Particle formation in ambient MALDI plumes.

The ablated particle count and size distribution of four solid matrix materials commonly used for matrix-assisted laser desorption ionization (MALDI) were measured with a scanning mobility particle sizer (SMPS) combined with a light scattering aerodynamic particle sizer (APS). The two particle sizing instruments allowed size measurements in the range from 10 nm to 20 μm. The four solid matrixes investigated were 2,5-dihydroxybenzoic acid (DHB), 4-nitroaniline (NA), α-cyano-4-hydroxycinnamic acid (CHCA), and sinapic acid (SA). A thin film of the matrix was deposited on a stainless steel target using the dried droplet method and was irradiated with a 337 nm nitrogen laser at atmospheric pressure. The target was rotated during the measurement. A large number of nanoparticles were produced, and average particle diameters ranged from 40 to 170 nm depending on the matrix and the laser fluence. These particles are attributed to agglomeration of smaller particles and clusters and/or hydrodynamic sputtering of melted matrix. A coarse particle component of the distribution was observed with diameters between 500 nm and 2 μm. The coarse particles were significantly lower in number but had a total mass that was comparable to that of the nanoparticles. The coarse particles are attributed to matrix melting and spallation. Two of the compounds, CHCA and SA, had a third particle size distribution component in the range of 10 to 30 nm, which is attributed to the direct ejection of clusters.