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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 evolu-
tion of LC-MS methods for BFRs is the development of compre-
hensive 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 exhibit-
ing 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 station-
ary 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 sol-
vents such as toluene, necessitating a solvent exchange step prior
to analysis. The same issue arises for BFRs isolated from environ-
mental 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 station-
ary phases and mass spectrometers have been astute in recogniz-
ing 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.
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.
biota (typically >90%). In addition, an inherent property of ali-
phatic 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 chroma-
tography-high resolution mass spectrometry (GC-HRMS) meth-
ods. These technological advances allow the resolving power of
contemporary LC stationary phases to be coupled with the sen-
sitivity and specificity of state-of-the-art mass spectrometers.
In addition, electrospray ionization (ESI), one of the most com-
monly used ionization mechanisms, is softer than electron ion-
ization (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 spectrom-
etry. 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 gener-
ally 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 com-
pounds, 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 pres-
sure 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.
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