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What is a lubricant?

A lubricant is defined as a petroleum-derived product used to control and reduce the friction and wear of moving machinery parts (e.g., in engines and turbines). The main purpose of lubricants  is to help protect and prolong the lifetime of the equipment.

Machinery and lubricants go hand in hand, as shown here.
Machinery and lubricants go hand in hand, as shown here.

These goals are accomplished in the following ways:

Lubrication by reducing friction and wear. The lubricant forms a film between the mechanical moving parts of the equipment. In this way the metal-to-metal contact and, thus, the wear is reduced.

Cooling by acting as a heat sink. This causes the heat to dissipate away from critical parts of the equipment so that deformation due to increased temperature is prevented.

Protection by building a film. This film is unaffected by oxygen or corrosive substances and therefore prevents metal damage and oxidation (rust) and therefore also prevents wear.

Types of lubricants

For the most part, lubricants consist of oils to which additives and other chemical substances are added. There are two common types of lubricants which are based on the origin of the oil:

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1. Lubricants based on mineral oils (Figure 1a) are the most commonly used type.

They are comprised of petroleum products (base stock) to which synthetic additives are added. These types of lubricants are used in applications where there are no high temperature requirements. Typical areas where mineral oil-based lubricants are used include: engines, hydraulics, gears, and bearings.

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b) Figure 1. Difference in the molecular structure found in lubricants: a) mineral oil and b) synthetic oil.

2. Lubricants based on synthetic oils (Figure 1b) are artificially developed substitutes for mineral oils.

They are less common and more expensive. Synthetic oils are specifically developed to create lubricants with superior properties to mineral oils. For example, heat-resistant synthetic oils are used in high performance machinery operating at high temperatures.



In the following table, different lubricant types with sub-classes are listed.

The physical properties of a lubricant (such as viscosity and density) mostly depend on the oil base stock, whereas the additives fine-tune the chemical properties, e.g., the acid number or base number. For each application, the oil is typically formulated to meet the physical and chemical properties required by the customer. Therefore, various types of oil exist (Table 1).

Table 1. Different lubricating oil types.

Lubricant type



Automotive oil

Engine oil

Gear oil

Transmission fluids

Industrial oil

Hydraulic oil

Turbine oil


Metal working fluids

Forming fluids

Cutting fluids

Near-infrared spectroscopy—an ASTM compliant tool to assess the quality of lubricants

Near-infrared spectroscopy (NIRS) has been an established method for fast and reliable quality control within the petrochemical industry for more than 30 years. However, many companies still do not consistently consider the implementation of NIRS in their QA/QC labs. The reasons could be either limited experience regarding application possibilities or a general hesitation about implementing new methods.

There are several advantages of using NIRS over other conventional analytical technologies. For one, NIRS is able to measure multiple parameters in just 30 seconds without any sample preparation! The non-invasive light matter interaction used by NIRS, influenced by physical as well as chemical sample properties, makes it an excellent method for the determination of both property types.

In the remainder of this post, available solutions for lubricants are discussed which have been developed according the NIRS implementation guidelines of ASTM E1655 (method development), ASTM D6122 (method validation), and ASTM D8340 (results validation).


Read our previous blog posts to learn more about NIRS as a secondary technique.

Benefits of NIR spectroscopy: Part 1

Benefits of NIR spectroscopy: Part 2

Benefits of NIR spectroscopy: Part 3

Benefits of NIR spectroscopy: Part 4

Applications and parameters for lubricant analysis with NIRS

The main NIRS application for lubricants is to easily monitor the oil condition, i.e., checking if the oil is still of suitable quality for proper lubrication of the equipment. Reducing unnecessary oil changes means significant cost savings. On the other hand, changing the oil too infrequently can result in possible damage of the equipment, leading to costly repairs. Therefore optimizing the usage of the lubricating oil is very important.

The following parameters can be correlated between NIRS and the values from a primary method: kinematic viscosityviscosity index, color, density, water content, TAN (total acid number), and TBN (total base number). A large set of samples provided by several different companies was used to develop working NIRS models of these parameters, including hydraulic oil, gear oil, and others. In some cases, it was not clear for what application the lubricant was used, so the exact identity of the oil was unknown.

The most relevant application notes for NIRS analysis of lubricants are listed below in Table 2.

Table 2. Metrohm’s NIRS solutions for lubricants including application details and benefits.
Parameter Reference method Norm NIRS Application Notes NIRS benefits
Acid number Titration ASTM D664







All parameters are measured simultaneously within a minute, without requiring any sample preparation or chemical reagents.

