High-Throughput Capillary Liquid Chromatography Using a Droplet Injection and Application to Reaction Screening.

The cycle time of a standard liquid chromatography (LC) system is the sum of the time for the chromatographic run and the autosampler injection sequence. Although LC separation times in the 1-10 s range have been demonstrated, injection sequences are commonly >15 s, limiting throughput possible with LC separations. Further, such separations are performed on relatively large bore columns requiring flow rates of ≥5 mL/min, thus generating large volumes of mobile phase waste when used for large scale screening and increasing the difficulty in interfacing to mass spectrometry. Here, a droplet injector system was established that replaces the autosampler with a four-port, two-position valve equipped with a 20 nL internal loop interfaced to a syringe pump and a three-axis positioner to withdraw sample droplets from a well plate. In the system, sample and immiscible fluid are pulled alternately from a well plate into a capillary and then through the injection valve. The valve is actuated when sample fills the loop to allow sequential injection of samples at high throughput. Capillary LC columns with 300 µm inner diameter were used to reduce the consumption of mobile phase and sample. The system achieved 96 separations of 20 nL droplet samples containing 3 components in as little as 8.1 min with 5-s cycle time. This system was coupled to a mass spectrometer through an electrospray ionization source for high-throughput chemical reaction screening.

The utilization of open-source microcontroller boards and single-board computers in liquid chromatography.

Electronic system control of analytical instrumentation remains a critical aspect of modern measurement science. Within the field of liquid chromatography (LC), this is especially relevant for automation, module operation, detection, data acquisition, and data analysis. Increasingly, home-built analytical tools used for liquid-phase separations rely upon open-source microcontrollers and single-board computers to aid in simplifying these operations. In this review, we detail literature reported within the past 5 years in which these types of devices were used to advance various aspects of the LC research field, including sample preparation, instrument control, and data collection.

High-Throughput Liquid Chromatographic Analysis Using a Segmented Flow Injector with a 1 s Cycle Time.

High-throughput screening (HTS) workflows are revolutionizing many fields, including drug discovery, reaction discovery and optimization, diagnostics, sensing, and enzyme engineering. Liquid chromatography (LC) is commonly deployed during HTS to reduce matrix effects, distinguish isomers, and preconcentrate prior to detection, but LC separation time often limits throughput. Although subsecond LC separations have been demonstrated, they are rarely utilized during HTS due to limitations associated with the speed of common autosamplers. In this work, these limits are overcome by utilizing droplet microfluidics for sample introduction. In the method, a train of samples segmented by air are continuously pumped into the inlet of an LC injection valve that is actuated once each sample fills the sample loop. Coupled with 2.1 mm diameter × 5 mm long columns packed with 2.7 µm superficially porous C18 particles operated at 5 mL/min, the injector enabled separation of 3 components at 1 s/sample and analysis of a 96-well plate in 1.6 min with <2% peak area relative standard deviation. Analyte-dependent carryover was minimized by including wash droplets composed of organic solvent in between sample droplets. High-throughput LC coupled with mass spectrometric detection using the segmented flow injector was applied to a screen of inhibitors of a cytochrome P450-catalyzed hydroxylation reaction. Measurements of the reaction substrate and product concentrations made using fast LC with the segmented flow injector correlated well with measurements made using a more conventional, 3 min LC method. These results demonstrate the potential for droplet microfluidics to be used for sample introduction during high-throughput LC analysis.

Development of a dual-electrospray ionization source with in-line absorbance-based voltage control.

Emitter tip arrays for electrospray ionization have been used for a variety of MS sample introduction purposes, including detection of multiple sample eluent streams and improved accuracy through parallel infusion of an internal standard. User control is typically required for targeted application of high voltage to specific channels to maximize analyte signal and minimize other background signals. In this communication, an automated approach to applying electrospray voltage only when a detectable analyte is present is described. An in-line absorbance detector is used to identify the presence of an analyte in the fluidic path between the sample introduction valve and the mass spectrometer. A Raspberry Pi-controlled system is then used to apply high voltage to a downstream emitter tip at the MS inlet following a delay volume between the detectors. Demonstration of this technique on two parallel sample channels is reported, including a pulsed voltage application to maximize signal when analytes elute on each channel simultaneously.

