The ever-changing needs of pharmaceutical laboratories are pushing analytical chemists to create better, more efficient characterization methods for their target compounds. Key to this process is the use of separation tools that provide sufficient resolution, speed, sensitivity, and robustness in these rigorous environments.
The most common separation tool in pharmaceutical analytical laboratories is liquid chromatography (LC). LC offers high resolution, flexibility, and a diverse group of chemistries that deliver the ability to separate a broad range of compounds. When coupled to mass spectrometry (MS), LC can provide valuable insight into a compound’s metabolism, modification, and exact composition.
LC-MS cannot, however, cover all compound types, and does not provide good differentiation of highly polar small molecules. Furthermore, when coupled to MS, nanoliter flow LC systems lack robustness.
Capillary electrophoresis (CE) represents an alternative tool that provides high-speed, high-resolution separations that excel at resolving polar-charged analytes. The ability to couple an inherently low-flow technique with MS represents a perfect complement to current LC technology, opening analytical avenues that previously were not easily accessible.
In the past, coupling CE with MS has proven difficult; with the advent of the CESI 8000 High Performance Separation-ESI Module with OptiMS Technology* (Beckman Coulter), the barriers to using this technique have been removed. This was accomplished by focusing on the interface between CE and MS.
CE should have a perfect marriage with MS if its inherent advantages of low flow and high resolution are maintained. For this reason, we selected an elegant and simple sheathless interface design. To execute this, a hydrofluoric acid-etched tip was utilized to create a porous region at the capillary outlet.
Figure 1 highlights each of the critical components of this porous sprayer: polyimide-coated silica separation capillary, porous segment, stainless-steel needle, static conductive liquid, and background electrolyte (BGE). Several features of this design make it ideal for this interface.
The capillary and sprayer are one complete piece, meaning there are no junctions or dead volumes. The capillary is not tapered as is common in nanospray sources, thus providing a robust system that does not clog.
This design neatly answers the CE-MS challenges posed by earlier designs.
- Sensitivity—provided by ultra-low flow, even in the mass-sensitive range, with no make-up liquid needed.
- Robustness—an open-tube separation column that maintains a constant inner diameter eliminates clogging common to conventional nanospray interfaces.
- Resolution—CE peak/spike shape is maintained by direct bulk-flow into MS without the dilution associated with sheath-flow designs.
- Electrochemical artifacts—the low ESI voltages needed to reduce oxidation/fragmentation en route to the MS inlet.
To test the capabilities of this system for the analysis of interesting pharmaceuticals, a forensic case was used as a benchmark. It is well established that there is an ongoing requirement in forensic casework to detect and quantify low levels of potent drugs and their metabolites. This need arises especially when there has been an extended period of time between administration of the drug and collection of the sample.