Heterogeneity Studies of a Single Particle/Fiber by Using Fourier Transform Infrared Micro-Sampling Photoacoustic Spectroscopy

Abstract

This paper reports the first investigation of the micro-sampling possibility of Fourier transform infrared photoacoustic spectroscopy (FT-IR PAS) in depth profiling of a single particle and a single fiber. The samples studied in this paper include a Bio-Bead™ (150 μm in diameter) coated with commercial strawberry flavor oil, a nylon wire with grease coating, and a piece of natural human hair (60 μn in diameter and 7 mm in length) with gel-coating. Major FT-IR PAS operation modes utilized in depth profiling of heterogeneous/layered macro-samples reported previously, such as (continuous-) rapid-scan, phase-, and nonphase modulation step scans, were used in this study. The software-based digital signal processing (DSP) algorithms for step-scan photoacoustic (fundamental and harmonic) signal modulation/demodulation were employed in the phase modulation experiments. Both modulation frequency variation and photoacoustic signal phase analysis methods were applied to elucidate detailed spectral depth profiling information from the raw data of these micro-samples. The results conclude that FT-IR micro-PAS offers a unique and complementary method in analysis of heterogeneous micro-samples with nondestructive, noncontact, and cost-effective advantages compared with conventional FT-IR micro-transmission/reflection/ATR approaches.