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Chinese Chemical Letters 21 (2010) 1358–1360

Selectivity of guanidinium ionic liquid for capillary gas

chromatography

Kai Lu, Li Zhen Qiao, Mei Ling Qi *, Ruo Nong Fu

Department of Chemistry, School of Sciences, Beijing Institute of Technology, Beijing 100081, China

Received 8 March 2010

Abstract

A guanidinium ionic liquid, N, N, N0, N0-tetrahexyl-N00, N00-dimethylguanidinium bis(trifluoromethane)sulfonylimide (THDMG-

NTf2), was synthesized and used as stationary phase for capillary gas chromatography. In comparison with imidazolium ionic liquid

stationary phase, the present new stationary phase exhibits quite different selectivity and behaves more like a low polar stationary

phase. The guanidinium ionic liquid of THDMG-NTf2 exhibited better separation of Grob test mixture than imidazolium ionic

liquid of 1-octyl-3-butylimidazolium bis(trifluoromethane)sulfonylimide (OBIM-NTf2). Solvation parameter model was also used

to evaluate the selectivity of THDMG-NTf2. Additionally, essential oil of Magnolia biondii Pamp was analyzed to further evaluate

the selectivity of THDMG-NTf2 for a sample of complicated components. Satisfactory separation of the essential oil was achieved

on a THDMG-NTf2 column (10 m) while using a commercial column (30 m) as reference. The present study shows that the

guanidinium ionic liquid possesses novel chromatographic selectivity and has great potential for wide applications.

# 2010 Mei Ling Qi. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.

Keywords: Guanidinium ionic liquids; Capillary gas chromatography; Stationary phase

Ionic liquids (ILs) as non-molecular solvents possess unique properties such as low vapor pressure, non-

flammability and good thermal stability and have attracted great interest of analysts in the past decade [1–4].

Imidazolium ILs as stationary phases for capillary gas chromatography (CGC) generally behave more like polar

ones and have two nitrogen atoms (positions 1 and 3) available for substituted groups. Compared to imidazolium

ILs, guanidinium ILs have much better thermal stability and organic solvents miscibility [4], which are ideal as

CGC stationary phases. Besides, structurally speaking, guanidinium ILs have six positions available for

substituting groups which provide hundreds of possible combinations for new guanidinium ILs stationary phases.

As a result of a recent survey, no reports on using guanidinium ILs as CGC stationary phases are available. In the

present work, a guanidinium IL stationary phase, N, N, N0, N0-tetrahexyl-N00, N00-dimethylguanidinium

bis(trifluoromethane)sulfonylimide (THDMG-NTf2), was synthesized and coated onto capillary columns. Grob

test mixture and essential oil of Magnolia biondii Pamp were used to examine the chromatographic selectivity of

THDMG-NTf2 as CGC stationary phase.

* Corresponding author.

E-mail address: mlqi@bit.edu.cn (M.L. Qi).

1001-8417/$ – see front matter # 2010 Mei Ling Qi. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.

doi:10.1016/j.cclet.2010.04.035

K. Lu et al. / Chinese Chemical Letters 21 (2010) 1358–1360 1359[(Fig._1)TD$FIG]

Fig. 1. Structure of THDMG-NTf2.

1. Experimental

THDMG-NTf2 (Fig. 1) was synthesized as follows [5]. At first, chloride salt (THDMG-Cl) was synthesized by

reacting (dichoromethylene)dimethylammonium chloride with excess dihexylamide and triethylamide in

dichloromethane. Then excess lithium bis(trifluoromethane) sulfonylimide was added to conduct ion exchange

reaction. THDMG-NTf2 was obtained as a yellow liquid and dissolved in dichloromethane at concentration of 0.25%

(w/v) and was pushed into the capillary column (10 m, 0.25 mm i.d.) which was pretreated by depositing sodium

chloride on the inner wall. After dichloromethane was vaporized at 40 8C, the coated column was conditioned from

30 8C to 160 8C at 1 8C/min and held at 160 8C for 7 h. The column efficiency was determined with about 1000 plates

per meter at 100 8C.

