by Vinh Quang Do, Heidi Fleischer, Dany Hoffmann and Kerstin Thurow
AL
Integration of a Dilution Module in a Mass Spectrometry-Based Online Reaction Monitoring System
Online reaction monitoring enables the study of reaction mechanisms and reaction kinetics through the monitoring of reactants, products and transient species, and can be used for real-time data acquisition1 without unnecessary interruptions or sample manipulation. Among the spectroscopic techniques used for online reaction monitoring, i.e., fluo- rescence, UV/VIS, IR, Raman, X-ray and nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry offers specific information regarding molecular mass and the structure of components in a reaction mixture with higher accuracy, sensitivity and speed.2
However, due to the high
molar concentration of reaction mixtures, sample overloading can occur, resulting in excitation of the detector operating range. Sample overload can also reduce lifetime and result in distorted mass spectra.3,4
One solution is to modify the ionization technique. Ionization techniques such as low-temperature plasma (LTP), extracted electrospray ionization (EESI) and Direct Analysis in Real Time (DART, marketed by JEOL [Peabody, Mass.] and IonSense [Saugus, Mass.]) analyze chemical reactions at molar concentrations.5
followed by thorough mixing.4 flows such as varying the time,6
Another approach is dilution of the analytical solution Dilution can be based on the ratio of input flow rate,7
frequency8 or volume.9
Clinton et al. used a microporous membrane interface to achieve high dilution ratios ([initial volume]/[final volume]).10
A donor solvent contain-
ing the sample was pumped to waste through the groove on one side of the membrane interface. Acceptor solvent was pumped through the other side of the membrane in the opposite direction into an atmospheric pressure chemical ionization-quadrupole mass spectrometer (APCI-MS). A dilution ratio of approx. five orders of magnitude was achieved for the Michael addition reaction of phenylethylamine (PEA) and acrylonitrile in ethanol. The analytical response was influenced by changes in pressure and flow rate of the donor and acceptor solvents.5
Dell’Orco et al. used HPLC pumps and a passive flow splitter to dilute the reaction mixture (piperidine-catalyzed Knoevenagel reaction) by approx. 3000 times. The important reaction intermediates and reaction kinetic time scale were identified, but quantitation of these species was still prob- lematic.3
Using an HPLC pump and a flow splitter coupled with a flow
injection analysis/atmospheric pressure chemical ionization (FIA/APCI) MS, Zhu et al. reported quantitative real-time monitoring of a model reac- tion performed at 1.63 mol/L, a concentration often required in process control.5
via a mixing tee for low ratio dilution.11,12 AMERICAN LABORATORY 36
For dilutions up to 100, 200, 500 or even 1000, another method includes use of an active splitter or mass rate attenuator (MRA), which can reach a dilution ratio of up to 100,000. Cai et al.4
replaced a passive splitting device
with an MRA in a purification system, which resulted in improved system plumbing, volume reduction of the collected fraction and reduced evapo- ration time to reclaim purified products. Toribio and Leister13,14
employed
MRA in the purification of combinatorial libraries, drug metabolites or characterizable impurities. MRA was used for sampling and transferring the analyte(s) to the dilution flow by Bristow et al.1
This make-up flow was
then sent to a portable, low-footprint mass spectrometer. Jeurissen et al.15 configured an MRA after the HPLC column for diluting extracts in an online HPLC detection system. The main usage of the MRA in this application was to lower the solution concentration to a suitable working range of the MS.
Reaction monitoring system For the current study, a mobile reaction system based on FIA was coupled
to an electrospray ionization/time-of-flight (ESI/TOF) MS.16 Analytical so-
lution from the microreactor outflow was injected after dilution into a carrier stream, which was coupled to the analyzer. The dilution module consisted of an MRA, dilution pump and compensation pump, and was able to perform dilutions at different ratios. Settings were determined automatically based on the desired dilution ratio and flow rate in the microreactor, which varies according to the reaction stages.
Hardware
WellChrom K-501 and Smartline 100 HPLC pumps (Knauer GmbH, Berlin, Germany) were used to transfer reactants/educts to the 11.2-mL meander reactor (Ehrfeld Mikrotechnik BTS, Wendelsheim, Germany). The reactor solu- tion temperature was controlled by the F26-HP refrigerated/heating circulator (Julabo, Allentown, Penn.). After exiting the microreactor, the solution was diluted at the dilution module, and then entered a 10-port switching valve via the two-position microelectric valve actuator (Valco Instruments, Houston, Texas). The solution was injected into the carrier stream controlled by the mzr-2942 microannular gear pump (HNP Mikrosystem GmbH, Schwerin, Germany) and CORI-FLOW model M13 mass flowmeter (Bronkhorst Mättig, Kamen, Germany). Finally, the sample was transferred to the TOF-MS G1969A (Agilent Technologies, Santa Clara, Calif.) for analysis.16,17
Dilution module Brum et al. and Browne et al. added a pump to the main stream
The dilution module (Figure 1) comprised the MRA-100-00 (Rheodyne, L.P., Rohnert Park, Calif.) and WellChrom K-501 HPLC pump. Output from the
JANUARY/FEBRUARY 2017
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