12
SeptumPurge Optimization
The septum purge (Figure 8) serves two func-
tions: to sweep septum bleed volatiles out of
the system and to reduce the potential for
sample backflash contaminating the carrier
gas inlet line. Optimization of the septum
purge flow rate is important, especiallywhen
the inlet is operated in the splitlessmode.
Most GCmanufacturers recommend that the
septum purge flow rate be set between 3 and
5cc/min. Flow rates exceeding 5cc/min.
should not be used because highly volatile
sample components could be preferentially
purged from the inlet liner buffer volume after
vaporization. Flow rates lower than 3cc/min.
can allow septum bleed to enter the inlet liner
and cause ghost peaks to appear on the chro-
matogram.
The septum purge flow ratemust be readjust-
ed each time the injection pressure is changed
bymore than 5psig.Most GCs have a low-
flow needle valve that makes septum purge
adjustments easy.
Figure 8.
Typical carrier gas flowpath in a
Varian split injector.
septum purge
out
carrier gas in
split liner
column
spring
split point
split vent out
ferrule
GC_EX00600
Rtx
®
-5 15m, 0.32mm ID, 1.50µm (cat.# 10266)
Sample:
50µg/mLPAH standard (cat.#31011 ) in hexane
Inj.:
1.0µLsplitless (hold 2min.),
4mm single gooseneck inlet liner w/FSwool (cat.# 22405)
Inj. temp.:
200°C
Carrier gas:
helium, constant pressure
Linear velocity: 76cm/sec.@ 40°C
Oven temp.:
40°C(hold 4min.) to 325°C@10°C/min. (hold 5min.)
Det.:
FID@350°C
Figure 9.
Injector temperature affects the recovery of highermolecular weight compounds.
GC_EX00601
1. naphthalene
2. acenaphthylene
3. acenaphthene
4. fluorene
5. phenanthrene
6. anthracene
7. fluoranthene
8. pyrene
9. benzo(a)anthracene
10. chrysene
11. benzo(b)fluoranthene
12. benzo(k)fluoranthene
13. benzo(a)pyrene
14. indeno(1,2,3-cd)pyrene
15. dibenzo(a,h)anthracene
16. benzo(ghi)perylene
Injector temp: 300°C
Injector temp: 200°C
