Sulfentrazone

Materials to be

analyzed

Soybean seed and processed parts (meal, hulls and oil);

wheat grain, forage, hay; corn grain, forage, stover and

processed parts (flour); rice grain, straw and processed

parts (hulls, bran and polished rice); sorghum grain, for-

age and stover; tobacco (green, dried, cigarette, and

smoke condensate); pea; alfalfa forage and hay; sun-

flower and processed parts (meal and oil); peanut nut-

meat and processed parts (meal and oil); sugarcane and

processed parts (refined sugar and molasses); and potato

tuber and processed parts (wet peel, flakes and chips).

Instrumentation

Gas-chromatographic determination for plant matrices.

1

Introduction

Chemical name

(IUPAC)

N -[2,4-dichloro-5-[4-(difluoromethyl)-4,5-dihydro-3-

methyl-5-oxo-1H-1,2,4-triazol-1-yl]phenyl]methane-

sulfonamide

Structural formula

S

N

H

CI

N

N

N

F

F

CI

O

O

O

Empirical formula

C

10

H

10

N

4

O

3

F

3

SCl

2

Molar mass

387.2

Melting point

121–123

◦

C

Physical state/odor

Light-tan powder with a musty odor

Vapor pressure

8

× 10

−10

mmHg (25

◦

C)

Water solubility

400 mg L

−1

(25

◦

C)

Specific gravity

1.21 g mL

−1

(20

◦

C)

Stability

Stable at pH 5–9

Other properties

Undergoes photolysis in water rapidly. The compound

is stable to photolysis in soil and is relatively persistent

in soil, with a field half-life (t

1/2

) of 121 days in sandy

soil and t

1/2

of 302 days in clay soil.

Handbook of Residue Analytical Methods for Agrochemicals.

C

2003 John Wiley & Sons Ltd.

Sulfentrazone

565

Use pattern

Sulfentrazone is a broad-spectrum, pre-emergent

herbicide that provides good control over broadleaf

weeds, grasses and sedges in crops and turf. The

metabolism of sulfentrazone in animals and plants

is similar. The major plant metabolite of sulfentra-

zone is 3-hydroxymethyl sulfentrazone (HMS). The

soybean tolerance of 0.05 mg kg

−1

includes residues

of sulfentrazone plus its major metabolite, HMS.

The rotational crop tolerance includes residues of

sulfentrazone and its major metabolites, HMS and

3-desmethylsulfentrazone (DMS). The tolerance levels

for cereal grains (excluding sweet corn) are as follows:

0.1 mg kg

−1

in grain, 0.2 mg kg

−1

in hay, 0.6 mg kg

−1

in straw, 0.2 mg kg

−1

in forage, 0.1 mg kg

−1

in stover,

0.15 mg kg

−1

in bran and 0.3 mg kg

−1

in hulls.

Regulatory position

The residue of interest includes the parent sulfentra-

zone, HMS, sulfentrazonecarboxylic acid (SCA) and

DMS.

Sulfentrazone-3-carboxylic acid (SCA)

S

HN

CI

N

N

N

F

F

O

HO

CI

O

O

O

1-[2,4-dichloro-5-[N-(methylsulfonyl)amino]phenyl]-

4-difluoromethyl-4,5-dihydro-5-oxo-1H-1,2,4-

triazole-3-carboxylic acid

3-Desmethylsulfentrazone (DMS)

S

N

H

CI

N

N

N

F

F

CI

O

O

O

N-[2,4-dichloro-5-[4-(difluoromethyl)-4,5-dihydro-

5-oxo-1H-1,2,4-triazol-1-yl]phenyl]methanesulfona-

mide

3-Hydroxymethylsulfentrazone (HMS)

