Patent Publication Number: US-2005120775-A1

Title: Systems and methods for detecting contaminants

Description:
CROSS REFERENCES TO RELATED APPLICATIONS  
      This patent application claims the benefit of U.S. provisional patent application No. 60/526,862 filed on Dec. 3, 2003, the contents of which are incorporated by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION  
      Semiconductor manufacturers continue to measure and control the level of contamination in the processing environment, especially during the critical steps of the photolithography processes. The typical means of determining the quality and quantity of contamination in gas samples in cleanroom manufacturing environments involves sampling air and purge gases, such as, for example, filtered and unfiltered air, clean dry air, and nitrogen. One of the most common base contaminants in the air worldwide is ammonia. Within a semiconductor fabrication facility ammonia commonly arises from wet benches, chemical vapor deposition (CVD), cleaners, Si 3 N 4  and TiN deposition, and people. And, outside the semiconductor fabrication facility, farms, fertilizer and sewers are major sources of ammonia.  
      Contaminant measurement systems used in semiconductor environments are typically of the continuous, or semi-continuous, sampling variety or of the fixed sampling interval variety. A continuous or semi-continuous sampling system is installed in a cleanroom and operates in a substantially ON state whenever-the facility is operating. Continuous and semi-continuous sampling systems are typically equipped with measurement and analysis systems for providing data on contaminant levels within the cleanroom. As such, continuous and semi-continuous sampling systems do not require removal from a cleanroom in order for an operator to obtain data regarding contaminant levels within the cleanroom environment. Since continuous and semi-continuous sampling systems produce results on site, they are often large, complex and expensive to acquire, operate and maintain. In addition, these systems require periodic calibrations to ensure that they operate properly in the environment. In contrast, samplers of the fixed interval variety are often portable and inexpensive since they only sample gases within a cleanroom. Fixed sampling interval devices are removed from the cleanroom for analysis of contaminants contained therein. Analysis can be performed by an end user of the devices or the sampling devices can be sent offsite for analysis. Fixed interval sampling devices are attractive to end users because they are relatively simple to operate; however, prior art fixed interval sampling devices have shortcomings.  
      For example, one traditional sampling approach for determining the concentration of cleanroom contaminants, especially ammonia, using fixed interval sampling devices has been through the use of wet impingers. These wet impinging approaches have certain drawbacks including accidental spillage of the scrubbing media (typically, deionized (DI) water or a DI water solution), accidental inversions of the impinger, and limits to sampling time (thereby imposing limits on the lower-detection-limit of the approach) due to natural evaporation of scrubbing media (DI water). In addition, wet impinger systems frequently require installation of the impinger by a highly trained technician. Another shortcoming associated with wet systems is that an end user typically has to wait more than a week to receive analysis results after sending the wet impinger system to an analysis facility. Furthermore, wet impinger systems can be prone to bacterial contamination of the liquid scrubbing media and wet impinger vials are often made of fragile glass or quartz making them prone to breakage during shipment. Consequently, there is an ongoing need for improvements in systems and methods for the measurement of contamination of gases used for industrial processes using fixed interval sampling devices that do not require wet impinger based sampling.  
     SUMMARY OF THE INVENTION  
      The present invention relates generally to systems and methods for obtaining a sample from a gas to measure one or more contaminants therein. In various embodiments, the present invention provides systems and methods for air sampling that facilitate reducing or eliminating the drawbacks associated with traditional wet impinger approaches to sampling.  
      In one aspect, the present invention provides a sampler where the collection material is a dry media. Accordingly, such dry samplers are liquid free avoiding the problems associated with liquid spillage and liquid evaporation inherent to wet impingers.  
      The systems and methods of the present invention are of particular use in the measurement of contaminants in gases used in the semiconductor industry. For example, filters used in the removal of contaminants in gases used in photolithography tools must be replaced on a regular basis to avoid contamination of the tool environment and degradation of semiconductor wafers being manufactured with the tool. By sampling the air circulated within the tool, data can be acquired to determine the need for filter replacement on a regular basis.  
      A preferred embodiment of the invention employs either passive or active sampling in which one or more sampling elements or pads are used to remove contaminants from a gas in contact with the sampling elements or pads.  
      In various embodiments, the present invention provides a dry sampler for ammonia with a lower detection limit (LDL) in the range between about 0.02 ppbV and 0.1 ppbV at a signal to noise ratio of at least about 2 and depending on the duration of the sampling.  
      In various embodiments, the present invention provides a dry sampler having one or more of a low ammonia background and detection limit of 0.1 ppbV for a 4 hour sampling time that corresponds to a 240 L collected volume; a low pressure drop across the dry sampler to enable, for example, use of current hand-held pumps for active sampling system configurations; high capture efficiency for ammonia.  
      A preferred embodiment of the invention includes a sampling system having a plurality of sampling or collection media to provide redundancy and/or to collect separate samples of different contaminants. For example, a first trap-collects one or more bases, a second trap collects acidic materials and a third trap collects organic materials. The different samplers or traps are contained in a single housing with inlet and outlet ports for the gas being sampled. The samplers can be a combination of wet and dry traps, or all contaminants can be collected using a plurality of dry traps.  
      A manifold is used to deliver gas samples through each of the traps and can include manual or automated valves to control fluid flow through the system. Control electronics are located within the housing to automate system operation and record performance data.  
      Analysis of the samples can be conducted using gas chromatography or mass spectrometry for organics and ion chromatography for acids and bases.  
      The foregoing and other features and advantages of the systems and methods for air sampling provided by various embodiments of the present invention will be apparent from the following more particular description of preferred embodiments as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.  
       FIG. 1  illustrates a schematic representation of a system for collecting and analyzing contaminants consistent with embodiments and aspects of the invention;  
       FIG. 2  contains a flow chart illustrating a method for using a dry sampler to measure contaminants;  
       FIGS. 3A and 3B  illustrate perspective views of the upper and side portion of a sampling system and or the lower portion of a sampling system, respectively;  
       FIG. 4  illustrates a side view of a dry sampler in accordance with an embodiment of the invention;  
       FIG. 5A  illustrates a schematic representation of the gas sample processing path for a dry sampler;  
       FIGS. 5B-5D  illustrate a perspective view, a top view, a cut-away view and a side view, respectively, of an exemplary entry manifold in accordance with a preferred embodiment of the invention.  
       FIGS. 5F-5H  illustrate a perspective view, a top view and a side view, respectively, of an exemplary exit manifold in accordance with a preferred embodiment of the invention;  
       FIG. 5I  illustrates a schematic representation of electrical components associated with a preferred embodiment of the invention;  
       FIGS. 6A and 6B  illustrate exploded cross-sectional views of various embodiments of a dry sampler in accordance with the present invention;  
       FIG. 7  is a photograph of an embodiment of an active dry sampler system configuration in accordance with the present invention;  
       FIG. 8  is a photograph of another embodiment of a dry sampler system configuration in accordance with the present invention;  
       FIG. 9  illustrates a schematic view of an embodiment for monitoring an air stream using a sampler for a filter installed on a semiconductor tool, namely, a coat/develop track in accordance with the present invention;  
       FIG. 10  is a graph for determining approximate sampling times for ammonia when using embodiments of dry samplers employing a passive sampling configuration in accordance with the present invention.  
       FIG. 11  illustrates ion chromatography elution profiles for a dry sampler in accordance with the present invention;  
       FIGS. 12A-12D  compare measurements of ammonia concentration in an air stream determined using a deionized (DI) water wet impinger to those obtained using various embodiments of a dry sampler in accordance with the present invention;  
       FIGS. 13A and 13B  schematically illustrate various embodiments of an “on-site” measurement of analyte concentration obtained from a dry sample in accordance with the present invention; and  
       FIG. 14  is a fluorometer calibration curve for ammonia detection using an embodiment of a dry sample in accordance with the present invention.  
       FIG. 15  illustrates a schematic representation of a controller used in embodiments of the invention;  
       FIGS. 16A-16D  contain an exemplary flow diagram illustrating a method for using a dry sampler to acquire contaminant data and for processing the acquired data to obtain a result;  
       FIG. 17A  illustrates a schematic representation for a handheld embodiment of a gas sampling system;  
       FIG. 17B  illustrates a handheld embodiment of a gas sampling system.  
       FIG. 18A  illustrates a detector for use in monitoring contaminants in a gas supply;  
       FIGS. 18B and 18C  illustrate the beat frequency and rate of change thereof for the detector of  FIG. 18A , respectively;  
       FIGS. 19A and 19B  illustrate alternative embodiments of detectors for monitoring gas supplies;  
       FIG. 20  illustrates a flow diagram illustrating an exemplary method of using a detector to monitor contaminants in a gas supply;  
       FIG. 21  illustrates an exemplary system for monitoring contaminants. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIG. 1  contains a schematic representation of a system for measuring and analyzing contaminants using a preferred embodiment of the invention. System  10  includes a cleanroom  12  having semiconductor fabrication equipment  14  operating therein, a gas sampling unit  16 , a gas sampler analysis facility  18 , an original equipment manufacturer (OEM), a communications network  22 , and communications links  24 . Cleanroom  12  is used in the manufacture of semiconductor devices such as silicon wafers. Within cleanroom  12 , one or more pieces of fabrication equipment  14  are operating. Examples of fabrication equipment  14  are, but are not limited to, photolithography machines and chemical vapor deposition systems. Gas sampling unit  16  is placed in cleanroom  12  to sample contaminants present therein. Gas sampling unit  16  may be coupled to one or more pieces of fabrication equipment  14  using, for example, Teflon tubing, or gas sampling unit  16  may be located such that it draws ambient cleanroom air through the sampling devices located therein. Gas sampling unit  16  contains one or more internal sampling devices to sample contaminants present in cleanroom  12 . For example, gas sampling unit  16  may contain sampling devices for measuring acids, bases and organic contaminants.  
      When gas sampling unit  16  is used, it samples a gas flow within cleanroom  12  for a determined period of time. When sampling is complete, inlet and outlet valves on gas sampling unit  16  are closed by the user, or alternatively may be closed automatically, to prevent additional gas from entering the unit. Then the user returns gas sampling unit  16  to analysis facility  18 . For example, a user may return gas sampling unit  16  to analysis facility  18  using a common carrier  26  such as Federal Express™, United Parcel Service™ (UPS), or by way of a governmental postal service.  
      Upon receipt, analysis facility  18  opens gas sampling unit  16  and analyzes the quantity and/or type of contaminants present inside each sampling device. When finished, analysis facility  18  replaces the used sampling devices with new devices and re-seals the unit. Now reconditioned, the gas sampling unit  16  is returned to the user to facilitate additional sampling of cleanroom  12 . Analysis facility  18  also provides sampling results to the user and/or OEM  20 . Sampling results may be conveyed to an operator of cleanroom  12  by way of network  22  and communications links  24 . OEM  20  may provide analysis facility  18  and cleanroom  12  with minimum contaminant levels as part of a cleanroom certification program. In such an instance, analysis facility  18  can inform cleanroom  12  as to whether it met the OEM criteria. In addition, analysis facility  18  may report the results directly to the OEM  20  so that it can maintain records pertaining to the certification of cleanroom  12  as suitable for the installation and operation of fabrication equipment  14 .  
