The%20Foundations:%20Classical%20Split%20and%20Splitless%20Injection

1
The Foundations Classical Split and Splitless
Injection

• Nicholas H. Snow
• Department of Chemistry
• Seton Hall University
• South Orange, NJ 07079
• snownich_at_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
SPLIT INJECTION

• High Temperature
• High Linear Velocity
• Rapid Transfer
• Bulk of Sample Wasted
• Split Ratio Important
• Liner Geometry

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
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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
SPLITLESS INJECTION

• High Temperature
• Low Liner Velocity
• Slow Transfer
• Bulk of Sample and Solvent to Column
• Many Factors Important

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

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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.
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Band Focusing Mechanisms

• Splitless injections involve slow transfer to
  column ---gt initial peaks are broad
• Need focusing
• cold trap
• solvent effects

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Cold Trap

• Initial column temperature cold enough to
  freeze analyte on column

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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
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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
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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.

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INITIAL COLUMN TMPERATURESOLVENT EFFECT
INJECTIONS
40oC
60oC
0
20
0
20
Time (min)
Time (min)
Solvent Cyclohexane (bp 81oC), Sample 10ppm
hydrocarbons
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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.
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INLET TEMPERATURECHROMATOGRAMS
2
70000
250oC
3
4
5
1
100oC
40000
TP 40oC initial, 1 min, 10oC/min
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INLET PRESSURE

• Linear Gas Velocity Increased Injector Col
  umn
• Analyte Boiling Point Increased

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PRESSURE PULSE

• Increased Pressure During Injection Only

Purge ON Time
150
Pressure (kPa)
50
0.75
20
Time (min)
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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
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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

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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.