Method and device for analyzing volatile substances in gases

The invention concerns a method and device for the automatic analysis of volatile substances in gases, in particular volatile substances in air, the method comprising the following successive steps: a) the gas to be analyzed is passed through an absortion solution or reaction solution via a sponging type exposure; and b) the solution is analyzed by a gas or liquid chromatography apparatus once the gases reach dissolved solution equilibrium with such absortion solution or reaction solution. According to the invention, samples as tiny as 10-100 microliters of the absortion solution or reaction solution are located in a depression in the bottom of a reaction recipient vessel and, when it has been acted on by the gas to be analyzed thereby additionally forming a reaction product mixed therein, is removed with a cannula and taken for chemical analysis via capillary gas chromatography (CGC) or high performance liquid chromatography (HPLC) equipment means.

BACKGROUND OF THE INVENTION 
The invention refers to a method and a device for automatically analyzing 
volatile substances in gases, in particular in air, the method comprising 
the following successive steps: 
a) the gas to be analyzed is passed through an absorption or reaction 
solution, 
b) the solution is analyzed by a chromatography method. 
Such methods and devices are known: An article of A. Gromping and K. 
Cammann "Development of an automated, quasi-continuous method for the 
simultaneous determination of nitrogen oxides, aldehydes, and ketones in 
air" published in the periodical INSTRUMENTATION SCIENCE & TECHNOLOGY 22 
(1994), pages 25 to 38 describes a method of this type according to which 
the gas to be analyzed is passed through a recipient (impinger) which is 
nearly completely filled with a reaction solution, the gas sparkling 
upwards through the solution. After the intended exposure time a part of 
the solution is pumped via ducts and by means of a peristaltic pump into a 
chromatograph while the remaining solution is withdrawn from the 
recipient. The latter is then cleaned and charged with fresh solution for 
the next analysis cycle. All these steps are automatically supervised and 
controlled by a sequential program. 
A main inconvenience of this method is the low sensitivity especially due 
to the following effects: 
a) The analysis by means of the two chromatographic methods commonly in 
use, i.e. the high resolution liquid chromatography (HPLC) and the 
capillary gas chromatography (CGC) allows to inject only about 20 .mu.l or 
5 .mu.l respectively of the absorption or reaction liquid into the 
chromatograph column. Taking into account a liquid volume of about 20 ml 
this represents only 1% or less of the entire sample volume and signifies 
a corresponding loss in sensitivity. 
b) The absorption or reaction recipient having a volume of several 
milliliter cannot be cleaned at reasonable expenditure to an extent which 
is necessary for a sensitive measurement. Therefore the memory effect is 
high. The memory effect is further increased by the duct through which 
after exposure the solution to be analyzed must flow towards the 
chromatograph. For this reason this duct must also be cleaned after each 
analysis cycle. Moreover the volume of the recipient cannot be reduced in 
size at will since the length of this duct imposes a lower limit of 
recipient volume. Thus, in each cycle 20 ml of solution should be pumped 
towards the chromatograph. Under these conditions concentrations of down 
to 50 ppb of formaldehyde had to be measured. 
The increasing pollution burden of closed rooms due to the improvements in 
the thermal isolation and the use of poisonous adhesives in the furniture 
industry and for posing wallpapers or carpets has created a need for a low 
cost measurement and supervision of even lower pollutant concentrations, 
especially the concentration of formaldehyde in the concentration range 
below 50 ppb. 
SUMMARY OF THE INVENTION 
It is therefore an object of the invention to propose a new method and a 
device adapted thereto allowing in a fully automatic manner the 
realisation of a sequence of analysis cycles which allows the extraction 
of a sample of gases to be analyzed, the adsorption of the sample in a 
reaction solution and finally the analysis of this solution and which 
permits measurement of very low concentration rates of pollutants, for 
example a formaldehyde concentration of 1 .mu.g/m.sup.3 or about 0.8 ppb. 
This object is achieved according to the invention by the method as 
defined in claim 1. The device for implementing this method is 
characterized by claim 3. Concerning features of a preferred embodiment of 
the method reference is made to claim 2. 
The invention will now be described in more detail by means of a preferred 
embodiment and the attached drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The invention is explained in the frame of formaldehyde detection in 
ambient air, but it can also be applied to other products, especially 
pollutants in air or another support gas. 
