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February / March 2012
Ionic Liquid Stationary Phases:
Application in Gas Chromatographic Analysis of Polar Components of Fuels and Lubricants
by Sam Whitmarsh, Investigational Analysis, BP Research and Technology Global Lubricants Technology, Pangbourne, Berkshire, RG8 7QR
samuel.whitmarsh@bp.com
The capillary gas chromatographic analysis of polar components within fuels and lubricants was investigated using commercially available ionic liquid coated columns. The improved temperature stability and high polarity of these phases relative to polyethylene glycol were studied in one and two dimensions. In one dimension, the reduction in hydrocarbon matrix interference presents opportunities for improved quantitation and less sample preparation. In two dimensions, a reduction in second dimension retention was observed as phase polarity was increased providing a reminder that column sets should be correctly matched to sample composition for effective separation.
Engine oils are complex mixtures of hydrocarbon base fluids which can act as lubricants by virtue of their viscometric properties. These fluids also act as carriers for a range of additive components which have a chemically active role in the engine. These additives function as viscosity modifiers, anti-oxidants, detergents, dispersants and surface active molecules which protect against wear. Analysis of these materials in both fresh and used lubricants is critical to the behavioural understanding of these materials for practical troubleshooting, performance monitoring and as indicators of the fundamental processes involved in internal combustion engines.
Gas chromatography is one of the primary analytical techniques involved in the investigation of these compounds. Whilst mass spectrometry and statistical analysis have enabled sophisticated deconvolution of overlapping peaks, it is still highly desirable to achieve a physical separation of the analyte of interest from the matrix and other signals. Evaluation of column chemistries which enable such separations continue to be an important part of laboratory development. Analytes of interest in lubricant samples are typically more polar that the hydrocarbon base fluids due to the degradation pathways of oxidation and nitration and because additive components tend to contain heteroatoms. Therefore, separation of these polar materials from the non-polar matrix would allow for the study of the analytes of interest with reduced interference from the matrix.
Ionic liquids are a class of compounds which
can be defined as organic salts that are liquid below 100°C. They find use in many areas of chemistry as highly tuneable non-molecular solvents in synthesis, electrochemistry and analytical chemistry [1]. The properties of extremely low vapour pressure even at high temperatures, high viscosity, high thermal stability and high polarity make them suitable as phases for gas chromatography [1]. The first commercial ionic liquid column was launched by Supelco in 2008 [2] and further development has yielded an additional five column phases. A range of columns are now available encompassing different phase types, from polyethylene glycol equivalent polarity with improved thermal stability (SLB-IL59) to phases with extremely high polarity (SLB-IL111).
This combination of high polarity and high temperature stability along with new selectivity has great potential to impact methods within our labs. This report describes initial work studying the application of these columns to commonly observed problems within the group. Namely: the removal of hydrocarbon matrices from analytes of interest in a single dimension and improved orthogonality for use in 2D GCxGC separations.
Experimental
Two capillary gas chromatographs were employed. Firstly, an Agilent 6890N (Agilent Technologies, Santa Clara, CA, USA) equipped with an ATAS Optic 4 PTV inlet (ATAS GL International B.V., Eindhoven, NL), Agilent 5973MSD single quadrupole EI mass
spectrometer (Agilent Technologies, Santa Clara, CA, USA) operated in positive ion mode and a nitrogen/phosphorus detector. Secondly, an Agilent 7890A (Agilent Technologies, Santa Clara, CA, USA) equipped with a split/splitless inlet, flame ionisation detector and Agilent G3486A capillary flow GCxGC modulator (bypassed for use in single dimension studies). System control and data processing were carried out using Agilent ChemStation (Agilent Technologies, Santa Clara, CA, USA).
Methods in the single dimension: A 0.1 µl injection volume was used for undiluted
samples (diesel, B20, gasoline) and 1.0 µl used for diluted samples (test mixes,
methanol extract, lubricant samples). Injections were made into a split/splitless injector with a single gooseneck liner containing quartz wool at 275°C. Split ratios of 200:1 were typically used. The column flow was 1 ml/min constant with helium as carrier gas. The temperature program was as follows: 50°C with hold for 5 minutes, then 8°C/min until maximum column temperature reached, then hold until 45 minutes total run time reached. Detection was carried out using either mass spectrometry (scan mode 20-500 AMU, 70 eV, interface constant 270°C), nitrogen phosphorus detection
(310°C, 60 ml/min air, 3 ml/min H2, make up flow 2 ml/min He and electrometer voltage auto optimised) or flame ionisation detection
(300°C, 40 ml/min H2, 400 ml/min air, make up flow 40 ml/min He).
Methods in two dimensions were as follows: Inlet and injection conditions as in single dimension with 500:1 split ratio. First
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