Two-dimensional gas chromatography (GC×GC) and GC×GC–time-of-flight (TOF)-mass spectrometry (MS) have received increasing attention as powerful techniques for separation and analysis of extremely complex mixtures. Over the past 20 years, solid-phase microextraction (SPME) has become a staple sample preparation technique for a large variety of GC analyses. With a few additional considerations, SPME is combined readily with GC×GC or GC×GC–TOF-MS to provide easily automated and solvent-free sample preparation. This combination provides up to six dimensions of separation power in analytical method development that can be used to provide the required selectivity or can cause unwanted selectivity through sample losses or slow kinetics. In this article, the combination of SPME with GC×GC and GC×GC–TOF-MS is discussed using mixtures of solvents as example analytes. Optimization of the injection process is emphasized, as extractions remain generally the same as in traditional SPME–GC. To ensure good peak shapes, the injection liner, carrier gas flow rate and column temperatures must be optimized carefully. The differences in carrier gas flow optimization between GC×GC with flame ionization detection and GC×GC–TOF-MS also are discussed.

Since the development of comprehensive two-dimensional gas chromatography (GC×GC) by Phillips and coworkers in 1990, and most notably in the past five years, GC×GC and multidimensional GC by heartcutting have become increasingly popular among analysts in a variety of disciplines.¹ A number of recent reviews describe advances in GC×GC that demonstrate its high selectivity and ability to separate extremely complex mixtures, so these are not repeated here.² GC×GC is moving rapidly from the realm of being a research technique for hard-core chromatographers and into use by the broader community. GC×GC coupled with time-of-flight-mass spectrometry (GC×GC–TOF-MS) provides perhaps the ultimate in chromatographic separation and identification power, combining the separation power of multidimensional GC with the sensitive detection and identification capability of electron impact TOF-MS. A recent article demonstrating GC×GC–TOF-MS in athletic doping control illustrates the separation, detection and quantification power of GC×GC–TOF-MS.³ Further selectivity and sensitivity may be achieved by adding a concentrating and selective extraction technique, such as solid-phase microextraction (SPME), to GC×GC analysis. In this article, we discuss the principles, advantages and challenges in combining SPME with GC×GC and with GC×GC–TOF-MS. While the same extractions can be used with both traditional and multidimensional GC, there are additional injection considerations that result from the two-column combination in multidimensional separations and the additional selectivity afforded by the two column system can also require rethinking of sampling procedures and extraction chemistry.

SPME was developed about 20 years ago as a solvent-free alternative to traditional extraction methods for the analysis of volatile organic contaminants in water.^4,5 Headspace SPME subsequently became the method of choice for volatile compounds such as solvents.⁶ Because of its ease of use and automation, SPME has become one of the most popular sample preparation methods for GC. Being a solvent-free extraction and injection technique, SPME has unique properties and capabilities as an injection method.⁷ Most notably, there is no interference in the chromatogram from the presence of the large solvent peak found in traditional liquid injection chromatograms. There is also none of the beneficial band focusing due to solvent effects, as there is no traditional solvent. The only band-focusing mechanism available is cold trapping at the head of the analytical column. A complete discussion of the beneficial solvent effects, and how to avoid deleterious effects involved in traditional liquid injections, is provided in the comprehensive text on injection by K. Grob,⁸ which is a must-read for any chromatographer seeking to optimize or troubleshoot GC injections. Lacking solvent effects, SPME–GC injections must be optimized carefully, depending upon analyte volatility and instrumental conditions with an eye toward minimizing width of the injected band.