Reinvestigation of phase-transfer-catalyzed chlorpyrifos synthesis

Article abstract of DOI:10.1021/op049929k

Production of chlorpyrifos via the phase-transfer-catalyzed reaction of 0,0-diethylphosphorochloridothioate and the sodium salt of 3,5,6- trichloropyridin-2-ol was reinvestigated. The formation of sulfotep (the major byproduct) and the yield are influence

Full text of DOI:10.1021/op049929k

Organic Process Research & Development 2004, 8, 680−684
Reinvestigation of Phase-Transfer-Catalyzed Chlorpyrifos Synthesis
H. Fakhraian,* A. Moghimi, H. Ghadiri, M. A. Dehnavi, and M. Sadeghi
Department of Chemistry, Imam Hossein UniVersity, Tehran, Iran
Abstract:
Different times of reaction and yields have been reported
Production of chlorpyrifos via the phase-transfer-catalyzed
reaction of O,O-diethylphosphorochloridothioate and the so-
dium salt of 3,5,6-trichloropyridin-2-ol was reinvestigated. The
formation of sulfotep (the major byproduct) and the yield are
influenced by the nature and concentration of the catalysts,
temperature, stirring rate, and time of the reaction. The
elucidation of the roles of different parameters influencing the
end of the reaction have permitted us to perform the synthesis
of chlorpyrifos on bench scale (0.3 M scale) under optimized
conditionssusing the minimum amounts of catalysts (0.5 mol
%)swith 92% yield and 98.5% purity.
concerning the reaction of NaTCP with DECTP performed
in a single-1-6 or a two-phase solvent system7-13 using several
types of catalysts with different concentrations (Table 1).
The reactive form of NaTCP is the pyridinate ion (TCP-),
the concentration of which is maximized in alkaline medium
at pH ≈ 10 (pKa of 3,5,6-trichloropyridin-2-ol was estimated
to be 9.8). The alkaline medium was mostly accomplished
and stabilized using boric acid/borate tampon (pKa ) 9.2).
Two important kinds of catalysts used that gave best
performance of the reaction have been tertiary amines (TA)
in conjunction with phase transfer catalysts (PTC). The TA
have been used to protect DECTP from hydrolysis, and the
PTC, to transport pyridinate ion (TCP-) through the organic
phase for reaction with DECTP.
Introduction
Chlorpyrifos (Dursban: O,O-diethyl-O-3,5,6-trichloro-2-
pyridylphosphorothioate) is a broad-spectrum, commercial
organophosphorus insecticide first elaborated and proposed
by R. H. Rigterink et al.1,2 The major route for the production
of chlorpyrifos consists of the reaction between O,O-
diethylphosphorochloridothioate (DECTP) and the sodium
salt of 3,5,6-trichloropyridin-2-ol (NaTCP).3-13
NaTCP was formed after the reaction of a mineral base
(such as NaOH) with 3,5,6-trichloropyridin-2-ol, the syn-
thesis of which via the CuCl-catalyzed reaction of trichlo-
roacetyl chloride and acrylonitrile has been recently re-
investigated.14,15
The main compounds used as the PTC have been BTEAC
(benzyl triethylammonium chloride) or BTMAC (benzyl
trimethylammonium chloride)6,10-13 and PG 26-2 surfactant
(produced from the reaction of di-sec-butyl phenol with
ethylene oxide and propylene oxide with an HLB value in
the range of 8-10) or other surfactants.5,9
Several types of TA with different amounts of steric
hindrance have been used in the chlorpyrifos synthesis, for
example, TMA (trimethylamine), DMAP (dimethyl amino
pyridine), TEDA (triethylenediamine), MI (methyl imida-
zole), and so on.
By using a two-phase solvent system (H2O/CH2Cl2) plus
a dual catalyst system (the tertiary amine and phase transfer
catalyst)swhich was first proposed and elaborated by
Kroposki et al.sthe hydrolysis of DECTP which causes
sulfotep ((C2H5O)2P(S))2O) formation was overcome, and
the product was readily separated in high purity (Scheme
1).
* Corresponding author. Fax: +98-7313938. E-mail: fakhraian@yahoo.com.
(1) Rigterink, R. H.; Kenega, E. E. J. Agric. Food Chem. 1966, 14, 4 (3),
304-306.
(2) Rigterink, R. H. Fr. 1,360,901; Chem. Abstr. 1964, 61, 16052b.
(3) Maurer, F.; Homeyer, B.; Stendel, W. Ger. Offen. DE 3,446,104; Chem.