S

HN

CI

N

N

N

F

F

HO

CI

O

O

O

N-[2,4-dichloro-5-[4-(difluoromethlyl)-4,5-dihydro-

3-hydroxymethyl-5-oxo-1H-1,2,4-triazol-1-

yl]phenyl]methanesulfonamide

566

Individual compounds

2

Method description

2.1

Method development history

The analytical method for sulfentrazone and its major plant metabolites originally in-
cluded only a single hydrolysis step and analysis by gas chromatography with electron
capture detection. During a radio-validation study (GC/ECD) analyzing plant sam-
ples from a metabolism study, it became apparent that the method did not account for
all of the conjugated HMS and that the SCA was not completely converted to DMS.
Consequently, the previous method was modified to include a more stringent hydrol-
ysis step, to free all conjugated analytes of concern and to convert SCA completely to
DMS. Additionally, a more specific detector was required to discriminate between the
residues of sulfentrazone and its metabolites, and the matrix components released dur-
ing the stringent hydrolysis step. A gas chromatograph equipped with an electrolytic
conductivity detector (ELCD) or a halogen-specific detector (XSD) was utilized.

2.2

Outline of method

The enforcement method began with an acetone–0.25 N HCl reflux (1 h), filtration and
concentration. The aqueous concentrate was acidified to 1 N, boiled under reflux (2 h),
and filtered. The sample was then passed through a C

8

solid-phase extraction (SPE)

cartridge and a silica gel SPE cartridge for clean-up. The eluate was concentrated
and the HMS analyte was derivatized with N,O-bis(trimethylsilyl)trifluoroacetamide
(BSTFA). The derivatized solution was passed through a second silica gel SPE car-
tridge for additional cleanup. The eluate was concentrated and brought to a final
volume with acetonitrile. Analysis was performed by gas chromatography (GC) with
a 35% or 50% phenylmethylsilicone megabore or narrow-bore column.

This enforcement method has been validated on the raw agricultural commodity

(RAC) and processed parts of various crops. For hay and straw matrices, the method
limit of quantitation (LOQ) was validated at 0.05 mg kg

−1

and the method limit of

detection (LOD) was set at 0.01 mg kg

−1

. For all other crop matrices, the LOQ was

validated at 0.025 mg kg

−1

and the LOD was set at 0.005 mg kg

−1

. The method flow

chart is presented in Figure 1.

3

Apparatus

AccessChrom or TurboChrom data acquisition software running on a MicroVax
Adapters, neoprene
Adapters, reducing
Balance, Analytical PM 2000, Mettler
Balance, top loading, Mettler
Boiling stones, Hengar
Buchner filter funnels, porcelain, 10.5-cm i.d., Coors
Capillary column, DB-5MS, 15 m

× 0.25-mm i.d., 0.25-µm, J&W Scientific

Sulfentrazone

567

Capillary column, DB-35MS, 15 m

× 0.25-mm i.d., 0.25-µm, J&W Scientific

Capillary column, DB-17, 30 m

× 0.54-mm i.d., 1-µm, J&W Scientific

Capillary column, DB-35, 30 m

× 0.54-mm i.d., 1-µm, J&W Scientific

Centrifuge tubes, 13-mL, graduated, 0.1 mL, Pyrex
Condensers, Pyrex, Graham coil, 41

× 500 mm with Ts 24/40 joint

Cylinders, graduated, 50-, 100-, 250-mL
Filter paper, Whatman No. 1, 11-cm diameter
Filter paper, Whatman GF/F (0.8 µm), 11-cm diameter

Matrix

Acetone--0.25 N HCI, Reflux 1 h

Filtration

Liquid

Evaporation

Aqueous

Acidify

Coupled Cartridges
for elution

1 N HCI, Reflux 2 h

C

8

Cartridge SPE

Silica Cartridge SPE

Acid

Discard

BSTFA Derivatization

Silica Cartridge SPE

GC/ELCD or XSD Quantitation

Concentration

Concentration

Solid

Discard

Filtration

Figure 1

Method flowchart for sulfentrazone determination.

