Cucurbit[n]urils as a new class of stationary phases for gas chromatographic separations

Journal of Chromatography A, 1334 (2014) 139–148

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Cucurbit[n]urils as a new class of stationary phases for gas chromatographic separations Pu Zhang, Shijia Qin, Meiling Qi ∗ , Ruonong Fu Key Laboratory of Cluster Science, Ministry of Education of China, Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials and School of Chemistry, Beijing Institute of Technology, Beijing 100081, China

a r t i c l e

i n f o

Article history: Received 27 December 2013 Received in revised form 29 January 2014 Accepted 30 January 2014 Available online 7 February 2014 Keywords: Cucurbit[7]uril Cucurbit[8]uril Stationary phases Capillary gas chromatography Separation performance

a b s t r a c t Cucurbit[n]urils (CBs) possess unique structures and physicochemical properties as well as excellent thermal stability. These characteristics concur to make them good candidates for stationary phases in capillary gas chromatographic (GC) separations. Herein, CB7 and CB8 in neat (CB7, CB8) and binary (CB7–CB8) forms were investigated for this purpose. After they were statically coated onto fused silica capillary columns, the CB columns were evaluated in terms of chromatographic parameters, separation performance, thermal stability and column repeatability. The columns had efficiencies ranging from 1060 to 2200 plates per meter determined by n-dodecane at 100 ◦ C and exhibited nonpolar to weakly polar nature. These CBs columns showed good separation performance for a wide range of analytes such as n-alkanes, aromatic hydrocarbons, esters, aldehydes, ketones, alcohols and the Grob mixture, and exhibited nice peak shapes for analytes that are liable to peak-tailing in GC analysis. The results also proved the good column repeatability and thermal stability of the CB columns. No noticeable decreases in both retention times and resolution or appreciable baseline drift were observed after the columns were conditioned up to 250 ◦ C (CB8 and CB7–CB8 columns) or 280 ◦ C (CB7 column). This work demonstrates the promising future of CBs as a new class of GC stationary phase. To the best of our knowledge, this is the first report on using CB stationary phases in capillary GC separations. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Cucurbit[n]urils (CBs, n = 5–8, 10) are pumpkin-shaped macrocyclic host molecules with a hydrophobic cavity and two identical openings that are lined with carbonyl groups. CBs possess unique features such as rigid cavity structure, high selective interactions with analytes via dipole-dipole, H-bonding and hydrophobic interactions [1–4], low toxicity [5] and high chemical and thermal stability [1,6]. For example, CB7 and CB8 are thermally stable up to around 400 ◦ C and their structures are shown in Fig. 1. Due to their fascinating features, researches on CBs are rapidly unfolding in supramolecular chemistry [4,6,7], drug delivery [8], sample preparation [9], chromatographic separation [10–15] and many others [16,17]. Although CBs are potentially as useful as other host molecules in chromatographic separation, only a few publications are available [10–15]. The reason for this may be mainly due to their poor solubility in ordinary organic solvents and chemical inactivity for derivatization, which have greatly limited researches and applications of CBs in chromatography.

∗ Corresponding author. Tel.: +86 10 68912668; fax: +86 10 68914780. E-mail address: [email protected] (M. Qi). 0021-9673/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.chroma.2014.01.083

In chromatographic separation, Feng and coworkers reported perhydroxyl-CB6 bonded silica stationary phase for hydrophilicinteraction chromatography [10] and CB7 as an additive in capillary electrophoresis for separation of positional isomers [11] and aristolochic acids in medicinal plants [12]. Kim and co-workers reported the CB6-anchored silica gel in liquid chromatography [13]. Li and co-workers used perhydroxy-CB6 [14] and CB7 [15] as stationary phase for packed gas chromatography (GC), in which CBs was dissolved in concentrated hydrochloric acid and coated onto white support for column packing, demonstrating their separation ability as stationary phases for packed GC separations. Briefly, the above researches, regardless of different chromatographic methods, demonstrate the potential for using CBs in chromatographic separation. In contrast to packed GC, capillary GC is characterized by high separation ability, sensitivity and column efficiency and has found wide applications in the analysis of volatile compounds in various samples. Multiple molecular interactions and high thermal and chemical stability have elicited CBs as ideal candidates for stationary phases in high-resolution capillary GC separations. However, no recent survey showed any available report on CBs as capillary GC stationary phases. As mentioned above, it is quite challenging to achieve this purpose owing to the limitations

