Qualitative and quantitative analysis of nanogram amounts of substances by transmission infrared spectroscopy with the use of silver island films was carried out. The effect of silver thickness, the comparison of several plates (Ge, BaF2, and ZnSe), the dynamic range of intensity enhancement vs. sample amount, and the enhancement stability after sample deposition were investigated extensively to determine the optimum conditions. The limits of detection (LOD) of several substances were also described. LOD values ranging from 5 ng/cm2 for polydimethylsiloxane to 10 ng/cm2 for diethylphthalate were obtained, and these values show a remarkable improvement in LOD in comparison with results from normal transmission spectroscopy. A quantitative study was also undertaken. A good linear relationship with respect to the amount of triphenyl phosphate, ranging from 25 to 250 ng/cm2, was obtained. The data were reproducible with a coefficient of variation of 15%, and this variation was comparable to that obtained with several surface-enhanced Raman scattering methods.

The intensity enhancement of infrared spectra with Ag island films in surface electromagnetic wave spectroscopy (SEWS), which is commonly associated with the ATR method, was observed with the use of an ordinary transmission configuration. We refer to this as “transmission SEWS” or “T-SEWS.” With 10 nm Ag, the magnitude of enhancement factor was about 200 for the NO2 symmetric stretching band at 1350 cm−1 with p-nitrobenzoic acid. In order to estimate its effectiveness as a qualitative analytical method, T-SEWS spectra were compared with normal transmission spectra with the use of several samples. The results showed that T-SEWS is useful in detecting and identifying nanogram amounts of substances which do not have reactive functional groups—e.g., triphenyl phosphate (TPP) or polydimethylsiloxane (PDMS), which do not have -COOH, -SO3H, -C=S, etc. A usable spectrum was observed from as little as 25 ng/cm2 of TPP with the use of a standard triglycine sulfate detector without a beam condenser. Combined with microspectroscopy, T-SEWS shows promise for picogram order analysis and chromatographic applications.

Improvement in the detection limit of the surface layer on polymer films has been demonstrated with the use of Ag films under Kretschmann's attenuated total reflection (ATR) geometry. A remarkable absorption enhancement for 2.5-nm-thick polydimethylsiloxane (PDMS) or for 4.0-nm-thick polycarbonate (PC) coated on polyethyleneterephthalate (PET) films was observed. The magnitude of the enhancement factor is 10 for PDMS and 2 for PC compared with results for the conventional ATR method. In the case of PDMS, the limiting thickness of the measurable surface layer is about 0.5 nm with the use of the spectral subtraction technique. The correlation between the difference in the enhancement factor and the enhancement mechanism is also discussed.

Identification of subnanometre surface layers on polymer films, which are only analysable by x-ray photoelectron spectroscopy (XPS) or secondary ion mass spectrometry (SIMS), have been demonstrated by the use of transmission surfaceenhanced infrared absorption (SEIRA) spectroscopy with the use of silver island lilms: 1.0 nm thick polydimethylsiloxane (PDMS) or 0.5 nm thick polymethylphenylsiloxane (PMPHS) surface layers, which are semisolid materials, on polyethyleneterephthalate (PET) films were successfully identifiable with the use of transmission SEIRA by measuring the PDMS or PMPHS that was transferred onto the silver-deposited BaF, substrates under pressure. This approach completely eliminates strong spectral interference from PET films by measuring the transferred surface layer and greatly improves the signal-to-noise ratio of the surface layer absorp tion with the use of silver island films. The detection limit of PDMS surface layers on PET films is -0.2 run. The present method gives a considerable amount of information about surfaces, such as chemical composition and chemical structure. This is a highly promising method for analysing subnanometre surface layers which consist of liquid or semisolid materials, such as oil, lubricant, plasticizer and surface-active agent on polymer materials.