Detection of fumigants

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The importance of having proper equipment for detection of fumigants (down to the threshold limits) cannot be overemphasized. Needless accidents have occured where personnel were unaware of the presence of a fumigant in the atmosphere; furthermore, relatively low concentrations can be hazardous and the health of workers can be adversely affected.

Several fumigants have little or no odour and even for those having a characteristic odour the sense of smell may not always be reliable AS a means of detection. For safety purposes it is considered essential to have detection equipment that will give reliable and immediate indication of toxic concentrations of fumigants. An outline of the priciples of atmospheric monitoring of toxic gases has been given by Thain (1980).

DETECTION DEVICES

A number of instruments or methods are available for the detection of fumigants:

Detector Tubes

Gas detector tubes for determining low levels of several gases are available on the market. These are sealed glass tubes filled with an appropriate indicator chemical to react with a particular gas and give a colour reaction. To make a determination, the seals are broken at each end of the tube and a definite volume of the atmosphere being sampled is drawn through by a handoperated or mechanical pump. The tubes are marked off in scale divisions and the concentration is determined according to the length of discolouration of the indicator for a given volume of atmosphere (see Chapter 6, Figure 18).

Detector tubes are simple, easy to use devices that can provide reasonably reliable, on-the-spot measurement of gas concentrations. Their accuracy may be in the range of 70 to 90 percent of the mean value if sampling is done carefully according to manufacturers' directions. For taking gas samples from difficult locations, extension tubes are available from manufacturers so that the detector tubes can be placed at the desired site.

In addition to these tubes, which give an immediate reaction, long duration tubes for monitoring various toxic gases throughout the normal work day are available. These tubes can be carried anywhere on a worker's clothing in a special holder, while a lightweight pump continuously draws a measured volume of air through the tube. At the end of the shift, the tube can be evaluated to give a time-weighted average (TWA) of exposure for the working day.

Handbooks that describe in detail the characteristics and capabilities of a wide range of detector tubes are available from some manufacturers (Leichnitz, 1979).

In making use of detector tubes some precautions should be noted:

- Tubes will deteriorate with age - some makes have a shelf life of two years when stored at room temperature; above 30C deterioration is more rapid.

- Direct sunlight can affect the properties of the tubes.

- At low temperatures, around freezing or below, tubes may not give reliable readings; they should be warmed to room temperature for best performance.

- Tubes may have cross-sensitivity to gases other than those for which they are designed. Information on cross-sensitivity should be obtained from the manufacturer.

Halide Leak Detector

This instrument, which is described more completely under methyl bromide in Chapter 6, is useful for indicating the presence and approximate concentration of methyl bromide, ethylene dibromide or other halogenated compounds in air. It has been used both as a leak detector to locate fumigant escaping from spaces under treatment and as a safety device around fumigation sites. It is also used to check atmospheres for halide fumigant that may desorb from treated commodities.

It should be noted that, while this device is useful for detecting low levels of halogenated fumigants, it may not be safe for detecting potentially harmful concentrations of them. The threshold limit values for a number of fumigants, including methyl bromide, ethylene dibromide and carbon tetrachloride, are below the limits of detection of this instrument.

Infra-Red Analysers

These are instruments that can measure concentrations of gases by the absorbing effect the gases have on a beam of infra-red radiation. Absorption is proportional to path length of the infra-red beam as affected by concentration of the gas. Fumigants have characteristic infra-red absorption spectra that allow both identification and quantitative analysis.

Instruments are available that are ruggedly constructed, but reasonably light and portable so that they can be used in the field for on-the-spot analysis. They are made with scales that read directly in ppm of the fumigant with a reasonable degree of accuracy. The ranges of detection for a number of fumigants are shown in Table 9. Infra-red analysers can be operated with a minimum of instruction by relatively untrained personnel for spot sampling or they can be used for continuous monitoring of atmospheres in the work place.

Although these instruments are relatively expensive, their capabilities for instantaneous detection of low levels of harmful gases may warrant their use in some situations.

Gas Chromatouraphs

Portable gas chromatography are manufactured that can be used for analysis of fumigants in field projects. These instruments also are expensive but they are very effective for both identifying and measuring concentrations of gases at both high and low levels. An instrument (shown in Figure 10) using direct air analysis, which can be easily operated with minimum instruction and can give results in the ppb range (0.001 mg/kg), is available on the market (Barker and Leveson, 1980).

