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?
9. Other top­ics of inter­est about GC-MS

In this post, we take a closer look at GC-MS and how it can be used to answer the ques­tion of “how much do we have?” or quantitation.

The good news is that it is not that much of a mys­tery and is fairly easy. Just as with all forms of quan­ti­ta­tive analy­sis, we have to have a clue of what com­pound we are look­ing for before we can quan­tify it. The quan­ti­ta­tion can­not begin unless we have com­pleted the qual­i­ta­tive deter­mi­na­tion. This is due to the fact that no GC-MS out of box from the ven­dor has a clue as to either the qual­i­ta­tive or quan­ti­ta­tive mea­sure. The machine has to be “taught” not only what it was look­ing for, but most impor­tantly how much we are look­ing at.

Here are some posts on cal­i­bra­tion in general:

When is a straight line a curve: Cal­i­bra­tion curve

Why do instru­ments need to be calibrated?

A rose by any other name??? More on Metrol­ogy and its nomenclature

How do they make the squig­gly lines turn into a magic num­ber: Area under the peak

A com­mon way to get more quan­ti­ta­tive infor­ma­tion out of a mass spec­trum is to cre­ate a stan­dard curve to com­pare the sam­ple to just as we would do in GC-FID. As we talked about above, this requires know­ing what is to be quan­ti­tated ahead of time, hav­ing a stan­dard avail­able, and design­ing the exper­i­ment specif­i­cally for this purpose.

A more advanced vari­a­tion on this the use of an inter­nal stan­dard which behaves very sim­i­larly to the ana­lyte (deuter­ated inter­nal stan­dard). The deuter­ate com­pound is dif­fer­ent by + or — mol­e­c­u­lar weight (MW). In the case of GC-MS it has char­ac­ter­is­tics so sim­i­lar to the drug of inter­est that it elutes (comes out) from the col­umn as a peak with char­ac­ter­is­tic ions. This is often an iso­topi­cally labeled ver­sion of the analyte.

Quan­ti­ta­tive work is typ­i­cally run in SIM mode.

Now lets look at some real data from a DUID-MJ case where quan­tifi­ca­tion occured.

Cal­i­bra­tion Curves

Under­ly­ing SIM data to the above cal­i­bra­tion curve

An exam­ple of a “bad” SIM where the ratios are out­side of limits

SIM based quan­ti­tion data of an unknown