Exam 3 Flashcards
What do the acronyms MALDI and DESI stand for?
Desorption Elector Spray Ionization
Matrix-assisted laser desorption/ionization
Imagine you work for a designer impostor perfume company and you were asked to copy a competitor’s popular new fragrance. You decide to collect 100% of the aromatic compounds from the perfume so that you can identify them and also determine their relative concentrations. What is the proper sample preparation method?
Purge and Trap
In an experiment that measured the concentration of cocaine in a drug sample, the following results were seen? Sample Mass(g) Rel. Std. Dev (%) 0.89 6.21 4.22 2.00 8.63 1.39 14.78 1.07
Calculate the average sampling concentration Ks?
What mass of sample is required for +/-1.50% sampling standard deviation?
How many samples of the size in part b are required to produce 90% confidence that the mean is known to within 1.00%?
a) Ks Avg 16.82
b) m=Ks/R^2 ANS: 7.48g
c) 8 samples of 7.48 g
A compound with a composition C6H12N2Br2 contains how many double bonds?
1
Be able to calculate the charge, mass of a protein and its standard deviation and average from a chart.
D
What is the retention index and how is it used in GC?
In gas chromatography, Kovats retention index (shorter Kovats index, retention index; plural retention indices) is used to convert retention times into system-independent constants. The index is named after the Hungarian-born Swiss chemist Ervin Kováts, who outlined this concept during the 1950s while performing research into the composition of the essential oils.[1]
Which of the following statements about Cl versus El is true?
Chemical ionization yields less fragmentation than electron ionization.
When using chemical ionization, the peak with the highest mass-to-charge ratio occurs at m/z 246. Which of the following statements is true?
The nominal mass of the compound is 245 amu.
Briefly explain selected reaction monitoring. Why is this technique used?
This is a method for high analyte selectivity it often uses tandem MS. You can have multiple guardpoles that become more selective.
……………………………
What is the advantage to using ion trap or orbitrap MS versus magnetic sector or transmission quadruple instruments? What is the primary challenge for the ion trap/orbitrap analyzers
- All ions are analyzed rather then over time as they enter the detector.
- Low pressure
The nominal mass of a molecule or ion is?
the integer mass of the species with the most abundant isotope of each of the constituent atoms
A mass spectral library match with quality of 90% or higher is all you need for the positive identification of a compound using GC-MS. True or False
False: You also need a retention time watch to confirm
Say you are analyzing C6H12N2Br2. At what m/z will you see the m+* peak? Predict the intensity of the M+1 peak relative to that of the M+* peak?
M+*=270
I=1.08(6) +.012(12)+.369(2)=7.36%
Describe how a quadrupole mass analyzer works?(generates a mass spec)
The quadrupole consists of four parallel metal rods. Each opposing rod pair is connected together electrically, and a radio frequency (RF) voltage is applied between one pair of rods and the other. A direct current voltage is then superimposed on the RF voltage. Ions travel down the quadrupole between the rods. Only ions of a certain mass-to-charge ratio will reach the detector for a given ratio of voltages: other ions have unstable trajectories and will collide with the rods. This permits selection of an ion with a particular m/z or allows the operator to scan for a range of m/z-values by continuously varying the applied voltage.[1] Mathematically this can be modeled with the help of the Mathieu differential equation.[3]
Explain why it is beneficial to use larger particles (~50 μm) for solid phase extractions, but smaller particles (~5 μm) for chromatography.
The use of larger particles in solid phase extractions allows samples to elute from the column without applying high pressure. The use of smaller particles in chromatography allows for more efficient separation of analytes, but requires high pressure to push the solvent through the column.
Chemical alteration of a molecule to make the molecule more easily detected.
derivatization
Destruction of organic matter in a sample using a liquid reagent prior to analysis of an inorganic component.
wet ashing
Procedure that uses a molten salt to dissolve an otherwise insoluble substance.
fusion
Oxidation of organic substances without the use of a liquid at high temperatures to leave behind any inorganic components for analysis.
dry ashing
The medium used for a fusion.
flux
A procedure that uses a small amount of chromatographic stationary phase to isolate an analyte from a solution prior to analysis.
Solid phase extraction
Concentrating trace analytes in a mixture prior to analysis.
preconcentration
Why do double-focusing mass spectrometers give narrower peaks and higher resolution than single-focusing instrument?
The resolution of a single focusing mass spectrometer is limited by the initial kinetic energy
spread of the sample molecules. This spread is minimized in a double focusing instrument by
accelerating the sample through an electrostatic analyzer, which limits the range of kinetic
energies of ions being introduced into the magnetic sector analyzer. Signicantly narrower
peaks are the result.
