillary, Tran et al. [3] included the effect of temperature on a range of compounds, including naphthalene, acenaphthene, ibuprofen, butylparaben, diethyl phthalate, monoethyl phthalate, amitriptyline, propranolol, ampheta- mine, all-trans-retinol, 13-cis-retinol, and dl-leucine-dl-phenylalanine. A few applications have employed conventional packed columns, although recent developments in new thermally stable stationary-phase materials have generated a renewed interest and the temperature stability of the different stationary-phase materials has been reviewed by Claessens and van Straten [43]. The new materials have included stable metal oxide materials, based on zirconia (Figures 18-4 and 18-5) and titania [44, 45] and hybrid phases com- bining silica and methylene or ethyl bridges [46]. These have been applied in a number of applications to pharmaceutical compounds (Table 18-1). One of the most interesting thermally stable groups of stationary phase materials has been the polybutadiene, carbon and phenyl-coated zirconia
ELEVATED TEMPERATURE AND PHARMACEUTICAL SEPARATIONS 819 Figure 18-4. The separation of barbiturates on (A) ODS at 30°C, (B) C-ZrO2 at 30°C, (C) ODS at 60°C, (D) C-ZrO2 at 60°C. Mobile-phase 20/80 acetonitrile. (Reproduced from reference 27, with permission. Copyright 2001, American Chemical Society.) phases developed by Carr and colleagues [48, 49]. They reported that at high temperatures, the zirconia material offered a much higher stability than silica- based columns [50]. Under these conditions the reduced solvent viscosity gave advantages as ﬂow rates as high at 5 ml/min were feasible [51]. The PBD zir- conia column has been used for the separation of tricyclic antidepressants [50] and lidocaine, quinidine, norephrine, tryptamine, amitriptyline, and nor- triptyline. Some selectivity changes with temperature were noted. The low vis- cosity at 100°C also enabled very small 1-µm particles to be used for the separation of benzdiazepines (Figure 18-6) [52]. Guillarme applied these techniques to the separation of a series of caffeine derivatives, including
820 TEMPERATURE AS A VARIABLE IN PHARMACEUTICAL APPLICATIONS Figure 18-5. The separation of therapeutic tricyclic antidepressants on PBD-coated zirconia at different temperatures. Solutes: 1, lidnocaine; 2, quinidine; 3, norephedrine; 4, tryptamine; 5, amitriptyline; 6, nortriptyline. (Reproduced from reference 52, with permission. Copyright 1997, American Chemical Society.) theobromine, theophylline, and caffeine when separation on a PBD zirconia column at 150°C took place in less than 1 minute compared to 7 minutes at 40°C on a Hypercarb column (Figure 18-7) [35]. In a recent study, Marin et al. [53] used a set of test compounds including amitriptyline, salicylic acid, and ibuprofen to compare the temperature stabil- ity of six stationary phases at temperatures up to 150°C.
SUPERHEATED WATER CHROMATOGRAPHY 821 TABLE 18-1. Pharmaceuticals Separated at Elevated Temperature Using Conventional Mobile Phases Stationary Analyte Mobile Phases Phase Temperature Reference Tricyclic — PBD zirconia 40–100°C 49 antidepressants Benzodiazepines Acetonitrile– PBD zirconia 100°C 53 water (nonporous) Barbiturates Acetonitrile– ODS + silica + 30–80°C 27 water PDB zirconia Basic 28 pharmaceuticals Vitamin A and Acetonitrile– Suplex pKb-100 25–60°C 47 retinoids water Figure 18-6. UPLC chromatography of benzodiazepines on a 14.5-cm × 50-µm column packed with 1-µm polybutadiene-encapsulated non-porous zirconia particles. Eluent pH 7 buffer–acetonitrile 68 : 22 at 100°C. Peaks: 1, uracil; 2, clorazepate; 3, ﬂuni- trozepam; 4, clonazepam; 5, chlordiazepoxide; 6, oxazepam; 7, clorazepate; 8, diazepam. (Reproduced from reference 53, with permission from Elsevier.) 18.4 SUPERHEATED WATER CHROMATOGRAPHY With an increased interest and awareness of the impact of society and indus- try on the environment, there has been a signiﬁcant attempt in recent years to reduce or replace the usage of organic solvents. Much early work in this area concentrated on the application of supercritical and subcritical carbon dioxide, but in recent years superheated (or subcritical/pressurized hot) water (SHW) has become of interest for both chromatography and extraction [43, 54]. The earliest work was reported by Guillemin et al. [55], who used the term thermal aqueous liquid chromatography. As well as using SHW for the separation of