568

Individual compounds

Flasks, filter, 250-mL
Flasks, round-bottom boiling, 500-mL, T

s 24/40 joint

Flasks, volumetric, 100-mL
Gas chromatograph [Hewlett-Packard (HP) 5890 or 6890 GC with HP 7673 or 6890

Series injector and O I Analytical Model 5220 electrolytic conductivity detector or
5360 halogen-specific detector; HP 5890 or 6890 equipped with HP 7673 or 6890
Series injector and HP 5970 or 5972 mass-selective detector]

Heating mantles, Glas-Col
Injection port insert, cyclo-double gooseneck, Restek
Magnetic stirrers, VWR, Model 200
Microsyringes, Hamilton
Mill, Hobart
Mill, Wiley
Multi-tube vortexer, VWR
N-EVAP evaporator, Organomation
Pipets, disposable and volumetric
Reservoirs, plastic, 75-mL
Robot Coupe vertical cutter/mixer
SPE cartridge, silica gel (1-g), J.T. Baker
SPE cartridge, C

8

(1-g), Varian

Teflon stirring bars, VWR
TurboVap Evaporator, Zymark
TurboVap vessels, 200-mL, Zymark
TurboVap vessel support rack, Zymark
Visiprep manifold, Supelco
Visidry vacuum manifold drying attachment, Supelco

4

Reagents

Acetone, Resi-Analyzed, J.T. Baker
Acetonitrile, Resi-Analyzed, J.T. Baker
BSTFA, Pierce
Ethyl acetate, Resi-Analyzed, J.T. Baker
Hexane, Resi-Analyzed, J.T. Baker
Hydrochloric acid (HCl) (36.5–38.0%), J.T. Baker
Methanol, Resi-Analyzed, J.T. Baker
Equivalent equipment and reagents may be substituted as appropriate, unless specified
otherwise in the method.

5

Sampling and preparation

Prior to analysis, samples should be chopped and finely pulverized with liquid nitrogen
using a large Hobart (forage, hay, fodder and straw samples) or a Wiley mill (grain
and seed samples). Recently, frozen crop matrices were processed more effectively
with a Robot Coupe vertical cutter/mixer without liquid nitrogen.

Sulfentrazone

569

6

Analytical procedures for nonoil crop matrices

6.1

Sample extraction, filtration and concentration

Weigh 10 g of the matrix into a 500-mL round-bottom boiling flask. For control
samples to be fortified, add an accurately measured volume of a standard solution
containing sulfentrazone, SCA and HMS uniformly to the matrix by syringe. Allow
the solvent to evaporate (ca 1 min). Add 150 mL of acetone–0.25 N HCl (3 : 1, v/v)
and a Teflon stirring bar or boiling stones. Place the round-bottom flask in a heating
mantle and attach the flask to a cooling condenser. Gently boil the solution under
reflux with stirring (if using a stirring bar) for 1 h.

Cool the sample extract to room temperature and filter the extract through a

Whatman No. 1 (11-cm) filter paper (pre-rinsed with 5 mL of acetone) into a filter
flask using a Buchner funnel and vacuum (15 inHg). Rinse the boiling flask with 2

×

25 mL of acetone and pass the rinsate through the post-reflux solid and filter paper.
Transfer the filtrate into a 200-mL TurboVap vessel. Rinse the filter flask with 5 mL of
acetone and add the rinsate to the TurboVap vessel. Concentrate the filtrate to

<25 mL

(not to dryness) using a TurboVap Evaporator (water-bath at 50

◦

C; increase the pres-

sure up to 30 psi as the volume decreases). All traces of acetone must be removed.

6.2

Second reflux (conversion of SCA to DMS and release
of conjugated HMS) and filtration

Transfer the aqueous concentrate into a 500-mL round-bottom boiling flask. Rinse
the TurboVap vessel with 2

× 5 mL of distilled water and add the rinsate to the round-

bottom boiling flask. Add 3.5 mL of concentrated HCl to the aqueous concentrate to
make the solution 1 N. Add a Teflon stirring bar or boiling stones. Place the round-
bottom flask in a heating mantle and attach the flask to a cooling condenser. Gently
boil the solution under reflux with stirring (if using a stirring bar) for 2 h.