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2.2. Methods

Fig. 1. Structures of CB7 (m = 2) and CB8 (m = 3), adapted from [1].

of CBs in solubility and their chemical inertness for derivatization. Though Kim and co-workers once successfully synthesized perallyloxy-CB6 as stationary phase for liquid chromatography [13], the quite low yield was a great limitation for its practical application. Here we report the first example of using CBs as stationary phases for capillary GC separations. CB7 and CB8 were investigated for this purpose. Three capillary columns coated with neat (CB7, CB8) or binary (CB7–CB8) stationary phases were investigated in terms of chromatographic parameters (column efficiency, McReynolds constants), separation performance for a wide range of analytes (n-alkanes, aromatic hydrocarbons, esters, aldehydes, ketones, alcohols and the Grob mixture), thermal stability and column repeatability.

2.2.1. Syntheses of CB7 and CB8 stationary phases The syntheses of CB7 and CB8 were performed following the references [18,19]. Briefly, glycoluril (71.4 g, 0.5 mol) and paraformaldehyde (30.1 g, 1.0 mol) were thoroughly mixed and added to a 500 mL round-bottomed flask. A concentrated HCl solution (110 mL) was then slowly added to the flask under rapid stirring at room temperature. Stirring was continued until the mixture set as a gel. Next, the mixture was heated to 100 ◦ C, resulting in a rapid dissolution of the gel, and refluxed for 20 h and then cooled to room temperature. After the solution was concentrated to 50 mL and poured into 500 mL water, a crystalline precipitate formed and was then filtrated. The obtained filtrate (A) and crystalline solid (B) were further purified as follows. The filtrate A was concentrated to 100 mL and added to an 800 mL solution of acetone–water (7:1). As a result, a white precipitate formed and was then filtered. This process was repeated two more times. The obtained white solid was then successively washed with methanol–water (1:1), water and acetone. The purified solid was collected by filtration and dried under vacuum and the final CB7 product was obtained. The solid B was successively washed with water and acetone and dried under vaccum. Then the obtained solid was dissolved in a 40 mL HCl solution (4 mol/L) and stirred for 30 min and filtered. This process was repeated one more time. The collected solid was successively washed with a 10 mL HCl solution and acetone, recrystallized in HCl solution (6 mol/L), washed with water and then filtered and dried under vacuum. Finally, the CB8 product was obtained as a white crystalline solid. The obtainment of CB7 and CB8 was confirmed by FT-IR and 1 H NMR, which was in good agreement with the data in the references. Then, the obtained products were finely powdered, sifted through a standard sieve of average pore size of 74 ␮m (200 mesh) and were used for the following capillary column preparation.

2. Experimental 2.1. Materials and equipment All the chemicals used in this work were of analytical grade. Benzene, toluene, ethylbenzene, o-xylene, m-xylene, pyridine, 1-butanol and 1-nitropropane were purchased from Alfa Chemical Co., Ltd (Tianjin, China). The analytes used for the evaluation of chromatographic performance including n-octane, n-decane, n-undecane, n-dodecane, n-tridecane, n-tetradecane, n-propyl acetate, n-butyl acetate, methyl hexanoate, methyl heptanoate, methyl nonanoate, methyl decanoate, methyl undecanoate, methyl dodecanoate, 1,4-butyrolactone, 1-pentanol, 1-hexanol, 1-heptanol, 1-octanol, 1-nonanol, 1decanol, 1-undecanol, pentanal, heptanal, benzaldehyde, salicylaldehyde, octanaldehyde, nonanal, 2-pentanone, acetophenone, 2-hexanone, 2,3-butanediol, 2,6-dimethylaniline, dicyclohexylamine, 2-ethylhexanoic acid, 2,6-dimethylphenol and dichloromethane were purchased from Beijing Chemical Reagent Company (Beijing, China). All the analytes were dissolved in dichloromethane at 1 mg/mL. Untreated fused-silica capillary column (0.25 mm i.d.) was purchased from Yongnian Ruifeng Chromatogram Apparatus Co., Ltd. (Hebei, China). A HP-5MS capillary column (12 m × 0.25 mm, i.d.) was purchased from Agilent Technologies. An Agilent 7890A gas chromatograph equipped with a split/splitless injector, a flame ionization detector (FID) and an autosampler was used for GC separations. All the separations were performed under the following GC conditions: nitrogen of high purity (99.999%) as carrier gas at a flow rate of 1 mL/min, injection volume of 0.2 ␮L, injection port at 250 ◦ C, split injection mode at a split ratio of 30:1 and FID as detector at 300 ◦ C.

