Environmental Engineering Reference
In-Depth Information
properties of MWNTs in comparison with activated charcoal. linear/branched alkanes and aromatics could be analyzed on
CNTs, contrary to charcoal where very strong adsorption and lack of selectivity were evident, and not even temperature over
300°C could desorb alkyl benzenes in a reasonable timeframe [35, 36].
saridara and Mitra reported the immobilization of CNTs on the inner wall of capillary columns by using Cvd to prepare
self-assembled CNTs in capillary tubes with ethylene as the carbon source. Radial-aligned MWNTs were grown in situ on the
wall of a stainless-steel tube, which was properly treated to generate a catalytically active surface. Importantly, the thickness of
the CNT coating, responsible for different retention capabilities, could be varied by tuning experimental conditions; cogenera-
tion of amorphous and nontubular carbon was overcome by selective annealing after assembling, due to the higher thermal
stability of CNTs. The column showed good thermal stability and satisfactory efficiency in the separation of C2-C6 alkenes.
It has to be underlined that in situ self-assembly enables preservation of CNTs. Nano characteristics may be lost due to agglom-
eration when nanotubes are packed as a powder. such synthetic route was studied further by the same research group. sWNTs
were Cvd-grown on an open tubular GC column by using ethanol and metallic cobalt as the carbon source and catalyst, respec-
tively. In contrast to a previous report, noodle-like structures of CNTs were observed. C2-C14 alkanes and polycyclic aromatic
hydrocarbons (PAHs) were separated, although the peak shape was far from ideal. Temperature up to 425°C could be reached
for eluting the highest molecular weight compounds, with no bleeding. Further optimization of CNT self-assembly via Cvd
was possible by a one-step process based on ethanol as the carbon source, yielding highly pure CNTs (Fig. 14.10). despite its
relatively short length (1.5 m), this column showed good chromatographic properties in terms of resolution and efficiency,
enabling analysis of compounds whose volatility is very different (Table 14.5) [6, 7, 37-39].
safavi et al. synthesized packed capillary columns by the online Cvd method. CNTs showed selectivity for separation of
gases. Ar, H
2
, and Co
2
were easily resolved on sWNTs, yielding better results than MWNTs, whereas no separation was pos-
sible on activated carbon and carbon nanofibers. Completely different procedures, based on chemical methods (i.e., oxidation,
acylation and nucleophilic substitution), were adopted to anchor CNTs on the inner surface of capillary columns. sWNTs-
CoCl, obtained by treating sWNTs-CooH with thionyl chloride, were covalently grafted on the column wall exposing free
amino groups previously inserted by amino silanization of the fused-silica capillary tube with 3-aminopropyl-trietoxysilane.
Interestingly, bonded sWNTs were end-to-end-linked forming a CNT network, as a result of the reaction between sWNTs-
CoCl and sWNT-CooH possibly present in the reaction medium. The column, tested for separation of light hydrocarbons,
alcohols, and aromatics, showed selective adsorption in terms of retention times, but resolution was far from optimal, with
marked band broadening and peak tailing [40].
stadermann et al. fabricated the sWNTs-based stationary phase of the microchannels consisting of a 100 m by 100 m
2
and a
50-cm-long microcolumn placed in a functional chip with an integrated resistive heater and interfaced with a flame ionization
detector (Fig. 14.8c). The device was initially patterned by photolithography and then the CNT film was grown in situ by Cvd,
with variable thickness being obtained depending on the growth time. The layer was quite uniform, despite the presence of
voids. In less than 1 min four C6-C12 alkanes, as well as a functional group test mixture (methanol, 2-pentanone, anisole,
decane), were separated at flow rates of 12-14 ml min
−1
. due to the small thermal mass, a fast temperature gradient of 60°C s
−1
was applicable, with short equilibration time (90 s) between each thermal gradient run. The micro-GC system was further
improved, in particular a smaller device was implemented (50 by 50 m
2
and a 30-cm-long channel) with a different chip
bonding technique and a new growth 183 recipe, obtaining a homogeneous CNT mat, important for minimizing band broad-
ening. such improvements led to better resolution and peak shape, as apparent from the separation of five alkanes in a frame
time of about 5 s. Control tests performed on the bare chip highlighted the chromatographic role of sWNTs [31, 41] (Fig. 14.11).
14.5.2
Chromatographic stationary phases with fuls
Chemical linking of C60 to polysiloxane by bond formation was undertaken to prepare a capillary column for separation
of PCbs. starting from functionalized FUls, the synthesis consists of a single simple reaction step involving the coupling of
polyaminopropyl(methyl)siloxane with the reactive ester cyclohexylaminocarbonyl-(N)-succinimyloxycarbonyl-1,2-
dihydro-[60] FUls. Increase in analyte retention was seen in going from tetra-ortho- to mono-ortho-substituted PCbs, as the
biphenyls with low number of chlorine atoms in the ortho position are liable to assume planar conformations; this promotes
charge transfer/dipole interactions between the solute and the sorbent. High-resolution separation of other high-boiling-point
compounds such as PAHs and phthalic esters was achieved on a polysiloxane capillary column modified with FUls. In this case
the polymeric phase was firstly chemically modified by electrophilic addition to allyl bromide, followed by reaction with
sodium azide and finally by 1,3-dipolar addiction with C60. C12-C32 n-alkanes and C1-C10 phthalic esters, as well as stan-
dard mixtures of PAHs, which eluted in the order of their increasing dispersion force, were nicely separated [42, 43].
As expected, stacking was one of the main interactions governing adsorption, as demonstrated by higher retention times of
methyl esters of unsaturated fatty acids in comparison with homologous saturated analytes. Also, alcoholic and aromatic
positional isomers were resolved, thus evidencing structural selectivity. FUls loading was an influencing parameter with regard
