Pestic. Sci. 1995,44, 249-253

Analysis, Sorption and Residues of a Homologous Series of Isothiocyanates on Wheat Eli Shaaya" & James M. Desmarchelierb*

* Department of Stored Products, ARO Volcani Centre, Bet-Dagan 50250, Israel

(Received 17 August 1994; accepted 28 February 1995)

CSIRO Division of Entomology, GPO Box 1700, Canberra ACT 2601, Australia

Abstract: A series of isothiocyanates (R-NCS, R, 1-6 carbon atoms) was applied in the vapour phase to wheat in sealed containers. The speed of sorption, as measured by the rate of loss from the intergranular air space, was inversely pro- portional to molecular weight; e.g. the time for 50% sorption of hexyl iso- thiocyanate was over 100 times longer than for the methyl analogue. Despite extensive sorption, most of the applied chemicals was recovered from the fumi- gated wheat seven days after dosing, indicating that sorption did not lead to extensive degradation. Purge and trap techniques proved suitable for recovery of low-boiling, but not high-boiling chemicals.

Key words : isothiocyanates, sorption, fumigaticn, residues, homologous series.

1 INTRODUCTION

The rate of sorption of fumigants is an important parameter in fumigation of commodities, as it influences the effective dosage (concentration x time) product of the fumigant. In this context, sorption is typically mea- sured from the decline of vapour phase concentrations of fumigants in sealed enclosures, and typically rep- resented by a plot of log (vapour concentration) versus time after dosing.' The effects of temperature and mois- ture on rate of sorption have been studied and reviewed, and sorption rates of different gases have been studied.' However, to our knowledge, no study has been per- formed on a homologous series. We therefore under- took a comparative study on rates of sorption of a homologous series of isothiocyanates to determine whether such rates correlated with properties such as molecular weight. Isothiocyanates were chosen for this study because of the pesticidal properties of these chemicals' and because of the potential use of methyl isothiocyanates as a fumigant for wheat.3

* To whom correspondence should be addressed.

2 MATERIALS AND METHODS

2.1 Chemicals

Chemicals were either purchased from Aldrich Chemi- cal Co. (methyl and ally1 isothiocyanate) or prepared by reaction of alkyl halide with potassium thiocyanate, fol- lowed by thermal isomerization and distillation. Product identity was confirmed by Gas Chromatography/Mass Spectrometry on a Finnigan Ion Trap, and separation on a DB624 capillary column (J&W 122-1 334).

2.2 Dosing and sorption

Procedures for sorption followed the protocol approved by the Sixth International Working Congress on Stored Products (H. J. Banks, pers. comm.). Wheat, in this case Australian Standard White wheat of 12% moisture content, was added to 120-ml flasks which were fitted with a Mininert valve. Jars were filled to 95% with wheat and another series of jars were filled to 20%. A small piece of filter paper was placed on top of the

249 Pestic. Sci. 0031-613X/95/$09.00 0 1995 SCI. Printed in Great Britain

250 Eli Shaaya, James M . Desmarchelier

wheat. Fumigant (5 pl) was injected onto this paper, with the exception that the solid methyl isothiocyanate was injected as a 500 g litre-' solution in methanol. Vapour samples (10 pl) were taken at timed intervals and analyzed by gas chromatograph, on a Varian Series 3300 chromatograph, fitted with a Thermionic Specific Detector. The column was 15 m x 0.53 mm internal diameter DB wax (J&W 125-7012) and the isothermal temperature ranged from 120°C, for methyl iso- thiocyanate, to 1 50°C, for hexyl isothiocyanate. All experiments were performed at 25°C in triplicate and the mean result recorded.

2.3 Residue studies

The methodology for residue studies on wheat followed the purge-and-trap procedure of Heikes and H ~ p p e r , ~ whereby grain in boiling water was purged with nitro- gen, with the exception that: (1) the trap comprised two tubes each containing 0.2 g activated charcoal; (2) the charcoal was extracted in 5 ml methanol in a sealed vial; and (3) the fumigants were trapped over 1-h, 2-h and 3 h periods, to ascertain completeness of recovery. Recoveries were also measured for fumigant-free wheat dosed with fumigant immediately prior to 'residue' determination. Procedures for extracting the fumigant sorbed on charcoal were to add the charcoal to a 10-ml vial containing 5 ml methanol, followed by sealing, shaking, and extraction after standing for 3 h and for 24 h.

