Correct Sample Prep: Key to an Accurate Particle Count

Oil testingCrude oil, petroleum, petroleum products, additives and lubricants are routinely analyzed for their elemental content using a variety of analytical techniques. Some tests may require only simple dilutions while others involve a series of complex sample decomposition before any kind of analysis may take place.

Failure to follow good laboratory practices and defined sample prep instructions can lead to inaccurate results, an unnecessarily large reproducibility estimate or false outlier data. Standardizing procedures for consistent results can help ensure proper sampling including:

  • A contaminated collection vessel. Accuracy is compromised when the vessel used for sample collection is contaminated.
  • Improper agitation to suspend particles in the sample. There are differences in the time required and method used based on the kind of contamination, mass, fluid type and time period a sample has been static.
  • Inadequate de-gassing. Sometimes little or no de-gassing is done to remove entrapped air after proper agitation. De-gassing can be performed by vacuum or an ultrasonic bath.
  • Allowing a de-gassed and agitated sample to rest for an extended period. Myth: If sample sits for an extended period following agitation, it will de-gas on its own. Gravity works to settle particles causing the sample to appear cleaner than it is.
  • Use of a high-speed magnetic stirrer.  A little stirring is fine. Too much speed while stirring however, drives particles out of the suspension to the inside wall of the vessel, effectively removing those particles from the sampling path. A magnetic stirrer should not be used with sample fluids containing ferrous metals as these particles will attach to the stir bar and skew your results. 

A Basic Approach for Petroleum Fluid Preparation

  1. Collect the sample in a clean vessel with a clean cap. This is very important. After sampling, either discard or clean the collection bottle thoroughly before re-use.  
  2. Agitate the sample ultrasonically for one minute. Ultrasonic agitation will disintegrate agglomerated particles. The ultrasonic bath fluid level must be halfway up the sample bottle. One minute is sufficient for most sampling scenarios. If you’re working with a viscous fluid, extend the time based on the level of viscosity.
  3. ‘Hand-shake’ the sample for two minutes. If the sample is viscous or has been static for an extended period, add more time to your agitation step. ISO recommends a minimum of one minute on a mechanical shaker (ISO 11171 Annex E, Section E4, 2010).
  4. Degas the solution in an ultrasonic bath. For this step, the ultrasonic bath fluid level should reach halfway up the sample bottle. Degas for 25-35 seconds for hydraulic fluids with a viscosity range of 10-50 cSt. For more viscous fluids (e.g., > 50 cSt) extend the de-gassing time to remove all entrapped air.
  5. Allow the sample to stand for 5 to 10 seconds to ensure there are no visible bubbles.
  6. Immediately introduce the sample to the counter to prevent settling of larger particles.

Particle Counting systems such as the HIAC™ PODS+ and HIAC 8011+ apply pressure to move the sample through the flow control and counting elements. While this pressurization is not an alternative for degassing a sample, it provides a secondary method for removing residual air bubbles from the sample.

Perfecting your Particle Count

Stir bars may be used to help keep 50 µm or larger particles suspended when measuring large sample volumes. The stir bar must be particle-free and your stirrer speed slow enough to prevent a ‘centrifuge effect’, but rapid enough to suspend the contaminants to be measured. As with any standardized procedure, variables for dealing with larger sample volumes should be minimized.

Following the steps in the right sequence will help ensure an accurate particle count and a consistently successful sampling process.