METHOD AND DEVICE FOR CONTINUOUS, AUTOMATIC AIR ANALYSIS

Abstract: A method and a device, a gas chromatograph, for continuous, automatic air analysis. The gas chromatograph is provided with a pre-concentrating step (1) which, besides the function of concentrating those substances that are to be analyzed in an air sample, is also functioning as a purification step. Because of this, the air present in the sample can be utilized as carrier gas through column (2) and detector (4). A separate carrier gas source or a complicated purification system is thus not required for using the atmospheric or ambient air as carrier gas.

Description: Method and device for continuous, automatic air analysis TECHNICAL FIELD The present invention relates to a continuously working, automatic gas chromatograph that is useful for qualified air analyses, for instance for the determination and identification of extremely small contents of impurities in air. The gas chromatograph of the inven¬ tion is small, compact, easily operated and portable and because of this, useful for qualitative and quantitative air analyses at the place of sampling, if so is desired. The invention particularly makes possible a simplified method for purification of the air in the sample to be used as carrier gas. BACKGROUND ART In a conventional gas chromatograph a definite amount of the sample is injected into a vaporization chamber for a quick vaporization thereof if the sample is in the Liquid state. It is then carried by a carrier gas into a chromatograph column and through a detector for the analysis of the sample components. If the sample is in the gaseous state already from the beginning, it is injected directly into the carrier gas flow. As carrier gas helium, argon or nitrogen is usually used. It has also been suggested to use air as carrier gas. As the carrier gas must be dry and free from.impurities, the air must in such a case be taken from a separate gas bottle or, when the atmospheric or ambient air is to be used, it must be puri¬ fied before the use as carrier gas. For such a purpose it has up to now been required a relatively complicated purification equipment, for instance as described in the U.S. patent 3,772,909. The gas chromatograph as a whole will then become relatively large and complicated. DISCLOSURE OF INVENTION The present invention makes possible a simplified method of puri- fying the atmospheric or ambient air for use as carrier gas through column and detector. As one " can utilize the air that is present at the very place of sampling and analysis, no separate gas source is required for the supply of carrier gas- This circumstance together with the fact that the gas chromatograph is battery operated and that you can go "down in scale", for instance by using a capillary column, contributes in making the gas chromatograph according to the invention small, simplified, portable and useful for field use. The characteristics of the invention will become apparent from the succeeding patent claims. When analyzing air one has previously been forced to carry out a preparatory step of sampling at the very place and to analyze the ' sample later on in a laboratory. As the gas chromatograph of the in- vention can be pre-calibrated for possible substances in the air and as it makes an analysis possible already at the sampling spot, a very quick answer is obtained to the question if a certain substance is present in the air at a certain place and to what extent the sub¬ stance is present, i.e. the percentage of substance. The gas chroma- tograph is of course also suitable for conventional laboratory use. As those substances that are to be determined are concentrated in a pre-concentrating step by means of an adsorbent before the analysis thereof, the gas chromatograph of the invention is, in spite of its compact construction, extremely sensitive and reliable and makes pos¬ sible analysis of for instance nerve gases, dangerous substances at places of work, such as benzene, toluene, xylene etc, in so low con¬ tents as 0.005 mg/m . Thus, the adsorbent in the pre-concentrating step has a double func¬ tion, viz. to concentrate those substances in the air that are to be determined and to purify the air for the use thereof as carrier gas. By arranging an air pump after the detector of the air analysis sys- tern, both a simplified sampling and a simplified purification of the air are obtained. The air pump is- working all the time and draws the air at an even speed through the adsorbent in the pre-concentrating step, through the column and the detector. As the air pump is posi¬ tioned after the detector, the risk of contamination of impurities deriving from the pump can be avoided. As one also makes use of a computer, suitably a microcomputer, for the control of the gas chromatograph and for the preparation and presentation of the measurement results, the apparatus becomes very easy to operate, and so it can be used by not qualified personnel for analysis at the very place of sampling. Thus, a highly field- adapted analytical instrument is obtained. BRIEF DESCRIPTION OF DRAWINGS The invention