822 TEMPERATURE AS A VARIABLE IN PHARMACEUTICAL APPLICATIONS Figure 18-7. Effect of temperature on the separation of caffeine derivatives on a Hypercarb column (1 mm × 100 mm). (a) Column at 100°C, mobile phase: acetonitrile; (b) Column at 180°C, mobile phase: water/acetonitrile 70/30. Samples: 1, hypoxantine; 2, theobromine; 3. theophylline; 4, caffeine 5, β-hydroxyethyltheophylline. (Reproduced from reference 35, with permission.) alcohols, carbohydrates, and phenol, they also looked at iprodine and used an on-line FID detector for analysis. Water has interesting and unusual thermal properties, which have only recently been signiﬁcantly exploited by chromatographers [56–58].As the tem- perature is increased, thermal motion weakens the hydrogen-bonding so that the polarity of water is reduced (Figure 18-8). At 200°C, water has a polarity similar to that of methanol; in addition, the viscosity also drops markedly with temperature and the diffusion rate increases. However, the vapor pressure remains low and by 250°C has only reached 30 bar, well within the normal
SUPERHEATED WATER CHROMATOGRAPHY 823 Figure 18-8. Effect of temperature on the relative permittivity of water. (Reproduced from reference 59, with permission.) capabilities of HPLC systems and markedly below the 300–350 bar usually needed in SFC. However, the density is largely unaltered so that the water in effectively incompressible. Hence the pressure applied to the system has a minimal effect, as long as it is sufﬁciently high to prevent gasiﬁcation, it does not inﬂuence separations. The principal advantages in the use of superheated water are that it is rel- atively easy to attain and the back-pressures required on the column are small. Thus even a modest length of narrow bore tubing can be employed to provide sufﬁcient resistance to prevent boiling in the column and at these pressures many conventional spectroscopic ﬂow cells can be used. Because of the high temperatures, there have been concerns about the thermal stability of the ana- lytes, but of the numerous examples, there have been few reports of instabil- ity or a tendency for accelerated hydrolysis or oxidation, of the reported examples, only aspirin has hydrolyzed. Compounds which might be expected to be labile to oxidation or hydrolysis, such as the paraben antioxidants, have chromatographed without problems even up to 200°C [59]. Because of its solvent properties, SHW up to 250°C has also been used for extractions mainly of environmental samples [59]. At higher temperatures >350°C, the critical point of water can be achieved, but by that point the con- ditions are severe and will probably cause analyte degradation. 18.4.1 Columns for Superheated Water Chromatography The principal limitation of the use of superheated water has been the ther- mal instability of conventional ODS-silica-based stationary phases, which are unstable above 70°C or 80°C. Early work concentrated on PS-DVB columns, which were stable up to 220°C. Then zirconia-based PBD and ODS bonded
824 TEMPERATURE AS A VARIABLE IN PHARMACEUTICAL APPLICATIONS phases became available with stabilities up to 140°C or 180°C, respectively (Section 18.3.4). In addition, PGC columns can be used up to 200°C, and many of these materials have been compared to conventional column materials [60, 61]. Hybrid phases, such as ODS-X-Terra, can also be employed up to 150°C. 18.4.2 Detectors in Superheated Water Chromatography Superheated water also offers some novel advantages in detection because the absence of an organic solvent reduces low-wavelength spectroscopic absorp- tion, eliminates the solvent peaks from NMR spectra, and eliminates the solvent signal from ﬂame detectors. This has enabled a wide range of unique HPLC detection methods to be employed. UV and ﬂuorescent spectroscopy can be employed down to 190 nm because there is no