Why do spectra sometimes show negative peaks?

Spectra sometimes show negative peaks due to the process of phase inversion in Fourier Transform Infrared (FTIR) spectroscopy.

In Fourier Transform Infrared (FTIR) spectroscopy, a technique used to obtain an infrared spectrum of absorption, emission, or photoconductivity of a solid, liquid or gas, negative peaks can appear. This is due to a phenomenon known as phase inversion. Phase inversion is a mathematical process that occurs during the Fourier transformation of the raw data.

The Fourier transformation is a mathematical method that converts a time-domain signal, which is a representation of a physical quantity as a function of time, into a frequency-domain signal, a representation of the same physical quantity as a function of frequency. This transformation is used in FTIR spectroscopy to convert the raw data, which is in the time domain, into a spectrum, which is in the frequency domain.

During the Fourier transformation, the raw data is multiplied by a phase factor, which is a complex number. This multiplication can cause the phase of the signal to invert, which means that the signal is flipped over the horizontal axis. When this happens, positive peaks become negative and vice versa.

This phase inversion is not a physical phenomenon, but a mathematical one. It does not indicate that the substance being analysed is absorbing negative amounts of light. Instead, it is a result of the mathematical processes used to convert the raw data into a spectrum.

In practice, phase inversion can be corrected by applying a phase correction to the raw data before the Fourier transformation is performed. This phase correction is a mathematical operation that adjusts the phase of the signal so that it does not invert during the Fourier transformation. After the phase correction is applied, the resulting spectrum will only have positive peaks.

In conclusion, negative peaks in a spectrum are not a cause for concern. They are simply a result of the mathematical processes used in FTIR spectroscopy and can be easily corrected.