Jeffrey Perkel has been a scientific writer and editor since 2000. He holds a PhD in Cell and Molecular Biology from the University of Pennsylvania, and did postdoctoral work at the University of Pennsylvania and at Harvard Medical School.
When most life scientists think of mass spectrometry, their minds likely wander into the ‘omics arena. After all, proteomics and metabolomics—not to mention glycomics and lipidomics—are largely driven by increasingly powerful and sophisticated mass spectrometers. Sporting high resolution and high mass accuracy, these finely tuned research instruments enable researchers to pin down subtle molecular differences between, say, normal and diseased tissues, and thus identify potential biomarkers.
Yet mass spectrometry is not reserved for research laboratories. Clinical labs use the technique as well, to quantify medically informative molecular species. In that sense, mass spec functions as a highly multiplexed alternative to immunoassays, only without the antibodies and associated risk of cross-reactivity, says Jim Langridge, director of health sciences at Waters.
But the questions these labs ask also are fundamentally different from basic-research mass spectrometry, and so, therefore, is their hardware.
Clinical needs
“In a research lab, you’re asking research questions, and the work you do is different from day to day,” says Tamara Smith, senior manager of global clinical markets at mass spec vendor SCIEX (formerly AB SCIEX). “In a clinical lab, it’s more of a factory-type setting, where day in and day out you are processing samples to get the same answers.”
Research labs often use mass spectrometers to drive molecular fishing expeditions, but clinical mass specs typically are used to quantify known, actionable compounds from biological fluids—usually one or more small molecules whose abundance yields either a disease diagnosis or informs treatment decisions, Langridge says.
Waters, for instance, has developed and sells the FDA-cleared MassTrak™ Immunosuppressants Kit for quantifying the immunosuppressant tacrolimus in the blood of kidney- and liver-transplant recipients—information that can help physicians direct subsequent patient dosing.
Similarly, SCIEX offers European customers a kit that measures amino acid and acyl-carnitine levels in newborns for the early detection of metabolic disorders, such as phenylketonuria. Physicians must follow-up positive results with more detailed analyses, but, says Smith, “In most cases, early detection significantly improves quality and length of life
Another difference between clinical and research labs is that clinical labs are highly regulated environments, Langridge notes, with very specific and validated workflows, documentation and quality-control requirements. As a result, clinical labs tend to be less interested in cutting-edge performance and more focused on reproducibility, robustness and reliability. “The customer profile changes from one of scientific discovery to generating results you can stand behind,” Langridge says.
Instrumentation options
Research laboratories interested in biomarker discovery typically focus on high-performance liquid-chromatography-coupled instruments capable of exceptional sensitivity, resolution and mass accuracy—configurations such as quadrupole-time-of-flight (qTOF) or Orbitrap mass spectrometers. These instruments all may be used to scan for specific metabolites, but their strength is in identifying and characterizing the unexpected.
Clinical labs, in contrast, typically use triple-quadrupole or tandem quadrupole-ion trap configurations, the better to quantify molecules of known clinical significance using multiple reaction monitoring (MRM) workflows. “Most of the work that’s done in clinical diagnostics is [identifying and quantifying] set targets,” Smith says, such as drugs or heavy metals. “You know exactly what you’re looking for.”
SCIEX offers both triple-quadrupole and quadrupole-ion trap (QTRAP®) systems in either of two sensitivity levels for clinical diagnostics, Smith says -- the API 3200MD™ and 3200MD QTRAP instruments, and the more sensitive Triple Quad™ 4500MD LC/MS/MS and QTRAP 4500MD LC/MS/MS. Of the two series, says Smith, the 4500MDs are faster and more sensitive – up to 100-fold more sensitive in the case of the QTRAP 4500MD -- but also more expensive, and customers can choose whichever instrument best suits their needs. (Hybrid quadrupole-ion traps can do everything a triple-quad can, Smith says, but with greater sensitivity and the ability to dig deeper into molecular structure.)
In Europe, Smith says, the newborn screening kit is validated for use on the 3200MD platform, while the company’s immunosuppressant kit is validated on the 4500MD series. “Customers make tradeoffs in terms of application,” she says.
Waters offers three triple quadrupoles for in vitro diagnostics use, says Langridge, the Acquity TQD, Xevo TQD and Xevo TQ-S. These, too, differ in price and sensitivity, with the top of the line being the Xevo TQ-S. “It can be up to an order of magnitude more sensitive, depending upon what you’re analyzing,” states Langridge.
But clinical performance is about more than just a good mass analyzer, Langridge says; also key is sample preparation—turning biofluids (typically blood or urine) into something the mass spec can actually analyze. Waters’ customers can use Tecan’s Freedom EVO workstation for large-scale, automated, microtiter plate-based sample preparation. Alternatively, Waters offers the MassTrak Online SPE Analyzer System that couples directly to the company’s Acquity UPLC liquid-chromatography system for serial sample preparation via solid-phase extraction.
The complete package
As a practical matter, there’s little to distinguish research- and clinical-grade mass spectrometers of the same class (such as triple quadrupoles), at least under the hood. But that’s not to say differences don’t exist. Among other things, says Smith, “clinical instrumentation is manufactured under tighter procedures and documentation to ensure the quality of the device, and there are strict requirements for reporting to users errors and bugs and fixing them for users in the field.”
That said, when it comes to picking an instrument, Langridge advises users to look at the big picture. The instrument itself is a one-time purchase, but there’s also service, reagents, software and support to consider. With health-care decisions on the line, instrument uptime and productivity are key. “What is probably most important,” he says, “is that it’s the complete package.”
Image: Scripps Center for Mass Spec