Fatty Acid Methods and Measurements

The Analytical Chemistry Division is working to develop natural-matrix and solution-based SRMs that are characterized for fatty acid composition. The natural-matrix SRMs are intended primarily for use as control materials, but other uses include method validation, the development of new analytical methods, and use as a component in achieving traceability of measurements. Solution SRMs can be used for calibration, as spiking solutions for fortification of samples, in studying extraction recoveries, for developing chromatographic separations, and as authentic standards for identifying constituents in more complex-matrix extracts.

Efforts at NIST to provide SRMs for fatty acids began in the mid-1990s after Congress passed the Nutrition Labeling and Education Act (NLEA), which mandated specifications for the labeling of processed foods. Among the requirements of this law, the fat content, calories due to fat, and saturated and unsaturated fatty acid content must be provided on the nutritional label. Reference materials with fatty acid data are needed to underpin the measurements of chemical composition upon which the labeling is based. The use of SRMs to support the measurement of fatty acids is relevant to a broad spectrum of applications including compliance with legislation and accuracy of nutritional labeling, food manufacturing tolerances (e.g., infant formulas), traceability of measurements for food exports, clinical nutritional measurements, and characterization of alternative energy sources (e.g., biodiesel).

Fatty acids are typically present in plant and animal tissues as triglycerides, which are saponified (hydrolyzed) and converted to fatty acid methyl esters (FAMEs) for analysis by gas chromatography. When this conversion is required, the fatty acids are reported as the triglycerides. Free fatty acids do not require saponification, but do require esterication prior to analysis by gas chromatography. For value assignment of fatty acid levels in SRMs, two independent methods are utilized. Multiple deuterated fatty acids are added to the samples for use as internal standards. If necessary, the samples are extracted and the triglycerides are converted to free fatty acids through saponification. The derivatized fatty acid mixtures are analyzed by either gas chromatography with flame ionization detection (GC-FID) or by GC with mass spectrometric detection (MS). 

Biodiesel materials represent a special case since the materials that are produced from plant or animal fats are converted to FAMEs during manufacturing. The analysis of biodiesel SRMs is simplified, since the materials can be spiked with the internal standards and injected directly without further processing.

Major Accomplishments:

• Twenty-six SRMs have been issued with certified, reference, or information values for fatty acids, including various oils and biodiesel, peanut butter, and baking chocolate; two SRMs have been issued with values for fat; and eight additional SRMs are under development, with values expected for fatty acids. 
• Multiple analytical methods have been developed for the measurement of fatty acids in plant, animal, and mixed-source materials.