In a complete High‑Performance Liquid Chromatography (HPLC) system, the infusion pump delivers mobile‑phase power, the chromatographic column achieves component separation, and the detector collects signals. The autosampler is responsible for delivering samples accurately and stably into the chromatographic flow path. It directly determines injection precision and data repeatability, serving as a critical component to guarantee reliable quantitative results. Compared with manual injection, autosamplers eliminate human‑operated deviations and support automatic operation of large‑batch sequences. They are widely adopted in pharmaceutical testing, food inspection, environmental monitoring, third‑party testing and other laboratories.
A complete HPLC autosampler mainly consists of seven key assemblies: sample tray module, injection needle and mechanical transmission mechanism, six‑port high‑pressure injection valve, sample loop, metering syringe pump, needle‑washing flow path, and electronic control workstation.
Sample tray module: Holds sample vials and wash‑solution vials. Some high‑end models are equipped with temperature‑controlled refrigeration modules to store thermolabile samples under low‑light and low‑temperature conditions and prevent degradation of analytes.
Mechanical transmission mechanism: A stepper motor drives three‑dimensional movement of the injection needle to precisely locate designated sample vials and needle‑wash ports with micron‑level positioning accuracy.
Six‑port high‑pressure injection valve: The core high‑pressure component of the autosampler. It has two positions: LOAD and INJECT for flow‑path switching. It withstands high HPLC system pressure and completes switching within milliseconds without interrupting mobile‑phase flow.
Sample loop: Stores sample solution to be injected. Common specifications are 20 μL, 50 μL and 100 μL, which is the key part controlling injection volume.
Metering syringe pump: Accurately aspirates and dispenses samples and controls sampling volume to ensure sampling precision.
Needle‑washing flow‑path system: Consists of wash‑solution bottles, washing pumps and waste‑liquid pipelines. It implements inner‑needle washing and outer‑needle washing to reduce sample carry‑over.
Electronic control and workstation: Receives sequence commands from chromatography software and controls all mechanical actions. Compliance‑oriented pharmaceutical models integrate audit trails to fully record injection sequences, parameter modifications and equipment alarms to satisfy GMP, CMA and CNAS data‑integrity requirements.
Full‑loop injection The sampling volume reaches more than three times the sample‑loop volume to fully displace the original solution inside the loop. It delivers optimal injection repeatability with RSD usually ≤ 0.3 %. The disadvantage is higher sample consumption. It is suitable for scenarios with sufficient sample quantity and high‑precision quantitative analysis. The actual injection volume equals the volume of the sample loop.
Partial‑loop injection (partial‑fill injection) Only a portion of the sample loop is filled, and the injection volume shall not exceed 50 % of the loop volume. The sampling volume is controlled by the metering pump. Part of the aspirated sample is discharged as waste liquid, resulting in sample consumption. Multiple injection volumes can be set for one fixed sample loop. This mode is widely used in domestic HPLC instruments such as the Elite EClassical 3200 autosampler.
Partial‑fill without waste injection (waste‑free injection) Small‑volume sample is aspirated and fully pushed into the flow path by wash liquid with almost no sample loss. It is ideal for precious and trace samples. Restricted by metering‑pump precision, the injection volume is generally recommended not to exceed 40 % of the sample‑loop volume.
Note: For partial‑loop and waste‑free injection modes, injection repeatability will degrade significantly if the set injection volume exceeds 50 % of the sample‑loop volume, which should be avoided in experimental setup.
For instrument acceptance, annual calibration and third‑party assessment, four core indicators of autosamplers are evaluated, which also serve as critical selection criteria for laboratories in different industries.
Injection repeatability (precision) Multiple consecutive injections are performed to evaluate the RSD of retention time and peak area. General requirements for pharmacopoeia‑compliant testing laboratories: peak‑area RSD ≤ 1.0 %, peak‑height RSD ≤ 2.0 %. This indicator directly reflects the stability of quantitative results and is particularly important for high‑volume pharmaceutical testing.
Sample carry‑over (cross‑contamination) After injecting a high‑concentration sample, inject blank solvent. The ratio of the analyte peak area detected in the blank run to the peak area of the high‑concentration standard solution is defined as carry‑over ratio. Pharmaceutical and customs testing attach great importance to this indicator to prevent cross‑contamination‑caused false‑positive results. Pharmaceutical laboratories commonly require carry‑over ≤ 0.01 %. Excessive carry‑over requires optimization of wash‑solvent composition or increase of washing cycles.
Injection linearity Vary the injection volume. The correlation coefficient r between peak area and injection volume shall be ≥ 0.995 to verify the sampling accuracy of the metering pump.
