In chromatography and micro GC in particular, external calibration is required.
For the analysis of gases in particular, standards are commonly presented in the form of pressure standard gas cylinders. This is the easiest way to calibrate your device.
However, it remains to define the composition of its standard bottle! The question is not so simple and different factors need to be taken into account.
Can I mix all the compounds in one bottle?
Yes, standard gases may contain 1 to more than 30 compounds in various concentrations. It is therefore not necessary to make bottles for each compound individually.
However, it is sometimes necessary to have several bottles to obtain all of its sample. This is particularly the case for compounds with reactivity, for example, some sulphur compounds.
These compounds may also have lower stability durations. It may therefore be interesting to have them in a separate cylinder in order to maintain longer stability for the majority of the other constituents.
The feasibility of a mixture also depends on the physicochemical properties of the gases.
Some compounds, such as solvents or hydrocarbons above C6, impose limitations on the production of gaseous standards. Mainly, some compounds, under normal conditions, are present in the liquid state such as solvents, or gas, but at low pressures such as toluene for example.
For the latter, the standard cylinders will therefore be available with a limited maximum concentration and/or a reduced maximum cylinder pressure and therefore a limited amount of standard gas available.
What matrix for my stallion bottle?
Where possible, it is recommended to select the same matrix as the sample as the background gas. The most common background gases are nitrogen, helium and argon. In microGC, depending on the application, the background gas may also be methane.
What concentration for my constituents?
Components in standard cylinders may have a concentration of ppb, ppm, %. Generally, the concentration is expressed in molar fraction as 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, only one calibration point per compound is often sufficient. A concentration value close to the expected normal value in its sample is then selected to obtain a quantification with the lowest uncertainty.
How many bottles are needed?
Wide range of concentration for the same compound:
As discussed earlier, in micro GC, often only one calibration point is sufficient. However, in more complex applications, especially if the variation in concentration of the same compound varies over a wide range, several calibration points are relevant.
In this case, two standards are usually used: a low and a high value. The low value, however, for microGC analysis is chosen not too close to the device's detection limit (LOD) in order to maintain 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 is intended to determine a methane production of 0-43%. The low calibration point can be set to 50 ppm and the high to 45%. The calibration curve is a straight line through zero.
Change of matrix during reaction:
In microGC and mainly on the molecular sieve column, the quantification of some compounds can be affected by a "matrix effect". Indeed, in the analysis of a process whose matrix between the beginning and the end passes from hydrogen to methane, it is noted that the quantification of nitrogen or CO2, for example, can be affected, even without any change in concentration for these compounds. This will be the subject of a subsequent article. In this case, it may be necessary to have several standard cylinders, one for each matrix of its application.
Some general remarks
Duration of validity
The certificate issued with the cylinder shall indicate a period of validity of the mixture.
A gas mixture can be stable for up to several years depending on its composition. Similarly, the stability of the mixture is guaranteed under storage conditions, usually between -10 and 50°C for example, in direct sun shelter.
Today, control of the entire production chain of the gas standards ensures stability over time, even for low levels.
Accuracy
The accuracy of the mixture is defined by the difference of achievement and uncertainty. The accuracy of the mixture is mainly due to two parameters:
A difference in concentration between the requested concentration and the realized concentration. This parameter depends on the production technique. A tolerance of 5% is commonly accepted.
Uncertainty: maximum difference between measured concentration and true concentration. This is indicated by a 95% confidence interval according to ISO 6141. This parameter depends on the capacity of the measurement technique. An uncertainty of 1% to 2% is generally accepted for chromatography measurements. Sometimes, depending on the compounds and the desired concentration, the minimum possible uncertainty is 5% or more.
NB: High precision bottles with less than 0.1% uncertainty are achievable. These, on the other hand, are much more expensive and generally not necessary for the calibration of our analysers.
Several levels of accuracy are generally available in gas tankers to cover all needs.
For the calibration of your microGC, a 5 to 10% implementation gap and an uncertainty of 1 to 2% are sufficient for a good calibration.
Bottle size
Standard bottles are available in different sizes from 1L to 50L with pressures up to 200 bars.
In microGC, although calibration is not very common because the analyser is known for its stability, one can move towards a small bottle. However, it is interesting to learn about the prices of several volumes because sometimes between a B10 and a B20, there is very little difference in price due to the complexity of making mixtures in a small volume, especially in low content (needs for successive dilutions, etc.).
Good practices
Beware of contamination!
The standard bottle as well as all elements from the bottle to the connection to your analyser can be contaminated. In fact, ambient air and humidity in particular, due to a phenomenon of backscattering are common contaminants. This is explained by the difference in partial pressure between gases on both sides of the walls.
In order to eliminate the risk of contamination, some good practices must be followed:
– Utiliser du matériel adapté à votre application : le choix du matériau ou du traitement peut avoir une incidence sur l’ensemble de l’application. Les matériaux doivent être non perméables, non poreux, non réactifs.
– S’assurer de l’étanchéité et de la propreté des éléments en contact avec le gaz en tout point de l’installation. Attention, nous vous déconseillons de vérifier l’étanchéité avec du liquide (type Snoop).
– Purger l’ensemble de votre installation lors de la mise en service afin d’éliminer l’air et les impuretés résiduelles.
Source image : Air Liquide
Attention aux mélanges à faible tension de vapeur !
Ces mélanges présentent un risque de condensation durant l’injection. Le chauffage des lignes de transfert est une bonne solution pour limiter ce risque et garantir l’intégrité du mélange injecté.
En résumé
Pour mon étalonnage en micro GC, dans l’idéal j’ai besoin de :
- 1 bouteille étalon
- avec tous les composés que je veux quantifier
- dans des concentrations proches de mon échantillon
- dans la même matrice que mon échantillon
- avec un manodétendeur adapté et dédié
Si mon application balaye une gamme de concentration très large pour mes composés, je devrais réaliser plusieurs points de calibration et donc à avoir plusieurs étalons gazeux.
Si je souhaite analyser des échantillons qui contiennent des teneurs importantes en phase gazeuse de composés non disponibles sous forme gazeuse ou dans des conditions très limitées, je devrais mettre en oeuvre une autre méthode de calibration.
D’autres méthodes de calibration sont effectivement disponibles, cela pourra faire l’objet d’un futur article !