2.2.2. Capillary column preparation Suspensions of the neat (CB7, CB8) and binary (CB7–CB8, 1:1) stationary phases were individually prepared at 0.25% (w/v). For Table 1 Probes used for McReynolds constants and their functions. Symbol

Probe

Function

X Y Z U S

Benzene n-Butanol 2-Pentanone 1-Nitropropane Pyridine

␲-Interaction H-acceptor and donor H-acceptor Dipole interaction Strong H-acceptor

Table 2 Column efficiencies of the CB and commercial columns. Column

Stationary phase

Column efficiency (plates/m)a

1 2 3 4

CB7 CB8 CB7-CB8 HP-5MS

1400 2200 1060 2749

a

n-dodecane at 100 ◦ C.

Table 3 McReynolds constants of the CB and commercial columns. Columns

X

Y

Z

U

S

Average polarity

CB7 CB8 CB7-CB8 HP-5MS

11 15 47 33

146 154 148 72

78 73 88 66

124 178 152 99

82 160 112 67

88 116 109 67

X : benzene, Y : 1-butanol, Z : 2-pentanone, U : 1-nitropropane, S : pyridine. Temperature: 120 ◦ C.

P. Zhang et al. / J. Chromatogr. A 1334 (2014) 139–148

600

CB7

CB8

1

700

400

600

2

3

300

4

5

6

pA

500

pA

800

1

141

2

500

3

400

200

300

100

200

4

5

6

100 0

5

10 15 Time ( min )

600

10 Time ( min )

15

CB7+CB8

500

2

300

4

3

400 pA

5

0

20

5

6

1

200 100 0

5

10 15 Time ( min )

20

Fig. 2. GC separation of n-alkanes on the CB7, CB8 and CB7–CB8 columns. Peaks: (1) n-octane, (2) n-decane, (3) n-undecane, (4) n-dodecane, (5) n-tridecane, (6) n-tetradecane. Oven temperature program: 40 ◦ C (1 min) to 130 ◦ C at 5 ◦ C/min.

800 2 5

1

200

2

3

5

400 200

0

1

800

2 3 Time ( min )

4

0

1

2 3 Time ( min )

4

5

CB 7 + CB 8

1

600

3 2

pA

4

1

600

3 4

400

CB 8

800

pA

600

pA

1000

CB7

4

5

400 200

0

1

2 3 Time ( min )

4

5

Fig. 3. GC separation of aromatic hydrocarbons on the CB7, CB8 and CB7–CB8 columns. Peaks: (1) benzene, (2) toluene, (3) ethylbenzene, (4) o-xylene, (5) m-xylene. Oven temperature program: 40 ◦ C (1 min) to 80 ◦ C at 10 ◦ C/min.

