1、 Calibration method for infrared spectrophotometer oil analyzer

（1） Preparation before calibration

Before calibration, it is necessary to ensure that the instrument is in a stable working state, the ambient temperature is controlled at 15-30 ℃, the relative humidity does not exceed 85%, and to avoid interference factors such as dust and vibration. At the same time, check the core components such as the instrument light source, detector, and colorimetric dish to ensure no damage or contamination; Prepare standard oils, solvents (such as carbon tetrachloride, n-hexane, etc.), pipettes, volumetric flasks, and other instruments required for calibration, and clean and dry the instruments to prevent cross contamination.

（2） Calibration Core Process

Baseline calibration: In the absence of samples, inject blank solvent into a colorimetric dish, place it in the instrument's optical path, and perform baseline scanning. Ensure that the absorbance fluctuation at characteristic wavelengths such as 2930cm ⁻¹ (CH ₂ stretching vibration), 2960cm ⁻¹ (CH ∝ stretching vibration), 3030cm ⁻¹ (aromatic hydrocarbon C-H stretching vibration) does not exceed ± 0.002 for the baseline. If it exceeds the range, the instrument needs to be readjusted or the solvent needs to be replaced.

Standard curve drawing: Select a standard oil solution with a suitable concentration gradient (usually 0-100mg/L), inject it into a colorimetric dish in sequence, and measure the absorbance at the set characteristic wavelength. Using oil concentration as the horizontal axis and absorbance as the vertical axis, the least squares method is used to fit the standard curve, requiring a correlation coefficient R ² ≥ 0.999 to ensure good linearity of the curve.

Precision and accuracy calibration: Repeat the measurement of the same concentration standard oil solution 6 times, calculate the relative standard deviation (RSD), and require RSD ≤ 3%; Simultaneously measure the quality control samples with known concentrations, calculate the relative error (RE), and ensure that RE is ≤± 5%. If it does not meet the requirements, recheck the standard solution preparation or instrument status.

2、 Key points for selecting standard oil products

（1） Match detection object

Select standard oil products based on the types of oil pollutants in the actual test samples. If detecting mineral oil in industrial wastewater and soil, n-hexadecane is preferred as the standard substance. Its carbon chain structure is consistent with the main components of mineral oil, and its infrared absorption characteristics match well; If testing samples containing animal and vegetable oils, olive oil or soybean oil should be used as the standard oil to avoid detection errors caused by differences in oil types.

（2） Ensure purity and stability

Standard oil products must comply with national measurement standards, with a purity of ≥ 99.5% and no obvious volatility or decomposition. Priority should be given to selecting certified standard substances (such as standard oil produced by the National Center for Standardization Research) to ensure accurate concentration and strong stability, and to avoid using self prepared mixed oil as a standard, as its complex composition can easily lead to calibration result deviations.

（3） Compatible with solvent characteristics

The selected standard oil should be miscible with the solvents used for testing (such as carbon tetrachloride and carbon disulfide), without any stratification or precipitation. For example, n-hexadecane has high solubility in carbon tetrachloride and does not react chemically with solvents, ensuring the uniformity of standard solutions; If incompatible oil products are used, it will cause distortion in absorbance measurement and affect calibration accuracy.