KEY
Questions – Test 2
1 Circle the best answer.
a.
Increasing the flow rate in a gas chromatograph always
increases resolution
decreases resolution
reduces the effects of diffusion
improves equilibrium between the mobile and stationary phase.
b.
Which is NOT true about a capillary column in GC
it is easy to overload the column
capillary columns tend to have poorer resolution that packed columns
very sensitive detectors are needed
they can be used with mass selective detectors
c.
As compared to HPLC, capillary electrophoresis is
slower and more easily interfaced to MS
more easily used for electrically neutral compounds
slower and has poorer resolution
faster and has higher resolution
d. Which factors affect m/e in a time-of-flight mass analyzer.
voltage and
length and
voltage and
magnetic field
time
length
voltage, time and
length
e. Which of the following is important for resolution in both electrophoresis and
chromatography.
electroosmotic flow
the length of the column
equilibrium between the mobile and stationary phase
longitudinal diffusion
f. Which of the following detectors does NOT destroy the sample?
mass selective detector
FID
thermal conductivity
flame photometric detector
g. Which of the following is NOT a common GC detector?
FID
UV detector
electron capture detector
thermal conductivity detector
h. Which of the following is not a strategy for improving resolution in GC?
use temperature program
change flow rate
change column
change detector
1
KEY
Questions – Test 2
i. What resolution is required to separate peaks with masses 114.53 and 114.55?
114.54
2863.5
11454
5727
j.
Which term(s) in the Van Deemter equation that depend on flow rate?
A and B
A and C
B and C
A and C
k. In a mass spectrometry, the usually sequence of events is:
ionization, acceleration to constant energy, mass analysis
acceleration to constant energy, mass analysis, ionization
acceleration to constant energy, ionization, mass analysis
ionization, mass analysis, acceleration to constant energy
l.
Which factor does NOT affect separation in electrophoresis?
charge
viscosity of solvent
equilibrium
voltage
m. Which type of compounds is expected to have mass spectra with prominent peaks
separated by 14 m/e units.
compounds with aromatic rings
compounds with long alkyl chains
compounds with double bonds
compounds that have been enriched with 14C
n. In which ionization method is the sample is mixed in 1000:1 ratio with low MW gas (CH4,
N2,etc) and very little fragmentation is observed in the mass spectrum.
EI
MALDI
ESI
CI
o. Which is NOT true of exclusion chromatography ?
separation is based on the size of the molecules
the method is commonly used to separate proteins
a given column can be used to separate any mixture as long as you wait long enough
there are some columns that are used for low molecular weight mixtures
p. Which of the mass analyzers below usually gives the best resolution?
quadrupole
TOF
double focusing
q.
r.
Which detector would NOT be appropriate with a capillary column?
FID
electron capture
thermal conductivity
detector
detector
Which is a common detector in mass spectroscopy?
photomultiplier
TCD
electron multiplier
MS-MS
MSD
FID
s.
Which of the following is TRUE for a mass spectrum?
the parent peak is always strong
the fragmentation pattern is not affected by the acceleration voltage
very accurate quantitative determinations are difficult
isotope effects are not important
t.
A process that is important for packed columns but NOT for capillary columns is
diffusion
equilibrium
volatilization
eddy currents
2
KEY
Questions – Test 2
u. Which of the following detectors would be especially sensitive to chlorine containing
compounds?
FID
thermal conductivity
UV
electron capture
v.
w.
Which is a MS ionization method that uses a laser in the ionization process?
EI
FAB
MALDI
CI
Which type of ionization uses a stream of high-energy ions to strike a target holding the
sample?
electron impact
MALDI
electrospray
FAB
x.
If a magnetic sector mass analyzer is tuned to pass ions with a m/e of 100, then by what
factor will the magnetic field have to be changed to pass ions with a m/e of 25?
2
½
1/4
4
y. Which statement is TRUE about capillary versus packed columns?
capillary columns are faster but have poorer resolution
capillary columns provides better resolution but requires very sensitive detectors,
a capillary column is longer than a packed column and therefore resolution is lower,
capillary columns provides better resolution but must be operated at higher temperatures
z. Which is the quantity that is the equilibrium constant between the mobile and stationary
phase?
capacity factor
retention time
selectivity constant
partition coefficient
aa. Which is FALSE about mass spectra?
a fragment that contains an odd number of N’s will have an m/e that is an odd number
the parent peak is often not the highest intensity peak in a mass spectrum
ions that have charges greater than one are not seen in mass spectra
a MALDI spectrum has little fragmentation
bb. Which of the following detectors would be especially sensitive to phosphorous containing
compounds?
FID
thermal conductivity
electron capture
Thermionic
cc. In a magnetic sector mass spectrometer, the mass/charge ratio that passes through the
