High-separation efficiency micro-fabricated multi-capillary gas chromatographic columns for simulants of the nerve agents and blister agents
© Li et al.; licensee Springer. 2014
Received: 13 November 2013
Accepted: 18 March 2014
Published: 8 May 2014
To achieve both high speed and separation efficiency in the separation of a mixture of nerve and blister agent simulants, a high-aspect-ratio micro-fabricated multi-capillary column (MCC, a 50-cm-long, 450-μm-deep, and 60-μm-wide four-capillary column) was fabricated by the application of the microelectromechanical system (MEMS) techniques. Mixtures of chemical warfare agent (CWA) simulants - dimethyl methylphosphonate (DMMP), triethyl phosphate (TEP), and methyl salicylate - were used as samples. The fabricated MCC allowed for the separation of all the components of the gaseous mixture within 24 s, even when the difference in boiling point was 4°C, as in the case of TEP and methyl salicylate. Furthermore, interfering agents - dichloromethane, ethanol, and toluene - were also included in the subsequent gaseous mixture samples. The boiling point of these six components ranged from 78°C to 219°C. All six components were clearly separated within 70 s. This study is the first to report the clear separation of gas mixtures of components with close boiling points. The column efficiency was experimentally determined to be 12,810 plates/m.
Since Terry's first report in 1979 , micro-fabricated gas chromatography (GC) columns have been developed for over 30 years. The new generation of GC columns has unique characteristics. Silicon is often used as a substrate for column fabrication. These GC columns come in small sizes with high-column efficiency  and differ significantly from packed or capillary columns, which are made of steel or silica [3, 4]. Thus, micro-fabricated columns are suitable for applications in hand-held GC systems . The structure of the GC column varies when fabricated via microelectromechanical system (MEMS) processes. For instance, since the depth and width of columns can be arbitrarily designed, the column structure can feature different aspect ratios. These flexibilities provide a new direction for research in this field.
Over the past 30 years, techniques for column fabrication have changed significantly. Wet etching was an important technique in early fabrication techniques . In 1998, Sandia National Laboratories reported the application of wet etching process to fabricate single open-tube columns with rectangular channels . However, precise regulation of concentrations and temperatures of etching solution were important factors that influenced structure formation. The chemical wet etching technique has not found widespread use because of its lack of control over the structure. To allow for better control of the column shape, the deep reactive-ion etching (DRIE) technique was developed. This technique prevents lateral etching of the silicon and results in highly anisotropic etch profiles at high etch rates . Etching capabilities can vary from <1 μm to >700 μm in depth in vertical sidewalls . Considering its many advantages, DRIE has become the workhorse of column fabrication.
Since the 9/11 attack, acts of terrorism have become a matter of significant concern to many countries. Chemical warfare agents (CWAs) constitute one class of such lethal weapons for potential use by terrorists. Rapid separation and identification of lethal gas in public space is a great challenge, especially in airports and subways. Previously, researchers have shown that micro-fabricated GC columns can separate the components of a mixture in a complex environment [10, 11]. For instance, MEMS-based semi-packed GC columns can separate environmental carcinogens with concentrations at the ppb level  with higher separation efficiency than commercial GC columns, and the total length of the GC column is only 2-m long.
To reduce the retention time of analytes, extra-short GC columns can be used. Agah et al. designed a high-speed signal open-tube GC column, through which components of the mixture were separated within 10 s. However, the separation efficiency and sample capacity of the fabricated column can be improved further. In 1975, Golay introduced the principle of multi-capillary columns (MCCs). MCCs demonstrated much higher sample capacities when compared with single capillary column [14, 15]. MEMS-based multi-capillary GC columns were subsequently designed. The sample capacity of MCC was ten times higher than in the single channel . However, for MCCs with a short length, the separation efficiency needs to be improved further. Our work focuses on improving separation efficiency by designing a column with a high aspect ratio.
In this study, MEMS techniques were applied in the fabrication of an MCC. Using the DRIE process, a 50-cm-long, 450-μm-deep, and 60-μm-wide four-capillary column was fabricated. The static coating method was used for coating the column with the stationary phase - dimethyl (94%) + vinyl (1%) + phenyl (5%) polysiloxanes (SE-54). Mixtures of DMMP, TEP, and methyl salicylate (representing CWAs) were used as samples to evaluate the efficiency of the column. Dichloromethane, ethanol, and toluene were added as interference components to the analytes to produce new sample mixtures.
