Category: Publications

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Parallel gradients as alternative to shifted gradients in 2D-LC

In pursuit of full usage of the two-dimensional separation space as prescribed by Giddings [1], the LC×LC chromatographic community is continuously scouting for new methods that yield fully orthogonal separations. With the rich and diverse LC toolkit of available retention mechanisms, chromatographers mainly focus on improving the compatibility of orthogonal – yet incompatible – separation methods. This has spurred the development of active-modulation techniques such as stationary-phase-assisted modulation [2] and active-solvent modulation (ASM) [3]. Ultimately, this angle of innovation is mainly driven by the selection of stationary -and mobile phases, as well as their underlying retention mechanisms.

Rather than fine-tuning selectivity, another branch focuses on tweaking retention factors. For LC×LC separations, this has led to the introduction of shifted gradients. Here, the second-dimension gradient is altered and adapted as a function of the first-dimension gradient program [4]. While extremely effective, the optimization of shifted-gradient assemblies introduces additional complexity to the already more-complex method development process for comprehensive 2D-LC.

The situation of LC×LC contrasts heavily with that of GC×GC. For GC×GC, orthogonal separations are extremely difficult if not impossible due to analyte volatility. As a consequence, wrap-around effects are frequently generated, yet this is rather seen as advantage than disadvantage.

Prof. Tadeusz Gorecki (University of Waterloo, Canada) thus set to investigate what would happen if the same approach would be applied in LC×LC [5]. The project was an international collaboration with Alshymaa Aly (Minia University, Egypt, and University of Waterloo, Canada), Prof. Andre de Villiers and Magriet Muller from Stellenbosch University in South Africa, and Bob Pirok from the CAST team at the University of Amsterdam in the Netherlands.

RPLC×RPLC separations were simulated based on experimental data using the MOREPEAKS framework (formerly PIOTR [6]). Predicted separations using shifted gradients and parallel gradients were compared. The results suggested that parallel gradients indeed may advantageous. To verify this assessment, optimized experimental methods were executed and the resulting separations compared.

Supported by both experimental data and theoretical simulations, the authors concluded that non-orthogonal separation mechanisms could still yield good separation methods in LC×LC.

References

[1] Two-dimensional separations: concept and promise. J.C. Giddings, Anal. Chem. 1984, 56(12), 1258A–1270A, DOI: 10.1021/ac00276a003

[2] Recent Developments in Two-Dimensional Liquid Chromatography: Fundamental Improvements for Practical Applications. B.W.J. Pirok, D.R. Stoll and P.J. Schoenmakers, Anal. Chem., 2019, 91(1), 240-263, DOI: 10.1021/acs.analchem.8b04841

[3] Active Solvent Modulation: A Valve-Based Approach To Improve Separation Compatibility in Two-Dimensional Liquid Chromatography. D.R. Stoll, K. Shoykhet, P. Petersson, and S. Buckenmaier, Anal. Chem. 2017, 89(17), 9260–9267, DOI: 10.1021/acs.analchem.7b02046

[4] Optimizing separations in online comprehensive two-dimensional liquid chromatography
B.W.J. Pirok, A.F.G. Gargano and P.J. Schoenmakers, J. Sep. Sci., 2018, 41(1), 68–98, DOI: 10.1002/jssc.201700863

[5] Parallel gradients in comprehensive multidimensional liquid chromatography enhance utilization of the separation space and the degree of orthogonality when the separation mechanisms are correlated. A.A. Aly, M. Muller, A. de Villiers, B.W.J. Pirok, T. Górecki, J. Chromatogr. A, 1628, 2020, 461452, DOI: 10.1016/j.chroma.2020.461452

[6] Program for the interpretive optimization of two-dimensional resolution. B.W.J. Pirok, S. Pous-Torres, C. Ortiz-Bolsico, G. Vivó-Truyols and P.J. Schoenmakers, J. Chromatogr. A, 2016, 1450, 29–37, DOI: 10.1016/j.chroma.2016.04.061

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Publications

Liquid Chromatography in the Oil and Gas Industry

With the petroleum industry representing roughly 40% of the chemical industry [1], the analysis of petroleum-related samples is of interest for the optimization of refining processes, studying environmental pollution or monitoring of other processes, such as biodegradation. While gas chromatography is commonly used for most petrochemical samples, liquid-phase separations may be employed for the difficult cases.

