THC-COOH Urine Extraction Using the Empore™ Membrane C18 Cartridge
- 5 days ago
- 9 min read
Authors: Xiaohui Zhang, Guotao Lu, Michael Apsokardu
Study summary: A validated solid-phase extraction (SPE) workflow using the Empore™ 3 mL C18 membrane cartridge (Model 4215SD) extracts the THC metabolite 11-nor-9-carboxy-Δ⁹-tetrahydrocannabinol (THC-COOH) from urine for GC-MS quantitation. After alkaline hydrolysis (10 M KOH, 60°C, 15 min) and pH adjustment to 4.0–4.5, urine is loaded onto the cartridge by gravity, washed with 50:50 methanol/water, and eluted with 75:25 n-hexane/ethyl acetate. After MSTFA+1% TMCS derivatization, THC-COOH is quantified by GC-MS using THC-COOH-d3 as the internal standard. Average recovery at 20 ng spike level: 89.0% (RSD 4.0%, n=6); all recoveries from 15 ng to 300 ng spike levels were above 83%. Calibration linearity: R² = 0.9998.
TL;DR: THC-COOH urine extraction is the standard for forensic cannabis confirmation, workplace drug screening, and DUI testing — but the glucuronide-conjugated metabolite requires hydrolysis, sensitive SPE, and GC-MS derivatization. The Empore™ 3 mL C18 (4215SD) membrane cartridge handles all three: 89% average recovery at the 20 ng level, all spike levels 15–300 ng above 83%, and a calibration R² = 0.9998. The cartridge's 8-layer pre-filter and bonded membrane design eliminate channeling and particulate shedding — common pain points in traditional loose-packed SPE forensic workflows.
Study at a Glance
Introduction: Cannabis Urine Testing Demand
The cannabis market is undergoing rapid expansion across regulated medical and recreational segments, and with that growth comes an increasing need for purified extracts and accurate quantitation of tetrahydrocannabinol (THC) and its metabolites. Solid-phase extraction (SPE) is one of the standard sample preparation methods for achieving both purification and concentration goals in THC analysis.
Empore™ is a membrane-type SPE that offers a number of advantages over traditional loose-packed SPE products. In the Empore™ membrane, sorbent particles are trapped in a bed of inert polymer matrix. Empore™ SPE cartridges specifically are made with a sealing ring that secures the SPE membrane to the bottom of a medical-grade polypropylene barrel. On top of the SPE membrane sits an 8-layer pre-filter, composed of polypropylene microfiber layers with different pore sizes.
Why this design matters for forensic urine analysis:
No channeling effects — uniform flow through the membrane gives reproducible recovery sample-to-sample
No shedding of fine particulates — eliminates a common source of GC-MS chromatographic interference and ion-source contamination in trace forensic analysis
Built-in 8-layer pre-filter — handles the colloidal and particulate load typical of post-hydrolysis urine matrices without bridging or clogging
In this work, a 3 mL C18 (Model 4215SD) Empore™ membrane SPE cartridge was used to extract the major urinary THC metabolite, 11-nor-9-carboxy-Δ⁹-tetrahydrocannabinol (THC-COOH), from urine. THC-COOH was then quantified by GC-MS following silylation with MSTFA + 1% TMCS.
Why Empore™ 4215SD for THC-COOH Urine Extraction
Forensic urine drug testing has demanding requirements: high reproducibility for chain-of-custody defensibility, clean extracts for low-level GC-MS confirmation, and a workflow that scales to high-volume laboratories. The Empore™ 4215SD cartridge addresses these directly:
Membrane format eliminates SPE-derived variability — compared to loose-packed cartridges where particle settling and bed packing affect recovery sample-to-sample, the Empore membrane gives uniform analyte retention from cartridge to cartridge
Medical-grade polypropylene barrel — minimizes contamination risk and ion-source background relevant to trace toxicology work
8-layer pre-filter handles hydrolyzed urine matrix — post-hydrolysis urine often contains colloidal protein and lipid debris that clogs traditional SPE; the integrated pre-filter handles this without a separate filtration step
Validated for the regulated THC-COOH workflow — recovery consistently above 83% across the 15–300 ng spike range, with R² = 0.9998 calibration linearity
Available through major scientific distributors — Fisher Scientific (13-110-013), VWR (76333-122), and Millipore-Sigma (66872-U) for rapid procurement by forensic and clinical labs
Materials
Reagents
THC-COOH standard solution — 100.0 ± 0.6 µg/mL (Sigma-Aldrich)
THC-COOH-d3 internal standard solution — prepared in methanol at 1,000 ng/mL
MSTFA + 1% TMCS — silylation/derivatization reagent (Sigma-Aldrich)
Artificial urine — pH 5.7, 500 mL (Fuzhou Feijing Biotechnology Co., Ltd.)
