INSTRUMENTAL ANALYSIS ACS
EXAM QUESTIONS AND ANSWERS
2026/2027
How to calculate signal to noise - ANSWERSS/N = mean/STDEV
-inverse of RSD
-STDEV: 5 (#STDEV gaussian curve)
-STDEV: peak-to-peak noise/5
S/N relationships - ANSWERS-increase S/N, increase run time
-decrease noise, increase S/N
Beer's Law - ANSWERS
Beer's Law Calculations - ANSWERS
Hard ionization vs soft ionization - ANSWERS-hard: no M+ peak (or small)
-soft: M+ peak is visible
MS: hard ionization sources - ANSWERS-impart high quantities of residual energy in
the subject molecule invoking large degrees of fragmentation
MS: hard ionization sources examples - ANSWERS-electron impact
Electron Impact (EI) - ANSWERS-Uses electron impact to ionize a molecule
-gas phase
-energetic electrons
-possibility of over fragmentation
-hard
-most common
EI advantages - ANSWERS-good stability
-universal ionization
-reproducible spectra
EI disadvantages - ANSWERS-MW can be hard to determine
-compatible only with volatiles (GCMS)
-Hard to make negative ions
MS: soft ionization sources - ANSWERS-processes which impart little residual energy
onto the subject molecule and as such result in little fragmentation
, MS: soft ionization sources examples - ANSWERS-CI
-ESI
-MALDI
Chemical ionization (CI) - ANSWERS-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
fragmentation in CI - ANSWERS-occurs if enough excess energy after the ionization of
the analyte
CI advantages - ANSWERS-MW info more readily available
-fewer fragments
-control specificity
CI disadvantages - ANSWERS-fragments not very reproducible
-compatible only with volatiles (GCMS)
Electrospray ionization (ESI) - ANSWERS-multiple charged ions
-can make positive or negative ions by taking on hydrogens or removing hydrogens
MALDI abreviation - ANSWERSMatrix assisted-laser desorption ionization
MALDI Theory - ANSWERS-non-volatile ionization
-don't want fragmentation
-TOF
-absorption of laser beam by matrix
-transfer energy form matrix to analyte
-desorption of both matrix and analyte
-transfer of proton to analyte from matrix
-large MW
MS Mass Analyzers - ANSWERSTOF, quadrupoles and traps
TOF - ANSWERS-fastest
-highest m/z range
-uses electric field to accelerate ions through the same potential, and then measures
the time they take to reach the detector
-lower mass reaches the detector first
-assumes that all ions have same voltage, velocity, position, and angle (not always true)
EXAM QUESTIONS AND ANSWERS
2026/2027
How to calculate signal to noise - ANSWERSS/N = mean/STDEV
-inverse of RSD
-STDEV: 5 (#STDEV gaussian curve)
-STDEV: peak-to-peak noise/5
S/N relationships - ANSWERS-increase S/N, increase run time
-decrease noise, increase S/N
Beer's Law - ANSWERS
Beer's Law Calculations - ANSWERS
Hard ionization vs soft ionization - ANSWERS-hard: no M+ peak (or small)
-soft: M+ peak is visible
MS: hard ionization sources - ANSWERS-impart high quantities of residual energy in
the subject molecule invoking large degrees of fragmentation
MS: hard ionization sources examples - ANSWERS-electron impact
Electron Impact (EI) - ANSWERS-Uses electron impact to ionize a molecule
-gas phase
-energetic electrons
-possibility of over fragmentation
-hard
-most common
EI advantages - ANSWERS-good stability
-universal ionization
-reproducible spectra
EI disadvantages - ANSWERS-MW can be hard to determine
-compatible only with volatiles (GCMS)
-Hard to make negative ions
MS: soft ionization sources - ANSWERS-processes which impart little residual energy
onto the subject molecule and as such result in little fragmentation
, MS: soft ionization sources examples - ANSWERS-CI
-ESI
-MALDI
Chemical ionization (CI) - ANSWERS-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
fragmentation in CI - ANSWERS-occurs if enough excess energy after the ionization of
the analyte
CI advantages - ANSWERS-MW info more readily available
-fewer fragments
-control specificity
CI disadvantages - ANSWERS-fragments not very reproducible
-compatible only with volatiles (GCMS)
Electrospray ionization (ESI) - ANSWERS-multiple charged ions
-can make positive or negative ions by taking on hydrogens or removing hydrogens
MALDI abreviation - ANSWERSMatrix assisted-laser desorption ionization
MALDI Theory - ANSWERS-non-volatile ionization
-don't want fragmentation
-TOF
-absorption of laser beam by matrix
-transfer energy form matrix to analyte
-desorption of both matrix and analyte
-transfer of proton to analyte from matrix
-large MW
MS Mass Analyzers - ANSWERSTOF, quadrupoles and traps
TOF - ANSWERS-fastest
-highest m/z range
-uses electric field to accelerate ions through the same potential, and then measures
the time they take to reach the detector
-lower mass reaches the detector first
-assumes that all ions have same voltage, velocity, position, and angle (not always true)