Abstr. 1986, 105, 153329a.
(4) Sato, Y. Jpn. Kokai Tokkyo Koho JP 07 82,284; Chem. Abstr. 1995, 123,
228516s.
Although several types of TA and PTC were used in the
chlorpyrifos synthesis, the advantage or disadvantage of each
case has not yet been discussed in detail. On the other hand,
according to a two-phase system (H2O/organic solvent),
efficient stirring of the reaction mixture is of main importance
in the accomplishment of the reaction and is influenced by
the rate of the stirrer and the shape of the reactor.
In this contribution, we attempt to reconsider the factors
leading to completion of the reaction and to define the
optimized conditions for yield and purity of the product.
(5) Gatling, S. C. Eur. Pat. Appl. EP 307, 501; Chem. Abstr. 1989, 111, 154104
j. U.S. Patent 4,814, 451; Chem. Abstr. 1989, 111, 214699 u.
(6) Freedman, H. H. U.S. Patent 3,972,887; Chem. Abstr. 1976, 85, 142996h.
(7) Sharvit, J.; Pereferkowitz, A. A. Israeli IL 62,545; Chem. Abstr. 1986, 105,
172716r.
(8) Kihara, K. Ger. Offen. DE 3,439,347; Chem. Abstr. 1985, 103, 196241s.
(9) Gatling, S. C.; Krumel, K. L. Eur. Pat. Appl. EP 307,502; Chem. Abstr.
1989, 111, 115588a. U.S. Patent 4,814,448; Chem. Abstr. 1989, 111,
97504p.
(10) Dow Chemica.l Co. Japan Kokai Tokkyo Koho 77 19,640; Chem. Abstr.
1978, 88, 6535s. Israeli IL 47,871; Chem. Abstr. 1980, 92, 110862q.
(11) Freedman, H. H.; McGregor, S. D.; Yoshimine, M.; Kroposki, L. M. U.S.
Patent 4,147,866; Chem. Abstr. 1977, 86, 43811h.
(12) Kroposki, L. M.; Yoshimine, M.; Freedman, H. H. Can. 1,018,163; Chem.
Abstr. 1978, 88, 22378y. Neth. Appl. 75 09,202; Chem. Abstr. 1978, 88,
22642e. U.S. Patent 4,028,439; Chem. Abstr. 1977, 87, 102435j. U.S. Patent
3,917,621; Chem. Abstr. 1976, 84, 121436q.
Results and Discussion
As mentioned earlier, a two-phase-catalyzed chlorpyrifos
synthesis requires an organic/H2O solvent system, a phase
transfer catalyst, and a tertiary amine.
Among the factors that influence the completion of the
reaction in the two-phase system is the addition sequence
(13) Kroposki, L. M.; Yoshimine, M. U.S. Patent 3,907,815; Chem. Abstr. 1976,
84, 43864m. U.S. Patent 4,016,225; Chem. Abstr. 1977, 87, 23070h.
(14) Fakhraian, H.; Moghimi, A.; Bazaz, A.; Hadj-Ghanbary, H.; Sadeghi, M.
Org. Process Res. DeV. 2003, 7, 329-333.
(15) Fakhraian, H.; Bazaz, A.; Hadj-Ghanbary, H. Org. Process Res. DeV. 2003,
7, 1040-1042.
680
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Vol. 8, No. 4, 2004 / Organic Process Research & Development
10.1021/op049929k CCC: $27.50 © 2004 American Chemical Society
Published on Web 06/09/2004

Table 1. Some of the reported data concerning the use of the solvent system, tertiary amine and phase transfer catalyst in the
chlorpyrifos synthesisa via the reaction of DECTP with NaTCP
solvent
system
tertiary amine
(mol %)
phase transfer catalyst
(mol %)
time
(h)
temp
(°C)
yield
(%)
purity
(%)
reference
MeCN
H2O
H2O
CH2Cl2
H2O/ CH2Cl2
H2O/Toluene
H2O/ CH2Cl2
H2O/ CH2Cl2
H2O/ CH2Cl2
H2O/ CH2Cl2
H2O/ CH2Cl2
H2O/ CH2Cl2
-
-
6
1.5
2
2
5
4.5
2
1
3
1
50
60
45
45
40
25
45
-
43
98
95
88
-
-
98.5
99
95
-
97
-
-
-
99.5
99
3
4
5
6
7
8
9
DMAP
-
DMAP (1)
-
TEDA (1)
-
DMAP (0.1)
DMAP (10)
TMDA (10)
TMA (10)
MI (1)
PG 26-2
BTEAC (30)
-
99
-
95.5
95.5
89
91
94
PG 26-2
BTMAC (10)
BTEAC (10)
BTEAC (10)
BTEAC (1)
BTEAC (1)
10, 11
10, 11
10, 11
12
-
-
42
42
1.5
1.5
94
97
TEDA (1)
13
a In most cited references the reaction has been performed in 0.05 M scale.
