Charged aerosol detection - factors affecting uniform analyte response

Significance of the Topic

Charged aerosol detection (CAD) in LC/UHPLC offers a universal, mass-flow sensitive detection method that delivers nearly uniform response across a wide range of non-volatile and many semi-volatile analytes. Unlike UV-based approaches, CAD does not rely on chromophores, enabling quantitation of compounds lacking UV absorbance and even of unknown analytes in pharmaceutical, industrial, and biological samples.

Objectives and Study Overview

This work examines the principles behind CAD’s uniform response, identifies factors that can perturb this uniformity—mobile phase composition, analyte volatility, salt formation, and density—and presents practical strategies to optimize CAD performance, including the use of inverse gradients, salt corrections, and evaporator temperature control.

Instrumentation

Key instrumentation and workflows include:

• Thermo Scientific Vanquish Flex UHPLC system with Dual Pump and Corona Veo CAD
• Vanquish Flex Split Sampler FT and Column Compartment H
• Inverse Gradient Kit for Vanquish Duo and Chromeleon CDS 7.2.8 with Inverse Gradient Wizard
• Typical flow injection and gradient conditions: capillary nebulization, evaporation temperatures 35–50 °C, data rates 5–20 Hz, filter settings 0.5–3.6 s

Main Results and Discussion

Uniform Response Demonstration:
Through flow injection analysis of 36 structurally diverse analytes (without chromatography) CAD showed < 6 % variability in response, confirming its near-identical mass-based sensitivity.

Extractables Analysis:
In a biopharmaceutical bag extractables study, CAD provided consistent quantitation of unknown contaminants, whereas UV response varied greatly with analyte extinction coefficients.

Inverse Gradient Compensation:
During reversed-phase gradients, changing organic content alters nebulization efficiency and CAD response. By adding a counter-gradient (inverse gradient) via a second pump, a constant composition reaches the detector, restoring uniform response. In a stressed paclitaxel impurity profile, inverse gradient reduced quantitation bias (~10 % difference) compared with uncorrected gradients. In tenofovir disoproxil fumarate analysis, inverse gradients eliminated baseline drift and yielded accurate impurity quantitation using a single calibrant.

Analyte Volatility and Evaporation Temperature:
Non-volatile analytes (boiling point > 400 °C, high enthalpy) exhibit uniform response, while semi-volatiles (e.g., oxalic acid) can show reduced signals. Lowering the CAD evaporation temperature broadens detection of semi-volatiles but may increase noise; higher temperatures suppress semi-volatile signals.

Salt Formation Effects and Corrections:
Ionizable analytes can form non-volatile salts with counterions from mobile phase additives or sample matrices, altering response proportionally to added molar mass. Using low-mass additives (formic acid, ammonium formate) minimizes this effect. Response normalization by correcting for salt molar mass restores uniformity, demonstrated for dopamine-HCl, guanidine-HCl, and in tenofovir impurity quantitation.

Intentional Salt Formation for Semi-volatiles:
Adding volatile modifiers (e.g., triethylamine) can convert semi-volatiles into non-volatiles. Oxalic acid response increased ten-fold when measured as its triethylammonium salt.

Analyte Density:
Density influences particle diameter to the one-third power and has minor impact; typically ignored unless analytes exhibit large density differences.

Benefits and Practical Applications of the Method

• Universal detection and quantitation of non-chromophoric or unknown compounds
• Accurate impurity profiling in pharmaceuticals using single calibrants
• Robust extractables/leachables screening without UV limitations
• Enhanced sensitivity for semi-volatiles via salt formation strategies

Future Trends and Applications

Ongoing research into aerosol formation mechanisms and gas-to-particle partitioning will refine CAD predictions. Integration of CAD with multi-detector systems (UV, MS) and advanced chromatographic workflows will expand applications in drug development, QA/QC, environmental testing, and industrial analytics. Novel detector designs and software tools will further simplify inverse gradient implementation and salt correction automation.

Conclusion

Charged aerosol detection offers an inherently uniform mass-based detection platform for a broad range of analytes. Employing inverse gradient compensation, optimized evaporation temperatures, salt correction, and intentional salt formation extends CAD’s applicability to volatile and ionizable compounds, enabling accurate, single-calibrant quantitation in diverse analytical contexts.

References

1. Gamache PH, ed. Charged Aerosol Detection for Liquid Chromatography and Related Separation Techniques. Wiley. 2011.
2. Thermo Fisher Scientific Application Note AN72594, Quantification of paclitaxel, its degradants, and related substances using UHPLC with charged aerosol detection. 2018.
3. Thermo Fisher Poster PO72726, Multi-detector set-up comprising UV/Vis, CAD, and single quadrupole MS for comprehensive sample analysis. 2018.