Characterization of semi-packed columns with different cross section in high-pressure gas chromatography.

Hydrodynamics, efficiency, and loading capacity of two semi-packed columns with different cross sections (NANO 315 µm x 18 µm; CAP 1000 µm x 28 µm) and similar pillar diameter and pillar-pillar distance (respectively 5 µm and 2.5 µm) have been compared in high-pressure gas chromatography. A flow prediction tool has been first designed to determine pressure variations and hold-up time across the chromatographic system taking into account the rectangular geometry of the ducts into the semi-packed columns. Intrinsic values of Height Equivalent to Theoretical Plate were determined for NANO and CAP columns using helium as carrier gas and similar values have been obtained (30 µm) for the two columns. Loading capacity of semi-packed columns were determined for decane at 70 °C using helium, and the highest value was obtained from CAP column (larger cross section and stationary phase content). Finally, significant HETP improvement (down to 15 µm) and peak shape were observed when carbon dioxide was used as carrier gas, suggesting mobile phase adsorption on stationary phase in high pressure conditions.

Dual detection chromatographic method for fast characterization of nano-gravimetric detector.

Nano-gravimetric detector (NGD) has been recently introduced as miniaturized gas chromatography detector. The NGD response is based on an adsorption-desorption mechanism of compounds between the gaseous phase and the NGD porous oxide layer. The NGD response was characterized by hyphenating NGD in-line with FID detector and a chromatographic column. Such method led to the full adsorption-desorption isotherms of several compounds in a single run. Langmuir model was used to describe the experimental isotherms, and the initial slope of the isotherm (Mm.KT) obtained at low gas concentration was used to compare the NGD response for different compounds (good repeatability was demonstrated with a relative standard deviation lower than 3%). The column-NGD-FID hyphenated method was validated using alkane compounds according to the number of carbon atoms in the alkyl chain and to the NGD temperature (all results agreed with thermodynamic relations associated to partition coefficient). Furthermore, relative response factor to alkanes, for ketones, alkylbenzenes, and fatty acid methyl esters have been obtained. These relative response index values led to easier calibration of NGD. The established methodology can be used for any sensor characterization based on adsorption mechanism.

Characterization of Nano-Gravimetric-Detector Response and Application to Petroleum Fluids up to C34.

A nano-gravimetric detector (NGD) for gas chromatography is based on a nanoelectromechanical array of adsorbent-coated resonating double clamped beams. NGD is a concentration-sensitive detector and its sensitivity is analyte-dependent based on the affinity of the analyte with the porous layer coated on the NEMS surface. This affinity is also strongly related to the NGD temperature (NGD working temperature can be dynamically set up from 40 to 220 °C), so the sensitivity can be tuned through temperature detector control. An adsorption-desorption model was set up to characterize the NGD response on a large set of n-alkanes from C10 to C22 at different NGD temperatures. For fast identification of petroleum mixture based on chromatogram fingerprint, a general strategy for NGD temperature program design was developed leading to a constant relative response factor between 0.96 and 1.03 for all the alkanes, and then chromatograms are very similar to those obtained with a flame ionization detector (FID). The analysis of a real petroleum fluid was also performed and compared to FID results: quantitative results obtained for all the analytes were satisfactory according to precision (<5%) and accuracy (average relative error = 4.3%). Based on such temperature control strategy, NGD sensitivity and the dynamic linear range can be adjusted and detection limits at a picogram level can be easily achieved for all n-alkanes.

Single-particle mass spectrometry with arrays of frequency-addressed nanomechanical resonators.

One of the main challenges to overcome to perform nanomechanical mass spectrometry analysis in a practical time frame stems from the size mismatch between the analyte beam and the small nanomechanical detector area. We report here the demonstration of mass spectrometry with arrays of 20 multiplexed nanomechanical resonators; each resonator is designed with a distinct resonance frequency which becomes its individual address. Mass spectra of metallic aggregates in the MDa range are acquired with more than one order of magnitude improvement in analysis time compared to individual resonators. A 20 NEMS array is probed in 150 ms with the same mass limit of detection as a single resonator. Spectra acquired with a conventional time-of-flight mass spectrometer in the same system show excellent agreement. We also demonstrate how mass spectrometry imaging at the single-particle level becomes possible by mapping a 4-cm-particle beam in the MDa range and above.

Frequency fluctuations in silicon nanoresonators.

Frequency stability is key to the performance of nanoresonators. This stability is thought to reach a limit with the resonator's ability to resolve thermally induced vibrations. Although measurements and predictions of resonator stability usually disregard fluctuations in the mechanical frequency response, these fluctuations have recently attracted considerable theoretical interest. However, their existence is very difficult to demonstrate experimentally. Here, through a literature review, we show that all studies of frequency stability report values several orders of magnitude larger than the limit imposed by thermomechanical noise. We studied a monocrystalline silicon nanoresonator at room temperature and found a similar discrepancy. We propose a new method to show that this was due to the presence of frequency fluctuations, of unexpected level. The fluctuations were not due to the instrumentation system, or to any other of the known sources investigated. These results challenge our current understanding of frequency fluctuations and call for a change in practices.

VHF NEMS-CMOS piezoresistive resonators for advanced sensing applications.

This work reports on top-down nanoelectromechanical resonators, which are among the smallest resonators listed in the literature. To overcome the fact that their electromechanical transduction is intrinsically very challenging due to their very high frequency (100 MHz) and ultimate size (each resonator is a 1.2 µm long, 100 nm wide, 20 nm thick silicon beam with 100 nm long and 30 nm wide piezoresistive lateral nanowire gauges), they have been monolithically integrated with an advanced fully depleted SOI CMOS technology. By advantageously combining the unique benefits of nanomechanics and nanoelectronics, this hybrid NEMS-CMOS device paves the way for novel breakthrough applications, such as NEMS-based mass spectrometry or hybrid NEMS/CMOS logic, which cannot be fully implemented without this association.

Modal control of mechanically coupled NEMS arrays for tunable RF filters.

A novel tuning strategy of nanoelectromechanical systems (NEMS)-based filters is proposed based on the modal control of mechanically coupled NEMS arrays. This is done by adjusting separately addressed distributed actuation and detection configurations proportionally to desired modal vectors. This control scheme enhances the global output signal, raising the power handling of the filter on all channels. Although the modal control of 1-D arrays exhibits narrow-band responses with adjustable resonance frequency, its application to 2-D arrays produces filters with both adjustable bandwidth and central frequency. One possible realization scheme is suggested by using electrostatically driven coupled NEMS arrays whose transduction gains are adjusted by changing the electrodes¿ bias voltages. Dispersion effects on both 1-D array and 2-D array frequency response are analytically expressed using eigenvalues perturbation theory. Based on these results, we show how to reduce their impact by appropriately choosing the coupling stiffness and the number of resonators.