Biogenic aldehyde determination by reactive paper spray ionization mass spectrometry

Ionization of aliphatic and aromatic aldehydes is improved by performing simultaneous chemical derivatization using 4-aminophenol to produce charged iminium ions during paper spray ionization. Accelerated reactions occur in the microdroplets generated during the paper spray ionization event for the tested aldehydes (formaldehyde, n-pentanaldehyde, n-nonanaldehyde, n-decanaldehyde, n-dodecanaldehyde, benzaldehyde, m-anisaldehyde, and p-hydroxybenzaldehyde). Tandem mass spectrometric analysis of the iminium ions using collision-induced dissociation demonstrated that straight chain aldehydes give a characteristic fragment at m/. z 122 (shown to correspond to protonated 4-(methyleneamino)phenol), while the aromatic aldehyde iminium ions fragment to give a characteristic product ion at m/. z 120. These features allow straightforward identification of linear and aromatic aldehydes. Quantitative analysis of n-nonaldehyde using a benchtop mass spectrometer demonstrated a linear response over 3 orders of magnitude from 2.5. ng to 5. μg of aldehyde loaded on the filter paper emitter. The limit of detection was determined to be 2.2. ng for this aldehyde. The method had a precision of 22%, relative standard deviation. The experiment was also implemented using a portable ion trap mass spectrometer

Distinguishing Amorphous and Crystalline Ice by Ultralow Energy Collisions of Reactive Ions

Ion scattering using ultralow energy projectiles is considered to be a unique method to probe the nature of molecular surfaces because of its capacity to probe the very top, atomically thin layers. Here, we examine one of the most studied molecular solids, water-ice, using this technique. When ice surface undergoes the amorphous to crystalline transition, an ultralow energy reactive projectile identifies the change through the reaction product formed. It is shown that ultralow energy (2, 3, 4, 5, 6, and 7 eV) CH₂⁺ (or CD₂⁺) collision on amorphous D₂O (or H₂O) ice makes CHD⁺, while crystalline ice does not. The projectile undergoes H/D exchange with the dangling −OD (−OH) bond present on amorphous ice surfaces. It is also shown that H/D exchange product disappears when amorphous ice is annealed to the crystalline phase. The H/D exchange reaction is shown to be sensitive only to the surface layers of ice as it disappears when the surface is covered with long chain alcohols like 1-pentanol as the ice surfaces become inaccessible for the incoming projectile. This article shows that ultralow energy reactive ion collision is a novel method to distinguish phase transitions in molecular solids

Atomically Precise Silver Clusters as New SERS Substrates

An atomically precise silver cluster, Ag₁₅₂ protected with thiolate ligands, was used as a surface-enhanced Raman scattering (SERS) substrate. The cluster shows intense enhancement of Raman signals of crystal violet with an enhancement factor of 1.58 × 10⁹. Adaptability of the substrate for a wide range of systems starting from dyes to biomolecules is demonstrated. Solid-state drop casting method was used here, and SERS signals were localized on the Ag₁₅₂ crystallites, confirmed from Raman images. Excellent periodicity of clusters, their plasmonic nature, and absence of visible luminescence are the main reasons for this kind of large enhancement. SERS was compared with smaller clusters and larger nanoparticles, and the size regime of Ag₁₅₂ was found to be optimum. Several control experiments were done to understand the SERS activity in detail. The method has wide adaptability as the cluster can be easily drop-casted on any surface like paper, cotton, and so forth to produce effective SERS media. The work suggests that atomically precise clusters, in general, can show SERS activity

Diffusion and Crystallization of Dichloromethane within the Pores of Amorphous Solid Water

Dichloromethane (CH₂Cl₂) thin films deposited on Ru(0001) at low temperatures (∼80 K or lower) undergo a phase transition at ∼95 K, manifested by the splitting of its wagging mode at 1265 cm^–1, due to factor group splitting. This splitting occurs at relatively higher temperatures (∼100 K) when amorphous solid water (ASW) is deposited over it, with a significant reduction in intensity of the high-wavenumber component (of the split peaks). Control experiments showed that the intensity of the higher wavenumber peak is dependent on the thickness of the water overlayer. It is proposed that diffusion of CH₂Cl₂ into ASW occurs and it crystallizes within the pores of ASW, which increases the transition temperature. However, the dimensions of the CH₂Cl₂ crystallites get smaller with increasing thickness of ASW with concomitant change in the intensity of the factor group split peak. Control experiments support this suggestion. We propose that the peak intensities can be correlated with the porosity of the ice film. Diffusion of CH₂Cl₂ has been supported by low-energy Cs⁺ scattering and temperature-programmed desorption spectroscopies

Zero Volt Paper Spray Ionization and Its Mechanism

The analytical performance and a suggested mechanism for zero volt paper spray using chromatography paper are presented. A spray is generated by the action of the pneumatic force of the mass spectrometer (MS) vacuum at the inlet. Positive and negative ion signals are observed, and comparisons are made with standard kV paper spray (PS) ionization and nanoelectrospray ionization (nESI). While the range of analytes to which zero volt PS is applicable is very similar to kV PS and nESI, differences in the mass spectra of mixtures are interpreted in terms of the more significant effects of analyte surface activity in the gentler zero volt experiment than in the other methods due to the significantly lower charge. The signal intensity of zero volt PS is also lower than in the other methods. A Monte Carlo simulation based on statistical fluctuation of positive and negative ions in solution has been implemented to explain the production of ions from initially uncharged droplets. Uncharged droplets first break up due to aerodynamics forces until they are in the 2–4 μm size range and then undergo Coulombic fission. A model involving statistical charge fluctuations in both phases predicts detection limits similar to those observed experimentally and explains the effects of binary mixture components on relative ionization efficiencies. The proposed mechanism may also play a role in ionization by other voltage-free methods