Question

3. How do you determine the relative percentages of substances in a mixture that you analyze by gas chromatography? Answer by describing a specific method (see Technique 22). (1 point)

Answer 1

Gas chromatogram has several sub-units and sequential processes related to them.

1. Sample injection into a box, which then vaporizes the sample.

2. Mixing analyte gas with an inert carrier gas ( the mobile phase ).

3. The sample then goes through a column containing a liquid stationary phase coated on walls. Different gases have a different degree of interaction with this stationary phase. This leads to the separation of analyte gases. Gases having stronger interaction with the stationary phase lag behind in the column.

Now comes the detection part, this can be done with a simple gas detector device, which when detects a certain gas produces an electrical signal. The intensity of the signal directly proportional to the amount of gas detected. But, this technique is not that accurate and reliable. Hence, a gas chromatograph is always coupled with another technique as a detection system.

Mass spectrometry is the best method. This receives the gas from gas chromatograph ( of course all analytes separated reach one by one ).

1. The gases are ionized in a chamber using electron beams.

2. These charged gases now move to a chamber with an electric field and get accelerated to high velocities.

3. Next, they are exposed to a magnetic field.

4. Moving charged particles now curve their path with a radius proportional to their mass/charge ratio.

5. The analytes now hit the detector at different times depending on their circular trajectory.

6. The detector is a photomultiplier tube which produces electrical signal corresponding to each ion bombarded.

7. A graph between intensity vs mass/charge ratio is obtained.

The more the amount of analyte, the more intense the peak. A graph is plotted between intensity and m/z. The most intense peak is assigned 100% abundance. Rest peaks are denoted relative percentages. Both quantitative and qualitative analysis is performed.