Use of N-(4-aminophenyl)piperidine derivatization to improve organic acid detection with supercritical fluid chromatography-mass spectrometry.

The analysis of organic acids in complex mixtures by LC-MS can often prove challenging, especially due to the poor sensitivity of negative ionization mode required for detection of these compounds in their native (i.e., underivatized or untagged) form. These compounds have also been difficult to measure using supercritical fluid chromatography (SFC)-MS, a technique of growing importance for metabolomic analysis, with similar limitations based on negative ionization. In this report, the use of a high proton affinity N-(4-aminophenyl)piperidine derivatization tag is explored for the improvement of organic acid detection by SFC-MS. Four organic acids (lactic, succinic, malic, and citric acids) with varying numbers of carboxylate groups were derivatized with N-(4-aminophenyl)piperidine to achieve detection limits down to 0.5 ppb, with overall improvements in detection limit ranging from 25-to-2100-fold. The effect of the derivatization group on sensitivity, which increased by at least 200-fold for compounds that were detectable in their native form, and mass spectrometric detection are also described. Preliminary investigations into the separation of these derivatized compounds identified multiple stationary phases that could be used for complete separation of all four compounds by SFC. This derivatization technique provides an improved approach for the analysis of organic acids by SFC-MS, especially for those that are undetectable in their native form.

Exploring Biopharmaceutical Analysis with Compact Capillary Liquid Chromatography Instrumentation.

A recent trend in the design of liquid chromatography (LC) instrumentation is the move towards miniaturized and portable systems. These smaller platforms provide wider flexibility in operation, with the opportunity for conducting analysis directly at the point of sample collection rather than transporting the sample to a centralized laboratory facility. For the manufacturing of pharmaceutical and biopharmaceutical products, these platforms can be implemented for process monitoring and product characterization directly in manufacturing environments. This article describes a portable, miniaturized LC instrument coupled to a mass spectrometer (MS) for characterization of a biopharmaceutical monoclonal antibody (mAb).

Navigating the Future of Separation Science Education: A Perspective.

A number of recommendations on how to improve the education and training of separation scientists were recently made by the National Academies of Sciences, Engineering, and Mathematics in their report, A Research Agenda for Transforming Separation Science. This perspective outlines how some of these recommendations may be fulfilled by examining trends in potential curriculum topics related to the field and new technological platforms for interactive content delivery. Identifying and adopting the best practices within these emerging educational directions will ensure the future success of the field.

A microfluidic chip for on-line derivatization and application to in vivo neurochemical monitoring.

Microfluidic chips can perform a broad range of automated fluid manipulation operations for chemical analysis including on-line reactions. Derivatization reactions carried out on-chip reduce manual sample preparation and improve experimental throughput. In this work we develop a chip for on-line benzoyl chloride derivatization coupled to microdialysis, an in vivo sampling technique. Benzoyl chloride derivatization is useful for the analysis of small molecule neurochemicals in complex biological matrices using HPLC-MS/MS. The addition of one or more benzoyl groups to small, polar compounds containing amines, phenols, thiols, and certain alcohols improves reversed phase chromatographic retention, electrospray ionization efficiency, and analyte stability. The current derivatization protocol requires a three-step manual sample preparation, which ultimately limits the utility of this method for rapid sample collection and large sample sets. A glass microfluidic chip was developed for derivatizing microdialysis fractions on-line as they exit the probe for collection and off-line analysis with HPLC-MS/MS. Calibration curves for 21 neurochemicals prepared using the on-chip method showed linearity (R2 > 0.99), limits of detection (0.1-500 nM), and peak area RSDs (4-14%) comparable to manual derivatization. Method temporal resolution was investigated both in vitro and in vivo showing rapid rise times for all analytes, which was limited by fraction length (3 min) rather than the device. The platform was applied to basal measurements in the striatum of awake rats where 19 of 21 neurochemicals were above the limit of detection. For a typical 2 h study, a minimum of 120 pipetting steps are eliminated per animal. Such a device provides a useful tool for the analysis of small molecules in biological matrices which may extend beyond microdialysis to other sampling techniques.

