98
One major advantage of EI generated spectra is their reproducibility. The fact that
EI at 70 eV transfers more energy than needed for ionizing a molecule, leads to
energy-release through fragmentation. The fragments generated inform about the
structure of the molecule, which is ideal for subsequent library searches. Today very
large curated commercial libraries are available, facilitating non-target analysis.
The downside of fragmentation-based analysis is that the signal of a given analyte
will be spread over all fragments, leading to a substantial reduction in the absolute
intensity. Therefore, if sensitivity is the ultimate goal, CI, which produces protonated molecular ions in a gas phase reaction, can be used instead. ESI is used for
coupling with LC. This is a soft ionization technique that desorbs protonated or
deprotonated ions from the liquid phase into the gas phase, in the process producing
few fragments if at all, but high intensity molecular ions instead. It accommodates
thermolabile, polar and ionized compounds and, since mass spectrometers measure
m/z, allows determining very big molecules that carry multiple charges. Depending
on the instrument, positive/negative switching can be done within one run, facilitating for instance the analysis of a metabolome. Identification and structural information is only available through additional MS/MS scans. The spectra generated can
then be matched against libraries that now become publicly available. Unlike in EI,
ion formation is a competitive acid-base reaction and hence depends on the matrix
surrounding the analyte, which can lead to ion suppression.
The most common mass spectrometers are either triple quadrupoles, typically
used for high sensitivity using multiple reaction monitoring (MRM), or highly accurate mass spectrometers such as orbitraps, Fourier-transform ion cyclotron resonance (FT-ICR) or time-of-flight (TOF) instruments, which provide high resolving
power. Today’s state-of-the-art mass spectrometers routinely reach >100,000 resolving power, and with that unsurpassed capabilities to detect unknown contaminants
in a high-throughput format either using data-dependent or data-independent MS/
MS approaches.
6.1.2 Effect-Directed Analysis (EDA)
Both environmental chemists and toxicologists intending to contribute to ecotoxicological risk assessment are concerned with the question of how best to identify the
chemicals present in a very complex mixture that cause adverse effects in an ecosystem. One approach is to develop a hypothesis of how this effect is caused and then
focus on the chemicals known to be linked to it, an approach called targeted analysis.
Another popular strategy, called effect-directed analysis (EDA) [17], works by
linking chemical analysis with a biological readout, which could be anything from
a response in a reporter gene assay to mortality observed in a toxicity test with a
model organism [18]. In this approach, chemical analysis focuses only on samples
or fractions that actually are causing an effect. This allows greatly reducing the
number of samples that need to be chemically analyzed. In the ideal case the measured activity can be explained by the chemicals found to be present in the sample.
K. J. Groh and M. J.-F. Suter
One major advantage of EI generated spectra is their reproducibility. The fact that
EI at 70 eV transfers more energy than needed for ionizing a molecule, leads to
energy-release through fragmentation. The fragments generated inform about the
structure of the molecule, which is ideal for subsequent library searches. Today very
large curated commercial libraries are available, facilitating non-target analysis.
The downside of fragmentation-based analysis is that the signal of a given analyte
will be spread over all fragments, leading to a substantial reduction in the absolute
intensity. Therefore, if sensitivity is the ultimate goal, CI, which produces protonated molecular ions in a gas phase reaction, can be used instead. ESI is used for
coupling with LC. This is a soft ionization technique that desorbs protonated or
deprotonated ions from the liquid phase into the gas phase, in the process producing
few fragments if at all, but high intensity molecular ions instead. It accommodates
thermolabile, polar and ionized compounds and, since mass spectrometers measure
m/z, allows determining very big molecules that carry multiple charges. Depending
on the instrument, positive/negative switching can be done within one run, facilitating for instance the analysis of a metabolome. Identification and structural information is only available through additional MS/MS scans. The spectra generated can
then be matched against libraries that now become publicly available. Unlike in EI,
ion formation is a competitive acid-base reaction and hence depends on the matrix
surrounding the analyte, which can lead to ion suppression.
The most common mass spectrometers are either triple quadrupoles, typically
used for high sensitivity using multiple reaction monitoring (MRM), or highly accurate mass spectrometers such as orbitraps, Fourier-transform ion cyclotron resonance (FT-ICR) or time-of-flight (TOF) instruments, which provide high resolving
power. Today’s state-of-the-art mass spectrometers routinely reach >100,000 resolving power, and with that unsurpassed capabilities to detect unknown contaminants
in a high-throughput format either using data-dependent or data-independent MS/
MS approaches.
6.1.2 Effect-Directed Analysis (EDA)
Both environmental chemists and toxicologists intending to contribute to ecotoxicological risk assessment are concerned with the question of how best to identify the
chemicals present in a very complex mixture that cause adverse effects in an ecosystem. One approach is to develop a hypothesis of how this effect is caused and then
focus on the chemicals known to be linked to it, an approach called targeted analysis.
Another popular strategy, called effect-directed analysis (EDA) [17], works by
linking chemical analysis with a biological readout, which could be anything from
a response in a reporter gene assay to mortality observed in a toxicity test with a
model organism [18]. In this approach, chemical analysis focuses only on samples
or fractions that actually are causing an effect. This allows greatly reducing the
number of samples that need to be chemically analyzed. In the ideal case the measured activity can be explained by the chemicals found to be present in the sample.
K. J. Groh and M. J.-F. Suter