Kinematic viscosity at 40 °C Viscosimeter ASTM D445
Kinematic viscosity at
100 °C
Viscosimeter ASTM D445
Viscosity index Calculation ASTM D2270
Color number Colorimeter ASTM D1500
Moisture content Karl Fischer titration ASTM D6304
Base number
ASTM D2896
Density Density meter ASTM D4052

Solutions by means of starter models—expedite and simplify quality control of lubricants

Lubricants keep our modern lives running smoothly. During use, the oil needs to be monitored to check if it still of good enough quality or whether it needs to be exchanged.

The data obtained here indicate that lubricants vary per application and per supplier. This means that there is still not sufficient information for each oil type and subtype to prepare a model robust enough to transform into a pre-calibration. However, if a partner provides the samples, a feasibility study can quickly indicate if the NIR spectra are able to be correlated to the primary method values.

Typically, several key parameters such as the acid and base numbers (AN and BN), viscosity, moisture content, color, and density are determined in the laboratory by various chemical and physical methods. These methods not only incur high running costs, they are also quite time consuming to perform.

NIRS on the other hand requires neither chemicals nor sample preparation, and provides results in less than a minute. This spectroscopic technique is also easy enough to be used by non-chemists. Furthermore, multiple chemical and physical parameters can be determined simultaneously. The combined benefits of this technology make NIRS the ideal solution for many daily QA/QC measurements or ad-hoc atline analysis.

Application example: starter model for lubricants with the NIRS DS2500 Liquid Analyzer

For lubricant analysis, determination of the acid number (ASTM D664), viscosity (ASTM D445), moisture content (ASTM D6304), and color number (ASTM D1500) require the use of multiple analytical technologies and, in part, large volumes of chemicals. The time to result can therefore be quite a long and costly process.

In this example, different lubricant samples were measured with a Metrohm NIRS DS2500 Liquid Analyzer in transmission mode over the full wavelength range (400–2500 nm). The built-in temperature controlled sample chamber was set to 40 °C to provide a stable sample environment. For convenience reasons, disposable vials with a pathlength of 8 mm were used, which made a cleaning procedure obsolete.


Learn more about the possibilities of petrochemical analysis with Metrohm NIRS DS2500 Analyzers in our free brochure below.

DS2500 Analyzer – Boosting efficiency in the QC laboratory with Near-Infrared Spectroscopy (NIRS)

Figure 2. Quality control of lubricants as performed by the Metrohm NIRS DS2500 Liquid Analyzer.

The obtained Vis-NIR spectra (Figure 2) were used to create prediction models for the determination of key lubricant parameters (such as those in Table 2). The quality of the prediction models was evaluated using correlation diagrams, which display the correlation between the Vis-NIR prediction and primary method values. The respective figures of merit (FOM) display the expected precision of a prediction during routine analysis (Figure 3).

Figure 3. Correlation plots and figures of merit (FOM) for different parameters measured in lubricants.

This solution demonstrates that NIR spectroscopy is excellently suited for the analysis of multiple parameters in lubricants in less than one minute without sample preparation or using any chemical reagents.

In case a large series of samples must be analyzed, there is also the possibility to measure lubricant samples in a fully automated way, as detailed in our free Application Note below.

Quality Control of Lubricants – Unassisted, rapid determination of the Acid Number by automated NIR spectroscopy according to ASTM E1655


Here, the samples were measured in transmission mode over the full wavelength range (400–2500 nm) using a NIRS XDS RapidLiquid Analyzer in combination with an 815 Robotic USB Sample Processor, which can carry a total of 141 samples (Figure 4).

Figure 4. Metrohm NIRS XDS RapidLiquid Analyzer equipped with a with 5.0 mm flow cell (left) and the 815 Sample Processor (right).



Near-infrared spectroscopy is very well suited for lubricant analysis. Available starter models are developed and validated in accordance with the ASTM guidelines. Positive aspects of using NIRS as an alternative technology to primary methods are the short time to result (less than one minute), no chemicals or other expensive equipment needed, and ease of handling so that even shift workers and non-chemists can perform these analyses in a safe manner.

Other installments in this series

This blog article was dedicated to the topic of lubricants and how NIR spectroscopy can be used as the ideal QC tool for the petrochemical / refinery industry. Other installments are dedicated to:


Wim Guns

International Sales Support Spectroscopy
Metrohm International Headquarters, Herisau, Switzerland