BS ISO 23345:2021 pdf download – Jewellery and precious metals — Non destructive precious metal fineness confirmation by ED-XRF
5 Apparatus
5.1 ED-XRF, with the following specifications:
— X-ray tube: adapted for precious metals analysis, minimum rating 50 kV and 40 W;
— primary filter: adapted for precious metals analysis;
— collimator: minimum 2 collimators, at least one <1 mm and one ≥1 mm;
— camera: with magnification of the image of the measuring area;
— detector: Si-PIN, SDD;
— energy resolution: ≤160 eV (for Kα of Mn line), the lower eV value the better;
— detectable elements: Z = 22 (Ti) to Z = 92 (U).
6 Procedure
6.1 Calibration
6.1.1 Calibration standards
To calibrate the instrument for a specific alloy, calibration standards with the following features shall be used:
— flat and clean surface of suitable size, with at least a diameter 5 times larger than the collimator used for measurement;
— sufficiently thick (at least 0,5 mm);
— homogeneous;
— known exact composition;
— composition of major and minor elements matching approximatively the sample to be analysed; trace elements may be ignored; a minimum of 3 standards shall be used, with the major element(s) composition covering a range up to 50 ‰.
— minor elements below 5 ‰ do not need to be present in the standards but shall be declared in the calibration; they may be calibrated using the instrument maker’s calibration. EXAMPLE For the measurement of an alloy with Au = 750 ‰, the fineness of Au in the calibration standards may be 730 ‰, 750 ‰, and 770 ‰.
6.1.2 Method calibration The ED-XRF instrument (5.1) shall be specifically calibrated for each sample to be analysed. Each standard shall be analysed at least on 5 different positions. The standard deviation obtained for the measurements of each calibration standard shall not exceed 1,2 ‰ for the major element. Inter-element interference corrections should be applied. Analysis of the calibration standards, the reference material and the samples shall be performed using the same collimator size and using the same parameters (tube voltage, current, etc.). The largest possible collimator compatible with the sample target area should be used.
6.2 Verification
6.2.1 Reference material To verify the calibration, a reference material with the following features shall be used:
— flat and clean surface of suitable size, with at least a diameter 5 times larger than the collimator used for measurement;
— sufficiently thick (at least 0,5 mm);
— homogeneous;
— known exact composition;
— not being used as calibration standard;
— certified reference material (CRM) and materials prepared under the ISO 17034 accreditation are preferred;
— the composition of the reference material should be matching as far as practicable that of the sample; for major elements there shall be no more than 10 ‰ absolute difference between the concentration in the sample and in the reference material; for minor elements, that difference shall be no more than 20 ‰ absolute difference; trace elements may be ignored. EXAMPLE If an alloy with a 750 ‰ Au – 240 ‰ Ag – 10 ‰ Cu is to be analysed, the concentrations in the reference material are between 740 ‰ and 760 ‰ for Au, between 260 ‰ and 220 ‰ for Ag, and between 30 ‰ and 0 ‰ for Cu.
6.2.2 Method verification Before each batch analysis, the calibration shall be controlled by analysing the reference material. The reference material shall be analysed at least on 5 different positions. The analysis time for each replicate shall not exceed the time used for the calibration. The standard deviation obtained for the measurements in the reference material shall not exceed 1,2 ‰ for the major element. The calibration is verified by comparing the fineness measured for the reference material (obtained by the mean value of the analyses) and its declared value. The difference, Δ RM , between those two values shall not be greater than 1,2 ‰ and will be taken into the uncertainty evaluation of the method.
If the standard deviation for the measurements or the difference between the fineness measured for the reference material and its declared value are outside the tolerances, the calibration shall be repeated.
The reference material shall be analysed at regular intervals under the same conditions to monitor the stability of the instrument.