Yes, Standard gases can contain from 1 to more than 30 compounds in various concentrations. It is therefore not necessary to make bottles for each compound individually.

However, sometimes it is necessary to have several bottles to obtain the whole sample. This is particularly the case for compounds which present a reactivity, certain sulphurous compounds for example.

These compounds may also have shorter stability times. It may therefore be interesting to have them in a separate bottle in order to keep a longer stability for the majority of the other constituents.

The feasibility of a mixture also depends on the physicochemical properties of the gases. 

Indeed, certain compounds, in particular solvents or hydrocarbons higher than C6, pose limitations to the realization of gas standard. Mainly, some compounds, in normal conditions, are present in liquid state such as solvents, or gaseous state but at low pressures as toluene for example.

For the latter, standard cylinders will therefore be available with a limited maximum concentration and/or a reduced maximum cylinder pressure and thus a limited available quantity of standard gas.

When possible, it is recommended to choose the same matrix as the sample as the background gas. The most common background gases are nitrogen, helium, argon. In micro GC, depending on the application, the background gas can also be methane in particular.

The constituents in the standard bottles can have a concentration in ppb, ppm, %. Generally, the concentration is expressed in molar fraction because this measurement is independent of pressure and temperature. It is also possible to find measurements in volume or mass fraction.

Micro GC is an analytical technique known for its linearity and stability. Thus, for many applications, a single calibration point per compound is often sufficient. One then chooses a concentration value close to the expected normal value in one’s sample in order to obtain a quantification with the lowest uncertainty.

Wide concentration range for the same compound :

As previously discussed, in micro GC, often a single calibration point is sufficient. However, in more complex applications, especially if the concentration variation of the same compound varies over a large range, it is relevant to perform several calibration points.

In this case, two standards are generally used: a low value and a high value. The low value, for a microGC analysis, is however chosen not too close to the detection limit (LOD) of the apparatus in order to preserve a quality calibration. Indeed, close to the LOD, the repeatability of the analysis is less good and will therefore have an impact on the accuracy of the calibration.

For example, the application aims to determine a methane production of 0 to 43%. The low calibration point can be set at 50 ppm and the high value at 45%. The calibration curve is a straight line passing through zero.

Matrix change during the reaction :

In microGC and mainly on the molecular sieve column, the quantification of certain compounds can be affected by a “matrix effect”. Indeed, when analyzing a process where the matrix between the beginning and the end changes from hydrogen to methane, we notice that the quantification of nitrogen or CO2, for example, can be affected, even without any variation in concentration for these compounds. This phenomenon will be discussed in a later article. In this case, it may be necessary to have several standard bottles, one for each matrix of its application.