Separation characteristics of phenyl-containing stationary phases for gas chromatography based on silarylene-siloxane copolymer chemistries

The solvation parameter model is used to characterize the retention properties of five open-tubular column stationary phases (ZB-5 ms, DB-5 ms, DB-XLB, DB-17 ms, and DB-35 ms) based on silarylene-siloxane copolymer chemistries at five equally spaced temperatures over the range 60-140 degrees C. System constant differences and regression models for varied compounds are used to establish the selectivity equivalence of the silarylene-siloxane copolymer stationary phases and to compare their separation characteristics with poly(dimethyldiphenylsiloxane) stationary phases containing a nominally similar concentration of phenyl groups. These studies demonstrate that ZB-5 ms and DB-5 ms are selectivity equivalent. DB-XLB is significantly more dipolar and polarizable than DB-5 ms. In general terms, the silarylenesiloxane copolymer stationary phases are slightly less cohesive and more dipolar and polarizable with similar hydrogen-bond basicity to the poly(dimethyldiphenylsiloxane) stationary phases they were designed to replace. None of the silarylenesiloxane copolymer or poly(dimethyldiphenylsiloxane) stationary phases are hydrogen-bond acidic. Selectivity differences between the two types of stationary phase are temperature dependent and tend to be smaller at higher temperatures within the temperature range studied. Consequently, selectivity differences cannot be globalized without reference to the temperature for the comparison.     

System constants for the bis(cyanopropylsiloxane)-co-methylsilarylene HP-88 and poly(siloxane) Rtx-440 stationary phases

The solvation parameter model is used to characterize the retention properties of the bis(cyanopropylsiloxane)-co-methylsilarylene, HP-88, and poly(siloxane), Rtx-440, stationary phases over the temperature range 60-140 degrees C. HP-88 is among the most cohesive, dipolar/polarizable and hydrogen-bond basic of stationary phases for open-tubular column gas chromatography. It has no hydrogen-bond acidity or capacity for electron lone pair interactions. It exhibits similar selectivity to the poly(cyanopropylsiloxane) stationary phase SP-2340. Rtx-440 is a low-polarity, low-cohesion stationary phase with a moderate capacity for dipolar/polarizable and hydrogen-bond base interactions. It has no hydrogen-bond acidity and possesses weak electron lone pair interactions. It has unique selectivity when compared against a system constants database for 28 common stationary phase compositions. Cluster analysis indicated that the poly(cyanopropylphenyldimethylsiloxane) stationary phase containing 6% cyanopropylphenylsiloxane monomer, DB-1301, the poly(dimethyldiphenylsiloxane) stationary phase containing 20% diphenylsiloxane monomer, Rtx-20, the poly(siloxane) stationary phase of unknown composition, DB-624, and DX-1 [a mixture of poly(dimethylsiloxane) and poly(ethylene glycol) 9:1] are the closest selectivity matches in the database. The selectivity of DB-1301 and Rtx-440 are very similar for solutes with weak hydrogen-bond acidity allowing one stationary phase to be substituted for the other with likely success. For strong hydrogen-bond acids, such as phenols, DB-1301 and Rtx-440 exhibit different selectivity.     

Evaluation of the separation characteristics of application-specific (volatile organic compounds) open-tubular columns for gas chromatography

The solvation parameter model is used to characterize the separation characteristics of two application-specific open-tubular columns (Rtx-Volatiles and Rtx-VGC) and a general purpose column for the separation of volatile organic compounds (DB-WAXetr) at five equally spaced temperatures over the range 60-140 degrees C. System constant differences and retention factor correlation plots are then used to determine selectivity differences between the above columns and their closest neighbors in a large database of system constants and retention factors for forty-four open-tubular columns. The Rtx-Volatiles column is shown to have separation characteristics predicted for a poly(dimethyldiphenylsiloxane) stationary phase containing about 16% diphenylsiloxane monomer. The Rtx-VGC column has separation properties similar to the poly(cyanopropylphenyldimethylsiloxane) stationary phase containing 14% cyanopropylphenylsiloxane monomer DB-1701 for non-polar and dipolar/polarizable compounds but significantly different characteristics for the separation of hydrogen-bond acids. For all practical purposes the DB-WAXetr column is shown to be selectivity equivalent to poly(ethylene glycol) columns prepared using different chemistries for bonding and immobilizing the stationary phase. Principal component analysis and cluster analysis are then used to classify the system constants for the above columns and a sub-database of eleven open-tubular columns (DB-1, HP-5, DB-VRX, Rtx-20, DB-35, Rtx-50, Rtx-65, DB-1301, DB-1701, DB-200, and DB-624) commonly used for the separation of volatile organic compounds. A rationale basis for column selection based on differences in intermolecular interactions is presented as an aid to method development for the separation of volatile organic compounds.     