To follow the reaction, 31P NMR was chosen as the most
appropriate technique to analyze the composition of the
phosphorus-containing compounds present in the organic
phase.
Scheme 1. Phase-transfer-catalyzed chlorpyrifos synthesis
In the first four experiments (Table 2, experiments 1-4),
the reaction was performed in H2O/CHCl3 at 60 °C using
triethylamine (TEA) as tertiary amine in the absence or
presence of BTEAC (phase transfer catalyst). In the absence
of BTEAC (Table 2, experiment 1), most of the starting
DECTP was found unreacted in the organic phase after 3 h.
Upon addition of BTEAC, the conversion rate was increased
(Table 2, experiment 2-4). In the presence of TEA, the rate
enhancement was BTEAC concentration dependent (1-4
mol %). The substitution of BTEAC for pyridinium chloride
(PTC) (Table 2, experiment 5) showed almost the same
conversion rate as that in experiment 4 except that a
considerable amount of undesirable byproduct, sulfotep
(12%), was formed.
The preliminary test with BTEAC (4 mol %) and TEDA
(2 mol %) indicated that TEDA (as tertiary amine) was much
better than TEA (Table 2, experiments 6 and 7). Under these
conditions and after 1 h, no starting DECTP was detected
in the 31P NMR spectrum of the organic phase, while 6%
sulfotep was formed.
of the starting materials and the quantities of the solvents
used.
The solutions of PTC, NaCl, NaOH, and boric acid in
H2O and TA in organic solvent (CH2Cl2 or CHCl3) were
separately prepared and mixed in the reactor which contains
NaTCP, before dropwise addition of DECTP dissolved in
organic solvent.
The total amounts of water and organic solvent used are
optimized for the synthesis of chlorpyrifos, respecting the
minimum volume of the reactor and other vessels required
for the separation, washing, and drying of the organic solvent.
The lesser amounts of water and organic solvent have
increased the viscosity of the mixture in the beginning of
the reaction and influenced the efficiency of the stirring and
separation of organic phase.
The purity of the product and the yield depend on other
different factors such as the PTC and TA and their respective
concentrations, the reaction temperature and time, the stirrer
type, and the stirring rate. Considering these factors, the types
of catalysts and their concentrations were first optimized on
0.003 M scale (Table 2, experiments 1-11). The optimized
parameters were applied to bench scale (0.3 M scale) (Table
2, experiment 12) where the stirring rate and the DECTP
addition rate were then optimized.
Among the TA used in the literature, DMAPsas a more
sterically hindered amineswas successfully applied in our
next experiments (Table 2, experiments 8-12). The replace-
ment of TEDA by DMAP caused the enhancement of
sulfotep byproduct formation from 6 to 12% while maintain-
ing the same amount of chlorpyrifos formed (88%). To
eliminate the byproduct formation via DECTP hydrolysis and
to keep the concentration of unreacted DECTP as low as
possible, the concentrations of both the phase transfer catalyst
and tertiary amine were reduced as was the reaction time
(Table 2, experiments 9-10).
