Instrumental Analysis ACS Exam Correct
100%
How to calculate signal to noise - ANSWER S/N = mean/STDEV
-inverse of RSD
-STDEV: 5 (#STDEV gaussian curve)
-STDEV: peak-to-peak noise/5
S/N relationships - ANSWER -increase S/N, increase run time
-decrease noise, increase S/N
Beer's Law - ANSWER
Beer's Law Calculations - ANSWER
Hard ionization vs soft ionization - ANSWER -hard: no M+ peak (or small)
-soft: M+ peak is visible
MS: hard ionization sources - ANSWER -impart high quantities of residual energy in the
subject molecule invoking large degrees of fragmentation
MS: hard ionization sources examples - ANSWER -electron impact
Electron Impact (EI) - ANSWER -Uses electron impact to ionize a molecule
-gas phase
-energetic electrons
-possibility of over fragmentation
-hard
-most common
EI advantages - ANSWER -good stability
-universal ionization
-reproducible spectra
EI disadvantages - ANSWER -MW can be hard to determine
-compatible only with volatiles (GCMS)
-Hard to make negative ions
MS: soft ionization sources - ANSWER -processes which impart little residual energy
onto the subject molecule and as such result in little fragmentation
MS: soft ionization sources examples - ANSWER -CI
-ESI
, -MALDI
Chemical ionization (CI) - ANSWER -First ionizes a molecular gas which in turn ionizes
the molecule of interest. A "gentler" method of ionization.
-soft
-electron beam creates ions
-proton transfer between reagent gas and the analyte
-gives MW and MW+1 or MW-1
Quadrupole mass analyzer - ANSWER -oscillating electrical fields to selectively stabilize
or destabilize the paths of ions passing through the field created b/w 4 parallel rods
-mass selective filter
photomultiplier tube - ANSWER -An apparatus that converts a photon of visible light into
an electrical pulse
-most sensitive photodetector
CCD - ANSWER -is a silicon-based multichannel array detector of UV, visible and near-
infra light
-sensitive to light
Beer's Law extension to mixtures of compounds - ANSWER -can extend to sample
containing several absorbing components if there are no interactions
-individual absorbances are additive
-low concentration analytes
-assumes one monochromatic wavelength
-
Sensitivity of absorbance methods - ANSWER -low sensitivity at low sample
concentrations
Sensitivity of Fluorescence methods - ANSWER -sensitivity is x1000 greater than
absorption
-leads to more accurate and precise results
-more expensive because of this
Ion Selective Electrodes (ISE) - ANSWER -transducer that converts the activity of a
specific ion dissolved in a solution into an electrical potential.
Electrochemical detector (HPLC) - ANSWER -analyte gets either oxidized or reduced at
the working electrode
-only responds to compound that can partake in redox reactions
-measures current when an electroactive solute emerges from the column and passes
over the working electrode.
100%
How to calculate signal to noise - ANSWER S/N = mean/STDEV
-inverse of RSD
-STDEV: 5 (#STDEV gaussian curve)
-STDEV: peak-to-peak noise/5
S/N relationships - ANSWER -increase S/N, increase run time
-decrease noise, increase S/N
Beer's Law - ANSWER
Beer's Law Calculations - ANSWER
Hard ionization vs soft ionization - ANSWER -hard: no M+ peak (or small)
-soft: M+ peak is visible
MS: hard ionization sources - ANSWER -impart high quantities of residual energy in the
subject molecule invoking large degrees of fragmentation
MS: hard ionization sources examples - ANSWER -electron impact
Electron Impact (EI) - ANSWER -Uses electron impact to ionize a molecule
-gas phase
-energetic electrons
-possibility of over fragmentation
-hard
-most common
EI advantages - ANSWER -good stability
-universal ionization
-reproducible spectra
EI disadvantages - ANSWER -MW can be hard to determine
-compatible only with volatiles (GCMS)
-Hard to make negative ions
MS: soft ionization sources - ANSWER -processes which impart little residual energy
onto the subject molecule and as such result in little fragmentation
MS: soft ionization sources examples - ANSWER -CI
-ESI
, -MALDI
Chemical ionization (CI) - ANSWER -First ionizes a molecular gas which in turn ionizes
the molecule of interest. A "gentler" method of ionization.
-soft
-electron beam creates ions
-proton transfer between reagent gas and the analyte
-gives MW and MW+1 or MW-1
Quadrupole mass analyzer - ANSWER -oscillating electrical fields to selectively stabilize
or destabilize the paths of ions passing through the field created b/w 4 parallel rods
-mass selective filter
photomultiplier tube - ANSWER -An apparatus that converts a photon of visible light into
an electrical pulse
-most sensitive photodetector
CCD - ANSWER -is a silicon-based multichannel array detector of UV, visible and near-
infra light
-sensitive to light
Beer's Law extension to mixtures of compounds - ANSWER -can extend to sample
containing several absorbing components if there are no interactions
-individual absorbances are additive
-low concentration analytes
-assumes one monochromatic wavelength
-
Sensitivity of absorbance methods - ANSWER -low sensitivity at low sample
concentrations
Sensitivity of Fluorescence methods - ANSWER -sensitivity is x1000 greater than
absorption
-leads to more accurate and precise results
-more expensive because of this
Ion Selective Electrodes (ISE) - ANSWER -transducer that converts the activity of a
specific ion dissolved in a solution into an electrical potential.
Electrochemical detector (HPLC) - ANSWER -analyte gets either oxidized or reduced at
the working electrode
-only responds to compound that can partake in redox reactions
-measures current when an electroactive solute emerges from the column and passes
over the working electrode.
