Polybrominated diphenyl ethers (PBDEs) are persistent organic pollutants with recognized adverse health effects. Because they are ubiquitous in the environment and can pass into the food chain, routine safety testing is critical. However, quantifying trace levels of structurally similar PBDEs in complex background matrices can be challenging. Advances in gas chromatography-tandem mass spectrometry (GC-MS/MS) are now enabling accurate quantification of PBDEs, helping food safety laboratories meet regulatory requirements and safeguard consumer health.
Challenges of PBDE quantification using traditional GC/MS
PBDEs are brominated hydrocarbons widely used as flame retardants in plastics, textiles, and electronics. When added to polymeric materials, they are physically blended rather than chemically bonded to the polymer matrix, making it possible for them to leach out into the environment over time. Their lipophilic nature means that PBDEs can bioaccumulate in the food chain to reach potentially harmful levels. They have been associated with a range of health risks, including endocrine disruption, neurodevelopmental toxicity, hepatic dysfunction, and cancer.1
While PBDEs with known toxicity have been banned and listed under the Stockholm Convention Inventory of Persistent Organic Pollutants,2 their potential to bioaccumulate and widespread historical use mean that the risk of human exposure is expected to continue for some time. Monitoring of PBDE levels in food is therefore crucial to gain a better understanding of the health risks these chemicals pose3 and ensure regulatory compliance.
The analytical techniques used to detect these compounds must be able to distinguish and quantify multiple PBDEs. There is a total of 209 possible PBDE congeners, which are structurally similar but vary in the number and position of bromine atoms in the phenyl rings. Given the volatility and polarity of these compounds, GC/MS is the analytical method of choice for PBDE analysis.4
Several challenges must be overcome when applying traditional GC/MS workflows to PBDE analysis. Analytical techniques must be capable of distinguishing between molecules of similar size and mass. High sensitivity is also vital: PBDEs can be present in food at trace levels (ppt), and it is necessary to detect and quantify ultralow concentrations of these analytes in a range of complex background matrices. Moreover, the reliability of quantification can be challenging for higher-molecular-weight PBDEs, which are prone to thermal degradation during GC analysis. It is therefore important for methods to be optimized to minimize the risk of compound degradation and matrix interference and maximize sensitivity and selectivity, allowing accurate and reliable quantification.
PBDE workflows must also be able to maintain this sensitivity and selectivity over time without requiring extensive system maintenance. The sample matrices analyzed are typically complex, and matrix co-extractives can accumulate in the GC/MS instrument’s injector port and analytical column head. This can degrade performance and increase maintenance frequency, which can have a negative effect on throughput and cost of analysis. In addition, without regular maintenance, matrix accumulation within the column and instrument can result in decreased measurement accuracy and reproducibility, potentially resulting in the need to repeat analyses. Fortunately, recent improvements in technology and software allow GC/MS workflows to achieve reliable quantification of PBDEs, ensuring that regulatory requirements can be easily met.
Enhancing sensitivity and chromatographic resolution
The principle of gas chromatography is to separate chemical components in a sample by their relative affinities for a stationary (solid) and mobile (gas) phase. Injected samples are first vaporized and are then carried by an inert gas (usually helium) through a column containing the stationary phase, where they are separated due to the difference in the time it takes to elute. One of the challenges associated with PBDE analysis is the quantification of highly brominated PBDEs of high molecular weight, as they are prone to degradation in the GC inlet and on the columns at longer retention times.
Developments in sample injection techniques have helped to overcome this challenge. Traditionally, the most commonly used approach has been split/splitless injection, in which analytes are vaporized immediately in the inlet liner. However, these high temperatures can cause thermal degradation of highly brominated PBDEs. Further, only small injection volumes can be used, which can limit detection sensitivity at trace levels. Programmable temperature vaporization (PTV) techniques enable higher injection volumes to be used, which can significantly improve sensitivity.4 This approach involves venting off the solvent while performing multiple injections, which increases the mass of analytes in the inlet liner. Thermal degradation can be reduced with improved insulation, enabling a uniform heating profile.
While improvements in injection methods can reduce degradation in the inlet, highly brominated PBDEs can also break down on the column. This is particularly problematic with longer columns due to the longer retention times. Long columns are often used for PBDE analysis as they have higher resolution, allowing better separation of congeners. PBDE congeners must be separated prior to MS, as many are isobaric and difficult to distinguish by their MS fragmentation patterns. Some congeners, such as the critical pair BDE-49 and BDE-71, are difficult to separate chromatographically, so high-resolution GC systems are needed in these workflows.
