Both aim to reduce pulsation, but with different emphases. Series-connected design excel at maintaining exceptional flow accuracy and stability under low flow rates and high pressure, making them ideal for applications requiring ultra-high baseline stability, such as conventional analysis and microfluidics. Parallel design demonstrates superior pulsation control at medium-to-high flow rates, support a wider flow range, and feature a more compact structure, making them ideal for semi-preparative, preparative, and high-throughput laboratories requiring rapid flushing.

This is primarily related to the complexity of the sample matrix and the effectiveness of pretreatment. If the analyzed sample matrix is complex (e.g., biological samples, environmental water samples) and not adequately pretreated, large molecular impurities and particulate matter in the sample can rapidly clog the filtration system and chromatographic column, leading to increased pressure fluctuations. In contrast, pure samples cause less system contamination and maintain relatively stable pressure. Additionally, variations in sample viscosity also affect pressure. High-viscosity samples exhibit greater resistance changes during flow through the chromatographic column, making pressure fluctuations more likely. It is recommended to optimize the pretreatment process for complex matrix samples by incorporating steps such as solid-phase extraction (SPE) and centrifugation to ensure thorough sample filtration.

At this stage, prioritize inspecting the analytical column and tubing system. First, disconnect the inlet of the analytical column and activate the pump to measure the pressure in the upstream tubing. If the pressure is normal, the issue lies with the analytical column. You may attempt reverse flushing using a compatible solvent (e.g., methanol-water = 90:10 for reversed-phase columns). If the pressure remains high after flushing, the column bed may be collapsed or the packing material degraded, necessitating replacement of the analytical column. If the upstream tubing pressure persists, check for tubing bends or residual impurities on the inner walls. The tubing can be disassembled and flushed with methanol or isopropanol, and new tubing should be replaced if necessary.

This situation is most likely caused by air bubbles within the system or uneven solvent mixing. First, check whether the mobile phase degassing is adequate. You can perform ultrasonic degassing again or activate the online degassing unit to extend the degassing time. Second, investigate whether there are air bubbles in the pump head by completely exhausting the air through the manual degassing function. If it is a gradient elution mode, confirm that the ratio valve is functioning properly. You can attempt manual mixing of the mobile phase and perform isocratic elution testing to observe whether the pressure remains stable.

Stepwise drift is primarily caused by significant UV absorption differences between two or more mobile phases during gradient switching, or by inadequate degassing and uneven mixing of the mobile phases. Optimization strategies: ① Perform thorough degassing (using ultrasonic degassing or online degassing) for all mobile phases to minimize bubble interference with the baseline; ② Increase the equilibration time before gradient elution to ensure complete mixing of mobile phases and full column equilibrium; ③ For cases with pronounced absorption variations, select mobile phase components with similar absorption wavelengths, or run under blank gradient conditions to obtain baseline spectra. Subsequent data processing software should then subtract the blank gradient baseline to enhance integration accuracy.

Core troubleshooting directions: ① Sample injection issues, such as failed injection or improperly switched injection valve; ② Chromatographic column issues, such as column blockage preventing sample elution or disconnected tubing between column and detector; ③ Detector issues, such as incorrect wavelength settings (not matching component absorption wavelengths) or no mobile phase flow in the detection cell. Troubleshooting steps: First, manually inject a standard sample to verify injection issues and check the injection valve status; then observe system pressure. Abnormally high pressure may indicate column blockage, requiring column flushing or replacement, along with tubing inspection; finally, verify detector wavelength parameters to ensure consistency with standard methods and check the detection cell for bubbles or blockages.

The primary causes include: ① Sampling system issues such as clogged sampling needles, inaccurate sample volume, or aged seals in the autosampler; ② Poor sample stability leading to degradation or polymerization in the solvent; ③ Inadequate mobile phase equilibrium and unstable system pressure. Solutions: First, check the sampling needle for patency, replace aged seals, and calibrate the autosampler. Second, evaluate sample stability—preparation and injection should be performed immediately if the sample is prone to degradation, or stabilizers should be added. Finally, extend the mobile phase equilibrium time and wait for system pressure stabilization before proceeding with sampling and analysis.

UHPLC represents an advanced upgrade of HPLC, utilizing higher pressure (>1000 bar) and smaller particle size packing materials (<2μm), offering faster throughput and enhanced sensitivity. However, their relationship is not one of "replacement" but rather "complementarity and evolution." UHPLC imposes higher demands on sample pretreatment, system maintenance, and operator expertise. Decision-making recommendations: ① For laboratories primarily engaged in routine quality control with fixed methodologies, mature HPLC systems provide greater stability and cost-effectiveness; ② For cutting-edge research requiring ultra-high-speed processing of large sample volumes (e.g., metabolomics) with adequate budget, UHPLC is a prudent choice; ③ Consider future compatibility, as some manufacturers' HPLC platforms can be upgraded with pumps and modules to support future UHPLC methods.