A solid phase extraction (SPE) column is an essential tool for sample preparation in the laboratory. It is widely used for sample purification and enrichment in fields such as food, environmental, and pharmaceutical analysis, effectively removing interferences and improving the accuracy of subsequent detection. Many beginners tend to have low recovery or poor purity due to improper procedures. This article, based on practical needs, provides a detailed breakdown of the SPE column operation process, key points, and problem‑solving strategies to help you master the correct usage quickly.
I. Prerequisites Before SPE Column Operation: Three Essential Preparation Steps
Proper preparation before operation directly affects the experimental results. Focus on verifying the compatibility of consumables, solvents, and tools to avoid subsequent errors.
SPE columns with different mechanisms are suitable for analytes with different polarities or charges. Using the wrong column leads to extraction failure. The common types and their applications are as follows:
Reversed‑phase columns (e.g., C18, C8): Retain non‑polar or moderately polar substances through hydrophobic interactions (e.g., pesticide residues, polycyclic aromatic hydrocarbons). Typical applications: enrichment of organic pollutants in environmental water samples, detection of veterinary drug residues in foods.
Normal‑phase columns (e.g., silica, Florisil): Retain polar substances through polar interactions (e.g., vitamins, phenols). Typical applications: removal of polar interferences from lipid samples, purification of herbicides from agricultural products.
Ion‑exchange columns (e.g., SCX, SAX): Retain charged substances through ionic interactions (e.g., amino acids, alkaloids, organic acids). Typical applications: extraction of drug metabolites from biological samples (serum, urine), separation of additives in foods.
Affinity columns (e.g., immunoaffinity): Retain specific target analytes through specific binding (e.g., aflatoxins, antibiotics). Typical applications: trace toxin detection in foods, precise removal of impurities in pharmaceuticals.
Also confirm the column capacity (e.g., 100 mg/3 mL, 500 mg/6 mL). The total amount of target analyte in the sample must not exceed the column capacity (typically 100 mg of sorbent can adsorb 1–10 mg of target). Overloading leads to extraction failure.
Solvents must be chosen according to the SPE column type and must be HPLC or analytical grade (to avoid interference from impurities). Their specific functions are:
Activation solvent: Removes column impurities (e.g., residual packing debris, preservatives) and activates the active sites of the sorbent.
Reversed‑phase columns: commonly use a “strong solvent (methanol) + weak solvent (ultrapure water)” combination.
Normal‑phase columns: use hexane, ethyl acetate.
Ion‑exchange columns: use a buffer with a certain ionic strength (e.g., 0.1 mol/L hydrochloric acid).
Loading solvent: Dissolves or dilutes the sample. Its polarity must match the activated sorbent to prevent premature elution of the analyte.
Reversed‑phase columns: the loading solvent should contain ≥ 80% water.
Ion‑exchange columns: adjust sample pH according to the analyte charge (e.g., adjust acidic analytes to pH 2 to enhance binding to cation exchangers).
Washing solvent: Elutes only weakly retained interferences. Its polarity should lie between that of the activation solvent and the elution solvent.
Reversed‑phase columns: use 5–10% methanol in water.
Normal‑phase columns: use 10–20% ethyl acetate in hexane.
Elution solvent: Must completely elute the target analyte from the sorbent.
Reversed‑phase columns: use a strong polar solvent (e.g., pure methanol, acetonitrile).
Normal‑phase columns: use a weak polar solvent (e.g., methanol, ethanol).
Ion‑exchange columns: use a high‑concentration salt solution (e.g., 1 mol/L sodium chloride) or an acid/base solution.
Core equipment: SPE vacuum manifold (for controlling solvent flow rate), vacuum pump (maintain negative pressure at 0.02–0.05 MPa to avoid adverse effects from too high or too low pressure).
Auxiliary tools: Micropipettes (1 mL, 10 mL, 50 mL sizes for accurate measurement), 0.22 μm/0.45 μm membrane filters (to remove suspended particles or precipitates from the sample, preventing column clogging), centrifuge tubes (for sample pre‑treatment such as centrifugation to remove impurities), collection tubes (must be compatible with the subsequent analytical instrument, e.g., 10 mL centrifuge tubes, 2 mL chromatography vials).