      Exemplary Method for Use  
       FIG. 2  illustrates an exemplary method for employing gas sampling unit  16  to measure contaminants. A user, or operator of cleanroom or semiconductor processing system  12 , connects gas sampling unit  16  to a gas supply within cleanroom  12  (per step  28 ). Gas sampling unit  16  is then activated for a determined time period (per step  30 ). When gas sampling unit  16  has operated for the desired time period, the user disconnects the unit and returns it to analysis facility  18  by common carrier (per step  32 ). The analysis facility  18  analyzes the contents of gas sampling unit  16  (per step  34 ) and sends the user, or customer, a report containing the results obtained from the unit (per step  36 ). In addition, the analysis facility  18  reconditions the gas sampling unit  16  so that it can be reused by the customer (per step  38 ). In order to recondition the gas sampling unit  16 , some components within the unit may have to be replaced, while other components can be reset mechanically, chemically and/or electronically without requiring replacement.  
      Exemplary Embodiment of Gas Sampling Unit  
       FIG. 3A  contains a perspective view of an embodiment of gas sampling unit  16 . Embodiments of the invention are designed to be man-portable and typically weigh less than  20  lbs. The gas sampling unit  16  of  FIG. 3A  includes a housing  50 , an inlet coupling  52 , a first bypass/purge valve  54 , a second bypass/purge valve  56 , a handle  58 , an ON/OFF switch  60 , an hour meter  62  and a fan grating  64 .  
      Housing  50  is adapted to sealably enclose internal parts and to protect them from incidental contact with foreign objects. In a preferred embodiment, housing  50  is made of aluminum; however, it can be made of, for example, plastic, composite, glass, and the like. In addition, housing  50  may be anodized or painted. Inlet coupling  52  is attached to a gas source and may consist of a NPT connector. First and second bypass/purge valves  54  and  56 , respectively, are used to allow the input gas flow to bypass sampling components in order to ensure adequate gas flow is present without risking contamination of the sampling components within the gas sampling unit  16 . Handle  58  provides a convenient and safe way for a person to transport gas sampling unit  16  without damage to the unit.  
      Embodiments of gas sampling unit  16  can be powered using external power sources such as alternating current (AC) obtained from a standard power receptacle or by way of internal power sources such as batteries. ON/OFF switch  60  is used to turn the unit on before sampling a gas flow in a cleanroom  12 , and ON/OFF switch  60  is used to turn the unit off when sampling is complete. Hour meter or clock  62  is used to indicate how long gas sampling unit  16  has been run. Hour meter  62  may serve only as an indicator or it may be configured to automatically turn off gas sampling unit  16  when a predetermined operating interval has been reached. Fan grating  64  houses the blades of an internal cooling fan (shown in  FIG. 5B ).  
       FIG. 3B  contains a perspective view of the bottom of gas sampling unit  16  showing feet  74 , a first outlet grate  70  and a second outlet grate  72 . Gas sampling unit  16  may include a plurality of feet for causing the base of the unit to stand a determined distance above a surface upon which the unit is supported, such as a floor or bench top. Feet  74  may be made of compliant material such as rubber, silicone, Teflon, and the like. First outlet grate  70  and second outlet grate  72  are designed to protect the blades of the first and second exhaust fans, respectively. If desired, first outlet grate  70  and second outlet grate  72  may be positioned above ventilated sections of cleanroom flooring for exhausting into a cleanroom exhaust system. In other implementations, gas sampling unit  16  may exhaust into an ambient environment within a cleanroom  12 .  
       FIG. 4  contains a side view of gas sampling unit  16  showing, among other things, entry manifold  80 , a first dry trap  82  and a second dry trap  84  that both collect bases such as ammonia, a first wet impinger  86  and a second wet impinger  88 , exit manifold  100 , pressure regulator  102  and vacuum pump  112 . Embodiments of gas sampling unit  16  may contain only dry traps, only Tenax traps, or a combination of dry traps and Tenax traps. In addition, alternative embodiments of the invention may include one or more wet traps configured in series or in parallel along with the dry media sampling devices above. Employing wet sampling media alongside dry sampling media in gas sampling unit  16  makes possible a comparison of results obtained using wet media against results obtained using dry media. Such a comparison may be useful for calibrating new dry media sampling technologies against more readily known wet media sampling technologies.  
       FIG. 5A  contains a schematic diagram showing the components of  FIG. 4  along with connections, fittings and additional components. A gas sample is received by way of sample inlet  87 . Sample inlet  87  may be made of, for example, Perfluoroalkoxy (PFA) tubing having a diameter of approximately 0.25 inches. The gas sample then passes through PFA tubing  79  to tee connector  88  and then to the input of first bypass/purge valve  54 . The first output of first bypass/purge valve  54  is coupled to a pre-purge bypass line  81  which runs to a first output of second bypass/purge valve  56 . The pre-purge bypass line  81  is used to direct an incoming gas sample directly to exit manifold  100  while bypassing entry manifold  80 . The second output of first bypass/purge valve  54  is coupled to an input port  81  of entry manifold  80  by way of tubing  79  and entry manifold connector  104 . In a preferred embodiment, entry manifold connector  104  is a ⅛″ NPT connector.  
      Entry manifold  80  is preferably machined from a material that will not outgas in the presence of anticipated contaminants contained in the gas sample. In a preferred embodiment, entry manifold is machined from analytical grade Teflon. Entry manifold  80  makes the gas sample available to sampling devices such as first dry trap  82 , second dry trap  84 , first Tenax trap  90 , second Tenax trap  92 , first wet impinger  86  and second wet impinger  88 . Entry manifold  80  may also include a plug  94  for providing access to internal gas passageways disposed therein. Entry manifold  80  may be coupled to the sampling devices using manifold output connectors  86 A-E, collectively  86 , and sampling device tubing  78 . Sampling device tubing  78  may consist of PFA tubing having a diameter on the order of ⅛″.  FIG. 5B  illustrates a perspective view of an embodiment of entry manifold  80  having a width of approximately 1.5 inches and a height of substantially 1.0 inches. Entry manifold  80  has input port  81  and a plurality of output ports  85 A- 85 F.  FIGS. 5C-5E  illustrate a top view, a cut away view and a side view of entry manifold  80 , respectively. The sampling devices are further discussed in conjunction with  FIGS. 6-14 .  
      The outputs of the sampling devices are coupled to exit manifold  100  by way of sampling device tubing  78  and exit manifold connectors  96 A-E, collectively  96 . In a preferred embodiment, exit manifold connectors  96  consist of ⅛″ tube to ⅛″ NPT male connectors and are made of Nylon. Exit manifold  100  may be equipped with threads for allowing exit manifold connectors  96  to be threadably inserted thereto. Exit manifold  100  may further have a ⅛″ NPT plug.  FIG. 5F  illustrates a perspective view of exit manifold  100 . Exit manifold includes a plurality of input ports  99 A- 99 F and an exit port  101 .  FIGS. 5G and 5H  illustrate alternative views of exit manifold  100 . Exit manifold  100  is still further coupled to the second output of second bypass/purge valve  56  by way of tubing  79  and exit manifold bypass connector  106 . A pressure regulator  102  is employed to ensure that a constant pressure is present in exit manifold  1100  during operation of the gas sampling unit  16 . The pressure regulator  102  may be a passive device or may be active. An active pressure regulator employs electrical signals for making measurements and for controlling the operation of the regulator.  
      The sampling components, tubing, couplers and connectors may be grouped and located proximate to each other in a sample compartment  101  within gas sampling unit  16 . For example, in the embodiment illustrated in  FIG. 3A , sample compartment  101  is located in the upper portion of the unit  16 , while the pump compartment  103  is located in the lower portion of the unit  16 .  
      A vacuum pump  112  is used to draw a gas sample through the pre-purge bypass line or through entry manifold  80 , the sampling components (collectively  82 ,  84 ,  86 ,  88 ,  90 ,  92 ) and exit manifold  100 . Vacuum pump  112  is electrically powered by way of batteries or conventional AC power obtained from a standard wall outlet. NPT fittings may be employed for coupling tubing  79  to the input of vacuum pump  112  and for directing the exhaust from the output of vacuum pump  112 . In a preferred embodiment vacuum pump  112  operating in conjunction with pressure regulator  102  maintains a pressure of 20 inches of mercury. A charcoal back diffusion trap  108  and check valve  110  may be employed to ensure that vacuum pump  112  cannot inject a gas sample into the sampling components by way of backflow. A solid state timer  114  may be used to control operation of vacuum pump  112 . Gas sampling unit  16  may be powered by way of standard AC voltage (typically 110 v or 220 v). Furthermore, a combination fuse and ON/OFF switch  60  may be used to turn the unit on and off. In addition, gas sampling unit  16  may include an hour meter  62  for tracking sample times and total usage of the unit. Gas sampling unit  16  may further include inlet louvers  118  for allowing cooling air into the unit and gas sampling unit  16  may include an exit filter  116  for removing airborne contaminants before exhaust air is released to the exterior of the unit.  
       FIG. 51  illustrates a schematic diagram showing electrical components used with gas sampling unit  16 . A wall plug  130  is electrically coupled to a power source so that power is supplied to ON/OFF switch  60 . ON/OFF switch  60  couples the-power, with and/or without attenuation, to solid state timer  114 , hour meter  62 , vacuum pump  112  and cooling fan.  132 .  
      Exemplary Dry Trap  
      A dry sampler of the present invention can be used in several system configurations including “active sampling” and “passive sampling”. As used herein, “active sampling” refers to the use of air moving device which utilizes an external source of energy coupled to the sampling system to deliver a gas sample to a collection material of a dry sampler of the sampling system. In comparison, passive sampling uses the energy of the gas sample itself to deliver a gas sample to a collection material of a dry sampler, for example, by diffusion. A dry sampler can also be used in sampler system configurations such as those found in co-pending U.S. application Ser. No. 10/395,834, the entire contents of which is herein incorporated herein by reference.  
       FIG. 6A  illustrates an exemplary embodiment of first and second dry traps  82  and  84 , respectively. Dry trap  82 ,  84  includes a perforated end cap  136 , one or more screens  138 , one or more collection pads or media element  148  for sampling, a support structure  140 , and a retaining ring  142  (such as, for example, an O-ring) in a sampler body  144 .  
      The end cap includes several channels  146  to allow passage of an air sample to the collection  148 . This embodiment is an example of a passive sampler in which contaminants are collected in a collection media by diffusion.  
       FIG. 6B  depicts a dual passive dry sampler configuration that includes a second separate stack  164  of end cap  160 , one or more screens  158 , one or more collection pads  156 , a support structure  152 , and retaining ring  154  in the sampler body  150 . In various embodiments, for example, one stack of a dual dry sampler configuration is used to sample a volume or air stream of interest while the other stack is used to sample ambient conditions.  
      Preferably, the perforations in the end cap and opening in the one or more screens are chosen to ensure the intake rate of the species of interest is diffusion limited. In various embodiments, an end cap or end cap and screen combination can be used to provide variable restriction of air flow into the sample. For example, in one embodiment, the intake rate for ammonia is about 16.7 cm 3 /min. utilizing an end cap with a total opening cross-sectional area of about 0.785 cm 2  composed of channels about 0.6 cm long and one stainless steel mesh screen with a total opening cross-sectional area of about 0.152 cm 2  composed of channels about 0.02 cm long. As is understood by those of skill in the art, intake rate varies with diffusion constant. The mass uptake can be estimated from Fick&#39;s law using the equation,  
               Q   =       D   ⁢           ⁢   A   ⁢           ⁢   C   ⁢           ⁢   t     L       ,           (   1   )             
 