Cool the sample extract to room temperature and filter the extract through a

Whatman GF/F (11-cm) fine filter paper (pre-rinsed with 5 mL of distilled water)
into a filter flask using a Buchner funnel and vacuum (15 inHg). Rinse the round-
bottom boiling flask with 2

× 10 mL of distilled water and pass the rinsate through

the post-reflux solid and filter paper. Transfer the filtrate into a 100-mL graduated
mixing cylinder. Rinse the filter flask with 2

× 10 mL of distilled water, and add the

rinsate to the mixing cylinder. Bring the volume up to 100 mL with distilled water.
Shake the sample and take a 5-g (50-mL) aliquot.

6.3

C

8

SPE cartridge

Place a C

8

cartridge (1-g, Varian) on a vacuum manifold and condition the column

with 6 mL of methanol followed by 6 mL of 0.25 N HCl. When conditioning SPE
cartridges, allow the first conditioning solvent to reach the top of the cartridge packing
before adding the second solvent. Maintain the flow rate through the C

8

cartridge at

570

Individual compounds

5 mL min

−1

by regulating the vacuum pump (5 inHg). The flow rate is more important

than the vacuum pressure. Close the cartridge and add an additional 3 mL of 0.25 N
HCl to the cartridge barrel. Attach a 75-mL plastic reservoir with an adapter to the top
of the C

8

cartridge. Transfer the 50-mL aqueous sample aliquot to the reservoir. Pass

the sample through the C

8

cartridge. Once the entire sample has passed through the C

8

SPE cartridge, use full vacuum briefly (2 min). Blow the cartridge completely dry with
nitrogen using a manifold drying attachment (30 psi for at least 30 min). Return the
C

8

SPE cartridge to the manifold and wash the cartridge with 6 mL of hexane–ethyl

acetate (19 : 1, v/v). Remove the C

8

cartridge and prepare the first silica gel cartridge.

6.4

C

8

SPE cartridge/first slica gel SPE cartridge

Place a silica gel cartridge (1-g, J.T. Baker) on the vacuum manifold and condition
with 6 mL of ethyl acetate followed by 6 mL of hexane. Do not allow the silica gel
cartridge to go dry at any time during this step. Maintain the flow rate through the
silica gel cartridge at 2 mL min

−1

by regulating the vacuum pump (5 inHg). Close the

cartridge and add 1 mL of hexane–ethyl acetate (7 : 3, v/v). Attach the C

8

cartridge to

the top of the silica gel cartridge with a reducing adapter. Add 3 mL of hexane–ethyl
acetate (7 : 3, v/v) to the C

8

cartridge. Open the connected cartridges and allow a

few drops to drip from the C

8

cartridge into the silica gel cartridge before applying

vacuum. This will help to prevent the silica gel cartridge from going dry. When the
first 3 mL have reached the top of the C

8

cartridge packing, add an additional 6 mL of

hexane–ethyl acetate (7 : 3, v/v). Allow the C

8

eluate to reach the top of the silica gel

cartridge packing. Remove the C

8

cartridge and discard. Wash the silica gel cartridge

with 3 mL of hexane–ethyl acetate (7 : 3, v/v). Elute and collect the analytes from
the silica gel cartridge with 6 mL of ethyl acetate in a 13-mL glass centrifuge tube.
Discard the silica gel cartridge. Evaporate the eluate under a slow nitrogen stream
(just enough to produce a ripple on the surface) in a water-bath (45

◦

C) to near dryness

(until a thin oily film remains; do not overdry).