Two-dimensional separation techniques using supercritical fluid chromatography.

High-resolution separation systems are essential for the analysis of complex mixtures in a wide variety of application areas. To increase resolution, multidimensional chromatographic techniques have been one key solution. Supercritical fluid chromatography provides a unique opportunity in these multidimensional separations based on its potential for high solvent compatibility, rapid duty cycles, and orthogonality to other separation modes. This review focuses on two-dimensional chromatography methods from the past decade that use supercritical fluid chromatography because of these advantages. Valving schemes and modulation strategies used to interface supercritical fluid chromatography with other liquid chromatography and gas chromatography techniques are described. Particular applications of multidimensional separations using supercritical fluid chromatography for the analysis of oils and chiral separations of pharmaceutical compounds are highlighted. Limitations of and a potential trajectory for supercritical fluid chromatography in this field are also discussed.

Strategies in developing high-throughput liquid chromatography protocols for method qualification of pharmacopeial monographs.

Method qualification is a key step in the development of routine analytical monitoring of pharmaceutical products. However, when relying on published monographs that describe longer method times based on older high-performance liquid chromatography column and instrument technology, this can delay the overall analysis process for generated drug products. In this study, high-throughput ultrahigh pressure liquid chromatography techniques were implemented to decrease the amount of time needed to complete a 24-run sequence to identify linearity, recovery, and repeatability for both drug assay and impurity analysis in 16 min. Multiple experimental parameters were tested to identify a range of experimental settings that could be used for the sequence while still maintaining this fast analysis time. The full sequence was replicated on a different system and with different columns, further demonstrating its robustness.

Portable capillary liquid chromatography for pharmaceutical and illicit drug analysis.

A newly developed portable capillary liquid chromatograph was investigated for the separation of various pharmaceutical and illicit drug compounds. The system consists of two high-pressure syringe pumps capable of delivering capillary-scale flow rates at pressures up to 10 000 psi. Capillary liquid chromatography columns packed with sub-2 µm particles are housed in cartridges that can be inserted into the system and easily connected through high-pressure fluidic contact points by simply applying a specific, predetermined torque rather than using standard fittings and less precise sealing protocols. Several over-the-counter analgesic drug separations are demonstrated, along with a simple online measurement of tablet dissolution. Twenty illicit drug compounds were also separated across six targeted drug panels. The results described in this study demonstrate the capability of this compact liquid chromatography instrument to address several important drug-related applications while simplifying system operation, and greatly reducing solvent usage and waste generation essential for onsite analysis.

Monitoring cell secretions on microfluidic chips using solid-phase extraction with mass spectrometry.

Microfluidics is an enabling technology for both cell biology and chemical analysis. We combine these attributes with a microfluidic device for on-line solid-phase extraction (SPE) and mass spectrometry (MS) analysis of secreted metabolites from living cells in culture on the chip. The device was constructed with polydimethylsiloxane (PDMS) and contains a reversibly sealed chamber for perfusing cells. A multilayer design allowed a series of valves to control an on-chip 7.5 µL injection loop downstream of the cell chamber with operation similar to a six-port valve. The valve collects sample and then diverts it to a packed SPE bed that was connected in-line to treat samples prior to MS analysis. The valve allows samples to be collected and injected onto the SPE bed while preventing exposure of cells to added back pressure from the SPE bed and organic solvents needed to elute collected chemicals. Here, cultured murine 3T3-L1 adipocytes were loaded into the cell chamber and non-esterified fatty acids (NEFAs) that were secreted by the cells were monitored by SPE-MS at 30 min intervals. The limit of detection for a palmitoleic acid standard was 1.4 µM. Due to the multiplexed detection capabilities of MS, a variety of NEFAs were detected. Upon stimulation with isoproterenol and forskolin, secretion of select NEFAs was elevated an average of 1.5-fold compared to basal levels. Despite the 30-min delay between sample injections, this device is a step towards a miniaturized system that allows automated monitoring and identification of a variety of molecules in the extracellular environment.

Advances in and prospects of microchip liquid chromatography.

