Prototype of an Interface for Hyphenating Distillation with Gas Chromatography and Mass Spectrometry

Abstract

Chemical analysis of complex matrices-containing hundreds of compounds-is challenging. Two-dimensional separation techniques provide an efficient way to reduce complexity of mixtures analyzed by mass spectrometry (MS). For example, gasoline is a mixture of numerous compounds, which can be fractionated by distillation techniques. However, coupling conventional distillation with other separations as well as MS is not straightforward. We have established an automatic system for online coupling of simple microscale distillation with gas chromatography (GC) and electron ionization MS. The developed system incorporates an interface between the distillation condenser and the injector of a fused silica capillary GC column. Development of this multidimensional separation (distillation-GC-MS) was preceded by a series of preliminary off-line experiments. In the developed technique, the components with different boiling points are fractionated and instantly analyzed by GC-MS. The obtained data sets illustrate dynamics of the distillation process. An important advantage of the distillation-GC-MS technique is that raw samples can directly be analyzed without removal of the non-volatile matrix residues that could contaminate the GC injection port and the column. Distilling the samples immediately before the injection to the GC column may reduce possible matrix effects-especially in the early phase of separation, when molecules with different volatilities co-migrate. It can also reduce losses of highly volatile components (during fraction collection and transfer). The two separation steps are partly orthogonal, what can slightly increase selectivity of the entire analysis.

Keywords: distillation; gas chromatography; gasoline; hyphenated techniques; online sample preparation.

Fig. 1. Schematic illustration of the multi-dimensional online separation system incorporating simple distillation and gas chromatography: (A) distillation system interfaced with gas chromatograph; (B) simplified schematic of the electronic control unit.

Fig. 2. Off-line GC-MS analysis of fractions of gasoline obtained during simple distillation. The blue bars highlight four chromatographic (TIC) features that clearly change in the course of distillation.

Fig. 3. Two-dimensional plots relating signal intensity with condensation time and GC-MS retention time: TIC and EICs for m/z 42, 92, and 120 (selected ions related to gasoline hydrocarbons). This figure presents the same data set as Fig. 2.

Fig. 4. Gas chromatograms (TIC) obtained during online coupling of microscale distillation with GC-MS, applied to analysis of a gasoline sample (25 mL)—first test without fraction dilution and without automated temperature control. The blue bars highlight four chromatographic features that clearly change in the course of distillation.

Fig. 5. Two-dimensional plot (TIC) obtained during online coupling of microscale distillation with GC-MS, applied to analysis of a gasoline sample (25 mL)—second test with fraction dilution and with automated temperature control.

Fig. 6. Temperature program applied by the hot plate during distillation (second online test). Red zones: hot plate on. Blue zones: hot plate off.

Fig. 7. Variability of the distillate output. Fractions were collected during 1-min intervals, and weighed. Experimental conditions are the same as in the second online test.

Fig. 8. Stability of the automated injection system (cf. Fig. 1). Peak areas of limonene and terpinene (standard spiked into diluent, 10⁻⁵ M). Labels: () limonene; (■) terpinene.