Raman captured-area fraction

The Raman captured-area fraction is the part of a specified complete spectral band's area that lies inside a chosen integration interval. For a nonnegative isolated background-corrected band $S(\tilde{\nu })$ with finite positive complete area,It lies between zero and one. It quantifies one mechanism of Raman integration-window bias: changing the center or width changes the fraction even when the complete area stays fixed. Background errors or overlapping bands can prevent measured intensity from representing the assumed isolated band.

For a Gaussian function centered at ${\tilde{\nu }}_0$ with width parameter $s$, substitution of $z=(\tilde{\nu }-{\tilde{\nu }}_0)/s$ giveswhere $\Phi$ is the cumulative distribution function of the standard normal distribution. A centered window with half-width $ks$ captures $2\Phi (k)-1$. Half-widths of one, two and three $s$ capture approximately68.27%,95.45% and99.73% of the complete Gaussian area. These percentages refer to this line shape, not every Raman spectrum.

For an original example, a window extending4cm⁻¹ either side of the fitted center captures about0.95450 when $s=2\mathrm{c}{\mathrm{m}}^{-1}$, but about0.81758 when $s=3\mathrm{c}{\mathrm{m}}^{-1}$. Even if the height decreases so the complete area is unchanged, the finite-window area falls. If one incorrectly uses the first fraction to correct the broader band, the estimated complete area is only $0.81758/0.95450\approx 0.85655$ of the true value.

Raman band-area extrapolation uses a justified fraction to estimate complete area as $A_{\mathrm{full}}=A_{[a,b]}/f$. For example, measured area10 in a centered two-width Gaussian window implies about10.477 in the same area units. The correction depends on the model and its parameters; it is not extra measured signal. At fixed $f$, an absolute area error is multiplied by $1/f$, while uncertainty in $f$ supplies another contribution. Large extrapolations from a narrow interval therefore need especially convincing evidence about the unobserved tails.

Collecting and retaining the full local spectra supports checks of center, width, background and neighboring bands instead of interpreting a single intensity summary alone. Primary manufacturer guidance makes this distinction between simple spectral-intensity images and fitted band parameters explicit.^[ref]

Raman band-area extrapolation estimates the complete area of a spectral band from a measured finite-window area and a model for the fraction outside that window. It uses the Raman captured-area fraction $f>0$:The extra area is inferred rather than observed. Its validity depends on the assumed line shape, width, center, background and separation from neighboring bands. Keeping these assumptions explicit prevents a corrected Raman band-area map from concealing the model dependence of its values.

For an original uncertainty example, let $A_{\mathrm{win}}=10.0$ in arbitrary area units with standard uncertainty0.20, and let $f=0.950$ with standard uncertainty0.010. The extrapolated area is about10.526. For independent inputs, the first-order combined standard uncertainty isin the same area units. Treating $f$ as exact would retain only the first term and understate the uncertainty in this example.

The fitted fraction and window area may be inferred from the same spectrum, so independence needs justification. The general first-order expression retains their covariance:The signs follow from the sensitivity coefficients $\partial A_{\mathrm{full}}/\partial A_{\mathrm{win}}=1/f$ and $\partial A_{\mathrm{full}}/\partial f=-A_{\mathrm{win}}/f^2$. The GUM's correlated-input propagation is the applicable measurement framework.^[ref]

A deterministic bound answers a different question. If $A_{\mathrm{win}}\in [9.8,10.2]$ and $f\in [0.94,0.97]$, positivity givesThis interval follows from allowed ranges and is not automatically a probabilistic confidence interval. Small $f$ makes both sensitivity coefficients large, so extrapolating from a small central portion can be unstable even when that portion is measured precisely.

Model inadequacy needs attention alongside parameter uncertainty. A background mistaken for a band tail, or an unresolved neighboring band, changes what is being extrapolated. Retaining the local Raman spectrum allows the fitted band parameters to be checked instead of relying solely on an intensity summary.^[ref]