Ethyl acetate, methanol, n-hexane — chromatographically pure (Thermo Fisher Scientific)
Potassium hydroxide (KOH) — 10 M solution, for hydrolysis
Glacial acetic acid — for pH adjustment to 4.0–4.5
All reagents analytical grade or higher
SPE Cartridge
Empore™ Membrane SPE Cartridge — 3 mL C18 (CDS Analytical)
Fisher Scientific PN: 13-110-013
VWR PN: 76333-122
Millipore-Sigma PN: 66872-U
Instrument
Agilent 7890B–5977B GC-MS (Agilent Technologies)
GC-MS Parameters
Injection & Column
Oven Temperature Program
Mass Spectrometer Quantitation Ions
Working & Calibration Solutions
Standard Working Solution
THC-COOH standard working solution was prepared in methanol at 100 ng/mL. The THC-COOH-d3 internal standard solution was prepared in methanol at 1,000 ng/mL.
5-Level Calibration Curve
Different volumes of THC-COOH working solution were added to six 20 mL vials such that the THC-COOH mass per vial was 3 ng, 15 ng, 30 ng, 60 ng, and 300 ng. To each vial, 50 µL of internal standard solution was added, the solutions were dried under a gentle stream of nitrogen at 40 °C, and then 100 µL of MSTFA + 1% TMCS derivatization reagent was added. Derivatization was carried out at 60 °C for 20 minutes. Each solution was transferred to a GC vial and diluted to 1.0 mL with n-hexane for GC-MS analysis.
Calibration linearity: The 5-level calibration curve from 3 ng to 300 ng yielded R² = 0.9998, demonstrating excellent linearity across two orders of magnitude — suitable for both screening and confirmation forensic workflows.
Validated Extraction Procedure
1. Sample Preparation & Hydrolysis
Place each 3 mL urine sample in a 20 mL sample tube. Add the appropriate volume of THC-COOH standard solution as the spike (validation used spike masses from 15 ng to 300 ng). Add 300 µL of 10 M potassium hydroxide solution to each sample, then hydrolyze in a water bath at 60 °C for 15 minutes. Remove from the water bath and cool to room temperature. Adjust the pH to 4.0–4.5 with glacial acetic acid.
⚠ Why hydrolysis is essential: The majority of urinary THC-COOH is excreted as the glucuronide conjugate (THC-COOH-glucuronide), which does not retain efficiently on C18 SPE. Alkaline hydrolysis with 10 M KOH cleaves the glucuronide bond and releases free THC-COOH for SPE recovery. Skipping or abbreviating this step will dramatically underestimate true THC-COOH levels in urine.
2. SPE Cartridge Activation
Pre-activate the Empore™ C18 (4215SD) cartridge with 150 µL methanol.
3. Sample Loading
Load the hydrolyzed, pH-adjusted urine sample onto the activated cartridge. The sample is allowed to flow down by gravity — no vacuum or positive pressure is required for the loading step.
4. Wash
Rinse the cartridge with 750 µL of methanol/water solution (50:50).
5. Drying
Dry the cartridge under vacuum for 5 minutes.
6. Elution
Elute with 800 µL of n-hexane/ethyl acetate solution (75:25 v/v).
