Recent Progress in Laser Analytics

Home , Franz Hillenkamp, Michael Barber (chemist), Renato Zenobi

KOLUMNE 417 CHIMIA 44 (1990) Nr.I~ (Ikzem""r)

Chimia 44 (/990) 417 424 <&') Schll'ei=. Chemiker- Verhand; ISSN 0009 4293

Analytical methods are on their way to mass spectrometry (MS). In 1946, William penetrate the biological sciences. In this E. Stephens (University of Pennsylvania in trend, laser technology plays an important Philadelphia) described a mass spectrome- role, especially in the form of laser-desorp- ter with time dispersion, followed by the tion mass spectrometry (LD-MS). ion velocitron of A.E. Cameron and D.£. The remarkable progress made in this Eggers. These devices represented early field is nicely demonstrated by a statement forms of the time-of-flight mass spectrom- made in 1986 by Frank H. Field, a specialist eter (TOF-MS) first described by the Swiss in the mass spectrometric investigation of R. Keller in 1949. biomolecules and Professor at Rockefeller The first commercially successful TOF- University in New York. Citing Professor MS was introduced by Bendix Corpora- In dieser Kalwnne schreibl Field: 'The mass region of real interest for tion, and it was based on the design re- Prof Dr. H. M. Widmer proteins lies between 40000 and 100000 ported in 1955 by William C. Wile)' and Forse/lUng Analylik Da, and one can only speculate as to l. H. McLaren (Bendix Aviation Corpom- Ciha-Geigy AG. FO 3.2 CH 4{)()2 Basel whether such monster gaseos ions could be tion). In these early days of TOF-MS, the regelmiissig eigene Meinungsarlike/ oder liidl Giiste produced. My personal feeling is that to do ions were generated by electron impact ein. lI11gemein inreressierende Ange/egenheilen der so may well require the discovery of some (El). The Bendix instrument had a 2 m long ",odemen Analyrik =u kOlllmenlieren. Eilllvendwl- flight tube and its mass range was limited xen aus dem Leserpl/hlikl/m sind nichtl/nerlVionization (LDI), An important improvement in the TOF- and was introduced by Michael Karas and MS instrumentation was introduced in Franz Hillenkamp (Institute for Medical 1973 by B.A. Mamyrin, v.J. Karatajeo, Physics, University of Munster) in 1987/ D. V. Shmikk, and V. Zagulin (Physical 88. Their invention made it possible to in- Technical Institute loffe, Leningrad) the vestigate biomolecules up to 300000 Da in inventors of the reflectron. Their mass re- the gas phase of a time-of-flight mass spec- flector enabled the focussing of ions with trometer (TOF-MS). the same mass, and, therefore, inhanced It is interesting to recall the different the mass resolution ofTOF·MS. steps that made this success come true. It is Parallel to these instrumental develop- again one of these cases demonstrating the ments, attempts were made to desorb and necessity of interdisciplinary influences for ionize nonvolatile and polar substances to Dr. Klaus Olaf Bomsell is the laser program project the advancement of analytical methodol- make them accessible for MS. The idea was leader in the Central Analytical Research Depart- ogies. At the same time, it is a typical to desorb such molecules directly from the ment of Ciha-Geixy Ltd., Basel. He studied at the Christian-Albrechts University in Kiel. Under model case for the instrumental develop- solid state into the gas phase and to ionize Prof. Edward M. Schlag at the Technical Univer- ments from a complicated physical instru- them in a second step in the gas phase. sity in Munich, hc rcceived his Ph. D. in 1987. He ment to a widely used analytical tool. It In 1960, Hans D. Beckel' and P. Schult: joined Ciha-Geigy in 1988 as a postdoctoral fellow and became a permanent cmployee in 1989. needed a fine-tuning through chemical ex- (University of Bonn) introduced thejield- perience to improve the efficiency of the desorption (FD) technique. Field desorp- method and to simplify the instrumental tion creates a direct and combined desorp- device, so that it became economical and tion and ionization from the condensed may be operated by relative unskilled per- phase. The primary excitation is achieved sonnel, a prerequisite for the wide-spread through low-energy ions and neutral parti- use of a technology, especially in medical cles such as Xe· and Xe. applications. In 1964, Joseph Berkowitz and William Laser techniques have long been used in A. Chupka used a laser beam directed at the analytical research, but seldom has a laser surface of graphite to study the mass spec- method reached whide applications. It re- trum of the vapor ejected from the solid mained the domain of specialists, partially material. Kenneth A. Lincoln (Naval due to the fact, that for many years lasers Radiological Defense Laboratory, San remained expensive and delicate to handle. Francisco) also used laser radiation to Dr. Martin Se/,iir rcccived his Ph. D. in 1988 from However, in the last few years, novel laser flash evaporate solid materials for the University of Berne, where he studied under instruments became available, among mass spectrometric investigations. Other Prof. Ernst Schumacher. He spent a postdoctoral them the simple diode and semiconductor desorption methods followed shortly. In year at the c:.lifornia Institute of Technology in Pasadcna, wherc hc investigated the photoelec- lasers, which found many applications in 1976, D. MacFarlane and D.£. Torgerson tronic spectra of van der Waals clusters in molecu- fiber optical sensors and other analytical (Cyclotron Institute, Texas A and M Uni- lar beams. He joined the Laser Laboratory of the fields. versity, College Station) published work Oha-Geigy, Ccntral Analytical Rescarch Depart- The most important analytical applica- on the plasma desorption (PO) of large ment, in July 1990. tions of laser methods are connected with molecules, based on the analyte surface KOLUMNE 4]8 CHIMIA 44 (1990) Nr.12 (DelClllher)

