CHEM 524 -- Course Outline (Part 14)

IX. Absorption Spectroscopy UV-VIS-near IR (Read Chap. 13)

A. Example instruments--commonality--similar optics/detector/sources cover overlapping regions, make for multi-use instruments

1. Single beam (very old designs) -- Spectronic 20, vis, non-scan

-- Beckman DU (prism) -- uv-vis (high throughput, prism), original non-scan

2. Multichannel single beam (modern approach, fast, low noise,

get full spectrum, fixed resolution)-- diode array, HP5481

3. Dual beam (baseline corrected, abs. converted, dual in time)-- Cary 14/17

4. Differential (output DA, amplify the modulated spectrum)-- JASCO-CD

B. Sampling--quartz cuvettes/ many solvents/ DL limitation (A > 10-5 possible)

C. Noise sources

--blank--shot noise unimportant, high light level, flicker to to sampling is problem

--sample--low absorb--flicker, high absorb shot plus dark noise dominate

D. Lambert-Beer Law and deviation from linearity

---non-zero intercept - blank off (cell match problem)

---Chem equilibrium, vary conc. -- Measure at isosbestic point

or over region of ~constant absorbance for both speices

---polychromatic radiation, measure at top of band, A ~ const.

Slit effects: plot A/Amvs. A at various s/Dl values (Fig. 13-14),

Minimize error: s < 0.1 Dl at lm for Aħ0.5%

---Stray radiation: r = ES/ER apparent transmittance reduced: T ' = (T+r)/(1+r) ~ T+r

E. Sampling Problems

---Non parallel beam (path vary, effective e OK if cell fixed), A too high

---Multiple reflect, over est. A: DA = 0.434 F(1-T2) F-fraction multiply reflected

-- Fluorescence, normally negligilbe unless very weak source or high A, like stray light

---Detection limit in conc. DL = ksbk/eb

F. Applications

--- Multicomponent Quant. Anal./Stoichiometry/Titration

--- Kinetics, Chromatography

--- Reflectance, CD, LD

G. Reading Specification Sheets -- seek out important characterisitics

--compare insturments on same basis, need to see how establish noise, dyn range etc.