Charge state distribution shifting of protein ions observed in matrix-assisted laser desorption ionization mass spectrometry

By using a new sample preparation method for matrix-assisted laser desorption ionization, a significant shift to lower mass-to-charge values can be obtained for many protein samples. The sample preparation technique involves the creation of a thin film of protein-doped α-cyano-4-hydroxycinnamic acid (CHCA) matrix formed in the presence of glycerol on top of a previously deposited pad of CHCA matrix. The higher charge states were not observed if the laser power was significantly above the threshold needed to produce protein molecular ions. Similar spectra were observed when samples were prepared in the presence of urea. The phenomenon was specific for the CHCA matrix because no effects were observed when sinapinic acid (3,5-dimethoxy-4-hydroxy-trans-cinnamic acid) and 2-(4-hydroxyphenylazo) benzoic acid matrices were used with the new sample preparation method.