Titania-Based Coating for Capillary Microextraction

Abstract

A method is presented describing in situ preparation of the titania-based sol-gel PDMS coating and its immobilization on the inner surface of a fused silica microextraction capillary. Sol-gel titania-poly (dimethylsiloxane) (TiO 2 -PDMS) coating was developed for capillary microextraction (CME) to perform on-line preconcentration and HPLC analysis of trace impurities in aqueous samples. The sol-gel titania-based coatings demonstrated strong pH stability and enhanced extraction capability over other commercially available GC coatings. Extraction characteristics of a sol-gel titania-PDMS capillary remained practically unchanged after continuous rinsing with a 0.1 M NaOH solution (pH=13) for 12 hours.

1 . A method of making a sol-gel titania-based coating comprising the steps of: mixing two or more suitable sol-gel precursors to form a sol-gel solution wherein a first of the two or more sol-gel precursors is titanium (IV) isopropoxide; hydrolyzing the sol-gel solution to form hydrolyzed products; polycondensating the hydrolyzed products to form a sol-gel network; incorporating chemically the remainder of the two or more sol-gel precursors in the sol-gel network; and surface bonding the sol-gel network to a substrate to form a surface bonded sol-gel coating thereon. 
   
   
       2 . The method according to  claim 1  wherein a second of the two or more sol-gel precursors is polydimethylsiloxane (PDMS). 
   
   
       3 . The method according to  claim 1  further comprising the step of deactivating residual silanol groups on the sol-gel coating with a deactivating agent. 
   
   
       4 . The method according to  claim 3  wherein the deactivating reagents are hexamethyldisilazane (HMDS) and polymethylhydrosiloxane (PMHS). 
   
   
       5 . The method according to  claim 4  wherein the ratio of HMDS to PMHS is about 4:1 (v:v). 
   
   
       6 . The method according to  claim 1  further comprising the step of thermal conditioning the coated substrate in an oven using temperature-programmed heating wherein the heat increments upward from about 40° C. to about 320° C. at an increment of about 1° C./minute followed by a holding at about 320° C. for about 3 hours. 
   
   
       7 . The method according to  claim 1  wherein the mixing step further includes the addition of a chelating agent to decelerate the sol-gel formation. 
   
   
       8 . The method according to  claim 7  wherein the chelating agent is selected from the group consisting of acetic acid, trifluoroacetic acid and metal beta-diketonates. 
   
   
       9 . The method according to  claim 8 , wherein the chelating agent is trifluoroacetic acid. 
   
   
       10 . A method of making a titania-based sol-gel coated capillary for microextraction of analytes in a sample medium comprising the steps of: preparing a sol solution comprising titanium (IV) isopropoxide; processing the sol solution to form a sol-gel extraction medium; filling a capillary with the sol-gel extraction medium wherein the sol-gel extraction medium chemically binds to the inner walls of the capillary to form a titania-based sol-gel coated capillary; and purging the capillary of unbound sol-gel extraction medium. 
   
   
       11 . The method according to  claim 10  wherein the sol solution further comprises PDMS as a sol-gel precursor. 
   
   
       12 . The method according to  claim 10  wherein the capillary remains filled with the sol-gel extraction media for at least about 15 minutes to facilitate the formation of a surface bonded sol-gel coating before the unbound sol-gel extraction medium is purged. 
   
   
       13 . The method according to  claim 10  wherein the step of purging the capillary of unbound sol-gel extraction medium is performed by applying helium pressure of about 20 psi for at least about 30 minutes. 
   
   
       14 . The method according to  claim 10  further comprising the step of conditioning the titanic-based sol-gel coated capillary in an oven using temperature-programmed heating wherein the heat increments upward from about 40° C. to about 320° C. at an increment of about 1° C./minute followed by a holding at about 320° C. for about 3 hours. 
   
   
       15 . The method according to  claim 14  wherein the thermal conditioning further includes; cooling the titania-based sol-gel coated capillary to room temperature following the 3 hour holding; rinsing the capillary with methylene chloride and methanol; and reheating using temperature-programmed heating wherein the heat increments upward from about 40° C. to about 320° C. at an increment of about 1° C./minute followed by a holding at about 320° C. for about 30 minutes.