Infrared Spectroscopy

Infrared Spectroscopy Introduction

IR spectroscopy measures the amount of infrared light absorbed over a range of wavelengths.  The typical range is 2.5 µm to 15 µm, or 4,000 to 670 cm-¹.  Infrared (IR) Spectroscopy is a valuable tool in characterizing functional groups present in organic and inorganic compounds.   Galbraith performs IR spectroscopy measurements using a Fourier-Transform Infrared (FT-IR) instrument.

Most IR testing falls into one of three categories:

  1. Comparison to a USP reference standard for the purpose of identity verification
  2. Comparison to a library of IR spectra for the purpose of identifying of an unknown compound
  3. Scan of a compound to acquire the IR spectrum, especially for the purpose of showing that a particular peak (functional group) is present in the scan


The following methods, given in USP <197>, are available at Galbraith Laboratories:

  • USP <197>A: Analysis using an Attenuated Total Reflectance (ATR) cell
  • USP <197>K: Analysis is conducted on a mixture of the compound pressed into a pellet of potassium bromide
  • USP <197>M: Analysis is conducted on a mixture of the compound dissolved in mineral oil
  • USP <197>F: Analysis is conducted on the compound that is suspended between sodium chloride or potassium bromide plates

Click here to log into the most current version of USP <197> (login required).


Samples are prepared according to the section of USP <197> chosen (see above).  For USP materials, the reference substance and sample will be dried prior to analysis if required.


IR scans are acquired over the range of about 2.5 to 15 µm.  If a comparison to a reference substance is required, an overlay of the unknown and the reference substance is made by the technician.


The reported information will be based on the nature of the order:

  1. If a pass/fail identification of the compound is required, the final report will only indicate the pass/fail outcome.
  2. If a comparison to library spectra is made, a copy of the unknown scan is provided along with a series of the best library matches.
  3. If a spectrum on a compound is ordered, a copy of the scan will be provided with the report.  The scan will contain labels of the major peaks.

Regulated Submissions

The general methodology is suitable for the analysis of regulated samples.  The method is considered validated to a reference substance, but not to the sample matrix unless a formal validation is conducted.  Click here to see more information on Method Validation.