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7/29/2019 Topic 13 Liquid Chromatography & Solid-Phase Extraction
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SKA6014ADVANCED ANALYTICAL CHEMISTRY
TOPIC 16
Liquid Chromatography and Solid-phase
Extraction
Azlan Kamari, PhDDepartment of Chemistry
Faculty of Science and Mathematics
Universiti Pendidikan Sultan Idris
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Basic LC Terminology Adsorption chromatography
The stationary phase is an adsorbent (like silica gel or any othersilica-based packing)
The separation is based on repeated adsorption-desorptionsteps.
Normal-phase chromatography The stationary bed is strongly polar in nature (e.g., silica gel),
and the mobile phase is nonpolar (such as n-hexane ortetrahydrofuran).
Polar samples are retained on the polar surface of the columnpacking longer than less polar materials.
Reversed-phase chromatography The stationary bed is nonpolar (hydrophobic) in nature, The mobile phase is a polar liquid, such as mixtures of water
and methanol or acetonitrile.
The more nonpolar the material is, the longer it will be retained.
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Size exclusion chromatography (SEC) column filled with material having precisely controlled pore
sizes, and the sample is simply sieved or filtered according toits solvated molecular size. Larger molecules are rapidly washed through the column;
smaller molecules penetrate inside the pores of the packingparticles and elute later.
Also called gel permeation chromatography (GCP)although the stationary phase is not restricted to a "gel"
Ion-exchange chromatography (IC) the stationary bed has a charged surface of opposite charge
to the sample ions. Used almost exclusively with ionic or ionizable samples. The stronger the charge on the sample, the stronger it will be
attracted to the ionic surface and thus, the longer it will taketo elute
The mobile phase is an aqueous buffer, where both pH and
ionic strength are used to control elution time
Basic LC Terminology
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Analytical Applications of LC The branches of the LC family:
Note this means analyte polarity
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Basic Mechanisms used in LC Separations
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High Performance Liquid Chromatography (HPLC)
HPLC utilizes a high-pressure liquid mobile phase (ca.100-300 bar) to separate the components of a mixture
These analytes are first dissolved in a solvent, and thenforced to flow through a packed small-particle
chromatographic column, where the mixture is resolvedinto its components
HP = high pressure and high performance
Resolution depends upon the extent of interactionbetween the solute components and the stationaryphase
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Differences between HPLC and Classical LC Small ID (2-5 mm), reusable stainless steel columns
Column packings with very small (3, 5 and 10 m) particlesand the continual development of new substances to be usedas stationary phases
Relatively high inlet pressures and controlled flow of themobile phase
Precise sample introduction without the need for largesamples
Special continuous flow detectors capable of handling smallflow rates and detecting very small amounts
Automated standardized instruments Rapid analysis
High resolution
From now on, LC refers to HPLC
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Advantages and Disadvantages of LC
Advantages:
Speed (minutes) High resolution Sensitivity Reproducibility
Accuracy Automation
Disadvantages: Cost Complexity Low sensitivity for some compounds Irreversibly adsorbed compounds not detected Co-elution difficult to detect
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More on Reversed-phase (RP) LC
RP is the most widely used mode of HPLC (75%?)
Separates molecules in solution on basis of theirhydrophobicity Non-polar stationary phase
Polar mobile phase
In practice: non polar functional group bonded to silica Stationary phase
functional group bonded to silica
this corresponds to a volume (Van deemter)
Alkyl groups ( C4, C8, C18)
retention increases exp. with chain length
Mobile Phases Polar solvent (water) with addition of less polar solvent (acetonitrile
or methanol)
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The Packed Column and the Stationary Phase
Packed LC columns, usually made of stainless steel and
carefully filled with material, are the heart of the LCexperiment
The stationary phase fills the column its properties arecritical to the separation
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Review of Molecular Interactions
The basis of separations (and most of chemistry)
Name Energy(kcal/mol)
Description
Covalent 100-300Hold molecules together,
orbital overlap
Ionic 50-200 Electrostatic attraction
Polar Hydrogen bonding Dipole-dipole -stacking
3-10Vary from electrostatic-typeinteractions (e.g. hydrogen
bonds) to much weaker
Non-Polar Van der Waals
(dispersion)1-5 Weak, induced dipole
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Retention Mechanisms in LC
HPLC is a dynamic adsorption process. Analyte molecules, whilemoving through the porous packing bead, tend to interact with thesurface adsorption sites. Depending on the HPLC mode, the differenttypes of the adsorption forces may be included in the retentionprocess
Hydrophobicinteractions are the main ones inreversed-phase
separations Dipole-dipole (polar) interactions are dominant innormal phasemode.
