Trapezoidal Microchannels Outperform Circular Designs in Breath-Analysis Chips

Exhaled human breath carries thousands of volatile organic compounds, and among the most diagnostically valuable of them are acetone and ethanol. Acetone is a well-established marker of diabetes, diabetic ketoacidosis, fasting, and ketogenic metabolism, while ethanol appears in breath through endogenous microbial fermentation as well as through diet and exposure, and has been linked to liver disorders, respiratory conditions, gastroenterological diseases, and certain cancers. Because these two oxygenated compounds share similar chemical characteristics and often coexist in biological samples, a portable analyzer that cannot cleanly separate them risks producing misleading results. A new computational study published in Frontiers in Sensors now shows that the shape of the microscopic channel inside a miniaturized gas chromatograph may be the deciding factor in whether such separation succeeds.

The research, conducted by Abdullah of the Department of Electronics Engineering at the Indian Institute of Technology Jodhpur, compares two straight micro gas chromatography columns that differ only in their cross-sectional geometry: one circular, one trapezoidal. Both columns were simulated in COMSOL Multiphysics under strictly identical operating conditions, with helium as the carrier gas and polyethylene glycol, or PEG, as the polar stationary phase suited to oxygen-containing compounds such as ketones and alcohols. Column lengths ranged from 10 to 100 centimeters, and hydraulic diameters spanned 50 to 600 micrometers. Performance was judged by the metrics chromatographers care about most: retention time, the number of theoretical plates, the height equivalent to a theoretical plate known as HETP, and the chromatographic resolution.

The numerical model itself was carefully grounded. The author solved the continuity and Navier-Stokes equations for the laminar, incompressible carrier-gas flow, with Reynolds numbers below 10, and coupled it to a transient convection-diffusion equation describing how acetone and ethanol molecules travel through the channel. Molecular diffusion coefficients were fixed at 2.5 x 10^-10 square meters per second for acetone and 2.0 x 10^-10 for ethanol, and the inlet injection was modeled as a Gaussian pulse at a concentration of 4.4 x 10^-5 moles per cubic meter. The crucial chromatographic interaction, the reversible partitioning of analytes between the flowing helium and the PEG-coated wall, was captured with a Robin boundary condition, using partition coefficients of 90 for acetone and 190 for ethanol, values held constant across every simulation so that geometry alone would determine the outcome.

Before any comparison was made, the model was validated against a previously published finite-element microGC simulation. The reproduced velocity field matched the published maximum velocity closely, 0.45 meters per second versus 0.48, and the outlet concentration profile for a hexane-benzene test mixture reproduced retention times of 21.40 and 28.89 seconds, within a few percent of the reference values. With roughly 1.2 million degrees of freedom per model, an ultrafine physics-controlled mesh with boundary-layer refinement, and mesh independence confirmed by outlet-concentration variations below 0.5 percent, the framework was judged accurate enough to capture the transport phenomena that govern separation in microchannels.

The hydrodynamic results followed classical microfluidic theory with striking clarity. In the circular channel with a fixed inner diameter of 250 micrometers, velocity stayed nearly constant at about 0.6 meters per second regardless of length, while pressure climbed from roughly 300 to nearly 500 pascals as the channel stretched from 20 to 100 centimeters, a 65 to 70 percent increase consistent with the Hagen-Poiseuille law. Diameter proved far more consequential than length: shrinking the circular channel from 600 micrometers down to 100 micrometers sent velocity soaring to about 9 meters per second and pressure drop up to roughly 80,000 pascals, changes of 95 to 97 percent, reflecting the inverse fourth-power dependence of pressure drop on diameter. The trapezoidal channel obeyed the same physics but with greater resistance; at a hydraulic diameter of 250 micrometers its velocity held near 0.4 meters per second while pressure rose almost eightfold, from about 500 to approximately 4,000 pascals, across the same length range.

