# Progressive Cellular Architecture in Microscale Gas Chromatography for Broad Chemical Analyses
## 4 Experimental Evaluation  
Prior to evaluating the PCA3 mv, the microfabricated separation columns were individually evaluated using a benchtop GC. After that, all the components were assembled into the PCA3 mv subsystem, which was tested for the sampling and separation of various analytes at different concentrations and humidity levels. The PCA3 mv was also tested for sample carryover from run to run. In principle, a variety of microfabricated detectors, such as capacitive, photoionization, and surface acoustic wave detectors can be used within the PCA3 mv [41; 42; 43; 44; 45; 46]. However, in this work, a commercial flame ionization detector (FID) was used to evaluate the preconcentration and separation. The FID does not broaden any peaks in a substantive way, and hence provides truer representation for the performance of the PCA3 mv [31]. Prior to conducting full functional testing, the performance of the microfabricated separation columns within the Cell 2 and Cell 3 monolithic chips was evaluated by connecting the separation column between the inlet and the FID of a benchtop GC (#Agilent 7890, Agilent, CA, USA). For this evaluation, the flow path through the on-chip preconcentrator was temporarily blocked by an external septum. Pre-mixed analytes in liquid form were injected and vaporized at the inlet of the benchtop GC using splitless injection. The analytes were routed into the chip at on-chip T-connector, i.e., T2 for Cell 2 and T3 for Cell 3 (Figure 2). The eluents from the separation column were measured by the FID. A typical Column2 separation of a mixture of isopropyl alcohol (IPA) and phosphonate esters is shown in Figure 6a, whereas that of a mixture of alkanes from C5 to C12 is shown in Figure 6b. Operated at 80 \({}^{\circ}\)C and a 0.8 sccm N2 flow rate, the IPA and phosphonate esters were separated within 220 s, whereas the alkanes were separated within 1000 s.  
The height equivalent to a theoretical plate (_HETP_) and the theoretical plate number (\(N\)) are measures of separation column efficiency that can be extracted from the chromatogram as follows [31]:  
\[N=5.54\bigg{(}\frac{t_{R}-t_{0}}{PWHH}\bigg{)}^{2} \tag{6}\]  
\[HETP=\frac{L}{N} \tag{7}\]  
where \(L\) is the length of the column. Higher \(N\) and lower _HETP_ indicate higher column efficiency. The _HETP_ of the microfabricated Column2 in this work reached a minimum of 0.14 mm at 0.4-0.5 sccm flow rate, as calculated using the C10 peak with a measured \(t_{R}=236.5\) s, a calculated \(t_{0}=13.4\) s, and a measured _PWHH_ = 8.26 s (Figure 6c). The value of \(t_{0}\) was calculated as the time for an unretained analyte to travel through the separation column and the guard column connections at the tested flow rate. The _HETP_ value corresponded to 4048 theoretical plates and 6747 plates/m. These values are comparable to other reported microfabricated columns [51; 52]. Column3 provided similar performance for lower volatility analytes. Figure 5: Photograph of the PCA3 mv implementation. The PCA3 mv subsystem has a footprint of 29.2 cm \(\times\) 35.6 cm. To test the fully assembled PCA3 mv, analytes were pre-mixed in liquid form, and injected into a first 2 L dilution bottle to form a relatively high-concentration vapor. The high-concentration vapor was subsequently diluted into lower concentrations by using a gas tight syringe to transfer an aliquot of the vapor into a second dilution bottle. Two guard columns were connected to the second dilution bottle: one was used for connecting to the PCA3 mv, whereas the other vented to the ambient air to prevent a vacuum in the bottle during sampling. While this setup was not able to provide precise concentrations in a sustained manner, it was appropriate for a preliminary evaluation. During sampling, the dilution bottle was connected to the sample inlet of the PCA3 mv, whereas the outlet of the PCA3 mv was temporarily blocked to prevent the entrance of the ambient air through Column3. During separation, the outlet of the PCA3 mv was connected to an FID of the benchtop GC.