6.5

Derivatization (silylation of 3-hydroxymethyl sulfentrazone)

Add 0.5 mL of acetonitrile and 100 µL of fresh BSTFA (Precaution: once the ampule
containing BSTFA is opened, the contents should be used within 10 min, since BSTFA
will absorb moisture) to the centrifuge tube containing the sample extract, stopper
the tube and vortex the sample for 15 s. Add 9.5 mL of hexane–ethyl acetate (9 : 1,
v/v) to make 10 mL. Cap the centrifuge tube and vortex the sample until the contents
are mixed (there should be no phase separation). If there is a phase separation, gently
warm the samples in a water-bath (45

◦

C) for 1 min. Vortex the sample again. If phase

separation persists, continue warming and vortexing the sample until the phases mix.

6.6

Second (post-derivatization) silica gel SPE cartridge

Place a silica gel cartridge (1-g, J.T. Baker) on a vacuum manifold and condition the
cartridge with 6 mL of ethyl acetate followed by 6 mL of hexane. Do not allow the

Sulfentrazone

571

cartridge to go dry at any time during this step. Maintain the flow rate through the
silica gel cartridge at about 2 mL min

−1

by regulating the vacuum pump (5 inHg).

Load the derivatized sample extract on to the cartridge. Rinse the centrifuge tube
twice, each with 3 mL of hexane–ethyl acetate (9 : 1, v/v), and add the rinsate to the
cartridge. Drain the rinsate to the top of the silica gel packing. Elute the analytes with
9 mL of ethyl acetate–hexane (1 : 1, v/v) into a 13-mL glass centrifuge tube. Discard
the silica gel cartridge. Add 1 mL of acetonitrile to the eluate. Evaporate the eluate
under a slow nitrogen stream (just enough to produce a ripple on the surface) in a
water-bath (45

◦

C) to near dryness (until a thin oily film remains; do not overdry).

Dilute the sample to the appropriate final volume with acetonitrile.

6.7

Analytical procedures for oily crop matrices

When analyzing oily crop matrices, e.g., sunflower seed and peanut nutmeat, the
above method for nonoil crop matrices needs to be slightly modified. Sample extracts
of the oily crop matrices need additional hexane and acetonitrile partitions prior to
the C

8

SPE cartridge. After acid reflux some of the sulfentrazone compounds tend to

adsorb on the oil drops which would not pass through the C

8

SPE cartridge. After the

second reflux with 1 N HCl, the entire sample extract is filtered, diluted and partitioned
with hexane. The hexane fraction is then partitioned with acetonitrile. The hexane is
discarded, the acetonitrile is concentrated to near dryness, and the container is used
to collect the eluate from the C

8

SPE cartridge in Section 6.4. The aqueous solution

is then passed through the C

8

SPE cartridge; the rest of the analytical procedures are

followed as described in Section 6.4.

6.8

Analytical procedures for crop refined oils

Crop refined oils should be dissolved in hexane and extracted in a separatory funnel
with 0.25 N HCl follow by an evaporation of residual hexane. Concentrated HCl is
then added to make the solution 1 N and the samples are boiled under reflux for 2 h.
The rest of the analytical procedures are followed as described in Section 6.4.

6.9

Instrumentation

GC was used to analyze the sample extracts. Three detection systems were used, two
for quantitation and one for analyte confirmation.

Gas chromatography/electrolytic conductivity detection (GC/ELCD) and gas

chromatography/halogen-specific detection (GC/XSD) are specific for halogenated
compounds and were effective for discriminating between sulfentrazone compounds
and the matrix components. Operating conditions are listed below.

