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Fixed liquid cell – Neat liquid samples or organic solutions. Cell windows are made of CaF2 (transmission range 70,000 – 1,110 cm-1) so NO acidic solutions are allowed. Acidic solutions and ammonium salts will destroy the CaF2 windows, but other solvents are applicable. For this application, more volatile solvents (some common examples given below) are better suited as these would typically evaporate, negatively impacting spectral collection, during cuvette, non-fixed/demountable liquid cell, or ATR sample acquisition. Transmission ranges for some solvents are provided in the table below.
- Chloroform
- Dichoromethane
- Hexanes
- Benzene
Concentrations can range as low as ppb (ng/mL) or 0.5%, depending on absorptive band strengths, but solutions in the ppm (mg/mL) range are recommended. If running a concentration series, low to high concentration is recommended. Usually, 5 – 50 µL is necessary to fill the cell (with some preliminary cell flushing). The cell should be filled horizontally, with the two retaining plugs pointed up. Syringes are employed to flush and fill the cell. To fill the cell with sample solution, one syringe pushes solution through the cell, while another syringe can be used to pull the out the passing solution. The cell should be filled and emptied of sample solution 1-2 times before filling the cell for analysis. The pulling syringe is then removed and that cell side plugged to retain the liquid sample between the two salt windows. Sample solution should then be pushed in until the area between the windows is filled with liquid and shows no bubbles. Once the cell is appropriately filled, the filling syringe is removed and the port plugged. The cell is then inserted vertically into the cell holder adaptor in the FT-IR. After analyzing the sample, the cell should be flushed with 2-3 volumes of blank solvent prior to refilling with another solution or emptying for the next user.
A background spectrum using a blank solvent (same as sample) should be acquired prior to the solution to better negate any solvent IR bands. After use, sample should be flushed with 2-3 volumes of blank solvent and the cell left open (briefly) to air to dry. Cells should be stored in an appropriate desiccated environment while not in use so the humidity does not detrimentally degrade the cell’s salt windows.
Solvents and Solvent Effects in Organic Chemistry, 4th Ed. Edited by Christian Reichardt and Thomas Welton. pg 561
Attenuated Total Reflection (ATR) – Solids (powder, crystal, films, etc.), liquids (neat), and solutions (ca. 1 ppm, pH range = 5-9) can be applied to the window. The ATR crystal is made of zinc selenide (accessible spectral range = 20,000 – 500 cm-1), so a wide variety of solvents can be applied. Particularly, aqueous and alcohol solvents are better suited for ATR analysis as the thin sample layer and methodology minimizes solvent bands that would normally dominate the IR spectrum in traditional absorbance/transmittance measurements.
A few mg of solid material, thin film, or 1-2 drops of liquid sample is sufficient for ATR spectral acquisition. The applicator should be fully depressed to minimize space between the sample and ATR crystal, being careful not to press too hard, as too much pressure could damage the crystal or distort the IR spectrum (reduce reproducibility).
For solids and neat liquids, a clean window with depressed applicator is recommended for the background spectrum. For solutions, a blank solvent with depressed applicator is recommended for the background spectrum. After use, the ATR crystal should be wiped clean with an isopropanol soaked Kimwipe and the applicator depressed (to protect the crystal).