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The Foundations: Classical Split and Splitless Injection Nicholas H. Snow Department of Chemistry Seton Hall University South Orange, NJ 07079 [email protected] Split and Splitless Split vaporize and remove most of the sample to waste Splitless

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The foundations classical split and splitless injection l.jpg

The Foundations: Classical Split and Splitless Injection

Nicholas H. Snow

Department of Chemistry

Seton Hall University

South Orange, NJ 07079

[email protected]


Split and splitless l.jpg
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 l.jpg
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 l.jpg

High Temperature

High Linear Velocity

Rapid Transfer

Bulk of Sample Wasted

Split Ratio Important

Liner Geometry

SPLIT INJECTION


Classical split ratio determination l.jpg
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?


Modern split ratio determination l.jpg
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 l.jpg
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 l.jpg
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


Split injection techniques l.jpg
Split Injection Techniques

  • Filled Needle

  • Cold Needle

  • Hot Needle

  • Solvent Flush



Summary split inlet l.jpg
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 l.jpg
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 l.jpg

High Temperature

Low Liner Velocity

Slow Transfer

Bulk of Sample and Solvent to Column

Many Factors Important

SPLITLESS INJECTION


Steps in a splitless injection l.jpg
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


Band broadening l.jpg
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.


Band focusing mechanisms l.jpg
Band Focusing Mechanisms

  • Splitless injections involve slowtransfer to column ---> initial peaks are broad

  • Need focusing

    • cold trap

    • solvent effects


Cold trap l.jpg
Cold Trap

  • Initial column temperature cold enough to “freeze” analyte on column


Initial column temperature l.jpg
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


Solvent effects l.jpg
Solvent Effects

  • Solvent is recondensed in the column

  • Long plug of liquid

  • Start column 30-50 degrees below normal boiling point of solvent



Solvent effects22 l.jpg
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.


Initial column tmperature solvent effect injections l.jpg
INITIAL COLUMN TMPERATURESOLVENT EFFECT INJECTIONS

40oC

60oC

0

20

0

20

Time (min)

Time (min)

Solvent: Cyclohexane (bp 81oC), Sample: 10ppm hydrocarbons


Inlet temperature reality l.jpg
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.


Inlet temperature chromatograms l.jpg

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


Inlet pressure l.jpg
INLET PRESSURE

  • Linear Gas Velocity Increased Injector Column

  • Analyte Boiling Point Increased


Pressure pulse l.jpg
PRESSURE PULSE

  • Increased Pressure During Injection Only

Purge “ON” Time

150

Pressure

(kPa)

50

0.75

20

Time (min)


Pressure pulse28 l.jpg
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


Optimization splitless injection l.jpg
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


References l.jpg
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.


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