The analysis of microplastics using laser direct infrared imaging (LDIR) is increasingly becoming the standard procedure in environmental and food analysis. Suitable filter membranes are crucial for this technique. They form the basis for precise chemical identification, high particle retention rates, and reproducible results.
In the application note „A Validated, Cost-Effective Alternative: Gold-Coated vs. Aluminum-Coated Membranes for LDIR Microplastic Analysis“ we collaborated with LHZ analytics to investigate the performance of aluminum-coated PETG membranes compared to established gold-coated membranes for use in microplastic analysis.
Membrane requirements in microplastic analysis
Membranes place high technical demands on LDIR-based analysis. Among other things, they must:
- be chemically resistant to solvents and reagents
- remain mechanically robust and manageable
- have optimal infrared-reflective properties
- have a suitable pore structure for microplastic sizes
- provide a smooth, even surface for reliable particle detection
Key finding: Comparison of aluminum vs. gold
The tests included chemical stability tests, spectral analysis evaluations, and particle recovery experiments:
Chemical stability
Gold-coated membranes exhibited very high chemical resistance across a wide pH range and against aggressive reagents. Aluminum-coated membranes, on the other hand, showed visual corrosion under strongly acidic or strongly alkaline conditions. However, they were fully suitable for neutral to mildly saline solutions, as frequently found in environmental and drinking water samples.
Spectral power
Measured quantitatively using the Hit Quality Index (HQI), which evaluates the match between a measurement spectrum and the best library hit, there were no significant differences between aluminum and gold coatings.
Particle recovery
Quantitative recovery tests with EasyMP™ reference particles (different types of polymers) also showed comparable recovery rates for both coating variants, with recoveries of up to 94% for larger microplastic particles.
Conclusion and practical relevance
Aluminum-coated PETG membranes are a robust, analytically valid, and cost-effective alternative to gold-coated filters when it comes to routine analyses in neutral or mild aqueous samples, as well as drinking water. However, for applications involving aggressive chemicals or extreme pH conditions, gold coating remains technically superior.
Download the complete application note free of charge.