WO2018193444A1

WO2018193444A1 - Combined volumetric - gravimetric system and method for preparation of gas mixtures

Info

Publication number: WO2018193444A1
Application number: PCT/IL2018/050424
Authority: WO
Inventors: Aharon BRAYER
Original assignee: Maxima Air Separation Center Ltd.
Priority date: 2017-04-16
Filing date: 2018-04-16
Publication date: 2018-10-25
Also published as: IL251746A0

Abstract

A system and method for preparation of gas mixtures that comprise a carrier gas and at least one other component is disclosed. The amount of the carrier gas is determined gravimetrically, while the amounts of the other components are determined volumetrically. The dilute component is introduced into a small container of known volume at a known pressure and temperature, and then transferred to the final container, while the carrier gas is introduced directly into the final container, where it is weighed. The system and method allows accurate and precise preparation of gas mixtures having concentrations down to the ppb level.

Description

COMBINED VOLUMETRIC - GRAVIMETRIC SYSTEM AND METHOD FOR

PREPARATION OF GAS MIXTURES

REFERENCE TO RELATED PUBLICATIONS

[0001] This application claims priority from Israel Pat. Appl. No. 251746, filed 16 April 2017, and which is incorporated by reference in its entirety.

FIELD OF THE INVENTION

[0002] This invention relates in general to systems and methods for preparing mixtures of gases. It relates in particular to a system and method that combines volumetric and gravimetric methods in order to produce the final gas mixture.

BACKGROUND OF THE INVENTION

[0003] There is an increasing need in modern industry, for example, in the semiconductor industry, for precise gas mixtures in which at least one component is present in a very low concentration - parts per billion (ppb) or even parts per trillion (ppt). Static techniques for preparation of such mixtures, i.e. techniques in which the concentration of the components of the mixture are determined from the amount in the container rather than from the rate of flow of the gas into the container, generally fall into three categories: gravimetric, in which the concentrations of the components are determined by weight; volumetric, in which they are determined by filling a container of fixed volume at a known pressure (generally atmospheric) and temperature; and manometric, in which the amount of each component is determined from the change in total pressure as each additional component is added to a high pressure cylinder.

[0004] Each of these methods has significant shortcomings. For example, gravimetric methods require special balances that have both high capacity and high sensitivity. Volumetric methods can suffer from errors due to introduction of small amounts of a low- concentration component into a carrier gas, and for highly dilute gases, require accurate and precise pressure measurements over a several orders of magnitude of pressure. Manometric methods suffer from the shortcoming that a constant temperature must be maintained in the cylinder as each component is added. A general review of static techniques for producing gas mixtures can be found in Naganowska-Nowak et al., Critical Rev. Anal. Chem. 2005, 35, 31.

[0005] ISO standard 6142-1 describes a gravimetric method for preparing calibration gas mixtures and for calculating the uncertainties in the concentrations of the components. While the standard does not explicitly state the minimum concentration for which the method is valid, the examples given therein imply that the method is unlikely to be useful at concentrations below the ppm level. Mixtures at lower concentrations would have to be prepared by dilution of the ppm-level standards, with the concomitant loss of precision and accuracy arising from the uncertainties added in the dilution step.

[0006] Several publications from the U.S. National Institute of Standards and Technology, for example, Rhoderick, G. C; Zielinski, W. L., Jr. Anal. Chem. 1988, 60, 2454, and Rhoderick, G. C. et al. Anal. Chem. 2014, 86, 2580, disclose gravimetric methods for producing gas mixtures at the ppb or ppt level. These methods involve introducing vapor from a known weight of a volatile liquid into a carrier gas. Since the volatile liquid is weighed separately, its concentration can be known precisely even at low concentrations. This method suffers from the disadvantage that it cannot be used to prepare gas mixtures in which one or more of the low-concentration components are gaseous at room temperature and pressure.

[0007] Thus, methods and systems for producing highly dilute gas mixtures with precise and accurate concentrations down to the ppb level in which the components can be any arbitrary combination of gases, remain a long-felt but as yet unmet need.

