Introduction to Pharmaceutical Analysis: UV-Vis Spectroscopy

Vocabulary flashcards covering the principles, equations, instrumentation, and pharmaceutical applications of UV-Visible spectroscopy as presented in Lecture 1.

Spectroscopy

The study of matter and its properties by investigating light, sound, or particles that are emitted, absorbed, or scattered by matter, used for identification and quantification.

Electromagnetic Spectrum (UV region for drugs)

The region where most drugs absorb light, typically ranging from $190$ to $390\,nm$.

Characteristic energy levels

Specific energy levels unique to each molecule that create an absorption spectrum acting as a "finger-print" for identification.

Intensity and broadness of peak

intensity is the quantity of substance in the sample, and the broadness of the peak is measured at half height

$$\lambda_{max}$$

The wavelength of maximum absorbance, which is inversely proportional to the energy of the transition.

Far-UV

The region of the ultraviolet spectrum between $100$ and $200\,nm$, where oxygen and simple molecules absorb radiation.

Middle/Near-UV

The region of the ultraviolet spectrum between $200$ and $380\,nm$.

Visible region

The region of the electromagnetic spectrum between $380$ and $740\,nm$ where compounds appear coloured.

Electronic transitions

The excitation of electrons to excited electronic states due to transitions in the electronic energy levels of molecular bonds.

Conjugation

The presence of alternating single and double bonds in a molecule, which lowers $\Delta E$ and increases the $\lambda_{max}$.

Vibronic transition

A transition from the ground state to any of the different vibrational levels, resulting in fine structure in a spectrum.

Chromophores

Groups or compounds that absorb light in the visible region, often characterized by highly conjugated $\pi$-systems.

Absorbance (A)

$A = \log\left(\frac{I_0}{I_t}\right)$, where $I_0$ is incident radiation and $I_t$ is transmitted radiation.

Beer-Lambert law

$A = \epsilon l c$ stating that absorbance is proportional to concentration ($c$), path-length in cm ($l$), and the molar extinction coefficient in mol/L ($\epsilon$).

Molar extinction coefficient ($\epsilon$)

A constant for a substance at a given wavelength, expressed in units of $L\,mol^{-1}\,cm^{-1}$.

$$A(1\%, 1cm)$$

The absorbance of a $1\%\,w/v$ solution ($1\,g/100\,mL$) in a $1\,cm$ pathlength, commonly used in Pharmacopoeias.

Bathochromic Shift (Red shift)

A shift of an absorption maximum towards a longer wavelength or lower energy.

Hypsochromic Shift (Blue Shift)

A shift of an absorption maximum towards a shorter wavelength or higher energy.

Hyperchromic Effect

An effect that results in increased absorption intensity.

Hypochromic Effect

An effect that results in decreased absorption intensity.

Auxochrome

Any atom or group which, when added to a chromophore, causes a bathochromic shift.

What causes bathochromic shifts?

• decreased symmetry

• changing solvents

• intermolecular interactions

• auxochromes

• pH change

Blank sample

A sample used in spectrophotometry to subtract the absorption of the solvent or background from the total measurement.

Monochromator

An instrumental component, such as a prism or diffraction grating, used to isolate a specific wavelength of light.

Henderson-Hasselbach equation for Acids (Spectroscopy)

$pKa = pH + \log\left(\frac{A_i - A}{A - A_u}\right)$, used to determine $pKa$ from pH-dependent UV shifts.

UV-Vis instrumentation

• light sources

  • deuterium lamp (180-350nm)

  • tungsten lamp (350-1000nm)

• monochromator

  • prism (slow)

  • diffraction grating (fast)

• detector

  • photo cell + photomultiplier (light photons → current → amplification → signal)

UV-Vis application

• quatitative measurements of drugs in formulations

• determination of physicochemical properties

• measuring release of drug from formulation

UV-Vis strengths and limitations

• simple, convenient and cheap quantitative technique with moderate sensitivity and selectivity, but limited to chromophores that absorb UV-Vis light.

• cannot analyse mixtures, and does not tell us which molecule is absorbing the light

UV-Vis sample preparation

• cell/cuvette with suitable transparency - quartz is preferred

• solvent with suitable transparency

• suitable concentration (absorbance < 2) - absorbance of blank sample is subtracted