EN 13805-2014 pdf download.Foodstuffs-Determination of trace elements – Pressure digestion.
WARNING — Perchloric acid shall never be used in pressure digestions, not even In small amounts.
The volume of acid required for the digestion depends on the nature of the sample material. Usually 3 ml of nitric acid (4.1) are sufficient to digest the required amounts. Systems with microwave-assisted heating usually require a higher amount of acid. For carbon-rich samples, it may be necessary to increase the amount of acid and to reduce the sample mass. 0,5 ml to 1 ml of hydrogen peroxide (4.5) can be added to prevent adhesion of the samples to the wall of the digestion vessel and to achieve complete mixing with the acid. Hydrogen peroxide causes increased gas formation when the acid reacts with the sample, thereby resulting In thorough mixing. Furthermore, less nitrogen oxides are generated.
Following the addition of the acid, thoroughly mix the sample. Avoid any adhesion of sample clumps to the wall of the digestion vessel. Before closing the vessels, cover the sample with acid. In the case of microwave- assisted digestion, the total volume of the liquid including the sample should correspond with the manufacturers information. It is recommended to perform a pre-reaction following the acid addition, at room temperature with the vessel open and loosely covered. In the case of reactive samples, this may be carried out overnight, Following the pre-reaction, dose the vessel as described by the vessel manufacturer,
For digestions where an addition of hydrochloric acid is necessary, e.g. when analysing the elements tin, antimony and iron, proceed as follows: the addition of at least 0,5 ml of hydrochloric acid (4.3) is required to prevent losses due to adsorption to the vessel wall and to keep the elements fully solved. First add nitric acid and mix with the sample. Add the hydrochloric acid only after the spontaneous reaction caused by the nitric acid is finished. Seal the digestion vessel immediately after addition of hydrochloric acid to make the resulting chlorine gas available for reaction.
The surface finish of the digestion vessels (e.g. roughness) is decisive for the adsorption to the vessel wall. Here, quartz vessels are preferable to digestion vessels made of fluoroplastics. Therefore, following each digestion, check and clean the vessels and remove any deposits on the vessel wall completely.
The digestion of some foodstuffs, e.g. containing silicates or titanium dioxide, may require the addition of hydrofluoric acid (4.4). When using hydrofluoric acid, the appropriate safety guidelines shall be followed. Only inserts of fluoroplastics may be used for digesting with hydrofluoric acid. Quartz glass is damaged irreversibly when it comes into contact with hydrofluoric acid.
6.3.3 Digestion temperature
Digest the samples at a temperature of at least 180 °C. Heat pressure vessels using microwave-assisted heating to the digestion temperature by applying a low heating rate with reduced microwave power, adapted to the reaction behaviour of the samples. Identify the required digestion temperature and in consequence the resulting completeness of the decomposition by the subsequent measurement procedure. Higher temperatures result in lower contents of residual carbon in the digestion solutions. Thereby the background interferences in ET-AAS and ICP-OES measurements are reduced. Interferences in e.g. chromium determination and influence on signal sensitivity for arsenic and selenium on ICP-MS measurements are reduced [7]. (8], [9] and trouble-free voltammetric measurements can be perfoi’med.
It has been found that the quality of the digestion improves with an increasing digestion temperature [101, [11]. If organic arsenic compounds are present in the food, a temperature of 320 C can be necessary if hydride MS is used for the subsequent determination of arsenic [121. The temperature of 320 C can be reached e.g. by pressure digestion apparatus with conventional heating (5.2.2). For the determination of selenium with hydride MS. the maximum temperature for digestion is 280 C [131.
For all steps of the digestion process, follow to the manufacturer’s safety provisions.
6.3.4 Digestion time
In the case of pressure digestion using conventional heating, the usual digestion time after attaining the final digestion temperature is at least 1,5 h. In the case of discrete pressure vessels, the complete time for digestion is up to 15 h. With microwave systems. the digestion time should be at least 20 mm after the digestion temperature has been attained.
6.3.5 CoolIng
To reduce the excess pressure, cool the still sealed pressure vessel to a temperature of less than 40 °C.
6.3.6 Treatment of the digestion
Once the digestion vessel has been cooled down, carefully open it whilst suctioning off the liberated gases. Depending on the construction of the pressure vessel, this is preceded by ventilation of the vessel in order to relieve any residual pressure. Afterwards, place the digestion vessel initially under a fume hood until no brown fumes are visible. It is recommended to degas the digestion solution in the ultrasonic bath. The digestion solution shall be clear and its volume shall be roughly the same as before the digestion. An obvious reduction in volume suggests that the pressure vessel was not tight. Repeat the digestion in such cases. Once the digestion solution has been brought to room temperature, make it up to a defined volume with water. Transfer this solution to vessels made of quartz glass (for the determination of mercury use appropriate vessels. e.g. quartz glass or fluorinated ethylene propylene (FEP)) or suitable plastics vessels (e.g. made of FEP or PFA).
NOTE Yellow coloured digestion solutions are caused by incompletely digested organic substances. They can be the result of too high a sample mass and/or too low a digestion temperature. Digestion temperatures exceeding 200 C usually do not result in yellow coloured digestion solutions. Blue coloured digestion solutions are the result of dissolved nitrogen oxides. Following dilution with water, the blue colour disappears.EN 13805-2014 pdf download.