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.