bombardment with high-energy heavy par- MPI and analyzed in a TOF-MS system. technology. The most astonishing results ticle fission products of mCf, and in the In an earlier Columna Analytica article were published by the groups around same year Alfred Benninghoven, D. Jaspers, (Chimia, 1988, 42, 147), Renato Zenobi, Michael Karas and Franz Hillenkamp and and W. Sichtermann (University of Kaln) Jong Hoon Hahn, and Richard N. Zare by Ronald C. Beavis and Brian T. Chait used lighter particles, such as fast alkali or (Department of Chemistry, Stanford Uni- (Department of Mass Spectrometry and rare-gase ions, to desorb ions form the versity) reviewed and illustrated the feasi- Gas Phase Ion Chemistry, The Rockefeller solid surface. The method was called sec- bility of such technologies. University, New York) and Robert J. Cot- ondary-ion mass spectrometry (SIMS). An The classical ionization, i.e. electron-im- ter (Department of Pharmacology of extension of these techniques was intro- pact (£1) and chemical-ionization (CI) Molecular Sciences, The John Hopkins duced in 1981 by Michael Barber, Robert techniques for gas-phase molecules are University, Baltimore). S. Bordoli, R. Donald Sedgwick, and An- suitable only for relatively small species. In Matrix-assisted LDI-MS is character- drew N. Tyler (Chemistry Department, contrast, the laser desorption techniques ized by several typical features. Singly University of Manchester), called fast- are applicable to large polar and thermo- charged molecular ions are in all cases the atom-hombardment (F AB). For many labile molecules, such as antibiotica, en- base peak of the analyte signal, and no years, SIMS and FA B were the most popu- zymes, and carbohydrates. fragments are observed above 1000 Da. lar methods for the desorption oflarge and In the early 1980's, laser desorption of Sometimes, multiple charged ions are ob- nonvolatile molecules, since these tech- intact organic molecules evolved. Al- served, which has not been reported before niques were easily adapted to the existing though various mass analyzers were used for the laser desorption, though for plasma sector and quadrupole mass spectrometers for this purpose, the time-of-flight ana- desorption MS of high-mass proteins. of numerous MS laboratories. Although lyzer proved to be the most suitable, since Multimers and doubly charged molecular FAB and SIMS were extended to desorb TOF-MS can record ions over a broad ions generally improve the molecular ion compounds above 10000 Da, these investi- mass range, therefore, extending the mass detection. A remarkable sensitivity is gations were generally handicapped by the range of previous laser-desorption mass demonstrated and only small samples are resulting week signals and the sensitivity spectrometry. However, laser desorption required for multiple analysis (sub- remained unsatisfactory for this upper also fits well with ion-cyclotron-resonance nanogram range). Furthermore, matrix- mass range. (lCR). David A. McCreary, E. B. Ledford, assisted LDI is also characterized by a low Around 1975 laser microprobe instru- Jr., and M. L Gross conducted some of the chemical-noise level. ments became available and were widely very first experiments in which samples The matrix material plays a crucial role used. Their characteristics is that they fo- were analyzed by ICR, also called Fourier- in the successful desorption and ionization cus the laser to a very small spot size on a Transform MS (FT-MS). of the macromolecules. Whereas Michael thin foil on which the sample is deposited In 1987, Kuni Tanaka, Y. ldo, S. Akita, Karas and Franz Hillenkamp used nicotinic on the opposite side of the foil. Heraeus Y. Yoshida, and T. Yoshida reported the acid as matrix material, Ronald C. Beavis successfully commercialized an instrument mass-spectral analysis of oligomers in and Brian T. Chait improved the ionization (LAMMA 500 and 1000) which became lysozyme with masses far above 10000 Da, efficiency for certain analytes by the use of very popular among the specialists. The using laser desorption from a matrix of sinapinic acid. LAMMA 500 used a Nd/YAG laser metal powder (Ni), finely dispersed in glyc- The matrix should be chosen so that it is focussed to a spot size of 0.1-1.0 Ilm erol. Laser mass spectrometry for the ex- able to absorb light. Aromatic systems (IOH-IOll W·cm~2). amination