characterization
Terms
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- The spectronic 20 spectrophotomer is used for
- to measure ability of a liquid sample to absorb light (in a visible region of electromagnetic spectrum) at a set wavelength
- To obatin chemical or physical information you must have:
-
Signal generator
input transducer
signal modifier
output transducer - SISO for spect 20
-
s-tungsten lap
I-measuring phototube
S-analog amplifier
O-analog abs meter - Sensitivity limits
- the abilit to separate signifcant data-containing signals from meaningless noise
- define senisitivity
- change in instrument response w/change in stimulus
- fo absorption spectroscopy, sensitivity formula is
-
s=da/dc=
a is sample absorbance valu
c is saple concentration - sensitivity is affected by (3 things)
-
-physical & chemical properties of the analyte
-ddetector response
-composition of sample matrix - sensitivity is determined from...
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the slope of the calibration curve:
absorbance vs concentration - At low concentrations, noise..
- limits the lowest concentration of analyte that can be detected. aka it has hit the detection limit.
- At high concentrations, noise..
- fixes the precision of measurements.
- for signal/noise signal enhancement, theres hardware that
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-filters-blocks out unwanted
-integrators-increases coherent nonrandom sigover random noise
-modulators-shifts import freq away from noice - for signal/noice signal enhancement, software...
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-boxcar averaging-lots of data points
-ensemble avearaging
-fourier transformations - define boxcar averaging
-
avg of a small series of data points to get one datapoint then use to get a curve.
mosteffectve w/fast signal - fourier transformation is used to
-
-convert time domain data f(t) into frequency domain data F(v)
-much faster than old frequ domain methods - FT's takes a single waveform and converts it into
- a single line at frequ vo w/an amp a.
- The FT conversion takes multicomponent waveforms that are generated by ____ & the sampling freq should by at least___
-
superposition of individual single frequ waves
&
2x that of the highest frequ component - define spectroscopy:
- the measurement & interpretation of electromagnetic radiation (energy)absorbed/emitted when mcules of a sample move from 1 allowed energy state to another
- electromagnetic radiation (light) has properties of:
-
-a microscopic particle having a discrete energy
-a wave extending over a broad area of space - light ineracts w/matter in only 3 fundamental ways:
-
reflectoin
transmission
absorption
=all 3 happen simutaneously - in the absorption process, the photon
- is consumed & its energy is converted into an atomic or mcular process
- in the absorption process its coverted into an atomic or mcular process meaning
-
-a nucleus spins in an altered manner
-a mcule vibrates differently
-an electron changes orbit - wavelength
- λ-is measured as distance between 2 crests
- period
-
T-is time elapsed between crests
T=1/V - frequency
- V=# of waves that pass a fixed point in a given unit of time
- amplitude
- A- is max magnitued of the displacement
- Intensity
- I=proportional to the amplitude squared
- A wave of electromagnetic radiation contains
-
electric field vectors
magnetic field vectors
plane polaraized light - electric field vectors
- have periodic maxima perpendicular to direction of propagation
- magnetic field vectors
- orthogonal to electric field vectors
- Plane polarized light
- has its periodic field oscillations occuring in a set plane
- The wavelength & frequency are related to energy E of a photon by Plank's constant,h, such that:
- E=hv
- In absorption or emission spectroscopy, the energy difference between the low energy state & high energy state is given by
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bohr frequency condition:
{}E=hv - The only true characteristic of a particular wave of electromagnetic radiation is
- frequency
- the index of refraction depends on the
- wavelength
- frequency of electromagmetic radiation remains
- constant, meaning photon energy is constant & waveletngth shorens as wave travels through matter
- short wavelength(which means---energy) waves interact w/electrons----than long wavelength waves
- high energy, more strongly
- greater interaction of the waves gives a
- larger n-index of refraction
- monochromatic light
- contains a single wavelength (or frequency) ex color, laser light
- polychromatic light
- contains spectral range of many wavelengths (ex white light)
- constructive interference
- in phase combination increases resulting amplitude
- desturctive interference
- out of phase combination decreaes resulting amp
- the phase angle defines
- period shift in periodic wave oscillations
- define NMR
- study of molcular sturcture by means of interaciton of radio frequency of e.r.w/a collection of nuclei immersed in a magnetic field,Ho
- quantum mechanics- for a particle to absorb a photon of e.r., it must
- first be in some uniform periodic motion
- frequency of motion for e.r.must exactly match frequ of----& is true of all absorption spectroscopies
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absorbed radiation
vmotion=vphoton