Derivatisation in GC/GCMS For Nonvolatile Drugs Analysis: Silylation, Acylation, Alkylation & More (FAQs + Case Studies)

Derivatisation in GC/GC-MS is essential for converting non-volatile or highly polar pharmaceuticals into volatile, stable, and analytically compatible derivatives suitable for chromatographic separation and detection.

Analysing nonvolatile compounds with GC (Gas Chromatography) 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. In this blog, I will discuss how GC can be adapted for nonvolatile drugs with case studies and FAQs.

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Analysing Nonvolatile Compounds With GC: GC Key Challenges

Gas Chromatography (GC) fundamentally relies on the ability of analytes to enter the gas phase and remain chemically intact throughout the chromatographic process. For reliable GC performance, analytes must meet two essential requirements:

1. Sufficient Volatility

Analytes must be capable of complete and rapid vaporisation in the injector at temperatures typically ranging from 200–300 °C (depending on method and matrix).
Compounds with:

• High molecular weights
• Strong intermolecular forces (e.g., hydrogen bonding)
• Low vapour pressures

Often fail to vaporise efficiently, resulting in incomplete transfer, peak tailing, discrimination, or total non-elution.

2. Adequate Thermal Stability

Analytes must withstand temperatures used in:

• Injection port
• Capillary column
• Detector interface

without undergoing thermal decomposition, isomerisation, or polymerisation. Many pharmaceutical molecules contain labile functional groups (e.g., esters, amides, carbamates, and peptides) that degrade at elevated temperatures, resulting in multiple secondary peaks or no detectable parent peak.

Why Most Pharmaceuticals Are Not GC-Amenable?

Many APIs and excipients are inherently:

• Nonvolatile (due to high polarity or high molecular mass)
• Thermolabile (susceptible to decomposition at >200 °C)

Because they fail to meet volatility and stability criteria, they are typically analysed by Liquid Chromatography (HPLC, UHPLC).

Why GC Is Still Valuable in Pharmaceutical Analysis?

Despite these limitations, GC offers several analytical strengths that make it indispensable for certain pharmaceutical applications:

Exceptional Separation Efficiency

GC columns (commonly coated fused-silica capillaries) provide highly efficient separations with theoretical plate counts exceeding 100,000, enabling resolution of closely related impurities and solvents.

High Sensitivity for Trace-Level Detection

GC coupled with FID, NPD, ECD, or MS achieves ppb–ppt level detection, making it ideal for:

• Residual solvent analysis (ICH Q3C)
• Volatile impurities
• Degradation products
• Extractables/leachables

Robustness for Routine Quality Control

GC systems offer stable retention times and consistent detector response, enabling:

• Routine QC testing
• Rugged validated methods
• Minimal method drift

Faster Run Times and Low Operating Costs

Compared with LC:

• GC often yields shorter analysis times (minutes instead of tens of minutes)
• No mobile-phase preparation is required
• Lower solvent consumption → reduced cost and environmental impact

Expert Tips

Although nonvolatile and thermolabile pharmaceuticals are typically incompatible with GC in their native form, the technique provides superior performance for volatile analytes, impurities, and trace contaminants. When used appropriately — sometimes with derivatisation to improve volatility and stability — GC remains a critical analytical tool in the pharmaceutical industry.

How can nonvolatile drugs be analysed using GC?

Non-volatile compounds can be analysed using GC using the derivatisation technique, such as Silylation, Acylation, Alkylation, etc.

Derivatisation is the chemical modification of nonvolatile compounds to improve their volatility, thermal stability, and detectability. The following derivatisation methods are widely used for non-volatile compounds by GC.

1. Silylation: Converts polar functional groups (e.g., -OH, -NH, -COOH) into trimethylsilyl (TMS) derivatives, enhancing volatility.
   • Reagents: BSTFA (N,O-bis(trimethylsilyl)trifluoroacetamide), MSTFA (N-methyl-N-trimethylsilyltrifluoroacetamide)
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 derivatised forms are then amenable to GC analysis with flame ionisation 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.

Silylation of Glycine Using BSTFA

Reactants:

• Glycine: HOOC–CH₂–NH₂
• BSTFA: (CH₃)₃Si–NH–CO–CF₃ twice substituted (donates TMS groups)

Typical Derivatisation Reactions used for analysis of nonvolatile compounds by GC/GC-MS:

HOOC–CH₂​–NH₂ +3BSTFA→ (CH₃)₃Si–OOC–CH₂​–N(Si(CH₃)₃)+3CF₃​CONH

The final product Glycine-bis(trimethylsilyl) ester/amine derivative is highly volatile, thermally stable → suitable for GC-FID or GC-MS.

R–CH(NH₂)–COOH+n(TMS–donor)⟶R–CH[N(TMS)]–COO–TMS+nBy-products.

Where n = number of reactive hydrogens:

• Carboxyl H → 1 TMS
• Amino H (2 hydrogens) → 2 TMS
• Side-chain functional groups (OH, SH, COOH) → additional TMS groups

Thus, many amino acids form tri- or tetra-TMS derivatives.

Silylation of Alanine Using MSTFA

CH₃–CH(NH₂)–COOH+3MSTFA→CH₃–CH[N(TMS)]–COO–TMS+3CF₃CONHCH₃

Expert Tips: Why Silylation is Used for GC?

• Creates highly volatile, thermally stable derivatives
• Reduces polarity
• Prevents tailing of amino acids on GC columns
• Improves sensitivity for GC–FID and GC–MS
• Widely used in amino acid profiling and impurity analysis

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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.

You may like:

1. Relative Response Factor (RRF) in Pharmaceutical Analysis

FAQs

Further Reading

• Poole, C.F. Gas Chromatography. Elsevier, 2012.
• Handbook of ANALYTICAL VALIDATION: Michael E. Swartz Ira S. Krull

Abbreviations:

• BSTFA = N,O-bis(trimethylsilyl)trifluoroacetamide
• MSTFA = N-methyl-N-trimethylsilyltrifluoroacetamide
• TMCS = trimethylchlorosilane as a catalyst)