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  • Bulletin 910

    T197910 1998 Sigma-Aldrich Co.

    IntroductionSolid phase extraction (SPE) is an increasingly useful sample preparation technique. With SPE, many of the problems associated withliquid/liquid extraction can be prevented, such as incomplete phase separations, less-than-quantitative recoveries, use of expensive,breakable specialty glassware, and disposal of large quantities of organic solvents. SPE is more efficient than liquid/liquid extraction,yields quantitative extractions that are easy to perform, is rapid, and can be automated. Solvent use and lab time are reduced.SPE is used most often to prepare liquid samples and extract semivolatile or nonvolatile analytes, but also can be used with solidsthat are pre-extracted into solvents. SPE products are excellent for sample extraction, concentration, and cleanup. They are availablein a wide variety of chemistries, adsorbents, and sizes. Selecting the most suitable product for each application and sample is important.

    Page

    Introduction 1

    Phase Types 2Reversed phase packingsNormal phase packingsIon exchange packingsAdsorption packings

    SPE Theory 3How compounds are

    retained by the sorbentReversed phase SPENormal phase SPEIon exchange SPESecondary interactionsThe role of pH in SPE

    How to Use SPE 6Selecting the proper

    extraction schemeThe five-step SPE method

    development processSample pretreatment options

    - Liquid samples- Solid samples

    SPE hardware and accessoriesfor processing samples

    9960028

    Guide to Solid Phase Extraction

    9950124

    9970312

    9940256

    9960029

  • SUPELCOBulletin 9102

    s SPE tubes that are packed with this material contain stainless steel or Teflon frits, required by US Environmental Protection Agency Contract LaboratoryProgram (CLP) pesticide methods.

    ss Highly crosslinked, neutral, specially cleaned styrene-divinylbenzene resin. Very high surface area, mean pore size 110-175.

    Silica-Based Packing 40m particles, 60 pores (unless otherwise noted).LC-18 octadecyl bonded, For reversed phase extraction of nonpolar to moderately polar compounds, such as

    endcapped silica antibiotics, barbiturates, benzodiazepines, caffeine, drugs, dyes, essential oils,fat soluble vitamins, fungicides, herbicides, pesticides, hydrocarbons, parabens, phenols,phthalate esters, steroids, surfactants, theophylline, and water soluble vitamins.

    ENVITM-18 octadecyl bonded, Higher phase coverage and carbon content than LC-18, greater resistance to extreme pHendcapped silica conditions, and slightly higher capacity for nonpolar compounds. For reversed phase

    extraction of nonpolar to moderately polar compounds, such as antibiotics, caffeine, drugs,dyes, essential oils, fat soluble vitamins, fungicides, herbicides, pesticides, PNAs,hydrocarbons, parabens, phenols, phthalate esters, steroids, surfactants, water solublevitamins. Also available in disk format.

    LC-8 octyl bonded, For reversed phase extraction of nonpolar to moderately polar compounds, such asendcapped silica antibiotics, barbiturates, benzodiazepines, caffeine, drugs, dyes, essential oils,

    fat soluble vitamins, fungicides, herbicides, pesticides, hydrocarbons, parabens, phenols,phthalate esters, steroids, surfactants, theophylline, and water soluble vitamins. Alsoavailable in disk format.

    ENVI-8 octyl bonded, Higher phase coverage and carbon content than LC-8, greater resistance to extreme pHendcapped silica conditions, and slightly higher capacity for nonpolar compounds. For reversed phase

    extraction of barbiturates, benzodiazepines, caffeine, drugs, dyes, essential oils,fat soluble vitamins, fungicides, herbicides, pesticides, PNAs, hydrocarbons, parabens,phenols, phthalate esters, steroids, surfactants, theophylline, water soluble vitamins.

    LC-4 butyldimethyl bonded, end- Less hydrophobic than LC-8 or LC-18. For extraction of peptides and proteins.capped silica (500 pores)

    LC-Ph phenyl bonded silica Slightly less retention than LC-18 or LC-8 material. For reversed phase extraction ofnonpolar to moderately polar compounds, especially aromatic compounds.

    HisepTM hydrophobic surface For exclusion of proteins in biological samples; retains small molecules such as drugsenclaved by a under reversed phase conditions.hydrophilic network

    LC-CN cyanopropyl bonded, For reversed phase extraction of moderately polar compounds, normal phase extractionendcapped silica of polar compounds, such as aflatoxins, antibiotics, dyes, herbicides, pesticides,

    phenols, steroids. Weak cation exchange for carbohydrates and cationic compounds.LC-Diol diol bonded silica For normal phase extraction of polar compounds.LC-NH2 aminopropyl bonded silica For normal phase extraction of polar compounds, weak anion exchange for

    carbohydrates, weak anions, and organic acids.LC-SAX quaternary amine bonded For strong anion exchange for anions, organic acids, nucleic acids, nucleotides, and

    silica with Cl- counterion surfactants. Capacity: 0.2meq/g.LC-SCX sulfonic acid bonded silica For strong cation exchange for cations, antibiotics, drugs, organic bases, amino acids,

    with Na+ counterion catecholamines, herbicides, nucleic acid bases, nucleosides, and surfactants.Capacity: 0.2meq/g.

    LC-WCX carboxylic acid For weak cation exchange of cations, amines, antibiotics, drugs, amino acids,bonded silica with catecholamines, nucleic acid bases, nucleosides, and surfactants.Na+ counterion

    LC-Si silica gel with no For extraction of polar compounds, such as alcohols, aldehydes, amines, drugs, dyes,bonded phase herbicides, pesticides, ketones, nitro compounds, organic acids, phenols, and steroids.

    Alumina-Based Packing Crystalline, chromatographic grade alumina, irregular particles, 60/325 mesh.LC-Alumina-A acidic pH ~5 For anion exchange and adsorption extraction of polar compounds, such as vitamins.LC-Alumina-B basic pH ~8.5 For adsorption extraction of polar compounds, and cation exchange.LC-Alumina-N neutral pH ~6.5 For adsorption extraction of polar compounds. With pH adjustment, cation or

    anion exchange. For extraction of vitamins, antibiotics, essential oils, enzymes,glycosides, and hormones.

