Effects of solvent flow, dopant flow, and lamp current on dopant-assisted atmospheric pressure photoionization (DA-APPI) for LC-MS. Ionizationvia proton transfer

Abstract

In this paper, the effects of solvent flow, dopant flow, and lamp power on proton transfer ionization in dopant-assisted (DA) atmospheric pressure photoionization (APPI) are investigated. A broad theoretical framework is presented, describing the primary photoionization process, the formation of protonated-solvent cluster ions, and the balance between analyte ion creation via proton transfer and loss via recombination. The principal experimental test system utilized methanol as the solvent, toluene as the dopant, and acridine as the analyte. Comparisons are made between acridine and a less basic compound, 9-methylanthracene (9-MA). Experimental determinations of the trends in the analyte MH ϩ signal and the total ion current (TIC) with variations in the subject parameters are provided. Experimental results and theory demonstrate that both the analyte signal and the TIC approach asymptotic limits with increases in dopant flow and/or lamp current (two factors which dictate the rate of photoion generation). The data show that these limits are lowered at higher solvent flow rates. These results are attributed to the recombination loss process, the rate of which increases with the second power of ion concentration. We deduce that the recombination rate constant increases with solvent flow rate, a consequence of the growth of ion-solvent clusters. Cluster growth is also believed to be a factor in the dramatic loss of sensitivity for 9-MA that occurs as the solvent flow is raised, because larger protonated-solvent cluster ions have greater solvation energies and may be unreactive with compounds having low gas-phase basicity and/or low solvation energy. (J Am Soc Mass Spectrom 2005, 16, 1275-1290) © 2005 American Society for Mass Spectrometry P hotoionization (PI) is the latest means of ionization to be incorporated into atmospheric pressure ionization (API) sources for liquid chromatography-mass spectrometry (LC-MS). The original motivation for the development of atmospheric pressure photoionization (APPI) sources was the demand for a method or device capable of expanding the range of compounds amenable to LC-MS to include nonpolar compounds not readily ionized by either electrospray [1,2] or atmospheric pressure chemical ionization (APCI) [3,4]. In recent years, two approaches towards utilizing PI at atmospheric pressure have emerged: dopant-assisted (DA) APPI [5] and direct APPI [6]. Recent review papers provide details of the two APPI-MS methods [7,8]. This paper is concerned mainly with DA-APPI.As is often the case for new technologies, the practical application of DA-APPI has outpaced the development of detailed knowledge regarding the mechanisms responsible for its performance. DA-APPI relies upon gas-phase ion-molecule reactions to place a charge on neutral analytes, so it is especially important that these reactions be well understood. The groups of Kostiainen and Bruins have completed several studies of the reaction chemistry of DA-APPI and the effects of solvent and dopant composition on the ionization effici...

APPI-MS: Effects of mobile phases and VUV lamps on the detection of PAH compounds

“…Although direct, single-photon ionization (SPI) of a compound [M] however, there is little published experimental evidence as to which solvents or solvent combinations are best suited for specific light sources. We recently observed that the solvent acetonitrile results in a lower analyte response compared with methanol [7], and this effect has also been observed with acetonitrile and chloroform [17,18]. This effect becomes more significant at high flow rates, and has recently been theorized by Kauppila…”

“…A similar effect has also been observed to occur in a low-pressure PPI source, where [M · ] ϩ ions occur close to the sampling cone and [M ϩ H] ϩ ions occur with greater abundance farther away from the cone [16]. With APLI, direct photoionization of [M] to [M · ] ϩ occurs within the ionization volume, and as the position of the laser beam is backed away from the sampling cone, then either the "slow" H-abstraction [16], or "fast" H ϩ -transfer reactions can result in generation of [M ϩ H] ϩ [17]. With APPI, the source of photons is relatively distant from the sampling cone, and thus chemical ionization can make significant contribution to ion generation.…”

“…Although direct, single-photon ionization (SPI) of a compound [M] results in the radical cation [M · ] ϩ , use of certain mobile phases can result in the adduct [M ϩ H] ϩ [16]. The notion that a solvent can play a significant role has been observed with APPI [4], DA-APPI [17], and APLI [3]; however, there is little published experimental evidence as to which solvents or solvent combinations are best suited for specific light sources. We recently observed that the solvent acetonitrile results in a lower analyte response compared with methanol [7], and this effect has also been observed with acetonitrile and chloroform [17,18].…”

“…The notion that a solvent can play a significant role has been observed with APPI [4], DA-APPI [17], and APLI [3]; however, there is little published experimental evidence as to which solvents or solvent combinations are best suited for specific light sources. We recently observed that the solvent acetonitrile results in a lower analyte response compared with methanol [7], and this effect has also been observed with acetonitrile and chloroform [17,18]. This effect becomes more significant at high flow rates, and has recently been theorized by Kauppila and Bruins to be a result of the large photoabsorption cross-section of acetonitrile at a light energy of 10.0 eV [18].…”

Atmospheric pressure photoionization mass spectrometry of polyisobutylene derivatives

“…As shown for APPI, the rate of photon absorption in the gas-phase [34,35] by the dopant molecules (I D , photons/s) in a mixture composed of dopant and solvent molecules with number densities of n D and n S (particles/cm 3 ), respectively, can be given by…”