solvent interference. Mass spectrometry is easy because the water provides good ionization. Flame ionization detection (FID) is of particular interest because potentially it offers a sensitive and universal detector. A number of different interfaces have been used, including heated capillaries, which have been examined by Miller and Hawthorne [62], Ingelse et al. [63], and others [64, 65], who separated a range of analytes including alcohols, amino acids, and phenols. An alternative method employing a cold nebuliza- tion of the eluent has been introduced by Bone et al. [66]. They were able to detect both aliphatic and aromatic alcohols, polymers, carbohydrates, parabens, and steroids. By using heavy water (deuterium oxide) as the eluent, on-line NMR spec- troscopic detection is simpliﬁed as negligible solvent signals are detected to interfere with the sample signals. This method can be used for drug analysis (Figures 18-9 and 18-10) [67]. By stopping the mobile-phase ﬂow, the peak can be held in the NMR spectrometer cell, thereby increasing sensitivity or enabling more complex data analysis, such as COSY. This method was also combined with on-line mass spectroscopy for a number of model drugs [68] and was used to understand the mechanism of an unexpected selective deu- terium exchange that occurred during the separation of some sulfonamides [69]. The combination of detectors using SHW as the eluent has been extended, and a train of four on-line detectors (UV spectroscopy, FT-IR, 1H- NMR, and MS) were applied to model pharmaceuticals [70] and ecdysteroids in plant extracts [71]. 18.4.3. Pharmaceutical Applications of Superheated Water Chromatography One attraction of SHW is that it can be used for reversed-phase separations and is therefore readily applicable to a wide range of pharmaceutical compounds including barbiturates, sulfonamides, analgesics and steroids (Table 18-2), and anticancer drugs, including 5-ﬂuorouracil, methotrexate, and
SUPERHEATED WATER CHROMATOGRAPHY 825 Figure 18-9. Separation of barbiturates on PS-DVB column at 200°C with water as the eluent. Samples; 1, barbitone; 2, phenobarbitone; 3, talbarbitone; 4, amylobarbitone; 5, heptabarbitone. (Reproduced from reference 59, with permission.) Figure 18-10. Stop ﬂow LC-NMR of heptabarbitone after separation as in Figure 18-9 with D2O as the eluent at 200°C. (Reproduced from reference 59, with permission.)
826 TEMPERATURE AS A VARIABLE IN PHARMACEUTICAL APPLICATIONS TABLE 18-2. Pharmaceutical Compounds Separated Using Hot and Superheated Water Analyte Mobile Phase Column Temperature Reference Barbiturates Deuterium oxide PS-DVB 200°C 69 Sulfonamide Buffered water PS-DVB 70–190°C 72 pH 3–12 Sulfonamide Deuterium oxide PS-DVB 160–200°C 71 Steroids Water Zirconia PDB 170–200°C 62 Analgesics Deuterium Novapak 80–130°C 70 paracetamol, oxide C18 caffeine, and phenacetin Anticancer drugs Water pH 11.5 PS-DVB Up to 160°C 73 and 3.6 Caffeine, Deuterium Oasis HLB 185°C 72 paracetamol, oxide amitriptyline, Xterra 85°C and phenacetin Paracetamol, Water Hypercarb, Up to 225°C 61 antipyrine, and PS-DVB caffeine and zirconia PBD Paracetamol, Water PS-DVB 75–185°C 74 salicylamide, methyl paraben, phenacetinethyl paraben. etoposide (Figure 18-11) [75]. The method could be applied to even relatively nonpolar pharmaceutical compounds, such as the steroids [62]. In a related study, Tajuddin and Smith demonstrated the on-line coupling of SHW extrac- tion with SHW chromatography for the separation of a series of pharmaceu- ticals [76]. The drugs could be sequentially released from the extraction by stepwise temperature increases (Figure 18-12). SHW has also been applied to the separation of nutraceuticals, natural products, and biochemicals, including the water-soluble vitamins, thiamine, riboﬂavin, and pyidoxine (Table 18-3) without signiﬁcant thermal degradation. 18.6 SUBAMBIENT SEPARATIONS As well as selectivity changes at low temperatures, such as those reported by Sander and Wise [78, 79] for the separation of PAHs (Figure 18-13), subam- bient column temperatures can also alter chromatographic separations, by reducing the rate of the racemization of enantiomers and structural isomeri-