To cover the applications of liquid chromatography in this context, MSc. graduate student Denice van Herwerden compiled this in a chapter as a contribution to the book Analytical Techniques in the Oil and Gas Industry for Environmental Monitoring [2].

In her review, Van Herwerden observed that LC is mainly applied for analysis of heavier petroleum fractions (due to their high boiling points), thermally labile compounds, and, for example, acidic compounds that would require a derivatization step prior to GC analysis. She also discussed applications where LC is used as a pre-separation technique to decrease the sample dimensionality prior to GC analysis. Denice found that, while both on- or off-line coupling may be used, the off-line coupling is recently more favored. 

Finally, van Herwerden addressed a limited number of applications of comprehensive two-dimensional liquid chromatography to the analysis of heavy-oil fractions and derivatives

Analytical Techniques in the Oil and Gas Industry for Environmental Monitoring (ISBN: 9781119523307) is a book edited by Melissa Dunkle and William Winniford. The book was published this August and includes 11 chapters. The chapter on LC applications, which was co-written by Bob Pirok and Peter Schoenmakers, can be found here.

References

[1] Hazardous Effects of Petrochemical Industries: A Review. A. Sharma, P. Sharma, A. Sharma, R. Tyagi and A. Dixit, Advances in Petrochemical Science, 2018, 3(2), 2–4, DOI: 10.19080/rapsci.2017.03.555607

[2] Liquid Chromatography: Applications for the Oil and Gas Industry, D. van Herwerden, B.W.J. Pirok, and P.J. Schoenmakers, Analytical Techniques in the Oil and Gas Industry for Environmental Monitoring, 2020, John Wiley & Sons, Inc., ISBN: 9781119523307, DOI: 10.1002/9781119523314.ch5

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Publications

Challenges in Obtaining Information from 1D- and 2D-LC

Earlier this June, CAST-member Bob Pirok (Van ‘t Hoff Institute for Molecular Sciences) and Johan Westerhuis (Swammerdam Institute for Life Sciences) published their vision on current challenges in data analysis in one-dimensional (1D) and two-dimensional (2D) chromatography [1].

In their article, the authors discuss the caveats of common data-analysis strategies that are typically employed in processing data obtained from 1D and 2D chromatography. The authors discuss the importance of data pre-processing and the associated challenges. Highlighting one of the conclusions of an earlier review [2], the authors again emphasized that no current studies provide an objective numerical comparison of background correction metrics.

image_2020-11-05_085441

Figure 1. Comparison of commonly applied methods to assess the area of a peak. Reprinted from [1] with permission.

Pirok and Westerhuis furthermore explained the difficulties with common curve resolution methods such as matched filtering (a.k.a. curve-fitting) and derivated-based approaches. While multi-dimensional separations increase the likelihood of resolution, the authors noted that this by no means eases the job of obtaining information of these datasets. The authors also discussed some key opportunities currently in the works by scientists around the globe. You can read the article freely here.

Figure 2. The availability of an additional dimension of data through the detector (in these case DAD) certainly helps to distinguish the peaks, but does not aid in easing extracting the information of the data.

References

[1] Challenges in Obtaining Relevant Information from One- and Two-Dimensional LC Experiments
B.W.J. Pirok & J.A. Westerhuis, LC-GC North America, 6(38), 2020, 8-14 [LINK]

[2] Recent applications of chemometrics in one- and two-dimensional chromatography
T.S. Bos, W.C. Knol, S.R.A. Molenaar, L.E. Niezen, P.J. Schoenmakers, G.W. Somsen, B.W.J. Pirok, J. Sep. Sci. 43(9-10), 2020, 1678-1727, DOI: 10.1002/jssc.202000011

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Publications

Applications of chemometrics in 1D and 2D chromatography

As we strive for more peak capacity to tackle the separation of the samples of tomorrow it is easy to forget that we should also still be able to retrieve the answer to our original question [1]. However, with the ever increasing complexity of our separation systems, the data we obtain from our experiments becomes similarly more sophisticated [2]. One modern example is an LC×LC-MS/MS system, which is capable of generating truly massive amounts of data per experiment.

