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CAPILLARY GC INLETS
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SAMPLE INJECTIONGoals
• Introduce sample into the column
• Reproducible
• No efficiency losses
• Representative of sample
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ShortConcentrated
LongDiffuse
Solute Bands
Same column, same chromatographic conditions
Influence of Injection Efficiency
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FIRST, SOME BASIC DEFINITIONS:
• Backflash
• Discrimination
• Electronic pressure control (EPC)
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BACKFLASHCause
• Vaporized sample expands 100 -1000 X
• Portions may leave the liner
• Occurs when vapor volume > liner volume
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Sample expands to fill injector Some sample flows out of injectorHeavier portions condense on the cool areasNext injections or carrier gas dislodges condensed sample Then sample enters the column
BACKFLASH
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10
20
30
40
50
60
70
80
90
50 100 150 200 250 300 350 400
Injection PortSet Point Temperature
350°C
35°COven
150°COven
300COven
Temperature in Gas Stream (°C)
Bottomof Septum
SyringeTip
Base ofInjection
Port
Temperature Profile of a Typical VaporizationInjector vs Oven Temperature
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BACKFLASHProblems
• Loss of sample
• Baseline interferences
• “Ghost” peaks
• Tailing solvent front
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BACKFLASHMinimizing
• Large volume liner
• Small injection volume
• Low expansion solvent
• Low injector temperature
• High carrier gas flow rates
• High head pressures
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DISCRIMINATION
• Injected sample ¹ Sample into the column
• Due to compound volatility differences
• Higher volatility = More into the column
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DISCRIMINATIONHydrocarbons at the Same Concentration
DB-1, 30 m x 0.32 mm ID, 0.25 um80-200°C at 10°/min, He at 30 cm/secSplit 1:100
2 4 6 8 10 12
C10
C11
C12C13
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DISCRIMINATIONConsiderations
• Efficient heat transfer to injected sample
• Efficient mixing of vaporized sample with carrier gas
• Column position in inlet
• Inlet versus syringe discrimination
• Consistent conditions are important
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ELECTRONIC PRESSURE CONTROLEPC
• Ability to program column head pressure
• Provides carrier gas flow or average linear velocity control during a run
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CARRIER GAS PROPERTIES
• Compressible
• Viscosity increases with temperature
• Solute diffusion coefficients (Dm) increase with temperature
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VELOCITY AND TEMPERATURE
• Average linear velocity decreases as column temperature increases
• Thus, average linear velocity changes during a temperature program
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VELOCITY AND PRESSURE
• For a temperature program of 100-300°C
• Need to increase pressure by up to 7 times to maintain constant u
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*Non-constant velocity or flow
EPC PRESSURE PROGRAMMINGIncrease Pressure During the GC Run
• Constant average linear velocity
• Constant flow
• Variable pressure program*
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0
50
100
150
200
250
300
0 5 10 15 20 25 30 35
Time
Ove
n T
emp
erat
ure
0
5
10
15
20
25
30
35
40
Pre
ssu
re
Constant PressureConstant Pressure
Constant FlowConstant Flow
Pressure ProgramPressure Program
Oven TemperatureOven Temperature
EPC PRESSURE PROGRAMMINGTypes of Programs
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EPC PRESSURE PROGRAMMINGBenefits
• Potential resolution improvements
• Reduce run times
• Lower elution temperatures
• Extend operating range for high temperature applications
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EPC PRESSURE PROGRAMMINGCautions
• Pressure programming may improve, degrade or not affect resolution
• Dependent on column temperature program
• Accurate predictions are not possible
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*Typically results in 10% run time decrease and 2-3% gain in resolution
EPC PRESSURE PROGRAMMINGStarting Points
• Try constant flow first*
• Then try higher final pressure and faster ramp rates
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EPC PRESSURE PULSINGFor Splitless Injections
• High head pressure at the start of the GC run
• Followed by rapid decrease to desired head pressure
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1. Minimizes sample expansion volume2. Rapid transfer of analytes into the column
EPC PRESSURE PULSINGBenefits
• Reduce loss of more volatile analytes1
• Reduce analyte decomposition in the injector2
• Better reproducibility with large volume injections
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Especially for earlier eluting peaks
EPC PRESSURE PULSINGCautions
• Peak broadening may occur
• A retention gap is recommended
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EPC PRESSURE PULSINGStarting Points
• Limit initial pressure to 2-3 times normal column head pressure
• Start with 0.25 min pulse time
• Pulse times >1.0 min usually result in distorted peaks
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EPC MAINTENANCELeaks
• Inability to reach set points
• Pressure/retention time fluctuations
• More sensitive to leaks than mechanical systems
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EPC MAINTENANCEMost Common Problems
• Need to occasionally zero EPC channels
• Electronic valves are more prone to plugging and sticking
• Contamination by high MW compounds at high concentrations
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OTHER INLET CONSIDERATIONS
• Silylation
• Glass or fused silica wool
• Cleaning and reusing liners
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GLASS WOOLConsiderations
• Always use deactivated (silylated) wool
• Borosilicate or quartz material?
