10
Table II shows typical expansion volumes for common sample solvents at a pressure of
10psig. These expansion volumes can be compared to the injector liner volumes inTable III
to determinewhether a specific combination of solvent, sample volume, and liner ID is com-
patible at this pressure. For example, 1µL of liquidmethylene chloridewill expand tomore
than 400µL of vapor in a heated injection port, and 2µL of liquidmethylene chloridewill
expand tomore than 800µL of vapor.A 4mm IDUniliner
®
direct injection liner can accom-
modate asmuch as 900µL of vapor before the vapor will backflash out of the injector liner
and cause gross solvent tailing. The liner can accept only half this volume of solvent vapor,
however, because it already contains carrier gas. Thus, the 400µL of vapor produced by
injecting 1µL ofmethylene chloride can be accommodated by a 4mm IDUniliner
®
liner, but
the 800µL of vapor produced by 2µL ofmethylene chloride cannot be accommodated.
Solventswith a larger expansion volume thanmethylene chloride alsowill exhibit backflash.
For example, 1µL of awater-based samplewould expand tomore than 1400µL of vapor and
would exceed the buffer volume in any of the direct injection liners.
Compress the sample vapor cloudwhenmaking large injections
Table IV further illustrates the importance ofminimizing sample volume to avoid backflash.
Values in this table show that when a 2µL injection ofmethylene chloride ismade in combi-
nationwith a typical column flow rate of 10cc/min., it would take approximately 1.7 seconds
for the entire vaporized sample to be carried from the injection port liner and into a 0.53mm
ID column. If a 5µL sample is used instead of a 2µL sample, it would take almost nine sec-
onds for the entire sample to enter the column.
In order accommodate the size of the vaporized sample, a slower rate of injection could be
used to allow the carrier gas flow to transfer the sample onto the column as the vapor cloud
is formed. Keep inmind that slow rates of injection cannot be reproduced consistentlywhen
performingmanual injections.
A better alternative formaking injections larger than 1–2µL is to use higher carrier gas pres-
sures in the injection port to compress the vaporized sample cloud. TableV shows how sol-
vent density, solvent molecular weight, and pressure affect sample vapor cloud volume. Note
that pressure (“P”) is in the denominator of the ideal gas law. This indicates that an increase
in column head pressurewill reduce the volume of the sample vapor cloud. Thus, by increas-
ing the injection port pressure from a typical 10psig to a pressure of 15psig ormore, the size
of the sample vapor cloud can be reduced, allowing rapid injections of large volume samples
to bemadewithout resulting solvent peak tailing or backflash.
Table II.
Typical expansion volumes for sample solvents.
Injection
Volume
Expansion Volume (vaporized, µL)*
(liquid) (µL) water carbon disulfide methylene chloride hexane
isooctane
0.1
142
42
40
20
16
0.5
710
212
200
980
78
1.0
1420
423
401
195
155
2.0
2840
846
802
390
310
3.0
4260
1270
1200
585
465
4.0
5680
1690
1600
780
620
5.0
7100
2120
2000
975
775
Table III.
Internal volumes of injector liners.†
Liner
Internal Volume (µL)
ID (mm)
Theoretical* Effective**
0.53
16
1.0
59
30
2.0
236
118
3.0
530
265
4.0
942
471
*Expansion volumes based on a 250°C injection port temperature and a 10psig head pressure.
*Total internal volume for a typical 75mm-long
liner.
**Liner volume available to accommodate an
injection (carrier gas present in liner).
†
FromGrob,
Split and Splitless Injection, 3rd ed.
