Development, Evaluation, and Application of a Gas Chromatography – Atmospheric Pressure Chemical Ionization Interface for Orbitrap Mass Spectrometers

Gas chromatography – mass spectrometry (GC-MS) is a valuable tool used by numerous scientific fields including forensics, medicine, and petrochemicals. Until recently, many GC-MS instruments have relied on either electron ionization or chemical ionization sources with low resolution mass analyzers. With improving technology, Thermo Fisher Scientific launched the Q Exactive™ GC hybrid quadrupole-Orbitrap MS, which utilizes these sources, while other companies also released GC-MS systems capable of exact mass measurements; however, these GC-MSs instead employ atmospheric pressure chemical ionization (APCI), which like other atmospheric pressure ionizations (API), provides “soft” ionization of analytes. Moreover, APCI extends the range of analytes that can be detected and identified compared to traditional ionization techniques. Thus, the combination of high-resolution mass spectrometry (HRMS) and APCI enables accurate yet versatile analyses. To utilize these advantages, an interface between a GC and a Thermo Scientific™ Ion Max housing unit was designed. This simple, yet temporary interface enables the reversible conversion of a liquid chromatography system to a GC-MS. The GC-APCI interface was tested against a commercially available atmospheric pressure photoionization (APPI) interface for three types of analytes that span the breadth of typical GC analyses: fatty acid methyl esters (FAMES), polyaromatic hydrocarbons (PAHs), and saturated hydrocarbons analyses. The GC-APCI-Orbitrap was found to have similar or improved performance to the APPI interface and other reported methods. Additionally, it was found that the interface consistently produced unique ions of these analytes, which were also detected using the APPI source. Using a Q Exactive™ Orbitrap, high-resolution and MS2 data were collected using standards of non-deuterated and select fully deuterated analytes. From these experiments, the data suggest unique, underreported mechanisms of GC-API due to in-source oxidation. Finally, the newly designed interface was used to characterize fatty acid composition of subcutaneous adipose tissue from broiler chicks in support of studying polyunsaturated fatty acid (PUFA) pathways. Specifically, it was determined that expression of three genes significantly and positively correlated with seven fatty acids during embryonic and post hatch development. Together, these experiments demonstrate the utility and capability of a GC-APCI-Orbitrap as an alternative to traditional GC-MS.