Brief introduction of large-scale instrument analysis method-gas chromatography (1)

1.

Gas chromatography is a rapid development after the 1950s to separate and analyze various components in complex mixtures

(1) The principle of the method

(2) The characteristics of gas chromatography

1) High separation efficiency

2) High selectivity

3) High sensitivity

4) Fast analysis speed

5) Less sample usage

6) Separation and detection can be completed at one time

However, the application of gas chromatography has its limitations, that is, it can only measure the amount of a single substance, not the total amount of certain congeners; in qualitative and quantitative analysis, the standard substance of the test substance is required as a control, while the standard Products are often not easy to obtain, which brings difficulties to qualitative identification

(3) Chromatogram and related terms

1) Chromatogram

2) Baseline

Noise: The fluctuation of the baseline in a short period of time, expressed as the peak of the fluctuation

Drift: The baseline changes slowly over a long period of time (such as tens of minutes)

3) Chromatographic peaks
.
When the carrier gas carries the sample components through the detector, the curve of the signal output by the detector over time is the chromatographic peak
.
The ideal chromatographic peak is a normal distribution function, which means that its peak shape is symmetrical
.

Peak height: the vertical distance from the highest point of the chromatographic peak to the bottom of the peak (the extension of the baseline below the peak), generally expressed by h
.

Width at half height: the width at 1/2 of the peak height, commonly used in cm
.

Peak Bottom: A straight line connecting from the start and end of the peak
.

(4) Chromatographic column
.
The choice of chromatographic column is the key link of gas chromatographic analysis
.

1) Pack the column
.
A chromatographic column packed with packing is called a packed chromatographic column
.
The inner diameter is preferably 2~4mm.
Increasing the column diameter will increase the longitudinal diffusion and reduce the column efficiency, which is not conducive to separation
.
The column length of 1~2m is generally used to separate high-boiling substances, and 3~4m is suitable for low-boiling substances
.
The column materials are stainless steel, copper, glass and polytetrafluoroethylene, and glass columns are commonly used
.
There are U-shaped and spiral-shaped columns
.

2) Capillary column
.
Also known as open-tube column or hollow capillary column
.
The inner diameter is 0.
2～0.
5mm
.
Glass capillary columns are more economical, have high column efficiency, and are widely used
.
However, due to the presence of adsorption and catalytic activity on its surface, it must undergo a series of treatments such as silanization, and it is easy to break, so special care should be taken during operation
.
In recent years, the emergence of quartz elastic columns completely overcomes the shortcomings of glass capillary columns
.
Quartz columns are greatly improved in terms of flexibility and inertness, are easy to install, and have good repeatability.
At present, they have been widely used in the analysis of organic pollutants in environmental samples
.