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the neat CB columns, the suspensions were prepared by dispersing 25 mg of CB7 or CB8 in 10 mL of dichloromethane under ultrasonication for 5 min; for the binary column, 12.5 mg of each CB was used instead. Prior to static coating, a fused-silica capillary (10 m × 0.25 mm, i.d.) was purged with nitrogen at 260 ◦ C for 2 h and pretreated with a saturated solution of sodium chloride in methanol. The pretreated capillary columns were individually coated with each of the suspensions by static coating method at room temperature. After the suspension was pumped into the pretreated capillary column, one end of the capillary column was sealed and the other end was connected to a vacuum system to gradually remove the solvent. The coated capillary column was then conditioned from 40 ◦ C to 180 ◦ C at 1 ◦ C/min and held at the high-end temperature for 7 h under a constant flow of nitrogen at 1 mL/min. Using this coating method, three capillary columns individually containing CB7, CB8 and CB7–CB8 stationary phases were prepared and denoted as CB7, CB8 and CB7–CB8 columns in the following sections, respectively.

of the stationary phase. The higher the value of the average polarity, the higher the polarity of the stationary phase. 3. Results and discussion 3.1. Chromatographic parameters of the CB capillary columns The column efficiencies expressed as the number of theoretical plates per meter (plates/m) determined by n-dodecane at 100 ◦ C on the CB columns are shown in Table 2. The CB columns achieved column efficiencies ranging from 1060 to 2200 plates/m that were much lower than that of the commercial column. This may originate from the fact that CBs are difficult to dissolve in ordinary organic solvents. However, it is known that a column with low efficiency does not mean poor resolving ability because resolution is determined not only by column efficiency but also by selectivity factor and retention factor, as revealed by the fundamental resolution equation shown later in this work. A column with low efficiency can also achieve good resolution if the stationary phase has a high selectivity factor for specific analytes. Undoubtedly, improvement in column efficiency for CB columns will favor resolution. Though difficult, there is still large room for the improvement by narrowing down the particle distribution range or even finding a way to improve the solubility of CB stationary phases in the solvent commonly used for column preparation. The McReynolds constants for CB columns are shown in Table 3. Generally, a GC stationary phase can be classified as non-polar,

2.2.3. Column efficiency and McReynolds constants Column efficiencies of the prepared CB columns were determined by n-dodecane at 100 ◦ C. McReynolds constants of the CB columns were determined at 120 ◦ C using the five probe compounds, namely benzene, n-butanol, 2-pentanone, 1-nitropropane and pyridine. The measured intermolecular interactions of the probes with a stationary phase are listed in Table 1 and together they provide a measure of the overall polarity or average polarity

600

600

CB 7

500

7 8 9

6

5

500

400 pA

pA

12

200

CB 8 3

5

6

9

7

400

3 300

2 1

4

8

300 4

200

100

100 0

2

4

6

8

10

12

14

Time ( min )

2

4

6

8

10

12

Time ( min )

700

CB 7 + CB 8

600 500 pA

0

5

400

6

7

8

10

8

9

3

300 1

200

2 4

100 0

2

4

6

12

14

Time ( min ) Fig. 4. GC separation of esters on the CB7, CB8 and CB7–CB8 columns. Peaks: (1) n-propyl acetate, (2) n-butyl acetate, (3) methyl hexanoate, (4) 1,4-butyrolactone, (5) methyl heptanoate, (6) methyl nonanoate, (7) methyl decanoate, (8) methyl undecanoate, (9) methyl dodecanoate. Oven temperature program: 40 ◦ C (1 min) to 160 ◦ C at 10 ◦ C/min.

P. Zhang et al. / J. Chromatogr. A 1334 (2014) 139–148

weakly-to-moderately polar or highly polar when average polarity is 1–100, 100–400 or over 400, respectively. Accordingly, the CB columns exhibit nonpolar to weakly polar nature in GC separations. 3.2. Separation performance Separation performance of the CB7, CB8 and CB7–CB8 columns was evaluated by GC separation of different analytes of great variety, including nonpolar (n-alkanes, aromatic hydrocarbons), low to medium polar (esters, aldehydes, ketones) and polar analytes (alcohols) and the Grob mixture consisting of 12 test analytes. 3.2.1. Separation of nonpolar analytes GC separations of nonpolar analytes including n-alkanes and aromatic hydrocarbons on the three CB capillary columns are shown in Fig. 2 and Fig. 3, respectively. As can be observed in Fig. 2, all of the n-alkanes were baseline resolved with nice peak shapes on the three columns and eluted in the order of their boiling points (b.p.). One interesting phenomenon occurred in n-alkanes that were retained less on CB8 column than on CB7 column. This may arise from their different cavity sizes. CB8 possesses a larger cavity that may allow the linear alkanes to get in and out of the cavity more freely and quickly, which leads to their shorter retention time. In contrast, the binary CB7–CB8 column showed slightly longer retention towards the analytes than either CB7 or CB8 column, which suggests the possible synergistic effect of the binary stationary phase. These results indicate that the geometric fitness of nonpolar analytes with the cavity of CB stationary phases may also play a role in their GC separation.

