Module 6 · Classes of poisons
Acidic, Neutral and Alkaline Drug Analysis
How drugs are partitioned by pKa across acidic, neutral and alkaline fractions before chromatographic analysis, with the Indian DFC case patterns that drive this workflow.
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How drugs are partitioned by pKa across acidic, neutral and alkaline fractions before chromatographic analysis, with the Indian DFC case patterns that drive this workflow.
The Indian SFSL bench separates drugs by acid-base chemistry before any instrument is switched on, and the dividing variable is pKa. A drug with a pKa below 7 is mostly un-ionised at acid pH and pulls cleanly into ether from a pH 2 to 3 aqueous. A drug with a pKa near 7 sits balanced at physiological pH and pulls into a moderately polar solvent at pH 7 to 9. A drug with a pKa above 7.5 is un-ionised only when the aqueous is basified to pH 9 to 10, and partitions into chloroform-isopropanol or hexane-ethyl acetate at that point. Three pH-controlled pulls on the same sample produce three analytically clean fractions, and a senior chemist at FSL Madhuban or CFSL Chandigarh can tell from the colour and the colour-spot result of each fraction roughly which drug class the case belongs to before LC-MS/MS confirmation.
The contrarian point is that this pKa logic is not a textbook abstraction. It is the chemistry running silently inside every SPE cartridge sold for clinical and forensic toxicology, the chemistry behind the buffer choice for any reversed-phase LC method, and the chemistry that decides why a single autopsy blood sample needs three separate aliquots if the analyst is screening for a barbiturate, a benzodiazepine and an opioid in one case. The drug-facilitated crime caseload at Indian SFSLs has shifted hard towards alprazolam in metro date-rape, zolpidem in domestic violence, datura on the Mumbai-Pune and Delhi-Agra highways, and dextromethorphan abuse from cough syrups like Phensedyl and Corex. Each of these falls into a different pKa bracket, and the fraction logic is what keeps the screening tractable.
Why the same drug pulls into ether at pH 2 and into chloroform at pH 10.
Every organic drug carries one or more ionisable groups. A weakly acidic group like the carboxylic acid in salicylic acid sits in its protonated HA form at low pH and in its anionic A minus form at high pH. A weakly basic group like the tertiary amine in morphine sits in its protonated BH plus form at low pH and in its free base B form at high pH. The un-ionised form is lipophilic and partitions readily into organic solvent. The ionised form is hydrophilic and prefers the aqueous. The pKa is the pH at which the two forms are present in equal concentration, and the Henderson-Hasselbalch equation gives the ratio at any other pH.
The rule the bench actually uses is simple. Bring the aqueous to two pH units below the pKa for an acid (un-ionised, extractable) or two units above the pKa for a base. For a barbiturate with pKa near 7.5, pH 2 to 3 puts the drug in its HA form and ether pulls it out cleanly. For an opioid with pKa near 8, pH 9.5 to 10 puts the drug in its free base form and chloroform pulls it out cleanly. For a benzodiazepine with pKa near 3 in the acidic and around 11 in the basic group, the molecule is best extracted at a near-neutral pH where neither group is fully ionised, which is why the neutral fraction exists at all.
Example drugs and the pKa values that decide which fraction they fall into.
| Acid | pH 2 to 3 | Diethyl ether or chloroform | Phenobarbital, pentobarbital, thiopental, aspirin, salicylic acid, ibuprofen, diclofenac, paracetamol, glutethimide, phenol, cresols | 3.0 to 9.5 (acidic groups dominate the partition behaviour at pH 2-3) |
| Neutral | pH 7 to 9 | Methyl tert-butyl ether, ethyl acetate or toluene-hexane | Diazepam, alprazolam, nitrazepam, lorazepam, clonazepam, midazolam, zolpidem, zaleplon, carbamazepine, phenytoin, methadone, dextropropoxyphene | 2.4 to 11.3 (weakly basic or amphoteric, neutral at physiological pH) |
| Alkaline | pH 9 to 10 | Chloroform-isopropanol (9:1) or hexane-ethyl acetate (7:3) | Morphine, codeine, heroin, 6-MAM, fentanyl, tramadol, methadone, amitriptyline, nortriptyline, imipramine, fluoxetine, sertraline, chlorpromazine, haloperidol, olanzapine, amphetamine, methamphetamine, MDMA, cocaine, LSD, atropine, scopolamine, quinine, nicotine | 7.6 to 10.4 (basic nitrogen, free base at pH 9-10) |
The acid fraction is dominated by sedative-hypnotic barbiturates and the analgesic-antipyretic group. Phenobarbital (pKa 7.4) is the workhorse anticonvulsant in Indian government neurological practice, the cheap legacy drug that still turns up in elderly suicides. Pentobarbital (pKa 8.1) and thiopental (pKa 7.6) appear in legacy euthanasia and veterinary contexts. Salicylates land here as aspirin hydrolysed in plasma to salicylic acid (pKa 3.0), and the Trinder reaction with ferric chloride gives the characteristic violet at the bench. NSAIDs like ibuprofen and diclofenac, the most prescribed analgesic combination in Indian outpatient practice, partition cleanly at pH 2.5. Paracetamol (pKa 9.5) partitions into the acid fraction despite its alkaline-leaning pKa because its phenolic group is weakly acidic and its low molecular polarity favours organic at pH 2.5. Above 150 mg per kg paracetamol is hepatotoxic, and the autopsy picture is fatty liver with centrilobular necrosis.
