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Introduction and Principles of Gas Chromatography
Introduction and Principles of Gas Chromatography
Jaap de Zeeuw
Restek, Middelburg, The Netherlands
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• Definition and Uses of Gas Chromatography
• GC Components and Types of Columns
• Factors Affecting Chromatographic Separation
• Basic Terminology and Theory
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Which Industries Use Chromatography?Which Industries Use Chromatography?
• Chemical/Petrochemical
• Clinical/Forensic
• Consumer Products
• Environmental
• Food
• Pharmaceutical
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Why Gas Chromatography?Why Gas Chromatography?
• Simple
• Cheap (can be automated)
• Short analysis times
• High Accuracy
• Qualitative and Quantitative analysis
• Applicable in % to ppb level
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General• Nitrogen
• Hydrocarbons
• PCBs*
• Tars, Oils, waxes
• Radon
• Olefins
• Sulfur components
• Ethylene Glycol
• Aldehydes
• Phenols & cresols
• Amines
• Organic sulfur
• Organo metallic compounds
• Oxygenates
Which components do we see in Natural Gas?Which components do we see in Natural Gas?
* Ref: California proposed Public Gas Warning List
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Important Separations in Natural Gas analysisImportant Separations in Natural Gas analysis
50 m x 0.32 mm CP-Sil 5 CB, 1.2 um
Helium, 160 kPa
40 °C, --> 200 °C, 5 °C/min
C8C9
C10C11
40 min
C6
C5
50 m x 0.32 mm CP-Sil 5 CB, 5 µm
40,2 °C
H2, 65 kPa; TCD
C4i-C4
C3
H2S
C2
CO2
CH4 + N2
10 min
•Methanol well separatedfrom matrix
•Symmetrical peak formethanol
•Methanol well separatedfrom matrix•Symmetrical peak for
methanol
Methanol 20 ppm
Natural gas peak
10 m x 0.53 mm CP-Lowox
150°C(2 min) -->200°C, 10 °C/min
Direct Injection, 50 µl
9 min
Hydrocarbons Nitrogen - methane
OxygenatesSulfur
RtxRtxRtxRtx----1 Thin1 Thin1 Thin1 Thin----film film film film Rt QRt QRt QRt Q----BONDBONDBONDBOND
RtxRtxRtxRtx----1 thick1 thick1 thick1 thick----filmfilmfilmfilm Polar phase (wax)Polar phase (wax)Polar phase (wax)Polar phase (wax)
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IUPAC Definition of ChromatographyIUPAC Definition of Chromatography
“A physical method of separating sample components from a mixture by selective adsorption or partitioning of the analyte between two phases:
a “mobile phase and a stationary phase”
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Chromatography PhasesChromatography Phases
Mobile Phases
• Liquids (methanol, water…)
• Changing dielectric strength, temperature, pH
• Gases (nitrogen, helium, hydrogen, argon)
Stationary Phases
• Solids (alumina, silica, polymers, carbon…)
• Adsorption chromatography
• Liquids (siloxanes, polyethylene glycols…)
• Partition (distribution) chromatography
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stationary phasesstationary phases• Specialized polymer chemists
• Lowest specifications possible
• Innovation, unique phase technologies
High purity /viscosity polymers for low-bleed and stable stationary phases
Commercial phases are very dirty..
