Elsevier

Journal of Chromatography A

Volume 1573, 26 October 2018, Pages 115-124
Journal of Chromatography A

Ultrafast separations via pulse flow valve modulation to enable high peak capacity multidimensional gas chromatography

https://doi.org/10.1016/j.chroma.2018.08.001Get rights and content

Highlights

  • A modulation period PM ≥ 50 ms via a pulse flow valve is implemented.

  • Comprehensive two- (GC × GC) and three-dimensional (GC3) gas chromatography are demonstrated.

  • Following pulse flow valve modulation, peak widths range from 8 to about 40 ms.

  • With GC × GC, a peak capacity of 20 is achieved using a PM of 500 ms.

  • A total peak capacity of 10,000 was achieved for GC3 in an 11 min separation.

Abstract

Ultrafast modulation with a modulation period PM ≥ 50 ms via a pulse flow valve is demonstrated for comprehensive two-dimensional gas chromatography (GC × GC) and comprehensive three-dimensional (3D) gas chromatography (GC3). Significant increases in peak capacity and peak capacity production are achieved for GC × GC and GC3 relative to previous studies due to using pulse flow valve modulation. Due to the nature of the “partial” modulation process, 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. In the GC × GC mode, a 115-component test mixture was evaluated using a PM of 500 ms, creating an apparent 2D peak width-at-base 2W with an average of 25 ms, producing a 2nc of 20. Based on the average 1W of 1.0 s for the 6 min first dimension 1D separation, an ideal peak capacity nc,2D of 7200 is achieved (1,200/min peak production). For a high-speed GC × GC separation (30 s run), a PM of 75 ms produced apparent 2W of 8 ms, ideal for the third dimension of a GC3 instrument. 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 evaluated in the GC3 mode: 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). For the 115-component test mixture with a 1PM of 1.2 s and a 2PM of 60 ms, average 1W of 3.2 s, 2W of 130 ms, and apparent 3W of 13 ms were produced, resulting in a 1nc of 210, 2nc of 9.2, and 3nc of 5, respectively. Hence, an ideal peak capacity, nc,3D of ∼10,000 for GC3 was achieved for the 11 min 1D separation window of the 115-component test mixture.

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,nc=t1W1

For GC × GC the ideal peak capacity is given by,nc,2D=n1cn2cwhere 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.

References (50)

  • C.E. Freye et al.

    Enhancing the chemical selectivity in discovery-based analysis with tandem ionization time-of-flight mass spectrometry detection for comprehensive two-dimensional gas chromatography

    J. Chromatogr. A

    (2018)
  • P.J. Marriott et al.

    Multidimensional gas chromatography

    TrAC Trends Anal. Chem.

    (2012)
  • K.M. Pierce et al.

    Review of chemometric analysis techniques for comprehensive two dimensional separations data

    J. Chromatogr. A

    (2012)
  • Z. Zeng et al.

    Interpretation of comprehensive two-dimensional gas chromatography data using advanced chemometrics, TrAC

    Trends Anal. Chem.

    (2014)
  • M.S. Klee et al.

    Evaluation of conditions of comprehensive two-dimensional gas chromatography that yield a near-theoretical maximum in peak capacity gain

    J. Chromatogr. A

    (2015)
  • W.C. Siegler et al.

    Increasing selectivity in comprehensive three-dimensional gas chromatography via an ionic liquid stationary phase column in one dimension

    J. Chromatogr. A

    (2010)
  • N.E. Watson et al.

    Targeted analyte deconvolution and identification by four-way parallel factor analysis using three-dimensional gas chromatography with mass spectrometry data

    Anal. Chim. Acta

    (2017)
  • C.E. Freye et al.

    High temperature diaphragm valve-based comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry

    Talanta

    (2016)
  • A.M. Muscalu et al.

    Evaluation of a single-stage consumable-free modulator for comprehensive two-dimensional gas chromatography: analysis of polychlorinated biphenyls, organochlorine pesticides and chlorobenzenes

    J. Chromatogr. A

    (2015)
  • J.V. Seeley et al.

    The multi-mode modulator: a versatile fluidic device for two-dimensional gas chromatography

    J. Chromatogr. A

    (2018)
  • L.M. Blumberg

    Flow optimization in one-dimensional and comprehensive two-dimensional gas chromatography

    J. Chromatogr. A

    (2018)
  • C.E. Freye et al.

    Comprehensive two-dimensional gas chromatography using partial modulation via a pulsed flow valve with a short modulation period

    Talanta

    (2018)
  • C.E. Freye et al.

    High temperature diaphragm valve-based comprehensive two-dimensional gas chromatography

    J. Chromatogr. A

    (2015)
  • C.E. Freye et al.

    Partial least squares analysis of rocket propulsion fuel data using diaphragm valve-based comprehensive two-dimensional gas chromatography coupled with flame ionization detection

    Talanta

    (2016)
  • R.B. Wilson et al.

    High throughput analysis of atmospheric volatile organic compounds by thermal injection – isothermal gas chromatography – time-of-flight mass spectrometry

    Talanta

    (2013)
  • Cited by (20)

    • Average theoretical peak time as a metric to analytical speed in one dimensional and multidimensional gas chromatographic separations

      2022, Journal of Chromatography A
      Citation Excerpt :

      As a result, when the analyst is acquiring a 1D chromatogram, direct measurement of the average peak width will give a direct response with respect to chromatographic speed. In addition, it makes the already common-sense metric used comparable for multidimensional techniques as discussed [30,33-39]. This implies that, in a GC × GC system, ATPT is dependent on the separation efficiency in both 1D and 2D.

    • A systematic investigation of comprehensive two-dimensional gas chromatography time-of-flight mass spectrometry with dynamic pressure gradient modulation for high peak capacity separations

      2020, Analytica Chimica Acta
      Citation Excerpt :

      The optimal pw must provide full modulation (i.e., no breakthrough) and produce narrow 2Wb for high peak capacity. A series of pw ranging from 0 ms to 1960 ms was evaluated so that data could be compared across all modes of DPGM: the full modulation mode, and partial modulation in the negative and positive modes [35,47,49,50], wherein the positive pulse mode indicates baseline between modulations concurrent with pw approaching PM. A PM of 2 s and initial Paux of 330.9 kPa were applied for each pw studied.

    • Development of gas chromatographic pattern recognition and classification tools for compliance and forensic analyses of fuels: A review

      2020, Analytica Chimica Acta
      Citation 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].

    View all citing articles on Scopus

    Selected paper from the 42nd International Symposium on Capillary Chromatography and 15th GCxGC Symposium, 13-18th May 2018, Italy.

    1

    These authors contributed equally to this work.

    View full text