Application Note EDXRF3272
Scope
The measurement of additive elements in new, fresh lubricating oils according to ASTM D7751 is demonstrated using the Rigaku NEX QC II+ EDXRF analyzer with empirical calibration.
Background
Lubricating oils are used in on-road and off-road engines, as well as in mechanical machinery, and are formulated for specific applications. Various additives are blended into base oil to enhance lube oil performance and provide the desired lubrication properties for each situation. Controlling additive elements when blending new, fresh lube oils is vital to ensure optimum lubricity while minimizing production waste and product rejection.
To meet industry needs, the Rigaku NEX QC II+ can be used for compliance with ASTM D7751 Standard Test Method for Additive Elements in Lubricating Oils by EDXRF Analysis. Its simple, intuitive, icon-driven touchscreen interface supports operators with an excellent tool for routine quality control when blending various lube oil formulations.
INTERNATIONAL NORMS
The Rigaku NEX QC II+ analyzer can be used to comply with ASTM D7751 and D6481 using empirical calibration.
Calibration
Empirical calibrations were built using a suite of 17 commercially available calibration standards. The suite of standards must be representative of the lube oil formulations to be analyzed. Empirical alpha corrections are applied to automatically compensate for variations in X-ray absorption and enhancement effects within the sample. A summary of the empirical calibrations is shown below.
| Element | Concentration range mass% |
| Mg | 0.020 – 0.400 |
| P | 0.020 – 0.250 |
| S | 0.005 – 2.000 |
| Cl | 0.001 – 0.050 |
| Ca | 0.002 – 0.450 |
| Zn | 0.040 – 0.265 |
| Mo | 0.004 – 0.0500 |
Repeatability
Representative samples from the calibration suite were selected to demonstrate typical instrument repeatability (precision). Ten repeat analyses of each sample were performed with the sample in a static position, using a total analysis time of 500 seconds per measurement.
The tables below show measurement standard deviation (σ). D7751 “little r" repeatability is expressed as 2.77σ at a 95% confidence level. Comparisons of the instrument 2.77σ values to the D7751 r values show repeatability results meet ASTM D7751 requirements.
| Sample: 2 Units: mass% |
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| Element | Standard value | Average value | Std. dev σ | Instrument 2.77σ | D7751 r |
| Mg | 0.0200 | 0.0190 | 0.0006 | 0.0017 | 0.0023 |
| P | 0.2000 | 0.1996 | 0.0002 | 0.0006 | 0.0018 |
| S | 1.3800 | 1.3786 | 0.0022 | 0.0061 | 0.0189 |
| Cl | 0.0280 | 0.0281 | 0.00009 | 0.0002 | 0.0007 |
| Ca | 0.0600 | 0.0599 | 0.0001 | 0.0003 | 0.0021 |
| Zn | 0.1750 | 0.1754 | 0.0001 | 0.0003 | 0.0049 |
| Mo | 0.0340 | 0.0342 | 0.00009 | 0.0002 | 0.0022 |
| Sample: 3 Units: mass% |
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| Element | Standard value | Average value | Std. dev σ | Instrument 2.77σ | D7751 r |
| Mg | 0.0450 | 0.0438 | 0.0012 | 0.0033 | 0.0003 |
| P | 0.1701 | 0.1754 | 0.0003 | 0.0008 | 0.0019 |
| S | 1.7550 | 1.7409 | 0.0036 | 0.0100 | 0.0232 |
| Cl | 0.0250 | 0.0249 | 0.0001 | 0.0003 | 0.0006 |
| Ca | 0.0300 | 0.0298 | 0.0001 | 0.0003 | 0.0015 |
| Zn | 0.2050 | 0.2049 | 0.0004 | 0.0017 | 0.0055 |
| Mo | 0.0220 | 0.0220 | 0.0001 | 0.0003 | 0.0014 |
| Sample: 13 Units: mass% |
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| Element | Standard value | Average value | Std. dev σ | Instrument 2.77σ | D7751 r |
| Mg | 0.2200 | 0.2188 | 0.0013 | 0.0036 | 0.0133 |
| P | 0.0500 | 0.0517 | 0.0001 | 0.0003 | 0.0015 |
| S | 2.0000 | 2.0019 | 0.0022 | 0.0061 | 0.0259 |
| Cl | 0.0130 | 0.0134 | 0.00005 | 0.0001 | 0.0005 |
| Ca | 0.3600 | 0.3589 | 0.0002 | 0.0006 | 0.0053 |
| Zn | 0.1300 | 0.1290 | 0.0002 | 0.0006 | 0.0039 |
| Mo | 0.0070 | 0.0067 | 0.00004 | 0.0001 | 0.0004 |
Conclusion
The results presented here demonstrate the ability of the Rigaku NEX QC II+ to measure additive elements in new, fresh lubricating oil formulations according to ASTM D7751 using empirical calibration. The NEX QC II+ features a simple, user-friendly touchscreen interface and requires no external PC, helping simplify operator training and routine analysis. Minimal maintenance needs also help reduce routine equipment maintenance.