    Florisil-Based Packing Magnesium silicate, 100/120 mesh particles.LC-Florisil For adsorption extraction of polar compounds, such as alcohols, aldehydes, amines,

    drugs, dyes, herbicides, pesticides, PCBs, ketones, nitro compounds, organic acids,phenols, and steroids.

    ENVI-Florisils For adsorption extraction of polar compounds, such as alcohols, aldehydes, amines,drugs, dyes, herbicides, pesticides, PCBs, ketones, nitro compounds, organic acids,phenols, and steroids.

    Graphitized Carbon-Based Packing Nonbonded carbon phase.ENVI-Carb nonporous, surface area For adsorption extraction of polar and nonpolar compounds.

    100m2/g, 120/400 meshENVI-Carb C nonporous, surface For adsorption extraction of polar and nonpolar compounds.

    area 10m2/g, 80/100 meshResin-Based Packing 80-160m spherical particles.ENVI-Chrom Pss For extraction of polar aromatic compounds such as phenols from aqueous

    samples. Also for adsorption extraction of nonpolar to midpolar aromatic compounds.

    SPE Phase TypesA

    dsor

    ptio

    nIo

    n Ex

    chan

    geR

    ever

    sed

    Phas

    eN

    orm

    al P

    hase

  • 3SUPELCOBulletin 910

    SPE TheoryHow Compounds Are Retainedby the Sorbent

    Reversed Phase(polar liquid phase, nonpolar modified solid phase)

    Hydrophobic interactionsl nonpolar-nonpolar interactionsl van der Waals or dispersion forces

    Normal Phase(nonpolar liquid phase, polar modified solid phase)

    Hydrophilic interactionsl polar-polar interactionsl hydrogen bondingl pi-pi interactionsl dipole-dipole interactionsl dipole-induced dipole interactions

    Ion ExchangeElectrostatic attraction of charged group on compoundto a charged group on the sorbents surface

    Adsorption(interactions of compounds withunmodified materials)

    Hydrophobic and hydrophilic interactions may applyDepends on which solid phase is used

    Reversed Phase SPEReversed phase separations involve a polar (usually aqueous;see Table A on page 8) or moderately polar sample matrix (mobilephase) and a nonpolar stationary phase. The analyte of interestis typically mid- to nonpolar. Several SPE materials, such as thealkyl- or aryl-bonded silicas (LC-18, ENVI-18, LC-8, ENVI-8,LC-4, and LC-Ph) are in the reversed phase category. Here, thehydrophilic silanol groups at the surface of the raw silica packing(typically 60 pore size, 40m particle size) have been chemicallymodified with hydrophobic alkyl or aryl functional groups byreaction with the corresponding silanes.

    CH3 CH3 Si-OH + Cl-Si-C18H37 Si-O-Si-C18H37 + HCl

    CH3 CH3Retention of organic analytes from polar solutions (e.g. water)onto these SPE materials is due primarily to the attractive forcesbetween the carbon-hydrogen bonds in the analyte and thefunctional groups on the silica surface. These nonpolar-nonpolarattractive forces are commonly called van der Waals forces, ordispersion forces. To elute an adsorbed compound from a re-versed phase SPE tube or disk, use a nonpolar solvent to disruptthe forces that bind the compound to the packing. LC-18 andLC-8 are standard, monomerically bonded silicas. Polymericallybonded materials, such as ENVI-18 and ENVI-8, result in a morecomplete coverage of the silica surface and higher carbon load-ing. Polymeric bonding is more resistant to pH extremes, and thusis more suitable for environmental applications for trapping or-ganic compounds from acidified aqueous samples. All silica-based bonded phases have some percentage of residual unreactedsilanols that act as secondary interaction sites. These secondaryinteractions may be useful in the extraction or retention of highlypolar analytes or contaminants, but may also irreversibly bindanalytes of interest (see Secondary Interactions on page 5).

    The following materials also are used under reversed phaseconditions: ENVI-Carb (carbon-based), ENVI-Chrom P (poly-mer-based), and Hisep (polymer-coated and bonded silica).Carbonaceous adsorption media, such as the ENVI-Carb mate-rials, consist of graphitic, nonporous carbon that has a highattraction for organic polar and nonpolar compounds from bothpolar and nonpolar matrices. The carbon surface is comprised ofatoms in hexagonal ring structures, interconnected and layered ingraphitic sheets. The hexagonal ring structure demonstrates astrong selectivity for planar aromatic or hexagonal ring-shapedmolecules and hydrocarbon chains with potential for multiplesurface contact points. Retention of analytes is based primarily onthe analytes structure (size and shape), rather than on interac-tions of functional groups on the analyte with the sorbent surface.Elution is performed with mid- to nonpolar solvents. The uniquestructure and selectivity of ENVI-Carb materials, compared tobonded alkyl-silicas, makes them an excellent alternative whenthe bonded silicas will not work for an application.Polymeric adsorption media such as the ENVI-Chrom P materialalso is used in reversed phase fashion. ENVI-Chrom P is astyrene/divinylbenzene material that is used for retaining hydro-phobic compounds which contain some hydrophilic functionality,especially aromatics. Phenols are sometimes difficult to retain onC18-modified silica under reversed phase conditions, mainly dueto their greater solubility in water than in organic matrices. TheENVI-Chrom P material has been shown to retain phenols wellunder reversed phase conditions. Elution steps can be done withmid- to nonpolar solvents, because the polymeric packing isstable in almost all matrices.Hisep is a hydrophobic (C18-like) bonded silica that is coated witha hydrophilic polymer and is typically used under reversed phaseconditions. The porous polymer coating prevents the adsorptionof large, unwanted molecules onto the silica surface. The pores inthe polymer allow small, hydrophobic organic compounds ofinterest (such as drugs) to reach the bonded silica surface, whilelarge interfering compounds (such as proteins) are shielded fromthe bonded silica by the polymer and are flushed through the SPEtube. SPE procedures on Hisep material are similar to those onLC-18.

