Although supercritical fluid chromatography (SFC) is not a new technique, preparative SFC is becoming increasingly more popular with advances in instrumentation, software and chemistry. Basic principles for isolating compounds with SFC are similar to the fundamental rules for large-scale preparative liquid chromatography. In this study, we illustrate the effect of flow rate, sample dissolution solvent, and column selectivity on the isolation of compounds from a mixture. Columns packed with Optimum Bed Density (OBD) Technology1 and with matching chemistry ensure easy and predictable scalability from the small scale to larger preparative separations.
1. Sample Preparation
- Compound Mixture 1: Coumarin, Ibuprofen, and Carbamazepine; 20 mg/mL each in methanol
- Compound Mixture 2: Ibuprofen and Carbamazepine; 20 mg/mL each in methanol
- Compound Mixture 3: Ibuprofen and Carbamazepine; 20 mg/mL each in DMSO (Dimethyl sulfoxide)
- Compound Mixture 4: 10 mg/mL Fenoprofen, 15 mg/mL Ketoprofen, 20 mg/mL Flavone, 20 mg/mL Ibuprofen, and 20 mg/mL Coumarin in methanol
2. Experimental Conditions
- Analytical chromatography was performed on a Waters Method Station SFC System.
Column: Viridis SFC 2-Ethylpyridine column, 5 μm, 4.6 x 150 mm, Part Number 186004937.
- Preparative chromatography was performed on a Waters Prep 100 SFC UV Directed System. The
- Viridis SFC 2-Ethylpyridine OBD column, 5 μm, 19 x 150 mm, Part Number 186004945
- Viridis SFC Silica OBD column, 5 μm, 19 x 150 mm, Part Number 186004918
- Viridis SFC 2-Ethylpyridine OBD column, 5 μm, 19 x 100 mm, Part Number 186004944
- Other Method Parameters:
- Column Temperature: 40 °C
- Cosolvent: Methanol
- Injection volume, Flow rate, and Gradients: Reported in figures
3. Representative Results
The flow rate used in preparative LC separations is limited by a number of factors including the length and diameter of the column, the particle size, and the system backpressure. Low viscosity carbon dioxide, the main component of the mobile phase in SFC, enables the use of higher flow rates than those generally used in liquid chromatography. The backpressure associated with column length is also lower, enabling usage of longer columns to increase resolution. Higher flow rates increase throughput by reducing the amount of time to complete a separation. As shown in Figure 1, the chromatographic profile is the same at different flow rates provided that the slope of the gradient expressed in percent change per column volume remains constant.. This is accomplished by decreasing the run time in the same proportion as the flow rate is increased.
Sample Dissolution and Loading Solvent
Isolating compounds from mixtures requires that the sample be completely soluble before injection. Methanol, the most common co-solvent in SFC, does not solubilize all samples at the high concentrations usually required for typical preparative separations. Large volumes of solvents like methanol can limit the mass capacity of the column due to strong solvent effects that can distort the chromatography. Modifier stream injection, the standard patented configuration of all Waters prep SFC systems, was designed to reduce this solvent effect. Dimethyl sulfoxide (DMSO) is used regularly in purification labs to solubilize many different types of compounds. Figures 2 and 3 illustrate how DMSO injections in SFC can result in higher loading and better resolution for achiral applications as compared to samples injected in methanol.
Note: For chiral applications, caution should be considered as DMSO can severely damage a traditional coated chiral stationary phase. If DMSO should be required for sample solubility, an immobilized chiral column should be selected.
In SFC, multiple columns are routinely tested to determine which provides the best resolution and peak shape for components in a mixture. Good resolution between compounds leads to isolations of target compounds with high purity. In the example below, the 2-ethylpyridine column shows excellent separation of all five compounds in the sample mixture. The same sample mixture analyzed on a silica column shows only three component peaks because it has a different selectivity. Coumarin and ibuprofen coelute at about 0.8 minutes and flavone and fenoprofen coelute slightly later. Flavone and ibuprofen also change elution order on the silica column. Ketoprofen is well-separated from all of the other peaks on both columns.
Crude sample mixtures are usually analyzed with a screening gradient that starts from a low percentage of organic solvent and ends at a higher percentage of organic solvent in a relatively short time. If the compounds of interest are well resolved, the gradient may be scaled directly for purification. When all compounds elute before the completion of the gradient and are fully resolved, a simple reduction in the length of the gradient is acceptable. The fast flow rates used in SFC combined with modified gradients considerably shorten the total time required for purification of target compounds.
Scaling separations requires matching column chemistry and particle size as well as appropriately scaled injection volumes and gradients. Waters preparative SFC columns are packed with OBD Technology, ensuring excellent bed stability and comparable performance to Waters analytical SFC columns. OBD preparative columns are packed to bed densities which closely match the equivalent analytical column. This innovative procedure produces preparative columns with excellent stability, reproducibility, and efficiency.
A loading study is performed at the small scale to determine how much sample mass can be loaded on the column. The goal is to maximize the load without compromising the resolution. Improved peak resolution leads to higher loads and better purity for isolated compounds. Higher column loading reduces the number of runs required for obtaining enough material for subsequent experiments. The chromatography from the loading study is depicted in Figure 5. The conservative 35 μL injection volume shows good resolution between all of the compounds in the mixture and is subsequently scaled for the preparative run on the large column. A total volume of 600 μL was injected on the 19 x 150 mm preparative SFC column. Figure 6 compares the chromatography using the small scale modified gradient at maximum load with the large-scale chromatography used for the isolation. The selectivity is identical, a critical factor when using analytical chromatography as a scouting tool for isolation and purification.The differences in retention time and resolution between the analytical and preparative chromatograms can be attributed to factors including system volume and injection mode. In particular, the patented modifier stream injection mode* is employed for all Waters preparative SFC systems, minimizing the solvent effect and therefore improving throughput and efficiency.
*US Patent # 6,576,125