572

Individual compounds

GC/ELCD instrument parameters
Instrument

HP 6890 gas chromatograph

Column

DB-35, 35% phenylmethylsilicone, 30 m

× 0.54-mm i.d.,

1.0-µm film thickness

Inlet

Splitless injection mode (cyclo-double gooseneck insert)

Detector

O I Analytical 5220 electrolytic conductivity detector,

halogen mode

Temperatures

Injection port

250

◦

C

Oven

180

◦

C/1 min (initial), 20

◦

C min

−1

(ramp), 260

◦

C/2 min

(hold), 5

◦

C min

−1

(ramp), 280

◦

C/4 min (final)

Reactor

900

◦

C

Column gas flow rate

He carrier gas, 16 mL min

−1

% 1-Propanol flow 37%

ELCD gas flow rates

H

2

+ carrier gas, unvented 135 mL min

−1

H

2

+ carrier gas, vented 85 mL min

−1

Injection volume

2 µL

GC/XSD instrument parameters
Instrument

HP 6890 gas chromatograph

Column

DB-17, (50% phenyl)silicone, 30 m

× 0.546-mm i.d.,

1.0-µm film thickness

Inlet

Splitless injection mode (cyclo-double gooseneck insert)

Detector

O I Analytical 5360 halogen-specific detector

Temperatures

Injection port

250

◦

C

Oven

180

◦

C/1 min (initial), 10

◦

C min

−1

(ramp), 260

◦

C/2 min

(hold), 5

◦

C min

−1

(ramp), 280

◦

C/5 min (final)

Reactor

1100

◦

C

Column gas flow rate

He carrier gas, 16 mL min

−1

XSD make-up flow rate

Air, 25 mL min

−1

Injection volume

2 µL

Operating conditions for spectral analyte confirmation
Instrument

HP 5890 or 6890 gas chromatograph

Column

DB-5MS, 5% phenylmethylsilicone, 15 m

× 0.25-mm

i.d., 0.25-µm film thickness

Inlet

Splitless injection mode (cyclo-double gooseneck insert)

Detector

HP 5970 or 5972 mass-selective detector

Temperature

Injection port

260

◦

C

Oven

120

◦

C/2 min (initial), 20

◦

C min

−1

(ramp), 280

◦

C/6 min

(final)

Detector

280

◦

C

Gas flow rate

He carrier gas, 1 mL min

−1

Injection volume

2 µL

Sulfentrazone

573

7

Method validation and quality control

7.1

Experimental design

The LOQ was validated by acceptable and reproducible recoveries of the respective
analytes from laboratory-fortified control samples. For hay and straw, the LOQ was
validated at 0.05 mg kg

−1

and the LOD was set at 0.01 mg kg

−1

. For all other matrices,

the LOQ was validated at 0.025 mg kg

−1

and the LOD was set at 0.005 mg kg

−1

. Each

analysis set contained a minimum of one control sample, one fortified control sample,
and several treated samples.

A calibration curve was generated for each analyte at the initiation of the analytical

phase of the study. Standard solutions for injection contained sulfentrazone, DMS
and/or derivatized HMS. Standard solutions were injected at the beginning of each
set of assays and after every two or three samples thereafter to gage the instrument
response.

7.2

Preparation of standards

Stock solutions of approximately 1 mg mL

−1

were prepared by dissolving the appro-

priate amounts of the analytical standards in acetonitrile. Working standard solutions
for fortification were prepared in volumetric flasks by appropriate dilutions of the
stock solutions for each analyte or combination of analytes. During analysis, SCA is
converted to DMS and HMS is derivatized; therefore, the analytical standard solu-
tions for quantitation and instrument calibration contained sulfentrazone, DMS and
derivatized HMS. A measured volume of a standard solution containing sulfentra-
zone, DMS and HMS (prepared from stock solutions) was derivatized simultaneously
with the samples.

7.3

Calculation

The amounts of sulfentrazone, SCA (analyzed as DMS), and HMS were quantitated by
an external standard calibration method. A computer spreadsheet program (Microsoft
Excel) was used for calculation and reporting.