 where Q is the mass uptake, D the diffusion coefficient, C the gas concentration, t sampling time, A the cross sectional area of the diffusion channel, and L is the length of the diffusion channel. 
 
      The collection pad comprises a collection material, which is preferably a treated fibrous media. For example, for ammonia sampling, acid treated quartz fibrous media is preferred. A wide variety of acids, of suitable pKa, can be used for ammonia sampling including, but not limited to, citric acid. Preferred acids include those which have a low ammonia background (such backgrounds can arise, for example, from reactions within the sampler and/or upon subsequent treatment and analysis). Collection materials include, but are not limited to those listed in Table  1 , which include ion exchange resins, zeolite, silica gel, and treated Tenax-TA®, in addition to treated fibrous media.  
      Prior art dry traps employ binders, such as for example glycerin, as coatings on the loosely woven fibrous media used therein. These prior art devices use binders to facilitate retention of acids contained within a sampled gas supply. In contrast, preferred embodiments use a more tightly woven fibrous media without binders. Employing binders in connection with preferred embodiments actually reduces the ability of the fibrous media to retain gas borne acids because the binders coat granules disposed along the fibers rendering them less available to contaminants.  
                       TABLE 1                                      Tested “dry” collection materials                                         Ion exchange               Acid treated quartz       Feature   resin   Zeolite-13x   Silica gel   Acid treated Tenax-TA   fibrous media               Presence of liquid   No   No   No   No   No       phase       Initial background   Medium   Medium   Low-High   Medium   Low       Capturing   Yes   Yes   Yes   Yes   Yes       efficiency       Detection limit   &gt;0.1 ppb   &gt;0.1 ppb   &gt;0.1 ppb   &gt;0.1 ppb   0.1 ppb       Flow (portable   Yes   Yes   Yes   Yes   Yes (up to 3       Pump, 1 lpm)                   lpm tested)       Bacterial   Not tested   Not tested   Not tested   Not tested   Not tested       degradation       Sample recovery   ˜50-70% (multiple   ˜50% (multiple   50-60%   Inconclusive results   close to 100%       efficiency   extractions)   desorptions)   (extraction)       Storage efficiency   Not tested   Failed   Not tested   Not tested   Passed lab tests       Analytical   DI extraction + IC   Thermodesorption + IC   DI extraction + IC   Thermodesorption + IC   DI extraction + IC       procedure       Reusable   No   Yes   No   Yes   No       Simplicity   No   No   No   No   Yes       Cost   Medium   High   Medium   High   Low                  
 
      In various embodiments, zeolites can be used as a collection material, specifically zeolites with mild “acidity” can be used for ammonia “scrubbing”. Temperature-programmed desorption (TPD) is a relatively simple and reproducible method for ammonia recovery in studies of zeolites acidity (see, for example, M. Niwa, N. Kanada, M. Sawa et all ” Temperature-programmed desorption of ammonia with readsorption based on the derived theoretical equation” J. Phys. Chem  1995 ,  99 , p. 8812 - 8816 ; and T. Masuda, Y. Fujikata, H. Ikeda et all “A method for calculating the activation energy distribution for desorption of ammonia using TPD spectrum obtained under desorption control conditions” Applied Catalysis A:General 162 (1997), p.29-40); the entire contents of both of which are hereby incorporated by reference). However, the TPD method contains a complex behavior relating to the readsorption and diffusion in the zeolite framework.  
      Silica gel, an amorphous form of silica, is another example of a collection material where electrically polar active sites on the surface provide a high affinity to polar molecules like ammonia. Polar adsorbents can be used for short duration sampling of gaseous challenges at relatively low humidity, so that the adsorbent does not become saturated with water vapor before sampling is complete. Among important advantages of silica gel is: desorption of contaminants can be easily accomplished with a variety of solvents such as alcohols and water and can be even automated through application of solid phase extraction (SPE) devices available on the market place (Perkin Elmer, Dynaterm and others). Application of other non-porous, fibrous systems followed by SPE is also feasible.  
      The results shown in Table 1 were obtained from collection materials handled as follows. For the zeolite data of Table 1, zeolite 13X, (sodium form, pellets) was received from Kurt J. Lesker Inc. and then ground and sieved to obtain 30/40 fraction. The sample of zeolite was pre-treated at 600° C. in a quartz boat with a flow of filtered clean dry air (CDA) overnight and then loaded into ½″ quartz tube (1.5-2 gr) with quartz wool to support the adsorption bed. An ammonia capacity test was conducted with a known ammonia challenge (with CDA and humidified CDA as make-up gas) and total molecular base real time monitor showed about 600 L/gr breakthrough volume (minimum sample volume collected should be about 240 L to reach LDL of 0.1 ppbV by IC). A limited series of TPD tests which immediately followed after loading the adsorption bed with a known amount of ammonia (VICI calibrator with permeation device, loaded about 0.3 μg of ammonia and He as the desorption gas was used to avoid secondary reactions of ammonia at high temperature on zeolite, 500° C.—desorption temperature) showed about 50% recovery of ammonia in a series of 2-3 desorption cycles. In a series of “storage efficiency” tests when “pre-baked” zeolite bed was loaded with ammonia (about 0.3 μg) and then stored in the capped tube overnight, TPD elevated levels of ammonia-as high as about 4-6 μg were measured, indicating that the source of the contamination apparently is in the zeolite. Repetitive tests consistently showed elevated levels of ammonia.  
      For the silica gel data of Table 1, silica gel was used in the form of packed so-called ORBOT™-507 tubes (Supelco). Ammonia capacity tests showed satisfactory results. The tubes had greater than 300 L/per tube “breakthrough volume” at a flow of 1 Lpm and challenge level about 20 ppb of ammonia. The “breakthrough volume” is defined as the total gas volume delivered through the tube at which trap collection efficiency drops below 100% for a particular contaminant of interest. Measurements of the ammonia background (extraction with DI water) of each part of unexposed tubes showed elevated and inconsistent ammonia levels between lots, which ranged from about 0.8 ppb to about 2.3 ppb (when derived from 240 L sample size and 10 cc of DI water for extraction). Measurements of extraction efficiency of ORBO-tube pre-loaded with ammonia (test similar to the above) showed about 50% efficiency in single extraction cycle.  
      Other non-porous media, for example, citric acid treated quartz fibrous media (Whatman) was deployed for ammonia “scrubbing” followed by DI water extraction and gradient IC analysis. Gradient elution was used for better separation of ammonia from sodium (see, for example,  FIG. 11 ). The VICI calibrator, fitted with an ammonia permeation device, controlled ammonia gaseous challenges (known concentrations of ammonia) and clean room ambient air with 30-35% relative humidity (RH) were used for ammonia exposure.  
      For example,  FIG. 7  is a drawing of one embodiment of a dry sampler of the present invention used in an active sampling configuration  200 , where a ⅞″ scale  201  is provided for reference. The collection pads are contained in the body  202  of a ½ inch nylon Swagelock™ type fitting. In use, one of the ends of the fitting  204  is connected to the volume to be sampled and the other end to an air moving device. In an active sampling system configuration, any support structure that may be used is chosen based on an acceptable pressure drop for the sampling system.  
       FIG. 8  is a drawing of one embodiment of a dry sampler of the present invention used in a passive sampling configuration  300 , where a ⅞″ scale  301  is provided for reference. The dry sampler  302  is placed in one arm of a Swagelock™ type T-union, a line  306  to the volume to be sampled is connected to another arm, and the last arm  308  is plugged. The dry sampler  302  depicted in  FIG. 8  is a dual-type dry sampler where the upper dry sampler  310  is arranged to sample ambient air whereas the lower dry sampler (not visible within the T-union) samples gas provided by the sample line  306 .  
      In other embodiments, a dry sampler is located directly in contact with the air stream or volume to be sampled (thereby, for example, avoiding sample line contamination); using either passive diffusion or an active flow to collect the sample. For example,  FIG. 9  schematically depicts the use of one or more dry samplers  402  to sample the air at various locations (upstream  403 , inter-stack or inter-filter  405 , and downstream  407 ) relative to a filter (or filter stack)  410  for the track  414  of a semiconductor processing system. Similar arrangements can be used to sample air in or being delivered to, for example, a filter cabinet or a semiconductor processing tool.  
      For example, the dry samplers of the present invention can be used to receive samples from ambient, between filter layers, and discharge and can be located within the filter body to avoid, for example, mechanical integration problems (e.g., obstructing the adjacent installed track filter). Dry samplers are preferably shipped sealed and are preferably automatically unsealed during installation (e.g., by removal of a cover screen or some other mechanism representative of installation unseals the sampler). In various embodiments, the installation process activates an electronic timer of the dry sampler system, which will start upon installation and trigger a visual and/or auditory alarm when a pre-set time (e.g., 30 days) is reached, and/or send a signal through a network connection to trip an alarm with either the track software or the fabrication facility manufacturing software control system, and/or send an email to the person who needs to know.  
      Preferably, the dry samplers of the present invention are allowed to sample a volume or air stream for a time sufficient to detect the molecule of interest at the detection level desired. The time sufficient to detect the molecule of interest at the detection level desired depends on the intake rate of a gas sample to collection material of the dry sampler. For example, where the molecule of interest is ammonia and the intake rate is about 16.7 cm 3 /min., an intake volume of about 240 L (corresponding to an intake time of about 4 hours for active sampling at a flow rate of about 1 lpm) is preferred to achieve a LDL of 0.1 ppbV, whereas an intake volume of about 1200 L is preferred to achieve a LDL of 0.02 ppbV.  
       FIG. 10  is a log-log plot  500  for determining approximate sampling times for ammonia for various embodiments of dry samplers employing a passive sampling configuration in accordance with various embodiments of the present invention. The plot  500  assumes a collection material treated with about 1.5 milligrams of citric acid (or its weight equivalent) and an intake rate of 16.7 cm 3 /min. to the collection material. The plot  500  gives an approximate value for the sampling time (x-axis in units of days) to reach a selected LDL (y-axis in units of μg/m 3 ). The plot  500  illustrates three general regions, a region where the sampling time is insufficient to reach the LDL  502 , a region where the collection material is overloaded  504  and a region where the LDL is reached but the collection material is not overloaded  506 . It is to be realized, however, that the lower bound of the overload region  504  depended on the amount of collection material used, where an increase in the amount of collection material used increases the sampling time that can be used before overload.  
      A wide variety of techniques can be used to analyze the collection material for the amount of the molecule of interest collected. In various embodiments, the molecule of interest, for example ammonia, is desorbed (for example, by thermal desorption) or extracted (for example, using DI water) from the collection material for analysis. Preferred analysis include, but are not limited to, gas phase chromatography mass spectrometry (GCMS) for molecules of interest desorbed into the gas phase, chemiluminescence (for example, for ammonia, by catalytic conversion of ammonia into NO, followed by chemiluminescent detection of the generated NO), fluorometry (for example, by addition of a suitable dye to the molecules of interest), and ion chromatography (IC) for molecules of interest extracted into a liquid phase. For example,  FIG. 11  depicts examples of various elution profiles  600  of a collection material where the molecule of interest, ammonia, was extracted with DI water and IC was used to determine the amount of ammonia collected, and thereby the average concentration of ammonia in the volume of air sampled. Illustrated in  FIG. 11  are set of profiles obtained without gradient elution  602 , and a set obtained with gradient elution  603 , where the first peak in each set  606 ,  607 , corresponds to sodium (Na) and the second peak  608 ,  609  corresponds to ammonia.  
       FIG. 7  and  FIG. 8  show general views of dry samplers deployed in these studies, some results of which are shown in  FIGS. 12A-12D  and Tables 4-10. The active dry sampling (ADS) used a citric acid treated quartz fibrous media pad and was connected to an external hand-held pump. The passive dry sampling configuration was placed into a stainless tee and was exposed to ammonia the same way as ADS but without an external pump (see, for example,  FIG. 8 ). The uptake rate of ammonia, 16.7 cm 3 /min was calculated based on the geometry of the flow restrictor (end cap) and, the diffusion coefficient of ammonia (W. J. Massman “A review of the molecular diffusivities of H2O, CO2,CH4, CO, O3, SO2,NH3 in air and nitrogen” Atmospheric Environment, 32(1998), p.1111-1127.). Comparative studies were conducted in an attempt to find quartz fibrous media with minimal ammonia background as well as citric acid. Measurements were carried out to establish the initial ammonia background after treatment with citric acid and “storage efficiency” of acid treated pads in common nalgene bottles in the lab environment, some results of which are shown in Table 2. The data indicated that citric acid pre-treated quartz fibrous media pads have low ammonia background comparable with the DI water used for extraction, and they also may be stored at room temperature without significant deterioration over the time period studied. Similar results were obtained by placing freshly prepared acid treated quartz fibrous media pads into a nylon housing (for example,  FIG. 7 ) and storing the assembly in the lab environment followed by extraction with DI, some results of which are shown in Table 3.  
                           TABLE 2                                  Concentration   Concentration       Batch of   Time    (mg/L)-Room   (mg/L)-Storage was       pads #1   delay (days)   temp. storage.   in the fridge.               stack of pads   0     0.004   0.004       in one container   2.8   0.002   0.003           2.8   0.008   0.008           4.8   0.004   0.007           5.8   0.001   0.003           6.8   0.001   0.001               Average   Average               0.003   0.003               Batch of   Time   Room   Refrigerator       pads #2   delay (days)   Temp (mg/L)   (mg/L)               Separately in   1.8   0.002   0.001       ind. containers   2.8   0.001   0.003           3.8   0.003   0.002               Average   Average               0.003   0.002                    
     
