Disposable polymeric devices used for direct sample pretreatment/signal enhancement and MALDI-MS analyses of biomolecules suffer from the accumulation of electric charge and related positive mass shifts due to photoelectric effects on non-conductive surfaces after irradiation with lasers. The effects of surface charging on the mass shift of protein spectra during MALDI-TOF/MS measurements on copolymeric sample array chips were studied. Methyl viologen is used to monitor the development of charge and is demonstrated to be an effective electron scavenger. The use of such reducible species leads to increased accuracy, signal homogeneity, and resolution for mass spectral measurements of proteins in mixtures with sinapinic and 2,5-dihydroxybenzoic (DHB) acids. This approach can be used on a wide range of nonconductive support materials. Copyright © 2007 John Wiley & Sons, Ltd.