Article pubs.acs.org/ac
Simultaneous Online Monitoring of Multiple Reactions Using a Miniature Mass Spectrometer Christopher J. Pulliam,† Ryan M. Bain,† Heather L. Osswald,† Dalton T. Snyder,† Patrick W. Fedick,† Stephen T. Ayrton,† Tawnya G. Flick,*,‡ and R. Graham Cooks†,§ †
Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States Department of Attribute Sciences, Amgen Inc., 1 Amgen Center Drive, Thousand Oaks, California 91320, United States § Center for Analytical Instrumentation Development, West Lafayette, Indiana 47907, United States ‡
S Supporting Information *
ABSTRACT: Advances in chemical sampling using miniature mass spectrometer technology are used to monitor slow reactions at a frequency of ca. 180 h−1 (on the Mini 12) with no sample carryover and with inline derivatization in the case of poorly ionizing compounds. Moreover, we demonstrate high reproducibility with a relative error of less than 10% for major components. Monitoring is enabled using a continuous-flow nanoelectrospray (CF-nESI) probe contained in a custom-built 3D-printed rotary holder. The holder position is automatically set using a stepper motor controlled by a microcontroller. Reaction progress of up to six reactions, including hydrazone formation and Katritzky transamination, can be monitored simultaneously without carryover for several hours.
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spectroscopy.21,22 Despite the rapid growth of process analytical technology, mass spectrometry has not yet become a widely accepted tool.5,11 One exception is the use of membrane introduction (MIMS), which has facilitated analysis of relatively volatile small molecules from solution including fermentors.23,24 One such early demonstration was the online reaction monitoring of the base-catalyzed methylation of cyclohexanone using a membrane interface coupled to a triple quadrupole mass spectrometer.23 Moreover, membranes allow for a wide variety of in situ time-resolved analysis including underwater analysis,24−26 atmospheric analysis,26 forensic air monitoring,27 bioreactor monitoring,28 and intravenous blood monitoring.29 Online MIMS technology has been the subject of several reviews.30,31 While many problems in environmental science, forensics, and bioreactions can be addressed by monitoring volatile analytes using MIMS, online monitoring of less volatile analytes is of much greater interest to process analytical chemists. Atmospheric ionization (e.g., electrospray ionization (ESI)32,33 and nanoESI (nESI) 34 ) and ambient ionization (e.g., desorption electrospray (DESI)35 and paper spray (PS)36,37) have enabled MS to be used for such analytes, the former methods often being coupled to chromatographic separation and the latter not. Time-resolved MS (tracking the evolution of a MS data set over time) has enabled benchtop mass
rocess analytical technology (PAT) is receiving growing attention, as evidenced by the nearly 100-fold increase in publications on the topic over the past two decades (61 publications 1995, 572 publications in 2015 (ISI Web of Science, Dec, 2016)). The Food and Drug Administration defines PAT as a system that aids in understanding and identifying critical points in a process.1−3 Under this broad definition, several technologies have been developed for in situ analysis of chemical reactions.1,3 Prominent among them are optical methods. PAT tools facilitate in situ chemical and physical analysis of pharmaceutical processes to aid experimental design, analysis, and control of large and small scale systems to ensure quality of the final product.4 In many cases, a single PAT tool cannot provide all necessary information on a process, so typically multiple instruments are employed to monitor different attributes of the system.3,5−8 Such a collection of PAT tools is sometimes referred to as a PAT toolbox. The toolbox is used often to understand attributes of crystallinity, chirality, and purity in bulk synthesis.5,9−16 Infrared (IR) and Raman spectroscopy provide quantitative and qualitative data about reaction progress in real time.10,17 Information about reaction kinetics, specific functional group transformations (e.g., formation of a carbonyl group), and the degree to which the reaction has proceeded can be gathered quickly from such data provided that there are unique spectral signatures for the species of interest. This is often the case. Many other techniques also exist for process analysis including nuclear magnetic resonance,18,19 liquid chromatography,20 and UV−vis © XXXX American Chemical Society
Received: January 10, 2017 Accepted: May 18, 2017 Published: May 18, 2017 A
DOI: 10.1021/acs.analchem.7b00119 Anal. Chem. XXXX, XXX, XXX−XXX
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Analytical Chemistry
Figure 1. (a) Samples delivered to the Mini 12 mass spectrometer for analysis of a single analyte. (b) Rotary sprayer for sequential analysis of multiple reaction vessels. (c) Typical scan table for the Mini when monitoring a reaction. (1*) Preionization, (2*) ionization/DAPI open, (3*) cooling, (4*) source trigger, (5*) ion detection, (6) save data. Steps 1−5 are repeated when averaging multiple scans.
