DIMETHOATE

Organic Service Branch II
OSHA Technical Center
Salt Lake City, Utah

1. General Discussion

   1.1 Background

   1.1.1 History of procedure

   This evaluation was undertaken to determine the effectiveness of the OVS-2 tube as a sampling device for dimethoate. It follows the procedure developed for several other organophosphorus pesticides (Ref. 5.1).

   1.1.2 Toxic effects (This Section is for information only and should not be taken as the basis of OSHA policy).

   The following paragraph is excerpted from the book OCCUPATIONAL DISEASE, A Guide To Their Recognition (Ref. 5.2).

   The organic phosphorus compounds act as irreversible inhibitors of cholinesterase, thereby allowing the accumulation of large amounts of acetylcholine. When a critical level of cholinesterase depletion is reached, usually about 20% of normal, symptoms and signs of acetylcholine accumulation poisoning become manifest. Symptoms may include blurred vision, weakness, nausea, headache, abdominal cramps, chest discomfort, and diarrhea. Signs may include miosis, muscle twitching, salivation, sweating, tearing, cyanosis, convulsions, and coma.

   Besides being absorbed following inhalation or ingestion, arganophosphorus pesticides are readily adsorbed through the intact skin (ref. 5.2). When a particular pesticide has a low dermal LD₅₀, a skin notation should be added to the TLV or PEL.

   Dimethoate has an acute oral LD₅₀ of 152 mg/Kg and acute dermal LD₅₀ of 353 mg/Kg for rats (Ref. 5.3).

   Due to these factors an arbitrary target concentration of 0.2 mg/m³, with a skin notation, ws chosen for dimethoate.

   1.1.3 Potential workplace exposure

   Domethoate is a systematic insecticide/acaricide used to spray walls of farm buildings to control houseflies and in the control of insects on ornamental plants, vegetables, fruits, and farm crops. no data is available on the extent of work place exposure (Ref. 5.4).

   1.1.4 Physical properties (Ref. 5.3, 5.4 and 5.5)

   CAS number:	60-51-5
   IMIS number:	D617
   Molecular weight:	229.3
   Molecular formula:	C5H12NO3PS2
   Melting point:	52 to 52.5°C at 101.3 kPa (760 mmHg)
   Solubility:	very slightly soluble in water freely soluble in most organic solvents, except saturated hydrocarbons.
   Chemical name:	Dimethoate
   Synonyms:	Phosphorodithioic acid, 0,0-Dimethyl S-[2-(methylamino)-2-oxoethyl]ester; 0,0-Dimethyl S-(N-Methylcarbamoylmethyl) Phosphorodithioate; Cygon; Fostion MM; Perfekthion; Rogor; Roxion
   Appearance:	white crystals
   Structure:

   
   

   1.2 Limit defining parameters

   The detection limit of the analytical procedure, including a 12.5:1 split ratio, is 0.078 ng per injection. This is the amount of analyte which will give a peak whose height is approximately five times the baseline noise.

2. Sampling Procedure

   2.1 Apparatus

   2.1.1 A sample is collected by using a personal sampling pump that can be calibrated to within ±5% of the recommended flow rate with the sampling device in line.

   2.1.2 OVS-2 tubes, which are specially made 13-mm o.d. They are packed with a 140-mg backup section and a 270-mg sampling section of cleaned XAD-2. The backup section is retained by two foam plugs and the sampling section is between one foam plug and a 13-mm diameter glass fiber filter. The glass fiber filter is held next to the sampling section by a polytetrafluoroethylene (PTFE) retainer. These tubes are commercially available.

   

   Figure 1.  OVS-2 Sampling Device

   2.2 Reagents

   No sampling reagents are required.

   2.3 Sampling technique

   2.3.1 Attach the small end of the OVS-2 sampling tube to the sampling pump with flexible plastic tubing such that the large front section of the sampling tube is exposed directly to the atmosphere. Do not place any tubing in front of the sampler.

   2.3.2 Attach the sampler vertically (large end down) in the employee's breathing zone in such a manner that it does not impede work performance.

   2.3.3 After sampling for the appropriate time, remove the sampling device and seal the tube with plastic end caps.

   2.3.4 Wrap each sample end-to-end with an OSHA seal (Form 21).

   2.3.5 Record the air volume for each sample and list any possible interferences.

   2.3.6 Submit at least one blank with each set of samples. Handle the blank in the same manner as the other samples, except that no air is drawn through it.

