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In-Process Control

Monitoring Peptide Synthesis in Real Time

Purpose

In-process controls (IPC) verify each synthesis step before proceeding, preventing accumulation of errors and ensuring final product quality.

IPC Methods Comparison

Method Monitors Time Sensitivity Quantitative
Kaiser test (ninhydrin) Free amine 5–10 min +++ Semi
Chloranil test Free amine 2 min ++ No
TNBS test Free amine 10 min ++++ Yes
UV monitoring (301 nm) Deprotection Real-time ++ Yes
Conductivity Coupling Real-time + Yes
IR spectroscopy Amide bond Real-time ++ Yes

Kaiser Test Protocol

Reagent Preparation

Reagent Composition
A 5 g ninhydrin in 100 mL ethanol
B 80 g phenol in 20 mL ethanol
C 2 mL 0.001 M KCN in 98 mL pyridine

Test

  1. Wash 5–10 resin beads with ethanol
  2. Add 2 drops each of A, B, C
  3. Heat to 100 °C for 5 min
  4. Read color:
Color Interpretation
Colorless or yellow ✅ No free amine — coupling complete
Blue or purple ❌ Free amine present — coupling incomplete
Light blue ⚠️ May need extended coupling

Automated IPC with UV Monitoring

Deprotection Monitoring

  • Continuous flow-through UV cell at 301 nm
  • Two peaks per deprotection cycle
  • Peak area correlates to amino acid loading
  • Trend: decreasing area = decreasing resin accessibility

Conductivity Monitoring

  • Measures ion concentration in coupling step
  • Drops sharply when coupling completes
  • Useful for first ~15 AA; less sensitive later

In-Process Release Criteria

Step IPC Check Pass Criteria
After resin loading Loading measurement Within ±10% target
After each coupling Kaiser test Negative (yellow)
After deprotection UV monitor Absorbance peak detected
Before cleavage Final cycle Kaiser Negative
After cleavage HPLC (crude) Purity ≥70% expected
After purification HPLC (pooled) ≥98% per target

🔗 Related: SPPS Process | Coupling Reaction | HPLC Analysis | Custom Peptide Synthesis at RPL