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A topic from the subject of Quantification in Chemistry.

  • 1 year ago
  • 9 min read
Quantification through Infrared Spectroscopy
Introduction:

Infrared spectroscopy (IR) is a powerful tool for the identification and characterization of molecules. It can also be used to quantitatively determine the concentration of a particular compound within a sample. This quantitative application is known as "quantitative IR spectroscopy."

Basic Concepts:
  • Infrared absorption: When a photon of infrared (IR) radiation interacts with a sample, it can excite the molecules by increasing their vibrational energy. The energy of the absorbed photon corresponds precisely to the energy difference between two vibrational energy levels within a specific vibrational mode of the molecule.
  • Beer-Lambert's Law: This law describes the relationship between the absorbance of a sample and the concentration of the analyte. It states that absorbance is directly proportional to both the concentration of the analyte and the path length of the light through the sample. A = εbc where:
    • A is the absorbance (unitless)
    • ε is the molar absorptivity (L mol-1 cm-1)
    • b is the path length (cm)
    • c is the concentration (mol L-1)
Equipment and Techniques:
  • FT-IR (Fourier Transform Infrared Spectroscopy): This is the most common type of IR spectrometer. It employs a Fourier transform to convert the time-domain interferogram signal into a frequency-domain spectrum.
  • FTIR Spectrophotometer: This instrument measures the intensity of IR radiation absorbed by a sample as a function of frequency or wavenumber.
  • Sample Preparation: Sample preparation methods vary depending on the sample's physical state:
    • Solids: Often prepared as a potassium bromide (KBr) pellet – the sample is finely ground and mixed with KBr, then pressed into a transparent disc.
    • Liquids: Typically analyzed using a liquid cell with a known path length.
    • Gases: Analyzed using a gas cell with a known path length.
Types of Experiments:
  • Qualitative Analysis: Used to identify the functional groups and components present in a sample by examining the characteristic absorption bands in the spectrum.
  • Semi-quantitative Analysis: Provides an estimate of the concentration of a compound by comparing the intensity of its absorption bands to a known standard.
  • Quantitative Analysis: Determines the exact concentration of a specific compound using Beer-Lambert's Law and a calibration curve (a plot of absorbance versus concentration).
Data Analysis:
  • Qualitative Analysis: Involves identifying the functional groups present based on the positions of absorption bands in the IR spectrum. Spectral libraries are commonly used for comparison.
  • Semi-quantitative Analysis: Involves comparing the peak intensities of the analyte to a standard, often using peak area or height measurements.
  • Quantitative Analysis: Requires constructing a calibration curve using samples of known concentrations and measuring the absorbance of an unknown sample at the same wavelength (or wavenumber). The unknown concentration is then determined by interpolation from the calibration curve.
Applications:
  • Organic and Inorganic Chemical Industries
  • Pharmaceutical Industry
  • Biochemical Analysis
  • Food Analysis
  • Polymer Analysis
  • Environmental Monitoring
Conclusion:

Quantitative infrared spectroscopy is a valuable technique for determining the concentration of compounds in various samples. Its widespread applicability across diverse fields stems from its ability to provide both qualitative and quantitative information about molecular composition.

Quantification through Infrared Spectroscopy
Introduction

Infrared (IR) spectroscopy is a powerful analytical technique used to quantify various compounds in a sample. It relies on the absorption of infrared radiation by molecules, causing vibrational transitions within their functional groups. The intensity of absorption is directly related to the concentration of the analyte.

Principle

When infrared radiation interacts with a molecule, it can be absorbed if the frequency of the radiation matches the natural vibrational frequency of a specific bond or functional group within the molecule. This absorption causes a change in the molecule's vibrational energy state. The amount of infrared radiation absorbed at a specific frequency is directly proportional to the concentration of the molecule containing that functional group, following Beer-Lambert's Law: A = εbc, where A is absorbance, ε is the molar absorptivity, b is the path length, and c is the concentration.

Calibration

To quantify a compound using IR spectroscopy, a calibration curve is typically established. This curve is created by measuring the absorbance (or % transmittance) of solutions with known concentrations of the analyte. The absorbance at a characteristic peak (or a specific range of peaks) is plotted against the concentration. This produces a standard curve used for quantitative analysis.

