Femtosecond pulse shaping adds a new dimension tomass spectrometryIgor Pastirk,1Xin Zhu,2Vadim V. Lozovoy,2and Marcos Dantus21Biophotonic Solutions, Inc., Okemos, Michigan 48864, USA2Department of Chemistry, Michigan State University, East Lansing, Michigan 48824, USA*Corresponding author: [email protected] 16 October 2006; revised 23 February 2007; accepted 28 February 2007;posted 1 March 2007 (Doc. ID 76092); published 12 June 2007Phase-shaped femtosecond laser pulses and mass spectrometry were implemented as a tool for improvingmolecular identification. We demonstrate that the specific lines in the mass spectra of several chemicalwarfare simulants are sensitive to the phase characteristics of the incident laser field. The deviation inthe relative yield of fragment ions observed upon pulse shaping (enhancement or suppression) adds a newdimension to mass spectrometry that improves molecular identification and can be used to quantitativelyanalyze mixtures of isomers. © 2007 Optical Society of AmericaOCIS codes: 320.5540, 300.6350.1. IntroductionDetection and identification of chemical species in realtime is of significant importance for both environmen-tal and security applications. Mass spectrometry (MS)is widely used for air quality monitoring, usually boost-ing its reliability by increasing the multidimensional-ity of the analysis. Compact, portable, backpack-sizedMS devices [1] are commercially available, althoughstill not providing zero false positives that are a pre-rogative in critical applications. Traditionally higherreliability and兾or confidence level (increased dimen-sionality of analysis) has been achieved by adding gaschromatography (GC) or another MS in the analyti-cal path, therefore providing two- (GC-MS) or three-(GC-MS-MS) dimensional analysis [2]. This approachincreases the reliability of results (low false positives)but inevitably increases the response and兾or analysistime to several minutes.It has been established that shaped femtosecondpulses can influence the relative yield of fragmentions [3– 6]. We report here on significant advances infast photonic identification of chemicals based on bi-nary shaped [7] femtosecond pulses. The method pre-sented here combines a femtosecond laser with aphase shaping module and a time-of-flight mass spec-trometer [5]. The advantage of the system is that thelaser-molecule interactions depend on the pulse char-acteristics and the electronic and molecular structureof the analyte. This dependence is reduced to a pa-rameter given by ␦ ⬅具Y ⫺ YTL典, which determines theaverage deviation for each fragment ion from a ref-erence, which we have chosen to be the relative yieldof that fragment obtained for transform-limited (TL)pulses.Previous work from our group focused on singulardifferences in the mass spectra resulting from theapplication of differently shaped ultrashort fields [5].The new approach presented here emphasizes theobserved deviation ␦ (enhancement or suppression)in the yield of particular fragment ion peaks as aresult of pulse shaping. Therefore we utilize not onlythe position and intensity of MS lines for identifica-tion, but also an average deviation for each spectralline upon pulse shaping. Here we present this newapproach of multidimensional mass spectrometry forreal-time analysis of warfare agent analogs.2. Experimental SectionExperiments were performed using regenerativelyamplified femtosecond pulses, with 35 fs pulse dura-tion, full width at half-maximum, produced by a com-mercial Ti:Al2O3laser system as described in Ref. 5.0003-6935/07/194041-05$15.00/0© 2007 Optical Society of America1 July 2007 兾 Vol. 46, No. 19 兾 APPLIED OPTICS 4041Phase distortions were measured and eliminated toobtain TL pulses by using the self-calibrating mul-tiphoton intrapulse interference phase scan (MIIPS)method [8]. Having TL pulses is a crucial step in thisapproach, as the MS obtained from TL pulse servesas an internal standard; furthermore, it ensures thereproducibility of the results. Femtosecond lasers,unlike nanosecond and picosecond sources, are proneto phase distortions, which cause temporal broaden-ing. This is why we stress the importance of correct-ing spectral phase distortions at the location wherethe laser interacts with the molecules. MIIPS en-sures that phase distortions inherent to the lasersystem, as well as distortions introduced by theoptics in the beam path, are all eliminated. WithMIIPS, obtaining TL pulses at the sample takesonly 20 s. MIIPS uses a pulse shaper with a liquid-crystal-based spatial light modulator that is situ-ated between the oscillator and the amplifier. Themeasurement of spectral phase distortions is ac-complished by introducing a reference phase func-tion. Once phase distortions are measured, they arecompensated by the pulse shaper, thus obtainingTL pulses. This setup preserves the total outputpower of the laser and produces accurate spectralphase modulation [9].The new dimension introduced in this work dependson causing significant and reproducible changes in thefragmentation pattern of molecules when studied bymass spectrometry. Pulse shapers use spatial lightmodulators to control the phase and amplitude of dif-ferent frequency components of the pulse. The shaperused for this work has 128 pixels, and each can intro-duce 100 different phases and 100 amplitude levels.The total number of different shaped pulses that canbe generated is 共100 ⫻ 100兲128⫽ 10512. In the samelaboratory, we have pulse shapers with 640 pixels,which can generate 102560different-shaped pulses.These numbers are well beyond astronomic propor-tions. In fact, the number of atoms in the universe isestimated at 1080. Clearly, in order to create a usefulapplication, the task is to reduce the number of shapedpulses that need to be evaluated to the absolute min-imum and to be certain that those shaped pulses canbe reproduced by every laser used for molecular iden-tification. During the past 3 years, we have been work-ing on exploring the minimum number of phasesrequired for molecular identification. The greatest re-ductions have resulted from two observations. First,amplitude modulation is similar to pulse attenuation,and we have found that pulse attenuation causes min-imal changes to the fragmentation pattern. Further-more, because multiphoton pulses depend on therelative phase of the different frequency components ina femtosecond pulse, we can control their amplitudewith