On-line monitoring of continuous flow chemical synthesis using a portable, small footprint mass spectrometer

Development of setups for real-time analysis of the effluent of a microreactor by mass spectrometry

Monitoring a synthesis reaction in real time could allow not only the detection of the intermediates involved in the synthesis, to better understand its mechanisms, but also the impurities. Spectroscopic methods could be performed but are not so performant when analyzing complex mixtures and could require specific properties for the detection of the molecules of interest, the presence of a chromophore moiety for example. Mass spectrometry (MS) may overcome these limitations and is able to reach the accuracy and sensitivity required to efficiently detect, quantify, identify, and characterize the reagents and species produced during the synthesis. This is why the hyphenation of a microreactor with MS has already allowed synthesis processes to be monitored, but most of the time it targets a specific reaction or compounds and involves solvents compatible with MS. In this study, a universal setup for the hyphenation of a microreactor with MS and based on two valves has been developed. This two-valve setup has proven itself for the analysis of molecules of different nature and hydrophilicity, soluble in a large number of solvents even in non-MS-compatible ones. The developed setup evidenced a good repeatability and a linear response for the detection of the studied compounds. In addition, the dilution step included in the two-valve setup allows the MS monitoring of compounds initially synthesized at different concentrations. Finally, it was successfully used to study an amination reaction allowing the detection of the reaction products in 4 min with good repeatability as RSD values of MS signals were lower than 17%.

Review of quantitative and qualitative methods for monitoring photopolymerization reactions

Authomatic in-situ monitoring and characterization of photopolymerization.

Simultaneous self-optimisation of yield and purity through successive combination of inline FT-IR spectroscopy and online mass spectrometry in flow reactions

Self-optimisation constitutes a very helpful tool for chemical process development, both in lab and in industrial applications. However, research on the application of model-free autonomous optimisation strategies (based on experimental investigation) for complex reactions of high industrial significance, which involve considerable intermediate and by-product formation, is still in an early stage. This article describes the development of an enhanced autonomous microfluidic reactor platform for organolithium and epoxide reactions that incorporates a successive combination of inline FT-IR spectrometer and online mass spectrometer. Experimental data is collected in real-time and used as feedback for the optimisation algorithms (modified Simplex algorithm and Design of Experiments) without time delay. An efficient approach to handle intricate optimisation problems is presented, where the inline FT-IR measurements are used to monitor the reaction’s main components, whereas the mass spectrometer’s high sensitivity permits insights into the formation of by-products. To demonstrate the platform’s flexibility, optimal reaction conditions of two organic syntheses are identified. Both pose several challenges, as complex reaction mechanisms are involved, leading to a large number of variable parameters, and a considerable amount of by-products is generated under non-ideal process conditions. Through multidimensional real-time optimisation, the platform supersedes labor- and cost-intensive work-up procedures, while diminishing waste generation, too. Thus, it renders production processes more efficient and contributes to their overall sustainability.

Graphical abstract

MINIATURIZATION IN MASS SPECTROMETRY

Expectations for continuous miniaturization in mass spectrometry are not declining for years. Portable instruments are highly welcome by the industry, science, space agencies, forensic laboratories, and many other units. All are striving for the small, cheap, and as good as possible instruments. This review describes the recent developments of miniature mass spectrometers and also provides selected applications where these devices are used. Upcoming perspectives of further development are also discussed. @ 2019 John Wiley & Sons Ltd. Mass Spec Rev.

Heterogeneous photocatalysis in flow chemical reactors

The synergy between photocatalysis and continuous flow chemical reactors has shifted the paradigms of photochemistry, opening new avenues of research with safer and scalable processes that can be readily implemented in academia and industry. Current state-of-the-art photocatalysts are homogeneous transition metal complexes that have favourable photophysical properties, wide electrochemical redox potentials, and photostability. However, these photocatalysts present serious drawbacks, such as toxicity, limited availability, and the overall cost of rare transition metal elements. This reduces their long-term viability, especially at an industrial scale. Heterogeneous photocatalysts (HPCats) are an attractive alternative, as the requirement for the separation and purification is largely removed, but typically at the cost of efficiency. Flow chemical reactors can, to a large extent, mitigate the loss in efficiency through reactor designs that enhance mass transport and irradiation. Herein, we review some important developments of heterogeneous photocatalytic materials and their application in flow reactors for sustainable organic synthesis. Further, the application of continuous flow heterogeneous photocatalysis in environmental remediation is briefly discussed to present some interesting reactor designs that could be implemented to enhance organic synthesis.

