Hobbies And Interests
Home  >> Science & Nature >> Science

How to Use FTIR in the Analysis of Pharmaceuticals

Fourier transform infrared spectroscopy (FTIR) is a handy tool for the analysis of pharmaceuticals, because you can use it to identify functional groups present in molecules from a sample or even to identify a molecule based on its "fingerprint". If you're taking an IR spectrum for the first time, you want to be careful about how you prepare the samples. The drug compound you want to analyze should be pure before you start. If the compound is a solid (as is true of most drug compounds at room temperature), you need to prepare what is called a cast film.

Things You'll Need

  • Gloves, goggles and lab coat (put these on first)
  • Fume hood
  • Scale
  • Sample you want to test
  • Spatula
  • Small test tube
  • Dichloromethane
  • Stopper
  • Two salt disks (NaCl)
  • Beaker and glass funnel
  • Tissues/Kimwipes
  • Pasteur pipette
Show More

Instructions

    • 1

      Transfer your materials to the fume hood. Dichloromethane is volatile, toxic, flammable and a possible carcinogen; don't attempt to use it outside of the fume hood. Make sure you are wearing double gloves.

    • 2

      Weigh out about ~50 mg of your compound and place it in the test tube. Add about ~0.25 mL of dichloromethane and stir with your spatula until it dissolves.

    • 3

      Stopper the test tube.

    • 4

      Put the salt disks in the glass funnel to hold them upright and put it in the beaker. Rinse the two salt disks with methylene chloride. DO NOT rinse the salt disks with water -- they are made of NaCl and will dissolve. Handle these disks only by the edges, as moisture from your gloves can leave prints on the surface of the disk. Also be aware that the disks are very brittle, and if dropped -- they are likely to break.

    • 5

      Place the disks on a folded tissue or Kimwipe and allow them to dry. It's best to use a Kimwipe or a tissue when removing the salt disks from the funnel, thereby minimizing possible exposure to methylene chloride.

    • 6

      Using a Pasteur pipette, add a drop of your dissolved sample to the surface of one of the disks. Don't touch the surface of the disk with the pipette -- you could scratch it.

    • 7

      Wait until the solvent in the drop evaporates completely. You should see a thin film remaining behind once the solvent has evaporated. If you cannot see a thin film of the compound, repeat the last step by adding another drop and wait for the solvent to evaporate again.

    • 8

      Slot the disk into the triangular sample holder in the FTIR device. Follow the manufacturer's instructions to operate the device and obtain the spectrum.

    • 9

      Look at your FTIR spectrum, once you've printed it out. You should see a ragged line near the top of the graph, which plunges into stalactite-like protrusions, called peaks. These peaks mark the frequencies at which the molecules in the sample strongly absorb IR light. Usually, they are given in terms of the wavenumber, which is just the inverse of the wavelength.

    • 10

      Note the position of the peaks, if you want to identify the functional groups in your molecule. Pay careful attention to peaks over 1500; from 1500 - 1000 is the so-called "fingerprint region" and is useful primarily for identifying compounds, since the pattern of peaks in this region varies widely and can thus be used as a "fingerprint" by matching it up with any spectra for other molecules that you already have on file.

    • 11

      Look for a broad, strong upside-down hill-shaped peak between 3200 and 3550; a peak of this kind marks the presence of an alcohol. A similar broad peak of only moderate intensity denotes a carboxylic acid; peaks for carboxylic acids are often so broad they may run all the way from 3500 down to 2500 or so. A carboxylic acid will also have a strong sharp peak somewhere in the 1850 to 1630 region. If you see this strong sharp peak WITHOUT the broad peak associated with a carboxylic acid, by contrast, your molecule contains a carbonyl group.

    • 12

      Look for a peak in the 2280 to 2100 region. A strong peak here denotes a nitrile -- a carbon triple-bonded to a nitrogen. A weaker or more moderate peak, by contrast, denotes an alkyne (two carbons triple-bonded to each other).

    • 13

      Look for a very strong peak in the 1300 - 1000 region. A strong peak here denotes the presence of a carbon single-bonded to an oxygen (as in the case of an ether or an ester). An ester will show this signal plus the signal for a carbonyl group.

    • 14

      Look for a sharp spike of moderate intensity above 3000 (usually between 3050 and 3150). A peak of this kind denotes the presence of an alkene (i.e. two carbon atoms double-bonded to each other).


https://www.htfbw.com © Hobbies And Interests