Application of Laser Multi-Photon Ionization to Trace Detection in Chromatography Victoria Fun-Young, Iris Litani-Barzilai, Valery Bulatov, Vladimir V. Gridin, and Israel Schechter Department of Chemistry, Technion - Israel Institute of Technology, Haifa 32000, Israel ABSTRACT Multi-photon ionization (MPI) has the potential to provide sensitive detection of a large variety of organic compounds. Coupling this technique with chromatographic methods, such as TLC and HPLC, may result in powerful analytical tools. Moreover, enhanced performance is expected when applying the resonant multi-photon ionization mode of operation. We examined the feasibility of utilizing MPI in conjunction with TLC and HPLC. The fast- conductivity method was applied, such that direct results can be obtained under ambient conditions. In particular, we focused on detection of polycyclic aromatic hydrocarbon mixtures, whereby direct MPI scanning of TLC plates were examined. HPLC detection The target The separation in Thin-Layer Chromatography is commonly observed by fluorescence or optical reflection data. The detection of non-fluorescent and/or colorless compounds is more difficult and uncertain. We suggest an alternative detection scheme, based upon the Laser induced Multiphoton Ionization (MPI) processes. Imaging Fluorescence (UV Absorbance) Detection PAH mixture solution TLC (HPLC) Analysis Multiphoton Ionization Fast Conductance Detection Figure 1 2 MPI-chromatogram obtained for n-hexane solutions of (a) pyrene, (b) 1-Brom-Pyrene and (c) their 1:1 mixture. Developed on Silica gel 60 precoated TLC plates by Cyclohexane for 20 min. Figure 3a Figure 3b Experimental setup Single-trace TLC analysis Mixture separation/TLC The mixture of Benzo(e)pyrene (I), Pyrene (II) and 1-Bromopyrene (III) in n-hexane 1:1:1 was developed by cyclohexane for 25 minutes. The green line presents the fluorescence intensity along the TLC plate, obtained at 254 nm. The yellow line shows the corresponding MPI signals. Observe reliable MPI detection associated with each TLC spot- location. A simple and low cost fast-conductance technique (Fig. 2a) provides a photocurrent read-out due to the Multi-Photon Ionization of trace compounds (Fig. 2b). Figure 2b Current Amplifier Storage Oscilloscope _ Power Supply + Nd-YAG 3 rd harmonic 355 nm XY- stage for TLC plate Figure 2a The same as in Fig. 4 but for a shorter (20 min) TLC development time. The MPI signal of pyrene is strong and readily observed. This demonstrates the situation where a poor fluorescing material can be detected by the MPI facility. Observe, however, that the MPI based separation of pyrene (II) and 1- Bromopyrene (III), seems incomplete. This is a result of the lower development time, since the “tale” of 1-Bromopyrene contributes to the MPI reading of the pyrene spot. MPI-FC and HPLC data of benzo(e)pyrene, as a function of elution time. The HPLC results are cross-referenced with the corresponding MPI data obtained from the filter substrates. Conclusions In order to apply the MPI detection in HPLC, the effluent was transferred to glass fiber filters and the corresponding MPI-FC readings were recorded. Figure 6 Note the remarkable correspondence of the time-resolved HPLC read-outs to the MPI-FC photocharges. The molecular selectivity of the MPI detection (in its resonant mode) is exemplified: benzo(e)pyrene is resonatively ionized at 337 nm, while perylene is not. This results in huge differences in the slopes. Figure 4 IIIIII  Multi-photon ionization (MPI) has the potential to provide sensitive and material selective detection of organic compounds.  Coupling this technique with chromatographic methods, such as TLC and HPLC, may result in powerful analytical tools slope = benzo(e)pyrene perylene slope = MPI signal, mV*  s Normalized Absorbance Figure 7 Figure 3c III II I Figure 5