Oxidation and Nitration in Used Oil Analysis

Both oxidation and nitration can be determined using the Fourier Transform Infrared (FTIR) method, which is also used to determine water content as discussed in a previous article. 

As you can see from the FTIR scan below both oxidation and nitration give individual peaks from which their level can be determined. There are various FTIR methods to produce this scan, some of which require the laboratory to have a sample of the unused oil for comparison purposes. 

If your laboratory uses this comparison FTIR method it is very important that the laboratory knows the correct grade of oil, as comparing the in-service oil to a different grade will produce incorrect results.

Oxidation & Nitration


Oxidation occurs when oil combines with oxygen. Under certain conditions this creates a wide variety of oxidation by-products. 

Once oxidation is initiated a chain reaction occurs which can rapidly destroy an oil’s usefulness.

Causes of oxidation:

  • Overheating
  • Over extended oil drains
  • Improper oil types 
  • Excessive blow-by
  • High Sulphur Level

Oxidation products can:

  • Form lacquer deposits
  • Corrode metal parts
  • Thicken oil beyond its ability to lubricate
  • Cause accelerated wear


Nitration by-products are formed during the fuel combustion process and most  are formed when an excess of oxygen is present.

Excessive Nitration can be caused by:

  • Heavy engine loads
  • Lean air/fuel ratio
  • Improper ignition timing
  • Improper ring sealing
  • Long drain intervals
  • Low temperature (Stop & Go operation)

Nitration by-products are highly acidic and can form deposits and varnish. They can also rapidly accelerate oxidation.

So when should you change the oil?

As in the case of all tests you should consult your oil supplier and/or testing laboratory to ascertain the appropriate alert levels, as these would have been determined from their technical experience and historical data.

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