State-of-the-Art in Capillary Liquid Chromatography: Now, Next, and How?

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The “State-of-the-Art in Capillary Liquid Chromatography” panel discussion at the 43rd International Symposium on Capillary Chromatography (ISCC 2019) in Fort Worth, Texas, USA, was a thoughtful dialogue on current challenges and potential future directions in the field. The session included a general overview of the current state of the field, key drawbacks preventing widespread use of capillary liquid chromatography (LC) columns, and how these challenges might be overcome. In this article, we highlight some of the common themes that were discussed as part of this panel.

The 43rd International Symposium on Capillary Chromatography (ISCC) and 16th GC×GC Symposium were held in Fort Worth, Texas, USA, from 12–17 May 2019. ISCC is the premier international meeting for pressure and electrodriven microcolumn separations and related techniques. This is the U.S. version of the highly successful meetings held in even years in Riva del Garda, Italy. The 44th ISCC and 17th GC×GC symposia will be held in Riva Del Garda from 24–29 May 2020 (iscc44.chromaleont.it). The next instalment of ISCC and GC×GC in 2021 will return to Ft. Worth from 9–14 May 2021. As these symposia continue to evolve, new means for information sharing and participation engagement have been incorporated. In 2019, a series of panel discussions were instituted in the programme.

At the “State-of-the-Art in Capillary Liquid Chromatography” panel discussion at ISCC 2019, more than 25 attendees participated in a thoughtful dialogue on current challenges and potential future directions in the field. The panellists for this discussion, moderated by James Grinias (Rowan University), included three experts in the area: Justin Godinho (Advanced Materials Technology), Fabrice Gritti (Waters Corporation), and Purnendu “Sandy” Dasgupta (University of Texas at Arlington).

The session began with a general overview of the current state of the field, with most agreeing that adoption of capillary liquid chromatography (LC) columns has been limited despite their multiple advantages over analytical-scale columns. Some of the key drawbacks preventing further use of capillary LC columns include: the considerations that must be made for instrument design and during column installation to prevent extracolumn volumes that broaden peaks; and the limited number of detection options that are compatible with capillary LC outside of mass spectrometry (MS). This is especially true with open-tubular LC (OT-LC) columns, which typically have inner diameters in the 1–5 µm range; the low flow rates that are used with these small diameter columns are generally incompatible with modern, commercial nano-electrospray ionization interfaces that make MS detection feasible. In general, industry leaders in the discussion concluded that the capillary LC segment of the overall LC column market is very low. These challenges identified in the opening minutes of the discussion framed the rest of the session in terms of how the community might work to overcome them and increase the use of the technique. In this article, we highlight some of the common themes that were discussed as part of this panel.

Capillary LC Columns

Inherently, the difference between traditional LC methods and capillary LC is the design of the column itself. Rather than a stainless steel analytical‑scale column with an inner diameter in the range of 1.0 to 4.6 mm, capillary columns are typically packed in fused silica capillaries with inner diameters under 500 µm. With these smaller dimensions, flow rates are significantly lower, which means that special pumps capable of delivering accurate flow rates at nL/min to low µL/min flow rates are required. Additionally, the reduced column volume means that extracolumn effects related to connecting tubing, injection, detection, and unions have a much larger impact on chromatographic efficiency than with a traditional column. This set of factors has traditionally meant that the installation and use of capillary LC columns required a higher level of expertise than an analytical‑scale column, which is one cause for their more limited usage. Attendees from chromatography manufacturers mentioned that capillary LC is only a very small segment of the overall LC market. This becomes a major challenge in advancing the technique, as major instrument and column manufacturers put fewer resources into developing capillary-scale technologies that lack a wide customer base to serve. Predominantly, capillary LC has developed in parallel with “omics” strategies that typically rely on coupling capillary LC columns with MS, and often relies on unique and specialized setups (1). Thus, while commercial column options are available, many researchers, especially academics, opt to pack their own capillary LC columns to meet their needs.

