Quantification
After mass spectrometry, we used Sciex Analyst software (Version: 1.6;
Framingham, MA) to integrate peaks manually. To determine analyte
concentrations, we first calculated peak area ratios between the target
compound and the matched IS. We then used the peak area ratios of
calibrants to interpolate linear regressions, establishing calibration
curves (Supplementary Tables 3 and 4). For the biphenyl data, where
there were high analyte concentrations in spiked and pregnant samples,
we created separate high and low calibration curves to target relevant
concentrations.
We defined each analyte’s observed reporting limit
(RLobs) as the lowest calibrant used in the calibration
curve. We calculated the limit of detection (LOD) by adding the mean of
the three blanks with three times the standard deviation of the blanks.
We compared the RLobs and LOD and used the higher of the
two as the reporting limit (RL). For calculations using analyte
concentrations, we substituted values below RL with RL/2.
We determined accuracy with the percent analyte recovered in the
spike-recovery experiment and used relative standard deviation (RSD) to
assess precision. We calculated percent recovery by comparing recovered
analyte mass in spiked samples with expected analyte mass, using the
following equation:
\begin{equation}
Percent\ Recovery=\frac{\text{ab}}{cb+de}\ \times 100\%\nonumber \\
\end{equation}where ‘a’ is the analyte mass fraction (ng/g) measured in a spiked
sample, ‘b’ is the mass (g) of that spiked sample, ‘c’ is the mean
analyte
mass
fraction (ng/g) measured in endogenous (i.e., un-spiked) samples, ‘d’ is
the analyte mass fraction (ng/g) in the calibrant used to spike the
sample, ‘e’ is the mass of the spike (g). We considered percent recovery
between 70 % and 120 % and relative standard deviation (RSD) values
below 15 % acceptable.