Back End to GC, LC

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MS originated as a stand-alone technique for volatile compounds. Next came the ability to volatilize high molecular weight materials through heating. The emergence of electron-impact ionization MS was a natural, as GC analysis requires volatilization. Furthermore, derivatization enabled GC analysis and hence MS as well, even on high molecular weight molecules containing several polar functional groups.

Early spectrometer detectors for GC were large magnetic sector instruments. They worked well for the time, but they were large, difficult to maintain, and their spectrum acquisition was slow, particularly in high-resolution mode. “The marriage of GC and magnetic sector instruments was never very comfortable,” comments Ian Jardine, chief technology officer at Thermo Fisher Scientific (San Jose, CA).

To the rescue came quadrupole MS, which was more compact and fasterscanning than magnetic sector spectrometers. Quads took off because they were sensitive, could keep pace with chromatography peaks, and gave quality spectra. Various improvements occurred during the 1980s, when Hewlett-Packard (now Agilent) could advertise a compact GC-quadrupole MS system for around $50,000. “HP revolutionized the GC-MS business because the price and performance were right for applications of the day,” Jardine says.

MS Detector / Flexar SQ 300 / PerkinElmer / www.perkinelmer.com By the 1990s, liquid chromatography was coming into widespread use. LC’s strength is its ability to separate nonvolatile compounds without derivatization. A challenge in coupling LC with MS was that LC’s complex buffer components deposited within the MS instrument. With the discovery that volatile mobile phases (e.g., ammonium acetate) did not foul MS instrumentation, the world appeared finally to be ready for LC-MS, but the lack of a suitable ionization method thwarted these efforts.

Momentous was the advent of electrospray ionization (ESI), for which John Fenn received the chemistry Nobel Prize in 2002. In ESI, the pressure drop from atmospheric to vacuum, with the addition of some heat, causes solvents to evaporate, leaving behind only the charged analyte.

The main drawback of ESI single quad systems is they do not induce fragmentation; hence, they are unsuited to structural analysis. Electron impact, by contrast, may or may not show a parent ion, but always provides fragmentation. The solution was to use multiple MS dimensions: The first quadrupole for parent ion selection, the second to fragment the analyte through interaction with a collision gas, and a third for detection. Hence, the term “triple quadrupole” or triple quad.

“LC-triple quad was a huge development because it provided separation by LC, high sensitivity ionization, molecular weight information, and structural information through fragmentation,” says Jardine, who estimates that half of all LS-MS setups in the life sciences are triple quad systems. The technique significantly improves signal to noise and limit of quantitation. “It came along just in time for pharmaceutical companies working with potent, low-dose compounds.”

The next major advance, in the 1990s, was quadrupole ion traps, which found greater application in LC-MS than GCMS. Developed by Thermo, ion traps sequester ions for a time and permit both molecular weight determinations and fragmentation in a single device. With their small size, sensitivity, and fast scanning, ion traps became the instruments of choice for peptide sequencing.

Benchtop LC-MS System / Exactive Plus / Thermo Fisher Scientific / www.thermoscientific.com High-resolution accurate-mass techniques were a subsequent game changer. Most elements possess a positive or negative molecular weight deficit relative to their nominal atomic masses. Thus, oxygen’s nominal atomic weight is 16, but its actual mass, based on carbon’s molecular weight of 12.0000, is 15.9994. MS instruments capable of mass resolution to about 5 ppm—four or five decimal places—can therefore provide not just accurate parent and fragment masses, but elemental composition. “When people talk about high resolution, they’re actually referring to accurate mass MS. The high resolution part is how you get to the accurate mass part,” Jardine explains. Since the early 2000s, accurate-mass MS has been available for fragments as well as parent ions. Quadrupole time-of-flight (QTOF) MS exemplifies this capability.

Further, more recent developments were the introduction, by Thermo in 2003, of Fouriertransform MS (FTMS), which surpassed any competing technology in resolution and mass accuracy. Two years later, Thermo replaced FTMS and its huge superconducting magnet with Orbitrap technology that, in addition to higher resolution and mass accuracy, was quantitative.

While GC-MS instrumentation is pretty much settled, at least for now, Jardine sees the evolution of LC-MS continuing. “Techniques like QTOF and Orbitrap will become smaller and more affordable. Just as single quad became a no-brainer for GC-MS, LC-MS/MS will become the standard for what people expect in an LC detector, that is, a black box.” The adoption of LC-MS in clinical diagnostics will also drive this trend, Jardine says.

Categories: INSIGHTS

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Published: April 1, 2013

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