Test 1 Flashcards
Spectroscopy
use of E/M energy, particles, or sound to study matter
Qualitative
What is present?
Quantitative
How much is present?
Limitations in using human eye
- Need standards for each unknown studied. 2. Uncolored solutions. 3. Variations in light source. 4. Variations in eye sensitivity. (color blindness)
All Instruments have:
source, monochromator, sample, detector, readout device
Types of Errors
- Random 2. Systematic
Random
(Indeterminate) Noise Example: Random variations in mass of a weighing boat.
Systematic
(Determinate) Instrumental bias, personal, method Example: Always dispensing total volume of a pipette even though pipette marked TD.
Absolute Error
E(abs)=X-Xt Xt accepted or true value and X is the measurement or average of several measurements.
Relative Error
E(relative)=(X-Xt)/Xt
Population
All measurements of an observable. Infiinite. (impossible to get)
Sample
A subset of all the possible measurements. Finite. (Your results)
Samples mean
Summation of all samples divided by number of samples.
Sample Std. Dev
Sx=sqrt(Summation of (xi-xmean)^2/(N-1))
Signal (S)
A voltage or current produced by the spectrometer in response to a change in the absorption or emission of photons by a sample. (Avg. peak height above avg. baseline)
Noise(N)
Extraneous and unwanted signals that are superimposed on the desired signal. N=max. std. dev of baseline.
SNR
Signal to Noise Ratio. Measure of quality of a signal in spectrum.
Spectra are…
variations in a voltage as wavelength is scanned.
Time…
elapses as a spectrum is scanned
Signal & Noise…
can be thought of variations in a voltage w/ respect to time or in terms of their frequency
Don’t confuse the frequency…
of a signal and noise with the frequency of the light being absorbed or emitted in the spectroscopy.
S/N need to observe a signal…
depends on frequency of noise and if some knowledge of the signal exists.
Types of Noise:
- Chemical 2. Instrumental 3. Thermal of Johnson 4. Shot 5. Flicker 6. Environmental
Chemical Noise
Noise arising from uncontrollable variables that affect chemistry of system being analyzed.
Instrumental Noise
Noise associated with components of instrument.
Shot Noise
Occurs whenever electrons are transferred across electrical junctions.
Thermal of Johnson Noise
Noise that contains all frequencies and arises from thermal motion of electrons in resistive elements of electric circuits.
Flicker Noise.
Noise whose frequency spectrum is inversely proportional to frequency, the cause of which is not well understood.
Environmental Noise
Noise from surrounding environment
S/N improvement
=sqrt(number of spectra averaged)
Uncertainty in adding or subtracting
=sqrt(Summation(uncertainty squared))
Uncertainty in multiplying or dividing (x=p*q/r)
Sx/x=sqrt(summmation((uncertainty/value)squared))
Uncertainty in exponential x=p^y
Sx/x=y*Sp/P
Uncertainty in logarithm x=ln(p)
Sx=Sp/P
TV
Total volume of, not in, glassware
SD
Smallest division of glassware
Method of least squares
A mathematical technique that draws the best line through data by minimizing the residuals.
If you blank…
(0,0) should be a data pt.
E/M Radiation
Beam of sub-atomic particles called photons that possess an oscillating electric and magnetic field
Frequency
(curly v) number of oscillations of the field per second. (Hz)
Period
(P) time between 2 successive maxima (s)
Wavelength
(lamba) distance between successive maxima (nm)
Velocity
(vi) rate at which a wavefront moves through a medium i. (m/s)
Wave number
(sigma) number of waves or oscillations per cm. (/cm)
Power
(P) energy that reaches an area per second (Watt)
Intensity
(I) Power per unit solid angle (Watts/steradian)
10-180 nm
Vacuum UV
180-400 nm
UV
400-700 nm
Visible
2.5-15 micron
InfraRed
.3-10 m
Nuclear Magnetic Resonance
Vacuum UV
10-180 nm
UV
180-400 nm
Visible
400-700 nm
InfraRed
2.5-15 micron
Nuclear Magnetic Resonance
.3-10 m
When two or more waves….
