Sample Preparation of Heavy Crude Oil v2
The digestion of heavy crude oil samples prior to trace metal analysis by inductively coupled plasma–optical emission spectrometry (ICP-OES) has traditionally been performed by dry ashing in a muffle furnace. The oil is burned off over a burner prior to ashing in a muffle furnace. The process takes several hours and requires continual operator attention. Loss of sample and cross contamination can occur due to foaming and splashing. Volatile analytes are lost and cannot be measured. While closed-vessel microwave digestion offers several advantages over dry ashing, a maximum sample weight of ~0.2 g is considered too small for representative analysis. Single Reaction Chamber (SRC) microwave digestion, however, can digest higher sample weights, enabling microwave digestion to be applied to the sample preparation of heavy oils.
Instrumentation
Milestone’s ultraWAVE SRC benchtop digestion system can digest up to 5 heavy oil samples at a time, at up to 300 oC and 199 bar pressure. Its high temperature and pressure capability enables the complete digestion of heavy oils, which makes sample introduction to the ICP-OES instrument easier and more routine. The ultraWAVE can digest 0.75 g of heavy oil (up to 5 samples simultaneously)—more than 3x higher sample weight than traditional closed-vessel digestion systems. Samples are simply digested in disposable glass vials; unlike dry ashing, there are no crucibles to clean.
Figure 1: ultraWAVE digestion program with 5 x 0.75 g heavy oil: Ramp to 250 oC over 30 min and hold for 10 min. Line T1 shows the actual digestion temperature achieved. The applied microwave power is automatically controlled by the system, so the actual digestion temperature precisely follows the programmed temperature profile. The outer temperature of the reaction chamber is also monitored (T2). Note that pressure (P) reached almost 100 bar during the run. This is due to the high weight and high organic content of the samples, which generates NOx and CO2. Pressure was released automatically at 55 min.
The system is fully automated: the sample is weighed into a vial, and 10 mL HNO3 is added and loaded into the ultraWAVE. Its walk away operation means labor costs are significantly reduced over dry ashing. The reaction chamber is pre-pressurized with N2, which eliminates cross contamination of loss of volatiles.
Figure 2: Sample rack being loaded and samples after digestion.
Conclusion
Sample digestion with the Milestone ultraWAVE takes around one hour including cool-down time. The digestion of heavy oils in the amounts described was sufficient for the application, with increased sample throughput and reduced labor costs compared to dry ashing. Risk of contamination and loss of volatile elements is eliminated, and the superior digestion quality allows for easier analysis by ICP-OES.
Milestone, Inc.
25 Controls Drive, Shelton, CT 06484
Tel. (203) 925-4240, toll free (866) 995-5100
Website: www.milestonesci.com
