In a series of posts, we are going to talk about Mass Spectrometry.

1. Introduction-The dif­fer­ent con­fig­u­ra­tions and the Elec­tron Impact process
2. What types of mass ana­lyz­ers are there?
3. What type of detec­tors are there?
4. What types of analy­sis can be done?
5. How do you read the output?
6. How do they come to a qual­i­ta­tive mea­sure using software?
7. How do they quan­ti­tate the results?
8. Do you need chro­matog­ra­phy if you are using Mass Spectrometry?

One of the lim­i­ta­tions of GC-MS work is actu­ally on the GC side. GC requires volatiles and can­not ana­lyze non-volatiles. So, what­ever is going to be ana­lyzed needs to be in the gaseous form. This is typ­i­cally done either pre-injection in a process called deriva­ti­za­tion or in the injec­tor itself which is heated and there­fore there is flash vapor­iza­tion of the sam­ple which puts it into a gaseous state.

This is opposed to Liq­uid Chro­matog­ra­phy which requires the com­pound that we want to ana­lyze to be dis­solved or soluble.

Once we over­come this lim­i­ta­tion on the pre-MS side of the prover­bial house, then we can look at the most typ­i­cal types of analy­sis run on EI-based Mass Spec­trom­e­try using a quadru­pole: Scan­ning ver­sus Selec­tive Ion Mon­i­tor­ing (SIM).

Both of these tech­niques have their own inher­ent strengths and their own inher­ent lim­i­ta­tions (weaknesses).

Think of scan­ning as like being in a big sun-room with a lot of win­dows. You have the abil­ity to look over very big win­dow out of a house. Because you have big win­dows, your eyes (which are really a detec­tor if you think about it) get a big pic­ture of every­thing out­side. A big overview where you can admire the view. As you have a large field of vision, you may be inspired by the over­all view as opposed to any­thing spe­cific. You may miss the details or some­thing selective.

To do a scan on using MS is just like this. Scan­ning is accom­plished by sys­tem­at­i­cally chang­ing the field strengths, thereby chang­ing the m/z value that is trans­mit­ted through the ana­lyzer. Just like look­ing at dif­fer­ent spe­cific spots with your eyes.

Strengths: It is a great method if you are ana­lyz­ing a true unknown, which is what we are mostly doing in foren­sic analy­sis. Because you look over a wider view, your chances of iden­ti­fy­ing and not miss­ing every­thing in the sam­ple is much better.

Lim­i­ta­tions:

Now, we have another method of analy­sis called Selec­tive Ion Mon­i­tor­ing (SIM).

In our anal­ogy, instead of hav­ing a bunch of win­dows look­ing over a very wide area, it is like only being able to look through a very lim­ited area not unlike a key­hole. As a result, you only get a very lim­ited amount of infor­ma­tion. How­ever, if you are only inter­ested get­ting that lim­ited amount of infor­ma­tion and you know exactly what you are look­ing for and where it is, then you aren’t incon­ve­nienced by the lim­ited view.

When you run in SIM, you are mea­sur­ing for a spe­cific ion. As a result other ions do not reach the detec­tor. Since it is just a mat­ter of whether or not the ion passes through, the analy­sis tube receives almost no con­t­a­m­i­na­tion, allow­ing it to endur­ing severe analysis.

Strengths: Because you are only look­ing at one very spe­cific spot and not every­where, then the chances of us over­look­ing any­thing in that one nar­row area is much, much less.

Lim­i­ta­tions: