Gas chromatography: how technologies help to solve problems

Gas chromatography is a conservative analytical method of chemistry, but nevertheless it constantly advances. In the course of the Ruscatalysis-2021 Congress the chief researcher of Boreskov Institute of Catalysis, Dr. Vladimir Sidelnikov spoke about the problems arising with the use of the method and their solutions due to the new separation technologies.

I came, I saw, I separated

Gas chromatography is a method of separation, identification, quantitative analysis and physicochemical studying of substances based on the distribution of the components of analyzed mixture between two immiscible phases relatively moving to each other, with gas being the mobile phase. As with any method, the gas chromatography utilization can involve certain difficulties that have to be resolved.

“Sometimes it happens that you introduce an extremely complex mixture of compounds into the column of gas chromatograph, and the chromatogram would feature many non-separated peaks, meaning that the column failed to do what it must”, — says Vladimir Sidelnikov.

As a variant of solution, the column can be very long and the temperature can be slowly elevated in the course of separation. Then it would seem that all peaks at the chromatogram are separated, but only at first sight — in the end they would not be separated fully. The thing is that the column simply can have a selectivity not sufficient for separation of the certain group of compounds.

In the 1960s the method of multidimensional chromatography appeared that helped to solve the problem but was very labor-consuming. The core of the method is that the first column separates the substances and then the non-separated group is introduced manually into the second column with necessary selectivity.

“The situation changed dramatically in the end of the past century when the two-dimensional complex chromatography appeared. A prerequisite for that was the creation of modulators — devices that excise the non-separated fragment from the first column, accumulate it and then transport it into the second column with different selectivity, which allows complete separation of the substances that were not separated in the first column”, explained the researcher.

Eventually, the scientists obtain a number of subsequent small chromatograms that, with the help of special software, allow receiving a single chromatogram in 3D or 2D form.

“This method significantly increases the separating characteristics of the chromatographic system. It allows structuring of the chromatographic space, which makes it possible to perform a group analysis when analyzing complex mixtures. And as a bonus we get an increase in sensitivity of the method by approximately two orders”, underlined Sidelnikov.

The method of two-dimensional complex chromatography allows working with extremely complex mixtures, for example, one of the first demonstrations of the separating capacity was analysis of the components of cigarette smoke. The chromatogram showed about 30 thousand peaks, which could never be achieved with only one column.

Still another advantage is a very high rate of separation. Separation of C₄ fraction in hydrocarbon mixture with the help of gas-adsorption chromatography takes 16-20 minutes. With the help of two-dimensional chromatography nine components of C₄ mixture were separated in two minutes. Sidelnikov noted that he has not met publications where such separation with complete resolution of chromatographic peaks would be made faster.

New solutions come to the aid

In the 1980s a study was published on the then fastest separation of the compound mixture — nine components were separated on the capillary column of 50 µm in diameter in just 0.6 s. However, the study results were not used in practice — there is not much of immobile liquid phase in such column, and it could be easily overloaded. A question came up on how to make a column that would keep the high rapidity but could work with large volumes?

One of the solutions is to make a polycapillar column (PCC). The researchers in Boreskov Institute of Catalysis were first to obtain the separations with the use of glass PCC consisting of about 1.5 thousands of capillaries. With the use of this column it is quite possible to separate, for example, 40 components from С₆ до С₁₂ in about 70 seconds.

“Transformation of the columns in the world occurs quietly and constantly. Quite recently a new technology appeared that is based on the methods of preparation of the micromechanics items. These items help us to significantly decrease the size of components including those for gas chromatographs”, says Vladimir Sidelnikov.

Such columns are made by etching of the silicon plates: they are etched by thin channels that are connected with a single channel, and then one plate is covered with another with the help of molecular adhesion. The inner surface of the channel is covered with the immobile liquid phase and a chromatograph tract is connected to it. This technology provides a possibility for preparation of the coin-sized columns, which leads to a fantastic decrease in the size of devices while their analytical characteristics remain acceptable.

As far as 3D printing is concerned, unfortunately, according to Sidelnikov, there is only one example of the printed column in the world so far – it is the element for analyzer of ethylene content in air. The problem is that the additive technologies do not allow creation of the details with typical sizes that meet requirements of gas chromatography. Nevertheless, scientists and engineers search for the way to use the 3D printing for making various elements of chromatographs, including columns.