Exam 1 Flashcards
What is Spectrometry
The reactions and measurements of Radiation Intensity
Assumptions of External Calibration
1.) There is no error in the [conc] of standards - careful prep of samples is required
2.) No errors in measurement - careful observation and measurement required
3.) Signal response is the same for standards and samples - done by instrument or blank measurement
OVERALL no CORRECTIONS
Assumes matrix effect are absent or have little impact in the analytical (use of certified pure substances)
Systematic Errors
from inaccurate gravimetric and volumetric measurements
Matrix Effect
Species not within blank, present in sample, will cause samples/standards different responses.
Differences in experimental variables at the time of measurement of blanks, standards and samples
Precision
Reproducibility of results from a measurement.
Represented as stdev, relative stdev,
standard error of mean, coefficient of variant and variance
Bias
A measure of the systematic error of an analytical method, technique, equipment or calibration. Proper instrument calibration and use blanks to reduce bias
Sensitivity
The ability to discriminate between small differences in analyte [conc] limited by the slope of the calibration curve and precision of the measurement.
Detection Limit
The minimum concentration or mass of an analyte that can be detected (Sm = signal(blank) + k * blank(stdev)
Dynamic Range
Concentration over which measurement is reliable
Selectivity
Freedom from interferences
What is more sensitive?
a.) slope= 3.0 E3 ; R^2 = 0.9393
b.) slope= 3.0 E4 ; R^2 = 0.8326
B.) - When talking about sensitivity, the slope express the sensitivity. Not the R^2
Standard Addition Method
Used to eliminate matrix in samples by spiking the sample with known aliquots of a standard
Must use if standard and unk. have drastically different solution environments
Standard add. Method: How does it work?
Constant is added to remove proportionality ( i.e. we are changing from proportionality to equaling)
Internal Standard Method (Quality Control Method)
I.S. should have similar properties to the test samples and standards; producing a distinct signal from both sample/standards
Added in constant amount
or present in excess, assumed to be constant
Corrects both measurement and instrument error.
Using a ratio of both sample/standard provides a better R^2
Signal
The measurement that contains the information of the analyte
Noise
Contains no information regarding the analyte and overall decrease accuracy and precision - overall limiting the amount of analyte that can be detected
Signal-to-Noise Ratio (S/N)
Best describes the quality of an analytical measurement of an analytical method or the performance of an instrument and is defined as the stdev of the measurement of the signal
Sources of Instrumental error
Faulty calibrations
Calibration errors in meters
Weights
volumetric glassware
Chemical Noise
Variations in experimental onditions cause a change in the chemistry of the analyte
Instrumental Noise
This is associated with components of the instrument
Thermal Noise / Johnson Noise
Caused by electron or charge agitation which leads to fluctuations
Measure results of signal changes in temp
Voltage Measurement
Bandwidth dependent
Shot Noise
Fluctuations of electrons in semiconductors
Dependent on Bandwidth
Hardware - Signal to noise enhancement
1.) Grounding/Shielding
2.) Electronic filtering
3.) Modulation
Grounding/Shielding
Surrounding critical instrument components with a conducting material attached to the ground to remove noise and electromagnetic radiation from the environment
Electronic Filtering
Removes frequencies that are different from the signal frequency (use low or high pass filter to reduce noise)
Modulation
Move signal to quieter region of the spectrum
Software - signal to noise enhancement
Ensemble Averaging
Smoothing
Digital filtering
Ensemble Averaging
Noise is often random, signal is not. By recording repetitive signals, random noise can be reduced
Entirely dependent on times measured (n)
Provides stronger signals
Smoothing
Averaging successive points from rough data - the more successive points, increases smooth character of line
Digital Filtering
Mathematically remove selected fragments
Signal is low so use a low pass filter to reduce background - converts from signal domain to frequency domain
True or False
Hardware is out of our control, we cannot change it, however we can control the software methods that can shape our data from hardware
Electronic Transition
When an electron is excited from one energy level to another in a molecule or atom.
When electrons jump from one energy level to another, they absorb energy equivalent to the energy difference between the two levels.
Excited Singlet State
The electron is promoted in the same spin orientation as it was in the ground state (paired)
Internal Conversion
The radiation less transition between energy states of the same spin
The intermolecular process in which a molecule crosses to a lower electronic state without emitting radiation
Intersystem Crossing (ISC)
radiation- less process involving a transition between two electrons states with different states spin multiplicity
Is the process in which a molecule in one spin state changes to another spin state with nearly the same total energy (e.g. single to triplet)
Nonradiative Relaxation
A step-wise loss of energy by an excited atom (no fluorescence)
Non-resonance Fluorescence
Due to vibrational energy loss, the emitted radiation has a lower energy level or longer wavelength
Resonance Fluorescence
Process in which the emitted and excited radiation have identical frequencies
Produced by atoms in the gaseous state with no vibrational energy levels
There is no radiative or vibrational energy loss upon excitation making the excited species fluoresce at the same energy as excitation
Singlet State
The spins of the electrons of an atom or molecule are all paired so there is not a net spin angular momentum
Triplet State
Is one in which the spins of the electrons of an atom or molecule are unpaired so that their spin angular moments add to give a net-zero moment
Vibrational Relaxation
is the process by which a molecule loses its excess vibrational energy without emitting radiation
External Conversion
Is a radiation- less process in which a molecule loses electronic energy while transferring that energy to the solvent or another solute
Quantum Yield
The fraction of excited molecules that undergo fluorescence
When does fluorescence occur?
