Introduction

Determination of the oxygen content in an organic sample is not ordered frequently. Our laboratory is equipped with a method to perform this analysis and we recommend this characterisation to complement C/H/N-results primarily for C/H-compounds and all hydrophilic substances which frequently contain significant amounts of water.

Oxygen analysis is don using the "EA 3000" (Eurovector) combined with a high temperature pyrolysis oven "HT 1500" (Hekatech). Pyrolysis takes place at almost 1500 °C in helium atmosphere. The reactor is made of glassy carbon and is halfways filled with glassy carbon shards. On top of these a layer of granulated carbon is serving as reducing agent.

Method

Oxygen determination using the HT1500/EA3000:

The  sample is weighed in using a silver capsule. 1 to 2.5 mg of material are required. After closing and folding the capsule it is placed in the autosampler.

Die Silberkapsel mit der Probe fällt in denen Pyrolyseofen wo sie bei 1.480 °C in reiner Helium-Atmosphäre zersetzt wird. Die Produktgase reagieren mit dem im Überschuss vorhandenen Kohlenstoff wobei der gesamte Sauerstoff als Kohlenmonoxid freigesetzt wird. 

The silver capsule is released and falls down into pyrolysis oven where it is digested at 1.480 °C in pure helium atmosphere. Fluid products react at granulated carbon phase where oxygen is set free in the form of carbon monoxide. Under the reaction conditions carbon dioxide and water are not stable so that a quantitative yield of CO for oxygen is assumed.

To decompose high melting oxides like SiO₂ teflon ([CF₂]_n) is added to the sample. The teflon decomposes and forms various aggressive species to set free the O from almost all compounds. Digestion by-products like HF and F₂ are among the product gases trasferred to the analysis system.

In addition to CO  important product gases observed are H₂ and N₂. Other small molecules like H₂S, CH₄ etc. are condensed in a cool trap at -196°C (liquid nitrogen) to condese HF, F₂ and other aggresive fluorine containing compounds that would decompose the chromatographic column. A trap filled with metallic calcium is another barrier for HF and F₂.

The gaseous products are sparated by gas chromatography at a silica based molecular sive column and detected and quantified at a TCD (thermal conductivity detector).

Blank values are recorded with empty silver vials.

For calibration we only used certified NIST-referenced standard reference materials namely acetanilide, benzoic acid and L-cystine.

Working range

Analysis returns the total amount of oxygen. 

The limit of quanitification at normal operating conditions is 0.05 wt-% O. The uncertainty is lower than 0.3 wt-% at O-levels of up to 50 wt-%.

Problems and Interferences

The mineralization of metallic or metal organic compounds can also be limited by interferences. Reduction of nickel- and ironcompounds should theoretically be possible ander the conditions of the process. Earth alkaline elements, lanthanides and actinides, silicon and other elements that form oxygen compounds which cannot be reduced by carbon are able to form stable oxides under the chemical conditions which leads to systematic errors which can even be stoichiometric loss compared to the expected oxygen content.

Important Advice

We need as much information about your sample as available to be able to provide you with best results according to the state of the art.

Declaration of the expected elemental composition (sum formula) is required for standard elemental analysis on preparative samples. Additionally we need to be informed about remarkable instability and/or sensitivity of the material to bring your samples into the analysis in the expected state (e.g. how to store, dry etc.).

With technical products we require knowledge about the assumed amount of analyte in the sample as well as known additives. If we know about the background of analysis (e.g. verify the limit of 0,2 w-% halogene in waste material) we can guarantee results fitting your purposes best.