[Hawach Scientific] SAX SPE Cartridge

[Hawach Scientific] SAX SPE Cartridge

 

 

Sorbent mass	Reservoir	Best for	Cat#
30 mg	1 mL	Method development, trace bioanalysis (low sample load)	15011089
100 mg	1 mL	Routine urine/plasma screening (1-2 mL sample)	15011090
200 mg	3 mL	Standard bioanalysis, moderate matrix load	15011091
500 mg	3 mL	Higher loading, dirty matrices (e.g., plasma, wastewater)	15011092
200 mg	6 mL	Low loading but larger reservoir (automation)	15011093
500 mg	6 mL	General purpose, environmental water (100-500 mL)	15011094
1 g	6 mL	High loading, large-volume water (>500 mL), dirty samples	15011095
1 g	12 mL	Large-volume biological fluids, preparative cleanup	15011096
2 g	12 mL	Very high loading, industrial scale	15011097

 

 

SAX SPE cartridges are strong anion exchange sorbents based on quaternary ammonium bonded silica, designed to selectively retain acidic and anionic compounds from biological and environmental samples.

The permanently charged quaternary amine group provides consistent, high-capacity anion exchange to selectively retain deprotonated acidic compounds. SAX is widely used for extracting acidic drugs, metabolites, carboxylic acids, sulfonates, phosphates, and other anionic analytes from urine, plasma, serum, and water – where reversed-phase sorbents like C18 may exhibit poor retention for ionized acidic compounds.

When to use SAX

• Your target analytes are acidic (carboxylic acids, sulfonic acids, phenols with pKa < 7)
• You need high and reproducible recovery from biological fluids or aqueous samples
• Reversed-phase gives poor retention or matrix interference due to ionization

SAX SPE Retention Mechanism

The primary retention mechanism of SAX is electrostatic (ionic) interaction. The positively charged quaternary ammonium group binds to negatively charged analytes (deprotonated carboxylic acids, sulfonates, phosphates). Under typical loading conditions (neutral to basic pH 6-8), acidic compounds are fully deprotonated and strongly retained.

Unlike weak anion exchangers (WAX), SAX remains fully ionized across the entire pH range (0-14) due to the quaternary amine structure (pKa > 14). This allows method development without concern for pH-dependent charge loss.

Key point: To ensure retention, the sample pH must be at least 2 units above the pKa of your acidic analyte. For carboxylic acids (pKa 4-5), load at pH 7-8.

SAX vs WAX vs C18 – Which One Fits Your Application?

SAX is preferred for strong acidic analytes, WAX for weak acids requiring pH-dependent selectivity, and C18 for neutral compounds.

Sorbent	Well – suited for	Primary Interaction	Limitation
SAX (quaternary amine)	Strongly acidic compounds (sulfonates, phosphates, carboxylic acids)	Pure ionic (permanently charged)	May retain neutral compounds via hydrophobic interaction in high-organic conditions
WAX (weak anion exchange)	Weakly acidic compounds (phenols, enols)	pH-dependent ionic	Requires careful pH control; loses charge at high pH
C18 (reversed-phase)	Neutral and non-polar compounds	Hydrophobic only	Poor retention for charged acidic drugs

Selection guide:
– Choose SAX for strong acidic analytes (e.g., sulfonated dyes, phosphorylated metabolites, carboxylic acid drugs).
– Choose NH2 for weak acids where you want to retain only deprotonated species and wash away neutrals.
– Choose C18 for neutral compounds or when your acidic analytes are not the primary target.

SAX SPE Cartridge Applications by Industry

Industry / Segment	Typical Matrices	Target Analytes
Pharmaceutical bioanalysis	Plasma, serum, urine	Acidic APIs, drug metabolites (e.g., NSAIDs, statins, barbiturates), glucuronides
Clinical toxicology	Urine, blood	Acidic drugs of abuse (e.g., THC-COOH), barbiturates, salicylates
Environmental water testing	Groundwater, surface water, wastewater	Anionic pesticides (e.g., glyphosate, 2,4-D), perfluorinated compounds (PFAS), phosphonates
Food safety residue analysis	Meat, milk, honey	Acidic veterinary drug residues (e.g., tetracyclines, quinolones), mycotoxins
Industrial & chemical	Process streams, wastewater	Sulfonated dyes, organic acids, anionic surfactants

Typical Analytical Workflows

• Pharmaceutical bioanalysis– Extraction of acidic APIs and metabolites from plasma for PK/PD studies
• Clinical & forensic toxicology– Drugs of abuse (THC-COOH, barbiturates) in urine, blood
• Environmental monitoring– Acidic pesticides, PFAS, phosphonates in water
• Food safety– Veterinary drug residues (e.g., tetracyclines, sulfonamides)
• Impurity profiling– Acidic degradants and by-products in drug substance

SAX SPE Workflow (Step-by-Step)

Goal: Retain acidic compounds, remove neutrals, elute with acidic or high-ionic-strength solvent.

Step	Solvent	Purpose	Critical tip
Condition	Methanol – water or dilute buffer (pH 6-8)	Activate sorbent	Do not let cartridge dry after conditioning
Load	Sample adjusted to pH 6-8 (≥ pKa + 2)	Retain deprotonated acidic analytes	Verify pH with meter; avoid high organic content
Wash	Water or dilute buffer (same pH)	Remove neutral interferences	For dirty samples, add 5-10% methanol to wash
Elute	2% formic acid in methanol, or 0.5 M ammonium acetate in 95% methanol/5% water	Displace anions, recover analytes	Collect eluate in low-retention tubes; may need evaporation for LC-MS

Common failure points:
– Loading pH too low – analyte not deprotonated – breakthrough
– Wash too aggressive (high organic) – loss of weak acids
– Elution solvent too weak (pure methanol) – incomplete recovery

Troubleshooting SAX SPE Problems

Problem	Likely Cause	Solution
Low recovery	Loading pH too low (analyte not charged); insufficient sorbent; elution solvent too weak	Increase sample pH to ≥ pKa + 2; use larger sorbent mass; elute with 2% formic acid in methanol or 0.5 M ammonium acetate in methanol
Breakthrough during load	Insufficient conditioning; flow rate too high; sorbent overloaded	Re-condition with methanol and re-equilibrate; reduce flow rate (≤5 mL/min); use larger sorbent mass
Dirty extracts / high background	Wash step too weak (interferences not removed)	Increase wash strength (e.g., add 5-10% methanol to wash); add an additional wash with 2% ammonium hydroxide in water for basic interferences
Ion suppression in LC – MS/MS	Matrix overload; co-elution of neutral interferences	Reduce sample equivalent load; add a mixed-mode wash before elution (e.g., 2% ammonium hydroxide in 20% methanol)
Poor reproducibility	Inconsistent conditioning or pH variation	Standardize conditioning steps; verify sample pH with meter; use certified SAX cartridges with batch-specific COA