Instrumental Analysis Exam 1

T/F A continuous source provides a constant output of radiation over a wide range of wavelengths.

True

T/F Fluorescence is a type of emission that originates from a triplet source.

False, originates from a singlet source (as opposed to phosphorescence, which emerges from a triplet source).

T/F The wavelength isolation device that has the narrowest bandpass is the filter.

False, a diffraction grating has the narrowest bandpass (highest wavelength resolution).

Filter (broadest) > Prism > Diffraction grating (narrowest)

T/F A bathochromic shift means a shift in wavelength to longer wavelengths (lower energy).

True, also known as a red shift. It is the opposite of a hypsochromic shift (blue shift).

T/F A heterogeneous sample has different compositions in different regions of the sample.

True, as opposed to a homogeneous sample.

T/F Intersystems crossing is an example of radiative process from a singlet to triplet excited state.

False, intersystems crossing is non-radiative (S_n → T_n). It involves a change in electron-spin multiplicity and does not emit a photon.

By contrast:

• Fluorescence: radiative, S₁ → S₀

• Phosphorescence: radiative, T₁ → S₀

T/F Amplification is a signal processing method designed to reduce the signal output.

False, amplification increases signal output.

T/F Samples for fluorescence are typically degassed to remove oxygen.

True, because O₂ can quench luminescence, reducing fluorescence intensity.

T/F Corrected spectra allow for data obtained from different instruments to be properly compared.

True

T/F Slits are examples of a type of filter.

False, slits are adjustable openings in a monochromator that limit the amount and spatial spread of light. Filters selectively transmit/reject light based on wavelength through absorption, reflection, or interference.

Phrase Q: A transducer with a built-in amplification system.

Photomultiplier, which utilize a photocathode (photoelectric effect) and (accelerated to) series of dynodes, eventually reaching the anode/detector

Phrase Q: The continuous source used for visible absorption (and near IR) measurements.

Tungsten lamp, which works by resistive heating

Fun fact: Tungsten-halogen lamps allow for operation at a higher filament temperature → higher light intensity (brightness) → use in car headlights

Phrase Q: A wavelength isolation device that reflects light based on Bragg’s Law.

Diffraction grating, whose resolving power increases with the number of illuminated grooves

Bragg’s Law: nλ = d(sin θ_incident ± sin θ_reflection)

Resolving power: R = (n = grating order) x (N = # grooves) = W(sin θ_incident ± sin θ_reflection) / λ

Phrase Q: A monochromatic source of electromagnetic radiation.

Laser

Phase Q: A filter composed of alternating layers of high and low refractive index materials to eliminate a very narrow wavelength region (i.e., effective transmittance is zero in that region).

Notch filter

Absorption v.s. absorbance

• absorption: results from interaction with ions or molecules, energy transfer from photons to atoms/ions/molecules in a substance

• absorbance: the quantitative measure of absorption that depends on medium composition and the length of the optical path, given by A (= εbc)

Excitation vs. Emission vs Absorption Spectra

• excitation spectrum: luminescence intensity as the excitation wavelength is scanned while the emission wavelength is fixed

• emission spectrum: luminescence intensity as the emission wavelength is scanned while the excitation wavelength is fixed

• absorption spectrum: absorbance as a function of incident wavelength, shows wavelengths that promote analyte from lower to higheer energy states

Chemiluminescence

A chemical reaction yields an electronically excited species that emits light and returns to the ground state

Reactants → Excited Product → Ground State Product

Given an absorption spectrum, how would you conduct the collection of emission and excitation spectra?

1. Emission Spectrum: Scan to the red of fixed λ_ex = λ_max of absorption (i.e. scan above).

2. Excitation Spectrum: Fix emission monochromator at λ_em = λ_ex,max and scan excitation wavelengths across absorption band.

Characterize the components of electromagnetic radiation

Sinusoidal oscillations at right angles to each other and in the direction of propagation.

• Plane x: direction of propagation

• Plane y: electric field

• Plane z: magnetic field

Sampling v.s. sample preparation

• sampling: obtain raw sample containing analyte from environment, may be homo- or heterogenous

• sample preparation: isolation of analyte from raw sample (purification/concentration) for downstream characterization and experimentation

Lasers as a source of luminescence measurements: Pros and Cons

Pros: high intensity, extremely precise specificity/wavelength

Cons: limited wavelength accessibility

Absorption spectrum collection: photodiode array (advantages) v.s. dual beam absorption spectrophotometer

Faster (collect data points simultaneously), signal averaging, high λ reproducibility

Ensemble averaging

Repetitive measurements or successive data sets over an entire spectral region are collected, summed together point by point, and finally divided by the number of scans

sqrt(# measurements) * S/N

Advantage: The impact of noise decreases, noise is averaged out of a set of datasets

Beer’s Law

A = εbc

Percent transmittance

Fraction of incident light transmitted by the matter

%T = P_in / P_out

A = -log(T) aka T = 10^-A

Purpose and operation of a monochromator

Entire dispersing module containing five components that work together to isolate the desired, narrow spectral band (ideally a single wavelength or frequency)

Five components of a monochromator

1. Entrance slit

2. Collimator (spread rays in parallel direction)

3. Wavelength isolator (dispersing)

4. Focusing element (reform image)

5. Exit slit

Single beam UV-Vis spectrophotometer box diagram

Fluorescence spectrophotometer box diagram

Three major/distinct differences in the instrumental components and setup between UV-Vis v.s. fluorescence spectrophotometers and their significance

1. in line v.s. 90º detector placement: For fluorescence, keeps it out of the direct path of the high-intensity excitation beam, which reduces contamination

2. 1 v.s. 2 (excitation/emission) monochromators: Luminescence requires either λ_excitation or λ_emission to be held constant.

3. continuous v.s. high-intensity light source: Luminescence requires sufficiently high-intensity light source to achieve excitation, as opposed to absorption prioritizing coverage of desired spectral region.

Signal

Measurable response produced by a generator that contains information about the identity, properties, or concentration of an analyte, represented by x-bar (mean)

Noise

Random, unwanted fluctuations or variations in an analytical signal, obscuring the true signal, represented by s (standard deviation)

Two major sources of noise found in analytical and instrumental methods

1. Instrumental

2. Chemical