SUBAMBIENT SEPARATIONS 827 Figure 18-11. Separation of the anticancer drugs: 5-ﬂuorouracil (5-FU), methotrexate (MTX), 7-hydroxymethotrexate (7-OH-MTX), and etopoxide (VP-16) using super- heated water and a PS-DVB column at 80°C. (Reproduced from reference 75, with per- mission. Copyright 2001, American Chemical Society.) sation, such as cis–trans interconversions. However, as backpressure rises, eluent viscosity increases and diffusion decreases. A classic example is the anomerization of the carbohydrates: Sucrose will give two broad peaks at room temperature but a single sharp peak by 50°C. At 10°C it will give two resolved peaks. Lower temperature separation will also result in the separation of the xylose-mannose and rhammnose-arabinose pairs on anion exchange chromatography [80]. Early work on the Pirkle chiral column with propanolol gave improved peak shapes and chiral resolution as the temperature was reduced from 21°C to −24°C [81]. Improved low-temperature chiral selec- tivity was also reported by Kersten [82] in the separation of beta-amino-3- pyridylpropanoic acid at subambient temperatures. Reducing the temperature to close to the freezing point of the eluent enabled isomeric dipeptides con- taining proline at the C-terminus to be resolved [83, 84]. Potential anti-arthritic protein kinase inhibitors also showed enhanced chiral resolution at subambient temperatures in a study on Chiralcel OD by Whatley [85]. Normal-phase separations at −30°C enabled the determination of the impu- rity proﬁle of a mesylated ester, which underwent in-column cyclization at room temperature, to be determined [86]. Subambient temperatures down to −10°C were used by LoBrutto et al. [87] to generate a single product from the on-column derivatization of an acetylenic aldehyde with diethylamine. “Supercritical” ﬂuid chromatography using carbon dioxide as the eluent is often carried out subcritically at 20°C or 25°C, because the more dense eluent
828 TEMPERATURE AS A VARIABLE IN PHARMACEUTICAL APPLICATIONS Figure 18-12. Separation of test mixture and fractions after extraction and trapping and sequential elution at increasing temperatures. Separation on PS-DVB column at 75–185°C at 15°C/min. Analytes: 1, paracetamol; 2, salicylamide; 3, caffeine; 4, methyl paraben; 5, phenacetin; 6, ethyl paraben. Separations: a, direct injection of original mixture of 1–6 without trapping; b, fraction untrapped at ambient temperature; c, frac- tion released from trap at 70°C; d, released at 90°C; e, released at 110°C. (Reproduced from reference 76, with permission from Royal Society of Chemistry.) TABLE 18-3. Superheated Water Chromatography of Nutraceuticals and Natural Products Analyte Mobile Phase Stationary Phases Temperature Reference Ginger Deuterium oxide Xterra RP 18 50–130°C 75 Ecdysteroids Deuterium oxide C8- XTerra or 160°C 73 C18 X-Terra Water-soluble Deuterium oxide PS-DVB Up to 200°C 76 vitamins Kava Deuterium oxide zirconia PBD 80–160°C 77
SUBAMBIENT SEPARATIONS 829 Figure 18-13. Separation of phase selectivity test mixture of phenanthro[3,4-c]phenan- threne (PhPh), 1,2:3,4:5,5:7,8-tetrabenzonaphthalene (TBN), and benzo[a]pyrene (BaP) on polymeric C18 phase (Vydac) at subambient temperatures. (Reproduced from reference 81, with permission. Copyright 1989, American Chemical Society.) enables a greater sample loading and more column interactions, especially useful in chiral separations [88]. These conditions can also change the selec- tivity in “entropically driven” chiral separations, resulting in a reversal of the elution order of some pharmaceuticals [89]. The use of carbon dioxide as the mobile phase also means that it is pos- sible to carry out assays considerably below subambient temperatures. At −50°C, Gasparrini et al. used a DACH-DNB column to resolve the enan- tiomers of the thermally enantiolabile 2-methyl-1-(2,2-dimethylpropanoyl) naphthalene, which can undergo rotation around the CO–CAr bond [54]. Reducing the temperature resulted in negligible degradation in column performance.
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