Doped amorphous silica nanoparticles as enhancing agents for surface-assisted time-of-flight mass spectrometry

Abstract

This article examines the use of doped amorphous silica nanoparticles for surface-assisted laser desorption/ionisation-time of flight-mass spectrometry (SALDI-TOF-MS) of hydrophilic and hydrophobic compounds. A range of particles with surface aliphatic carboxylic, aminophenyl, phenyl or aminopropyl groups have been produced and these have been doped with carbon black, polyaniline or graphite. The effects of surface groups and dopants on the laser desorption/ionisation process were studied. The key factor in effective LDI was the presence of carbon black dopant carrying carboxyphenyl or phenyl residues for positive and negative ion formation. The second key factor was the presence of hydrophilic surface functional groups for hydrophilic amino acid analytes for their detection in positive or negative mode as protonated or de-protonated species respectively whereas hydrophobic surfaces were need for ionisation via cationisation for the hydrophobic analyte squalene. The mechanism for LDI of these particles appears to involve initial adsorption of the analyte onto the surface of the particle, formation of primary ions via adsorption of laser UV irradiation by carboxyphenyl residues attached to the carbon black network which act in an equivalent way to the matrix in matrix-assisted LDI. This is followed by reaction of the primary ions with neighbouring adsorbed analyte molecules. The latter are then released possibly via thermal desorption following proton donation or acceptance from/to via surface residues such carboxylate groups associated with the carbon black within the dopant. Alternatively in the absence of such proton donor/acceptor residues as with hydrophobic particles, the primary ions are released from the particles during desorption and form cation adducts as sodiated and potassiated species in the gas phase above the surface.

Detection of nitro-organic and peroxide explosives in latent fingermarks by DART- and SALDI-TOF-mass spectrometry

“…The enhanced signal intensity with this powder compared with other commercial dusting agents has also been observed for drugs [8] and nicotine [9,10] in pre-dusted lifted latent fingermarks using SALDI-MS. Its effect on the spectrums of endogenous constituents in latent fingrmarks has also been described [13].…”

“…In our experiments it is assumed that similar adsorption of the explosives onto the ROAR particles occurred rapidly to produce a complex of sufficient stability to resist evaporation of the explosives under the high vacuum conditions within the mass spectrometer and that under laser irradiation the residues present on the surface of the particles assisted ionization of the adsorbed explosives producing the negative ions detected for the nitro-organic explosives or positive ions for the peroxide explosive TATP. The mechanism for Surface Assisted Laser Desorption/Ionization of polar and non-polar analytes using these particles has recently been described [13]. The SALDI-MS of the aliphatic explosives were characterized by a lack of the de-protonated molecular ions and peaks probably due loss of nitro groups and the presence of low mass peaks at m/z 46 and/or 62 due to (NO 2 + ) and (NO 3 + ) ions, respectively.…”

Separation of fingerprint constituents using magnetic silica nanoparticles and direct on-particle SALDI-TOF-mass spectrometry

“…1(c) and (d). These observations can be explained by a proposed mechanism [42] which involves the initial adsorption of the analyte onto the particle surface. Ionic interactions enable the analyte to remain close to the particle surface.…”

“…In an earlier SALDI-TOF-MS study with carbon black-doped silica particles, amino acids analysed were best detected in positive ion mode and squalene was not detected in negative mode [42]. A recent review also reported that lipids were more easily detected and with greater sensitivity in positive mode than in negative mode [43].…”

“…Proton or electron donation occurs between ionised particle surface groups and analyte molecules on the surface during secondary processes. The APTES-DP used were washed to pH 5 and their effectiveness as a LDI-enhancing agent may be derived from the full protonation of both carboxylic acid and amino residues on the particles following this treatment which may facilitate analyte-particle binding interactions and subsequent proton donation on laser irradiation [42]. The ionised analyte is then released from the surface via thermal effects and electrostatic attraction towards the detector.…”