Fig. 1. Apparatus for stop-flow selectivity enhancement with a dual-column ensemble using an IIL stationary phase-coated column and a Rtx-1 column. C₁ and C₂ are the columns; V₁, stop-flow valve; I, split inlet; P₁, pressure gauge; FID, flame ionization detector; DMS, differential mobility spectrometer.

Fig. 2. Height equivalent to a theoretical plate vs. average linear carrier gas (H₂) velocity for (a) 2.8-m long partially cross-linked IIL stationary phase-coated column and (b) a 2.8-m long completely cross-linked IIL stationary phase-coated column for experimental data using n-decane at 50 °C.

Fig. 3. Chromatograms of the 20-component test mixture using H₂ carrier gas at 50 °C: (a) commercially-coated Rtx-1 column; (b) IIL stationary phase-coated column; (c) column ensemble. Peak numbers correspond to compound numbers in Table 1. Chromatograms were obtained at carrier gas velocities of 54, 53, and 54 cm s⁻¹, respectively.

Fig. 4. Band trajectory plots indicating band position vs. time and the corresponding chromatograms for the 20-component test mixture with no stop-flow pulses (a) and two stop-flow pulses as indicated by the vertical broken lines (b). See text for details. Peak numbers correspond to compound numbers in Table 1.

Fig. 5. Chromatograms of the 13-component test mixture of the markers found in the headspace of U.S. currency using H₂ carrier gas at 50 °C: (a) IIL stationary phase-coated column; (b) commercially-coated Rtx-1 column; (c) column ensemble. Peak numbers correspond to compound numbers in Table 2. Chromatograms were obtained at carrier gas velocities of 50, 52, and 50 cm s⁻¹, respectively.

Fig. 6. Band trajectory plots indicating band position vs. time and the corresponding chromatograms for the 13-component test mixture of the markers found in the headspace of U.S. currency with no stop-flow pulses (a) and two stop-flow pulses as indicated by the vertical broken lines (b). See text for details. Peak numbers correspond to compound numbers in Table 2.

Fig. 7. Contour plots for the co-eluting components of the 20-component text mixture from the IIL column—DMS system accompanied by a vertically-oriented FID trace showing the one-dimensional separation. The contours show ion current (color) as a function of retention time (vertical axis) and compensation voltage (horizontal axis) for the positive ion channel (a) and a portion of the negative ion channel (b) with a detector RF voltage of 1500 V and an operating temperature of 120 °C. Contour (c) shows the separation of peak pair 16 and 17 with a detector Rf voltage of 1200 V and an operating temperature of 60 °C.

Fig. 8. Contour plots showing the separation of the co-eluting components from the 13-component mixture of the markers found in the headspace of U.S. currency. Component peak pairs 2/4 (a) and 3/7 (b) were separated using a detector Rf voltage of 1300 V and an operating temperature of 120 °C, while component pair 8/12 was separated using a detector Rf voltage of 1500 V and an operating temperature of 120 °C.

Components and retention factors for the 20-component test mixture

Components and retention factors for mixture that are used as a fingerprint for the headspace vapor above U.S. currency.