High-throughput analysis of growth promoters by LC-MS/MS using a novel tube plasma ionisation source

Background: The use of estrogenic compounds to promote growth in livestock animals is prohibited in the European Union. They are typically determined by gas chromatography coupled to mass spectrometry (GC–MS) using the electron ionisation source (EI). However, the high fragmentation observed by EI requires large injection volumes to reach the analytical requirements. Also, the need for time-consuming derivatisation makes this approach not desired for routine analysis. Besides, liquid chromatography-mass spectrometry (LC-MS) methodologies using electrospray ionisation (ESI) shows also inefficient ionisation and high matrix effects and thereby, reliable alternatives are needed to improve the throughput of the analysis of estrogenic compounds. Results: In this work, a recently described ionisation source called tube plasma ionisation (TPI) is used to overcome the limitations of LC-MS determination of growth promoters, as it enhances the range of ionisable compounds, covering both polar and non-polar substances. After careful evaluation of source and MS related parameters and optimization of an HPLC separation based on a water/methanol gradient programme, a comparison was made to a commercial ESI source. TPI showed lower detection limits instrumentally (0.007 to 5.1 μg L⁻¹ compared to 0.112 to 394 μg L⁻¹ with ESI) but also in both bovine urine and meat matrices. Additionally, the sensitivity of the determination is increased when using TPI (median: 0.66) over ESI (median: 0.20). The novel approach shows high linearity, precision (both intra- and inter-day) and trueness and significantly increases the analytical workflow compared to ESI, allowing a proper quantification of the compounds in complex matrices. Significance: The relevance of this works lies on the capabilities of the LC-MS system using TPI to outperform ESI, ensuring the monitoring of growth promoters in livestock animal matrices at ultra trace levels. This new source not only guarantees a proper analytical performance, but it also helps to simplify time-consuming and tedious sample treatment steps such as derivatisation for low-polar compounds.