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Related Concept Videos

¹H NMR of Conformationally Flexible Molecules: Variable-Temperature NMR01:15

¹H NMR of Conformationally Flexible Molecules: Variable-Temperature NMR

The axial and equatorial protons in cyclohexane can be distinguished by performing a variable-temperature NMR experiment. In this process, except for one proton, the remaining eleven protons are replaced by deuterium. The deuterium substitution avoids the possible peak splitting caused by the spin-spin coupling between the adjacent protons. The remaining proton flips between the axial and equatorial positions.
¹H NMR of Labile Protons: Temporal Resolution01:10

¹H NMR of Labile Protons: Temporal Resolution

Protons bonded to heteroatoms such as nitrogen and oxygen exhibit a range of chemical shift values. This is due to the varying degree of hydrogen bonding between the proton and the heteroatom in other molecules. The extent of hydrogen bonding affects the electron density around the proton, thereby giving different chemical shift values for the protons in the proton NMR spectrum.
The –OH proton in alcohols typically appears in the range of δ 2 to 5 ppm but can vary depending on the specific...
¹H NMR of Labile Protons: Deuterium (²H) Substitution00:48

¹H NMR of Labile Protons: Deuterium (²H) Substitution

This lesson illustrates the role of deuterium substitution in simplifying the NMR spectrum of compounds comprising labile protons. One method employed is the use of deuterium. Amongst the three isotopes of hydrogen, deuterium (2H) has a nucleus composed of one proton and one neutron. When the D2O solvent is added to a pure dry ethanol solution, its labile proton is substituted with deuterium.
UV–Vis Spectrometers01:14

UV–Vis Spectrometers

The absorbance of UV and visible (UV–visible) radiations is measured using a UV–visible spectrophotometer. Deuterium lamps, which emit UV radiation, and tungsten lamps, which produce radiation in the visible region, are used as light sources in UV–visible spectrophotometers. A monochromator or prism is used for diffraction grating, i.e., to split the incoming radiation into different wavelengths. A system of slits is used to focus the desired wavelength on the sample cell. Samples for...
Mass Spectrometry: Complex Analysis01:21

Mass Spectrometry: Complex Analysis

Mass spectrometry is an important technique for the identification of pure compounds. However, it has some limitations for the analysis of complex mixtures, often due to excessive fragmentation making the spectrum too complicated to decipher. Mass spectrometry can be combined with suitable separation methods in sequence, forming hyphenated methods, which are useful in the analysis of complex mixtures.
GC–MS is a powerful hyphenated method commonly used in forensics and environmental...

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Related Experiment Video

Updated: Jun 16, 2026

Measuring Dissolved Methane in Aquatic Ecosystems Using An Optical Spectroscopy Gas Analyzer
05:00

Measuring Dissolved Methane in Aquatic Ecosystems Using An Optical Spectroscopy Gas Analyzer

Published on: July 26, 2024

Methanol detection under variable humidity conditions using an improved wavelength modulation spectroscopy method

Rui Wang1, Liqun Sun1, Yanfei Xiong2

  • 1State Key Laboratory of Precision Measurement Technology and Instruments, Department of Precision Instruments, Tsinghua University, Beijing 100084, China.

Spectrochimica Acta. Part A, Molecular and Biomolecular Spectroscopy
|June 13, 2026
PubMed
Summary

This study introduces an improved wavelength modulation spectroscopy method for accurate methanol detection, even with high humidity. The new technique effectively suppresses water vapor interference, enabling reliable measurements.

Keywords:
First-harmonic (1f) signalLogarithmic transformationMethanolWavelength modulation spectroscopy (WMS)

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Infrared Degenerate Four-wave Mixing with Upconversion Detection for Quantitative Gas Sensing
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Infrared Degenerate Four-wave Mixing with Upconversion Detection for Quantitative Gas Sensing

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Last Updated: Jun 16, 2026

Measuring Dissolved Methane in Aquatic Ecosystems Using An Optical Spectroscopy Gas Analyzer
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Measuring Dissolved Methane in Aquatic Ecosystems Using An Optical Spectroscopy Gas Analyzer

Published on: July 26, 2024

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Infrared Degenerate Four-wave Mixing with Upconversion Detection for Quantitative Gas Sensing
10:42

Infrared Degenerate Four-wave Mixing with Upconversion Detection for Quantitative Gas Sensing

Published on: March 22, 2019

Area of Science:

  • Analytical Chemistry
  • Spectroscopy
  • Environmental Monitoring

Background:

  • Methanol is a hazardous volatile organic compound.
  • Reliable methanol detection is challenging due to ambient conditions, especially water vapor interference.
  • Existing methods struggle with accuracy and robustness in variable humidity.

Purpose of the Study:

  • To develop an improved wavelength modulation spectroscopy (WMS) method for methanol detection under variable humidity.
  • To establish a methanol measurement technique robust against water vapor interference.
  • To enhance the accuracy and reliability of methanol sensing in challenging environments.

Main Methods:

  • Proposed an improved WMS method utilizing first-harmonic (1f) signal analysis.
  • Applied logarithmic transformation to transmitted intensity for a linear 1f signal-concentration relationship.
  • Introduced a point-selection strategy to minimize water vapor interference by targeting specific wavelengths.

Main Results:

  • Achieved excellent linearity (R² = 0.999) in calibration experiments.
  • Determined a minimum detectable methanol concentration of 182 ppb with an 8.2 s integration time.
  • Demonstrated stable methanol concentration retrieval with a maximum relative error of 3.1% across different humidity levels.

Conclusions:

  • The proposed 1f-based WMS method offers a reliable and effective solution for methanol detection in fluctuating humidity.
  • This method significantly outperforms conventional 2f-based approaches in accuracy, detection limit, and humidity robustness.
  • The technique enables direct concentration retrieval of methanol by suppressing water vapor interference.