       
         
           
               
               
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                   
               
               
                   
                 1 day 
                 5 days 
               
            
           
           
               
               
               
            
               
                 Dry impinger 
                 Concentration, mg/L 
               
               
                   
               
            
           
           
               
               
               
            
               
                 Blank 
                 0.005 
                 0.008 
               
               
                 1 
                 0.004 
                 0.003 
               
               
                 2 
                 0.004 
                 0.002 
               
               
                 3 
                 0.003 
                 0.002 
               
               
                 4 
                 0.003 
                 0.004 
               
               
                 5 
                 0.002 
                 0.004 
               
               
                 6 
                 0.002 
                 0.004 
               
               
                 7 
                 0.003 
                 0.005 
               
               
                 8 
                 0.002 
                 0.002 
               
               
                 9 
                 0.003 
                 0.003 
               
               
                 10  
                 0.003 
                 0.003 
               
               
                   
               
            
           
         
       
     
       FIGS. 12A-12D  and tables 4-10 present the results of several comparisons ammonia concentration measurements obtained using traditional DI water wet impingers to those using various embodiments of dry sampler and dry sampler system configurations of the present invention. As can be seen from the slope of the plot of ammonia concentration determined by a wet impinger technique versus that obtained using a dry sampler technique of the present invention, a high degree of proportionality is observed. In  FIGS. 12A-12D  the filled symbols represent values based on actual measurements and the solid line a linear fit to the plotted data and the axis in  FIGS. 12A-12D  are in units of parts-per-billion (ppb) by volume. Ammonia concentrations were determined by IC and for the dry samplers, ammonia was extracted from the collection material with DI water and the resultant solution subjected to IC to determine ammonia concentrations.  
      The data in  FIG. 12A  is a plot  700  comparing ammonia concentrations as determined by a wet impinger (y-axis values) and a dry sampler (x-axis values) using various embodiments of active sampling. The active dry sampling (ADS) data is an average of measurements obtained using various flow rates to the dry sampler: 1 standard liters-per-minute (1 μm) for 4 hours; 2 lpm for 2 hours; and 3 lpm for 1 hour. The first two data points  702 ,  704  represent ammonia concentration measurements of dry air from a calibrator, and the third data point  706  represents ammonia concentration measurements of ambient room air having a relative humidity (RH) of about 30-35%. The slope of the linear fit to the data in  FIG. 12A  was about 0.93. The data plotted in  FIG. 12A  is tabulated in Table 4.  
                           TABLE 4                                      Wet Impinger   Dry sampler (ADS)                         Sample size   Concentration, ppb                                 7   0.9 +− 0.1   0.8 +− 0.2       7   5.9 +− 0.2   5.3 +− 0.2       7   12.9(ambient) +/− 1.2   12.2(ambient) +/− 1.8                  
 
      The data in  FIG. 12B  is a plot  710  comparing ammonia concentrations as determined by a wet impinger (y-axis values) and a dry sampler (x-axis values) using an embodiment of passive sampling. This passive dry sampling (PDS) data was obtained using sampling times sufficient to sample at least a total sample volume of 240 L. The first two data points  712 ,  714  represent ammonia concentration measurements of dry air from a calibrator, and the third data point  716  represents ammonia concentration measurements of ambient room air having a relative humidity (RH) of about 30-35%. The slope of the linear fit to the data in  FIG. 12B  was about 0.95.  
      The data in  FIG. 12C  is a plot  720  comparing ammonia concentrations as determined by a wet impinger (y-axis values) and a dry sampler (x-axis values) using an embodiment of passive sampling. This PDS data was obtained using sampling time of 10 days. The first two data points  722 ,  724  represent ammonia concentration measurements of dry air from a calibrator, and the third data point  726  represents ammonia concentration measurements of ambient room air. The slope of the linear fit to the data in  FIG. 12C  was about 0.97.  
      The data in  FIG. 12D  is a plot  730  comparing ammonia concentrations as determined by a wet impinger (y-axis values) and a dry sampler (x-axis values) using an embodiment of passive sampling. This PDS data was obtained using sampling times sufficient to sample at least a total sample volume of 240 L. The first two sets of data points  732 ,  734  represent ammonia concentration measurements of clean dry air from a calibrator, and the third data point  736  represents ammonia concentration measurements of ambient room air having a relative humidity (RH) of about 30-35%. The slope of the linear fit to the data in  FIG. 12D  was about 1.14.  
      The results of further comparisons of ammonia concentration measurements obtained using traditional DI water wet impingers to those obtained using various embodiments of dry sampler and dry sampler system configurations of the present invention are given in Tables 5-10 below.  
               TABLE 5                          Cleanroom Ambient (RH ˜ 35%), Ammonia Concentration                             Dry Sampler,               Active Sampling           Configuration -   Wet Impinger -           IC analysis   IC analysis       N   (ppbV)   (ppbV)                                  1   12.1   13.1        2   12.7   13.7        3   14.8   14.3        4   12.8   11.0        5   11.9   10.1        6   13.4   11.5        7   9.6   8.3        8   10.5   8.2        9   13.6   15.5       10   18.6   18.6       11   12.8   12.1       12   10.7   9.9       13   12.0   10.4       14   11.1   9.0       15   11.6   9.4       16   11.2   9.4       Average   12.5   11.5       Sigma   2.1   2.9       +−%, 1 sigma   17%   25%       Bias    8%                  
 
