Presentation on theme: "The Foundations: Classical Split and Splitless Injection"— Presentation transcript:
1 The Foundations: Classical Split and Splitless Injection Nicholas H. SnowDepartment of ChemistrySeton Hall UniversitySouth Orange, NJ
2 Split and Splitless Split Splitless Both use the same hardware vaporize and remove most of the sample to wasteSplitlessvaporize and transfer most of the sample to the column; use cold trapping and solvent effects to focus bandsBoth use the same hardware
3 Split Inlet Use for higher concentration samples ppm and above hot inlet; vaporize samplemix with carrier gasuse purge valve to “split” the samplesplit ratio is criticalplace fraction of sample on column
4 SPLIT INJECTION High Temperature High Linear Velocity Rapid Transfer Bulk of Sample WastedSplit Ratio ImportantLiner Geometry
5 Classical Split Ratio Determination Measure column flow from tmFc = pr2L/tmMeasure purge vent flow using flow meterFsSplit Ratio = Fs / FcWhat 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 directlyColumn flow is calculated from inlet conditions and column dimensionsadd equation herePurge flow adjusted to desired value
8 Advantages of Split Inlets Reduced sample size (narrow bands)Fast inlet flow rate (narrow bands)Dirty samples OKSimple to operate (best for isothermal GC)Inject “neat” samplesExcellent interfacing
9 Disadvantages of Split Inlets Nonlinear splittinghigh molecular weights can be lost preferentiallyThermal degradationhot metal surfaces can lead to reactionSyringe needle discriminationTrace analysis limitedppm detection limits with FID
12 Summary - Split Inlet Simple Hot vaporizing technique syringe discrimination (best to use autosampler)liner discriminationuse glass wool (deactivated)shape of liner may be criticalBest for “neat” or concentrated sampleshigh ppm or higher
13 Splitless Inlet Inject sample into hot inlet without “purge” 95% of sample enters columnSame hardware as split except linerMore variablessolvent, splitless time, initial column temperatureOpen purge valve after short timeBetter sensitivity
14 SPLITLESS INJECTION High Temperature Low Liner Velocity Slow Transfer Bulk of Sample and Solvent to ColumnMany Factors Important
15 Steps in a Splitless Injection Purge valve is off; column is coldInject samplefast autosampler injection bestslower injections have been proposedFlow through inlet is slow; slow transfer to cold columnAfter sec, open purge valve - cleans inletTemperature program column
16 BAND BROADENING Time Space (solvent effect) Thermal Focusing Time Grob, K., Split and Splitless Injection in Capillary GC, Huthig, 1993, pp ,
17 Band Focusing Mechanisms Splitless injections involve slow transfer to column ---> initial peaks are broadNeed focusingcold trapsolvent effects
18 Cold TrapInitial column temperature cold enough to “freeze” analyte on column
19 INITIAL COLUMN TEMPERATURE 20oC0oC40oChexane, heptane500 ppb10 min extractionFiber: PDMS 100 mLinermmoCPinj: 1 bar(g)-20oC-40oC
20 Solvent Effects Solvent is recondensed in the column Long plug of liquidStart column degrees below normal boiling point of solvent
29 OPTIMIZATION SPLITLESS INJECTION Can Be DifficultMinimize Transport Time (high linear velocity)Maximize Thermal Focusing (low initial column temperature)Maximize “solvent effect” (low initial column temperature)Chemistry remains a factor
30 REFERENCESGrob, 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.
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