Infrared oil analyzer is a precision environmental monitoring instrument based on the principle of infrared spectral absorption, specifically used for quantitative analysis of mineral oil (total petroleum hydrocarbons) content in water. Its core technology lies in the precise measurement of the vibrational energy of specific chemical bonds.

1、 Core working principle: Infrared absorption and Lambert Beer law

The basic working principle of the instrument follows two core physical and chemical laws:

Molecular vibration absorption principle: Mineral oil is mainly composed of hydrocarbons (CH3-, - CH ₂ -, aromatic rings, etc.). When the sample is irradiated with infrared light (mid infrared light of a specific wavelength), the C-H bonds of methyl (- CH3) and methylene (- CH ₂ -) in the oil molecules selectively absorb infrared light energy that is consistent with their vibration frequency, and transition from the ground state to the excited state.

Lambert Beer's law: This law states that the absorbance (A) of infrared light at a specific wavelength is proportional to the concentration (c) of the absorbing substance in the sample and the optical path length (L), i.e. A=ε cL (where ε is the absorption coefficient). This is the cornerstone of quantitative analysis. By measuring the absorbance, the concentration of oil can be calculated back.

2、 Key spectral analysis technique: Three wavelength measurement method

To accurately measure complex oil mixtures, modern infrared oil meters commonly use three wavelength measurement technology to eliminate interference from other substances and achieve standardized measurements.

Measurement wavelength (2930cm ⁻¹, approximately 3.41 μ m): This is the characteristic absorption peak of the stretching vibration of the C-H bond in methylene (- CH ₂ -), which is the main wavelength for measuring mineral oil content. The absorbance of the sample here directly reflects the main information of oil concentration.

Reference wavelength (3030cm ⁻¹, approximately 3.30 μ m): This is the characteristic absorption peak of the stretching vibration of the C-H bond on the aromatic ring. Measuring the absorbance at this wavelength helps to evaluate the composition of the oil and calibrate the main wavelength measurement, especially when the oil contains a high amount of aromatic hydrocarbons.

Compensation wavelength (approximately 2600cm ⁻¹): This region usually does not have obvious oil characteristic absorption peaks. The purpose of setting this wavelength is to deduct background interference, such as slight changes in solvents (carbon tetrachloride or tetrachloroethylene), scattering from the sample cell wall, or the influence of suspended particles.

Finally, the absorbance of the sample is calculated using the formula: A=A ₂₉③₀ - [A ∝₀③₀× (α/β)] (where α and β are instrument calibration coefficients). This method effectively eliminates the interference of non hydrocarbon substances, ensuring the accuracy of measurement results and adaptability to different oil products.

Summary: Infrared oil meters accurately capture the infrared characteristic absorption of C-H bonds in oil molecules, and use three wavelength spectral analysis technology for calibration and anti-interference. Finally, based on Lambert Beer's law, high-precision quantitative analysis of mineral oil in water is achieved, which is a method in the field of environmental monitoring.