12
Figure 10.
Uniliner
®
injector liner for
Agilent 6890GCs (cat.# 21054).
enlarged view
Figure 9.
4µL samples can be rapidly injected via an autosampler if EPC is
used to compress the sample vapor cloud—note the improved solvent peak shape.
Direct injection of 4µL hydrocarbons (EPA/WisconsinDROMix, cat.# 31064)
using an autosampler.
1 2 3 4 5 6 7 8 9 10
1 2 3 4 5 6 7 8 9 10
*EPC program: the head pressurewas initially set at 23psi, held for
10 sec., then decreased to 8.2psi (80cm/sec.@ 40°C).
1. decane (C10)
2. dodecane (C12
3. tetradecane (C14)
4. hexadecane (C16)
5. octadecane (C18)
6. eicosane (C20)
7. docosane (C22)
8. tetracosane (C24)
9. hexacosane (C26)
10. octacosane (C28)
30m, 0.53mm ID, 1.5µmRtx
®
-1 column (cat.# 10170)
Conc.:
114µg/mL inCH
2
Cl
2
Oven temp.:
40°C (hold 2min.) to 310°C@ 12°C/min. (hold 8min.)
Inj./Det. temp.:
280°C/310°C
Carrier gas:
helium
Linear velocity: 80cm/sec. set@ 40°C
FID sensitivity:
8x10
-11
AFS
no electronic pressure control
electronic pressure control*
If large samplesmust be used for
direct injection, an injection port
system that incorporates a septum
purge line is preferred over one
without a septum purge.A septum
purgewill sweep away any excess
solvent or sample vapor that flashes
out of the top of the injection port
liner. Problems associatedwith
backflash, such as peak tailing and
ghost peaks, will beminimized.
However, lowmolecular weight or
low-boiling point sample compo-
nents also can be swept away in the
septum purge if they experience
backflash problems similar to the
sample solvent. This can lead to
irreproducible peak areas for early-
eluting compounds, and poor quan-
titative accuracy.
HOT
tech
tip
Optimizing Direct injectionWhen Using Agilent 6890
EPCGCs
The pneumatics in anAgilent 6890EPCGC are not the same as in previousmodels of
Agilent GCs. The EPC design includes one pressure sensor upstream of the injection port
and a second pressure sensor downstream of the injection port, at the split vent. Because a
traditional Uniliner
®
direct injection liner seals to the analytical column, there is no down-
stream flow to the split vent and therewill be a difference in the pressure that ismeasured
between the two sensorswhen setting up for direct injection. The upstream sensor will over-
compensate for the difference and a high-pressuremalfunctionwill result.
To prevent this problem, we have designed aUniliner
®
liner with a unique geometry (Figure
10) that allows you tomake direct injections using yourAgilent 6890EPCGC.A small hole
in the upper part of theUniliner
®
liner allows a portion of the carrier gas to escape from the
liner and equalize the pressure at the second sensor, thereby eliminating pressuremalfunc-
tions. The design of theUniliner
®
liner forAgilent 6890GCs requires no software or hardware
modifications, or flow adjustments. For optimum performance, keep the purge on and use a
very low split flow (3 to 5mL/min. or less).
Summary
Direct injection offers higher sensitivity, less potential for adsorption of active compounds,
and greater simplicity, relative to splitless injection. Less adsorption of sample components,
in turn, reduces the need for columnmaintenance. Excellent performance and low cost of
the equipment needed also are strong arguments for adopting this technique.
