High-Temperature Liquid Chromatography and the Hyphenation with Mass Spectrometry Using High-Pressure Electrospray Ionization

Optimizing transfer and dilution processes when using active solvent modulation in on-line two-dimensional liquid chromatography

Two-dimensional liquid chromatography (2D-LC) is becoming increasingly popular for the analysis of complex samples, which is partly due to the recent introduction of commercial 2D-LC systems. To deal with the mobile phase incompatibility between highly orthogonal retention mechanisms, such as hydrophilic interaction liquid chromatography (HILIC) and reversed-phase LC (RPLC), several strategies have been introduced over the years. One of these strategies is active solvent modulation (ASM), a valve-based approach allowing the on-line dilution of the effluent eluting from the first dimension before transfer to the second dimension. This strategy has gained a lot of attention and holds great potential, however, no clear guidelines are currently in place for its use. Therefore, this study aims to investigate how the ASM process can be optimized when using highly incompatible LC combinations, such as HILIC and RPLC, in a simplified selective comprehensive 2D-LC set-up (sHILIC x RPLC) to suggest guidelines for future users. Using a representative sample, the dilution factor (DF), the duration of the ASM phase, the filling percentage of the sample loops, and their unloading configuration are investigated and optimized. It is observed that a DF of 10 with an optimal ASM phase duration, a sample loop filling of maximum 25%, and an unloading configuration in backflush mode, result in the best peak shapes, intensities, and recoveries for early eluting compounds, while keeping the total analysis time minimal. Based on these results, some general recommendations are made that could also be applied in other 2D-LC modes, such as comprehensive 2D-LC (LC x LC), heart-cutting 2D-LC (LC-LC), and other chromatographic combinations with mobile phase incompatibility issues.

Increase the flow rate and improve hydrogen deuterium exchange mass spectrometry

Reversed-phase peptide separation in hydrogen deuterium exchange (HDX) mass spectrometry (MS) must be done with conditions where the back exchange is the slowest possible, the so-called quench conditions of low pH and low temperature. To retain maximum deuterium, separation must also be done as quickly as possible. The low temperature (0 °C) of quench conditions complicates the separation and leads primarily to a reduction in separation quality and an increase in chromatographic backpressure. To improve the separation in HDX MS, one could use a longer gradient, smaller particles, a different separation mechanism (for example, capillary electrophoresis), or multi-dimensional separations such as combining ion mobility separation with reversed-phase separation. Another way to improve separations under HDX MS quench conditions is to use a higher flow rate where separation efficiency at 0 °C is more ideal. Higher flow rates, however, require chromatographic systems (both pumps and fittings) with higher backpressure limits. We tested what improvements could be realized with a commercial UPLC/UHPLC system capable of ∼20,000 psi backpressure. We found that a maximum flow rate of 225 µL/min (using a 1 × 50 mm column packed with 1.8 µm particles) was possible and that higher flow rate clearly led to higher peak capacity. HDX MS analysis of both simple and particularly complex samples improved, permitting both shorter separation time, if desired, and providing more deuterium recovery.

High-temperature liquid chromatography for evaluation of the efficiency of multiwalled carbon nanotubes as nano extraction beds for removal of acidic drugs from wastewater. Greenness profiling and comprehensive kinetics and thermodynamics studies

The retention behavior of a series of acidic drugs, namely ketoprofen (KET), naproxen (NAP), diclofenac (DIC), and ibuprofen (IBU), on the heat-resisting ZORBAX 300SB-C₁₈ column, was studied thermodynamically using high-temperature liquid chromatography (HTLC). A perfect correlation of the compounds' lipophilicity and the calculated thermodynamic indicators evidenced its contribution to the retention behavior. Besides, the steric fitting has a subsidiary effect on IBU retention. Isocratic HTLC separation of the four compounds was achieved using an aqueous mobile phase containing 30% acetonitrile-0.2% acetic acid-0.2% triethylamine at 60 °C. This method has been utilized to monitor the adsorption efficiency of multiwalled carbon nanotubes (MWCNTs) for the removal of the four NSAIDs from water. Different variables affecting the remediation process have been optimized such as the time of contact, pH, ionic strength, temperature, and the mass of MWCNTs. The kinetics and thermodynamics of the adsorption were investigated. The adsorption was evidenced to take place via pseudo-second-order kinetics and the intraparticle diffusion is the rate-controlling step. The thermodynamic investigation showed that the adsorption process is exothermic and enthalpy-driven, and the adsorption is more extensive at a lower temperature. The MWCNTs showed excellent adsorption efficiency of about 76.4 to 97.6% at the optimum conditions. The obtained results are promising and encouraging for the full-scale application of MWCNTs for remediation of NSAIDs-related water pollution. The green analytical chemistry metric "AGREE" and the analytical eco-scale score tool confirmed that the developed protocol is greener and more favorable to the environment and user than most of the reported literature.

Enhancement of fucosylated N-glycan isomer separation with an ultrahigh column temperature in porous graphitic carbon liquid chromatography-mass spectrometry

Due to the heterogeneous and isomeric nature of glycans, the development of an advanced separation of distinct glycan isomers is essential for glycan research and application. In this study, we utilized porous graphite carbon (PGC) chromatography for the separation of isomeric oligosaccharides without reduction or chemical derivatization at 190 °C in a custom-built heating oven. Furthermore, the fine structures of glycan isomers could be identified by using ultrahigh temperature PGC liquid chromatography mass spectrometry (UHT-PGC-LCMS). A nonreduced hydrolyzed dextran was applied to verify the performance of UHT-PGC. When the temperature of the PGC column was increased from 25 to 190 °C, the liquid chromatography separation power of the nonreduced dextran ladder significantly increased. The advantage of the UHT-PGC column was its high peak capacity with gradient elution in 10 min at 190 °C, 6700 psi, and a 250 μL/min flow rate for native glycan analysis. Four synthetic Lewis antigen isomers were used to elucidate the separation effectiveness in UHT-PGC. Moreover, mass spectrometry-based sequencing to generate specific diagnostic ions from the four synthetic Lewis antigens was used to predict isomeric glycans based on the relative intensity ratio (RIR) of diagnostic ions. The intensities of the diagnostic ions of synthetic isomers were used to identify each isomer of the fucosylated glycan. The results clearly showed that terminal Lewis A and X residues were in the 3- and 6-arms of N-glycan, respectively.

A Plug-and-Play High-Pressure ESI Source with an Emitter at Ground Potential and Its Application to High-Temperature Capillary LC-MS

A new high-pressure ESI source that can be readily used for commercial API mass spectrometers in a plug-and-play manner without any modification on the ion sampling interface is introduced. The emitter can be operated at ground potential, and the positive mode electrospray is generated by applying a negative high potential to the counter electrode. A shielding electrode effectively shields the opposing electric field and improves the ion transmission. This feature facilitates the direct connection of the ESI emitter to the electrically grounded components. The application of the present ion source to the high-temperature (>100 °C) capillary liquid chromatography for high-speed separation of peptide and proteins is demonstrated using a monolithic polymeric column.