Laboratory diagnostics: Maximizing sensitivity of a Q-Exactive Orbitrap mass spectrometer for untargeted metabolomics of dried blood spots

Abstract

Biological samples consist of an unknown, but large, number of regulated metabolites. It is of great interest scientifically and clinically to determine the composition and dynamics of this “metabolome”. Metabolomics is the systematic study of the metabolome and requires the development of reliable and sensitive methods. High resolution mass spectrometry is widely used, often in combination with liquid chromatography. It is essential to optimize instrumental parameters for each specific application. In this project, mass spectrometric parameters were optimized in order to detect as many compounds as possible and maximize signal intensity in dried blood spots, a sample material used in newborn screening and increasingly used in laboratory diagnostics. The following parameters were optimized on a Q Exactive Orbitrap mass spectrometer: voltage applied to the spray liquid in the ionization source (electrospray voltage), distance between the electrospray needle and the MS inlet (electrospray needle position), mass resolving power of the mass spectrometer (resolution), automatic gain control target value (which controls the number of ions to be injected into the Orbitrap), and mobile phase flow rate. The following values were chosen for each parameter: electrospray voltage 3.5 kV, electrospray needle position C, resolution 70 000, and automatic gain control target value 1 000 000 ion counts. Mobile phase flow rate was increased from 150 to 300 µL/min, leading to a 50 % reduction of the initial analysis time, without compromising sensitivity. In certain cases, the optimal values derived for parameters differed unexpectedly from theoretical expectations. This emphasizes the importance of thorough parameter optimization during method development. Using a suboptimal value will decrease the number of detected compounds. This may have serious consequences, especially if compounds which are not detected are clinically relevant. The present work indicates that if extensive optimization of parameters is done during method development, diagnostic opportunities will improve.