In addition, wheat (40 g) previously dosed with hexyl isothiocyanate at 40 mg kg-' was extracted in meth- anol (60 ml), and after 24 h extraction, 1 pI of super- natant, was injected into the gas chromatograph. Column conditions were 90°C for 2 min, followed by temperature programming at 50°C min-' to 140°C and a further holding period of 3 min.

The two extraction procedures used for hexyl iso- thiocyanate, namely the purge-and-trap procedure and methanol extraction, were also tested on another fumi- gant of low volatility, chloropicrin. This was chosen both because of its low volatility and because it is some- times added to the widely used fumigant, methyl bromide.

3 RESULTS

3.1 Sorption studies

The sorption of isothiocyanates in wheat is shown in Fig. I(a)-(e) where lines of best fit are drawn through the data. As expected,' sorption of each fumigant was faster in containers 95% full of wheat than in containers 20% full. Typical sorption graphs' which plot log (concentration) versus time show an initial rapid drop,

TABLE 1 Time to 50% Sorption of Alkyl Isothiocyanates, RCNS, on

Wheat of 12% Moisture Content at 25°C

Time to 50% sorption for givenJilling ratio (min)

Substituent ( R ) 20% full 95% full

Methyl 28 Ethyl 51 Ally1 240 Pentyl 390 Hexyl 960

6 30 50

330 780

followed by a decline that is linear, or close to linear. The rapid initial decline was observed with the iso- thiocyanates (Fig l(a)-(e)), but the subsequent plots were not linear. Rather, the rate of sorption slowed after c. 90% of the fumigant has been sorbed (log (C/C,) < - 1, where C , is the applied concentration).

For containers 20% full, corresponding times to 50% sorption increased from 28 min to 960 min (Table 1). Thus, the effect of increasing chain length was to slow the rate of sorption by factors which ranged up to 130 in the case of methyl versus hexyl isothiocyanate.

3.2 Methods of analysis

The instrument response of vapour concentration of each isothiocyanate was linear over a tested range of 0.4-40 mg litre-', and the signal to noise ratio at the lowest tested dose considerably exceeded a factor of 10.

Recoveries by the purge-and-trap procedure are listed in Table 2. In no case did the second trap contain appreciable quantities of any fumigant, indicating that the first trap was adequate. However, the procedure of refluxing for 1 h was inadequate for all isothiocyanates except methyl isothiocyanate. A second hour of reflux- ing significantly increased recovery of ethyl, ally1 and pentyl isothiocyanate (Table 2). Extraction of the fumi- gants from charcoal into methanol was largely complete after 3 h (Table 2), but increasing the time of extraction to 24 h increased the amount extracted by approx- imately 10% (Table 2).

Recoveries of hexyl isothiocyanate by the purge-and- trap technique were poor. However, recoveries of this chemical, applied to 40 mg kg-' and extracted from the whole grain into methanol, were quantitative (see Table 3). In a comparative study on recovery of another non- volatile fumigant, recoveries of chloropicrin by the purge-and-trap technique were 37%, but exceeded 95% by the method of methanol extraction.

The method of Heikes and Hopper? with some modifications, was thus validated for isothiocyanates with chains containing one to five carbon atoms, but

1.6

0.0

0 40 80 120 160

Time (h)

I Fig. 1. Sorption of a homologous series of alkyl isocyanates

I T 1 I I I

0.0 1 7 40 80 120 160

Time (h)

.

