12 research outputs found
Resolution modeling of length tuning in gas chromatography
Tunable selectivity provides a relatively simple and inexpensive way to manipulate peak positions and gain resolution in chromatographic separations. Length tuning utilizes two columns of different polarities connected in series. Selectivity is manipulated by changing the relative lengths of the two columns. However, a direct correlation is not seen between relative length and effective contribution due to gas compression effects. Rather, a direct correlation is observed between the carrier gas transport time through a segment of column (relative to the total carrier gas transport time) and the effective contribution of that segment. This relationship has been used to predict retention data for analytes in a target mixture, and to determine the combination of columns that would result in the best resolution overall. Further examination reveals that at a given length fraction, the effective contributions of the columns in series are independent of inlet pressure. The relative resolution, a measure of peak separation independent of peak width, is thus constant. In contrast, the resolution calculated using the Purnell Equation does depend on inlet pressure, in accordance with the plate height measured for each individual column and its fractional contribution to a tandem-column separation. Retention factors, peak widths, and plate heights for aromatic molecules of varying functionality, and homologous series of alkanes and alcohols were measured over a range of pressures using both individual columns and several different tandem-column combinations. Measured values of overall plate height and resolution closely matched theoretical predictions and it was determined that theoretical surface plots could be used to accurately predict optimal length fractions for separation
Advances in ultra-high-pressure and multi-dimensional liquid chromatography instrumentation and workflows
The present contribution discusses recent advances in ultra-high-pressure liquid chromatography (UHPLC) and multi-dimensional liquid chromatography (MDLC) technology. First, new developments in UHPLC column technology and system design are highlighted. The latter includes a description of a novel injector concept enabling method speed-up, emerging detectors, and instrument diagnostics approaches. Next, online MDLC workflows are reviewed and advances in modulation technology are highlighted. Finally, key applications published in 2020 are reviewed
Comparison of Design Approaches for Low-Cost Sampling Mechanisms in Open-Source Chemical Instrumentation
Robotic positioning systems are used in a variety of chemical instruments, primarily for liquid handling purposes, such as autosamplers from vials or well plates. Here, two approaches to the design of open-source autosampler positioning systems for use with 96-well plates are described and compared. The first system, a 3-axis design similar to many low-cost 3D printers that are available on the market, is constructed using an aluminum frame and stepper motors. The other system relies upon a series of 3D printed parts to achieve movement with a series of linker arms based on Selective Compliance Assembly Robot Arm (SCARA) design principles. Full printer design files, assembly instructions, software, and user directions are included for both samplers. The positioning precision of the 3-axis system is better than the SCARA mechanism due to finer motor control, albeit with a slightly higher cost of materials. Based on the improved precision of this approach, the 3-axis autosampler system was used to demonstrate the generation of a segmented flow droplet stream from adjacent wells within a 96-well plate
Measurement and Modeling of Extra-Column Effects Due to Injection and Connections in Capillary Liquid Chromatography
As column volumes continue to decrease, extra-column band broadening has become an increasingly important consideration when determining column performance. Combined contributions due to the injector and connecting tubing in a capillary LC system were measured and found to be larger than expected by Taylor-Aris theory. Variance from sigma-type and tau-type broadening was isolated from eluted peaks using the Foley-Dorsey Exponentially Modified Gaussian peak fitting model and confirmed with computational fluid dynamics. It was found that the tau-type contributions were the main cause for the excessive broadening because of poorly-swept volumes at the connection between the injector and tubing. To reduce tau-type contributions (and peak tailing), a timed pinch mode could be used for analyte injection
Pushing the boundaries of chromatography and electrophoresis—Honoring James Jorgenson on his 65th birthday
A low-cost, open-source digital stripchart recorder for chromatographic detectors using a Raspberry Pi
Rowan Faculty Panel on Open Access
In this panel discussion, Rowan University faculty share their perspectives on open access and open access publishing
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 ÎĽm particles, covering capillary inner diameters
from 10 to 75 ÎĽm. 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
Comparison of Design Approaches for Low-Cost Sampling Mechanisms in Open-Source Chemical Instrumentation
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 ÎĽm 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