will now be described in more detail with reference to the attached drawings, where Fig 1 is a schematic diagram of the over-all construction of a gas chromatograph according to the invention; Fig 2 shows a suitable embodiment of the injector device; Fig 3 shows a suitable embodiment of the detector device; and Fig 4 is a principle outline of the working mode of the gas chromato¬ graph. BEST MODE FOR CARRYING OUT THE INVENTION The gas chromatograph of Fig 1 consists of an injector 1 with an ad¬ sorbent, also functioning as concentrating step, a chromatograph co¬ lumn 2 and a detector 4 disposed in a thermostat-controlled, with a fan provided oven 3, an air pump 5, a microcomputer 6 and a power supply unit 7, for instance a battery. The injector 1 (see Fig 2) suitably consists of a solid adsorbent material 8 in powder form, put between two glass-wool plugs 9 in a thin-walled glass tube 10 that is provided with an electric resistance wire in the form of a heating coil 11 wound round the tube. The hea- ting coil connection to the power supply is designated 12. One end of the tube is connected directly to the chromatograph column 2 by means of a kind of quick-connective coupling via a rubber sealing joint 13. The other end opens directly to the ambient air but is so devised that it may be connected by means of a tubing, suitably of Teflon, to another air sample. The entire adsorbent tube is dis- connectable, and so it can be used for sampling separately from the gas chromatograph, for instance at exposition measurements. The ad¬ sorbent material inside the tube is chosen taking into consideration what air contaminants there are to be analyzed. However, it must stand being heated during the heat desorption. Liquids, for instance high temperature boiling silicone o l, may also be used as adsorbent material. The column 2 is suitably a capillary column in the form of a glass tube helix, the coil thereof having a diameter of 11 cm and a relative¬ ly large tube inner diameter (about 0.4 mm).The inner surface has been deactivated and coated with a stationary phase with temperature stabi¬ lity even when air is used as carrier gas. The stationary phase has a more or less slowing down effect on the components of the sample, and so they will leave the column separated from each other, making possible a separate detection thereof. * The oven 3 is designed to hold exactly the column 2 and the detector 4. The oven is well insulated with glass-wool or a similar fibre material and provided with a lining of thin aluminium foil so that the heat capacity will become low. The heating is carried out electri- cally by means of a resistance wire. The temperature is measured with a resistance sensor and is controlled by the microcomputer. There is a fan in order to avoid gradients and obtain a more effec¬ tive temperature control. The detector 4, connected directly to the column 2, is placed inside the oven 3. The choice of detection principle is made taking into consideration what substances there are to be analyzed. It is desirable, however, that the detector has a small dead volume, small size and high sensitivity for the substances in question. It should also function without detector gases. There are several detectors that meet said requirements, for instance the photoionization detec¬ tor and piezoelectric crystals coated with a specific liquid film. O P The detector may be so constructed that, besides being used for gas chromatography as has been described, it can also be used as a direct- -reading detector. In the latter case the sample air is directly furnished into the detector without previously passing the pre-concen- tratihg step and the chromatograph column. In this case there is obtained an instantaneous value of how much of impurities the sample air. contains. A suitable embodiment of the detector 4 is shown in Fig 3. The detec- tor is concentrically built in a holder 14, which by means of a double-threaded fixing screw 19, screw joints 25 and with inter¬ mediate insulations 15 of Teflon fixes an UV~lamp 16, a detector cell 17 and an anode 18 in a gastight way to each other. The sample air is drawn into the detector, either through a column connecting element 23 or through a direct-delivering tube 24. (Only one of said connections has an open communication with the sample air, and the shut-off device or switching device is positioned where the sample air is furnished into the gas chromatograph.) The air sample is brought centrically into the detector cell 17 through a tubular part 22 of the anode 18 and exhausted by suction through a tube 20. The two tubes 20 and 24, that are insulated with Teflon tubing 21, are also electric conduits for the ionization current that arises between the detector cell and the anode, i.e. the detector signal. In order to tighten the column connection to the detector there is also provided an 0-ring gasket of rubber 26 between the anode 18 and the fixing screw 19 (between the column connecting element 23 and the insulation 15). The air pump 5 is connected to the detector 4 via flow resistances and buffer volumes, in which way a continuous