01. Mobile phase
GC uses gas as the mobile phase, also known as carrier gas. The commonly used carrier gases include helium, nitrogen, and hydrogen. Compared with HPLC, GC has fewer types of mobile phases and a smaller range of options. The main function of the carrier gas is to introduce the sample into the GC system for separation, and its impact on the separation results is limited.
In HPLC, there are many types of mobile phases that contribute greatly to the separation results. Looking at it from a different perspective, optimizing the operating parameters of GC is relatively simpler than HPLC. In addition, the cost of GC carrier gas is lower than that of HPLC mobile phase.
02. Fixed phase
Due to the relatively limited types of carrier gases in GC, its separation selectivity is mainly changed by different stationary phases, especially in packed column GC where the stationary phase is often composed of a carrier and a fixed liquid coated on its surface, which has a decisive impact on separation. Therefore, this has led to the development and research of a wide variety of GC stationary phases. So far, there are hundreds of GC stationary phases available for us to choose from, but there are only a dozen commonly used HPLC stationary phases.
Therefore, LC largely relies on selecting different mobile phases to change separation selectivity. Of course, there are only a dozen commonly used stationary phases for capillary GC. In practical analysis, GC generally selects a carrier gas and optimizes separation by changing the chromatographic column (i.e. stationary phase) and operating parameters (column temperature, carrier gas flow rate, etc.), while LC often optimizes separation by changing the type and composition of the mobile phase and operating parameters (column temperature, mobile phase flow rate, etc.) after selecting the chromatographic column.
03. Analysis Object
The samples that can be directly separated by GC are volatile and thermally stable, with a boiling point generally not exceeding 500 ℃. According to relevant data statistics, 20% to 25% of known compounds can be directly analyzed by GC, while the rest can be analyzed by LC in principle. That is to say, GC has far fewer analysis objects than LC.
It should be pointed out that some samples that cannot be directly analyzed by GC can also be indirectly analyzed by GC through special injection techniques such as headspace injection and pyrolysis injection. For example, the cracking chromatography of polymer materials is like this. This to some extent expands the scope of GC analysis objects. In addition, GC is more suitable for gas analysis than LC.
04. Testing Technology
There are various detection techniques commonly used in GC, such as thermal conductivity detector (TCD), flame ionization detector (FID), electron capture detector (ECD), nitrogen phosphorus detector (NPD), etc. Among them, FID responds to most organic compounds and has a high sensitivity, with a minimum detection limit of up to nanograms.
However, there is no highly sensitive detector with such good universality in LC. The commonly equipped LC instruments for commodities are UV Vis absorption detectors and refractive index detectors (RI). The former is far less versatile than FID in GC, and the latter has lower sensitivity and is not suitable for gradient elution. Of course, both GC and LC have some highly sensitive selective detectors,