Sample thickness in infrared spectroscopy is a delicate balance that can significantly influence the resulting spectrum. A sample that's too thick can lead to excessive absorption of infrared radiation, resulting in peak saturation. This can obscure finer details, making it challenging to discern closely spaced peaks or weaker absorption bands. Conversely, an overly thin sample might produce weak, barely discernible peaks. Achieving the optimal thickness ensures that the spectrum is neither too intense nor too weak, allowing for a clear and detailed representation of the sample's molecular structure. Sample preparation techniques, such as using specific pellet sizes or attenuated total reflectance (ATR) methods, help in achieving this balance, ensuring that the spectrum obtained is both clear and informative.

While infrared spectroscopy is a powerful tool for identifying many functional groups, it does have its limitations. The technique relies on the change in dipole moment as molecules vibrate. Functional groups or bonds that don't induce a significant change in dipole moment during their vibrations might not produce strong absorption peaks. Symmetrical molecules, for instance, might not exhibit significant peaks because their vibrations don't alter the dipole moment. However, for a vast majority of organic compounds, infrared spectroscopy provides a wealth of information, revealing the presence of diverse functional groups through their characteristic peaks.

A reference spectrum serves as a foundational benchmark in infrared spectroscopy. When analysing an unknown sample, its spectrum can display numerous peaks corresponding to various functional groups. By juxtaposing this spectrum with a reference spectrum, chemists can discern which peaks are inherent to the sample and which might be due to impurities or other external factors. Furthermore, in the realm of organic chemistry, where myriad compounds can have similar functional groups, a reference spectrum aids in pinpointing the exact compound by comparing the entire spectrum, especially the unique "fingerprint" region. In essence, it provides a roadmap, guiding chemists towards accurate compound identification.

Water vapour's influence on infrared spectroscopy is multifaceted. Water molecules have distinct vibrational modes that absorb infrared radiation, leading to characteristic peaks in the spectrum. When analysing samples, especially in open-air environments, the presence of water vapour can introduce these peaks, potentially overlapping with the sample's peaks and complicating analysis. This is particularly evident in the O-H stretching region, where water shows strong absorption. To mitigate this, samples are often dried thoroughly, and the spectrometer chamber is purged with dry air or nitrogen. Some advanced spectrometers even have built-in algorithms to subtract the water vapour spectrum from the obtained spectrum, ensuring clarity in the results.

The infrared spectrum's division into "functional group" and "fingerprint" regions is a reflection of the spectrum's inherent information-rich nature. The "functional group" region, typically between 4000 cm^-1 and 1500 cm^-1, is so named because it predominantly displays absorption peaks corresponding to specific functional groups. These peaks arise due to vibrations, such as stretching or bending, of bonds within these groups. For instance, carbonyl groups (C=O) and hydroxyl groups (O-H) have characteristic peaks in this region. By analysing these peaks, chemists can deduce the types of functional groups present in a molecule. On the other hand, the "fingerprint" region, which spans from 1500 cm¹ to 500 cm^-1, contains a complex array of peaks unique to each individual compound. This region is invaluable for compound identification, especially when comparing an unknown compound's spectrum to reference spectra.