SUMMARY OF THE INVENTION

[0008] The invention disclosed herein is designed to meet this need. The method disclosed herein combines volumetric and gravimetric methods for preparing gas mixtures in which the amount of analyte gas is determined from its pressure in a vessel of known volume and temperature, while the amount of carrier gas is determined gravimetrically. A system for preparing gas mixtures according to this method is also disclosed.

[0009] It is therefore an object of this invention to disclose a volumetric-gravimetric method for producing a mixture of an analyte gas and a carrier gas, wherein said method comprises: (a) preparing a volume of said analyte gas at a known temperature and pressure, thereby obtaining A moles of said analyte gas; (b) weighing an amount of a carrier gas, thereby obtaining C moles of said carrier gas; and, (c) mixing said analyte gas and said carrier gas, thereby obtaining a gas mixture characterized by a mole ratio -^.

[0010] It is a further object of this invention to disclose such a method, wherein said mole ratio is less than 10^"3. In some embodiments of the method, said mole ratio is less than 10^"6. [0011] It is a further object of this invention to disclose the method as defined in any of the above, wherein said step of preparing a volume of said analyte gas comprises preparing a volume of said analyte gas at a known temperature and pressure calculated to provide a predetermined number of moles A of said analyte gas, and said step of weighing said carrier gas comprises calculating a weight of said carrier gas equivalent to a predetermined number of moles C of said carrier gas.

[0012] It is a further object of this invention to disclose the method as defined in any of the above, wherein said step of preparing a volume of said analyte gas at a known temperature and pressure comprises: (a) obtaining an analyte gas vessel of known volume; (b) determining a temperature within said analyte gas vessel; (c) calculating, from said known volume and determined temperature, a calculated gas pressure within said analyte gas vessel equivalent to a predetermined number of moles A of said analyte gas; and, (d) introducing a quantity of said analyte gas into said analyte gas vessel sufficient to raise the pressure within said analyte gas vessel by said calculated gas pressure.

[0013] It is a further object of this invention to disclose the method as defined in any of the above, wherein said step of preparing a volume of said analyte gas at a known temperature and pressure comprises: (a) obtaining an analyte gas vessel of known volume; (b) determining a temperature within said analyte gas vessel; (c) calculating, from said known volume and determined temperature, a calculated gas pressure change within said analyte gas vessel equivalent to transfer into or out of said analyte gas vessel of a predetermined number of moles A of said analyte gas; (d) introducing a quantity of said analyte gas into said analyte gas vessel; and, (e) releasing analyte gas from said analyte gas vessel until the pressure within said analyte gas vessel drops by said calculated gas pressure change.

[0014] It is a further object of this invention to disclose the method as defined in any of the above, wherein said method comprises: (a) evacuating a gas mixture vessel (320) having a known volume; (b) weighing said evacuated gas mixture vessel; (c) calculating said number of moles A of analyte gas and C of carrier necessary to provide a predetermined mole ratio at a predetermined pressure and temperature within said gas mixture vessel (320); (d) introducing at least one analyte gas from an analyte gas source (310) into at least one analyte gas vessel (100) of known volume; (e) measuring temperature and pressure of said at least one analyte gas within said at least one analyte gas vessel; (f) closing said analyte gas vessel to said source; (g) opening said analyte gas vessel to said gas mixture vessel; (h) flowing analyte gas from said analyte gas vessel into said gas mixture vessel until the pressure in said analyte gas vessel drops by an amount corresponding to delivery of A moles of analyte gas; (i) closing said analyte gas vessel to said gas mixture vessel; (j) opening said gas mixture vessel to a carrier gas source (300); and, (k) flowing carrier gas from said carrier gas source to said gas mixture vessel while monitoring its weight until a weight of carrier gas corresponding to C moles of carrier gas has been introduced into said gas mixture vessel.

[0015] It is a further object of this invention to disclose the method as defined in any of the above, wherein said method is performed on a system, and said method comprises at least one step selected from the group consisting of: (a) evacuating at least part of said system prior to said step of preparing a volume of analyte gas; (b) flushing at least part of said system with said carrier gas prior to said step of preparing a volume of analyte gas; and, (c) prior to said step of preparing a volume of analyte gas, performing at least one purge/flush/purge cycle comprising: (i) evacuating at least part of said system; (ii) flushing at least part of said system with said carrier gas; and, (iii) evacuating at least part of said system subsequent to said step of flushing.