Ionic interactions are responsible for the retention inion-exchangechromatography
Retention in LC is competitive:Analyte molecules compete with the eluent molecules for theadsorption sites. So, the stronger analyte molecules interact withthe surface, and the weaker the eluent interaction, the longeranalyte will be retained on the surface
http://hplc.chem.shu.edu/HPLC/glossary/g_fgh.htmlhttp://hplc.chem.shu.edu/HPLC/glossary/g_pr.htmlhttp://hplc.chem.shu.edu/HPLC/glossary/g_mno.htmlhttp://hplc.chem.shu.edu/HPLC/glossary/g_ijl.htmlhttp://hplc.chem.shu.edu/HPLC/glossary/g_ijl.htmlhttp://hplc.chem.shu.edu/HPLC/glossary/g_ijl.htmlhttp://hplc.chem.shu.edu/HPLC/glossary/g_ijl.htmlhttp://hplc.chem.shu.edu/HPLC/glossary/g_mno.htmlhttp://hplc.chem.shu.edu/HPLC/glossary/g_pr.htmlhttp://hplc.chem.shu.edu/HPLC/glossary/g_pr.htmlhttp://hplc.chem.shu.edu/HPLC/glossary/g_pr.htmlhttp://hplc.chem.shu.edu/HPLC/glossary/g_fgh.html7/29/2019 Topic 13 Liquid Chromatography & Solid-Phase Extraction
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Retention Mechanisms in LC
Remember the elution order!
Normal-phase vs. reversed-phase LC
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Physical Properties of Stationary Phase Particles
HPLC separations are based on the surface interactions,
and depends on the types of the adsorption sites (surfacechemistry). Modern HPLC adsorbents are the small rigidporous particles with high surface area.
Key parameters: Particle size: 3 to 10 m Particle size distribution: as narrow as possible, usually within 10%
of the mean
Pore size: 70 to 300
Surface area: 50 to 250 m2/g Bonding phase density (number of adsorption sites per surface
unit): 1 to 5 per 1 nm2
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Electron micr ophotograph of spherical and ir regular sili ca particles. [W.R.Melander, C.Horvath,
Reversed-Phase Chromatography, in HPLC Advances and Perspectives, V2, Academic Press, 1980]
The Most Popular Particle: Silica
Macroporous spherical silica particle. [K.K.Unger,
Porous silica, Elsevier, 1979]
Different morphology for different applications:
Different chemistry:
Si OH Si OH O
H
H
Si
OH
OH
Free Silanol Adsorbed Water Geminal Silanol
Si
O
Si
O
DehydratedOxide Siloxane
O
H
O
H
Si
Si
O
Bound andReactiveSilanols
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Chemical Modifications to Silica Silica (or zirconia, or alumina) by itself cannot do the job needed
by modern LC users it must be functionalized and modified to
suit the analytical problem
Residualsilanols
S iO
S iO
S iO
S iO
SO
S i
S i OS i
OS iO H
S
O HO H
OH i
i
OO
Diagram from Crawford Scientific
Functionalizedgroups
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Chemical Modifications to Silica Groups are usually attached via reaction
of an organosilane (which can be pre-
polymerized in solution) Besides attaching groups, it is also
possible to polymerize the silica (or theattached group)
Purpose:stability at low pH, more
coverage High-carbon load
Monomeric phases are morereproducible (easier reactions to control)
Monomeric phases are also knownas sterically-protected
Endcapping: fully react the silicasurface, remove silanols and theiracidity, more coverage
Diagram from K. A. Lippa et al., Anal. Chem.2005, 77,7852-7861
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Common LC Stationary PhasesName Structure Description
Silica Normal phase, for separating polar, non-ionicorganics
PropylReversed-phase, for hydrophobic interaction
chromatography (proteins, peptides)
C8 Reversed-phase, like C18 but less retentive,used for pharmaceuticals, steroids,nucleotides
C18Reversed-phase, retains non-polar solutes
strongly. When bonded to 300A silica can beused for large proteins and macromolecules
CyanoReversed-phase and normal-phase, more
polar than C18, unique selectivity
AminoReversed-phase, normal-phase, and weak
anion exchange. RP used to separatecarbohydrates
Si C3H7
Si C8H17
Si C18H37
Si CH2CH2CH2CN
Si CH2CH2CH2NH2
Si OH
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Common LC Stationary Phases
Name Structure Description
PhenylReversed-phase, retains
aromatic molecules. Alsoused for HIC (proteins)
Diol
Both reversed-phase andnormal-phase utility. Used for
RP SEC, also used for NPseparations as a more robustalternative to silica (not ruined
by trace water)
Nitro
Normal-phase, separates
aromatic and alkene-containing moleculesSi NO2
Si O
OH
OH
Si CH2CH2CH2
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Polar Stationary Phase Interactions
Sorbents Interactions
CN
NH2
2OH
Dipole/Dipole
Hydrogen-Bonding
Hydrogen-Bonding
OH
Si NH
H
SiN
OH
C
OSi
OH
OOH
H
Source: Crawford Scientific.
