Analyzing Lubricant Additive Elements Under Ambient Air

Background

ARL QUANT’X EDXRF spectrometer

Wear and heat between constantly moving and contacting surfaces inside precision-engineered equipment is reduced using high quality lubricants. Today’s lubricants are formulated far beyond simple petroleum base oils through a complex range of specialty organometallic additives, and they considerably enhance the performance and lifespan of the equipment by also preventing rust, reducing oxidation and sealing moving parts against contaminants such as dirt, dust and water.

Fast, repeatable and accurate measurement of additive metallic and other elemental constituents from parts-per-million to percentage levels is needed to control the quality and blending operations of high-performance lubricants. High-end energy dispersive X-ray Fluorescence (EDXRF) analysis is an established, easy-to-run and cost effective technique for lubricant quality control applications, giving simultaneous analysis of many elements ranging from low to high concentrations and requiring almost no sample preparation. Analysis of common lubricant additive components such as calcium (Ca), phosphorous (P), barium (Ba), and zinc (Zn) in an air atmosphere is focused on in this study. This article especially reports on the attainable detection limits.

Instrument

The ARL QUANT’X EDXRF spectrometer used for this application is provided with the new generation of Silicon Drift Detector (SDD) and a 50 kV, 50 W silver target X-ray tube. The sample is excited by primary filtered radiation used by the ARL QUANT’X spectrometer. The peak-to-background for elements from F to Am, optimized by a set of nine specially designed filters, ensures that the ARL QUANT’X spectrometer can be easily adapted per application or element range.

Sample Preparation

Lubricant measurement is done by moving 3 grams of product into a 4-micron polypropylene film sealed sample cup of 32 mm outer diameter.

Excitation Conditions

The excitation condition to carry out the analysis is shown in Table 1. The most optimal condition for every element is created using four separate filters. For every condition, a live time of 100 seconds is used. A typical spectrum acquired using condition Mid Zc of a lubricant sample containing the elements of interest (P, Zn, Ca and Ba) at 90 ppm is shown in Figure 1. Zinc is excited by optimizing condition Mid Zc.

Calibration

Linear calibration curves linking net intensities to concentrations are established using standards prepared using a Conostan AM 900ppm standard, diluted using 75 cSt blank oil (also from Conostan). To set up the curves and to find the detection limit, three concentration levels 23 ppm, 45 ppm, 90 ppm and a blank are prepared. The calibration curves acquired for phosphorous (P) and zinc (Zn) are shown in Figures 2a and 2b, respectively. Root Mean Square Errors (RMSE) of 1.2 ppm (P) and 0.27 ppm (Zn) are achieved. The RMSE values obtained for the other elements of interest are also shown in Table 2.

Validation and Repeatability

Five more XRF cups are filled with 3 grams of the 23 ppm standard and measured to get a repeatability value at 23 ppm concentration. Table 3 shows the results. The relative error is lower than 5% besides phosphorous, which has a 23 ppm concentration close to the limit of determination.

Limit of Detection

Ten XRF cups are filled with 3 grams of the blank oil to find the Limit of Detection (LoD). The LoD is measured to be three times the standard deviation for each element. The results are shown in Table 2.

Conclusion

Accurate, fast and repeatable control of finished lubricant product quality is provided due to these distinct capabilities of the ARL QUANT’X EDXRF spectrometer. Particularly valuable and convenient is the ability of the instrument to measure lubricant samples under ambient air offering savings on costly helium gas consumption.

Table 1. Excitation condition used for additive elements in lubricants.

Condition Voltage (kV) Current (mA) Atmosphere Live Time (s) Analytes
Low Za ii 10 Auto Air 100 P
Low Zb 10 Auto Air 100 Ca
Mid Za 18 Auto Air 100 Ba
Mid Zc 30 Auto Air 100 Zn

Table 2. Concentration range, RMSE and LoD values for additive elements in lubricants.

P Ca Zn Ba
Line
Concentration range [ppm] 0 - 90 0 - 90 0 - 90 0 - 90
RMSE [ppm] 1.2 0.14 0.27 1.3
LoD, 100s live time [ppm] 11 0.9 0.2 3

Typical spectrum for Zn at 90 ppm in lubricant oil using condition Mid Zc in an air atmosphere.

Figure 1. Typical spectrum for Zn at 90 ppm in lubricant oil using condition Mid Zc in an air atmosphere.

Table 3. Repeatability values for additive elements in lubricants.

P
ppm
Ca
ppm
Zn
ppm
Ba
ppm
AM 23 ppm R 1 17.0 24.0 22.5 26.1
AM 23 ppm R 2 20.0 24.0 22.9 26.3
AM 23 ppm R 3 17.5 24.0 22.9 27.5
AM 23 ppm R 4 18.7 23.5 22.8 25.2
AM 23 ppm R 5 17.2 23.4 22.6 25.1
Average 18.1 23.8 22.7 26.0
Std. Dev. 1.2 0.3 0.2 1.0
Rel. Error (%) 6.9 1.3 0.8 3.7

Calculated versus given concentrations in the case of phosphorous (P).

Figure 2a. Calculated versus given concentrations in the case of phosphorous (P).

Calculated versus given concentrations in the case of zinc (Zn).

Figure 2b. Calculated versus given concentrations in the case of zinc (Zn).

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific - Elemental Analyzers.

For more information on this source, please visit Thermo Fisher Scientific - Elemental Analyzers.

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