UV-Visible Spectrophotometer Instrumentation Part 1 PDF

UV-VISIBLE SPECTROPHOTOMETER INSTRUMENTATION PART 1 By Nancy Tyrer and Ela Kogut Edits by Katie Rankin Readings: Fundamentals of Analytical Chemistry, Chapter 25 Chemical Analysis, Chapter 9 Lab Manual, Experiment 3 Cary Double Beam UV-Visible Spec...

UV-VISIBLE SPECTROPHOTOMETER INSTRUMENTATION PART 1 By Nancy Tyrer and Ela Kogut Edits by Katie Rankin Readings: Fundamentals of Analytical Chemistry, Chapter 25 Chemical Analysis, Chapter 9 Lab Manual, Experiment 3 Cary Double Beam UV-Visible Spectrometer https://www.youtube.com/watch?v=O39avevqndU 2  Radiation Sources 3 Radiation source requirements:  Provide enough radiant energy (Po) over spectral region where absorption is measured (high incident power)  Maintain constant Po (incident power)  Stable over measurement time How is the above achieved?  Regulation of the power source  Double beam design where Po to P is ratioed in-time (nulls out instability as Po and P are affected the same)  Categories of Radiation Sources 4 Line Source:  Emits individual λ’s characteristic of the element the cathode is made from (resonance λ’s)  Example: Hallow cathode lamp (element dependent) → AAS Continuous Source:  Emits all λ’s at a constant Po over a wide spectral range.  Examples:  W-I2 (tungsten/halogen) → visible region (380- 780 nm)  D2 (deuterium) → UV region (180-380 nm)  Tungsten/Halogen (Quartz/Halogen) Lamp 5 Visible source: 240-2500 nm Design:  Sealed quartz (can withstand high temp.)  Evacuated bulb  Thin coiled wire of W (filament)  Small vapour of I2 (g)  Regulated AC power source Operation:  Po is proportional to the filament temperature, which is proportional to the current flow  Constant stable current passes through the filament causing it to become white hot at ~ 3200 oC  Light from lamp = continuous spectrum (UV/visible/IR)  Tungsten/Halogen Lamp 6 Filament Reactions: a) W(s) + applied voltage → W(g) (sublimation) b) I2(g) + O2(g) + W(g) → W-Ix(g)-Ox(g) (variety of tungsten oxyhalide complexes) c) W-Ix(g)-Ox(g) + filament → W(s) + I2(g) + O2(g) (redeposition)  Tungsten/Halogen Lamp 7 Advantages of the lamp: 1. Longer lifetime due to regeneration process (10,000 hours vs. 1000 hours) 2. Run at higher temperatures (2000 – 3300 K) due to quartz envelope  Maximize Po  Shifts spectral range closer to UV region  Deuterium Lamp 8 UV source: 190-350 nm Spectrum of deuterium lamp.  Deuterium Lamp 9 Operation:  Current regulated power source provides 100 W  Mechanical aperture between anode and cathode constricts the discharge to a narrow path  Lifetime: 2000 hours Reactions: a) D2o(g) + electrical energy → D2* 𝒉𝒉𝒉𝒉 b) D2* → D + D + photons (E = ) 𝝀𝝀𝑼𝑼𝑼𝑼 c) D + D → D2(g)  Lenses 10  Optical component with two or more refracting surfaces that are highly transmissive (cannot absorb UV or visible radiation)  Made of quartz or silica Function:  Collimate light (produce parallel beam of light)  Focus light to an image (focal point) by: a) Collecting radiation b) Condensing radiation to focal point Collimating Lens Focusing Lens  Front Faced (Concave) Mirrors 11  Made of vacuum-evaporated coatings of aluminum or other reflective metals on highly polished substrates (glass)  Further coated with SiO2 or MgF2 to prevent oxidation of aluminum (front surface coated) Function of reflective surface is to: 1. Collimate radiation into parallel beams 2. Steer/change direction of radiation 3. Focus the radiation onto a focal point  Wavelength Selection 12  Wavelength selectors restrict the radiation being measured to a narrow band that is absorbed or emitted by the analyte.  Narrow bandwidths increase the likelihood of adherence to Beer’s Law (assumes radiation is monochromatic).  Each selector has a nominal effective bandwidth.  