Impact disintegration of submicron clusters – molecular dynamics simulation 1) Department of Materials Science & Engineering, University of Virginia 2) Department of Chemistry, Arizona State University Leonid Zhigilei, 1) Sergei Aksyonov, 2) Peter Williams, 2) Yasushi Katsumi 1) Snapshots from MD simulations of impact disintegration of sub-micron clusters 20 ps 30 ps 5 ps 40 ps 50 ps 70 ps 100 ps 150 ps 10 ps 15 ps 80 ps 50 ps 30 ps 20 ps Impact velocity of 2000 m/sImpact velocity of 500 m/s Potential Energy[eV per molecule] Initial binding energy is 0.2 eV/molecule, potential energy of a gas-phase molecule is 0 eV. Red color corresponds to the molecules compressed by the shock wave that propagates through the cluster from the impact region. Only half of the simulated system is shown so that the distribution of the potential energy inside the cluster can be monitored. Introduction: Impact Desolvation of Electrosprayed Clusters Computational Method: Molecular Dynamics Technique Molecular dynamics simulation technique is used in this work to obtain detailed information on the processes involved in the cluster impact desolvation under experimental conditions realized in IDEC. In order to expand the length- and time- scales of the simulations up to the ones comparable to the experimental conditions, we apply the breathing sphere model [3,4], which describes the cluster material with molecular rather than atomic resolution. The parameters of inter-particle interaction in the breathing sphere model are chosen to reproduce the properties of water and glycerol clusters used in the IDEC experiments. First simulations are performed for clusters of up to 300,000 constituent particles, impacting a rigid target with an incident velocity of m/s. A visual picture of the cluster impact can be correlated with the evolution of the physical parameters of the involved processes (the energy and pressure evolution during the impact) and final characteristics of the ejected species (velocity and angular distributions). 40 nm The widely diverse world of clusters is populated by species of very different size – from several atoms to millions of molecules. Here we consider the largest ones – micron/sub-micron size clusters. Such clusters have found an analytical application in the novel ionization method – Impact Desolvation of Electrosprayed Clusters (IDEC) [1]. The charged clusters are produced by electrospray of the analyte solution in vacuum. They are subsequently accelerated through a potential drop of several kV and directed towards a solid target. The collision with the target surface is believed to lead to the cluster explosion and release of analyte ions, which can be subsequently extracted and analyzed. 49th Conference on Mass Spectrometry and Allied Topics, Chicago, Illinois, May , 2001 Summary and Future Work [1] S. A. Aksyonov and P. Williams, Impact Desolvation of Electrosprayed Clusters (IDEC) - a New Ionization Method for Mass Spectrometry of Large Biomolecules, Rapid Commun. Mass Spectrom., submitted (2001). [2] J. F. Mahoney, J. Perel, T. D. Lee, P. A. Martino, P. Williams, J. Am. Soc. Mass Spectrom. 3, 311 (1992). [3] L.V. Zhigilei, P. B. S. Kodali, and B. J. Garrison, J. Phys. Chem. B 101, 2028 (1997); ibid., 102, 2845 (1998). [4] L.V. Zhigilei and B. J. Garrison, J. Appl. Phys. 88, 1281 (2000). 500 m/s impact velocity (sub-sonic in water environment): The cluster compression at the impact does not lead to the shock wave formation. Rather, the pressure is released by an acoustic wave propagating back and forth within the cluster and by spreading out the cluster material on the surface. The cluster does not lose its integrity and a relatively small number of single molecules and small clusters is ejected. The analytes are unlikely to be ejected and desolvated in this regime. The existence of the threshold acceleration voltage needed for detection of the analyte ion signal in IDEC can be related to the strong dependence of the character of the impact-induced processes on the cluster impact velocity.  Initial results of the presented molecular-level computer simulation study demonstrate the ability of the technique to provide insight into microscopic mechanisms of the cluster impact desolvation phenomenon.  For impact velocities higher than the speed of sound in the cluster material, simulations predict that the formation of an intra-cluster shock wave leads to the cluster disintegration and efficient transfer of the impact energy into the energy of the lateral expansion of the cluster material.  Only a small fraction of the impact energy transfers into heat, allowing for survival of relatively large molecular clusters. Thus fragile biomolecules can also survive the impact desolvation.  