Time-Resolved Surface Temperature Measurement of MALDI Matrices under Pulsed UV Laser Irradiation Antonis Koubenakis, ² Vladimir Frankevich, Juan Zhang, and Renato Zenobi* Department of Chemistry and Applied Biosciences, Swiss Federal Institute of Technology (ETH), ETH-Ho ¨nggerberg, CH-8093 Zu ¨rich, Switzerland ReceiVed: December 11, 2003; In Final Form: February 16, 2004 Time-resolved examination of the surface temperature was performed for various MALDI matrices, irradiated by UV (λ ) 355 nm, τ ) 5 ns) laser pulses. The temperature was measured by detecting the emitted blackbody radiation from the sample surface. The maximum surface temperature was observed to occur during the falling edge of the laser pulse, with a delay of 2 ns with respect to the laser pulse peak. Its value was found to depend on the kind of the matrix, the thickness of the sample and the laser fluence. For 2,5-dihydroxybenzoic acid, the most popular matrix, the peak temperature was found to be ∼850 and 1100 K at the lower (10 J/m 2 ) and the higher (100 J/m 2 ) fluences, respectively, used in this work. Introduction When irradiating organic crystals with UV laser pulses, electronically excited molecules exchange vibrational energy and/or transfer it to the surrounding medium with a half-life on the order of picoseconds. 1 The deposited thermal energy causes an increase of the solid temperature. On the basis of fluorescence measurements on matrix-assisted laser desorption ionization (MALDI) matrices, 2 it has been reported that a large percentage of the initially absorbed UV laser energy (∼80%) is converted to thermal energy. The lifetime of this process has been estimated to be 0.5-1 ns. 3 Because of the low thermal con- ductivity of organic solids, large temperature changes may be attained in the solid for nanosecond laser pulses that are usually employed in the MALDI. A high temperature of the system may play a key role for various processes that take place after the absorption of the laser energy in MALDI. It has been argued on the basis of experiments and molecular dynamics (MD) simulations that the temperature increase during laser irradiation is important for the mechanisms of molecular ejection in MALDI. Depending on the rate of the temperature change in the solid, the ejection/desorption can be characterized as either a thermal surface vaporization 4 or a phase explosion. 5 Furthermore, the temperature increase can also affect the thermal degradation and the conformation of the biomolecules inside clusters of the ejected material. 6 Finally, ion formation in MALDI can be influenced in part by the temperature increase. Theoretical calculations predict that the probability of ion formation through thermoionization paths can become signifi- cant as the matrix temperature increases to 2000 K. 7,8 Despite the importance of the issue, there has not been any experimental study of the time evolution of the matrix surface temperature, during and after the laser irradiation. Such informa- tion can be useful especially for the theoretical models and MD simulations as well as for interpretation of some of the mass spectrometry results. In this work, we report the time-resolved measurement of the surface temperature of two well-known MALDI matrices, 2,5-dihydroxybenzoic acid (2,5-DHB) and nicotinic acid (NA), during the irradiation with UV (λ ) 355 nm, τ ) 5 ns) laser pulses at various fluences. The dependence of the peak surface temperature on sample thickness was also examined for 2,5-DHB, NA, 2-(4-hydroxyphenylazo)-benzoic acid (HABA), 2,4,6-trihydroxyacetophenone (THAP), dithranol, and ferulic acid. The experiments are based on the detection of the blackbody radiation emitted by the matrix surface at elevated temperature. Previous works on metal substrates 9,10 have shown that blackbody radiation emitted from hot irradiated surfaces can be used for a noncontact, fast (nanosecond time scale) detection of the temperature during and after the laser irradiation pulse. Experimental Section The apparatus used for this experiment is shown schematically in Figure 1. The samples are irradiated by a Nd:YAG (Minilite, Continuum, CA/USA) laser beam (λ ) 355 nm, full width at half maximum (fwhm) ) 5 ns). The pulse duration of the laser beam was measured using a high-speed silicon PIN photodiode (DET200, ThorLabs, NJ/USA) with a rise/fall time e 1 ns and a digital oscilloscope (9350C, Lecroy, Switzerland) with a bandwidth of 500 MHz (2-ns time resolution). The laser beam is gently focused onto the sample surface to a spot size of about 3 mm × 1 mm. The laser fluence was varied using attenuators and/or filters. The values of the laser fluences ranged from 10 to 100 J/m 2 . The reported values are uncorrected for scattering/ reflection losses at the sample surface. All experiments are performed at fixed angles of incidence (45°) and detection (0°), with respect to the surface normal of the sample. The initial temperature of the sample was at approximately 300 K (room temperature). All experiments were performed at atmospheric pressure (p ) 1 atm); i.e., the results may relate to atmospheric pressure MALDI more directly than to MALDI experiments carried out in a vacuum. The blackbody radiation emitted from the sample surface was detected by a fast infrared InGaAs-PIN photodiode (model G8376-03, Hamamatsu, Herrsching, Germany) with a spectral response range of ∼0.9-1.7 μm. The detector’s responsivity * Author to whom correspondence may be addressed. E-mail: zenobi@ org.chem.ethz.ch. ² Current address: Hellenic Air Force, 115 ΠM Akrotiri, Chania, Crete/ Greece. 2405 J. Phys. Chem. A 2004, 108, 2405-2410 10.1021/jp037811k CCC: $27.50 © 2004 American Chemical Society Published on Web 03/09/2004