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The liquid-phase detection methods for the degradation products of polylactic acid (PLA) need to be selected based on the degradation mechanism (hydrolysis, enzymatic degradation, or thermal degradation) and the target products (monomers, oligomers, or other small molecules):
1) Reverse-phase high-performance liquid chromatography (RP-HPLC)
Chromatographic column:
C18 column (such as Agilent ZORBAX SB-C18, 250 × 4.6 mm, 5 μm) is suitable for most polar degradation products.
For oligomers analysis, a wide pore diameter column (such as 300 Å) can be chosen.
2) Hydrophilic interaction liquid chromatography (HILIC)
Applicable scenarios: When the retention of high-polarity small molecules (such as lactic acid, glycolic acid) on the reversed-phase column is too weak.
Chromatographic column: Amino column or silica gel column (such as Waters XBridge HILIC).
Mobile phase: Acetonitrile/water (containing 10 mM formic acid ammonium, pH 3.0), isocratic or gradient elution.
3) High-performance liquid chromatography-mass spectrometry (HPLC-MS)
Advantages: Provide molecular weight and structural information, especially suitable for oligomer identification.
Ionization mode:
Electrospray ionization (ESI): Negative ion mode for detecting lactic acid ([M-H]⁻, m/z 89) and oligomers.
Atmospheric pressure chemical ionization (APCI): Suitable for weakly polar degradation products.
4) Ion chromatography (IC)
Applicable objects: Acidic products generated by hydrolysis (lactic acid, acetic acid, etc.).
Chromatographic column: Anion exchange column (such as Dionex IonPac AS11-HC).
Mobile phase: KOH gradient elution (0–20 mM, 15 min).
5) Gel permeation chromatography (GPC)
Objective: Determine the molecular weight distribution of degradation products (especially oligomers).
Chromatographic column: PLgel Mixed-D column (THF as mobile phase) or water-based GPC column (such as TSKgel SuperMultiporeHZ-N).
