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    WHO Pesticide Residues Series, No. 1

    1971 EVALUATIONS OF SOME PESTICIDE RESIDUES IN FOOD

    THE MONOGRAPHS

    The evaluations contained in these monographs were prepared by the
    Joint Meeting of the FAO Working Party of Experts on Pesticide
    Residues and the WHO Expert Committee on Pesticide Residues that met
    in Geneva from 22 to 29 November 1971.1

    World Health Organization

    Geneva

    1972

                     
    1 Pesticide Residues in Food: Report of the 1971 Joint Meeting of
    the FAO Working Party of Experts on Pesticide Residues and the WHO
    Expert Committee on Pesticide Residues, Wld Hlth Org. techn. Rep.
    Ser., No. 502; FAO Agricultural Studies, 1972, No. 88.

    These monographs are also issued by the Food and Agriculture
    Organization of the United Nations, Rome, as document AGP-1971/M/9/1.

    FAO and WHO 1972


    HYDROGEN PHOSPHIDE

    Hydrogen phosphide, there referred to as phosphine, was evaluated at
    the 1965 Joint Meeting (FAO/WHO 1965c), reviewed and re-evaluated in
    1966 and 1967 (FAO/WHO 1967b, 1968b) and the recommendations for
    tolerances revised in 1969 (FAO/WHO 1970b). Reference should be made
    to Appendix IV which includes Section 3 of the report of the 1971
    meeting (FAO/WHO 1972a) where general principles concerning fumigant
    residues are discussed.

    IDENTITY

    The data here considered refer to hydrogen phosphide from tablets,
    pellets or paper packets containing aluminium phosphide from which the
    gas is released on exposure to the atmosphere. These preparations
    contain other materials formulated to control the evolution of the
    hydrogen phosphide and to prevent combustion.

    RESIDUES IN FOOD AND THEIR EVALUATION

    Use Pattern

    Post-harvest use on dry foodstuffs

    The fumigant is used in many countries for controlling insects in many
    types of dry stored foods. The main use is for cereal grains in bins,
    in boxcars or on floors when the preparations are added during loading
    or are inserted into the bulk. Stacks of bagged or packaged
    commodities can also be fumigated in sealed stores or under a covering
    of gas-proof sheets. In the United States uses with processed food for
    human consumption is subject to recommendations that no unreacted
    aluminium phosphide come into contact with such food and that none of
    the food will be offered to the consumer before 48 hours airing. In
    the United Kingdom it is also recommended that the fumigant
    preparation will be applied in a manner that permits all powder
    residues to be removed after the treatment.

    Residues in foods

    The types of residues may be considered as follows:

    Unreacted aluminium phosphide. The powdery residue consists mainly
    of aluminium hydroxide but typically contains about 5% of the original
    content of aluminium phosphide, This appears to be occluded within the
    particles of aluminium hydroxide, liberating the fumigant very slowly
    into the atmosphere unless it is disturbed. By following recommended
    practices, these residues are avoided.

    Residues of unreacted phosphine. The information previously reviewed
    indicated that all traces of sorbed phosphine are eliminated very
    rapidly from fumigated food by normal aeration.

    Non-volatile residues. Early studies, as reported by Dieterich et
    al. (1967), concluded that there was virtually no loss of phosphine in
    or reaction with exposed food. It was claimed that practically 100% of
    the phosphine applied in a closed system could be recovered.

    In similar experiments, Berck (1968) was unable to recover all the
    phosphine applied. The losses appeared to be related to the nature of
    the food, its moisture content, the temperature and the exposure time.
    Berck inferred that there was some breakdown on, or reaction with, the
    food. Other experiments of the same type have provided evidence
    supporting the likelihood that there is a small amount of reaction in
    foods, typically of the order of 1 ppm (Heseltine, 1970).

    The high background of phosphorus compounds naturally present in food
    makes it difficult to determine phosphorus-containing reaction
    products in food by conventional chemical analysis. This difficulty
    can be overcome in experiments using radioactively-labelled phosphine.
    Several laboratories are at present active in such investigations.

    Robinson and Bond (1970) used 32P-labelled phosphine derived from
    labelled aluminium phosphide to demonstrate the presence of phosphorus
    residues after treatment of wheat, flour, insects and cystine. The
    radioactive residue on wheat and flour could not be removed by
    thorough aeration or by heating at baking temperature. It was shown to
    be largely water soluble and paper chromatography was used to identify
    the main products as hypophosphite and phosphite. It was concluded
    that the oxidation of phosphine to the lower oxyacids of phosphorus
    was mainly a surface phenomenon and in the normal course of air
    oxidation these residues would eventually all appear as
    ortho-phosphate. Deposition of oxidation products of phosphine was
    found also to occur on glass and other surfaces. Reduction of cystine
    in vitro was found to take place but at a very slow rate. In these
    investigations no attempt was made to ensure that all of the
    radioactivity was accounted for.

