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: $\displaystyle 79 + 79 = 158$ $\displaystyle \color{red}{79 + 81} = 160$ $\displaystyle \color{red}{81 + 79} = 160$ $\displaystyle 81 + 81 = 162$ ...
Stanislav Bashkyrtsev's user avatar
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 ...
TAR86's user avatar
  • 6,851
11 votes
Accepted

Can we recycle garbage with the principles of mass spectrometry?

Yes it is possible, but is very expensive and would be orders of magnitude more costly than what people are willing to pay for recycled materials. Let me give two data points to explain why it is ...
Curt F.'s user avatar
  • 21.9k
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 ...
andselisk's user avatar
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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 ...
Buttonwood's user avatar
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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 ...
Karsten's user avatar
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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 ...
Buck Thorn's user avatar
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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,...
Jan's user avatar
  • 67.3k
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 ...
andselisk's user avatar
  • 37.4k
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}...
Joe's user avatar
  • 181
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 ...
Stanislav Bashkyrtsev's user avatar
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 ...
Oscar Lanzi's user avatar
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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 ...
AChem's user avatar
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7 votes

Why is the molecular peak not observed in the mass spectrum of 2,2-dimethylbutane?

The answer probably has to do with how the mass spectrum was obtained. Most mass spectra that are provided online, including the ones on the NIST WebBook, are electron ionisation mass spectra. That ...
orthocresol's user avatar
  • 71.1k
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 ...
Buttonwood's user avatar
  • 28.4k
7 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 ...
Curt F.'s user avatar
  • 21.9k
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 ...
Mathew Mahindaratne's user avatar
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 ...
Oscar Lanzi's user avatar
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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. ...
Oscar Lanzi's user avatar
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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 ...
Buttonwood's user avatar
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6 votes
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How to retrieve Dalton value from m/z value?

IUPAC is wrong Martin's answer is useful for its pointer to the IUPAC Gold Book, which is one of the most authoritative sources for chemical nomenclature around. However, on this particular question,...
Curt F.'s user avatar
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6 votes
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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 ...
orthocresol's user avatar
  • 71.1k
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 ...
Curt F.'s user avatar
  • 21.9k
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 ...
Jan's user avatar
  • 67.3k
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 ...
Nicolau Saker Neto's user avatar
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 ...
AChem's user avatar
  • 39.1k
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, ...
andselisk's user avatar
  • 37.4k
5 votes

How to retrieve Dalton value from m/z value?

As a mass spectrometrist, I agree with Curt. IUPAC is wrong, and you can convince yourself of this by going back to SI units. Mass is in kilograms, charge is in coulombs (technically Amp*seconds), ...
Peter OConnor's user avatar
5 votes

Can we recycle garbage with the principles of mass spectrometry?

The preparative mass spectrometry is field of active research, trying to overcome many obstacles. One of them is capturing the species (not only atoms, but almost anyting, undestroyed), with the hot ...
ssavec's user avatar
  • 4,497

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