The%20Foundations:%20Classical%20Split%20and%20Splitless%20Injection - PowerPoint PPT Presentation

Title: 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
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
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

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 slow transfer to
  column ---gt 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
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 Col
  umn
• 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.