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APPLICATION NOTE 43158
Lubricating oil analysis according to ASTM D5185
using the Thermo Scientific iCAP 7400 ICP-OES
Authors
Introduction
Nora Bartsch,
Application Specialist,
Thermo Fisher Scientific,
Bremen, Germany
Analysis of used lubricating oil for wear metals, contaminants and additive
elements is a valuable diagnostic tool to schedule preventative maintenance
of engines and machinery. The cost of unscheduled maintenance can be
high, not only in materials and labor, but also in lost profits due to down
time. Once the oil has been sampled, analysis by ICP-OES is very useful
for aiding with maintenance scheduling, basing decisions on the results of
analysis. ICP-OES is an ideal technique due to its high temperature source
which dissociates any organometallic compounds such as zinc dialkyldithiophosphates, an additive used as anti-wear in motor oil, and also has the
ability to handle difficult organic solvent matrices. This allows the oil to be
directly aspirated into the instrument after a simple dilution, negating the need
for any time consuming digestion sample preparations and consequently
enabling faster turnaround times.
Keywords
Additives, Contaminants,
Used oil, Wear metals
Goal
This application note describes
the analysis of oils in accordance
with standard test method ASTM
D5185 for “Determination of Additive
Elements, Wear Metals, and
Contaminants in Used Lubricating
Oils and Determination of Selected
Elements in Base Oils by ICP-OES”.
The Thermo Scientific iCAP 7400
ICP-OES Radial used for the analysis
combines high matrix tolerance and
reduced matrix-based interferences
with a simple and efficient sample
introduction design for optimized
analyses.
Standard method ASTM D5185
Scope
The standard method ASTM D5185 is for “Determination
of Additive Elements, Wear Metals, and Contaminants
in Used Lubricating Oils and Determination of Selected
Elements in Base Oils by ICP-OES”. A total of
22 elements can be determined by this test method
and it is generally used as a rapid screening method
to monitor the condition of the equipment using the
oil and to define when preventative action is needed.
The metallic analytes must be oil soluble for accurate
quantification. The quantification of insoluble particles
such as small particles (greater than a few micrometers)
of metal dislodged from a mechanical part is not possible
when using this method and any attempt to do so will
result in low recoveries. This is due to the plasma not fully
atomizing larger particles. It should also be highlighted
that obtaining a representative sample would be difficult
in such cases.
Summary of test method
An aliquot of a homogenized sample is diluted by weight
with a suitable solvent (mixed xylenes). Standards are
prepared in the same manner. The concentration of
metals within a sample is then determined by direct
analysis using ICP-OES.
Instrumentation
The Thermo Scientific™ iCAP™ 7400 ICP-OES Radial
was chosen for the analysis. The radial instrument
configuration was selected for its high matrix tolerance
and reduced matrix interferences. The iCAP 7400
ICP-OES is fully compatible with the Teledyne CETAC
ASX-1400 Stirring autosampler which ensures good
homogeneity of the solutions analyzed.
Method development
Reagents
The following reagents and standards were used in this
work: Xylene (Fisher Scientific, Loughborough, UK);
Conostan® base oil (Conostan® SCP SCIENCE,
Baie- D’Urfé, Canada); S21 Conostan oil-based standard
900 mg·kg-1 (Ag, Al, B, Ba, Ca, Cd, Cr, Cu, Fe, Mg, Mn,
Mo, Na, Ni, P, Pb, Si, Sn, Ti, V, Zn); Conostan oil-based
standard 5000 mg·kg-1 S; Conostan oil-based standard
5000 mg·kg-1 Y.
The following standard reference materials (SRM) and
samples were analyzed: wear metals in lubricating oil
NIST SRM 1084a (approximate concentrations
100 mg·kg-1); wear metals in lubricating oil NIST SRM
1085b (approximate concentrations 300 mg·kg-1);
Quartz 7000 oil (Total); Quartz 7000 oil (Total) 200 hours
use; Quartz 7000 oil (Total) 400 hours use.
Sample and standard preparation
Prior to any sample or stock standard being diluted, it
was homogenized by sonication. For very viscous oils,
the sample can be pre-heated to 60 °C.
The yttrium oil-based standard was diluted (by weight)
in xylene to give a final concentration of 10 mg·kg-1. This
solution was used for all dilutions, the yttrium being used
as an internal standard. For all samples and standards,
the final solution contained 10% oil (by weight) to ensure
that differences in viscosity were minimized. To achieve