This review focuses on recent advances in the field of microfluidic liquid chromatography from January 2013 through April 2015. Articles are organized by the type of stationary phase support focusing on device fabrication, column preparation, and use for specific applications. Additionally, a comprehensive table comparing chromatographic figures of merit for the work described is included as Appendix A as a reference for readers.

An Inexpensive, Open-Source USB Arduino Data Acquisition Device for Chemical Instrumentation.

Many research and teaching labs rely on USB data acquisition devices to collect voltage signals from instrumentation. However, these devices can be cost-prohibitive (especially when large numbers are needed for teaching labs) and require software to be developed for operation. In this article, we describe the development and use of an open-source USB data acquisition device (with 16-bit acquisition resolution) built using simple electronic components and an Arduino Uno that costs under $50. Additionally, open-source software written in Python is included so that data can be acquired using nearly any PC or Mac computer with a simple USB connection. Use of the device was demonstrated for a sophomore-level analytical experiment using GC and a CE-UV separation on an instrument used for research purposes.

Repetitive mild traumatic brain injury impairs norepinephrine system function and psychostimulant responsivity.

Traumatic brain injury (TBI) is a complex pathophysiological process that results in a variety of neurotransmitter, behavioral, and cognitive deficits. The locus coeruleus-norepinephrine (LC-NE) system is a critical regulator of arousal levels and higher executive processes affected by TBI including attention, working memory, and decision making. LC-NE axon injury and impaired signaling within the prefrontal cortex (PFC) is a potential contributor to the neuropsychiatric symptoms after single, moderate to severe TBI. The majority of TBIs are mild, yet long-term cognitive deficits and increased susceptibility for further injury can accumulate after each repetitive mild TBI. As a potential treatment for restoring cognitive function and daytime sleepiness after injury psychostimulants, including methylphenidate (MPH) that increase levels of NE within the PFC, are being prescribed "off-label". The impact of mild and repetitive mild TBI on the LC-NE system remains limited. Therefore, we determined the extent of LC-NE and arousal dysfunction and response to therapeutic doses of MPH in rats following experimentally induced single and repetitive mild TBI. Microdialysis measures of basal NE efflux from the medial PFC and arousal measures were significantly lower after repetitive mild TBI. Females showed higher baseline PFC-NE efflux than males following single and repetitive mild TBI. In response to MPH challenge, males exhibited a blunted PFC-NE response and persistent arousal levels following repetitive mild TBI. These results provide critical insight into the role of catecholamine system dysfunction associated with cognitive deficits following repeated injury, outcome differences between sex/gender, and lack of success of MPH as an adjunctive therapy to improve cognitive function following injury.

Supercritical Fluid Nanospray Mass Spectrometry: II. Effects on Ionization.

Nanospraying supercritical fluids coupled to a mass spectrometer (nSF-MS) using a 90% supercritical fluid CO2 carrier (sCO2) has shown an enhanced desolvation compared to traditional liquid eluents. Capillaries of 25, 50, and 75 µm internal diameter (i.d.) with pulled emitter tips provided high MS detection sensitivity. Presented here is an evaluation of the effect of proton affinity, hydrophobicity, and nanoemitter tip size on the nSF-MS signal. This was done using a set of primary, secondary, tertiary, and quaternary amines with butyl, hexyl, octyl, and decyl chains as analytes. Each amine class was analyzed individually to evaluate hydrophobicity and proton affinity effects on signal intensity. The system has shown a mass sensitive detection on a linear dynamic range of 0.1-100 µM. Results indicate that hydrophobicity has a larger effect on the signal response than proton affinity. Nanospraying a mixture of all amine classes using the 75 µm emitter has shown a quaternary amine signal not suppressed by competing analytes. Competing ionization was observed for primary, secondary, and tertiary amines. The 75 and 50 µm emitters demonstrated increased signal with increasing hydrophobicity. Surprisingly, the 25 µm i.d. emitter yielded a signal decrease as the alkyl chain length increased, contrary to conventional understanding. Nanospraying the evaporative fluid in a sub-500 nm emitter likely resulted in differences in the ionization mechanism. Results suggest that 90% sCO2 with 9.99% methanol and 0.01% formic acid yielded fast desolvation, high ionization efficiency, and low matrix effect, which could benefit complex biological matrix analysis.