Describe the similarities and differences between transmission quadrupole mass spectrometers and magnetic sector mass spectrometers.
The quadrupole mass analyzer is a “mass filter”. Combined DC and RF potentials on the quadrupole
rods can be set to pass only a selected mass-to-charge ratio. All other ions do not have a stable
trajectory through the quadrupole mass analyzer and will collide with the quadrupole rods.,
never reaching the detector.
A magnetic sector alone will separate ions according to their mass-to-charge ratio. However, the
resolution will be limited by the fact that ions leaving the ion source do not all have exactly the
same energy and therefore do not have exactly the same velocity. This is analogous to the chromatic
aberration in optical spectroscopy. To achieve better resolution, it is necessary to add an
electric sector that focuses ions according to their kinetic energy. Like the magnetic sector, the
electric sector applies a force perpendicular to the direction of ion motion, and therefore has the
form of an arc.
- How does the reflectron help improve resolution in a TOF mass spectrometer?
The time of flight mass spectrometer separates ions on the basis of velocity by monitoring the time
it takes for them to drift through a field-free flight tube. In order for the mass separation to be
accurate, ions must be introduced into the flight tube in a small pulse and ions of the same m/z
must have the same kinetic energy to that they have the same velocity. In practice, ions are
produced with a small spread in KE leading to a small spread in velocity. The reflectron serves to
minimize the impact of this spread in velocities on the resolution of the measurement. It does so
by presenting an increasing electric field in such a way that slower moving ions penetrate less
deeply into the reflectron and faster moving ions penetrate more deeply before being redirected
toward the detector. As a result of this slight difference in flight distance, the packet of ions ends
up being more focused once it reaches the detector.
Suggest a reasonable combination of ionization source, mass analyzer and detector for the
following applications:
a. The determination of heavy metal contamination in drinking water.
b. The identification of a bacterium with a mass of 30 kDa.
c. Molar mass and structural identification of a newly developed steroid-based antibiotic.
a. Reasonable combination would be ICP source, quadrupole, and electron multiplier
b. Reasonable combination would be MALDI source, TOF, and electron multiplier
c. Reasonable combination would be EI/CI source, quadrupole, and electron multiplier
In terms of utility, the mating of LC with MS holds tremendous promise. Why is it difficult to
interface and LC with a MS? How has this problem been addressed to lead to commercial LC-MS
instruments?
The key challenge in interfacing LC and MS is the very different conditions at which each
instrument operates. In traditional HPLC, eluent exits the column at mL/min flow rates, resulting in
a large amount of material exiting the LC in a short time. If all of this material were introduced into
the MS, it would be impossible to maintain the high vacuum conditions required to provide a large
mean free path for the ions produced in the MS.
One of major developments in the coupling of MS to LC is the electrospray ionization source.
In electrospray, the sample solution eluting from the LC flows through a needle which is subject to
a large electric field. As solution leaves the needle, it obtains a charge. Electrostatic repulsion
causes the charged stream to break into smaller charged droplets, which continue to “explode”
until solvent is essentially evaporated and ionized analyte remains. When coupled with a sampler
and skimmer, these ions can be extracted into the MS.
MALDI is a very common ionization source for TOF mass analyzers, why is this the case? How
could an ionization source like electrospray be paired with a TOF? What benefits would
electrospray ionization provide compared to MALDI?
MALDI: The analyte is dispersed in a MALDI matrix (a molecule that readily sublimes when it
absorbs energetic photons) and deposited onto a target. The target is irradiated with a laser
pulse, resulting in absorption and sublimation of the matrix (including the analyte) and
ionization. The result is the formation of intact molecular ions, some of which may be multiply
charged. Benefits: Soft ionization source. Good for molar mass determination. Challenges:
Pulsed source, need mass analyzer that can handle pulsed introduction. Need appropriate
matrix.
Electrospray: The sample solution flows through a needle which is subject to a large electric field.
As solution leaves the needle, it obtains a charge. Electrostatic repulsion causes the charged
stream to break into smaller charged droplets, which continue to “explode” until solvent is
essentially evaporated and ionized analyte remains. This is a more energetic ionization
source, capable of producing multiply charged ions and fragments. Benefits: Continuous
source. No additional sample handling steps. Challenges: May lead to complex spectra.
Need to remove some sample to produce lower pressure for mass analyzer.