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Ionic Stationary Phase Interactions
Sorbents InteractionsPRS
CBA
SAX
Electrostatic
Electrostatic
Electrostatic
H3+N
SO3-Si
Si
H3+N
O-
O
N+(CH3)3Si
-O3S
Source: Crawford Scientific.
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Non-Polar Stationary Phase Interactions
Sorbents Interactions
C8
PH
C2
van der Waals
van der Waals
van der Waals
Si
Si
Si
Source: Crawford Scientific.
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A Good Choice of Stationary Phase Depends on
the Analyte
NNHH22
NNHH33++
NNHH22
Functionality Analyte Mechanism
Hydrophobic
H-Bonding
Ionic
Non-Polar
Polar
Ion-Exchange
Source: Crawford Scientific.
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More Subtle Effects
Shape selectivity (correlates with stationary phase order),
temperature, coverage (and the role of bonding chemistry):
Diagram from K. A. Lippa et al., Anal. Chem.2005, 77,7852-7861
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More Subtle Effects
The effects of
temperature on theorder of thestationary phaseare often surprising:
Diagram from K. A. Lippa et al., Anal. Chem.2005, 77,7852-7861
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Chiral Stationary Phases
Interactions between chiral analytes (enantiomers andmolecules with more than 1 chiral center) and chiral
stationary phases are also possible Normal-phase is most common because of binding modes
A. Berthod, Chiral Recognition Mechanisms,Anal. Chem. 78, 2093-2099 (2006).
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Chiral Stationary Phases
Interactions between chiral analytes and chiral stationaryphases are also possible.
Common chiral stationary phases:
Name Chiral Recognition MechanismAnalyte and Mobile Phase
Requirements
Protein
based
Hydrophobic and electrostatic
interactions
Analyte must ionize, helpful if it
contains an aromatic. RPonly.
CyclodextrinInclusion complexation, H-
bondingPolar and aromatic groups, RP
and NP.
Polymer-
basedcarbohydrat
es
Inclusion interactions, attractiveinteractions
H-bonding donors/acceptors,steric bulk at chiral center, RPand NP.
PirkleH-bonding, interactions,dipole-dipole interactions
H-bonding donor/acceptors,mostly NP.
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A Chiral LC Separation Example: separation of
naproxen enantiomers
Chiral AGP column AGP = 1-acid glycoprotein
(orosomucoid), 181 aminoacid residues and 14 sialicacid residues
Isocratic (no change in mobile
phase composition duringseparation)
O
HO
(S)
O
(S)-naproxen
O
HO
(R)
O
(R)-naproxen
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Ion Chromatography (IC)
Form of LC, also known as ion-exchange chromatography Basic mechanism is electrostatic exchange:
Source: Rubinson and Rubinson, Contemporary Instrumental Analysis, Prentice Hall Publishing.