II. Core SPE Column Operations: Two Mechanisms with Different Procedures (Key Points Included)
The essence of SPE is selective adsorption and desorption. Based on whether the sorbent retains the target or the impurities, the operation is divided into a “four‑step method” and a “three‑step method”. Beginners often confuse them, so it is important to distinguish and follow them strictly.
(A) Mechanism 1: Sorbent retains the target (commonly used, e.g., enrichment of pesticide residues)
This is suitable for low‑concentration analytes that need enrichment, or samples with many interferences that require purification (e.g., pesticide residues in environmental water, trace components in pharmaceuticals). The procedure has four steps; each step critically affects the recovery of the target.
Procedure: ① Pass 3–5 bed volumes (BV; e.g., for a 3 mL column, use 9–15 mL) of a strong solvent through the column at a flow rate of 1–2 drops/second. ② Then pass 3–5 BV of a weak solvent to equilibrate the sorbent to a polarity consistent with the loading solvent.
Purpose: Remove column impurities and activate the active adsorption sites of the sorbent, preparing for subsequent analyte adsorption.
Critical note: After activation, always keep the sorbent covered by solvent (liquid level 1–2 mm above the sorbent bed). If the sorbent dries out and cracks, its adsorption capacity drops sharply, and reactivation is required.
Procedure: ① Slowly add the pre‑treated sample (already filtered and pH‑adjusted) to the column using a pipette. ② If the sample volume is large (e.g., 100 mL water sample), turn on the vacuum pump and control the flow rate ≤ 5 mL/min.
Purpose: Allow the analyte to fully contact the sorbent and be selectively adsorbed on its surface.
Critical note: ① Flow rate must not be too high (> 5 mL/min leads to “breakthrough” – the analyte passes through without being adsorbed, greatly reducing recovery). ② If the sample is turbid or contains precipitates, it must be passed through a 0.45 μm filter beforehand; otherwise the particles will clog the sorbent pores, causing extremely slow flow or even stopping it.
Procedure: ① Add 3–5 BV of washing solvent at a flow rate of 1–2 drops/second. ② After washing, turn on the vacuum pump for 10–15 seconds to remove residual washing solvent.
Purpose: Elute weakly retained interferences (e.g., pigments in food samples, proteins in biological samples) while retaining the target.
Critical note: The strength of the washing solvent must be determined by preliminary experiments (e.g., test gradient concentrations of 3%, 5%, 10% methanol in water). Too high a strength will elute the target; too low a strength will not effectively remove interferences. Do not over‑dry the sorbent during aspiration.
Procedure: ① Add 1–3 BV of elution solvent. Allow the solvent to fully wet the sorbent; soak for 1–2 minutes to enhance desorption. ② Pass the elution solvent through at 1–2 drops/second and collect the eluate in a clean collection tube. ③ For higher recovery, repeat elution once (using fresh elution solvent) and combine the two eluates.
Purpose: Completely elute the adsorbed target for subsequent analysis (e.g., HPLC, GC).
Critical note: The elution volume should be kept at 1–5 mL to avoid excessively long concentration times (e.g., nitrogen evaporation). If the analyte is volatile or easily oxidised, concentrate it immediately after collection or add a stabiliser (e.g., 0.1% formic acid to prevent degradation).
(B) Mechanism 2: Sorbent retains impurities (e.g., removal of pigments from food samples)
This is suitable for samples with high analyte content and few impurities, where fast collection of the analyte is desired (e.g., purification of fructose and glucose from foods, extraction of water‑soluble vitamins from agricultural products). The procedure has only three steps, and the key is to collect the effluent immediately.
Pre‑treat the sorbent with the appropriate solvent (e.g., ethyl acetate for a normal‑phase column) to equilibrate the column polarity, keeping the sorbent wet throughout.
Procedure: Slowly add the sample solution to the column. From the moment the sample first contacts the sorbent, place a collection tube under the column outlet and control the flow rate at 1–2 drops/second. The target passes through the column with the solution, while the impurities are retained on the sorbent.
Purpose: Quickly separate the analyte from interferences, avoiding analyte loss due to transient adsorption on the column.