Improvements in column design, such as the incorporation of phenyl groups into the polymer backbone to improve thermal stability, are now enabling good chromatographic resolution of all PBDEs on shorter columns without compromising the detection of higher brominated compounds. These improvements in column performance allow increased throughput with a lower cost per sample. Technological advances in GC/MS are therefore overcoming the challenges to PBDE separation; however, accurate quantification also relies on highly advanced MS systems able to reliably distinguish and quantify all congeners.
Sensitive and selective PBDE analysis using advanced MS
To quantify analytes at ultralow levels, PBDE workflows require exceptionally high sensitivity and selectivity, and improvements in GC/MS design are helping analysts push the limits of what is possible. Improved analyte selectivity can be achieved using tandem MS (MS/MS, or MS2), a technique that involves fragmenting the initially generated precursor ions in a second round of MS (MS2). This two-stage approach means that if two precursor ions have identical mass-to-charge ratios, they can still be distinguished based on the fragmentation patterns of the product ions in MS2. Tandem MS therefore enables greater accuracy to be realized with lower detection limits. The benefits are particularly apparent when analyzing samples containing complex matrices, which is often the case in PBDE analysis. Improving selectivity with tandem MS therefore reduces the potential for false positives caused by matrix compounds.
Sensitivity can also be improved by increasing the efficiency of analyte ionization. Some MS instruments now employ advanced electron ionization (AEI) sources that produce more tightly focused ion beams, permitting better sensitivity for trace analysis. Increasing efficiency in this way allows lower injection volumes to be used, which minimizes the potential for matrix contamination and reduces maintenance requirements. Instruments with a robust AEI source are thus well-suited to routine food safety workflows as it is possible to achieve high sensitivity with minimal maintenance.
Combining advanced GC-MS/MS technology and software for accurate PBDE quantification
GC-MS/MS instruments with a robust AEI source provide high sensitivity and selectivity for quantitative PBDE analysis. However, these workflows can be complex, and for routine food safety testing it is important to streamline processes to maximize throughput. Advances in software are now allowing workflows to be optimized efficiently.
GC-MS/MS instruments can be used alongside intelligent software solutions to automate and optimize tandem MS for PBDE analysis. These analytical systems can be configured by analyzing a sample containing the relevant PBDEs to identify the relevant precursor ions and product ions. Methods can be enhanced by performing multiple runs to identify the optimal collision energy for each PBDE. This permits the development of an efficient, automated method for the routine analysis of test samples. These intelligent software solutions can respond to changes in experimental conditions and even compensate for shifting retention times and aging columns. Such systems increase the reliability of PBDE analysis and, when used in concert with advanced GC-MS/MS technology, can provide highly accurate quantification at trace levels.
Some GC-MS/MS instruments combine the latest improvements in technology with these software solutions in a single system. For example, the Thermo Scientific TSQ9000 triple quadrupole GC-MS/MS system (Thermo Fisher Scientific, Waltham, MA) combines high-performance GC technology with an advanced tandem MS system equipped with an AEI source, and uses an intelligent software platform to automate and optimize the method. The high sensitivity and selectivity of the systems, coupled with their reliable performance with low maintenance, mean that advanced GC-MS/MS can be a highly effective solution for routine PBDE testing in food.
- Linares, V.; Belles, M. et al. Human exposure to PBDE and critical evaluation of health hazards. Arch. Toxicol. 2015, 89(3), 335–56.
- Guidance for the inventory of polybrominated diphenyl ethers (PBDEs) listed under the Stockholm Convention on POPs.
- Boucher, B.; Ennis, J. et al. A global database of polybrominated diphenyl ether flame retardant congeners in foods and supplements. J. Food Compos. Anal. 2018, 69, 171–88.
- Abdallah, M.A.-E. Advances in instrumental analysis of brominated flame retardants: current status and future perspectives. Int. Sch. Res. Not. 2014, 2014, 6518.
Paul Silcock is senior manager, Product Marketing, Thermo Fisher Scientific, Tudor Rd., Manor Park, Runcorn, WA7 1TA, U.K.; tel.: +44 (0)1928 534 050; www.thermofisher.com