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                   
               
               
                 About 5 ppbV Calibrator output with CDA, Ammonia Concentration 
               
            
           
           
               
               
               
            
               
                   
                 Dry Sampler, 
                   
               
               
                   
                 Active Sampling 
                 Wet Impinger - 
               
               
                   
                 Configuration - 
                 IC analysis 
               
               
                 N 
                 IC analysis (ppbV) 
                 (ppbV) 
               
               
                   
               
            
           
           
               
               
               
            
               
                 1 
                 5.2 
                 5.0 
               
               
                 2 
                 5.5 
                 5.1 
               
               
                 3 
                 5.5 
                 5.7 
               
               
                 4 
                 5.0 
                 6.4 
               
               
                 5 
                 5.1 
                 5.8 
               
               
                 6 
                 5.5 
                 6.8 
               
               
                 7 
                 5.1 
                 5.5 
               
               
                 Average 
                 5.3 
                 5.8 
               
               
                 Sigma 
                 0.2 
                 0.7 
               
               
                 +−%, 1 sigma 
                   4% 
                 11% 
               
               
                 Bias 
                 −8% 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 7 
               
             
            
               
                   
               
               
                   
               
               
                 About 1 ppbV Calibrator output with CDA, 
               
               
                 ammonia concentration 
               
            
           
           
               
               
               
            
               
                   
                 Dry Sampler, 
                   
               
               
                   
                 Active Sampling 
                 Wet Impinger - 
               
               
                 N 
                 concentrations 
                 Concentrations 
               
               
                   
               
               
                  1 
                 Ppb 
                 ppb 
               
               
                  2 
                 1.2 
                 1.1 
               
               
                  3 
                 0.9 
                 1.1 
               
               
                  4 
                 2.1 
                 1.7 
               
               
                  5 
                 1.0 
                 1.1 
               
               
                  6 
                 0.8 
                 0.7 
               
               
                  7 
                 0.8 
                 0.7 
               
               
                  8 
                 1.7 
                 1.1 
               
               
                  9 
                 0.8 
                 0.5 
               
               
                 10 
                 0.6 
                 0.4 
               
               
                 11 
                 0.7 
               
               
                 12 
                 0.8 
                 0.6 
               
               
                 13 
                 0.6 
                 0.6 
               
               
                 14 
                 0.8 
                 0.6 
               
               
                 15 
                 0.5 
                 0.5 
               
               
                 Average 
                 0.9 
                 0.8 
               
               
                 Sigma 
                 0.4 
                 0.4 
               
               
                 +−%, 1 sigma 
                 47% 
                 0.5 
               
               
                 Bias 
                 18% 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 8 
               
             
            
               
                   
               
               
                   
               
               
                 Clean room ambient RH ˜35%, 21° C. 
               
               
                 Ammonia concentration 
               
            
           
           
               
               
               
            
               
                   
                 Dry Sampler - 
                   
               
               
                 Wet 
                 Ammonia 
               
               
                 Impinger 
                 concentration 
                 Sampling time 
               
               
                 (ppbV) 
                 (ppbV) 
                 days 
               
               
                   
               
            
           
           
               
               
               
            
               
                 13.1 
                   
                   
               
               
                 13.7 
               
               
                 14.3 
                 12.3 
                 6 
               
               
                 14.7 
                 13.0 
                 6 
               
               
                 10.1 
                 12.4 
                 6 
               
               
                 11.5 
               
               
                 8.3 
               
               
                 8.2 
               
               
                 11.6 
               
               
                 11.7 
                 12.6 
                 7% bias vs. wet 
               
               
                   
                   
                 impinger 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 9 
               
             
            
               
                   
               
               
                   
               
               
                 Challenge from VICI calibrator, CDA-make-up gas 
               
            
           
           
               
               
               
            
               
                   
                 Dry Sampler - 
                   
               
               
                   
                 Ammonia 
               
               
                 Wet Impinger 
                 concentration 
                 Sampling time 
               
               
                 (ppbV) 
                 (ppbV) 
                 days 
               
               
                   
               
            
           
           
               
               
               
            
               
                 8.6 
                 6.90 
                 4 
               
               
                 8.4 
                 7.10 
                 4 
               
               
                 7.8 
                 6.6 
                 4 
               
               
                 6.8 
                 7.8 
                 4 
               
               
                 7.6 
                 7.8 
                 4 
               
               
                 7.8 
               
               
                 7.8 
                 7.2 
                 −8% bias vs. wet 
               
               
                   
                   
                 impinger 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 10 
               
             
            
               
                   
               
               
                   
               
               
                 Challenge from VICI calibrator, 
               
               
                 CDA-make-up gas passive sampling configuration 
               
            
           
           
               
               
               
            
               
                   
                 Dry Sampler - 
                   
               
               
                   
                 Ammonia 
               
               
                 Wet Impinger 
                 concentration 
                 Sampling time 
               
               
                 (ppbV) 
                 (ppbV) 
                 days 
               
               
                   
               
               
                 0.7 
                 calibrator 
                   
               
               
                 1.2 
               
               
                   
                 0.9 
                 9 
               
               
                 1.1 
                 1.0 
                 9 
               
               
                 0.7 
                 1.1 
                 9 
               
               
                 0.9 
               
               
                 1.1 
               
               
                 0.5 
               
               
                 0.9 
                  1.00 
                 14% bias vs. wet 
               
               
                   
                   
                 impinger 
               
               
                   
               
            
           
         
       
     
      In various embodiments, the present invention provides methods of determining the concentration of a molecule of interest (analyte) using fluorometric analysis. In various embodiments, an analysis kit is provided including a dry sampler and a flourometer for measuring analyte concentration.  FIG. 13A  depicts one comparison of  10  IC and fluorometric analysis and  FIGS. 13A and 13B  schematically depict one embodiment of on “on-site” measurement of analyte concentration using a fluorometer  800 ,  810 . For example, one or more dry samplers  8 . 01  can be configured to sample in an active sampling configuration  802 , passive sampling configuration  803  (e.g., by diffusion), or both. After a sampling time, the collection material is prepared for analysis  804 , for example, by IC, fluorometry  805 , etc. Results of a comparison of an IC analysis  806  and a fluorometric analysis  808  of a sample are also shown in  FIG. 13A .  
      In various embodiments, molecules of interest are extracted from the collection material of the dry sampler and complexed with a suitable dye, if needed, for fluorometric analysis. This solution  812  is placed in a cuvette or sample cell  814  and irradiated with excitation light  816  from a source of excitation light  818 , which can be specific wave lengths selected by a filter  819  from a broader band of light  821 . The excitation light excites the dye molecules to fluoresce and the emitted light  822 , specific to the compound of interest, is detected by a detector  830 , such as, for example, a photomultiplier tube (PMT). Preferably, the fluorometer  810  is portable such that it can be carried or used with a cart  840 .  
       FIG. 14  presents the results of several measurements (filled symbols  902 ) of known ammonia concentrations in air (x-axis values in units of ppbV) for a 4 hour active sampling (about 1 lpm flow rate) to produce a calibration curve (polynomial fit represented by solid line  904 ) for the fluorometer. The data in  FIG. 14  is for a TD-700 Laboratory Fluorometer (Turner BioSystems, Inc. Sunnyvale, Calif. 94085) using 500 nm excitation of about 3 ml of solution to which a dye of OPA was added. The y-axis represents the resultant fluorescence intensity observed in arbitrary units.  
      One challenge in development of a sampling device and approaches in embodiments using off-site analysis is to make sure that after a field sampling session the sample will “survive” travel back to the analytical lab. The data in Tables 11 and 12, respectively, are from two types of “travel” measurements conducted on various embodiments of dry samplers of the present invention using citric acid treated quartz fibrous media as a collection material. The measurement results shown in Table 11 are for a Type A measurement comprising: initial pre-loading of ammonia in the lab, “travel” and comparison of the results (this type of measurement can eliminate site specific issues AMC vs. airborne PM). The measurement results shown in Table 12 are for a Type-B test comprising: simultaneous sampling with wet impinger and dry sampler.  
                                   TABLE 11                                   Pre-loaded amount                           (no “travel”)   After “travel”       Sampling        TAT,    In mg/L (10 cc   In mg/L (10 cc       device   Location   days   extraction)   extraction)   Comments                  1″ SS-“active”   Arizona (March)   5   0.002    0.006 (unloaded blank)   2 different caps used (this                   (unloaded blank   0.004 (unloaded blank)   design no longer tested).       ½″ nylon housing-“active”   Arizona (April)   6   0.095   0.092   capped       ½″ nylon housing-“active”   Arizona (May)   5   0.004   0.006   capped       ¾″ polypropylene   Arizona (May)   5   0.023   0.025   Were placed in Nalgene       passive “Ogawa”                   Bottles                  
 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 12 
               
               
                   
                   
               
               
                   
                   
               
               
                   
                   
                 Concentration 
                 Concentration 
               
               
                   
                   
                 (“dry” impinge) 
                 (“wet” impinge) 
               
               
                   
                   
                 (ppb) 
                 (ppb) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Blank 
                 0.1 
                 0.1 
               
               
                   
                 CDA pre-filter 
                 1.8 
                 2.9 
               
               
                   
                 CDA pre-filter 
                 1.8 
                 3 
               
               
                   
                 Volume collected, L 
                 480 
                 240 
               
               
                   
                   
               
            
           
         
       
