Application of Gas Chromatography in Coating Instrument Analysis

Gas chromatography is a very important analytical method in coating instrument analysis. Now I will briefly introduce some basic knowledge of gas chromatography in coating applications.

Basic knowledge 1) Basic information of gas chromatograph On a gas chromatogram, we can at least get the following data.

Dead time: The time required for inert gas to flow through the column to the detector.

Retention time: The time from the injection of the sample into the vaporization chamber to the peak of the chromatographic peak.

Adjust retention time: retention time minus difference in dead time.

Chromatogram Peak Height, Half Peak Height: Chromatogram peak width, half width (width at half height)

The quotient squared of retention time and half width is multiplied by 5.54 times, which is the number of theoretical plates. If you adjust the retention time, you can calculate the number of effective plates. The length of the column divided by the number of plates gives the height of the plate. In general, a packed column of 2 meters in length has an effective number of plates of about 2000 and an effective plate height of about 1 mm.

2) Gas chromatographic operating conditions Carrier gas: Normally nitrogen and helium. Hydrogen is also a good carrier gas, but special care must be taken when using it. The flow rate of the carrier gas affects the separation efficiency of the column and the separation time of the sample. A 2-meter-long packed column, with a typical pre-column pressure of 0.2 MPa. The precolumn pressure of the capillary column is approximately the same as the former. Hydrogen flame detectors also use combustion gas hydrogen and oxyfuel gas.

Vaporization chamber: In general, the temperature of the vaporization chamber is 50°C higher than the temperature of the sample. For example, the analysis of TNR samples is about 200-250°C. However, attention should be paid to easily decomposable samples such as diacetone alcohol. Even if the boiling point is as high as 168°C, the analysis temperature can only be set to 120°C.

Column temperature: Usually 50 °C below the boiling point of the main component. Frequently used program temperature column temperatures.

Detector temperature: Under consideration of not allowing any components of the analyte to condense with the water produced by the combustion, the low is not required. Generally between 100-200 °C.

3) Chromatographic Qualitative Methods For the separation of unknown mixed samples, the best method is gas chromatograph-mass spectrometry. But when there is no such condition, it can also be qualitative. There are several basic methods. a, Qualitative multi-column, repeated measurements of the peak time of the same sample with different columns, preferably a capillary column, the same retention time, can be qualitative. The reference is generally benzene; b, qualitative relative retention time. That is, a benchmark is used for comparison. The quotient for adjusting the retention time of the two as the compound alpha value can be qualitative; c, the Kovats method, that is, the logarithm of the retention time of the normal paraffin trim is used as the coordinate to measure the retention position of the compound. The above methods have a lot of work, of course, it is best to use technology with mass spectrometry!

4) There are four methods for chromatographic quantitative methods:

a. The correction factor method takes into account the detector's response to the compound and introduces a correction factor. In fact, it is necessary to configure a mixture of different concentrations of the sample and the reference substance, and calculate the relative correction factor of the compound through the peak area.

b. The normalization method considers that the ratio of the area of ​​each peak in the sample is equal to the mass percentage of each component.

c, internal standard method In the sample by adding a known material as an internal standard, the quality of the known internal standard, the quality of the sample, the measured area of ​​each peak, you can obtain the mass ratio of each component.

d, The external standard method uses the compound of known content formulated as a standard to determine the content of the compound of the test substance. This requires considerable injection techniques.