Patent Application
20090250349
Solid phase microextraction (SPME) was developed in 1989 by Belardi and Pawliszyn to facilitate rapid sample preparation both for the laboratory and field analysis. It provided a simple and efficient solvent-free method for extraction and preconcentration of analytes from various sample matrices. In SPME, a sorptive stationary phase coating, (either on the outer surface of a fused silica fiber or on the inner surface of a fused silica capillary) actually serves as the extraction medium in which the analytes get preferentially sorbed and preconcentrated. Polymeric surface coatings are predominantly used in conventional fiber-based SPME as well as in the newly materialized in-tube SPME, also referred to as capillary microextraction (CME). A number of new polymeric coatings have recently been developed. Besides polymeric coatings, SPME fibers have also been prepared by using nonpolymeric materials or by gluing reversed-phase HPLC particles onto SPME fiber surface.
The stationary phase coating play a fundamentally important role in the SPME analysis. Because the stationary phase coating is the key component of the SPME device, further development and growth of SPME will greatly depend on new breakthroughs in the areas of stationary phase development and coating technology.
Sol-gel chemistry offers an effective methodology for the synthesis of macromolecular material systems under extraordinarily mild thermal conditions (often at room temperature) which greatly simplifies the job to carry out sol-gel reactions within small-diameter fused silica capillaries by eliminating the procedural complications as well as the need for enhanced technological and safety requirements for carrying out reactions under elevated temperatures. Sol-gel process provides a facile mechanism to chemically bind the in situ created sol-gel stationary phase coatings to the inner walls of the capillary made out of an appropriate sol-gel-active material. Due to this chemical bonding, sol-gel coatings possess significantly higher thermal and solvent stabilities compared with their conventional counterparts. Sol gel chemistry has thus opened a whole new approach to column technology for analytical separations and sample preconcentrations. The sol-gel approach can be applied to create both silica-based stationary phases as well as the newly emerging transition metal oxide-based stationary phases. Furthermore, sol-gel chemistry provides an opportunity to create advanced material systems and to use their properties to achieve enhanced performance and selectivity in analytical separations and sample preconcentration.
Sol-gel organic-inorganic hybrid materials provide desirable properties that are difficult to achieve using either purely organic or purely inorganic materials. This opportunity is being explored in the filed of microcolumn separations and sample preparation through creation of hybrid organic-inorganic stationary phases in the form of surface coatings and monolithic beds. In 1993, a procedure was developed for the preparation of a thin layer of silica gel with chemically bonded C18 moieties on the internal wall of fused-silica capillaries for reversed-phase high-performance liquid chromatography. Colon and Guo in 1995 implemented sol-gel stationary phase for open tubular liquid chromatography and electrochromatography. Malik and coworkers introduced sol-gel coated column for capillary GC and sol-gel coated fibers for solid-phase microextraction (SPME). Following this, other groups also got involved in sol-gel research aiming at developing novel stationary phases for solid-phase microextraction and solid-phase extraction. Sol-gel SPME fibers demonstrated superior performance compared with conventional fibers by exhibiting high thermal stability (up to 360° C.) and solvent stability due to chemical bonding between the sol-gel coating and the fiber surface. They also showed better selectivity and extraction sensitivity toward various analytes, less extraction time due to fast mass transfer and extended lifetime. Recently, sol-gel capillary microextraction (CME) was reported by Malik and coworkers. In this in-tube format, sample extraction was accomplished using a sol-gel coating created on the inner surface of a fused silica capillary.
The existing stationary phases are predominantly silica-based. In spite of many attractive material properties (e.g., mechanical strength, surface characteristics, catalytic inertness, surface derivatization possibilities, etc.), silica-based stationary phases have some inherent shortcomings. The main drawback of silica-based stationary phases is the narrow range of pH stability. Under extreme pH conditions silica-based stationary phases become chemically unstable, and their stationary phase properties are compromised. For example, silica dissolves under alkaline condition, and their dissolution process starts at a pH value of about 8. Under highly acidic pH conditions, silica-based bonded stationary phases become hydrolytically unstable. Therefore, developing stationary phases with a wide range of pH stability is an important research area in contemporary separation and sample preparation technologies. Transition metal oxides (zirconia, titania, etc.) are well known for their pH stability, and appear to be logical candidates for exploration to overcome the above-mentioned drawbacks inherent in silica-based stationary phases.
