# Progressive Cellular Architecture in Microscale Gas Chromatography for Broad Chemical Analyses
## 4 Experimental Evaluation
Alkanes C\({}_{5}\)–C\({}_{7}\) were separated in Cell 2, whereas alkanes C\({}_{8}\)–C\({}_{15}\) were separated in Cell 3. Figure 7: Temperature control for the preconcentrators and the separation columns. analytes with molecular weights ranging from 32-212 g/mol and vapor pressures ranging from 0.005-231 mmHg. Phosphonate esters are typically used as simulants for chemical warfare agents due to similarities in their molecular structures and chemical properties [53]. The high polarity of the phosphonate esters results in their strong adsorption on surfaces along the flow path and can manifest as the absence of a detectable peak or other atypical response [54]. In tests performed with the PCA3 mv, the phosphonate esters were successfully separated and detected. As expected, however, the retention behaviors were different from the nonpolar analytes. The vapor pressure of DMMP is 0.96 mmHg, which is lower than those of octane, nonane, and decane (14.05 mmHg, 4.45 mmHg, and 1.43 mmHg, respectively). However, the DMMP peak appeared in Cell 2, whereas the octane, nonane, and decane peaks appeared in Cell 3. This indicated that DMMP broke through the Carbograph(tm) 2 sorbent in Precon3 more easily than nonpolar analytes of comparable vapor pressure. The vapor pressure of DEMP is 0.01 mmHg, similar to that of tetradecane. In the experimental results (Figure 9), DEMP eluted in Cell 3 with a retention time similar to undecane, which was slower than expected. In addition, the DEMP peak presented a large tail. These effects were likely caused by the high adsorption of the phosphonate ester on the surface of fluidic channels with insufficient deactivation. Bracket tests comprised of three consecutive cycles were intended to show sample carryover from run to run. These included a clean nitrogen sample in the first cycle, then a mixture of IPA, benzene, and C\({}_{10}\) in the second, and again a clean nitrogen sample in the third. As evident in Figure 11, the chromatogram from the third cycle matched that of the  
Figure 10: PCA3 mv chromatogram of 30 min sampling of 200 ppb of aromatics. Acetone was separated in Cell 1, 2-butanone, benzene, toluene, and ethylbenzene were separated in Cell 2, and ethylbenzene, o-xylene, and mestilbene were separated in Cell 3. \[HETP=\frac{L}{N} \tag{7}\]0 The PCA3 mv chromatogram of 30 min sampling of 1 ppm of alcohols and phosphonate esters. Methanol, ethanol, and IPA were separated in Cell 1, 1-butanol and DMMP were separated in Cell 2, and 1-butanol and DEMP were separated in Cell 3.  
first, indicating that the IPA, benzene, and C\({}_{10}\) collected in the second cycle were removed from the PCA3 mv with minimal sample carryover from run to run. In the second cycle, the benzene peak in Cell 2 had a height of 94 pA, whereas in the first and third cycles, the peak heights at the retention time of benzene in Cell 2 were 3 pA and 3.6 pA respectively. The benzene peak in Cell 2 had \(\approx\)4% carryover. The impact of humidity was evaluated using mixtures of IPA, benzene, C\({}_{10}\) (each 100 ppb) in air with 0 and 80% relative humidity (RH). Based on a humidity calculator [55], the sample with 80% RH was prepared by injecting 30.5 \(\mu\)L of high-purity, double-distilled water (#WX0003-6, Sigma Aldrich, MO, USA) into the 2 L dilution bottle and then allowing the water enough time to vaporize and reach equilibrium before sampling. The VOCs used in this set were selected to allow Cell 1 to be evaluated with IPA, Cell 2 to be evaluated with benzene, and Cell 3 to be evaluated with C\({}_{10}\). The resulting chromatograms (Figure 12) showed that the retention peaks of benzene and C\({}_{10}\) were not affected by the humidity: the measured peak areas of benzene and C\({}_{10}\) showed <2% difference between the chromatograms at 0 and 80% RH. In principle, the FID is not responsive to water. However, in the run at 80% RH, two additional peaks were observed in Cell 1 and Cell 2, likely resulting from possible contaminants in the water that was used for humidity.