    Normal Phase SPENormal phase SPE procedures typically involve a polar analyte,a mid- to nonpolar matrix (e.g. acetone, chlorinated solvents, andhexane), and a polar stationary phase. Polar-functionalized bondedsilicas (e.g. LC-CN, LC-NH2, and LC-Diol), and polar adsorptionmedia (LC-Si, LC-Florisil, ENVI-Florisil, and LC-Alumina) typi-cally are used under normal phase conditions. Retention of ananalyte under normal phase conditions is primarily due to interac-tions between polar functional groups of the analyte and polargroups on the sorbent surface. These include hydrogen bonding,pi-pi interactions, dipole-dipole interactions, and dipole-induceddipole interactions, among others. A compound adsorbed bythese mechanisms is eluted by passing a solvent that disrupts thebinding mechanism usually a solvent that is more polar than thesamples original matrix.The bonded silicas LC-CN, LC-NH2, and LC-Diol have shortalkyl chains with polar functional groups bonded to the surface.These silicas, because of their polar functional groups, are muchmore hydrophilic relative to the bonded reversed phase silicas. Aswith typical normal phase silicas, these packings can be used toadsorb polar compounds from nonpolar matrices. Such SPEtubes have been used to adsorb and selectively elute compoundsof very similar structure (e.g. isomers), or complex mixtures orclasses of compounds such as drugs and lipids. These materials

  • SUPELCOBulletin 9104

    also can be used under reversed phase conditions (with aqueoussamples), to exploit the hydrophobic properties of the small alkylchains in the bonded functional groups.The LC-Si material is underivatized silica commonly used as thebackbone of all of the bonded phases. This silica is extremelyhydrophilic, and must be kept dry. All samples used with thismaterial must be relatively water-free. The functional groups thatare involved in the adsorption of compounds from nonpolarmatrices are the free hydroxyl groups on the surface of the silicaparticles. LC-Si may be used to adsorb polar compounds fromnonpolar matrices with subsequent elution of the compounds inan organic solvent that is more polar than the original samplematrix. In most cases, LC-Si is used as an adsorption media,where an organic extract is applied to the silica bed, the analyteof interest passes through unretained, and the unwanted com-pounds adsorb onto the silica and are discarded. This procedureis usually called sample cleanup.LC-Florisil and ENVI-Florisil SPE tubes are packed with amagnesium silicate that is used typically for sample cleanup oforganic extracts. This highly polar material strongly adsorbs polarcompounds from nonpolar matrices. The ENVI-Florisil SPE tubesare made with either Teflon or stainless steel frits, a configurationnecessary for environmental procedures specified in US EPAmethods. ENVI-Florisil is specifically tested for low backgroundsvia GC analysis.LC-Alumina SPE tubes are also used in adsorption/samplecleanup-type procedures. The aluminum oxide materials caneither be of acidic (Alumina-A, pH ~5), basic (Alumina-B, pH~8.5), or neutral (Alumina-N, pH ~6.5) pH, and are classified ashaving Brockmann Activities of I. The activity level of the aluminamay be altered from grade I through grade IV with the controlledaddition of water, prior to or after packing this material into tubes.

    Ion Exchange SPEIon exchange SPE can be used for compounds that are chargedwhen in a solution (usually aqueous, but sometimes organic).Anionic (negatively charged) compounds can be isolated onLC-SAX or LC-NH2 bonded silica cartridges. Cationic (positivelycharged) compounds are isolated by using LC-SCX or LC-WCXbonded silica cartridges. The primary retention mechanism of thecompound is based mainly on the electrostatic attraction of thecharged functional group on the compound to the charged groupthat is bonded to the silica surface. In order for a compound toretain by ion exchange from an aqueous solution, the pH of thesample matrix must be one at which both the compound of interestand the functional group on the bonded silica are charged. Also,there should be few, if any, other species of the same charge asthe compound in the matrix that may interfere with the adsorptionof the compound of interest. A solution having a pH that neutral-izes either the compounds functional group or the functionalgroup on the sorbent surface is used to elute the compound ofinterest. When one of these functional groups is neutralized, theelectrostatic force that binds the two together is disrupted and thecompound is eluted. Alternatively, a solution that has a high ionicstrength, or that contains an ionic species that displaces theadsorbed compound, is used to elute the compound.

    Anion Exchange SPEThe LC-SAX material is comprised of an aliphatic quaternaryamine group that is bonded to the silica surface. A quaternaryamine is a strong base and exists as a positively-charged cationthat exchanges or attracts anionic species in the contactingsolution thus the term strong anion exchanger (SAX). The pKaof a quaternary amine is very high (greater than 14), which makes

    the bonded functional group charged at all pHs when in anaqueous solution. As a result, LC-SAX is used to isolate stronganionic (very low pKa, 2) compounds, as long as the pH of the sample is one at whichthe compound of interest is charged. For an anionic (acidic)compound of interest, the pH of the matrix must be 2 pH unitsabove its pKa for it to be charged. In most cases, the compoundsof interest are strong or weak acids.Because it binds so strongly, LC-SAX is used to extract stronganions only when recovery or elution of the strong anion is notdesired (the compound is isolated and discarded). Weak anionscan be isolated and eluted from LC-SAX because they can beeither displaced by an alternative anion or eluted with an acidicsolution at a pH that neutralizes the weak anion (2 pH units belowits pKa). If recovery of a strongly anionic species is desired, useLC-NH2.The LC-NH2 SPE material that is used for normal phase separa-tions is also considered to be a weak anion exchanger (WAX)when used with aqueous solutions. The LC-NH2 material has analiphatic aminopropyl group bonded to the silica surface. The pKaof this primary amine functional group is around 9.8. For it to beused as an anion exchanger, the sample must be applied at a pHat least 2 units below 9.8. The pH must also be at a value wherethe anionic compound of interest is also charged (2 pH unitsabove its own pKa). LC-NH2 is used to isolate and recover bothstrong and weak anions because the amine functional group onthe silica surface can be neutralized (2 pH units above its pKa) inorder to elute the strong or weak anion. Weak anions also can beeluted from LC-NH2 with a solution that neutralizes the adsorbedanion (2 pH units below its pKa), or by adding a different anion thatdisplaces the analyte.

