Mass Spect:Isotopes - Online Tutor, Practice Problems & Exam Prep

Isotopes play a crucial role in mass spectrometry, particularly in identifying molecular structures. The M+1 peak, primarily from carbon-13, allows for estimating the number of carbon atoms in a compound using the equation: $N^1=\frac{h}{M}100/1.1$. The M+2 peak indicates the presence of halogens like chlorine and bromine, with distinctive ratios aiding in structure determination. The nitrogen rule helps deduce the number of nitrogen atoms based on the molecular weight's parity.

The (M + 1) Peak

In this video, we're going to cover the role of isotopes in mass spectrometry. So before we go any further, let's just all remember that an isotope would be an atom that has the same number of protons, meaning it's the same element, but has a different number of neutrons. What that means is that isotopes are going to have different weights from each other depending on which isotope it is. Now why would that be important for mass spectrometry? Because, guys, we're analyzing weight, so I need to know if there's atoms out there that are the same atom with different weights. I need to take that into consideration. Right? So we're going to talk about a few different types of peaks that result purely because there are isotopes present. So the first one is the M plus one peak. So we've talked a lot about M-one, M- a lot of numbers, but we haven't talked yet about the pluses. Pluses happen because of isotopes. Okay? And the M plus one peak is probably the most famous situation where this happens, and it's due to the isotope of carbon-13. Remember that carbon is usually 12, but it turns out that 1.1% of all the carbon in the universe, in fact, 1% of all the carbon that's making up your body has an extra neutron in it. It's carbon-13. So what this is going to do is it's going add a small, very small, because it's only 1%, but distinctive M plus one peak that's proportional in size to the number of carbons in your compound. So remember in our intro video how we were looking at methane and methane had an mz=16, but you saw this tiny little peak at 17. And I told you don't worry about it yet. That is where it was coming from. It was coming from the carbon-13 isotope. Well, guys, it turns out that this isn't something that you just have to mentally know. It's also something you need to be able to do some calculations based on because it's such a consistent ratio that we can actually build equations to solve problems with this.

The equations that we're going to cover today are: 1, calculating the height of an M+1. So sometimes you're going to be asked to estimate how tall would this M+1 be based on what the structure is. And we're never you're not expected to get it perfectly right because this is an approximation, but we can get pretty darn close. The second equation we're going to cover is using the N plus one peak, the height of it, to go back and look at how many carbons did we originally have in our compound.

N plus one is going to be calculated as the number of carbons multiplied by the percentage 1.1. Now you might be asking, "Johnny, if the chances of having carbon-13 are 1.1%, doesn't that mean that the M plus 1 is always going to be 1.1 percent of the original?" And that's not true at all. Actually, the M plus 1 peak gets bigger and bigger depending on the number of carbons we have in our structure. The reason is simple. Something like methane, which has only one carbon in it, what are the chances of having a carbon-13 in methane? Exactly 1.1%. So we would be multiplying 1 carbon times 1.1 and we would get an estimated height of the N plus one peak at 1.1%. Makes sense. But how about a molecule like decane? Guys, remember decane has 10 carbons in it. So doesn't it have a higher chance of having a carbon-13 in it? Sure. Because now any of those carbons could be a carbon-13. Not just one of them. It could be any of them and it's going to increase the weight for the whole molecule. So that's why you have to multiply that 1.1 percentage by the total number of carbons in the molecule, so in this case, 10 times 1.1, guess how tall that peak is going to be. Well, you do the math. You could type it into your phone or your calculator. That's fine. It's going to be 11%. Guys, that's what's seen and observed in the mass spectrum. For methane, look how tiny that N plus one peak is. For decane, look how much bigger it is. Why is it so much bigger? I'm sorry. I'm right in the way. Why is it so much bigger, guys? Because now there's all those different chances. Basically, what it's saying is that 11 out of 100 times, one of your carbons is going to be a carbon-13 because you have so many different carbons there that the chances, the probability of one of those carbons being heavier just increased by 10 versus the first one. Making sense so far?

The equation for calculation is fairly straightforward. You multiply the M/N plus one peak by 100 and then divide by 1.1 to get the total number of carbons in your compound. This approximation however might not be accurate for large molecules that also contain isotopes of nitrogen, sulfur, or phosphorus. Now let's continue and move on to the M plus 2 peak.