2. Results and discussion

Grob test mixture was used to evaluate the selectivity of the guanidinium IL stationary phase and the results were

shown in Fig. 2A. The elution order and resolution of the components with narrow and symmetrical peaks indicated

the unique selectivity of THDMG-NTf2 different from the corresponding imidazolium IL stationary phase of 1-octyl-

3-butylimidazolium bis(trifluoromethane)sulfonylimide (OBIM-NTf2) in Fig. 2B. Especially, it can be observed that

the components in the Grob mixture eluted chiefly by their melting points on THDMG-NTf2, suggesting the lower

polarity of the guanidinium IL. Moreover, the short retention of 2, 3-butanediol (strong H-bond compound)

demonstrated the low H-bond adsorption of THDMG-NTf2.

[(Fig._2)TD$FIG]

Fig. 2. Separation of Grob test mixture by (A) THDMG-NTf2 column (10 m � 0.25 mm � 0.16 mm) and (B) OBIM-NTf2 column

(10 m � 0.25 mm � 0.16 mm). Peaks: (1) n-decane, (2) n-undecane, (3) 2, 3-butanediol, (4) 1-nonanal, (5) 1-octanol, (6) 2, 6-dimethylphenol,

(7) 2, 6-dimethylaniline, (8) 2-ethylhexanoic acid, (9) methyl decanoate, (10) dicyclohexylamine, (11) methyl undecanoate, (12) methyl

dodecanoate. GC conditions: 50 8C for 2 min, 5 8C/min to 150 8C; carrier gas, nitrogen, 1 ml/min; injection, 250 8C, split (46:1); FID, 260 8C.

K. Lu et al. / Chinese Chemical Letters 21 (2010) 1358–13601360[(Fig._3)TD$FIG]

Fig. 3. Separation of the essential oil of Magnolia biondii Pamp by (A) THDMG-NTf2 column (10 m � 0.25 mm � 0.16 mm) and (B) commercial

HP-INNOWax column (30 m � 0.25 mm � 0.25 mm). Peaks of solvent and five main chemical components: (s) hexane, (1) b-pinene, (2)

eucalyptol, (3) 3,7-dimethyl-1,6-octadien-3-ol, (4) carypphyllene, (5) 2,6,10-dodecatrien-1-ol. GC conditions: (A) 30 8C for 3 min, 2 8C/min to

40 8C, 5 8C/min to 150 8C; (B) 50 8C for 5 min, 2 8C/min to 60 8C, 20 8C/min to 120 8C and held for 10 min, 25 8C/min to 190 8C; carrier gas,

nitrogen, 1 ml/min; injection, 250 8C, split (46:1); FID, 260 8C.

To investigate the molecular interactions between the stationary phase and analytes, solvation parameter model of

THDMG-NTf2 was determined as follows: 70 8C, log k = � 3.31 � 0.14 + 1.29S + 1.47A + 0.59B + 0.69L (n = 39,

R2 = 0.99); 100 8C, log k = � 3.19 � 0.06E + 1.25S + 1.25A + 0.32B + 0.55L (n = 38, R2 = 0.98). Dipole-type

interaction (S) and H-bond basic interaction (A) as the main molecular interactions of THDMG-NTf2 stationary

phase were both lower than that of imidazolium ionic liquids [2], suggesting that THDMG-NTf2 behaves more like a

low polar stationary phase. This can also explain why the components of Grob mixture eluted earlier with narrow peak

shapes on guandinium IL than on imidazolium IL.

To further evaluate the selectivity of guanidinium ILs for more complicated samples, the essential oil of M. biondii

Pamp was analyzed on the THDMG-NTf2 column (10 m � 0.25 mm � 0.16 mm) and a commercial HP-INNOWax

column (30 m � 0.25 mm � 0.25 mm) used for comparison, respectively, and the results were shown in Fig. 3.

Though the length of the guanidinium IL column was much shorter than the commercial one, satisfactory separation of

the components in the essential oil on the guanidinium IL column was achieved in comparison with the commercial

one. It can be reasonably expected that much better separation can be achieved on a longer column of THDMG-NTf2.

The above results demonstrate the novel chromatographic selectivity of guanidinium ILs and the great potential for

further intensive studies and wide applications.

3. Conclusions

The guanidinium IL of THDMG-NTf2 as CGC stationary phase exhibits unique selectivity which is quite different

from that of imidazolium ILs. Satisfactory separations of Grob mixture and essential oil of M. biondii Pamp

demonstrate the great potential of guanidinium ILs as CGC stationary phases for wide applications.

Acknowledgment

The authors thank the National Nature Science Foundation of China (No. 20675007).

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