Anal. Chem. 2006, 78, 6384-6390
Single-Walled Carbon Nanotubes Used as Stationary Phase in GC Li-Ming Yuan,* Chao-Xing Ren, Li Li, Ping Ai, Zhi-Hong Yan, Min Zi, and Zheng-Yu Li
Department of Chemistry, Yunnan Normal University, Kunming 650092, P.R. China
Single-walled carbon nanotubes (SWNTs) have high surface area, high adsorption ability, and nanoscale interactions. In this study, capillary columns including SWNTs, ionic liquid (IL), and IL + SWNTs for GC were prepared. The separation results showed that SWNTs possessed a wide selectivity toward alkanes, alcohols, aromatic compounds, and ketones, and a SWNT capillary column was a very useful GC column for the separation of gas samples. Coating the IL stationary phase on the SWNT capillary column, the SWNTs were able to improve chromatographic characteristic of ionic liquid. Comparing the IL coated on three graphite carbon black capillary columns, which were prepared by dynamic coating, static coating, and chemical bonding the Carbopack C with on SWNTs capillary column, the capacity factors were much higher on the SWNT column. The SEM showed that SWNTs could be bonded to the inner surface of capillary tubing, and most of them were linked end-to-end to form a layer of network structure of skeletons resulting in a high surface area, which increased the interactions between stationary phase and analytes. This is the first single-wall carbon nanotubes bonded to the fused-silica capillary tubing. In the first approach, SWNTs assist ionic liquid with enhanced chromatographic characteristic in GC. This work indicates that SWNTs make it possible to extend the application range on the newly prepared chromatographic stationary phases for GC. Since carbon nanotube (CNT) was discovered in the early 1990s, they were of great interest because of their extraordinary mechanical, thermal, and electronic properties. CNTs exist as single-walled nanotubes (SWNTs) or multiple-walled nanotubes and also as open- or close-ended structures of different morphology and diameter. SWNT consists of a grapheme sheet rolled into a cylinder with a typical diameter on the order of 1 nm. It has stimulated intensive research into potential high-impact applications such as nanoelectronic devices, catalyst supports, biosensors, and hydrogen storage.1,2 This development provides good opportunities for the development of higher performance separation techniques that utilize the nanoscale interactions. For example, self-assembled carbon nano* To whom correspondence should be addressed. Telephone: 86-8716633873. Fax: 86-871-5518946. E-mail:
[email protected]. (1) Ajayan, P. M. Chem. Rev. 1999, 99, 1787-1799. (2) Wei, B. Q.; Vajtai, R.; Jung, Y.; Ward, J.; Zhang, R.; Ramanath, G.; Ajayan, P. M. Nature 2002, 416, 495-496.
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tubes were used in long capillary tubes for the development of gas chromatography columns;3,4 To determine an environmental sample, carbon nanotubes were used as adsorbents.5,6 An organic polymer monolithic stationary phase embodying single-walled carbon nanotubes was used for µHPLC and capillary electrochromatography.7 The carbon nanotubes have shown great potential in separation science and techniques. Ionic liquids (ILs) are salts that are liquid at low temperature ( IL + SWNT capillary column > IL capillary column. The separations for a gas mixture of methane, ethene, and ethyne on the SWNT capillary column, and for another mixture (14) Nota, G.; Goretti, G. C.; Armenante, M.; Marino, G. J. J. Chromatogr. 1974, 95, 229-231.
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Figure 1. Picture of SWNT capillary column (7.5 m × 0.25 mm i.d.) (A), IL capillary column (10 m × 0.25 mm i.d.) (B), and IL + SWNT capillary column (10 m × 0.25 mm i.d.) (C). Table 1. Characteristics for Three Capillary Columns in GC
column
column dimension (m × mm)
SWNTs
7.5 × 0.25
IL
10 × 0.25
IL + SWNTs
10 × 0.25
capacity factor k′
temp (°C)
opt linear velocity (cm/s)
column efficiency (plates/m)
compound tested
6.13 5.71 1.80 2.51 2.01 1.91
110 130 80 120 100 55
21.9 20.5 13.8 14.5 19.1 20.7
660 620 1680 1440 2270 2010
n-decane n-undecane 1-butanol naphthalene 1-butanol benzene
Figure 2. Van Deemter plots of SWNT capillary column (7.5 m × 0.25 mm i.d.) for n-decane at 110 °C (A), IL capillary column (10 m × 0.25 mm i.d.) for 1-butanol at 100 °C (B), and IL + SWNT capillary column (10 m × 0.25 mm i.d.) for 1-butanol at 100 °C (C) in GC. Split ratio, 80:1, FID.
