Use of sulfoxides for testing ionization detector system

A method of simulating a positive response by volatile organophosphorus ers to an ionization detector system which comprises forwarding to the ionization detector system a gaseous stream comprising water vapor and a di(lower alkyl) sulfoxide or a cyclic sulfoxide represented by the formula I: ##STR1## wherein R is hydrogen or a lower alkyl group, in an amount sufficient to elicit a positive response by the ionization detector system.

BACKGROUND OF THE INVENTION 
This invention relates to the use of sulfoxides such as dimethyl sulfoxide 
or tetramethylene sulfoxide to simulate a positive response of volatile 
organophosphorus esters to an ionization detector system. 
An ionization detector system has been developed as an alarm system to 
selectively respond to volatile chemical agents in the atmosphere, such as 
toxic organophosphorus esters, for example, iso-propoxymethylphosphoryl 
fluoride (hereinafter "GB"), pinacolymethylphosphoryl fluoride 
(hereinafter "GD") or O-ethyl S-(2-diiso-propylamino)ethyl 
methylphosphonothioate (hereinafter "VX"). 
It would be desirable to test periodically the ionization detector alarm 
system by use of relatively non-toxic volatile chemical reagents which 
would simulate the response of the toxic organophosphorus esters to the 
ionization detector system. 
SUMMARY OF THE INVENTION 
In accordance with this invention, there is provided a method of simulating 
a positive response by volatile organophosphorus esters to an ionization 
detector system which comprises forwarding to the ionization detector 
system a gaseous stream comprising water vapor and a di(lower alkyl) 
sulfoxide or a cyclic sulfoxide represented by the formula I: 
##STR2## 
wherein R is hydrogen or a lower alkyl group, in an amount sufficient to 
elicit a positive response by the ionization detector system.

