Introduction and Principles of Gas Chromatography Jaap de Zeeuw Restek, Middelburg, The Netherlands Jaap.dezeeuw@restek.com
Definition and Uses of Gas Chromatography GC Components and Types of Columns Factors Affecting Chromatographic Separation Basic Terminology and Theory
Which Industries Use Chromatography? Chemical/Petrochemical Clinical/Forensic Consumer Products Environmental Food Pharmaceutical
Why Gas Chromatography? Simple Cheap (can be automated) Short analysis times High Accuracy Qualitative and Quantitative analysis Applicable in % to ppb level
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? * Ref: California proposed Public Gas Warning List
Important Separations in Natural Gas analysis Hydrocarbons Rtx-1 Thin-film Nitrogen - methane Rt Q-BOND C5 C6 50 m x 0.32 mm CP-Sil 5 CB, 1.2 um Helium, 160 kpa 40 C, --> 200 C, 5 C/min C8 C9 C10 C11 40 min Sulfur Rtx-1 thick-film Oxygenates Polar phase (wax) CO2 H2S 50 m x 0.32 mm CP-Sil 5 CB, 5 µm 40,2 C H2, 65 kpa; TCD Natural gas peak 10 m x 0.53 mm CP-Lowox 150 C(2 min) -->200 C, 10 C/min Direct Injection, 50 µl CH4 + N2 C2 C3 Methanol 20 ppm i-c4 C4 10 min 9 min Methanol well separated from matrix Symmetrical peak for methanol
IUPAC 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
Chromatography 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
stationary phases Commercial phases are very dirty.. Specialized polymer chemists Lowest specifications possible Innovation, unique phase technologies High purity /viscosity polymers for lowbleed and stable stationary phases
GC Components and Types of Columns Components of a Gas Chromatograph Types of GC Columns Types of GC Capillary Columns
Components of a Gas Chromatograph
Gas Purification Equipment Triple Filter 12
New Filter Triple filter O2 indicator Used Filter New O2 Indicator Saturated O2 Indicator Color change
New Filter Triple filter H2O indicator Used Filter New H2O Indicator Saturated H2O Indicator Color change
Components of a Gas Chromatograph
Types of GC columns
Types 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)
Capillary 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 380 o C(400 o C) 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
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
Compounds 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
Sample 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
Peak width of eluting component σ injection + σ column + σ detection = Σ peak + + = + + =
Sample 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..
Sample Transfer How 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
Analyte 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
Analyte focusing: example focused peaks Smeared peaks Not enough retention for focusing
Solvent-Focusing in splitless injection 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.
Splitless Injection at oven temperature 20 C BELOW BP of solvent The solvent effect makes sure all peaks are focused 2 4 6 8 10
Focusing in Splitless injection No Focusing: Long Initial Sample Band, broad peaks Focusing: Correct solvent peak and narrow peaks 2 4 6 8 10
Split/Splitless injection system Silicone septum
Split 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
Impact of Split Ratio Increasing the split ratio decreases the peak area, if all other variables are equal. 25:1 Split 50:1 Split SPLIT Column Head Pressure Total Flow Septum Purge Split Vent Purge Vent 0 10 0 10
Factors impacting the separation
Non-Column Factors Affecting Separation Carrier gas: type & linear velocity Temperature Injection bandwidth
Gas Carrier and Linear Velocity Hydrogen : 40-50 cm/sec Helium : 25-35 cm/sec Nitrogen : 10-15 cm/sec
Carrier 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
Hydrogen 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)
Column 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?
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 SHORTER the column..
Examples 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
Column 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
Internal 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 %
Basic Terminology and Theory Resolution (R) Theoretical Plates (N eff ) Height Equivalent to a Theoretical Plate (HETP) Phase Ratio (ß) Retention (Capacity) Factor (k) Retention Time (t) Column Selectivity Factor (α)
The Resolution R s Quality of Separation between 2 Peaks Dimensionless Parameters: - Selectivity - Retention - Efficiency
Resolution Resolution depends on: α : Selectivity k : Retention Factor N th : Plate Number
Impact of Nth on resolution Increase N: Longer column Smaller Internal diameter Impact of Higher N using 2x smaller diameter, same length
Impact of k on resolution Increase k: use thicker film (same column dimensions) decrease oven temperature (ever 15C, k changes a factor 2) CH4 Impact of using 2x thicker film, Same column dimensions and temp.
Impact of α on resolution Increase alpha: use different stationary phase (same column dimensions) CH4 Impact of using different phase with higher selectivity
Effective Theoretical Plates (N eff ) N eff = 16 t' ( ) W R b 2 Dead Time W b Adjusted Retention Time = Retention Time Dead Time W b = Width between tangents of a peak at baseline intercept t' R
Theoretical plates 1 touch = 1 plate This nr is the number of touches of a component with the stationary phase, while it moves through the column.
