(Translator's Note: The provided source document contains a mismatch between the title and the body text. The title discusses upgrading a standard LC to an HPLC, while the body text is a comprehensive guide on HPLC injection procedures and maintenance. The translation below accurately reflects the original body text provided.)
Injection is the "throat" of the entire HPLC analysis workflow. The process of samples entering the chromatographic flow path from the sample vial relies entirely on the injection system. Many people think injection is just simply placing the sample vial and clicking "run." In fact, a large number of details can directly affect data reproducibility. Improper operation can not only invalidate an entire batch of sample experiments but also wear out the injection valve seal, block the sample loop, and shorten the service life of expensive chromatographic columns.
I. Standard Operating Procedure for HPLC Injection
The entire injection operation is divided into three major modules: pre-instrument preparation, core instrument operation, and post-experiment wrap-up and maintenance. None of these steps can be omitted.
Pre-instrument preparation is an important step to reduce later failures. Many LC anomalies have already laid hidden dangers before the sample is even loaded onto the instrument.
1) Sample Pretreatment
Samples must be fully dissolved, with no visible precipitation. For reversed-phase systems, use a 0.22 μm HPLC-specific filter membrane; for normal-phase systems, use a 0.45 μm filter membrane. Filtration is a mandatory requirement to protect the column. Even if the sample solution looks clear and transparent, it must be filtered to remove fine particulate impurities. This prevents particles from scratching the column head frit, which would cause column pressure to continuously rise and column efficiency to rapidly decline. After filtration, the sample can be centrifuged at 3000 r/min for 5 minutes to shake out micro-bubbles in the solution.
2) Standardized Sample Vial Handling
Use LC-specific screw-thread sample vials, and control the sample liquid level at 1/2 to 2/3 of the vial body. If the liquid level is too low, the autosampler needle can easily draw air; if overfilled, it is prone to leakage during operation, causing cross-contamination. Bottle caps and injection septa should be dedicated to one vial per set, and mixing is strictly prohibited. Replace septa promptly if scratches or damage appear. When placing vials on the sample tray, ensure the vial label faces outward, and verify the sample tray position numbers to prevent placing samples in the wrong position.
3) Instrument Status Verification
Confirm that the mobile phase reserve is sufficient and has been filtered and degassed; check that there are no residual bubbles in the tubing; wait for the system baseline to stabilize, with baseline noise ≤ 0.1 mV and drift ≤ 0.5 mV/h; ensure the column oven has reached the set experimental temperature, and the instrument status displays "Ready" before proceeding to edit the injection sequence.
1) Workstation Method Parameter Settings
Enter the complete method in the chromatography workstation: injection volume, needle wash solvent, needle wash times, needle wash time, flow rate, column temperature, and other parameters. When using a sample loop for quantitative analysis, it is generally recommended that the injection volume not exceed 50% of the loop capacity. For example, for a 20 μL loop, control the injection volume to ≤ 10 μL to maximize injection reproducibility.
2) Establish the Sample Run Sequence
Carefully fill in the sample number, sample tray position, and sample name. For data file naming, it is recommended to use the format "Date-Project-Sample Number" to facilitate subsequent data traceability and searching, avoiding the confusion of a large number of data files.
3) Pre-Run Hazard Inspection
Check the waste bottle liquid level; it should not exceed the 80% mark. If the waste liquid is too full, back-siphoning can occur, contaminating the injection valve and the entire flow path. Confirm that the autosampler compartment door is fully closed and not loose.
4) Start the Sequence and Monitor Operating Status in Real Time
Click to start the sequence. Observe the system pressure for a slight fluctuation and a slight baseline rise during the first injection; this indicates that the sample has been successfully injected into the flow path. If the pressure does not move at all and the baseline has no response, immediately stop the sequence and troubleshoot the sample aspiration and injection valve faults. Do not continue running samples.
Many laboratories shut down directly after experiments, ignoring injection system maintenance. Over time, this leads to ghost peaks, cross-contamination, and sample loop blockage.
1) Injection Needle Cleaning and Maintenance
After the entire sequence has run, replace the needle wash bottle with a cleaning solution of 90% water + 10% methanol. Execute the automatic needle wash program, repeating the needle wash 3-5 times to flush out samples adsorbed on the needle tube wall, preventing samples from drying and crystallizing to block the needle.
2) Targeted Flushing of the Flow Path and Injection Valve
If high-salt buffer systems, high-viscosity, or strongly polar samples were analyzed this time, do not directly flush with pure organic phase. First, flush the injection valve, sample loop, and flow path with pure water or a low-salt solution for 10-15 minutes to fully flush away salt substances. Then switch to methanol or acetonitrile for storage flushing to prevent salt crystallization from damaging components.
3) Consumables and Data Organization
Back up all experimental data immediately to prevent data loss due to computer failure; remove all sample vials from the sample tray, clean, dry, and store them; wipe the autosampler compartment to remove splashed residual solvents.
II. Four Core Pitfalls to Avoid in HPLC Injection
Point 1: Strictly Prevent Cross-Contamination to Ensure Data Purity
Cross-contamination is a headache in LC quantitative experiments. High-concentration standard residues carried over into low-concentration samples directly cause results to be falsely high.
