High‐temperature silylation in the production of inert glass capillary columns. From the first experiments to the chemical mechanism

Abstract

SummaryThis retrospective article describes the beginnings and the development of silylation methods with diverse silazanes and siloxanes at high temperatures, and stresses their role in producing modern capillary columns characterized by a high degree of thermostability and adsorptive and catalytic inertness. Particular emphasis i s placed on the outstanding contributions to this field of Kurt Grob, whose persilylation techniques provided the basis for the expansion of high resolution capillary gas chromatography. The BeginningsWhen we became engaged in the production of glass capillary columns at the chromatography laboratory of the Leipzig University Chemistry Department 18 years ago this meant making a virtue out of necessity: complex isomeric mixtures of different polarities had to be separated and identified. It was therefore necessary to use capillary columns and the Kovats retention index, although neither suitable gas chromatographs nor capillary columns were commercially available. Thus we set about building ourown device. Thanks to the generous support of our colleagues DoringandEstel, ofVEB Leuna-Werke, GDR, who had built a glass-drawing machine after Desty [l] and were quite experienced in "carbonization", introduced by Grob [2] as a method for pretreating capillary columns, we succeeded in separating hydrocarbon mixtures on home-made capillary columns after a few months of work. Soon other pretreatments were tried. Yet the reproducibility of the retention indices obtained on glass capillary columns pretreated with different methods was poor even for aromatics or weakly polar compounds, and the influence of adsorption on the active support produced for stable film formation became obvious. Grob later commented on this in a review [3] entitled the role of column technology in capillary gas chromatography:"During the first 15 years (1962)(1963)(1964)(1965)(1966)(1967)(1968)(1969)(1970)(1971)(1972)(1973)(1974)(1975)(1976)(1977) of capillary column work our glass columns suffered from excessive activity. This was demonstrated, for instance, by the behavior of classical stationary phases, such as SE-30. The pure phase could be heated to 4OOOC with very modest degradation. In packed columnsit could be used up to 300-350°C. As a coating in capillary columns, however, it showed severe bleeding when heated beyond 25OOC. This is a result of destabilization by the active support surface."While deactivation procedures involving blocking of the surface silanol groups by an organic salt [4-61 or polyethylene glycol phases [7-101 improved the situation they did not result in a decisive breakthrough.What prevented thecapillarycolumnfrom being used more commonly in GC was pointed out by Grob very clearly as early as 1974 [ll]. The summary of this article read as follows:"The paper discusses various requirements which must be fulfilled by good quality, high resolution GC-columns. These requirements are: low adsorption on the solid support, stability of retention and polarity, thermal stability, controlled acidity, ...

Polymer-Coated Fibrous Stationary Phases in Packed-Capillary Gas Chromatography

“…1,2 Various deactivation and polymer-coating processes have been also proposed for the preparation of GC columns with good separation performance and stability. [3][4][5][6] In contrast to the successful applications and subsequent commercialization of these polymer-coated columns, such as wall-coated, support-coated and pellicular-coated, the reports for the fibrous polymer-packed stationary phases, however, have been limited, except for characterizing the surface of the fibers in inverse GC (IGC). 7,8 In general, the surface characteristics of the fiber have been analyzed with IGC technique by injecting the standard samples into a fiber-packed column and measuring the elution behavior.…”

The Column in Gas Chromatography

“…The most widely used methods include coating with the thermal degradation products of stationary phases [192][193][194], high temperature silylation [195] and reaction with silicon hydride polysiloxanes [196,197]. No universal method of deactivation exists, but some techniques have emerged as more useful than others.…”

“…High temperature silylation with disilazanes, disiloxanes, or cyclosiloxanes, thermal degradation of polysiloxanes, and the dehydrocondensation of silicon hydride polysiloxanes are now established as the most effective procedures for masking silanol groups on deionized glass and fused silica surfaces for subsequent coating with nonpolar and moderately polar polysiloxane stationary phases [140,146,[195][196][197][198]. Some typical reagents are shown in Figure 2.6 in which the R group is varied to change the wettability of the glass surface and is generally the same as the substituent attached to the polysiloxane backbone of the stationary phase [199][200][201][202][203].…”

Columns: Packed and Capillary; Column Selection in Gas Chromatography

“…A variety of agents and procedures have been explored for deactivation purposes (60)(61)(62)(63)(64)(65)(66)(67)(68)(69)(70)(71)(72)(73)(74). For subsequent coating with nonpolar and moderately polar stationary phases such as polysiloxanes, fused silica has been deactivated by silylation at elevated temperatures, thermal degradation of polysiloxanes and polyethylene glycols, and the dehydrocondensation of silicon hydride polysiloxanes (71,(75)(76)(77)(78)(79).…”