exit slit is
proportional to the magnetic field strength
proportional to the magnetic field strength squared
proportional to the accelerating voltage
proportional to the accelerating voltage squared
dd. In a time-of-flight MS the mass/charge ratio is
proportional to the time of flight
proportional to the time of flight squared
inversely proportional to the time of flight
inversely proportional to the time of flight squared
3
KEY
Questions – Test 2
2 In one or two sentences, discuss each of the following.
a. Why is mass spectroscopy done in vacuum?
to prevent recombination of ions
b. What is the difference between a packed column and a capillary column?
a capillary column is an open bore tube with stationary phase on the inside
diameter. A packed column is a much wider tube that is filled with particles coated with
the stationary phase. The capillary column generally has better resolution because of
the great reduction in eddy diffusion
c. How does the mobile phase flow rate affect resolution in chromatography?
increasing the flow rate reduces contribution to broadening from diffusion effects
but increases contribution to broadening because of poor equilibrium. Decreasing the
flow rate has the opposite effects.
d. How does column temperature affect resolution in HPLC, GC?
Temperature affects retention times because equilibrium constants(partition
coefficients change) . Increasing temperature also increases diffusion. In GC
decreasing temperature almost always improves resolution at the expense of longer
retention times.
e. From the periodic table one finds that the mass of Br is 79.9. For a typical bromine
compound there is no peak at 79.9 in the mass spectrum.
The mass on the periodic table is not the mass of any particular isotope. It is the
weighted average for all Br isotopes.
f. Why is the parent peak not usually the highest m/e peak in mass spectroscopy?
Most mass spectra for small molecules are done by high impact electron
ionization which tends to fragment molecules into many small pieces.
g. What is gradient elution and why is it used?
Gradient elution is used in HPLC to decrease analysis time while maintaining
resolution. In gradient elution the solvent composition is changed during the run.
3 For each of the following mixtures, indicate an appropriate type of chromatography or
electrophoresis. Also indicate an appropriate detector (GC, normal phase HPLC, reverse
phase HPLC, GPC, CZE, CIEF, CGE, IC, TLC)
a.
a mixture of proteins
GPC (UV detector) - detection will be difficult unless concentration is high. A
fluorescent tag may me necessary
b. a perfume
GC (FID)
d. a mixture of amino acids
IC (conductance)
normal phase HPLC (UV)
e. a mixture of C8 to C14 alkanes
reverse phase HPLC (refractive index detector)
f. a mixture of anions
IC (conductance)
4
KEY
Questions – Test 2
4 (6) Identify the compound that gives the following mass spectrum. The only atoms that
could be present are C, H, O, N, Cl, Br. Explain your answer by identifying the fragments.
m/e
35
36
37
47
relative
m/e
abundance
5
51
2
82
2
84
18
85
relative
abundance
32
12
92
5
48
49
13
49
86
87
58
2
50
5
88
17
CH2Cl2
m/e
relative
abundance
m/e
35
36
37
47
5
2
2
18
Cl
35
H Cl
37
Cl
C35Cl
51
82
84
85
abundance
32
12
92
5
CH237Cl
C35Cl2
C35H2Cl2
CH35Cl37Cl
48
49
13
49
CH35Cl
C Cl, CH235Cl
86
87
58
2
CH235Cl37Cl
CH37Cl2
50
5
CH37Cl
88
17
CH237Cl2
35
37
relative
5 (10) Consider the chromatogram shown below.
45
D
40
C
35
A
30
25
20
E
15
B
10
5
0
0
20
40
60
80
100
120
time (sec)
5
140
160
180
200
KEY
Questions – Test 2
a. Roughly predict the appearance of a chromatogram for the same mixture if the column was
changed from non-polar to polar. Consider retention time, peak width and peak amplitude.
Assume that the detector responds equally well for each component.
The components would probably come out in the reverse order. The peaks
coming out later would be broader because of diffusion.
b.
Why can’t you say conclusively that component D is the most abundant?
The peak area depends on the amount of material and on the sensitivity. The
detector may be more sensitive to component D than to other components
c. Assuming that the figure is a gas chromatogram, what could be done to improve the
resolution?
change column, reduce temperature
d. Assuming that the column is non-polar, which peak corresponds to the most polar
component of the mixture?
A
e. Estimate the resolution for the peaks C and D in the chromatogram.


2 t DR  t RC
280  60
RS 

1
WD  WC
16  24
6.(5)For a given column H = 2 mm. Assume that you want to separate two components with a
resolution of 2.0. How long does the column have to be to separate two components that
have a selectivity factor of 1.03. Assume that the capacity factor for component B = 2.
kB  2
Rs  2
2
    1  kB 
N  16  Rs  
 

   1   kB 
2
L  N H
  1.03
H  2  mm
2
N  1.697  10 5
L  339.488 m
7 (12) Fill in the table
Detector
UV absorption
refractive index
flame photometric
detector
fluorescence
thermal conductivity
electrochemical
HPLC or GC
HPLC
HPLC
GC
types of samples that can be detected
compounds that absorb UV
universal
organics
HPLC
GC
HPLC
fluorescent compounds
universal
compounds that are electrochemically active.
6