Comprehensive two-dimensional gas chromatographic separations with a temperature programmed microfabricated thermal modulator (original) (raw)
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A low power, high-speed miniaturized thermal modulator for comprehensive 2D gas chromatography
2010 IEEE 23rd International Conference on Micro Electro Mechanical Systems (MEMS), 2010
In comprehensive two-dimensional gas chromatography (GC×GC), a modulator is placed at the juncture between two separation columns to focus and re-inject eluting mixture components, and thereby enhance the selectivity, sensitivity, and analyte capacity. Here, we present the design, fabrication, and testing of a two-stage microscale thermal modulator (µTM). The µTM cryogenically trap analytes eluting from the first column and thermally inject them into the second column. For this operation, each stage is periodically heated to 200 o C for 100 ms and then cooled to-50 o C for a few second. Preliminary results using a conventional capillary column interfaced to the µTM demonstrate successful modulation of a mixture of alkanes with a sensitivity enhancement as high as 24 folds.
Sensors and Actuators B: Chemical, 2013
In comprehensive two-dimensional gas chromatography (GC × GC), thermal modulation is an important process to enhance the detectability of a volatile organic compound analyte and compound separation capacity. For increased detectability, we explore a method to enhance the temperature uniformity of a microfabricated thermal modulator with an area-adjusted air-gap spacer. The area-adjusted spacer controls the spatial distribution of heat transfer to increase the temperature uniformity of the analyte passing through the device's channel. This enables higher analyte peak-amplitude enhancement (PAE) during thermal modulation, thereby increasing detectability of the analyte. The influence of varying spacer area on temperature uniformity was characterized by simulation, and its effect on PAE was experimentally estimated. With an optimized spacer design, we achieved a 25% increase in PAE (34-42) for 10 ppm n-octane vapor.
Extending the Upper Temperature Range of Microchip Gas Chromatography Using a Heater/Clamp Assembly
2017
Miniaturization of gas chromatography (GC) instrumentation is of interest because it addresses current and future issues relating to compactness, portability and field application. While incremental advancements continue to be reported in microchip GC, the current performance is far from acceptable. This lower performance compared to conventional GC is due to factors such as pooling of the stationary phase in corners of non-cylindrical channels, adsorption of sensitive compounds on incompletely deactivated surfaces, shorter column lengths and less than optimum interfacing to injector and detector. In this work, a microchip GC system was developed that solves the latter challenge, i.e., microchip interfacing to injector and detector. A microchip compression clamp was constructed that seals injector and detector fused silica interface tubing to inlet and outlet ports of the microchip channels with minimum extra-column dead volume, and that allows routine operation at least up to 300 ºC. The compression clamp was constructed of a low expansion alloy, Kovar™, to minimize leaking due to thermal expansion mismatch at the interface during repeated thermal cycling. A 5.9 m channel with a cross-section that approximately matches a 100 µm i.d. cylindrical fused silica column was fabricated in a silicon wafer using wafer bonding and deep reactive ion etching (DRIE) and coated statically with a 1% vinyl, 5% phenyl, 94% methylpolysiloxane stationary phase. High temperature separations of C10-C40 n-alkanes and a commercial diesel sample were demonstrated using the system under both temperature programmed GC (TPGC) and thermal gradient GC (TGGC) conditions. TGGC analysis of a complex essential oil sample was also demonstrated.
Analytical Chemistry, 2011
The following text and figures provide details about the μTM module assembly, the effect of changing the modulation offset on the PAE of n-hexane and n-octane, the time to steady-state values of T stage-min and T stage-max , and the relationship between the injected mass and PAE for n-octane µTM module assembly. The μTM chip with inlet/outlet capillaries was placed with its Pyrex surface face-up on a chip-carrier printed circuit board (PCB1) having metal traces leading to connector-pin sockets. A droplet of epoxy adhesive was dispensed at each of the four corners of the μTM chip and cured to permanently bond the chip to PCB1. Wire bonds were then used to connect the contact pads on the μTM chip to the sockets on PCB1. A Si spacer was aligned to the thin-film heater pattern of each μTM stage under a stereo microscope and bonded with photoresist to allow for removal and recycling of the spacers. Each spacer is a rectangular Si piece fabricated by DRIE that consists of a base layer, two rails along opposite sides, and a square mesa in the center, whose height is less than that of the rails. Each spacer was bonded (via the rails) to the Pyrex surface of the μTM beneath the heater for each stage to fix the air gap between the top surface of the mesa and the Pyrex. This air gap is a critical feature that governs the heat transfer between the Pyrex surface of the μTM and the top surface of the TE cooler. This study used a spacer design that forms a relatively narrow air gap of 19 µm, which promotes faster cooling at the expense of heating power consumption. The μTM module assembly employed a second custom-made PCB (PCB2) with a rectangular cutout in its center and a set of thin metal traces leading to electrical connector pins arranged in a pattern that matched the sockets on PCB1. The TE cooler was sealed to the underside of PCB2 using an O-ring and a screw clamp and was positioned so that the top stage protruded through the cutout with its top surface slightly above the top surface of PCB2. A thin layer of heat-sink compound was
Journal of Chromatography A, 2017
Miniaturization of gas chromatography (GC) instrumentation is of interest because it addresses current and future issues relating to compactness, portability and field application. While incremental advancements continue to be reported in microchip GC, the current performance is far from acceptable. This lower performance compared to conventional GC is due to factors such as pooling of the stationary phase in corners of non-cylindrical channels, adsorption of sensitive compounds on incompletely deactivated surfaces, shorter column lengths and less than optimum interfacing to injector and detector. In this work, a microchip GC system was developed that solves the latter challenge, i.e., microchip interfacing to injector and detector. A microchip compression clamp was constructed that seals injector and detector fused silica interface tubing to inlet and outlet ports of the microchip channels with minimum extra-column dead volume, and that allows routine operation at least up to 300 ºC. The compression clamp was constructed of a low expansion alloy, Kovar™, to minimize leaking due to thermal expansion mismatch at the interface during repeated thermal cycling. A 5.9 m channel with a cross-section that approximately matches a 100 µm i.d. cylindrical fused silica column was fabricated in a silicon wafer using wafer bonding and deep reactive ion etching (DRIE) and coated statically with a 1% vinyl, 5% phenyl, 94% methylpolysiloxane stationary phase. High temperature separations of C10-C40 n-alkanes and a commercial diesel sample were demonstrated using the system under both temperature programmed GC (TPGC) and thermal gradient GC (TGGC) conditions. TGGC analysis of a complex essential oil sample was also demonstrated.