    Cation ExchangeThe LC-SCX material contains silica with aliphatic sulfonic acidgroups that are bonded to the surface. The sulfonic acid group isstrongly acidic (pKa 14) or weak cationic (moderately highpKa,

  • 5SUPELCOBulletin 910

    In many cases, the analyte in ion exchange SPE is eluted in anaqueous solution. If you must use an acidic or basic solution toelute an analyte from an SPE tube, but the extracted sample mustbe analyzed in an organic solvent that is not miscible with water,try to elute the compound with acidic methanol (98% methanol/2% concentrated HCl) or basic methanol (98% methanol/2%NH4OH). The methanol can be evaporated quickly, and thesample may be reconstituted in a different solvent. If you need astronger (more nonpolar) solvent to elute the analyte from theSPE tube, add methylene chloride, hexane, or ethyl acetate to theacidic or basic methanol.

    Secondary InteractionsThe primary retention mechanisms for compounds on the SPEmaterials are described above. For the bonded silicas, it ispossible that secondary interactions will occur.For reversed phase bonded silicas, the primary retention mecha-nism involves nonpolar interactions. However, because of thesilica particle backbone, some polar secondary interactions withresidual silanols such as those described for normal phaseSPE could occur. If a nonpolar solvent does not efficiently elutea compound from a reversed phase SPE packing, the addition ofa more polar solvent (e.g. methanol) may be necessary to disruptany polar interactions that retain the compound. In these cases,methanol can hydrogen-bond with the hydroxyl groups on thesilica surface, thus breaking up any hydrogen bonding that theanalyte may be incurring.The silanol group at the surface of the silica, Si-OH, can also beacidic, and may exist as an Si-O- group above pH 4. As a result,the silica backbone may also have cation exchange secondaryinteractions, attracting cationic or basic analytes of interest. In thiscase, a pH adjustment of the elution solvent may be necessary todisrupt these interactions for elution (acidic to neutralize thesilanol group, or basic to neutralize the basic analyte). This can bedone by using acidic methanol (98% MeOH:2% concentratedHCl) or basic methanol (98% MeOH:2% concentrated NH4OH), orby mixtures of these with a more nonpolar, methanol-misciblesolvent.Normal phase bonded silicas will exhibit primary polar retentionmechanisms via the bonded functional group, but also can havesome secondary nonpolar interactions of the analyte with thesmall alkyl chain that supports the functional group. In this case,a more nonpolar solvent, or a mix of polar and nonpolar solvents,may be needed for elution. As with the reversed phase silicas,secondary polar or cation exchange interactions of the adsorbedcompound may occur with the silica backbone.Ion exchange bonded silicas can provide secondary nonpolarinteractions of analytes with the nonpolar portions of their func-tional groups, as well as polar and cation exchange interactionsof the analyte with the silica backbone. A delicate balance of pH,ionic strength, and organic content may be necessary for elutionof the analyte of interest from these packings.

    The Role of pH in SPESolutions used in SPE procedures have a very broad pH range.Silica-based packings, such as those used in HPLC columns,usually have a stable pH range of 2 to 7.5. At pH levels above andbelow this range, the bonded phase can be hydrolyzed andcleaved off the silica surface, or the silica itself can dissolve. InSPE, however, the solutions usually are in contact with thesorbent for short periods of time. The fact that SPE cartridges aredisposable, and are meant to be used only once, allows one to useany pH to optimize retention or elution of analytes. If stability of theSPE cartridge at an extreme pH is crucial, polymeric or carbon-

    based SPE materials such as ENVI-Chrom P or ENVI-Carb maybe used. These materials are stable over the pH range of 1-14.For reversed phase SPE procedures on bonded silicas, if trap-ping the analyte in the tube is desired, the pH of the conditioningsolution and sample (if mostly or entirely aqueous) should beadjusted for optimum analyte retention. If the compound ofinterest is acidic or basic you should, in most cases, use a pH atwhich the compound is not charged. Retention of neutral com-pounds (no acidic or basic functional groups) usually is notaffected by pH. Conversely, you can use a pH at which theunwanted compounds in the sample are retained on the SPEpacking, but the analyte of interest passes through unretained.Secondary hydrophilic and cation exchange interactions of theanalyte can be used for retention at a proper pH. (For more detail,see Secondary Interactions).For adsorption media (e.g. ENVI-Carb and ENVI-Chrom P) thatare used under reversed phase conditions, a pH should bechosen to maximize retention of analytes on the sorbent as withreversed phase bonded silicas. Elution is usually done with anorganic solvent, so pH is usually not a factor at this point.Surprisingly, phenols retain better on ENVI-Chrom P when ap-plied in solutions at a neutral pH, where phenols can be charged,than at an acidic pH levels where they are neutral. This shows thatadsorption media may have different selectivities than the bondedsilicas for certain compounds, and that a range of pH levels of thesample and conditioning solutions should be investigated whenusing these materials.In normal phase SPE procedures on bonded silicas or adsorp-tion media, pH is usually not an issue, because the solvents usedin these processes are typically nonpolar organic solvents, ratherthan water.Retention in ion exchange SPE procedures depends heavily onthe pH of the sample and the conditioning solutions. For retentionof the analyte, the pH of the sample must be one at which theanalyte and the functional groups on the silica surface arecharged oppositely. For further details, see Ion Exchange onpage 4.

