30
August/September 2011
Ionic Liquids:
Sensitivity Enhancement in Headspace Gas Chromatography
Emma Hollender and Edward W. Hammersley* Research Analytics, Pfizer Global Research and Development, Sandwich, Kent, CT13 9NJ *Corresponding author Email:
edward.hammersley@pfizer.com
A relatively new application for ionic liquids, as solvents in static Headspace Gas Chromatography, is further investigated. Two ionic liquids were studied and the response towards a range of low boiling point residual solvents increased compared to when dimethyl acetamide was used as the headspace diluent. An attempt to correlate results with a suitable solvent model was made. The results highlight the potential of ionic liquids to enhance headspace sensitivity for residual solvent analysis.
Ionic liquids provide a wide spectrum of functions for chemical applications including: MALDI mass spectrometry matrices for the analysis of large biomoleculesi solvents’ii for paintiii
, ‘Green
and even performance additives . Gas Chromatography is a
relatively recent application for ionic liquids, in which novel ionic liquid stationary phases for capillary columns have been investigatediv
.
An ionic liquid can be defined as an organic salt, liquid under 100°C. Ionic liquids comprise of an organic cation, commonly based on nitrogen containing aromatic species and an anion such as Cl-
, BF4-
What makes ionic liquids so appealing for industrial applications are their physiochemical properties; these can be ‘tuned’ by alteration of alkyl chain length or associated anionvi
(Figure 1). They have a
wide liquid range, low thermal pressure, high viscosity, high conductivity, ability to dissolve many organic and inorganic compounds and high thermal stabilityvii
.
Static Headspace Gas Chromatography is a popular method for residual solvent analysis. Enhancement of headspace sensitivity is necessary because stringent guidelines exist for residual solvents in pharmaceutical productsviii
. In a previous experiment, the performance of
the ionic liquid [BMIM][BF4] (1-butyl-3- methylimidazolium tetrafluoroborate) was compared to DMSO (dimethylsulfoxide) as a
Aromatic ∏-∏ interactions with imidazole ring
or PF6-v
. Dispersion interactions with alkyl chain Figure 1: Illustration indicating various intermolecular interactions with ionic liquids using [BMIM][PF6] as an example
diluent for high boiling point residual solvent analysis, the ionic liquid produced responses five- fold greaterix
. In the following experiment the performance
of two ionic liquids: [BMIM][PF6] (1-butyl-3- methylimidazolium hexafluorophosphate)
and [BMIM][BF4] (1-butyl-3- methylimidazolium tetrafluoroborate) as static headspace solvents was determined with an aim to increase sensitivity. The sensitivity towards a range of low boiling point residual solvents was studied and the response compared to the common headspace solvent DMAC (N,N- dimethylacetamide).
Experimental Chemicals and reagents Analytical reagent grades of all residual solvents and ionic liquids were used and purchased from Sigma Aldrich.
Standard and Sample Preparation A stock solution was made up as follows:
500µL of the following residual solvents were added to a 50 ml volumetric flask: methanol, n-pentane, ethanol, diethyl ether, acetone, propan-2-ol, t-butanol, n-propanol, ethyl acetate, dichloromethane, chloroform, cyclo-hexane, 1,4-dioxan, 4-methyl-2- pentanone, toluene and n-butyl acetate. The diluent used was DMAC.
Polar and hydrogen bonding interactions with Anion (anion dictates hydrogen bond basicity)
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