|dc.description.abstract||The fragmentation processes in the mass spectra of
a series of organophosphorus, organochlorine, thio and
dithiocarbamate as well as a number of miscellaneous pesticides
have been studied i n detail by using the Bendix timeof-
flight, MS-12 single-focussing and MS-30 double-focussing
mass spectrometers. Interpretation of all the spectra have
been presented; their mode s of dissociation elucidated,
aided by metastable transitions wherever possible and the
structures of the various f ragmentation species postulated
wherever f easible. The fragmentation mechanisms are based
on the concepts of inductive, resonance and steric ef~ects.
Multiple bond cleavages accompanied by simultaneous bond
formation and rearrangement reactions involving cycli c t r ansition
states have clarified t he formation of various ions .
Due emphasis has been placed on the effect of the functional
groups or substituents in altering the mass spectral behaviour
of the pesticides as they form the basis for the identifi cation
of the otherwise identical pesticides.
The organophosphorus pesticides which have been
studied include i) the phosphates (eg: DDVP and Phosdrin );
ii) phosphorothionates (eg: Parathion, 0-2, 4 dichloro
phenyl 0, O-diethyl thionophosphate); iii) phosphorothioites
(eg: Tributyl phosphorotrithioite); i V) phosphorothioates
(eg: Ethion) and v) phosphorodithioates (eg: Carbophenolthion).
Cleavages and rearrangements of the ester moiety dominate the spectrum of phosdrin while that of DDVP is
dominated by t he fragmentation modes of the (OH30)2P=0
moiety. Fragmentation §f the (CH30)2P=S characterises
the spectrum of (OH30)2"P -Cl while cleavages of the
(C2H50 )2P=S species mark the spectra of parathion and
0-2, 4- di chlorophenyl O, O-diethyl thiophosphate. The 0(,
cl eavages of the thioether f unction rather than
cleavages of the (C2H50)2P=S signify the spectrum of carbophenolthion.
Tributyl phosphorotrithioite behaves more like
an aliphatic hydrocarbon than like the corresponding phosphites.
The isopropyl and butyl esters of 2, 4 dichlorophenoxy
acetic acid show cleavage and rearrangement ions
typical of an ester. In spite of its structural similari ty
to pp' - DDT and pp' - DDD, Kalthane has a completely
different mass spectral behaviour due to the influence of
its hydroxyl function.
The thiocarbamate pesticides studied include Eptam
and Perbulate. Both are structurally similar but having
different alkyl substituents on nitrogen and sulphur. This
structurQlsimilarity leads to similar types of (N-C), (O-S)
and (S-alkyl cleavages). However, perbulate differs from
Eptam in showing a rearrangement ion at mle 161 and in forming
an isocyanate ion as the base peak. In Eptam the base
peak i s the alkyl ion. The dithiocarbamate, Vegadex,
resembles the thiocarbamates in undergoing simple cleavages but it differs from them in having a weak parent ion;
in the formation of its base peak and in undergoing a series
of rearrangement reactions.
The miscellaneous pesticides studied include
1-Naphthalene acetic aCid- methyl ester, Fiperonyl butoxide
and Allethrin. The ester i s stable to electron impact and
shows only fewer ions. Piper onyl butoxide, a polyether,
shows characteristics of an et her, alcohol and aldehyde .
Allethrin is regarded as an ester of the type R-C-O-R1 with
R being a substituted cyclopr opane moiety and o Rt, a
substituted cyclopentenone mOiety. Accordingly it shows
cleavage ions typical of an aliphatic ester and undergoes
bond ruptures of the cyclic moieties to give unusual ions.
Its base peak is an odd electron ion, quite contrary to