ANALYSIS OF IN-SERVICE OILS FOLLOWING ASTM D5185 WITH ICP-OES/AES
Globally, heavy machinery is used in a variety of areas, including construction and mining. As the size, complexity, and cost of the equipment increases, breakdowns can be costly, both in equipment repair and lost revenue. Therefore, preventive maintenance is paramount, but taking equipment off-line for unneeded maintenance can be just as costly. As a result, a diagnostic must be used to determine the optimum time to perform maintenance, both to maximise up-time and minimise breakdowns and repairs.
Engine oil is a key component for keeping heavy machinery up and running and serves as a diagnostic of the health of an engine. While many aspects of in-service oils need to be monitored, the metal content serves as a key indicator of the engine’s condition. While sudden spikes in the concentration of a certain metal can indicate the imminent failure of a component, it is more important to monitor the trends of the metal content in the oil over time, as increasing metal levels in the oil indicate when maintenance is needed. Because of the importance of using in-service oil analysis as a diagnostic for heavy equipment maintenance, ASTM created a method specifi cally addressing this analysis: D51851
.
When monitoring wear metals in oils, typical concentrations which serve as diagnostics are greater than 1 ppm, making ICP-OES the preferred analytical technique (as specifi ed in D5185) due its speed, matrix tolerance, and ability to easily and accurately measure these concentrations. To ensure high quality data, method D5185 contains stringent criteria, although, through years of implementation, labs have developed their own criteria, facilitating the analysis of a large number of samples while still maintaining high-quality data. Table 1 shows several parameters specifi ed in D5185 along with their common implementations.
Labs may analyse anywhere from less than 50 to hundreds of in-service oil samples per day. Although simultaneous ICP-
Table 1. Parameters of ASTM D5185 and Common Implementation
Parameter Elements
Sample Preparation Internal Standard QC Frequency QC Limits
Specifi ed in D5185
Al, Ba, B, Ca, Cr, Cu, Fe, Pb, Mg, Mn, Mo, Ni, P, K, Na, Si, Ag, S, Sn, Ti, V, Zn
By weight Cd, Co, or Y
Every 5 samples + 5%
Common Implementation
Elements important to the components being tested
By volume Co
Every 10-25 samples + 10%
OES instruments are ideal for high throughput labs, labs with lower daily sample requirements can opt for lower cost, slower simultaneous or hybrid scanning ICP-OES instruments to meet their requirements. This study will focus on the analysis of in-service oils using both a truly simultaneous ICP-OES with enhanced sample introduction capabilities to maximise sample throughput and a hybrid scanning ICP-OES with conventional sample introduction, a more cost-effective solution for labs with lower sample requirements.
Experimental Samples and Sample Preparation
In-service oil samples were prepared by diluting 10x with V-Solv (a modifi ed version of kerosene, with the lower molecular weight compounds removed), spiked with cobalt (Co) at 40 ppm. The Co serves as an internal standard, and spiking into the diluent is the fastest, most accurate way to add it to all standards and samples.
Quantitative measurements were made against external calibration curves consisting of a 75 cSt base oil blank and three V-23 oil stock solutions at 50, 100, and 500 ppm, along with a metals additive oil standard containing Ca at 5000 ppm and Mg, P, and Zn at
1600 ppm each. A QC standard consisting of 50 ppm for all wear metals and the metal additive standard for the additive elements was analysed every 11 samples.
Instrument Conditions All analyses were performed on either a PerkinElmer Avio®
500
(simultaneous) or Avio 200 (hybrid scanning) ICP-OES, using the conditions and parameters in Table 2. The position of the plasma and carbon Swan bands within the plasma is critical to prevent carbon deposition on the injector, so nebuliser fl ow was adjusted accordingly. The read time range is shorter on the Avio 200 due to its enhanced sensitivity.
To maximise sample throughput on the simultaneous system, a CETAC ASX-1400 autosampler was used in conjunction with a CETAC ASXpress. The autosampler stirs the diluted samples just before analysis, while the ASXpress increases sample throughput through the incorporation of a valve-and-loop. Although the ASXpress can also be used on the hybrid scanning system, it was not used in this work to better mimic what a low-volume lab may use. Instead, rapid pumping between samples was used.
Results and Discussion
With the instrumental parameters in Table 2, the sample-to-sample time for a hybrid scanning ICP-OES is 5 minutes. For a low-volume, in-service oil lab running up to 50 samples a day, this analytical speed is all that is required.
However, many labs following ASTM D5185 are high-throughput environments requiring maximum sample throughput. With a simultaneous ICP-OES (Avio 500), sample uptake and washout require more time than the actual measurement time. These times were minimised with the ASXpress, which uses a loop and vacuum
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