How to Find Absorbancy From a Transmittance Spectrometer

Chemical elements and mixtures possess certain characteristics with respect to the ability to absorb light for a specific wavelength. Absorbance measurements are valuable in various fields of chemistry and molecular biology to identify and determine the quantity of components in a mixture. For example, concentration of DNA is determined by measuring light absorbance using the 260nm ultraviolet wavelength. Inverse to absorbance is light transmittance, or the amount of light that passes through a substance. Beer's Law provides a mathematical relationship between transmittance and absorbance. You can use Beer's law to determine absorbance from transmittance data.

Things You'll Need

• Scientific calculator
• Spectrometer

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Instructions

• 1

  Using a spectrometer, measure transmittance of the substance tested. This value will be displayed on the transmittance spectrometer. Transmittance is defined as the ratio of light transmitted (I) by the substance over initial light intensity (Io), or T= I/ Io. For example, if the spectrophotometer display shows a value of 50%, this equals 0.5 transmittance.

• 2

  Calculate the inverse value for transmittance. This value is equivalent to 1/T equal to Io/I. Using the calculator, the inverse value is calculated as one divided by transmittance. The example provided in step 1 would be 1/0.5 = 2.

• 3

  Calculate the absorbance value from the inverse value of transmittance using LOG. Beer's Law describes the relationship between absorbance (A) and transmittance (I/Io) with the formula A = Log (Io/I) where Io/I equals 1/T. To determine absorbance, enter the value for 1/T and press the LOG button. The example provided would be A= log 2 = 0.3 absorbance units.