Regulatory compliance is at the heart of environmental analyses throughout the world. While some regulatory systems provide more flexibility in the methodology used for measuring contaminants, the goal is to gain reliable information that can be used for decision-making. Oil and grease measurement is a simple and important measurement of water quality, but time consuming1. Methods such as US EPA 1664 (A and B) or ISO 11349 are used around the world to extract oil and grease from water samples and gravimetrically measure the amount present after the solvent is eliminated2. As laboratories around the world, in commercial and government sectors seek to implement simpler and more cost effective technologies to gain efficiency in the determination of total hexane extractable material (HEM) or silica gel treated n-hexane extractable material (SGT-HEM) (non-polar) material from aqueous wastewater samples there are some key aspects to consider. And while liquid-liquid extraction (LLE) remains a widely accepted technique, it is cumbersome and prone to numerous limitations and pitfalls due to the complex nature of wastewater sample matrices. The goal for most labs seeking to automate this procedure remains consistent regardless of geographical region, an automated method consistent with regulatory requirements, meeting or exceeding the quality control of existing methods and offering lower cost and faster turnaround time.
First some facts about US EPA Method 1664 and a review of recent advancements…
- As outlined in the introduction of the method…The US EPA withdrew all oil and grease methods using chlorofluorocarbon-113 (CFC-113 & Freon-113) as acceptable extraction solvents in the final rule published March 7, 2007. As such EPA Method 1664, Revision B (Feb. 2010): n-hexane extractable material (HEM; oil and grease) and silica-gel treated n-hexane extractable material (SGT-HEM; non-polar material) is the only US EPA approved method for the measurement of oil and grease in wastewaters.
- Method 1664B is a performance-oased method applicable to the measurement of the method defined parameters HEM and SGT-HEM from aqueous matrices and requires the use of n-hexane as the extraction solvent, gravimetry, as the determinative technique and includes additional QC procedures designed to monitor precision and accuracy.
- The 'B' modifications promulgated in February 2010 were designed to provide additional information for labs to improve performance and more importantly define modifications that are now allowed for nationwide use without prior EPA review (40 CFR 136.6) and to also define dearly any modifications that are not allowed...These 'modifications' are found in section 1.7 of the method and take into account advances occurring in analytical technology, provide options to improve separations and consider the reduction of overall costs for measurements.
- Options now included: Continuous Liquid-Liquid Extraction (CLLE) and Solid Phase Extraction (SPE) as accepted separation techniques as well as Kudema-Danish as an alternative concentration technique.
The method also specifies that:
- 'Alternate determinative techniques, such as immunoassay and infrared spectroscopy, and changes that degrade method performance or change the chemistry of the method including the use of extraction solvents other than n-hexane (85% minimum purity, 99% min. saturated C6 isomer, residue less than 1 mg/L) are not allowed.'
Solid phase extraction was incorporated into method 1664 for a variety of reasons including:
- Less solvent is used, reducing waste disposal and improving technician safety
- Emulsions do not form, reducing variability and speeding completion of the extraction step
EPA method 1664 B allows the use of a co-eluting solvent such as methanol so long as this solvent is immediately discarded and not collected. The methanol step is essential in order to maximize the recoveries of HEM and SGT-HEM using n-hexane. The reason for this is that residual water tends to encompass oil and grease molecules not allowing all of the oil and grease molecules to be extracted by the hexane solvent. Figure 1, steps 1 through 4 shows schematically what is occurring at a molecular level.