by Robert L. Stevenson
AL
Advances in Separation Science at Pittcon 2016
While spectacular developments in chemical imaging and spectroscopy continue to entice the market, separation science remains a key enabling technology. Increasing demands for higher sample throughput and data quality have led to automated methods for solid-phase extraction, dried blood-spot analysis and analytical separations with LC/MS. This review details a selection of separation science products featured at Pittcon 2016, held March 6–10, in Atlanta, Georgia.
Particle size analysis The CPS Disc centrifuge from CPS Instruments (Prairieville, La.) provides
a robust and convenient method for the high-resolution measurement of multi-modal distributions of particles. This is important in drug devel- opment, especially in formulation, and in the analysis of nanomaterials. Particles in the 20-nm to 40-μm range that differ in size by less than 5%, and often as little as 1.5%, can be separated via sedimentation velocity into bands and measured photometrically. The output looks like a chro- matogram with individual peaks. Monomers are resolved from dimers, trimers, etc.
The centrifuge rotates at ~18,000 rpm. Particles settle in the fluid ac- cording to Stokes’ law. A broad polydisperse mixture of sizes smears out across the detection window, but if the sample is a mixture of particle size families, each with narrow dispersity, they focus into narrow bands. The position of the bands is recorded. Run time is 3–40 min, run-to-run precision is ±0.5% and the chamber must be cleaned between runs.
Gas chromatography Chromatography analyzers from IUT Medical (Berlin, Germany) use photo-
ionization detection (PID) and ion mobility spectroscopy (IMS) detection. Not all applications need separations; IMS alone is often sufficient. Assay of volatile organic compounds (VOCs) is an exception. The preferred ap- proach is GC with PID detection. Since the analytes in a VOC sample have different risk profiles, they need to be treated individually. For example, an infrared detector can be used for the detection of fumigation gases, which are found in containers used for the ocean transport of foods. IUT’s ethylene-oxide analyzer uses GC as an inlet to reject matrix interferences. It is portable and provides real-time results essential to sterilization work.
JEOL (Peabody, Mass.) added a combination electron ionization/pho- toionization source to the AccuTOF-GCx high-resolution TOF/MS. The instrument provides multi-dimensional GC separations for complex samples such as fuels. Photoionization is particularly useful for the
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detection of aromatics, including polynuclear aromatic hydrocarbons in environmental samples.
MS detectors for GC Combining mass spectra with retention time in GC/MS is the most popular way to identify analytes. For unknowns, accurate-mass MS can be supple- mented with multiple-reaction monitoring.
Thermo Fisher Scientific (Waltham, Mass.) introduced the Q Exactive GC Orbitrap GC-MS/MS, which combines GC with high-resolution accurate- mass (HRAM) Orbitrap mass spectrometry.
The Orbitrap provides a mass resolving power of 100,000 at m/z 272, suf- ficient to differentiate between molecular formulas that have the same number of nuclides. In addition, the quadrupole stage transports the ions to a collision cell for fragmentation. The pattern provides further dis- criminating power between potential isomers. Mass range is 50–3000 m/z. Since the GC is the inlet, this seems more than enough range, especially for multiply charged ions.
As with many other multi-stage MS detector schemes, non-ionized components entering the MS are deflected by bends in the ion path. Ion sources include both positive and negative chemical ionization and electron impact. The Q Exactive should appeal to scientists working with volatile natural products, metabolites, unknowns such as leachables, and to those doing initial environmental surveys.
Element- and group-specific detectors Detector Engineering & Technology (Walnut Creek, Calif.) presented sys- tems that use catalytic combustion or thermionic emission to detect and often differentiate between isomers. For example, catalytic combustion ionization detection responds selectively to ignition of methylene groups (–CH2
–). This is a low-cost way to check fuel quality. Thermionic emission
with electrically heated ceramic beads converts nitrogen-, phosphorus- or oxygen-containing compounds to electrical current. These detectors are robust and quantitative.
Detectors from O.I. Analytical (College Station, Texas), a Xylem brand, include an electrolytic conductivity detector (ELCD), photoionization detector (PID), halogen-specific detector (XSD) and pulsed-flame pho- tometric detector (PFPD) (see below). U.S. EPA Methods 502.2 and 8021 specify tandem PID and ELCD for the assay of volatile aromatic and chlo- rinated compounds in drinking and wastewater.
MAY 2016
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