Área de trabajo: Análisis de Alimentos y Bebidas.
Título: Análisis Isotópico de Agua, Jugo de Fruta y Vino usando IRMS.
Título original: Isotope Analysis of Water, Fruit Juice and Wine Using the Thermo Scientific GasBench II IRMS.
Autor: Andreas W. Hilkert, Hairigh Avak Thermo Fisher Scientific, Bremen, Germany.
The measurement of the 18O/16O ratio of water is one of the most important applications of isotope ratio mass spectrometry. Very diverse applications, ranging from studies of variations in natural abundance in the hydrologic cycle to authenticity control on beverages to the use of 18O enriched tracers in human and animal metabolic studies, demand a general purpose analytical solution which can provide high precision/high accuracy/ high throughput analysis of 18O/16O in an enormous range of substrates. Continuous flow techniques, in which the sample is entrained in a carrier gas, have been shown to allow high throughput analysis from elemental analyzers and gas chromatographs, but for high precision work, the dual inlet system has remained the instrument of choice. The development of the technique of repetitive loop injection has allowed us to close the gap between the dual inlet system and continuous flow inlet systems.
Conditions Borosilicate sample bottles are washed with diluted acid, then washed twice with deionized water and dried overnight at 70 °C. After adding 500 μL of sample into the open vials using a disposable syringe they are sealed with new septa. Residual air in the vials is removed from the sample vials by an automated autosampler-assisted flushing procedure, which uses a mixture of 0.3% to 0.5% CO2 in He. Flushing is carried out at a flow of 100 mL/min for 5 minutes. The 0.3% to 0.5% CO2 in the flushing He stream is used as equilibration gas. Measurement is carried out after an equilibration time of 24 h at 24 °C. The temperature stability required for 18O equilibration is ± 0.1 °C (the temperature dependence is ~ 0.25 ‰ per 1 °C). By helium the sample is moved through the water trap, filling the sample loop and leaving to vent. Water vapor is quantitatively removed on-line from the gas stream. The sampling loop aliquots 100 μL samples of the headspace into an isothermal gas chromatograph, where CO2 is separated from any other gas species. The use of repetitive loop injection (1–2 min per replicate) allows to approach a precision comparable to that of a dual inlet system. Some of the analysis presented have been prepared by an older, manual method injecting 500 μL of aqueous sample through the septum into 10 mL screwcap Exetainers, prefilled with a commercial mixture of 0.2% up to 1% CO2 in helium (99.996% purity).
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