Crosslinked versus Chemically Bonded Phases

The practice of capillary GC has been enriched by the advances made in the immo- bilization of a thin film of a viscous stationary phase coated uniformly on the inner wall of fused-silica tubing. At present, two pathways are employed for the immo- bilization of a stationary phase: free-radical crosslinking and chemical bonding.

By immobilizing a stationary phase by either approach, the film is stabilized and is not disrupted at elevated column temperatures or during temperature programming.

Thus, less column bleed and higher operating temperatures can be expected with a phase of this nature, a consideration especially important in GCMS. A column containing an immobilized stationary phase is also recommended for on-column injection and large volume injectors/cool on-column inlets where large aliquots of solvent are injected without dissolution of the stationary phase. Likewise, a column having an immobilized phase can be backflushed to rinse contamination from the column without disturbing the stationary phase (Section 3.11.7).

3.11.5.1 Crosslinking of a Stationary Phase

The ability of a polymer to cross-link is highly dependent on its structure. The overall effect of crosslinking is that the molecular weight of the polymer steadily increases with the degree of crosslinking, leading to branched chains until eventually a three-dimensional rigid network is formed. Since the resultant polymer is rigid, little opportunity exists for the polymer chains to slide past one another, thereby increasing the viscosity of the polymer. On treatment of a crosslinked polymer with solvent, the polymer does not dissolve, but rather a swollen gel remains behind after decantation of the solvent. Under the same conditions, an uncrosslinked polymer of the same structure would dissolve completely. In summary, the dimensional stability, viscosity, and solvent resistance of a polymer are increased as a result of crosslinking. The mechanism for crosslinking 100% dimethyl polysiloxane is described below where R^žandγ(gamma radiation) are free-radical initiators:

CH₃ Si CH3

O

CH₃ Si CH2

O

•

CH₃ Si CH2

O

CH2

Si O

CH₃

→

CH₃ Si CH₃

O

CH₃ Si CH₂

O

•

R + or →

γ

•

+ 2RH or H2

The sequence of photographs presented in Figure 3.40 permits a helpful visualization of the crosslinking process for several stationary phases, 100%

dimethylpolysiloxane (SE-30), and trifluoropropylmethylpolysiloxane, as a function of increasing degree of crosslinking by ⁶⁰Co gamma radiation (114).

In Figures 3.40c,d the conversion of DC200 (a silicone oil) and OV-101 (a polysiloxane fluid) to crosslinked gel versions as a function of radiation dosage is illustrated.

Madani et al. provided the first detailed description of capillary columns where polysiloxanes were immobilized by hydrolysis of dimethyl and diphenylchlorosi- lanes (115,116). Interest increased when Grob found that the formation of cross- linked polysiloxanes resulted in enhanced film stability (117). Blomberg et al.

illustrated in situ synthesis of polysiloxanes with silicon tetrachloride as a precur- sor, followed by polysiloxane solution (118,119). Since then, various approaches for crosslinking have been investigated. These include chemical additives such as organic peroxides (120–128), azo compounds (82,129,130), ozone (131), and gamma radiation (77,132–135). Several different peroxides have been evaluated;

dicumyl peroxide is the most popular. However, peroxides can generate polar decomposition products that remain in the immobilized film of stationary phase.

Moreover, oxidation may also occur, which increases the polarity and decreases the thermal stability of a column. These adverse effects are eliminated with azo species as free radical initiators. Lee et al. have crosslinked a wide range of stationary phases, from nonpolar to polar, in their studies using azo-tert-butane

0.25 0.50 1.0 1.5 MRAD

SE – 30, NEAT

(a)

FIGURE 3.40 Effect of gamma radiation on degree of crosslinking of (a) SE-30 (polydimethylsiloxane; (b) OV-215 [trifluoropropylmethylpolysiloxane (Reference 133)];

(c) DC 200 (a silicone oil); (d) conversion of OV-101 (a polydimethylpolysiloxane fluid) to a gum similar to OV-1. [Parts (a) and (b) reproduced from the Journal of Chromatographic Science by permission of Preston Publications, a Division of Preston Industries, Inc. and reprinted with permission of Preston Publications, Inc.]

OV – 215, NEAT

1 5 10 50 MRAD

(b)

(c)

FIGURE 3.40 (Continued )

(d )

FIGURE 3.40 (Continued )

and other azo species (82,129,130). If an azo compound or a peroxide exists as a solid at room temperature, the agent is spiked directly into the solution of the stationary phase used for coating of the column. On the other hand, for free- radical initiators that are liquid at ambient temperature, the column is first coated with stationary phase, then saturated with vapors of the reagent (47,82).

Gamma radiation from a⁶⁰Co source has also been used (77,114,132–135) as an effective technique for crosslinking polysiloxanes. In a comparative study of gamma radiation with peroxides, Schomburg et al. (77) noted that each approach immobilized polysiloxanes, but that the formation of polar decomposition prod- ucts is avoided with radiation. Radiation offers the additional advantages of the crosslinking reaction occurring at room temperature, and columns can be tested both before and after immobilization of the stationary phase.