[0016] It is a further method to disclose the method as defined in any of the above, wherein said analyte gas is gaseous at standard temperature and pressure.

[0017] It is a further object of this invention to disclose a system for volumetric-gravimetric preparation of a gas mixture comprising at least one analyte gas and at least one carrier gas, comprising: (a) an analyte gas manifold (10) comprising an input side and an output side; (b) weight measuring means (20); and, (c) a connection system (30); wherein:

[0018] said analyte gas manifold is configured to determine volumetrically an amount of said analyte gas;

[0019] said weight measuring means is configured to determine gravimetrically an amount of said carrier gas; and,

[0020] said connection system is configured to provide fluid connection: (a) between said input side of said analyte gas manifold and a source of analyte gas (310); (b) between said output side of said analyte gas manifold and a gas mixture container (320) in which said gas mixture is to be stored; and, (c) between a source of carrier gas (300) and said gas mixture container.

[0021] It is a further object of this invention to disclose such a system, wherein said analyte gas manifold comprises: (a) at least one analyte gas vessel (100) having a known volume; and, (b) pressure and temperature measuring means (110) in fluid connection with said at least one analyte gas vessel, said pressure and temperature measuring means configured to measure pressure and temperature within said at least one analyte gas vessel.

[0022] It is a further object of this invention to disclose the system as defined in any of the above, wherein said analyte gas manifold comprises at least one component selected from the group consisting of: (a) a purge line (120) configured to provide fluid connection between said analyte gas manifold and atmosphere; and, (b) a vacuum line (130) configured to provide fluid connection between said analyte gas manifold and a source of vacuum.

[0023] It is a further object of this invention to disclose the system as defined in any of the above, wherein said analyte gas manifold comprises a valve system and a tubing system configured to allow gas flow through said analyte gas manifold to bypass said at least one analyte gas vessel.

[0024] It is a further object of this invention to disclose the system as defined in any of the above, wherein said weight measuring means comprises a scale configured to weigh standard gas cylinder to within a predetermined accuracy and precision.

[0025] It is a further object of this invention to disclose the system as defined in any of the above, wherein said connection system is configured to provide fluid connection between said source of carrier gas and said input side of said analyte gas manifold.

[0026] It is a further object of this invention to disclose the method as defined in any of the above, performed on the system as defined in any of the above.

BRIEF DESCRIPTION OF THE DRAWING

[0027] The invention will now be described with reference to the drawing, wherein FIG. 1 presents a schematic diagram of a typical embodiment of the system of the invention herein disclosed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] In the following description, various aspects of the invention will be described. For the purposes of explanation, specific details are set forth in order to provide a thorough understanding of the invention. It will be apparent to one skilled in the art that there are other embodiments of the invention that differ in details without affecting the essential nature thereof. Therefore the invention is not limited by that which is illustrated in the figure and described in the specification, but only as indicated in the accompanying claims, with the proper scope determined only by the broadest interpretation of said claims. In some cases, for clarity or conciseness, individual components of the system disclosed herein, or individual steps of the process disclosed herein, are discussed separately. Nonetheless, any combination of elements (system components or method steps) disclosed herein that is not self- contradictory is considered by the inventors to be within the scope of the invention.

[0029] As used herein, the term "carrier gas" refers to the gas or mixture of gases that comprises the major fraction of the final gas mixture.

[0030] As used herein, the term "analyte gas" refers to the component or components present in low concentration in the final gas mixture. The analyte gas may comprise a single substance that is gaseous at standard temperature and pressure (25 °C and 1 atm), or a mixture of such substances. In some embodiments of the invention, the analyte gas may comprise vapors of a volatile liquid or solid, a mixture of such vapors.

[0031] As used herein, the term "gauge" is used generically to describe any apparatus that measures pressure and or temperature. Unless a specific type of pressure or temperature measurement apparatus is described, the term is used without limitation regarding the particular type of device used.