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Typical IC Results
Example:an isocratic method formonovalent cations in ammoniumnitrate based explosives
Detection limits 50-100 ppb, maxworking range 40 ppm
Method: Sample Loop Volume: 50 L
Columns: IonPac CS3 Analytical,IonPac CG3 Guard Eluent: 25 mM HCl, 0.1 mM DAPHCl,
4% Acetonitrile Eluent Flow Rate: 1.0 mL/min Suppressor: Cation MicroMembrane
Suppressor (CMMS) Regenerant: 100 mMTetrabutylammonium Hydroxide
Detector: Conductivity, 30 S fullscale
Injection Volume: 50 L
From Dionex Application Note 121R
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Mobile Phases in LC
Mobile phases differ for each LC mode Normal phasesolvents are mainly nonpolar Reversed-phaseeluents are usually a mixture of water with some
polar organic solvent such as acetonitrile. Size-exclusionLC has special requirements for mobile phases
Must dissolve polymers Must also suppress all possible interactions of the sample
molecule with the surface of the packing material
The type and composition of the mobilephase (eluent) is one of the variablesinfluencing LC separations
Desirable properties: Purity Detector compatibility Solubility of the sample
Low viscosity Chemical inertness Reasonable price
http://hplc.chem.shu.edu/HPLC/glossary/g_mno.htmlhttp://hplc.chem.shu.edu/HPLC/glossary/g_pr.htmlhttp://hplc.chem.shu.edu/HPLC/glossary/g_s.htmlhttp://hplc.chem.shu.edu/HPLC/glossary/g_s.htmlhttp://hplc.chem.shu.edu/HPLC/glossary/g_s.htmlhttp://hplc.chem.shu.edu/HPLC/glossary/g_s.htmlhttp://hplc.chem.shu.edu/HPLC/glossary/g_pr.htmlhttp://hplc.chem.shu.edu/HPLC/glossary/g_pr.htmlhttp://hplc.chem.shu.edu/HPLC/glossary/g_pr.htmlhttp://hplc.chem.shu.edu/HPLC/glossary/g_mno.html7/29/2019 Topic 13 Liquid Chromatography & Solid-Phase Extraction
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Isocratic elution: the eluent composition remains constant as it
is pumped through the column during the whole analysis.Gradient elution: the eluent composition (and strength) issteadily changed during the run.
Control of Eluent Polarity
time
%m
obilephase
kkNR
s11
4
*
*11
4 k
kNR
s
where k* is the k at the midpoint of the column
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LC Instrumentation
Pumps, Mixers
and InjectorsColumn Detector(s) Computer
LC Instrumentation
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LC Instrumentation
The Agilent 1100, a typical modern LC system
Solvent reservoirs
Solvent degasser
Pump
Autosampler
Column oven
DAD
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Review: The Purpose of Key LC Components
column separation chemistry
detectorsignal transductionamplification/scalingfiltering
A/Ddata acquisitiondigitization
tubing to detector flow cell
analog output
digital output
chromatogramdigital processingdata analysis
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The LC Pump(s)
Modern pumps have the following parameters:Flow rate range: 0.01 to 10 ml/min
Pressure range from 1-5,000 psiPressure pulsations : less than1 %
Types of PumpsConstant pressure pumps
Constant flow pumps
Reciprocating Piston Pump (90% of HPLCs)
small internal volumepulsed flow
Syringe type pumps (Displacement Pumps)limited solvent capacity
Pneumnatic Pumps (pressure)
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Temperature Control in LC
Thermoelectric heating/cooling
the ability of a surface toproduce or absorb heatwhen current is appliedacross the junction of twodissimilar conductors orsemiconducted
The effect can be reversed (i.e.heating turned to cooling) byreversing the DC currentthrough the junction
Also known as the Peltier effectafter its 1834 discoverer, aFrench watch maker
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Overview of LC Detectors
Common HPLC detectors
Refractive Index UV/Vis
Fixed Wavelength Variable Wavelength Diode Array
Fluorescence Detector
Less common: Conductivity Mass-spectrometric (LC/MS)
Evaporative light scattering (ELSD)
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Desirable Features of an LC Detector
1. Low drift and noise level2. High sensitivity (ability to discriminate between
small differences in analyte concentration)3. Fast response4. Wide linear dynamic range
5. Low dead volume6. Cell design that eliminates remixing of separated
bands7. Insensitivity to changes in types of solvent, flow
rate, temp8. Operational simplicity and reliability9. Non-destructive
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Baseline Noise and Drift
D t t R
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Detector Response
The definition of detector response depends on whether
it is mass sensitive or concentration sensitive
Mass sensitive mV/mass/unit time
R = hw/sM
Concentration sensitive mV/mass/unit volumeR = hwF/sM
h = peak height mV
W = width at .607 of heightF = flow rate
M = mass of solute
s = chart speed
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Cell Efficiency
Example:
column 15,000 plates15 cm long
2 min tR2 ml at 1 ml /min
peak width of 80uL
flow cell of 20 ul
only four measurements
Things to note:parallel light beam
flow cell volume
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Ultraviolet/Visible Spectroscopic Detectors
infrared (IR) 2,500 - 50,000 nm
near infrared 800 - 2,500 nm
visible 400 - 800 nm
ultraviolet (UV) 190 - 400 nm
Any chemical compound could interact with the electromagnetic field. Beam of the
electromagnetic radiation passed through the detector flow-cell will experience
some change in its intensity due to this interaction. Measurement of this changes is
the basis of the most optical HPLC detectors.