    Disney and Fowler (1971a, 1971b) have also provided interim reports on
    a study of the reaction of 32P-labelled phosphine with whole wheat.
    Their results are in agreement with Robinson and Bond. They also
    demonstrate the important effect of moisture content as well as
    exposure period on the amount of the residue which cannot be removed
    by prolonged aeration. After exposure of wheat to a phosphine
    concentration of 3 mg per litre at 25% the residue (calculated as
    phosphorus) varied between 1.2 ppm for a five-day exposure of wheat of
    10% moisture content and 18.4 ppm for a 14-day exposure at a moisture
    content of 19.7%. These treatment levels are several times higher than
    encountered in practical fumigations when it can be inferred that the
    total residue is unlikely to exceed 1 or 2 ppm. Autoradiograms of
    sections of whole wheat showed most of the radioactivity in the outer
    layers and in the crease. It was also found that approximately 70% of
    the residue is extractable from whole grain by hot water.

    A brief summary is available of a third investigation in which
    32P-labelled phosphine was used to treat wheat, flax and rapeseed
    (Tkachuk, 1971). Approximately 50% of the added 32PH3 formed
    non-PH3 residues. In wheat the distribution of residues was
    approximately 85, 12 and 4% in the bran, endosperm and germ fractions.
    In wheat bran approximately 56 and 11% of the residues are
    water-soluble and appear to be hypophosphite and pyrophosphate; the
    remaining 33% are water-insoluble and have not been identified.

    Robison and Hilton (1971) have developed a method to determine traces
    of phosphine released from zinc phosphide in sugar-cane by
    gas-chromatography. Part of the released phosphine reacted
    irreversibly with sugar-cane and, in a separate investigation with
    32P-labelled phosphine added to sugar-cane in aqueous acid, about
    30% of the phosphine reacted irreversibly to form water-soluble
    compounds of phosphorus while another 10% remained irreversibly bound
    in fibre. It was suggested that reaction occurred to phosphorus
    oxyacids which would be water-soluble and that a portion of the acid
    may have formed insoluble iron or aluminium salts in the fibre.

    The general problem of residues in cereals after the use of phosphine
    has been discussed by Robinson (1971a).

    The experimental work with 32P-labelled phosphine has been
    criticized (Rauscher, 1971) on the grounds that this behaves
    differently from inactive phosphine. Robinson and Bond (1970) and also
    Disney and Fowler (1971a, 1971b) had already considered this
    possibility and were satisfied that there was no significant
    difference. In view of the criticisms, however, carefully designed
    experiments to establish the validity of the work with the labelled
    phosphine were performed in both laboratories. Accounts have been
    presented by Robinson (1971b) and by Disney and Fowler (1971c) to a
    Joint FAO/IAEA Panel (FAO/IAEA 1971) which endorsed the conclusion
    that the data obtained by the use of 32P-labelled phosphine were not
    invalidated by the use of the radioactive material.

    Evidence of residues in food in commerce or at consumption

    The methods used to examine samples of grain in commerce for phosphine
    usually also determine that evolved from any residual aluminium
    phosphide present and the small amounts reported, usually below 0.1
    ppm, must normally be derived from the latter source, thus accounting
    for the occasional higher sample in a consignment of grain.

    Methods of residue analysis

    The method developed by Bruce, Robbins and Tuft (1962) hydrolyses
    aluminium phosphide in the presence of dilute sulfuric acid to form
    phosphine. The liberated phosphine is driven out by nitrogen gas into
    scrubbers, and the contents of the scrubbers are analysed by bromine
    oxidation of phosphine to phosphoric acid which is determined
    colorimetrically. A limit of detection in grain below 0.005 ppm is
    claimed.

    The method by Heseltine (1963), based in part on the above method,
    depends upon reaction with acid potassium permanganate and a
    colorimetric determination of the phosphate as the blue reduction
    product of the phosphomolydate. The limit of determination for
    phosphine residues in grain is 0.01 ppm.

    Gas-chromatography has been used to determine concentrations of
    phosphine in air; but it is difficult to apply such methods to the
    determination of residues. Robinson and Hilton (1971) described a
    procedure for determining zinc phosphide in sugar-cane. Phosphine was
    released by suspension of the sample in aqueous acid and toluene in a
    sealed flask and the amount in the toluene layer was determined by
    gas-chromatography using a photometric detector. A considerable loss
    of phosphine occurred and calibration appeared difficult. A correction
    factor of 1.7 was applied to the results to give an accuracy of  10%.
    Absolute sensitivity of the gas-chromatographic determination was very
    high (20 pg of phosphine) and minimum amounts of phosphine equivalent
    to about 0.005 ppm in sugar-cane were reported.

    National tolerances

    Hydrogen phosphide (as reported to the meeting)

    Several countries have 0.1 ppm for raw cereals but West Germany has
    adopted a figure of 0.05 ppm. For grain immediately before milling
    Belgium and the Netherlands reduce this figure to 'zero'.

    The United States of America has a tolerance in or on processed foods
    of 0.01 ppm. The regulation requires that the finished food should be
    aerated for 48 hours before it is offered to the consumer and the
    formulation containing aluminium phosphide must on no account be used
    so that it or its unreacted residues will come into contact with any
    processed food.

    Appraisal

    Preparations of aluminium phosphide which evolve hydrogen phosphide by
    reaction with moisture in the surrounding atmosphere are used for the
    post-harvest fumigation of a wide range of produce including processed
    foods.