traverse the same space the resultant field at any time is the sum of the fields of individual waves
Wave equation
Y=A*sin(2pi*frequency*time+phase angle)
Destructive interference
phase angle differs by +/- pi A=A1-A2
Constructive interference
equal phase angles, A=A1+A2
Plane Polarized
All electric fields of a beam of photons lie in one plane
Monochromatic
All photons have same frequency
Coherent
All photons have same frequency and phase angle
Energy
of a single photon is dependent of it’s frequency E=hv
Velocity
v=frequency*wavelength Vvac=c=3*10^8 m/s
Velocity is always slower…
in a medium
frequency is always…
constant from one medium to another
Refractive index
ni measure of interaction of medium i with E/M Radiation
ni=
c/vi=wavelength vacuum/wavelength
Refractive index….
is usually wavelength dependent and reported at a wavelength
Refraction
change in direction of beam of E/M radiation on passing from one medium into another with different refraction index. sin(theta1)/sin(theta2)=refraction index1/refraction index2=v1/v2=wavelength1/wavelength2
Reflection
angle of incidence equals angle of reflection
Intensity of reflection
Ireflected/Iincident=(n2-n1)^2/(n1+n2)^2
Diffraction
process in which beam of parallel waves of radiation is bent as it passes by a sharp barrier or through narrow opening
Diffraction equation
CF=BCsin(theta) DE=OD*sin(theta) n*wavelength=BCsin(theta)=BC8DE/OD constructive when n=0(equal path length) or n=1 (path length differ by one wavelength)
Absorption
removal of select frequency from a beam of radiation as it passes through a medium
Types of Absorption
Electronic, vibrational, and magnetic
Electronic Absorptio
H(1s)+hv->H(2s) Uv, Vis
Vibrational absorption
HCl+hv-> IR
Magnetic Radiation
H(^) +hv-> ESR, NMR
Photons are produced…
When excited molecules and atoms return to lower energy levels
Absorbance equation
A=log(Pincident/Ptransmitted)=Molarabsorptivity*pathlength*concentration
Emission
Fluorescence power=constant*concentration
Thermal Radiation
Radiation emitted from a solid when heated to incandescence
Difference between absorption setup and fluorescence
both source-wavelength selector-sample-detector-readout, but fluorescence has additional wavelength selector after sample
Discontinuous Spectrum
Tall narrow peaks (think sodium spectra)
Continuous Source Spectrum
Think SWCNTs ROlling peaks
Effective bandwidth
full width of emission peak at half its height
H2 or D2 light source
continuous light source, electrical excitation of H2 or D2 at low pressure UV 160-375 nm
Tungsten Filament
Electrically heated W filament (320-2500) Vis-IR
Halogen Lampe
W lamp with Halogen gas that minimizes W deposition on glass
Xenon Arc lamp
electrical discharge in Xe 250-600 UV-Vis
Nernst Glower
electrically heated rare earth oxides, ZrO2 and Y2O3 320-20000 nm Vis-IR
LED
incoherent semiconductor that emits narrow band of light when electrically biased in forward direction 420-720 Vis 10 x’s more efficient than W in Vis blue peak ~450, phosphorescence peak ~550
Metal Vapor Lamp
Gaseous metal atoms at low P excited with electrical discharge
Hollow Cathode Lamp
excited metal atoms fro a sepecial geometry cathode (AA spectroscop)
Laser
light amplification by stimulated emission of radiation coherent light source inverted population needed for laser to operate
Absorption Filter
Colored glass which absorbas a select band of light and transmits others. Bandwidth ~ 40 nm
Interference Filter
transparent dielectric filter sandwiched between two partially reflective surfaces. Constructive and destructive interference causes selective transmittance. Bandwidth ~5 nm. m*wavelength=2*T ^^T is thickness of dielectric ^^wavelength dielectric*ndielectric=wavelength vacuum