7. Concentration & Derivatization
Concentrate the eluate under a gentle stream of nitrogen at 40 °C until dry. Add 100 µL of MSTFA + 1% TMCS derivatization reagent, then heat at 60 °C for 20 minutes. Transfer the derivatized solution to a GC vial and dilute to 1.0 mL with n-hexane.
8. GC-MS Analysis
Inject onto the GC-MS using the parameters listed in the GC-MS Parameters section above. Quantify THC-COOH against the 5-level calibration curve using THC-COOH-d3 as the isotope-dilution internal standard.
Recovery Validation Results
Recovery validation was performed at two separate study designs: (1) a high-replicate study at a single 20 ng spike level (n=6) to assess intra-batch precision, and (2) a multi-level study from 15 ng to 300 ng (n=2 per level) to assess recovery across the calibration range.
Table 2 — Spike Recovery at 20 ng Level (n = 6 replicates)
Table 3 — Spike Recovery Across 15–300 ng Range (n = 2 replicates per level)
Validation outcome: Across both study designs and all spike levels from 15 ng to 300 ng, individual THC-COOH recoveries were > 83% — satisfactory for routine forensic toxicology and cannabis urine testing applications.
Where This Method Fits
The validated 89% average recovery, >83% all-level recovery, and R² = 0.9998 calibration make this Empore 4215SD cartridge method applicable across the full spectrum of regulated urine cannabinoid testing:
DUI / driving-under-the-influence cannabis confirmation — confirmation testing for THC-positive DUI screens, supporting court-defensible chain-of-custody workflows
Workplace drug screening — regulated and non-regulated employer drug testing, including safety-sensitive occupations and pre-employment screening
U.S. DOT-mandated transportation drug testing — Department of Transportation programs covering commercial drivers, pilots, train operators, pipeline workers, and maritime crew
Drug court & probation monitoring — court-ordered urine testing for cannabis abstinence verification
Substance abuse treatment compliance — addiction recovery program monitoring and clinical substance use disorder verification
Military drug testing — armed forces routine and for-cause cannabis screening
Clinical toxicology — emergency department, pain management clinic, and addiction medicine cannabis use assessment
Conclusion
A simple and effective method for the enrichment and extraction of the tetrahydrocannabinol metabolite THC-COOH from urine has been demonstrated using the Empore™ 4215SD 3 mL C18 membrane solid-phase extraction cartridge.
THC-COOH-spiked urine samples were processed through hydrolysis, SPE purification, concentration, and derivatization, then quantified by GC-MS with isotope-dilution against THC-COOH-d3 internal standard. Validation achieved:
89.0% average recovery at the 20 ng spike level (RSD 4.0%, n = 6)
All recoveries > 83% across spike levels from 15 ng to 300 ng
R² = 0.9998 linearity across the 5-level calibration curve (3–300 ng)
Clean GC-MS chromatograms suitable for routine forensic confirmation
These results demonstrate that the Empore™ 4215SD C18 cartridge is suitable for the extraction and purification of THC-COOH in urine, supporting forensic toxicology, workplace drug screening, DUI confirmation, drug court monitoring, clinical toxicology, and other regulated cannabinoid testing applications.
Frequently Asked Questions
What is THC-COOH and why is it the target metabolite for urine drug testing?
THC-COOH (11-nor-9-carboxy-Δ⁹-tetrahydrocannabinol) is the major inactive metabolite of THC excreted in urine. Because it persists in urine for days to weeks after cannabis use — much longer than parent THC itself — it is the standard target analyte for forensic urine drug testing, workplace drug screening, and DUI cannabis confirmation. Unlike active THC, THC-COOH does not produce psychoactive effects, but its presence and concentration provide reliable evidence of past cannabis exposure.
What recovery does the Empore C18 cartridge achieve for THC-COOH in urine?
At a 20 ng spike level with n=6 replicates, the Empore C18 (4215SD) cartridge achieved an average THC-COOH recovery of 89.0% with an RSD of 4.0%. Across additional spike levels from 15 ng to 300 ng, all individual recoveries exceeded 83%. The 5-level calibration curve from 3 ng to 300 ng showed excellent linearity (R² = 0.9998), confirming the method is suitable for routine forensic urine analysis.
