Faster-Firing Lasers Yield Faster Mass Spectrograms

Hank Hogan

Exploiting advances in laser technology, researchers at Northeastern University’s Barnett Institute in Boston boosted the speed of a matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (Maldi-Tof) instrument tenfold. That increase, noted Barry L. Karger, director of the institute, opens up a number of potential applications, including tissue imaging and the rapid quantification of the proteins in biological samples.

The 2-kHz Maldi-Tof mass spectrometer collects data 10 times faster than commercially available axial systems. The speed refers to the repetition rate of the Nd:YAG laser (black rectangular box near the monitor in the iddle of the picture). Image courtesy of Northeastern University's Barnett Institute.

In Maldi-Tof, researchers fire laser pulses at a sample, ionizing molecules and freeing them for analysis. Compared with other techniques, the method better preserves the structure of biomolecules and produces simpler, easier to understand spectra. For these and other reasons, it is used in the analysis of peptides, proteins and DNA fragments.

The rate at which it generates data, however, can leave something to be desired. Tissue imaging, for example, can require hours or days.

The speed of Maldi-Tof is a function of the repetition rate of the laser in the system. In the past, only relatively slow nitrogen lasers had the pulse-to-pulse energy consistency needed for the technique. Now, however, solid-state ultraviolet lasers can be used.

The researchers took advantage of this technological development. They used a 355-nm tripled Nd:YAG laser from MeshTel of Genoa, Nev., operating at a repetition rate of 2 kHz in an axial configuration.

They employed various innovations in exploiting the laser’s repetition rate. They used an AP200 digitizer from Acqiris USA of Monroe, N.Y., to perform real-time signal processing, thereby eliminating the need to transfer large amounts of data to disk. They altered the way the sample stage moved, switching from stop-and-go to continuous motion. To avoid drilling through the sample, they jiggled the position of the laser pulses on the target by some 50 µm at about 20 times a second using a scanning mirror driven by a galvanometer from Cambridge Technology Inc. of Cambridge, Mass.

“We used a scanning mirror not only to provide small oscillations, but also basically to turn the beam off between spots. So it was like a shutter,” said Eugene Moskovets, a principal research scientist at the institute. Moskovets and former institute staff scientist Jan Preisler, now at Masaryk University in Brno, Czech Republic, played a key role in designing and building the instrument.

The researchers compared results from their new instrument with those from a similarly configured commercial unit. As expected, they collected spectra 10 times faster. Their instrument had a mass resolution of 10,000, versus 14,000 for the commercial device. The detection limit of the two was about the same, at 200 amol of the Glu fib peptide.

Karger said the next step will be high-speed tandem mass spectrometry, which would enable biomolecular quantification. He noted, however, that the increased speed alone should be useful.

“For people doing Maldi who have a large number of samples they want to run,” he said, “this could be very interesting.”

Analytical Chemistry, online Dec. 23, 2005, doi:10.1021/ac051393t.