26 votes
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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: $\displaystyle 79 + 79 = 158$ $\displaystyle \color{red}{79 + 81} = 160$ $\displaystyle \color{red}{81 + 79} = 160$ $\displaystyle 81 + 81 = 162$ ...
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14 votes
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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 ...
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  • 6,758
11 votes
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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 ...
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  • 20.3k
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 ...
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  • 35.7k
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 ...
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  • 23.8k
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 ...
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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 ...
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  • 18.9k
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,...
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9 votes
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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 ...
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  • 35.7k
8 votes
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What is the highest relative abundance for diatomic bromine?

In $\ce{Br2}$, the possible combinations of isotopes are: $\ce{^{79}Br-^{79}Br} : \mathrm{m/z}~158$ $\ce{^{79}Br-^{81}Br} : \mathrm{m/z}~160$ $\ce{^{81}Br-^{79}Br} : \mathrm{m/z}~160$ $\ce{^{81}Br-^{...
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  • 10.5k
8 votes
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How can I reverse engineer a commercial cleaning solution's "other ingredients"?

Reverse engineering, or deformulation, is a common industry practice. This is a very common type of request for commercial analytical laboratories, and is generally not cheap. You can discover just ...
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  • 11.8k
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}...
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  • 181
7 votes
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In a mass spectrum, how can the sum of relative abundances be greater than 100?

2.3:10, 23:100 and (23/123):(100/123) are all correct values for the ratio of the relative abundance of B-10 to B-11. However, only the last value, (23/123):(100/123) or 18.7:81.3 shows the ...
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  • 82.2k
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 ...
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  • 66.4k
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 ...
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  • 23.8k
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 ...
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7 votes
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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 ...
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6 votes

Mass Spectrum and Molecular Ion peaks

The major isotopes of chlorine are 35 and 37. If 4 chlorines are present in a molecule, then the following isotopic combinations of chlorine are possible 35, 35, 35, 35 35, 35, 35, 37 35, 35, 37, 37 ...
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6 votes
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Why would the 2+ molecular ion of butane have a mass spectrum peak at 29?

The mass spectrometer observes the mass to charge ratio (m/z). Most of the time, small organic molecules like butane will be singly charged by the ionizer, which means that the mass can be read off ...
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  • 10.5k
6 votes

How were the absolute masses of atoms calculated before the mass spectrometers?

Absolute mass can be determined electrochemically, once the coulomb is defined in terms of numbers of electrons, as through the Millikan oil-drop experiment, to be $6.24×10^{18}$ electrons per ...
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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 ...
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  • 66.4k
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 ...
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  • 33k
6 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 ...
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  • 20.3k
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, ...
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6 votes
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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 ...
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  • 41.1k
6 votes
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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. ...
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  • 41.1k
5 votes
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What is 'Retention period' in chromatography and what is its importance?

To answer your three questions in series: 1) The time is a measure of how strong the interaction of a compound is with the column used. The stronger the interaction the longer the compound will stay ...
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5 votes
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How does bombarding a molecule with electrons cause ionization?

This type of ionization is known as electron ionization (there are many types of ionization methods used for MS). One possible reaction for EI is: $$\ce{M +e- ->M^{•+} +2e-}$$ source If the ...
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