What is needed, then, is an improved product and a method of use of such product, for increasing the sample concentration sensibility in Chromatographic and electrophoretic separation techniques including but not limited to GC, HPLC, CZE, and CEC.
The present invention relates to zirconia-based stationary phases for chromatographic separations and sample preconcentrations. Zirconia has many unique properties that make it attractive as a chromatographic support material. Zirconia's most notable properties are: it's excellent chemical and pH stability, inertness, and unique surface chemistry. In the present method, sol-gel chemistry is used to exploit these properties by in situ preparation of zirconia-based stationary phases within separation columns and extraction tubes. The utility of the created columns and extraction tubes are demonstrated by experimental results on gas chromatographic separations obtained on sol-gel zirconia coated capillary columns and capillary microextraction results obtained on sol-gel zirconia-coated capillaries.
A sol solution coated column is developed. The column comprising a vessel having a bore, and an inner surface, where the sol solution molecules are chemically bonded onto the inner surface. The sol solution is made from a stationary phase mixture comprising zirconium solution, and an organic compound. A first portion of the stationary phase is a thin layer, chemically bonded to the inner surface of the capillary, and a second portion of the stationary phase is a residual solution, which residual solution is then expelled, leaving a prepared column for sample preconcentration.
For a fuller understanding of the nature and objects of the invention, reference should be made to the following detailed description, taken in connection with the accompanying drawings, in which:
a is CME-GC analysis of PAHs using a sol-gel zirconia-PDMDPS coated capillary before rinsing with NaOH.
b is CME-GC analysis of PAHs using a sol-gel zirconia-PDMDPS coated capillary rinsed with NaOH for 24 hours.
For the preparation of sol-gel-coated capillaries according to this invention, a cleaned and hydrothermally treated fused-silica capillary is filled with a specially designed sol solution using a helium pressure-operated filling/purging device. The key ingredients of the sol solution include appropriate amounts of: sol-gel precursor such as zirconium (IV) butoxide; a stationary phase coating such as silanol-terminated poly copolymer (dimethyldiphenylsiloxane); a solvent such as methylene chloride; a chelating reagent such as acetic acid; a deactivating reagent such as poly(methylhydrosiloxane); or a deactivating reagent such as hexamethyldisilazane. After filling, the sol solution is allowed to stay inside the capillary for 10-15 minutes. During this time, an organic-inorganic hybrid sol-gel network evolves in the sol solution within the confined environment of the fused-silica capillary, and a thin layer of the evolving sol-gel stationary phase chemically bonds to the capillary walls as a result of condensation reaction with the silanol groups on the capillary inner surface. After the residence time, the residual sol solution is expelled from the capillary under helium pressure. The sol-gel-coated capillary is further purged with helium for 1 hour and conditioned in a GC oven using temperature programming (from 40° C. to the final temperature (e.g., 300° C.) at 1 C./mm). The capillary is held at the final temperature under helium purge. The final conditioning temperatures used for the two types of sol-gel coatings are determined by their thermal stability. The coated capillary is then rinsed with a suitable solvent (e.g., methylene chloride) and purged with helium. The column is ready to use.
The sol-gel zirconia coated capillaries can be used either as a separation column for chromatographic (HPLC or GC) and electrophoretic (CE or CEC) techniques or as extraction micro tubes for sample preconcentration (online or offline) of trace analytes for subsequent analysis by GC, HPLC, SFC, CZE or CEC. For online sample preconcentration and HPLC analysis of trace analytes, the prepared coated capillary is installed in the HPLC as the injection loop for sample extraction. While the dilute sample solutions are injected into the loop, the analyte is extracted by the stationary phase on the surface of the capillary. The sample solution is allowed to reside inside the loop for some time until it reaches the equilibrium. After removing the sample matrix from the capillary, the extracted analytes are injected into the separation column by using a mobile phase rich in organic component (e.g., acetonitrile). Low parts per billion level detection limits are achieved in HPLC-UV/vis using sol-gel zirconia coated extraction capillary. The GC separation of a natural gas sample into its components in less than one minute are also conducted.
