Instrumental Analysis ACS Exam Questions with
Answers
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
fragmentation in CI - ANSWER --occurs if enough excess energy after
the ionization of the analyte
CI advantages - ANSWER --MW info more readily available
-fewer fragments
-control specificity
CI disadvantages - ANSWER --fragments not very reproducible
-compatible only with volatiles (GCMS)
Electrospray ionization (ESI) - ANSWER --multiple charged ions
-can make positive or negative ions by taking on hydrogens or removing
hydrogens
MALDI abreviation - ANSWER -Matrix assisted-laser desorption
ionization
MALDI Theory - ANSWER --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 - ANSWER -TOF, quadrupoles and traps
TOF - ANSWER --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)
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
Answers
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
fragmentation in CI - ANSWER --occurs if enough excess energy after
the ionization of the analyte
CI advantages - ANSWER --MW info more readily available
-fewer fragments
-control specificity
CI disadvantages - ANSWER --fragments not very reproducible
-compatible only with volatiles (GCMS)
Electrospray ionization (ESI) - ANSWER --multiple charged ions
-can make positive or negative ions by taking on hydrogens or removing
hydrogens
MALDI abreviation - ANSWER -Matrix assisted-laser desorption
ionization
MALDI Theory - ANSWER --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 - ANSWER -TOF, quadrupoles and traps
TOF - ANSWER --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)
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
