Keep Pushing

During the last few years, the limits of liquid chromatography have been ‘pushed’, specifically in terms of:

1. UHPLC instrumentation: Recently, four 2.1 mm ID columns were coupled in series to give a total length of 60 cm and run at inlet pressures as high as 2,000 bar (1). This approach is very promising and should enable chromatographers to either reduce their analysis times or to deliver higher resolution by using longer columns. However, there may be some limitations regarding the stability of the packed beds, due to the high pressures involved. 
2. Column packing: A recent breakthrough was the random packing of 1 m long x 75 μm ID capillary columns with sub-2 μm fully porous particles, halving current plate heights. This performance was achieved by combining high slurry concentrations to minimize heterogeneity in trans-column flow, and using ultrasound sonication to reduce large voids in the bed volume (2). 
3. Analytical column imaging: For the first time, 3D image reconstruction of the actual bed of narrow-bore 2.1 mm ID columns was achieved using focused ion beam scanning electron microscopy (FIB-SEM) (3). This achievement opens new avenues for optimizing packing procedures.
4. Easy-to-use microfluidic LC-MS devices: 300 μm separation channels (straight or serpentine tubes with a rectangular cross-section area) can be packed with sub-2 μm particles as efficiently as standard UHPLC columns and connected to a mass spectrometer with minimum post-column dispersion. They provide a ten-fold increase in sensitivity (4).
5. UHPLC and SFC column performance: A new column technology is based on the suppression of heat transport between the column and its surroundings (5). Natural air convection and air conduction are eliminated by applying a high air-vacuum, and electromagnetic radiation is minimized by wrapping the column in a low–emissivity material. This approach allows users to bridge the gap between GC- and LC-like separations by using low-density supercritical fluids, while maintaining column efficiency (6). 
6. 3D-printing: We are at a cornerstone for column technology: classical column slurry packing with silica-based particles is reaching its limit, so focus is turning to the design of ordered structures by 3D-printing technologies (7). Two-photon polymerization appears the most promising, because its feature size can be around 1 μm, while its build range remains of the order of a few centimeters.

As for the future...

Chromatography is no longer considered a science in most academic institutions, and most chemistry students do not graduate with a deep fundamental knowledge of liquid chromatography. As this workforce faces complex analytical challenges in metabolomics, proteomics, genomics, and quality control in general, they will need instruments adapted to their qualifications, such as:

1. Easy-to-use, integrated, and self-control preparation/separation/detection systems. For instance, process analytical technology will enable the user to meet critical quality attributes rapidly and with a low rate of rejection by continuously controlling the critical process parameters. 
2. More universal stationary phases (multi-mode chromatography), which allow the single-run separation of a large variety of compounds (hydrophobic, polar, ionizable, and so on).
3. New optimization software, enabling the user to rapidly develop an analytical method by accurately predicting retention and maximizing chromatographic resolution.
4. High-resolution (LC-MS → LC2-MS → LC3-MS?), high-throughput separation techniques to handle increasingly complex samples. 
5. Efficient data processing, data storage, data handling, and data sharing in the cloud and via the internet.