Electric Field-Modulated Electrospray Ionization Mass Spectrometry for Quantity Calibration and Mass Tracking

Analyses conducted by electrospray ionization (ESI) mass spectrometry (MS) typically entail performing a number of preparatory steps, which include quantity calibration and mass calibration. Quantity calibration can be affected by signal noise, while mass calibration can be affected by instrumental drift if analyses are performed over an extended period of time. Here, we present two methods for achieving these calibrations using modulation of electrospray plume by alternating electric fields and demodulating the resulting MS ion currents. For this purpose, we use an ESI source fitted with three ring electrodes between the electrospray emitter and the mass spectrometer's inlet. One of these electrodes is supplied with a sine electric signal. Optionally, a nanoESI emitter is also placed between the ring electrodes and the mass spectrometer's orifice to supply calibrant ions. The ion currents, recorded with this setup, present wave-like features. In the first variant, using a triple quadrupole mass analyzer, the ion currents are subjected to data treatment by fast Fourier transform (FFT), and the resulting FFT magnitudes are correlated with analyte concentrations to produce a calibration plot. In the second variant, using a quadrupole time-of-flight mass analyzer, the mass spectra recorded at the analyte ion current maxima are mass-checked using the m/z value of the internal standard (injected via nanoESI emitter), which appears predominantly in the time intervals corresponding to the analyte ion current minima. The setup has been characterized using simulation software and optimized. Overall, the method enables the preparation of quantity calibration plots and monitoring (minor) m/z drifts during prolonged analyses.

On the potential of micro-flow LC-MS/MS in proteomics

INTRODUCTION

Due to its excellent sensitivity, nano-flow liquid chromatography tandem mass spectrometry (LC-MS/MS) is the mainstay in proteome research; however, this comes at the expense of limited throughput and robustness. In contrast, micro-flow LC-MS/MS enables high-throughput, robustness, quantitative reproducibility, and precision while retaining a moderate degree of sensitivity. Such features make it an attractive technology for a wide range of proteomic applications. In particular, large-scale projects involving the analysis of hundreds to thousands of samples.

AREAS COVERED

This review summarizes the history of chromatographic separation in discovery proteomics with a focus on micro-flow LC-MS/MS, discusses the current state-of-the-art, highlights advances in column development and instrumentation, and provides guidance on which LC flow best supports different types of proteomic applications.

EXPERT OPINION

Micro-flow LC-MS/MS will replace nano-flow LC-MS/MS in many proteomic applications, particularly when sample quantities are not limited and sample cohorts are large. Examples include clinical analyses of body fluids, tissues, drug discovery and chemical biology investigations, plus systems biology projects across all kingdoms of life. When combined with rapid and sensitive MS, intelligent data acquisition, and informatics approaches, it will soon become possible to analyze large cohorts of more than 10,000 samples in a comprehensive and fully quantitative fashion.