Pharmaceutical analysis
UV-Vis Spectrum Plotter
Plot absorbance spectra, find λmax and peaks, and fit calibration curves.
Sample
Recorded on the lab card. The range limits which readings are plotted and searched.
Leave blank to start at the first reading.
Leave blank to end at the last reading.
Spectrum readings
Wavelength (nm) | Absorbance. Rows can be in any order — they are sorted for plotting only.
| # | Wavelength (nm) | Absorbance | Delete |
|---|---|---|---|
| 1 | |||
| 2 | |||
| 3 | |||
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| 5 |
Spectrum graph
Drag across the graph to zoom into a wavelength window.
No graph yet
Enter at least 2 complete readings (wavelength and absorbance) to plot a spectrum.
Laboratory calculation card
Enter at least 2 complete readings to find λmax and build the lab card.
A = ε · b · c
A is absorbance (no units), ε the molar absorptivity (L mol⁻¹ cm⁻¹), b the path length (cm, usually 1) and c the concentration (mol/L). Absorbance is also A = log₁₀(I₀/I) = −log₁₀ T, so 1 A means 10% of the light gets through and 2 A means 1%.
λmax = the measured wavelength with the highest absorbance
Peaks are readings higher than the reading before them whose level then falls — endpoints excluded, a flat top counted once at its first point. Prominence = peak height − the higher of the two bases, where each base is the lowest reading between the peak and the nearest higher reading on that side (or the end of the data).
m = Σ(c − c̄)(A − Ā) / Σ(c − c̄)² b = Ā − m·c̄
Through origin: m = Σ(c·A) / Σc² b = 0
R² = 1 − SSres / SStot SSres = Σ(A − A_fit)² SStot = Σ(A − Ā)²
SE(m) = s(y/x) / √Σ(c − c̄)² s(y/x) = √(SSres / (n − 2))
Unknown: c = (A − b) / m Original = c × dilution factor
For a line forced through the origin, s(y/x) uses n − 1 degrees of freedom and SE(m) = s(y/x) / √Σc². R² is still computed about the mean absorbance so the two fits can be compared.
Keep learning
References
- Harris DC, Lucy CA. Quantitative Chemical Analysis. 10th ed. New York: W. H. Freeman (Macmillan Learning); 2019.
Common questions
λmax depends on the solvent, pH and the instrument's slit width, and it can only be found at the wavelengths you measured. A 5 nm scan step cannot locate a maximum more precisely than about ±2.5 nm. Rescan near the peak with a 1 nm step, in the solvent the monograph specifies.
Straight segments show exactly what you measured and are the default. The smoothed line is a monotone cubic, which never rises above or dips below neighbouring readings, so it cannot invent a peak — but it is still a drawing aid. λmax, peaks and every number in the report use only the readings as entered.
How far a peak stands above the surrounding spectrum. A small noise bump on the side of a large band has a tiny prominence even if its absorbance is high. Set a minimum prominence (for example 0.02 A) to ignore bumps that are within your instrument's noise.
With 3 points almost any smooth curve looks straight, and a single bad standard can hide. Five or more concentrations spread across the working range let you see curvature and outliers in the residuals, which is what a linearity claim actually needs.
No. R² measures scatter about the line, not whether a straight line is the right model — a gently curving calibration can still give R² of 0.998. Look at the residuals in the report: they should be small and randomly scattered, not a systematic arch.
Only when you have evidence the intercept is zero — the blank reads zero and the intercept's confidence interval includes zero. Forcing it otherwise biases every result, most at low concentrations. The ordinary fit is the safer default.
Not as a validated result. Linearity is only demonstrated between the lowest and highest standards. Dilute the sample (and enter the dilution factor) or prepare a standard that brackets it, then re-measure.
Prepared by the PharmaWallah student team. Verify results with your textbook or instructor. Report an error · Editorial policy