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Trace Detection of Organic and Bioorganic Compounds - Narcotics and Explosives - in Liquids and Atmospheric Air

Novel Method Based on Laser Desorption of Ions from Microscopically Rough Solid Surfaces and Techniques Elaboration


Instrumentation / Detection Devices

Chemistry / Analytical Chemistry


Because of the increasing threat from world terrorism and continuing problems with illicit drugs, the effective detection of explosives, war gases, and drugs has taken on a new urgency. The goal of this project is to develop a novel laser analytical method for ultra-sensitive detection of trace compounds in ambient air. A prototype instrument for trace narcotics and explosives detection will be built. Preliminary results show that the proposed method has an excellent promise of achieving a much higher sensitivity than existing detectors, that may be equal to, or better than, that of trained dogs. In addition, the method can be used as with gas chromatography or for condensed phase analysis.

State of the art. The most generally applicable and powerful approach for the detection of explosives and drugs is trace detection in which the chemical signatures of the explosives or drugs are detected. Commonly, ambient air or particulates are sampled. Trace detection in ambient air represents a formidable analytical problem. First of all, the vapor pressures of many explosives and drugs are very low. Second, the actual concentration of these compounds in air is often much lower. The reasons for this include dilution, vapor sticking to surfaces, and wrapping or other types of deliberate concealment.

