27
votes
Accepted
Why is the probability for the signal at 160 in the bromine mass spectrum twice as high as for the signals at 158 and 162?
All possible arrangements of $\ce{Br2}$ molecule:
$\displaystyle 79 + 79 = 158$
$\displaystyle \color{red}{79 + 81} = 160$
$\displaystyle \color{red}{81 + 79} = 160$
$\displaystyle 81 + 81 = 162$
...
- 1,567
14
votes
Accepted
Why is a very small peak with larger m/z not considered to be the molecular ion?
Without knowing more details, it is hard to guess, but at this $m/z$, it seems likely that the peak is the result of one $^{12}\ce{C}$ being substituted by one $^{13}\ce{C}$. It is more useful to ...
- 7,016
11
votes
Which unit should I use for m/z?
TL;DR: According to current IUPAC recommendations, $m/z$ is an abbreviation for a dimensionless quantity.
Use of thompson unit is indeed currently discouraged.
There is an overview provided in ...
- 38.5k
11
votes
Why is a very small peak with larger m/z not considered to be the molecular ion?
The peak at $m/z = 59$ with lower intensity in respect to the one at $m / z =58$ (the molecular ion) is not overseen. Mass spectroscopy is capable to deliver information about the isotopic ...
- 31.3k
11
votes
Why are isotopes an issue in reading mass spectra?
Here is the "periodic table for biomolecules" (leaving out hydrogen, please ignore silicon):
The pie chart shows the isotope distribution. Of the elements C, N O, P, S and H, sulfur has the highest ...
- 42.3k
10
votes
How do I figure out how many hydrogens my compound actually has using a mass and NMR spectrum?
Being an NMR fan myself I would inspect that NMR spectrum:
The integrals suggest you have 11 $\ce{^1H}$ or a multiple thereof (the number under each peak is the normalized integral, which is ...
- 22.9k
10
votes
How do I figure out how many hydrogens my compound actually has using a mass and NMR spectrum?
I would probably also use the method Buck has suggested, but let’s say the NMR broke down or somebody is measuring a $\ce{^13C}$ of $\pu{2.5mg}$ meaning it will be blocked until tomorrow; in this case,...
- 68.7k
9
votes
Accepted
Behavior of Natural Abundance and Mass Spec Ratios
You are probably mixing natural abundance (NA) and relative abundance (RA).
In mass spectrometry RA is a more valuable parameter as it can be directly obtained as the $y$-coordinate of a plotted mass ...
- 38.5k
8
votes
Why is the probability for the signal at 160 in the bromine mass spectrum twice as high as for the signals at 158 and 162?
A way to understand this that may be familiar is that of the Punnett square from biology, since the two isotopes have nearly 50/50 split in nature.
\begin{array}{c|cc}
& \ce{^{79}...
- 181
8
votes
Difference between HCD and CID collision induced dissociations?
Brief overview of CID vs. HCD
"Collision-induced dissociation" or CID is a much older and more general term in mass spectrometry than HCD. HCD is a vendor-specific term invented to describe a new ...
- 22.6k
8
votes
Accepted
Relative abundance of mixture in mass spectra
There are universal detectors (e.g. CAD) with which you can compare signals of different substances. But MS detectors are not universal — meaning that high intensity of the signal doesn't necessarily ...
- 1,567
8
votes
Accepted
Does the MS oxygen peak at m/z = 16 correspond to $\ce{O+}$ or $\ce{O2^{2+}}$?
Most likely the signal is predominantly or entirely $\ce{O^+}$.
The question is whether two $\ce{O^+}$ ions will bind to each other forming a diatomic ion in the gas phase. A rough energy balance ...
- 62.5k
8
votes
Does the MS oxygen peak at m/z = 16 correspond to $\ce{O+}$ or $\ce{O2^{2+}}$?
This is a very tricky molecule. A single m/z value cannot tell you whether this is singly ionized or doubly charged on a low resolution mass spectrometer. One would look at mixed isotope patterns of ...
- 42.6k
7
votes
Mass spectrum analysis
Generally speaking: knowing the history of your sample (e.g. earlier steps of synthesis allowing / excluding elements that could numerically fit into the list of suggested fragments) and additional ...
- 31.3k
7
votes
How do I figure out how many hydrogens my compound actually has using a mass and NMR spectrum?
