Mass spectrometry (MS) is an analytical tool used in chemical analysis to identify unknown compounds, quantify known compounds, and investigate the chemical properties of target analytes. A basic mass spectrometer consists of an ion source, a mass analyzer, and a detector, and each part has a variety of different technologies that can be leveraged to tailor the analysis for very specific applications. The type of MS is typically defined by the mass analyzer, and there are a variety of mass analyzers to choose from: time-of-flight, magnetic/electric sector, linear quadrupole, quadrupole ion trap, and ion cyclotron resonance, to name a few; and each mass analyzer has a variety of sub-types to enhance or change the functionality. This blog will focus on triple-quadrupole mass spectrometry.
Quadrupole Mass Analyzers
A quadrupole consists of paired metal rods arranged symmetrically around an axis. An electric field is generated when DC electrical voltage is applied across a pair of rods, and opposing polarity DC voltage is applied to the other paired rods. By adding RF voltage to both sets of rods, oscillations can be induced in ions travelling through the instrument. By moderating the applied RF voltage (and thus the electric field), which ions make it through the system and which are filtered out can be adjusted. Single-quad (common parlance) mass spectrometers use one quadrupole to filter ions prior to detection, which offers basic qualitative and quantitative capabilities.
Sometimes, a single-quad instrument can’t provide the information needed, and so the ions are collided with a gas to induce fragmentation, and then these fragments are measured by another mass analyzer in tandem. This is referred to as tandem MS (or MS/MS), which can provide more details about the analyte through its fragmentation pattern. The precursor ion is selected by the first mass analyzer, fragmented in the collision chamber using a gas, and then the product ions are selected in the second mass analyzer. This process is so specific, that ion concentrations can be measured with high sensitivity and specificity.
Triple-quad mass spectrometry technology was invented by Christie G. Enke and Richard A. Yost in the late 1970’s. For a basic triple-quad instrument, two linear quadrupole analyzers are connected by a third quadrupole, where collision gas is introduced to fragment the ions. Ions from analytes not of interest are discarded in the first quadrupole, the second quadrupole fragments the chosen analyte and directs the fragment ions into the third quadrupole, where they are scanned in the same manner as a single-quad instrument. This produces an MS/MS spectrum that can be used to confirm the identity of analytes. Alternatively, the third quadrupole can be set to monitor only specific ions. This greatly increases the sensitivity and specificity of the analyzer, which is great for quantitative analysis.
What can triple-quad mass spectrometry be used for?
Because the triple-quad mass spectrometry offers good sensitivity and selectivity, the biggest area of applications involves the analysis of trace amounts of chemicals. Pesticides in food, nitrosamine impurities in pharmaceuticals, and dioxin analysis are all applications that demand high sensitivity and specificity. Triple-quads can also be good research instruments, because the pattern produced by scanning the fragment ions in the third quadrupole can provide identifying information for an analyte of interest. The same data can also be used to elucidate properties about the chemical.
Where can I get more information?
For a deeper insight into the power and precision of triple-quad mass spectrometry, refer to our JMS-TQ4000GC product page. Or contact a member of the JEOL USA team today if you have any questions. There is also a good C&EN article that explains the history of the triple-quad. Finally, the original paper about the invention of the triple-quad by Enke and Yost can be found here.