Isomeric differentiation of polycyclic aromatic hydrocarbons using silver nitrate reactive desorption electrospray ionization mass spectrometry.

RATIONALE

Polycyclic aromatic hydrocarbons (PAHs) are nonpolar and difficult to detect by desorption electrospray ionization. We present a new detection method based on cationization with silver ions, which has the added advantage of being able to differentiate PAHs with the same mass but different structure.

METHODS

9,10-Diphenylanthracene and triptycene, in addition to four different groups of PAH isomers: (1) anthracene and phenanthrene, (2) pyrene and fluoranthene, (3) benz[a]anthracene, benz[b]anthracene (tetracene), and chrysene (4) benzo[a]pyrene and benzo[k]fluoranthene, were deposited on a paper surface and bombarded with methanol droplets containing silver nitrate. The resulting microdroplets entered a quadruple mass spectrometer for mass analysis.

RESULTS

The mass spectrum shows PAH, Ag + OH + PAH, and Ag(PAH)(n) (n = 1, 2) (and PAH + O(2) in the case of benz[b]anthracene) ions. PAHs having a bay structure, such as phenanthrene, showed a different tendency to interact with silver ions from those PAHs having a linear arrangement of the fused benzene rings, such as anthracene. The ratios of the PAH peak intensity to that of Ag-PAH, Ag + OH + PAH, Ag(PAH)(2), and PAH + O(2) were used to differentiate the PAH isomers sharing the same molecular formula with different structures. For isomeric mixtures the PAH to Ag + OH + PAH ratio was found to be the most useful parameter. The uncertainty in the mole fraction of an isomeric mixture was ±0.09, ±0.13, ±0.25, and ±0.1 for phenanthrene-anthracene, fluoranthene-pyrene, benz[a]anthracene-chrysene, and benzo[a]pyrene-benzo[k]fluoranthene, respectively.

CONCLUSIONS

A simple method has been developed for the detection of PAHs in desorption electrospray ionization mass spectrometry based on Ag(I) cationization. The method showed a capability to differentiate PAHs isomers (having the same molecular mass) in isomeric mixture with an uncertainty in the mole fraction of about ±0.1. At high inlet temperature and voltage, this method showed better sensitivity but less ability to differentiate between isomeric species.