Rapid Identification of Adulteration in Extra Virgin Olive Oil via Dynamic Headspace Sampling and High-Pressure Photoionization Time-of-Flight Mass Spectrometry

Discriminating extra virgin olive oils from common edible oils: Comparable performance of PLS-DA models trained on low-field and high-field 1H NMR data

INTRODUCTION

Olive oil, derived from the olive tree (Olea europaea L.), is used in cooking, cosmetics, and soap production. Due to its high value, some producers adulterate olive oil with cheaper edible oils or fraudulently mislabel oils as olive to increase profitability. Adulterated products can cause allergic reactions in sensitive individuals and can lack compounds which contribute to the perceived health benefits of olive oil, and its corresponding premium price.

OBJECTIVE

There is a need for robust methods to rapidly authenticate olive oils. By utilising machine learning models trained on the nuclear magnetic resonance (NMR) spectra of known olive oil and edible oils, samples can be classified as olive and authenticated. While high-field NMRs are commonly used for their superior resolution and sensitivity, they are generally prohibitively expensive to purchase and operate for routine screening purposes. Low-field benchtop NMR presents an affordable alternative.

METHODS

We compared the predictive performance of partial least squares discrimination analysis (PLS-DA) models trained on low-field 60 MHz benchtop proton (1H) NMR and high-field 400 MHz 1H NMR spectra. The data were acquired from a sample set consisting of 49 extra virgin olive oils (EVOOs) and 45 other edible oils.

RESULTS

We demonstrate that PLS-DA models trained on low-field NMR spectra are highly predictive when classifying EVOOs from other oils and perform comparably to those trained on high-field spectra. We demonstrated that variance was primarily driven by regions of the spectra arising from olefinic protons and ester protons from unsaturated fatty acids in models derived from data at both field strengths.

Hydrogen-Assisted Photoionization and Its Use in Promoting Mass Spectrometry Analysis of VOCs

As a simple soft ionization method, photoionization (PI) is often coupled with mass spectrometry (MS) for the direct analysis of volatile organic compounds (VOCs). PI enables selective ionization of analytes, but the ion yield is generally not high due to the limited light intensity of the ultraviolet lamp. Here, a hydrogen-assisted photoionization (HAPI) strategy was developed and integrated into a miniature ion trap mass spectrometer. In particular, hydrogen was introduced as a versatile buffer gas to facilitate both photoionization and ion trap operation. This can increase the ion yields by up to 2 orders of magnitude compared to conventional PI-MS, with a low hydrogen consumption (less than 100 μL) for each analysis. The generation of protonated ions indicates a specific photochemical process in HAPI, which has also been studied and initially revealed. The detection of various VOCs and plant volatile gases confirmed the versatility and practicality of the HAPI technology.

Detection of walnut oil adulterated with high-linoleic acid vegetable oils using triacylglycerol pseudotargeted method based on SFC-QTOF-MS

Authentication of walnut oil (WO) is challenging due to the adulteration of high-linoleic acid vegetable oils (HLOs) with similar fatty acid composition. To allow the discrimination of WO adulteration, a rapid, sensitive and stable scanning method based on supercritical fluid chromatography quadrupole time-of-flight mass spectrometry (SFC-QTOF-MS) was established to profile 59 potential triacylglycerol (TAGs) in HLOs samples within 10 min. Limit of quantitation of the proposed method is 0.002 µg mL^-1 and the relative standard deviations range from 0.7% to 12.0%. TAGs profiles of WO samples from various varieties, geography origins, ripeness, and processing methods were used to construct orthogonal partial least squares-discriminant analysis (OPLS-DA) and OPLS models that were highly accurate in both qualitative and quantitative prediction at adulteration levels as low as 5% (w/w). This study advances the TAGs analysis to characterize vegetable oils and holds promise as an efficient method for oil authentication.

Determination of oxidative deterioration in edible oils by high-pressure photoionization time-of-flight mass spectrometry

Since lipid oxidation often causes serious food safety issues worldwide, determination of oil's oxidative deterioration becomes quite significant, which still calls for efficient analytical methods. In this work, high-pressure photoionization time-of-flight mass spectrometry (HPPI-TOFMS) was firstly introduced for rapid detection of oxidative deterioration in edible oils. Through non-targeted qualitative analysis, oxidized oils with various oxidation levels were successfully discriminated for the first time by coupling HPPI-TOFMS with the orthogonal partial least squares discriminant analysis (OPLS-DA). Furthermore, by targeted interpretation of the HPPI-TOFMS mass spectra and the subsequent regression analysis (signal intensities vs TOTOX values), good linear correlations were observed for several predominant VOCs. Those specific VOCs were promising oxidation indicators, which would play important roles as TOTOX to judge the oxidation states of tested samples. The proposed HPPI-TOFMS methodology can be used as an innovative tool for accurate and effective assessment of lipid oxidation in edible oils.

Rapid Determination of Ethyl Carbamate in Chinese Liquor via a Direct Injection Mass Spectrometry with Time-Resolved Flash-Thermal-Vaporization and Acetone-Assisted High-Pressure Photoionization Strategy

Ethyl carbamate (EC), a carcinogenic compound, is naturally produced in fermented foods and alcoholic beverages. Rapid and accurate measurement of EC is necessary and important for quality control and safety evaluation of Chinese liquor, a traditionally distilled spirit with the highest consumption in China, but it remains a great challenge. In this work, a direct injection mass spectrometry (DIMS) with time-resolved flash-thermal-vaporization (TRFTV) and acetone-assisted high-pressure photoionization (HPPI) strategy has been developed. EC was rapidly separated from the main matrix components, ethyl acetate (EA) and ethanol, by the TRFTV sampling strategy due to the retention time difference of these three compounds with large boiling point differences on the inner wall of a poly(tetrafluoroethylene) (PTFE) tube. Therefore, the matrix effect of EA and ethanol was effectively eliminated. The acetone-assisted HPPI source was developed for efficient ionization of EC through a photoionization-induced proton transfer reaction between EC molecules and protonated acetone ions. The accurate quantitative analysis of EC in liquor was achieved by introducing an internal standard method (ISM) using deuterated EC (d₅-EC). As a result, the limit of detection (LOD) for EC was 8.88 μg/L with the analysis time of only 2 min, and the recoveries ranged from 92.3 to 113.1%. Finally, the prominent capability of the developed system was demonstrated by rapid determination of trace EC in Chinese liquors with different flavor types, exhibiting wide potential applications in online quality control and safety evaluation of not only Chinese liquors but also other liquor and alcoholic beverages.