of large molecules, thus, came nicely fulfill this requirements. But until Another approach evolving around to ages, supported by improvement in laser now only a few molecules are known which 1978 by was the resonant and non-reso- technology and high-speed electronic explode in crystalline form when exposed nant multiphoton ionization (MPI). Origi- recording techniques. to laser light. Under these conditions, a nally, the sample was introduced from the Lasers may desorb neutral species, in- collective evaporation of the irradiated mi- vapor phase over a liquid effluents through cluding intact molecules, an advantage cro volume occurs. Obviously, in the laser a thin nozzle to generate a supersonic that was fully realised when matrix-as- process the lattice vibration of the matrix is molecular beam by expansion into the vac- sisted laser desorption/ionization was in- excited after the relaxation of the deposited uum. The molecules of interest were then troduced by Michael Karas and Franz Hi!- energy, leading to a desintegration of the ionized through a two photon process and lenkamp in 1988, using a time-of-flight mi- crystal lattice. At the same time proton- extracted into a TOF-MS. The two-step croprobe instrument (LAMMA 1000) with transfer reactions occur through photoion- laser ionization was preferred over a direct a frequency-quadrupled ND/YAG laser at ization processes in the condensed phase ionization, because only two photons have 266-nm wavelength for ion desorption/ leading to a simultaneous desorption and enough energy to ionize the molecule. Us- ionization. The sample solutions (5 x 10-6 ionization. However, the theoretical pro- ing two photons of different color it is pos- to 10-7 M) were mixed with equal amounts cess is still unknown. sible to ionize in an extremly soft mode. of 5 x 10-2 M solutions of nicotinic acid as Carbohydrates, i. e. polysaccharides, However, the target molecule must contain matrix. Less than I III of the combined polypeptides and proteins, and oligonucle- a chromophore to ionize. Maximum reso- 'solution were dripped onto an Ag target otides have been successfully investigated lution of the wavelength spectra was ob- and dried under vacuum. This rather by this matrix-assisted LDI-MS technique. tained, when the evaporated molecules simple sample preparation lead to a final Karas et al. published investigations of were cooled in an expanding beam of neu- sample-matrix solid mixture covering I to lysozyme with a molecular weight of 14306 tral gas. Pioneers in this field were groups 5 mm2 of the probe metal surface. The Da, p-Iactoglobulin A (18277 Da), porcine around Richard N. Zare (Chemistry De- probe was then introduced into the vac- trypsin (23463 Da), and albumin (67000 partment, Stanford University), David M. uum system of the microprobe analyzer Da). In 1990, Ronald C. Beavis and Brian Lubman (Chemistry Department, Univer- and irradiated with the laser light. The to- T. Chait published a study on the rapid and sity of Michigan, Ann Arbor), and Edward tal time for sample preparation, insertion sensitive analysis of protein mixtures in- W. Schlag and Jiirgen Grotemeyer (Tech- into the vacuum chamber, and recording cluding commercial bovine milk and hu- nical University of Munich). of the spectra amounted to several min- man breast milk. Later, two-step laser procedures were utes, and single spectra could be registered Most recently, K. Olaf Boernsen, Martin also performed with solid samples. In these at a rate of ~ 5-10 per min. Schiir, and H. Michael Widmer (Analytical experiments, a CO2-laser was used to de- This technique represented a remarkable Research, Ciba-Geigy Ltd., Basel) using sorb the species deposited on a flat surface. technological breakthrough, since it al- sinapinic acid as matrix material, demon- The desorbed molecules were subsequently lowed the mass spectral investigations of strated that this technique also works for introduced into a supersonic molecular biopolymers in the mass range up to sulfonic-acid compounds. With other mass beam with carrier gas and then ionized by 300000 Da. Several groups adopted the spectrometric methods this substance class KOLUMNE 419 CHIMIA 44 (1990) Nr.12 ([)~/~mh~r) could not be efficiently ionized. With most - N. C. Fenner, N. R. Daly, Rev. Sci. In- - H. D. 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Astrochem