Other Detectors

A number of new devices are being developed for estimating exposure of individuals to toxic gases (McCammon, 1979). These devices have some distinct advantages for personal protection because they are small, lightweight and can be located in the immediate breathing area of the worker. A whole air sampler known as "Critical Orifice Personal Sampler" has been successfully tested for several years and is commercially available. This is an evacuated stainless steel container with a valve allowing air to enter through a micronsize critical orifice so that an 8-hour sample can be collected. Once the sample has been collected, the valve is closed and the sample returned to a laboratory for analysis. Several passive monitors that collect samples onto a collection medium are becoming available. A pocket-size gas chromatograph that will provide real-time warning to acute exposures and will accumulate a worker's 8-hour TWA exposure is in the developing stages.

Table 9. DATA FOR ANALYSING FUMIGANTS WITH AN INFRA-RED GAS ANALYZER1

Fumigant Analytical Wavelength in Microns Min. Detectable Concentration at 20.25 metres2 Max. Detectable Concentration at 20.25 metres2 Max. Detectable Concentration at 0.75 metres2
    (ppm) (ppm) approx. (ppm) approx.
Carbon disulphide 4.55 0.5 > 1,450 > 39,200
Carbon tetrachloride 12.6 0.06 > 32 > 870
Acrylonitrile 10.5 0.4 > 330 > 8,900
Chloropicrin 11.5 0.05 > 50 > 1,350
Ethylene dibromide 8.4 0.1 > 300 > 8,100
Ethylene dichloride 8.2 0.3 > 550 > 14,800
Methyl bromide 7.6 0.4 > 945 > 25,530
Sulphuryl fluoride 11.5 0.1 > 57 > 1,543
Phosphine3 10.1 1.0 1,000  

1 Foxboro Analitical Company, South Norwalk, CT. 06856, U.S.A.
2 Path lenght of gas cell
3 Phosphine can be detected at 4.3 microns in concentrations as 0.3 ppm; hovever, carbon dioxide also absorbs at this same wavelenght.


4. Field determination of fumigants

A number of instruments are available on the market for analysis of fumigants under practical operating conditions. Determinations may be conducted at regular intervals both in the free space and in the commodity. Used in conjunction with integrated concentration - time products, as described in Chapter 2, such analyses enable the operator to monitor concentrations throughout a treatment and know when the desired measure of treatment has been attained in all parts of the system. The fumigation may then be terminated at the appropriate time. Apart from the determination of full fumigant concentrations during actual exposure, much of the equipment may also be used to measure the success of the aeration process as indicated by the presence or absence of residual vapours. Some of the equipment may also be used for the purpose of detecting leaks from the structure during treatment.

The methods discussed in this chapter are generally applicable to a variety of fumigants. Specialized procedures are given in Chapter 6 under the headings of the particular fumigants concerned.

Thermal conductivity analysers

In recent years a portable instrument known as the thermal conductivity analyser or meter has been used extensively for fumigant determination, principally with methyl bromide. This was first developed for practical use by Phillips and Bulger (1953).

PRINCIPLE OF OPERATION

The basic principle underlying this instrument is that when a constant electric current is passed through a wire, the final equilibrium temperature of the wire is affected by the composition of the gas surrounding it. If the composition of the gas is changed, the equilibrium temperature of the wire will alter. This in turn will alter the resistance of the wire.

In a thermal conductivity apparatus for gas analysis, a Wheatstone bridge circuit is used to measure the imbalance caused by passing gas over the detector filaments. There are usually four or eight filaments in the same number of cells. Half of the cells are used for passing the fumigant/air mixture and the other half, in which only air is present, are used as a control. When an electric current is passed through the filaments, the whole bridge is balanced if the composition of the gases surrounding all the filaments is the same throughout. If the cells surrounding the detecting filaments are filled with a different gas mixture, the bridge becomes unbalanced; the extent of this can be measured by a galvanometer. By calibration with known concentrations of a given gas the galvanometer readings can be transposed into the units of concentration desired, such as 9 per m3.

STANDARD EQUIPMENT

A commercial thermal conductivity analyses has the following components:

1. Four tungsten filaments in as many cells, a pair providing each arm of the bridge. The cells are bored in a brass block. Two cells are used as a control to hold the standard gas, which is air, and the other two are incorporated in the sampling train of the gas-air mixture undergoing analysis.

