Definition
A chromatograph is an analytical instrument based on the principle of chromatographic separation for the separation, qualitative and quantitative analysis of components in complex mixtures. The core process is to divide the components in the mixture between the stationary phase and the mobile phase, and due to the different partition coefficients between the two phases, the components move at different rates driven by the mobile phase to achieve separation. This technology is widely used in laboratory testing in various fields such as environmental monitoring, food safety, petrochemical, and materials science.
Principle
The working principle of chromatographs is based on the theory of chromatographic separation. When the sample is introduced into the system, it enters the column with the mobile phase. The column is filled or coated with a stationary phase, and each component has a different retention time due to the difference in adsorption, partitioning, ion exchange, or size rejection between the stationary phase and the mobile phase. Components with shorter retention times flow into the column first, and longer retention times flow out after physical separation. The separated components enter the detector, which is converted into an electrical signal that is recorded as a chromatogram by the data processing system for further analysis.
The basic retention time relationship can be expressed as:tR = t0(1 + k), among themtRretention time for components,t0Time to die,kis the capacity factor.
Measurement method
Chromatographic analysis typically involves sample preparation, injection, separation, detection, and data processing. First, the sample undergoes appropriate pretreatment, such as extraction, filtration, or derivatization, to suit the instrument requirements. The injection methods include manual injection and automatic injection, and the accuracy and reproducibility of the injection volume must be ensured. The separation process is performed in a column to optimize the separation by adjusting parameters such as mobile phase composition, flow rate, and column temperature. Detectors are selected according to the characteristics of the components, and the common types include ultraviolet-visible light detectors, fluorescence detectors, index refractive detectors, etc. Ultimately, the peak area or peak height in the chromatogram is used for quantitative analysis, and the retention time is used for qualitative reference.
Influencing factors
Chromatographic results are influenced by a variety of factors. The column's performance is key, including stationary phase type, column efficiency, column length, and internal diameter. The properties of the mobile phase, such as composition, purity, pH, and flow rate, directly affect the selectivity and efficiency of the separation. Temperature control affects the separation kinetics, and fluctuations in column temperature can lead to drift in retention time. Consistency in injection techniques is critical to analytical reproducibility, and excessive injection volumes can cause column overload. The sensitivity and linear range of the detector determine the quantitation accuracy. In addition, sample matrix interference, system piping residues, and operating environmental conditions can also introduce errors.
Application
In the field of environmental monitoring, chromatographs are used to detect organic pollutants such as PAHs and pesticide residues in water, soil, and air. In terms of food safety, it is suitable for the analysis of food additives, preservatives, toxins and flavor substances. In the petrochemical industry, it is used for hydrocarbon composition analysis and quality control of refined oil. In materials science, it can characterize the molecular weight distribution of polymers and the composition of additives. In addition, chromatography also plays an important role in forensic identification, cosmetic testing, and industrial process monitoring.
Selection
When choosing a chromatograph, consider your analytical needs and system configuration. Define the properties and concentration range of the target compounds to determine the appropriate chromatographic type, such as gas chromatography for volatile compounds and liquid chromatography for thermally unstable or high-boiling substances. The detector should be matched to the UV absorption, fluorescence, or electrochemical properties of the components. Column selection is a matter of separation selectivity and should refer to the literature or standard methods for similar compounds. The degree of automation of the system, sample throughput, data processing software capabilities, and subsequent maintenance costs are also factors that need to be evaluated in the actual selection. It is recommended to refer to the instrument requirements in national standards, industry specifications or international standards (such as ISO, ASTM) and combine them with actual samples for verification testing.