One theoretical plate Two Theoretical plates Three Theoretical plates Carrier gas
Theoretical plates 1 touch = 1 plate 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
Height Equivalent to a Theoretical Plate (h) h = L N H depends on Flow and Column ID Lower h = Improved Separation Basic equation used in GC: Van Deemter equation
Van 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
The A - term Contribution to peak broadening due to different path length (eddy diffusion) For capillary columns A = 0
The B - term Contribution to peak broadening due to multidirectional diffusion in the Gas phase Indirect proportional to flow rate
The B - term Contribution to peak broadening due to multidirectional diffusion in the Gas phase Indirect proportional to flow rate
The B - term Contribution to peak broadening due to multidirectional diffusion in the Gas phase Indirect proportional to flow rate
The C - term Resistance to mass transfer in the liquid phase Direct proportional to flow rate
The C - term Resistance to mass transfer in the liquid phase Direct proportional to flow rate
The C - term Resistance to mass transfer in the liquid phase Direct proportional to flow rate
The C - term Resistance to mass transfer in the liquid phase Direct proportional to flow rate
Van Deemter: there is an optimal flow
Van deemter: Gas Carrier and Linear Velocities Van Deemter Plot 1.0 N 2 HETP (mm) 0.6 He H 2 0.2 10 20 30 40 50 60 70 Average Linear Velocity (cm/sec)
Stationary phase Film Thickness Kapacity factor K and Retention
Retention (Capacity) Factor : k Practical the most effective separation occurs when the k value for an analyte is minimal 5.
Retention (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
Film Thickness and Beta (phase ratio) 0.25 μm 1.0 μm 3.0 μm 0.5 min 2 min k is linear with Film thickness 6 min
Phase Ratio (ß) β = mobile phase volume stationary phase volume = column radius 2x film thickness 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
Film Thickness Effects : 0.25µm Rtx-1 30m, 0.32mm ID, 0.25µm Rtx-1 70ºC isothermal 1. 1-butanol 1-6 2. benzene 3. 2-pentanone 7 4. C 7 8 K C10 = 4.5 5. 1-nitropropane 6. pyridine 9 7. C 8 8. C 9 9. C 10 5.5 min 2 4 6 [min]
Film Thickness Effects: 1.0µm Rtx-1 30m, 0.32mm ID, 1.00µm Rtx-1 70ºC isothermal 1-6 7 8 K C10 = 18 9 1. 1-butanol 2. benzene 3. 2-pentanone 4. C 7 5. 1-nitropropane 6. pyridine 7. C 8 8. C 9 9. C 10 19 min. 0 10 20 [min]
Film Thickness Effects : 3.0µm Rtx-1 30m, 0.32mm ID, 3.0µm Rtx-1 70ºC isothermal 1,2,3 6 5 4 7 K C10 = 54 8 1. 1-butanol 2. benzene 3. 2-pentanone 4. C 7 5. 1-nitropropane 6. pyridine 7. C 8 8. C 9 9. C 10 55 min. 0 16 20 24 4 8 12 [min] Peak 9 elutes at 55 min..
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
Column Selectivity Chemical Composition of Phases Rtx -1 Stationary Phase 100% dimethylpolysiloxane
Column Selectivity Chemical Composition of Phases Rtx -5 Stationary Phase 5% diphenyl 95% dimethylpolysiloxane
Column Efficiency, Selectivity, and Peak Symmetry Not Efficient, not Selective Not Efficient, but Selective Efficient, but not Selective Efficient and Selective
Examples of selectivity..
Selectivity via geometry Separation of Para - en Meta Xylenes P M O P + M O P + M O P M O P M O Squalane 100 % methyl Rtx-1 50 % phenyl 50 % methyl Rtx-17 PEG Rtx-Wax Rt-TCEP Non-Polar Polar
Shape selectivity Using selected cyclodextrins as modifiers p-xylene m-xylene o-xylene
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
(GC-MS) Presence of Diethylene Glycol and Ethylene Glycol in Toothpaste Column: Stablewax Dissolves : likes like
Hydrocarbons on 100% PDMS n-c4 Column n-c5 C3 n-c6 n-c7 : 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; n-c9 n-c10 n-c8 0 50 100 150
PAHs using an Rxi-17Sil MS (30m x 0.25mm x 0.25μm) 1 2 3 4 5 6 7 8 14,15,16 benzo(b)fluoranthene benzo(k)fluoranthene benzo(j)fluoranthene 9 10 12/13 11 17 21/22 18 20 23 24 25 19 26 27
Detection in GC