Comparison of Positive and Negative Ion Clusters of Methanol and Ethanol Observed by Low Temperature Secondary Ion Mass Spectrometry

Abstract

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...

Low temperature secondary emission mass spectrometry. Cryobiological applications

“…51 This was observed for water solutions of the primary alcohols methanol 57,58 (T e = -138ºC, C e = 77%) 73 and ethanol 50,57,58 (T e = -125ºC, C e = 92.5%), 73,84 which showed a threshold for transformation of the spectral pattern from the featureless to cluster-type at temperatures close to C e (reproducible with temperature cycling around this value). As the liquid eutectic must contain a certain amount of water, abundant sets of water-alcohol clusters were recorded in parallel with pure alcohol clusters.…”

“…The liquid state is formed under LT mass spectrometric conditions due to melting of the sample during thawing or due to formation of a supercooled liquid during cooling (a characteristic, for example, of glycerol). The mechanisms of ion formation in such a case do not differ much from those in ambient liquid matrix FAB; the spectra are of good quality and contain clusters 24,25,[49][50][51]57,58 and there is a strong dependence of the spectral pattern on the viscosity of the liquid, whose increase with decrease of temperature hinders ion production. 26 3.…”

“…[23][24][25][26] The time of thawing (usually 10-15 minutes) is sufficient for recording of a number of good spectra, providing rapid spectrum scanning and recording facilities are available. 57,58 The advantage of this method is the rapid and relative ease in obtaining some information about the sample of interest. The disadvantages are also obvious: the short time available for spectra recording, the impossibility of temperature control or "on-line" temperature measurement.…”

“…45,47 The other source of mixed clusters may be a common liquid solution formed due to melting of the eutectic phase while still at sub-zero temperatures. 51,57,58 Layered (sandwich-type) nanostructures as a source of mixed clusters will be discussed separately below, as, in their case, some peculiarities of the mechanisms of secondary ion formation interfere strongly with the process of cluster formation. 48 The phase diagram shown in Figure 4 is of the simplest type, which corresponds to separation of a solution into crystallites (including finely dispersed crystallites in the eutectic phase) of the pure components only.…”

“…In particular, the melting of the eutectic phase can be observed only for systems whose eutectic temperature lies below the mentioned value. 50,51,57 The lifetime of the liquid phase formed due to melting, when possible, is also restricted at reduced pressures. 58 Meanwhile, even within these limitations, useful information on phase transitions (melting, boiling, sublimation, devitrification and crystallization, dehydration of crystalline hydrates) can be obtained by means of LT-FAB during the variation of the temperature of the sample.…”