Fourier-transform infrared spectroscopy (FTIR)

Fourier-transform infrared spectroscopy (FTIR) is a technique used to obtain an infrared spectrum of absorption or emission of a solid, liquid or gas. ’Fourier-transform’ in FTIR stands for a mathematical process used to convert the raw data into the actual spectrum. FTIR spectrometer is the instrument that is used to perform FTIR spectroscopy. The FTIR spectrometer at hand is BRUKER TENSOR II. The operational information provided below is intended for the same. The instrument under possession has 2 modes, transmission mode and ATR mode. The sample preparation and spectral acquisition for each are discussed separately.

POWERING ON THE INSTRUMENT

  1. POWERING ON THE INSTRUMENTPower on the instrument followed by the computer system.
  2. On the instrument, check the humidity indicator. The indicator is a red light, which blinks or stays on in the presence of excess moisture. If this indeed is the case, switch off the instrument and refresh the desiccant. Ensure that the indicator is off before proceeding.
  3. Open OPUS software, password is ’OPUS’. In the screen that opens, there is a solid circle at the bottom right corner of the window. if it is green, Instrument is ready for measurements. If not, click on it. This will open a window. Now click on ’ATR’ and then run PQ and OQ tests. Quite often, this turns the solid circle to green,.

TRANSMISSION MODE

  1. Light of wavelengths from 400 cm−1 to 4000 cm−1 is passed through the sample. Depending on property of the sample, different proportion of incident radiation for each wavenumber is absorbed by the sample. The resulting plot of absorbance vs wavenumber is called as absorbance spectra. Similarly, the plot of transmission vs wavenumber is called as transmission spectra.
  2. In order to take measurements, pellets consisting of mixture of KBr and sample powder is made. The sample preparation procedure is as follows.  (a) A 0.33 wt% mixture of sample powder in KBr is made. To ensure that the mixture is homogenized, grind it using mortar-pestle. (b) 200 mg of this mixture is taken in a mold and pelletized using hydraulic press. The obtained pellet should be transparent, fracture free disc of uniform thickness. If a KBr pellet turns out to be milky white, it means it has absorbed excess moisture from the atmosphere.
  3. Procedure for pellet preparation for background measurement is as follows.
    (a) Before measuring the spectra of the sample, background spectra needs to be measured. Since the matrix here is KBr, spectra of pure KBr pellet is considered as background spectra.
    (b) To prepare the pure KBr pellet, 200 mg of pure KBr powder is taken in a mould and pelletized using hydraulic press.
  4. The prepared pellet is carefully placed in the pellet holder. The pellet holder’s ring is gently tightened and placed in the instrument.
  5. Performing measurements
    (a) For taking background measurement, place pure KBr pellet in the instrument.
    (b) In the OPUS software, on the top bar, click Measure –> Advanced Measurement. This will open a window named “Measurement”, containing parameters. In Basic –> Experiment , load ’MIR_TR.xpm’ file. The xpm files contain the parameters applied during the measurement. The parameters can be edited and saved as a new xpm file.
    (c) In Measurement window click, Basic –> Background Single Channel. This initiates the measurement. Progress bar in the lower region of the window indicates the progress of the measurement.
    (d) After the background measurement has been measured. Remove the Pure KBr pellet, clean the pellet holder and place the sample pellet in its place.
    (e) Inorder to perform sample measurement, open the ’Measurement’ window, click, ’Basic’ -> ’Sample Single Channel’. This starts the sample measurement. Progress bar indicates the progress of the measurement.

ATR MODE

1. ATR can be a delicate instrument. Please read the ATR manual before operating it. If the Germanium crystal option is to be used, read the manual once more with more care.
2. In the ATR mode, sample is placed above the crystal in contact with it. The available crystal options are Diamond and Germanium. The infrared rays pass through the crystal and onto the interface between the crystal and the sample. There it undergoes total internal reflection. However
while doing so, the IR beam, travels few micrometers into the sample, allowing the absorption of different wavenumbers of IR. The receiver records the transmitted spectra thus giving the absorbance and transmittance spectra.
3. Sample preparation of rock samples involves grinding them into fine powder.
4. Sample measurement
(a) Place the powder on the crystal in such a way that the entirety of the crystal is covered by it. Ensure that the thickness of the sample layer on the crystal is atleast 1 mm thick.
(b) Press down on the sample using ATR’s anvil. Doing this puts pressure on the sample powder, increasing the sample’s contact area with the crystal, resulting in better IR absorption and hence better spectra. However, applying too much pressure could damage the crystal. To prevent damage, ensure the red dot of the pressure indicator of the anvil is centered.
(c) In OPUS software
i. Click ’Measure’–>’Advance Measurement’. This will open a window named ’Measurement’.
ii. In ’Measurement’ –> ’Basic’ –> ’Experiment’, load ’MIR_ATR.xpm’.
iii. In ’Measurement’ –> ’Basic’ click ’Background Single Channel’. Check progress bar for measurement status.
iv. Inorder to perform sample measurement, open the ’Measurement’ window, click, ’Basic’ -> ’Sample Single Channel’. Now, a preview of the sample spectra is displayed. Note that the preview spectra wont be recorded as part of the measurement. Once satisfied with the preview spectra, click ’Start Measurement’. Progress bar indicates the progress of the measurement.
(d) As soon as the measurement is done, anvil’s pressure is to be released and the crystal cleaned. This minimizes the risk of crystal damage.
5. Background measurement
(a) For samples of powdered rock, Spectra of air is taken as background measurement. To measure the spectra, simply clean the ATR crystal and run the measurements.
(b) Background measurement in performed before sample measurements

WORKING WITH THE SPECTRA

After measuring the spectra, the data is stored as a 0 file. OPUS provides several abilities to work with the measured spectra. This includes, peak picking, spectrum search, quick compare, calculator, baseline correction. Furthermore, the spectral data can be copied into an excel file as shown in the following example.

  1. A spectrum file can consist of several data blocks. Here kaolinite.0″1 is the called spectral file and AB, SSC, RSC, HISTORY are considered as data blocks.
  2. On the spectrum file, ’Right click’ -> ’Properties’ -> ’AB’. Now, a subwindow of table containing spectral data is displayed. This data can be copied into an excel file. Note that ’AB’ here refers to absorbance. Another common data block is Transmittance which, is indicated by ’TR’.
  3. Apart from spectral data, ’properties’ allows access to several information including data parameters and instrument parameters.

FURTHER READING

This very basic introduction intends to be a starting point to get started with FTIR. It is strongly recommended to go through following documents before actually working with the instrument.
1. Platinum ATR accessory – user instructions
2. Base package manual for OPUS software