There are several major benefits for LIAD. First, compared to electrospray ionization, LIAD is not limited to just acidic or basic analytes, making it especially useful for complex mixtures. Also, LIAD typically produces minimal fragmentation or thermal damage from the laser, because the laser is directed at the back side of the foil, which does not contain the sample, so there is no direct exposure. On striking the foil the laser produces an acoustic wave that travels through the foil and desorbs neutral analytes previously deposited on the other side (see Figure). The acoustic wave travels faster than the heat wave so that the sample is desorbed into the gas phase before the heat wave can produce thermal damage. The most important benefit of LIAD is that ionization is separate from desorption, thereby enabling ionization to be tailored to specific analytes. Because the solvent evaporates before desorption, the only interaction that occurs after desorption is between the analyte and reagent ions (i.e., carrier gas molecules that have been charged by the high-voltage electrode. Analyte ions are then injected into the mass spectrometer for detection.
Figure 1. Schematic diagram of an atmospheric pressure laser-induced acoustic desorption chemical ionization (AP/LIAD-CI) source coupled to a 9.4 T Fourier transform ion cyclotron resonance mass spectrometer. The inset show the mass spectra obtained for a petroleum model compound (ellipticine) following LIAD with the corona discharge on. Source: Leonard Nyadong et al, Atmospheric Pressure Laser-Induced Acoustic Desorption Chemical Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry for the Analysis of Complex Mixtures, Anal. Chem., 2011, 83 (5), pp 1616–1623.
Petroleum crude oil presents high compositional complexity, with up to tens of thousands of different elemental compositions in a single sample. Analysis of the saturated hydrocarbon components is additionally challenging due to their low polarity and facile fragmentation. In conjunction with Fourier transform ion cyclotron resonance mass analysis, LIAD of crude oil enables resolution and identification of saturated hydrocarbons not accessed by electrospray ionization.
This technique can be used with the following instruments:
- 14.5 Tesla FT-ICR MS (Actively Shielded)
- 21 Tesla FT-ICR MS (Actively Shielded)
- 9.4 Tesla FT-ICR MS (Passively Shielded)
For more information please contact Amy McKenna, Manager, ICR User Program.