Atmospheric‐pressure laser ionization: a novel ionization method for liquid chromatography/mass spectrometry

Abstract

We report on the development of a new laser-ionization (LI) source operating at atmospheric pressure (AP) for liquid chromatography/mass spectrometry (LC/MS) applications. APLI is introduced as a powerful addition to existing AP ionization techniques, in particular atmospheric-pressure chemical ionization (APCI), electrospray ionization (ESI), and atmospheric pressure photoionization (APPI). Replacing the one-step VUV approach in APPI with step-wise two-photon ionization strongly enhances the selectivity of the ionization process. Furthermore, the photon flux during an ionization event is drastically increased over that of APPI, leading to very low detection limits. In addition, the APLI mechanism generally operates primarily directly on the analyte. This allows for very efficient ionization even of non-polar compounds such as polycyclic aromatic hydrocarbons (PAHs). The APLI source was characterized with a MicroMass Q-Tof Ultima II analyzer. Both the effluent of an HPLC column containing a number of PAHs (benzo[a]pyrene, fluoranthene, anthracene, fluorene) and samples from direct syringe injection were analyzed with respect to selectivity and sensitivity of the overall system. The liquid phase was vaporized by a conventional APCI inlet (AP probe) with the corona needle removed. Ionization was performed through selective resonance-enhanced multi-photon ionization schemes using a high-repetition-rate fixed-frequency excimer laser operating at 248 nm. Detection limits well within the low-fmol regime are readily obtained for various aromatic hydrocarbons that exhibit long-lived electronic states at the energy level of the first photon. Only molecular ions are generated at the low laser fluxes employed ($1 MW/cm 2 ). The design and performance of the laser-ionization source are presented along with results of the analysis of aromatic hydrocarbons.

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

“…Generation of large clusters is facilitated by the large binding energy of neutral CH 3 OH to protonated solvent clusters, e.g., CH 3 OH binding with [CH 3 OH] n H ϩ is Ϫ33 kcal/mol for n ϭ 1, Ϫ22 kcal/mol for n ϭ 2, and Ϫ16 kcal/mol for n ϭ 3 [42].…”

“…In many cases atmospheric pressure photoionization (APPI) [1,7] has demonstrated extended linear dynamic range [11], enhanced sensitivity and thus lower detection limits [6,9,[12][13][14][15], and reduced or no off-line sample cleanups [6,9] in comparison with direct APCI or ESI. Adding a dopant (dopant-assisted, DA) to the mobile phase in many cases can further increase sensitivity [2].A related analytical technique is atmospheric pressure laser ionization (APLI), which has shown excellent sensitivity for certain non-or low-polar compounds [3]. The enhanced sensitivity is a direct result of the high photon flux associated with a laser system, often several orders of magnitude higher than the noncoherent light sources, e.g., lamps used with APPI.…”

“…A related analytical technique is atmospheric pressure laser ionization (APLI), which has shown excellent sensitivity for certain non-or low-polar compounds [3]. The enhanced sensitivity is a direct result of the high photon flux associated with a laser system, often several orders of magnitude higher than the noncoherent light sources, e.g., lamps used with APPI.…”

“…Three conclusions from this investigation are: (1) methanol photoionization leads to protonated methanol clusters that can result in chemical ionization of analyte molecule; (2) use of the Ar lamp often results in greater signal and S/N; (3) acetonitrile photoionization is less efficient and resulting clusters are too strongly bound to chemically ionize the analyte efficiently, so that analyte ion formation is dominated by direct photoionization. (J Am Soc Mass Spectrom 2007, 18, 589 -599) © 2007 American Society for Mass Spectrometry M ethods to detect nonpolar compounds by liquid chromatography-mass spectrometry (LC-MS) have advanced significantly over the last few years, owing to development of newer source technologies [1][2][3] and improved choices of mobile phase(s) for chromatographic separation [4,5]. Recent work has focused on the direct comparison of different sources (APPI, APCI, and ESI) with specific target compounds, e.g., polyaromatic hydrocarbons [6,7], hydrophobic peptides [8], pesticides [9,10], as well as fatty acids and lipids [4,5].…”

“…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].…”