Using the Avio 200 ICP-OES for Analysis of Oil Additives

Knowing and controlling the concentrations of certain elements while mixing base oils and additives for use as lubricants is important to achieve optimal performance and longer engine life. As this process is so crucial, ASTM has developed a technique to monitor this procedure - ASTM method D4951, which covers boron (B), barium (Ba), copper (Cu), calcium (Ca), molybdenum (Mo), magnesium (Mg), phosphorus (P), zinc (Zn), and sulfur (S).

However, all of these elements do not need to be measured all of the time – the additives and blends are specified to meet specific performance specifications, which can differ among oil types, based on their final use.

Oil blending facilities usually test 10 - 40 samples per day to support manufacturing. Due to the small number of elements, the low sample load, and fairly high concentrations, analysis is frequently performed either by ICP-OES or X-ray, with each method having its own benefits and limitations.

This article explores how PerkinElmer’s Avio™ 200 ICP Optical Emission Spectrometer¹ (ICP-OES), which overcomes limitations of other X-ray analyses and ICP-OES systems and X-ray analyses, is used to analyze additives in new oils.

Experimental

Samples and Standards Preparation

New oil samples were acquired and diluted 10x by weight with V-Solv™ containing 40 ppm cobalt (Co) as an internal standard. Calibration curves were created from a 75 cSt base oil as a blank, two V-23 stock solutions at 500 and 100 ppm (PerkinElmer), and a Metals Additives Standard (MA4) (PerkinElmer) which contains Ca at 5000 ppm as well as Zn, Mg, and P at 1600 ppm.

The blank and all standards were made the same way as the samples. During sample analysis, the MA4 standard was used as the QC sample.

Instrument Conditions

All analyses were conducted on the Avio 200 ICP-OES operating in radial mode, using the conditions listed in Table 1 together with the analytes and wavelengths shown in Table 2. The nebulizer gas flow was modified so that the tip of the “green bullet” in the central channel was just below the top flat plate, as illustrated in Figure 1.

Table 1. Avio 200 ICP-OES instrumental parameters and conditions.

Table 2. Analytes and wavelengths.

Figure 1. Correct position of green carbon "bullet" just below the second plate of Avio’s unique Flat Plate plasma technology, when aspirating V-Solv.

Results and Discussion

Five new oil samples with varying formulations were tested, and the concentrations compared to those achieved from X-ray analysis using ASTM D6443. The results are shown in Table 3, indicating that both analyses delivered similar results, demonstrating agreement of the results with X-ray.

Additionally, the relative standard deviations (RSDs) for each measurement were normally 1% or less, with the exception of the low-level analytes (Mg in Oil 3 and 5), where the low intensities caused statistically slightly higher RSDs.

Compared to X-ray analysis, the Avio 200 ICP-OES provides a number of advantages including sample throughput, price, ease of use, and the ability to measure varied oil formulations with calibration standards that do not have to be matched to the composition of each oil.

A downside to X-ray analysis of additives in oils is that it suffers from interferences, requiring either physical or mathematical corrections, both of which demand running additional standards. However, the ICP-OES does not suffer from interferences for this analysis, resulting in faster, simpler, and more accurate measurements.

Another key difference between ICP-OES and X-ray is the ability to measure high-level samples. In X-ray methods, additional sample dilutions have to be done, while on the Avio 200 ICP-OES the same sample can be run in attenuation mode, which selectively curbs the signal of only the analyte of interest, without influencing other analytes.

Table 3. Results from five new oil samples with different formulations.

As a standard blending plant may run around 20 samples per day, a 20-sample stability run was conducted where each oil sample was run, followed by the QC sample (MA4) after every five samples. This sequence was repeated until each oil sample was analyzed four times.

All QC values were within ± 2%, with the first and last QC samples shown in Table 4. These results show the stability of the methodology, enabling a standard oil blending plant to easily conduct their daily analyses using the Avio 200 ICP-OES.

A number of instrumental design considerations of the Avio 200 ICP-OES contribute to its stability. First, the vertical torch means that all non-ionized sample will drain back down the torch instead of pooling in the injector, which causes faster upsurge of carbon on the injector and torch.

The shear gas cuts off the end of the plasma, preventing deposition on the axial window. Even though axial mode is not used in this analysis, this feature enables the Avio to be used for other analyses without having to worry about cone maintenance or deposition.

Finally, Avio’s unique Flat Plate™ plasma technology, which forms the plasma (Figure 1), is very robust and needs no cooling, which results in better stability. In addition, the flat plates reduce argon consumption: only 11.2 L/minute of total argon were used for these analyses.

Another significant use of the ICP for an oil blending plant is to verify other additives used in the oil formulation (such as dispersants, detergents, and viscosity improvers) for elemental contamination that could alter the additive levels in the end product.

Table 4. First and last QC check samples of a 20-sample analytical run.

Conclusion

This research has demonstrated the ability of the Avio 200 ICP-OES to analyze a number of lubricating oil samples for oil additives in compliance with ASTM method D4951. Instrumental design considerations in the Avio 200 ICP-OES enable stable and accurate analysis of various oil formulations against an external calibration curve, which do not have to be matched to the specific oil compositions.

Consumables Used

References

1. http://www.perkinelmer.com/product/avio-200-icp-optical-emission-spectrometer-avio200

This information has been sourced, reviewed and adapted from materials provided by PerkinElmer Inc.

For more information on this source, please visit PerkinElmer Inc.