EN 14538 Fat and oil derivatives - Fatty acid methyl ester (FAME) - Determination of Ca, K, Mg and Na content by optical emission spectral analysis with inductively coupled plasma (ICP OES)
6 Sampling
Samples shall be taken as described in EN ISO 3170 or EN ISO 3171 and/or in accordance with the requirements of national standards or regulations for the sampling of automotive diesel fuel. Bottles made of plastic (PE or PTFE) preferably should be used.
7 Preparation of the calibration solutions and the blank solution
7.1 General
In order to avoid inhomogenities, the standard solutions (4.3) should be shaken vigorously before use. The masses given in 7.3, 7.4, 7.5 and 7.6 correspond to a nominal element content of 500 mg/kg per element in the standard solutions (4.3) and (4.4). Calculate the exact concentrations for the calibration solutions, taking into account the exact weights. All prepared solutions shall be homogenized by vigorous shaking.
NOTE 1 As a strong general recommendation, the solutions used for calibration should be freshly prepared. If that is not possible, the samples should be stabilized using a suitably small amount of a stabilizing agent like 2-Ethylhexanoic acid, which has been shown to be free of the elements under investigation. Experience from daily practice has shown that such stabilized samples can be used for about 14 days. However, there is also sufficient evidence that calibration standard (7.3) in particular is not sufficiently stable, so that it is strongly recommended to prepare fresh solutions for each new calibration.
NOTE 2 The dilution factors for the blank solution, the calibration solutions and sample solution should be selected such that the viscosities are as close to one another as possible. This can often be achieved by using a somewhat higher dilution factor.
7.2 Blank solution
Approximately 30 g paraffin oil (4.1) is weighed, with a precision of 0.01 g, into a 250 ml PE bottle (5.2), filled up with kerosene (4.2) to a total sample mass of approximately 100 g, with a precision of 0.01 g.
7.3 Calibration solution with a nominal element content of 0.5 mg/kg
For each element, approximately 0.1 g standard solution (4.3) is weighed, with a precision of 0.0001 g, into a 250 ml PE bottle (5.2). Then approximately 30 g paraffin oil is added, with a precision of 0.01 g. Subsequently, kerosene (4.2) is added to make up to a total sample weight of approximately 100 g, with a precision of 0.01 g. Special attention shall be executed in using this calibration solution, as it is not as stable as the other ones.
7.4 Calibration solution with a nominal element content of 1 mg/kg
For each element, approximately 0.2 g standard solution (4.3) is weighed, with a precision of 0.0001 g, into a 250 ml PE bottle (5.2). Then approximately 30 g paraffin oil is added, with a precision of 0.01 g. Subsequently, kerosene (4.2) is added to make up to a total sample weight of approximately 100 g, with a precision of 0.01 g.
7.5 Calibration solution with a nominal element content of 5 mg/kg
For each element, approximately 1 g standard solution (4.3) is weighed, with a precision of 0.0001 g, into a 250 ml PE bottle (5.2). Then approximately 30 g paraffin oil is added, with a precision of 0.01 g. Subsequently, kerosene (4.2) is added to make up to a total sample weight of approximately 100 g, with a precision of 0.01 g.
7.6 Calibration solution with a nominal element content of 10 mg/kg
For each element, approximately 2 g standard solution (4.3) is weighed, with a precision of 0.0001 g into a 250 ml PE bottle (5.2). Then approximately 30 g paraffin oil is added, with a precision of 0.01 g. Subsequently, kerosene (4.2) is added to make up to a total sample weight of approximately 100 g, with a precision of 0.01 g.
8 Calibration
8.1 General
The ICP OES spectrometer setup and instrument check is performed according to the instructions from the manufacturer. A separate calibration function for each element under investigation shall be established.
8.2 Calibration
The calibration of the ICP OES spectrometer shall be done by the measurement of the blank solution (7.2) and of the standard solutions (7.3 to 7.6). For the determination of the elements the wavelengths recommended in 5.3 shall be used. It is important to ensure that the wavelengths used in calibration also match exactly the ones used in the measurement.
For each element under investigation, a calibration curve is constructed using linear regression with concentration as independent variable (X) and signal as dependent variable (Y). This can either be done manually or with use of a computer. The calibration curve will have the form Y = m x X + b, where m is the slope and b is the Y-intercept for the regression line.
8.3 Check of calibration
The calibration curves shall be checked in regular intervals. In practice, at least two points of each calibration curve have to be checked every day. If the results of this examination differ by more than the repeatability (see 12.1) from the results obtained during the calibration experiment, a new calibration curve shall be established (see also Note 1 under 7.1).
