No abstract

We demonstrated a minimally destructive two-tier approach for multielement forensic analysis of laser-printed ink. The printed document was first screened using a portable-X-ray fluorescence (XRF) probe. If the results were not conclusive, a laser microprobe was then deployed. The laser probe was based on a two-pulse scheme: the first laser pulse ablated a thin layer of the printed ink; the second laser pulse at 193 nm induced multianalytes in the desorbed ink to fluoresce. We analyzed four brands of black toners. The toners were printed on paper in the form of patches or letters or overprinted on another ink. The XRF probe could sort the four brands if the printed letters were larger than font 20. It could not tell the printing sequence in the case of overprints. The laser probe was more discriminatory; it could sort the toner brands and reveal the overprint sequence regardless of font size while the sampled area was not visibly different from neighboring areas even under the microscope. In terms of general analytical performance, the laser probe featured tens of micrometer lateral resolution and tens to hundreds of nm depth resolution and atto-mole mass detection limits. It could handle samples of arbitrary size and shape and was air compatible, and no sample pretreatment was necessary. It will prove useful whenever high-resolution and high sensitivity 3D elemental mapping is required.

Laser-excited-atomic-fluorescence at the single excitation wavelength of the ArF laser was demonstrated for numerous analytes. These analytes were present in the material plumes produced by pulsed laser ablation of ceramic and polymeric targets. Under minimally destructive conditions, the fluorescence spectra were orders of magnitude brighter than the corresponding laser-induced breakdown spectra. Simultaneous emissions from Al, Ca, Co, Cr, Cu, Fe, In, Mg, Mn, Na, Pb, Sn, and Si were observed. The technique was applied to four analytical problems: the analysis of dried paint for trace lead when mg g À1 detection limits were achieved; the analysis of valuable potteries when two lookalike specimens were differentiated based on practically non-destructive single-shot analysis; the elemental analysis of ink when lines written with different pens could be discriminated without discernable sample destruction even under the microscope; and the analysis of electrode-plastic interfaces when the detection sensitivity was comparable to SIMS. The fluorescence intensity varied with the fluence and the timing of the ArF laser pulse in ways suggestive of particulates in the plume.

In pulsed laser ablative sampling, photoexcitation of the nascent plume can be nonselective because the energy structure is band-like. As the plume disperses, the bands taper to discrete levels and analyte emission lines become narrow. Multi-analyte fluorescence at a single excitation wavelength is therefore possible.