Calculator inputs
Example data table
| Sample | Wavelength (nm) | Raw A | Blank A | Corrected A | Path Length (cm) | Concentration | Dilution | Coefficient |
|---|---|---|---|---|---|---|---|---|
| Caffeine Standard | 272 | 0.864 | 0.012 | 0.852 | 1.00 | 0.05 mmol/L | 1.0 | 17,040 L·mol⁻¹·cm⁻¹ |
| Dye Solution A | 520 | 0.620 | 0.010 | 0.610 | 1.00 | 0.040 mmol/L | 1.0 | 15,250 L·mol⁻¹·cm⁻¹ |
| Protein Fraction | 280 | 0.410 | 0.015 | 0.395 | 1.00 | 0.80 g/L | 2.0 | 0.988 L·g⁻¹·cm⁻¹ |
Formula used
A = measured absorbance
A = -log10(T)
A = 2 - log10(%T)
Acorrected = Araw - Ablank
cmeasured = centered / dilution factor
A = εlc for molar absorptivity
ε = A / (l × c)
A = alc for mass absorptivity
a = A / (l × c)
Aprojected = coefficient × target path length × target concentration
In this page, l is path length in centimeters, and concentration is converted to a compatible base before computing the coefficient.
How to use this calculator
- Select whether you want molar absorptivity or mass absorptivity.
- Choose the signal input mode: absorbance, transmittance, or percent transmittance.
- Enter a direct absorbance value, or provide transmittance data.
- Add blank absorbance if you want baseline correction.
- Enter path length, concentration, and the concentration unit.
- Provide molecular weight whenever you need unit conversion between molar and mass concentration.
- Enter a dilution factor if the analyzed sample was diluted before measurement.
- Optionally add target concentration and target path length for forward projection.
- Click Calculate coefficient to show results above the form.
- Use the CSV or PDF buttons to export the generated report.
FAQs
1) What is an absorbance coefficient?
It is a proportionality constant that links absorbance to concentration and path length. In chemistry, it often means molar absorptivity or mass absorptivity in the Beer-Lambert relationship.
2) What is the difference between molar and mass absorptivity?
Molar absorptivity uses molar concentration and reports units of L·mol⁻¹·cm⁻¹. Mass absorptivity uses mass concentration and reports units of L·g⁻¹·cm⁻¹. The correct choice depends on your laboratory concentration basis.
3) Why should I enter a blank absorbance?
Blank correction removes baseline signal from the solvent, cuvette, or reagents. This improves accuracy by isolating the analyte response before calculating the coefficient.
4) Can I use transmittance instead of absorbance?
Yes. The calculator converts transmittance or percent transmittance into absorbance automatically using logarithmic relationships, then applies blank correction and the Beer-Lambert equation.
5) Why does dilution factor matter?
If the measured sample was diluted, its actual concentration in the cuvette is lower than the stock value. The coefficient must use the concentration present during the measurement.
6) Why does path length affect the result?
Absorbance increases with optical path length. A longer cuvette allows light to travel through more sample, so the Beer-Lambert equation directly includes path length.
7) When do I need molecular weight?
You need molecular weight when converting between mass concentration and molar concentration. It lets the calculator align your entered units with the coefficient basis you selected.
8) Why is a negative corrected absorbance invalid here?
A negative corrected value means the blank exceeded the measured signal. That usually indicates an incorrect blank, a very weak sample, or measurement noise that must be reviewed first.