Selectivity equivalence of poly(ethylene glycol) stationary phases for gas chromatography

The solvation parameter model is used to study differences in selectivity for poly(ethylene glycol) stationary phases for packed column (Carbowax 20M) and fused-silica, open-tubular column (HP-20M, AT-Wax, HP-INNOWax and DB-FFAP) gas chromatography. All phases are dipolar, strongly hydrogen-bond basic with no hydrogen-bond acidity and of moderate cohesion. No two phases are exactly alike, however, and selectivity differences identified with cavity formation and dispersion interactions, n- and pi-electron pair interactions, dipole-type interactions and hydrogen-bond interactions are quantified by differences in the system constants at a fixed temperature where retention occurs solely by gas-liquid partitioning. The system constants vary linearly with temperature over the range 60-140 degrees C (except for n- and pi-electron pair interactions which are temperature invariant) facilitating a general comparison of the importance of temperature on selectivity differences for compared phases. From a mechanistic point of view it is demonstrated that selectivity differences can result from chemical differences between the poly(ethylene glycol) stationary phases and from differences in the relative contribution of interfacial adsorption to the retention mechanism. The latter depends on both system properties and solute characteristics.     

Selectivity assessment of popular stationary phases for open-tubular column gas chromatography

The solvation parameter model is used to study the influence of temperature and composition on the selectivity of nine poly(siloxane) and two poly(ethylene glycol) stationary phase chemistries for open-tubular column gas chromatography. A database of system constants for the temperature range 60-140 degrees C was constructed from literature values with additional results determined for HP-50+, DB-210, DB-1701, DB-225 and SP-2340 columns. The general contribution of monomer composition (methyl, phenyl, cyanopropyl, and trifluoropropyl substituents) on the capacity of poly(siloxane) stationary phases for dispersion, electron lone pair, dipole-type and hydrogen-bond interactions is described. The selectivity coverage of the open-tubular column stationary phases is compared with a larger database for packed column stationary phases at a reference temperature of 120 degrees C. The open-tubular column stationary phases provide reasonable coverage of the range of dipole-type and hydrogen-bond base interactions for non-ionic packed column stationary phases. Deficiencies are noted in the coverage of electron lone pair interactions. None of the open-tubular column stationary phases are hydrogen-bond acids. The system constants are shown to change approximately linearly with temperature over the range 60-140 degrees C. The intercepts and slopes of these plots are used to discuss the influence of temperature on stationary phase selectivity.     

Insights into the retention mechanism of neutral organic compounds on polar chemically bonded stationary phases in reversed-phase liquid chromatography

The solvation parameter model is used to characterize the retention properties of a 3-aminopropylsiloxane-bonded (Alltima amino), three 3-cyanopropylsiloxane-bonded (Ultrasphere CN, Ultremex-CN and Zorbax SB-CN), a spacer bonded propanediol (LiChrospher DIOL) and a multifunctional macrocyclic glycopeptide (Chirobiotic T) silica-based stationary phases with mobile phases containing 10 and 20% (v/v) methanol-water. The low retention on the polar chemically bonded stationary phases compared with alkylsiloxane-bonded silica stationary phases arises from the higher cohesion of the polar chemically bonded phases and an unfavorable phase ratio. The solvated polar chemically bonded stationary phases are considerably more hydrogen-bond acidic and dipolar/polarizable than solvated alkylsiloxane-bonded silica stationary phases. Selectivity differences are not as great among the polar chemically bonded stationary phases as they are between the polar chemically bonded phases and alkylsiloxane-bonded silica stationary phases.     