[0032] Reference is now made to FIG. 1, which presents a schematic diagram of one typical non-limiting embodiment of the system of the current invention. The system comprises an analyte gas manifold 10, weight measuring means 20, and a connection system 30 that is configured to connect the analyte gas manifold to the source of the components of the gas mixture (30A) and the container in which the gas mixture is to be stored (30B).

[0033] Analyte gas manifold 10 comprises at least one analyte gas vessel 100 having a known volume, and various gas lines and connections as described in detail below. In the embodiment shown in FIG. 1, the manifold comprises four analyte gas vessels (100A - D). In preferred embodiments in which the manifold comprises more than one analyte gas vessel, the vessels are of different volumes. As a non-limiting example, a system that comprises four analyte gas vessels might have vessels of volumes 10 cm 3 , 50 cm 3 , 300 cm 3 , and 2250 cm 3. The user of the system can thereby fill whichever vessel is most appropriate for the desired level of dilution of the analyte gas in the final gas mixture. The analyte gas vessels may be made of any material that is inert to the gases being used and sufficiently resistant to deformation that the volume will not change significantly over the pressure range being used. As a non-limiting example, the analyte gas vessels may be made of stainless steel. [0034] In some preferred embodiments of the invention, flow of gas through the analyte gas manifold is monitored and/or regulated by use of flow meters or controllers. In the embodiment of the system illustrated schematically in FIG. 1, the flow meters/controllers are marked by reference numbers of the form R nn, where "nn" represents two digits; for example, flow controllers R 52 and R 54 control the flow of gas through vessel 100D.

[0035] In some preferred embodiments of the invention, the analyte gas vessels are in thermal contact with means for regulating their temperature. Any appropriate temperature regulating means known in the art may be used.

[0036] Each analyte gas vessel is in fluid connection with means 110 for measuring the pressure and temperature within the vessel. In preferred embodiments of the invention in which the manifold comprises multiple vessels, each analyte gas vessel is connected to a separate pressure/temperature measuring means (110A - D in the embodiment illustrated in FIG. 1). Any type of pressure/temperature gauge known in the art that is appropriate for use with the gases introduced into the manifold and for the temperatures and pressures of the gas in the vessel may be used. Although in the embodiment illustrated in the figure, an integrated temperature/pressure gauge is shown, embodiments in which the temperature and pressure measurement devices are separate are considered by the inventor to be within the scope of the invention. In typical embodiments of the system, the pressure measurement device (e.g. a manometer) has an accuracy and precision of at least 1 mb, which corresponds to an uncertainly of 0.002% at typical pressures (50 bar) at which the final gas mixture is prepared.

[0037] The gas flow in the manifold is through tubes made of a material that is inert to the gases being used, which are welded together or connected using standard connectors in order to provide the fluid connections herein described. The tubes should have an inner diameter sufficient not to limit the conductance unduly. In typical embodiments of the invention, standard ¼" - ½" tubing constructed of a material that is inert to the gases in the mixture is used, and the tubing is connected either with standard tube connectors or by welding. Non- limiting examples of typical tube material include stainless steel, copper, and Monel.

[0038] In preferred embodiments of the invention, the manifold additionally comprises lines for purging and/or flushing the system. In the embodiment shown in FIG. 1, the manifold comprises purge line 120 which can be opened to atmosphere, and vacuum line 130 which is connected to a vacuum source. Any type of pump (e.g. a mechanical pump) that is capable of bringing the system to the desired level of vacuum may be used. [0039] Each vessel can be independently isolated from the rest of the system, and from the pressure/temperature gauge with which it is in fluid contact. Typically, this is done by a plurality of valves. A non-limiting example of such a valve system is illustrated schematically in FIG. 1. As an illustrative example, in the embodiment shown in FIG. 1, vessel 100D is connected on its input side to valves V51 and V17, which can open or close it to sources of analyte and carrier gases, respectively, as explained in detail below, and on its output side by valves V54, V55, and V56, which can open or close it to purge line 120, vacuum line 130, and output connection system 30B, respectively. In addition, vessel 100D can be opened or closed to pressure/temperature gauge HOD via valve V04. Analogous valve systems are constructed independently for the remaining analyte gas vessels. Any type of valve appropriate for use with the analyte and carrier gases and the pressures and pressure differences to be produced in the system may be used. In typical embodiments of the invention, needle valves are used to introduce analyte gases into the small vessels because of their ability to deliver gas at a known accurate rate of release, and ball valves in the mixing section because they open and close quickly and provide a tight seal. The use of manually operated valves, mechanically operated valves (non-limiting examples of which include electrically operated valves and electropneumatic valves), and any combination thereof is considered by the inventor to be within the scope of the invention.

