GB2106265A - Measuring concentration of elements in gases by spectrometry - Google Patents

Abstract

For rapid-response measurement of the concentration of an element present in a gas as a constituent of particles therein, eg for monitoring the concentration in air of toxic metals like beryllium, successive fixed volumes of the gas are directed as a jet through the injection hole in the wall of the atomisation tube of an electrothermal atomic absorption spectrometer (ETAAS) so that entrained particles are collected on its inside surface by impaction. After each collection the tube is heated to atomise the collected particles, measurement of the element is effected by conventional AAS technique, and the tube is thereafter cleansed. The measurements are preferably standardised after every so many collections by injection of a known amount of the element as an aerosol.

Description

SPECIFICATION Improvements in or relating to measuring the concentration of elements present in gases as constituents of particles therein This invention relates to measuring the concentration of elements present in gases as constituents of particles therein, and has one application in measuring the concentration of hazardous metals present as particles in the atmosphere, e.g. lead or beryllium. In this Application the term "metal particle" includes the metal in elemental or compound form.

In situations in which exposure to metals in the atmosphere is an occupational hazard, it is normal practice to monitor the blood and/or urine levels of the specific toxic metal in individual workers.

Atmospheric metal concentrations are normally measured by collecting particles or vapours over an extended period of perhaps eight hours, during which the individual will be exposed to the hazard in a normal working day. In both cases the level of exposure is assessed retrospectively and the individual receives no warning of an accidental major exposure to a toxic hazard. It would clearly be valuable to provide an automatic rapidresponse monitor which would give a fast indication of excessive exposure to particular metals. This might be a continuous monitor, or possibly a monitor giving discontinous but frequent readings, say every few minutes, of exposure levels.Such an instrument could also be used to measure the levels of toxic metals or other substances released in a particular industrial process, or as a means of regularly measuring such concentrations in stack gases released to the environment.

The present invention provides a method and apparatus suitable for such monitoring, using electrothermal atomic absorption spectrometry (ETAAS) as the quantitative analytical method.

In a known form of ETAAS apparatus, for example, the Pye Unicam SP9, the sample to be analysed, e.g. a drop of a solution containing the element or elements of interest, is introduced into a graphite atomisation tube via an injection hole in the wall of the tube. The tube is heated (by passing current therethrough in a non-oxidising atmosphere) to atomise the element or elements, and a light beam is directed along the tube bore, through the thus-atomised material, to effect analysis by the well known method of atomic absorption spectrometry. Apparatus of this kind is described, for example, in "Atomic Absorption Spectroscopy" by Morris Slavin, John Wiley and Sons, Second Edition, 1978, at pages 59-66.

According to the present invention, a method of measuring the concentration of a given element, e.g. a metal, present in a gas as a constituent of particles present in the gas, e.g. in the atmosphere, comprises the steps of directing a known volume of the gas as a jet on to a surface, whereby entrained particles are collected on said surface by impaction thereon, thereafter heating the surface to atomise the collected particles, and determining the amount of the collected element by atomic absorption spectrometry of the atomised particles.

The above steps may form a repeated subcycle, the determined amount of the substance being presented after each sub-cycle, e.g. on a chart recorder, and the surface being cleansed of said substance between sub-cycles. A complete cycle of operation may comprise impacting a known quantity of the substance on to the surface after each given number of said sub-cycles in order to standardise the determinations. The known quantity may be in the form of an aerosol produced from a standard solution of the substance by nebulisation.

In one form of the invention the known volume of the gas is directed by a nozzle as a jet through a hole in the wall of the electrothermal atomisation tube of an ETAAS apparatus, whereby entrained particles are collected on the inside wall of the tube by impaction thereon, the tube being thereafter heated to atomise the collected particles and the amount of the collected substance being determined by a method of atomic absorption spectrometry which comprises directing a light beam lengthwise through the heated tube. The aforesaid standardisation may be effected by directing the aerosol into the atomisation tube via the same nozzle as is used to introduce the gas whose content of the substance is to be measured.

The jet may be formed by applying suction, e.g.

by means of a vacuum pump, to the atomisation tube whereby the particle-bearing gas, or the calibration aerosol, as the case may be, is drawn through the nozzle.

The present invention also provides apparatus for use in a method as aforesaid.

To enable the nature of the present invention to be more readily understood, attention is directed, by way of example, to the accompanying drawings wherein: Fig. 1 is a simplified diagram of the furnacehead of an ETAAS apparatus including, in crosssection, an atomisation tube and adjacent nozzle for use in the present invention.

