Page 12 - Fundamental Guide to GCMS
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Chapter 1  Basics of GC
                                                               Figure 5: Molecular Application Range of GC
                                                               2019/11/27 ver.

                However, not all analyses can be performed using GC.
            Figure 2 shows an illustration of the approximate molecular
            range of GC and LC. The molecular application range of GC
            is limited to small, volatile, non-polar and thermally stable
            compounds. Compounds that are not of this nature, such as
            semi-volatile, involatile and/or thermally-labile compounds, can   pyrolysis   LC
            go through various methods (e.g. pyrolysis, thermochemolysis   Molecular Weight  HT-GC  thermochemolysis
            and derivatization) to ensure compatibility for separation and
            analysis through GC. Derivatization, for example silylation           derivatization
            and acylation, is commonly performed to reduce polarity, and   GC
            increase thermal stability and volatility.
                Besides GC, there are several other chromatographic and
            separation techniques such as LC (including ion exchange, size   Figure 2. Molecular application range of GC.
            exclusion and column chromatography), supercritical fluid
            chromatography (SFC) and capillary electrophoresis. The choice
            of technique generally depends on the nature of the sample   With lesser variables and components such as solvents, columns
            and the type of analysis required. GC and LC, the two most   and operating pressure, the GC system is less complex and
            widely-used chromatographic techniques used in the industry,   requires less maintenance and operating cost. In all, it is a
            are frequently being compared. A concise comparison of GC and   powerful technique and can achieve separation, identification
            LC and the key features and differences are listed in Table 1.  and quantification of mixtures and compounds. GC can be
                GC is usually the method of choice due to its easy   coupled with various detection methods and the following
            operation, high speed, resolution, sensitivity and reproducibility.   section describes the instrumentation and components of GC.

            Table 1. Comparison of GC and LC.
                                                  GC                                      LC
                                • Partition between the gaseous mobile phase & solid/  • Partition between the liquid stationary & mobile phase
                                liquid stationary phase                 • Polarity of compound
              Separation principle
                                • Polarity of compound                  • Other separation modes (e.g. ion exchange, size
                                • Boiling point of compound             exclusion, reverse-phase) are available
              Mobile phase      Inert gas                               Solvent / buffer (liquid)
                                                                        Require high pressure to operate
              Pressure requirements  Requires lower pressure to operate (max 1200 kPa)
                                                                        HPLC & UHPLC* (30 – 140 MPa)
                                • Volatile
                                                                        • Soluble in liquid phase
              Sample requirements  • Thermally stable
                                                                        • Not limited by molecular weight or polarity
              (Compound nature)  • Low molecular weight
                                                                        (e.g. Proteins, peptides, amino acids and polymers)
                                (e.g. alcohols, short-chain fatty acids, pesticides and VOCs)
                                Consists of MS or ionization and conductivity detectors   Consists of MS or spectroscopy techniques (e.g. UV,
                                (e.g. flame ionization, thermal conductivity, photo-  fluorescence, refractive index, conductivity & light
              Type of detectors
                                ionization, electron capture & barrier discharge ionization)  scattering)
                                • generally destructive in nature       • generally non-destructive in nature
            *HPLC: High Performance Liquid Chromatography     UHPLC: Ultra-High-Performance Liquid Chromatography

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