Saba Jazeeli, Senior Product Manager at IDEX Health & Science, discussed a new method by which vacuum degassing can be better controlled and the impact this could have on high-performance liquid chromatography (HPLC) efficiency and optimizing processes in analytical labs.
1. There are a variety of features that impact the analytical performance of HPLC systems. Can you outline the challenges of dissolved air in the mobile phase?
The impact of outgassing on an HPLC method depends on the type of mixing system: low pressure, which mixes solvents before the pump using a proportioning valve, or high pressure, which mixes solvents after the pump. Low pressure mixing systems are more susceptible to bubble formation, as the mixed mobile phase is supersaturated before it reaches the pump and outgassing is initiated in the low-pressure region between the proportioning valve and the pump inlet. Nucleation sites and inlet check valves can further cause bubble formation due to nucleation and turbulence.
Although high pressure mixing systems use a separate pump for each solvent, mixing occurs ahead of the injection valve and there is potential for cavitation and malfunction of the inlet check valve of each individual pump, especially in the case of gravity-operated check valves. Additionally, mixing takes place at high pressure, so any dissolved gas will remain in the solution until the pressure drops, which occurs after the mobile phase leaves the column. Outgassing before the mobile phase enters the detector or the detector flow cell can cause errors in detection due to increased baseline noise and even form false peaks.
Reducing the dissolved air from the mobile phase is therefore crucial to the stability of both low and high-pressure HPLC system flow rates, as well as to mobile phase composition and the accurate identification of separated compounds.
2. Tubular-based degassers have been in use for some time. Are there any alternative devices to tubular-based degassers that produce the same, if not better results?
Since most HPLC systems have integrated degassers, their longstanding functionality means models have remained largely unchanged over the decades.
All current HPLC degassers operate at a single fixed setpoint for the applied vacuum of all methods and flow rates. This value has typically been set to 50 mmHg absolute pressure, but other pressure/ vacuum levels are also used. Vacuum degassing systems that are set to a single high vacuum setpoint (e.g. 50 mmHg) are designed specifically to meet the degassing requirements of the upper flow rate range of the individual HPLC system. Additionally, current systems make use of as many as six vacuum degassing channels, so irrespective of the system flow rate, there will be variable efficiencies and unnecessary demand on the vacuum pump. The ability to vary the vacuum applied to the chambers by the vacuum pump not only relieves this demand but will ensure the constant performance of HPLC separations.
This level of control is possible using the new control methodology along with the film degasser to provide constant performance across almost the entire flow rate range of the HPLC system. Liquid flows across a Teflon™ AF film in a thin layer, then dissolved gas migrates through the degassing film. Simplifying the complex and variable design elements of tubular-based vacuum degassers, such as membrane wall thickness, tubing ID (fluid diffusion path), and length of the tube, will improve efficiency and significantly reduce flow restriction.
3. Can you provide some insight into the reasons for developing this latest degasser?
Today, most degassers use tubular Teflon™ AF or polytetrafluoroethylene (PTFE) membranes. While in-line degassing prevents most issues surrounding solvent outgassing, as currently practiced, tubular degassers operating at a single, fixed vacuum setpoint do not address all the challenges associated with the removal of excess dissolved air from the mobile phase.
A key reason for designing this degasser was to replace the current tubular design and decrease membrane wall thickness. By improving the vacuum degasser flow channel design, users can achieve higher efficiency degassing, in a more streamlined device. In HPLC separations, the reduction of dissolved air from the mobile phase is of critical importance to the baseline and pump flow rate stability, therefore minimizing the movement of solvent across the membrane (pervaporation) to reduce concentration changes in mixed mobile phase. The reduction of dissolved air is also important to the chromatographic separation stability and proper identification of compounds separated by the HPLC method. The key drivers for our latest product were to firstly reduce compositional changes to the mobile phase and secondly maintain sufficient degassing. We have redesigned the degasser to improve efficiency and keep pace with liquid chromatography (LC) and HPLC technology innovation. The reduction of pervaporation from pre-mixed mobile phases also leads to a more consistent chromatography.
Another consideration that led to the development of the degasser was that current users are restricted by the intricacies of tubular-based degasser design. Currently, numerous iterative design elements such as wall thickness, length and ID of tubing, all with the correct combination of parameters, are needed for optimal performance for each instrument type and design. Additionally, productivity and efficiency are affected as current HPLC instruments are optimized for a wide range of flow rates. At low flow rates, optimization and adjustments are likely to be needed for each HPLC system and method. The universality and flexibility afforded by flat film degassers, as a single solution, eliminates concerns over multiple vacuum chambers for various HPLC applications and improves consistency in performance.
4. What are the advantages of flat film technology for scientists in the laboratory?
Reducing the diffusion path to improve degassing efficiency is challenging in tubular-based membranes. There are two diffusion paths involved in in-line degassing (ignoring vacuum side diffusion). First, dissolved gasses in the mobile phase must diffuse to the surface of the membrane or film. Secondly, the dissolved gasses must diffuse through the membrane (flat film or tubing wall). Wall thickness cannot be too thin, or the degassing coil will kink during manufacturing. The diffusion path in the fluid cannot be so small that flow restriction within the degasser becomes problematic if it is reduced by minimizing the ID of the tubing, flow restriction increases dramatically. A key advantage of new film membrane degasser is the ability to create a shorter diffusion path in both the fluid and the membrane (flat film). It can be made much thinner than the wall of a tube which must be coiled, making the film proportionally more efficient.
With an intelligent vacuum control protocol, the degasser operates at the highest possible pressures, minimizing the solvent loss to the laboratory atmosphere.
Also, the universality, with increased solvent compatibility, and flexibility afforded by flat film degassers, as a single solution, eliminate concerns over multiple iterative vacuum chambers for various HPLC applications and improve consistency in performance. Furthermore, in terms of practical use, the simple design of flat film degassers, with no fittings or connections, minimizes the likelihood of system failure and improves the integrity of the device.
For more information on the Flat Film Membrane Degasser, please visit here.
About the Author
Saba Jazeeli is a senior product manager working for IDEX Health & Science in Rohnert Park, CA, specializing in the analytical instrumentation markets. Saba has over twelve years of experience developing and commercializing the next generation platforms spanning across multiple industries; most recently for the ever-evolving HPLC market. Saba has her Bachelors in Engineering from Bangalore University, India and an MBA with special emphasis on Technology Management, from the New Jersey Institute of Technology.