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Noncovalent interaction of methylene blue dye cation (MB + ) with single walled carbon nanotubes (CNT) is characterized by molecular dynamics (MD) simulation, quantum chemical calculations, and laser desorption/ionization (LDI) mass spectrometry. The MD simulation of the (MB + ) n −CNT (n = 1−10) complexes in water demonstrates that the MB + cations are adsorbed on the nanotube surface in the monomeric form. MD reveals both parallel and perpendicular orientations of the MB + tricyclic plane in relation to the long axis of CNT when placed in the water environment. The interaction energy between the components of the complex in the perpendicular conformation, as determined by quantum chemical calculations at the DFT/M05-2X/6-31++G(d,p) level of theory, explains why the bending of the MB + cation at the sulfur atom weakens the π-system of bonds and allows for the perpendicular orientation to occur. It is also found that the adsorbed MB + induces positive electrostatic potential around the adjacent semicylindrical segment of the nanotube. The mainly monomolecular adsorption of the MB + cations at the CNT surface leads to the absence in the LDI mass spectra of (MB + ) n −CNT of features corresponding to products of the reduction of MB + commonly observed in the LDI mass spectra of crystalline dyes.

Frozen solutions of a cryoprotector, glycerol, in water were studied by low temperature fast-atom bombardment mass spectrometry in the temperature range from -196" to 0 "C. Strong dependence of the mass spectral pattern on the water-glycerol ratio and temperature of the sample was observed. The evolution of mass spectra of the frozen 2000: 1 water-glycerol mixture with increase of the sample temperature is reproducible: in the lower portion of the temperature range (-196" to -120°C) the spectra contain a set of cluster ions, (H20),H+, (n = 1-15), characteristic of pure water; then the spectra show a superposition of peaks due to water clusters and glycerol itself, the latter with characteristic changes with temperature rise: degraded "peak at every mass" pattern in the -120" to -80 "C range followed by a build-up of glycerol clusters, G,,H+, (n = 1-3) in the -80 "C to -55°C range. At approximately -55°C to -50°C a sublimation of water component occurs, detected by [H20]+' ion, and the remainder of the spectral pattern coincides with that of pure glycerol up to ambient temperature. Mixed water-glycerol clusters were never observed. An explanation of such spectral behavior is proposed based on ideas about the morphology of the frozen solutions, characterized by water microcrystallites separated by eutectic channels containing cryoprotector. The independent sputtering of the two substances from different domains of the frozen sample surface is discussed.The freezing of a sample allows one to obtain secondary-emission mass spectra from substances which exist in the gaseous or liquid state at room temperature.'-'' The overwhelming majority of low temperature mass spectrometric studies carried out to date have been devoted to individual substances. Whereas the low temperature technique seemed to open up exciting possibilities for mass spectrometric studies of solutions, especially water solutions of both inorganic and organic compound^,^^-'^ the recent interest in the electrospray method" which provides great scope for studies of different types of solutions under nearly ambient conditions, have diverted the interest from frozen samples. While, in the above-listed papers the freezing of liquids was used simply to produce a suitable sample, one of the specific fields of application of low temperatures is cryobiology, and connected with it the study of the freezing of water-cryoprotector mixtures for the cryopreservation of biological material. It is known that one of the reasons for damage to cells and tissues during freezing is the crystallization of water, and the main purpose of cryoprotectors is to prevent the formation of large water crystals and to change the temperature parameters of the freezing process. Glycerol, widely used as a matrix in fast-atom bombardment (FAB) and liquid secondary ion mass spectrometry (SIMS), is also a widely used cryoprotector. A large set of mass spectrometric information concerning the properties of glycer01~-~.'~ and over a wide temperature range has already been acquired. In the pr...

