Optofluidic ring resonators for new gas chromatography applications

Gas analysis has become increasingly important for environmental monitoring, homeland security, and medical diagnosis. For example, a handheld gas analyzer could warn soldiers of the existence of toxic chemical agents on the battlefield. Researchers recently found that trace levels of biomarkers in the exhaled breath correlate with various cancers, diabetes, and many metabolic disorders. A portable gas analyzer at the point-of-care could provide rapid disease diagnosis through non-invasive breath analysis.

Among gas analysis methods, gas chromatography (GC) is one of the most powerful and routinely used techniques in analytical chemistry labs. The interaction between vapor molecules and the polymer coating of the GC column causes variations in retention time within the column. As a result, gas chromatography can separate and detect hundreds of chemical compounds within a vapor mixture. However, bench-top GCs are usually bulky, costly, and power-intensive. To help overcome these issues, a collaboration between the University of Missouri and ICx Technologies has used innovations in photonics and microfluidics to develop an optofluidic ring resonator (OFRR) based micro-GC (μGC) analyzer: a rapid, sensitive, low-cost, and portable sensing device.^1–4

Figure 1 illustrates the concept of the OFRR μGC analyzer, which is a thin-walled fused silica capillary with a diameter of approximately 100μm and wall thickness of 2–3μm. The interior surface of the capillary is coated with a thin layer (∼100nm) of polymer to interact with vapor molecules flowing through the OFRR. The circular cross-section of the capillary forms an optical ring resonator, where total internal reflection of the light along the curved inner/outer boundary supports whispering-gallery modes (WGMs), resonances that occur within circular objects. Since the capillary wall is sufficiently thin, the evanescent field of the WGMs has a presence in the polymer layer. When an analyte flows through the OFRR capillary, the interaction between the vapor molecules and the polymer causes the polymer thickness and refractive index (RI) to change, resulting in a WGM spectral shift. In the OFRR μGC, a tunable diode laser is used to excite the WGM through tapered optical fibers in contact with the OFRR, which determines the detection locations along the OFRR. This on-column detection capability allows the acquisition of information regarding vapor analyte concentration and adsorption/desorption kinetics.

Figure 1. Conceptual illustration of the optofluidic ring resonator (OFRR) micro-gas chromatography (μGC) analyzer. WGM: Whispering-gallery mode.

We used the OFRR μGC for explosive detection using 2,4-dinitrotoluene (DNT) as a proof-of-concept analyte. Detection of explosives is challenging. For instance, they have extremely low volatility, so highly sensitive sensors are required. Furthermore, in field applications, trace amounts of explosives are usually mixed with many other vaporous chemicals with much higher volatilities, leading to false positive and false negative signals. However, our μGC can separate explosives from interfering chemical compounds and detect each analyte in a sample, providing a solution to the specificity problem.

The inset of Figure 2 (left) shows the detection of pure DNT vapor at room temperature, where the entire sensing process was completed within one minute. First, a solid phase microextraction (SPME) fiber extracts DNT vapor from the equilibrium headspace. The vapor is sent through a GC injector, which is connected to the OFRR capillary inlet. The SPME extraction time is proportional to the mass of DNT vapor injected into the OFRR capillary. Figure 2 (left) plots the OFRR sensor response to various amounts of DNT, and it shows excellent linearity and good repeatability. According to the calibration of SPME extraction mass with a gas chromatograph/mass spectrometer (GC/MS), the OFRR μGC analyzer has a detection limit of approximately 200pg of DNT vapor. Figure 2 (right) presents the separation and detection of DNT vapor from two interfering analytes, nitrotoluene and triethyl phosphate (TEP). The four traces of independent analyses show reliable retention times obtained for each analyte. As shown in Figure 3, in addition to explosive detection, we have also demonstrated the capability of the OFRR μGC analyzer to efficiently analyze complex vapor samples with different volatilities and polarities.

Figure 2. OFRR μGC separation and detection of the explosive 2,4-dinitrotoluene (DNT) from interfering chemical compounds. (left) DNT alone. (right) DNT mixed with nitrotoluene and triethyl phosphate (TEP). SPME: Solid phase microextraction.

Figure 3. Chromatogram of a vapor mixture containing 12 chemical compounds with different volatilities and polarities. DMMP: Dimethyl methylphosphonate. DEMP: Diethyl methylphosphonate. DMNB: 2,3-dimethyl-2,3-dinitrobutane.

In summary, OFRR technology shows great promise in the development of low-cost portable gas analyzers that have rapid response times, high sensitivity, and high specificity. Future work will include detection of more challenging explosive compounds, such as 2,4,6-trinitrotoluene (TNT), cyclotrimethylenetrinitramine (RDX), and pentaerythrite tetranitrate (PETN), mixed with common interferents; detection and analysis of specific biomarkers in human exhaled breath; and engineering development toward multi-dimensional separation and on-chip integration.

The authors thank the National Science Foundation for support (ECCS-0729903).