Flashcards in Chapter 13 Deck (35):
affect of high concentration on Beer's law
solute-solute, solute-solvent, and H bond interactions change how molecule absorbs.
problem with chemical dissociation
individual components follow beer's law, but total concentration does not
Beer's law only strictly applies with ? sources
radiation source usually employed
polychromatic source with grating or filter
why is the lamda max chosen as the wavelength when plotting concentration vs absorbance
has least variation from adjacent wavelengths. produces a more linear plot of conc. vs absorbance
problems with stray light
limits maximum absorbance
two types of instrument limitations
stray light, mismatched cuvettes
as concentration increases, deviation from stray light decreases/increases
type one noise influence on transmittance
limited readout resolution, heat detector johnson noise, dark current and amplifier noise. Not dependent of transmittance.
type two noise influence on transmittance
photon detector shot noise
type three noise influence on transmittance
cell positioning uncertainties, source flicker. Linearly dependent on transmittance.
type one noise is a usually a result of ?
lower slit width =
high absorbance, greater resolution, but increased noise
false peaks a result of
transmission of stray radiation
light source for UV
h2 or D2 (D2 preferred, much brighter)
Use ? cells in UV region
quartz (fused silica)
mechanism of production of coninuum spectrum
formation of an excited molecular
species followed by dissociation of the excited
molecule to give two atomic species plus an ultraviolet
deuterium wavelength range
used for visible and near-infrared wavelengths. 350-2500nm range. blackbody approximate output (temperature dependent)
add small amount of iodine to tungsten lamp. extends life of source as W can redeposit. higher source intensity.
Beers law generates linear plot when...
concentrations of analytes are low and using a monochromatic source
Problem with polychromatic sources
Analyte can have different molar absorptivity at the different wavelengths. Makes plot less linear if molar absorptivities differ with wavelength.
why match cells
so analyte and blank have same reflection
When to calibrate at 0% transmittance
dark current (no light passing)
When to calibrate at 100% transmittance
noise from transfer of electrons across a junction
components of Single beam instrument
source, filter or monochromator, cel, photodetector, amplifier, readout
Double beam in space components
source, filter or monochromator, lens, beam splitter, reference cell, sample cell, 2 photodetectors, difference amplifier, readout
double beam in time components
source, filter or monochromator, sector mirror, reference cell, sample cell, grid mirror, one photodetector, amplifier, readout
double beam in space photodetector
takes ratio of P and P0 (T=P/P0)
double beam in time example instrument
most commonly used spec? range?
spec 20. 400 to 900nm
double dispersing instrument
has 2 monochromators