containing n-decane, n-undecane, 1-octanol, n-decanoic acid methyl ester, and 2.6-dimethylaniline on IL and IL + SWNT capillary columns, are shown in Figure 3. For all analytes, complete resolution was attained. Comparing the results on the IL + SWNT capillary column with those on the IL capillary column, the IL + SWNT capillary column was more effective. The SWNT capillary column not only can be used to separate gas sample but also can be used to enhance the column resolution of IL. Typical reproducibility of those columns in retention time measured as relative standard deviation was IL + static coated Carbopack > IL + dynamic coated Carbopack capillary columns. The high capacity factor on the IL + SWNT capillary column reflects the strong interactions of the SWNTs. To study the mechanism and the reason for this resolution enchancement, some segments were cut from the SWNT, bonded Carbopack, and static coated Carbopack capillary columns. The samples were cut open to expose the inside surfaces, which were analyzed by scanning electron microscope. The SEM images (Figure 5A f B) showed that the size of SWNTs was similar to that reported in the literature,12 and besides, SWNTs could be bonded to the inner surface of capillary tubing, and most of them were linked end to end to form a layer of network structure of skeletons due to the reactions between SWNTs-COCl and Analytical Chemistry, Vol. 78, No. 18, September 15, 2006
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Figure 4. Chromatograms of some mixtures on three capillary columns in GC. Split ratio, 80:1, FID. SWNT capillary column (7.5 m × 0.25 mm i.d.): (A) T ) 70 °C, v ) 18.2 cm/s; (D) T ) 120 °C, v ) 19.7 cm/s; (G) T ) 80 °C, v ) 19.0 cm/s; (J) T ) 90 °C, v ) 22.3 cm/s; (M) T ) 100 °C, v ) 20.2 cm/s; (P) T ) 80 °C, v ) 22.0 cm/s. IL capillary column (10 m × 0.25 mm i.d.): (B) T ) 50 °C, v ) 22.0 cm/s; (E) T ) 70 °C, v ) 21.0 cm/s; (H) T ) 45 °C, v ) 20.8 cm/s; (K) T ) 40 °C, 21.8 cm/s; (N) T ) 40 °C, v ) 22.8 cm/s; (Q) T ) 30 °C, v ) 20.4 cm/s. IL + SWNT capillary column (10 m × 0.25 mm i.d.): (C) T ) 50 °C, v ) 22.0 cm/s; (F) T ) 70 °C, v ) 21.0 cm/s; (I) T ) 45 °C, 20.8 cm/s; (L) T ) 40 °C, v ) 21.8 cm/s; (O) T ) 40 °C, v ) 22.8 cm/s; (R) T ) 30 °C, v ) 20.4 cm/s.
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Figure 5. SEM images showing the SWNTs and graphite carbon black film on the capillary tubing. (A,B) SEM images of the inner surface of the SWNT capillary column; (C) SEM image of the inner surface of the bonded Carbopack capillary column; (D) SEM image of the inner surface of the static coated Carbopack capillary column.
CONCLUSIONS
Table 3. Evaluation of Capacity Factors for Column-Column Reproducibility capacity factor (k′) column
n-undecane
1-heptanol
1 2 3
1.24 1.17 1.18
17.40 15.94 16.27
SWNTs-COOH. As shown in Figure 5C f D, the amount of attached graphite carbon black in the columns was much more on the bonded Carbopack column compared to the static coated Carbopack column. Among the three columns, the SWNT capillary column possessed the biggest surface for the coated IL stationary phase. Therefore, this is one possible reason that the IL + SWNT capillary column has the highest resolution ability. To evaluate the column-to-column reproducibility, three IL + SWNT capillary column were prepared under identical condition. The capacity factors were obtained with undecane and heptanol used as tested analytes (Table 3). The results showed that the technique was reliable and reproducible. 6390
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SWNTs possess a wide selectivity toward alkanes, alcohols, aromatic compounds, ketones, et al. The SWNT capillary column was a very useful GC column for the separation of gas samples. When the IL stationary phase was coated on the inner wall of a SWNT capillary column, the SWNTs were able to assist ionic liquid with enhanced chromatographic characteristics, because the SWNTs formed a layer of network structure of skeletons in the inner wall of capillary tubing, resulting in a high surface area, which increased the interactions between stationary phase and analytes. This research will lead to the development of a wide range of chromatographic columns with improving selectivity. ACKNOWLEDGMENT The work is supported by the National Natural Science Foundation, TRAPOYT, and Yunnan Province’s Natural Science Foundation of China.
Received for review April 9, 2006. Accepted July 14, 2006. AC060663K
