Quasi-continuous infrared matrix-assisted laser desorption electrospray ionization source coupled to a quadrupole time-of-flight mass spectrometer for direct analysis from well plates

Abstract

High-throughput screening (HTS) is a technique mostly used by pharmaceutical companies to rapidly screen multiple libraries of compounds to find drug hits with biological or pharmaceutical activity. Mass spectrometry (MS) has become a popular option for HTS given that it can simultaneously resolve hundreds to thousands of compounds without additional chemical derivatization. For this application, it is convenient to do direct analysis from well plates. Herein, we present the development of an infrared matrix-assisted laser desorption electrospray ionization (IR-MALDESI) source coupled directly to an Agilent 6545 for direct analysis from well plates. The source is coupled to a quadrupole time-of-flight (Q-TOF) mass spectrometer to take advantage of the high acquisition rates without sacrificing resolving power as required with Orbitrap or Fourier-transform ion cyclotron resonance (FTICR) instruments. The laser used for this source operates at 100 Hz, firing 1 pulse-per-burst, and delivers around 0.7 mJ per pulse. Continuously firing this laser for an extended duration makes it a quasi-continuous ionization source. Additionally, a metal capillary was constructed to extend the inlet of the mass spectrometer, increase desolvation of electrospray charged droplets, improve ion transmission, and increase sensitivity. Its efficiency was compared with the conventional dielectric glass capillary by measured signal and demonstrated that the metal capillary increased ionization efficiency due to its more uniformly distributed temperature gradient. Finally, we present the functionality of the source by analyzing tune mix directly from well plates. This source is a proof of concept for HTS applications using IR-MALDESI coupled to a different MS platform.

Keywords: IR-MALDESI; Q-TOF mass spectrometer; ambient ionization; direct analysis.

FIGURE 1

Center top: Quasi‐continuous infrared matrix‐assisted laser desorption electrospray ionization (IR‐MALDESI) source. (A) 2970 nm laser with a visualization of the burst rate (100 Hz). (B) Electrospray orthogonal to the sample and coaxial to the inlet of the mass spectrometer. Caffeine was added to the electrospray to monitor the variability of the signal over time. (C) The geometry used to align the emitter tip (E = 15 mm), laser (L = 4 mm), and well plate (H = 14 mm) relative to the inlet of the mass spectrometer. (D) Fully automated X, Y, and Z stages controlled by a Newport XPS‐RLD controller using the in‐house developed program RastirZ. (E) A diagram of the metal capillary dimensions used to replace the glass dielectric ion transfer capillary within the quadrupole time‐of‐flight (Q‐TOF)

FIGURE 2

(A) Scheme of the communication between the different electronic components of the quasi‐continuous infrared matrix‐assisted laser desorption electrospray ionization (IR‐MALDESI) source. An Arduino Nano is used to synchronize axes motion with triggering the laser using the in‐house developed program RastirZ. (B) Screenshot of the RastirZ graphical user interface (GUI). This GUI easily enables the user to fire the laser and jog the axis, move to a desired position, or set up a raster scan pattern. Q‐TOF, quadrupole time‐of‐flight

FIGURE 3

Synchronization timing for collecting infrared matrix‐assisted laser desorption electrospray ionization (IR‐MALDESI) data with the Agilent quadrupole time‐of‐flight (Q‐TOF). The Q‐TOF acquisition signal is set to 3 scans/s and the width of each pulse is 100 μs. The Arduino to XPS‐Controller signal has a pulse width of 30 ms. The XPS‐Controller signal has a pulse width of 20 ms. The Q‐TOF start and stop signals each have a pulse width of 500 ms. The laser pulses have a burst width of 10 ms firing 1 PPB at 100 Hz with 80% duty cycle. The yellow section shows the acquisition clock while the Q‐TOF is recording data.

FIGURE 4

(A) Extracted ion chromatogram (EIC) and spectrum of reserpine acquired with the metal and glass capillaries by electrospray ionization (ESI). (B) EIC and spectrum of leucine enkephalin acquired with the metal and glass capillaries by ESI. (C) Spectra of the main ions in the tune mix showing mass accuracy acquired with the metal capillary by ESI

FIGURE 5

(A) The burst mode consists of firing the laser for a short period of time intermittently. (B) The quasi‐continuous mode consists of firing the laser for a long period of time. This mode demonstrates the capability of the source to ionize an analyte quasi‐continuously.