Not all polysiloxanes can be directly or readily crosslinked. The presence of methyl groups facilitates crosslinking. Consequently, the nonpolar siloxanes exhibit very high efficiencies and high thermal stability. However, as the popu- lation of methyl groups on a polysiloxane phase decreases and as these groups are replaced by phenyl or more polar functionalities, crosslinking of a polymer becomes more difficult. Incorporation of vinyl or tolyl groups into the synthesis of a polymer tailored for use as a stationary phase for capillary GC overcomes this problem. Lee (126,129) and Blomberg (127,128) have successfully synthesized and crosslinked stationary phases of high phenyl and high cyanopropyl con- tent that also contain varying amounts of these free-radical initiators. Colloidal particles have also been utilized to stabilize cyanoalkyl stationary-phase films

for capillary GC (136). More recently, favorable thermal stability and column inertness were obtained by a binary crosslinking reagent, a mixture of dicumyl peroxide and tetra(methylvinyl)cyclotetrasiloxane (137).

Developments in the crosslinking of polyethylene glycols have been slower in forthcoming, although successes have been reported. Immobilization of this phase by the following procedures increases its thermal stability and its compatibility and tolerance for aqueous solutions. DeNijs and de Zeeuw (138) and Buijten et al. (139) immobilized a PEG in situ, the latter group with dicumyl peroxide and methyl(vinyl) cyclopentasiloxane as additives. Etler and Vigh (140) used a com- bination of gamma radiation with organic peroxides to achieve immobilization of this polymer, while Bystricky selected a 40% solution of dicumyl peroxide (141).

George (7) and Hubball (142) have successfully crosslinked PEG using radia- tion; in Figure 3.41 an array of vials of Carbowax 20M after receiving various dosages of gamma radiation is pictured, indicating that crosslinking has been achieved (vial D). The chromatographic separation of a cologne in Figure 3.42 was generated on a capillary column containing Carbowax 20M crosslinked by gamma radiation and indicates acceptable thermal stability to 280^◦C. Horka and colleagues described a procedure for crosslinking Carbowax 20M with pluriiso- cyanate reagents (143). Thermally bondable PEGs and polyethyleneimines have been popular phases and yield chromatographic selectivity similar to those of the traditional PEGs. Despite these efforts, the upper temperature limit of PEG columns generally remains below 300^◦C.

3.11.5.2 Chemical Bonding

Since the early nineties, column manufactures have devoted extensive resources to acquiring technology for the development of chemically bonding a stationary-phase

FIGURE 3.41 Effect of gamma radiation on degree of crosslinking of Carbowax 20M.

0 5 10 15 20 25 30 35 40 45 TIME (minutes)

FIGURE 3.42 Chromatogram illustrating the separation of a cologne sample on a capil- lary column (15 m×0.32-mm-i.d., 0.25-µm film) containing Carbowax 20M crosslinked by gamma radiation; column conditions: 40^◦C (2 min) at 6^◦C/min to 280^◦C. Det: FID, 25 cm/s He.

film to the inner wall of a fused-silica capillary. As the term suggests, an actual chemical bond is formed between fused silica and the stationary phase. The foun- dation of this procedure was first reported by Lipsky and McMurray (144) in their investigation of hydroxy-terminated polymethylsilicones and was later refined by the work of Blum et al. (145–149), who employed OH-terminated phases for the preparation of inert, high-temperature stationary phases of varying polarities. The performance of hydroxy-terminated phases has also been evaluated by Schmid and Mueller (150) and Welsch and Teichmann (151). Other published studies include the behavior of hydroxyl phases of high cyanopropyl content by David et al. (152) and trifluoropropylmethylpolysiloxane phases by Aichholz (153).

In the chemical bonding approach to stationary phase immobilization, a cap- illary column is coated in the conventional fashion with an OH-terminated poly- siloxane and then temperature-programmed to an elevated temperature, during which time a condensation reaction occurs between the surface silanols residing on the fused-silica surface and those of the phase. It is important to note here that both deactivation and coating are accomplished in a single-step process and result in the formation of a Si–O–Si bond more thermally stabile than the Si–C–C–Si bond created via crosslinking. Crosslinking of the stationary phase is not a nec- essary requirement. However, if a stationary phase contains a vinyl group (or another free-radical initiator), crosslinking can occur simultaneously. Phases that cannot be crosslinked during the bonding process can be crosslinked afterward with an azo compound, for example. Grob, after observing the increased inertness and thermostability of OH-terminated phases, commented that they might reflect a “revolution in column technology” (154).

Since the mid-1980s, immobilization of polysiloxanes and polyethylene gly- cols has no longer been a subject of rapid advancements reported in the literature;

a procedural blend of polymeric synthesis, crosslinking, and/or chemically bond- ing is utilized by column manufacturers today, as the fixation of these stationary phases via crosslinking and/or chemical bonding for capillary GC is now a well-defined and a matured technology. A capillary column containing such a stationary phase is the resultant of elegant pioneering efforts of people such as

M. L. Lee, L. Blomberg, K. Grob and his family members, G. Schomburg and their colleagues, and many, many others too numerous to mention here. Atten- tion has now shifted to such areas as silphenylene(arylene) phases for GCMS, MS-grade phases, stationary-phase selectivity tuned or optimized for specific applications, multidimensional chromatographic techniques, and immobilization of chiral stationary phases, which are discussed in Section 3.11.6.