The sol-gel-coated capillary is a small diameter cylindrical vessel having a bore, and an inner surface, where the sol-gel polymeric molecules are chemically bonded onto the inner surface. The sol solution comprises a mixture of zirconium precursor (e.g., zirconium alkoxide) and an organic compound, where the solution undergoes the following procedure: sol solution forms a thin layer, chemically bonded to the inner surface of the capillary, leaving behind a residual solution; The residual solution is then expelled; the capillary is heated in an oven within a temperature range of 250° C. to 300° C., thereby forming a prepared column for sample preconcentration or analytical separation.
All CME-GC experiments are performed on a Shimadzu Model 14A capillary GC system equipped with a flame ionization detector (FID) and a split-splitless injector. On-line data collection and processing are done using ChromPerfect (version 3.5) computer software (Justice Laboratory Software, Denville, N.J.). A Fisher Model G-560 Vortex Genie 2 system (Fisher Scientific, Pittsburgh, Pa.) is used for thorough mixing of various sol solution ingredients. A Microcentaur model APO 5760 microcentrifuge (Accurate Chemical and Scientific Corp., Westbury, N.Y.) is used to separate the sol solution from the precipitate (if any) at 13,000 rpm (15,682 g). A Barnstead Model 04741 Nanopure deionized water system (Barnstead/Thermodyne, Dubuque, Iowa) was used to obtain ˜16.0 M Ω water. Stainless steel mini-unions (SGE Incorporated, Austin, Tex.) are used to connect the fused silica capillary GC column with the microextraction capillary, also made of fused silica. An in-house-designed liquid sample dispenser (
Fused-silica capillary (320-μm i.d.) with an outer protective polyimide coating may be purchased from Polymicro Technologies Inc. (Phoenix, Ariz.). Naphthalene and HPLC-grade solvents (methylene chloride, methanol) may be purchased from Fisher Scientific (Pittsburgh, Pa.). Hexamethyldisilazane (HMDS), poly (methylhydrosiloxane) (PMHS), ketones (valerophenone, hexanophenone, heptanophenone, and decanophenone), aldehydes (nonylaldehyde, n-decylaldehyde, undecylic aldehyde, and dodecanal), polycyclic aromatic hydrocarbons (PAHs) (naphthalene, acenaphthene, ydroxyl, phenanthrene, pyrene, and 2,3-benzanthracene), may be purchased from Aldrich (Milwaukee, Wis.). Silanol-terminated Poly (dimethyldiphenylsiloxane) copolymer (PDMDPS) may be purchased from United Chemical Technologies, Inc. (Bristol, Pa.).
A sol solution is used to create the coating. As seen in Table I, the key ingredients of the sol solution used are: a sol-gel precursor such as zirconium (IV) butoxide; a stationary phase coating such as silanol-terminated poly copolymer (dimethyldiphenylsiloxane); a solvent such as methylene chloride; a chelating reagent such as acetic acid; a deactivating reagent such as poly(methylhydrosiloxane); or a deactivating reagent such as hexamethyldisilazane.
