Presentation on theme: "The Foundations: Classical Split and Splitless Injection Nicholas H. Snow Department of Chemistry Seton Hall University South Orange, NJ 07079"— Presentation transcript:
The Foundations: Classical Split and Splitless Injection Nicholas H. Snow Department of Chemistry Seton Hall University South Orange, NJ
Split and Splitless Split –vaporize and remove most of the sample to waste Splitless –vaporize and transfer most of the sample to the column; use cold trapping and solvent effects to focus bands Both use the same hardware
Split Inlet Use for higher concentration samples ppm and above hot inlet; vaporize sample mix with carrier gas use purge valve to “split” the sample –split ratio is critical place fraction of sample on column
SPLIT INJECTION High Temperature High Linear Velocity Rapid Transfer Bulk of Sample Wasted Split Ratio Important Liner Geometry
Classical Split Ratio Determination Measure column flow from t m –F c = r 2 L/t m Measure purge vent flow using flow meter –Fs Split Ratio = F s / F c What are the problems with these measurements? Do we really ever know how much we injected? Does the exact injection volume matter?
Modern Split Ratio Determination EPC systems measure pressures and flows directly Column flow is calculated from inlet conditions and column dimensions –add equation here Purge flow adjusted to desired value
Advantages of Split Inlets Reduced sample size (narrow bands) Fast inlet flow rate (narrow bands) Dirty samples OK Simple to operate (best for isothermal GC) Inject “neat” samples Excellent interfacing
Disadvantages of Split Inlets Nonlinear splitting –high molecular weights can be lost preferentially Thermal degradation –hot metal surfaces can lead to reaction Syringe needle discrimination Trace analysis limited –ppm detection limits with FID
Summary - Split Inlet Simple Hot vaporizing technique –syringe discrimination (best to use autosampler) –liner discrimination use glass wool (deactivated) shape of liner may be critical Best for “neat” or concentrated samples –high ppm or higher
Splitless Inlet Inject sample into hot inlet without “purge” 95% of sample enters column Same hardware as split except liner More variables –solvent, splitless time, initial column temperature Open purge valve after short time Better sensitivity
SPLITLESS INJECTION High Temperature Low Liner Velocity Slow Transfer Bulk of Sample and Solvent to Column Many Factors Important
Steps in a Splitless Injection Purge valve is off; column is cold Inject sample –fast autosampler injection best –slower injections have been proposed Flow through inlet is slow; slow transfer to cold column After sec, open purge valve - cleans inlet Temperature program column
BAND BROADENING Time Space (solvent effect) Thermal Focusing Grob, K., Split and Splitless Injection in Capillary GC, Huthig, 1993, pp , Time Space Focusing
Band Focusing Mechanisms Splitless injections involve slow transfer to column ---> initial peaks are broad Need focusing –cold trap –solvent effects
Cold Trap Initial column temperature cold enough to “freeze” analyte on column
INITIAL COLUMN TEMPERATURE 40 o C 20 o C0oC0oC -20 o C-40 o C hexane, heptane 500 ppb 10 min extraction Fiber: PDMS 100 m Liner mm o C Pinj: 1 bar(g)
Solvent Effects Solvent is recondensed in the column Long plug of liquid Start column degrees below normal boiling point of solvent
Refocus moderate volatility compounds near column head Require solvent to wet stationary phase Use non-polar solvent with non-polar stationary phase, etc.
INITIAL COLUMN TMPERATURE SOLVENT EFFECT INJECTIONS 020 Time (min) 020 Time (min) 40 o C60 o C Solvent: Cyclohexane (bp 81 o C), Sample: 10ppm hydrocarbons
INLET TEMPERATURE REALITY Set Point 350 o C Distance from Septum (mm) Carrier Gas Temperature ( o C) Klee, M.S., GC Inlets: An Introduction, Hewlett Packard, 1991, p. 42.
INLET TEMPERATURE CHROMATOGRAMS o C 100 o C 1. octane 2. decane 3. tridecane 4. tetradecane 5. pentadecane HP Pinj = 5.0 psi HP5 30m x 0.25mm x 0.25 mm Transfer: 280 o C TP: 40 o C initial, 1 min, 10 o C/min
INLET PRESSURE Linear Gas Velocity Increased Injector Column Analyte Boiling Point Increased
PRESSURE PULSE Increased Pressure During Injection Only Time (min) Pressure (kPa) Purge “ON” Time 20
PRESSURE PULSE No Pulse 10 psi pulse octane 2. decane 3. tridecane 4. tetradecane 5. pentadecane HP Pinj = 5.0 psi HP5 30m x 0.25mm x 0.25 mm Transfer: 280 o C Pressure increased to 15 psig during splitless period TP: 80 o C initial, 1 min, 10 o C/min
OPTIMIZATION SPLITLESS INJECTION Can Be Difficult Minimize Transport Time (high linear velocity) Maximize Thermal Focusing (low initial column temperature) Maximize “solvent effect” (low initial column temperature) Chemistry remains a factor
REFERENCES Grob, K. Split and Splitless Injection in Capillary GC, 3rd. Edition, A. Huethig, Klee, M.S., GC Inlets: An Introduction, Hewlett Packard, Stafford, S.S., Electronic Pressure Control in Gas Chromatography, Hewlett Packard, 1993.