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.

Quantitative multiplexed elemental (C, H, N and S) detection in complex mixtures using gas chromatography.

We present a novel and single detection approach that enables sensitive, accurate and compound-independent quantification of N, S and H in the individual compounds present in complex samples. Integration of the whole chromatographic profile gives the total content of the elements. Simultaneous universal detection is also achieved using the C profile.

Isotope Dilution Mass Spectrometry for Highly Precise Determination of Dissolved Inorganic Carbon in Seawater Aiming at Climate Change Studies.

Dissolved inorganic carbon (DIC) is one of the most important parameters to be measured in seawaters for climate change studies. Its quantitative assessment requires analytical methodologies with overall uncertainties around 0.05% RSD for clear evaluation of temporal trends. Herein, two alternative isotope dilution mass spectrometry (IDMS) methodologies (online and species-specific) using an isotope ratio mass spectrometer (IRMS) and two calculation procedures for each methodology have been compared. As a result, a new method for the determination of DIC in seawaters, based on species-specific IDMS with isotope pattern deconvolution calculation, was developed and validated. A 13C-enriched bicarbonate tracer was added to the sample and, after equilibration and acidification, the isotope abundances at CO2 masses 44, 45, and 46 were measured on an IRMS instrument. Notably, early spiking allows correcting for evaporations and/or adsorptions during sample preparation and storage and could be carried out immediately after sampling. Full uncertainty budgets were calculated taking into account all the factors involved in the determination (initial weights, concentration and isotope abundances of standards, and final IRMS measurements). The average DIC value obtained for CRM seawater agreed very well with the certified value. Propagated precision obtained ranged from 0.035 to 0.050% RSD for individual sample triplicates. Reproducibility, assessed by three independent experiments carried out in different working days, was excellent as well (-0.01% and 0.057%, error and full combined uncertainty, respectively). Additionally, the approach proposed improves on established methods by simplicity, higher throughput (15 min per sample), and lower volume requirements (10 mL).

Modification of a commercial gas chromatography isotope ratio mass spectrometer for on-line carbon isotope dilution: Evaluation of its analytical characteristics for the quantification of organic compounds.

We describe the instrumental modification of a commercial gas chromatography isotope ratio mass spectrometer (GC-IRMS) and its application for on-line carbon isotope dilution. The main modification consisted in the addition of a constant flow of enriched (13)CO2 diluted in helium after the chromatographic column through the splitter holder located inside the chromatographic oven of the instrument. In addition, and in contrast to the conventional mode of operation of GC-IRMS instruments where the signal at m/z 45 is amplified 100-fold with respect to the signal at m/z 44, the same signal amplification was used in both Faraday cups at m/z 44 and 45. Under these conditions isotope ratio precision for the ratio 44/45 was around 0.05% RSD (n=50). The evaluation of the instrument was performed with mixtures of organic compounds including 11 n-alkanes, 16 PAHs, 12 PCBs and 3 benzothiophenes. It was observed that compounds of very different boiling points could be analysed without discrimination in the injector when a Programmable Temperature Vaporizer (PTV) injector was employed. Moreover, the presence of heteroatoms (Cl or S) in the structure of the organic compounds did not affect their combustion efficiency and therefore the trueness of the results. Quantitative results obtained for all the analytes assayed were excellent in terms of precision (<3% RSD) and accuracy (average relative error≤4%) and what is more important using a single and simple generic internal standard for quantification.