Semi-Quantification

Importance of the Topic

The analysis of complex hydrocarbon mixtures such as gasoline poses significant quantitation challenges due to the large number of volatile components and frequent coelution of isomers.
Semi-quantification is a practical approach that groups structurally similar analytes and estimates their concentrations against calibration curves of representative compounds.
This strategy accelerates routine screening in quality control and regulatory compliance where full resolution of every isomer is impractical.

Objectives and Study Overview

The primary objective is to demonstrate a semi-quantitative method for determining aromatic hydrocarbons in finished gasoline by GC-MS using LECO Pegasus software.
The study follows ASTM Method D5769 and applies selected masses to approximate concentrations of C10, C11, and C12 benzene isomers.
A representative calibration compound (1,2-diethylbenzene for C10 aromatics) is used to generate response curves, which are then extended to structurally similar analyte groups.

Methodology and Instrumentation

The method involves rapid gas chromatographic separation under conditions optimized for maximum resolution of aromatic isomers.
Selected ion monitoring is used for semi-quantification, choosing characteristic m/z values for each carbon series:

• C10 aromatics: m/z 134
• C11 aromatics: m/z 148
• C12 aromatics: m/z 162

Calibration curves are prepared for key reference compounds prior to data processing.

Instrumentation

The analysis is performed on a Pegasus GC-TOF MS system equipped with dedicated semi-quantification software.
Data processing methods allow the assignment of quantification masses and calibration curves directly within the instrument software.

Results and Discussion

Chromatograms reveal resolved peaks for a wide range of aromatics in the C6–C12 range.
Semi-quantitative results yield a total aromatic content of approximately 41.85 wt% for the test gasoline sample.
Individual group summaries include:

• C10 benzene isomers: ~0.74 wt%
• C11 benzene isomers: ~0.86 wt%
• C12 benzene isomers: ~0.01 wt%

Minor deviations arise when analyte response factors differ significantly from the chosen reference; multipliers can correct for known response disparities.

Benefits and Practical Applications

Semi-quantification drastically reduces analysis time by avoiding the need for individual calibration of every isomer.
The approach aligns with regulatory methods such as ASTM D5769 for rapid screening of benzene, toluene, and total aromatics in gasoline.
Results are sufficiently accurate for routine quality control, environmental testing, and process monitoring.

Future Trends and Opportunities

Advances in mass spectrometer scan rates and deconvolution algorithms will improve resolution of coeluting components.
Integration of machine learning models may refine response factor predictions across broader compound classes.
Expanded compound libraries and automated multiplier generation could enhance the accuracy of semi-quantitative workflows.

Conclusion

Semi-quantification on the Pegasus GC-TOF MS provides a robust, efficient means to estimate aromatic content in gasoline.
By grouping structurally related isomers and using representative calibration curves, laboratories can meet regulatory requirements with reduced resource investment.
Future software and hardware innovations will further streamline this approach.

Reference

• ASTM Method D5769: Rapid Determination of Benzene, Toluene, and Total Aromatics in Finished Gasoline
• LECO Application Note #203-821-094