Ultra-Low P, S, Cl Analysis in Liquids with a Benchtop WDXRF Spectrometer
Sep 17 2019 Read 363 Times
Phosphorous, sulfur and chlorine are commonly analysed elements in liquids such as petroleum products and organic solvents (e.g., phosphorous is added to lubricating oils to prevent wear of gears under high pressure). Sulfur content in automotive fuel oils is regulated by the US environmental protection agency (EPA) and directives such as EURO VI to minimise air pollution due to automotive vehicles. Chlorine is often known for corroding oil processing installations and liquid transmission pipelines.
To better meet demands of such applications, the Rigaku Supermini200 benchtop wavelength dispersive X-ray fluorescence (WDXRF) spectrometer’s analysing crystal set has been newly reconfigured to optimise performance for trace level analysis of P, S and Cl.
XRF analysis allows rapid and precise elemental characterisation of such samples with minimum sample preparation. Laborious sample treatment such as acid digestion or serial dilution are not required. Liquid samples are simply poured into a plastic cup covered with film for measurement and results can be obtained in minutes.
The Supermini200 benchtop sequential WDXRF spectrometer is designed specifically to deliver excellent performance while eliminating typical installation requirements such as cooling water, special power supply and large floor space.
Equipped with a unique air cooled 200 watt X-ray tube, two detectors and three analysing crystals, the Supermini200 spectrometer is capable of measuring elements from oxygen to uranium. Analysis of P, S and Cl is optimized with the RX9 analysing crystal. Standardless semi-quantitative analysis is also available allowing quick and simple characterisation of non-routine samples.
Calibration and measurement
Six calibration samples were prepared by diluting a NIST traceable lubricating oil standard obtained from AccuStandard®
Test samples with P, S, Cl at 0.5, 1 and 5 mg/kg were prepared by mixing and diluting 3 single element standards obtained from ASI Standards (PMO1C-1000, S(PS)MO1C-1000, ClMO1C-1000). Short term reproducibility test was performed by consecutively running 20 aliquots. A long-term reproducibility test was performed by running 2 aliquots per day (morning and afternoon) over 10 days. Instrument was shut down for one week between short and long terms tests.
Short and long term analysis results for P, S, Cl test samples at 0.5, 1 and 5 mg/kg are shown in Tables 1, 2 and 3 respectively.
The largest S result difference between two consecutive short term measurements of the 5 mg/kg sample is 0.4 mg/kg, between run 11 and 12. ASTM D2622-16 prescribes that the difference between successive results for a 5 mg/kg sample may exceed 0.5 mg/kg only in one case in twenty. For ISO 20884:2011, the criterion is 1.8 mg/kg. Results show that the instrument meets precision requirements of both test methods. Sulfur levels of 0.5 and 1 mg/kg are below the lower range covered by either test method.
Chlorine standard test method JPI-5S-73-2014, issued by the Japanese Petroleum Institute, requires that analysis result difference between successive results is less than 1.3 ppm with 95% probability. This criterion is met with chlorine levels of 0.5 and 1 mg/kg, below the lower range covered by the test method.
The results demonstrate that the Supermini200 spectrometer is able to analyse low level P, S and Cl in liquids in a single run, meeting precision required by test methods such as ASTM D2622-16, ISO 20884:2011 and JPI-5S-73-2014. Measurement results can be obtained in minutes with quick and easy sample preparation allowing processing of large quantities of samples. Minimal installation prerequisites, and no requirements of expensive consumables and dangerous acids, contributes to safety and low cost of ownership.
ASTM D2622 (2016), Standard Test Method for Sulfur in Petroleum Products by Wavelength Dispersive X-ray Fluorescence Spectrometry
ISO 20884 (2011), Petroleum products - Determination of Sulfur Content of Automotive Fuels - Wavelength Dispersive X-ray Fluorescence Spectrometry
JPI-5S-73 (2014), Fuel Oils - Determination of Chlorine Content - Wavelength Dispersive X-ray Fluorescence Spectrometer
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