Analysing of Nonvolatile Non-Volatile Compounds With GC: Challenges and Solutions

Analysing non-volatile compounds with Gas Chromatography (GC) can be challenging for chromatographers. However, many non-volatile compounds can be effectively analysed on GC after converting them into volatile derivatives, enabling successful separation and detection.

Gas Chromatography (GC) is a powerful analytical technique widely used in pharmaceutical quality control and research. While it’s most suitable for volatile and thermally stable compounds, modern advancements in derivatisation and instrumentation have made it possible to extend GC applications to nonvolatile pharmaceuticals as well. This blog explores how GC can be adapted for nonvolatile drug compounds, the key challenges involved, and the innovative solutions enabling accurate analysis.

Related: How To Control Impurities In Pharmaceuticals: Get Mastery In …

Why Analyse Nonvolatile Pharmaceuticals with GC?

Nonvolatile drugs, including many active pharmaceutical ingredients (APIs) and excipients, are traditionally analysed using Liquid Chromatography (LC). However, GC offers unique advantages:

• High separation efficiency
• Superior sensitivity for trace-level detection
• Robustness for routine analysis
• Faster run times with lower solvent consumption

To harness these benefits, nonvolatile compounds must be chemically modified to make them GC-compatible.

Key Challenge: Volatility and Thermal Stability

GC requires analytes to be:

• Volatile enough to vaporise in the injection port
• Thermally stable at the column’s operating temperature

Nonvolatile pharmaceuticals typically fail on both counts. They may decompose under heat or have high molecular weights that prevent vaporisation.

Solution: Derivatisation

Derivatisation is the chemical modification of nonvolatile compounds to improve their volatility, thermal stability, and detectability. Common derivatisation methods include:

1. Silylation: Converts polar functional groups (e.g., -OH, -NH, -COOH) into trimethylsilyl (TMS) derivatives, enhancing volatility.
   • Reagents: BSTFA, MSTFA
2. Acylation: Reduces polarity by forming esters or amides.
   • Reagents: Acetic anhydride, trifluoroacetic anhydride
3. Alkylation: Replaces acidic hydrogens with alkyl groups.
   • Reagents: Diazomethane, methyl iodide

These derivatized forms are then amenable to GC analysis with flame ionization detectors (FID) or mass spectrometry (GC-MS)

Case Studies

1. Amino Acids and Peptides: After derivatisation (e.g., silylation), GC-MS can quantify amino acids in biological fluids or pharmaceutical formulations.
2. Steroidal Compounds: Cholesterol and its derivatives are nonvolatile but can be efficiently analysed using GC after derivatisation with MSTFA.
3. Drugs with Carboxylic Acid Groups: NSAIDs like ibuprofen can be alkylated to form methyl esters, making them suitable for GC analysis.

Limitations and Considerations

While derivatisation makes GC accessible for nonvolatile pharmaceuticals, it comes with trade-offs:

• Time-consuming sample prep
• Potential for incomplete derivatisation
• Reagent purity and moisture sensitivity
• Extra method validation steps

Conclusion

Despite being traditionally reserved for volatile compounds, GC has found a place in the analysis of nonvolatile pharmaceuticals through the use of derivatisation and advanced instrumentation. With proper method development, it offers a robust, sensitive, and reproducible approach for analysing complex pharmaceutical matrices.

As pharmaceutical analysis continues to evolve, GC will remain a critical tool, especially as detection technologies improve and workflows become more automated.

FAQs

Further Reading

• Poole, C.F. Gas Chromatography. Elsevier, 2012.