An ensuing discussion related to the observation that many academic researchers choose to pack their own capillary LC columns and the overall reproducibility of this column format. Recent research efforts to correlate packing techniques with both column efficiency and bed morphology, which can now be directly measured through confocal laser scanning microscopy (2–4), have provided insight into strategies for preparing capillary LC columns that provide very low theoretical plate heights, and are very reproducible (5). The general conclusions of this work suggested the implementation of high particle slurry concentrations, high pressures, and sonication during the packing process to obtain efficient capillary columns (6). Investigations of bed morphology have found that these packing parameters minimize particle size segregation and void formation throughout the bed structure that can hurt column performance (7,8). Other research groups have also identified column frits to be a key source of broadening in modern LC columns (9,10). In packed capillary LC columns, the strategy for trapping particles in the column is different than with analytical-scale columns, and many researchers have found success using polymeric potassium silicate frits for this purpose (11,12). While these frits still maintain high efficiency, the panellists did identify the adsorption of trace analytes in biological samples onto the frits as one issue of concern to consider when using this approach.

The other column format that was heavily discussed during this session was OT capillary LC columns. Historically, the key challenge of utilizing OT-LC columns was limited loadability, as few detectors had sufficient sensitivity to detect the low level of analytes that could be injected onto the column without suffering significant band broadening due to overloading (13). However, as MS detectors continue to improve in sensitivity, this issue is becoming less of a concern. Additionally, the implementation of porous layer open‑tubular (PLOT) columns in LC has provided increased loadability to these types of columns, without losing the efficiency benefits gained with the OT format (14–16). In PLOT-LC columns, a thin layer of silica or polymer monolith is bonded to the inner wall of the fused silica capillary, and then functionalized with the desired stationary phase. The increased surface area provided by this monolith allows for more analyte to be injected onto the column while only minimally increasing mass transfer broadening due to the added stationary phase support. Multiple research groups across the world have made advances in the preparation and use of these columns in recent years, which indicates they may be a key strategy in further expanding the implementation of capillary LC into analytical laboratories, although this growth will always be related to the aforementioned sensitivity issue in detectors. Furthermore, to truly achieve the full benefit of the OT-LC format, the total inner diameter of the column should not be larger than 10 to 15 µm at a maximum (17). Because this dramatically reduces the overall column volume, the considerations for extracolumn broadening described above become even more important.

OT-LC columns have also been utilized for ion chromatography (IC) (18). Many of the same challenges for other chromatography modes still exist when using OT columns in IC, but other unique opportunities are also present. Because of the ionic nature of the analytes separated in IC, contactless conductivity detection can also be used, which provides a zero‑dead volume detection scheme without many of the issues that arise from coupling capillaries to MS (19,20). Also, a method for attaching latex anion exchangers directly to particles for stationary phase functionalization was described during the discussion (21,22). This technique was originally developed for IC, but could provide opportunities for other chromatography modes in OT-LC as well.

Coupling Capillary LC with MS

The other key discussion topic through the panel session on capillary LC was the need for improved detector interfaces, specifically with MS detectors. Because most capillary LC separations are coupled to MS, the interface between these two instruments is key to this analytical technique. Many in the audience discussed their frustrations with the current options that are available for coupling capillary LC with electrospray (or nanospray) ionization (ESI), and the general consensus among session attendees was that this is a fundamental hurdle to expanding capillary LC in the separations community. This interface is critically important to realize the sensitivity enhancements of nanospray regimes. There are a number of commercial platforms that are designed to make this coupling simpler for users, but the number of column options that can be used with these integrated spray devices is low. Again, the challenges of having major instrument manufacturers develop new methods for simplifying this interface were discussed, with the lack of a major capillary LC user base providing minimal business incentive to pursue this area. Rather, smaller start-up companies have taken the lead in this area, including 908 Devices and their ZipChip platform. This product involves a monolithically integrated emitter tip on a microfluidic device (23), which reduced the need for coupling two fused silica capillaries (column and emitter tip) together and avoided potential silica fines that lead to clogs and higher back pressures. Others in the audience noted that the clogging of tips over time is a major issue in “omics” research where hundreds of samples are analyzed in a sequence; an in-line UV trace is consistent across the samples, but the MS signal slowly degrades over time due to clogging and degradation of the spray emitter tip. Because the highest signal enhancement occurs at the lowest flow rates (24), this issue will become critically important with smaller column and tip diameters that provide sub-10 nL/min flow rates, a regime that is not easily achieved with commercial equipment.

