Evaluation of functional characteristics of lactose by inverse gas chromatography

Significance of the topic

The selection and characterization of lactose as a pharmaceutical excipient are critical for ensuring consistent drug performance and manufacturing reproducibility. Differences in crystalline and amorphous forms, as well as production routes (spray drying vs milling), directly influence surface energetics, moisture sensitivity and downstream processing behavior. Inverse gas chromatography (IGC) offers a non-destructive approach to quantify dispersive and specific surface interactions, supporting QA/QC and formulation design.

Objectives and overview of the study

This work evaluated multiple batches of lactose monohydrate and spray-dried amorphous lactose from different suppliers. Key aims were to compare surface energy components (dispersive and acid–base) across production methods, and to assess the impact of elevated relative humidity (90% RH at 313 K) on surface properties over time. The study probed batch variability, stability under humid heat, and implications for pharmaceutical processing.

Methodology and instrumentation

IGC in infinite dilution mode was used to determine surface energetics, supplemented by BET nitrogen physisorption for total surface area.

• Samples: Six lactose batches—three spray-dried (amorphous) and three milled/sieved (crystalline with amorphous defects).
• Probes: Non-polar (n-pentane to n-octane), polar (diethyl ether, chloroform, acetone, benzene).
• Chromatographic conditions: 303 K analysis; sample columns conditioned at 313 K; helium carrier at 2 mL/min; Shimadzu QP-2010 GC-MS.
• B.E.T. analysis: Micromeritics Pulse Chemisorb 2700 at 77 K over three pressures for surface area.

Main results and discussion

Significant batch variations were observed: spray-dried lactose displayed higher dispersive surface energy, reflecting its thermodynamically unstable amorphous state. Milled monohydrate showed intermediate values; mechanical activation during milling likely increased surface defect sites. Specific acid–base interactions (‘GSP) varied most for DMV spray-dried samples, indicating manufacturing-dependent surface acidity.

Under 90% RH at 313 K, monohydrate lactose’s dispersive energy steadily declined, suggesting reorganization or recrystallization of amorphous defects. Its electron-acceptor (acidic) sites diminished rapidly, whereas spray-dried lactose exhibited increased surface acidity over time, underlining form-dependent hygroscopic reactivity.

Benefits and practical applications of the method

IGC provides precise, reproducible measurement of surface energy components with minimal sample preparation and non-destructive analysis. It can detect subtle batch-to-batch differences and predict stability under storage or processing conditions, aiding excipient selection for direct compression, inhalation carriers, and wet granulation.

Future trends and potential applications

Combining IGC with real-time moisture sorption analysis and advanced imaging could deepen understanding of dynamic surface transformations. Extending this approach to other excipients and co-processed powders will support design of robust formulations. Integration with machine learning may enable predictive modeling of excipient behavior under varying environmental stresses.

Conclusion

This study demonstrated that IGC effectively differentiates surface energetics of lactose forms and monitors humidity-induced changes. Manufacturing route and moisture exposure critically influence dispersive and specific interactions, with direct relevance for pharmaceutical performance and stability. IGC is a valuable tool for excipient quality control and formulation optimization.

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