The sol solution is prepared in a clean polypropylene centrifuge tube by dissolving the following ingredients in methylene chloride (1 mL): 10-15 μL of zirconium (IV) butoxide (80% solution in 1-butanol), 80-100 μL of silanol-terminated poly (dimethyldiphenylsiloxane) copolymer (PDMDPS), 80 μL of poly (methylhydrosiloxane) (PMHS), 10 μL of 1,1,1,3,3,3-hexamethyldisilazane (HMDS), and 2-4 μL of acetic acid. The dissolution process is aided by thorough vortexing. The sol solution is then centrifuged at 13,000 rpm (15,682 g) to remove the precipitates (if any). The top clear sol solution is transferred to a clean vial and is further used in the coating process. Using pressurized helium (50 psi) in the filling/purging device, a hydrothermally treated fused silica capillary (200 cm) is filled with the clear sol solution, allowing it to stay inside the capillary for a controlled period of time (typically 30 min). After that, the free portion of the solution is expelled from the capillary, leaving behind a surface-bonded sol-gel coating on the capillary inner walls. The sol-gel coating is then dried by purging it with helium. The coated capillary is further conditioned by temperature programming from 40° C. to 150° C. at 1° C. /min and held at 150° C. for 300 min. Following this, the conditioning temperature is raised from 150° C. to 320° C. at 1° C./min and held at 320° C. for 120 min. The extraction capillary is further rinsed with 3 mL of methylene chloride to clean the coated surface and conditioned again from 40 to 320° C. at 4° C. /min. While conditioning, the column is constantly purged with helium at 1 mL/min. The conditioned capillary is then cut into 10 cm long pieces that are further used to perform capillary microextraction.
PAHs, ketones and aldehydes are dissolved in methanol to prepare 0.1 mg/L stock solutions in silanized glass vials. For extraction, fresh samples with ppb level concentrations are prepared by diluting the stock solutions with deionized water.
A gravity-fed sample dispenser for capillary microextraction may be constructed by in-house modification of a Chromaflex AQ column (Kontes Glass Co.) consisting of a thick-walled glass cylinder coaxially placed inside an acrylic jacket. The inner surface of the thick-walled glass column is deactivated by treating with a 5% v/v solution of HMDS in methylene chloride followed by overnight heating at 100° C. The column is then cooled to ambient temperature, thoroughly rinsed with methanol and liberal amounts of deionized water, and dried in a helium flow. The entire Chromaflex AQ column is subsequently reassembled.
To perform capillary microextraction, a previously conditioned sol-gel extraction capillary (10 cm×320 μm i.d.) is vertically connected to the bottom end of the empty sample dispenser. The aqueous sample (50 mL) is then placed in the dispenser from the top, and allowed to flow through the extraction capillary under gravity. While passing through the extraction capillary, the analyte molecules are sorbed by the sol-gel zirconia-PDMDPS coating residing on the inner walls of the capillary. The extraction process is continued for 30-40 min for equilibrium to be established. After this, the microextraction capillary is purged with helium at 25 KPa for 1 min and connected to the top end of a two-way mini-union connecting the microextraction capillary with the inlet end of the GC column. Approximately 6.5 mm of the extraction capillary resides in the connector, and the same length of GC column from other side of the connector. The installation of the capillary is completed by providing a leak-free connection at the bottom end of the GC injection port so that top 9 cm of the extraction capillary remaines inside the injection port. The extracted analytes are then thermally desorbed from the capillary by rapidly raising the temperature of the injector (up to 300° C. starting from 30° C.). The desorption is performed over a 8.2 min period in the splitless mode whereby the released analytes are swept over by the carrier gas into the GC column held at 30° C. during the entire injection process, facilitating effective solute focusing at the column inlet. The split vent remains closed throughout the entire chromatographic run. On completion of the injection process, the column temperature is programmed from 30° C. to 320° C. at rate of 20° C./min. Analyte detection is performed using a flame ionization detector (FID) maintained at 350° C.
Capillary microextraction (in-Tube SPME) uses a stationary phase coating on the inner surface of a capillary to overcome a number of deficiencies inherent in conventional fiber-based SPME such as coating scraping, fiber breakage, and possible sample contamination. In this format, the stationary phase coating is protected by the fused silica tubing against mechanical damage. The in-tube format also provides flexibility and convenience to the microextraction process since the protective polyamide coating on the outer surface of fused silica capillary remains intact. Inner surface-coated capillaries provide a simple way to perform extraction in conjunction with a gravity-fed sample dispenser (
In the present application, a sol solution is used to create the sol-gel zirconia-PDMDPS coating. Zirconium (IV) butoxide (80% solution in 1-butanol) is used as a sol-gel precursor. It serves as a source for the inorganic component of the sol-gel organic-inorganic hybrid coating, and delivers it through hydrolytic polycondensation reactions. It also provides active hydroxyl groups to facilitate its chemical bonding to other sol-gel-active ingredients of the sol solution within the capillary, as well as to the silanol groups of the fused silica surface.