I like both answers provided before me where one has used exclusive use of internet to suggest structure by NMR spectrum, and the other has used thorough analysis of mass spectrum. Although these two ...
- 42.1k
7
votes
Accepted
Peak at m/z = 39 in mass spectrum of 3-chloropropene
It would indeed be $\ce{C3H3^+}$ assuming predominant isotopic species. Although there can be other isomers, the cyclic isomer, the cyclopropenyl cation is strongly stabilized by aromaticity.
As PLD ...
- 62.5k
7
votes
Accepted
Why do dihaloalkanes lose both halogen atoms during mass spectrometry?
It isn't just both bromine atoms that are lost, although that might be expected in mass spec given the relative weakness of the carbon-bromine bond. You also lose a hydrogen atom. And that is key.
...
- 62.5k
7
votes
Resolution in mass
The mass resolution $R$ of a mass spectrometer is defined as
$$R = \frac{m}{\Delta{}m} $$
with $\Delta{}m$ the mass difference between two masses still be resolved, $\Delta{}m = m_1 - m_2$. Thus, if ...
- 31.3k
6
votes
Accepted
In a mass spectrum, are there peaks corresponding to a combination of 13C and bromine isotopes?
It does appear. The graphs you're looking at online probably just dumbed it down.
Here's some proof if you like, a compound synthesised by yours truly, with molecular formula $\ce{C7H7BrN2O2}$. I ...
- 71.9k
6
votes
Accepted
How to evenly ionize in mass spectrometry
I know that in mass spectrometry they charge different molecules and then run them through a magnetic field and then see where they land.
That is indeed how some mass spectrometers work, but not most ...
- 22.6k
6
votes
What is the relative size of the (M+2) peak?
Let’s assume your compound is $\ce{C_nH_xO_y}$. Thankfully, both hydrogen and oxygen are elements that only have one significant naturally occuring isotope. Therefore, we can treat the entire ...
- 68.7k
6
votes
Accepted
What is the relative size of the (M+2) peak?
You are correct on all accounts.
To a very good approximation, molecules can be thought as made of elements (with their respective isotope distributions) combining completely independently. You can ...
- 29.2k
6
votes
Why are isotopes an issue in reading mass spectra?
You have already got an answer by Dr. Karsten, but let me add that modern mass spectra have become sensitive beyond imagination.
Secondly, the resolution of mass is no longer a problem. Fourier ...
- 42.6k
6
votes
How is the distance between m/z axis values in mass spectrums called?
I wanted to back up my comment and make sure it's justified, so I decided to flip through Gross'Mass spectrometry: a textbook [1] and it looks like there are indeed basically two possible variations, ...
- 38.5k
6
votes
Examples of compounds that are hard to ionise or which decompose easily upon ionisation in mass-spectrometer
In a sense, everything you put into a mass spectrometer decomposes; the (mass/charge ratio of the) fragments are actually valuable in distinguishing different isomers.
That being so, extensive ...
- 62.5k
5
votes
What is the difference between a time of flight mass spectrometer and a spectrometer that uses a magnetic field?
Both types of mass spectrometers separate different chemical species based on the mass-to-charge ratio.
The time-of-flight mass spectrometer (TOF MS) is based on the ions being released at a given ...
- 35.3k
5
votes
Accepted
Why does 2-chloropropane's mass spectrum have a peak at 27?
Here is a little mechanism I made up about it. Not quite sure if its correct, seems to fit the mass spectrum though.
Definitely have a look at McLafferty classical textbook. Maybe you will find the ...
- 2,804
5
votes
Is there a way to search for adducts by exact mass?
I guess that would be what you are looking for:
http://www.chemcalc.org/mf_finder/mfFinder_em_new
This one gives you theoretical combinations of atoms, that fit the mass.
You could also check the ...
- 419
5
votes
How Does Benzyl Alcohol Produce a Peak at m/z = 79?
Interesting question.
The cycloheptatriene - norcaradiene tautomerization is a common equilibium. Usually the norcaradiene is present in small amounts, but as long as some is present the reaction ...
- 85.4k
5
votes
Accepted
How can the formula of WD-40 remain a trade secret?
"How can it remain secret?". Trivial answer is - it can't (it isn't).
WD40 is a Fast Moving Consumer Good (FMCG). It is also a reasonably cheap product - as an industrial chemist this suggests at ...
- 121
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