(Editor’s Note: This is an online-only article attributed to the December/January 2019 issue.)
Food contamination is a serious problem on a global scale. Monitoring the presence of persistent organic pollutants, such as dioxins, is crucial to reduce risk to human health and maximize food safety. Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are a common contaminant and have been shown to exert a number of toxic responses, causing reproductive and developmental problems, immune system damage, and hormone interference, and can be carcinogenic. Detection and quantification of dioxins in feed and food supplies is of vital importance, as more than 90 percent of human exposure to dioxins is through food, in particular meat and dairy products, and fish and shellfish.
Prevention and Control of Contamination
Many national authorities have programs in place to monitor and protect the food supply. The World Health Organization (WHO) has carried out human-based risk assessments aimed at evaluating the potential health risk and prevention and control of environmental exposure to these pollutants, and has set toxic equivalency factors. Dioxins are restricted internationally under the Stockholm Convention. The European Union has also made efforts to limit human exposure, and has set maximum levels in feed and food.
These toxins are formed when organic compounds are incinerated in the presence of chlorine and accumulate in the food chain, mainly in the fatty tissue of animals. They aren’t intentionally produced, and are formed as a by-product of industrial-chemical processes such as chemical manufacture and thermal processes. Due to the bio-accumulative nature of these compounds, it is essential to monitor them at ultra-trace levels in food and environmental samples in order to achieve worldwide regulatory compliance.
Food Safety at IQSTAP
Scientists at the Institute of Quality Standard and Testing Technology for Agro Products (IQSTAP) in Beijing, China, are working to advance the analysis of dioxins by developing new methods for the detection and quantification of the most toxic PCDDs and PCDFs in feed and food samples. They analyze feed and agricultural products as well as environmental samples that can affect these products.
However, this is a challenge that comes with several complications, including the need for high sensitivity and accuracy, as well as matrix interference. The uniqueness of dioxin analysis means that each dioxin laboratory is faced with the complexity of the sample matrix, the effective purification of the sample, and the quasi-deterministic quantitative problem of the target compound.
IQSTAP uses what is considered to be the international gold standard method for detecting PCDDs and PCDFs—isotope dilution high-resolution gas chromatography (GC) coupled with high-resolution magnetic mass spectrometry (MS). The European Commission established that GC-MS/MS may be used as a confirmatory method for PCDD and PCDF detection.
Putting APGC to the Test
The stricter regulatory oversight on contaminants in food has resulted in the need for greater sensitivity from analytical techniques like GC-MS. Additionally, reducing the injected volume of samples has been shown to minimize matrix effects and the contamination on instrumentation—two factors that directly affect dioxin analysis.
As a result, IQSTAP developed a GC-MS/MS with atmospheric pressure gas chromatography (APGC) technique to analyze 17 of the most toxic PCDDs and PCDFs. APGC-MS/MS was found to be a very sensitive detection system for the accurate determination of dioxins and furans at regulatory levels. Additionally, IQSTAP determined that using APGC provides numerous advantages, in comparison to traditional dioxin analysis methodology.
The sensitivity of the technique allows detection of contaminant limits at ultra-trace levels in the most complex samples, achieving compliance with regulatory limits on the presence and quantity of toxic PCDDs and PCDFs. In addition, because IQSTAP scientists could inject less sample matrix, the APGC technique reduced the effects of contamination on the instrumentation—and therefore increased uptime.
The technique also reduced the cost of tests for contaminants due to its improved sensitivity and selectivity—using only half the materials needed with the previously used techniques and analyzing them at a lower concentration. The ability to eliminate the matrix effect also reduced the need for time-consuming purification steps, helping IQSTAP to provide faster results for time-sensitive analyses.
Overcoming the Limitations of Electron Ionization
Traditionally, electron ionization (EI) based systems have been used for the determination of dioxins and other persistent organic pollutants, this technique however, produces significant amounts of fragment ions. But some of the compounds of interest for IQSTAP scientists were not suitable for ‘hard’ ionization analysis, such as EI. One of the key limitations to this technique is selectivity, as it gives rise to identical ions for different compounds. This technique can make it challenging to see the whole molecule, for example, it can result in the same fragment for compounds with different numbers of bromine atoms.
However, APGC results in a “soft” ionization process where the molecule is ionized by either proton transfer or charge transfer, rather than by direct electron bombardment. This soft ionization with APGC was extremely beneficial for the IQSTAP team’s work because of the reduced fragmentation for many compounds. This can result in higher sensitivity and specificity, therefore simplifying precursor ion selection in MS/MS analyses. The APGC soft ionization method is suitable for the analysis of many easily degradable compounds. That’s particularly helpful with dioxin analysis, where EI has significant limitations.
The IQSTAP scientists also found the analysis of samples by APGC allows for improved selectivity when generating multiple reaction monitoring transitions in comparison to the significant fragmentation experienced with EI gas chromatography. Operating the GC system at atmospheric pressure provides increased scope for ionization mode optimization—namely charge and proton transfer.
The technique has ability to have both GC and LC coupled to the same mass spectrometer, and has a relatively quick and simple changeover from LC to GC and vice versa—which is important since many analyses cross the boundary between LC and GC.
Shaping the Future of Food Safety
Continuous changes in research into dioxins drive the need for ever-lower limits of detection and quantification. APGC has proved to provide the speed, selectivity, and sensitivity needed for dioxin analysis. Researchers at IQSTAP concluded that their results with APGC are encouraging for food safety, and plan to continue to improve the analysis of other persistent organic pollutants using APGC and subsequent compliance in dioxin analysis.
Dr. Li is an assistant professor IQSTAP. Reach her at firstname.lastname@example.org.