    SPE packing embedded in glass fiber matrix

    Typical SPE Tube and Disk

    SPE Tube

    SPE ENVI-Disk

    713-0479, G000071

    Syringe barrel-like body(usually polypropylene,sometimes glass)

    Frits, 20m pores(usually polyethylene,sometimes Teflonor stainless steel)

    Luer Tip

    SPE Packing

  • SUPELCOBulletin 9106

    How to Use SPESolid phase extraction is usedto separate compounds ofinterest from impurities in threeways. Choose the most appro-priate scheme for your sample:

    Selective Extraction. Select an SPE sorbent that will bind selectedcomponents of the sample either the compounds of interest or thesample impurities. The selected components are retained when thesample passes through the SPE tube or disk (the effluent will containthe sample minus the adsorbed components). Then, either collect theadsorbed compounds of interest through elution, or discard the tubecontaining the extracted impurities.

    Select the ProperSPE Tube or Disk

    Add the Sample

    G000025A

    G000022A

    STEP

    1

    G000016A

    SCH

    EME

    1

    G000026A

    STEP

    2

    G000023AST

    EP 3

    G000024A

    STEP

    4

    STEP

    5

    Wash the Packing Elute theCompounds of Interest

    G000019

    Key to Processes

    Condition the SPETube or Disk

    = Matrix

    = Impurity

    = Compound of interest

    = Solvent A

    = Solvent B

    = Solvent C

    SPE Is aFive-Step ProcessThe SPE process provides samplesthat are in solution, free of interferingmatrix components, and concentratedenough for detection. This is done infive steps (summarized here and de-scribed on the next two pages).

    l For reversed phase, normal phase,and ion exchange SPE procedures,all five steps typically are needed.

    l For some sample cleanup proce-dures, only the first three steps mayapply. Steps 1 and 2 are the sameas shown. However, in step 3, theanalyte is collected in the effluent asthe sample passes through the tube;interfering impurities remain on thesorbent.

    Selective Elution. The adsorbed compounds of interest are eluted ina solvent that leaves the strongly retained impurities behind.

    G000018A

    SCH

    EME

    3

    Selective Washing. The compounds of interest and the impuritiesare retained on the SPE packing when the sample passes through;the impurities are rinsed through with wash solutions that are strongenough to remove them, but weak enough to leave the compoundsof interest behind.

    G000017

    SCH

    EME

    2

    Sorb

    ent

  • 7SUPELCOBulletin 910

    Note: An SPE disk is recommended for largevolume samples, samples containing highamounts of particulates, or when a high flow rateis required during sampling.

    Selecting an SPE Tubeor Disk: SizeSelecting SPE Tube SizeIf Your Sample Is . . . Use Tube Size . . .

    < 1mL 1mL

    1mL to 250mL andthe extraction speedis not critical 3mL

    1mL to 250mL and afast extraction procedureis required 6mL10mL to 250mL andhigher sample capacityis needed 12, 20, or 60mL< 1 liter and extractionspeed is not critical 12, 20, or 60mLSelecting SPE Disk SizeIf Your Sample Is . . . Use Disk Size . . .100mL to 1 liter 47mm

    >1 liter and highersample capacityis needed 90mm

    Selecting an SPE Tube:Bed WeightReversed Phase, Normal Phase,and Adsorption-Type Procedures:The mass of the compounds to be extractedshould not be more than 5% of the mass ofthe packing in the tube.

    In other words, if you are using a100mg/1mL SPE tube, do not loadmore than 5mg of analytes.

    Ion Exchange Procedures:You must consider ion exchange capacity.

    l LC-SAX and LC-SCX tubes have~0.2meq/gram of sorbent capacity(1 meq = 1mmole of [+1] or [-1]charged species).

    l LC-NH2 and LC-WCX tubes: ionexchange capacities should bedetermined for your own application.

    Selecting an SPE Tube:Sorbent Type(Note: Refer to schematic on page 12.)Is your sample matrixaqueous or organic?If aqueous:Is your analyte of interest more soluble inwater or in organic solvents(e.g., hexane or dichloromethane)?If more soluble in water, is your analytecharged or neutral?Charged:If weakly anionic (-) and acidic,

    use an LC-SAX or LC-NH2 tube.If strongly anionic (-) and acidic:

    - and you want to recover theextracted analyte,

    use an LC-NH2 tube.- and you do not want to recover

    the extracted analyte,use an LC-SAX tube.

    If weakly cationic (+) and basic,use an LC-SCX or LC-WCX tube.

    If strongly cationic (+) and basic:- and you want to recover the

    extracted analyte,use an LC-WCX tube.

    - and you do not want to recover theextracted analyte,

    use an LC-SCX tube.Neutral:If analytes are difficult to extract usingreversed phase packings (e.g. alcohols,sugars, glycols), try an ENVI-Carb orENVI-Chrom P tube, or try to removeinterferences by reversed phase extractionor by using an LC-SAX or LC-SCX tube.

    If more soluble in organics, is your analytecharged or neutral?Charged:Try reversed phase or ion exchangeextraction.

    Neutral:Try reversed phase extraction.If organic:Try any of the following.

    Concentrate analyte by evaporation.Evaporate to dryness and reconstitutewith another solvent.Use SPE.

    Is the organic solvent polar and water-miscible (e.g. methanol or acetonitrile) ormid- to nonpolar and not water-miscible(e.g. dichloromethane or hexane)?Refer to Table A.

