COMPARISON OF THE NOVEL UNISPRAY ION SOURCE TO A RANGE OF ALTERNATIVE ATMOSPHERIC PRESSURE IONIZATION SOURCES

Importance of the Topic

Atmospheric pressure ionization techniques play a critical role in mass spectrometry workflows, enabling sensitive detection of a wide variety of small molecules. The development of novel sources such as UniSpray can expand analytical coverage, improve response factors for challenging analytes, and complement existing methods in fields ranging from environmental monitoring to pharmaceutical and petrochemical analysis.

Objectives and Study Overview

This study compares the performance of the UniSpray ionization source against established atmospheric pressure ionization techniques—electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), atmospheric pressure photoionization (APPI) and atmospheric solids analysis probe (ASAP). Various compound classes including pesticides, polycyclic aromatic hydrocarbons (PAHs), polymer additives, flame retardants, cosmetics allergens, fatty acid methyl esters (FAMEs), engine oil additives and OLED materials are examined to assess sensitivity, linear dynamic range and operational parameters.

Methodology

Sample Preparation and Chromatography

• Standard mixtures prepared at relevant concentration levels (0.1–1.0 µg/mL for small molecules; 1 mg/mL for crude oil samples).
• UHPSFC separation using supercritical CO₂ (Solvent A) and methanol with additives (Solvent B) on an ACQUITY HSS C18 SB column at 40 °C and 150 bar with a 2 µL injection.

Ionization Conditions

• UniSpray impactor voltages evaluated at 0.5, 1.0 and 3.0 kV.
• APCI corona currents tested at 1, 5, 10 and 12 µA.
• APPI and ASAP corona currents assessed at 1 and 12 µA.

Used Instrumentation

High-resolution mass spectrometry was performed on a SYNAPT G2-Si HDMS system with ion mobility enabled. Key parameters included a 50 V cone voltage, source temperature of 120 °C, IMS wave velocity of 1000 m/s, wave height of 40 V and cell pressure at 3.3 mbar. Data were processed using MassLynx v.4.1.

Main Results and Discussion

Response Enhancement

• UniSpray displayed significantly higher signal intensities for oilfield additives, with up to 17-fold increase for 12OH imidazoline and six-fold for C12 quaternary ammonium salt compared to ESI across 10 ppt to 2 ppm.
• In polymer additive mixtures, UniSpray provided the best responses for the majority of compounds, outperforming APCI, APPI and ASAP in many cases.

Selectivity Across Compound Classes

• PAH analysis revealed APPI as the most effective source, while UniSpray exhibited minimal response for radical cation formation in this class.
• For OLED phosphorescent compounds and other aromatic materials, UniSpray and ESI generated comparable isotopic clusters, with APCI also providing similar intensity.

Adduct-Dependent Optimization

• Optimal impactor voltage varied with adduct type: protonated species benefited from higher voltages (~3.0 kV), whereas sodiated adducts showed better response at lower voltages (~0.5 kV).

Linearity and Calibration

• Both imidazoline and quaternary ammonium analytes demonstrated excellent linearity over four orders of magnitude, validating UniSpray for quantitative applications.

Benefits and Practical Applications

• UniSpray expands the analytical toolbox for mass spectrometrists, particularly enhancing detection of nonpolar and surface-active analytes that are challenging for ESI alone.
• Complementary use of APCI, APPI and ASAP alongside UniSpray ensures broad coverage of diverse compound classes in complex samples such as crude oil fractions, environmental pollutants and advanced materials.

Future Trends and Applications

• Integration of UniSpray with ion mobility separation may offer improved isomer resolution and structural characterization in complex matrices.
• Development of tailored impactor geometries and voltage profiles could further optimize sensitivity for specific adduct types or molecular classes.
• Expanded application in metabolomics, lipidomics and biopharmaceutical analysis as the technique matures and commercial availability increases.

Conclusion

UniSpray has been shown to deliver substantial improvements in ionization efficiency for a variety of small molecule classes, particularly oilfield additives and polymer stabilizers. While not universally superior for all analytes, its complementary performance alongside ESI, APCI, APPI and ASAP makes it a valuable addition to comprehensive mass spectrometry workflows.

References

1. Bajic S. An Aerodynamic Perspective on Impactor API Sources, Waters Poster PSTR134897766, ASMS 2016.
2. Lubin A., Geerinckx S., Bajic S., Cabooter D., Augustijns P., Cuyckens F., Vreeken R.J.J. Chromatographia A 2016, 1440, 260.