Sample preparation
2.3.1. Qualitative analysis
As porous surface for the fingerprints deposition, it was selected
white office paper in sheets of A4 size with a paper weight of 75 g/
m2. Two female and three male subjects were chosen to deposit the
fingerprints. Each subject deposited by rotation the fingerprints of
all fingers on four different sheets of paper, two for each hand,
divided into 30 quadrants (5 columns 6 rows). On the first sheet,
natural fingermarks were collected; on the second sheet charged
fingerprints were deposited after running the fingers on the areas
of maximum secretion of the face, such as the forehead, nose or
chin. This procedure was repeated six times, waiting half an hour
between two subsequent depositions, to ensure the regeneration
of natural secretions on the hands of the donor. The columns of the
sheet corresponded to the days in which the analyses were made
and the rows corresponded to the different solutions with which
the fingerprints were treated.
Once the deposition was completed, the sheets were cut to
separate the quadrants with only one fingerprint deposited. Each
quadrant was then immerged in a different solution (S1, S2, S3 or SD)
within a plastic tray until it was completely soaked, then air-dried
for a few seconds and finally placed in an oven for 3 min at 170 8C.