Another area that was discussed in terms of coupling chromatography with MS was supercritical fluid chromatography (SFC). With the increase of SFC using analytical-scale columns over the past several years, the idea of renewed interest in capillary SFC was discussed. Capillary SFC was developed throughout the 1980s and early 1990s, typically using an OT format and coupled to a flame ionization detector (FID) (25). As back pressure regulator (BPR) technology improved, and the desire in industry to utilize analytical‑scale columns that many consider to be “more robust”, capillary SFC decreased in usage compared to the more traditional SFC in use today. However, with the growing importance of coupling capillary columns to MS, new opportunities may exist. SFC generally provides improved sensitivity with MS compared to LC based on enhanced desolvation in ESI (26). SFC is also gaining attention as major industries become increasingly conscious about their environmental footprint, and try to decrease the amount of toxic solvents used for chemical analysis, with capillary flow rates reducing solvent usage even further. Attendees discussed the opportunities that exist now to explore packed capillary SFC techniques based on these various factors and expand its use relative to the other capillary LC methods in use today.

Moving Capillary LC Forward

Apart from technology and business aspects of capillary LC, two other factors related to the growth of the field were also identified: lack of education, and the need for a “champion” of the technique. In terms of education, most chromatographers are trained on analytical‑scale instruments and columns, and are thus more familiar with and confident using these platforms.
Because dead volumes in fittings and connections are critically important when using capillary LC columns, many users find that the skills that are sufficient for analytical-scale columns can still lead to poor peak shapes and lower column efficiencies when using capillary LC. The need for members of the academic community to better introduce and instruct young analytical scientists on the use of capillary LC was discussed during the panel. However, as most students take jobs in industry following their education, they will mainly continue to use analytical-scale instrumentation and must have more exposure to those platforms. Thus, determining when and if capillary LC will take a more prominent role at various pharmaceutical and chemical manufacturers will help guide expanded use of the technique in analytical chemistry education.

Related to guiding the field as a whole, the perceived lack of a “champion” of capillary LC was identified. The case of solid-phase microextraction (SPME) was used to demonstrate how having a champion in the field consistently present to explain the advantages of the technique can help increase its use by others in the analytical community. Because capillary LC is a wide field, with many researchers investigating specific areas, it has been difficult to identify one single person who can promote the technique and its benefits to others. The panellists did indicate that there are many experts and leaders within the capillary LC community (a recent review on the field can be found in reference 27), and determined that this may be the reason why one single person has not been able to fully embrace all aspects of capillary LC and promote its use over other chromatographic techniques. Instead, the challenge was made to the ISCC community as a whole to become a group of “champions” who work together to encourage increased use and adoption of capillary LC as a separations technique to others in the analytical chemistry world.

Conclusions

Upon completing the panel discussion, both attendees and participants were enthusiastic about participating in other panel discussions throughout the rest of the meeting and encouraged holding a similar session at future ISCC meetings. The challenge of promoting capillary LC as an important technique that falls between traditional, analytical-scale LC and direct infusion MS systems remains, but many clear opportunities to overcome this issue were determined at this event. First and foremost, improvements in the quality, robustness, and simplicity of electrospray interfaces to capillary columns are sorely needed, but whether the advances will come from academic or government laboratories or industry remains to be seen as the smaller market size provides less incentive for major instrumentation companies to pursue this area. To help grow this market and increase usage of capillary LC, a convincing “champion” or “team of champions” is needed to unambiguously demonstrate the reliability, reproducibility, and quantification capabilities of capillary LC–MS across a wide range of application areas. One approach to achieving this goal is the potential development of training laboratories based on academic–industry partnerships that provide collaborative research centres where these capabilities can be validated and used to solve the most challenging problems at the frontier of analytical research. On the industry side, input and commitment from both instrumentation and chromatography column manufacturers will be key to forward movement. From these partnerships, the hope is that new ideas and strategies for the next generation of capillary LC instruments can be identified and used to convince manufacturers of the key concepts needed in future instrumentation. The dialogue on these topics, and many others in the field, will continue at the upcoming ISCC and GC×GC symposia-and we all hope to see you there and have you join the conversation!

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James Grinias is an assistant professor at Rowan University.

Justin Godinho is a research scientist at Advanced Materials Technology, Inc.

Fabrice Gritti is a principal consulting scientist in the Instrument/Core Research/Fundamental department of Waters Corporation.

Purnendu K. (Sandy) Dasgupta is the Hamish Small Chair in Ion Analysis at the University of Texas at Arlington.

Kevin Schug is a Full Professor and the Shimadzu Distinguished Professor of Analytical Chemistry in the Department of Chemistry & Biochemistry at The University of Texas at Arlington.