9
Figure 8 shows a pesticide analysis performed on a direct injection system fittedwith a low-
quality septum, thenwith a high-qualityThermolite
®
septum. Notice that outgassed products
from the low-quality septum interferewith peaks 3, 4, and 5, andwith peaks 18 and 19. Use
of a packed purged injector or a splitless injector (either of which incorporates a septum
purge system) greatly diminishes this problem. To ensure that your analyses are free of sep-
tum contaminants, however, it iswise to always use high-quality septa, evenwith a septum
purge system. Formore information on the advantages of using high-quality septa, request
our
Guide toMinimizing SeptaProblems
(Lit. cat. # 59886).
Keep injection volumes small whenmaking direct injections
In general, analysts should strive to keep sample injection volumes as small as possiblewhen
using direct injections, ideally less than 1µL. Sample sizes greater than 1µL increase the
probability of backflash, inwhich a portion of the vaporized sample can spill out of the top
of the injection port liner. Sample backflash can lead to tailing peaks, split peaks, and poor
resolution. If the sample volumemust exceed 1-2µL, higher injection port pressures or slow-
er rates of injection canminimize backflash.
Figure 8.
Low-quality septawill cause ghost peaks when usedwith direct injections.
Low-quality septum
High-qualityThermolite
®
septum
1
2
3
4
5
6
7
8 9
10
11
12,13
14
15
16
17
18
19
20
21
22
4
8
12 16 20 24 28 32 36 min.
1
2
3
4
5
6
7
8
9
10
11
12,13
14
15
16
17
18
19
20
21
22
1. 2,4,5,6-tetrachloro-
m
-xylene
2.
a
-BHC
3.
b
-BHC
4.
g
-BHC
5.
d
-BHC
6. heptachlor
7. aldrin
8. heptachlor epoxide
9.
g
-chlordane
10. endosulfan I
11.
a
-chlordane
12. dieldrin
13. 4,4'-DDE
30m, 0.53mm ID, 0.50µmRtx
®
-5 column (cat.# 10240)
1.0µL direct injection of 50pg pesticide standard using a
Uniliner
®
liner
Oven temp:
150°C to 275°C@
4°C/min. (hold 15min.)
Inj./Det. temp.:
250°C/300°C
Carrier gas:
helium
Linear velocity: 40cm/sec. (flow rate: 10cc/min.)
ECD sensitivity: 8x10-11AFS
14. endrin
15. endosulfan II
16. 4,4'-DDD
17. endrin aldehyde
18. endosulfan sulfate
19. 4,4'-DDT
20. endrin ketone
21. methoxychlor
22. decachlorobiphenyl (IS)
4
8
12 16 20 24 28 32 36 min.
septum bleed