Fig. 2 is a graph showing the variation in minimum collection efficiency with nozzle-to-tube spacing in Fig. 1.

Fig. 3 is a typical recorder trace of lead monitored from a laboratory atmosphere using the present method.

Fig. 4 is a bar diagram of the variation of atmospheric lead in a city-centre atmosphere over a period of hours as determined by the present method.

Referring to Fig. 1, the gas-tight furnace-head 1 of an ETAAS apparatus, suitably a Pye Unicam SP9, is indicated by the interrupted lines. Within the furnace-head 1 is mounted a conventional graphite atomisation tube 2, shown in crosssection, having an injection hole 3 in its wall for the introduction of samples to be measured. In accordance with the present invention there is mounted adjacent the hole 3 (which may need to be enlarged) a nozzle 4 located at the end of a tube 5. A gas whose metal-particle content is to be measured, e.g. atmospheric air, is admitted to the far end of tube 5. A vacuum pump 6 and flowmeter 7 are connected to the furnace-head 1.

In operation the gas is drawn through the nozzle 4 as a result of the pressure-reduction in head 1 produced by pump 6, forming a jet which is directed on to the inside wall of tube 2 opposite hole 3. A proportion of the metal particles entrained in the jet is collected on the wall of impaction. After a known volume of gas has passes through the nozzle, as measured by the flowmeter and determined by the duration of the flow, vacuum-pumping is discontinued and the tube 2 is heated to atomise the metal particles in a conventional manner, e.g. by passing current lengthwise through the tube, the amount of metal present being measured by the ETAAS in the usual manner (involving directing a light beam lengthwise through the heated tube) and recorded on a chart recorder (not shown).In order to avoid loss of atomised metal during atomisation (and hence loss of sensitivity) by reverse flow along nozzle 4 (as a result of purge gases, e.g. N2 or Ar, introduced into the furnace-head during atomisation in accordance with the normal mode of operation of the ETAAS apparatus), it is preferred to close tube 5 by a valve (not shown) during atomisation. Normally "gas-flow" conditions are used during atomisation, but for enhanced sensitivity the apparatus may be operated under "gas-stop" conditions, i.e. without the purge gas flow through tube 2 itself during the measurement, despite the resulting reduction in tube life.

Fig. 2 shows the effect of the nozzle dimensions and position on the minimum efficiency (emit) of particle collection for two nozzle internal diameters (ID) over a range of distances between the nozzle tip and the outer surface of tube 2. The positive distance values indicate that the nozzle is inserted into the tube. It is seen that the nozzle can be removed several millimetres away from the tube without serious loss of collection efficiency, so that the nozzle can remain in position during the atomisation part of the cycle without thermal damage thereto by the hot tube. The efficiency values in Fig. 2 represent the minimum efficiency of the impaction collection process itself, since in the experiment used to measure this efficiency some particles were lost in the pipes etc. preceding the nozzle and the efficiencies shown do not allow for these losses.The collection efficiency of the impaction process itself varies between about 60 and 80%, depending on the design of the arrangement.

Suitably tube 5 is made of aluminium and provided with a copper nose in which a tantalum nozzle 4 is fixed by an over-lapping tight fit.

Although shown in Fig. 1 as having a uniform bore, the nozzle arrangement can take other forms, e.g. it may taper progressively from tube 5 to the nozzle tip to avoid perturbing the resulting jet or causing particles to lodge in the tube.

The apparatus is operated on a sub-cycle which comprises collecting the particles followed by atomisation and measurement, with a conventional cleansing of the tube 2 (by a short elevated heating in a purging gas flow) between sub-cycles. In order to effect standardisation or calibration of the measurements, an aerosol of the relevant metal is produced using a conventional pneumatic atomic absorption benuliser, as used in flame atomisation spectrometry, and the aerosol is injected as a jet through the nozzle 4 in the same manner as the gas being measured, every few cycles. Other standardisation methods may be used instead.

In one embodiment of the invention, using a Pye Unicam SP9 instrument adapted as hereinbefore described, collection times of from 12 seconds to more than 1 hour can be used at flow-rates of the order of 10 1 /min. In a typical complete cycle, standardisation is obtained by a 7 second nebulisation and is performed every five or ten sub-cycles. A single analysis, i.e. atomisation and simultaneous measurement, takes approximately 1 min 40 sec., and the response time of the arrangement is therefore of the order of 2-2.5 min depending on the selected collection time.

Typical detection limits for the present embodiment are shown in the following Table and are significantly lower than the medically permissible threshold limiting values (TLV) for relevant elements. The results are for a collection time of 1 min and a flowrate of 100 1/min.

