Page 10 - Fundamental Guide to GCMS
P. 10

Chapter 1  Basics of GC












            - Equation 1
            Partition Coefficient of Compound
              Concentration of Compound in Stationary Phase
            =
               Concentration of Compound in Mobile Phase
                The separation of compounds in a GC mainly depends on   as column type and dimensions, carrier gas type and flow,
            two factors: (i) the polarity (i.e. interaction of the compound   temperature and the analytical GC method used. Given the
            with the stationary phase in the GC column) and (ii) the boiling   same analytical parameters, the same chromatogram and RT of
            point of the compound (i.e. volatility). The lower the boiling   compounds are typically expected. However, in reality, RTs of
            point, the higher tendency of the compound being in the   the same compound separated by GC columns of even the same
            gaseous phase; thus, it travels faster in gaseous form through   manufacturing batch may differ slightly. Therefore, the use of
            the column. Compounds with similar boiling points, elute   relative retention or retention indices, a standardized form for
            with respect to their polarity, where compounds of weaker   comparison of GC retention data, can facilitate and allow GC to
            interaction with stationary phase elute first. Compounds with   be an excellent tool for qualitative analysis. This standardized
            similar polarity elute in order of increasing boiling points, where   system is used for the comparison between different analytical
            compounds of lower boiling points have a shorter retention   parameters, and also for analysis conducted by different GC
            time.                                              systems and laboratories. The use of retention indices in GC for
                The time taken for the compound to travel from the   qualitative analysis is well-established and is extensively used
            injection port to the detector is termed as retention time (RT).   in many industries such as flavor and fragrance, pesticide and
            RT for a specific compound depends on various factors such   environmental testing.


               Learn More: How to calculate relative retention?
                   Absolute RTs are affected by many parameters. The   For unretained compounds, the time taken for it to elute is
               use of other retention values which are less dependent   termed as the gas hold-up time (RT0). The time difference
               on parameters such as column dimension, carrier gas flow   between the peak of an unretained compound and any
               and analytical conditions are preferred. Therefore, relative   compound is the adjusted retention time (RT’).
               retention, which depends only on the ratio of partition   Relative retention can be calculated as shown below:
                                                               Adjusted Retention Time (RT’) = Retention Time (RT) - Gas Hold-Up
               coefficient of the compounds, are commonly used for this
                Figure 3: Relative Retention
               purpose.                                        Time (RT 0)
                2019/11/10 ver.
                CHROMATOGRAM                                   Relative Retention (α)
                                   Reference  Target               RTtarget’
                                  Compound  Compound           =
                                                                  RTreference’
                                                                   Partition Coefficient of Target Compound
                 Detector Signal  Injection Unretained peak    =   Partition Coefficient of Reference Compound
                                                               where adjusted retention time of reference and target
                                                               compound is RTreference’ and RTtarget’ respectively

                                                     Time        Click here  to learn more on retention
                                                                           index (Kováts Index) and
                             RT    ’                           Linear Retention Index (LRI).
                               reference


                      RT          RT target ’
                       0

                For unretained compounds, the time taken for it to elute is termed as the gas hold-up
                time (RT 0 ). The time difference between the peak of an unretained compound and any
                compound is the adjusted retention time (RT’). Relative retention can be calculated as
                shown below:
      10
                Adjusted Retention Time (RT’) = Retention Time (RT) - Gas Hold-Up Time (RT 0 )
                Relative Retention (α)
                  RTtarget’  Partition Coefficient of Target Compound
                =       =
                 RTreference’  Partition Coefficient of Reference Compound
                where adjusted retention time of reference and target compound is RT reference ’ and RT target ’
                respectively
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