As presented in Table 2, 0.5 mol % of DMAP and
BTEAC and a 15-min reaction time were enough for the
maximum conversion rate of DECTP with the minimum
formation of sulfotep at 60 °C using H2O/CHCl3 as solvent
system (Table 2, experiment 10). These optimized conditions
were then applied to the H2O/CH2Cl2 solvent system, and it
Vol. 8, No. 4, 2004 / Organic Process Research & Development
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681

Table 2. Proportional composition (%)a of the organophosphorus compounds in the organic phase of the
phase-transfer-catalyzed reaction of NaTCP with DECTP under different conditions
solvent
system
tertiary
amine (mol %)
phase transfer catalyst
(mol %)
time
(h)
temp
(°C)
sulfotep
(53.8 ppm)
DECTP
(69.8 ppm)
chlorpyrifos
(60.9 ppm)
experimentb
1
2
3
4
5
6
7
8
H2O/CHCl3
H2O/CHCl3
H2O/CHCl3
H2O/CHCl3
H2O/CHCl3
H2O/CHCl3
H2O/CHCl3
H2O/CHCl3
H2O/CHCl3
H2O/CHCl3
H2O/CH2Cl2
H2O/CH2Cl2
TEA (1)
TEA (1)
TEA (1)
TEA (1)
TEA (1)
TEA (2)
TEDA (2)
DMAP (2)
DMAP (1)
DMAP (0.5)
DMAP (0.5)
DMAP (0.5)
-
3
3
3
3
3
1
1
1
0.5
0.25
0.5
2
60
60
60
60
60
60
60
60
60
60
40
40
1
-
-
-
12
-
6
12
8
8
90
78
61
41
32
68
6
-
3
3
6
9
22
39
59
56
32
88
88
89
89
94
100
BTEAC (1)
BTEAC (2)
BTEAC (4)
Pyridine-HCl (4)
BTEAC (4)
BTEAC (4)
BTEAC (4)
BTEAC (0.5)
BTEAC (0.5)
BTEAC (0.5)
BTEAC (0.5)
9
10
11
12
0
0
0
a
b
Based on 31P NMR spectroscopy. Experiments 1-11 and experiment 12 were performed in 0.003 and 0.3 M scale, respectively.
was realized that the production of sulfotep could be
prevented under such conditions (Table 2, experiment 11)
under which the reaction temperature was stabilized by the
reflux condition of CH2Cl2 (40 °C). Use of a more appropri-
ate industrial solvent such as ethylene dichloride (CH2ClCH2-
Cl) did not afford this opportunity.
Thus, the optimized conditions used in experiment 11
were applied to the bench scale (0.3 M scale) production of
chlorpyrifos, where in 2 h complete conversion of DECTP
was observed without any formation of sulfotep (Table 2,
experiment 12).
The mixing and DECTP addition rates were optimized
in bench scale so that the temperature was stabilized at 30
°C and the mixture became homogeneous. The surface area
separating the two phases should be large enough to provide
the best mixing with a suitable stirrer. In 0.003 M scale, the
reaction, performed in a three-necked 25-mL flask equipped
with a magnetic stirrer, lasted 30 min. However, on 0.3 M
scale, in a 1000-mL reactor equipped with a mechanical
stirrer, the complete conversion required 2 h, after which
the mixture became completely clear as a sign of the end of
reaction. The reactor used in bench scale (0.3 M scale) had
10 cm as the internal diameter and 20 cm as height.
Consequently, it seems that a suitable proportional ratio of
internal diameter to height of the reactor should be at least
0.5. The mixing rate and the shape of the mechanical stirrer
(one or a multiple set of four-blade turbines, etc.) determine
Figure 1. 1H (a), {1H}13C (b), and 31P (c) NMR spectra of
chlorpyrifos sample (in CDCl3).
the time of the reaction and control the formation of the
byproducts. We used a mechanical stirrer with one four-blade
turbine and a rate of 1500 rpm. Using other mechanical
stirrers with more efficient shapes can afford the same results
in lesser time.
constants between different protons and phosphorus atoms
(see Experimental Section).
Retention times of chlorpyrifos and 2,3,5,6-tetrachloro-
pyridine (used as the internal standard) determined by HPLC
in the conditions stated in Experimental Section are respec-
tively 4.49 and 3.38 min.
The 13C NMR characteristics of chlorpyrifos are outlined
in Table 3 and Figure 2. It was found that 3JCH3-P was greater
than 2JCH2-P: 8 vs 5 Hz. CH3 and CH2 carbons further couple
to adjacent protons. The same feature was observed concern-
3
2
The 1H, 13C, and 31P NMR spectra have demonstrated the
absence of other organic impurities and of sulfotep as the
main byproduct (Figure 1).
ing the coupling constants JC3-P and JC2-P. The aromatic
carbons C2, C3, C5, and C6 couple both with phosphorus
and C4-H hydrogen, giving rise to dd peaks (Figure 2). C4
carbon resonance appears at 141.1 ppm as does a dd. The
The fine structure of the peaks in different regions in the
1H and 31P NMR have permitted us to determine the coupling
3
3
2
greater JC2-H and JC6-H values relative to those of JC3-H
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Vol. 8, No. 4, 2004 / Organic Process Research & Development

Table 3. 13C NMR characteristics of chlorpyrifos (in CDCl3)
Experimental Section
General Procedures. NMR spectra were recorded on a
1
Bruker DPX-250 instrument (250 MHz for H, 62.5 MHz
for 13C and 100 MHz for 31P), and CDCl3 was used as
solvent; chemical shifts were reported in δ (ppm) from TMS
(1H and 13C) and 85% H3PO4 (31P) with downfield shifts
positive. Electronic ionization GC-MS spectra were re-
corded on a Varian (SATURN 4D) spectrometer with
capillary column (DB-5MS, 0.1 µ, 30 m × 0.250 mm). Only
m/z values having intensities of more than 20% were given,
and retention time was reported using temperature program-
ming (100-250 °C, 10 °C/min) with He flow rate of 10 mL/
min. IR spectra were recorded on a Perkin-Elmer 783
instrument using KBr pellet. Melting point was obtained on
a Mettler FP61 apparatus. HPLC analysis was performed
using a Cecil 1100 instrument.