DETAILED DESCRIPTION OF THE INVENTION 
As used herein, by the term "volatile organophosphorus esters" is meant the 
toxic volatile derivatives of methylphosphonic acid including GB, GD, and 
derivatives of methyldichlorophosphine such as VX. GB and GD may be 
prepared by reaction of methylphosphonyl difluoride with iso-propyl 
alcohol and pinacolyl alcohol, respectively. VX may be prepared by 
conversion of methyl dichlorophosphine into 
ethyl-(2-diiso-propylamino)ethyl methylphosphonate which, in turn, is 
mixed with a sulfur source to form VX. 
By the term "lower alkyl" as used herein is meant straight and branched 
chain alkyl groups of one to six carbon including methyl, ethyl, propyl, 
butyl, pentyl, hexyl and the corresponding branched-chain isomers thereof 
such as iso-propyl, tert- or sec-butyl, iso-valeryl and iso-hexyl. 
Typical suitable di(lower alkyl) sulfoxides include dimethyl sulfoxide, 
diethyl sulfoxide, di-n-propyl sulfoxide, diiso-propyl sulfoxide, 
di-n-butyl sulfoxide, di-sec-butyl sulfoxide, di-n-pentyl sulfoxide, 
di-n-hexyl sulfoxide, methyl ethyl sulfoxide, and n-propyl sec-butyl 
sulfoxide. Di(lower alkyl) sulfoxide are commerically available but may be 
prepared by oxidation of the corresponding sulfide with, for example, 
meta-chloroperbenzoic acid. 
Typical suitable cyclic sulfoxides represented by the formula I include 
tetramethylene sulfoxide, 3-methyl-, 3-ethyl, 3-n-propyl-, 3-n-butyl, 
3-n-pentyl, or 3-n-hexyl-tetramethylene sulfoxide. Tetramethylene 
sulfoxide is commerically available. The 3-substituted tetramethylene 
sulfoxides may be prepared by peracid oxidation of the corresponding 
3-(lower alkyl) substituted tetramethylene sulfide which is conveniently 
obtained by a catalytic reduction of the corresponding 3-(lower alkyl) 
substituted thiophene. For example, the 3-(lower alkyl) substituted 
thiophenes can be prepared by contact of the appropriate alkadiene with 
sulfur for a short time at elevated temperatures. Thus, isoprene 
(2-methylbutadiene) is contacted with sulfur for about 2 seconds at 
566.degree. C. to form 3-methylthiophene. 
The instrument known as an ionization detector system operates at or near 
atmospheric pressure to detect trace (10 to 100 ppb) toxic impurities, 
e.g., GB, GD, or VX in a moist air stream. The moist air stream, heated to 
about 60.degree. C. (the normal operating temperature of the system) is 
pumped into the system and passes over a source of ionization radiation, 
normally a beta-ray source, such as titanium tritide coated foil. The 
ionized air contains primary ions (N.sub.2.sup.+, O.sub.2.sup.+, and 
O.sub.2.sup.-) formed near the beta-ray source by electron impact or 
attachment. A sequence of ion-molecule reactions follow and equilibrium 
between ionic clusters is rapidly established. The mixture of air and ions 
is drawn through a series of baffles to a detector in the form of a 
Faraday Cup ion collector. On the end of the Faraday Cup is a grid to 
allow the air to exit. The beta-ray source is in electrical contact with a 
center manifold stud and can be biased either positively, negatively, or 
maintained at zero potential with respect to the collector. The collector 
responds to the ions by producing a current which is conveniently measured 
by a picoammeter. Compounds, such as volatile toxic organo-phosphorus 
esters and the sulfoxides useful in this invention elicit a positive 
response at the collector part of the ionization detector system. By the 
term "positive response", as used herein in reference to compounds such as 
the sulfoxides useful in this invention, is meant an ion current produced 
at the collector by the ionized gaseous stream containing water and a 
sulfoxide. The ion current produced should be easily and reproducibly 
measured compared to the background at compund concentrations of less than 
about 100 parts per billion (ppb). 
Volatile toxic organophosphorus esters such as GB, GD, and VX are 
sensitively detected by the ionization detector system in that (1) GB, GD, 
or VX produce a signal of about 2 to 4 volts at concentrations of less 
than about 100 ppb, and (2) there is a reasonably linear relationship 
between the signal and the concentration of, for example, GB, GD, or VX. 
The sulfoxides useful in the present invention also are sensitively 
detected by the ionization system. 
To practice the method of this invention, trace concentrations (less than 
100 ppb) of the sulfoxides usful in this invention are prepared in a 
continuous flow by using air dilution of the vapor from the sulfoxides of 
interest and water vapor. 
Based on the response of known concentrations of dimethyl sulfoxide, 
diethyl sulfoxide, and tetramethylene sulfoxide (at constant flow rates), 
we have shown that there exists a linear relationship between the 
logarithm of the concentration of the ions reaching the detector (and 
hence the current) and the inverse of the square root of the molecular 
weight of the sulfoxide. Each of the three above-listed sulfoxides 
followed this linear relationship, and produced an ion current at the 
picoammeter of the ionization detector system equivalent to a 3 volt 
response. 
The concentrations of other sulfoxides useful in this invention required to 
give a positive response of 3 volts are shown in Table I hereinbelow. 
Based on the data for the sulfoxides useful in this invention, the 
calculated concentrations of GB, GD, and VX which would be expected to 
give a positive response of about 3 volts are 4.8, 3.4, and 1.2 ppb, 
respectively. In making these calculations, it is assumed that the 
sulfoxide oxygen and phosphoryl oxygen have similar basicities and that, 
therefore, a sulfoxide and organophosphonate ester of the same molecular 
weight would give the same quantitative response at the ionization 
detector of this invention. In an actual test using the ionization 
detector system described hereinabove, a concentration of GB of 6.4 ppb 
gave a voltage reading of 2.2.+-.0.2 volts, a concentration of 6.0 ppb of 
GD gave a voltage reading of 4.0.+-.0.2 volts, and a concentration of VX 
of 1.7 ppb gave a voltage reading of 3.7.+-.0.2 volts. 
Thus, the sulfoxides useful in this invention are excellent substitutes for 
the toxic organophosphorus ester in the ionization detector system. The 
sulfoxides useful in the present invention gave a positive response in the 
ionization detector system described hereinabove that is both 
qualitatively and quantitatively equivalent to the behavior of 
organophosphorus esters including GB, GD, and VX. 
TABLE I 
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Molecular Weight 
Concentration 
Compound (g/mole) (ppb) 
______________________________________ 
(C.sub.2 H.sub.5).sub.2 SO 
106 10.4 
(C.sub.3 H.sub.7).sub.2 SO 
134 4.8 
(C.sub.4 H.sub.9)SO 
162 3.2 
(CH.sub.2).sub.4 SO 
104 4.4 
______________________________________ 
The selection of the sulfoxide to use for testing the ionization detector 
system for response to a particular organophosphorus ester will depend 
upon the sensitivity desired. The sulfoxide should have a vapor pressure 
at the temperature for testing the ionization detector alarm system so as 
to produce a concentration in the vapor state which will give an 
acceptable positive response at the detector. Thus, under arctic 
conditions, one should select a cyclic sulfoxide of the formula I, 
whereas, in tropical climates, one should select one of the many di(lower 
alkyl) sulfoxides useful in this invention. The physical properties of the 
sulfoxides (vapor pressure, molecular weight) provides a basic for 
limiting the size of the di(lower alkyl) groups and lower alkyl group, R, 
in the sulfoxides useful in this invention. 
Trace concentrations of samples of the sulfoxides may be prepared by air 
dilution of the sulfoxide of interest. The concentration of water vapor in 
the gaseous stream forwarded to the ionization detector system should be 
at least about 3 ppm.