Modern Extraction: SPE, SPME and Accelerated Solvent Extraction

LLE did not fail, it stopped making economic sense. A Stas-Otto on 100 g of viscera consumed 300 to 500 mL of chloroform, took a working day, produced an emulsion, and still carried lipids into the chromatograph.

Four pressures pushed Indian labs forward. Solvent: an SPE cartridge for a urine screen uses about six mL of methanol and water, against the hundred-plus mL LLE demanded. CPCB hazardous waste rules and chlorinated solvent disposal cost made the economics obvious. Recovery: SPE recoveries for opioids, amphetamines and benzodiazepines sit between 80 and 95 percent on mixed-mode cartridges, against 50 to 75 percent for LLE. Cleanliness: matrix effects on electrospray are the biggest source of failed LC-MS/MS validation, and SPE removes phospholipids and salts that suppress ionisation. Automation: a 96-well SPE plate runs unattended overnight on a Hamilton or Tecan deck, which is how NDTL Delhi manages high-throughput anti-doping workloads.

The effect on LOD is real. LLE plus GC-MS for morphine in urine sat at roughly 50 ng/mL in the early 2000s. The same urine on Oasis MCX SPE plus LC-MS/MS at AIIMS Forensic now comes in at 1 to 2 ng/mL, easily inside SAMHSA and WADA cut-offs.

SPE is a chromatographic process in a single column. A bed of bonded silica or polymer sits in a polypropylene barrel between two frits. The cartridge goes on a vacuum manifold, sample is pulled through, washes follow, and the analyte is eluted.

Reverse-phase C18 is the default, retaining hydrophobic analytes from an aqueous load and releasing them on methanol. C8 and phenyl cover mid-polarity. The 1990s innovation was the mixed-mode sorbent: a reverse-phase backbone with cation- or anion-exchange groups on the same particle. Strata-X-C, Oasis MCX and Bond Elut Certify dominate Indian alkaline-drug clean-up, retaining basic drugs like morphine, methamphetamine, MDMA and tramadol by both hydrophobic and electrostatic binding and releasing them on ammoniated methanol that neutralises the protonated nitrogen. Mixed-mode anion exchange handles acidic drugs like barbiturates and salicylates.

1. 1. Condition
   Wet the sorbent with methanol, then equilibrate with water or aqueous buffer matched to the sample pH. The bed must not run dry between condition and load, or recovery drops sharply.
2. 2. Load
   Apply the prepared sample. Urine goes on with a phosphate buffer adjustment. Plasma is protein-crashed with cold acetonitrile, centrifuged, and the supernatant diluted with buffer before loading. Flow rate sits at 1 to 2 mL per minute.
3. 3. Wash
   Pull water or a weak organic wash (5 percent methanol, or a low-pH buffer for cation exchange) through the cartridge to remove salts, sugars and weakly retained matrix. Wash strength sits just below elution strength.
4. 4. Elute
   Switch to a clean tube. Methanol releases hydrophobic analytes from C18. Methanol with 2 to 5 percent ammonia releases alkaline drugs from mixed-mode cation exchange. Methanol with 2 percent formic acid releases acidic drugs from mixed-mode anion exchange. The eluate is dried under nitrogen and reconstituted in mobile phase.

The 96-well plate format runs ninety-six samples in parallel on a single manifold. NDTL Delhi runs anti-doping urines on 96-well Oasis MCX plates into LC-MS/MS, and CFSL Chandigarh adopted 96-well SPE for narcotic throughput around 2018. Indian manufacturers like Chromatopak and Genaxy now stock generic cartridges at a third of imported price, so the economic argument against SPE no longer applies.

SPME pushes the argument further. No solvent at all. A coated fused silica fibre, around one centimetre long, sits inside a steel syringe needle. The fibre is exposed to the sample or its headspace, analytes partition into the coating until equilibrium, the fibre is retracted, and the assembly goes into a heated GC injector for thermal desorption.

Coating chemistry decides application. PDMS is the non-polar default for volatiles like ethanol, methanol, chloroform and light hydrocarbons. Polyacrylate is the polar coating for phenols and anilines. DVB/CAR/PDMS gives a broad range and is the workhorse for hair drug work and fire debris.

Forensic uses cluster in three buckets. Blood alcohols: older Conway and Widmark methods have been displaced by headspace SPME plus GC-FID. The fibre sits over a sealed vial of blood with an internal standard, equilibrates at 60 degrees Celsius, and desorbs into the GC. Methanol, the killer in every hooch tragedy, runs on the same fibre. Arson debris: charred debris is warmed in nylon bags and a DVB/CAR/PDMS fibre traps petrol, kerosene or diesel from the headspace. CFSL Hyderabad, FSL Mumbai and the Maharashtra ATS forensic wing have standardised headspace SPME for arson. Hair: SPME from a digested hair matrix gives a clean extract for cocaine, opiates and cannabinoids without the lipid background LLE drags along, used at NIMHANS Bengaluru and AIIMS Forensic for chronic-exposure questions.

ASE attacks the solid matrices SPE and SPME cannot handle. Load the sample into a stainless steel cell, pressurise to around 1500 psi with the chosen solvent, heat to 100 to 200 degrees Celsius, and the solvent stays liquid above its boiling point. Higher temperature drops viscosity, raises diffusion and weakens analyte-matrix binding. A Soxhlet that needed eighteen hours and 250 mL on a 10 g sample collapses into a 10 to 20 minute cycle using 10 to 15 mL.

The Dionex ASE 350 is the instrument most Indian forensic labs run; the Buchi SpeedExtractor is the alternative at NIPER. Pesticide residue in adipose tissue, liver and kidney is the dominant use at AIIMS Forensic and FSSAI-affiliated state labs. PCBs in fat run on hexane-acetone ASE. Drugs in hair run on methanol ASE. ASE also handles bone marrow, brain tissue and embalmed viscera, usually followed by SPE clean-up.

Preparative TLC uses a thick silica layer (1 to 2 mm), accepts milligram loadings, and lets the analyst scrape a band and elute it for follow-on spectroscopy or LC-MS. Slow and manual, but it isolates intact natural products without the on-column degradation that some glycosides and labile alkaloids show on HPLC.

HPTLC is the quantitative form. Smaller silica particles give sharper bands. Automated band application by a Camag Linomat 5 places sample tracks of reproducible width, and a densitometric scanner gives a readout comparable to HPLC. NIPER Mohali, the Bose Institute and the CCRAS network use HPTLC for herbal characterisation and plant toxins in cases that cross from AYUSH into medico-legal.

Dried blood spot (DBS) sampling started as a neonatal screening tool in the 1960s and crossed into forensic toxicology in the last fifteen years. A finger-prick produces around 50 microlitres of capillary blood, spotted on Whatman 903 paper, air-dried and shipped at ambient temperature. A punched disc is extracted into methanol for LC-MS/MS. No venepuncture, no cold chain. AIIMS Forensic and Lady Hardinge have published DBS methods for paracetamol, valproate and common paediatric overdose drugs.

A modern extraction realises its sensitivity advantage only when QC runs alongside every batch. Spike recovery: a fortified blank matrix is taken through the full extraction, and recovery is calculated. Method recovery between 70 and 110 percent is the acceptance band. Deuterated internal standards: the per-deuterated form (d3-morphine, d5-amphetamine, d6-cocaine, d9-THC) is added at loading. It tracks every loss, matrix effect and ionisation suppression, and the response ratio gives accurate quantitation even when absolute recovery is variable. CRMs from Cerilliant, LGC and NPL India sit in every batch as the bench check.