     
      In the data of Table  12 , the clean dry air (CDA) unit was purged for approximately 4 hours before sampling started. The inlet line was also purged for 4 hours at approximately 3 lpm and post filtered at approximately 0.5 lpm. Approximately one foot of tubing was attached to the sample port and the teflon manifold, the end of the manifold was open with a one foot length of tubing used as a diffusion barrier. Approximately 1 lpm excess flow was used. Additionally, two dry impingers were used in this measurement. Total flow from the pre-filter port was approximately 7.5 lpm. The inlet line to the CDA unit was brown in color with the following markings: “Parker Parfiex pure air tubing, Pat6⅜″ OD 350 wp, 029317 . . . .” The pumps used for the ADS configuration for the dry sampler were 4 L pumps, the flow rate was about 2 lpm and a sampling time of about 256 minutes.  
      Exemplary Wet Impinger  
      As previously discussed, wet impingers may be employed in embodiments of gas sampling unit  16  when it is desirable to compare results obtained using dry traps to results obtained using wet media. When wet impingers are employed in gas sampling unit  16 , they may be connected in series as shown in  FIG. 5A . Wet impingers  86  and  88  contain deionized water and are used for determining nitrogen containing acidic species in a gas sample. For example, nitric acid (HNO 3 ) may be measured as NO 3   −  ion and nitrous acid (HNO 2 ) measured as NO 2   − . In particular, wet impingers  86  and  88  are used to provide a result indicative of the difference between a measured value of ionic NO x  and an actual value associated with atmospheric ionic NO x . Atmospheric NO x  represents the total of ionic and non-ionic NO x  present in the air within a cleanroom, for example. In contrast, atmospheric ionic NO x  (NO x   − ) is the total of nitric acid (HNO 3 ) measured as the NO 3   −  ion and nitrous acid (HNO 2 ) measured as NO 2   − . Virtual NO x   −  represents the small fraction of non-ionic NO x  solubilized by the series wet impingers  86  and  88  as ionic, respectively. Virtual NO x   −  is a generated result caused by the interaction between atmospheric (non-ionic) NO x  and water in the impingers. Virtual ionic NO x  is formed by way of impingers  86  and  88  as follows: 
 
NO 2 (gas=g)→NO 2 (water=w)   (1) 
 
NO(g)→NO(w)   (2) 
 
NO 2 ( w )+NO 2 ( w )→←N 2 O 4 ( w )   (3) 
 
N 2 O 4 (W)+H 2 O→HNO 2 +HNO 3 ( w )   (4) 
 
NO 2 ( w )+NO( w )→←N 2 O 3    (5) 
 
N 2 O 3 ( w )+H 2 O→2HNO 2 ( w )   (6) 
 
 Note that in Equation 4 the acids dissociate as follows: 
 
HNO 2 →H + +NO − 
 
HNO 3 →H + +NO −   3  
 
 Thus, in passing through first wet impinger  86 , a small fraction of atmospheric (non-ionic) NO x  will be converted to virtual ionic NO x  therein. The gas sample leaving first wet impinger  86  will contain almost the same amount and composition of atmospheric (non-ionic) NO x  as the gas sample which entered it. Second wet impinger  88  will contain substantially the same amount of virtual NO x  (i.e. the amount of ionic NO x  that was generated from conversion of non-ionic NO x  in the impinger ) as was measured in first wet impinger  86 . In contrast, the actual amounts of atmospheric (ionic) NO x  will be effectively (i.e. substantially 99%) retained by the first wet impinger  86 . Subtracting the measured amount of virtual NO x  from the amount of ionic NO x  retained in first impinger  86  produces a result indicative of the amount of atmospheric (ionic) NO x  that was present in the sampled gas volume. 
 