Rate of Diffusion ( 10E-8) sec
Fluorescence excitation spectra
-Where is fluorescence and why?
Fluorescence is on the right of excitation because of energy loss from vibrational relaxation
As a result, fluorescence bands occur at longer wavelengths than absorption bands
How does Temperature Affect Quantum Yield?
QY decreases with the increase in temp, due to an increase in frequency of collision at high temps and a high probability of deactivation by external conversions
How does Viscosity Affect Quantum Yield?
Increase in viscosity, reduces the frequency of collisions and increase the fluorescence. Increases QY
How does Structure Affect Quantum Yield?
Aromatic functional groups, pi-pi* transitions, fused-ring structures are rigid and produce more fluorescence. Less rigid aromatic structures undergo high frequency vibrations and loss in energy. I
I.e. More rigid/support, higher fluorescence = Increased QY
What is Fluorescence dependent on?
Structure
Energy
Time
How do Halogens decrease fluoresce?
Halogens decrease fluorescence via heavy-atom effect. Cause orbital spin interactions resulting in high intersystem crossing effect and increase the rate of triplet formation .
Halogens are useful for slowing down fluorescence to get to phosphorescence (slows down transition - 2 secs)
Effect on QY: Decreases by keeping excited species longer (Quenching Fluorescence) past the point of fluorescence.
What do carboxylic acid and carbonyl groups do to fluorescence ?
Inhibit due to lower n-pi* transition energy than pi-pi*
Overall don’t want lone pairs, as they decrease fluorescence
Absorbance
Absorbance is always a fraction Po/P
Specifically, abs is related to ratio of Incident and transmitted beam, Max you can get is 1. Overall absorbance can be limited by concentration
Po is incident beam power and P after passing through the medium
A = Po/P
A= Ebc
What is Directly proportional to fluorescence?
F = Kc
Fluorescence intensity is directly correlated to concentration
How low can fluorescence be detected?
Can detect low pico molar range
Spectrometry
Measurement of radiation intensity with an electronic transducer
Wave Model
Describes electromagnetic radiation as a wave characterized by wavelength, frequency, velocity and amplitude
Particle Model
Electromagnetic radiation is a stream of discrete particles/wave packets of energy called photons
Frequency (Weird V)
Number of oscillations of the radiations/sec
Wavelength
Linear distance between 2 successive equivalent points
What is important about Frequency?
The frequency of a beam of radiation is determined by the source and remains fixed and invariant (CONSTANT)
In vacuum frequency = 3.0 * 10^8 m/s
Frequency is not proportional to wavelength as it is constant
What happens when a beam is impeded? i.e. does it have an effect on Frequency, Velocity or Wavelength
Frequency is always constant
Velocity and Wavelength decrease
What is the frequency or wavelength of radiation related to?
Energy differences between states
What happens when an atom or ion changes its state?
It absorbs or emits an amount of energy exactly equal to the energy difference between the states
Constructive Wave
2 or more waves in phase with each other
The amplitude of the resultant wave is additive and greater than the individual waves
Add both energies from the waves to provide a stronger wave
Destructive Wave
2 or more waves travel in opposite directions (out of phase) to each other
The amplitude of the resultant wave is less than (dampened) the individual waves
more noise (measuring too low of amplitude)
What is the lifetime of excited vibrational transition?
10E-12 sec
What is the lifetime of electronic transition?
10E-8 sec
Emission Spectrum Vocab:
Lines?
Bands?
Continuum?
Lines - From individual atomic particles behaving independently of each other (Brine spectrum examples - Na, K, Ca)
Bands - From small molecules or gaseous radicals is due to numerous vibrational levels superimposed on each other (Brine spectrum examples - CaOH)
Continuum - A feature of the temperature of the emitting surface rather than the material composing the surface (Dependent on Radiation sources from instrument, i.e. The sun, tungsten lamp, deuterium lamp)
Continuum Radiation Sources
Xenon arc
Carbon arc
Tungsten lamp
Nernst glower
UV Absorption Spectra Vocab:
Line Spectra
Band Spectra
Continuum
Line Spectra
- Absorption of Monoatomic vapor
- Fewer number of possible energy states
Band Spectra
-Absorption of Polyatomic molecules
-Greater number of possible energy levels in molecule than isolated atoms
Continuum
-Interaction with solvent molecules leads to broadening
Sources of Band Broadening
1.) Pressure Broadening
2.) The Uncertainty Effect
3.) Doppler Effect
4.) Electric and Magnetic Effect
Pressure effect
Due to collisions between atoms of the same kind and with different atoms
The Uncertainty Principle
Spectra lines have finite width because the lifetimes of the upper and lower states of the transition are finite which leads to uncertainty in energy
The energy of a particle can be known with zero uncertainty only if it is observed for an INFINTE period
Doppler Effect
High frequency: Decrease in wavelength
Low frequency: Increase in wavelength
Electric and Magnetic Effect:
Cannot do anything to fix this.
Can cause variations of varying electromagnetic radiation changes energy levels; leading to Band Broadening.
When atomic vapor is exposed to a strong magnetic field, a splitting of the electronic energy levels of the atom occurs and leads to the formation of several abs lines for each transition
Effect of temperature on atomic spectra
Temp causes efficient atomization and increase in the number of excited atoms
Leads to line broadening due to particles traveling at higher velocities
Temperature increases the ratio of the number of excited and unexcited atomic particles in an atomizer