3.11.5.3 MS-Grade Phases versus Polysilarylene or Polysilphenylene Phases

Many column manufacturers offer what has become known as “MS columns,”

namely, columns that generate lower bleed than do regular or conventional polysiloxane equivalents. Lower bleed is highly desirable in GCMS analyses because complications such as bleed ions, misidentifications of compounds and errors in quantitation, to name few, are avoided. A MS-grade column may con- tain (1) a higher-molecular-weight polymer obtained by a fractional procedure of the corresponding starting stationary-phase version; (2) a polymer result- ing from a crosslinking of a higher-molecular-weight fraction of the starting polymer; (3) a “crossbonded” polymer resulting from a condensation reaction of a hydroxyl-terminated polymer of either a conventional polysiloxane or a higher-molecular-weight fraction with fused silica where crosslinking may or may not have occurred, as discussed in the previous section; or (4) the sta- tionary phase may be a polysilarylene-siloxane, often referred to as an arylene phase, or as a polysilphenyl-siloxane, often referred to as a phenylene phase.

These latter phases are inherently more thermally stable because of the pres- ence of aromatic rings in the polymer chain as depicted in Figure 3.43 and represent an upgrade in thermal performance over the corresponding polysilox- ane counterpart, but one may also notice slight differences in selectivity due to different chemistry employed both in the deactivation procedure and also in a possible synthesis of the phase itself. Thus, cross-reference column charts should be carefully examined when comparing columns offered by various ven- dors when selecting a capillary column for an application requiring low column bleed level.

FIGURE 3.43 Structures of 5% phenyl – 95% methylpolysiloxane and 5% phenylpoly- silphenylene– siloxane as stationary phase.

3.11.5.4 Solgel Stationary Phases

Sol-gel is basically a synthetic glass with ceramic-like properties. The process- ing consists of hydrolysis and condensation of a metal alkoxide, specifically, tetraethoxysilane to form a glassy material at room temperature. Further modifi- cation of this material with a polymer (stationary phase) is used to prepare phases for capillary columns; there has been keen interest in this process (155–160). The final sol-gel product retains the properties of the polymer as well as properties of the sol-gel component. The sol-gel material is able to covalently bond to fused silica, yielding a strong bond, which means better thermal stability and less column bleed. In addition, the molecular weight of the stationary phase is stabilized via end-capping chemistry, providing protection from degradation and potential further condensation. At the present time, two sol-gel phases have been developed, a SolGel-1 ms derived from 100% dimethylpolysiloxane and the other, SOLGEL-WAX, which has PEG in the matrix. A cross-sectional view of a SOLGEL-WAX column along with a corresponding view of a conventionally coated capillary is presented in Figure 3.44; an application of a separation with this type of column appears in Figure 3.45.

FIGURE 3.44 Comparison of cross section of a SOLGEL-WAX Column with a con- ventional wax-type column. (Reproduced with permission of SGE International Pty.)

FIGURE 3.45 Separation of industrial solvents on a SOLGEL-WAX column. Column:

30 m×0.32-mm-i.d., 0.5-µm film: Temperature conditions: 35^◦C (3 min) at 15^◦C/min to 230^◦C. Det: FID, 1.84 mL/min, 30 cm/s He, split injection (83 – 1) 240^◦C (Reproduced with permission of SGE International Pty.)

3.11.5.5 Phenylpolycarborane-Siloxane Phases

This classification of phases can be traced back to the previous use of a carborane- type phase termed Dexsil, which was widely utilized as a stationary phase in packed columns for high-temperature separations because of its excellent thermal stability. The carborane network has been incorporated into the backbone of phenylpolysiloxane phases having either a 5% or 8% phenyl content, providing unique selectivity for selected applications. In Figure 3.46, the structure of this modified polysiloxane is presented. A capillary column containing this stationary phase exhibits high selectivity for difficult-to-separate Aroclor 1242 congeners because of the carborane functionality in the polysiloxane polymer. An example of this is shown in Figure 3.47, where the carborane phase interacts preferentially with ortho-substituted PCB congeners, namely, congeners 28 and 31.

FIGURE 3.46 Structure of a phenylpolycarborane– siloxane stationary phase.

FIGURE 3.47 Chromatographic separation of Aroclor 1242 on a capillary column con- taining HT8 phenyl polycarborane– siloxane as stationary phase; column: 50 m×0.22- mm-i.d., 0.25-µm film; temperature conditions: 80^◦C (2 min) at 30^◦C/min to 170^◦C, then 3^◦C/min to 300^◦C. Det: ECD, 1.84 mL/min, 40 psi He, split injection. (Reproduced with permission of SGE International Pty.)

3.11.6 Specialty Columns