Near Infrared (NIR) Strong Field Ionization and Imaging of C60 Sputtered Molecules: Overcoming Matrix Effects and Improving Sensitivity

A review on quantitation-related factors and quantitation strategies in mass spectrometry imaging of small biomolecules

Accurate quantitative information of the analytes in mass spectrometry imaging (MSI) is fundamental for determining the accurate spatial distribution, which can provide additional insight into the living processes, disease progression or the pharmacokinetic-pharmacodynamic mechanisms. However, performing a quantitative analysis in MSI is still challenging. This review focuses on the quantitation-related factors and recent advances in the strategies of quantitative MSI (q-MSI) of small molecules. The main quantitation-related factors are discussed according to the new investigations in recent years, including the regionally varied extraction efficiencies and ionization efficiencies, signal-concentration regression functions, and the repeatability of surface sampling/ionization methods. Newly developed quantitation strategies in MSI based on aforementioned factors are introduced, including new techniques in standard curve calibration with normalization to an internal standard, kinetic calibration, and chemometric methods. Different strategies for validating q-MSI methods are discussed. Finally, the future perspectives to q-MSI are proposed.

Laser Desorption Combined with Laser Postionization for Mass Spectrometry

Lasers with pulse lengths from nanoseconds to femtoseconds and wavelengths from the mid-infrared to extreme ultraviolet (UV) have been used for desorption or ablation in mass spectrometry. Such laser sampling can often benefit from the addition of a second laser for postionization of neutrals. The advantages offered by laser postionization include the ability to forego matrix application, high lateral resolution, decoupling of ionization from desorption, improved analysis of electrically insulating samples, and potential for high sensitivity and depth profiling while minimizing differential detection. A description of postionization by vacuum UV radiation is followed by a consideration of multiphoton, short pulse, and other postionization strategies. The impacts of laser pulse length and wavelength are considered for laser desorption or laser ablation at low pressures. Atomic and molecular analysis via direct laser desorption/ionization using near-infrared ultrashort pulses is described. Finally, the postionization of clusters, the role of gaseous collisions, sampling at ambient pressure, atmospheric pressure photoionization, and the addition of UV postionization to MALDI are considered.

On the SIMS Ionization Probability of Organic Molecules

The prospect of improved secondary ion yields for secondary ion mass spectrometry (SIMS) experiments drives innovation of new primary ion sources, instrumentation, and post-ionization techniques. The largest factor affecting secondary ion efficiency is believed to be the poor ionization probability (α⁺) of sputtered material, a value rarely measured directly, but estimated to be in some cases as low as 10^-5. Our lab has developed a method for the direct determination of α⁺ in a SIMS experiment using laser post-ionization (LPI) to detect neutral molecular species in the sputtered plume for an organic compound. Here, we apply this method to coronene (C₂₄H₁₂), a polyaromatic hydrocarbon that exhibits strong molecular signal during gas-phase photoionization. A two-dimensional spatial distribution of sputtered neutral molecules is measured and presented. It is shown that the ionization probability of molecular coronene desorbed from a clean film under bombardment with 40 keV C₆₀ cluster projectiles is of the order of 10^-3, with some remaining uncertainty arising from laser-induced fragmentation and possible differences in the emission velocity distributions of neutral and ionized molecules. In general, this work establishes a method to estimate the ionization efficiency of molecular species sputtered during a single bombardment event. Graphical Abstract <!-- [INSERT GRAPHICAL ABSTRACT TEXT HERE] -->.