The amount of sample injected was determined by the following equation:

Amount of sample injected (mg)

=

initial aliquot weight (mg)

final sample extract volume (µL)

× sample extract volume injected (µL)

An equation representing area versus concentration was determined using a standard
linear regression analysis applied to the injection standards yielding a slope m and an
intercept b. The following equation was then used to calculate the concentration of

574

Individual compounds

the sample injected from the area measured:

Concentration of sample (ng µL

−1

)

=

area of sample

− b

m

The amount of analyte (in nanograms) detected in a sample injection was calculated
by multiplying the concentration calculated above by the injection volume. Then the
concentration detected (in ppm) was determined by dividing this result by the amount

Table 1

Recoveries from fortified samples

% Recovery (average

± SD)

Fortification levels

No. of

Matrix

(mg kg

−1

)

analyses

Sulfentrazone

SCA

HMS

Soybean seed

0.025

4

80

± 7

NA

82

± 14

Soybean hulls

0.025

1

75

NA

80

Soybean meal

0.025

1

76

NA

70

Soybean refined oil

0.025

3

73

± 8

NA

90

± 14

Corn grain

0.025

3

91

± 11

87

± 1

79

± 2

Corn forage

0.025

2

85

82

76

Corn fodder

0.025, 0.05

3

87

± 9

75

± 6

76

± 3

Corn flour

0.025

1

99

102

98

Rice grain

0.025, 0.05

3

104

± 10

97

± 18

89

± 3

Rice straw

0.05, 0.5

2

98

125

86

Rice hulls

0.025

1

95

99

98

Rice bran

0.025

1

77

69

72

Rice, polished

0.025

1

118

72

81

Sorghum grain

0.025

2

95

82

95

Sorghum forage

0.025, 0.05

2

89

86

96

Sorghum fodder

0.025, 0.1

2

84

76

73

Wheat grain

0.025

2

96

120

83

Wheat forage

0.025, 0.1

2

91

89

83

Wheat hay

0.05, 0.2

2

88

89

85

Wheat straw

0.05, 0.5

2

87

114

102

Pea

0.025

4

93

± 18

70

± 9

87

± 12

Alfalfa forage

0.0125, 0.025, 0.05, 0.25

10

105

± 17

93

± 15

90

± 13

Alfalfa hay

0.025, 0.25, 0.2, 1.0

10

75

± 9

82

± 12

85

± 15

Sunflower seed

0.05, 0.5

8

77

± 14

86

± 15

89

± 15

Sunflower meal

0.05, 0.5

6

84

± 6

73

± 9

74

± 5

Sunflower refined oil

0.05, 0.5

6

90

± 5

103

± 8

77

± 4

Sugarcane

0.025, 0.05

5

82

± 3

70

± 6

74

± 3

Refined sugar

0.025, 0.05

2

91

99

87

Molasses

0.025, 0.05

2

94

96

95

Peanut

0.025, 0.05, 0.1, 0.2

7

68

± 5

77

± 5

84

± 7

Peanut meal

0.025

1

68

64

74

Peanut refined oil

0.025, 0.05

3

100

± 11

86

± 10

84

± 7

Potato

0.05, 0.5, 1.0

16

100

± 7

106

± 11

87

± 11

Potato flakes

0.05

1

76

120

84

Potato wet peels

0.25, 0.5

2

121

91

93

Potato chips

0.05, 0.25, 0.5

3

99

± 4

97

± 19

82

± 16

Sulfentrazone

575

of sample injected:

Detected or uncorrected ppm (ng mg

−1

)

=

conc. of sample (ng µL

−1

)

× inj. volume (µL)

amount of sample injected (mg)

No correction for molecular weights was necessary for derivatized HMS, because
the injection standards were derivatized simultaneously with the samples. However,
a correction factor was needed for calculating the recovered amount of SCA since
the SCA was quantitated as DMS. The correction factor (molecular weight ratio)
between SCA and DMS was 1.12 (417/373; 417

= molecular weight of SCA and

373

= molecular weight of DMS). To calculate the amount of SCA, use the above

equation, which will yield DMS (ng), then multiply that value by 1.12 to convert to
nanograms of SCA.

