11
Expansion Volume (µL)*
at various column headpressures
Solvent
Density (g/mL) MolecularWeight
5psig
l0psig
15psig
heptane
0.68
100
219
174
145
hexane
0.66
86
245
196
163
pentane
0.63
72
280
224
186
toluene
0.87
92
303
242
201
ethyl acetate
0.90
88
328
261
217
chloroform
1.49
119
400
319
266
methylene chloride
1.33
85
500
399
332
methanol
0.79
32
792
629
525
water
1.00
18
1776
1418
1179
Table IV.
Injection speedmust be based on volume of solvent and column flow rate.
InjectionRate =
[solvent/sample expansion volume (cc) - injector liner buffer volume (cc)]
column flow rate (cc/min.)
Injection Expansion
Injection Time (sec.) for 0.53mm IDColumns
Volume
Volume Flow rate:
5cc/min.
10cc/min.
20cc/min.
30cc/min.
(liquid)
(vaporized)
Injection:
DI
OC DI
OC DI
OC DI
OC
0.1µL
38µL
0.5 0.5 0.5 .23 0.5 .11 0.5 .08
0.5µL
194µL
0.5 2.3 0.5 1.2 0.5
.58 0.5 .39
1.0µL
388µL
1.3 4.7 0.67 2.3 0.5 1.16 0.5 .78
2.0µL
779µL
3.3 9.3 1.7 4.7 0.84 2.3
0.6 1.6
5.0µL
1952µL
17.4 23.4 8.7 11.7 4.4 5.9
2.9 3.9
Table V.
The ideal gas law indicates that an increase in pressure greatly reduces sample expansion volume.
Use these formulas to calculate values not listed in
Table IV orV:
Solvent/sample expansion volume (v)=
nRT / P
n = number ofmoles of sovent and sample.
[volume (mL) xdensity (g/mL)] /molwt (g/mole)
R= gas law constant
82.06cc atm/mole °K
T= absolute temperature of injector (°K)
(°K= °C+ 273)
P = absolute column headpressure (atm) + 1 atm
Injector liner volume** =
p
r
2
L
p
= 3.14
r = liner internal radius (cm)
L= liner length (cm)
**Also use this formula to determine capillary column
internal volume.
DI = direct injection
OC= on-column injection
*Expansion volumes determined using a 1.0µL injection volume, a 250°C injection port temperature,
and a headpressure of 5, 10, or 15psig (common operating pressures for 30m columns having IDs of
0.53, 0.32, or 0.25mm, respectively). For 2µL injections, double the volumes.
Electronic pressure control and direct injection
SomeGCs offer electronic pressure control (EPC), which can be used tomomentarily pulse
the pressure in the injection port, briefly causing an increase in carrier gas velocity during
the initial injection period. Notice the reduction in solvent tailingwhen a 4µL sample of a
hydrocarbonmix inmethylene chloridewas analyzed usingEPC (Figure 9).A rapid injec-
tionwasmade using anAgilent 7673 autosampler into an injection port capable of EPC. The
injection port pressurewas increased from 8psig to 23psig just prior to the injection andwas
held at 23psig for 10 seconds after the injectionwasmade. The solvent peak shape, and sep-
aration of early-eluting compounds from the solvent, are greatly improved byEPC.