Types of selectors: a) Filters  Absorption  Bandpass b) Monochromators  Reflection gratings  Diffraction gratings  The wavelength selector used in the UV-Vis spectrometers is a Czerny-Turner monochromator  Czerny-Turner Monochromator 13 Function: Optical system that receives polychromatic radiation from the source, disperses the radiation into individual λ’s and projects λ’s onto the exit slit Location in the instrument: Sample Readout Source Monochromator Detector Holder Device https://www.youtube.com/watch?v=1pIjSuK23RM&t=32s  Czerny-Turner Monochromator Components 14 1. Entrance slit: Controls Po (amount of polychromatic radiation) 2. Collimating mirror: Collects and directs radiation to grating 3. Diffraction grating: Separates polychromatic radiation into individual λ’s 4. Focusing mirror: Projects radiation to the exit slit 5. Exit slit: Allows only a narrow bandwidth of λ’s through to sample holder  Slits 15  Slits are used to isolate a small area of source radiation (entrance slit) or attenuated radiation (exit slit)  A slit in an instrument is a carefully machined metal with sharp edges.  Slit jaws must be parallel and perfectly lined up  Entrance Slit 16  The entrance slit of a monochromator receives continuous polychromatic radiation from the source  Entrance slit dictates the magnitude of Po  Polychromatic radiation enters the entrance slit, and is directed to the grating via the collimating (concave) mirror  Diffraction Grating 17  Grating consists of a hard optically flat polished reflective mirrored surface with many thousands equally spaced parallel grooves etched onto it’s surface Design:  Made from glass blanks which have a reflective surface onto which very narrow grooves are formed using a diamond tool mounted to a ruling engine  UV-visible spectrometer: 1200 -1400 grooves/mm  Diffraction Grating 18 These minute, periodic grooves diffract, or disperse the incident polychromatic light. When incident light hits the diffraction grating, it is dispersed away from the grating surface at an angles dependent on the individual wavelengths.  Diffraction Grating Surface 19  Diffraction Grating 20  Polychromatic light falling onto the grating is fanned out and diffracted at different angles depending upon the λ  shorter λ’s → blue light diffracts at smaller angles (less) than red light which diffracts at large angles Angular dispersion of the incident light is repeated over and over again at all grooves on the grating  Operation of Diffraction Grating 21  Rays come in contact with one another (no longer travelling in parallel)  Most rays are destroyed by destructive interference (out of phase)  Constructive interference results in monochromatic beams reinforcing each other (in phase) Final result → Dispersion of polychromatic radiation into individual wavelengths (monochromatic radiation)  Exit Slit 22  The individual wavelengths are then projected to the exit slit via the focusing (concave) mirror  The exit slit isolates a narrow band of dispersed radiation  Spectral bandpass (in nm) leaving the exit slit depends on the grating dispersion, width of exit slit and position of the grating Exit slit  Effective Bandwidth 23  Impossible to obtain spectrally pure λ’s from a monochromator  Instead, a band of λ’s will emerge from the exit slit, depending upon the dispersion of the grating, exit slit width and position of the grating  We set a nominal λ: Nominal λ Effective bandwidth (spectral bandpass):  Width of a band of λ’s at ½ peak height (intensity) of the nominal λ  Range of λ‘s that fills the exit slit at a given λ setting  Slit Width Determination 24  Exit slit can be adjusted to limit the bandwidths passing through  As slit width is narrowed, the radiant power reaching the detector will decrease  Tradeoff between the wavelength resolution and intensity of light  Effective bandwidth → full resolution between 2 λ‘s and enough radiant power reaching the detector Spectral Spectral Spectral Bandwidth Bandwidth Bandwidth 2nm 1 nm 0.5 nm  Summary of Monochromator Operation 25 Watch this video: Tunable Monochromator; Spectroscopy. Jan 14, 2008. YouTube. http://www.youtube.com/watch?v=1pIjSuK23RM  References 26  Tyrer, N.; Kogut, E. CHEM25415 Lecture on UV-Visible Spectrometer Instrumentation. Presented at Sheridan College, Brampton, ON, Fall 2012.  Tyrer, N. CHEM25415 Instrumental Analysis 1 Laboratory Manual; Sheridan College: Brampton, ON; Experiment 3.  Skoog, D. A.; West, D. M.; Holler, F. J.; Crouch, S. R. Fundamentals of Analytical Chemistry, 9th ed.; Brooks/Cole: California, 2014; Chapter 25.  Rouessac, F.; Rouessac, A. Chemical Analysis, 2nd ed.; John Wiley & Sons: New Jersey, 2007; Chapter 9.