“…Potentially subtle variations in ink and/or paper of stamps with otherwise identical designs may affect their value and desirability to philatelists and investors or reveal them to be forgeries or reprints. Because of its ability to nondestructively reveal elemental compositions, proton-induced X-ray emission spectrometry (PIXE) has long been an applicable technique for analysis of a variety of historical inks and papers (Cahill et al, 1981;Johanson, 1989;Demortier, 1991;Hassanzadeh et al, 1999) and has been used for analysis of postage stamps from the technique's early days to the present (e.g., Cahill et al, 1981;Malmqvist, 1986;Almquist, 1985Almquist, , 1986Almquist, , 1988Johanson et al, 1986;Hall and Lee, 1990;Johanson, 1992;Cheng et al, 1996Cheng et al, , 1998Pillay, 2000;Oliaiy et al, 2009;Voltr and Nejedly, 2012). …”