Development of New Modulation Methods Using the Pulse Valve Modulator for Multidimensional Gas Chromatography

Abstract

The potential efficiency and applicability of gas chromatography is advanced through the continued development and use of the pulse valve modulator. New advancements in modulation techniques are reported with the pulse valve modulator concurrently with the use of commercial chemometric algorithms and new data visualization techniques. First, the application of partial modulation in the negative pulse mode (NPM) is demonstrated. The NPM is developed for high speed, one-dimensional gas chromatography (1D-GC), comprehensive two-dimensional (2D) gas chromatography (GC×GC), and comprehensive three-dimensional gas chromatography (GC3). This modulation technique is shown to be more beneficial than previous work with partial modulation in the positive pulse mode (PPM). The NPM produces comparable analyte peak widths-at-base (wb) as the PPM but does so with greater S/N and with less data processing required. In the 1D-GC mode, 8 analytes are baseline resolved (Rs ≥ 1.5) in a 200 ms window, providing a peak capacity, nc, of 14 at unit resolution (Rs = 1.0). Demonstrating this higher efficiency to a GC×GC, a 20-component test mixture is separated. Analytes were separated on the second dimension column, 2D, with 2wb ranging from 7 to 12 ms, providing an exceptional 2D peak capacity, 2nc of ~12 using a PM of 100 ms. Next, the NPM is applied to a GC3 with time-of-flight mass spectrometry detection (TOFMS). Narrow third dimension, 3D, peaks 3wb ~ 15 ms were obtained, resulting in a GC3 peak capacity, nc,3D, of ~35,000, in a 45 min separation. While high peak capacity has been produced, there are challenges that needed to be addressed regarding the use of commercially available data analysis techniques to ensure method translation can occur. The NPM data has the appearance of 2D separations superimposed on top of the 1D separation, which is unique. Multivariate curve resolution – alternating least squares (MCR-ALS) is used to demonstrate the potential for method translation with the NPM technique. A mixture of 15 similar analytes was isothermally separated to purposefully create several scenarios of peak overlap within 20 s. Despite the high degree of overlap, the NPM data is shown to be amenable to MCR-ALS, with all 15 analytes able to be decomposed, separated, and identified. The potential for quantification is demonstrated for two representative analytes, with percent deviation values of 5.6% (± 2.2%) for 1-hexene, and 1.8% (± 3.4%) for 2-pentanone. Next, the pulse valve modulator was used in the full modulation mode to demonstrate further advancements and quantifiable gains in S/N with GC3 instruments. The detector response enhancement factor (DREF) is a quantifiable measure of the increase in S/N ratio of analyte signal as the number of dimensions is increased. We calculated DREF values of ~ 37 for dodecene and ~ 21 for tridecane. Additionally, the 2D×3D peak capacity was demonstrated at 30 peaks per 1.5 s. Last, a method to visualize the 2D×3D chromatogram is presented, in which four successive modulations are lined up and one can “see” analytes arriving at and eluting from the TOFMS detector.