600

3.2.2. Separation of low to medium polar analytes Analytes of low to medium polarity primarily cover compounds containing oxygen atoms without active hydrogen, such as esters, aldehydes and ketones, etc. Figs. 4–6 show the results for GC separations of esters, aldehydes and ketones on the three CB columns, respectively. As shown in Fig. 4, all the esters were well resolved with nice peak shapes except peak 4 and exhibited shorter retention on CB8 column than on CB7 column. The explanation given above for the linear alkanes also applies here.

800

300

4

5

400

6

300

5

200

2 3

500 pA

3

1

600

6 1

CB 8

700

2

400 pA

Fig. 3 shows the GC separations of aromatic hydrocarbons on CBs columns. The elution order for the first three analytes, namely benzene, toluene and ethylbenzene, were in good agreement with their b. p. order, but the elution order for the last two analytes was against their b. p. order, that is, m-xylene (b.p. 139.1 ◦ C) eluted behind o-xylene (b.p. 144.4 ◦ C). The reason for this may be due to their different interactions with the CBs stationary phases. In this case, the molecular geometry of m-xylene probably fitted better with the cavity of CB7 or CB8 than that of o-xylene. Moreover, aromatic hydrocarbons showed slightly longer retention times on CB8 column than those on CB7 column, which was quite different from the retention behaviors of the linear alkanes. This may be attributed to the better fitting of these aromatic analytes in the larger cavity of CB8. The analytes retained shorter on CB7 column but achieved remarkable resolution and sharper peaks. In brief, the above results suggest that for nonpolar analytes, the CB stationary phases exhibit varied retention behaviours and separation performance towards analytes of different geometry or size via shape- fitting interactions as well as dispersion interactions.

CB 7

500

143

4

200 100

100 0

2

4

6

8

Time ( min ) 600

0

2

4

6

8

Time ( min )

CB 7+ CB 8

500

pA

400

3

2

5

6

300 4

1

200 100 0

2

4

6

8

10

Time ( min ) Fig. 5. GC separation of aldehydes on the CB7, CB8 and CB7–CB8 columns. Peaks: (1) pentanal, (2) heptanal, (3) benzaldehyde, (4) octanal, (5) salicylaldehyde, (6) nonanal. Oven temperature program: 40 ◦ C (1 min) to 120 ◦ C at 10 ◦ C/min.

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700

500

CB7

600

CB 8 2

400

1

1 2

400

3

300

3

pA

pA

500

300

200

200

100

100 0

2

4

6

8

2

4

6

8

Time ( min )

Time ( min )

500

0

CB7+CB8 1

400

3

2

pA

300 200 100

0

2

4

6

8

10

Time ( min ) Fig. 6. GC separation of ketones on the CB7, CB8 and CB7–CB8 columns. Peaks: (1) 2-pentanone, (2) 2-hexanone, (3) acetophenone. Oven temperature program: 40 ◦ C (1 min) to 80 ◦ C at 5 ◦ C/min.

Fig. 5 shows that all the aldehydes were baseline separated on the three columns and achieved relatively sharp peaks on the CB7–CB8 column. Though slight peak tailings existed for some of the analytes on the neat CB columns, the peak shapes were good enough for the direct analysis of aldehydes with no need for derivatization. In addition, the CB7 and CB8 columns shared almost the same retention times for the aldehydes and a similar result was also observed with ketones as shown in Fig. 6. This may be due to their analogous dipole-dipole interactions with aldehydes and ketones. The binary CB7–CB8 column showed longer retention for aldehydes and ketones than CB7 and CB8 columns, showing the same trend as it did for other analytes indicated above. Longer retention of analytes on the binary column suggests the possible synergistic effect of CB7 and CB8, leading to their stronger interactions with analytes.

