Analysis of Brominated Flame Retardants by Liquid Chromatography Mass Spectrometry
A wide variety of brominated flame retardants (BFRs) are currently used in industry and commerce. Use of these compounds has increased exponentially in the past 50 years as a result of strict regulations regarding the flame retardancy of consumer products. Roughly 40% of all flame retardants on the market are brominated. Some of these compounds have the potential to be persistent, toxic, bioaccumulative, and are amenable to long range transport. In addition, the occurrence, distribution, and fate of many of these compounds in the environment remain largely unknown.
Polybrominated diphenyl ethers (PBDEs) remain the most widely studied of the BFRs, despite the penta- and octa-formulations being banned in Europe and voluntary cessation of production in North America. With the exception of the fully-substituted decabromodiphenyl ether (BDE-209), the PBDEs are easily determined by gas chromatography-mass spectrometry (GC-MS) and are now routinely measured in a wide range of environmental matrices. Due to its unique chemical and physical properties, including high molecular weight, poor solubility, and sensitivity to heat and light, accurate determination of BDE-209 remains a significant challenge. A host of other BFRs are not readily amenable to analysis by GC-MS and pose an analytical challenge as a result of their physical properties. Although their chemical structures appear quite simple, BFRs such as hexabromocyclododecane (HBCD), 1,2,5,6-tetrabromocycloctane (TBCO) and tetrabromoethylcyclohexane (TBECH) thermally isomerize and partition poorly on GC stationary phases. HBCD is one of the most widely used BFRs with production globally in excess of 20,000 tons; HBCD is the primary flame retardant used in the extruded and expanded polystyrene foams used as thermal insulation in buildings, as well as in upholstery fabrics. Some laboratories continue to report HBCD concentrations as the sum of the three predominant isomers based on analysis by GC, i.e., the sum of α-, β- and γ-HBCD. These nonisomer specific analyses preclude thorough investigation of environmental pathways, and potential shifting of isomer profiles during manufacture or cycling in the environment. Differences in pathways of HBCD in the environment are evidenced by the predominance of γ-HBCD in the technical mixture and in sediment, while α-HBCD is dominant in biota (typically >90%). In addition, an inherent property of aliphatic BFRs is that they exist as diastereomers. Therefore, the study of enantioselective accumulation of BFRs in food chains requires separation of the individual enantiomers.
The last decade has been a period of extraordinary progress in development of LC-MS technology. As a result, detection limits of some LC-MS methods are on a par with those of gas chromatography-high resolution mass spectrometry (GC-HRMS) methods. These technological advances allow the resolving power of contemporary LC stationary phases to be coupled with the sensitivity and specificity of state-of-the-art mass spectrometers. In addition, electrospray ionization (ESI), one of the most commonly used ionization mechanisms, is softer than electron ionization (EI) used in GC-MS. Robust LC-MS methods for analysis of BFRs, including HBCD and tetrabromobisphenol-A (TBBPA), are now routinely used in analytical laboratories. Most methods for analysis of BFRs are based on negative ion mass spectrometry. Despite these advances, significant analytical challenges remain in LC-MS methods development. LC-MS continues to be susceptible to matrix effects, and the technique still generally lacks the retention time reproducibility of GC-MS methods. The use of isotopically-labeled internal standards is effective in minimizing matrix effects, but investigations of new chemicals continue to be plagued by a paucity not only of labeled compounds, but authentic native standards.
Other challenges of LC-MS analysis of BFRs can include poor ionization efficiency and limited fragmentation. In the case of TBCO and TBECH, both ESI and atmospheric pressure chemical ionization (APCI) result in weak molecular ions or molecular ion adducts. Adequate detectability of the compounds can be achieved by monitoring the Br- ions in selected ion monitoring (SIM) mode; however, this approach negates the advantages of a triple quadrupole mass spectrometer, in that the power of tandem MS techniques cannot be exploited. Atmospheric pressure photoionization (APPI) is the latest ionization technique developed for LC-MS; in fact, the impetus behind development of APPI was the need to extend the range of compounds beyond those only amenable to ESI or APCI. Typical variations of the technique are based on vaporization of the liquid sample (similar to APCI), combination with a dopant, and subsequent ionization resulting from gas phase reactions initiated by photons from a krypton discharge lamp. APPI has shown great potential for analysis of compounds across a broad range of polarities, but particularly for nonpolar analytes. The method is also reportedly less susceptible to matrix effects than ESI and APCI.
Progress in LC-MS methods development continues as lessons learned from investigations of individual compounds are applied to subsequent generations of BFRs. A new challenge in the evolution of LC-MS methods for BFRs is the development of comprehensive methods for concurrent analysis of multiple compound classes. The primary challenge in development of comprehensive methods is identification of suitable LC stationary phases coupled with MS ionization techniques applicable to compounds exhibiting a broad range of chemical and physical characteristics. The LC stationary phase must provide adequate separation among compounds that can exhibit dramatically different retention behaviors; key factors include particle size, pore size, and stationary phase chemistry. In addition, even individual isomers within the same compound class can exhibit significantly different mass spectrometric response factors. A further convoluting factor is the limited solubility of BFRs in typical reversed phase (RP) HPLC mobile phases. Many BFR standards are marketed in nonpolar solvents such as toluene, necessitating a solvent exchange step prior to analysis. The same issue arises for BFRs isolated from environmental samples using conventional column cleanup methods, in that these techniques frequently culminate in the extracts being concentrated in nonpolar solvents amenable to analysis by GC.
Ultimately, partnerships among experts in the field of analytical standards, separation science, and mass spectrometry will yield viable comprehensive methods for BFRs. In the past few years, suppliers of analytical standards and manufacturers of LC stationary phases and mass spectrometers have been astute in recognizing trends in analysis of compounds of potential environmental concern, and correspondingly have been proactive in developing technologies of great value to the toxics research and monitoring community.
Dr. Chris Marvin is a Research Scientist for Environment Canada, Burlington, Ontario. His research interests include new and emerging environmental contaminants, occurrence and fate of contaminants in the Great Lakes, and LC/MS methods development.