    If polar:Dilute with water to

  • SUPELCOBulletin 9108

    Accurately transfer the sample to thetube or reservoir, using a volumetricpipette or micropipette. The sample mustbe in a form that is compatible with SPE.Total sample volume can range from mi-croliters to liters (see step 1). When exces-sive volumes of aqueous solutions areextracted, reversed phase silica packingsgradually lose the solvent layer acquiredthrough the conditioning process. This re-duces extraction efficiency and samplerecovery. For samples >250mL, add smallamounts of water-miscible solvents (up to10%) to maintain proper wetting of re-versed phase packings. Maximum samplecapacity is specific to each application andthe conditions used. If recoveries are lowor irreproducible, test for analyte break-through using the following technique:Attach two conditioned SPE tubes of thesame packing together using an adapter.Pass the sample through both tubes. Whenfinished, detach each tube and elute itseparately. If the analyte is found in theextract of the bottom tube, the samplevolume is too great or bed weight is toosmall, resulting in analyte breakthrough.To enhance retention of appropriate com-pounds on the packing, and elution orprecipitation of unwanted compounds, ad-just the pH, salt concentration, and/or or-ganic solvent content of the sample solu-tion. To avoid clogging SPE tube frits orthe SPE disk, pre-filter or centrifugesamples prior to extraction if possible.Slowly pass the sample solution throughthe extraction device, using either vacuumor positive pressure. The flow rate canaffect the retention of certain compounds.Generally, the flow rate should not exceed2mL/min for ion exchange SPE tubes,5mL/min for other SPE tubes, and may beup to 50mL/min for disks. Dropwise flow isbest, when time is not a factor.For some difficult sample matrices, addi-tional pretreatment may be necessary. SeeSample Pretreatment on the next page.

    G000023B G000024B

    Table A. Characteristics of Solvents Commonly Used in SPEPolarity Solvent Miscible in Water?Nonpolar Strong Weak Hexane No

    Reversed Normal Isooctane NoPhase Phase Carbon tetrachloride No

    Chloroform NoMethylene chloride (dichloromethane) NoTetrahydrofuran YesDiethyl ether NoEthyl acetate PoorlyAcetone YesAcetonitrile YesIsopropanol Yes

    Weak Strong Methanol YesReversed Normal Water Yes

    Polar Phase Phase Acetic acid Yes

    G000025B

    If compounds of interest are retainedon the packing, wash off unwanted,unretained materials using the samesolution in which the sample wasdissolved, or another solution that willnot remove the desired compounds.Usually no more than a tube volume ofwash solution is needed, or 5-10mL forSPE disks.To remove unwanted, weakly retainedmaterials, wash the packing with solutionsthat are stronger than the sample matrix,but weaker than needed to remove com-pounds of interest. A typical solution maycontain less organic or inorganic salt thanthe final eluant. It also may be adjusted toa different pH. Pure solvents or mixtures ofsolvents differing sufficiently in polarity fromthe final eluant may be useful wash solu-tions (see Table A).If you are using a procedure by whichcompounds of interest are not retained onthe packing, use about one tube volume ofthe sample solvent to remove any residual,desired components from the tube, or5-10mL to remove the material from a disk.This rinse serves as the elution step tocomplete the extraction process in thiscase.

    Rinse the packing with a small volume(typically 200L to 2mL depending onthe tube size, or 5-10mL depending onthe disk size) of a solution that removescompounds of interest, but leavesbehind any impurities not removed inthe wash step. Collect the eluate andfurther prepare as appropriate.Two small aliquots generally elute com-pounds of interest more efficiently thanone larger aliquot. Recovery of analytes isbest when each aliquot remains in contactwith the tube packing or disk for 20 sec-onds to 1 minute. Slow or dropwise flowrates in this step are beneficial.Strong and weak elution solvents foradsorbed compounds in SPE are describedin Table A.

    STEP

    3

    STEP

    4

    STEP

    5

    Add the Sample Wash the Packing Elute the Compounds of Interest

  • 9SUPELCOBulletin 910

    Sample PretreatmentIn addition to ensuring proper pH of thesample (see The Role of pH in SPE onpage 5), you should consider other samplepretreatment needs. The following sectiondescribes how some difficult sample matri-ces should be pretreated before being ap-plied to the SPE device:

    LiquidsBiological MatricesSerum, plasma, and whole blood: Serumand plasma samples may not need to bepretreated for SPE. In many cases, however,analytes such as drugs may be protein-bound,which reduces SPE recoveries. To disruptprotein binding in these biological fluids, useone of the following methods for reversedphase or ion exchange SPE procedures:

    l Shift pH of the sample to extremes (pH9) with acids or bases in theconcentration range of 0.1M or greater.Use the resulting supernatant as thesample for SPE.

    l Precipitate the proteins using a polarsolvent such as acetonitrile, methanol,or acetone (two parts solvent per onepart biological fluid is typical). After mix-ing and centrifugation, remove the su-pernatant and dilute with water or anaqueous buffer for the SPE procedure.

    l To precipitate proteins, treat the biologi-cal fluid with acids or inorganic salts,such as formic acid, perchloric acid,trichloroacetic acid, ammonium sulfate,sodium sulfate, or zinc sulfate. The pH ofthe resulting supernatant may be ad-justed prior to use for the SPE proce-dure.

    l Sonicate the biological fluid for 15 min-utes, add water or buffer, centrifuge, anduse the supernatant for the SPE proce-dure.

    Urine: Urine samples may not require pre-treatment for reversed phase or ion exchangeSPE, but often is diluted with water or a bufferof the appropriate pH prior to sample addition.In some cases, acid hydrolysis (for basiccompounds) or base hydrolysis (for acidiccompounds) is used to ensure that the com-pounds of interest are freely solvated in theurine sample. Usually a strong acid (e.g.concentrated HCl) or base (e.g. 10M KOH) isadded to the urine. The urine is heated for 15-20 minutes, then cooled and diluted with abuffer, and the pH adjusted appropriately forthe SPE procedure. Enzymatic hydrolysisthat frees bound compounds or drugs alsomay be used.

    Cell Culture MediaCell culture media may be used without pre-treatment. Some methods may require dilu-tion of the media with water or buffer at theproper pH to ensure that the analyte is freelysolvated in the sample. If a particulate-ladencell culture medium is difficult to pass throughthe SPE device, it may need to be vortexedand centrifuged prior to SPE. Most SPE pro-cedures for cell culture media are done usingreversed phase or ion exchange methods.

    MilkMilk generally is processed under reversedphase or ion exchange SPE conditions. Thesample may be diluted with water, or withmixtures of water and a polar solvent such asmethanol (up to 50%). Some proceduresmay require precipitation of proteins by treat-ment with acid (typically HCl, H2SO4, or trichlo-roacetic acid). After precipitation, the sampleis centrifuged and the supernatant is used forSPE.