 Exemplary Tenax Trap 
 
      First Tenax trap  90  and second Tenax trap  92  are used to retain non-acids and non-bases which typically consist of condensables, organic compounds, and refractory compounds. Refractory compounds are typically a non-volatile residue left from a photochemical reaction. Tenax traps  90 ,  92  may be custom fabricated for use in gas sampling unit  16  or they may be purchased as an off-the-shelf item. By way of example, an embodiment of gas sampling unit  16  employs Perkin Elmer Supelco Tenax traps.  
      Exemplary Controller Embodiments of gas sampling unit  16  may further include a controller for operating components such as solid state timer  114 , vacuum pump  112 , pressure regulator  102 , computer controllable entry and exhaust manifolds, computer controllable pre-purge bypass valves, data displays, network interfaces, and the like.  FIG. 15  illustrates an embodiment of a controller  920  in the form of a general-purpose computer that executes machine-readable instructions, or function-executable code, for performing control of gas sampling unit  16 . The exemplary computer  920  includes a processor  902 , main memory  905 , read only memory (ROM)  906 , storage device  908 , bus  910 , display  912 , keyboard  914 , cursor control  916 , and communication interface  918 .  
      The processor  902  may be any type of conventional processing device that interprets and executes instructions. Main memory  905  may be a random access memory (RAM) or a similar dynamic storage device. Main memory  905  stores information and instructions to be executed by processor  902 . Main memory  905  may also be used for storing temporary variables or other intermediate information during execution of instructions by processor  902 . ROM  906  stores static information and instructions for processor  902 . It will be appreciated that ROM  906  may be replaced with some other type of static storage device. The data storage device  908  may include any type of magnetic or optical media and its corresponding interfaces and operational hardware. Data storage device  908  stores information and instructions for use by processor  902 . Bus  910  includes a set of hardware lines (conductors, optical fibers, or the like) that allow for data transfer among the components of computer  920 .  
      The display device  912  may be a cathode ray tube (CRT), liquid crystal display (LCD) or the like, for displaying information to a user. The keyboard  914  and cursor control  916  allow the user to interact with the computer  920 . In alternative embodiments, the keyboard  914  may be replaced with a touch pad having function specific keys. The cursor control  916  may be, for example, a mouse. In an alternative configuration, the keyboard  914  and cursor control  916  can be replaced with a microphone and voice recognition means to enable the user to interact with the computer  920 .  
      Communication interface  918  enables the computer  920  to communicate with other devices/systems via any communications medium. For example, communication interface  918  may be a modem, an Ethernet interface to a LAN (wired or wireless), or a printer interface. Alternatively, communication interface  918  can be any other interface that enables communication between the computer  920  and other devices or systems.  
      By way of example, a computer  920  consistent with the present invention provides a gas sampling unit  16  with the ability to communicate over network  22  while operating in a cleanroom  12 . Alternatively, network  22  may convey signals to gas sampling unit  16  for remotely turning the unit on at a determined time and for remotely turning the unit off when a determined sampling interval has been concluded. In addition, computer  920  may be used to calibrate components within dry sampler  16 . The computer  920  performs operations necessary to complete desired actions in response to processor  902  executing sequences of instructions contained in, for example, memory  905 . Such instructions may be read into memory  905  from another computer-readable medium, such as a data storage device  908 , or from another device via communication interface  918 . Execution of the sequences of instructions contained in memory  905  causes processor  902  to perform a method for controlling gas sampling unit  16 . Alternatively, hard-wired circuitry may be used in place of or in combination with software instructions to implement the present invention. Thus, the present invention is not limited to any specific combination of hardware circuitry and software.  
      Exemplary Method for Using Gas Sampling Unit  
       FIGS. 16A-16C  contain flowcharts illustrating an exemplary method for using gas sampling unit  16  for measuring contaminants present in a cleanroom  12 . In  FIG. 16A , the method begins when analysis facility  18  initializes gas sampling unit  16  for making contamination measurements (per step  930 ). Next, a pre-shipment checkout/test of gas sampling unit  16  may be performed by analysis facility  18  (per step  932 ). The gas sampling unit  16  is shipped to a customer site via common carrier (per step  934 ). Upon receipt, the customer removes gas sampling unit  16  from a reusable shipping container used by analysis facility  18  (per step  936 ). The customer places gas sampling unit  16  in a cleanroom environment (per step  938 ) and connects the unit to a gas supply (per step  940 ). Embodiments of sampling unit  16  are designed to be used by relatively unskilled workers and therefore do not require specialized training procedures. Next, the customer places first and second bypass purge valves  54 ,  56  in bypass position (per step  942 ) to purge gas sampling unit  16  using the cleanroom  12  gas supply line (per step  944 ). The customer then places first and second bypass purge valves  54 , 56  into run position (per step  946 ) and allows the gas sampling unit  16  to operate for a determined sampling interval (per step  948 ,  FIG. 16B ). Gas sampling unit  16  may be operated anywhere from a few hours to several weeks depending on the types of contaminant measurements being made.  
      Gas sampling unit  16  receives a gas sample (per step  950 ) and passes the sample through dry traps  82 , 84 , Tenax traps  90 , 92  and wet traps  86 , 88  (per step  952 ). Solid state timer  114  indicates to the customer that the end of the sampling interval has been reached (per step  954 ). The customer turns off gas sampling unit  16  in response to the readout of solid state timer  114 ; or alternatively, gas sampling unit  16  may automatically turn off when the end of the desired sampling interval is reached (per step  956 ). The customer disconnects gas sampling unit  16  from the gas supply and places the unit into the reusable shipping container (per step  958 ). The customer places a pre-printed return shipping label on the container and ships the unit back to analysis facility  18  (per step  960 ).  
      Analysis facility  18  receives gas sampling unit  16  and logs the unit&#39;s arrival into its inventory management system (per step  962 ). The dry sampler is opened by a technician and the contents of the sampling components is analyzed using methods known in the art (per step  964 ,  FIG. 16C ). The analysis facility  18  generates a report containing results of measurements taken by gas sampling unit  16  while installed at the customer site (per step  966 ). The report is sent to the customer via hardcopy and/or in electronic format such as email (per step  968 ). In addition, analysis facility  18  may send a copy of the results to a third party such as a certification or standards setting organization or government entity having some type of oversight authority for the customer site (per step  970 ). An alternative embodiment of analysis facility  18  can maintain a web site containing the results of sampling data taken by gas sampling unit  16 . Secure access by way of passwords or other security means known in the art may be used to prevent unauthorized access to data on the web site.  
      The analysis facility reconditions gas sampling unit  16  so that it can be reused by a customer for taking additional contamination measurements (per step  972 ). The analysis facility returns the dry sampler to the customer (per step  974 ) and the customer uses gas sampling unit  16  to perform additional contamination measurements (per step  976 ).  
       FIG. 16D  illustrates an exemplary method for measuring the level of ionic NO x  in a gas sample using an embodiment. A gas sample containing atmospheric NO x  is directed through a first wet impinger (step  980 ). By way of example, if the gases are thought of as balls, there are 100 balls of NO x , 1 ball of NO 3  and 1 ball of NO 2  entering first wet impinger. The first wet impinger retains all ionic NO x  and retains only a small percentage of atmospheric NO x  (step  982 ). Using the ball example, the first impinger will retain 1 NO 3 -ball and 1 NO 2 -ball and will only retain 1 NO x  ball. The gas is then directed from first wet impinger through second wet impinger which retains the same small percentage of atmospheric NO x  as was retained by the first wet impinger (step  984 ). For example 1 NO x  ball would be retained. The retained NO x  in the second impinger is subtracted from the retained NO x  in the first impinger to get the amount of ionic NO x  in the gas flow (step  986 ).  
      Embodiments of the gas sampling unit may take many forms. For example, a first alternative embodiment of a gas sampling unit may take the form of a handheld gas sampling unit  1000 .  FIG. 17A  illustrates a schematic representation of components that can be used in a handheld unit  1000 . For example, handheld unit  1000  may include a sample inlet  1002 , an overpressure/overflow control  1004 , one or more user operated valves  1006 , an entry manifold  1008 , one or more dry traps  1110 , an exit manifold  1112 , electrical supply and control components  1114 , a battery  1116  and an optional AC power source  1118 .  
      Sample inlet  1002  can include a coupling capable of allowing a gas sample to pass therethrough. For example, sample inlet  1002  may include an NPT connector adapted to mateably receive a gas supply line. An optional overpressure/overflow control  1004  may be inserted between sample inlet  1002  and user operated valves  1006  to prevent an overpressure or overflow condition within handheld unit  1000 . User operated valves  1006  can be operated to allow passage of a gas sample into entry manifold  1008 , or the valves  1006  can be operated to prevent passage of the gas sample. In addition, valves  1006  can be configured to cause a gas sample to flow directly to exit manifold  1112  via bypass channel  1005  without passing through entry manifold  1008  and or dry traps  1110 A-F. Bypassing entry manifold  1008  is useful for purging the gas sample line prior to passing a gas sample through dry traps  1110 A-F. In a preferred embodiment of handheld unit  1000 , valves  1006  are operated by way of controller  1115  operating within electrical supply and control components  1114 .  
      Entry manifold  1008  receives the gas sample and distributes it to dry traps  1110 A-F. The embodiment illustrated in  FIG. 17A  includes six dry traps; however, handheld unit  1000  may operate with substantially any number of dry traps depending on the type and concentration of contaminants being measured. Exit manifold  1112  receives the gas sample that was distributed by entry manifold  1008  and generates a single output gas stream. The output gas stream can be coupled to a sample outlet connector located on the exterior of handheld unit  1000 . The sample outlet connector facilitates gas passage through handheld unit  1000  without allowing the gas sample to be exhausted into the ambient environment in which handheld unit  1000  is operating.  
      Handheld unit  1000  may also include an electrical supply and control components subsystem  1114 . The electrical subsystem can include a controller  1115 , power regulation and distribution modules, alarm indicators, error sensors, cooling fans, and the like.  
      For example, in a preferred embodiment of handheld unit  1000 , electrical subsystem  114  measures sample times, measures and controls flow rates, measures gas pressures, temperatures, and humidity levels, controls valves  1006 , controls entry manifold  1008 , controls exit manifold  1112 , logs performance data, receives and logs user input data such as date, time, sampling location, and the operator&#39;s name. Using electrical subsystem  1114 , handheld unit  1000  can be configured such that a user presses a single button to make measurements after connecting a gas line to the input of handheld unit  1000 . The controller  1115  operating within handheld unit  1000  can facilitate preprogrammed operation by a user. For example, controller  1115  can be programmed to allow a gas sample to pass through only a subset of dry traps  1110 A-F.  
      Handheld unit  1000  may further include an internal power source such as battery  1116 . Battery  1116  may consist of one or more replaceable batteries such as disposable alkaline batteries or it may consist of a rechargeable battery such as a lithium ion, nickel metal hydride, and the like. Handheld unit  1000  may also include a connector for coupling an external power source thereto such as, for example, an AC power source.  
       FIG. 17B  illustrates an exemplary embodiment of a handheld unit  1000  that includes some, or all, of the components illustrated in  FIG. 17A . Handheld unit  1000  includes a case  1120  having an upper surface  1122  and a lower surface  1124  opposedly mounted from the upper surface  1122 , a first side  1126  and a second side  1128  opposedly mounted from the first side  1126 , a third side  1130  and a fourth side  1132  opposedly mounted from the third side  1130 . Upper surface  1122  may include an On/OFF switch  1134  for powering handheld unit  1000  on and for powering the unit off after a sampling interval is completed. An ON LED  1136  and OFF LED  1138  may be used for informing a user about the status of handheld unit  1000 . A valve control button  1140  may be located on upper surface  1122  for letting a user place the handheld unit  1000  in a run mode or in a bypass/purge mode. Handheld unit  1000  can also include one or more displays for providing operational information to a user. For example, handheld unit  1000  can have a timer display  1142  for displaying a running time of the unit. A diagnostic display  1144  may provide information as to the unit&#39;s operational status such as flow rates, gas sample temperature, valve status, and the like. Handheld unit  1000  also includes a gas sample inlet port  1148  and a gas sample exhaust port  1150 .  
      Embodiments of handheld unit  1000  can also utilize a replaceable dry trap module  1154 . Replaceable module  1154  can be inserted into housing  1120  by way of a dry trap receptacle  1152 . Receptacle  1152  includes an opening, for example in first side  1126 , into which the replaceable module  1154  is inserted. Once inserted into housing  1120 , dry traps contained in replaceable module  1154  are positioned so that a gas sample passes therethrough to measure contaminants in the gas sample. A controller  1115  can be adapted to read measurement data from the dry traps for reporting to a user using results display  1146 .  
      While embodiments of the sampling system described thus far employ a combination of dry traps, Tenax traps and wet impingers, other types of detectors can be employed. For example, embodiments can utilize sensors that change their output response when contaminants are detected. An example of a sensor having this type of characteristic is a surface acoustic wave (SAW) sensor. Sensors such as these can have surface coated materials that facilitate the retention of contaminants thereon. Contaminants retained on the detection surface of a sensor preferably form non-volatile residues which can subsequently be detected. Buildup of non-volatile residues on the detection surface may be representative of the concentration of molecular contamination in the airstream in the vicinity of the detection surface. These sensors can be used at input, or upstream, sampling locations, at output, or downstream, sampling locations, or at mid-point, or inter-stack, sampling locations with respect to filter systems operating in a cleanroom.  
      The actual formation rate of non-volatile residue onto the detection surfaces of an upstream and downstream detector not only depends on the varying molecular contamination concentrations in the respective airstreams, but also depends on other factors such as temperature, humidity, and the amount of material previously formed onto the detection surfaces; all of which change over time, creating artifacts in the measured signals. For example, the rate non-volatile residue forms onto the upstream detection surface exposed to upstream air having a constant concentration of molecular contamination may change significantly over time. For example, a significant drop in the formation rate may result from a change in temperature or humidity. Also, a generally decreasing artifact in the formation rate may be observed due to changes in the detection surfaces over time caused by previously formed material.  
      Similar trends may be observed in the rate of non-volatile residue formation on a downstream, or post-filtering, detection surface. However, a generally decreasing artifact, similar to an artifact accumulating on an upstream detection surface typically appears at a later time in the measured formation rate on the downstream detection surface because the rate of material formation on the downstream detection surface is less than the formation rate on the upstream detection surface as long as a filter system is operating. Also, superimposed onto these artifacts for the downstream detector are changes in the concentration of molecular contamination in the airstream resulting from changes in filter efficiency.  
      Referring to  FIG. 1   8 A, a detector  1200 , which may be used as upstream detector, mid-stack detector, or downstream detector, includes a detection surface  1202 , which is exposed to an incoming air stream  1204  including molecular contamination  1206 . In a presently preferred embodiment, detection surface  1202  is formed from a piezoelectric crystal  1208  and is configured as a mass microbalance resonator sensor, as described in W. D. Bowers et al., “A 200 MHz surface acoustic wave resonator mass microbalance,” Rev. Sci. Instrum., Vol. 62, June 1991, which is herein incorporated by reference. The frequency of vibration is related to the way the crystal is cut and to the amount of mass formed on detection surface  1202 . Leads  1210 ,  1212  are used to apply time-varying electrical signals to piezoelectric crystal  1208 . Leads  1210 ,  1212  are also used to detect a shift in the resonant frequency of the detector. Alternatively, detector  1200  may be configured as a delay line, as described in H. Wohltjen et al., “Surface Acoustic Wave Probe for Chemical Analysis,” Analytical Chemistry, Vol. 51, No. 9, pp. 1458-1475 (August 1979).  
      Even when detection surface  1202  is exposed to an air stream having a constant concentration of molecular contamination, the measured rate of change of formed non-volatile residue will vary depending on environmental conditions, e.g., temperature and humidity. The amount of deposited (formed) material will also depend on other parameters, e.g., the amount of material previously deposited onto the surface. The detectivity D(t) of a detection surface may depend on 
 
D( t )=K 1 .S(T, RH, R, A( t ))   (1) 
 
 where, K 1  is a constant, S(T, RH, R, A(t)) is a “sticking coefficient” that depends on (among other things) the temperature (T), the relative humidity (RH), the reactivity (R) of the surface with the molecular contamination, and A(t) which is the effective surface area of the detection surface which decreases over time (t). The sticking coefficient (S) represents a probability that molecular contamination in the vicinity of the detection surface will condense from the gas-phase and adhere onto the detection surface. 
 
