[A brief history of the development of chromatography and its enlightenment to us].

This article describes the brief history of chromatography and clarifies a few confusions in some Chinese publications on the history of chromatography. Based on the analysis of several milestones in the development of chromatographic techniques, we summarized the enlightenment that the history tell us:(1) fundamental research has always been the source of advancements in science and technology, including chromatography; (2) the needs of social development have been the main driving force for the innovations in chromatographic methods; (3) interdisciplinary study has been an important approach to technological innovation; and (4) an unremitting persistence and courage to explore are the necessary conditions for technical innovation. Finally, the future development prospects of chromatography are also briefly discussed.

Graphitic carbon nitride as high-resolution stationary phase for gas chromatographic separations.

This work presents the first example of utilization of graphitic carbon nitride (g-C3N4) as stationary phase for capillary gas chromatographic (GC) separations. The statically coated g-C3N4 column showed the column efficiencies of 3760 plates/m and weak polarity. Its resolving capability and retention behaviours were investigated by using the Grob test mixture, and mixtures of diverse types of analytes, and structural and positional isomers. The results showed superior separation performance of the g-C3N4 stationary phase for some critical analytes and preferential retention for aromatic analytes. Specifically, it exhibited high-resolution capability for aromatic and aliphatic isomers such as methylnaphthalenes and dimethylnaphthalenes, phenanthrene and anthracene and alkane isomers. In addition, g-C3N4 column showed excellent thermal stability up to 280°C and good repeatability with relative standard deviation (RSD) values less than 0.09% for intra-day, below 0.23% for inter-day and in the range of 1.9-8.4% for between-column, respectively. The advantageous separation performance shows the potential of g-C3N4 and related materials as stationary phase in GC and other separation technologies.

Features of a truxene-based stationary phase in capillary gas chromatography for separation of some challenging isomers.

Herein we report the first example of exploring truxene-based derivatives for gas chromatographic (GC) separations. The fabricated thiophene-functionalized truxene (TFT) column exhibited weak polarity and efficiencies as high as 4000plates/m for 0.250mm i.d. columns. TFT column showed preferential retention for halogenated and alkyl benzene analytes, and especially, high resolving capability for the xylene isomers, di- and trichlorobiphenyls (di-CB and tri-CB) isomers. Interestingly, its unique retentions for the latter analytes were found to be closely related with their dihedral angles and the locations of chlorine atoms. This finding on the retention trend has not been reported in GC separations, which may provide a new perspective in elucidating retention behaviours for specific analytes. Moreover, TFT column exhibited high thermal stability up to 320°C and excellent repeatability. This work demonstrates the promising future of truxene derivatives in the separation science.

Cyclotriveratrylene as a new-type stationary phase for gas chromatographic separations of halogenated compounds and isomers.

Cyclotriveratrylene (CTV) is reported here for the first time as stationary phase for capillary gas chromatographic (GC) separations. CTV stationary phase showed weak polarity comparable to the conventional 5% phenyl polysiloxane stationary phase but exhibited different retention behaviours and higher resolution for some of the indicated analytes. Most importantly, CTV stationary phase exhibited preferential selectivity for halogenated compounds, positional and geometrical isomers. Effect of column temperature on retention and thermal stability of CTV column were also investigated. Moreover, CTV capillary column showed good repeatability in terms of run-to-run, day-to-day and column-to-column. The unique physicochemical features and efficient separation ability for analytes of close properties show the potential of CTV as a new-type stationary phase in GC as well as separation science.

Performance of graphene sheets as stationary phase for capillary gas chromatographic separations.

This work presents the investigation of graphene as a new type of stationary phase for capillary GC separations. Graphene capillary column (0.25 mm, i.d.) achieved column efficiency of 3100 plates/m determined by n-dodecane at 120 °C. The obtained McReynolds constants suggested the weakly polar nature of graphene sheets as GC stationary phase. As evidenced, graphene stationary phase differs from the conventional phase (5% phenyl polysiloxane) in its resolving ability and retention behaviors, and achieved better separation for the Grob and other mixtures. The advantages of graphene stationary phase may mainly originate from its specific π-π stacking interaction as well as H-bonding interaction. Furthermore, graphene column exhibited good repeatability with relative standard deviation (RSD%) in the range of 0.01-0.07% for run-to-run and 2.5-6.7% for column-to-column, respectively.

