A ROBIN is not just for Christmas: Is the rotating ball interface here to stay this time?

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  • Published: Jan 1, 2011
  • Author: Steve Down
  • Channels: Base Peak
thumbnail image: A ROBIN is not just for Christmas: Is the rotating ball interface here to stay this time?

The rotating ball interface refuses to roll over and die

In 2000, a novel interface for MALDI mass spectrometry was introduced by Kermit Murray and his group at Emory University, which had the potential to revolutionise sample introduction. The rotating ball inlet (ROBIN) incorporated a novel way to transfer sample from atmospheric pressure to the vacuum of a mass spectrometer.

An analyte-matrix solution was delivered by a capillary to the surface of the ball which was in contact with a polymer gasket. Upon rotation of the ball, the sample was moved past the gasket into a vacuum chamber for laser irradiation and analysis. The system could operate continuously for 20-30 hours with a capillary flow rate of 2 µL/min before the ball had to be cleaned.

Since then, there have been a few different versions of the rotating ball inlet but it does not seem to have captured the imagination of mass spectrometry practitioners or instrument manufacturers.

Undeterred, a team of scientists has devised a new version of the rotating ball inlet for use in surface-assisted laser desorption/ionisation (SALDI) mass spectrometry. In SALDI MS, the properties of the solid substrate influence the desorption/ionisation process, unlike in MALDI MS.

The inlet was designed and implemented by Jan Sunner from the University of Oklahoma, USA, Sergey Alimpiev, Vladimir Karavanskii and Yaroslav Simanovsky from the Prokhorov General Physics Institute of Russian Academy of Sciences, Alexander Grechnikov from the Vernadsky Institute of Geochemistry and Analytical Chemistry of Russian Academy of Sciences and Sergey Nikiforov from Advanced Energy Technology Ltd., Moscow.

A stainless steel ball was situated between two PTFE ring seals between the atmospheric pressure side and the vacuum side (at 10-7 Torr) of the system. The region between the seals was held at 0.05 Torr.

Small sections on opposite sides of the ball were coated with a silicon substrate. A sample solution was electrosprayed onto the silicon spot on the atmospheric side and the ball was rotated 180° into the vacuum region by a stepping motor after which a laser was fired for sample desorption/ionisation and analysis in a time-of-flight mass spectrometer.

An opportunity for the rotating ball inlet: an interface for LC and SALDI MS

The initial sample deposition mode onto the ball, electrospray, gives a clue as to the overall aim of the researchers. They hope to see the rotating ball inlet as an interface between liquid chromatography and SALDI MS to complement their recent development of a GC-SALDI MS interface. Certainly, the performance with a set of standard compounds such as arginine, reserpine, atenolol and chloropyramine seems to suggest that this is not just a pipedream.

One of the major problems in systems such as this is reproducibility and the type of substrate applied to the ball is critical. A porous silicon substrate produced by anodic etching of an n-type silicon wafer required about 600 laser shots to reach maximum sensitivity. In contrast, an amorphous silicon substrate produced by standard RF sputtering required no such activation and gave reproducible results. The relative standard deviation was less than 10% for 20 replicate analyses over 1 week.

The performance is undoubtedly helped by the fact that the silicon substrate and adsorbed sample made no contact with the seals as the ball was rotated, unlike in previous rotating ball designs.

The SALDI ion signal intensities showed little variation for electrospray deposition in positive or negative polarities and the electrospray solvent had no effect. The researchers suggested that solvent was removed from the ball in the intermediate pressure region between the PTFE seals as it was rotated.

The sample was deposited over an area of 5-20 mm2 on the silicon substrate and the laser was scanned over an area of 1.25 x 1.25 mm2 to increase the ion signal intensity, resulting in an enhancement of about 200-fold compared with single spot analysis.

Under standard electrospray conditions, during which 30-100 fmol compound was deposited, detection limits of 3-50 fg were achieved, which are at least one order of magnitude lower than those of commercial LC/MS instruments. Sensitivities could be increased further by widening the laser scan area even more.

Decreases in the sensitivity on the amorphous silicon substrates induced by the use of aqueous solvents were reversed by treatment with a hydrofluoric solution followed by laser/water vapour activation, so that the coated balls could be reused.

The positive ion SALDI mass spectra of the standard compounds all produced protonated molecules and the degree of fragmentation varied between compounds, as expected, but was greater for porous silicon than amorphous silicon.

With a sample turnaround time of just 5 seconds, no matrix effects, negligible carryover from laser shot to laser shot, and high sensitivity, the rotating ball inlet has great potential as an interface for LC-SALDI MS.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

 The little-used rotating ball interface for laser desorption/ionisation mass spectrometers has been rejuvenated by scientists in the USA and Russia, to produce detection limits at least one order of magnitude less than commercial LC/MS instruments when used for SALDI MS

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