13C NMR
characteristics CH3 CH2 C2
C3
C4
C5
C6
δ (ppm)
15.8 65.8 150.8 120.4 141.1 126.7 143.9
J
C-H (Hz)
127, 149,
2.5 4.5
9
6
4
7
173
3
9
J
C-P (Hz)
8
5
∼0.5 ∼0.5 ∼0.5
Preparation of O,O-Diethyl-O-3,5,6-trichloro-2-py-
ridylphosphorothioate (Chlorpyrifos). Into a 1000-mL,
three-necked, double-glass-walled reactor (with water at
desired temperature circulating between the walls), equipped
with a mechanical stirrer (made up of one four-blade turbine),
a condenser, a dropping funnel, and a thermometer, was
placed 73.2 g (0.33 mol) of NaTCP. Then, two solutions of
3.48 g (0.057 mol) of boric acid, 1.59 g (0.027 mol) of NaCl,
1.89 g (0.048 mol) of NaOH, and 0.342 g (1.5 mmol) of
BTEAC in 375 mL of watersthe pH of this solution was
9.9 at 25 °Csand 0.183 g (1.5 mmol) of DMAP in 150 mL
of CH2Cl2 were placed into the reactor. The temperature of
the reaction mixture was stabilized at 30 °C using a water
circulator, while stirring at 1500 rpm, and a solution of 56.1
g (0.3 mol, 46.8 mL) of DECTP in 105 mL of CH2Cl2 was
added dropwise during 15 min. The reaction mixture was
maintained at 40 °C, and the vigorous stirring continued.
The reaction mixture turned to a milky viscous suspension
within 30 min. The stirring was continued for a further 1.5
h, until the mixture became completely clear. The organic
and aqueous layers (pH ) 9.2 at 25 °C) were separated. The
unreacted NaTCP was recovered from the aqueous layer by
filtration. The organic layer was washed with water (2 × 50
mL) and dried by 30 g of CaCl2. After vacuum stripping,
the product was found to consist of 94 g containing 98.5%
chlorpyrifos with 0.015% acidity (FAO specifications for
technical chlorpyrifos concerning purity and acidity are
respectively 94% and 0.1%).16 The overall yield based on
Figure 2. 13C NMR spectra of chlorpyrifos sample (in CDCl3)
and the multiplicity of the different peaks.
2
and JC5-H conform to the C-H coupling constants in
aromatic compounds.21
Summary
In summary, both yield and purity of chlorpyrifos
produced by the reaction of DECTP with NaTCP using a
phase-transfer-catalyzed system are influenced by different
factors such as the catalysts, temperature, and stirring of the
reaction mixture. The tertiary amine catalyst should act as a
protecting agent to prevent the DECTP hydrolysis (and
sulfotep formation) and, at the same time, act as a suitable
leaving group to permit the reaction of DECTP with NATCP
to occur. TEA was a good protecting agent but an undesirable
leaving group. Therefore, despite a very slow reaction in the
presence of TEA, sulfotep was not formed, except in the
presence of pyridinium chloride that catalyzed its formation.
On the other hand, both DMAP and TEDA were suitable as
temporary protecting agents and better leaving groups facing
NaTCP. Therefore, in the presence of DMAP or TEDA, the
reaction time must be at minimum to prevent DECTP
hydrolysis. This was accomplished by choosing a suitable
reactor and stirring time.
The 0.5 mol % of DMAP and BTEAC were enough for
the reasonable conversion rate with the minimum formation
of sulfotep in 60 °C (using CHCl3/H2O as solvent system).
Performing the reaction under optimized conditions concern-
ing the phase transfer catalyst and tertiary amine at 40 °C
(using CH2Cl2/H2O as solvent system) eliminated the forma-
tion of sulfotep and caused the formation of chlorpyrifos on
bench scale (0.3 M scale) after 2 h with 92% yield and 98.5%
purity.