The uncorrected ppm of the fortified control samples was divided by the fortification

level and multiplied by 100% to calculate the method recovery (%). The following
equation was used:

Method recovery (%)

=

uncorrected mg kg

−1

− control mg kg

−1

fortification level (mg kg

−1

)

× 100

The LOD was calculated as the concentration of analyte (ppm equivalent) at one-fifth
the area of the LOQ level standard, or one-fifth the LOQ, whichever was larger.

7.4

Time required for analysis

For a set of 10 samples, the analytical method can be completed within 16 laboratory
hours from the time of sample weighing to GC injection.

7.5

Accuracy and precision

The accuracy and precision of the analytical methods were determined by the average
and standard deviation of individual method recoveries of the fortified-control sam-
ples in 40 different matrices (see Table 1). These methods were also demonstrated
to be very rugged based on the results of accuracy and precision for a variety of crop
matrices.

8

Important points

After the initial extraction with acetone–0.25 N HCl, all traces of acetone must be
removed using a TurboVap Evaporator. Traces of solvent can lead to analyte loss
through the SPE cartridge(s).

576

Individual compounds

The proper elution and wash solvent composition and the volume and flow rate

through the cartridges must be determined. The SPE steps are critical to the separation
and cleanup of the sample extract. Listed brands for C

8

and silica gel cartridges should

be used, if possible.

After passing the sample solution through the C

8

cartridge, the cartridge and mani-

fold must be completely dry. Extend the drying time if necessary. Rinsing the manifold
with acetone prior to elution is a good practice. Traces of aqueous solution may in-
terfere with subsequent derivatization.

BSTFA should be used within 10 min after opening the ampule to ensure complete

derivatization. BSTFA readily absorbs moisture, which will interfere with derivati-
zation.

If final sample solutions will be stored for several days, the derivatization of the

HMS metabolite may reverse. If the derivatization has reversed, the HMS method
recovery would be low and an additional broad peak (underivatized HMS) would be
visible after the derivatized HMS peak. In this case, add 10 µL of fresh BSTFA to
the final sample solution in the GC vial, vortex the sample for several seconds and
re-inject the sample solution.

Optimizing the GC instrument is crucial for the quantitation of sulfentrazone

and its metabolites. Before actual analysis, the temperatures, gas flow rates, and
the glass insert liner should be optimized. The injection standards must have a low
relative standard deviation (

<15%) and the calibration standards must have a cor-

relation coefficient of at least 0.99. Before injection of the analysis set, the column
should be conditioned with a sample matrix. This can be done by injecting a ma-
trix sample extract several times before the standard, repeating this ‘conditioning’
until the injection standard gives a reproducible response and provides adequate
sensitivity.

Operation of the ELCD and XSD instruments must be optimized for greatest

sensitivity. Operating the ELCD instrument in tandem with another detector
may cause a decrease in sensitivity. More recently, liquid chromatography/mass
spectrometry (LC/MS) and liquid chromatography/tandem mass spectrometry
(LC/MS/MS) have been evaluated as possible alternative methods for sulfen-
trazone compounds in crop matrices. The LC/MS methods allow the chemical
derivatization step to be avoided, reducing the analysis time. However, the final
sample extracts, after being cleaned up extensively using three SPE cartridges,
still exhibited ionization suppression due to the matrix background. Acceptable
method recoveries (70–120%) of sulfentrazone compounds have not yet been ob-
tained.

9

Storage stability

Storage stability studies for sulfentrazone compounds on crop matrices showed a
pattern of stability for at least 3–38 months, depending on the study program or the
maximum sample storage interval for the study.

Sulfentrazone

577

Acknowledgments

The author gratefully thanks J.R. Arabinick, D. Baffuto, G.P. Barrett, J. Carroll, J.F.
Culligan, J.M. Fink, D.J. Letinski, E.M. McCoy, M.C. Reel, N.A. Shevchuk, C.M.
Suero, and M. Xiong for their help with sample preparation and analysis.

Andrey Chen

FMC, Princeton, NJ, USA