A number of analytical techniques are being used in commercial trace detectors. These include ion mobility spectrometry (IMS), electron capture, surface acoustic waves, chemiluminescence, and others. The most successful of these is IMS. Trace detectors are used in a variety of settings, such as airports, security checkpoints, border crossings, searches of buildings, etc. The market is large and growing rapidly. It is estimated that over 1,000,000 explosives detectors will be needed worldwide. This commercial success is due largely to the desperate need for security measures. However, current technologies, including IMS, all have serious analytical shortcomings. In particular, the sensitivity for ambient air analysis is not high enough. This is the reason why it is necessary to swipe luggage at airports as particulates contain much more of the target compounds. The limitations of current technology is well illustrated by the fact that trained dogs can find hidden explosives and drugs, when detectors cannot. A well-known example of this is searching for landmines. Dogs are indeed about 100 times more sensitive that current detectors. Dogs are also very good at discriminating one scent from another. IMS detectors, in contrast, have a relatively high frequency of false alarms. The dogs also respond faster, which means that they can follow a scent to its source, something that no instrumental technique can yet do. Clearly, improvements in the sensitivity, reliability of identification, and speed of analysis are desperately needed for trace detectors of explosives, drugs, and nerve gases. Such improvements are likely to revolutionize the applicability and power of trace detectors.
Mass spectrometry. The state of the art laboratory-based technique for trace analysis is mass spectrometry (MS). In combination with gas chromatography (GC) and pre-concentration of analyte during sampling, MS can be as much as 100 times more sensitive that dogs. In mass spectrometry, analyte molecules are ionized, and the ions separated in a mass analyzer according to their masses. A number of different mass analyzers are being used. The most commonly used ionization method is electron ionization (EI) in which molecules in the gas-phase are bombarded with high-energy electrons. MS owes its remarkable sensitivity to the facts that the efficiency of making ions is high, about 0.01% and that single ions can be detected. MS owes its ability to reliably identify compounds largely to the highly efficient separation of ions in the mass analyzer. However, MS traditionally requires very bulky and expensive instrumentation, as well as long analysis times (for GC). If the principles of mass spectrometry could be applied in field-deployable detectors, very large sensitivity improvements would be achieved. In the last decade, there has been much progress in making mass spectrometers compact and lightweight enough to be field-deployable. Possible, the most attractive design uses time-of-flight (TOF) mass analyzers. These have the simplest possible construction. They are inherently very sensitive, since they allow for all ions to be detected, and have high resolving power. Unfortunately, TOF analyzers are also very difficult to use with present ionization methods, such as EI, for gaseous compounds.
The opportunity of the present proposal derives from the discovery of the novel way to ionize gas-phase molecules. As in traditional mass spectrometry, gas-phase analytes are admitted into an ion source. However, instead to ionizing the molecules in the gas-phase, they are adsorbed on a special type of surface and ionized while on the surface. The preformed ions are then desorbed with a pulsed laser. The surfaces are chemically modified, microscopically rough, ionization surfaces, commonly of carbon or silicon. A patent is pending for this analysis method (J. Sunner, S. Alimpiev, S. Nikiforov, “ Method and Apparatus to Produce Gas Phase Analyte Ions ” filed on February 27, 2002 in the US Patent and Trademark Office, N 10/083.647). The method has several important advantages.
Sensitivity. Our preliminary research has demonstrated an ionization efficiency and sensitivity (<105 molecules/cm3) for explosives (trinitrotoluene - TNT) and narcotics (cocaine) that are as high as for EI mass spectrometry. It is remarkable that already very early in its development this ionization method shows a sensitivity that is comparable to state-of-the-art laboratory mass spectrometry, a technique that has matured for decades.
Pre-concentration of analytes. Analytes are effectively pre-concentrated on the ionization surface and they can be accumulated for an extended time. This can be used to effectively remove excess water and other more volatile compounds.
No matrix ions. In EI, all compounds present in the ion source, such as the oxygen and nitrogen in air samples, are ionized. These “matrix ions” are present in large excess. In the surface ionization method, ions are produced only of (selected) organic molecules.
Perfect match to TOF analyzers. The surface method of producing ions is perfectly matched to TOF analyzers. By having all ions start from a well-defined surface at the same time, the full potential of TOF analysis for a high resolving power is realized. The resolving power of the TOF analyser is high, and about 100 times higher than in IMS.
Selectivity. The surface ionization is highly selective, for example for basic compounds, and can be modified to promote the ionization of certain compounds.
Low pumping requirement. Because, sampling and ionization are separate processes, pumping requirements in ambient air analysis can be dramatically reduced.
Condensed phase analysis. Also condensed-phase samples can be analysed. This includes organics and bioorganic compounds.
Together, the advantages of surface-assisted ionization of gaseous compounds, makes for an extremely exciting opportunity for trace detectors for explosives and drugs. There is an excellent chance that the novel detectors will have much-improved analytical characteristics. The impact of such a detector technology could be huge. Imagine, for example, the impact of an ability to find explosives carried by a person in a crowd!
The proposed project should attain the following objectives:
Specific Objective 1: New knowledge on the fundamental processes underlining the analysis method – sorption, ionization, and desorption - will be obtained. Research is required on the following tasks: a) The role of surface substrate material and of surface roughness must be better understood. Rough surfaces of different origin and of different surface morphology as well as surfaces with submicron structural heterogeneity will be investigated. Further, we will investigate different micro- and macro-rough sorbents, particularly with different specific value of roughness, porosity and presence or absence of regular small-structure heterogeneity. To create such small-structure heterogeneity on the surface of semiconductors and graphite, anode and plasma-ion etching methods will be used. The structure of the ionization surfaces will be characterized using scanning electron microscopy and X-rays fluorescence methods.
b) Chemical modifications to the ionization surfaces that improve ionization efficiency, as well as pulse-to-pulse reproducibility, will be developed (Task 2). The surfaces will be modified by deposition of thin layers of organic high-molecular compounds to create surfaces with desired chemical and analytical characteristics. We propose to use surface modifiers with functional groups, which differ in polarity, polarizability, acidity, basicity.
c) Application to new classes of compounds of the laser-assisted desorption method will be explored. Thus, the surfaces will be optimized for different ionization modes, efforts will be made to extend the upper mass limit, primarily for protein analysis, and surface modifications that may allow for the degree of ion fragmentation to be varied will be explored. d) The ionization efficiency will be measured for different analytes from different types of surfaces. The measurements will be performed for both positive and negative ionization modes. e) The role of laser field parameters for the efficiency of ion formation on the rough surfaces and for the desorption process will be elucidated by using different laser systems (Task 4). The importance of wavelength of the laser, pulse energy, pulse shape, and pulse duration will be characterised.
Specific Objective 2: A method of analysis of organic compounds, capable of detecting large molecules with a high sensitivity will be developed. The application to the analysis of biochemical compounds, such as proteins, will be greatly empowered by the interfacing to high pressure liquid chromatography (HPLC) and capillary electrophoreses (CE);
Specific Objective 3: A prototype of the device for detecting narcotics and explosives in ambient air will be built. A prototype interface, that samples trace compounds from ambient air will be developed.

The work schedule is designed for three years and requires 385 man-months. The estimated effect of “weapon” scientists is 196 man-months. The group of people working on the proposed project includes highly qualified professionals in different areas – laser physics and spectroscopy, surface physics, classic mass-spectrometry, and analytical chemistry - as well as specialists in a field of gas-surface interactions and development of sensitive surface coating for solid state sensors. Half of group members are “weapons” scientists. The group possess all the necessary knowledge and some preliminary results as well as the major equipment to complete the project within the time frame indicated. The project is supported by an international collaborator, Professor Jan Sunner at Montana State University in USA, who is a specialist in analytical molecular spectroscopy.