2. A galvanometer from which readings are made.

3. A potentiometer for current control across the filaments.

4. A separate gas passage for drawing samples through the cells, with inlet and outlet connexions.

5. A source of electric current, which may be provided by batteries contained in the instrument or by connexions to outlets from the local main supply. Direct current of 6 volts is used in the instrument and, if the main supply is used, transformers and rectifiers are needed.

In addition some instruments may contain one or more of the following pieces:

1. An aspirator with a rubber hand bulb for drawing a constant flow of the sample across the cells.

2. An electrically driven pump for the same purpose as in (1).

3. A flowmeter for use when the mechanical pump is used.

4. A guard tube to hold soda-asbestos, or similar material, used to remove water vapour and carbon dioxide from the incoming samples.

In some instruments the reference cells are permanently closed, whilst in others they are open. Sometimes the reference cell is protected by a guard tube, but this is sealed off when sampling begins.

Ideally, it is desirable to take only small samples at a time and an instrument operated by a hand bulb ensures this. When a large structure is undergoing treatment, however, samples have to be drawn from considerable distances and mechanical pumps are necessary.

In practice, the thermal conductivity analyses is unsuitable for use with mixtures of fumigants. It may be possible to calibrate the instrument to indicate concentrations of a mixture in a flask or chamber containing the gaseous mixture only, but in the presence of material being fumigated, the various components of the mixture would be sorbed at different rates and the readings would not provide an accurate indication of the relative proportion of each fumigant present in the free air.

TYPES OF THERMAL CONDUCTIVITY INSTRUMENT

There is a range of instruments available on the market, which vary in accuracy and cost according to the quality of the components incorporated. A bulletin by Heseltine (1961) described in detail the construction and operation of a battery-operated meter, now obtainable commercially, which is of sufficient accuracy to be used in the laboratory and the field. This is shown in operation in Figure 11. A hand bulb is used for aspiration of the sample. (The bulletin contains comprehensive information on many aspects of the use of thermal conductivity meters and anyone employing this technique extensively is advised to study it).

Instruments of the type illustrated in Figure 12 are suitable for largescale fumigations where accuracy beyond + 0.5 mg/l is not required. They may be operated from the local electricity supply or, for short periods, by a car battery.

A small hand-operated analyses (Figure 13) is available at low cost. This also has a rubber bulb for manual extraction of samples and is powered by an "A" dry battery (6 volts). This is accurate to + 1 mg/l and may be used for periodic checks of gas concentrations in commercial treatments. In field practice this instrument has been found to be reliable as long as the checks discussed below are carried out regularly.

CALIBRATION

It is most important that the thermal conductivity analyser be calibrated frequently against a known concentration of the fumigant or fumigants for which it is being used.

Kenaga (1958) described a simple apparatus, using carbon tetrachloride as the standard gas, for the calibration of the thermal conductivity instrument for various fumigants. Carbon tetrachloride gives the same galvanometer reading as methyl bromide, and since it can initially be measured as a liquid at ordinary temperatures, it is more suitable for calibration.

It is also advisable to check periodically the performance of a thermal conductivity analyses under field operating conditions by taking a series of samples for chemical analysis and comparing the results with instrument readings corresponding in position and time to the origin of the samples. The instrument is then adjusted according to the results of the chemical analysis.

For further details of the design and operation of these analysers and their application in the field for the determination of methyl bromide and other fumigants, the following references may be consulted: Phillips and Bulger (1953); Phillips (1957a); Kenaga (1958); Heseltine et al 1958); Monro et al (1953); Heseltine (1961); Koucherova and Lisitsyn (1962); USDA (1976).

Interference refractometers

Instruments designed to utilize differences in the refractive index of gases have been employed successfully for determining fumigant concentrations. In this type of equipment, parallel light from 8 collimator is divided into two beams by two slits and passed through two tubes, into one of which has been introduced the gas mixture under test. The tubes are closed by opticallyworked glass plates. On emerging from the tubes the two separate beams are brought together by a lens and thus produce in the focal plane of the lens very fine vertical fringes, which can be viewed through an eyepiece. After the zero reading has been set in both tubes in ordinary air, a sample of the atmosphere containing the fumigant under test is drawn into one of the tubes by the squeeze bulb and the difference in the refraction of the gases in the two tubes, as shown by a shift in the fringes, is measured on the scale. By suitable calibration of the readings for a particular fumigant gas, the percentage concentration in the atmosphere under analysis may be easily measured. For greater accuracy in making readings, some operators have found that insertion of a piece of glass capillary tube in the bulb tube will regulate the inflow of gas so that the chosen fringe does not move off the scale. Since the brightness of a fringe can vary according to its position on the scale, this ensures that the same selected fringe is used at all times. An instrument of this type is illustrated in Fig. 14.

