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Mar 1, 2011 (Vol. 31, No. 5)

Getting Up to Speed with Modern LC-MS

More Sophisticated Instruments Streamline Practices but Introduce New Challenges in the Process

  • This isn’t your advisor’s LC-MS. It’s faster, more specific, more sensitive, more reliable, more automatable. It’s the go-to instrumentation of choice for bioanalysis and drug discovery. Yet, more sophisticated instrumentation brings with it more challenges, and leaves you to struggle with more difficult tasks, like how to use it for (top-down) proteomics; to mitigate—or at least measure—the effect of the matrix; to do both quantitative and qualitative on the same instrumentation; to get your runs even faster, or automate the process; or to figure out what that contaminant in your drug actually is.

    The advent of mass spectrometry has allowed scientists to find miniscule quantities of a biological substance, even as liquid chromatography runs became abridged. “That’s important if you’re trying to look at compounds that are dosed at very low levels,” such as inhalants that are designed not to have any systemic exposure, says Robert Plumb, senior manager in the pharmaceutical business operations division at Waters.

    Yet compounds in urine, plasma, and other biological matrices once removed by long, elegant chromatography runs now compete with the analyte for the mass spectrometer’s ionizing source—a phenomenon termed ion suppression. That can lead to irreproducible signal from the sample, and hence, a less robust assay, unless the matrix effect can be tamed and quantified.

    To determine the matrix effect, which is required for assays in support of regulatory submissions for preclinical and clinical drug applications, scientists would typically have to reconfigure the plumbing of their mass spectrometer. Measurements would have to be taken of combinations of different samples—analyte (with or without an internal standard), a solvent blank, and the matrix itself—injected through the mass spectrometer both through the LC column as well as postcolumn, and retention times determined for each. This data is then combined through an algorithm in internal or external software to determine to what effect the matrix is suppressing or enhancing the signal of the analyte.

    Waters has introduced new instrumentation—Xevo TQ and Xevo TQ-S—with IntelliStart technology, “which enables us to automatically mix different samples within the chromatography system without having to disconnect the system, make changes to the plumbing of the system, or reconfigure the system,” Plumb explains. Software built into the system crunches the data, determines the matrix factor, and incorporates it into the instrument operations.

    Of course, it’s always best to minimize matrix effects in the first place. At the end of last year, Waters introduced the Ostro 96-well plate with a special membrane that extracts phospholipids—one of the principle causes of ion suppression—from plasma and serum. The company also offers the Acquity UPLC system, which can generate high-resolution chromatography and work at different pHs, says Plumb, “enabling you to modify the chromatographic separation to minimize the interference of any of the endogenous materials with your analyte of interest.”

  • Oh, The Pain of LC-MS

    Click Image To Enlarge +
    Agilent’s Poroshell 120, 2.7 µm particles have a 1.7 µm solid silica core with a 0.5 µm porous outer layer, which offers the benefits of sub-two-micron performance in terms of high resolution and high-speed separations.

    Joan Stevens avoids some of the difficulties seen with sub-2 µm columns in the ultra-high pressures of UPLC—in particular the increased tendency to plug with “dirty” samples—by running reversed-phase LC through Agilent Technologies’ superficially porous Poroshell columns.

    “It has mass transfer such that it acts very much like a sub-2 µm particle LC column, but it doesn’t have the back pressure associated with it,” she says. The efficient mass transfer equates with faster analysis time, allowing for more samples in a shorter amount of time—yet with “optimum resolution so you have selectivity between your peaks of interest.”

    Stevens, a sample-preparation applications scientist at Agilent, works on detection of multisuite medications by LC-MS/MS—a relatively new field. With nearly 100 million people of the United States’ steadily-aging population reliant on a broad range of therapeutics, the demand on the clinics that manage those patients is huge and steadily increasing, she points out.

    Urine is routinely tested to make sure patients are not abusing the substances they’ve been prescribed, as well as to make sure they’re complying with their regimens. To meet that burden, the accredited laboratories that analyze these samples day to day require “less expensive, faster, more accurate and reproducible methods for monitoring the presence and concentration of pain-management medications.”

    The urine sample is cleaned up off-line by solid-phase extraction using Agilent’s Bond Elut Plexa™ PCX prior to injection into the LC. The amide-free cationic-exchange resin removes neutral and acidic compounds, yet has minimal binding sites for endogenous interferences, reducing the matrix effect by allowing concentration of basic analytes such as most pharmaceuticals and their metabolites.

    Plexa beads are polymeric and can therefore, be dried, and use a very small bed mass—which “minimizes the amount of solvent that you need to use and also quickens the sample-prep portion of the cleanup from the urine sample,” Stevens notes. “And the lovely part about this is that it can be automatable…when those samples come in they can actually be placed on an instrument and then be transferred right over to the LC-MS.”

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