Instrumental Methods Exam 3

Question 1

intersystem crossing

Answer 1

transition from a singlet to a triplet state

Question 2

fluorescence

Answer 2

emission of a photon (after excitation with a photon) all states are singlet states

Question 3

Phosphorescence

Answer 3

emission of a photon from a triplet state to a singlet state

Question 4

internal conversion

Answer 4

moving between vibrational modes in one electronically excited state to a mode in another

Question 5

external conversion

Answer 5

radiation-less decay, usually results from collisions

Question 6

quantum efficiency of fluorescence

Answer 6

tells us the likelihood of observing fluorescence as a function of rates of decay pathway

Question 7

combination modes

Answer 7

arise if a molecule has more than one mode

Question 8

time domain spectroscopy

Answer 8

measures variations of electric field strength over time

Question 9

Michelson interferometer

Answer 9

a device that slows down the signal

Question 10

throughput or Jaquinot advantage

Answer 10

all light gets used, no loss in slits, minimal loss in optics

Question 11

speed of acquisition “multiplex advantage”

Answer 11

easier to get more measurements quickly

Question 12

raman spectroscopy

Answer 12

tells us about the vibrational modes of molecules but uses excitation source in VIS or near IR region, different vibrations are active

Question 13

mass spectrometry

Answer 13

generates ions or ionic fragments of molecules and analyzes then as a function of mass to charge

Question 14

electron impact ionization

Answer 14

electrons generate via thermoelectric effect from hot filament, accelerate across a moderate potential, and collides with and ionizes analyte

Question 15

chemical ionization

Answer 15

ionizes some reagent ions which then react with and ionize analyte

Question 16

common reagent ions

Answer 16

small molecules that do not tend to fragment

Question 17

field desorption

Answer 17

electrode is dipped in sample held at high potential, mechanism is unknown but it has no fragmentation

Question 18

MALDI

Answer 18

(matrix assisted laser desorption ionization) analyte embedded in sample matrix, laser shines on sample matrix, ionizes matrix which transfers charge to analyte, no fragmentation

Question 19

electro spray ionization

Answer 19

liquid sample mixed with analyte, evaporation of solvent, coulomb explosion turns to charged analyte

Question 20

quadrupole

Answer 20

selects m/z using 4 modes in pairs, each with an oscillating voltage so that only certain m/z have stable trajectories

Question 21

magnetic sector analyzer

Answer 21

ions are accelerated across a potential and focused into a magnetic field, radius of curve shows m/z

Question 22

double focusing

Answer 22

uses electric field to select particular KE - can improve resolution greatly

Question 23

time of flight

Answer 23

detects m/z as a function of velocity (arrival time)

Question 24

faraday cup

Answer 24

current flows through large resistor to neutralize accumulated positive charges - generated measurable voltage

Question 25

electrochemical measurements

Answer 25

take advantage of reduction / oxidation reaction in which electrons are transferred between reactants

Question 26

potentiometry

Answer 26

measures the strength of the driving force behind electrons

Question 27

electrolytic cell

Answer 27

cells with current driven backwards

Question 28

thermodynamics of cell potentials

Answer 28

measured voltage implies concentration are not at equilibrium

Question 29

pH probe

Answer 29

glass membrane, constant H+ on the inside, variable H+ in analyte causes potential to develop across membrane

Question 30

alkaline error

Answer 30

anytime the [H+] is similar to the positively charged alkali metals- probe will respond to those and report a lower pH than actual

Question 31

acid error

Answer 31

at very low pH, you have populated all available sites, reports higher pH than actual

Question 32

elution

Answer 32

the process by which compound become separated as they move through a column - driven by differential partitioning

Question 33

chromatogram

Answer 33

plot of peak intensity vs time

Question 34

band broadening