Theoretically, an instrument employing this principle gives an absolute reading and is not subject to variable conditions, such as variations in voltage or the failure of component parts to function accurately, which may be encountered with other types of instruments. In practice, such an instrument is simple to operate and readings are reproducible under uniform conditions. However, in common with all apparatus used under field conditions, initial accurate calibration is essential.

Instruments that give different concentration ranges and different degrees of sensitivity are available, the price increasing with the sensitivity of the equipment.

Detector tubes

Glass detector tubes used for determining the concentrations of a wide range of gases in air are available on the market. These tubes are particularly suitable for use with fumigants which may present a fire hazard under conditions in which a device such as the halide lamp (discussed below) would present a hazard.

The use and accuracy of two makes of these tubes for a number of different gases have been discussed in detail by Dumas and Monro (1966). A more complete description of glass detector tubes has been given in Chapter 3. The employment of the tubes with a number of fumigants is discussed under the heading of each particular gas in Chapter 6.

Colour indicators

Colour indicators have been developed commercially for fumigant determination, more particularly with the use of ethylene oxide as a sterilizing agent. These indicators are tapes placed in or on the material being sterilized, or they may be small sachets containing chemicals which react proportionately to the intensity or duration of exposure. An automatic toxic gas detector that utilizes indicator tapes has been developed for HCN and other toxic gases. This detector is described in more detail in Chapter 6.

Heseltine and Royce (1960) described the application of sachets for both ethylene oxide and methyl bromide fumigations. According to these authors, the sachets used for ethylene oxide may be inspected for the appropriate colour change, either through a window in the treatment chamber or by withdrawal. The methyl bromide sachets give no direct colour reaction and it is necessary to carry out a titration following withdrawal from the fumigation system. With both gases the sachets may be used to determine if a desired concentration x time product has been reached in any part of the system thus ensuring that control of the insects or other organisms has been achieved.

Lamps

Detector lamps are used, at present, exclusively for halogenated hydrocarbon fumigants. Their best use is for detecting leaks from the system and as a safety check during aeration. They are employed mainly for methyl bromide determination and are discussed under this fumigant in Chapter 6 (see Table 10).

Halide meters

Instruments for measuring low concentrations of halogenated vapours in air, utilizing the principle of photometry, are commercially available.

The intensity of the blue lines of the copper spectrum, produced in an electric arc between two electrodes, is continuously measured with a photo-electric photometer using a blue-sensitive phototube fitted with a blue glass colour filter. Halide vapour coming in contact with the hot tip of the copper electrode reacts to form a copper halide, which vaporizes at the temperature of the electrode and is carried into the arc. The intensity of the blue spectrum is proportional to the concentration of halide vapour present.

These instruments are primarily designed for measuring halogenated hydrocarbons in air from 0 to 500 parts per millions (ppm) with 10 percent accuracy. They are used mainly for safety purposes, but Roth (personal communication, 1967) has found an instrument of this type useful for measuring concentrations of ethylene dibromide up to 7 mg/l in commercial fumigations. For higher concentrations a simple dilutionsampling technique is necessary.

Infra-red (IR) analysers

These instruments, described more fully in Chapter 3, can be used for analysing the high concentrations of fumigants needed for insect control as well as the lower levels that may contaminate atmospheres in the work place. They are portable, battery powered, direct reading and have no flame; they can be used safely in dusty atmospheres and are useful for determining whether a space is safe for occupancy. The use of an IR analyser to measure fumigant concentrations in experimental fumigations has been described by Wetzel et al (1977) and Webley et al (1981).

Gas chromatographs

Portable gas chromatographs are available for gas analysis in field operations (see Figure 10). This instrument is suitable for the high concentrations used for insect control as well as for low concentrations around the threshold limit value for human health (Bond and Dumas, 1982). Although these instruments are expensive, they are accurate and relatively easy to use under field conditions.


