Using Laser-Induced Acoustic Desorption/Electrospray Ionization Mass Spectrometry To Characterize Small Organic and Large Biological Compounds in the Solid State and in Solution Under Ambient Conditions

Abstract

We have coupled laser-induced acoustic desorption (LIAD) with electrospray ionization (ESI) mass spectrometry (LIAD/ESI/MS) to characterize molecules in the solid state and in solution under ambient conditions. To perform an LIAD/ESI analysis, the sample droplet is deposited on the surface of a thin aluminum foil by a micropipette; the rear side of the foil with the sample spot is then irradiated with a pulse from a Nd:YAG IR laser. The resulting shockwave and heat cause the sample on the rear side to change from the condensed phase to the gas phase. The desorbed species then move upward to enter an ESI plume to react with charged solvent species (methanol- and water-related ions and droplets), forming singly or multiply charged analyte ions. A quadrupole/time-of-flight (Q-TOF) mass analyzer attached to the LIAD/ESI source detects the analyte ions to obtain an ESI-like mass spectrum. Both small organic and large biological compounds (including amino acids, peptides, and proteins) were successfully ionized and detected by the LIAD/ESI/MS system. Although native and denatured myoglobin ions were both detected from a liquid sample solution, only the denatured myoglobin ions were detected from a dried sample.

Surface analysis of lipids by mass spectrometry: More than just imaging

“…Desorbed analytes are then ionized by subsequent interaction with the ion source. A variety of ionization techniques have been coupled with LIAD including chemical ionization [270], ESI [271] and APCI [272]. This desorption mechanism is not dependant on laser absorption by the analyte and is suitable for analysis of thermally labile and non-volatile analytes.…”

“…Although only non-polar lipids have been analyzed to date, other intact biomolecules such as peptides and proteins have been detected [271], suggesting LIAD will also be suitable for the analysis of additional lipid classes.…”

What can we learn from ambient ionization techniques?

“…Sampson et al proposed LAESI and IR LADESI "may be better described as IR-MALDESI" (MALDESI ϭ matrix-assisted laser desorption electrospray ionization) because of the use of water as a matrix [25]. More recently, ESI was coupled to laser-induced acoustic desorption (LIAD) where a laser beam is directed at the back of a substrate surface containing the sample and desorption occurs due to the action of the propagating acoustic wave [26].…”

Analysis of Amphiphilic Lipids and Hydrophobic Proteins Using Nonresonant Femtosecond Laser Vaporization with Electrospray Post-Ionization

“…However, in milk, the distribution likely corresponds to the multiple charge states of the proteins γ-casein (mass = 24,010 Da) and α-lactoalbumin (mass = 14,175 Da). The charge state distributions for γ-casein and α-lactoalbumin were also observed by other groups that analyzed milk samples using laser-based methods [8,32,33]. In the mass spectrum shown in Figure 5a, the instrument parameters were adjusted to provide a mass spectrum with the highest resolution in the lower m/z region.…”

“…However, the peaks labeled 6, 7, 8, and 13 observed at m/z 426, 442, 501, and 783, respectively, in Figure 5 are only observed when fs lasers were used to induce vaporization, and were not detected in the ns laser desorption measurements [8,9,[32][33][34]. Therefore, these peaks detected are not unique to the brand or fat content of milk analyzed but appear to be unique to the nonresonant fs vaporization of milk.…”