    The powder remaining after the use of the fumigant preparation is
    mainly aluminium hydroxide, but may contain a small part, up to about
    5%, of the original content of aluminium phosphide. This presents no
    hazard if good practice is followed. The normal cleaning of cereals
    before milling is effective in eliminating almost all of this powder
    and it has been widely accepted that a residue in a raw cereal of 0.1
    ppm, determined and expressed as hydrogen phosphide would yield a
    residue in bread and other food ready for consumption of a level at or
    below that which can be determined by current methods of analysis
    (0.01 ppm). For other foods which cannot be so cleaned before
    processing good practice requires that the fumigant preparation
    residue does not come into contact with the food. In these
    circumstances any residue of unreacted hydrogen phosphide is rapidly
    reduced below 0.01 ppm. Nevertheless tolerances are required to ensure
    that good practices are observed.

    It has been established that under normal fumigation conditions there
    is some, albeit a very small amount of, reaction of phosphine on or in
    cereals, leaving a non-volatile product which will probably not exceed
    a few parts per million. On present evidence a large part of this
    small residue consists of the lower oxyacids of phosphorus. which, in
    the normal course of air oxidation, can be expected to appear as
    ortho-phosphate. The nature of the reaction or breakdown products in
    food which has been exposed to phosphine requires further
    investigation and studies using isotopically labelled phosphine are in
    progress.

    1.   It is recommended that the present tolerance of 0.1 ppm in raw
         cereals be confirmed.

    2.   On the understanding that, if good practice is followed, any
         residue of hydrogen phosphide, present as such or derived from
         any aluminium phosphide, is reduced below the limit of
         determination by present methods (0.01 ppm), it is recommended
         that the tolerance of 0.01 ppm in flour, other milled cereal
         products, breakfast cereals, dried vegetables and spices should
         be confirmed and extended to include nuts, groundnuts, dried
         fruit, cocoa beans and other similar foods known to be fumigated
         with hydrogen phosphide.

    Further work desirable

    1.   Further elucidation of the nature of the reaction or breakdown
         products of hydrogen phosphide in grain.

    2.   Further data on the residues, if any, determined and expressed as
         hydrogen phosphide, in products known to be fumigated
         commercially with hydrogen phosphide.

    REFERENCES

    Berck, B. (1968) Sorption of phosphine by cereal products. J. Agr.
    Food Chem., 16: 419-425

    Bruce, R. B., Robbins, A. J. and Tuft, T. O. (1962) Phosphine residues
    from Phostoxin-treated grain. J. Agr. Food Chem., 10: 18-21

    Dieterich, W. H., Mayr, G., Hildt K., Sullivan, J. B. and Murphy, J.
    (1967) Hydrogen phosphide as a fumigant for foods, feeds and processed
    food products. Residue Reviews 19: 135-149

    Disney, R. W. and Fowler, K. S. (1971a) Phosphorus-32-labelled
    phosphine in the determination of fumigation residues in grain. Proc.
    2nd Int. Congr. Pestic. Chem., Tel Aviv, Israel, (in press)

    Disney, R. W. and Fowler, K. S. (1971b) Residues in cereals exposed to
    hydrogen phosphide. Working paper for FAO/IAEA Panel, Vienna

    Disney, R. W. and Fowler, K. S, (1971c) The possibility of isotope
    exchanges or other interfering reactions occurring during the
    determination of residues from fumigation with 32P-labelled
    phosphine. Working paper for FAO/IAEA Panel, Vienna

    FAO/IAEA (1971) Tracer aided studies of the fate and significance of
    foreign substances in food and agricultural environment. Proceedings
    of a combined panel and research coordination meeting. Vienna

    Heseltine, H. K. (1963) Determination of phosphine. Pest Infestation
    Research, Agricultural Research Council, London

    Heseltine, H. K. (1970) Pest Infestation Research, Agricultural
    Research Council, London

    Robinson, J. R. (1971a) Hydrogen phosphide residues in cereals.
    Working paper for FAO/IAEA Panel, Vienna

    Robinson, J. R. (1971b) Phosphorous residues from 32-P-phosphine: an
    artifact? Working paper for FAO/IAEA Panel, Vienna

    Robinson, J. R. and Bond, E. J. (1970) The toxic action of phosphine
    Studies with 32-PH3: terminal residues in biological materials.
    J. stored Prod. Res., 6: 133-146

    Robison, W. H. and Hilton, H. W. (1971) Gas-chromatography of
    phosphine derived from zinc phosphide in sugar-cane. J. Agr. Food
    Chem., 19: 875-878

    Tkachuk, R. (1971) Phosphine fumigation of wheat - residue formation.
    Cereal Sci. Today, 16: 307. (Abstract of paper presented at Amer,
    Ass. Cereal Che., Ann. Met Meeting, Dallas)
    


    See Also:
       Toxicological Abbreviations
       Hydrogen phosphide (FAO/PL:CP/15)
       Hydrogen phosphide (FAO/PL:1967/M/11/1)
       Hydrogen Phosphide (FAO/PL:1969/M/17/1)