Ambient Mass Spectrometry the Surface of a Sample (8)

DETECTION TECHNOLOGIES (typically nitrogen or helium) to form a plasma REVIEW of excited-state atoms and ions, and these de- sorb low–molecular weight molecules from Ambient Mass Spectrometry the surface of a sample (8). Other closely related methods have also been introduced. Desorp- R. Graham Cooks,1* Zheng Ouyang,1 Zoltan Takats,1,2 Justin M. Wiseman1 tion atmospheric pressure chemical ionization (DAPCI) (9), a variant of DESI that uses gas- A recent innovation in mass spectrometry is the ability to record mass spectra on ordinary samples, phase projectile ions generated by an atmospher- in their native environment, without sample preparation or preseparation by creating ions outside ic pressure corona discharge in the vapor of the instrument. In desorption electrospray ionization (DESI), the principal method described here, toluene or another compound, produces ions by electrically charged droplets are directed at the ambient object of interest; they release ions from the a heterogeneous (gaseous ion/adsorbed analyte) surface, which are then vacuumed through the air into a conventional mass spectrometer. Extremely charge-transfer mechanism. Electrospray-assisted rapid analysis is coupled with high sensitivity and high chemical specificity. These characteristics are laser desorption/ionization (ELDI) (10) uses a advantageously applied to high-throughput metabolomics, explosives detection, natural products laser for the desorption of neutral molecules discovery, and biological tissue imaging, among other applications. Future possible uses of DESI for from an ambient surface and uses charged drop- in vivo clinical analysis and its adaptation to portable mass spectrometers are described. lets produced by electrospray for post-desorption ionization of the ablated neutral molecules. In he application of mass spectrometry though it did not have unimpeded access to the atmospheric solids analysis probe (ASAP) (11), (MS) to the identification of chemical sample nor the lack of sample preparation that another variant on atmospheric-pressure DI Tcompounds in a mixture, including deter- characterize more recent methods. methods for solids analysis, a heated gas jet is mining the structural composition of large Recently, a new family of techniques has directed onto the sample surface, and desorbed biomolecules, depends on much more than the emerged that allows ions to be created under species are ionized by corona discharge in the resolving power of the analyzer used for discrim- ambient conditions and then collected and an- gas phase. inating mass/charge (m/z) ratios. Often, the main alyzed by MS. In the desorption electrospray Here we focus on the DESI method, on which limitation is getting the sample of interest into the ionization (DESI) method (5),afinesprayof there is the most literature, while noting cases in vacuum environment of the spectrometer in the charged droplets hits the surface of interest, which the applications of the other methods yield form of ions suitable for mass analysis. This from which it picks up small organic molecules comparable results. The ambient ionization meth- problem was solved, for the case of samples in the and large biomolecules, ionizes them, and de- ods retain the signature advantages of MS— solution phase, with the introduction of electro- livers them—as desolvated ions—into the mass speed, chemical specificity, low detection limits, sprayionization(ESI)(1). ESI is a method where spectrometer. DESI can be considered an atmo- and, via the MS/MS experiment, applicability to the solution is nebulized to create a fine spray of spheric pressure version of SIMS, being especial- complex mixtures. However, these characteristics droplets under conditions in which solvent ly close to versions that use C60 projectiles (6) are now implemented in a direct experiment that evaporation occurs as the droplets traverse the or massive water clusters as primary impacting requires no sample preparation. Applications to atmospheric interface, hence introducing molec- particles (7). In the DART (direct analysis in high-importance samples—such as traces of ular ions into the analyzer. real time) method, an electrical potential is ap- explosives on luggage, drug metabolites in urine, A critical development for the analysis plied to a gas with a high ionization potential lipids in intact tissue, and active ingredients in of condensed-phase samples was that of the desorption/ionization (DI) methods, where molecules embedded in a substrate and introduced Electrical potential into the vacuum system are rapidly desorbed and ionized using energetic charged particles or Solvent kV laser beams. High-energy sputtering methods Vacuum interface & mass spectrometer Nebulizing Charged solvent such as SIMS (secondary ion MS) (2) can be used gas to produce intact molecular ions. Larger mole- spray cules such as proteins are also amenable to DI methods if they are embedded in a frozen solvent Gas jet (typically ice) or in an ultraviolet (UV)-absorbing Analytes matrix that can be rapidly volatilized with a laser Analyte pulse, as in MALDI (matrix-assisted laser