Environmental Engineering Reference
In-Depth Information
(a)
(b)
Microconcentrator
Sensor or
gas
chromatograph
Hexane
Air or gas
sample
Ethanol
Electronics for
thermal desorption
(c)
Toluene
Commercial
GC injector
A
B
Vent
FID
Deactivated
silica capillary
leads
Dual
diaphragm
valve injection
Voltage
grounding
leads
V1
V2
1
3
Time (min)
Hydrogen
carrier
gas
V
Variable
AC power
supply
(0-120 V)
Microfabricated SWNT column
50 cm, 100 μm × 100 μm
figure 14.8
(a) online monitoring with a CNT microtrap; (b) chromatogram generated by a CNT microconcentrator; (c) microfabricated
sWNT column. Reproduced with permission from Refs. [31-33]. Ref. [31] © ACs Publications, Ref. [32] © science direct Elsevier, and
Ref. [33] © RsC Publication.
table 14.4 breakthrough times (min) for voCs and svoCs with different types of Cnts, where the concentration of the
analytes was in the range of 9-12 ppm at a flow rate of 3 ml/min, and the amount of sorbent in the microtrap was 20 mg
Type of microtrap
dichloromethane
Hexane
Toluene
o-nitro phenol
Naphthalene
MWNT-1C
a
5
15
40
50.0
120.0
sWNT
b
10
20
50
30.0
90.0
MWNT-1
c
2
5
6
5.0
15.0
Carbopack
2
5
6
5.0
12.0
Reproduced with permission from Refs. [32-34]. © science direct Elsevier.
a
Cleaned MWNTs (Fig. 14.3b).
b
Commercially available purified sWNTs.
c
synthesized MWNTs with significant amount of impurities (Fig. 14.3a).
Carbopack. This is mainly because of the nonnanotubular nature of adsorption. once the impurities were removed, the break-
through volume increased quite dramatically. sWNTs showed the highest adsorption capacity in terms of breakthrough volume.
This was attributed to the higher aspect ratio due to the smaller diameter, which led to a higher specific area. For example, the
breakthrough time of dichloromethane (dCM) on sWNTs was five times higher than on Carbopack or unpurified MWNTs.
CNTs were also excellent for sampling small molecules such as CH
4
to large svoC molecules [32, 33].
The effect of CNT functionalization on gas-phase preconcentration was also studied. Carboxylated CNT showed signifi-
cantly different retention characteristics. The breakthrough time for polar compounds increased dramatically—nearly 300% for
ethanol, 200% for dCM, and 200% for propanol. This was because the presence of polar functional groups led to strong inter-
actions with polar compounds. Therefore, CNTs aid preconcentration for a wide range of polar and nonpolar compounds, and
mixed sorbent beds may be used for a wide range of compounds [34].
The phenomenon of desorption is complementary to adsorption. Quantitative desorption is important for accurate quantita-
tion as well as for the reuse of expensive CNTs. desorption efficiency often determines the range of compounds, especially the
larger molecules that can be used with a microconcentrator. Rapid desorption from a microtrap is also essential for producing
a sharp concentration pulse to serve as an injection for gas chromatography (GC) or GC/mass spectrometry (Ms), and for
