Atmospheric Pressure Photoionization-Mass Spectrometry with a Microchip Heated Nebulizer

Abstract

A novel, microfabricated heated nebulizer chip for atmospheric pressure photoionization-mass spectrometry (APPI-MS) is presented. The chip consists of fluidic and gas inlets, a mixer, and a nozzle etched onto silicon wafer that is anodically bonded to a Pyrex glass wafer, on which an aluminum heater is sputtered. A krypton discharge lamp is used as the source for 10-eV photons to initiate the photoionization process. Dopant, delivered as part of the sample solution, is used to achieve efficient ionization. The use of the microfabricated heated nebulizer with APPI in the analysis of four analytes is demonstrated, and the spectra are compared to those obtained with a conventional APPI source. Ionization in positive and negative ion modes was successfully achieved and the spectra were mainly similar to those obtained with conventional APPI, indicating that the ionization in microfabricated and conventional APPI sources takes place by the same mechanisms. The flow rates with conventional APPI are approximately 100 muL/min, whereas the microchip heated nebulizer allows the use of flow rates 0.05-5 muL/min, thus being compatible with microfluidic separation systems or micro- and nano-LC. A stable signal was demonstrated throughout a 5-h measurement, which proved the excellent stability of the micro-APPI. The same heated nebulizer chip can be used for weeks.

Atmospheric pressure photoionization proton transfer for complex organic mixtures investigated by fourier transform ion cyclotron resonance mass spectrometry

“…Acidic species can deprotonate to form (M Ϫ H) Ϫ and positive electron affinity compounds can capture thermal electrons and form M -• . Kostiainen et al studied negative ion formation mechanisms by investigating ionization efficiency and ion type (even-or odd-electron ion) for analytes of different polarity in various solvents [3,7]. Traldi et al reported negative ion formation by resonant electron capture from thermal electrons originating from metal surfaces and dopant [8].…”

Microchip technology in mass spectrometry

“…Also, the flow rates applicable to current commercial APCI and APPI ion sources are high, typically 50-1,000 mL/min, thus excluding analysis with minimal sample volumes. For these reasons, a miniaturized heated nebulizer was developed for microchip-based APCI (Ö stman et al, 2004; Ö stman et al, 2006a,b) and APPI (Kauppila et al, 2004a). Later, the same microchip was used as a sonic spray (SSI) (Pol et al, 2007) or thermospray (TSI) ion source (Keski-Rahkonen et al, 2008).…”

“…The thermal nebulizer chips incorporate fluidic inlets, a mixer, a heater, and a spraying nozzle so that only fluidic connectors needed to be manually assembled afterwards. The first prototype of the chip was successfully applied to the ionization of small molecules by APCI (Ö stman et al, 2004) and APPI (Kauppila et al, 2004a).…”

“…Microchip atmospheric pressure photoionization (mAPPI) (Kauppila et al, 2004a) employs the same heated nebulizer microchip as mAPCI, but the corona discharge needle is replaced by a krypton discharge photoionization lamp similar to that in conventional APPI (Robb, Covey, & Bruins, 2000). In addition to a nebulizer gas, a dopant (e.g., toluene, acetone, anisole) is introduced to the heated nebulizer chip to enhance the ionization efficiency.…”

Analysis of selective androgen receptor modulators by gas chromatography-microchip atmospheric pressure photoionization-mass spectrometry

“…In mass spectrometry, the majority of the effort has been put to miniaturization of ion sources, especially electrospray ionization (ESI) [4]. However, other miniaturized ion sources, e.g., the heated nebulizer microchip for atmospheric pressure chemical ionization ( APCI) [5] and atmospheric pressure photoionization ( APPI) [6], and the nebulizer microchip for sonic spray ionization (SSI) [7], have been reported as well. The heated nebulizer microchips can be used to connect a capillary liquid chromatograph (cap LC) [8,9] or a gas chromatograph (GC) [9 -11], with any mass spectrometer with an atmospheric pressure ionization (API) source, or as a heated sprayer in desorption atmospheric pressure photoionization [12].…”