   2.3.7 Submit any bulk samples for analysis in a separate container. Do not ship bulk samples with the air samples.

   2.4 Desorption efficiency

   A 13-mm glass fiber filter and a amount of XAD-2 adsorbent equal to the sampling section (270 mg) of an OVS-2 tube were placed in each of nine 4-ml vials. They were divided into three groups of three vials. These groups were liquid spiked respectively with 9.5, 19 and 38µL of 5.15 mg/mL solution of dimethoate in toluene by spiking the glass fiber filter. These amounts represent 0.5×, 1.0×, and 2.0× the target concentration. They were then sealed with PTFE-lined septa and allowed to equilibrate overnight in a drawer at room temperature. The tubes, along with a blank tube, were desorbed with 2.0 mL of the desorbing solution, and analyzed as in section 3. The average desorption efficiency was 97.1%. The results are listed in Table 2.4.

   Table 2.4
   Desorption Efficiency

   Sample #	Amount Spiked, µg	Amount Found, µg	% Recovered
   D1	48.9	46.8	95.7
   D2	48.9	47.9	98.0
   D3	48.9	50.2	102.3
   		Average of 0.5× PEL= 98.7%
   D4	97.9	93.5	95.5
   D5	97.9	92.8	94.8
   D6	97.9	98.2	100.3
   		Average of 1.0× PEL = 96.9%
   D7	195.7	186.1	95.1
   D8	195.7	189.0	96.6
   D9	195.7	187.3	95.7
   D10	0.0	0.0	Blank
   		Average of 2.0× PEL = 95.8%

   
   

   2.5 Retention efficiency

   Four OVS-2 tubes were each liquid spiked with 19 µL (1× PEL) of 5.15 mg/mL solution of dimethoate in toluene by spiking the glass fiber filter. These were allowed to equilibrate overnight in a drawer at room temperature and then 480 L of humid air (~80% relative humidity) were drawn through each tube at 1.0 L/min. The tubes, along with a blank tube, were desorbed with 2.0 mL of desorbing solution, and analyzed as in Section 3. No analyte was observed in backup sections. The results are listed in Table 2.5.

   Table 2.5
   Retention Efficiency

   Sample #	Amount Spiked, µg	Amount Found, µg	% Recovered
   R1	97.9	92.3	94.3
   R2	97.9	95.3	97.3
   R3	97.9	95.2	97.2
   R4	97.9	98.0	100.1
   R5	0.0	0.0	BLANK
   		Average = 97.2%

   
   

   2.6 Sample storage

   Eight OVS-2 tubes were each liquid spiked with 19 µL (1× PEL)of 5.15 mg/mL solution in toluene by spiking the glass fiber filter. These tubes were allowed to equilibrate overnight in a drawer at room temperature and then 480 L of humid air (~80% relative humidity) were drawn through each tube at 1.0 L/min. The eight tubes were divided into two groups of four tubes each. The fires group was stored in a drawer at ambient temperature, the second group was stored in a freezer (-5°C). After seven days they were extracted and analyzed as in Section 3. No analyte was observed n backup sections. The results are given in Tables 2.6.1, and 2.6.2.

   Table 2.6.1
   Ambient Storage

   Days Stored	Amount Spiked, µg	Amount Found, µg	% Recovered
   7	97.9	94.1	96.1
   7	97.9	96.3	98.4
   7	97.9	97.2	99.3
   7	97.9	97.0	99.0
   7	0.0	0.0	BLANK
   		Average = 98.2%

   
   

   Table 2.6.2
   Freezer Storage

   Days Stored	Amount Spiked, µg	Amount Found, µg	% Recovered
   7	97.9	96.7	98.8
   7	97.9	98.7	100.8
   7	97.9	96.4	98.5
   7	97.9	99.2	101.3
   		Average = 99.9%

   
   

   2.7 Recommended air volume and sampling rate

   2.7.1 The recommended air volume is 480 L.

   2.7.2 The recommended flow rate is 1.0 L/min.

   2.8 Interferences (sampling)

   It is not known if any compounds will interfere with the collection of dimethoate. Any suspected interferences should be reported to the laboratory with submitted samples.

   2.9 Safety precautions (sampling)

   2.9.1 Attach the sampling equipment in such a manner that it will not interfere with work performance or employee safety.

   2.9.2 Follow all safety practices that apply to the work area being sampled.

3. Analytical Procedure

   3.1 Apparatus

   3.1.1 A GC equipped with an FPD. A Hewlett-Packard 5890A GC (capillary equipped with both an FPD operating in the phosphorus mode and a Hewlett-Packard 7673A Autosampler was used in this evaluation.

   3.1.2 A GC column capable of separating dimethoate from any interferences. A 30 m × 0.32 mm i.d. (0.5 µm film) DB-210 capillary column was used in this evaluation.

   3.1.3 An electronic integrator or some other suitable means to measure detector response. A waters 860 Networking Computer System was used in this evaluation.

