Ultrafast separations via pulse flow valve modulation to enable high peak capacity multidimensional gas chromatography☆
Introduction
Comprehensive two-dimensional (2D) gas chromatography (GC × GC) was conceptually introduced by Giddings [1] and experimentally pioneered by Liu and Phillips [2]. The transition from the initial instrumentation design to the modern GC × GC platform provides a highly effective instrument for the separation of complex samples [[3], [4], [5], [6], [7], [8], [9]]. In tandem with the GC × GC instrumentation development, a large amount of effort has been directed toward developing data analysis tools to convert the data into information [[10], [11], [12], [13], [14], [15]]. Many reviews cover GC × GC instrumental development [[16], [17], [18], [19]], as well as improvements to data analysis approaches [16,17,20,21]. The goal of GC × GC as with any analytical technique is to provide the information required to meet the requirements of the analysis, which often necessitates providing the greatest amount of information within the shortest period of time. For any separation technique, this goal is highly correlated to providing a high peak capacity or peak capacity production (i.e., peak capacity per unit time). When compared to one-dimensional (1D) GC, GC × GC theoretically can provide a ∼10-fold improvement in peak capacity due to addition of the second separation dimension (2D) [22]. Furthermore, the addition of the 2D separation provides increased chemical selectivity. Analytes that may be overlapped on the first separation dimension (1D) have the opportunity to be resolved on the 2D separation.
Despite the benefits of GC × GC compared to 1D-GC (increased peak capacity and chemical selectivity), there remains a high likelihood of having unresolved analytes using GC × GC. This may be overcome by increasing the peak capacity of the separation, but this may negatively impact the sample throughput cycle (i.e. run time and cool down time) which may be of concern when multiple samples require analysis. In order to further enhance the chemical selectivity, it is intriguing to consider higher order instruments, especially comprehensive multidimensional separations. There have been a limited number of comprehensive three-dimensional (3D) gas chromatography (GC × GC × GC) reports, herein referred to as GC3 [[23], [24], [25], [26]], and based upon the exceptional promise demonstrated, further development of GC3 is warranted. Indeed, the addition of a third dimension (3D) composed of an ionic liquid stationary phase was leveraged in order to fully resolve several phosphonated compounds (i.e. chemical warfare simulants) [24]. More recently, GC3 was coupled to a TOFMS, providing another dimension of selectivity and peak identification power [25,26].
While GC3 is an attractive instrumental platform for advanced GC-based separations due to the increased selectivity provided by the three separation dimensions, prior research has excessively traded peak capacity (and peak capacity production) for the increase in selectivity via the 3D dimension. In the first report, a peak capacity of 3500 (58 peaks/min) was demonstrated [23] during a total separation run time of 60 min. Peak capacity production was improved 3-fold in the subsequent report, with approximately the same peak capacity obtained in 20 min resulting in a peak capacity production of 180 peaks/min [24]. A third report resulted in a similar peak capacity of 7000 (140 peaks/min) [25], which is very similar to state-of-the art GC × GC [5,22,[27], [28], [29], [30]]. However, one might expect the peak capacity (and peak capacity production) of GC3 to exceed that of GC × GC by a significant amount, similar to how GC × GC has ∼10-fold more peak capacity compared to 1D-GC [22]. However, in this study, we do not take into account geometrical, statistical, and column set orthogonality considerations [[31], [32], [33], [34], [35]] as these considerations and their effect on experimentally realized peak capacity have not been sufficiently developed in detail for three-dimensional systems. Rather, we make comparisons using an expression for ideal peak capacity as defined by Giddings [1], which is simply the product of the peak capacity for each dimension, for clarity and consistency with our previous work. We acknowledge that the ideal capacity numbers we report, while instructive to see improvements in instrumental performance, will be higher than would be obtained if corrected by a currently unavailable theory. In order to understand where GC3 has room to improve, and what can be done to experimentally facilitate the improvement, recent advances in modulator technology and the factors that govern 3D peak capacity, nc,3D, must be considered. A promising development in modulation concept and technology [36] to address this challenge for GC3 has recently been reported, in which pulse flow valve modulation provided a PM ≥ 50 ms concurrent with creating narrow peak widths ≥ 15 ms [37]. Indeed, it is intriguing to consider using pulse flow valve modulation, linking the 2D–3D separations to facilitate more optimal separations conditions in GC3, and substantially higher nc,3D.