Quantitative Analysis

Once a calibration curve is established, the absorbance of an unknown sample is measured at the same characteristic peak(s). The concentration of the analyte in the unknown sample is then determined by comparing its absorbance to the calibration curve using interpolation or regression analysis. The accuracy depends on the linearity of the Beer-Lambert Law within the concentration range used.

Advantages
  • Non-destructive: Samples can be recovered after analysis.
  • Versatile: Applicable to a wide range of compounds, both organic and inorganic.
  • Provides structural information: Peak positions and intensities offer insights into the functional groups present.
  • Relatively simple and inexpensive compared to other techniques like NMR or Mass Spectrometry.
  • Fast analysis time: Measurements are usually quick.
Limitations
  • Sensitivity limitations: May not be suitable for very low concentrations.
  • Interfering peaks: Overlapping peaks from different components in a mixture can complicate analysis.
  • Sample preparation: Requires careful sample preparation to obtain accurate results, often requiring solutions of the analyte.
  • Calibration curve dependence: The accuracy of quantification is highly dependent on the quality of the calibration curve.
Applications

Infrared spectroscopy finds widespread use in various quantitative applications in chemistry, including:

  • Determining the concentration of organic compounds in mixtures (e.g., in environmental monitoring, pharmaceutical analysis).
  • Analyzing impurities in products (e.g., quality control in manufacturing).
  • Monitoring chemical reactions (e.g., kinetics studies by observing changes in peak intensities).
  • Identifying unknown compounds (qualitative analysis, though less precise than other techniques for ID).
  • Polymer analysis (studying composition and degradation).
Experiment: Quantification through Infrared Spectroscopy
Objective

To determine the concentration of an unknown solution using infrared (IR) spectroscopy and the Beer-Lambert Law.

Materials
  • Infrared (IR) spectrometer
  • Unknown solution of analyte (specify analyte, e.g., acetone in carbon tetrachloride)
  • Reference solutions of known concentrations of the analyte (at least 5 solutions spanning a suitable concentration range)
  • IR-transparent cuvettes (e.g., NaCl or KBr)
  • Pipettes and volumetric flasks for accurate solution preparation
  • Solvent (if applicable, specify solvent, e.g., carbon tetrachloride)
Procedure
  1. Prepare a series of reference solutions of known concentrations of the analyte in the chosen solvent. Ensure accurate preparation using appropriate volumetric glassware. Record the exact concentrations.
  2. Obtain a background spectrum using an empty cuvette filled with the pure solvent. This corrects for solvent absorption.
  3. Fill a clean, dry cuvette with each reference solution and the unknown solution. Thoroughly wipe the outside of the cuvette to prevent scattering.
  4. Place each cuvette in the IR spectrometer and record its spectrum. Ensure consistent path length for all measurements.
  5. Identify a suitable absorption peak for the analyte that is not overlapped significantly by solvent or other components. Record the absorbance at this peak for each solution.
  6. Plot a calibration curve of absorbance (y-axis) versus concentration (x-axis) for the reference solutions. A linear relationship should be observed if the Beer-Lambert Law is applicable. Determine the equation of the best-fit line (e.g., using linear regression).
  7. Using the equation of the calibration curve, determine the concentration of the analyte in the unknown solution based on its measured absorbance at the chosen peak.
Key Considerations
  • Prepare the reference solutions carefully to ensure accuracy and precision in concentration.
  • Use clean and dry cuvettes to avoid contamination and ensure reproducible results.
  • Record the spectra at a consistent temperature to minimize variations in absorbance.
  • Choose an absorption peak that shows a linear response to concentration within the Beer-Lambert Law's limitations.
  • Ensure the path length of the cuvette remains constant throughout the experiment.
  • Account for potential interferences or overlapping peaks that could affect the accuracy of the measurements.
Data Analysis

The concentration of the unknown solution will be determined from the calibration curve. Report the concentration along with any uncertainties or limitations of the analysis. Include the calibration curve plot, along with the equation of the best fit line and the R-squared value as evidence of the linearity and quality of the data.

Significance

Infrared spectroscopy is a valuable quantitative technique used extensively in various fields, including chemistry, environmental science, and materials science, for determining the concentration of substances in solutions or mixtures.

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