The material of a welded filter (also known as a chromatographic protection filter/online filter, whose core function is to filter particles and impurities in the sample/mobile phase, protect the chromatographic column and detector) directly determines its chemical compatibility, filtration efficiency, sample adsorption, and service life, thereby affecting the accuracy, repeatability, and stability of chromatographic analysis. The physical and chemical properties of different materials vary significantly, and their adaptability to different scenarios and performance are also vastly different. The specific impacts are as follows:
1、 Chemical compatibility of materials: avoid contamination and filter failure
Chemical compatibility is the core prerequisite for selecting welding filter materials, which directly determines whether it can adapt to the mobile phase and sample system, avoiding self degradation or release of impurities to contaminate the sample:
Stainless steel (316L/Hastelloy):
Strong corrosion resistance, suitable for most non corrosive systems (such as organic solvents, neutral/weak acid alkali samples, aqueous mobile phases), is a commonly used material in chromatographic analysis. However, in highly corrosive environments (such as mobile phases containing high concentrations of hydrochloric acid and chloroform, or alkaline mobile phases with pH>12), stainless steel may be corroded, releasing metal ions (such as Fe ³ ⁺, Cr ⁶⁺), which will adsorb onto the stationary phase of the chromatographic column, resulting in decreased column efficiency, peak tailing (such as alkaline compound peak splitting), and contamination of the detector (such as the ion source of mass spectrometry detectors).
Polytetrafluoroethylene (PTFE):
Strong chemical inertness, almost resistant to all acids, bases, and organic solvents (including concentrated nitric acid, tetrahydrofuran, etc.), no leaching pollution, suitable for strongly corrosive samples/mobile phases (such as ion chromatography, heavy metal detection in environmental pollutants, strong acid/strong alkali sample analysis). However, PTFE has low mechanical strength and is prone to deformation and rupture in high-pressure chromatography systems (pressure>20MPa), leading to filter failure.
Polypropylene (PP):
Resistant to neutral/weak acid-base and some organic solvents (such as methanol and acetonitrile), with low cost, suitable for water organic phase mobile phase systems of conventional reverse phase chromatography (such as conventional component detection in food and medicine). But it is prone to swelling and deformation in strong organic solvents such as dichloromethane and toluene, releasing low molecular weight polymer impurities, leading to baseline drift and ghost peaks.
Ceramics (alumina/zirconia):
Resistant to high pressure and high temperature (up to 300 ℃), with good chemical stability, suitable for high-temperature chromatography, ultra-high pressure liquid chromatography (UHPLC) systems, or complex samples containing small amounts of particulate matter (such as environmental water samples and biological samples). However, ceramic materials are brittle and easily damaged by impact, and may dissolve in strongly alkaline mobile phases (pH>13), releasing aluminum ions that affect analysis.
2、 Material filtration efficiency and pore size matching: protecting the core of the chromatographic column
The core function of a welded filter is to filter particulate matter (such as suspended impurities in the sample, small particles in the mobile phase, and fragments of chromatographic column packing), and the pore structure and mechanical strength of the material directly affect the filtration efficiency:
Stainless steel filters: mostly sintered, with uniform pore size (commonly 0.22 μ m, 0.45 μ m), high filtration accuracy, and effective interception of small particles; High mechanical strength, suitable for high-pressure chromatography systems (pressure ≤ 40MPa), less prone to pore size deformation due to pressure fluctuations, and stable filtration efficiency for long-term use. Suitable for routine analysis with low sample viscosity and no strong corrosion (such as drug content determination, food additive detection), it can effectively protect the chromatographic column sieve plate from clogging.
PTFE/PP filters: mostly membrane filters with wide pore size distribution and slightly lower filtration accuracy than stainless steel, but have better permeability to viscous samples (such as biological fluids and polymer solutions) and are less prone to clogging. But PTFE is prone to deformation under high pressure, leading to an increase in pore size and a decrease in filtration efficiency; After swelling in organic solvents, the pore size of PP filters may change, which may not effectively intercept particulate matter and increase the risk of chromatographic column blockage.
Ceramic filter: uniform pore size and strong rigidity, high filtration efficiency, suitable for high-pressure and high-temperature scenarios (such as supercritical fluid chromatography SFC), can maintain stable pore size for a long time. However, ceramic filters have low porosity and low flux for high viscosity samples, which can easily lead to pressure increase due to particle accumulation and require regular cleaning.
3、 Material sample adsorption: avoiding analysis errors
The difference in adsorption capacity of different materials for sample components can directly lead to sample loss, reduced peak area, or tailing, affecting quantitative accuracy:
Stainless steel filter: It has weak adsorption for non-polar compounds (such as hydrocarbons and fat soluble vitamins), but has a certain adsorption effect on polar compounds (such as organic acids and phenols) and metal chelates (such as EDTA complexes), which may lead to tailing of peak shape and poor repeatability of peak area. For example, when analyzing phenolic components in traditional Chinese medicine, the adsorption of stainless steel filters can lead to lower detection results.
PTFE filter: With strong surface inertness and extremely low adsorption for the vast majority of compounds (including polar, non-polar, acidic, and alkaline compounds), it has almost no sample loss and is the preferred material for trace analysis and polar compound analysis (such as trace VOCs detection in the environment and hormone analysis in biological samples).
PP filter: It has slight adsorption for non-polar compounds and lower adsorption for polar compounds than stainless steel. However, when analyzing low concentration samples (such as ng/mL level), the errors caused by adsorption cannot be ignored and are not suitable for trace analysis.
Ceramic filter: It has a certain adsorption capacity for polar compounds (such as alcohols and amines), especially in the aqueous mobile phase, where the adsorption effect is more obvious. It is suitable for the analysis of non-polar, high concentration samples (such as hydrocarbon separation in petroleum products).