Gas Chromatographic Applications for a High-Performance Cyanopropylphenyl-Substituted Polydimethylsiloxane Column Technology

Many generic stationary phases are available for use in contemporary capillary columns for gas chromatography. These phases are usually based on siloxane or polyethylene glycol chemistries and can be employed in a wide range of chromatographic applications. When these applications involve more reactive analytes or sample matrices, such as highly polar, acidic, or basic compounds, or when extra selectivity is required to achieve better separation, the issues of column inertness, chromatographic performance, and long-term stability become more critical.

An advanced low-bleed, high-performance siloxane-based polymer containing 14% cyanopropylphenyl-substituted polydimethylsiloxane has been developed for use as a stationary phase in gas chromatography. This stationary phase was found to exhibit very good resistance to hydrolysis and a high maximum operating temperature of 280 °C. It is also highly inert and offers bleed of less than 0.5 pA at 250 °C.

The addition of the cyanopropyl functional group provides a higher degree of polarity when compared to that of phenyl-based columns. This results in unique separations for solutes amenable to strong dispersion, dipole–dipole, and proton donor/acceptor interactions such as aromatics, alcohols, aldehydes, chlorinated compounds, and nitrogen-containing compounds. The column is able to produce good separation between vinyl chloride and methanol, with vinyl chloride eluting first; benzene and 1,2-dichloroethane; the successful separation of volatile and semivolatile hydrocarbons; alcohols; oxygenated compounds commonly seen in the biooxidation process; and common impurities/by-products in emulsion polymers. Limitations involve substantially lower chromatographic efficiency when the oven temperature is less than 30 °C, and some observed reactivity toward acids and volatile amines.

The columns are available in 0.25 mm, 0.32 mm, and 0.53 mm internal diameters, with lengths ranging from 15 to 30 m and film thicknesses of up to 1 μm.

Background

The most important parameter in gas chromatography is selectivity.1 Due to the limited choices available for the mobile phase in gas chromatography, the column stationary phase is the main mechanism for influencing selectivity.

Despite its importance, developments in advanced stationary phases have been rather slow.1 One of the reasons is the availability of high-performance gas chromatographic columns such as polydimethylsiloxane (PDMS). PDMS is widely used in GC as a result of its high degree of inertness and high diffusivity, which facilitates fast mass transport. Separation needs, however, cannot be fulfilled solely by this class of column. There is a strong demand for low-bleed, highly inert stationary phases, particularly in the mid- to high-polarity range such as those provided by phenyl or cyano substitution. A major limitation with the conventional cyanophenyl-containing phases is the formation of “ghost” peaks when performing temperature-programmed analyses. Ghost peaks are bleed products produced by the stationary phase that are trapped upon cooling of the oven at the end of a temperature cycle. The formation and intensity of the ghost peaks are a function of the absolute bleed rate of the polymer.

Through a proprietary polymer synthesis and deactivation process, a new generation of cyanopropylphenyl stationary phases, which claim to eliminate many of the deficiencies often encountered in their predecessors, has recently been developed and commercialized. The VF-1701ms™ stationary phase (Varian Inc., Middelburg, The Netherlands) contains 14% cyanopropylphenyl and 86% PDMS with thermal stability up to 280 °C. A number of columns with different dimensions were provided to the authors’ team for application developments.

The performance of this class of column was evaluated. GC applications of industrial significance were also investigated to illustrate the utility of the column technology.

Experimental

An Agilent HP-6890N gas chromatograph (Agilent Technologies, Wilmington, DE) equipped with two split/splitless injectors, a flame ionization detector (FID), and a Dielectric Barrier Detector (Advanced Industrial Chemistry Corp., Albuquerque, NM) was used. Structural elucidation was conducted using an Agilent HP-5890 Series II gas chromatograph coupled to an HP-5972B mass selective detector. An LTM™ A-68 low-thermal-mass module (RVM Scientific, Santa Barbara, CA) was also utilized for application development.

Columns used in the evaluation included: 1) 30-m, 0.25-mm-i.d., 1-μm VF-1701ms; 2) 30-m, 0.32-mm-i.d., 1-μm VF-1701ms; 3) 5-m, 0.25-mm-i.d., 0.25-μm VF-1701ms; and 4) 30-m, 0.53-mm-i.d., 1-μm VF-1701ms. The gas chromatographic conditions are explained in the following sections.

Benzene, toluene, ethyl benzene, xylenes (BTEX), and purgeable and extractable chlorinated compounds

  • Split/splitless injector in split mode with split ratio of 15:1 and 1-μL injection
  • Column: 30-m, 0.25-mm-i.d., 1-μm VF-1701ms
  • Carrier gas: helium, with average linear velocity of 45 cm/sec
  • Oven temperature profile: 40 °C for 3 min, 15 °C/min–250 °C for 15 min
  • Detector: Agilent HP-5972B mass selective detector in SCAN mode from 15 to 350 amus.

Alcohols and oxygenated compounds in water

  • Split/splitless injector in split mode with split ratio of 15:1 and 1 mL headspace injection (15-mL sample into 20-mL headspace vial at 70 °C for 15 min)
  • Column: 30-m, 0.25-mm-i.d., 1-μm VF-1701ms
  • Carrier gas: hydrogen, with average linear velocity of 60 cm/sec
  • Oven temperature profile: 40 °C for 1 min, 15 °C/min–250 °C for 15 min
  • Detector: FID at 250 °C, hydrogen: 45 mL/min, air: 450 mL/min, and nitrogen as auxiliary at 30 mL/min.

Amines and alkanolamines

  • Split/splitless injector in split mode with split ratio of 15:1 and 1-μL injection with an Agilent HP-7683B autoinjector 
  • Column: 30-m, 0.25-mm-i.d., 1 μm VF-1701ms
  • Carrier gas: hydrogen, with average linear velocity of 60 cm/sec
  • Oven temperature profile: 40 °C for 1 min, 30 °C/min–250 °C for 15 min
  • Detector: FID at 250 °C, hydrogen: 45 mL/min, air: 450 mL/min, and nitrogen as auxiliary at 30 mL/min.

Standards used for the evaluation were prepared from chemicals obtained from Aldrich Chemicals (Oakville, Ontario, Canada) and Supelco Canada (Oakville, Ontario, Canada), or were produced at the local manufacturing site at Fort Saskatchewan, Alberta, Canada.