Technical note: Analysis of biological substances in ink fingerprint by desorption electrospray ionization mass spectrometry

As a custom in China, a visible fingerprint pressed with red seal ink on a signature represents personal confirmation of a document. This custom makes ink fingerprints important evidence for individual identification in the forensic practice of questioned document examination. Recently, forged ink fingerprints using stamps or silica gel fingerprint models have emerged. Consequently, detecting and profiling the pattern of biological substances in visible fingerprints on questioned documents is crucial to prove that a fingerprint was imprinted by its real owner. To solve this problem, a desorption electrospray ionization mass spectrometry (DESI-MS) method was developed to detect the biological substances in ink fingerprints on paper. In the positive ion mode, more signals were detected, but the interference from the seal inks could not be ignored. In the negative ion mode, the ion detected in the sweat latent fingerprints at m/z 187 presented high chemical specificity for MS imaging. It was most likely a borate compound in human sweat. Using this biomarker, the fingerprint pattern was successfully profiled and the hand-imprinted fingerprint was clearly distinguished from the stamp-imprinted one. This method can help in determining the authenticity of ink fingerprints on questioned documents in forensic practice.