Additional compliance‑oriented performance (mandatory for pharmaceutical and forensic testing) ① Temperature‑controlled function: For preserving thermolabile analytes; ② Alarm sensors: Alarms for missing sample vials and liquid leakage; ③ Audit trail: Complete records of injection sequences and parameter modifications with non‑erasable logs for GMP and CNAS‑compliant data traceability; ④ Mechanical safety protection: Protection logic to prevent piercing vials or damaging injection needles when vials are misplaced.
Laboratories comply with Chinese Pharmacopoeia and GMP regulations. Carry‑over performance, injection repeatability and audit‑trail function are mandatory requirements. Complex matrices and wide concentration ranges in Chinese herbal samples easily cause cross‑contamination; therefore sufficient inner‑ and outer‑needle washing functions are required. The autosampler shall support long‑term large‑batch sequence runs, software role‑based permission management and full traceability of raw data. A refrigerated sample tray is optional for thermally unstable herbal components.
Test reports carry legal validity. Cross‑contamination must be strictly controlled to avoid false‑positive judgments. High injection repeatability is required. All injection logs and sequence parameters shall be exportable and archivable. Instruments need to adapt to diverse sample types and support both quantitative analysis of major components and trace contaminant detection.
Laboratories handle high sample throughput and prioritize durability and stability during continuous injection. Models with flexible adjustable washing programs and easy maintenance are preferred. Audit‑trail function is configured according to business requirements.
Laboratories prioritize operational flexibility and prefer waste‑free injection for precious, limited‑volume samples. Extended functions such as online dilution, automatic standard addition and online derivatization are desirable.
Deteriorated peak‑area repeatability Common causes: insufficient washing or improper wash‑solvent leading to carry‑over; tiny blockage of injection needle; low liquid level in sample vials; worn metering‑pump sealing gasket. Troubleshooting sequence: check washing procedure → inspect injection needle → examine pump sealing components.
High sample carry‑over Optimization priorities: select suitable wash‑solvents with good solubilizing properties; increase inner‑ and outer‑needle washing cycles; regularly sonicate the injection needle; replace the needle‑seat gasket if necessary.
Injection needle piercing sample vials Some domestic autosamplers lack sample‑vial presence sensors. Misplaced vials may result in broken glass vials and damaged injection needles. In daily operations, ensure vials are placed upright and caps are not over‑tightened.
Liquid leakage Aged needle‑seat gaskets are the most frequent cause of leakage. Replace gaskets periodically and check whether pipeline joints are loose.
Practical maintenance advice: After experiments, thoroughly wash the injection needle and flow path with appropriate solvents. Do not leave salt‑containing solutions inside the injection flow path for extended periods, as salt precipitation will abrade sealing parts and injection needles.
An autosampler does not merely “inject samples into the instrument”. It constitutes the first barrier to guarantee quantitative reliability for the whole HPLC system. Instrument selection shall not rely solely on specification‑sheet parameters; instead, industry regulatory requirements, sample characteristics and sample throughput shall be comprehensively considered. Regular maintenance and intermediate performance checks are essential to sustain accurate and reliable analytical results over long‑term operation.
Q1: Why does the autosampler exhibit poor repeatability in sample injection results?
A1: The following factors may be responsible:
① The injection needle may contain air bubbles or be partially clogged; it should be flushed or replaced;
② The sample vial may be improperly sealed, leading to volatilization or contamination;
③ There may be residual material or the sample loop may not be fully filled;
④ Temperature fluctuations can affect sample stability. It is recommended to perform a cleaning procedure, inspect the vial gasket, verify the temperature control, and conduct a system suitability test.
Q2: How can sample cross-contamination be prevented?
A2: Cross-contamination often results from inadequate cleaning. To prevent this:
① Make full use of the instrument's internal and external wall cleaning functions (e.g., the patented cleaning technology in the S3200L);
② Optimize the cleaning protocol by increasing the volume and frequency of the cleaning solvent;
③ Inject a blank solvent sample after analyzing high-concentration samples; ④ Regularly replace or clean the injection needle and flow path components.
Q3: The autosampler is frequently triggering a liquid leakage alarm; how should this be handled?
A3: First, verify the authenticity of the alarm:
① Check the needle holder, connecting tubing, and valve interfaces for any signs of liquid leakage;
② Confirm whether the waste liquid container is full. If the alarm is a false positive, it may be due to sensor contamination or malfunction; in such cases, the sensor should be cleaned or repaired. The S3200L series features dual-probe leakage detection technology, enabling more accurate early warnings; however, regular maintenance of the flow path sealing integrity is still required.