A major obstacle to preparing zirconia-based sol-gel materials using zirconium alkoxides (e.g., zirconium butoxide) is the rapid rate of sol-gel reactions undergone by zirconium alkoxides. Even if the solution of zirconium alkoxide is stirred vigorously, the rate of these reactions is so high that large agglomerated zirconia particles precipitate out immediately when water is added. Such fast precipitation makes the reproducible preparation of zirconia sol-gel materials difficult to achieve. The fast precipitation problem may be addressed by dissolving zirconium propoxide in a non-polar dry solvent like cyclohexane. The hydrolysis is performed by exposure of the coatings prepared from the solution to atmospheric moisture. The hydrolysis rate of zirconium alkoxide can also be controlled by chelating with ligand-exchange reagents like acetic acid and valeric acid. The addition of acetic acid modifies the structure of zirconium alkoxide by replacing the alkyl group by acetyl group, which slow down the reactivity of alkoxide toward water. β-diketones such as acetylacetonate and acetoacetate may also be used as chelating agents leading to the segregation of the β-diketone ligands on the surface of the growing particle, with subsequent particle growth restricted to those sites not occupied by the chelating ligands. Triethanolamine and 1,5-diaminopentane have also been investigated as chelating agents for zirconia sol-gel reactions.
In the present application, the hydrolysis rate of zirconium butoxide was controlled by glacial acetic acid, which served as a chelating agent as well as a source of water released slowly through the esterification with 1-butanol. Silanol-terminated poly (dimethyldiphenylsiloxane) copolymer is used as a sol-gel active organic ligand, which is chemically incorporated into the sol-gel network through polycondensation reactions with hydrolysis products of the zirconium butoxide precursor. This advantageous chemical incorporation of an organic component into the sol-gel network leads to the formation of an organic-inorganic hybrid material system that can be conveniently used for in situ creation of surface coating on a substrate like the inner walls of the fused silica capillary. The organic groups also help to reduce the shrinkage and cracking of the sol-gel coating. Furthermore, sol-gel process can be used to control the porosity and thickness of the coating and to improve its mechanical properties. The poly (methylhydrosiloxane) (PMHS) and 1,1,1,3,3,3-hexamethyldisilazane (HMDS) serve as deactivating reagents to perform derivatization reactions mainly during thermal conditioning of the sol-gel stationary phase following the coating procedure.
One of the most important undertakings in CME is the creation of a surface-bonded stable stationary phase coating on the inner walls of a fused silica capillary. The sol-gel Zirconia-PDMDPS coating presented here is generated via two major reactions: (1) hydrolysis of a sol-gel precursor, zirconium (IV) butoxide, and (2) polycondensation of the precursor and it hydrolysis products. Condensation of these reactive species between themselves and with the other sol-gel-active ingredients in the coating solution, including silanol-terminated PDMDPS, lead to the formation of an organic-inorganic hybrid polymer that are ultimately chemically bonded to the capillary inner walls through condensation with the silanol groups residing on the capillary inner surface.
The hydrolysis of the zirconium (IV) butoxide precursor is represented by the following equation showing the hydrolysis of zirconium (TV) butoxide precursor.
Hydrolytic polycondensation reactions for sol-gel-active reagents are well established in sol-gel chemistry, and constitute the fundamental mechanism in sol-gel synthesis. The condensation between sol-gel-active zirconia and silicon compounds is also well documented. One of the possible routes of polycondensation reactions is represented in the following reaction showing condensation reactions leading to the formation of zirconia based hybrid organic-inorganic material with chemically incorporated poly(dimethyldiphenylsiloxane).
The silanol groups on the inner surface of the fused silica capillary can also undergo condensation reaction forming a chemical anchor between the fused silica capillary surface and the sol-gel polymeric network evolving in the close vicinity of the surface. This is illustrated in the following reaction showing chemical anchoring of sol-gel zirconia-PDMDPS coating onto the fused silica capillary inner walls via condensation reactions.