Table Element Wavelength Detection (nm) limit (ng/m3) Be 234.9 2.1 Cd 228.8 5.5 Cu 324.7 47.0 Fe 248.3 2.5 Pb 283.3 36.0 Mn 279.5 5.8 Ag 328.1 4.1 Fig. 3 shows a typical chart recorder trace for the measurement of lead in a laboratory atmosphere; the large peaks are the standardisation values obtained by introducing an aerosol produced from a standard lead solution containing 0.2 mg/l for 7 sec and represent approximately 1 ng of lead. The collection time for each atmospheric measurement was 0.2 min at 9 I/min, i.e. a volume of 1.81 of air, and the peaks show a lead content of about 0.3 ng of lead in this volume, equivalent to 0.17,ug/m3. Fig. 4 shows the measurement of lead in a city-centre street, demonstrating higher levels at the time of peak rush-hour travel between 5.00 and 6.00 pm, which gradually diminish as the traffic levels fall later in the evening.

As will be seen, the present invention need require only relatively small modifications to standard ETAAS apparatus, and for this reason only these modifications have been described. For operation on in a cyclical manner as described, an automatic control system for the steps of the cycle and sub-cycle are required, but the design of control systems of this general type is familiar to those skilled in the instrumentation art.

In a modified form of the invention, the particles are charged electrostatically before forming the jet, e.g. by inserting a high-voltage electrode in tube 5, which is then made of insulating material, so that collection is by a combination of impaction and electrostatic precipitation. However the additional complication of this form of the invention has not been found necessary for applications of interest, impaction alone being sufficient, and so is not preferred.

Claims (14)

Claims

1. A method of measuring the concentration of a given element in a gas as a constituent of particles present in the gas comprising the steps of directing a known volume of the gas as a jet on to a surface, whereby entrained particles are collected on said surface by impaction thereon, thereafter heating the surface to atomise the collected particles, and determining the amount of the collected element by atomic absorption spectrometry of the atomised particles.

2. A method as claimed in claim 1 wherein the element is a metal.

3. A method as claimed in claim 1 or claim 2 wherein said steps form a repeated sub-cycle, the determined amount of the element being presented after each sub-cycle and the surface being cleansed of said element between subcycles.

4. A method as claimed in claim 3 wherein a known quantity of the element is impacted on to the surface after a given number of said subcycles in order to standardise the determinations.

5. A method as claimed in claim 4 wherein the known quantity is in the form of an aerosol produced from a standard solution of the element by nebulisation.

6. A method as claimed in any preceding claim wherein the surface is the inside wall surface of the heatable atomisation tube of an electrothermal atomic absorption spectrometry apparatus and said jet is directed through a hole in said wall.

7. A method as claimed in any of claims 2 to 6 wherein the gas is air and the metal is a toxic metal including beryllium or lead.

8. Apparatus for measuring the concentration of a given element present in a gas as a constituent of particles present in the gas comprising an electrothermal atomic absorption spectrometer including a heatable atomisation tube having an injection hole in the wall, a nozzle located adjacent said injection hole to direct a jet of said gas therethrough, and means for producing a flow of said gas through the nozzle to form said jet, whereby particles entrained in the jet are collected on the inside wall surface of the tube by impaction.

9. Apparatus as claimed in claim 8 wherein the nozzle extends into a gas-tight casing containing the atomisation tube, and the means for producing said flow of gas comprises a pump arranged to draw gas from within the casing.

1 0. Apparatus as claimed in claim 9 comprising a flowmeter connected in series with said pump.

11. Apparatus as claimed in any of claims 8 to 10 comprising means for producing an aerosol containing a known concentration of the element and for injecting said aerosol into the atomisation tube through said nozzle for standardising the spectrometer measurements.

12. Apparatus as claimed in any of claims 8 to 10 comprising atuomatic control means arranged to operate the apparatus according to a repeated sub-cycle comprising the steps of directing a constant volume of the gas through the nozzle, determining the elemental content in each said volume and presenting the thus-determined amount of the element, said control means being further arranged to cause cleansing of the atomisation tube between sub-cycles.

1 3. Apparatus as claimed in claim 12 wherein said control means is further arranged to cause the introduction of a known quantity of the element into the atomisation tube after a given number of said sub-cycles in order to standardise the determinations.

14. A method of measuring the concentrations of a given element present in a gas as a constituent of particles present in the gas substantially as hereinbefore described with reference to the accompanying drawings.

1 5. Apparatus for measuring the concentration of a given element present in a gas as a constituent of particles present in the gas substantially as hereinbefore described with reference to the accompanying drawings.