Why does THC-COOH urine extraction require alkaline hydrolysis with KOH first?
Most THC-COOH in urine exists as a glucuronide conjugate (THC-COOH-glucuronide) rather than the free acid. Direct SPE extraction would miss the conjugated fraction. Alkaline hydrolysis with 10 M KOH at 60°C for 15 minutes cleaves the glucuronide bond, releasing free THC-COOH for SPE retention. After hydrolysis, the sample is cooled and the pH adjusted to 4.0–4.5 with glacial acetic acid to put the analyte in its protonated form for optimal C18 retention.
What is the Empore 4215SD cartridge and how does it differ from traditional SPE?
The Empore 4215SD is a 3 mL C18 membrane SPE cartridge in which sorbent particles are trapped within an inert polymer matrix, with a sealing ring securing the membrane to the bottom of a medical-grade polypropylene barrel. On top of the SPE membrane is an 8-layer pre-filter composed of polypropylene microfiber layers with different pore sizes. This design eliminates channeling effects and prevents shedding of fine particulates — both common limitations of traditional loose-packed SPE cartridges that can cause poor reproducibility and chromatographic interferences in trace forensic analysis.
Why is derivatization with MSTFA + 1% TMCS required for GC-MS analysis of THC-COOH?
THC-COOH has a free carboxylic acid (-COOH) group and a free phenolic hydroxyl group, both of which are highly polar and would produce poor GC peak shape, low sensitivity, and possible degradation. MSTFA (N-Methyl-N-trimethylsilyl-trifluoroacetamide) with 1% TMCS catalyst silylates these polar groups, converting them to TMS-derivatives. The resulting TMS-THC-COOH is volatile, stable, and produces sharp Gaussian peaks ideal for GC-MS quantitation. The validation in this work used 100 µL of MSTFA + 1% TMCS at 60 °C for 20 minutes.
What is the Empore 4215SD part number across major distributors?
The Empore 4215SD 3 mL C18 membrane SPE cartridge is available through multiple distributor channels:
CDS Analytical model number: 4215SD
Fisher Scientific PN: 13-110-013
VWR PN: 76333-122
Millipore-Sigma PN: 66872-U
The cartridge is the same product across all distributor SKUs.
What deuterated internal standard is used for THC-COOH quantitation?
THC-COOH-d3 (deuterated THC-COOH with three deuterium atoms) is used as the internal standard for isotope-dilution quantitation. In this validation, THC-COOH-d3 was prepared in methanol at 1,000 ng/mL, and 50 µL was added to each calibration vial. The TMS-derivative of THC-COOH-d3 has qualifier and quantifier ions at m/z 374, 476, and 491, with a retention time of 12.09 minutes under the GC conditions used.
What forensic and cannabis-testing applications does this method support?
The Empore 4215SD method supports a range of forensic and cannabis-related urine drug testing applications: DUI / driving-under-the-influence cannabis confirmation, workplace drug screening, federal DOT-mandated transportation drug testing, drug court and probation monitoring, substance abuse treatment compliance, military drug testing, and clinical toxicology cannabis use assessment. The 89% average recovery and >83% recovery across all spike levels make it suitable for both screening and confirmation laboratory workflows.
References
Marilyn, et al. "Blood Cannabinoids. I. Absorption of THC and Formation of 11-OH-THC and THCCOOH During and After Smoking Marijuana." Journal of Analytical Toxicology, 1992.
Giroud, C., et al. "Delta(9)-THC, 11-OH-Delta(9)-THC and Delta(9)-THCCOOH plasma or serum to whole blood concentrations distribution ratios in blood samples taken from living and dead people." Forensic Science International, 2001, 123(2-3): 159.
Zhang X., Lu G., Apsokardu M. CDS Analytical Application Note #254: Extraction of THC Metabolites in Urine using the Empore™ Membrane C18 Extraction Cartridge. CDS Analytical, LLC.
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