Residues of the Fumigant Phosphine in Arid Vegetable Foods

Ein Bericht aus unserem Laboralltag

Ellen Scherbaum, Dr. Roland Perz, Erika Caspart, Anja Barth, Anne Wolheim and Dieter Köhl // Translated by Anja Barth and Catherine Leiblein

 

SummaryPicture.

After establishing an analytical method that is sensitive enough to detect the fumigant phosphine (limit of determination 0.1µg/kg), 101 samples of cereals, legumes, oil seeds and spices were analyzed for residues of phosphine.
Residues of hydrogen phosphide were traceable in 24.7 % of these samples, albeit with concentrations well below the official maximum residue limits (MRLs).
Residues were more often found in legumes, spices and nuts than in cereals. In these cases the origin and mode of transportation could be an issue, because phosphine is frequently employed in sea containers.
Residues of phosphine were traceable in as many as 31 % of the conventional food samples, whereas significantly fewer cases of organic food samples (12 %) were affected.  However, hydrogen phosphide is not permitted in organic cultivation, so these findings are not to be expected.   It was conspicuous, however, that the amount of residue concentration detected in the conventional and organic products was similar. This indicates either mingling of organic and conventional products or illegal application. Hence, investigations are continuing.

 

Background

Fumigant

Fumigants are gaseous substances used for pest control in buildings, rooms or containers. These fumigants are intended to rid stored commodities of mites, insects or other unwanted pests. This should take place both between and inside product particles to make sure the product is pest-free and to meet all requirements of quality.
The most noted field of application are sea containers. In addition, onshore warehouses are also fumigated with phosphine to destroy pests. Because of the high toxicity, approval for the opening of containers and rooms may only be granted to specially trained personnel. Suppliers and importers are obligated to announce and declare gas-flushed containers [3].

 

Phosphine

Phosphine is a colorless gas in which typical impurities cause an odor of garlic or fish. Due to its high toxicity the usage is allowed only for trained professionals.
In Germany some products with the active substances phosphine and its salts aluminum phosphide and magnesium phosphide are registered for use on coffee, cocoa, fatty seeds, dried fruit, legumes and on stored cereals (Federal Office of Consumer Protection and Food Safety, BVL). Zinc phosphide is only permitted as a rodenticide (against rodents). The dry salts aluminum phosphide and magnesium phosphide are stable, but with moisture from air or crops they gradually release phosphine.
Phosphine in a pure state is considered highly toxic, highly flammable, corrosive and pollutive. Moreover, it is self-igniting in air.

 

Table 1: Brief characterization of the possible active substances [2]

Name

Magnesium phosphide

Aluminum phosphide

Phosphine

Synonyms

trimagnesium diphosphide

 

phosphane. hydrogen phosphide. phosphene

Chemical formula

Mg3P2

AIP

PH3

CAS-number

12057-74-8

20859-73-8

7803-51-2

PubChem

61546

30332

24404

Brief description

grey-green pellets with garlic-like odor

dark grey to yellowish, crystalline solid

inflammable, toxic, color- and odorless gas

Molar mass

134.86 g/mol

50.90 g/mol

34.00 g/mol

State of aggregation

solid

solid

gaseous

Density

2.16 g/cm3

2.42 g/cm3

1.53 g/cm3

Melting point

> 750 °C

1800 °C

-133.8 °C

Solubility

decomposition in water

slow decomposition in water

330 mg/L (20 °C) in water

 

Phosphine affects the central nervous system and irritates the lungs. It is considered very toxic for fish. Using phosphine as a pesticide has caused intoxication in humans, as well as symptoms of intoxication like nausea, vomiting, numbness and spasms, which can result in death. Chronic poisonings are not noted, though, because minor doses are constantly detoxicated in the blood [4].

 

Legal aspects:

For the sum of phosphine and phosphides, maximum residue limits from 0.01 to 0.1 mg/kg foodstuff were set for conventional foods (see table 2).
For organic foods, application of phosphine and phosphides is not designated; therefore, residues should not be found.

 

Table 2: Reg. (EC) No 149/2008 Commission Regulation for phosphine and phosphide

Group of food

Maximum residue limit
sum (mg/kg)

Fruit

0.05

Vegetables

0.05

Potatoes

0.01

Legumes, dried
excluding peas

0.05
0.1

Oil seeds,
excluding sunflower seeds
excluding rape seed

0.05
0.1
0.1

Cereals

0.1

Tea, coffee, cocoa

0.05

Hops

0.02

Spices

0.05

Sugar plants

0.01

 

Establishment of an analytical method

Back in 2003 Swiss colleagues published a gas chromatographic method, where an aqueous solution of sulfuric acid was added to dry foodstuff. Phosphine was thus released as a gas into the head-space above the sample and injected  into a gas chromatographic system with a flame photometric detector (see info box for head-space technology).
The results from Switzerland demonstrated that the limits of determination and quantification have to be very low, because the concentrations measured in samples varied from 0.3 µg/kg to 2.5 µg/kg only [1].
Based on this publication, the Pesticide Residue Laboratory of CVUA Stuttgart has set up a head-space GC method with mass spectrometric detection, where a limit of determination of 0.1 µg/kg is accomplished.  The conditions for analyzing samples with this method have been met this year.

