Impurities Analysis in Sunflower Oil by HR ICP-OES Application Note · PlasmaQuant 9100 Elite

Introduction

Sunflower oil is a vegetable oil that is commonly used as main component or supplement in food, such as frying oil, salad dressing, baby food, margarine, or mayonnaise. Furthermore, it is used as an additive in cosmetics, biodiesel, varnish, textiles, and pharmaceuticals. Sunflower oil is among the most widely produced vegetable oils and is obtained from the seeds of the sunflower plant. The production includes the initial oil extraction by pressing (cold or hot) and refining processes such as solvent extraction, degumming, bleaching, and dewaxing. Refining sunflower oil increases its heat stability for frying and cooking purposes but is known to remove nutritional components (fatty acids, phospholipids, etc.), resulting in lower nutritional value and different sensory properties, such as color and flavor of the final product. Refining procedures such as transesterification or hardening aim to alter other properties, such as melting behavior, and involve the use of catalysts.

As a major constituent of food, sunflower oil is subject to many regulations concerning food safety, for example, decree 1881/2006 of the European Union, which defines limits for toxic heavy metals (e.g., Pb, Cd, Hg) in foodstuffs. On this basis, trace impurities originating from cultivation farming (fertilizers, air pollution), oil extraction, and refining processes (catalysts) need to be analyzed for purposes of food safety. At the same time, nutritional properties need to be ensured by monitoring the contents of macro minerals like phosphorus, calcium, or magnesium.

Due to its high viscosity and potential crystallinity, sunflower oil cannot be analyzed neatly for its elemental contents. Potential methods for sample preparation are mineralization by acidic closed vessel digestion or direct dilution in a suitable organic solvent. For closed vessel digestion, only small amounts of sample can be used, which is due to the high pressures during the digestion process originating from the high carbon content of the sample. Eventually, the resulting high dilution factors compromise the limits of detection, which is a limitation for trace elemental analysis. The approach of direct dilution in an organic solvent, such as kerosene, requires only low dilution factors and therefore enables sub-ppb limits of detection in challenging sample matrices, such as sunflower oil. The fully organic sample matrix of kerosene/sunflower oil, however, requires a highly robust plasma system as well as the highest spectral resolution in order to resolve analyte lines from the extensive unspecific background signal originating from kerosene.

The methodology presented here uses a fivefold dilution of sunflower oil in kerosene that is to be investigated for arsenic, calcium, cadmium, mercury, magnesium, phosphorous, and lead on a high-resolution ICP-OES, the PlasmaQuant 9100 Elite, that is equipped with an organic sample introduction kit. The exceptionally high resolution in combination with the powerful software tool CSI (correction of spectral interferences) allows for interferencefree analysis of traces of heavy metals in complex spectra featuring a high unspecific background. Furthermore, the dedicated introduction kit for organic samples in combination with the robust plasma of the HF Generator makes it possible to run 100% organic solvents with high precision and robustness.

In addition, the routine analysis of sunflower oil products largely benefit from the quality and ease of the baseline fit (ABC - automatic baseline correction), which improve precision and single-digit ppb detection limits for all elemental impurities.