The Medicinal Chemistry in the Era of Machines and Automation: Recent Advances in Continuous Flow Technology

Medicinal chemistry plays a fundamental and underlying role in chemical biology, pharmacology, and medicine to discover safe and efficacious drugs. Small molecule medicinal chemistry relies on iterative learning cycles composed of compound design, synthesis, testing, and data analysis to provide new chemical probes and lead compounds for novel and druggable targets. Using traditional approaches, the time from hypothesis to obtaining the results can be protracted, thus limiting the number of compounds that can be advanced into clinical studies. This challenge can be tackled with the recourse of enabling technologies that are showing great potential in improving the drug discovery process. In this Perspective, we highlight recent developments toward innovative medicinal chemistry strategies based on continuous flow systems coupled with automation and bioassays. After a discussion of the aims and concepts, we describe equipment and representative examples of automated flow systems and end-to-end prototypes realized to expedite medicinal chemistry discovery cycles.

Continuous-flow synthesis and application of polymer-supported BODIPY Photosensitisers for the generation of singlet oxygen; process optimised by in-line NMR spectroscopy

Commercial polystyrene Merrifield-type resins have been post-synthetically functionalised with BODIPY photosensitisers via a novel aryl ester linking strategy in continuous-flow. A unique synthetic advantage of post-synthetically modifying heterogeneous materials in flow was identified. The homogeneous analogues of the polymer-supported BODIPYs were synthesised and used as reference to assess photophysical properties altered by the polymer-support and linker. The homogeneous and polymer-supported BODIPYs were applied in visible-light photosensitisation of singlet oxygen for the conversion of α-terpinene to ascaridole. Materials produced in flow were superior to batch in terms of functional loading and photosensitisation efficiency. Flow photochemical reactions generally outperformed batch by a factor of 4 with respect to rate of reaction. The polymer-supported BODIPY resins could be irradiated for 96 h without loss of photosensitising ability. Additional material synthetic modification and conditions optimisation using an in-line NMR spectrometer resulted in a 24-fold rate enhancement from the initial material and conditions.

Ligand Exchange/Scrambling Study of Gold(I)-Phosphine Complexes in the Solid Phase by DESI-MS Analysis

Only a few analytical techniques are available for the characterization of mechanochemical synthetic reaction products. We demonstrate here that DESI-MS is a powerful technique for this purpose, combining the selectivity of MS-based assays with the simplicity and in situ analysis capability of ambient ionization methods. In this work, we report that auranofin, a gold-based drug, and its precursor triethylphosphine gold(I) chloride undergo a complex array of ligand exchange/scrambling reactions with thiol-containing amino acids in the solid state. The products were readily characterized by DESI-MS analysis from the solid-phase reaction, clearly exhibiting ligand exchange and scrambling, with independent confirmation by solid state ¹³C-NMR. The thioglucose and triethylphosphine moieties exchanged with cysteine and its derivatives, whereas the glutathione replaced 2,3,4,6-tetra-o-acetyl-β-1-D-glucopyranose only. It was concluded that ligand exchange and scrambling reactions can be carried out in the solid state, and some of the unique products reported in this study can be conveniently prepared through mechanochemical synthesis in good yields (> 98%), as demonstrated by synthesis of (L-cysteinato-S)-triethylphosphine gold(I) from triethylphosphine gold(I) chloride and L-cysteine.

Continuous flow chemistry: where are we now? Recent applications, challenges and limitations

A general outlook of the changing face of chemical synthesis is provided in this article through recent applications of continuous flow processing in both industry and academia. The benefits, major challenges and limitations associated with the use of this mode of processing are also given due attention as an attempt to put into perspective the current position of continuous flow processing, either as an alternative or potential combinatory technology for batch processing.

Coupling and optimisation of online nuclear magnetic resonance spectroscopy and mass spectrometry for process monitoring to cover the broad range of process concentration

Real time online monitoring of chemical processes can be carried out by a number of analytical techniques, including optical and vibrational spectroscopies, nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS). As each technique has unique advantages and challenges, combinations are an attractive option. The combination of a 500-MHz ¹ H NMR and a small footprint mass spectrometer to monitor a batch reaction at process concentration was investigated. The mass spectrometer was coupled into the flow path of an online reaction monitoring NMR. Reaction mixture was pumped from a 100-ml vessel to an NMR flow tube before returning to the vessel. Small aliquots were diverted into a sampling make-up flow using an active flow splitter and passed to the mass spectrometer. Advantages of the combination were observed. ¹ H NMR was ideal for quantitation of high level components, whereas MS showed a greater capability for detecting those at low level. In preliminary experiments MS produced a limited linear relationship with concentration (0.02% to 2% relative concentration, 0.01 mg/ml-1.25 mg/ml), because of signal saturation at the higher concentrations. NMR was unable to detect components below 0.1% relative to concentration maximum. Optimisation of sample transfer to the MS extended the linearity to 10% relative to the concentration maximum. Therefore, the combination of online NMR and MS allows both qualitative and quantitative analysis of reaction components over the full process range. The application of the combination was demonstrated by monitoring a batch chemical reaction and this is described. Copyright © 2016 John Wiley & Sons, Ltd.