0.0 r I I 1 8 I I I 0 40 80 120 160

Time (h)

I I I I I I 1 -1 0.0 ' 0 40 80 120 160

Time (h)

252 Eli Shaaya, James M. Desmarchelier

TABLE 2 Recovery of Alkyl Isothiocyanates from Spiked Samples by Purge-and-Trap Techniques

Recovery (Yo of applied) after extraction in methanol for

Substituent Trapping period Recovery in each time period (R) (h) (YO of recovered alkyl isothiocyanate) 3 h 24 h

Methyl 0- 1 100 78 87

Ethyl 0-1 78 88 97

1 -2 0 2-3 0

1-2 22 2-3 0

1-2 53 2-3 0

1-2 16 2-3 0

1 -2 0

Allyl 0- 1 47 61 69

Pentyl 0-1 84 14 82

Hexyl 0-1 100 17 19

was unsuitable for hexyl isothiocyanate and for chloro- picrin, presumably because of their low volatilities.

3.3 Residues

Recoverable residues of isothiocyanates, after one week of fumigation, are outlined in Table 3. For each fumi- gant, recoverable residues always exceeded 38% of the applied amount, and reached 90% for ethyl iso- thiocyanate. In the case of hexyl isothiocyanate, results from the purge-and-trap method must be treated with caution, because of the low recovery from spiked samples (Table 2). However, recoverable residues by methanol extraction were high, and this method also resulted in quantitative recovery from spiked samples.

TABLE 3 Residues on Fumigated Wheat, One Week after Dosing, by

Purge-and-Trap Techniques

Residue, corrected for recovery

Recovery (% of dose) from spiked

Substituent sample jar jar ( R ) (%I 20% full 95% full

~~ ~ ~

Methyl 87 42 46 Ethyl 97 88 90 Allyl 69 60 64 Pentyl 82 40 36 Hexyi 19” 94“ 128”

1 OOb 87b

Unsuitable, due to low percentage recovery. bAfter extraction of whole grain into methanol.

4 DISCUSSION

Three items are discussed, namely the analytical meth- odology of the purge-and-trap procedure, the corre- lation between rate of sorption and substituent in a homologous series, and the residues in grain resulting from fumigation with isothiocyanates.

The purge-and-trap procedure involves purging vapours in water with nitrogen through a reflux con- denser into a trap. Low recoveries of chemicals with low vapour pressures, such as hexyl isothiocyanate, were presumably due to condensation on the condenser. This, at least, is a plausible explanation for the low recoveries of chloropicrin and hexyl isothiocyanate by the purge-and-trap method. The method of solvent extraction into methanol, which is a method used for grain protectants,’ proved suitable for the high-boiling chemicals chloropicrin and hexyl isothiocyanate, where interferences from the solvent and solvent extracts were negligible. Solvent extraction for compounds of lower molecular weight, however, could cause problems of interference. It may be that simple solvent extraction procedures could complement purge-and-trap tech- niques. In any case, purge-and-trap techniques which pass fumigant through a condenser are suitable for some fumigants.

The influence of the carbon chain in a homologous series of isothiocyanates on rate of sorption are quite pronounced. This is the first direct study relating sorp- tion to substituent in a homologous series, and it is clear that rate of sorption decreases with increasing molecular weight.

Methyl isothiocyanate has been examined as a poten- tial grain f ~ m i g a n t . ~ Our studies indicate that such a use would pose a potential problem with residues. At

Sorption of a homologous series of isothiocyanates on wheat 253

the very least, a withholding period considerably greater than one week would be required before residues decayed to levels near the limit of detection.

ACKNOWLEDGEMENTS

The partners to the Stored Grain Research Laboratory, where the work was performed, are thanked for finan- cial assistance. Le Trang Vu is thanked for technical assistance.

REFERENCES

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2. Fenwick, G. R., Heaney, R. K. & Mullin, W. J. Critical Reviews in Food Science and Nutrition, 18 (1983) 123-201.

3. Ducom, V., Proc. 6th Inter. Work. Con$ Stored Products, Canberra, Australia, 1994, ed. E. Highley (in press).

4. Heikes, D. L. & Hopper, M. L. J. Assoc. Off Anal. Chem.,

5. Desrnarchelier, J. M., Bengston, M., Connell, M., Minett, W., Moore, B., Phillips, M., Snelson, J., Sticka, R. & Tucker, K. Pestic. Sci., 8 (1977) 473-83.

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