Measuring and testing – Instrument proving or calibrating – Gas or liquid analyzer
Reexamination Certificate
2002-03-26
2004-07-13
Raevis, Robert (Department: 2856)
Measuring and testing
Instrument proving or calibrating
Gas or liquid analyzer
Reexamination Certificate
active
06761056
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process and a device for producing a gas mixture which contains at least one gaseous component, in particular for producing a calibration gas.
The use of gases with defined concentrations of admixtures is required both in various technical devices and as calibration standards for analysis methods.
BACKGROUND OF THE INVENTION
In all analytical devices, calibration represents the most important step with a view to obtaining correct results from the analysis. Although the apparatus settings of an analysis device may have a decisive influence on the sensitivity and accuracy of an analysis result (and therefore the detection limit), correctness can only be ensured by comparison with a calibration specimen with a known content.
In the environmental analysis field, the need for global, time-independent comparability of analysis results, means that correctness is a crucial parameter if it is to be possible to understand ecological processes and understand global mass streams. Nowadays, the analysis of gaseous pollutants down to the lower ppt range no longer constitutes any problem for many analysis devices, for example those used in gas chromatography. However, the problem of appropriate calibration of such analyses remains substantially unresolved.
A continuous stream of the calibration gas is required in order to filter out any conditioning phenomena. The requirements of trace analysis of gaseous pollutants make continuously flowing certified test gases from pressurized-gas vessels the optimum calibration medium. The high price, caused by complex stabilization and production, and the long delivery times, which are caused by production factors, and also the high minimum volumes of the individual vessels make these processes appear too expensive for many users. Moreover, not all gas mixtures can be stabilized in corresponding pressure systems, with the metal surfaces which they employ. In particular, polar components can only be produced in stabilized form with very great difficulty by this route.
An ideal calibration specimen generally contains an accurately defined concentration of the analyte, distributed as homogeneously as possible in the same matrix gas which also surrounds the analysis specimen. However, a calibration specimen of this type can rarely be achieved, in particular in the field of environmental analysis, since it is almost impossible to obtain the uncontaminated matrix required for production of the calibration specimen as a blank and dilution medium.
There is another problem with regard to the analysis of gaseous specimens. Although in this case it is relatively easy to obtain the uncontaminated matrix, for example synthetic air, in this case the production of corresponding standard specimens represents a problem.
Gravimetric methods of producing gas mixtures are extremely complex and can only be undertaken by manufacturers of test gases with considerable financial outlay. This results in problems on account of the masses of the vessels which hold the gas mixture, which are high in relation to the matrix gases, but very particularly in relation to the trace components.
In the case of volumetric production, it must be possible for small volumes of a trace component, which is often in liquid form, to be introduced reproducibly into a very large volume of the matrix gas. There are problems with the stability of the gas mixtures on account of the low density of the gaseous analyte. Adsorption and desorption on the surfaces of the equipment which is in contact with the test gas, which surfaces are very large in relation to the mass of the trace components, lead to the risk of the concentration changing, for example on account of wall desorption and wall adsorption effects.
Commercially available calibration systems which attempt to achieve this have various weak points:
a short operating range of at most two decades,
problems with low concentrations,
limits on the number of components which can be mixed and their ratios,
the minimum achievable concentration range is excessively high.
It is also known for the pure components to be introduced continuously into a continuously flowing gas stream and to be reproducibly homogenized therewith. In this case, critical nozzles, permeation devices or controlled incoming flows of test gases of higher concentrations may be suitable for introduction. However, this does not solve the above problems.
Since, by definition, the detection limit of all analytical devices is decisively dependent on the reproducibility of the individual results, any improvement in the calibration also improves the detection limit and therefore considerably increases the capacity of the overall analysis device.
DE 198 58 366 A1 has disclosed a process of the type described in the introduction in which a capillary diffusion metering system is used in order to establish a defined mixing ratio of a carrier gas and the components which are of interest. Then, the mixture is passed over a trap, through which a purge-gas stream is passed, the components which are of interest being transferred into this stream. However, the accuracy which can be achieved is by no means sufficient for some applications.
U.S. Pat. No. 5,400,665 discloses a process for producing an analysis gas stream in which a liquid mist comprising small drops of a sample which is to be analysed is produced, solvent being eliminated, before the mist is introduced into a carrier gas stream, by heating the mist. In this case, the mist droplets are produced by means of a piezoelement. Apart from the fact that the mist does not contain substantially uniform droplets, but rather, at a frequency of 1.3 MHz, 70% of the droplets are smaller than 13 &mgr;m, with the larger droplets being separated out under the force of gravity, this process is not suitable for the production of a gas mixture which contains at least one gaseous trace component in a predetermined concentration, especially since there is no quantitative metering of the sample material.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a process which makes it possible to prepare a gas mixture with a known concentration of volatile components on-line even. It is a further object of the invention to provide a process which allows to prepare a gas mixture with a very low concentration range of volatile components. It is still a further object of the invention to provide a process for preparing a gas mixture usuable as calibration standard or for industrial processes.
It is also an object of the invention to provide a device which makes it possible to prepare a gas mixture with a known concentration of volatile components on-line even and especially one having with a very low concentration range of volatile components.
The invention concerns a process for producing a gas mixture which contains at least one gaseous trace component in a predetermined concentration, comprising the steps of:
continuously conveying a matrix gas stream with a predetermined quantitative flow;
introducing at least one trace component into the matrix gas stream in a predetermined quantity per unit volume of matrix gas;
wherein the at least one trace component is introduced in the form of successive microdroplets of substantially the same size and is evaporated in the matrix gas to form the gas mixture, the microdroplets being delivered from at least one nozzle, which is in each case made to contract by means of a piezoelement which is triggered by metering pulses which are generated according to an intended delivery quantity per unit time.
The invention further comprises a device for producing a gas mixture which contains at least one gaseous trace component in a predetermined concentration, comprising a matrix gas source, downstream of which there is a mass flow controller, and at least one source for a component, and a metering device for the at least one trace component, wherein the metering device is a microdroplet-metering device with at least one nozzle which releases successive individual microdrop
Albinus Thomas
Schram Jurgen
Gerstel Systemtechnik GmbH & Co.
Patterson Thuente Skaar & Christensen P.A.
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