High-performance temperature-programmed microfabricated gas chromatography columns
Journal of Microelectromechanical Systems, 2000
This paper reports the first development of high-performance, silicon-glass micro-gas chromatography ( GC) columns having integrated heaters and temperature sensors for temperature programming, and integrated pressure sensors for flow control. These 3-m long, 150-m wide and 250-m deep columns, integrated on a 3.3 cm square die, were fabricated using a silicon-on-glass dissolved wafer process. Demonstrating the contributions to heat dissipation from conduction, convection, and radiation with and without packaging, it is shown that using a 7.5-mm high atmospheric pressure package reduces power consumption to about 650 mW at 100 C, while vacuum packaging reduces the steady-state power requirements to less than 100 mW. Under vacuum conditions, 600 mW is needed for a temperature-programming rate of 40 C min. The 2300 ppm/ C TCR of the temperature sensors and the 50 fF/kPa sensitivity of the pressure sensors satisfy the requirements needed to achieve reproducible separations in a GC system. Using these columns, highly resolved 20-component separations were obtained with analysis times that are a factor of two faster than isothermal responses.
Journal of Chromatography A, 2014
This work focuses on the development and experimental evaluation of micromachined chromatographic columns for use in a commercial gas chromatography (GC) system. A vespel/graphite ferrule based compression sealing technique is presented using which leak-proof fluidic interconnection between the inlet tubing and the microchannel was achieved. This sealing technique enabled separation at temperatures up to 350 13°C on a GC column. This paper reports the first high-temperature separations in microfabricated chromatographic columns at these temperatures. A 2 meter microfabricated column using a double Archimedean spiral design with a square cross-section of 100 µm x 100 µm has been developed using silicon microfabrication techniques. The microfabricated column was benchmarked against a 2 meter 100 m diameter commercial column and the performance between the two columns was evaluated in tests performed under identical conditions. High temperature separations of simulated distillation (ASTM2887) and polycyclic aromatic hydrocarbons (EPA8310) were performed using the GC column in temperature programmed mode. The demonstrated μGC column along with the high temperature fixture offers one more solution towards potentially realizing a portable GC device for the detection of semi-volatile environmental pollutants and A c c e p t e d M a n u s c r i p t explosives without the thermal limitations reported to date with GC columns using epoxy based interconnect technology.
Analytical Chemistry, 2005
An instrument for comprehensive two-dimensional gas chromatography (GC×GC) is described using an electrically heated and air-cooled thermal modulator requiring no cryogenic materials or compressed gas for modulator operation. In addition, at-column heating is used to eliminate the need for a convection oven and to greatly reduce the power requirements for column heating. The single-stage modulator is heated by current pulses from a dc power supply and cooled by a conventional two-stage refrigeration unit. The refrigeration unit, together with a heat exchanger and a recirculating pump, cools the modulator to about-30°C. The modulator tube is silicalined stainless steel with an internal film of dimethylpolysiloxane. The modulator tube is 0.18 mm i.d. × 8 cm in length. The modulator produces an injection plug width as small as 15 ms. Comprehensive two-dimensional gas chromatography (GC×GC) has emerged as a powerful method for the separation of complex mixtures of volatile and semivolatile organic compounds. 1-7 The method has been used for petroleum-product analysis, 8-12 essential oils separations, 13,14 environmental analysis, 15-17 and pyrolysis GC
Talanta, 2009
A new growth recipe for producing carbon nanotubes (CNTs) combined with a new bonding technique was implemented in a microfabricated gas chromatography (micro-GC) chip. Specifically, the micro-GC chip contained a 30-cm (length) microfabricated channel with a 50 microm x 50 microm square cross-section. A CNT stationary phase "mat" was grown on the bottom of the separation channel prior to the chip bonding. Injections onto the micro-GC chip were made using a previously reported high-speed diaphragm valve technique. A FID was used for detection with a high-speed electrometer board. All together, the result was a highly efficiency, temperature programmable (via low thermal mass, rapid on-chip resistive heating) micro-GC chip. In general, the newly designed micro-GC chip can be operated at significantly lower temperature and pressure than our previously reported micro-GC chip, while producing excellent chemical separations. Scanning electron microscopy (SEM) images show a relatively thin and uniform mat of nanotubes with a thickness of approximately 800 nm inside the channel. The stationary phase was further characterized using Raman spectroscopy. The uniformity of the stationary phase resulted in better separation efficiency and peak symmetry (as compared to our previous report) in the separation of a mixture of five n-alkanes (n-hexane, n-octane, n-nonane, n-decane and n-undecane). The on-chip resistive heater employing a temperature programming rate of 26 degrees C/s produced a peak capacity of eight within a 1.5-s time window.