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GLASS WOOLLiner Packing Recommendations
• Amount, size and placement must be consistent for consistent results
• Can be broken upon installation into the liner, exposing active sites
• Liner deactivation with glass wool plug in place is ideal
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GLASS WOOLPlacement in Liner
• Near top of liner:
• Wipes syringe needle of sample
• Can improve injector precision
• Helps to prevent backflash
• Near bottom of liner:
• Helps in volatilization of high MW components
• Increases mixing
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LINER DEACTIVATION
• Prior to deactivation, surface must be cleaned with an acid leach step:
• Place liner in clean test tube
• Cover liner with 1N HCl or HNO3 solution
• Soak for at least 8 hours (overnight is preferred)
• If acid solution is highly discolored, replace with clean solution and
continue to soak until no color change is noted
• Do not soak liners for longer than 24 hours
• Rinse with deionized water followed by methanol
• Dry the liner at 100-150°C. Do not exceed 150°C.
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LINER DEACTIVATION
• Solution Silylation Procedure
• Place liner in screw cap test tube
• Cover liner with 10% TMCS or DMCS in toluene
• Tightly seal with Teflon-lined cap
• Allow to stand for at least 8 hours
• Remove from solution and thoroughly rinse with toluene, then methanol
• Dry the liner at 75-100°C
• NOTE: Several liners can be done in one test tube, but rotate the tube several times to ensure that all surfaces are
exposed to the solution.
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COMMON INJECTION MODES
• Vaporization Injection Modes
• Cold Injection Modes
• Large Volume Injection Modes
COLD INJECTION MODES
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COLD INJECTION MODES
• Thermally unstable solutes
• Low volatility solutes
• Dilute samples (on-column)
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COLD INJECTION MODES
• PTV and Cold On-column injection
• The sample is injected into a controlled temperature environment
• The solute bands need refocusing prior to beginning the separation
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COLD ON-COLUMN INJECTION
• Clean samples
• Thermally labile solutes
• High boiling solutes
• Refocusing required
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J&W COLD ON-COLUMN INJECTOR DESIGN
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GROB COLD ON-COLUMN INJECTOR DESIGN
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Cool TowerNeedle Guide
Duckbill Valve(Isolation Valve)
Septum purge out
Cryogenic cooling(optional)
Carrier in
Column
Heater block
Spring
Insert
GC
Septum
For Autoinjection
COLD ON-COLUMN INJECTION PORT
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SECONDARY COOLING
• Some means used to cool a small section at the front of the column:
Forced air
Liquid N2 or CO2
Column extraction from oven
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J&W COLD ON-COLUMN INJECTOR DESIGNSample Refocusing
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GROB COLD ON-COLUMN INJECTOR DESIGNSample Refocusing
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Cool TowerNeedle Guide
Duckbill Valve(Isolation Valve)
Septum purge out
Cryogenic cooling(optional)
Carrier in
Column
Heater block
Spring
Insert
GC
Septum
For Autoinjection
COLD ON-COLUMN INJECTION PORTSample Refocusing
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SECONDARY COOLING Benefits
• Warmer initial oven temperatures
• Shorter cool-down between analyses
• Shorter analysis times
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SECONDARY COOLING Considerations
• Potential peak shape problems
Split peaks
Band broadening
• Retention gap should be used
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*see Knaus, Fulleman & Turner, HRC 4(1981)643
At Injection (To)
Needle At Injection (To)
Carrier Gas
Carrier Gas
Solvent Vapors
Column
Column
Temperature ProfileHigh volatility solute
Low volatility solute
“SECONDARY COOLING” CAN CAUSE SPLIT PEAKS*!
