ENVIRONMENTAL ANALYSIS I. Solid Phase Extraction fundamentals of SPE

SPE versus HPLC Elution Chromatography: V r = V m + KV s 0.2 << K << 20 Extraction Chromatography: K = > 1000 Retention K = < Elution [stat] [matrix] [stat] [eluent]

SPE versus HPLC v HPLC ä continuous elution v SPE ä on-the-phase retention ä off-the-phase digital elution v Phase reproducibility ä top complaint in both techniques

Comparison of SPE and HPLC

Phase Usage in SPE and HPLC

Definition of SPE Separation or removal of an analyte or analytes from a mixture of compounds by selective partitioning of the compounds between a solid phase (sorbent) and a liquid phase (solvent).

SPE Analyte Partitioning Immiscible Phases Immiscible Phases

Why Silica? v Controlled particle/pore size v Controlled surface area v Surface can be modified v Mechanical strength/rigidity v Chemical stability v Wide solvent compatibility

Silica Surface Variations SiOH SiOHO H H Si OH OH Free silanolAdsorbed water Geminal silanol SiloxaneBound silanols

The Silica Surface

Monochlorosilane Chemistry R 3 SiCl + O Si O Si O OHOH O Si O Si O OHO Si RR R

Trichlorosilane Chemistry RSiCl 3 H 2 O RSi(OH) 3 OSiOSiO R OH R OH + OH Si O Si OO Si O OHOH OHSi R O Si OO Si O Si O O SiHO R OSiO HO OH R

Silica Particles (1)

Silica Particles (2)

ISOLUTE Bonded Silicas  50  m, 60 Å ä Narrow particle size distribution ä Easy, Rapid Flow v 550 m 2 /g Surface Area ä High Capacity v Rigid Silica Particles ä Dimensionally Stable ä Rapid Equilibration ä Small bed (void) volume (1.2mL/g) v Sorbent ä Silica pretreatment ä Proprietary Bonded Phase Washing

Particle Size Distribution I S T ISOLUTE C18 (EC), BATCH NO Particle Size,  m Number of Particles Narrow range of particle sizes Negligible fines

Particle Size Distribution of another Manufacturer Particle Size,  m Number of Particles Broad range of particle sizes Significant fines

Impact of Fines When fines are present, smaller particles tend to agglomerate together To the right is a sorbent bed having a narrow particle size distribution

Impact of Fines The presence of fines results in poor flow characteristics through the column... versus even flow characteristics for a column without fines present

Poor Flow Characteristics Conditioning and equilibration solvents don’t saturate areas having a higher pressure drop... versus a well conditioned and equilibrated bed without fines present

Sample Loading Analytes are likely to channel through a bed with poor flow characteristics... versus being retained in a tight band

Drying Time The presence of fines results in longer drying times Versus a column where there is even gas flow

Drying Time, C18(UC)s ISTBrand ABrand B Drying Time, min Water, grams

Analyte Elution Greater solvent volumes are required to elute analytes... versus when solvent can flow through the bed evenly Larger elution volumes result in a less concentrated analyte and poorer detection limits

O OH Silica Surface

Porous Silica

O Si O O O O O O O O O O O O O O HO O O Si O HO Si O O O OH Ideal C18 Surface Ideal C18 Surface

Bonded Silica Surface Residual silanols Si O Si O Si O Si O S O Si Si O Si O Si OH S OH OH OH i i O O pH<3

Polar Secondary Interactions Si O Si O Si O Si O Si O Si Si O Si O Si OH Si O H O O H H O H NH 3 + O O pH<3

Ionic Secondary Interactions O Si O Si O Si O Si O Si O Si Si O Si O Si O Si O O-O- O pH>7 O H3NH3N +

Multiple Interactions Si O Si O Si O Si O Si O Si Si O Si O Si OH Si O H O O H H SO 3 - NH O O

SPE Mechanism Selection (1) NH 2 NH 3 + NH 2 Functionality AnalyteMechanism Hydrophobic H-Bonding Ionic Non-Polar Polar Ion-Exchange

Analyte Characteristics v Functional groups v Solubilities v pKa values... pKa Database v v Chromatographic behavior Hydrophobic H-Bonding CationicAnionic OH HN NH 2 SH NH 3 + NH 2 + NH + COO - S O O O - P O O - O -

Molecular Interactions

SPE Mechanism Selection (2) OH HN NH 2 SH N H 3 + NH 2 + NH + S O O O - P O O - O - NR 3 + Aqueous Non-Polar Solvent Aqueous (Low Ionic Strength) Aqueous (Low Ionic Strength) Hydrophobic H-Bonding Cations Anions Non-Polar Polar Cation Exchange Anion Exchange C18C8 C2CH PH CN SI 2OH NH2 CBA PRS SCX NH2SAX AnalyteMatrixSorbent