2D plot shows separation of benzyl alcohol, benzene and butylbenzene using remote NMR/MRI with a monolithic chromatography column. Horizontal axis corresponds to the NMR chemical shift, vertical axis represents the transit time of compounds undergoing chromatographic separation. |
By
pairing an award-winning remote-detection version of NMR/MRI technology
with a unique version of chromatography specifically designed for
microfluidic chips, researchers with the U.S. Department of Energy
(DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) have opened
the door to a portable system for highly sensitive multi-dimensional
chemical analysis that would be impractical if not impossible with
conventional technologies.
Alexander
Pines, a faculty senior scientist in Berkeley Lab’s Materials Sciences
Division and the Glenn T. Seaborg Professor of Chemistry at the
University of California (UC) Berkeley, is one of the world’s foremost
authorities on nuclear magnetic resonance (NMR) and its daughter
technology, magnetic resonance imaging (MRI). In this latest
development, he led a collaboration in which a remote detection NMR/MRI
technique that can rapidly identify the chemical constituents of samples
in microfluidic “lab-on-a-chip” devices was used to perform analyses in
a micro-scale monolithic chromatograph column.
“We
have presented the first demonstration that a monolithic chromatograph
column can be used to separate small molecules on a timescale that is
compatible with NMR/MRI detection, an important first step to portable
chromatographic devices,” says Vikram Bajaj, a project scientist in the
Pines’ group who is the corresponding author of a paper describing this
work in Analytical Chemistry.
The
Analytical Chemistry paper is titled “Remotely Detected NMR for the
Characterization of Flow and Fast Chromatographic Separations Using
Organic Polymer Monoliths.” Co-authoring the paper with Pines and Bajaj
were Thomas Teisseyre, Jiri Urban, Nicholas Halpern-Manners, Stuart
Chambers, and Frantisek Svec.
Vikram Bajaj (left) and Alexander Pines developed a remote NMR/MRI technology that received a 2011 R&D 100 Award. Photo: Roy Kaltschmidt, Berkeley Lab |
Chromatography
is one of the indispensable tools of chemistry. By dissolving sample
into a fluid—called the mobile phase—and flushing it through a
solid medium—called the stationary phase—chemists can separate the
sample’s constituent chemical species—called analytes—for
identification and measurement, as well as for purification purposes.
Analytes will be separated on the basis of how fast each individual
species diffuses through the stationary phase.
“The
coupling of our remote NMR/MRI technology with monolithic
chromatography columns in a microfluidic chip enables us to obtain high
resolution, velocity-encoded images of a mobile phase flowing through
the stationary phase,” Bajaj says. “Our technique provides both
real-time peak detection and chemical shift information for small
aromatic molecules, and demonstrates the unique power of magnetic
resonance, both direct and remote, in studying chromatographic
processes.”
The
coupling of remote NMR/MRI to chromatography was made possible by the
polymer monolithic column, a technology developed by Analytical
Chemistry paper coauthor Frantisek Svec, a chemist who directs the
Organic and Macromolecular Synthesis facility at Berkeley Lab’s
Molecular Foundry, a DOE nanoscience center. In conventional
chromatography, the stationary phase column is typically filled with
porous polymer beads or some other discrete medium whose physical or
chemical properties modulate the diffusion rates of analytes passing
through. In Svec’s stationary phase, a chromatography column is filled
with a monolithic solid polymer—meaning it is a single, continuous
piece—that is perforated throughout with nanoscopic pores.
“Polymer
monoliths as a separation media can be compared to a single large
particle that does not contain inter-particular voids,” Svec says. “As a
result, all the mobile phase must pass through the stationary phase as
convective flow rather than diffusion during chromatographic processes.
This convective flow greatly accelerates the rate of analyte
separation.”
The
remote NMR/MRI technology whose development was led by Pines won a 2011
R&D 100 Award. These awards, known as the “Oscars of Innovation,”
recognize the year’s 100 most significant proven technological advances.
Through a combination of remote instrumentation, JPEG-style image
compression algorithms and other key enhancements, this remote NMR/MRI
technology can zoom in on microscopic objects of interest within a
sample flowing through the columns of a microfluidic chip with
unprecedented spatial and time resolutions.
“Our
remote NMR/MRI technology enables time-resolved imaging of
multi-channel flow, dispenses with the need for large and expensive
magnets for analysis, allows us to analyze complex and unprocessed
mixtures in one pass, and adds portability to NMR/MRI,” Bajaj says.
Frantisek Svec, a chemist with Berkeley Lab’s Molecular Foundry, has developed unique polymer monoliths as a separation media for microfluidic chromatography. Photo: Roy Kaltschmidt, Berkeley Lab |
The
key to the success of remote NMR/MRI technology is the decoupling of
the NMR/MRI signal encoding and detection phases. NMR/MRI signals arise
from a property found in the atomic nuclei of almost all molecules
called “spin,” which makes the nuclei act as if they were bar magnets
with poles that point either “north” or “south.” Obtaining an NMR/MRI
signal from a sample depends upon an excess of nuclear spins pointing in
one direction or the other. In a conventional NMR/MRI set up, in which
the signal encoding and detection phases take place within one machine,
this require the presence of a powerful external magnetic field. The
remote NMR/MRI technology developed by Pines and his group, in which
NMR/MRI signal encoding and detection are carried out independently, can
detect NMR/MRI signals without the need of such a strong magnet, yet it
still provides the same outstanding sensitivity of conventional
NMR/MRI.
“With
our remote NMR/MRI technology and the polymer monoliths of Frank Svec’s
group, we were able to look inside optically opaque microfluidic
columns and measure the velocity of the flowing fluid during a
chromatographic separation,” Bajaj says. “We were also able to
demonstrate in-line monitoring of chromatographic separations of small
molecules at high flow rates.”
Results
using the remote NMR/MRI technique with the polymer monoliths showed a
much better ability to discriminate between different analytes at the
molecular level, Bajaj says, than comparable analysis using spectrometry
based on either mass or optical properties. This paves the way for
multidimensional analysis, in which the result of a chromatographic
separation would be encoded into an NMR/MRI signal by charge, size, or
some other factor and stored. The encoded fluid would then be run
through a second separation and those results would also be encoded into
an NRM/MRI signal and stored.
“This
would allow us to create a multidimensional chromatography experiment
that does not require the fluid volume to be physically partitioned,”
Bajaj says. “The fluid would, quite literally, be partitioned in the
magnetic degrees of freedom instead.”