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Positively and negatively charged cluster ions were produced from methanol and ethanol samples under low temperature secondary ion mass spectrometric conditions. Neat alcohol clusters [ The comparatively large number of reports of studies of alcohol clusters by various mass spectrometric techniques 1-13 establishes them as one of the favourite subjects of study of hydrogen bonded clusters of organic molecules. Low temperature (LT) fast-atom bombardment (FAB) or secondary ion mass spectrometric (SIMS) techniques allow cluster production from the condensed state. The LT FAB mass spectra of alcohols have already been reported in a number of pioneering studies in the case of methanol 1,2 and ethanol.3 The main objects of interest in these studies were the protonated neat alcohol clusters M n .H + ] (always formed due to practically unavoidable water traces in the alcohol), with m = 1 for ethanol and m ≤ 3 for methanol, were recorded in these works but not discussed in detail. To our knowledge, all data reported previously were related to positive ions.In the present work the LT SIMS mass spectra of methanol and ethanol, recorded with much higher sensitivity, resolution and mass range than before, are reported. Negatively charged alcohol cluster ions are studied for the first time and their formation is discussed. EXPERIMENTALMeasurements in the LT SIMS mode were performed using a ZAB-SEQ instrument (Micromass, Manchester, UK). The energy of the bombarding Cs + ions was 15 keV.A droplet of an alcohol with 5 µL volume was deposited on the stainless steel probe tip and frozen in a small vessel filled with liquid nitrogen. The vessel was moved then to the immediate vicinity of the entrance of the direct inlet system in such a way that the probe tip was kept in the dry vapours of nitrogen during its quick transfer to the vacuum lock. Pumping to high vacuum before starting the measurements took no longer than about one minute. The spectra were recorded at a rate of 5 s per scan.The thawing of the sample allowed us to monitor changes in the spectra with increasing temperature. Under the conditions applied, the alcohol samples 'survived' in the ion source for 5-10 min.Sample temperature was not measured directly in the present experiments. The temperature range in which abundant alcohol clusters are observed in the spectra can be derived from our previous work (using an LT FAB ion source, described elsewhere) 4,14 and suggests a range of -140 °C to -100 °C for methanol and -130 °C to -110 °C for ethanol. Analytical grade ethanol and methanol samples ('Reanal', Hungary) were used without additional purification. RESULTS AND DISCUSSION General features of the LT SIMS mass spectra of alcoholsThe general features of changes in the LT SIMS mass spectra of ethanol and methanol as a function of temperature were the same as those described in the previous publications on LT FAB of alcohols. [1][2][3][4][5][6] In the lowest temperature range (starting from liquid nitrogen temperature) the LT SIMS spectrum has a noisy, featureless cha...

Some features of a 'matrix suppression effect' caused by ionic surface-active compounds under fast-atom bombardment (FAB) liquid secondary ion mass spectrometry (LSIMS) are being revised. It is shown that abundant transfer of the glycerol matrix molecules to the gas phase does occur under FAB-LSIMS of ionic surfactants, contrary to popular belief. This process can be obscure because of the dependence of the charge state of the glycerol-containing cluster ions on the type of ionic surfactant. It is revealed that, while glycerol matrix signals are really completely suppressed in the positive ion mass spectra of cationic surfactants (decamethoxinum, aethonium), abundant deprotonated glycerol and glycerol-anion clusters are recorded in the negative ion mode. In the case of an anionic surfactant (sodium dodecyl sulfate), on the contrary, glycerol is completely suppressed in the negative ion mode, but is present in the protonated and cationized forms in the positive ion mass spectra. It is suggested that such patterns of positive and negative ion FAB-LSIMS spectra of ionic surfactants solutions reflect the structure and composition of the electric double layer formed at the vacuum-liquid interface by organic cations or anions and their counterions. Processes leading to the formation of the glycerol-containing ions preferentially of positive or negative charge are discussed. The most obvious of them is efficient binding of glycerol to inorganic counterions of the salts Cl S or Na R , which is confirmed by data from quantum chemical calculations. The high content of the counterions and relatively small content of glycerol in the sputtered zone may be responsible for the charge-selective suppression of neat glycerol clusters of opposite charge to the counterions. In the case of a mixture of cationic and anionic surfactants the substitution of inorganic counterions by organic ones was observed. The dependence of the exchange rate in the surface layer is not a linear function of the bulk solution concentration, and an effect of abrupt recharging of the surface can be registered. No both positively or negatively charged pure glycerol and glycerol-inorganic counterion clusters are recorded for the mixture. Correlations between the mass spectrometric observations and some phenomena of surface and colloid chemistry and physics are discussed.

Satellite [M + 2](+*) and [M + 3](+) peaks accompanying the common peak of the protonated molecule [M + H](+) that are known to indicate the occurrence of a reduction process were observed in the fast atom bombardment (FAB) mass spectra of imidazophenazine dye derivatives in glycerol matrix. The distribution of the abundances in the [M + nH](+) peak group varied noticeably for different derivatives. This indicated different levels of the reduction depending on the different structure variations of the studied molecules. In the search for correlations between the mass spectral pattern and the structural features of the dyes, ab initio HF/6-31++G** quantum chemical calculations were performed. They revealed that the abundances of the [M + 2](+*) and [M + 3](+) ions show growth proportional to the decrease of the energy of the lowest unoccupied molecular orbital, i.e. proportional to the increase of the electron affinity of the dye molecule. A method for rapid screening of reductive properties of sets of dye derivatives on the basis of the FAB mass spectral data is discussed.