BS ISO 20620:2021 pdf download – Fertilizers and soil conditioners — Determination of total nitrogen by combustion
2 Normative references
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ISO 14820-2:2016, Fertilizers and liming materials — Sampling and sample preparation — Part 2:Sample preparation
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The sample is combusted at a high temperature of 900 °C or above in the presence of oxygen. Following the reduction of formed nitrogen oxides to elemental nitrogen and the removal of any interfering products of combustion, nitrogen is measured with a thermal-conductivity detector.
5.1 Automatic nitrogen analyser, based on combustion methods.
5.2 Analytical balance.
The accuracy of the balance is a function of the analyser used and the required weighed portions. The resolution should be 0,1 % or better of the weighed portion.
5.3 Auxiliary devices for sample preparation, for example:
been obtained for the same sample material when analysed in succession when the sample has been sufficiently prepared. Different types of apparatus are available on the market. Operation and performance criteria should be based on the respective operation manuals.
7.2 Reference curve Perform calibration as required for the specific type of analyser and according to the respective operation manuals (e.g. after replacement of the combustion tube, reagent or similar) by performing measurements as described in 7.4. Weigh in an appropriate amount of standard substances repeatedly as appropriate for the respective types of apparatus to obtain a reference curve.
7.3 Inspection and calibration Use an appropriate standard substance or in-house secondary standard to review the good working order of the apparatus and the reference curve. Preferably, a certified standard should be used. Frequency of inspection is a function of the analyser used. Use of quality control charts is also advised to monitor trends in instrument performance.
7.4 Measurement Weigh a portion of the sample in a suitable holder as specified for the type of nitrogen analyser used. The amount should be such that the absolute amount of nitrogen is in the middle range of the reference curve. Use approximately the threefold amount of combustion agent. Enter the required data (weighed portion, sample identification) into the analyser (or a control computer) depending on the type of apparatus. Feed the weighed-in sample to the analyser and start combustion. Perform at least three single determinations.
8.1 Calculation Use blank samples to background correct prior to analysing the calibration standards and calculating the calibration curve. Use the apparatus-specific program to calculate the reference curve or the drift correction for the samples. Calculate per cent nitrogen for each sample using the apparatusspecific software. 8.2 Expression of results Report the mean value for the replicates of the sample provided that each replicate is in good agreement. Express the results as per cent nitrogen.
Grubbs and Cochran tests were used to identify outliers. The recovery for each sample was calculated for samples with noted %N values. Repeatability (r), reproducibility (R), standard deviation of repeatability (s r ) and standard deviation of reproducibility (s R ) are listed in Table 1. Table 2 lists the composition and physical state of each fertilizer used in the ring-study. Additional data for the evaluation of total nitrogen by combustion in fertilizers is available in AOAC Official Method 993.13  . Data for the determination of total nitrogen in urea by combustion is available in ISO 22241-2:2019, Annex B.BS ISO 20620 pdf download.