ion Desorption desorption/ionization) (3). Although vacuum conditions are a simple choice for creating and Substrate maintaining ions, this environment is not abso- lutely necessary. Ions can in fact be generated in Vacuum interface & air; an atmospheric pressure version of the He/N 2 Discharge chamber Heating Metastable mass spectrometer MALDI experiment (4) was an important He/N2 progenitor of ambient MS experiments, even Analyte 1Purdue University, Department of Chemistry, West Lafay- ette, IN 47907, USA. 2Institute of Structural Chemistry, Ion/neutral Chemical Research Centre of the Hungarian Academy of separation Sciences, Pusztaszeri ut 59-67, Budapest, Hungary. *To whom correspondence should be addressed. E-mail: Fig. 1. DESI (upper) and DART (lower) analyses for ambient high-throughput mass spectrometric [email protected] analysis of unprepared samples (skin, bricks, urine spots, clothing, tissue, etc.). 1566 17 MARCH 2006 VOL 311 SCIENCE www.sciencemag.org SPECIALSECTION pharmaceutical tablets—have quickly followed intact molecular ions. This feature is associated low fmol range have been reported for proteins by the introduction of the methodology. with fast collisional cooling of nascent ions at using DESI (13)andELDI(10). atmospheric pressure and ambient temperature. DESI operating characteristics can be chosen Implementation to favor the ionization of small or large molecules. The essential aspects and the simple instrumen- Analytical Performance and Characteristics Small molecules are often seen well in the positive tation of the DESI experiment are illustrated in The type of ions observed in DESI, DART, and ion mode by using a spray voltage of 5 kV, a tip-to- Fig. 1. The condensed phase–to–gaseous ion other ambient MS methods depends on the sample distance of 5 mm, and an incident angle of transfer that is a feature of DI methods is nature of the sample, substrate, and reagent. For 40- to the surface normal. Oligosaccharides and achieved by using charged droplets (for larger example, the explosive RDX (hexahydrotrinitro- proteins require smaller tip-to-sample distances molecules) or primary ions (for smaller mole- 1,3,5-triazine) is observed as the chloride adduct and steeper impact angles. These conditions are cules), either of which is produced by ESI (1). [RDX + Cl]– when electrosprayed with a dilute associated with the need to have droplets hit this The sample of interest is in the solid phase— HCl solution, but is observed as the protonated type of sample to cause ionization (‘‘droplet pick- not in the solution phase as it is in ESI—and, in molecule [RDX + H]+ when sprayed with pure up’’ mechanism). Both small and macromolecule contrast to the established DI methods like SIMS water. The ionized molecules observed in the analytes were examined from a variety of surfaces and MALDI, the sample is not under vacuum. mass spectrum are conveniently mass-selected including paper, plastics, and glass surfaces, with Figure 1 also summarizes the DART experiment, and individually examined by recording their no significant differences in the spectra. Polymer which differs because it uses a gas rather than a dissociation products in the form of MS/MS analyses included determinations of molecular solvent to generate the energetic agents that de- spectra. Figure 2 illustrates the DESI mass spec- weights of industrial polymers such as poly- sorb and ionize the analyte molecules. trum of a dry urine spot on paper (2 mLof ethylene glycol (14), as well as those of proteins DESI applies to both large and small mol- urine), showing the complex nature of this mix- and oligosaccharides (13). ecules. The charged microdroplets used as pro- ture. Even minor components can be identified Complex processes are involved in producing jectiles in DESI pick up proteins and other large by recording their MS/MS spectra; for example, gas-phase ions from condensed-phase samples biomolecules from the surface, ionize them, and the isolation of the ion with m/z 0 214 and the through impact of charged aqueous droplets, transport them to the mass spectrometer. This measurement of its product spectrum allows its gas-phase ions, or metastable atoms. It is well process gives the mass spectra of proteins in the identification as aspartyl-4-phosphate. Exper- known that molecules at surfaces under vacu- solid phase, which typically closely resemble the iments of the type illustrated in Fig. 2 can be um can be simultaneously desorbed and ionized ESI spectra of protein solutions. In addition, gas- performed at a rate of one per second. There is by charge transfer (electron, proton, or other ion) phase solvent ions in the spray protonate or no preparation of the biological fluid other than using primary ions with low translational energies. otherwise react with analyte molecules on the its deposition on the surface. This low-energy heterogeneous process known as surface, generating ions from compounds that Not only is DESI a very rapid method, but it chemical sputtering (15) occurs in the ambient DI have low desorption energies, including volatile is well suited to trace analysis.

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