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GC Components and Types of ColumnsGC Components and Types of Columns
• Components of a Gas Chromatograph
• Types of GC Columns
• Types of GC Capillary Columns
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Components of a Gas ChromatographComponents of a Gas Chromatograph
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12
Gas Purification EquipmentGas Purification Equipment
Triple Filter
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Saturated O2 Indicator
New O2Indicator
Triple filter
O2 indicator
Triple filter
O2 indicator
Color change
New Filter Used Filter
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Triple filter
H2O indicator
Triple filter
H2O indicator
Used FilterNew Filter
New H2O Indicator
Saturated H2O Indicator
Color change
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Components of a Gas ChromatographComponents of a Gas Chromatograph
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Types of GC columnsTypes of GC columns
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Types of GC ColumnsTypes of GC Columns
Packed Capillary
Length, [meters] 1-6 5 - 150
ID, [millimeters] 0.53-4 0.1 - 0.53
Theoretical plates 5,000 (2m) 120,000 (30m)
Capacity [ng] 10,000 50 (0.25mm ID)
Amount of Liquid phase 1-30 % 0.1-7.0 μm
Price €100 €400 (30m, 0.25 mm ID)
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Capillary Column MaterialsCapillary Column Materials
Fused Silica
• Synthetic, amorphous glass with very low (<1ppm) metallic oxide impurities
• Protective outer coating of polyimide resin imparts flexibility but coil diameter is limited
• Excellent inertness and useable up to approximately 380oC(400oC)
Metal Tubing-MXT
• MXT, stainless steel, surface coated, Siltek deactivated• Can be coiled to small diameter• Almost as inert as fused silica but useable up to approx. 450ºC
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Types of GC Capillary Columns Types of GC Capillary Columns
WCOT (Wall Coated Open Tubular)• Partition chromatography
• Typical phases: Siloxanes and Polyethylene glycols
• 0.10 to 0.53mm internal diameters
PLOT (Porous Layer Open Tubular)• Adsorption chromatography
• Gases, light hydrocarbons/solvents analysis
• Adsorbents: molecular sieve, porous polymers, alumina (<1 um particle diameter)
• 0.25 to 0.53mm internal diameter
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Compounds Amenable to Gas ChromatographyCompounds Amenable to Gas Chromatography
• Should be thermally stable
• Should be un-reactive and non-absorptive to chromatographic system
• Should be volatile at temperatures below 350-400°C
• Presence of polar groups reduces volatility
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Sample TransferSample Transfer
Injection: how the sample is transferred to the column
• As a liquid via syringe
• Non-liquid techniques
• Purge & trap
• Headspace
• Gas sample loop
•NOTE: It is critical to get the sample into the column in a focused band…•NOTE: It is critical to get the sample into the column in a focused band…
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Peak width of eluting componentPeak width of eluting component
σ injection + σ column + σ detection = Σ peak
+
++
+ =
=
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Sample introductionSample introduction
Need to realize smallest possible sample band on the capillary column..
Injection Techniques commonly used:
• Split
• Splitless
• Thermal desorption (PTV)
• Headspace
• On-Column
% - 5 ppm levels
Small amount injected
Narrow injection band
ppm-ppb levels
Large amount injected
Must use a focusing mechanism..
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Sample TransferHow to Get a Focused Initial Band
Sample TransferHow to Get a Focused Initial Band
• Analyte Focusing
• Higher boiling components
• Increase retention (stationary phases type, film and temperature)
• Solvent Focusing
• All components, but must elute later then solvent itself
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Analyte focusingAnalyte focusing
• If analytes are high boiling, they will be focused by the retention of the stationary phase
• If analytes have lower boiling points this will show itself as “smeared” peaks
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Analyte focusing: exampleAnalyte focusing: example
“Smeared” peaksNot enough retention for focusing
“focused” peaks
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• In splitless injection some solvent condensation is required to create the ”solvent” effect.
• This solvent will trap (= focus) compounds and makes sure that a narrow band is formed.
• Realization: Oven temperature during injection must be 20°C below the Bp of the solvent.
Solvent-Focusing in splitless injection
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Splitless Injection at oven temperature 20°C BELOW BP of solventSplitless Injection at oven temperature 20°C BELOW BP of solvent
The “solvent effect” makes sure all peaks are focused
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No Focusing:
Long Initial Sample Band, broad peaks
Focusing:
Correct solvent peak and narrow peaks
Focusing in Splitless injectionFocusing in Splitless injection
2 4 6 8 10
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Split/Splitless injection systemSplit/Splitless injection system
Silicone septum
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Split injection ConsiderationsSplit injection Considerations
• 1-3 mL/min into the column
• 10-200 mL/min in the inlet (mostly split vent flow)
Split Ratio:Column flow rate : 1 mL/min
Split vent flow rate : 100 mL/min
Split ratio = Column Flow rate = 1 Split-vent + Column flow rate 100+1
Split ratio ~ 1:100
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0 10
25:1 Split
0 10
50:1 Split
Impact of Split Ratio Impact of Split Ratio
Increasing the split ratio decreases the peak area, if all other variables are equal.