flow is obtained through the system. It is suitably of plunger type in order to achieve a sufficient suction height. The microcomputer 6 is to control the oven temperature, the air pump and the heating of the adsorbent and to register detector signal and time, prepare and present measured values and results. The micro¬ computer is suitably of CMOS-type, which makes possible a battery operation of the gas chromatograph for about eight hours. Additional equipment comprises among other things a key set for control instructions to the gas chromatograph, a display and a sound alarm device for direct-indication of detector signal, oven tempera¬ ture etc and connecting taps for recorder, printer, external source of power supply and other fittings. WORKING MODE AND APPLICABILITY The gas chromatograph is working continuously in accordance with the following (cf. Fig 4): A; The sample air is drawn in through the injector 1 during a pre- set time. The air contaminants will get caught on the adsorbent. B; The adsorbent is heated during a short while. This is done almost instantaneously. It is then getting cooler and is functioning according to A; C; Substances that are desorbed, when the adsorbent is heated are drawn into the column 2, where they are subject to chromatography in the usual way but where the air is functioning as carrier gas. The adsorbent, which is now cold, purifies new sample air from contaminants, i.e. at the same time as the just desorbed sub¬ stances are subject to chromatography, new sample is furnished to the adsorbent. D; The detector 4 registers each of the substances individually as they leave the column. E; The microcomputer 6 prepares the detector signal and presents the result qualitatively and quantitatively. The next work cycle starts with the adsorbent being heated again (B; ) . It can be observed that the air pump is working all the time and is drawing the air at an even speed through the adsorbent in the pre-concentrating step, the column and the detector. The per se previously known principle of concentrating the air contaminants on an adsorbent before the chromatography and detec¬ tion will here make the detection limit of the detector about 100 times better. This means that contents of about 0.01 nig/m ( benzene, toluene or xylene) can be determined by means of a photoionization detector without difficulty. The good separation ability of the capillary column makes it possible to determine for instance each of the xylene isomers individually. Every amount determination that is carried out, becomes an average value of the content in question during the sampling period con- cerned. A time period of 15 minutes is usual for industrial hygie¬ nic measurements. By making the adsorbent tube connection in such a way that it is easily disconnectable, it can be used for exposition measurements, in which case sample air is collected from the breathing zone by means of a conventional, by a person carried air pump. As an alternative it is possible to carry the gas chromatograph about having a sampling hose, for instance of Teflon, opening out into the breathing zone. The described embodiment of the gas chromato- graph is appreciated to have a weight of about 4 kilogrammes, and in this case it can be battery operated for about 4 hours. The measurement results that are collected, during for instance every quarter of an hour, are stored in a storage device and can later on be taken out and then plotted/tabulated. The gas chromatograph can, as has previously been pointed out, also be so constructed that the detector can be used as a direct-reading instrument. To be able to change-over the gas chromatograph in an easy way to a direct-reading instrument is especially valuable at leak searches and so-called emission measurements at working places. It is also useful when one tries to make as quickly as possible a survey of the variations of a known substance in a room. It can also be used for comparison measurements, for instance when one wishes to make a comparison with the background signal in a gas chromatogram. A microcomputer for control, preparation etc gives a great flexi¬ bility. It may for instance have stored values for retention times ( identifiers), sensibility factors for the detector and hygienic limit values for those substances that the analysis con¬ cerns. Besides, alarm functions can be released when limit values have been exceeded. It has capacity to run two or more measuring systems in the gas chromatograph at the same time, for instance for specific detectors or a direct-reading, quick detector. The presentation of measurement results can be done in several ways by means of the microcomputer, for instance with a direct-reading recorder/display or a table over collected measurement values. The greatest advantage with the computer is that it controls the gas chromatograph in such a way that it becomes an easily opera¬ ted measuring system. O P