Column selection considerations in compact capillary liquid chromatography.

Recent years have seen significant advances in compact, portable capillary LC instrumentation. This study explores the performances of several commercially available columns within the pressure and flow limits of both the columns and one of these compact LC instruments. The commercially available compact capillary LC system with UV-absorbance detector used in this study is typically operated using columns in the 0.15-0.3 mm internal diameter (i.d.) range. Efficiency measurements (i.e., theoretical plates, N) for six columns with i.d.s in this range and of varying lengths and pressure limits, packed with stationary phases of different particle diameters and morphologies, were made using a mixture of standard alkylphenones. Kinetic plot comparisons between columns that vary by one (or more) of these parameters are described, along with calculated kinetic performance and Knox-Saleem limits. These theoretical performance descriptions provide insight into optimal operating conditions when using capillary LC systems. Based on kinetic plot evaluation of available capillary columns in the 0.2-0.3 mm i.d. range with a conservative upper pressure limit of 330 bar packed with superficially porous particles, a 25 cm column could generate ∼47,000 plates in 7.85 min when operated at 2.4 µL/min. For comparison, more robust 0.3 mm i.d. columns (packed with fully porous particles) that can be operated at higher pressures than can be provided by the pumping system (conservative pump upper pressure limit of 570 bar), a ∼20 cm column could generate nearly 40,000 plates in 5.9 min if operated at 6 µL/min. Across all capillary LC columns measured, higher pressure limits and shorter columns can provide the best throughput when considering both speed and efficiency.

Morphology and separation efficiency of low-aspect-ratio capillary ultrahigh pressure liquid chromatography columns.

We derive a quantitative relationship between the bed morphology and the chromatographic separation efficiency of capillary columns packed with sub-2 µm particles, covering capillary inner diameters from 10 to 75 µm. Our study focuses on wall effects and their impact on band broadening at increasing column-to-particle diameter (aspect) ratios. We approach these complex effects by a morphological analysis of reconstructed column segments composed of several thousand particles that were imaged by confocal laser scanning microscopy. Radial interparticle porosity profiles including wall effects are quantified through an integral porosity deviation, a scalar measure that proves to be a general descriptor of transcolumn porosity heterogeneity. It correlates with the associated transcolumn eddy dispersion, which dominates band broadening in the capillaries and is visualized in the plate height curves by a simple velocity-proportional term. Our comprehensive approach identifies the packing structure features that contribute to decreased efficiency as reflected, e.g., in subtle variations of the wall effect at different aspect ratios, or a particle size-segregation effect in larger-diameter columns as a result of an increased number of packing voids near the wall-bed interface.

Supercritical Fluid Nanospray Mass Spectrometry.

Supercritical fluids are typically electrosprayed using an organic solvent makeup flow to facilitate continuous electrical connection and enhancement of electrospray stability. This results in sample dilution, loss in sensitivity, and potential phase separation. Premixing the supercritical fluid with organic solvent has shown substantial benefits to electrospray efficiency and increased analyte charge state. Presented here is a nanospray mass spectrometry system for supercritical fluids (nSF-MS). This split flow system used small i.d. capillaries, heated interface, inline frit, and submicron emitter tips to electrospray quaternary alkyl amines solvated in supercritical CO2 with a 10% methanol modifier. Analyte signal response was evaluated as a function of total system flow rate (0.5-1.5 mL/min) that is split to nanospray a supercritical fluid with linear flow rates between 0.07 and 0.42 cm/sec and pressure ranges (15-25 MPa). The nSF system showed mass-sensitive detection based on increased signal intensity for increasing capillary i.d. and analyte injection volume. These effects indicate efficient solvent evaporation for the analysis of quaternary amines. Carrier additives generally decreased signal intensity. Comparison of the nSF-MS system to the conventional SF makeup flow ESI showed 10-fold signal intensity enhancement across all the capillary i.d.s. The nSF-MS system likely achieves rapid solvent evaporation of the SF at the emitter point. The developed system combined the benefits of the nanoemitters, sCO2, and the low modifier percentage which gave rise to enhancement in MS detection sensitivity.