Localized surface plasmon resonance (LSPR) has attracted interest because of its intriguing physical phenomena and is being explored for a wide range of applications including sensing, photocatalyst, display, and more. The localized field enhancement enables spectroscopy techniques like surface-enhanced Raman scattering (SERS) and surface-enhanced infrared absorption (SEIRA) to detect and identify compounds, even in small amounts. For our investigation of the few-molecules region, we employed a novel statistical model to analyze the bianalyte spectra. Comprehensive electromagnetic simulations were carried out to locate areas of high electric field enhancement and gather relevant statistical information, facilitating the quantitative estimation of molecules. Subsequently, the SEIRA technique is introduced as a complementary method alongside SERS. When the substrate operates in the infrared region, strong electric field enhancement is generated by the lightning-rod effect. A new analytical model is proposed to provide further explanation of the field enhancement on the SEIRA substrate.