The Foundations: Classical Split and Splitless Injection - PowerPoint PPT Presentation

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The Foundations: Classical Split and Splitless Injection

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  1. The Foundations: Classical Split and Splitless Injection Nicholas H. Snow Department of Chemistry Seton Hall University South Orange, NJ 07079 snownich@shu.edu

  2. 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

  3. 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

  4. High Temperature High Linear Velocity Rapid Transfer Bulk of Sample Wasted Split Ratio Important Liner Geometry SPLIT INJECTION

  5. Classical Split Ratio Determination • Measure column flow from tm • Fc = pr2L/tm • Measure purge vent flow using flow meter • Fs • Split Ratio = Fs / Fc What are the problems with these measurements? Do we really ever know how much we injected? Does the exact injection volume matter?

  6. 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

  7. Flow Equations

  8. 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

  9. 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

  10. Split Injection Techniques • Filled Needle • Cold Needle • Hot Needle • Solvent Flush

  11. Split Inlet Discrimination

  12. 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

  13. 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

  14. High Temperature Low Liner Velocity Slow Transfer Bulk of Sample and Solvent to Column Many Factors Important SPLITLESS INJECTION

  15. 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 30-60 sec, open purge valve - cleans inlet • Temperature program column

  16. BAND BROADENING • Time • Space (solvent effect) • Thermal Focusing Time Space Focusing Grob, K., Split and Splitless Injection in Capillary GC, Huthig, 1993, pp. 19-29, 322-36.

  17. Band Focusing Mechanisms • Splitless injections involve slowtransfer to column ---> initial peaks are broad • Need focusing • cold trap • solvent effects

  18. Cold Trap • Initial column temperature cold enough to “freeze” analyte on column

  19. INITIAL COLUMN TEMPERATURE 20oC 0oC 40oC hexane, heptane 500 ppb 10 min extraction Fiber: PDMS 100 m LinermmoC Pinj: 1 bar(g) -20oC -40oC

  20. Solvent Effects • Solvent is recondensed in the column • Long plug of liquid • Start column 30-50 degrees below normal boiling point of solvent

  21. Solvent Effects

  22. Solvent Effects • Refocus moderate volatility compounds near column head • Require solvent to wet stationary phase • Use non-polar solvent with non-polar stationary phase, etc.

  23. INITIAL COLUMN TMPERATURESOLVENT EFFECT INJECTIONS 40oC 60oC 0 20 0 20 Time (min) Time (min) Solvent: Cyclohexane (bp 81oC), Sample: 10ppm hydrocarbons

  24. INLET TEMPERATUREREALITY Set Point 350oC Distance from Septum (mm) Carrier Gas Temperature (oC) Klee, M.S., GC Inlets: An Introduction, Hewlett Packard, 1991, p. 42.

  25. 1. octane 2. decane 3. tridecane 4. tetradecane 5. pentadecane HP 5890-5972 Pinj = 5.0 psi HP5 30m x 0.25mm x 0.25 mm Transfer: 280oC INLET TEMPERATURECHROMATOGRAMS 2 70000 250oC 3 4 5 1 100oC 40000 TP: 40oC initial, 1 min, 10oC/min

  26. INLET PRESSURE • Linear Gas Velocity Increased Injector Column • Analyte Boiling Point Increased

  27. PRESSURE PULSE • Increased Pressure During Injection Only Purge “ON” Time 150 Pressure (kPa) 50 0.75 20 Time (min)

  28. PRESSURE PULSE 20000 1. octane 2. decane 3. tridecane 4. tetradecane 5. pentadecane HP 5890-5972 Pinj = 5.0 psi HP5 30m x 0.25mm x 0.25 mm Transfer: 280oC 5 No Pulse 4 3 2 1 40000 10 psi pulse Pressure increased to 15 psig during splitless period TP: 80oC initial, 1 min, 10oC/min

  29. OPTIMIZATIONSPLITLESS 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

  30. REFERENCES • Grob, K. Split and Splitless Injection in Capillary GC, 3rd. Edition, A. Huethig, 1993. • Klee, M.S., GC Inlets: An Introduction, Hewlett Packard, 1991. • Stafford, S.S., Electronic Pressure Control in Gas Chromatography, Hewlett Packard, 1993.