Perhydroxylcucurbit[6]uril as a highly selective gas chromatographic stationary phase for analytes of wide-ranging polarity.

This work describes the separation performance of a perhydroxylcucurbit[6]uril stationary phase for capillary gas chromatography. The perhydroxylcucurbit[6]uril stationary phase exhibits higher resolving capabilities for critical analytes with better peak shapes than cucurbit[6]uril and conventional stationary phases. The advantageous separation feature of the perhydroxylcucurbit[6]uril stationary phase may originate from its unique structure and favorably balanced interactions with the analytes. Also, the perhydroxylcucurbit[6]uril column shows good separation repeatability with relative standard deviations in the range of 0.01-0.13% for intraday, 0.37-0.82% for interday, and 1.0-4.7% for column-to-column repeatability.

Separation performance of cucurbit[7]uril in ionic liquid-based sol-gel coating as stationary phase for capillary gas chromatography.

Here we report the separation performance of a new stationary phase of cucurbit[7]uril (CB7) incorporated into an ionic liquid-based sol-gel coating (CB7-SG) for capillary gas chromatography (GC). The CB7-SG stationary phase showed an average polarity of 455, suggesting its polar nature. Abraham system constants revealed that its major interactions with analytes include H-bond basicity (a), dipole-dipole (s) and dispersive (l) interactions. The CB7-SG stationary phase achieved baseline separation for a wide range of analytes with symmetrical peak shapes and showed advantages over the conventional polar stationary phase that failed to resolve some critical analytes. Also, it exhibited different retention behaviors from the conventional stationary phase in terms of retention times and elution order. Most interestingly, in contrast to the conventional polar phase, the CB7-SG stationary phase exhibited longer retentions for analytes of lower polarity but relatively comparable retentions for polar analytes such as alcohols and phenols. The high resolving ability and unique retention behaviors of the CB7-SG stationary phase may stem from the comprehensive interactions of the aforementioned interactions and shape selectivity. Moreover, the CB7-SG column showed good peak shapes for analytes prone to peak tailing, good thermal stability up to 280°C and separation repeatability with RSD values in the range of 0.01-0.11% for intra-day, 0.04-0.41% for inter-day and 2.5-6.0% for column-to-column, respectively. As demonstrated, the proposed coating method can simultaneously address the solubility problem with CBs for the intended purpose and achieve outstanding GC separation performance.

Calix[4]pyrroles: highly selective stationary phases for gas chromatographic separations.

Calix[4]pyrroles offer a great potential as stationary phases for gas chromatography (GC) due to their unique structures and physicochemical properties. Herein we present the first report of using two calix[4]pyrroles, namely meso-tetra-cyclohexylcalix[4]pyrrole (THCP) and meso-octamethylcalix[4]pyrrole (OMCP). These stationary phases were statically coated onto capillary columns and investigated in terms of column efficiency, polarity, separation performance, thermal stability and repeatability. The columns achieved column efficiencies of 2200-3000plates/m and exhibited nonpolar nature with an average polarity of 67 for THCP and 64 for OMCP, respectively. THCP stationary phase shows high selectivity for analytes of different polarity and exhibits nice peak shapes, especially for aldehydes, alcohols and anilines that are prone to severe peak tailing in GC analysis. Interestingly, THCP stationary phase possesses superior resolving ability for aniline and benzenediol positional isomers while OMCP shows preferential selectivity for nonpolar analytes such as hexane isomers. Moreover, calix[4]pyrrole columns also have good thermal stability up to 260°C and repeatability with a relative standard deviation (RSD%) of less than 0.10% for run-to-run and less than 5.2% for column-to-column. This work demonstrates the unique separation performance of calix[4]pyrroles and their promising future as a new class of GC stationary phases.

Separation performance of cucurbit[8]uril and its coordination complex with cadmium (II) in capillary gas chromatography.

Here we report the investigation of using cucurbit[8]uril (CB8) and its coordination complex with cadmium (II) (CB8-Cd) as stationary phases for capillary gas chromatography (GC). The prepared capillary columns of CB8 and CB8-Cd stationary phases achieved column efficiency of 2200plates/m and 1508plates/m, respectively, and showed weak polarity based on the measured McReynolds constants. Their separation performance was investigated by GC separation of mixtures of different types while a commercial column was used for comparison. The CB8 stationary phase achieved high resolution for a wide range of analytes from nonpolar to polar while the CB8-Cd stationary phase exhibited good separation mainly for nonpolar to weak polar analytes. The CB stationary phases differ from the commercial one in terms of retention behaviors and resolving ability due to their different molecular interactions with analytes. Moreover, energy effect on the retention of analytes on CB8 and CB8-Cd stationary phases was examined, showing that retention on CB8 column was determined mainly by enthalpy change for polar analytes and by both enthalpy change and entropy change for weak polar analytes whereas retention on CB8-Cd column was mainly controlled by entropy change. This work demonstrates the great potential of CB8 and CB8-Cd stationary phases as a new type of GC stationary phases in GC analysis.