The (M + 2) Peak

All right guys, so why would we ever get an M+2 peak? Well, it turns out that there are two atoms that because of their isotopic ratios, they happen to give very distinctive M+2 peaks. And those two atoms are the halogens chlorine and bromine. Okay? Now why do they give N+2? Because the way they naturally occur in the universe is in isotopes that have a difference of 2 in weight. So let's talk about chlorine first. So guys, you might remember from your periodic table or whatever that the atomic weight or the molecular weight of chlorine is actually about 35.5. But the reason that it's 35.5 is because about three-fourths of it is chlorine 35 and about one-fourth of it is chlorine 37. So about 75% chlorine 35 and about 25% chlorine 37. So if you average those together, you get 35.5. Now you don't have to do that math. It's fine. But the interesting thing is that on a mass spectrum, this is going to give us a very interesting ratio where it's going to look like an approximate 3 to 1 ratio on a mass spectrum. That means wherever your molecular ion is, if you add 2 to that molecular ion, you're likely to see a peak that's about one third of the size of the molecular ion. I'm not going to write mass to charge numbers because it doesn't matter. But let's say that this is your molecular ion here. So I'm just going to put that this is my molecular ion. Well, then I would expect to find an N+2 that is about a third of the size. This would be my N+2 peak. And why is that? Well, once again, because about 75% of my chlorine atoms were chlorine 35, and about 25% are giving it the extra plus 2 weight of 37. Now the reason that this is helpful for us guys is because when you see that very distinctive 3 to 1 ratio, that's N+2, you automatically know that there's a chlorine in your molecule. So it's a really easy read. It's like oh, I know there's a chlorine here because I have this pattern and this pattern only happens with chlorine. Awesome. So that's chlorine. And it turns out bromine also has an N+2, but it's different because the isotopic abundance is different. It turns out that bromine on the periodic table has a weight of about 80. But it's not really 80. What it really is is 50% is 79 and about 50% is 81. So basically, about half of it is 79, and about half of it is 81. So they blend that together to get the number 80. So what this means is that for bromine, we're going to get an N+2 about a one to one ratio. Now I do see that the percentages are a little bit off, it's like 50.7, 49.3, but guys, those are so close that we're just going to call it 1 to 1. I hope you're okay with that. Let's say this is my molecular ion here. So this would be my molecular ion. What I would expect is if there's a bromine present, then I should have an N+2 that is about the same exact height and it's 2 units over. And this would be due to the bromine. So this would be my N+2, and I know that there's a bromine there because that means that about half of it was bromine 79 and about half of it was bromine 81. Now really quickly, I know that I drew them differently, so bring your eraser because what I'm trying to show is that the 81 increases by 2 units, that's why I'm putting N+2. This is your N+1 that I'm not really drawing that part. But over here, we should move this peak to be right here. So you can see how it's 2 units shifted, not just 1. What I'm trying to show you is that your M+2 should be about a third of the size with chlorine and it should be about the same exact size with bromine. So if you see this pattern, you know a bromine is present. If you see that pattern, you know chlorine is present. And by the way, I'm ignoring the N+1. I'm not even worrying about that because this I'm not talking about N+1 right now, I'm just talking about N+2. Would there be some N+1? Sure. But we're not talking about that right now. Cool. Awesome. So now we're done with N+2. The biggest thing is that you know how to identify it, whether it's chlorine or bromine. Let's move on to the nitrogen rule.

The Nitrogen Rule

Guys, the nitrogen rule actually has nothing to do with isotopes. I'm just throwing it in here because it's another very helpful form of structure determination for mass spectrometry and it's usually taught in the same area as isotopes, so we might as well just learn it here. So guys, nitrogen has a property that's different from carbon that makes it helpful for mass spectrometry, which is that carbon always likes to form 4 bonds. Right? So since it likes to form 4 bonds, if a molecule is made just out of carbon or hydrogen, carbon and hydrogen, you know that it's going to be an even number for the molecular weight because you always have those 4 bonds. So it's always going to be C1H4 or C2H6. There are always multiples of like 2 or 4. But nitrogen likes to form 3 bonds, meaning that the molecular weight could be odd or could be even and that's going to tell us how many nitrogens are present. So what the nitrogen rule says is that an even molecular weight of our parent ion, basically the molecular ion, indicates an even number of nitrogen present. Whereas an odd molecular weight of our parent ion indicates an odd number of nitrogens present. So for example, the number one odd number being 1, Right? If there's 1 nitrogen present, that means that nitrogen wants to have 3 bonds, meaning that when I pile together my molecular weight, I'm going to always have to add 3 at the end or some odd number of bonds. So my entire molecular weight is likely to be odd if there's one nitrogen present. Whereas, if there's an even number, so for example, 2 bonds, well, now we've got 2 nitrogens. Those 2 nitrogens like to form 3 bonds each, so 3 + 3 equals 6, so it's an even number again. So one of the easiest ways to look at this is just these two molecules. We've got butane, which we saw earlier has an M_z of 58. But if you take the butane and you replace one of the carbons with an NH₂, what's going to happen is that your molecular weight now turns odd because this nitrogen likes to form 3 bonds. So what's happening is that now you're just adding 1 you're adding a molecular weight of 1 to it. Now you might be saying, Johnny, how does that math work? Well, first of all, you can calculate it yourself. You'll see that I'm right. What's happening guys is that carbon weighs 12. Nitrogen weighs 14. So by turning 1 of the carbons into nitrogen, how much molecular weight should I be adding? 2. I should be adding 2. But then there's a difference. Carbon likes to have how many Hs? It likes to have 4. Nitrogen likes to have how many Hs? 3. So what that means is that when you add it all together, every carbon you add should be adding something around 16 if you were to do a CH₄. But every nitrogen you add should be adding about 17 because it's a little bit heavier, but it likes to have fewer hydrogens. So basically, that's the gist of it. But what you should know is that if you have an odd number of molecular weight, then it should be an odd number of nitrogens. Then if you have an even number, then you have an even number of nitrogens, even if that means 0. 0 is also an even number. So it just could mean I don't have any nitrogens present. Awesome guys. So let's go ahead and flip the page and do some practice.

