How is mass spectrometry used for quantitative analysis?





      Researchers are not always interested in obtaining full mass 
spectra in cases where the compounds are already known.  Rather, they may want to 
confirm the presence of specific substances or measure how much is present.  This 
is commonly done in environmental pollutant work and in pharmacokinetic studies 
where the goal is quantitation at very low concentrations in complex mixtures.  
The mass spectrometer is set to monitor only m/z values of ions representative of 
the molecules of interest so that valuable detection time is not wasted.  In many 
cases, the form of ionization is chosen to favor production of a single type of 
ion, thus maximizing sensitivity by keeping the ion signal in a single m/z value.  In Figure 16, only the signals at m/z 158 and m/z 160 were monitored during a gas chromatographic run.  This procedure is known as selected ion monitoring (SIM) and is different from the situation shown schematically in Figure 15 where compete mass spectra are obtained and used to construct selected ion profiles.  





      A quantitative mass spectral analysis of the dimethyl 
derivative of 5-fluorouracil in blood plasma is illustrated in Figure 16.  

In this analysis, the response from the substance of interest is measured 
relative to that from an internal standard added to the sample.  The top trace 
shows the SIM profile of the molecular ion at m/z 158. The compound of interest 
produces the center peak.  Two minor peaks arise from other sample components 
that also produce ions at m/z 158.  The lower trace is the SIM profile of the 
same molecule having 15N substituted for the normal 14N at both nitrogen 
positions.  Since both nitrogens are substituted, the signal from its molecular 
ion is observed at m/z 160, 2 Da higher.  Although both the sample and standard 
have the same retention time, they are detected separately by virtue of their 
different masses.  Standards may be a closely related substance or may be 
chemically identical 
                        
                       
                       
                       
       
but synthesized by substituting a isotope of one of the 
elements as in this example.

 
 


       
                     
                  
                    
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