the mass spectrometer where they are analyzed. Unlike traditional ESI, nESI has much lower flow rates (ca. 500 nL/ min), does not require nebulizing gas, and typically has a higher ionization efficiency, thus reducing ion suppression when analyzing complex mixtures. Moreover, nESI has virtually no carryover; hence, rapid multisample analysis can be performed.57 In spite of these advantages, the use of miniature mass spectrometers to perform online reaction monitoring is limited to a few cases.53,54
spectrometers to be used to perform rapid, online quantitative and qualitative analysis directly from solution-phase reactions without prior separation using an assortment of ionization techniques, usually ESI.12,13,32,34,38 Air and water sensitive organometallic reactions have been monitored by ESI and inductive-ESI (iESI) mass spectrometry.12−15 Reaction kinetics,13,39 mechanisms,40−44 and catalyst degradation7,45 can also be monitored using mass spectrometry. Notably, iESI, has demonstrated reasonable resistance to process chemist conditions which typically plague ESI such as high salt content. It has been proposed that iESI may undergo a “self-cleaning” process which aids in clog prevention during online chemical analysis.12 An overview of current progress in MS reaction monitoring was presented in recent reviews.46−48 Miniature mass spectrometers bring many of the advantages of a benchtop mass spectrometer with the added benefits of portability, a small footprint, and lower power consumption49−52 but have some cost in terms of performance. A growing number of portable mass spectrometers are being used to perform online chemical analysis from continuous flow reactors.53,54 Early examples of reaction monitoring using a portable/miniature instrument include use of the Mini 10.5 instrument to monitor traces of benzene in the air as well as to monitor chemical reactions using membrane and atmospheric pressure interfaces.41,55 Recent versions of miniature mass spectrometers have been used to analyze trace and bulk material using PS as well as nanoelectrospray (nESI).49,51 nESI is particularly useful because it provides a steady spray without transferring large amounts of material to the miniature instrument.34,56 nESI is performed by spraying the analyte solution from a narrow capillary that has been pulled to a sharp tip. Once the solution has been loaded, an electrode provides high voltage (2 kV) and generates a spray of small charged droplets which are desolvated as they travel to
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EXPERIMENTAL SECTION Hydrazone Formation. All solutions were purchased from Sigma-Aldrich (St. Louis, MO USA). Phenylhydrazine was prepared at a stock concentration of 185 mM in 1 mL of methanol. Solutions of aldehydes were prepared by separately dissolving p-tolualdehyde, p-ethylbenzaldehyde, p-isopropylbenzaldehyde, p-hydroxylbenzaldehyde, p-anisaldehyde, and pdimethylaminobenzaldehyde in methanol to generate a stock concentration of 90 mM in 1 mL of methanol. All aldehydes were prepared to a final concentration of 3 mM, and phenylhydrazine was prepared to a final concentration of 6 mM in 30 mL of methanol. This reaction was performed with 2 equiv of phenylhydrazine. The use of excess phenylhydrazine assured complete reaction, but its presence was apparent in the monitoring data since complete reaction did not result in full disappearance of phenylhydrazine. Katritzky Transamination Synthesis. The Katritzky transamination was performed by combining 2,4,6-triphenylpyrylium tetrafluoroborate (60 mmol in 1 mL of acetonitrile (ACN)) and 4-methoxyaniline (80 mmol in 2 mL of ACN). The two solutions were diluted in 27 mL of methanol.58 Mini 12 Instrumentation. A home-built mass spectrometer, the Mini 12,49 was used in all experiments. The Mini 12 mass analyzer is a rectilinear ion trap which enables access to B
DOI: 10.1021/acs.analchem.7b00119 Anal. Chem. XXXX, XXX, XXX−XXX
Analytical Chemistry
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RESULTS AND DISCUSSION Multisprayer spray ionization techniques can be used in two modes. In the first method, only one sprayer is used at a time, and the others remain off until supplied with a high voltage. The second mode involves all of the emitters spraying simultaneously in a parallel fashion. The sequential mode was chosen for experiments where electrophoresis and traditional liquid chromatography tandem mass spectrometry (LC-MS/ MS) experiments were carried out with the use of an internal standard.60,61 Micromass Inc. has developed a multiemitter ionization source for high throughput HPLC analysis.67 The parallel mode of operation was chosen for other studies because it provides a method of high throughput analysis and mass spectrometric integration of a number of different experiments.62,63 In this study, we will demonstrate simultaneous multisample reaction monitoring of multiple hydrazoneforming reactions and a Katritzky transamination reaction using a custom multisprayer source coupled to the Mini 12 mass spectrometer. Online monitoring directly from a reaction flask was explored using a single hydrazone formation reaction: 1a + 2 to produce 3a (see Scheme 1, Table 1). Reagent 2 (185 mM) was injected
the same scan features as does a commercial linear ion trap, but the instrument has a smaller footprint and a lower power consumption (∼50 W).49 This instrument gives peaks in the low mass range (
