5.2: Identification of Drug samples by infrared spectroscopy

C2: Identification of Drug samples by infrared spectroscopy

Safety Notes: The compounds used in these experiments as "unknown" samples are drug compounds and care must be taken to avoid ingestion and skin contact.

Introduction

To obtain an infrared spectrum, it is necessary to prepare a solid sample appropriately. In this case you will prepare potassium bromide discs of the unknown drug samples. The KBr disc has the advantage (over Nujol mulls) that the spectrum should contain no interfering peaks (apart from water from poorly dried sample or KBr). Consult a demonstrator for guidance on the preparation of a potassium bromide disc if you are unfamiliar with the procedure outlined below.

Procedure

1. Ensure that the instrument is switched on and that it has warmed up for at least a few minutes.
2. Make sure that the sample compartment is empty and close the lid.
3. Select the number of scans (usually ten is adequate for routine work).
4. Select ‘background’ on the menu and scan.

Preparation of KBr Disk

1. Take the spectroscopic grade KBr from the oven and cool in the desiccator.
2. Using an agate mortar and pestle grind a few mg of the drug sample to a fine powder then mix with about a 10-fold excess of dry KBr and grind the mixture to give a well mixed powder. Keep the mixture as dry as possible during this process.
3. Check that the ‘IR Die Kit’ is complete (plunger, top and bottom die, a die holder and an anvil.
4. Press the die holder onto the anvil ensuring a proper fit.
5. Lower the base die, dull side down into the die holder to a depth of about 50-mm.
6. Place sufficient of the ground sample/KBr mixture to completely cover the polished surface of the die and tap gently to produce an even layer on the base die.
7. Lower the top die (polished side down) onto the powder.
8. Slide the plunger onto the die and press gently so the die slides to the bottom.
9. Place the assembled ‘IR Die Kit’ into the hydraulic press and tighten the top screw until it touches the top of the plunger.
10. Connect the anvil to the vacuum pump.
11. Close the hydraulic release valve on the side of the press and gently pump the handle until the pressure gauge reads between 8 and 10 tons and leave for 30 seconds.
12. Open the hydraulic release valve gently and disconnect the vacuum from the anvil when the pressure falls to zero.
13. Loosen the top screw and remove from the press.
14. Turn the ‘IR Die Kit’ upside down and carefully remove the anvil.
15. CAREFULLY remove the disc from the press and transfer to a sample holder.
16. Clean the ‘IR Die Kit’ components with a tissue and check that all parts are present. Take care not to scratch the polished surfaces.
17. If the dies or plunger stick in the die holder contact the instructor.

Running the Spectrum

1. Place sample into the beam of the FT-IR spectrometer and scan the spectrum from 4000 cm-1 to 600 cm-1 .
2. Annotate the spectrum with name, date, compound, and phase (in this case KBr Disk).
3. Having obtained the infrared spectra, use the IR Viewmaster computer program to identify the unknown drugs. Consult the detailed instructions on the use of this program next to the computer.

How to Interpret an IR Spectrum

1. Note the conditions under which the spectrum was obtained.
2. Remember that the absence of peaks may be as useful as the presence of peaks.
3. Do not attempt to identify all the peaks. Go for the large peaks first.
4. Many sharp peaks of medium or strong intensity throughout the spectrum generally indicate an aromatic compound.
5. Examine (\(4000-2000 cm^{-1}\))

Table 3: Region 1 (\(4000-2000 cm^{-1}\))

Wavenumber (\( cm^{-1}\))	Bond	Notes
3600-3300	O-H	Alcohols, phenols
3500-3300	N-H	Amines
3100-3000	C-H	Aromatic rings
2980-2800	C-N	Unsaturated C-H (e.g. alkanes) for CH3, CH2, and CH
2280-2200	C=N	Nitriles

6. Examine (\(2000-1500 cm^{-1}\))

Table 4: Region 2 (\(2000-1500 cm^{-1}\))

Wavenumber (\( cm^{-1}\))	Bond	Notes
1760-1690	C=O	The most intense band in this region. Indicative of aldehydes, ketones, esters etc.
1680-1610	C=C	Alkenes (less intense and sharper than C=O)
1360-1180	C-N	Amines, amides
1750-1650	N=O	Intense and sharp band with second band in region 3

7. Examine (\(1500-650 cm^{-1}\))

Table 5: Region 2: the fingerprint region (\(1500-650 cm^{-1}\))

Wavenumber (\( cm^{-1}\))	Bond	Notes
1460	CH3	Medium intensity peaks
1370	CH2	Medium intensity peaks

8. Tabulate your results and make the appropriate deductions after consulting a detailed correlation table.

Thinking about Making Valid Measurements

Infrared spectroscopy is a technique often used to identify organic substances. Why is it a more useful technique than ultraviolet spectroscopy?

• Large peaks below the bottom of the chart or with ‘squared tips’ indicate that the sample is too thick or concentrated so a new KBr disk needs to be produced.
• If the spectrum is weak and very noisy there is not enough sample. The KBr disk needs to be remade.
• If the baseline cannot be adjusted to 90% transmittance then the NaCl or KBr disk is fogged or scratched and needs to be replaced or remade. Improving Your Practical Technique
• The instrument should be allowed to warm up before use.
• A wavelength check should be carried out to check the instrument is correctly set up using a polystyrene standard film.

Improving Your Practical Technique

• The instrument should be allowed to warm up before use.
• A wavelength check should be carried out to check the instrument is correctly set up using a polystyrene standard film.