1
the amount of DECTP used was calculated to be 92%. H
NMR (CDCl3): 1.43 (t, JH-H ) 7.5 Hz, JH-P ) 2 Hz, 3H,
CH3), 4.41 (m, JH-H ) 7.5 Hz, JH-P ) 10 Hz, 2H, CH2),
7.86 (s, 1H, C5HCl4) ppm (in agreement with lit.17). 13C NMR
(CDCl3): 15.8, 65.8, 120.4, 126.7, 141.1, 143.9, 150.8
ppm,31P NMR (CDCl3): 60.98 (m, J ) 10 Hz) ppm (lit.18
61.53 ppm). Mp ) 43.5-44.5 °C (lit.19,20 42-43.5 °C). GC-
MS: retention time ) 13 min; m/z (intensity (%)): 47 (39),
(16) Technical Chlorpyrifos, Full Specification WHO/SIT/21R3, 1999.
(17) Babad, H.; Herbert, W. Anal. Chim. Acta 1968, 41, 259-268.
(18) Mortimer, R. D.; Dawson, B. A. J. Agric. Food Chem. 1991, 39, 911-
916.
(19) Brust H. F. Down Earth 1966, 22, 21-22.
(20) See: http://www.chlorpyrifos.com/Science/pcprop.htm.
(21) Eberhard, B.; Wolfgang, V. Carbon-13 NMR Spectroscopy, 3rd ed.;
VCH: New York, 1987; pp 134-146
Vol. 8, No. 4, 2004 / Organic Process Research & Development
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683

65 (31), 97 (92), 98 (26), 107 (22), 109 (22), 197 (59), 199
(56), 208 (26), 210 (25), 258 (71), 260 (48), 286 (40), 288
(35), 314 (100), 316 (71), 350 (27), 352 (27). IR (KBr
pellet): 644 (m), 669 (m), 710 (m), 739 (m), 840 (vs), 959
(vs), 1016 (vs), 1053 (s), 1082 (s), 1160 (s), 1233 (m), 1263
(m), 1328 (m), 1402 (vs), 1538 (m), 2910 (w), 2970 (w),
3025 (w) cm-1.
HPLC Analysis of Chlorpyrifos Sample. The sample
was dissolved in acetonitrile solution containing 2,3,5,6-
tetrachloropyridine (mp ) 90-91 °C) as internal standard.
The chlorpyrifos content was determined by a HPLC
instrument equipped with a pumping system able to maintain
a pressure of 11 Mpa, a UV spectrophotometer detector (with
1.0 AUFS sensitivity), and a reverse-phase column (C-18,
250 mm × 4.6 mm). A mixture of acetonitrile (82 mL), water
(17.5 mL), and acetic acid (0.5 mL) was used as the mobile
phase with the flow rate of 2 mL/min at 30 °C. All solvents
were HPLC grade, and chlorpyrifos of known purity was
used as standard. The UV absorbance was measured at 300
nm.
solution was prepared as the calibration solution using 80
mg of the chlorpyrifos sample, the purity of which was to
be determined.
A 10-µL amount of each calibration and sample solution
with sufficient interval time was injected, and the peak area
of chlorpyrifos with that of internal standard (R) was
compared. Then, using the following formula, the purity of
the chlorpyrifos sample was determined.
chlorpyrifos content (g/kg) ) (R2 x m1 x P)/(R1 x m2)
R1 ) ratio of chlorpyrifos peak area to internal
standard peak area for calibration solution
R2 ) ratio of chlorpyrifos peak area to internal
standard peak area for sample solution
m1 ) mass of chlorpyrifos standard in the calibration solution
m2 ) mass of chlorpyrifos sample in the sample solution
P ) purity of chlorpyrifos standard (g/kg)
Acknowledgment
The work was supported by the Iranian Ministry of
Industry (Grant No. 78210111076). We are grateful to Mr.
Akbar Mirzaei for constructive discussions.
The internal standard solution was made by dissolving
150 mg of 2,3,5,6-tetrachloropyridine in acetonitrile and
diluting to 250 mL using a volumetric flask. The calibration
solution was prepared by weighing 80 mg of chlorpyrifos
standard into a 50-mL glass-stoppered conical flask and
mixing with 25 mL of internal standard solution. The sample
Received for review April 6, 2004.
OP049929K
684
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Vol. 8, No. 4, 2004 / Organic Process Research & Development