As chromatographers, we tend to forget the data analysis and tend to rely on commercial software packages. However, as we continue to produce more efficient separation systems, the field of chemometrics is doing its best to keep up. With the technological capabilities of computer systems increasing by the day, the field of chemometrics is unsurprisingly very active.

CAST members Tijmen Bos, Wouter Knol, Leon Niezen and Stef Molenaar reviewed these recent works in their review recently published in Journal of Separation Sciences [3]. In their review, the young authors divided the work in literature into a number of categories data pre-processing (including retention-time alignment), data analysis (peak detection, information extraction, etc.), quantitative approaches and method optimization.

While the authors addressed the multivariate approaches used to tackle highly complex data, the review also focused on developments in the processing and use of day-to-day data such as obtained in 1D chromatography.

Most reported methods were developed to tackle a specific challenge in a data set and comparisons with other approaches supported by numerical data have rarely been reported.

Bos et al.

Within the subject of data preprocessing (background correction, signal smoothening, etc.) the authors found a rather large number (>10) of new methods published in the last few years. Yet, the authors surprisingly observed a complete lack of objective comparisons of these approaches. Consequently, it appears to be rather unclear which of the by now many existing approaches is suitable for the chromatographer in the routine lab. Another conclusion was that modern peak-alignment strategies are not robust for elution-order shifts.

The authors furthermore noted that this issue culminated into issues with data processing, information extraction and – ultimately – method optimization.

For the latter focal point within chemometrics, optimization strategies, the authors found that a shift in attention may be in order. While a large number of optimization strategies are reported, most studies do not take into account the validity of the actual optimization criteria. Reciting also the message from earlier works [4], the authors noted that for optimization in chromatography, more attention should be given to the quality descriptors. 

The authors devoted a lot of attention to also explain every core chemometric approach to help non-experts to understand the importance and significance of each of the featured methods. Image shows and example of an explanation of fundamental signal processing methods. Reproduced with permission from [3].

The review contains a massive table with all recent and important applications and developments within chemometrics for chromatography. The different works are sorted by category, including background correction, peak alignment, peak detection and quantification.

The review was initiated within the collaboration with Agilent Technologies through the University Relations program. The work was published open-access and is available download here.

Researchers Bos, Knol, Niezen and Molenaar are part of the UNMATCHED project, which is supported by BASF, DSM and Nouryon, and receives funding from the Netherlands Organization for Scientific Research (NWO) in the framework of the Innovation Fund for Chemistry and from the Ministry of Economic Affairs in the framework of the “PPS‐toeslagregeling”. 

References

[1] Practical Approaches for Overcoming the Challenges of Comprehensive Two-Dimensional Liquid Chromatography, B.W.J. Pirok and Peter J. Schoenmakers, LC-GC Europe, 2018, 31, 242–249, [LINK].

[2] Recent Developments in Two-Dimensional Liquid Chromatography: Fundamental Improvements for Practical Applications, B.W.J. Pirok, D.R. Stoll and P.J. Schoenmakers, Anal. Chem., 2019, 91(1), 240-263, DOI: 10.1021/acs.analchem.8b04841

[3] Recent applications of chemometrics in one- and two-dimensional chromatography, T.S. Bos, W.C. Knol, S.R.A. Molenaar, L.E. Niezen, P.J. Schoenmakers, G.W. Somsen, B.W.J. Pirok, J. Sep. Sci. 43(9-10), 2020, 1678-1727, DOI: 10.1002/jssc.202000011

[4] Optimizing separations in online comprehensive two-dimensional liquid chromatography, B.W.J. Pirok, A.F.G. Gargano and P.J. Schoenmakers, J. Sep. Sci., 2018, 41(1), 68–98, DOI: 10.1002/jssc.201700863

The Authors

Tijmen Bos

Wouter Knol

Leon Niezen

Stef Molenaar

Researchers Bos, Knol, Niezen and Molenaar were part of the UNMATCHED project, which was supported by BASF, DSM and Nouryon, and received funding from the Dutch Research Council (NWO) in the framework of the Innovation Fund for Chemistry and from the Ministry of Economic Affairs in the framework of the “PPS‐toeslagregeling”. You can read more about them and find their contact info on the Team page.