1.7 - Sample Preparation by Oxidation

Sample oxidation also called sample combustion is an effective method for LSC (liquid scintillation counting) sample preparation. The technique is principally used for ¹⁴C and ³H and, more recently, also for ³⁶Cl samples¹. In the combustion process the organic sample is completely converted to carbon dioxide and water. If ¹⁴C and ³H radionuclides are present in the sample, the combustion products will contain ¹⁴CO₂ and ³H₂O. Since all organic compounds in the sample are oxidized to carbon dioxide and water, problems in the LSC measurement such as insolubility, chemiluminescence and colour-quenching are eliminated. Organic sample oxidation starts by decomposition of the sample under high temperature leading to release of volatilized gaseous molecules. These molecules are then oxidized in the reaction with the oxygen. The simplified reactions of the organic compounds are:

C + O₂ → CO₂

4H + O₂ → 2H₂O

The sample oxidation process involves 1) water production and incorporation into emulsifying cocktail such as the Hidex 600 OX Tritium cocktail and, 2) production of CO₂ and absorption into LSC cocktail such as the 600 OX Radiocarbon (Hidex). The CO₂ is absorbed by amine in a reaction where carbamate is formed for permanent CO₂ trapping. The 600 OX Radiocarbon is able to absorb 2.8 mmol of carbon dioxide into 1 ml of cocktail and, therefore, 15 ml of cocktail used in one reaction can absorb approximately 0.5 g of carbon. Furthermore, the 600 OX Radiocarbon cocktail includes the scintillators making the cocktail instantly ready for the LSC measurement.

Applications

The most common applications and sample types are: 1) Animal tissue in ADME (absorption, distribution, metabolism, and excretion) studies in development of pharmaceuticals, 2) Soil and sediment samples in environmental fate studies in agrochemical industry, 3) Plant samples in plant biology research and, 4) Concrete in nuclear power plant decommissioning. Furthermore, increasing environmental awareness have raised new applications along increased research on 5) Carbon sequestration and 6) Microplastics cycle in food chain.

Hidex 600 OX Oxidizer instrument is a state-of-art automated sample oxidizer that utilizes preheated furnace where the samples are loaded in a quartz glass spoons using a PLC (programmable logic controller) controlled machine automation. The samples are combusted in 900 °C temperature under controlled oxygen flow and the oxidation is finalized using copper oxide and platinum catalysts. The produced combustion gases, CO₂ and H₂O, are bubbled through the cocktails in the LSC vials. The automated instrument includes several sensors for monitoring and control of the sample oxidation process. For example, a gas tightness test is performed before every sample to ensure that all the combustion gases are collected in the cocktail and not leaked out from the system. The oxygen consumption is measured in real-time and O₂ input flow is automatically increased if the consumption is highly increased. The instrument also provides a QA (quality assurance) data from every sample combustion. The data includes measured values from several different sensors, such as, oxygen input flow, gas output flow, temperature of the furnace and combustion time

Advantages of sample combustion

Automated sample oxidation is the method of choice for variety of sample types especially when dozens of samples per day are processed. Hidex 600 OX Oxidizer has a six samples automated sample changer that enables combustion of a batch of six samples while the user can prepare (e.g. tissue slicing and weight measurement) the next batch of six samples simultaneously. There are several advantages of automated sample combustion, such as, 1) Rapid sample processing, (1-4 minutes per sample), 2) Minimal manual hands-on time, 3) Any sample matrix containing carbon and/or hydrogen can be combusted, 4) Sample can be wet, dry or freeze-dried and, 5) Ideal for single- and dual-label ¹⁴C and ³H samples. Furthermore, the sample oxidation method based on preheated furnace and catalysts is superior compared to flame oxidation for combusting of a relatively large size (1.5-2 g) samples such as soil and sediment that requires prolonged heating time for complete oxidation of all the carbon from the excess of inorganic non-combustible material.

1. Itoh et al., Determination of 36Cl in biological shield concrete using pyrohydrolysis and liquid scintillation counting. Analyst, 2002, 127, 964–966

2. L´Annunziata, Handbook of radioactivity analysis, Elsevier, 2012, ISBN: 978-0-12-384873-4