Alprazolam in tea, datura on the highway, zolpidem at home, cough syrup at the bus stop.
The Indian DFC caseload has its own signature. Metro SFSLs in Delhi, Mumbai, Bengaluru and Hyderabad see alprazolam dropped into tea, coffee or soft drinks in date-rape cases. The drug works because it dissolves invisibly, has no marked taste at low concentration, and produces anterograde amnesia at doses below 1 mg. The victim remembers being at a coffee shop or in a hotel room and then nothing until the next morning, and a sexual assault complaint follows. The toxicology requisition typically arrives 12 to 48 hours after the event. Blood is the more reliable matrix when collection is prompt; hair becomes the matrix of choice for delayed reporting.
Highway robbery cases on the Mumbai-Pune NH48, the Delhi-Agra NH19 and the Bengaluru-Chennai NH48 stretch follow a different pattern. The drug is datura, the alkaloid is a mixture of atropine and scopolamine and hyoscyamine, and the vehicle is a dosa or a lassi or a cigarette offered to a tired driver at a roadside dhaba. The victim is rendered confused, amnesic and physically compliant within 20 to 40 minutes, robbed of cash and valuables, and dumped in the car off the highway shoulder. The clinical picture is the anticholinergic toxidrome: dilated pupils, dry hot skin, tachycardia, urinary retention, confusion and amnesia. Toxicology runs the gastric aspirate and urine through alkaline-fraction extraction, and atropine and scopolamine confirm on LC-MS/MS.
Zolpidem domestic violence and intra-family rape cases are a quieter but growing category. The drug is slipped into a glass of milk at bedtime and the alleged assault occurs during the 6 to 8 hour sleep window. Most state SFSLs do not yet run dedicated zolpidem methods and escalate to CFSL or AIIMS.
Dextromethorphan and codeine abuse from cough syrups is the fourth pattern. Phensedyl and Corex were the two leading codeine syrups before the 2016 ban on the codeine-promethazine combination, but generic formulations re-emerged and smuggling routes from Bangladesh continue. The toxicology picture is positive codeine and morphine, with dextromethorphan and dextrorphan when the brand contains it.
Immunoassay on the rapid bench, LC-MS/MS on the confirmation bench.
A modern Indian SFSL workflow starts with an immunoassay screen and ends with a mass-spectrometric confirmation. The screen is a hospital ER drug-of-abuse panel run on EMIT, FPIA or CLIA platforms covering opiates, amphetamines, cocaine metabolite, benzodiazepines, barbiturates and tricyclics. Sensitivity is good (200 to 1000 ng/mL in urine) and the result is available within an hour. The problem is cross-reactivity. A benzodiazepine immunoassay calibrated to oxazepam will under-detect alprazolam and clonazepam, and a positive opiate result does not distinguish morphine from codeine from heroin metabolite.
Confirmation is by LC-MS/MS with two MRM transitions and an isotopically labelled internal standard. The Indian SFSL routine for an opioid panel uses morphine-d3, codeine-d3 and 6-MAM-d3 as internal standards, and reports concentration against a six-point calibration curve. SAMHSA-style cut-offs commonly used at Indian SFSLs are morphine 2 ng/mL serum or 300 ng/mL urine, cocaine 5 ng/mL serum, amphetamine and methamphetamine 10 ng/mL serum, benzodiazepines 5 ng/mL serum (often pushed to 1 ng/mL for DFC casework), barbiturates 100 ng/mL serum.
Colour spot tests remain on the legacy bench at several Indian SFSLs as a fast presumptive class identification on the alkaline fraction residue. Marquis reagent gives a purple to violet colour with opioids, orange to brown with amphetamines, and red-orange with mescaline. Mecke gives green to blue with opioids. Mandelin gives green with amphetamines, blue with methadone and orange-brown with ketamine. Forrest reagent gives red with tricyclic antidepressants. The colour tests are not confirmatory, but they steer the analyst to the right LC-MS/MS method within 30 seconds.
The Indian institutional landscape has CFSL Chandigarh, Hyderabad and Pune at the top tier, NDTL Delhi for WADA-accredited anti-doping, AIIMS Forensic Medicine for medico-legal toxicology, FSL Madhuban, FSL Kalina, and the state SFSLs in Lucknow, Bhopal, Kolkata, Bengaluru and Thiruvananthapuram, all running LC-MS/MS DOA panels. NDPS Act 1985 prosecutions, particularly Section 21 for trafficking and Section 27A for financing, drive much of the casework.
Why a single autopsy blood concentration is not enough for tricyclics.