3.2.3. Separation of polar analytes Alcohols are typically polar compounds and liable to peak tailing in GC analysis since they exhibit strong H-bonding interactions with a polar stationary phase or with trace active sites on the inner wall of a fused-silica capillary column. Therefore, it is hard to achieve sharp peak shapes for alcohols in GC separations. Fig. 7 shows the GC separations of alcohols on the CBs columns. Clearly, excellent resolution and nice peak shapes were achieved for all the alcohols on the three columns, which probably resulted from the moderate H-bonding interactions of CBs with the alcohols and the inertness of the prepared columns. Briefly, CB stationary phases

show advantages for separation of alcohols over other GC stationary phases such as ionic liquids [20,21] and calix[4]arene [22], in which severe peak tailing for alcohols were clearly shown. The above results demonstrate the great potential of CB stationary phases for alcohols and other H-bonding analytes in analytical practice.

3.2.4. Separation of the Grob mixture The Grob mixture containing 12 test analytes is a wellrecognized test mixture for the comprehensive assessment of separation performance of a GC column as well as the chromatographic system. Resolution, elution order and peak shape of the analytes in the mixture are indicative of their retention behaviours on the given column as well as system conditions. Fig. 8 shows the chromatograms for separation of the Grob mixture on the CB columns and a commercial column of comparable polarity used for comparison. As shown, though the CB columns had much lower efficiency than the commercial one, they achieved high resolution for most of the analytes and exhibited different selectivity for some of the analytes in comparison to the commercial column. Generally, analytes as proton–donor such as 1-octanol, 2,6-dimethylphenol and 2-ethylhexanoic acid retained relatively longer on the CB columns than on the commercial one. In addition, the CB columns achieved much better resolution for 2-ethylhexanoic acid than the commercial column. The above results demonstrate the high selectivity of the CB stationary phases for the indicated analytes, suggesting some advantages over the commercial stationary phase.

P. Zhang et al. / J. Chromatogr. A 1334 (2014) 139–148

600

CB8

CB7

1

600 2 5

4

1

6

2

7

pA

3

6

pA

400

200

4

3

400

7

5

200

0

2

4

6

8

0

10

2

4

Time ( min )

800

6

8

10

12

Time ( min )

CB7+CB8

600

pA

145

6 1

400

4

3

2

7

5

200

0

2

4

6

8

10

12

Time ( min ) Fig. 7. GC separation of alcohols on the CB7, CB8 and CB7-CB8 columns. Peaks: (1) 1-pentanol, (2) 1-hexanol, (3) 1-heptanol, (4) 1-octanol, (5) 1-nonanol, (6) 1-decanol, (7) 1-undecanol. Oven temperature program: 40 ◦ C (1 min) to 160 ◦ C at 10 ◦ C/min.

The three CB columns also exhibited different resolving performance for some of the components. For example, three of the analytes, namely n-undecane (peak 3), 1-nonanal (peak 4) and 1-octanol (peak 5), achieved nearly baseline resolution on CB8 column but coeluted on CB7 column. In addition, 2, 6-dimethylphenol (peak 6) and 2, 6-dimethylaniline (peak 7) were only partially separated on CB8 column but baseline resolved on the other two columns. Also, dicyclohexylamine (peak 10) and methyl undecanoate (peak 11) were well resolved on CB8 and CB7-CB8 columns but coeluted on CB7 column. Additionally, in contrast to the other two columns, CB7 column achieved excellent resolution and good peak shape for 2-ethylhexanoic acid (peak 8), a strong H-donor that is prone to peak tailing in GC separation. For the elimination of peak-tailing of such analytes, derivatization is often required prior to GC analysis. With respect to elution order, CB7 column exhibited reversal elution order for the pair n-decane (peak 1) and 2,3-butanediol (peak 2) in comparison to CB8 column. Though this observation is so far hard to explain, it is worth to explore in future work. In general, for separation of the Grob mixture, CB7–CB8 column achieved the best resolution for nearly all the analytes though this column exhibited lower column efficiency than CB7 and CB8 columns. Noticeably, the binary CB7–CB8 stationary phase behaved quite different from CB7 and CB8 stationary phases in terms of resolution, retention and elution order, indicating the possible existence of synergistic effect in the binary phase. It is known that higher column efficiency provides a higher theoretical plate number that favors the chromatographic separation, but column efficiency is not the sole factor determining the final resolution. As stated by the fundamental resolution equation (1), resolution

for a separation process is actually a comprehensive result of three main factors, namely column efficiency (N), selectivity factor (˛) and retention factor (k). R=

