Page 13 - Instrumental Analysis - Pharm D Clinical- 07-PA403

UV-visible Spectrophotometry

Absorbance and Transmittance

• Transmittance (T) is the ratio of the transmitted light’s intensity, It, to that
incident on the sample, I0. Therefore, T has the range 0 to 1.

• Multiplying the transmittance by 100 gives the percent transmittance (%T).

T = ─I─t %T = 100 x T = ──Itx 100
I0 I0

• As the absorbance (A) of a solution increases, its transmittance decreases.

A = ‒ log T = ‒ log ── I=t log ── I0 T = 10-A
I0 It

• Both A and T are dimensionless (have no units).

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Fundamental Laws

Lambert’s Law

I0 It

Pathlength
(b)

• Lambert’s law is defined as follows: when a monochromatic
light passes through a solution, absorbance of light by this
solution is directly proportional to its thickness (pathlength).

• Absorbance (A) is directly proportional to pathlength (b)

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