Why does the retention rate decrease over time
Basic terms of gas chromatography
This page gives an overview of the relevant basic terms in gas chromatography. The Gas chromatography (GC) is a chemical analysis method from the field of chromatography. The aim of gas chromatography is to separate mixtures of substances that can be vaporized without decomposition into their components.
GC column and separation principle
The following explanations describe by far the most widespread form of gas chromatography, as it can be found in almost every chemical analysis laboratory (high-resolution capillary gas chromatography, liquid / gaseous).
A modern GC column consists of a very thin, long capillary made of quartz glass. The outside diameter of this hollow tube is about 500 µm, the inside diameter 100-320 µm and the length 25-50 m. The outside of the quartz glass is provided with a very thin polyimide layer, which makes the capillary much more flexible, less brittle and sufficiently flexible to be able to to be wound up on a spool with a diameter of approx. 15-20 cm. The metallic support inside the coil is called a cage.
The inside of the capillary is coated with a liquid, highly viscous substance that serves as the stationary phase. They are typically polyorganosiloxanes, but a large number of other, very special stationary phases are available for special separation problems. The layer thickness is about 1 µm.
During operation, the GC column is permanently flowed through by the mobile phase, which in the GC is called Carrier gas referred to as. Typical carrier gases are hydrogen, helium and nitrogen. Since the thin capillary column offers a not insignificant resistance to the flowing gas, the carrier gas must have a certain resistance form be pressed through the column (about 0.5-1.3 bar).
The time that the carrier gas needs to flow through the entire column is called the dead time of the system (approx. 1 min). Under ideal conditions, the carrier gas does not interact with the stationary phase at all, but the dead time only depends on the inlet pressure and flow resistance of the column. In practice, the ideal conditions are met so precisely with real carrier gases that no deviations can be measured.
In contrast to the carrier gases, most chemical substances interact with the stationary phase, i.e. they stay in the stationary phase for a certain time. The duration of your stay in the stationary phase is added to the duration of your stay in the mobile phase (dead time), so you need longer overall to pass the entire GC column. The term retention is originally derived from the fact that the stationary phase holds back the analyte for a certain period of time. Nowadays, however, the term retention time is used in a simplified manner for the time that the analyte needs to pass through the column and this therefore includes the dead time. Therefore the terms are defined as follows:
- Retention time: Time it takes for an analyte to pass through the column. This corresponds to the time difference between injection and detection. [tR = ts + t0]
- Net retention time: Is the time in which an analyte stays in the stationary phase. [ts]
- Dead time: Is the time in which an analyte stays in the mobile phase. [t0]
The retention of a substance by the stationary phase is essentially determined by three aspects:
- Strength of the interaction of the substance with the stationary phase ("tendency to remain in the stationary phase")
- Boiling point of the substance ("tendency to remain in the mobile phase")
- Diffusion properties of the substance ("mobility in the stationary and mobile phase")
In many cases, a special interaction between the substance to be analyzed and the stationary phase is used to separate substances. The strength of the interactions between the sample components and the stationary phase is determined both by their structure and by their functional groups. In the case of non-polar substances, only dispersion interactions (van der Waals bond) occur, while polar separation phases can also enter into polar interactions, e.g. hydrogen bonds or donor-acceptor bonds. The latter separate according to the principle: opposites attract. This means that separation phases, which e.g. are able to take up hydrogen for hydrogen bonding, separate substances which can provide hydrogen for bonding (e.g. alcohols). Enantiomers, for example, which do not differ in their boiling points and would therefore have the same retention times, can also be separated by their interactions of different strengths with special derivatives of cyclodextrins.
Categories: Chromatography | Physical analysis method
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