    Water SamplesDrinking water, groundwater, and waste-water samples may be extracted directly bySPE, as long as they are not heavily ladenwith solid particles. Groundwater and waste-water samples might need to be filtered priorto the SPE procedure. Filtering may reducerecoveries if compounds of interest are boundto the removed particles. If possible, do notfilter the sample. Pass the unfiltered sampledirectly through the SPE device and, duringelution, allow the solvent to pass through theparticles on the adsorbent bed. This will im-prove recoveries, since particle-bound com-pounds of interest will be recovered using thisprocess. In most cases, water samples areused with reversed phase or ion exchangeSPE procedures.

    Wine, Beer, and Aqueous BeveragesAqueous and alcoholic beverages may beprocessed for SPE without pretreatmentunder reversed phase or ion exchange con-ditions. For reversed phase procedures, ifalcohol content is high, dilution with water orbuffer to

  • SUPELCOBulletin 91010

    Hardware and Accessories for Processing SamplesSPE tubes can be processed individuallyusing a single tube processor (Figure A) orwith a syringe and an adapter (Figure B).The liquid sample is placed in the SPEtube, and the processor or syringe is usedto provide positive pressure to force theliquid through the tube. Positive pressurefrom an air or nitrogen line also may beused to force the solutions through thetube.

    A solution also can be processed througha single SPE tube using a vacuum flaskand rubber stopper (Figure C). The vacuumpulls a solution through the SPE tube. Thesolution then can be collected in a test tubelocated inside the flask.Several SPE tubes can be processed us-ing a centrifuge (Figure D). The solutionsare placed in the SPE tubes and the centri-fuge forces the solutions through the tubesinto test tubes. Appropriate spin rates mustbe determined; they can vary dependingon the type and mass of the packing in thetube and the volume of sample. For sampleaddition, follow the recommended flow ratesdiscussed in step 3 (page 8).

    Figure A. Single Tube Processor

    Figure B. Process UsingApplied Pressure

    713-0539

    Rotate KnurledKnob for

    Slow FlowDepress Plungerfor Rapid Flow

    Solvent

    Positive Pressurefrom Syringe Positive Pressurefrom Air or N2 Line

    94-0143

    10-100ccSyringe

    TubeAdapter

    SampleSolution

    AirorN2

    SampleSolution

    1. Add conditioning solvents,sample solution, or elutingsolvent to SPE tube

    2. Place SPE tubein centrifuge tube

    3. Load multipletubes in centrifuge

    4. Centrifuge

    5. Elution solvent is forcedthrough SPE tube

    94-0145

    Figure D. Processing Several Tubes Using a Centrifuge

    Multiple tubes can be processed simulta-neously using a 12- or 24-port vacuummanifold. Supelco offers two types ofVisiprepTM SPE vacuum manifolds a stan-dard lid version and a disposable linerversion.

    l Our standard lid manifold (Figure E)has unique flow control valves that al-low easy control of flow through eachSPE tube. Reusable stainless steelneedle guides direct the sample intothe glass basin below.

    l Valves in the lid of the DL manifold(Figure F) contain a disposable Teflonliner that directs the sample into theglass basin. The liner is convenientlydisposable, is inert, and prevents cross-contamination in critical applications.

    Both types of vacuum manifolds have asolvent resistant main vacuum gauge andvalve used to monitor the vacuum andrelease the vacuum during processing.

    9950133

    Figure E. Visiprep VacuumManifold with Standard Lid

    94-0144

    Figure C. ConfigurationUsing a Vacuum Flask

    Packing Bed

    Collectiontest tube

  • 11SUPELCOBulletin 910

    SPE disks can be pro-cessed on a vacuumfiltration flask-typeassembly (Figure K).The ENVI-Disk Clamp(Figures L and M) isdesigned to eliminatepotential leakage thatis often observed withconventional flaskclamps. The ENVI-Disk Holder (Figure N)is used when astandard filtration flaskassembly is not avail-able.

    The Preppy vacuum manifold (Figure H)is our simplest and most economical mani-fold. It too enables the analyst to simulta-neously prepare up to 12 samples.

    Figure H. Preppy Vacuum Manifold

    9970312

    The Preppy manifold consists of a chemi-cal-resistant cover and gasket, a glassbasin, a vacuum release vent, 12 indi-vidual flow control valves with knurled tops,and stainless steel solvent guide needles.Two optional collection vessel racks areavailable: one holds both 1mL and 4mLautosampler vials, and the other holds15mL or 20mL vials. Adapters are avail-able for the 1mL-4mL vessel rack to holdmini-centrifuge tubes. An optional vacuumgauge/bleed valve assembly can be in-stalled to allow precise control of thevacuum used with the Preppy manifold.SPE tubes can be processed individuallyor can be combined using an adapter toprovide different selectivities. Small vol-umes are processed directly in the SPEtube. Larger volumes can be accommo-dated by using a reservoir with an adapter.For very large samples, a large volumesampler is available, which allows unat-tended sample processing (Figure I).

    Figure I. VisiprepLarge Volume Sampler

    VisidryTM drying attachments (Figure J),available with 12 ports and 24 ports, can beused to dry the tubes or evaporate andconcentrate collected samples during theSPE procedure.

    Figure J. Visidry Drying Attachment

    9960059

    9960280 9950117

    9950147

    Figure M. Figure N.

    Figure K.

    For more information on these accessories,please refer to Supelco's general catalog.

    8960035

    9950132

    Figure F. Visiprep VacuumManifold with Disposable Liner

    Unwanted solvents that collect in the bot-tom of the glass basin are pulled continu-ously through the gauge into the vacuumpump trap (Figure G) located between thepump and the manifold. This minimizescontamination by preventing buildup ofunwanted waste solutions in the basin.The manifolds are equipped with an ad-justable collection rack system that is placedinside the glass basin. The racks can beadjusted easily to accommodate manytypes and dimensions of collection ves-sels, such as small test tubes (10mm),large test tubes (16mm), volumetric flasks(1mL-10mL), and many types ofautosampler vials.

