Article pubs.acs.org/ac
Near- and Supercritical Water as a Diameter Manipulation and Surface Roughening Agent in Fused Silica Capillaries Pavel Karásek, Josef Planeta, and Michal Roth* Institute of Analytical Chemistry of the ASCR, v. v. i., Veveří 97, 60200 Brno, Czech Republic ABSTRACT: The prospects of near- and supercritical water for treatment of the inner surfaces of fused silica capillaries have been tested employing an in-labassembled apparatus. Unlike all other agents used for the purpose, water cannot introduce any undesirable heteroatoms to the treated surface. Theoretical background for this work comes from the well-known fact that water near its critical point can solubilize silica. The results show that depending on the temperature, water flow rate, flow mode, and exposure time, high-temperature water has wide-ranging effects on both the surface roughness and the internal diameter profile along the length of the treated capillary. By judicious selection of the operating conditions, tapered capillaries of various profiles for applications in electromigration techniques can be prepared with relatively high reproducibility. The water-treated fused silica capillaries with uniform internal diameter appear to be useful for preparation of monolithic silica capillary columns.
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water,14 SCW is a potentially dangerous and difficult-to-handle medium. However, unlike other silica surface treatment agents employed before and mentioned above, SCW has a significant virtue in that it cannot pollute the treated surface with residual heteroatoms that may be difficult to remove or highly undesirable in the intended use of the treated FSC. Moreover, the properties of SCW as a solvent and/or reaction medium can be tuned in wide limits through changes in operating temperature and pressure, thus making SCW a highly adaptable medium.15,16 In addition, because of the ability of SCW to dissolve fused silica, SCW also appears to be potentially useful for reproducible preparation of tapered FSCs. Tapered FSCs have some potential applications, e.g., in electromigration techniques,17,18 that have not yet been properly verified because of the lack of suitable methods for versatile production of tapered FSCs. To illustrate the potential of SCW as a green agent for FSC diameter manipulation and/or surface treatment, this contribution presents the results obtained from three different modes of SCW flow through the FSC.
ince their introduction as column material for capillary gas chromatography,1 fused-silica capillaries (FSCs) have become reliable workhorses in both chromatographic and electromigration techniques of analytical separations. Depending on the particular application, it is almost always necessary to modify the chemistry and/or roughness of the inner surface or, sometimes, to turn a cylindrical FSC into a tapered one. Several reviews are available of the large field of FSC surface modification and/or roughening procedures;2−4 during the last two decades, the majority of FSC surface treatment applications have shifted from chromatographic to electromigration methods. To name but a few agents used for treatment of the inner surfaces of FSCs, one should mention, e.g., microcrystalline NaCl layer to facilitate coating FSCs with ionic liquids,5 ammonium hydrogen difluoride for FSC applications in electrochromatography,6 application of Grignard agents to chlorinated surface silanols to form Si−C bonds on the inner surface of FSC,7 or 2-chloro-1,1,2-trifluoroethyl methyl ether used to prepare nanowires inside narrow-bore FSCs,8,9 apart from the age-long application of concentrated aqueous solution of sodium hydroxide to etch the glass or silica surfaces. The latter procedure has also been used in modifications involving the use of potassium hydroxide instead of NaOH and the use of methanol as a solvent instead of water.10 Among the prospective compounds for FSC surface treatment, there is a highly powerful and, to our knowledge, as yet unexplored agent, namely, near- and supercritical water (SCW). SCW is known to dissolve both quartz11 and amorphous silica,12 and high-temperature, high-pressure aqueous solutions of SiO2 play an important role in geochemical processes.13 Because of the high critical temperature (373.946 °C) and critical pressure (220.64 bar) of © 2012 American Chemical Society
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EXPERIMENTAL SECTION Apparatus for SCW Treatment of Fused Silica Capillaries. Figure 1 shows both the schematic diagram (a) and photograph (b) of the apparatus. The essential components of the setup included two JASCO PU-980 HPLC Pumps (Jasco International Co., Tokyo, Japan), a Received: October 1, 2012 Accepted: December 3, 2012 Published: December 3, 2012 327
dx.doi.org/10.1021/ac302849q | Anal. Chem. 2013, 85, 327−333
Analytical Chemistry
Article
prevent the polyimide coating of FSC from oxidative degradation by atmospheric oxygen at high operating temperatures. Each thermostated metallic block had six heaters (120 V, 150 W, Easytherm.cz, Polička, Czech Republic) and two temperature sensors (Pt100, JUMO s.r.o., Brno, Czech Republic) combined with a temperature control unit (dTRON316, JUMO s.r.o.) for uniform heating and precise control of temperature (±0.1 °C, up to 500 °C). The vertically positioned cell mentioned above was intended for future applications of the apparatus in SCW treatment of larger objects. In the current application to FSCs, the cell served as a large reservoir of thermally equilibrated SCW upstream of the capillary to be treated. The temperature and pressure ranges of the applications described below were 250−420 °C and 300− 500 bar, respectively, with a large number of applications having been carried out at 400 °C and 400 bar. Materials. Water was purified with a reverse osmosis system Ultra Clear UV (SG Wasseraufbereitung and Regenerierstation GmbH, Barsbüttel, Germany). Immediately before use, water was thoroughly stripped with helium (Linde Gas a.s., Prague, Czech Republic, purity 4.8) to remove the dissolved oxygen and carbon dioxide. During the experiments, the oxygen content was monitored and kept below 0.1 mg/L. Undeactivated, polyimide-coated FSCs of various dimensions (100 μm i.d., 360 μm o.d., Part No. 160-2634-10, Agilent Technologies, Waldbronn, Germany and 500 μm i.d., Polymicro Technologies, Phoenix, USA) were used as received and cut to pieces of desired lengths. For the initial test experiments with SCW action on outer surfaces of FSCs, the capillaries were mechanically stripped off the protective polyimide coating. Optical Microscopy. The evaluation of diameter changes in the SCW-treated FSCs was carried out with a BX-51 optical microscope (Olympus, Prague, Czech Republic) configured to provide images in either transmitted or reflected light and equipped with image processing software (Quick Photo Camera 2.3 + Deep Focus 3.1). Safety Considerations. Supercritical water is a hightemperature, high-pressure, and highly compressible medium. The apparatus should be covered with shatterproof panels to protect the operator against accidental release of overheated steam. Protective clothing and eye protection are absolutely necessary. Regular checking and/or replacement of all gaskets and ferrules is strongly recommended.