[0040] In preferred embodiments of the invention, the analyte gas manifold also comprises bypass lines and valves that permit gas to enter and leave the manifold without entering the analyte gas vessel. For example, in the embodiment shown in FIG. 1, the manifold comprises a bypass line that can be opened or closed with valve V18 can direct gas to exit connection 30B without passing through vessel 100D, as well as bypass lines upstream of vessel 100D that can direct flow to the purge line via valve V52 or to the vacuum line via valve V53. In the embodiment shown in FIG. 1, the manifold additionally comprises valve V57 that allows, when valve V18 is open, a flow of gas from input connection 30A to the vacuum line, bypassing the analyte gas vessel. As shown in the figure, in embodiments that comprise more than one analyte gas vessel, the manifold includes analogous valve systems for each vessel.

[0041] The system additionally comprises weighing means 20 for weighing the gas mixture vessel. The weighing means may be a balance for weighing gas cylinders of any appropriate type known in the art. The weighing means must have the capacity to weigh the gas mixture vessel (e.g. a typical gas cylinder) both when full and when empty and to have a precision and accuracy sufficient to limit the uncertainty in the measurement to an acceptable value. As a non-limiting example, in a typical embodiment of the invention, a commercially available bench scale comprising a 20" x 23" (51 cm x 58 cm) stainless steel platform and having a capacity of 135 kg (300 lb) and a readability of 1 g (0.002 lb) is used.

[0042] The system also comprises a connection system 30. The connection system is divided into connections for the input side of the manifold (30A) and connections for the output side of the manifold (30B) that are configured to connect the manifold to sources of analyte gas (310) and carrier gas (300) and to the gas mixture vessel (320) that will contain the final gas mixture prepared using the inventive system. The connection system may comprise any appropriate kind of connectors and, if necessary, valves and tubing, that can transfer the gases from their source to the manifold and from the manifold to the gas mixture vessel. In preferred embodiments of the invention, the connection system includes pressure gauge HOE in fluid connection with the gas mixture vessel that is configured to measure the total pressure within the gas mixture vessel.

[0043] As a non-limiting example, FIG. 1 presents schematically one embodiment of the connection system. In the embodiment illustrated in the figure, the input connection comprises independent valves and tubing that can be used to connect the manifold to a source or multiple sources of analyte gas. For example, in the embodiment shown, the connection system comprises valves V40 and V40.1 for connecting analyte gas source 310A to vessel 100A; valves V30 and V30.1 for connecting analyte gas source 310B to vessel 100B; valves V20 and V20.1 for connecting analyte gas source 310C to vessel lOOC; valves V50 and V50.1 for connecting analyte gas source 310D to vessel 100D; and valves V10 and V10.1 for connecting carrier gas source 300 to the manifold. While the figure implies separate valves for each vessel 310A - D used as an analyte gas source and for vessel 300 used as a carrier gas source, the connection system may comprise a single valve with branched piping, a valve manifold, or any other arrangement of connections between the analyte gas manifold and the gas source or sources that is convenient for the user.

[0044] On the output side of the analyte gas manifold, the connection system 30B is designed to connect the manifold to gas mixture vessel 320 (the gas mixture vessel will typically be a standard high-pressure gas cylinder). In the embodiment shown in FIG. 1, valve V49 controls the connection between analyte gas vessels 100A and 100B and the manifold output; valve V29, the connection between analyte gas vessel lOOC and the manifold output; and valve V59, the connection between analyte gas vessel 100D and the manifold output. In the embodiment illustrated in the figure, a connection between carrier gas source 300 and the manifold output can be established through any of valves V29, V49, or V59 by proper opening and closing of the intermediate valves in the system, bypassing or passing through the analyte gas vessels as desired by the user. The output connection system can also be connected to the purge and/or vacuum lines; e.g., in the embodiment shown in FIG. 1, valve V70 provides such a connection.