    9130181

    Figure G. SPE Vacuum Pump Trap

    9950140

    7130554

    Figure L.

  • SUPELCOBulletin 91012

    BULLETIN 910

    BLQ

    Choosing the proper SPE device for yourapplication depends on:

    l Sample volumel Degree of contaminationl Complexity of sample matrixl Quantity of compounds of interestl Type and solvent strength of sample matrix

    TrademarksENVI, Hisep, Preppy, Supelclean, Visidry, Visiprep Sigma-Aldrich Co.Florisil U.S. Silica Co.Teflon E.I. du Pont de Nemours & Co., Inc.

    Technical ServiceIf you need help in choosing the proper devices for your samplepreparation applications, please contact our Technical Servicechemists at 800-359-3041 or 814-359-3041.

    PatentsVisidry Drying Attachment US patent 4,810,471; other patents pending.Visiprep Vacuum Manifold US patents D.289,861; 4,810,471; other patents pending.

    Sample Characteristics Determine Your SPE Procedure

    Charged

    Water

    Aqueous Organic

    Organics(e.g., hexane or dichloromethane)

    Neutral Neutral

    TryReversed

    Phase

    Is yoursample matrix:

    Is your analyte ofinterest moresoluble in:

    Is your compound:

    Is yourcompound:

    Is yourcompound a:

    Do you wantto recover youranalyte from theSPE packing?

    Anionic(acidic)

    Cationic(basic)

    Weak Anion Strong Anion

    LC-SAX or LC-NH2

    Yes No

    LC-NH2 LC-SAX

    Charged

    Try ReversedPhase or

    Ion Exchange

    Yes No

    LC-WCX LC-SCX

    Weak Cation Strong Cation

    Difficult to trap(e.g., alcohols, sugars, glycols):

    Try to trap interferencesby Reversed Phase orLC-SAX or LC-SCX.

    LC-SCX or LC-WCX

    Is the organic solvent:

    Polar (e.g., methanol or acetonitrile)

    Mid- to Nonpolar(e.g., Dichloromethane, hexane)

    Dilute with waterand follow aqueous

    sample scheme

    Try Normal Phase orevaporate to dryness,

    reconstitute in water or polarsolvent, then dilute with

    water and follow aqueoussample scheme.

    Use:Use:

    Use: Use: Use:Use:

    Is your compound a:

    For more information, or current prices, contact your nearest Supelco subsidiary listed below. To obtain further contact information, visit our website (www.sigma-aldrich.com), see the Supelco catalog, or contactSupelco, Bellefonte, PA 16823-0048 USA.ARGENTINA Sigma-Aldrich de Argentina, S.A. Buenos Aires 1119 AUSTRALIA Sigma-Aldrich Pty. Ltd. Castle Hill NSW 2154 AUSTRIA Sigma-Aldrich Handels GmbH A-1110 WienBELGIUM Sigma-Aldrich N.V./S.A. B-2880 Bornem BRAZIL Sigma-Aldrich Quimica Brasil Ltda. 01239-010 So Paulo, SP CANADA Sigma-Aldrich Canada, Ltd. 2149 Winston Park Dr., Oakville, ON L6H 6J8CZECH REPUBLIC Sigma-Aldrich s.r.o. 186 00 Praha 8 DENMARK Sigma-Aldrich Denmark A/S DK-2665 Vallensbaek Strand FINLAND Sigma-Aldrich Finland/YA-Kemia Oy FIN-00700 HelsinkiFRANCE Sigma-Aldrich Chimie 38297 Saint-Quentin-Fallavier Cedex GERMANY Sigma-Aldrich Chemie GmbH D-82041 Deisenhofen GREECE Sigma-Aldrich (o.m.) Ltd. Ilioupoli 16346, AthensHUNGARY Sigma-Aldrich Kft. H-1067 Budapest INDIA Sigma-Aldrich Co. Bangalore 560 048 IRELAND Sigma-Aldrich Ireland Ltd. Dublin 24 ISRAEL Sigma Israel Chemicals Ltd. Rehovot 76100ITALY Sigma-Aldrich s.r.l. 20151 Milano JAPAN Sigma-Aldrich Japan K.K. Chuo-ku, Tokyo 103 KOREA Sigma-Aldrich Korea Seoul MALAYSIA Sigma-Aldrich (M) Sdn. Bhd. SelangorMEXICO Sigma-Aldrich Qumica S.A. de C.V. 50200 Toluca NETHERLANDS Sigma-Aldrich Chemie BV 3330 AA Zwijndrecht NORWAY Sigma-Aldrich Norway Torshov N-0401 OsloPOLAND Sigma-Aldrich Sp. z o.o. 61-663 Pozna PORTUGAL Sigma-Aldrich Quimica, S.A. Sintra 2710 RUSSIA Sigma-Aldrich Russia Moscow 103062 SINGAPORE Sigma-Aldrich Pte. Ltd.SOUTH AFRICA Sigma-Aldrich (pty) Ltd. Jet Park 1459 SPAIN Sigma-Aldrich Quimica, S.A. 28100 Alcobendas, Madrid SWEDEN Sigma-Aldrich Sweden AB 135 70 StockholmSWITZERLAND Supelco CH-9471 Buchs UNITED KINGDOM Sigma-Aldrich Company Ltd. Poole, Dorset BH12 4QHUNITED STATES Supelco Supelco Park Bellefonte, PA 16823-0048 Phone 800-247-6628 or 814-359-3441 Fax 800-447-3044 or 814-359-3044 email:[email protected] H

    Supelco is a member of the Sigma-Aldrich family. Supelco products are sold through Sigma-Aldrich, Inc. Sigma-Aldrich warrants that its products conform to the information contained in this and otherSigma-Aldrich publications. Purchaser must determine the suitability of the product for a particular use. Additional terms and conditions may apply. Please see the reverse side of the invoice or packing slip.


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