Selection of analytical column
Carrier: Hydrogen (Linear velocity varied from
Initially, we tested a 0.53-mm-i.d. polydimeth-
30 to 185 cm/s.)
ylsiloxane (DB1) column at a length (15 m) typi-
cally provided by the manufacturer to see if the
Makeup: Nitrogen (38 mL/min)
Method 8330 analytes were resolved. With the ex-
Oven program: 100C for 2 min, 10C per min
ception of HMX, which did not produce a peak,
ramp to 200C, 20C per min to 250C, hold for 5
the 8330 analytes eluted as individual peaks in or-
min.
der of decreasing vapor pressure, indicating that
Detector temperature: 300C.
this column provides adequate resolution for
these analytes. However, the additional analyte
Temperature programs for confirmation columns
are given in captions later in this pa-
per and in Appendix B.
7000
6000
RESULTS AND DISCUSSION
5000
GC setup
4000
Injection port
3000
The GC injection port is frequently
2000
the site of loss of thermally labile ana-
1000
lytes (Grob 1994). Deactivation of the
liners reduces loss (Levsen et al. 1993),
0
0
5
10
15
but is a laborious process, involving
Time (minutes)
acid soaking, water rinsing, silaniza-
a. Linear velocity = 44 cm/s.
tion in an inert atmosphere, and sol-
vent rinsing. In the last few years,
7000
deactivated injection-port liners have
6000
become commercially available; these
5000
have greatly facilitated the analysis of
labile analytes.
4000
Trace analysis by GC may be per-
3000
formed by splitless injection or direct
2000
injection. Splitless injection is not
generally appropriate for reactive or
1000
high boiling compounds such as ex-
0
plosives due to adsorption, conden-
0
5
10
15
sation, and discrimination against
Time (minutes)
high boiling compounds in the injec-
b. Linear velocity = 76 cm/s.
tion port. However, splitless injection
7000
is required for narrow-bore capillary
6000
columns. Wider-bore capillary col-
umns (0.32- and 0.53-mm i.d.) permit
5000
direct injection in which all sample
4000
and solvent is transferred to the col-
3000
umn. The column press-fits into the
hourglass-shaped end of the glass in-
2000
let liner, eliminating contact with
1000
metal parts in the injector. Direct injec-
0
tion liners are commercially available
0
5
10
15
that fit split/splitless ports or packed
Time (minutes)
column injection ports modified to ac-
c. Linear velocity = 126 cm/s.
cept wide-bore capillary columns. We
chose to use deactivated direct injec-
Figure 1. Calibration standard (from set 1 in Table 4) analyzed using
tion Uniliners available from Restek.
DB-1 at three carrier gas velocities. Higher linear velocities resulted in
Uniliners are available for both split/
higher HMX peak heights. Linear velocities correspond to those listed in
splitless and packed ports.
Table 5.
5
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