Nanojoules, nanoliters and nanosecond calorimetry

A nano-scale time-resolved calorimetric experimental approach is described using a photoacoustic cell made from narrow-bore flexible fused-silica capillary tubing. The fused silica capillary permits the transmission of an excitation source from the deep UV to the near infrared. Methods to couple the excitation energy into the sample and the acoustic energy away from the sample to the detector are discussed. To demonstrate the unique capabilities of the fused silica capillary cell, the excited triplet state energy (E-t) and lifetime (tau) of benzophenone (Bp) in the presence and absence of a quencher was measured. Irradiation of an air-saturated acetonitrile solution of Bp in the nano-scale photoacoustic cell, (i.e., absorption of ca. 100 nJ in a sample volume of 30 nl), yielded E-t = 69.5 +/- 1.4 kcal/mol and a tau of a few hundred nanoseconds. In the presence of potassium iodide (KI), a Bp*(3) quencher, the lifetime was significantly reduced. The rate of tripler quenching by KI (k(q) = 3.9 +/- 0.2 x 10(9) M-1 s(-1)) was determined from the slope of the Stern-Volmer plot of 1/tau observed versus quencher concentration. (C) 1999 Elsevier Science S.A. All rights reserved.