Everyone understands the maxim “garbage in, garbage out.” For most analytical methods, sample preparation is a given, an inextricable workflow component. The more careful the sample preparation, the greater the chance that the instrument will return a result that, if not exactly the desired one, is at least true.
With respect to sample prep, MS has become a victim of its own success.
According to Robert Classon, MS business development manager at Shimadzu Scientific Instruments (Columbia, MD), as MS becomes faster, more sensitive, and more specific, effects that may not have been obvious with lesser spectrometers can show up unexpectedly, particularly with dilute samples.
“When analyzing therapeutic drugs, using the most sensitive instrument on the market would require diluting samples, which increases the risk of losing trace components to contact surfaces, experiencing contamination in the mobile phase, or matrix interferences. Users could detune the instrument for lower sensitivity, but that is like buying a Lamborghini just for driving to the corner store.”
Removal of specific matrix and sample interferences improves the sensitivity on standard MS instrumentation by reducing the background chemical noise ionsuppressing interferences.
Arguably the greatest contribution to online MS sample prep takes the form of restricted access media (RAM) columns, which incorporate dual separation modes such as size exclusion plus reverse phase. Several marketed RAM columns will, for example, remove proteins from biological fluids while retaining drugs and drug metabolites. One type employs large silica particles bonded to octadecylsilyl groups with an outer hydroxymethyl cellulose surface and controlled pore openings. The pores are too small for proteins, which are washed through, but capture and concentrate smaller organic molecules.
Benefits of RAM columns compared with solid-phase extraction include lower cost, no sample handling losses, and less sample exposure to contaminants and oxygen. “You don’t have to pre-wet the cartridge, elute with different solvents, or evaporate or reconstitute with mobile phase,” Classon notes. “You just inject the sample, wash away impurities, and chromatograph as you would a normal sample.” RAM columns also perform chromatofocusing and shorten run times considerably, resulting in better signal to noise and higher sensitivity.
Another novel, interesting approach to sample prep in MS is the combination of ion exchange with reverse phase, which removes specific compound classes from complex samples. For example, a MASK (methacrylic acid 3-sulfopropyl ester potassium salt) column made from ethylene dimethacrolate beads surfacetreated with a sulfopropyl ester excludes anionic species, including humic acids, at neutral pH. MASK columns are ideal for pretreating water samples for environmental analysis. Common in soil and water, humic acids comprise thousands of different structures that cause poor sensitivity for many target analytes and result in short HPLC column life. “MASK column pretreatment eliminates these problems while reducing background noise considerably,” Classon explains.
The case for automation
High sample throughput coupled with sample complexity and the growing scrutiny around results provides a cogent argument for automating MS sample prep.
Today’s liquid handling systems incorporate almost any rapid sample cleanup method, including solid-phase extraction, liquid-liquid extraction, affinity purification, and others, observes Tony Mamone, senior marketing manager at Tecan Group (San Jose, CA). Capabilities are not a concern.
The $64,000 question is therefore not a liquid handler’s capabilities, but its economic justification.
Throughput is the first consideration. A technician can prepare one plate per week as fast as any robot when programming and setup are included in the comparison. At one plate per day, considering the benefits of a liquid handler is not out of the question. Once throughput reaches five or ten plates per day, automation should be automatic.
And not simply for reasons of speed. Automation frees operators from the boredom of repetitive tasks and babysitting instrumentation. For managers, this translates to having another set of hands for less routine tasks. More important, automation provides consistency of results, for the preparation of both samples and standards. Automation systems perform record keeping, sample tracking, and bar coding and maintain the security of samples and results.
Finally, automation brings versatility. Tecan sells to a variety of industries, but even within companies, automation tasks can change on a dime. “People may be up to their eyeballs in one method today but could be working on something entirely different the following week. Automation provides a means of adapting to changing workflows.”
Liquid handling sample preparation can also help laboratories recoup the value of their investments in hardware and method development. “People invest half a million dollars in a spectrometer, start using it, and realize that it works much better when their samples are clean,” Mamone says. “The way they get a payback on an expensive MS is by feeding it samples.” Even a relatively expensive liquid handling system can pay for itself, and the MS, by supporting higher throughput.
How small can you get?
One of the recurring themes of this column is miniaturization—“the shrinking instrument.” At some level, at some stage, all miniaturization involves performance trade-offs. It’s rare to find cutting-edge miniaturization that does not. At Pittcon 2013, Microsaic Systems (Woking, Surrey, UK) unveiled the world’s first full-featured, fully integrated mass spectrometer based on MEMS (microelectromechanical systems). Also known as micromachines, MEMS were conceived in the late 1980s at Imperial College London and about half a dozen Swiss and German universities. MEMS have evolved alongside the capabilities of micro- and nanofabrication technology (think of how computers have advanced in the last 20 years). As expected, nanofabrication terminology has changed somewhat as miniaturization techniques improved. Today one speaks of NEMS (nanoscale) and MEMS composing a continuum of miniaturization.
Microsaic’s chip-based instrument, the Microsaic 4000 MiD, weighs in at 27 lb. and offers what the company claims is the smallest full-featured, UHPLC-ready MS on the market. In other words, no compromises.
This level of miniaturization relies on three MEMS components. Spraychip® is a microspray atmospheric pressure ion source that is positioned freely with respect to the vacuum interface, known as Vac-Chip. This replaceable, easily cleaned component supplies an ion flux from Spraychip to Ionchip®, the microengineered quadrupole filter. According to Microsaic, Ionchip delivers the performance (resolution, mass range, etc.) of full-size quadrupoles but is easier to manufacture. Together these components—the guts of an MS of any size—were designed for plug-and-play operation and end-user maintenance.
According to company CEO Colin Jump, “The core technologies are chip-scale versions of traditional mass spec components that can be interchanged rapidly by the user. This modular approach allows users to maintain the system without the need for expensive service contracts and time-consuming call-outs.”
For additional resources on mass spectrometers, including useful articles and a list of manufacturers, visit www.labmanager.com/ms