[0045] In preferred embodiments of the invention, the carrier gas source is connected such that it can be placed in fluid connection with the lines used to transport analyte gases to the gas mixture vessels in order to flush the analyte gas lines with carrier gas (shown in FIG. 1 as vessel 300A). Additionally or alternatively, the carrier gas source can be connected directly to the gas mixture vessel, bypassing the analyte gas manifold. As non-limiting examples of configurations that are considered by the inventors as being within the scope of the invention, in some embodiments of the invention, a single carrier gas source vessel is connected to a line with valve connections configured to direct the gas flow either through the analyte gas manifold or directly into the gas mixture vessel as desired by the user. In other embodiments, a plurality of carrier gas source vessels is used, with at least one carrier gas vessel configured to direct gas through the analyte gas manifold and at least one other configured to direct gas directly to the gas mixture vessel. The carrier gas source may be located in the vicinity of the analyte gas manifold, or it may be remotely located. A direct connection of the carrier gas vessel to the gas mixture vessel is shown in FIG. 1 as connection 300B.

[0046] It will be clear to one of ordinary skill in the art that alternative arrangements of the connections in connection system 30 are possible. The exact arrangement of the valves in the connection system will depend on the specific use to which the system is put, and the specific gas mixtures that the user wishes to produce.

[0047] The system herein disclosed thus enables a user to prepare a gas mixture that contains at least one component (analyte gas) in very low concentration by measuring the amount of analyte gas volumetrically to high precision and measuring the amount of carrier gas gravimetrically to high precision, thereby overcoming the difficulties involved in attempting to prepare such mixtures by volumetric or gravimetric methods alone. Since the system can be connected to a vacuum source and then be isolated from atmosphere, the probability that impurities will enter the system and hence the final gas mixture is very small. [0048] It is also within the scope of this invention to disclose a volumetric-gravimetric method for producing gas mixtures. The method comprises determining volumetrically an amount of analyte gas by a volumetric method; determining gravimetically an amount of a carrier gas; and mixing the analyte and carrier gases to produce the gas mixture. The method can be used to produce gas mixtures having mole ratios (moles of analyte gas : total moles of mixture) on the ppm or even ppb level.

[0049] In order to assist a person of ordinary skill in the art to perform the inventive method, a detailed description of one non-limiting preferred embodiment of the method is presented here. The method begins with the evacuation of a gas mixture vessel 320. In preferred embodiments of the invention, the entire apparatus on which the gas mixture is prepared undergoes at least one evacuation/flush/evacuation cycle, where the flush step comprises flowing an inert gas or the carrier gas used in the mixture through the system.

[0050] At least one analyte gas is introduced from an analyte gas source 310 into at least one analyte gas vessel 100 of known volume, and the temperature and pressure of the analyte gas within the analyte gas vessel are determined. In preferred embodiments of the invention, the pressure and temperature within the analyte gas vessel are allowed to stabilize after the analyte gas is introduced. In some embodiments of the method, the temperature within the analyte gas vessel is controlled by a heating or cooling system of any appropriate type known in the art. In other embodiments of the system, the analyte gas vessel is allowed to equilibrate at room temperature after the analyte gas is added.

[0051] The analyte gas vessel is then closed to the source and opened to a gas mixture vessel 320, and part or all of the gas allowed to flow from the analyte gas vessel to the gas mixture vessel. The number of moles A of analyte gas and the number of moles C of carrier gas to be mixed in order to provide a mixture with the desired mole ratio (i.e. analyte gas concentration) -^-^ is calculated. The pressure drop in the gas analyte vessel corresponding to the delivery of A moles of gas is calculated from the known temperature and volume of the analyte gas vessel. The analyte gas vessel is then opened to the gas mixture vessel until the pressure has dropped by the calculated amount. In typical embodiments of the invention, calculating the required pressure drop is calculated by using the ideal gas law, which will generally give results of sufficient accuracy. If the user so desires, equations that include empirical adjustments to the ideal gas law (e.g. the van der Waals equation) may be used. [0052] In cases in which the mixture comprises more than one analyte gas, in some embodiments of the invention, the above procedure is performed separately for each analyte gas. In other embodiments of the invention, the different analyte gases are introduced into a single analyte gas vessel at a pressure ratio corresponding to the desired molar ratio in the final gas mixture. The analyte gas vessel containing the mixture is then opened to the gas mixture vessel and the pressure allowed to drop by the previously calculated amount.

