All spectrometry questions Flashcards
What is a photomultiplier tube and how does it work?
A photomultiplier tube, useful for light detection of very weak signals, is a photoemissive device in which the absorption of a photon results in the emission of an electron. These detectors work by amplifying the electrons generated by a photocathode exposed to a photon flux.
What are some alternatives to a photomultiplier tube?
Alternatives would be a phototube or photodiode.
What is a Czerny-Turner grating monochromator and what does it do?
This is figure 20-9 from Harris showing the Czerny-Turner grating monochromator. This device allows to select wavelengths for spectrometry experiments. The grating is moveable so that each wavelength within a range (here l1 and l2) can exit individually.
What is an alternative to Czerny-Turner grating monochromator?
Prisms are used, but much more seldom.
A compound with molecular mass 292.16 g/mol was dissolved in a 5-mL volumetric flask. A 1.00-mL aliquot was withdrawn, placed in a 10-mL volumetric flask, and diluted to the mark. The absorbance at 340 nm was 0.427 in a 1.000-cm cuvet. The molar absorptivity at 340 nm is e340 = 6130 M-1cm-1.
a. Calculate the concentration of compound in cuvet.
See Harris, problem 18-18. c=6.97x10-5 M
A compound with molecular mass 292.16 g/mol was dissolved in a 5-mL volumetric flask. A 1.00-mL aliquot was withdrawn, placed in a 10-mL volumetric flask, and diluted to the mark. The absorbance at 340 nm was 0.427 in a 1.000-cm cuvet. The molar absorptivity at 340 nm is e340 = 6130 M-1cm-1.
b. What was the concentration of compound in the 5-mL flask? (2p)
b. c=6.97x10-4 M
Molecular ion.
The molecular ion is the initial ion produced from a molecule (M) in an ion source of a mass spectrometer. Ion sources apply various ionization mechanisms, for example
electron impact (EI), in which an electron hits the molecule and produces an odd electron molecular ion, M+˖.
b. TIC
TIC is the abbreviation for “Total ion chromatogram” or “Total ion current” and is the initial result/graph after a MS experiment. It represents the total number of ions (y-axis) detected over time (x-axis).
c. MALDI
c. MALDI is the abbreviation for “Matrix assisted laser desorption ionization” which is another example for ionization applied in ion sources. It is a soft ionization method often combined with a time of flight analyzer, MALDI-TOF. Analytes are mixed with a crystalline UV-absorbing compound (matrix) and then subjected to a laser beam whose energy results in the production of [M+H]+ or [M-H]-.
d. Analyzer
“Analyzer” in the context of mass spectrometry is the device in the instrument that filters ions produced in the ion source with regard to their mass to charge ratio, m/z. Each value of m/z reaches the detector one at a time.
f. SIM
f. SIM is the abbreviation for “Selected ion monitoring” and is a mass spectrometry acquisition mode in which only a few selected ions are transmitted/detected by the instrument, as opposed to all ions in the full spectrum range, SCAN. This mode of operation typically results in significantly increased sensitivity.
How do you determine structures for compounds based on mechanisms to significant fragment ions and relative intensity table m/z for EI-MS
How do you determine a composition of a compound whos mass spectrum in show in a table?
Answer: See problem 22-20 in Harris!
Soft ionization techniqeues, when do you choose a positive and negative mode? Why?
If it’s an acid like OH then it can easily be turned into it’s corresponding base and therefore negative mode be preferred. If it’s a base like O it can be used as positive mode.
How can you predict the m/z value for a molecular ion that is singly charged when using soft ionization tecniques either positive or negative mode?