In this report, the performance of pulse flow valve modulation with GC × GC is improved upon [37], and lessons learned in the current GC × GC study guide the optimal design for GC3. Due to the nature of the “partial” modulation process using the pulse flow valve, the separation dimension following pulse flow valve modulation is not a traditional chromatogram, rather requires data processing to convert the data to expose the encoded chromatographic information, producing “apparent” chromatographic peaks. The basic principles of the data processing are presented for GC × GC and GC3. For the GC × GC studies, we continue to optimize the separations based on the previous work with the pulse flow valve [37], with the overall goal to improve peak capacity on the 2D separation without reducing the peak capacity of the 1D separation. Additionally, we assess the pulse flow valve as an ultra-fast modulator for high-speed GC × GC. Using the knowledge gained from this high-speed GC × GC experiment, the pulse flow valve was implemented as the second modulator in GC3. Three samples were used to evaluate the GC3 instrument: a simple mixture containing 18 compounds (to illustrate basic concepts), the 115-component test mixture (to determine peak capacity figures-of-merit), and a diesel spiked with 8 polar compounds (to illustrate chemical selectivity benefits of GC3).
Section snippets
Basic concepts guiding instrument design
The best practice of GC × GC relies on maximizing peak capacity [38,39]. The peak capacity, nc,1D, in a 1D separation at unit resolution, Rs = 1, is defined as the separation time, 1t, which is the portion of the total 1D separation used to determine peak capacity referred to herein as tsep, divided by the average width-of-base (4σ), 1W, given by,
For GC × GC the ideal peak capacity is given by,where 1nc and 2nc are the 1D and 2D peak capacities, respectively. Eq. (2) can be
Instrumental summary
The GC × GC and GC3 instruments were both evaluated using a flame ionization detector (FID). The instrumental platforms consisted of an Agilent 6890 GC (Agilent Technologies, Palo Alto, CA, USA). The stock electrometer for the Agilent FID was replaced with a high-speed electrometer built in-house allowing the data to be collected at 100 kHz, with the data boxcar averaged to 10 kHz. The electrometer was interfaced to a data acquisition board, and the resulting data was collected using an in-house
GC × GC studies
The GC × GC-FID chromatogram for the 115-component test mixture using a modulation period, PM, of 500 ms in the raw data vector form is shown in Fig. 2A. The raw data for three representative analytes (methyl decanoate, pentadecane, dodecanol) is provided in Fig. 2B which range from relatively unretained on 2D to moderately retained. In the raw data vector form, each sharp decrease in signal from a given 1D peak represents the 2D analyte response (i.e., vacancy chromatography), nominally in the
Conclusion
Past development and applications of GC3 instrumentation demonstrated great promise based on the superior chemical selectivity provided relative to GC × GC. But this increase in selectivity previously came at the cost of reduced peak capacity and peak capacity production. To address this challenge, it has been demonstrated herein that the use of a high temperature diaphragm valve modulator in conjunction with a recently introduced pulse flow valve modulator, it is now possible to take advantage
Acknowledgements
We would like to thank Stan Stearns of Valco for providing high temperature diaphragm valves.
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2020, Analytica Chimica ActaCitation Excerpt :The concept was based on pioneering work by Cai and Sterns, who developed the concept of “partial modulation” [143]. Recent work with pulse valve modulation has evolved from earlier efforts with partial modulation [144,145] to tune the pressure conditions and the pulse width for a given PM to operate the pulse valve with a 100% duty cycle (full modulation), referred to as dynamic pressure gradient modulation (DPGM), resulting in fast, high peak capacity separations [146]. Other unique flow modulators with high duty cycles have been developed and applied to fuel separations [147,148].
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Selected paper from the 42nd International Symposium on Capillary Chromatography and 15th GCxGC Symposium, 13-18th May 2018, Italy.
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These authors contributed equally to this work.