Although mass spectrometry (MS) dates back to 1897, and high-performance liquid chromatography (LC) to the 1970s, scientists are still developing methods to enhance their capabilities. Rapid advancements in the pharmaceutical and biotechnology fields are the main drivers behind new developments. Many efforts are stemming from academic labs, which are opening new avenues for these technologies.
LC/MS is now a multibillion dollar business with tandem LC/MS comprising the majority sector. In this article, a handful of researchers provide a preview of some of the topics that will be covered at “Pittcon 2009” next month.
A researcher in the University of Michigan’s chemistry department is developing analytical methods to study changes in metabolites within pancreatic islets of Langerhans cells. Charles Evans, Ph.D., says that this involves developing new liquid-chromatography methods, miniaturizing LC separations using capillary columns and multidimensional separation.
“By developing higher-resolution separation methods, we can more effectively take a complicated sample and break it up into all its components,” he says.
Islet cells contain beta cells, which are responsible for secreting insulin and maintaining constant blood glucose levels. “We are studying the metabolites present in these beta cells, which may be involved in pathways associated with diabetes development when they fail and are unable to secrete adequate insulin quantities.”
The multidimensional, 2-D separation involves two different liquid chromatography columns. A standard separation is performed first on one column and the fractions collected. These fractions are further analyzed in the second column; the columns must be different.
“Select two different columns that have as little correlation as possible between the separation on the first dimension as on the second dimension,” Dr. Evans suggests. For example, perform reverse-phase separation as one dimension and ion-exchange separation as the second dimension.
He says that the metabolites they are primarily interested in detecting are those in central carbon metabolism, glycolysis, and the TCA cycle. Additional energy transfer metabolites like NAD, NADH, ATP, and ADP can also be measured.
“One change we detect is that there are more central carbon metabolites present in the cells as a response to stimulation with glucose. We also detect changes in the energy metabolites. We’re still sorting out what all these changes mean,” states Dr. Evans.
The overall goal of this research is to develop methods to help improve the understanding of the biochemical mechanisms that underlie Type 2 diabetes. “We’re hoping that being able to measure these compounds will detect changes associated with diabetes along the course of the disease.” Dr. Evans adds that this method could have potential use for many other components—whether in disease research or understanding biochemical mechanisms that change based on environmental factors.
Ionic liquids have an advantage over organic solvents like methanol in that they don’t denature proteins as much and enable protein separation in their native confirmation. This is the driving force behind the research of Neil Danielson, Ph.D., professor of chemistry at Miami University, Ohio.
“We have been separating small molecules like caffeine and aromatic carboxylic acids by reversed-phase liquid chromatography (RPLC) using ionic liquid modifiers like ethylammonium formate (EAF) and methylammonium formate (MAF) as replacements for methanol in the mobile phase.”
Furthermore, Dr. Danielson has discovered that MAF has an advantage over EAF in that it has a lower viscosity, which makes it more compatible in terms of pressure limitations. “Organic solvents have a very low viscosity, below one centipoise (cP). MAF has a viscosity around 9 or 10 cP, which is really good for ionic liquids. This gives us better efficiency in LC if we work with a less viscous mobile phase.”
Polarity of the mobile and stationary phases, as well as that of a sample, are all involved in controlling retention of a sample compound and the separation of a mixture of sample compounds.
“We can control the polarity of the mobile phase—the amount of MAF in water. This ratio will vary the polarity, which is effective in controlling the retention of many pharmaceuticals. If we increase MAF, this lowers the retention.”
In addition, his group discovered that using low levels of MAF (1%) to replace methanol, for certain compounds like warfarin, the methylammonium ion formed an adduct and enabled easier qualitative identification by RPLC with MS detection.
When using MAF at higher concentrations (5–20%), Dr. Danielson says his group is able to control retention of water-soluble vitamins and antibacterials. “This is quite novel in that we are the first to show this ionic liquid as an organic solvent replacement in LC/MS is compatible because it’s forming volatile components in the interface.”
He adds that a compelling use of MAF is as a major mobile-phase component of LC/MS that has been shown to provide an advantage for separation of proteins as well as antibacterials. However, he says, “we’re still searching for a clear advantage for the use of MAF for small molecules.”