Column Head Pressure
Total Flow Septum Purge
Split Vent Purge Vent
SPLIT
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Factors impacting the separationFactors impacting the separation
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Non-Column Factors Affecting SeparationNon-Column Factors Affecting Separation
• Carrier gas: type & linear velocity
• Temperature
• Injection bandwidth
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Gas Carrier and Linear VelocityGas Carrier and Linear Velocity
Hydrogen : 40-50 cm/sec
Helium : 25-35 cm/sec
Nitrogen : 10-15 cm/sec
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Carrier Gas and Linear VelocityCarrier Gas and Linear Velocity
• Isothermal Analysis
• Hydrogen is 2x faster than helium and 4x faster than nitrogen
• Temperature Programmed Analysis
• Need to optimize temperature program to get SAME elution temperatures;
• Also here Hydrogen is 2x faster then helium
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Hydrogen is of interestHydrogen is of interest
• Fastest analysis
• Availability ( can be generated)
• Need less sample for same signal (maintenance)
Deal with safety issues
• Setting of constant flow (impossible to built up high H2 levels
• Use H2-detection (will cost $, but makes safety officer happy)
• Use metal (MXT) columns (also VERY inert)
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Column lengthColumn length
10/15 m < 10 components and Fast analysis
25/30 m 10 - 20 components
50/60 m complex mixtures: > 20 components
Most widely used is 30m
Do we need extreme LONG columns?
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Summary:Effect of Length
Summary:Effect of Length
• Retention time proportional to length in isothermal analysis but not proportional in temperature program analysis
• Gain in resolution is not double, but € are
The BETTER the
Chromatographer….. ... the SHORTERthe column..
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Examples where we need LONG columnsExamples where we need LONG columns
• Detailed Hydrocarbon Analysis (>300 components)
• Bio-ethanol (to get isobutane-methanol separation)
• Separation of PCB isomers (209 congeners)
• Separation of Cis and Trans FAME isomers
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Column Internal DiameterColumn Internal Diameter
0.53 mm
• High flow and loadability
• Direct injection via insert or valve (analyzer)
• TCD detection
• Retention gap for On-column
0.32 mm
• On-column injection;
• Thick films are possible
• Electronic Gas / Pressure Control
0.25 mm
• Ideal for split and splitless injection
• Relatively high plate number
0.18/0.15/0.10 mm
• Short analysis times
• Low bleed / GC x GC
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Internal diameterInternal diameter
0.10mm 0.15/0.18mm 0.25mm 0.32mm 0.53 mm
Practically the following dimensions are used :
0.10 <1 %
0.15/0.18 mm 3 %
0.25 mm 45 %
0.32 mm 35 %
0.53 mm 10 %
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Basic Terminology and TheoryBasic Terminology and Theory
• Resolution (R)
• Theoretical Plates (Neff)
• Height Equivalent to a Theoretical Plate (HETP)
• Phase Ratio (ß)
• Retention (Capacity) Factor (k)
• Retention Time (t)
• Column Selectivity Factor (α)
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The Resolution RsThe Resolution Rs
� Quality of Separation between 2 Peaks
� Dimensionless
� Parameters:
- Selectivity
- Retention
- Efficiency
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ResolutionResolution
Resolution depends on:
α : Selectivity
k’ : Retention Factor
Nth : Plate Number
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Impact of Nth on resolutionImpact of Nth on resolution
Increase N:
Longer column
Smaller Internal diameter
Impact of Higher N using 2x
smaller diameter, same length
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Impact of k on resolutionImpact of k on resolution
Increase k:
use thicker film (same column dimensions)
decrease oven temperature (ever 15C, k changes a factor 2)
CH4Impact of using 2x thicker film,
Same column dimensions and
temp.