Selectivity equivalence of two poly(methylphenylsiloxane) open-tubular columns prepared with different deactivation techniques for gas chromatography

The solvation parameter model is used to characterize the retention properties of a poly(methylphenylsiloxane) column Rxi-50 over the temperature range 60-240 degrees C. The smooth variation of the system constants with temperature affords a general picture of how the relative importance of the different intermolecular interactions change with temperature. The system constants and retention factors for varied compounds are compared with those for Rtx-50 prepared with a similar stationary phase but using a different surface deactivation technique. The two columns are shown to be nearly selectivity equivalent. The Rtx-50 column is slightly more cohesive, dipolar/polarizable and hydrogen-bond basic than Rxi-50, while Rxi-50 is slightly more electron lone pair attractive and hydrogen-bond acidic. Only the difference in hydrogen-bond acidity can be identified with some certainty as related to the difference in deactivation processes. For compounds with a separation greater than 0.2 retention factor units on Rtx-50, it should be relatively straightforward to achieve an acceptable separation for the same compounds on Rxi-50.     

Insights into the Retention Mechanism on a Pentafluorophenylpropylsiloxane-Bonded Silica Stationary Phase (Discovery HS F5) in RP-LC

The solvation parameter model is used to elucidate the retention mechanism of neutral compounds on the pentafluorophenylpropylsiloxane-bonded silica stationary phase (Discovery HS F5) with methanol-water and acetonitrile-water mobile phases containing from 10 to 70% (v/v) organic solvent. The dominant factors that increase retention are solute size and electron lone pair interactions while polar interactions reduce retention. A comparison of the retention mechanism with an octadecylsiloxane-bonded silica stationary phase based on the same silica substrate and with a similar bonding density (Discovery HS C18) provides additional insights into selectivity differences for the two types of stationary phase. The methanol-water solvated pentafluorophenylpropylsiloxane-bonded silica stationary phase is more cohesive and/or has weaker dispersion interactions and is more dipolar/polarizable than the octadecylsiloxane-bonded silica stationary phase. Differences in hydrogen-bonding interactions contribute little to relative retention differences. For mobile phases containing more than 30% (v/v) acetonitrile selectivity differences for the pentafluorophenylpropylsiloxane-bonded and octadecylsiloxane-bonded silica stationary phases are no more than modest with differences in hydrogen-bond acidity of greater importance than observed for methanol-water. Below 30% (v/v) acetonitrile selectivity differences are more marked owing to incomplete wetting of the octadecylsiloxane-bonded silica stationary phase at low volume fractions of acetonitrile that are not apparent for the pentafluorophenylpropylsiloxane-bonded silica stationary phase. Steric repulsion affects a wider range of compounds on the octadecylsiloxane-bonded than pentafluorophenylpropylsiloxane-bonded silica stationary phase with methanol mobile phases resulting in additional selectivity differences than predicted by the solvation parameter model. Electrostatic interactions with weak bases were unimportant for methanol-water mobile phase compositions in contrast to acetonitrile-water where ion-exchange behavior is enhanced, especially for the pentafluorophenylpropylsiloxane-bonded silica stationary phase. The above results are compatible with a phenomenological interpretation of stationary phase conformations using the haystack, surface accessibility, and hydro-linked proton conduit models.     

Evaluation of the separation characteristics of application-specific (pesticides and dioxins) open-tubular columns for gas chromatography

The solvation parameter model is used to characterize the retention properties of four application-specific open-tubular columns (Rtx-CLPesticides, Rtx-OPPesticides, Rtx-Dioxin and Rtx-Dioxin2) at five equally spaced temperatures over the range 60-140 degrees C. Cluster analysis is used to compare the system constants to a database of forty open-tubular columns characterized according to the same method. System constants differences and retention factor correlation plots are then used to determine selectivity differences between the application-specific columns and their nearest neighbors identified by cluster analysis. The Rtx-CLPesticides and Rtx-OPPesticides columns are shown to belong to the selectivity group containing poly(dimethylmethyltrifluoroprpylsiloxane) stationary phases with Rtx-OPPesticides having a similar selectivity to a poly(dimethylmethyltrifluoropropylsiloxane) stationary phase containing 20% methyltrifluoropropylsiloxane monomer (DB-200) and Rtx-CLPesticides separation properties for a stationary phase containing less than 20% methyltrifluoropropylsiloxane monomer. The Rtx-Dioxin and Rtx-Dioxin2 columns are located in the selectivity group dominated by the poly(dimethyldiphenylsiloxane) stationary phases containing less than 20% diphenylsiloxane monomer. The Rtx-Dioxin and Rtx-Dioxin2 columns are shown to be selectivity equivalent to a (5% phenyl) carborane-siloxane copolymer stationary phase (Stx-500) and a second generation silarylene-siloxane copolymer stationary phase containing dimethylsiloxane and diphenylsiloxane monomers (DB-XLB), respectively.     