Propose the number of carbons for a compound that exhibits the following peak in its mass spectrum: 

(M)+• at m/z = 72, relative height = 38.3% of base peak

(M+1)+• at m/z = 73, relative height = 1.7% of base peak 

• A. 3
• B. 4
• C. 5
• D. 7

Predict the approximate height of the (M + 1) peak for the molecule icosane, molecular formula C₂₀H₄₂.

• A. 11
• B. 44
• C. 50
• D. 22

1,4-dichlorobenzene is used as an insecticide and has the formula C₆H₄Cl₂. What would be the approximate relative heights of M, M+2, and M+4 peaks. For simplicity assume that all carbon atoms are ¹²C and the ³⁵Cl:³⁷Cl ratio is 75:25.

• A.
• B.
• C.
• D.

Here’s what students ask on this topic:

What is the significance of the M+1 peak in mass spectrometry?

The M+1 peak in mass spectrometry is significant because it helps in estimating the number of carbon atoms in a compound. This peak arises due to the presence of the carbon-13 isotope, which is about 1.1% abundant. The height of the M+1 peak is proportional to the number of carbon atoms in the molecule. The equation used to calculate the number of carbons (N₁) is:

$N{1}_{}=\frac{hM100}{1.1}$

where h is the height of the M+1 peak and M is the height of the molecular ion peak.

How do chlorine and bromine isotopes affect the M+2 peak in mass spectrometry?

Chlorine and bromine isotopes significantly affect the M+2 peak in mass spectrometry due to their natural isotopic ratios. Chlorine has two main isotopes, Cl-35 and Cl-37, in a 3:1 ratio, leading to an M+2 peak that is about one-third the height of the molecular ion peak. Bromine has isotopes Br-79 and Br-81 in a nearly 1:1 ratio, resulting in an M+2 peak of similar height to the molecular ion peak. These distinctive patterns help identify the presence of chlorine or bromine in a compound.

What is the nitrogen rule in mass spectrometry?

The nitrogen rule in mass spectrometry states that a molecule with an odd molecular weight has an odd number of nitrogen atoms, while a molecule with an even molecular weight has an even number of nitrogen atoms, including zero. This rule is based on the fact that nitrogen forms three bonds, which affects the overall molecular weight. For example, a molecule with one nitrogen atom will have an odd molecular weight, while a molecule with two nitrogen atoms will have an even molecular weight.

How can you estimate the number of carbon atoms in a molecule using the M+1 peak?

To estimate the number of carbon atoms in a molecule using the M+1 peak, you can use the following equation:

$N{1}_{}=\frac{hM100}{1.1}$

Here, N₁ is the number of carbon atoms, h is the height of the M+1 peak, and M is the height of the molecular ion peak. This equation leverages the 1.1% natural abundance of carbon-13 to provide an estimate of the number of carbon atoms in the molecule.

Why do isotopes create different peaks in mass spectrometry?

Isotopes create different peaks in mass spectrometry because they have different masses due to varying numbers of neutrons. For example, carbon-12 and carbon-13 are isotopes of carbon with masses of 12 and 13 atomic mass units, respectively. When a molecule contains these isotopes, it results in additional peaks (e.g., M+1, M+2) in the mass spectrum. These peaks help in identifying the presence and quantity of specific isotopes within the molecule, aiding in the determination of its molecular structure.