Realistic representation of metal or glycerol target, incorporation of large bioorganic molecules into clusters, analysis of the energy transfer to the intramolecular vibrational modes during the shock heating, more detailed investigation of the velocity and angular distributions of the secondary clusters and molecules are among the directions for further mechanistic investigation of IDEC ionization technique. 500 m/s 2000 m/s The advancement and optimization of this promising ionization method could be facilitated by a better understanding of the impact phenomenon and the conditions that analyte molecules experience during the cluster disintegration. In the present study the molecular dynamics simulation technique is used to obtain a molecular-level picture of the cluster impact phenomenon. The conclusions from the simulations are related to the experimental data [1] and to the predictions from an analytical shock wave model [2]. Results of the Simulations 2000 m/s, impact velocity (super-sonic in water environment):  The formation and propagation of an intra-cluster shock wave is the dominant process that defines the character of the initial cluster energy redistribution.  The expansion of the shock-compressed region of the cluster provides an efficient channel of energy transfer from the kinetic energy in the impact direction (red line) to the energy of expansion parallel to the surface plane (blue line). By the end of the simulation, the energy of the lateral expansion corresponds to ~80% of the impact energy.  The region of the cluster that is more distant from the impact area remains significantly colder, does not undergo complete vaporization, and produces relatively large secondary molecular clusters. One can speculate that large analyte molecules located in this part of the cluster are likely to survive the impact and contribute to the mass spectrometry signal.  Although the system is far from thermal equilibrium at any time, we can use the tangential component of the molecular velocities to estimate “thermal contribution” to the total kinetic energy of the system. The maximum “thermal K.E.” reaches ~ 11% during the impact, but quickly drops to ~ 1.2% as a result of cooling due to the expansion of the cluster material.  Nevertheless the temperature increase is sufficient for nearly complete vaporization of a layer of cluster material adjacent to the surface. The expansion of the vapor propels the rest of the cluster material away from the target surface.  High, up to 3000 m/s, maximum velocities of the lateral expansion and moderate, hundreds of m/s, velocities in the direction normal to the surface result in a broad angular distribution of the ejected species. Primary clusters  It is found that primary water clusters of small size and high kinetic energy produce only low weight (<400 m/z) ions of electrolyte complexes in IDEC mass spectrum.  There is an optimal (heavier) cluster size and a minimum kinetic energy that lead to appearance of abundant molecular ions of big mass ( m/z) analyte (oligonucleotide) in IDEC mass spectrum.  Clusters of too big mass and/or too low kinetic energy produce no IDEC spectrum.  Exact measurement of cluster mass and energy is still impeded by technical problems (high pressure caused by evaporating liquid, unstable character of cluster beam etc.), nevertheless it is possible to draw a qualitative analogy between experiment and molecular dynamic simulation. Secondary ions  Applying the retarding potential to the grid above the target we can analyze the kinetic energy of the ions produced by the impact.  It was observed that mass spectrum is still observed even at retarding potential up to 600 V. Such ions cannot be produced in single impact event, because they would be destroyed in the collision with damping gas in the quadrupole interface (between IDEC ionization chamber and ortho-TOF analyzer).  This can be explained by secondary cluster appearance. Such secondary clusters, having the same average momentum and heavier mass than free ions, can penetrate through the retarding potential and decay into free ions downstream. Such ions will survive in the damping interface and will be seen in the spectrum.  Taking into account that fragmentation potential of the most organic molecules is about 50 eV we can conclude that secondary clusters have average size of about 10 molecules. Experimental Results For technical realization of the electrospray in vacuum for IDEC-MS please see poster TPA-010, S. A. Aksyonov and P. Williams, “Optimizing the liquid cluster source for IDEC mass spectrometry”, Tuesday 8:45 am – 3:00 pm