Cucurbit[6]uril in combination with guanidinium ionic liquid as a new type of stationary phase for capillary gas chromatography.

Cucurbit[n]urils (CBs) possess unique structures and selective interactions with analytes of different variety and high thermal stability and show a great potential as stationary phases for capillary gas chromatography (GC). This work presents the first description of CB6 in combination with a new guanidinium-based ionic liquid (GBIL) as the GC stationary phase by using sol-gel coating method for the preparation of the CB6-GBIL and GBIL columns. Introduction of GBIL greatly improved the solubility of CB6 in the sol solution and successfully made the use of CBs for the intended purpose feasible. The CB6-GBIL and GBIL columns had average polarity of 179 and 280, respectively, suggesting their medium polar nature. The CB6-GBIL column achieves good resolution for a wide range of analytes and exhibits different retention behaviors and resolution for some of the analytes from the GBIL column and the commercial column. Moreover, it also shows good thermal stability up to 250°C. The proposed method integrates the full advantages of CBs, ionic liquids and sol-gel coating method and provides an efficient and feasible way for the wider application of CBs in separation science.

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

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.

Dithienyl benzothiadiazole derivatives: a new type of stationary phases for capillary gas chromatography.

Dithienyl benzothiadiazole (TBT) derivatives are characteristic of specific molecular structures and excellent thermal stability, offering the great potential for their use as stationary phases for capillary gas chromatography (GC). However, no related publications are available to date. Here we first report the exploration of two TBT derivatives, namely, 4,7-bis(5-dodecylthiophen-2-yl) benzothiadiazole (TBT-C12, a new derivative), and 4,7-di(thiophen-2-yl) benzothiadiazole (TBT), for the purpose. First, TBT-C12 and TBT were synthesized and statically coated onto capillary columns. Both of the as-prepared columns were characterized as nonpolar nature by the determined McReynolds constants. The results showed that both TBT-C12 and TBT columns achieved excellent separations for n-alkanes, esters and polycyclic aromatic hydrocarbons (PAHs), and the TBT-C12 column exhibited better thermal stability (up to 280°C) than the TBT column. Moreover, a comparative study between the TBT-type columns and a commercial column was also made, suggesting the better selectivity of the TBT-type stationary phases toward n-alkanes and PAHs than that of the commercial one. Additionally, thermodynamic parameters suggested that the retention behaviors of n-alkanes and PAHs on the TBT-C12 column was mainly controlled by entropy change (ΔS). In conclusion, this work demonstrates the excellent chromatographic performance of TBT derivatives as a new type of GC stationary phases and their potential in separation science.

Monolithic graphene fibers for solid-phase microextraction.

Monolithic graphene fibers for solid-phase microextraction (SPME) were fabricated through a dimensionally confined hydrothermal strategy and their extraction performance was evaluated. For the fiber fabrication, a glass pipeline was innovatively used as a hydrothermal reactor instead of a Teflon-lined autoclave. Compared with conventional methods for SPME fibers, the proposed strategy can fabricate a uniform graphene fiber as long as several meters or more at a time. Coupled to capillary gas chromatography (GC), the monolithic graphene fibers in a direct-immersion (DI) mode achieved higher extraction efficiencies for aromatics than those for n-alkanes, especially for polycyclic aromatic hydrocarbons (PAHs), thanks to π-π stacking interaction and hydrophobic effect. Additionally, the fibers exhibited excellent durability and can be repetitively used more than 160 times without significant loss of extraction performance. As a result, an optimum extraction condition of 40°C for 50min with 20% NaCl (w/w) was finally used for SPME of PAHs in aqueous samples. For the determination of PAHs in water samples, the proposed DI-SPME-GC method exhibited linear range of 0.05-200µg/L, limits of detection (LOD) of 4.0-50ng/L, relative standard deviation (RSD) less than 9.4% and 12.1% for one fiber and different fibers, respectively, and recoveries of 78.9-115.9%. The proposed method can be used for analysis of PAHs in environmental water samples.