Current status and future prospects for enabling chemistry technology in the drug discovery process

This review covers recent advances in the implementation of enabling chemistry technologies into the drug discovery process. Areas covered include parallel synthesis chemistry, high-throughput experimentation, automated synthesis and purification methods, flow chemistry methodology including photochemistry, electrochemistry, and the handling of "dangerous" reagents. Also featured are advances in the "computer-assisted drug design" area and the expanding application of novel mass spectrometry-based techniques to a wide range of drug discovery activities.

A benchtop NMR spectrometer as a tool for monitoring mesoscale continuous-flow organic synthesis: equipment interface and assessment in four organic transformations

An approach is reported for monitoring continuous-flow reactions by means of a low-field benchtop NMR spectrometer.

Online quantitative mass spectrometry for the rapid adaptive optimisation of automated flow reactors

An automated continuous reactor for the synthesis of organic compounds, which uses online mass spectrometry (MS) for reaction monitoring and product quantification, is presented.

Determining the suitability of mass spectrometry for understanding the dissolution processes involved with pharmaceutical tablets

RATIONALE

A current challenge for analytical chemists is the development of the measurement systems and approaches required to understand dynamic processes such as tablet dissolution. The design and development of oral tablets could be improved by the availability of detailed information about the rates of release of the individual tablet components. Small footprint mass spectrometry (MS) systems are gaining use for on-line reaction monitoring because of their ability to rapidly determine multiple reactant, intermediate, and product species. We have therefore assessed the utility of such MS systems to the study of dissolution processes.

METHODS

Aqueous dissolution media containing phosphate and other non-volatile buffer salts were pumped from a standard USPII dissolution vessel through an active splitter and back. The splitter sampled the dissolution stream and diluted it into a make-up flow which was pumped to a small single quadrupole mass spectrometer. Single ion monitoring was used to quantify the ions of interest. Three different bio-relevant dissolution media were studied to gauge the effect of the sample matrix.

RESULTS

Individual dissolution profiles were obtained from a tablet containing three drugs, and lactose as the soluble filler. This was successfully demonstrated with three different bio-relevant media designed to reflect the pH of the different sections of the human gastro-intestinal tract. Component concentrations as low as 0.06 µg/mL (representing 1% dissolution) were detected. The MS dissolution profiles correlated with the visual observation of tablet dissolution. MS gave linear responses with concentration for the individual components, although analysis of the tablet solution indicated that ion suppression is an area for further investigation.

CONCLUSIONS

An on-line MS system was used to determine the individual dissolution profiles of three drugs and lactose as they were released from the same tablet. The level of each of these components in solution was determined every 10 seconds, and each had a similar release profile. The dissolution profiles were determined using inorganic buffer solutions at three different bio-relevant pHs.

Understanding bacterial resistance to antimicrobial peptides: From the surface to deep inside

Resistant bacterial infections are a major health problem in many parts of the world. The major commercial antibiotic classes often fail to combat common bacteria. Although antimicrobial peptides are able to control bacterial infections by interfering with microbial metabolism and physiological processes in several ways, a large number of cases of resistance to antibiotic peptide classes have also been reported. To gain a better understanding of the resistance process various technologies have been applied. Here we discuss multiple strategies by which bacteria could develop enhanced antimicrobial peptide resistance, focusing on sub-cellular regions from the surface to deep inside, evaluating bacterial membranes, cell walls and cytoplasmic metabolism. Moreover, some high-throughput methods for antimicrobial resistance detection and discrimination are also examined. This article is part of a Special Issue entitled: Bacterial Resistance to Antimicrobial Peptides.

Watching polymers grow: real time monitoring of polymerizations via an on-line ESI-MS/microreactor coupling

A microreactor/electrospray ionization MS coupling is introduced that allows us to observe and optimize polymerization reactions with respect to chain length and endgroup patterns in real time under synthesis conditions, which was not possible before.