   3.1.4 Volumetric flasks, pipets, and syringes for preparing standards, making dilutions and performing injections.

   3.1.5 Vials, 2-mL, and 4-mL, with PTFE-lined caps.

   3.1.6 Mechanical shaker.

   3.2 Reagents

   3.2.1 Hydrogen, air and nitrogen, GC grade.

   3.2.2 Dimethoate. A 99% pure standard from EPA was used in this evaluation.

   3.2.3 Toluene. The toluene used in this evaluation ws purchased from Burdick and Jackson.

   3.2.4 Tributyl phosphate. The tributyl phosphate was purchased from Aldrich Chemical Company Inc.

   3.2.5 Desorbing solution. If an internal standard is used, the desorbing solution is prepared by adding 8 µL of tributyl phosphate to 100 mL of toluene. Otherwise, toluene is used.

   3.3 Standard preparation

   Prepare stock standards by adding either toluene or desorbing solution (if an Internal standard is used) to preweighed amounts of dimethoate. Prepare working range standards by dilution stock solutions with either toluene or desorbing solution. Store stock and dilute standards in a freezer.

   3.4 Sample preparation

   3.4.1 Transfer the 13-mm glass fiber filter and the 270-mg sampling section of the tube to a 4-mL vial. Place the fires foam plug and the 140-mg backup section in a separate vial. A small glass funnel can be used to facilitate the transfer of the adsorbent. Discard the rear foam plug. do not discard the glass sampling tube; it can be reused after it has been cleaned by surfactant or solvent washing.

   3.4.2 Add 2.0 mL of either toluene or desorbing solution (if an internal standard is used) to each vial and seal with a Teflon-lined cap.

   3.4.3 Shake the vials on a mechanical shaker for half and hour.

   3.4.4 If necessary, transfer aliquots of the samples to the vials used in GC analysis. In this evaluation the samples were transferred to 2-mL glass vials, sealed with PTFE-lined septa and loaded on the automatic sampler.

   3.5 Analysis

   3.5.1 Instrument conditions

   Column:	DB-210, 30 m × 0.32 mm i.d., 0.5 µm film
   Injector temperature:	200°C
   Detector temperature:	250°C
   Column temperature:	160°C
   Head Pressure:	7.5 psi
   Temperature program:	hold initial temp 1 min, increase temp at 16°C/min to 200°C, hold final temp. 7 min
   FPD conditions:
   hydrogen flow rate:	200 mL/min
   air flow rate:	100 mL/min
   nitrogen flow rate:	30 mL/min
   Injection volume:	1 µL
   Split ratio:	12.5:1
   Retention time:	4.5 min (tributyl phosphate)
   	7.5 min (dmethoate)

   
   

   3.5.2 Chromatogram

   

   Figure 2.  Chromatogram of Dimethoate at the target level.

   3.5.3 Measure detector response using a suitable method such as electronic integration.

   3.6 Interferences

   3.6.1 Any collected compound which produces an FPD response and has a similar retention time as dimethoate is a potential interference.

   3.6.2 GC conditions may generally be varied to circumvent interferences.

   3.6.3 Retention time on a single column is not proof of chemical identity. Analysis by an alternate GC column, high performance liquid chromatograph (HPLC) and confirmation by mass spectrometry are additional means of identification.

   3.7 Calculations

   3.7.1 An internal standard (ISTD) calibration method is used. A calibration curve may be constructed by plotting concentration of analyte per mL versus ISTD-corrected response of standard concentration (µg/mL) of dimethoate. Bracket the samples with prepared analytical standards over a range of concentrations.

   3.7.2 Determine the µg/mL of dimethoate in both sections of each sample and blank from the calibration curve. If dimethoate is found on the backup section, it is added to the amount found on the front section. Blank corrections should be performed before adding the results together.

   3.7.3 Determine the air concentration by using the following formula.
   
   

   where:	24.46 = molar volume (liters) at 101.3 kPa (760 mmHg) and 25°C
   	229.3 = molecular weight of dimethoate

   

   

   3.8 Safety precautions (analytical)

   3.8.1 Avoid skin contact and air exposure to dimethoate.

   3.8.2 Avoid skin contact with all solvents.

   3.8.3 Wear safety glasses in laboratory.

4. Recommendation for Further Study

   This method should be fully validated.

5. References

   5.1  Burright, D.; Methods #62, "Chlorpyrifos, DDVP, Diazinon, Malathion, and Parathion"; OSHA Analytical Laboratory, unpublished, 1986.

   5.2  "OCCUPATIONAL DISEASE, A Guide to their Recognition"; U.S. Department of Health, Education, and Welfare; Public Health Service, Public Health Service Publication No. 1097, U.S. Government Printing Office: Washington, D.C., 1964; p 245.

   5.3  Sax, N. Irving, Dangerous Properties of Industrial Materials. Van Nostrand Reinhold Company, 1981; p 608.

   5.4  "Farm Chemicals Handbook"; Meister Publishing Co.: Willoughby, OH, 1986; p C113.

   5.5  Windholz, M., Budavari, S., Blumetti, RF., and Otterbein, E., The Merck Index, 10th ed., Merck & CO., Inc., Rahway, N.J., 1983; p469.