Separation performance of guanidinium-based ionic liquids as stationary phases for gas chromatography.

Room temperature ionic liquids (RTILs) as stationary phases for gas chromatography (GC) have made great achievements in both research and applications over the last decades. Until now, all of the RTIL stationary phases reported have involved imidazolium, ammonium, pyrrolidinium, and phosphonium-based RTILs, and however, no publications are available using guanidinium-based ionic liquids (GBILs) as GC stationary phases except two preliminary reports from our group. In the present work, three hexaalkyl GBILs stationary phases, namely N, N,N',N'-tetramethyl-N″, N″-dioctylguanidinum hexafluophosphate (DOTMG-PF(6)), N,N,N',N'-tetramethyl-N″, N″-dioctylguanidinium bis (trifluoromethylsulfonyl) imide (DOTMG-NTf(2)), and N,N,N',N'-tetraoctyl-N″, N″-dimethylguanidinium bis (trifluoromethylsulfonyl) imide (TODMG-NTf(2)), were synthesized and used as stationary phases for GC separation after they were statically coated onto the inner walls of fused-silica capillary columns. The evaluation of DOTMG-PF(6) and TODMG-NTf(2) as GC stationary phases is reported here for the first time, whereas additional results on the DOTMG-NTf(2) stationary phase are added here on the basis of our previous report. In this work, McReynolds constants and Abraham solvation system constants are used to evaluate the average polarity and the solvation properties of the GBILs stationary phases for GC separation, respectively. The results show that the GBILs stationary phases exhibit medium polarity with an average polarity of 293-390, and that the major molecular interactions of the GBILs with analytes are dipole/polarizable interactions, H-bond basicity and dispersion forces, etc. After this, the separation performance and thermal stability of the GBILs stationary phases were evaluated, showing that these stationary phases achieve excellent separation for analytes of great variety covering hydrocarbons, alcohols, esters, aldehydes, ketones, amines, amides and aromatics, and exhibit different retention behaviors from the most widely used GC stationary phases such as polyethylene glycol (PEG-20M) and 5% phenyl-95% dimethylpolysiloxane (SE-54) in terms of resolution and elution order, and good thermal stability (at least up to 250°C). The present work demonstrates that the GBILs stationary phases possess excellent separation performance and thermal stability, and may be applicable as a new type of GC stationary phases for separation of complex samples.

[Evaluation of two hydroxyl-terminated monocationic ionic liquid stationary phases with high thermal stability for capillary gas chromatography].

Novel hydroxyl-terminated ionic liquids of 1-(6-hydroxyhexyl)-3-butyl imidazolium bis(trifluoromethyl) sulfonylimide (HHBIM-NTf2) and 1-(8-hydroxyoctyl) -3-butyl imidazolium bis(trifluoromethyl) sulfonylimide (HOBIM-NTf2) were synthesized as stationary phases for capillary gas chromatography (GC). Besides, 1-octyl-3-butyl imidazolium bis(trifluoromethyl) sulfonylimide (OBIM-NTf2) was used as the reference. Selectivities and thermal stabilities of HHBIM-NTf2, HOBIM-NTf2 and OBIM-NTf2 were evaluated by means of the separation of Grob test mixture and positional isomers after the columns were conditioned up to 160 degrees C, 250 degrees C and 300 degrees C, respectively. As a result, baseline separations of Grob test mixture and alcohol mixture and better peak shapes were observed with HHBIM-NTf2 and HOBIM-NTf2. Importantly, baseline separation of dimethoxybenzene isomers was also achieved with HHBIM-NTf2 and HOBIM-NTf2. Satisfactory selectivity of HHBIM-NTf2 still remained even after conditioned at 300 degrees C for 8 h. The results for thermal stability showed that as GC stationary phases, HHBIM-NTf2 and HOBIM-NTf2 were stable at least up to 300 degrees C and 250 degrees C, respectively. The hydroxyl-terminated monocationic ionic liquids possess excellent selectivity and thermal stability and are alternative candidates for GC stationary phases.

[Recent advances of gas chromatography].