Claims: Patent claims: 1. A. method relevant to a gas chromatograph for continuous, auto¬ matic, qualitative and quantitative air analysis, for instance for the determination and identification of extremely small contents of substances present in air, the gas chromatograph comprising an in¬ jector, a pre-concentrating step, a chromatograph column, a detec¬ tor, a computer for the control of the gas chromatograph, prepara¬ tion and presentation of analysis results and a power supply unit, c h a r a c t e r i z e d in that the sample air is being passed continuously and at an even speed through the injector, the pre- concentrating step which comprises an adsorbent, the column and the detector, during which period of time substances present in the air are adsorbed on the adsorbent in the pre-concentrating step at the same time as the air is purified and can be used as carrier gas through the column and the detector, and then the adsorbent is shortly heated so that the substances that have become concentrated on the adsorbent in the pre-concentrating step are desorbed and are carried by carrier gas, consisting of sample air purified in the pre-concentrating step, through column and detector for analysis thereof at the same time as new sample air is being passed through the injector and the adsorbent in the pre-concentrating step; this course of activity steps during said period of time being repeated, in which way a continuous sampling and analysis method is achieved. 2. Method according to claim 1, c h a r a c t e r i z e d in that the sample air is being drawn continuously and at an even speed through injector, pre-concentrating step, column and detector by means of an air pump disposed after the detector, air purified in the pre-concentrating step carrying substances along with it that have been concentrated and vaporized and now is to be analyzed. 3. A device for carrying out the method according to claim 1 or 2 comprising an injector (1), also functioning as pre-concentrating step for those substances that are to be analyzed as the injector comprises an adsorbent (8) that can be shortly heated in order to desorb the substances adsorbed thereon; a temperature-controlled. - URf _-OMPI • " v. " IPO fan-provided oven (3) comprising a capillary column (2) and a detec¬ tor (4); a microcomputer (6) for the control of the device and pre¬ paration and presentation of analysis results; and a power supply device (7), c h a r a c t e r i z e d in that there is provided after the detector (4) an air pump (5) that is drawing the sample air at an even speed via flow resistances and buffer volumes through the injector (1), the capillary column (2) and the detector (4), the air that is present in the sample forming a carrier gas through the capillary column and the detector for those substances that are to be analyzed. 4. Device according to claim 3, -c h a r a c t e r i z e d in that the injector (1) comprises an adsorbent material (8) in powder form which is held togethe ' r by means of two glass-wool .plugs (9) in a thin-walled glass tube (10), which is provided with a helix-formed resistance wire (11) tightly wound round it, the ends of the - resistance wire being fixed to the glass tube by means of two metal rings (12), which at the same time are electric connecting taps and through which a current can be led and in a short while heat the resistance wire, the glass tube and the adsorbent material. 5. Device according to claim 3 or 4, c h a r a c t e r i z e d in that the detector (4) is constructed from a detector cell (17) in the form of a hollow cylinder, an anode (18) that is concentric with the detector cell and comprises a tubular part (22) surrounded by the detector cell, an UV-lamp (16), a holder (14), which holds and is concentric with the detector cell and the anode and fixes the UV-lamp, the detector cell and the anode in a gas-tight way to each other by means of screw joints (19, 25) and intermediate insulations (15), for instance of Teflon, an element (23) connecting the column (2) to the detector (4 ) and provided with a through-hole for the delivery of sample air from the column to the detector, •a tube (24), which is fixed to the anode and in communication with the ambient air for direct delivery of sample air, a suction tube (20), which is connecting the cavity of the detector cell with the air pump (5), both the suction tube (20) and the direct-delivering tube (24) being insulated with for instance Teflon tubing (21) and constituting electric conduits for the ionization current that arises between the detector cell and the anode, and a device positioned outside the detector (4), which makes switching over or shutting off possible and with the help of which the sample air, that is drawn in by means of the air pump ( 5) and furnished to the cavity of the detector cell through the tubular part (22) of the anode (18), can be taken either from the column (2) through the column connecting element (23) or directly from the ambient air through the direct-delivering tube (24) to be exhausted through the suction tube (20) after the detection thereof. _OM

Inventors: RITTFELDT LARS FOLKE, RITTFELDT, LARS, FOLKE
Applicants: RITTFELDT LARS FOLKE, RITTFELDT, LARS, FOLKE
Application year: 1982
Main classification: G01N31
IPC codes: G01N30