Strong adsorptive interactions, typical of zirconia surface with polar solutes, can be moderated by surface derivatization and shielding. Like silica-based sol-gel coatings, the surface hydroxyl groups of sol-gel zirconia coating can be derivatized using reactive silicon hydride compounds such as alkyl hydrosilanes and hexamethyldisilazane (HMDS). In this work, a mixture of polymethylhydrosiloxane (PMHS) and hexamethyldisilazane (HMDS) was used for this purpose: the underlying chemical reactions are schematically represented in the following reaction showing deactivation and shielding of sol-gel zirconia-PDMDPS coated surface using PMHS and HMDS.
Such a surface bonded sol-gel polymeric stationary phase can be created by simply filling a fused silica capillary with the properly designed sol solution, and allowing the solution to stay inside the capillary for a short period (e.g., 10-30 min), and subsequently expelling the liquid content of the capillary under an inert gas pressure. Sol-gel zirconia-PDMDPS-coated capillaries allow the extraction of analytes belonging to various chemical classes.
Experimental data illustrating CME-GC of polycyclic aromatic hydrocarbons (PAHs) using a sol-gel zirconia-PDMDPS coated capillary is shown in
CME-GC experiments are performed on an aqueous sample with low ppb level analyte concentrations. Experimental data presented in Table II shows that CME-GC with a sol-gel zirconia-PDMDPS coating provides excellent run-to-run repeatability in solute peak areas (3-7%) and the used GC column provided excellent repeatability in retention times (less than 0.2%).
1) Napnthalene (2) n-Decylaldehyde (3) Undecylic aldehyde (4) Valerophenone (5) Dodacanal (6) Hexanophenon (7) Fluorene (8) Heptanophenone (9) Phenanthrene (10) Pyrene (11) 2,3 Benzanthracene.
In capillary microextraction technique the amount of analyte extracted into the stationary phase depends not only on the polarity and thickness of the stationary phase, but also on the extraction time.
Sol-gel zirconia-based hybrid organic-inorganic stationary phase coating is developed for use in microextraction. Principles of sol-gel chemistry were employed to chemically bind a ydroxyl-terminated silicone polymer (polydimethyldiphenylsiloxane, PDMDPS) to a sol gel zirconia network in the course of its evolution from highly reactive alkoxide precursor undergoing controlled hydrolytic polycondensations reactions. For the first time, sol-gel zirconia-PDMDPS coating is employed in capillary microextraction. The newly developed sol-gel zirconia-PDMDPS coating demonstrated exceptional pH stability and its extraction characteristics remained practically unchanged after rinsing with a 0.1 M solution of NaOH (pH=13) for 24 h. Solventless extraction of analytes may also be carried out by passing the aqueous sample through the sol-gel extraction capillary for approximately 30 min. The extracted analytes are efficiently transferred to a GC column via thermal desorption, and the desorbed analytes are separated by temperature programmed GC. Efficient CME-GC analyses of analytes belonging to various chemical classes are achieved using so-gel Zirconia-PDMDPS capillaries. Parts per trillion (ppt) level detection sensitivities are achieved for polar and nonpolar analytes. Sol-gel zirconia-PDMDPS coated microextraction capillary shows remarkable run-to-run and repeatability and produces peak area RSD values in the range of 1.24-7.25%.
It will be seen that the objects set forth above, and those made apparent from the foregoing description, are efficiently attained and since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matters contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.
It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.
This application is a continuation of U.S. application Ser. No. 11/491,786, filed Jul. 24, 2006, which is a continuation of U.S. application Ser. No. 10/704,766, filed Nov. 10, 2003, now abandoned, which claims the benefit of U.S. Provisional Application Ser. No. 60/319,680, filed Nov. 8, 2002.
| Number | Date | Country | |
|---|---|---|---|
| 60319680 | Nov 2002 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11491786 | Jul 2006 | US |
| Child | 12463156 | US | |
| Parent | 10704766 | Nov 2003 | US |
| Child | 11491786 | US |
