Higher Efficiency in Sample Preparation
Coupling of Sample Preparation Techniques with Ultra-High Performance Liquid Chromatography and SuperFluid Chromatographycritic
Due to a consistent need in rise of efficiency and sensitivity in analytical processes, the online coupling of sample preparation with liquid chromatography has been gaining increasing attention. The scientists from South China University of Technology, Guangzhou, published a review about recent advances of sample preparation coupling techniques with two of the recently most preferred fluid chromatography techniques.
Of the four essential steps in an analytical procedure: sampling, sample preparation, determination of the target compound and data handling, sample preparation is still considered as the bottleneck of the entire process. Thus, innovations to reduce preparation time as well as analytical errors are required.
The selected analytical techniques
Ultra-High Performance Liquid Chromatography (UHPLC) has, in comparison to a common Liquid Chromatography (LC), columns with smaller particles and works with ultra high pressure. That enables a higher efficiency, sensitivity and analysis speed. Alas, due to the complexion of the apparatus, the requirements for the sample parameters are much stricter than in the common LC, which makes the UHPLC harder to couple.
Supercritical Fluid Chromatography (SFC) is an alternative to the LC and an extension of the Gas Chromatography. A supercritical fluid, with low viscosity and high diffusity, like supercritical carbon dioxide, is used as mobile phase. With that, elevated separation efficiency, higher mobile phase velocity, less organic solvent consumption and shorter separation time can be achieved.
By automatizing online coupling in both high efficiency and high speed techniques, several advantages spoken above are to be enhanced. The authors present numerous sample preparation coupling techniques which have a rising relevance in analytical research.
Coupling with UHPLC
The most common and significant online couplings of Ultra-high performance liquid chromatography (UHPLC) are with solid phase extraction (SPE) and in-tube solid phase microextraction (IT-SPME). Which, when coupled to UHPLC and applied hyphenated with mass spectroscopy, allow a way simplified analysis of water-, biological-, and solid samples.
Analytical methods to screen water samples for estrogens, endosulfan isomers and pesticides are already established.
Methods to identify and quantify drugs were developed for biological samples. Blood, plasma, serum, urine, milk, biogas and cells can be analysed within minutes, sparing the usual pre-extraction preparation. Solid samples that can be screened include soils, mainstream cigarette smoke, indoor dust, cooked foods or dried fruit.
IT-SPME-UHPLC is applied in aqueous samples. For instance, to detect and quantify pollutants in water samples. In this approach smaller amounts methanol were required to elute analytes, which helps developing a green analytical chemistry.
It is to consider, that especially in the online coupling of UHPLC with SPE there are certain obstacles to overcome. The first bottleneck of the high pressure in UHPLC, which would cause damage on solid phase extraction materials, is solved by a system of valve switching. The other bottleneck is the high elution strength of SPE. This would lead to a volume overload in the UHPLC column, but can be prevented by dilution.
Coupling with SFC
The Practice of online coupling techniques with supercritical fluid chromatography isn’t as comprehensive as of those with UHPLC. The two sample preparation methods, however being used in coupling are the Supercritical Fluid Extraction (SFE) and Solid Phase Extraction (SPE). SFE and SFC are very suitable for coupling, since both methods use supercritical fluids in both extraction solvent and mobile phase. Another benefit is that after the extraction step, no preconcentration is needed before injecting the analyte into chromatographic column.
The applications for this online coupling method include the detection of phospholipids from dried plasma or of biomarkers in dried serum. Furthermore, methods for the analysis of more complex samples and their components like ketamine metabolites in dried wine or polyacrylic aromatic compounds in soil are established. The system shows high efficiency and high sensitivity, with processing times of around 15 to 20 minutes.
The other relevant method, namely coupling SFE with SPE is in its configuration similar to SPE-UHPLC. Here a preconcentration in three steps is required before the analysis step. The separation of sample solution in the chromatographic column, as well as the drying step, enable the processing of aqueous samples. This has not been possible before, since the fusion of water and supercritical carbon dioxide is problematic.
The review mentions several methods using which, water samples can be analyzed for contaminants like pesticides, phenolic compounds, polyacrylic aromatic hydrocarbons and more. All of those techniques have been developed over ten years ago and still, unfortunately there has been no further development in that field.
New trends and perspective
The scientists suggest that one of the significant flaws in the online coupling is a poor analytical throughput. Therefore enhancement in this area is required. Two possibilities to achieve this goal are presented. Firstly, reduction of analysis time, either by increasing the efficiency of sample preparation or by decreasing the time in the chromatographic column and apparatus. Secondly, parallelising sample preparation and analysis by using two SPE columns and hence sparing preparation and re-equilibration time.
Another objects involved in discussion are more efficient extraction materials. As the common ones cannot satisfy the requirements in sensitivity and selectivity, newly reported and applied extraction materials have been presented. Nanofiber polymers, multi-walled carbon nanotubes and molecularly imprinted polymers are named as materials attracting attention, due to their higher applicability for analytical purposes.
It is hoped and expected, that with rising reception and further development, these promising techniques will earn a simpler handling and a broader practise in the following years.
Yanshan Liang, Ting Zhou: Recent advances of online coupling of sample preparation techniques with ultra‐high performance liquid chromatography and supercritical fluid chromatography. Journal of separation science (2018); DOI: [10.1002/jssc.201800721]