Chen, H; Inoue, T; Ogawa, T
Use of laser two photon ionization as a technique for determining anthracene (120127) on solution surfaces was investigated. Radiation corresponding to the third harmonic, 355 nanometers, from an NdYAG laser operating at 10 hertz was focused on the surface of solutions of anthracene in water, ethylene-glycol, 2,2,4-trimethylpentane (isooctane), dodecane, hexane, cyclohexane, or toluene by a quartz lens. The laser energy varied up to 8 millijoules per pulse. The stainless steel container which held the samples served as the negative electrode. A disk electrode was located 6 millimeters above the solution surface and was positively biased by a high voltage power supply unit at voltages ranging up to 3 kilovolts. Photoionization signals generated in response to the laser radiation were recorded by the electrode system. After amplification by a current amplifier, they were fed into a digital storage scope, and analyzed by a microcomputer. Photoionization charges were determined from the data. The magnitude of the photoionization charges was proportional to the fourth power of the laser pulse energy, confirming that the photoionization process involved two photons. The photoionization charge was not linearly proportional to the bias voltage on the disk electrode, but tended to plateau at high voltages. The photoionization charge for polar solvent surfaces increased with increasing polarity, as denoted by their dielectric constants. For nonpolar solvents, the photoionization charge increased with increasing electron mobility. Detection limits for anthracene on the surface of the various solvents were: water 0.53 nanogram per milliliter (ng/ml), ethylene-glycol 55ng/ml, isooctane 44ng/ml, dodecane 120ng/ml, hexane 130ng/ml, cyclohexane 530ng/ml, benzene 140ng/ml, and toluene 210ng/ml. Calibration plots were linear over a concentration range spanning one to three orders of magnitude above the detection limit of each solvent. The authors conclude that for analytical purposes, this technique is best suited for determining anthracene on water surfaces. Anthracene cannot be determined in bulk water samples using this technique because of large leakage currents.