On-line solid phase extraction–liquid chromatography, with emphasis on modern bioanalysis and miniaturized systems

• A wide variety of on-line SPE–LC systems for bioanalysis exist.
• Application areas are proteomics, metabolomics, PTM analysis and many others.
• Nano and capillary dimensions are increasingly commonplace and routine.
• Separation principles/phases such as HILIC, PGC and RAM are employed and coupled.
• Phase incompatibilities can occur, some principles/phases are easier to couple than others.

On-line solid phase extraction (SPE)–liquid chromatography (LC) allows for automated, sensitive, precise and selective bioanalysis. It is a common feature in miniaturized- or nano LC systems, which are well suited for applications requiring high sensitivity and/or treatment of limited samples (laser micro-dissection samples, rare cancer stem cells, etc.). Traditionally, particles with reversed phase (RP) functional groups are used for the columns in SPE–LC systems. There is however an expanding diversity in SPE–LC combinations applied to meet today's bioanalytical challenges. Current online SPE–LC combinations employ, e.g. porous graphitic carbon (PGC) and hydrophilic interaction liquid chromatography (HILIC) materials for metabolomics and glycomics, restricted access media (RAM) columns coupled with nano LC for peptidomics, immunoaffinity trap columns for targeted proteomics and metal oxide affinity phases for phosphopeptide analysis. However, issues can arise when combining different phases in on-line SPE–LC, e.g. due to solvent incompatibilities between enrichment/separation principles and sample solvent requirements. Consequences can be low recovery and poor resolution, or need for additional instrumentation. On-line SPE–LC with very narrow columns (10–20 μm inner diameters) can be appropriate to obtain maximum sensitivity and information. In such highly miniaturized systems, non-particulate columns are arguably more suited (e.g. monolithic or porous layer open tubular (PLOT) columns) as e.g. hardware contributions resulting in extra column volumes are reduced. Basic SPE–LC systems can be configured/modified to perform quite complex analytical operations, and certain columns, configurations and hardware can improve robustness.

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