2010-Liquid Chromatographic and Mass Spectrometric Analysis of Dyes for Forensic Purposes

Liquid Chromatographic and Mass Spectrometric Analysis of Dyes for Forensic Purposes

Keith R. Beck, North Carolina State University


Fibers are a significant portion of trace forensic evidence. In the laboratory, forensic scientists compare fibers from a crime scene to determine if they are consistent with those from a suspect or his/her surroundings. The sequence of this analysis involves the use of light microscopy, polarized light microscopy, Fourier Transform Infrared spectroscopy (for fiber identification), visible microspectrophotometry (for color) and thin layer chromatography (TLC) of extracted dyes.
To improve the quality of information obtained about dyes, chromatographic separations have moved from TLC to high performance liquid chromatography (HPLC) with a diode array detector (DAD). The DAD is important because a full visible spectrum for each peak can be extracted from the acquired data file.
Recently, quadrupole mass spectrometers have become more common as HPLC detectors for the
analysis of dyes for forensic purposes. Because of the physics involved in measuring mass to charge ratio (m/z), mass accuracies from quadrupole mass filters are good to only unit mass. To improve m/z accuracy, time-of-flight (TOF) mass analyzers were developed. In a LC/TOF mass spectrometer, the eluate from the LC column is vaporized and ionized in the source. Ions are accelerated into the mass spectrometer, focused by ion guides and then pulsed into a flight tube. The time that ions take to pass from the pulser to the detector is dependent on m/z. Because of advances in electronics, these times can be measured very accurately, which allows the determination of m/z to four decimal places. These “exact” masses are significant because they allow the prediction of molecular formulas for the ions. As a result, TOF mass spectrometers have become excellent qualitative tools but, to our knowledge, have not been used to analyze dyes for forensic purposes.
The data being reported here have been developed in the initial stages of research supported by a National Institute of Justice (NIJ) grant and a National Science Foundation (NSF) grant (see acknowledgements). The grants have different objectives, but both involve the use of TOF mass spectrometric analysis of (different) dyes. Ultimately, our laboratory will establish an unprecedented comparative finished fiber analytical database (COMFFAD).
The NIJ grant will employ LC/TOF, a novel fiber preparation method employing cryomicrotomy for cross-sectioning fibers to facilitate minute loss of fiber (e.g. less than 0.2 mm) during analysis, and development of direct mass spectral analysis of
dyes found in the cross-section using a technique called TOF Secondary Ion Mass Spectrometry (TOF SIMS). If successful, the latter method would eliminate the need for extraction of dyes from the fiber.
The NSF grant will investigate different dyes and will also include standard forensic microscopic and FTIR analysis of a comprehensive physical library of automotive fabrics (the De Leo Detroit collection9) that contains practically all domestic and imported cars from 1955 to the present day. The results reported here describe extraction of selected dyed polyester (PET) and nylon fibers and comparison of the chromatograms and spectra of those extracts with data obtained directly from the commercial dyes.