√   k  N ˛−1 4

˛

k+1

(1)

From the equation, it can be noted that resolution approaches zero as the selectivity factor ˛ approaches unity (i.e., coelution). Selectivity mainly depends on the nature of the stationary phase, which governs peak spacing and the degree to which peak maxima are separated. To some extent, ˛ plays a key role in efficiently improving the resolution. For some of the aforementioned analytes, the CB7–CB8 column exhibited greater k and ˛ values than the other columns and thus achieved best resolution as a comprehensive result of these three factors.

3.3. Thermal stability of the CB capillary columns Thermal stability of the CB capillary columns was investigated by GC separation of different mixtures after the columns were conditioned up to higher temperatures. Evaluation of thermal stability of a capillary column by GC separation of a mixture instead of a single compound can provide more information concerning retention times and resolution of analytes as well as baseline drift. As such, a GC column can be regarded as having good thermal stability if it still exhibits excellent resolving ability and smooth baseline after it is subjected to a much higher conditioning temperature for a long time period.

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300

600

CB7

CB8

500 200

3

pA

9

12

3

300

7

2

0

8

5

100

10 15 Time ( min )

400

20

25

200

5 2

100

3

11

10 15 20 Time ( min )

1

300

53 7

9

11 12 10

200

25

20

HP-5MS

2

100

5

10 15 Time ( min )

6+4

8

1

0

67

5

400

10 12 pA

4

8

500

9 300

10 12 11

2 0

CB7+CB8

9

4 6 5

200

100

pA

1

400

10+11

6 4+5 7

pA

1

0

8 5

10 15 Time (min)

20

25

Fig. 8. GC separation of the Grob mixture on the CB7, CB8 and CB7-CB8 columns. Peaks: (1) n-decane, (2) 2, 3-butanediol, (3) n-undecane, (4) 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. Oven temperature program: 40 ◦ C (1 min) to 140 ◦ C at 5 ◦ C/min.

600 500

6 7 8 9

5

400

200

3

300

12

200

4

4

100

100 0

2

6 7 8 9

5

400

3 12

300

2 8 0 ºC

500

pA

pA

600

2 5 0 ºC

4

6

8

10

12

14

Time ( min )

0

2

4

6

8

10

12

14

Time ( min )

Fig. 9. Thermal stability of CB7 capillary column for GC separation of esters after the column was conditioned up to 250 ◦ C and 280 ◦ C for 4 h, respectively. Peaks: (1) n-propyl acetate, (2) n-butyl acetate, (3) methyl octanoate, (4) 1, 4-butyrolactone, (5) methyl heptanoate, (6) methyl nonanoate, (7) methyl decanoate, (8) methyl undecanoate, (9) methyl dodecanoate. Oven temperature program: 40 ◦ C (1 min) to 160 ◦ C at 10 ◦ C/min.

Fig. 9 shows the thermal stability of CB7 column for GC separation of esters after the column was conditioned up to 250 ◦ C and 280 ◦ C for 4 h, respectively. As shown, the column did not show appreciable baseline drift or decrease in retention time and resolution, suggesting that CB7 column is thermally stable at least up to 280 ◦ C. Fig. 10 describes the thermal stability of CB8 column for separation of n-alkanes after it was conditioned up to the indicated temperatures, showing smooth baseline and nice resolution of the analytes. The seemingly decreases in retention times in Fig. 10 in comparison to Fig. 2 mainly originated from their different oven temperature programs. In addition, the observed slight fronting in

Fig. 10 as well as in Fig. 2 for CB8 probably resulted from the overloading of the column and can be eliminated by minimizing the injected amount of the sample. Fig. 11 shows the thermal stability of the CB7–CB8 column for separation of aromatic hydrocarbons after it was conditioned up to the indicated temperatures, demonstrating that the column remains thermally stable up to 250 ◦ C without obvious baseline drift and changes in retention times and resolution. The above results show that CB7 column shows relatively higher thermal stability (at least up to 280 ◦ C) than CB8 and CB7–CB8 columns with regard to retention times, resolution and baseline drift.

