Charged Aerosol Detection: Factors Affecting Uniform Analyte Response

Significance of the Topic

Charged aerosol detection provides a near-universal response for nonvolatile and semivolatile analytes in liquid chromatography by generating size-dependent charged particles that are independent of chemical structure. This feature makes CAD a powerful tool for quantifying diverse analytes with a single calibrant, addressing key challenges in pharmaceutical, environmental and food analysis where compounds vary widely in volatility, polarity and ionization behavior.

Objectives and Study Overview

This study evaluates the factors that influence response uniformity in the charged aerosol detector. Key goals include

• Identifying the impact of mobile phase composition during gradient elution
• Determining volatility thresholds for analyte loss in the spray and drying process
• Exploring intentional salt formation to convert semivolatiles into nonvolatile forms
• Assessing the minor effect of analyte density on detector response

Flow injection analysis was performed on a chemically diverse set of 58 analytes to quantify these effects and establish best practices for consistent CAD performance.

Methodology and Instrumentation

Samples were introduced by flow injection at 0.5 µg per injection with and without triethylamine in the mobile phase to probe salt formation effects. A Thermo Scientific Vanquish Flex Quaternary UHPLC system was employed, featuring:

• Quaternary pump with an inverse gradient capability
• Split sampler with a 25 µL loop
• Corona Veo charged aerosol detector with 35 °C evaporation temperature

Constant composition at the detector inlet was maintained via a make-up gradient from a second pump. Data collection was performed at 5 Hz with a 0.5 s filter for high-resolution response profiles.

Main Results and Discussion

The CAD demonstrated less than 6 percent relative standard deviation across 36 diverse analytes after purity correction. Key findings include:

• Mobile phase organic content affects droplet formation and can introduce up to 6 percent variability if not controlled by an inverse or make-up gradient
• Analyte volatility correlates with boiling point and enthalpy of vaporization. Cutoffs around 400 °C boiling point or 65 kJ/mol enthalpy separate semi-volatile from nonvolatile behavior, with volatile analytes losing signal at higher evaporation temperatures
• Intentional salt formation using low-molar mass additives such as formic acid or ammonium formate increases the effective mass of semivolatile and volatile compounds, improving response uniformity after mathematical correction
• An analyte density correction based on the cube root of solute density has negligible impact for most compounds but can be applied for analytes with extreme densities

These observations underscore the importance of evaporation temperature, mobile phase control and strategic use of additives to achieve the intrinsically uniform response of CAD.

Benefits and Practical Application

Charged aerosol detection with optimized conditions offers:

• Single-standard quantification for complex mixtures, reducing calibration burden
• Enhanced sensitivity and stability for semivolatile drug metabolites, extractables and degradants
• Compatibility with inverse gradient workflows to eliminate bias from solvent composition changes
• Improved method transferability across laboratories and instruments

Future Trends and Potential Uses

Emerging directions include:

• Advanced modeling of spray-drying and gas-to-particle partitioning to predict individual analyte response
• Integration of machine learning algorithms for real-time correction of evaporation and salt formation effects
• Development of multi-pump inverse gradient systems for seamless method adaptation in high-throughput environments
• Expansion of CAD to volatile compound analysis through novel derivatization and additive strategies

Conclusion

The charged aerosol detector achieves fundamentally uniform response across a broad chemical space when key parameters are controlled. By maintaining constant mobile phase composition at the detector inlet, operating at the lowest practical evaporation temperature and leveraging low-molar additives for salt formation, practitioners can fully exploit the CAD’s universal quantitation capability. These guidelines enable robust method development for diverse applications in analytical chemistry.

References

1 Charged Aerosol Detection for Liquid Chromatography and Related Separation Techniques Gamache P H Wiley Chapters 1 and 3
2 Thermo Fisher Scientific Technical Note 72806 Charged Aerosol Detection – factors affecting uniform analyte response 2018
3 Thermo Fisher Scientific Application Note 72594 Quantification of paclitaxel, its degradants, and related substances using UHPLC with charged aerosol detection 2018
4 Thermo Fisher Scientific Application Note 72869 A multi-detector set-up comprising UV/Vis, charged aerosol and single quadrupole mass spectrometric detection for comprehensive sample analysis 2018
5 Thermo Fisher Scientific Application Note Quantitation of tenofovir and impurities in multi-component drug products by ternary gradient reversed phase chromatography with charged aerosol detection 2019