Testing oils with ICP spectroscopy - Some hidden risks
On our last case file, we noted the controversy about detecting metals in oil, and how sample preparation was so important to the ICP (Inductively Coupled plasma spectroscopy test, the most widely used metals test in oil analysis programs today. What we did not stress was why that is the case. Let’s take a look at the way an oil sample is prepared the ICP spectrometer.
Agitation
An experienced lab will first agitate the sample thoroughly, and get any material that has settled to the bottom of the sample bottle during transit to be distributed as well as possible throughout the bottle.
Dilution
Next, a sample of the oil is pipetted to a test tube. This may be done manually or using an autodilutor system- the typical volume of oil selected is between 0.3 and 1.0 ml.
The next step is to dilute the sample with a suitable diluent. The usually ratio is 10:1, though that can also be as low as 5:1. The ICP system is set up to test a sample in the 9 to 14 ml volume total range.
Aerosol
The sample is racked and set up on the instrument. After calibration, the autosampler siphons the sample, (the volume is again somewhere between 9 and 14 ml) and sends it to the nebulizer, a device designed to convert the liquid sample to a aerosol spray, to be carried up the torch. 99% of the sample goes into the drain, only 1% of the aerosol is carried up! Finally the aerosol is energized to plasma state using high radio frequency (RF) and the argon flame.
Looking at the above steps, it should be obvious why very fine particles are detected most reliably with this method. For a larger particle (a 5micron diameter particle and greater) there are several “loss mechanisms” paths even before the sample is energized! These are:
- Failure to agitate the sample bottle prior to test
- Delay in pipetteing sample to test tube (larger particles settle quickly)
- Small volume of sample used relative to overall sample
- Diluent effects (lower viscosity causes even more rapid settling out of particles of interest
- Nebulizer ratio- 99% of that small diluted sample is sent to the waste drain! Large particles will be in that drain fluid- not in the spray.
- Finally, if in the off chance a large particle makes it to the flame, there is only a finite amount of energy available to energize it-if it is not enough, the particle may only partially energize or not at all- therefore no signal is detected.
So there you have it- there are multiple ways of missing larger particles. So to recap: If you ask for metals analysis as part of your program, you must insist on some additional test to look for those larger particles(Our recommendation). If you just want the dissolved metals trend, then pay the money to have acid digestion preparation.
“Do it right or not at all”-DPW