Retention gap is usually a necessity
SECONDARY COOLINGPeak Splitting
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THE RETENTION GAP
• A 1 to 2 mtr length of uncoated but deactivated fused silica tubing, attached to the front of the column with a press-fit connector, is usually adequate. The diameter is usually equal to or greater than that of the column.
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COLD ON-COLUMN INJECTION REFOCUSING
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Volatiles Focused by Solvent
High Boilers Focused by Stationary Phase
Solvent Evaporating from the rear of the flooded zone
Solvent Evaporates
Flooded zone forms in the retention gap
Retention gap
Stationary Phase
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SYRINGE TECHNIQUE: ON-COLUMN INJECTIONPlunger-in-needle type of syringe may be preferred.
Manufacturer’s recommendations should be consulted. Fewer problems are encountered with small injections. Outside of needle must be dry.. avoid sample contact between the needle and the column; this would result in a “smeared” band. Retention gap useful and often critical…see “On Column Injection in Capillary Gas Chromatography”, K. Grob, Huethig Publishing, 1988
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PTV INJECTORS
• Lower thermal mass
• Rapid heating and cooling
• Lower internal volume
• Packing options
• Split vent timing
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Split LineCapillary Column
Insert(vaporization Chamber)
Glass Wool / Packing
Carrier Gas
Heating Coil
Seal
Septum PurgeSeptum
Cooling Gas
Cooling Gas
PTV INJECTION PORT
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Capillary Column
Carrier Gas
Heating Coil
Cooling DeviceSplit Line
Injection Needle
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PTV INJECTION MODES
• Cold on-column
• Cold split
• Cold splitless
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COLD ON-COLUMN Using PTV Injector
• Same rules that exist for non-PTV applications still apply:
Secondary cooling can be used
Sample refocusing still required
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COLD SPLIT VS COLD SPLITLESSWhich To Use?
• The same general concepts that apply to standard vaporization techniques also apply to Cold PTV injection techniques:
Split: General use and sample screening
Splitless: Trace level samples
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PTV COLD INJECTION TECHNIQUESBenefits Over Standard Vaporization Techniques • Reduced inlet discrimination
• Reduced decomposition of labile analytes
• Less risk of backflash contamination*
• Larger injection volumes
• *More prevalent with Splitless injections
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PTV COLD INJECTION CONDITIONSDuring Injection
PTV inlet PTV inlet ColdCold ColdColdModes Modes Split Split SplitlessSplitless
Liner temperature <<Solvent BP <<Solvent BP
Purge/Split flow OFF OFF
Delay before NONE NONEinlet heating
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PTV inlet PTV inlet ColdCold ColdColdModes Modes Split Split SplitlessSplitless
Liner temp program Maximum rate To max temp in<80 sec
Column temperature Solute dependent <Solvent BP
Purge/Split flow Split-ratio dependent Off until finaltemp reached
PTV COLD INJECTION CONDITIONSDuring Inlet Heating
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n-Paraffin standard showing distillation range from C6 to C110 on DB-HT Sim Dist
Column: DB-HT Sim Dist5 m x 0.53 mm I.D., 0.15 µm
Carrier: Helium at 18 mL/min, measured at 35°COven: - 30 - 430°C at 10°/minInjector: OPTICTM PTV
55 - 450°C at 2°/sec0.5 µL of about 2% n-Parraffins in CS2
Detector: FID, 450°CNitrogen makeup gas at 15 mL/min
Time(minutes)0 5 10 15 20 25 30 35 40 45 50
