Anal. Chem. 2010, 82, 621–627
Microplasma Discharge Ionization Source for Ambient Mass Spectrometry Joshua M. Symonds,† Asiri S. Galhena,‡ Facundo M. Ferna´ndez,*,‡ and Thomas M. Orlando*,†,‡ School of Physics and School of Chemistry and Biochemistry, Georgia Institute of Technology, 901 Atlantic Drive NW, Atlanta, Georgia 30332 In this paper, we demonstrate the first use of a microplasma ionization source for ambient mass spectrometry. This device is a robust, easy-to-operate microhollow discharge that enables ambient direct analysis of gaseous, liquid, and solid-phase samples with minimum requirements in terms of operating power and high purity gas consumption. The initial performance of the microplasma device has been evaluated by ionizing samples containing dimethyl sulfoxide (DMSO), dimethylformamide (DMF), methyl salicylate, caffeine, L-leucine, L-histidine, loratadine, ibuprofen, acetaminophen, acetylsalicylic acid, and cocaine in various forms. These molecules are diverse in nature, but almost all have relatively high proton affinities. Thus, the major species observed in all obtained mass spectra corresponded to protonated molecules. Though these microplasmas are known to produce significant densities of metastable species and electrons with mean energies greater than several electronvolt, minimal fragmentation was observed. Background spectra showed prominent signals corresponding to H+(H2O)2 ions and a distinct lack of H3O+. Small water cluster ions are likely the dominant proton transfer agents, giving rise to mass spectral data very similar to that obtained using other plasma-based ambient ionization techniques. The simplicity, low cost, low power, low rate of gas consumption, and possibility of being batchfabricated, makes these microplasma devices attractive candidates as ion sources for miniaturized mass spectrometry and other field detection applications. The first report of desorption electrospray ionization (DESI) by Cooks et al. in 20041 triggered immense interest in further exploration of alternative “ambient” approaches for generating ions directly from sample surfaces in open air. As a result, more than two dozen other ambient ionization techniques have since been developed.2-4 These new methods of producing ions have simpli* Corresponding author. Phone: 404-894-4012 (T.M.O.); 404-385-4432 (F.M.F.). Fax: 404-385-6057 (T.M.O.); 404-385-3399 (F.M.F.). E-mail: thomas.orlando@ chemistry.gatech.edu (T.M.O.);
[email protected] (F.M.F). † School of Physics. ‡ School of Chemistry and Biochemistry. (1) Takats, Z.; Wiseman, J. M.; Gologan, B.; Cooks, R. G. Science 2004, 306, 471–473. (2) Van Berkel, G. J.; Pasilis, S. P.; Ovchinnikova, O. J. Mass Spectrom. 2008, 43, 1161–1180. (3) Venter, A.; Nefliu, M.; Cooks, R. G. Trends Anal. Chem. 2008, 27, 284– 290. 10.1021/ac901964m 2010 American Chemical Society Published on Web 12/18/2009
fied the mass spectrometric analytical procedure, thus increasing throughput. Several of these new ambient ionization methods employ atmospheric plasmas, including direct atmospheric pressure photoionization (DAPPI),5 direct analysis in real time (DART),6 flowing atmospheric pressure afterglow (FAPA),7 plasmaassisted desorption ionization (PADI),8 desorption atmospheric pressure photoionization (DAPCI),9 low-temperature plasma (LTP) ionization,10 and dielectric barrier discharge ionization (DBDI).11 All these techniques are well understood from the analytical point of view. However, the physiochemical processes and mechanism(s) governing desorption/ionization are not so well-known and remain an active area of research. Furthermore, these plasma sources typically require relatively high operation voltage and power due to their macroscopic nature, thus somewhat complicating their use in the field of portable mass spectrometry. In addition, none of the plasma-based ambient desorption/ionization techniques allow the sufficient spatial control necessary to distinguish chemical features with less than a few hundred micrometers resolution. Approaches using laser desorption/ionization techniques achieve better resolution, but the available ion densities are limited and the comparative costs are higher.12 The plasma physics community has been actively engaged in developing inexpensive low-temperature microplasmas and plasma needles for industrial applications.13 Microplasmas are weakly ionized discharges that represent a new and fascinating realm of plasma science. The “pd scaling” of the well-known Paschen law suggests reasonably low voltage operation of microplasma devices at atmospheric pressure for cavities of sufficiently small (
