Evaporative cooling

Klots, Cornelius E.

doi:10.1063/1.449615

Cited by 359 publications

(193 citation statements)

References 23 publications

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“…For droplets, the surface energy reduces the vaporization enthalpy from that of the bulk. For neutral water clusters, the evaporation enthalpy of a single water molecule from a cluster of size n can be estimated from the bulk enthalpy of vaporization less a surface energy term which scales as n Ϫ1/3 [66]. Using a value for the bulk enthalpy of vaporization of hexagonal ice of 11.2 kcal/mol at zero K, the energy required to evaporate a single water molecule from water clusters from n ϭ 20 -50 is estimated to vary from 9.2 to 9.7 kcal/mol.…”

Section: Internal Energy Depositionmentioning

confidence: 99%

Nonergodicity in electron capture dissociation investigated using hydrated ion nanocalorimetry

Leib

Donald

Bush

et al. 2007

J. Am. Soc. Mass Spectrom.

118

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Hydrated divalent magnesium and calcium clusters are used as nanocalorimeters to measure the internal energy deposited into size-selected clusters upon capture of a thermally generated electron. The infrared radiation emitted from the cell and vacuum chamber surfaces as well as from the heated cathode results in some activation of these clusters, but this activation is minimal. No measurable excitation due to inelastic collisions occurs with the low-energy electrons used under these conditions. Two different dissociation pathways are observed for the divalent clusters that capture an electron: loss of water molecules (Pathway I) and loss of an H atom and water molecules (Pathway II). For Ca(H 2 O) n 2ϩ , Pathway I occurs exclusively for n Ն 30 whereas Pathway II occurs exclusively for n Յ 22 with a sharp transition in the branching ratio for these two processes that occurs for n Ϸ 24. The number of water molecules lost by both pathways increases with increasing cluster size reaching a broad maximum between n ϭ 23 and 32, and then decreases for larger clusters. From the number of water molecules that are lost from the reduced cluster, the average and maximum possible internal energy is determined to be ϳ4.4 and 5.2 eV, respectively, for Ca(. This value is approximately the same as the calculated ionization energies of M(H 2 O) n ϩ , M ϭ Mg and Ca, for large n indicating that the vast majority of the recombination energy is partitioned into internal modes of the ion and that the dissociation of these ions is statistical. For smaller clusters, estimates of the dissociation energies for the loss of H and of water molecules are obtained from theory. For Mg(H 2 O) n 2ϩ , n ϭ 4 -6, the average internal energy deposition is estimated to be 4.2-4.6 eV. The maximum possible energy deposited into the n ϭ 5 cluster is Ͻ7.1 eV, which is significantly less than the calculated recombination energy for this cluster. There does not appear to be a significant trend in the internal energy deposition with cluster size whereas the recombination energy is calculated to increase significantly for clusters with fewer than 10 water molecules. These, and other results, indicate that the dissociation of these smaller clusters is nonergodic. . The effectiveness of the bottom-up method for complex samples can be enhanced by using multidimensional separations. Clemmer and coworkers elegantly demonstrated that combining on-line liquid chromatography (LC) with ion mobility spectrometry and MS can greatly improve separations without increasing analysis times over LC/MS alone [2][3][4]. In contrast, the "top-down" approach to protein characterization has the advantage that de novo sequencing, including the identification and structural localization of labile posttranslational modifications, can be done directly on protein mixtures without proteolysis [5,6]. This top-down approach has greatly benefited from the development of electron capture dissociation (ECD), a method pioneered by McLafferty and coworkers [6][7][8][9]. In a typical ECD experim...

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Section: Internal Energy Depositionmentioning

confidence: 99%

Nonergodicity in electron capture dissociation investigated using hydrated ion nanocalorimetry

Leib

Donald

Bush

et al. 2007

J. Am. Soc. Mass Spectrom.

118

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“…In this model, water molecule loss from a large water cluster results in the partitioning of kT ‫ء‬ and (3/2)kT ‫ء‬ into translational and rotational modes, respectively, for a total of (5/2)kT ‫ء‬ for each water molecule lost. The effective temperatures of the newly formed cluster (as opposed to that of the precursor) is used for the energy partitioning values because this effective temperature should best match that of the products for dissociation through a loose transition-state with no reverse activation barrier [43]. The average internal energy of each cluster formed upon sequential water molecule evaporation is given by…”

Section: Recombination Energiesmentioning

confidence: 99%

“…Although there are several models to obtain the energy partitioned into translational energy [44 -46], the Klots cluster evaporation model [43] is used because it includes both translations and rotations. In this model, water molecule loss from a large water cluster results in the partitioning of kT ‫ء‬ and (3/2)kT ‫ء‬ into translational and rotational modes, respectively, for a total of (5/2)kT ‫ء‬ for each water molecule lost.…”

Section: Recombination Energiesmentioning

confidence: 99%

Measuring the extent and width of internal energy deposition in ion activation using nanocalorimetry

Donald

Williams

2010

J. Am. Soc. Mass Spectrom.

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The recombination energies resulting from electron capture by a positive ion can be accurately measured using hydrated ion nanocalorimetry in which the internal energy deposition is obtained from the number of water molecules lost from the reduced cluster. The width of the product ion distribution in these experiments is predominantly attributable to the distribution of energy that partitions into the translational and rotational modes of the water molecules that are lost. These results are consistent with a singular value for the recombination energy. For large clusters, the width of the energy distribution is consistent with rapid energy partitioning into internal vibrational modes. For some smaller clusters with high recombination energies, the measured product ion distribution is narrower than that calculated with a statistical model. These results indicate that initial water molecule loss occurs on the time scale of, or faster than energy randomization. This could be due to inherently slow internal conversion or it could be due to a multi-step process, such as initial ion-electron pair formation followed by reduction of the ion in the cluster. These results provide additional evidence for the accuracy with which condensed phase thermochemical values can be deduced from gaseous nanocalorimetry experiments. (J Am Soc Mass Spectrom 2010, 21, 615-625) © 2010 American Society for Mass Spectrometry C apture of a low-energy electron by a multiply charged peptide or protein can result in extensive and relatively non-selective fragmentation of the amide backbone, from which information about the amino acid sequence [1-3], post-translational modification sites [3][4][5], and higher-order structure [2,6,7] can be obtained. Similar fragmentation can be obtained by transferring an electron to the peptide or protein cation from a molecular anion [8] or from atoms [9,10]. These methods are being applied to both "bottom up" and "top-down" approaches to protein analysis.An important parameter in understanding how activated ions fragment is the extent of internal energy that is deposited in the activation step. For electron capture dissociation (ECD) [1-6], the recombination energy (RE) resulting from capture of an electron from a multiply charged peptide or protein has been estimated to be between 4 and 7 eV [1,11,12], based in part on known or estimated thermochemical data. Dissociation may occur from excited electronic states or from ground states [13,14], so that the internal energy deposited into an ion may not be a singular value.Some ions have been used as chemical thermometers to measure internal energy deposition [15][16][17][18]. For example, dissociation of the molecular ion of n-butylbenzene results in formation of product ions at m/z 91 and 92, the ratio of which provides information about the average internal energy of the activated precursor [15,16]. Ions that dissociate via sequential fragmentation and that have known threshold dissociation energies and similar dissociation entropies have also been used [18 -...

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“…Extreme cases are, for example, microcanonical ensembles and evaporative ensembles. 11 Let us examine microcanonical ensembles and the meaning of "temperature" for them, and then consider the relation of such temperature(s) to the possibility of negative heat capacities.…”

Section: Heat Capacities Normal and Otherwisementioning

confidence: 99%