 

Head-space technology in gas chromatography

Picture.

Analysis known as head-space technology is a convenient option for detecting volatile compounds. For this purpose, only the vapor space above the sample is analyzed instead of preparing a sample extract with solvents. The sample is sealed in a 20 mL head-space vial and heated. In the vapor space above the sample, usually at an increased temperature, an equilibrium of volatile components adjusts between the vapor space and the sample, depending on the type and concentration of the analyte. For measurement purposes only the gaseous phase is injected. This method presents some sort of gas extraction. 

 

It is a very convenient method for pharmaceutical, medical, environmental and food samples, because analytes traceable with gas chromatography are completely separated from their matrix [5].

 

First results

To get an overview of the residue situation, 101 samples of cereals, spices, oil seeds, legumes of conventional production and, to a smaller extent, of organic production were analyzed. In 25 of the tested samples residues of phosphine were detectable.  However, as expected, concentrations were well below official maximum residue limits. Legumes, spices and nuts showed more positive results than cereals. In these cases, the origin and mode of transportation could be an issue, because phosphine is frequently employed in sea containers.
In contrast to fruits and vegetables, for many other products information regarding the country of origin is not required for the packaging or price tag. Thus, in many cases, the country of origin for the samples analyzed is “not specified”.

 

Excel-diagramme: Phosphine concentration  in conventional foodstuff.

Figure 1: Number of conventional and organic foodstuff samples with and without residues of phosphine.

 

A detailed presentation of the results for conventional foodstuffs is found in Table 3 and, for organic foodstuffs, in Table 4.

 

Table 3: Results for phosphine in conventional foodstuffs

Foodstuffs

Country of origin

Number of samples analyzed

Thereof positive

Concentration

(µg/kg)

Wheat

Germany

6

1

1.4

not specified

1

0

 

Rye

Germany

3

0

 

Barley

Germany

1

0

 

not specified

1

0

 

Oat

Germany

2

0

 

Rice

Greece

1

1

0.1

Italy

1

0

 

not specified

5

0

 

Buckwheat

Germany

1

0

 

Millet

not specified

1

1

1.5

Cereal products

France

1

0

 

Legumes

Italy

3

1

0.7

Canada

1

1

0.2

not specified

8

1

0.4

Turkey

9

6

0.1 to 0.6

Oil seeds

not specified

1

0

 

Turkey

1

0

 

Nuts

not specified

4

4

0.9 to 3.7

Spices

not specified

13

4

0.7 to 3

Hungary

1

0

 

USA

1

0

 

Syria

1

1

1.4

Tea

China

1

0

 

Total conventional

 

68

21 (31%)

0.1 to 3.7

 

Residues of phosphine were traceable in 31 % of the conventional foodstuff samples, whereas detection in 12 % of the organic foodstuffs were significantly fewer. However, hydrogen phosphide is not permitted in ecological cultivation, so in this case no residues should have been found.  It was notable that the level of concentrations detected in conventional and organic products were in the same range. This indicates either mingling of organic and conventional products or illegal application.

 

Excel-diagramme: Phosphine concentration in organic foodstuff.

Figure 2: Number of samples with and without phospine residues in organic foodstuffs.

 

Table 4: Results for phosphine in organic foodstuffs

Foodstuffs

Country of origin

Number of samples

Positive findings

Concentration (µg/kg)

Wheat

Germany

3

1

1.1

Austria

1

0

 

Rye

Germany

1

0

 

Barley

Germany

2

0

 

Oat

Germany

2

0

 

Rice

not specified

5

1

0.35

Millet

Austria

1

0

 

not specified

1

0

 

Legumes

China

2

0

 

Germany

2

0

 

Canada

3

0

 

not specified

2

1

0.29

Turkey

3

1

0.12

Oil seeds

Germany

1

0

 

Kazakhstan

1

0

 

not specified

3

0

 

Total organic

 

33

4 (12%)

0.3 to 1.1

 

Investigations are continuing.

 

Acknowledgement:

We would like to thank Mr. Thomas Amrein from Coop Swiss for his support and knowledge exchange, as well as for the possibility to participate last-minute in a method validation ring test.

 

References:

[1] Richard Amstutz, Anton Knecht and Daniel Andrey: Detection of phosphine residues in (organic) cereals, Mitteilungen aus Lebensmitteluntersuchung und Hygiene, Band 94, 6/2003, Seite 603-608
[2] Wikipedia, Magnesiumphosphid, Aluminiumphosphid, Phosphorwasserstoff
[3] Wikipedia, Begasungsmittel
[4] www.umweltlexikon-online.de
[5] www.chemgapedia.de

 

Picture credits:

Leercontainer, C. Nöhren, Pixelio.de, Image-ID=120730.

Headspace-GC-Vials, Anja Barth, CVUA Stuttgart.

 

Artikel erstmals erschienen am 11.10.2012