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Impact of α on resolutionImpact of α on resolution
Increase alpha:
use different stationary phase (same column dimensions)
CH4Impact of using different phase
with higher selectivity
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Effective Theoretical Plates (Neff)Effective Theoretical Plates (Neff)
Dead
Time Wb
Neff = 16 ( )t'W
R2
b
t'R
•Adjusted Retention Time = Retention Time – Dead Time•Wb = Width between tangents of a peak at baseline intercept
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Theoretical platesTheoretical plates
• This nr is the “number” of “touches” of a component with the stationary phase, while it moves through the column.
1 touch = 1 plate
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One theoretical plate
Two Theoretical plates
Three Theoretical plates
Carrier gas
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Theoretical platesTheoretical plates
• This nr is the “number” of “touches” of a component with the stationary phase, while it moves through the column.
• The more “touches”, the more plates, the better the separation
• Impacted by:
• Column diameter
• Column length
1 touch = 1 plate
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Height Equivalent to a Theoretical Plate (h)Height Equivalent to a Theoretical Plate (h)
H depends on Flow and Column ID
Lower h = Improved Separation
=hL
N
Basic equation used in GC: Van Deemter equation
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Van Deemter equationVan Deemter equation
H = A + B/u + Cu
H = height of a theoretical plate
U = average linear gas velocity
A, B, C = different contribution factors to peak broadening
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The A - termThe A - term
Contribution to peak broadening due to different path length (eddy diffusion)
For capillary columns A = 0
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The B - termThe B - term
Contribution to peak broadening due to multidirectional diffusion in the Gas phase
Indirect proportional to flow rate
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The B - termThe B - term
Contribution to peak broadening due to multidirectional diffusion in the Gas phase
Indirect proportional to flow rate
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The B - termThe B - term
Contribution to peak broadening due to multidirectional diffusion in the Gas phase
Indirect proportional to flow rate
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The C - termThe C - term
Resistance to mass transfer in the liquid phase
Direct proportional to flow rate
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The C - termThe C - term
Resistance to mass transfer in the liquid phase
Direct proportional to flow rate
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The C - termThe C - term
Resistance to mass transfer in the liquid phase
Direct proportional to flow rate
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The C - termThe C - term
Resistance to mass transfer in the liquid phase
Direct proportional to flow rate
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Van Deemter: there is an optimal flowVan Deemter: there is an optimal flow
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Van deemter: Gas Carrier and Linear VelocitiesVan deemter: Gas Carrier and Linear Velocities
1.0
0.6
0.2
HETP (
mm
)
Van Deemter Plot
Average Linear Velocity (cm/sec)
N2
He
H2
10 20 30 40 50 60 70
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Stationary phase Film ThicknessKapacity factor “K” and RetentionStationary phase Film ThicknessKapacity factor “K” and Retention
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Retention (Capacity) Factor : kRetention (Capacity) Factor : k
Practical the most effective separation occurs when the k value for an analyte is minimal 5.
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Retention (k) can be influencedRetention (k) can be influenced
• By film thickness
Retention is LINEAR with film thickness
• By temperature
Every 15°C change in oven temp. the k will change about a factor 2
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Film Thickness and Beta (phase ratio)Film Thickness and Beta (phase ratio)
0.25 μm 1.0 μm 3.0 μm
k is linear with Film thickness
6 min0.5 min 2 min
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Phase Ratio (ß)Phase Ratio (ß)
β =column radius
2x film thickness
mobile phase volume
stationary phase volume =
Phase ratio is important if you want to change column internal diameter;For the most easy method conversions, one should try to keep the phase ratio the same
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Film Thickness Effects : 0.25µm Rtx-1Film Thickness Effects : 0.25µm Rtx-1
2 4 6 [min]
1-6
78
9
30m, 0.32mm ID, 0.25µm Rtx-1
70ºC isothermal 1. 1-butanol 2. benzene 3. 2-pentanone 4. C7
5. 1-nitropropane 6. pyridine7. C8
8. C9
9. C10 5.5 min
K C10 = 4.5
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1. 1-butanol 2. benzene 3. 2-pentanone 4. C7
5. 1-nitropropane 6. pyridine7. C8
8. C9
9. C10 19 min.
1-6
7
89
Film Thickness Effects: 1.0µm Rtx-1Film Thickness Effects: 1.0µm Rtx-1
K C10 = 18
30m, 0.32mm ID, 1.00µm Rtx-1
70ºC isothermal
[min]0 10 20
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4 8 12 [min]
7
8
16 20 24
6
5
4
1,2,3
Film Thickness Effects : 3.0µm Rtx-1Film Thickness Effects : 3.0µm Rtx-1
Peak 9 elutes at 55 min..
