14 August / September 2019
SFC-MS versus LC-MS - advantages and challenges
by Gesa J. Schad, Shimadzu Europa GmbH, 47269 Duisburg, Germany
For decades, chromatographers have been interested in the technique of supercritical fluid chromatography (SFC), primarily due to the rapid separations and complementary chromatographic selectivity, as well as its high ‘green credentials’ compared to LC. Progress in embedding SFC into routine use has been slowed by the lack of reliable SFC instrumentation. However, instrument manufacturers have recently started to invest in the development of equipment that is advanced and reliable enough to meet the demanding expectations of routine analytical laboratories.
SFC instrumentation in terms of ease-of- use and performing method development is similar to liquid chromatography (LC), reducing the barrier for user acceptance. SFC offers several potential advantages over LC regarding complementary selectivity and rapid analysis time as well as reduced solvent consumption with an environmental benefit of using CO2
. For SFC to be accepted as a
valuable alternative to (U)HPLC, hyphenation to mass spectrometry (MS) is essential. Preliminary investigations have reported major gains in sensitivity when using a zero- split interface design, and in some cases a significant decrease of matrix effects with SFC-MS compared to LC-MS [1, 2].
This paper provides an overview of the advantages to be expected when hyphenating SFC with MS, and also considers the challenges that may be encountered in SFC-MS method development.
Overcoming traditional notions
One of the biggest challenges in establishing SFC as a routine analytical technique is overcoming the initial reservations people may have when looking at something they consider to be a niche technique for expert users. However, advances in technology over recent years and an increase in research in this area [3- 5] are helping to remove these preconceptions. For example, Dispas et al published a paper in 2018 on a ‘First inter-laboratory study of a Supercritical Fluid Chromatography method for the determination of pharmaceutical impurities’ [5] that demonstrates the applicability and transferability of an SFC method with good reproducibility, as required in quality control laboratories. It shows that with development of new generation equipment reliability of the instrumentation is no longer an issue.
Similarities and differences
Scientists who are experienced in the use of (U)HPLC equipment will find that in terms of usability and application, SFC is very similar. When the two techniques are compared, many similarities are apparent: both can be used in different modes based on stationary and mobile phase characteristics, organic modifiers and additives are used to adjust selectivity, silica based porous or fused core particles can be employed and separations are run in either gradient or isocratic mode.
Some differences should also be considered, such as the compressibility of the mobile phase. In SFC, unlike LC, small changes in pressure affect fluid density and can have a strong effect on analyte retention. This is why a reliable backpressure regulator to keep the system pressure stable is a crucial part of an SFC system.
Advantages of SFC
There are also definite advantages to consider when looking at SFC for routine use, such as the lower viscosity of the mobile phase, resulting in the possibility to run higher flow rates and therefore deliver higher throughput. Cost per sample can be significantly reduced, in addition to the green credentials of the technique and its advantages in separation of isomers [6].
It also offers complimentary selectivity compared to the standard reversed phase (RP) LC approach. Peaks show very different elution patterns, even when choosing the same column, as can be seen in Figure 1 [7], showing MRM chromatograms of the analysis of 442 pesticides using a C18 column in SFC-MS (a) and LC-MS (b) respectively [7].
Figure 1: Comparison of elution pattern of 442 pesticides in SFC-MS vs. LC-MS (reproduced from [7]).
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