Name: 化學203.有機光譜學。講座05。同位素的品質、丰度和質譜法。 (Chem 203. Organic Spectroscopy. Lecture 05. Isotopic Masses, Abundances, and Mass Spectrometry)
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So, today what I'd like to talk about is two

of three really important concepts in mass spect.

we talked about how the technique worked.

We introduced one big idea, and the big idea was

In the mass spect, we got that ion from kicking

out an electron, and then we talked about CI

and other soft ionization techniques like Mol D

and we said the big idea there is you get an ion

by adding a proton or adding a sodium to the molecule.

in mass spect really aren't very hard, and in fact,

nothing is hard about mass spect except we think differently

than we've sort of gotten used to thinking as organic chemists.

Ever since you were, well, taking high school chemistry,

you got used to the molecular weights of compounds

and calculating them based on the average mass,

in other words, carbon S, carbon 12, and carbon 13,

But here, each ion is being separated individually.

In other words, you're separating each ion

from each ion, which means you're going isotopomer

from isotopomer, and so we need to get used to the idea

of the individual isotopes, and we'll explore some

the fact different elements have different abundances

Carbon, for example, has 1.1% of its carbons as C13,

Sulfur has a little bit of S34, in addition to S34,

and we'll explore the implications of this,

and then in the last lecture, we'll talk about fragmentation.

We'll discuss fragmentation as it pertains to EI mass spect,

which is really a subject or a special topic in its right.

in the electrospray ionization last time, so an example

of fragmentation of a quasi molecular ion

Remember the type of molecular ion you get

and that's essentially just protic chemistry.

What we saw was a reaction that involving a leaving group

Leaving, that's chemistry that you've really been used

to since your sophomore year, but when we talk

about fragmentation and EI mass effect, we'll be talking

about fragmentation of radical cat ions, and given the fact

you don't generally get a lot of fragmentation.

Fragmentation isn't super important there,

but in EI mass effect, where you have tons of fragmentation,

you often can't even see your molecular ion.

So, I've given a couple of resources here,

which you'll using in your homework assignments.

These are also linked to the webpage for the course,

so you don't have to type in all of this stuff into the computer.

You can just click on the links that I've provided,

and as you'd go through the homework, you'll get familiar

with using these tools and choosing proper settings.

What I'd like to do now and today, is to really talk

about the concepts behind these first two issues over there,

isotopic matches and isotopic abundances.

And I thought maybe a place to explore this,

since we were talking about mass spect techniques last time,

and we talked about a variety of techniques, EI mass spect,

ESI mass spect, CI mass spect, is to introduce a variant of any

any of those ionization techniques that's often referred

to HR mass spect or high resolution mass spect.

In general, when you get a mass spectrum,

the mass that you get is good to within a few tenths

like in one particular exercise, I'll show you later on,

You might say, okay, that number 1239.2 is probably good

However, with specialized instrumentation and calibration,

So, with better instrumentation and calibration,

you can often get masses to very high precision with mass

to charge ratios to, I'll say high precision,

typically may 5 millimass units or 5 parts per million

or better, and I'll show you what I mean by that,

and in the case of a technique called ion cyclotron residence

mass spectrometry, sometimes even one magnitude better

So for example, by five parts per million,

I mean if I had a mass of 300.0000, then the mass

of a small molecule may be like a steroid or something

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化學203.有機光譜學。講座05。同位素的品質、丰度和質譜法。 (Chem 203. Organic Spectroscopy. Lecture 05. Isotopic Masses, Abundances, and Mass Spectrometry)

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compound

molecule