[0053] Once the analyte gas has been added to the gas mixture vessel, the analyte gas vessel and gas mixture vessel are isolated, e.g. by closing the valves that place them in fluid connection with the system used to prepare the mixture, and the system purged, preferably by at least one evacuation/flush/evacuation cycle.

[0054] The gas mixture vessel containing the analyte gas is weighed. In preferred embodiments of the invention, a scale or balance having a precision of 0.05% or better is used. Carrier gas is then added to the gas mixture vessel. In some embodiments, carrier gas is introduced directly into the gas mixture vessel. In other embodiments, at least part of the carrier gas flows through the same tubing that carried the analyte gas to the gas mixture vessel in order to flush any analyte gas that might have been adsorbed or adsorbed on the walls of the tubing or valves, or was trapped within the system, into the gas mixture vessel. While the gas mixture vessel is being filled with carrier gas, its weight or mass is monitored. In typical embodiments of the invention, the gas mixture vessel sits on a balance or scale to enable monitoring the total weight (gas + cylinder). When the weight of carrier gas in the gas mixture vessel reaches the weight of C moles of carrier gas (i.e. the amount needed to produce a mixture having the desired mole ratio), the transfer of carrier gas is stopped, and the gas mixture vessel closed to the system. Note that as with the analyte gas, the carrier gas may itself be a mixture. The components can be added individually, or a mixture of carrier gases can be prepared, and the mixture added to the gas mixture vessel.

[0055] In preferred embodiments of the method, the system disclosed above is used to produce the gas mixture, but it is emphasized that the method is not necessarily limited to being performed on the specific system disclosed herein. When the inventive method is performed by using the system herein disclosed, a standard gas cylinder containing a gas mixture having an analyte gas at the ppm - ppb level can be produced typically within an hour to an hour and a half.

Claims

We claim:

1. A volumetric-gravimetric method for producing a mixture of an analyte gas and a carrier gas, wherein said method comprises:

preparing a volume of said analyte gas at a known temperature and pressure, thereby obtaining A moles of said analyte gas;

weighing an amount of a carrier gas, thereby obtaining C moles of said carrier gas; and, mixing said analyte gas and said carrier gas, thereby obtaining a gas mixture characterized by a mole ratio .

^J A+C

2. The method according to claim 1, wherein said mole ratio is less than 10^" .

3. The method according to claim 2, wherein said mole ratio is less than 10^~6.

4. The method according to claim 1, wherein said step of preparing a volume of said analyte gas comprises preparing a volume of said analyte gas at a known temperature and pressure calculated to provide a predetermined number of moles A of said analyte gas, and said step of weighing said carrier gas comprises calculating a weight of said carrier gas equivalent to a predetermined number of moles C of said carrier gas.

5. The method according to claim 1, wherein said step of preparing a volume of said analyte gas at a known temperature and pressure comprises:

obtaining an analyte gas vessel of known volume;

determining a temperature within said analyte gas vessel;

calculating, from said known volume and determined temperature, a calculated gas pressure within said analyte gas vessel equivalent to a predetermined number of moles A of said analyte gas; and,

introducing a quantity of said analyte gas into said analyte gas vessel sufficient to raise the pressure within said analyte gas vessel by said calculated gas pressure.

6. The method according to claim 1, wherein said step of preparing a volume of said analyte gas at a known temperature and pressure comprises:

obtaining an analyte gas vessel of known volume;

determining a temperature within said analyte gas vessel; calculating, from said known volume and determined temperature, a calculated gas pressure change within said analyte gas vessel equivalent to transfer into or out of said analyte gas vessel of a predetermined number of moles A of said analyte gas; introducing a quantity of said analyte gas into said analyte gas vessel; and,

releasing analyte gas from said analyte gas vessel until the pressure within said analyte gas vessel drops by said calculated gas pressure change.