Figure 1. Schematic diagram (a) and photograph (b) of the apparatus for SCW applications: (1) water reservoir, (2) modifier/organic reservoir (for future applications), (3) helium tank, (4) high-pressure pumps, (5) pressure sensor, (6) preheater coil, (7) extraction cell housing, (8) fused-silica capillary, (9) in-cell etching output (for future applications), (10) liquid cooling system, (11) fused-silica restrictor, (12) sample/waste collection vial, (13) PC control units and pressure indication, (V1) main control valve, (V2) output control valve, (V3) discharge valve (dynamic mode), and (V4) PC-controlled quick valve (semidynamic mode).
water preheater unit, a vertically positioned cylindrical cell (2 different sizes, 110 mm long ×10 or 20 mm i.d.) mounted in a thoroughly insulated heating/thermostating jacket made of a massive circular aluminum block, another special thermostated unit designed to house any length of FSC above 25 cm, a 250cm length of 1/16 in. i.d. stainless-steel tubing immersible into a cryostat to cool the aqueous effluent, and stainless-steel needle valves (SSI, Supelco Part. No. 58789, Supelco, Bellefonte, PA) for the system outlet controls and purge nitrogen gas flow control. The cell was rated for a maximum operating pressure of 750 bar, and it was made from the corrosion-resistant alloy Inconel 625. One of the two pumps was intended to deliver additional fluid (e.g., aqueous H2O2, acids, bases) in future applications of the setup. The water pump could be operated in a constant flow rate mode or in a constant pressure mode; in the latter case, the water flow rate could be adjusted by the outlet needle valve or, more accurately, by the FS restrictor (25 to 75 μm i.d.). The thermostat for FSC was made of two massive aluminum blocks adhering closely to each other for efficient transfer of heat. One of the blocks had both straight and circle-shaped grooves in it to house the FSC to be treated. If necessary, a gentle stream of preheated nitrogen gas could be passed through the grooves to
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RESULTS AND DISCUSSION Outer Surfaces of FSCs. Prior to the experiments with inner surfaces of FSCs, the initial tests of the apparatus were carried out with outer surfaces of FSCs. To this purpose, 7 cm long pieces of 500 μm i.d. fused silica capillary were mechanically stripped off the polyimide coating with a razor blade and exposed to near- or supercritical water in the cylindrical cell of the apparatus for a constant time period of 20 min at temperatures of 250−400 °C. To keep the water liquid at subcritical temperatures, a constant pressure of 300 bar was employed. To illustrate the effect of operating temperature, Figure 2 shows the transversal images of the exposed FSCs in transmitted light (magnification 50×) as well as details of the outer surfaces of the FSCs (magnification 500×). With the rising temperature, the FSC surface becomes progressively more coarse-grained indicating the increasing ability of water to corrode the silica surface. Although the experiments with outer surfaces were not pursued any further in this work, the results 328
dx.doi.org/10.1021/ac302849q | Anal. Chem. 2013, 85, 327−333
Analytical Chemistry
Article
ratio is important as, together with the operating pressure around 400 bar, it provides for a flash replacement of the SCW inside the FSC without any apparent decrease in the system pressure, thus enabling a virtually isobaric operation and a constant driving force for the SCW replacement. During the 0.1 s period when the valve is open, the amount of SCW passing through the system is 40 μL, corresponding to about five typical volumes of the treated FSC. The flash replacement of SCW inside the FSC was usually repeated 10−50 times. Figure 3a
Figure 2. Effect of static exposure to subcritical water (
