Design and evaluation of a novel pulsed radio frequency glow discharge instrument for imaging applications

Complete text of publication follows. Glow discharge optical emission spectroscopy (GD-OES) is a powerful tool with detection limits in the μg g-1 range and is most commonly applied to the bulk analysis of conductive solids. Moreover, radio-frequency powered glow discharges (rfGD) allow the analysis of non-conductive samples, such as thin films, polymers, and glass. Qualitative and compositional depth profiling of layered surfaces has become a major area of application with depth resolution in the nm range. Recently, it was demonstrated that the rfGD can also provide laterally resolved elemental information along a sample surface. In the present study, a novel pulsed rfGD instrument coupled to a monochromatic imaging spectrometer (MIS) will be described that enables the direct analysis of targets such as minigels or microarrays. The cathode diameter of the GD instrument significantly exceeds that of conventional sources. Model proteins were separated by gel electrophoresis, stained with silver-enhanced colloidal gold, and transferred to a blotting membrane. Surface elemental imaging was performed with the rfGD. Here, the analytes are subjected to sputtering and excitation by the glow discharge source. The rf energy is applied in short pulses (<300 μs) to reduce the time of diffusion of sputtered atoms before they emit. Moreover, a pulsed discharge limits the sample surface temperature and enables direct analysis of fragile targets such as membranes or gels. Limits of detection in the femtomole range were obtained (e.g. bovine serum albumin LOD = 0.05 ng / 1mm2 spot). This technique allows the identification of biological metal associations, e.g. the study of metalloproteins after separation under native conditions.