CONTAMINATION

The invention relates to an instrument for determination of contamination, intended to detect and identify toxic contamination.

Contemporary instruments for determination of contamination enable detection and identification of a majority of organic substances that are considered to be highly toxic. Highly toxic chemical substances (chemical weapons, toxic industrial substances) are nowadays detected mostly with detectors based on IMS (ion mobility spectrometry) technology and DMS (differential mobility spectrometry) technology.

The chamber of an IMS detector is divided into two areas. The first area is the area from the semi-permeable membrane to the dosing mesh, in which ionisation is performed using a β or a radiation source, and the second area is a drift area from the dosing mesh to the collective electrode, which contains a set of metal rings. Upstream of the radiation source, the mesh is supplied with high voltage (generally 1.5 kV to 3 kV), while the metal rings, subsequently from the source to the collective electrode, have decreasing potentials. Consequently, the field is shaped so that ions move from the ionisation area on linear paths to the collective electrode. Most gaseous substances are characterised by different mobility, and, consequently, the time of flow of ions through the drift area is different, which enables their identification.

A DMS detector is made of opposing ceramic plates with electrical elements applied to them; the electrical elements, in turn, are supplied with high voltage of high frequency. As a result of the electrical field created within the volume of the detector, segregation takes place on the collective electrode. The observed segregation of ions present in the flowing gas results from their different mobilities in fields of lower and higher intensity. The mobility of ions depends on their weight, the charge of the flowing ions and the speed of the flowing gas. As a result of the variable electric field applied to the electrodes, ions whose mobility does not meet the conditions for stable flow through the detector are captured. Given the relationship between the mobility of ions of particles migrating through the active volume of the spectrometer and the value of the compensated field, a certain ion filter is created.

In the case of detection of H-type substances, such as sulphur mustard, nitrogen mustard and Lewisite, it is particularly important to maintain very low condensation of steam in the air. Correct detection of those substances requires a relative humidity below 1% at 30°C. This is because the presence of steam reduces sensitivity to the tested substance and even leads to a loss of detection capacity at high concentrations.

The measurement instruments, based on classical linear ion mobility spectrometers (IMS), contain assemblies with two air loops: the so-called external and internal loop. Both loops are connected in an exchanger through a separating membrane.

The flow rates in the loops are set so that the concentration of the tested sample downstream of the membrane is high and generally equal to 0.2 to 1 1/min.

This design of the air loops is aimed at eliminating steam on the detector side. The filter contains molecular sieves, which absorb steam from the air, and in the exchanger, there is a membrane that lets the tested substances (the ones to be detected) into the internal loop and isolates steam. Transport of the tested substance through the membrane takes place by way of diffusion; consequently, the quantity of the medium that can pass to the other side (to the internal loop) is limited and depends on the physico-chemical characteristics of the membrane.

In the case of low flow rates, it can be assumed that the concentration of the tested substance on the side of the internal loop, upstream of the IMS, is virtually the same as the concentration in the external loop, at the inlet. An increase in the value of the flow rate in the internal loop results in dilution of the tested sample, because the tested substance settles on the internal loop filter.

This results in a significant reduction of the detection threshold of both H- type and G-type (phosphoroorganic poisons) substances. In assemblies where the flow rate through the detector should be higher than 1.5 1/min., this solution does not meet the conditions for rapid gas exchange; consequently, this type of gas system is ineffective. Such a high flow rate would lead to significant dilution of the sample, because transport through the membrane is limited. This would cause a significant decrease of sensitivity of such an analyser.

In the case of flows exceeding 1.5 1/min., instruments with DMS/FAIMS detectors are used. A known solution of the Owlstone company has a membrane-free assembly with loop mixing. The membrane is replaced with a three-way pipe and the filter - with molecular sieves, whose role is to absorb steam without absorbing the tested substance.

Unfortunately, this solution is ineffective at high air humidity (above 50% at 20°C), because the relative humidity in the internal loop exceeds the permissible limit values at which detection of such substances as sulphur mustard is possible.

The degree of dilution generally ranges from several to several dozen.

Because the sieves absorb only steam, the concentration of the tested substance upstream of the FAIMS detector is virtually the same as the concentration at the inlet, and the concentration of steam is significantly lower than the concentration at the inlet, but often higher than required for detection of sulphur mustard at a low level; moreover, such a solution requires frequent replacement of the sieves.

Depending on the type of tested substance, high humidity may hinder DMS analysis or may have no impact on the results.

The object of the invention is to develop an instrument that enables detection and determination of contamination at both high and very low air humidity.

An instrument for determination of contamination, according to the invention, comprising a DMS chamber and a drying assembly with molecular sieves, characterised in that it has two DMS chambers, between which there is an exchanger with a semi-permeable membrane. Between the exchanger and the second DMS chamber, there is a reducer, and the outlet of the other chamber is connected, through a pump and a filter with molecular sieves, to the inlet of the other chamber and, in parallel, through a pump and a carbon filter, to the outlet of the exchanger. On the other hand, the inlet of the exchanger is connected to an air intake, whose outlet is connected to the inlet of the first chamber.

Chamber I serves the purpose of detection and identification of gaseous substances, for which the presence of a large quantity of steam (from 5% to 98% relative humidity) in the gaseous medium does not affect the result of detection, while the membrane constitutes a significant barrier, which considerably reduces the detection capability of chamber II. Such substances include phosphoroorganic substances with large molecules, e.g. Vx.

The instrument enables simultaneous analysis of four channels:

positive and negative ions in chamber I and chamber II.

This solution expands the range of detected substances (to include, e.g. chloride), while increasing the selectivity of detected substances and significantly improving the sensitivity of the analyser.

In the case of high concentrations of phosphoorganic substances, the barrier effect increases the range of concentrations detected.

An embodiment of the instrument for determination of contamination is presented in its design diagram drawing.

The first assembly of the instrument is the air intake JL, into which a gas stream is delivered through the inlet 2. The gas stream is moved into chamber I 3 and then passes through the exchanger 4, which comprises the semi-permeable membrane 5, and reaches chamber II 6. At the inlet of chamber I 3, there is the inlet tower 7; at the outlet of chamber I 3, there is the outlet tower 8; and at the inlet of chamber II 6, there is the inlet tower 9. Both towers, located at the chamber inlets, are equipped with gas ionisation sources, which are not shown. In the inlet tower 9, there is a reducer 10, through which the gas sent from the exchanger is fed into chamber II 6. On the outlet of the chamber 6, there is the outlet tower 11 , in which the outlet of chamber II 6 is connected, through the pump 12 and the filter with molecular sieves 13, to the inlet of chamber II 6 and, in parallel, through the pump 14 and the carbon filter 15, to the outlet of the exchanger. The second outlet of the exchanger is connected to the air intake. The air intake assembly comprises the sensor _. 16, which indicates the difference between the pressure of the surrounding air and the pressure downstream of the air intake inlet, the valve 17, the block of sensors .18 that determine the temperature, the humidity and the ammonia content in the air, the pump 19 and the carbon filter 20. Between the inlets and outlets of the chambers, and the inlet and outlet of the reducer, there are the pressure difference sensors 21, 22, and 23.