Critical note: Do not wait until all sample has been loaded before collecting. Otherwise, part of the analyte may be temporarily adsorbed, reducing recovery.
Procedure: After loading, continue to pass 3–5 BV of washing solvent (with polarity matching the loading solvent, e.g., pure water) through the column. Collect all effluent (including the effluent from the loading step) and combine for subsequent analysis.
Purpose: Completely flush out any residual analyte remaining in the column.
III. Five Common Problems and Solutions for Beginners (Including Causes of Low Recovery)
Many people encounter low recovery, poor purity, or column clogging after operation. These are often caused by improper attention to detail. Corresponding solutions are given below.
Main causes: ① Sorbent dried out after activation, losing active sites. ② Loading flow rate too high, causing breakthrough. ③ Elution solvent too weak, incomplete desorption. ④ Column capacity overloaded.
Solutions: ① Keep the sorbent covered with solvent after activation; if it dries, reactivate. ② Reduce loading flow rate to ≤ 5 mL/min or use gravity loading (no vacuum). ③ Increase elution solvent strength (e.g., for reversed‑phase, use a methanol/acetonitrile mixture), increase elution volume, or extend soaking time. ④ Reduce sample load or use a larger SPE column (e.g., change from 100 mg/3 mL to 500 mg/6 mL).
Main causes: ① Washing solvent too weak, failing to remove weakly retained interferences. ② Inadequate sample pre‑treatment, containing pigments, suspended particles, etc.
Solutions: ① Increase washing solvent strength (e.g., for reversed‑phase, raise methanol concentration from 5% to 10%) or increase washing volume. ② Filter the sample through a 0.22 μm filter before loading, or pre‑treat by liquid‑liquid extraction (e.g., with ethyl acetate) to remove some interferences in advance.
Main causes: ① Sample contains many suspended particles or precipitates that block the sorbent pores. ② Vacuum pump negative pressure too high, compacting the sorbent.
Solutions: ① Centrifuge the sample (5000 rpm, 5 minutes) and use the supernatant, or filter through a 0.45 μm filter before loading. ② Reduce vacuum pressure to 0.02–0.03 MPa to avoid sorbent compaction (compaction is irreversible; the column must be replaced).
Main causes: ① Inconsistent solvent flow rates across operations. ② Incorrect solvent volumes (e.g., using only 2 BV for activation instead of 3–5 BV). ③ Different soaking times during washing/elution.
Solutions: ① Use the vacuum manifold valve to precisely control flow rate, or use a pipette to push solvent at a constant rate. ② Strictly follow “3–5 BV” and measure solvents accurately with a graduated tube or pipette. ③ Keep soaking times consistent (e.g., fixed at 1 minute).
Main causes: ① Sample pH unsuitable, leading to degradation during extraction (e.g., acidic analyte in alkaline medium). ② Long storage after elution, causing oxidation or volatilisation.
Solutions: ① Adjust sample pH according to analyte properties before loading (e.g., acidic analytes to pH 2–3, basic analytes to pH 8–9). ② After elution, immediately concentrate and reconstitute, or add a stabiliser (e.g., vitamin C for easily oxidised substances; store volatile substances sealed at low temperature), and perform subsequent analysis as soon as possible.
IV. Application Scenarios of SPE Columns
When the correct procedure is mastered, SPE columns can be widely used for sample preparation in many fields:
Food testing: Remove pigments and proteins from honey; enrich veterinary drug residues (e.g., fluoroquinolones, sulfonamides) from chicken or fish.
Environmental monitoring: Enrich polychlorinated biphenyls (PCBs) and organophosphorus pesticides from river or lake water samples; extract heavy metals (e.g., Pb, Cd, Hg) from soils using chelating SPE columns.
Pharmaceutical field: Extract antibiotic or anticancer drug metabolites from serum or urine; purify traditional Chinese medicine extracts (e.g., astragaloside from Astragalus) by removing polysaccharides, tannins, and other interferences.
Agricultural product testing: Remove chlorophyll interferences from vegetables (e.g., spinach, lettuce); enrich patulin, carbendazim, and other mycotoxins and pesticide residues from fruits (e.g., apples, grapes).