6
7
8
9
10
16
1112
70
50
40
9080
60
14
18
20 24 2830
32
110
HIGH TEMP SIM DIST ANALYSISn-Paraffins
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LARGE VOLUME INJECTION
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LARGE VOLUME INJECTIONBenefits
• Enhanced detection and recognition
• Quicker more accurate sample preparation
• Better coupling to automated systems
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LARGE VOLUME INJECTION
• PTV
• Cool On-Column
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ON-COLUMN LARGE VOLUME INJECTION
• Longer retention gap
• Solvent removal
• Very large volumes
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On-columninjector Detector
Solvent vaporexit
restriction
Retention Gap
RetainingPrecolumn
AnalyticalColumn
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PTV LARGE VOLUME INJECTION
• Solvent elimination split/splitless
• Rapid at once
• Controlled injection rate
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PTV SOLVENT ELIMINATION INJECTION Conditions During Injection
PTV Inlet PTV Inlet Solvent Elimination Solvent Elimination Solvent EliminationSolvent EliminationModes Modes Split Split SplitlessSplitless
Liner Temperature <Solvent BP <Solvent BP (vapor-pressure dependent)
Purge/Split Flow 100-1000 mL/min 100-1000 mL/min
Delay before 5-30 sec 5-30 secinlet heating
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PTV Inlet PTV Inlet Solvent Elimination Solvent Elimination Solvent EliminationSolvent EliminationModes Modes Split Split SplitlessSplitless
Liner temp program Maximum rate To max temp in <80 sec
Column temperature Solute dependent <Solvent BP
Purge/Split flow split-ratio dependent Off until final inlettemp reached
PTV SOLVENT ELIMINATION INJECTION Conditions During Inlet Heating
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Larger inner diameter PTV liners can retain large volumes
Approximate volumes retained (hexane)
Liners packed with glass wool plug (3 cm)C.A. Cramers, H.G. Janssen, H.G.J. Mol, U.A.T. Brinkman,
J. High Resolution Chromatography
Liner Diameter Liner Diameter VolumeVolume
1.2 mm 20 µL
2.2 mm 65 µL
3.4 mm 150 µL
RAPID “AT ONCE” INJECTIONMaximum Sample Volumes
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LARGE VOLUME INJECTION CONSIDERATIONS
• Use only high purity solvents (GC/MS grade)
• Store small volumes of solvent (£ 500 mL)
• Optimize inlet conditions to prevent solvent breakthrough during injection (PTV)
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ON-COLUMN LARGE VOLUME INJECTION
• Most inert
• Less thermal stress
• Volatile analysis
• Very large volume
• Ruggedness (“clean samples”)
• Speed control of injection
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PTV LARGE VOLUME INJECTION
• Dirty samples
• Easy optimization (“at once” and controlled injection)
• Packing selectivity
• Labile analysis
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Sample
Volatile Range Organics
Purge and Trapor
Static Headspace
Concentrated
ThermallyStable
PTV Splitor
On-Column
High BoilingComponents
No
No
Concentration
ThermallyStable
Yes
High BoilingPoint
Splitless
PTV Splitlessor
On-Column
Dilute
No Yes
NoYes
NoYes
Split
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GO FORTH AND INJECT!!!
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J&W Scientific Technical Support
800-227-9770 (phone: US & Canada)*
302-993-5304 (phone)*
* Select option 4, then
option 2.916-608-1964 (fax)www.agilent.com/chem
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Wrap-up E-Seminar Questions
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week. Please check our website frequently at:
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