What do the stars have in store for chemists next month? As a service to its readers and as a bold attempt to boost circulation, Chimia is now introducing a new feature - Astrochem. The connection between astrology and chemistry goes back centuries, to the very beginnings of alchemy. Alchemists in medieval times would never undertake an experiment without having first ascertained that the constellations were favourable. More recent research has discovered highly significant correlations between the time of birth of successful chemists and the positions of the planets. Now we have persuaded that eminent scientist and scholar Dr. Wolfgang von Hochnebel to cast a horoscope for our readers for the coming month. Here are his results.

~ Aries 21.3 - 20.4 ~ Libra 24.9. - 23.10. Too hasty acid extraction may cause you to lose much of your An organophosphorus compound may turn out to be an unexpec- yield. tedly poor nucleophile, ~ Taurus 21.4. - 20.5. ~ Scorpio 24.10. - 22.11. Hindered rotation about a single bond could lead to a surprising Distrust a result that seems to fit your theory, There may be a result. mistake somewhere. Try to seek it out. M. Gemini 21.5. - 21.6. ~ Sagittarius 23.11. - 21.12. Unexpected difficulties in a well planned synthesis can probably Your enantiomeric excess may be disappointing, but do not give be overcome by attention to detail. up too easily. ~ Cancer 22.6. - 22.7. ~ Capricorn 22.12. - 20.1. A misunderstanding with a colleague could cause irritation on React calmly if your manuscript is judged harshly by a referee. It both sides. may lead to an unforeseen advantage for you in the end, ~ Leo 23.7. - 23.8. ~ Aquarius 21.1. -18.2. Two short communications may be more advantageous than Attendance at a seminar may give you an interesting idea. Follow one full paper. Do not postpone a decision any longer. it up. ~ Virgo 24.8. - 23.9. B Pisces 19.2. - 20.3. A harmonious time for Beckmann rearrangements. But special An enzymatic reaction may turn out to be much more complicated care with the hydrolysis step may be advisable. than it seemed at first sight. Inadequate purification may be the cause.