Key Issues in Miniaturization of Gas Chromatography and Liquid Chromatography

Abstract: The benefits brought by the miniaturization of chromatographic instruments are not only the improvement of the separation efficiency per unit length, but also the maintenance or even improvement of the total separation capacity; not only the miniaturization of the separation system or a part, but the overall miniaturization ; Not only the improvement of mass sensitivity, but the maintenance or improvement of concentration sensitivity; not only the low consumption of energy and substances, but the convenience and friendliness of use; not only the reduction of the overall size, but more importantly, the whole machine Increased stability and reliability!

In the miniaturization process of chromatographic instruments, the size reduction must not only consider the material properties and manufacturing possibilities, but also consider in principle the series of problems brought about by the size reduction. These problems include: (1) whether the number of molecules allocated in the separation system is greater than 106, because only greater than 106 can obtain data that meets the statistical results; (2) the reduction in the size of the separation channel naturally improves the efficiency of the unit column length, However, the reduction of the total length may make the total separation efficiency much lower than conventional instruments; (3) For mass-sensitive detectors, whether the number of molecules that reach the detector per unit time after passing through the separation column meets the minimum number required by the detection principle; (4) For the concentration detector, whether the number of molecules reaching the detection cell can meet the number of molecules that meet the statistical law; (5) whether the applied energy density in the micro-area exceeds the limit that the detected molecule can withstand; (6) The transportation and control of trace mobile phase; (7) Due to the reduction of material size, the surface layer oxidation or corrosion affects the device function. Finally, the benefits of miniaturization of chromatographic instruments are not only the improvement of the separation efficiency per unit length, but also the maintenance or even improvement of the total separation capacity; not only the miniaturization of the separation system or a part, but the overall miniaturization ; Not only the improvement of mass sensitivity, but the maintenance or improvement of concentration sensitivity; not only the low consumption of energy and substances, but the convenience and friendliness of use; not only the reduction of the overall size, but more importantly, the whole machine Increased stability and reliability!

The above 7 issues are discussed below.

(1) The basic principle of chromatographic separation is that there are a number of molecular groups that meet the statistical law after continuous two-phase distribution and molecular collision, and the difference in their distribution coefficients is used to achieve the purpose of separation. This is a macro parameter. When the number of molecules is lower than this number, it will deviate from the statistical law and appear the so-called fluctuation phenomenon. The smaller the number of molecules, the more severe the fluctuation. When the number of molecules is less than 103, there is no accurate chromatographic retention law, so the separation law in the macro sense is lost. Generally, the number of molecules guaranteed to conform to the statistical law is 106.

For example, a packed capillary liquid chromatography (μ2HPLC) column or a capillary electrophoresis column with an internal diameter of 30 μm, if the separation efficiency of 100,000 / m and 400,000 / m are maintained, the injection volume without overload when direct injection is 40pL = 10-12L) and 115pL, the total number of molecules is 112 × 1012 ～ 112 × 1014 and 415 × 1010 ～ 415 × 1012. Samples with a content as low as 1 to 0.01 μL / L (for μ2 HPLC) or as low as 20 to 0.2 μL / L (for CE) cannot meet the requirement of 106 molecules, and the above problems will occur during the separation process. Therefore, the above separation system can have a repeated retention time when separating samples with a concentration higher than this index. If you consider the limitations of detection [see (3) and (4) below], packed columns with coarse inner diameters in trace analysis are always better than miniature columns.

In order to be able to carry out trace analysis, micro-separation analysis systems often use sample pre-concentration technology to compensate for the lack of concentration sensitivity. But the technology developed for this is also applicable to conventional separation and analysis systems, which can also improve the sensitivity of conventional instruments, unless the sample volume is strictly limited.

(2) The chromatographic column theory 45 years ago has pointed out that the smaller the inner diameter of the capillary open column, or the smaller the particle size of the packed column, the higher the separation efficiency of the chromatographic column. The same is true for capillary electrophoresis, but there are some differences in theory, such as heat dissipation and plug flow characteristics. The fine-diameter separation column commonly used in miniaturization is not a patent for micro-instruments, and the high column efficiency that can be achieved is not recently recognized. If this kind of equivalent internal diameter chromatography column is used in existing conventional instruments, and the sampling technique and detector are appropriately improved, the column efficiency of the same unit length as that of micro chromatography or chip electrophoresis will be obtained, and at the same time, there will be a very high efficiency. High total separation efficiency, because the length of the separation column in conventional instruments is rarely limited, and high separation efficiency is really meaningful. Therefore, the short separation columns used in microchromatography and chip capillary electrophoresis have the characteristics of rapid separation, which is not its true advantage, because the same size separation column can achieve the same effect on conventional chromatography and capillary electrophoresis, respectively.

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