7. The method according to claim 1, wherein said method comprises:

evacuating a gas mixture vessel (320) having a known volume;

weighing said evacuated gas mixture vessel;

calculating said number of moles A of analyte gas and C of carrier necessary to provide a predetermined mole ratio at a predetermined pressure and temperature within said gas mixture vessel (320);

introducing at least one analyte gas from an analyte gas source (310) into at least one analyte gas vessel (100) of known volume;

measuring temperature and pressure of said at least one analyte gas within said at least one analyte gas vessel;

closing said analyte gas vessel to said source;

opening said analyte gas vessel to said gas mixture vessel;

flowing analyte gas from said analyte gas vessel into said gas mixture vessel until the pressure in said analyte gas vessel drops by an amount corresponding to delivery of

A moles of analyte gas;

closing said analyte gas vessel to said gas mixture vessel;

opening said gas mixture vessel to a carrier gas source (300); and,

flowing carrier gas from said carrier gas source to said gas mixture vessel while monitoring its weight until a weight of carrier gas corresponding to C moles of carrier gas has been introduced into said gas mixture vessel.

8. The method according to any one of claims 1 - 7, wherein said analyte gas is gaseous at standard temperature and pressure.

9. The method according to any one of claims 1 - 7, wherein said method is performed on a system, and said method comprises at least one step selected from the group consisting of: evacuating at least part of said system prior to said step of preparing a volume of analyte gas; flushing at least part of said system with said carrier gas prior to said step of preparing a volume of analyte gas; and,

prior to said step of preparing a volume of analyte gas, performing at least one purge/flu sh/purge cycle comprising:

evacuating at least part of said system;

flushing at least part of said system with said carrier gas; and,

evacuating at least part of said system subsequent to said step of flushing.

10. The method according to claim 9, wherein said analyte gas is gaseous at room temperature and pressure.

11. A system for volumetric -gravimetric preparation of a gas mixture comprising at least one analyte gas and at least one carrier gas, comprising:

an analyte gas manifold (10) comprising an input side and an output side;

weight measuring means (20); and,

a connection system (30);

wherein:

said analyte gas manifold is configured to determine volumetrically an amount of said analyte gas;

said weight measuring means is configured to determine gravimetrically an amount of said carrier gas; and,

said connection system is configured to provide fluid connection:

between said input side of said analyte gas manifold and a source of analyte gas (310);

between said output side of said analyte gas manifold and a gas mixture container

(320) in which said gas mixture is to be stored; and,

between a source of carrier gas (300) and said gas mixture container.

12. The system according to claim 11, wherein said analyte gas manifold comprises:

at least one analyte gas vessel (100) having a known volume; and,

pressure and temperature measuring means (110) in fluid connection with said at least one analyte gas vessel, said pressure and temperature measuring means configured to measure pressure and temperature within said at least one analyte gas vessel.

13. The system according to claim 11, wherein said analyte gas manifold comprises at least one component selected from the group consisting of:

a purge line (120) configured to provide fluid connection between said analyte gas manifold and atmosphere; and,

a vacuum line (130) configured to provide fluid connection between said analyte gas manifold and a source of vacuum.

14. The system according to claim 11, wherein said analyte gas manifold comprises a valve system and a tubing system configured to allow gas flow through said analyte gas manifold to bypass said at least one analyte gas vessel.

15. The system according to claim 11, wherein said weight measuring means comprises a scale configured to weigh standard gas cylinder to within a predetermined accuracy and precision.

16. The system according to claim 11, wherein said connection system is configured to provide fluid connection between said source of carrier gas and said input side of said analyte gas manifold.

17. The method according to any one of claims 1 - 7, performed on the system according to any one of claims 11 - 16.

18. The method according to claim 8, performed on the system according to any one of claims 11 - 16.

19. The method according to claim 9, performed on the system according to any one of claims 11 - 16.

20. The method according to claim 10, performed on the system according to any one of claims

11 - 16.