Assessment of the selectivity equivalence of DB-608 and DB-624 open-tubular columns for gas chromatography

The solvation parameter model is used to characterize the selectivity of DB-608 and DB-624 open-tubular columns at five equally spaced temperatures over the range 60 to 140 degrees C. The system constants for the DB-608 and DB-624 columns were used as selectivity parameters to search a database of open-tubular columns to identify columns with similar selectivity. The search was refined using the absolute deviation of the system constants and retention factor regression models for varied compounds. For method development it is shown that the selectivity of the poly(cyanopropylphenyldimethylsiloxane) stationary phase containing 6% cyanopropylphenylsiloxane monomer (DB-1301) is equivalent to DB-624 and the poly(dimethyldiphenylsiloxane) stationary phases containing either 50 or 65% diphenylsiloxane monomer (Rtx-50 and Rtx-65) are suitable choices for DB-608.     

Characterization of immobilized artificial membrane (IAM) and XTerra columns by means of chromatographic models

Immobilized artificial membranes (IAMs) prepared from phosphatidylcholine analogs are used as stationary phases in liquid chromatography systems to model drug partitioning between an aqueous phase (mobile phase) and a cell membrane (IAM column). Two different chromatographic models, which describe retention as a function of solute and column-mobile phase properties, have been applied to characterization of an IAM and two reversed phase C18 columns (Waters XTerra MSC18 and XTerra RP18) with acetonitrile-water mobile phases. The comparison of the results shows that the phosphatidylcholine group makes IAM column more polar than both XTerra columns, specially in terms of hydrogen-bond acceptor ability. XTerra RP18 is slightly more polar than XTerra MSC18 because of the presence of the embedded carbamate polar group.     

Advances in sol-gel based columns for capillary electrochromatography: sol-gel open-tubular columns

The development of sol-gel open-tubular column technology in capillary electrochromatography (CEC) is reviewed. Sol-gel column technology offers a versatile means of creating organic-inorganic hybrid stationary phases. Sol-gel column technology provides a general approach to column fabrication for microseparation techniques including CEC, and is amenable to both open-tubular and monolithic columns. Direct chemical bonding of the stationary phase to the capillary inner walls provides enhanced thermal and solvent stability to sol-gel columns. Sol-gel stationary phases inherently possess higher surface area, and thus provide an effective one-step alternative to conventional open-tubular column technology. Sol-gel column technology is applicable to both silica-based and transition metal oxide-based hybrid stationary phases, and thus, provides a great opportunity to utilize advanced material properties of a wide range of nontraditional stationary phases to achieve enhanced selectivity in analytical microseparations. A wide variety of stationary phase ligands can be chemically immobilized on the capillary inner surface using a single-step sol-gel procedure. Sol-gel chemistry can be applied to design stationary phases with desired chromatographic characteristics, including the possibility of creating columns with either a positive or a negative charge on the stationary phase surface. This provides a new tool to control electroosmotic flow (EOF) in the column. Column efficiencies on the order of half a million theoretical plates per meter have been reported for sol-gel open-tubular CEC columns. The selectivity of sol-gel stationary phases can be easily fine-tuned by adjusting the composition of the coating sol solution. Open-tubular columns have significant advantages over their packed counterparts because of the simplicity in column making and hassle-free fritless operation. Open-tubular CEC columns possess low sample capacity and low detection sensitivity. Full utilization of the analytical potential of sol-gel open-tubular columns will require a concomitant development in the area of high-sensitivity detection technology.