P. Zhang et al. / J. Chromatogr. A 1334 (2014) 139–148

1

1200 1000

2 5 0 ºC 2

2 8 0 ºC

2

600 3

600

3

pA

800 pA

1

800

400

4

400

5

147

4

5

6

6

200

200 0

2

4

6

8

10

0

12

2

4

6

8

10

12

Time ( min )

Time ( min )

Fig. 10. Thermal stability of CB8 capillary column for GC separation of n-alkanes after the column was conditioned up to 250 ◦ C and 280 ◦ C for 4 h, respectively. Peaks: (1) n-octane, (2) n-decane, (3) n-undecane, (4) n-dodecane, (5) n-tridecane, (6) n-tetradecane. Oven temperature program: 40 ◦ C (1 min) to 160 ◦ C at 5 ◦ C/min.

1200 1000

1

3

2

800

4

5

2 8 0 ºC

1

300 250

600

pA

pA

350

2 5 0 ºC

200

400

150

200

100

2

345

50 0

1

2

3

4

5

0

1

2

3

4

5

Time ( min )

Time ( min )

Fig. 11. Thermal stability of CB7-CB8 capillary column for GC separation of aromatic hydrocarbons after the column was conditioned up to 250 ◦ C and 280 ◦ C for 4 h, respectively. Peaks: (1) benzene, (2) toluene, (3) ethylbenzene, (4) o-xylene, (5) m-xylene. Oven temperature program: 40 ◦ C (1 min) to 80 ◦ C at 10 ◦ C/min. Table 4 Repeatability of CB7 columns. Analyte

Methyl nonanoate Methyl decanoate Methyl undecanoate Methyl dodecanoate Dodecane Tridecane Tetradecane

Run-to-run (%, n = 6)

Column-to-column (%, n = 3)

Mean

RSD

Mean

RSD

7.34 8.60 9.80 11.00 10.85 13.50 16.02

0.01 0.01 0.01 0.01 0.01 0.01 0.01

7.26 8.48 9.70 10.92 10.78 13.43 15.94

2.1 1.8 1.5 1.4 2.3 1.9 1.6

3.4. Column repeatability Column repeatability of CB columns was evaluated in terms of run-to-run and column-to-column and the results are listed in Table 4. Since the same column preparation method was used for CB7, CB8 and CB7–CB8, for clarity, Table 4 only lists the column repeatability for CB7 column in retention times of esters and alkanes by relative standard deviation (RSD%). As shown in Table 4, the RSD values for run-to-run and column-to-column repeatability were below 0.01% and 2.3%, respectively, indicating the good repeatability of CB columns in terms of GC separation and column preparation. 4. Conclusions This work presents the first example of using CBs as GC stationary phases for capillary GC separations. The results demonstrate

their high selectivity and good resolution towards a wide range of analytes from nonpolar to polar and show advantages for separation of analytes prone to peak-tailing in GC analysis such as alcohols and aldehydes. The high selectivity of CB stationary phases may stem from their unique structures and specific molecular interactions with different analytes. Additionally, CB columns also show good column repeatability and thermal stability. CB7 column exhibits higher thermal stability (up to 280 ◦ C) than the rest. Undoubtedly, there is still much room for further improvement of the column efficiency either by narrowing down the particle distribution range or by finding an efficient way to make CBs dissolved in a solvent for column preparation. This work demonstrates the great potential of CB stationary phases for high-resolution GC separation of analytes ranging from nonpolar to polar analytes in practical samples.

Acknowledgements The authors gratefully acknowledge the financial support by the National Natural Science Foundation of China (21075010) and the 111 Project B07012 in China.

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