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2D IR response-factor transferability means that a response ratio characterized under one set of conditions remains applicable to another measurement. It is an assumption to test when using a 2D IR response-factor ratio to infer populations from spectral amplitudes. A common detector gain can cancel from a ratio, while different gains or optical responses for its two components do not.

In the positive-component measurement model, writeThe shared survival factor $P$ cancels. If conditions change the responses to ${\alpha }^{\prime }=g_D\alpha$ and ${\beta }^{\prime }=g_C\beta$, then ${\kappa }^{\prime }=(g_C/g_D)\kappa$. Equality of $g_C$ and $g_D$ is a substantive requirement, not a consequence of both signals coming from the same instrument.

An original error calculation supposes a reference established ${\kappa }_0=2$, but the unknown measurement actually has ${\kappa }_1=3$. For true population fraction $q=0.20$, its observed amplitude ratio is $C/D=3(0.2)/0.8=0.75$. Using the old response ratio givesan overestimate even with noiseless detection. More generally, with $\lambda ={\kappa }_1/{\kappa }_0$,A small response-ratio mismatch $\delta \ln \kappa$ therefore gives $\hat{q}-q\simeq q(1-q)\delta \ln \kappa$.

The optical reason to check transfer is visible in the primary response theory: different pathways contain different transition-dipole products, and their amplitudes include relaxation factors. Pulse bandwidth, polarization and the selected 2D IR waiting time help determine which response is recorded.^[ref] This does not establish a universal two-population model for every 2D IR cross peak.

A 2D IR response-factor validation mixture supplies independently known composition under the new conditions. Test whether the old response ratio predicts its amplitudes, and repeat over an appropriate composition range if matrix effects or nonlinearity are plausible. Reusing an unknown's fitted fraction to validate the factor that produced it would be circular.

Retain the amplitude definition and phase convention as well as the instrument settings. The phase-controlled peptide experiment measures phases and amplitudes, so a signed or complex component need not obey the positive-component equations above.^[ref] Successful transfer concerns a specified model and observable, not every intensity visible in a spectrum.