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.