W3 L4 - Pharm Analysis - EMR/UV (Notes made) Flashcards
(18 cards)
What is electromagnetic radiation and which part is useful in pharmaceutical analysis?
EMR spectrum: gamma, x-rays, UV, visible, infrared, microwaves, radio waves
UV, visible, and IR (200–1000 nm) used in pharma
Light = electric and magnetic fields vibrating at right angles
Speed of light (c) = λ × f; as f ↑, λ ↓
- λ = length of once cycle, lamda
- f = frequency (cycles per sec)
How are wavelength, frequency, and energy related in light?
- E = hf
- h = Plank’s constant
- Higher frequency → shorter λ → higher E
How does light behave like both a wave and a particle?
Light travels as waves but also as photons (energy packets)
Each photon’s energy depends on its wavelength, λ
Absorption vs. Emission
Absorption: Electron gains energy by absorbing a photon→ excited state
Emission: Electron returns to lower excited state or even ground state→ releases energy
Photon energy must match energy gap
What types of spectroscopy are used in drug analysis?
UV-Vis: electronic transitions
UV (ultraviolet) and visible light have enough energy to excite electrons.
An electron in a low-energy orbital (usually the ground state) absorbs a photon and jumps to a higher-energy orbital (excited state)
IR: vibrational transitions
IR light doesn’t have enough energy to excite electrons.
Instead, it makes chemical bonds vibrate (stretch, bend, twist).
Fluorescence: emission from excited electrons
Molecular Excitation
Energy absorbed/emitted = E₁ − E₀
E₀ (Ground State)
E₁ (Excited State)
Molecules have discrete electronic, vibrational, and rotational energy levels
What is the relative energy required for different transitions?
In order of energy needed for each transition:
Electronic > Vibrational > Rotational
UV (for electronic transition) needs more energy (short λ) than visible or IR (for vibrational transitions)
What types of electron transitions are important in UV spectroscopy?
Transitions occur between:
σ → σ* (high E, 150 nm)
n → σ* (180 nm)
π → π* (254 nm)
n → π* (290 nm)
Only π → π* and n → π* transitions useful for drug analysis (>200 nm)
Why is UV absorption only useful above 200 nm?
- Far UV (<200 nm) → unusable due to interference
- Drug analysis relies on π bonds and lone pairs (n) in conjugated systems
- A conjugated system is a molecule or part of a molecule where:
Double bonds (π bonds) and single bonds alternate.
OR, double bonds are next to atoms with lone pairs (non-bonding electrons, denoted as n).
This arrangement allows electrons to be delocalized (spread out) across multiple atoms.
Why do some drugs absorb UV light better than others?
Menthol: No double bonds → no UV absorption
Benzocaine: π bonds + lone pairs → strong UV absorption
How does conjugation affect UV absorbance?
- Bathochromic shift (red shift): more conjugation → higher λmax
- A conjugated molecule absorbs light more easily and at a longer wavelength, which is why more conjugation leads to a bathochromic (red) shift in the spectrum.
- Extended π systems (e.g., β-carotene) = easier electron excitation
- In molecules with more conjugation (alternating double and single bonds), the electrons are more spread out.
This makes it easier for electrons to get excited (they need less energy).
Lower energy = longer wavelength light absorbed.
What happens to absorption as conjugation increases?
Longer conjugation → lower energy gap → longer λmax
E.g., (CH=CH)n system shifts from 275 to 380 nm as n ↑
What are chromophores and auxochromes, and how do they affect UV spectra?
Auxochrome: Group with lone pairs (e.g., -OH, -NH₂)
Increases absorption intensity (hyperchromic shift)
Shifts λmax to longer wavelength
Chromophore: Group that absorbs UV/Vis (e.g., double bond)
Can we identify a drug just by its UV spectrum?
UV spectra often too complex for definitive identification
Still useful for quantitative analysis
What are the main parts and function of a spectrophotometer?
Lamps: Deuterium (UV), Tungsten (visible)
Monochromator: selects specific λ
Detector: measures intensity difference (I₀ vs I)
How does the Beer-Lambert Law relate absorbance to concentration?
- tells us how much light is absorbed by a solution.
- absorbance = log(I₀/I) = εbc
I₀ = light intensity that enters sample
I = lower intensity that leaves sample
ε (epsilon) = molar extinction coefficient
b = path length (cm)
c = conc (mol/L)
How is Beer-Lambert Law used in the BP for drug concentration?
BP uses A(1%,1cm) values to simplify calculations – a standard absorbance value if the solution has 1% w/v (1 g in 100 mL) and measured in a 1 cm cuvette.
Equation can be simplified for concentration in g/100mL
When does the Beer-Lambert Law not work perfectly?
Assumptions:
Molecules absorb independently
Homogenous solution
Deviations due to:
High concentration
Turbidity, fluorescence, photodegradation, etc.
Use standards (e.g. K₂Cr₂O₇) for checking