Materials and reagents
A solution of SE-54 (5% phenyl, 1% vinyl, 94% dimethyl polysiloxane) was purchased from Sigma-Aldrich (St. Louis, MO, USA) for use as the stationary phase. The internal unions were purchased from VICI (Valco Instruments Co., Schenkon, Switzerland), and the fused silica tubing was purchased from SGE (SGE Analytical Science, Ringwood, VT, Australia). All analytes were purchased from J&K Scientific Ltd. (Beijing, China). Samples (mixture of gases) were generated by a MF-3C dynamic vapour generator, where the analyte-solvent mixtures were injected into a vaporising chamber. Two digital mass flow controllers in the vapour generator regulated the concentration of the sample.
The MCC was deactivated with octamethylcyclotetrasiloxane (D4) before coating with the stationary phase. Since silanol (Si-OH) groups can attract moisture on the surface through hydrogen bonding and influence column performance, D4 was used to remove Si-OH groups and inactivate the surface of the column [18, 19]. D4 was injected into the MCC and both ends of the column were sealed. To ensure complete deactivation, the column was placed in an oven at 400°C for 90 min. After deactivation, the GC column was washed with methylene chloride (1 mL) while using N2 as carried gas at 220°C for 60 min to remove all residues.
Results and discussion
Film thickness of the stationary phase
where Ccs is the coating solution concentration; ρstatonary phase is the stationary phase density; and w and h are the channel width and height, respectively. In this experiment, the film thickness was controlled to approximately 1 μm using static coating. Figure 3 shows the film thickness in the middle of the channel.
Theoretical determination of column efficiency
where d f is the stationary phase thickness; w and h are the channel width and height, respectively; D g and D s are the binary diffusion coefficients in the mobile and stationary phases, respectively; and f1 (varies between 1 and 1.125) and f2 (varies between 0 and 1) are the Gidding-Golay and Martin-James gas compression coefficients, respectively.
where is the peak variance; u0 is the average linear gas velocity; and are the cross-sectional height σ h and width σ w variances normalised by the average height h0 and average width w0, respectively; and k0 is the retention factor in a capillary with some cross-sectional area. In this equation, the first term refers to the HETP of a capillary whose dimensions are the average of the dimensions of all capillaries in the bundle . This value is directly expressed by Equation 2. The second and third terms account for the band broadening caused by non-uniformity in the channels. In this experiment, since each of the four channels has a width of 60 μm and a depth of 450 μm, σ h and σ w are equal to 0. Thus, the second and third terms are cancelled and the HETP of an MCC is equal to its single capillary; the sample capacity is simply multiplied by the number of capillaries in the bundle .
Experimental determination of column efficiency
where is the width of the peak at half height. The number of plates for methyl salicylate is 6,410 plates. With a 1-m length, the theoretical number of plates is 12,810 plates/m. The main advantage of short-length GC columns is its ability to separate components in a short period of time. Using Equation 4, the shorter retention time of peak, the lower plate number is worked out. Meanwhile, the resolution also deceases when components are eluted quickly from the column. In our design, we optimise MEMS-based MCC separation conditions by striking a balance between the time required for separation, and a rational resolution and plate number.
Chromatographic separation of mixture components
Separation of six components in MCC
Retention time (min)
Number of plates/m
In this work, the MEMS technique was used to fabricate a MCC column which was 50-cm long. By applying the DRIE technique, a 60-μm-wide and 450-μm-deep MCC was fabricated; these dimensions resulted in an aspect ratio of 7.5:1. The resulting MCC achieved both high speed and high separation efficiency in separating nerve and blister agent simulants. This study is the first to report MCC etching at such high depths.
Flow splitters were installed at the inlet and outlet of the MCC. By simulating the flow of carrier gas through the column, the gas flow was shown to be equally divided between the capillaries of the MCC. To evaluate the effects of interfering components, we mixed three commonly used chemicals with the simulants. The boiling points of the six components ranged from 78°C to 219°C. This study is the first to report a successful separation of gas mixtures containing components with close boiling points.
This short length of the MCC ensured that components of the mixture were rapidly separated, i.e. within 70 s. The number of plates was determined to be 12,810 plates/m. The results indicate that the proposed MCC will find applications as a new generation of GC columns. The present study also features several limitations. First, fabrication of the MCC entails high costs. Furthermore, a smaller GC system requires miniaturisation of its component devices. Production of MCCs in a batch-to-batch manner may help reduce costs for commercialisation.
This work was supported by the National Science Foundation of China via Grant Nos. 61176066 and 61101031. It was also supported by the National High-Tech Research & Development Program (Grant No. 2014AA06A510).
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