5. Fumigant residues

When a pesticide residue remains in food or food products, several factors will determine its importance as a hazard to human health. The average fraction of the total diet likely to contain food with this residue is important, as well as the nature and toxicity of the residue itself.

The following definitions of the terms used in work on pesticide residues are given by the joint FAD/WHO Committee on Pesticide Residues (FAD/WHO 1965a):

Residue: a pesticide chemical, its derivatives and adjuvants in or on a plant or animal. Residues are expressed as parts per million (ppm) based on fresh weight of the sample.

Food factor: the average fraction of the total diet made up by the food or class of foods under discussion. Details of the diet of a country may be obtained from the FAO food balance sheets or other similar data.

Acceptable daily intake: the daily dosage of a chemical which, during an entire lifetime, appears to be without appreciable risk on the basis of all the facts known at the time. "Without appreciable risk" is taken to mean the practical certainty that injury will not result even after a lifetime of exposure. The acceptable daily intake is expressed in milligrammes of the chemical, as it appears in the food, per kilogramme of body weight (mg/kg/day).

Permissible level: the permissible concentration of a residue in or on a food when first offered for consumption, calculated from the acceptable daily intake, the food factor and the average weight of the consumer. The permissible level is expressed in ppm of the fresh weight of the food.

Tolerance: the permitted concentration of a residue in or on a food, derived by taking into account both the range of residue actually remaining when the food is first offered for consumption (following good agricultural practice) and the permissible level. The tolerance is also expressed in ppm. It is never greater than the permissible level for the food in question and is usually smaller.

Nature of fumigant residues

The kind of residue left after a fumigation may consist of original fumigant, reaction products formed by a combination of fumigant with components of the commodity or end products of a formulation that generates the fumigant.

Unreacted fumigant can remain in some materials for appreciable periods after the treatment. Usually, the amount remaining decreases progressively with time; however, some highly sorptive fumigants such as carbon tetrachloride, ethylene dibromide and hydrogen cyanide may persist in some materials for weeks or months after airing (Amuh, 1975; Jagielski et al, 1978; Lindgren et al, 1968).

Some fumigants react with components of commodities to form new compounds. Ethylene oxide can combine with the chlorides and bromides in food to form toxic chlorohydrins and bromohydrins (Scudamore and Heuser, 1971). Methyl bromide is decomposed in wheat to form several non-toxic derivatives (Winteringham et al, 1955) and hydrogen cyanide can combine with sugars in dried fruit to form laevulose cyanohydrin (Page and Lubatti, 1948). Other fumigants may also react with materials being fumigated.

In addition to residue from the fumigant, some by-products from formulations such as aluminium phosphide and calcium cyanide can leave residue on food materials. An ash-like residue of aluminium hydroxide, along with a small amount of undecomposed aluminium phosphide, is left after phosphine is generated. Calcium cyanide leaves a residue of calcium hydroxide after hydrogen cyanide is released.

Significance of fumigant residues

The residues remaining in treated materials after a fumigation may be of significance both as an occupational hazard to workers and others exposed to desorbing gas and as a hazard to consumers eating treated foods.

Although desorbing fumigant may not be considered a residue in the usual sense, appreciable amounts can remain for long periods of time and create hazards for personnel in the immediate vicinity. When treated goods are kept in confined spaces, such as airtight bins or a ship's hold, the residual fumigant can be of considerable consequence. There is great concern over the possibility of long-term effects that may develop from exposure to desorbing fumigant.

Some fumigant may remain in food materials and reach the ultimate consumer. Attention has been focused on residues of pesticides in food in recent years because of the harmful effects they may have on human beings. Concern over toxic chemicals in food has been heightened by sensitive detection methods that show traces of residue not previously suspected. The significance of very low levels of some compounds is not known. However, it is believed that the human body can tolerate small amounts without adverse effects. Therefore, residue tolerances are established on the basis of extensive investigation of toxic hazards.

It should be pointed out that numerous surveys for fumigant residues on food have shown only low levels in just a few samples and that cooking normally reduces these to even lower levels. However, concern has been expressed for fumigated foods that are not normally cooked and special recommendations have been given for these situations (FAD/WHO, 1980). In addition, residues that affect food quality through offensive odours or other factors may be of significance. The effect of fumigants on food quality has been reviewed by Plimmer (1977).

Good fumigation practice will normally require that treatments should be conducted in such a way as to keep residues to the lowest possible level.


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