Name Chromophore Wavelength [nm] Molar extinction, eacetylide -C=C 175-180 6,000
Aldehyde -CHO 210 1,500
amine -NH2 195 2,800
azo -N=N- 285-400 3-25
bromide -Br 208 300
carboxyl -COOH 200-210 50 - 70
disulphide -S-S- 194 5,500
ester -COOR 205 50
ether -O- 185 1,000
ketone >C=O 195 1,000
nitrate -ONO2 270 12
nitrile -C=N 160 -
nitrite -ONO 220 - 230 1000-2000
nitro -NO2 210 strong
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Fixed / Variable Wavelength Detectors
Mercury vapor lamp emit very intense light at 253.7nm. By filtering out all other emitted wavelengths,manufacturers have been able to utilize this 254 nmline to provide stable, highly sensitive detectorscapable of measuring subnanogram quantities ofany components which contains aromatic ring. The254 nm was chosen since the most intense line ofmercury lamp is 254 nm, and most of UV absorbingcompounds have some absorbance at 254 nm.
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Diode Array Detectors
Diode array detectors can acquire all UV-Visible
wavelengths at once.
Advantages:
Sensitivity
(multiplex) Speed
Disadvantages:
Resolution
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Other Detectors
Fluorescence Detector
Electrochemical Detector
Evaporative Light Scattering
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Putting it All Together: LC Method Development
The importance without a good method: Co-elution can be missed Unable to detect/assay key components
Basic consequences of method changes:
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Choosing an LC Approach
Goals of a separation:
Resolution (Rs) > 1.5 Short separation time (5-30 minutes)
Good quantitative precision/accuracy
Acceptable backpressure
Narrow peaks Minimal solvent use
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Overall Strategy
First select an
appropriate method If LC is best, then
determine nature ofthe sample
Exploratory RP runs,
i.e. fast simplegradients with C18phases, are usuallyhelpful in assessingretention and polarity
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Solid-phase Extraction (SPE)
What is SPE?
The separation of an analyte or analytes from a mixture ofcompounds by selective partitioning of the compoundsbetween a solid phase (sorbent) and a liquid phase (solvent)
Comparison with conventional liquid-liquid extraction (e.g.
the organic sep funnel approach): SPE: selective towards functional groups (better)
LLE: selective towards solubility
SPE: more choices because no miscibility (better)
LLE: must avoid miscible solvents SPE: concentrates analytes (better)
LLE: can concentrate analyte after stripping
The Typical SPE Process
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The Typical SPE Process
Conditioning: solvates the sorbent
Equilibration: removes excess conditioning solvent,matches with analytical conditions (prevents shock)
SampleApplication
InterferenceElution
AnalyteElution
ColumnConditioning
ColumnEquilibration
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Solid-phase Extraction
Conditioning the cartridge:
Not conditioned Conditioned
SPE cartridges have a range of chemistries that are oftensimilar to those of LC stationary phases, but are optimizedfor adsorption/desorption
Solid-phase Extraction
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Solid-phase Extraction
Automated SPE systems for sample cleanup the SparkSymbiosisTM
Can be hyphenatedwith LC, MS, NMR,
etc or used as astand-alone samplepretreatment