Chemistry: analytical and immunological testing – Including sample preparation – Liberation or purification of sample or separation of...
Reexamination Certificate
1999-04-06
2002-07-02
Warden, Jill (Department: 1743)
Chemistry: analytical and immunological testing
Including sample preparation
Liberation or purification of sample or separation of...
C436S177000, C436S157000, C210S767000, C210S774000, C210S775000, C210S149000, C210S176000, C422S105000
Reexamination Certificate
active
06413781
ABSTRACT:
BACKGROUND OF THE INVENTION
Detection of trace amounts of illicit substances such as explosives and narcotics is an ever more critical element of combating terrorism and contraband. However, the exacting operating requirements a detector must meet in order to be useful for these purposes severely limit the number of suitable technologies.
The primary operating requirement is sensitivity. Many of the materials targeted by law enforcement or security screenings are present in the gas phase at very low fractional molecular concentrations. Table 1 shows approximate values of room-temperature vapor pressures for common explosives and cocaine, reports of which often vary by as much as an order of magnitude.
TABLE 1
vapor pressure in air
explosive
at room temperature (atm)
glycerol trinitrate (“NG”)
10
−7
2,4,6 trinitrotoluene (“TNT”)
10
−8
1,3,5-trinitro-1,3,5-triazacyclohexane
10
−12
(“RDX”)
pentaerythritol tetranitrate (“PETN”)
4 × 10
−13
C-4
10
−14
(due to plasticizer and RDX)
cocaine
10
−10
Realistic use in demanding security environments, entailing screening thousands of containers per day, would further require that each determination be completed rapidly, in well under a second. For many law enforcement situations, a serviceable illicit substance detector would necessarily be portable.
Many sensing devices have been proposed for detecting trace amounts of explosives or drugs in security or law enforcement contexts. However, none has combined the sensitivity to detect constituents present at concentrations as low as 10
−12
atm with the requisite rapidity and portability.
For example, a bioluminescence-based explosives detection and identification system capable of detecting constituents in air having a fractional molecular concentration on the order of 10
−14
has been proposed. However, the required processing time is on the order of several minutes. (See, e g, E. M. Boncyk in
Proc.
3
rd Int. Symp. on Analysis and Detection of Explosives
, Mannheim-Neuostheim, Germany, 4.1-40.14 [1989].)
Ion mobility spectrometry (“IMS”) has found wide application as a relatively quick and accurate technology for detecting explosives and illicit drugs. The nominal sensitivity of IMS-based systems ranges from about 10
−12
to 10
−14
atm. As is typical of gas-phase sensors, the minimum level of a target compound detectable by this technique is limited by false positives and interference from other gaseous constituents rather than by the inherent capability of the sensor.
The reliance of standard IMS on the ion charge-to-mass ratio to differentiate constituents predisposes it to false positives when used to detect explosives or drugs. For example, the ion mobility spectrum obtained from methamphetamine, a product of cocaine decomposition, overlaps on the time axis with that due to a common skin conditioner ingredient, so that this ingredient provokes a false positive by an ion mobility spectrometer configured to detect methamphetamine. Introducing an ionizable vapor dopant that neutralizes the problematic skin conditioner ingredient but not molecules of explosives or methamphetamine—which have exceptionally large electron or proton affinities—mitigates this difficulty, but at the expense of some increase in system complexity. Impurities remaining in the sensor from a previous screening are another significant source of error in IMS-based systems.
Also, substances such as RDX and PETN having vapor pressures near the lower limits of detectability by IMS can be detected by this method only after several seconds of sampling. Such an interval is unacceptably long for high-volume applications, such as comprehensive passenger screening at airports.
BRIEF DESCRIPTION OF THE INVENTION
Objects of the Invention
It is, accordingly, an object of the present invention to provide method and apparatus for enhancing the capability of detectors with respect to trace constituents.
It is another object of the present invention to provide method and apparatus or eliminating interfering background impurities prior to subsequent downstream detection.
It is another object of the invention to provide method and apparatus for rapidly detecting trace constituents.
It is another object of the invention to reduce the occurrence of false positives in ion mobility spectrometry systems.
It is another object of the invention to provide method and apparatus that allow easy and quick clearing of a sensor system.
It is yet another object of the invention to provide suitable method and apparatus for detecting illicit drugs and explosives and decomposition products thereof in law enforcement and security environments.
SUMMARY OF THE INVENTION
The invention provides method and apparatus for propelling a target chemical constituent, or equivalently a set of constituents, along a pathway by applying a time-varying temperature profile along the pathway so as to effect a dynamic Soret effect. The temperature profile impressed upon the pathway creates at least one region over which temperature varies with position, so as to produce a warmer zone and a cooler zone situated consecutively along the path. In accordance with the Soret effect, components present at dilute concentration in a carrier medium segregate in the temperature gradient according to their respective molecular weights. Components having molecular weights greater than that of the carrier medium accumulate in the cooler zone, whereas components having higher molecular weights diffuse toward the warmer zone. In moving to establish this thermally driven concentration gradient, each component advances toward the appropriate portion of the temperature profile at a respective net average velocity known to those of skill in the art as its Soret velocity.
In accordance with the invention, the region of temperature variation is displaced along the pathway at a wave velocity, so as to generate a time-varying temperature profile. As the local temperature changes, the segregated dilute components move so as to preserve or reestablish the thermally induced concentration gradient. Thus the components are conveyed along the pathway with the moving region of temperature variation. The quantity of a particular constituent that is pumped down the path depends on the temperature gradient, the absolute value of the wave velocity and its relative value compared to the constituent's Soret velocity, and also the diffusion coefficient of the constituent in the carrier medium.
In one embodiment, the invention provides a dynamic thermophoretic concentrator for separating a target chemical constituent from a mixture of components on the basis of diffusion coefficient by using alternate forward and backward motion of the temperature profile along the pathway, thereby accumulating an ultimate concentration of the target constituent greater than its initial concentration in the mixture by a factor up to ten, 100, 10
3
, 10
4
or even greater. Because most components have very similar Soret velocities, as a practical matter the distribution of a constituent across a given moving temperature profile depends mainly on its diffusion coefficient. Particles having small diffusion coefficients, correlating with large particle sizes, are concentrated in the cooler portion of the temperature profile more compactly, and thus transported at a greater flux by the time-varying profile; the degree of localization drops rather abruptly with increasing diffusion coefficient, so that smaller constituents are distributed more evenly throughout the region of varying temperature and less efficiently transported. The diffusion coefficient at which the flux declines can be shifted to higher values by increasing the temperature difference between the warmer and cooler extremes. For a given temperature gradient, the basic shape of the flux-diffusivity function changes with the wave velocity.
In accordance with the invention, the temperature profiles and wave velocities used for forward and backward motion are chosen in conjunction to enhance
Geis Michael W.
Kunz Roderick R.
Stern Margaret B.
Cole Monique T.
Massachusetts Institute of Technology
Warden Jill
LandOfFree
Thermophoretic pump and concentrator does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Thermophoretic pump and concentrator, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Thermophoretic pump and concentrator will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2819321