When the two geometries were put head to head on the acetone-ethanol separation task, the differences became dramatic. The straight circular column delivered theoretical plate numbers between 0.5 and 3.9 thousand plates, HETP values of 0.027 to 0.035 centimeters, and resolutions below 1.25, all achieved at a modest pressure drop of 280 to 500 pascals and a brisk flow rate near 0.5 meters per second. The trapezoidal column, by contrast, reached up to 125,000 theoretical plates, cut HETP to between 0.010 and 0.015 centimeters, and pushed resolution as high as 1.90, comfortably above the threshold needed to distinguish two neighboring peaks. Retention times in the trapezoidal design stretched from 4.2 to as much as 39 seconds, compared with 3.0 to 8.2 seconds in the circular channel, reflecting the longer effective residence of analytes interacting with the wall.

The physical explanation lies in how each cross-section shapes the flow. Circular channels produce perfectly symmetric parabolic Poiseuille profiles, with velocity highest at the center and zero at the wall, which is efficient for transport but leaves analytes spending unequal times at different radial positions, promoting band broadening. Non-circular geometries disturb this symmetry, and the trapezoidal shape in particular offers a higher surface-to-volume ratio and a more favorable flow distribution, enhancing mass transfer between the mobile and stationary phases and reducing the spread of the analyte bands. The concentration profiles told the same story: in the trapezoidal channel, residence times increased by roughly 80 to 100 percent and peak concentrations dropped by about 60 to 70 percent relative to the circular case, evidence of stronger dispersion and diffusive mixing that ultimately sharpens the separation.

Length and diameter also mattered differently in the two designs. In the circular column, extending the channel from 10 to 100 centimeters raised acetone retention from 3.2 to 7.3 seconds and ethanol from 3.5 to 8.2 seconds, while plate numbers grew four- to five-fold, from 700 to 2,900 for acetone and 900 to 4,300 for ethanol. In the trapezoidal column, the same length increase multiplied retention times ten- to twelve-fold, with acetone rising from about 3 to 33 seconds and ethanol from roughly 4 to nearly 38 seconds, and plate numbers exploded by a factor of 40 to 50, from about 2,000 to 90,000 for acetone and 3,000 to 125,000 for ethanol. For both geometries, increasing the diameter from 50 micrometers produced an exponential rise in retention time and plate count up to around 300 micrometers, after which performance saturated, suggesting an optimum window for future designs.

The author emphasizes that none of these gains came from different operating conditions. Both models used the same hydraulic diameter, length, carrier-gas flow rate, stationary-phase film thickness, inlet concentration, Gaussian pulse injection, boundary conditions, and material properties, so the divergence in retention, plate number, HETP, and resolution is attributable purely to cross-sectional geometry. The price of the trapezoid’s superior performance is real but quantified: a pressure drop of roughly 3.8 x 10^3 pascals and a lower average carrier-gas velocity of about 0.35 meters per second, consequences of the modified cross-section at the same volumetric flow rate.

The implications reach well beyond acetone and ethanol. As breathomics advances toward early diagnosis of diabetes, lung and colorectal cancers, liver and neurodegenerative diseases, and infectious conditions, reviews in the field have repeatedly called for higher selectivity, standardized sampling, and proper separation of chemically similar compounds before detection. Miniaturized gas chromatography, with its microfabricated columns, small injectors, and mini-detectors, promises laboratory-grade separation in a portable package, and the rise of machine learning, electronic noses, and wearable sensors only increases the demand for high-resolution separation on a chip. The study points to the trapezoidal cross-section as a promising geometry for such devices, and the author’s stated next step is to fabricate an optimized trapezoidal column on a silicon wafer using MEMS technology, validate its chromatographic performance experimentally, and integrate it into a microGC system for rapid VOC detection in breath samples.

Subject of Research: Comparative simulation of circular and trapezoidal micro gas chromatography column geometries for acetone-ethanol separation in breath analysis

Article Title: Performance comparison of straight circular and trapezoidal µGC columns for acetone–ethanol separation

Article References: Abdullah (2026). Performance comparison of straight circular and trapezoidal µGC columns for acetone–ethanol separation. Frontiers in Sensors, 7, Article 1900358. https://doi.org/10.3389/fsens.2026.1900358

Image Credits: AI Generated

DOI: 10.3389/fsens.2026.1900358

Keywords: micro gas chromatography, breath analysis, volatile organic compounds, acetone, ethanol, trapezoidal microchannel, COMSOL simulation, theoretical plates, HETP, chromatographic resolution, MEMS, PEG stationary phase