Mass Spectroscopy
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What is the difference between molecular weight and exact mass?
-Molecular weight takes isotope ratios into account -Exact mass is only equivalent to the highest abundance isotope
What results in more deviation between exact mass and MW?
Additional Carbons
How do you calculate the molar mass of two isotopes when given their percent abundance?
Multiply the percent as a decimal by the MW and then add the values obtained by the isotopes together to get the molar mass
How do you measure the mass of individual molecules?
-Create an ion of the molecule -Accelerate the ion (w accelerating plates) -Bend the ions’ trajectory in a magnetic field -Measure where/when the ion hits the detector
Charged particles moving through a magnetic field change their ______ based on their ______.
Trajectory, mass to charge ratio
Due to their relative instability, the actual number of molecular ions observed is often ______.
Small
What is the base peak?
Tallest peak, always set to 100%
What is the parent ion?
The compound being analyzed
What is a fragment?
Smaller ions resulting from broken bonds in M (molecular ion)
What is the parent peak?
Heaviest peak that is the farthest to the right
What is the rule of 13?
-Divide molecular ion by 13 to get the # of Carbons -Subtract the molecular ion by the # of Carbons x12 to get the # of Hydrogens
How do you find the percent of the M+1 peak?
1.1 x # of Carbons
What is the formula for DOU?
[(2C-H-X+N)/(2)] + 1
What does it mean if [M] is an even number?
molecular ion contains an even number (or zero) N’s
Which isotopes have a 3:1 ratio?
35Cl and 37Cl
Which isotopes have a 1:1 ratio?
79Br and 81Br
Why are the peaks for Carbon 13 so small?
13 Carbon only occurs about 1% of the time (very rare!)
How do we determine the percent abundance when there are multiple isotopes of [M]?
-Figure out if Cl or Br is involved -Using those known ratios, create a polynomial by foiling (ex. (3x + y) (3x + y) ) to get ratios -Divide each number by the HIGHEST coefficient and multiply by 100 to get the percent abundance
Only ____ charged fragments are detected using this ionization method.
Positively
We want to place the positive FC by the atom that can …
Stabilize w resonance