The M+1 peak is caused and shows the relative abundance of ¹³C

¹³C is found in relative abundance of 1.1%

So, if M+1 has an \(\frac{m}{z}\) value of 4.4, that means there are 4 carbons since \(\frac{4.4}{1.1}\) = 4

Imagine a compound containing 2 carbon atoms. Either of them has an approximately 1 in 100 chance of being ¹³C.

There's therefore a 2 in 100 chance of the molecule as a whole containing one ¹³C atom rather than a ¹²C atom - which leaves a 98 in 100 chance of both atoms being ¹²C. That means that the ratio of the height of the M+1 peak to the M+ peak will be approximately 2 : 98. That's pretty close to having an M+1 peak approximately 2% of the height of the M+ peak.

Normally there is only a M peak shown, since we are given simplified graphs but if shown the complete then we can see a tiny peak immediately next to the M peak, called the M+1 peak