K C10 = 54
1. 1-butanol 2. benzene 3. 2-pentanone 4. C7
5. 1-nitropropane 6. pyridine7. C8
8. C9
9. C10 55 min.
30m, 0.32mm ID, 3.0µm Rtx-1
70ºC isothermal
0
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Stationary Phase: Column Selectivity (α)Stationary Phase: Column Selectivity (α)
• Interactions with the stationary phase
• pi – pi
• Van der Waals (london)
• Hydrogen bonding
• Depends upon the Chemical composition of the phase
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Column SelectivityChemical Composition of PhasesColumn SelectivityChemical Composition of Phases
Rtx®-1 Stationary Phase100% dimethylpolysiloxane
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Column SelectivityChemical Composition of PhasesColumn SelectivityChemical Composition of Phases
Rtx®-5 Stationary Phase5% diphenyl
95% dimethylpolysiloxane
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Column Efficiency, Selectivity, and Peak SymmetryColumn Efficiency, Selectivity, and Peak Symmetry
Not Efficient, but Selective
Efficient and SelectiveEfficient, but not Selective
Not Efficient, not Selective
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Examples of selectivity..
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Selectivity via geometrySeparation of Para - en Meta XylenesSelectivity via geometrySeparation of Para - en Meta Xylenes
100 % methyl
Rtx-1
50 % phenyl
50 % methyl
Rtx-17
PEG
Rtx-WaxRt-TCEPSqualane
P
P
P + M P + MPMM
M
OO
O
O
O
Non-Polar Polar
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Shape selectivityShape selectivity
Using selected cyclodextrins as modifiers
p-xylene
m-xylene
o-xylene
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Basic rule in GC stationary phase..Basic rule in GC stationary phase..
Solubility of sample component in the stationary phase based upon “likes dissolve likes.”
“ choose a stationary phase that “looks like” the components you want to separate..”
Hydrocarbons 100% PDMS Rtx-1
Aromatic subst. Phenyl subst. PDMS Rtx-5, 17, 35 RT-Dioxins
Halogenates Arom. Fluorimated-phenyl Rtx-440, Cl-Pesticides, Rtx-200
Solvents Cyano /phenyl Rtx-1301/624
Alcohols PEG Stablewax
Double bonds Cyano propyl Rt 2330, 2460
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(GC-MS) Presence of Diethylene Glycol and Ethylene Glycol in Toothpaste (GC-MS) Presence of Diethylene Glycol and Ethylene Glycol in Toothpaste
Column: Stablewax
Dissolves : ‘likes like”
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Hydrocarbons on 100% PDMSHydrocarbons on 100% PDMSC
3n-C
4
n-C
5
n-C
6
n-C
7
n-C
8
n-C
9
n-C
10
0 50 100 150
Column : 100 x 0.25 mm Rtx-1 PONA CB, tuned 5%phenyl PDMS
Oven : 5°C, 10 min -> 50°C, 5°C/min, 54 min, --> 200°C, 1.3 °C/min
Carrier gas : He, 24 cm/s, 39.3 Psi; Injection Split, 1 : 150; Detection : FID;
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PAHs using an Rxi-17Sil MS (30m x 0.25mm x 0.25μm)PAHs using an Rxi-17Sil MS (30m x 0.25mm x 0.25μm)
benzo(b
)flu
ora
nth
ene
benzo(k
)flu
ora
nth
ene benzo(j
)flu
ora
nth
ene
12
3
45
6
78
910
11
12/13
14,15,16
17
18
19
20
21/22
2324
25
26
27
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Detection in GCDetection in GC