Sophisticated Method for the Masses
Once the domain of do-it-yourself Ph.D. scientists who spent years studying its intricacies, mass spectrometry (MS) is continuing to go “down market,” says Alessandro Baldi, Ph.D., business manager for MS at PerkinElmer (Waltham, MA).
“MS is morphing from a research tool into an analytical instrument as its user population evolves,” he says. “We’re seeing more QC people and fewer scientists. MS has become part of an enterprise’s quality process.” A good deal of this work involves workflows dedicated to a small number of targets rather than obtaining the big picture on a sample. These specific analytes might include chemical precursors, products, impurities and side products, or ratios of two or more of these.
Within this context, time-of-flight (TOF) instruments play a critical role. TOF works by imparting equal energies to all fragments. Since smaller particles thereby travel faster, their masses are determined by how quickly they travel from the source to the detector.
TOF provides exquisite mass accuracy and resolution at very high dynamic range and response linearity within a spectrum. These qualities, Dr. Baldi says, allow analysts to focus on very specific mass ranges and eliminate signals outside that window. “That’s why TOF shines for target characterization.” TOF’s other great benefit for this application is fast spectrum acquisition, up to fifty per second. Vendors will often couple instruments with software packages that supply results at a glance, which makes TOF a method of choice for rapid, routine screening for the presence or absence of particular masses in many samples.
Scientists traditionally viewed highresolution/ accurate mass TOF-MS as a qualitative technique, notes Dominic Gostick, Ph.D., senior director for academic business at AB SCIEX (Framingham, MA). “Other mass spec methods were better known for quantitation.”
Today’s instruments, particularly MS/MS TOF mass spectrometers, are routinely used in proteomics, protein discovery, lipids, metabolomics, and drug and drug metabolite identification. Pharmaceutical companies, in particular, use these instruments to quantify and positively identify known and unknown compounds.
Another relevant TOF workflow is the identification of unknown compounds in drug metabolism assays, drugs-ofabuse testing, foods and beverages, or environmental samples. Because of its highly discriminatory mass detection, TOF can distinguish, based on isotope ratios, whether a pharmaceutical was manufactured at an approved facility or by counterfeiters.
Dr. Baldi describes a project he worked on years ago, analyzing the residue from a perfume vial found on a sunken Roman ship. “The analysis of the terpenes showed that the 14C ratio was completely off. It was amazing.”
Accurate determination of isotope distributions also enables TOF instruments to distinguish between two compounds of equal molecular weight, providing unequivocal identification.
Ionization of MS samples is critical, since the instruments detect only ions. Numerous ionization techniques exist for solid and liquid samples prepared manually or obtained from the back end of a chromatograph. Most require exposing the sample to high temperature, radiation, or a stream of high-velocity gas. Direct analysis in real time (DART) directly ionizes samples, with no preparation, at atmospheric pressure and under gentle conditions. DART is suitable for the direct analysis of pharmaceuticals, chemicals, and even samples of dried blood, with no sample prep.
Diab Elmashni, senior marketing manager for LC and LC/ MS at Thermo Fisher Scientific (San Jose, CA), also notes that the “big trend” in MS these days is more routine analysis.
Instruments have become easier to use in response. A great deal of usability resulted in instrument design, innovations in ionization and detection, and the general miniaturization of instrumentation components. Specialists accustomed to room-size spectrometers of twenty years ago would marvel at how MS is now employed as a detector for liquid and gas chromatography. A related capability is the rapid expansion of MS targets from smallish organic molecules to macromolecules.
The greatest strides in usability, says Mr. Elmashni, have been in software design. Software opens up a mass spectrometer’s full range of features to expert users while simplifying protocols for routine users through various levels of permissions. These allow the lab director or high-end operator to tweak the instrument as needed, or to write methods, while simplifying the interface for technicians through a nonalterable interface.
Similarly, software templates have simplified the process of running samples and analyzing data by taking over routine tasks such as data entry and report generation. “A lot of customers want simply to walk up to an instrument, introduce the samples, and come back in fifteen minutes to read the report,” Mr. Elmashni tells Lab Manager.