      Molecular contamination  1206  may simply condense from the gas-phase onto the surface of detector  1200  to form a non-volatile residue  1214 . Also, selective adsorption of a particular class of molecular contamination may be achieved, e.g., by applying to the detection surface a thin film (coating) of a selectively adsorbing material. Thus, when exposed to molecular contamination, some of the molecular contamination may be adsorbed on the detection surface of the detector as a non-volatile reaction product residue  1216 . The deposited non-volatile residue  1214  and non-volatile reaction product residue  1216  increase the mass on the detection surface that is measured as a resonant frequency shift.  
      The decrease in frequency following an increase in mass (Am) of an oscillating crystal, is given by:  
               Δ   ⁢           ⁢   f     =         2.3   ×     10     -   6         A     ·     Fo   2     ·   Am             (   2   )             
 
 where Δf is the change in frequency, F o  is the fundamental frequency applied to the crystal, Δm is the change in mass of the detection surface caused by formed non-volatile residue, and A is the area of the detection surface (typically on the order of 1 cm 2 ). This can be summarized by 
 
Δf==K.Am   (3) 
 
 where K is a constant (K=2.3×10 −6 F o   2 /A). The resonant frequency as a function of time (f(t)) can be expressed as 
 
 f ( t )=F o −K.M( t )   (4) 
 
      Accordingly, the variation in the mass over time (M(t)) can be expressed in terms of the resonant frequency  
               M   ⁡     (   t   )       =       fo   -     f   ⁡     (   t   )         K             (   5   )             
 
      Therefore, under ideal conditions changes in frequency are proportional to the change in mass of the detection surface. Such piezoelectric sensors are useful, at least in part, because of their small size and low detection limits (the lowest detection level is generally in the ppb region, with linearity to ppm). Low-level detection sensitivity is especially important in an environment in which low levels of molecular contamination are severely detrimental (e.g., in a semiconductor device fabrication area or a work environment that may be subject to gas-phase contaminants that are corrosive or toxic at low concentration levels). Such detectors should typically be replaced after a period of about six months to one year, depending on the molecular contamination concentration exposure level.  
      Referring to  FIGS. 18B and 18C , the measured beat frequency of detector  1200  changes over time as non-volatile residues form on surface  1202 . The rate of change in the beat frequency provides a measure of the rate of change in the formed mass over time (Δm).  
      An effective efficiency (E eff (t)) may be determined by measuring the resonant frequency f(t) in  FIG. 18A  to determine the amount of non-volatile residue that has  10  formed on a detection surface and by using equation (6), below.  
                   E   eff     ⁡     (   t   )       )     =           M   upstream     ⁡     (   t   )       -       M   downstream     ⁡     (   t   )             M   upstream     ⁡     (   t   )                 (   6   )             
 
 An alternative measure of effective efficiency E′ eff (t)) may be determined from the measured rate of change of the resonant frequency (Δf;  FIG. 18B ) and by using equation (7), below.  
                   E   eff   ′     ⁡     (   t   )       )     =         Δ   ⁢           ⁢       M   upstream     ⁡     (   t   )         -     Δ   ⁢           ⁢       M   downstream     ⁡     (   t   )             Δ   ⁢           ⁢       M   upstream     ⁡     (   t   )                   (   7   )             
 
      In all of the following detectors a membrane is used to control exposure of the collecting media to the molecular contamination and makes possible quantitative measurements. The collecting media (e.g., activated carbon, reagent solution, or water) varies, depending on the kind of molecular contamination that is to be monitored.  
      For monitoring volatile organic contaminants, such as toluene, benzene, and vapors of other low boiling point solvents, a detector  1200  (e.g., an Organic Vapor Monitor available from 3M Company of St. Paul, Minn., under the Brand Nos. 3500, 3510, 3520, and 3530, and described in H. C. Shields et al., “Analysis of ambient concentrations of organic vapors with a passive sampler,” APCA Journal, Vol. 37, No. 9, (September 1987), wherein is herein incorporated by reference), may be used as upstream detector or downstream detector.  
      Referring to  FIG. 19A , detector  1218  includes a detection surface  1220 , which is exposed to an incoming air stream  2004  including molecular contamination  1224  through a housing  1226  with a perforated face  1228  and through a diffusion barrier  1230  (e.g., a precalibrated semi-permeable membrane). Detector  1218  also includes a spacer  1232 , and a charcoal sorbent pad  1234 . The diffusion barrier creates a concentration gradient from its surface to the carbon sorbent pad.  
      In use, detectors are respectively positioned upstream or downstream of a filter to be monitor and left in place for a preselected period of time (t). After the preselected period, the detectors are sealed and typically taken to a lab for extraction of the adsorbed species. The pad is immersed in a solvent containing, for example, 1 μL of a 1.0 mg/mL cyclooctane/carbon disulfide solution. After a preselected period, the extract is decanted into a vial and reduced at ambient temperature and pressure in a low velocity fume hood. the final volume typically ranges from 0.5 mL to 51 μL. Sample volumes of 1-3 μL are injected into a gas chromatograph/mass spectrometer (e.g., a Hewlett-Packard 5992A GC/MS), which separates and identifies the adsorbed species. The identity of the molecular contamination and the collected masses of the respective components are used to calculate the concentration of the molecular contamination.  
      Molecular contamination  1224  contacts the detection surface of monitor  1218  by diffusion. At the surface of the screen the molecular contamination concentration is the air concentration (C) and at the sorbent pad the concentration is effectively zero. From Frick&#39;s First Law of Diffusion, it can be determined that  
             C   =     m     t   ·   u   ·   r               (   8   )             
 
 where C is the molecular contamination concentration, m is the mass of substance adsorbed onto the sorbent pad, t is the sampling interval, u is the uptake rate, and r is the recovery coefficient (a factor used to adjust for incomplete extraction of a substance from the sorbent pad). The uptake rate (u) and the recovery rate (r) have been measured and published for a large number of organic vapors (e.g., 3M #3500 Organic Vapor Monitor Sampling Guide (Occupational Health and Safety Products Division/3M; December 1992) and 3M #3500 Organic Vapor Monitor Analysis Guide (Occupational Health and Safety Products Division/3M; 1981), both of which are herein incorporated by reference). 
 
      A filter monitor useful for monitoring formaldehyde has a similar construction as detector  1218 , except the adsorbent material is coated with a solution reactive with formaldehyde (e.g., an organic passive monitor available from Advanced Chemical Sensors Co. 4901 North Dixie Hwy. Boca Raton, Fla. 33431). Formaldehyde contamination in an air stream passes through a diffusion barrier and forms a non-volatile residue on the adsorbent material. The mass of formaldehyde formed on the adsorbent material may then be measured after a preselected exposure period in a manner similar to that described above in connection with the 3M filter monitor.  
      Referring to  FIG. 19B , a detector  1236 , which may be used as an upstream detector, mid-stack detector, or downstream detector  1240 , includes a detection surface  1238 , which is exposed to an incoming air stream  1240  including molecular contamination  1242 . Detector  1236  includes a housing  1244  containing adsorbent media  1246  (e.g., activated carbon particles with or without a reagent) and a diffusion barrier  1248  that creates a diffusion gradient between the air stream and the adsorbent media. The adsorbent media is the same as that used in the gas-phase filter to be monitored. In this way detector  1236  adsorbs the same gas-phase contamination as the filter with a similar sensitivity. This provides a highly accurate determination of the filter&#39;s performance. The adsorbed contamination is extracted in the same way as described above in connection with other adsorbent detectors. Embodiments of detectors for performing performance monitoring of air filters are further described in U.S. Pat. No. 5,856,198 entitled Performance Monitoring of Gas Phase Air Filters, the contents of which are herein incorporated by reference.  
      Referring to  FIG. 20 , in a presently preferred embodiment, the performance of a gas-phase filter is monitored as follows. The amount of non-volatile residue formed on the upstream detection surface is determined (per step  1250 ). The amount of non-volatile reside formed on the downstream detection surface is determined (per step  1252 ). The effective efficiency is determined based on a comparison of the amount of non-volatile residue formed on the upstream and downstream detection surfaces (per step  1254 ). The determined effective efficiency is compared against a predetermined threshold (per step  1256 ). If the effective efficiency is greater than the threshold (per step  1258 ), a flag variable is set to 0 (per step  1260 ) and the monitoring process is repeated. If the effective efficiency is less than the threshold and the flag variable is currently not equal to 1 (per step  1262 ), the flag is set to one and the monitoring process is repeated (per step  1264 ). If, on the other hand, the effective efficiency is less than the threshold the flag variable is equal to 1 (per step  1262 ), then a signal is produced indicating that the filter should be replaced (per step  1266 ).  
      In an alternative embodiment, the flag variable may be compared against an integer greater than 1 (per step  1262 ) and the flag variable may be increased incrementally (per step  1264 ) to enhance the accuracy of verification procedure (per steps  1258 - 1264 ) before the filter replacement signal is produced (per step  1266 ).  
       FIG. 21  illustrates a schematic representation of a system  1300  for monitoring contaminants in a reactive gas using, among other things, a surface acoustic wave (SAW) detector. System  1300  includes an inlet  1304 , a chamber  1306  containing a media  1312 , an outlet  1308 , an inlet sample port  1314 , a mid-stack sample port  1316 , an outlet sample port  1318 , a sample manifold  1320 , a SAW detector  1322 , a detector manifold  1326 , a detector control line  1324 , a dry sampler  1330 , a dry sampler input line  1328 , a dry sampler output line  1332 , an analyzer  1336 , an analyzer input line  1334  and an analyzer output line  1338 .  
      An input gas sample  1302  passes through inlet  1304  and into chamber  2606 . Contaminants present in gas sample  1302  are removed using media  1312  to produce an outlet gas sample  1310 . A sample manifold  1320  may route a portion of a gas to SAW detector  1322  using inlet sample port  1314 , mid-stack sample port  1316  or outlet sample port  1318 . SAW detector  1322  may accumulate contaminants on a surface of the detector. As contaminants build up, an output signal associated with SAW detector  1322  will change. When contaminant levels on the surface of SAW detector  1322  reach or exceed a determined threshold, detector manifold  1326  may be activated, or controlled, by detector control line  1324 .  
      When activated, detector manifold  1326  may allow a gas sample to pass through analyzer line  1334  before entering analyzer  1336 . Analyzer  1336  may be a gas chromatograph or other analysis tool capable of determining a contaminant present in the gas sample. Analyzer  1336  may have an output line  1338  for exhausting the gas sample.  
      Detector manifold  1326  may route the gas sample through dry sampler input line  1328  to dry sampler  1330 . Dry sampler  1330  may include any combination of dry traps, Tenax traps, wet impingers, and/or SAW detectors  1322  for collecting and measuring contaminants in the gas sample. Dry sampler  1330  may include an outlet  1332  for exhausting a gas sampler after passing through dry sampler  1330 .  
      The detector can be used to initiate sampler system operation, or alternatively can be used to terminate sampler operation and indicate to the user that the sampler contents are ready to be analyzed.  
      In view of the wide variety of embodiments to which the principles of the present invention can be applied, it should be understood that the illustrated embodiments are exemplary only, and should not be taken as limiting the scope of the present invention. The claims should not be read as limited to the described order or elements unless stated to that effect. Therefore, all embodiments that come within the scope and spirit of the following claims and equivalents thereto are claimed as the invention.