Prohibited operations: Running low-concentration samples directly after high-concentration samples; repeatedly mixing bottle cap septa; setting too few needle wash times.
Practical specifications: After testing high-concentration samples and standards, increase the needle wash times to 5-8 times and extend the needle wash soaking time; select a mixed solvent of methanol and acetonitrile with strong elution capability as the needle wash solvent; before use, ultrasonically clean sample vials in methanol and dry them before loading samples; replace septa immediately if wear or stains appear, as septa are a high-frequency source of cross-contamination.
Point 2: Control Injection Precision to Ensure Result Reproducibility
A large peak area RSD is often not an instrument hardware failure but rather inadequate details in the sample introduction process.
Prohibited operations: Residual bubbles inside the sample; excessive difference in elution strength between the sample solvent and the mobile phase; injection volume exceeding the sample loop load.
Practical specifications: After sample filtration, centrifuge or gently flick the vial wall to expel bubbles; try to use the initial mobile phase ratio as the sample dissolution solvent; if solubility is limited, try to control the organic phase difference within 20% to avoid solvent effects and prevent peak fronting or splitting; do not exceed the maximum allowable injection volume of the sample loop to prevent quantification distortion.
Point 3: Protect Injection Components to Extend the Service Life of the Chromatograph and Column
Injection valve seals and sample loops are consumable parts. Standardized operation can greatly reduce replacement frequency.
Prohibited operations: Loading samples directly onto the instrument without filtration; ignoring seal wear; long-term failure to disassemble and clean the sample loop.
Practical specifications: Sample filtration is a mandatory bottom line; even if the solution is clear, it cannot be skipped; the injection valve rotor seal is generally recommended to be inspected and replaced after 2,000-5,000 injections; once abnormal pressure fluctuations or leakage at the injection port occur, immediately stop the machine and replace; it is recommended to disassemble and clean the sample loop once a month to remove adsorbed impurities on the tube wall.
Point 4: Experimental Safety Operating Specifications
Prohibited operations: Disassembling and assembling injector components under pressure; handling toxic samples without protection; pouring experimental waste liquid directly into drains.
Practical specifications: When disassembling tubing or servicing the injection valve, be sure to release system pressure first to prevent high-pressure solvent splashing; operate toxic and corrosive samples in a fume hood throughout, wearing nitrile gloves and safety goggles; all experimental waste liquids must be collected by category and handed over to compliant institutions for disposal.
III. Common Phenomena, Causes, and Quick Solutions Related to HPLC Injection
|
Problem Phenomenon |
Primary Suspected Cause |
Quick Solution Steps (in order of priority) |
|
Peak area fluctuates greatly, poor reproducibility |
Sample liquid contains bubbles/insufficient liquid level, incomplete needle wash |
1. Centrifuge sample to remove bubbles; 2. Add sample to 1/2-2/3 liquid level; 3. Increase needle wash times |
|
Peak shape splitting, fronting, tailing |
Solvent effect, sample not fully dissolved |
1. Re-prepare sample using initial mobile phase ratio; 2. Ultrasonically dissolve sample and filter again |
|
System pressure continuously increases |
Sample particles blocking guard column/inline filter |
1. Replace guard column/inline filter element; 2. Filter all samples again before loading |
|
No peak/very low response after injection |
Sample drawn empty/degraded, injection needle blocked |
1. Check sample vial liquid level, confirm injection needle position; 2. Verify sample stability |
|
High baseline noise, increased ghost peaks |
Injection valve contamination/cross-contamination, impure mobile phase |
1. Clean injection valve + sample loop; 2. Replace with high-purity mobile phase, re-degas |
|
Abnormal pressure fluctuation, going up and down |
Injection valve seal wear/bubbles in tubing |
1. Replace seal; 2. Empty bubbles in tubing and injection valve |
Standardize Injection to Make HPLC Analysis Twice as Effective
In HPLC analysis, 70% is operation and 30% is instrument, and the injection operation is precisely the "throat" of the entire experiment. Internalizing the standardized process into operating habits and keeping the pitfall-avoidance points firmly in mind will not only yield experimental chromatograms with symmetrical peaks, precise data, and excellent reproducibility, but will also greatly reduce instrument failures, extend the service life of core components such as columns and injection valves, and improve experimental efficiency.
Details determine success or failure. Start with every standardized injection to make your HPLC analysis work free from data anomalies and instrument failures, and handle everything with ease!
Many analysts focus their energy on optimizing mobile phase ratios and adjusting gradient programs, yet overlook the details of injection operation, repeatedly encountering data anomalies and consuming a lot of time on repeated retesting. Solidifying the entire injection process into daily operating habits will not only yield chromatograms with symmetrical peaks and qualified RSD, but also reduce instrument failures, lower the wear and tear on columns and injection valve accessories, and improve overall laboratory work efficiency.
This article was compiled by Elite LC After-Sales Technical Support and is applicable as an operation reference for most domestic HPLC autosamplers. The workstation interfaces of different brands may vary slightly, but the operating logic is the same.
