Direct infusion of an analyte with an ISQ EC or ISQ EM single quadrupole mass detector

Significance of the Topic

Liquid chromatography–mass spectrometry (LC–MS) workflows often require fine-tuning of ion source and acquisition parameters to achieve optimal sensitivity and accuracy. Direct infusion using a syringe pump streamlines method development and quality control for single quadrupole detectors by enabling immediate adjustment of source settings independent of chromatographic separation.

Objectives and Study Overview

This technical note describes step-by-step procedures to configure direct infusion and infusion into HPLC eluent streams using external syringe pumps with Thermo Scientific ISQ EC and ISQ EM single quadrupole mass spectrometers. Both manual setups and the use of Chromeleon CDS for automated source optimization are presented.

Methodology and Instrumentation

The infusion setups employ PEEK and Teflon FEP tubing, fingertight fittings, unions, and optional PEEK tee pieces. Two infusion modes are covered:

• Direct infusion: the syringe pump connects the sample directly to the HESI source grounding union.
• Eluent stream infusion: a PEEK tee piece integrates the sample flow into the HPLC effluent before MS detection.

System configurations in Chromeleon CDS link or manually define flow rates to Autospray algorithms. Typical MS parameters include vaporizer temperatures of 227–250 °C, ion transfer tube at 250 °C, sheath gas at 8–42.9 psig, auxiliary gas 1–4.8 psig, and a spray voltage of 3000 V. Data are acquired in profile mode over m/z 500–2000 with 1 s dwell.

Main Results and Discussion

Myoglobin (20 pmol/μL) was infused at 10 μL/min in 50% methanol, 0.05% formic acid. Direct infusions revealed clear charge envelopes for apo-myoglobin, holo-myoglobin, and free heme. Infusion into a 400 μL/min acetonitrile–formic acid eluent displayed only the apo-protein envelope, likely due to 40-fold dilution. Deconvolution tools in Chromeleon CDS and Biopharma Finder provided accurate molecular weights.

Benefits and Practical Applications

• Rapid source parameter optimization without full LC runs.
• Flexible switch between direct infusion and online eluent infusion.
• Enhanced throughput for protein and small molecule method development.
• Integration with Chromeleon CDS enables automated parameter screening.

Future Trends and Potential Applications

Emerging developments may include automated syringe pump control within chromatographic sequences, high-throughput multiplexed infusion approaches, real-time quality monitoring in regulated environments, and deeper integration with advanced deconvolution and data analysis platforms. Adaptation to different ionization sources and miniaturized flow regimes will further extend applicability.

Conclusion

A straightforward syringe pump infusion system for Thermo Scientific ISQ EC/EM single quadrupole detectors offers a fast, reproducible approach for method optimization and analyte screening. Both direct and eluent-stream infusion modes facilitate sensitive detection and accurate mass measurements with minimal system modifications.

Reference

1. Thermo Scientific Technical Note 73669: The benefits of adding single quadrupole mass detection to liquid chromatography.
2. Thermo Scientific Technical Note 73670: Quality control of oligonucleotides with a single quadrupole mass spectrometer.