How to conduct a GC measurement

# Preparations

- Check gas connections.
- TCD-Device: make sure, carrier gas is turned on.
- FID-Device: ignite FID, detector must have reached a temperature of 150°C ! For details see chapter 2.3.4

# Set measurement parameters

Clicking „New measurement“ shows the parameter window:

Fig. 6: Parameter window

The temperature program of the column furnace is set in the upper section of the window. Basically, two different types of measurement can be configured here:

- Isothermal measurement: The temperature of the separation column remains constant during the measurement.
- Dynamic temperature measurement: In many cases, the task on hand makes it necessary to progressively increase the column temperature during a measurement. A total of four parameters need to be entered in this context:

1. The initial temperature at which the measurement is starting.

2. The final temperature at the end of the heating phase.

3. The start of the heating phase, indicating the length of the preceding, isothermal phase.

4. The heating rate, i.e. the rate in degrees/minute at which the temperature is to rise during the heating phase.

The individual values of the temperature program can be set with the arrow keys next to each field. The entered values are checked for plausibility, and dependent values are corrected accordingly. For example, if the final temperature you have selected is lower than the initial temperature, then the initial temperature is lowered automatically to the same value as the final temperature

The minimum temperature of the detector is 150°C. At higher final temperatures, the detector is heated accordingly before commencement of the measurement. The temperature of the injector is 30°C higher than the selected final temperature. You can change this property by unchecking the “Determine Injector temperature automatically” checkbox. In this case, the detector temperature has to be entered manually. Click on OK to accept the displayed values, click on Cancel to abort preparations for the measurement.

The column furnace, detector and injector now need time to reach the set temperatures. Several different messages can appear now, depending on the instantaneous temperature of the furnace.

Input of user data

The lower section of this window contains several text fields where important information concerning the measurement can be entered. However, this information is optional and does not affect the measurement itself and can also be changed subsequently if required. The detector type is set by the system and cannot be changed

Loading and saving methods

By pushing the “Save method” button the contents of the parameter window can be stored into a method file (file extension .gcm) while the “Load method” button allows for restoring these values from file. In both cases the usual file dialog is used. It is advised to have a separate folder containing method files. Instructions for sample preparations can be integrated into the HTML – help system.

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# Preparations for measurement

Closing the parameter window with the „OK“ button initiates the start procedure of the measurement.

Dialog prepare measurement

Before the actual measurement can take place, it is necessary for the column and the detector to reach their final temperatures. You will be notified about this in the same window. You can also start the measurement immediately, in this case, however, measurement is made with an invalid temperature program.

After the dialog has changed to the following message, all temperatures are valid.

Dialog ready for injection

Now it is time to fill the microliter syringe with the substance to be analysed and adjust the volume to the required value. Once all the preparations for the measurement have been completed, click on “start meas.”. In the picture then a number appears displaying a countdown sequence which gives you 10 seconds time to insert the needle of the syringe through the septum. Once the countdown has elapsed to zero, inject the test substance. Then withdraw the microliter syringe from the injector. If a high injector temperature has been selected, it is advisable to insert the needle of the syringe as late as possible - depending on how practised you are - in order to prevent premature vaporisation of the test substance from the needle. In this context, also refer to the chapter 3.2 Notes on Injection

The actual measurement procedure can now be commenced. The components of the test mixture drift at different rates through the separation column. Their emergence from the column is registered electronically and recorded at certain time intervals. At the same time, these values are plotted as a characteristic on the screen. The display of this characteristic can be adjusted during the measurement.

Without intervention by the operator, the device automatically terminates the measurement after the scheduled time. However, you can terminate the measurement manually at any time by clicking “Stop measurement”.

In the case of TCD units, it is also possible to shift the reference line with the buttons labelled “baseline up” and “baseline down”, e.g. to correct drifts caused by the detector. This function is not available for FID units, due to their negligible drift.

# Ignition of the FID

The functional principle of the FID is based on ionisation of hydrocarbon fragments inside a hydrogen flame. Prior to any measurement the FID therefore has to be ignited. As the hydrogen flame is operated with an excess of combustion air, ignition is only successful if the excess of air is reduced for a short time. Ignition can be visualized because the detector current changes dramatically in the course of this process. Simply start a new measurement before ignition and take a closer look at the resulting curve, which is didactically interesting too:

Igniting the Flame Ionisation Detector

· Wait until the detector temperature is higher than 100 °C.

· Memorize (or better write down!) the position of the hydrogen valve.

· Turn the valve ¼ turn counter clockwise.

· Fan the flame of a gas lighter over the top of the FID. Ignition is indicated by a small oxyhydrogen explosion.

· Don’t forget to turn back the hydrogen valve to its former position.