8
ID, carrier gas flow rate, and injection port liner configuration. Table II lists approximate
splitless hold times for various column IDswhen operatedwith helium or hydrogen. The
splitless hold timewill decrease as either the column ID or column flow rate increases.
Setting an optimal splitless hold time also is dependent on the choice of sample solvent and
the sample size. UseTable III to estimate the volume of vapor producedwhen using differ-
ent solvents at different pressures. The volume of vapor cloud formed should be divided by
the column flow rate to determine the approximate time needed to keep the solenoid valve
closed for complete sample transfer. The calculated splitless hold time also should be evalu-
ated to provide the optimum response for the sample analytes. If the solenoid valve is
Table II.
Typical splitless hold times.
ColumnFlowRate
SampleTransferTime*
Column ID HoldTime
He
H
2
He
H
2
0.18mm
2min.
0.3cc/min.
0.6cc/min.
2.7min.
1.4min.
0.25mm
1min.
0.7cc/min.
1.4cc/min.
1.2min.
0.6min.
0.32mm 0.75min.
1.2cc/min.
2.4cc/min.
0.7min.
0.4min.
0.53mm 0.5min.
2.6cc/min.
5.2cc/min.
0.3min.
0.2min.
*2µL of liquidmethylene chloride expanded to 0.8mL vapor at 250°C (10psig headpressure).
Table III.
Solvent expansion volumes.
ExpansionVolume inµL
at various columnheadpressures
Solvent
Density (g/mL)
MW 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
H
2
O
1.00
18
1776
1418
1179
The expansionvolumeswere determinedusing a 1.0µL injectionvolume, a 250°C injectionport tem-
perature, and a headpressure of 5, 10, or 15psig (common operating pressures for 30m columns hav-
ing IDs of 0.53, 0.32, or 0.25mm, respectively). For 2µL injections, double the expansion volumes.
Use these formulas to calculate values not listed inTable III:
Expansion volume =
nRT / P
n = number ofmoles of solvent and sample.
= [volume (mL)
×
density (g/mL)] / mol. wt. (g/mole)
R= gas law constant
= 82.06cc atm/mole °K
T= absolute temperature of injector (°K)
(°K= °C+ 273)
P = absolute column headpressure = gauge pressure (atm) + 1 atm
atm= psig
×
0.06804 atm / psig
injector liner volume* =
π
r
2
L
π
= 3.14
r = liner internal radius (cm)
L= liner length (cm)
*Also use this formula to determine capillary column internal volume.
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