The recent advances and features of gas chromatography (GC) are briefly described. GC is a considered as a mature technique, and is applied in various fields. Now, in addition to stationary phases and columns, the investigations are focused on comprehensive two-dimensional gas chromatography (GC x GC), fast GC, portable GC, and micro GC (microGC). In the research of stationary phases, the room temperature ionic liquids continue its development, and modified cyclodextrins are other projects of GC stationary phases studied. Today a lot of chromatographers prefer to use commercial capillary GC columns rather than the home made one, and most of the stationary phases of commercial capillary GC columns used are 5% phenyl polydimethylsiloxane.

[Evaluation of chromatographic performance of polymerized ionic liquid stationary phase for capillary gas chromatography].

The selectivity and thermal stability of ionic liquids as the stationary phases for capillary gas chromatography (CGC) have attracted much attention of researchers in recent years. In this study, 1-vinyl-3-benzyl imidazolium-bis(trifluoromethane-sulphonyl)imidate (VBIm-NTf2) was synthesized and polymerized (PVBIm-NTf2) in a CGC column. In comparison with VBIm-NTf2, PVBIm-NTf2 exhibits much better thermal stability and chromatographic selectivity, and achieves satisfactory resolution for Grob test mixture, alcohols mixture, esters mixture and aromatics mixture with narrow and symmetric peak shapes. The satisfactory resolution and selectivity of the polymerized column still remain after conditioned at 250 degrees C for 6 h. Additionally, the Abraham solvation parameters of PVBIm-NTf2 were determined and the interactions between the stationary phase and solutes were elucidated. The present work demonstrates that the polymerization is an effective way to improve the selectivity and thermal stability of common ionic liquids as CGC stationary phases.

Analysis of the volatile compounds in Ligusticum chuanxiong Hort. using HS-SPME-GC-MS.

A headspace solid-phase microextraction (HS-SPME) method followed by gas chromatography-mass spectrometry (GC-MS) is described for the analysis of volatile compounds in the dry rhizome of Ligusticum chuanxiong Hort. Three types of SPME fibers including PDMS, PDMS-DVB and DVB-CAR-PDMS were investigated and the best extraction was achieved with the mixed fiber DVB-CAR-PDMS. Parameters for HS-SPME in terms of temperature and time, sample amount and particle size, and desorption time were also investigated. A polar capillary column was used for the chromatographic separation. As a result, 73 compounds were determined and identified by the HS-SPME-GC-MS method with at least 20 more compounds than those in the methods available. Comparison was made between HS-SPME-GC-MS and steam distillation (SD)-GC-MS methods. Using much less sample amount, shorter extraction time and simpler procedure, HS-SPME method can achieve similar results with those by SD. In conclusion, the present method is simple, rapid and effective and can be used for the analysis of volatile compounds in medicinal plants.

Headspace solvent microextraction-gas chromatography-mass spectrometry for the analysis of volatile compounds from Foeniculum vulgare Mill.

A novel and rapid headspace solvent microextraction followed by gas chromatography-mass spectrometry (HSME-GC-MS) for the analysis of the volatile compounds of Foeniculum vulgare Mill is described. HSME parameters including extracting solvent, extraction temperature and time, headspace volume and particle size were optimized. As a result, benzyl alcohol was finally used for the extraction at 70 degrees C for 20 min with headspace volume of 12.1 ml and particle size of 120 mesh. Under the determined conditions, the powered samples of Foeniculum vulgare Mill were directly applied for the analysis. A comparison of HSME-GC-MS, solid phase microextraction (SPME)-GC-MS and steam distillation (SD)-GC-MS methods was made and showed that the HSME-GC-MS method was simple, inexpensive and effective and can be used for the analysis of volatile compounds in traditional Chinese medicines (TCMs).

Solid-phase microextraction-gas chromatographic-mass spectrometric analysis of volatile compounds from Curcuma wenyujin Y.H. Chen et C. Ling.

A solid-phase microextraction coupled with gas chromatography-mass spectrometry (SPME-GC-MS) for analysis of the volatile compounds from Curcuma wenyujin Y.H. Chen et C. Ling is described. SPME parameters (fiber type, extraction temperature and time, headspace volume and desorption time) and GC conditions were tested. The powdered sample of C. wenyujin Y.H. Chen et C. Ling was directly analyzed by SPME-GC-MS and 72 compounds were identified. The results from SPME-GC-MS were compared with those obtained from steam distillation gas chromatography-mass spectrometry (SD-GC-MS) with a good agreement. The results show that SPME-GC-MS method is a fast, simple and efficient way for the analysis of volatile components from traditional Chinese medicines (TCMs).