A post-mortem toxicology report on a tricyclic antidepressant is interpretive as much as analytical. Amitriptyline, nortriptyline, imipramine and the other lipophilic basic drugs in the alkaline fraction undergo post-mortem redistribution. After death the equilibrium between drug bound in organ stores (liver, lung, myocardium) and drug free in blood breaks down. The bound drug leaches out and concentrates in the central blood compartment. A blood sample drawn from the heart cavity at autopsy can read two to ten times higher than a peripheral femoral blood sample from the same body.
The correction is to draw both. The Indian standard autopsy protocol for a suspected drug overdose specifies cardiac and peripheral (femoral vein) blood as separate aliquots. The toxicologist reports both and the cardiac-to-peripheral ratio. A ratio close to 1.0 indicates minimal redistribution. A ratio of 3 to 10 indicates substantial redistribution and the peripheral value is used for cause-of-death interpretation. Amitriptyline is the textbook example, with reported cardiac-to-peripheral ratios up to 8 to 10. Methadone and propoxyphene also redistribute heavily.
Vitreous humour offers a redistribution-resistant matrix. The vitreous is anatomically isolated from the blood circulation, equilibrates slowly with blood, and is clear and protein-poor. Vitreous concentrations are usually lower than blood but the ratio is more stable. An Indian lab interpreting a suspected tricyclic overdose increasingly asks for peripheral blood, cardiac blood and vitreous humour as a three-matrix panel.
Where the fraction logic actually fails and how the chemist recovers.
The acid fraction has two recurring problems. The first is paracetamol, which despite its pKa of 9.5 partitions well into the acid fraction because of its low molecular polarity. A chemist who assumes paracetamol must be in the alkaline fraction will miss it. The second is the volatility of ether. Diethyl ether evaporates rapidly, the volume in the separating funnel drops between shakes, and the bench fix is to use the funnel quickly, vent often, and keep the ether bottle in a cold-water bath until use.
The neutral fraction has the opposite problem. Benzodiazepines and zolpidem are not particularly soluble in any single solvent at neutral pH, and recovery depends on the choice of organic. Methyl tert-butyl ether (MTBE) gives the best general benzodiazepine recovery. Ethyl acetate works for the more lipophilic family members. The bench rule is MTBE first-line, ethyl acetate as second-line; never substitute chloroform here because it partitions badly at neutral pH for these analytes.
The alkaline fraction problem is the emulsion. A blood or liver homogenate basified with NH4OH and shaken with chloroform-isopropanol will emulsify in nearly every case, and the emulsion can take 30 to 60 minutes to break by gravity. The bench fix is the ammonium sulphate variant of the Stas-Otto method: saturate the alkaline aqueous with solid (NH4)2SO4 before adding chloroform, wait 30 minutes for protein precipitation, and decant the clear supernatant. This single step is the difference between 30 percent and 70 percent recovery for diazepam, amitriptyline and quinine in a liver homogenate.
The colour test failure mode is reagent age. Marquis loses its formaldehyde within a few months and an older reagent gives false negatives. The bench rule is to prepare Marquis fresh every quarter, store in a dark bottle, and run a positive control every working day.
A drug with pKa 8 is to be extracted from a blood sample. At which pH does the drug partition most efficiently into chloroform?
The neutral fraction is the benzodiazepine fraction. Diazepam (pKa 3.4) was the classical Roche Valium, still on Indian shelves and still abused. Alprazolam carries two pKa values (2.4 acidic and 11.3 basic), making it amphoteric and best extracted at pH 7 to 8. Alprazolam is the single most common DFC drug in Indian metro date-rape and unsuspected-administration cases. Nitrazepam (pKa 3.2 and 10.8), lorazepam, clonazepam and midazolam complete the benzodiazepine panel. Zolpidem (pKa 6.2) is the Z-hypnotic with a strong DFC footprint in domestic violence and intra-family rape cases. The neutral fraction also catches carbamazepine and phenytoin, the two anticonvulsants most likely to overshoot the therapeutic window in non-compliant epileptic patients.
The alkaline fraction is the largest by analyte count and carries the basic drugs that dominate Indian forensic drug casework. The opioid family runs from morphine (pKa 8.0 and 9.9) and codeine (8.2) through heroin (7.6) and its primary metabolite 6-monoacetylmorphine (8.1, the analyte that proves heroin specifically) to the synthetic opioids fentanyl (8.4), tramadol (9.4) and methadone (9.2). Tricyclic antidepressants amitriptyline (9.4), nortriptyline, imipramine and clomipramine appear in suicidal overdose, and their cardiotoxicity makes them clinically important. SSRIs fluoxetine (10.0) and sertraline are the modern replacement. Antipsychotics chlorpromazine (9.3), haloperidol (8.3), olanzapine and risperidone come up in psychiatric casework. Amphetamines (10.0), methamphetamine (10.4) and MDMA (10.4) are the ATS class. Cocaine (8.7) and its metabolite benzoylecgonine appear in metro recreational casework. LSD (7.8), atropine and scopolamine (both around 9.7), quinine and nicotine complete the alkaline panel.