What resolution is necessary to differentiate between: a. C4H6O2 \+ (m/z = 86.0367) and C5H10O \+ (m/z = 86.0731) b. A substitution of a methyl group for a proton on a protein with m/z of 11,500 Da c. ArCl+ and As+
Resolution = m/∆m (a) m = (86.0367+86.0731)/2 = 86.0549 m/∆m = 86.0549/(86.0731 - 86.0367)= 2364 (b) m/∆m = 11,500/(15.04 - 1.01) = 820 (c) m = (75.401 + 74.921)/2 = 75.161 m/∆m = (75.161)/( 75.401 - 74.921) = 157
Explain the steps in an MS-MS experiment. Why are MSn
experiments valuable?
In tandem in space instruments, two independent mass analyzers are used in two different
regions in space. This is a rather straight-forward way to do tandem ms and some
conventional mass spectrometers can be converted to tandem instruments. The advantages
are that it is relatively easy to take all the different types of spectra (product ion, precursor ion,
neutral loss, multidimensional. The disadvantages are that the efficiency can be very low and
thus the sensitivity can be low. Tandem in time instruments form the ions in a certain spatial
region and then at a later time expel the unwanted ions and leave the selected ions to be
dissociated and mass analyzed in the same spatial region. The efficiency can be fairly high
and the process can be repeated many times. It is, however, only straight forward to take
product ion spectra. Both approaches require quite expensive instrumentation.
Consider the peak at m/z 84 below in the spectrum for methylene chloride. Identify the ions that
result in the four peaks of m/z greater than 84 in the mass spectrum and justify why the peaks
have the relative sizes that they have.
Relative abundances: 12C 98.9%, 13C 1.1%: 35Cl 75.8%, 37Cl 24.2%.
m/z = 84 due to 35Cl2
12CH2
+
m/z = 85 due to 35Cl2
13CH2
+ Size = 2(1.1/98.9) times as large as 84 peak
m/z = 86 due to 37Cl35Cl12CH2
+
Size = 2(24.2/75.8) times as large as 84 peak
m/z = 87 due to 37Cl35Cl13CH2
+
Size = 2(24.2/75.8) x(1.1/98.9) times as large as 84 peak
m/z = 88 due to 37Cl2
12CH2
+
Size = (24.2/75.8) x(24.2/75.8) times as large as 84 peak
How does a quadrupole mass analyzer function to allow only a narrow range of m/z to transmit?
As they move through the mass filter, ions are subject to both AC and DC potentials. Depending
on their size, ions may be influenced differently by the RF and DC components. For example,
heavy ions are least influenced by the RF component, while light ions are most influenced by the
RF. The balance between DC and RF determines whether an ion will have a stable path. Spectra
can be scanned by systematically adjusting either the RF or DC voltages.
Describe the similarities and differences between quadrupole ion trap and Fourier transform ICR
mass spectrometers.
A quadrupole ion trap is similar to a linear quadrupole filter except it as a spherical 3-
dimensional configuration. By a combination of fields, ions are temporarily stored within the
trap. They are then released sequentially by increasing the radio frequency voltage applied to
the ring electrode. The ejected ions then strike a detector. A plot of detector signal vs. the
radio frequency voltage, related to the m/z value, is the mass spectrum. In an FT ICR
instrument, ions are trapped in a cell by an electric trapping voltage and a magnetic field.
Each ion assumes a circular motion in a plane perpendicular to the direction of the field. The
cyclotron frequency depends on the inverse of the m/z value. In modern instruments a radio
frequency pulse that increases linearly in frequency is employed. A time domain image
current is generated after termination of the pulse. Fourier transformation of the time decay
signal yields the mass spectrum.
Describe the interface between the ICP torch and the mass spectrometer in ICPMS
The interface consists of a water-cooled metal cone with a tiny orifice in its center. The region
behind the cone is maintained at a pressure of about 1 torr by pumping. The hot gases from
the ICP impinge on the cone, and a fraction of these gases pass through the orifice where
they are cooled by expansion. A fraction of the cooled gas then passes through a second
orifice into a region that is maintained at the pressure of the mass spectrometer. Here, the
positive analyte ions are separated from electrons and negative ions by a suitable field and
are accelerated into the mass spectrometer itself.
What types of interferences are encountered in atomic mass spectrometry?
Isobaric interferences are encountered when the isotopes of two elements have the same
mass. A second type of spectroscopic interference occurs from molecular species that have
the same mass as that of an analyte ion. A third type of interference is from matrix species
that combine with the analyte and reduce the analyte signal as a result.
Why is an internal standard often used in quantitative analysis by ICPMS?
An internal standard is often used in preparing calibration curves for ICPMS in order to
compensate for random error from instrument drift and noise, torch instabilities, and some
matrix effects. The internal standard chosen should be an element that is absent from the
sample, but that has an atomic mass and ionization potential close to that of the analyte.
