The primary drawback of GC/MS is that it is only suitable for volatile compounds.  Non-volatile compounds can be made volatile via derivatization. Derivatization can be time consuming, yield various “derivatization efficiencies”, and is not suitable for several complex large biopolymers such as proteins or large oligonucleotides. Traditionally, polar compounds were analyzed by LC techniques using polar solvents.

In the quest for making mass spectrometry suitable to polar, non-volatile compounds, a variety of techniques became popular in the 1980’s. These techniques included Plasma Desorption Time-of-Flight (Cf252-TOF), Matrix Assisted Laser Desorption Time-of-Flight (MALDI-TOF), and Fast Atom Bombardment (FAB). Of these techniques, only FAB was slightly amenable to LC. FAB primarily generated singly charged protonated (or deprotonated) species. Such species were ripe for fragmenting and most triple quadrupoles sold in 1990 had a FAB option.

Atmospheric Ionization (API) techniques, such as Electropray (ESI) and Atmospheric Pressure Chemical Ionization (APCI), were discovered in the late 1980’s. Like FAB, both of these techniques primarily yield singly charged ions. Although this technique has the advantage of simplicity, a fingerprint for qualitative identification was not apparent. API sources coupled with single quadrupoles can provide spectra by colliding the parent molecules via Collisional Induced Dissociation (CID) with solvent and/or gas molecules in the source. This technique is known as “source CID”.  Although this produces a fingerprint, the fingerprint can be hidden by the spectrum produced by everything else ionized in the source.  MS/MS provides a means of isolating the parent ion from all coeluters (except isobars) prior to fragmentation. Because of this capability, triple quadrupoles created a niche with the advent of API techniques, and source CID later became known as “poor man’s MS/MS”.

Aside from providing fingerprint spectra free of coeluting interferences, the specificity with parent-to-product transitions was harnessed for quantitative applications. The technique, whereby the first quadrupole isolated a parent (precursor) ion, followed by CID with a collision gas in the second quadrupole, followed by analysis of a single ion in the third quadruple is known as “Selection Reaction Monitoring” (SRM).  Because two or three specific fragment or “product” ions were monitored, the technique is also known as Multiple Reaction Monitoring (MRM). SRM and MRM are essentially synonymous today.

Over the last 10 years, triple quadrupole production has increased tremendously mainly due to the speed, selectivity, specificity, and sensitivity of SRM for quantitative bioanalyses applications.  Ion traps and quadrupole-TOF instruments have applied their inherent advantages of full scan MS/MS primarily, but not always, for qualitative applications. 

Historically, the gains in triple quadrupole mass spectrometry have advanced in two main areas: sensitivity and robustness. Pharmaceutical companies require millions of bioanalyses assays to be run for both preclinical and clinical assays. The speed, selectivity, specificity and sensitivity of the triple quadrupole using API techniques have led to an explosion of triple quadrupole sales over the last decade. Like the general trend in the electronics industry, triple quadrupole instruments today are more robust, more sensitive, smaller in size, easier-to-use, and less expensive than 10 years ago.

Aside from these gains, perhaps the most interesting change with triple quadrupoles used for quantitative bioanalyses has come with the introduction of the Thermo Scientific TSQ Quantum series of mass spectrometers.  Aside from being robust, sensitive, small and relatively inexpensive, the TSQ Quantum is the first commercial triple quadrupole providing higher resolution precursor ion selection. Prior to the TSQ Quantum, triple quadrupoles have been able to select precursor ions with unit resolution only. With its hyperbolic quadrupoles, now it is possible to select a precursor ion with a tolerance of +/- 0.1 amu or less. Isobaric coeluting interferences which may cause false positives at unit resolution will not interfere with a bioanalysis, provided its mass is > 0.1 amu different from the target molecule. With the TSQ Quantum, such potential interferences are “selected out” by the first quadrupole. This is the first innovation in several years in triple quadrupoles addressing the “specificity” of the SRM transition, and not merely addressing robustness and sensitivity.  

This new technique of Highly Selected Reaction Monitoring (H-SRM) can greatly reduce chemical noise which will, in turn, yield lower levels of quantitation via higher signal-to-noise ratios. Most importantly, H-SRM has the capability to completely “save” an analysis, assay, or study from complications or errors arising from coeluting isobaric interferences. H-SRM provides both an “insurance policy” against interferences and lowers quantitation levels. It is the state of the art technique in bioanalytical quantitative analyses via triple quadrupoles today.