A plasma sample is spiked with ¹³C-labeled cortisol before extraction. Lab A achieves 40% recovery; Lab B, running the same sample, achieves 90% recovery. Which outcome is correct?
ALab B's result is more accurate because higher recovery means less analyte was lost
BBoth labs produce the same result because the ratio of labeled to unlabeled cortisol is preserved regardless of recovery
CLab A's result is less accurate because uneven losses distort the ratio at low recovery
DLab B's result is more accurate only if the spike was added after extraction
This is the self-correcting core of IDMS. Because labeled and unlabeled cortisol are chemically identical, any loss during extraction affects both equally — the ratio stays constant whether recovery is 40% or 90%. Lab A and Lab B get the same answer. Option A is the intuitive but wrong response: in classical analytical methods, higher recovery means lower loss and better accuracy, but IDMS breaks this assumption by making the measurement recovery-independent.
Question 2 Multiple Choice
Why might a deuterium-labeled internal standard introduce bias in LC-ESI-MS even when fully equilibrated with the native analyte?
ADeuterium-labeled compounds have a different molecular formula, making mass-based distinction unreliable
BDeuterium substitution can alter polarity and chromatographic retention slightly, causing the labeled and unlabeled analyte to elute at different times and experience different matrix ion suppression
CThe mass spectrometer cannot distinguish a 4-dalton mass shift from isobaric interferences
DDeuterium labels undergo back-exchange in aqueous solution and are converted to the unlabeled form
Replacing hydrogen with deuterium changes bond strength and slightly alters polarity — a phenomenon called the deuterium isotope effect. In LC-ESI-MS, even a one-second difference in retention time means the labeled and unlabeled compound are in slightly different matrix environments when they enter the ion source, producing different ion suppression. The result is a biased ratio. ¹³C labels avoid this entirely because replacing ¹²C with ¹³C changes mass without altering any bond or polarity property.
Question 3 True / False
When an isotopically labeled analog is spiked into a sample and fully equilibrated before any processing, the measured isotope ratio is independent of extraction recovery.
TTrue
FFalse
Answer: True
This is the defining property of IDMS. Full equilibration ensures that labeled and native analyte enter every separation and cleanup step as a uniform mixture. Whatever fraction is lost, it is the same fraction for both species, leaving the ratio unchanged. The ratio encodes concentration without depending on how much was recovered.
Question 4 True / False
Isotope dilution mass spectrometry eliminates most sources of analytical error, making it an absolute measurement that requires no calibration.
TTrue
FFalse
Answer: False
IDMS is extraordinarily accurate but not error-free. Its self-correcting property only works if the spike is fully equilibrated with the native analyte before any separation. If the native analyte is protein-bound while the spike is free in solution, they will not experience the same losses, biasing the ratio. Additionally, deuterium isotope effects can introduce chromatographic artifacts. IDMS is designated a definitive method because its errors are small and well-characterized — not because they are zero.
Question 5 Short Answer
Why must the isotopically labeled spike be added to the sample and fully equilibrated before any extraction or cleanup steps, rather than added afterward as a final calibrant?
Think about your answer, then reveal below.
Model answer: The spike must enter the workflow with the native analyte so both experience identical losses. If added after extraction, the spike bypasses all the steps that could cause variable recovery, and the ratio no longer reflects how much analyte was actually in the original sample. Full pre-extraction equilibration is the mechanism that makes the ratio invariant to recovery.
IDMS works because the labeled and unlabeled analyte act as a single co-extracted pool — the ratio at the detector mirrors the ratio in the original spiked sample regardless of losses. Adding the spike after extraction means it only 'sees' the final step, leaving all prior variable losses uncompensated. This is why incomplete equilibration (e.g., analyte bound in a matrix, spike free in solution) is the primary failure mode for IDMS.