The air in a room 1 to be analyzed is sucked by means of a sucking pump 2 
through a reaction recipient 3. The gas inlet 4 and the gas outlet 5 of 
this recipient are arranged in such a way that the gas to be analyzed is 
injected through a thin pipe 10 into an absorption and reaction solution 
which is located in a tiny depression 6 at the bottom of the recipient, 
such solution contained in a reaction recipient with said volatile 
substances and said gases reaching dissolved solution equilibrium with the 
absorption or reaction solution while a negative sucking pressure is 
applied to a headspace volume above the absorption or reaction solution. 
The inlet and the outlet can be disposed horizontally or along inclined 
lines in such a way that the pipe 10 extends rectilinearly towards the 
depression 6. 
Above this depression the recipient is obturated by a septum 7 through 
which a cannula 8 can penetrate into the recipient. The recipient is for 
example made of a glass material which is inert with respect to the gas to 
be analyzed and to the reaction solution. The cannula is coupled to a 
transfer system 9 which has been shown here symbolically as a piston-type 
syringe but in reality the transfer system is automated and allows defined 
quantities of reaction products and cleaning liquids to be injected into 
the recipient or of exposed reaction liquid to be extracted therefrom in 
order to be transferred into an automated liquid chromatography column. 
The transfer system and the analysis column are known Per se and are for 
example commercialised under the designation SIL-7A by Shimadzu 
Corporation. In this commercial apparatus a plurality of test tubes each 
filled with an analysis sample are analyzed under the control of an 
automated sequence controller. A cannula penetrates through the septum of 
a tube, extracts a sample and transfers it into a liquid chromatography 
column in which the sample is analyzed separately by means of a convenient 
detector. After the cleaning of the cannula the next-following tube is 
treated. 
The device according to the invention operates preferably as follows: At 
first the transfer system withdraws via the cannula 8 a certain quantity 
of cleaning liquid from a supply tank (not shown) and pours this liquid 
into the reaction recipient 3. By operating the sucking pump the cleaning 
liquid is distributed in the recipient in such a manner that the entire 
inner surface is rinsed. Thereafter the cleaning liquid is extracted 
through the cannula from the recipient and is discharged. The device is 
now ready to receive a well defined quantity of an acid solution of 
dinitrophenylhydrazine through the cannula 8. Thereafter the air or gas 
sample is applied. After the exposure which might last for seconds, 
minutes or hours, as the case may be, the admission of the air to be 
analyzed is interrupted. The next step is constituted by a complete 
withdrawal of the solution via the cannula 8 and by its transfer to a 
dosing system of a liquid chromatograph column allowing to measure the 
content in pollutants. This is the end of the cycle, and a new cycle 
begins. as explained above by cleaning the cannula and the reaction 
recipient, by filling in fresh reaction solution into the depression 6 and 
by again cleaning the cannula 8. 
In the depression an extremely small reaction solution quantity of for 
example only 10 to 100 .mu.l can be charged which after the exposure is 
transferred totally through the cannula into the chromatograph. Due to 
this small quantity of solution which has become possible by the absence 
of the duct towards the chromatograph and by the direct transfer through 
the cannula, the pollutant concentration in the solution becomes higher 
and the cleaning of the recipient and the cannula is notably less 
complicated and consequently the memory effect is reduced to an extent 
that repeatable measurements of concentrations of for example formaldehyde 
in the range of 1 .mu.g/m.sup.3 or even below this value can be obtained. 
Since the entire cycle is performed without human intervention, a large 
variety of analysis requirements can be fulfilled by an appropriate 
programming of the transfer system. This is true for example for the 
duration of the exposure and, as the case may be, for the waiting time 
between two successive sample extractions, as well as for the type and 
quantity of the reaction solution. Thus a complex and variable measurement 
program can be pre-programmed and is performed automatically without human 
intervention. 
The device according to the invention can be realised by modifying the type 
SIL-7A injector mentioned above by simply withdrawing from the injector 
the rack intended to receive the plurality of test tubes and by mounting 
instead in a defined location the reaction recipient shown in the 
drawings. The transfer system then must only be guided to this location 
and to the tanks for the cleaning liquid and for the absorption or 
reaction solution or its components respectively instead of a plurality of 
test tubes awaiting in the rack successively their analysis. In the frame 
of the invention more than one reaction recipient 3 can be foreseen, 
especially two such recipients which are operated alternately, at each 
instant one of them being in the exposure phase and the other in the 
preparative phase for the next exposure. This allows a supervision of the 
air in a room or in the outer environment practically without 
interruption. 
The invention is not restricted to the device shown in detail and to the 
corresponding method. Thus other pollutants than formaldehyde such as 
acetaldehyde, acrolein and acetone can be detected with the device 
according to the invention, and these pollutants can further be contained 
in other support gases than air. As the case may be, the reaction product 
can be a composition other than dinitrophenylhydrazine.