6640 ACIDIC HERBICIDE COMPOUNDS*

6640 A. Introduction

1. Sources and Significance

Agricultural chemicals used for weed control can be found as

contaminants in various aquatic systems.1 Although formerly

only two chlorinated phenoxy acid herbicides have been among

the substances regulated by the U.S. Environmental Protection

Agency in drinking water, and thus were routinely measured,

several other types of carboxylic acid compounds used for their

toxic effects now are under regulatory control.2

2. Selection of Method

The micro liquid-liquid extraction gas chromatographic

method presented here is applicable to the determination of salts

and esters of analyte acids. The form of each acid is not routinely

distinguished by this method. Results are calculated and reported

for each listed compound as the total free acid. Since only the

free acid is quantified, the herbicide Lactofen will be quantified

as Acifluorfen because their structures represent different esters

of the same acid herbicide. Additionally, this method is unable to

quantify the degradation products of Dacthal separately from the

parent compound unless the optional wash step is performed. If

this wash step is not performed, all forms of Dacthal (the parent

compound and the mono- and di-acid degradation products) will

be quantified as Dacthal.

3. Sampling and Storage

Collect grab samples in accordance with conventional sampling

practices (see Section 1060B), using amber glass containers with

polytetrafluoroethylene (PFTE) lined screw caps and capacities of at

least 40 mL. Add sodium sulfite crystals (approximately 2 mg/40

mL sample) if residual chlorine is present in the sample. Fill sample

bottles but take care not to flush out the sodium sulfite. Because the

target analytes of this method are not volatile, it is not necessary to

ensure that the sample bottles are completely headspace-free. Ice

samples during shipment so that the temperature does not exceed

10°C during the first 48 h after collection. Confirm that samples are

at or below 10°C when they are received at the laboratory. Store

samples in the laboratory at or below 6°C and protect from light

until extraction. Do not freeze samples.

Because of the several pH adjustments made to the samples in

the course of this method, the addition of organic or inorganic

biocides, including hydrochloric acid, has been omitted. The

analyst should be aware of the potential for the biological degradation of the analytes. Extract samples as soon as possible and

within no more than 14 d.

4. References

1. U.S. ENVIRONMENTAL PROTECTION AGENCY. 1990. National Pesticide

Survey: Summary Results of EPA’s National Survey of Pesticides in

Drinking Water Wells. EPA 57019-90-015, U.S. Environmental Protection Agency, Cincinnati, Ohio.

2. U.S. ENVIRONMENTAL PROTECTION AGENCY. 1992. National Primary

Drinking Water Regulations; Synthetic Organic Chemicals and Inorganic Chemicals; Final Rule. 40 CFR Parts 141 and 142, Part 111;

Federal Register 57, No.138.

6640 B. Micro Liquid-Liquid Extraction Gas Chromatographic Method

1. General Discussion

a. Application: This is a gas chromatography (GC) method for

the determination of chlorinated acids in drinking water. It can

be used to measure various acidic organic compounds and their

corresponding acid salts, although the form of each acid is not

differentiated and the calculated amount of each is expressed as

free acid. Accuracy, precision, and detection level data have

been generated in reagent water and finished ground and surface

waters for compounds listed in Table 6640:I. Also see 6640A.2.

b. Principle: A 40-mL sample is adjusted to pH 12 with 4N

sodium hydroxide and is kept for 1 h at room temperature to

hydrolyze derivatives. NOTE: Because the chlorphenoxy acid

herbicides are formulated as a variety of esters and salts, the

hydrolysis step is required and may not be skipped. After hydrolysis, an optional wash step using a 90:10 hexane:methyl

tert-butyl ether (MtBE) mixture may be performed as a sample

cleanup and to remove Dacthal. It is only necessary to perform

this wash if the quantitation of the Dacthal degradation products

is desired. The aqueous sample then is acidified with sulfuric

acid to pH 1 and extracted with 4 mL of MtBE that contains

the internal standard. The chlorinated acids, which have been

partitioned into the MtBE, then are converted to methyl esters by

derivatization with diazomethane. The target esters are separated

and detected by capillary column gas chromatography using an

electron capture detector (GC/ECD). Analytes are quantified

using an internal-standard-based calibration curve.

c. Interferences: Clean all glassware meticulously by washing

with detergent and tap water, rinsing with tap water, and rinsing

with reagent water. A final rinse with solvents may be needed. In

* Approved by Standard Methods Committee, 2006.

Joint Task Group: 21st Edition—Russell Chinn (chair), Wenta Liao, David J.

Munch, Theresa A. Prato.

1
place of a solvent rinse, heat nonvolumetric glassware in a

muffle furnace at 400°C for 2 h. Do not heat volumetric glassware in an oven above 120°C. Store glassware inverted or

capped with aluminum foil. Method interferences may be caused

by contaminants in solvents, reagents (including reagent water),

sample bottles and caps, and other sample-processing hardware

that lead to discrete artifacts and/or elevated base lines in the

chromatograms. Interferences by phthalate esters can pose a

major problem in pesticide analysis when an electron capture

detector (ECD) is used. These compounds generally appear in

the chromatogram as large peaks. Common flexible plastics

contain varying amounts of phthalates that are easily extracted or

leached during laboratory operations. Cross-contamination of

clean glassware routinely occurs when plastics are handled during extraction steps, especially when solvent-wetted surfaces are

present. Interferences from phthalates can best be minimized by

avoiding the use of plastics in the laboratory. Exhaustive purification of reagents and glassware may be required to eliminate

background phthalate contamination.1,2 Routinely demonstrate

that all sources of reagents are free from interferences, i.e., less

than one-third the method reporting limit (MRL) for each target

analyte, under the conditions of the analysis, by analyzing laboratory reagent blanks. Do not subtract blank values from sample

results.

Use a grade of sodium sulfate suitable for pesticide residue

analysis. If the suitability of the available sodium sulfate is in

question, extract and analyze a laboratory reagent blank to test for

interferences before sample processing. Matrix interferences may

be caused by contaminants that are extracted from the sample. The

extent of matrix interferences will vary considerably from source to

source, depending upon the water sampled. Also, contaminants in

local ground water and surface water may interfere with the analysis

of Dalapon on the primary column. A slow initial temperature

program can ensure separation of the interferent from Dalapon.

Confirm analyte identifications with the confirmation column specified, another column that is dissimilar to the primary column, or by

GC/MS if the concentrations are high enough.

d. Method detection level (MDL): Method detection levels are

compound-, instrument-, and matrix-dependent. The MDL is

defined as the statistically calculated minimum amount that can

be measured with 99% confidence that the reported value is

greater than zero.3 Experimentally determined MDLs for the analytes are provided in Table 6640:I. The MDL differs from, and is

usually lower than (but never above), the minimum reporting level

(MRL). The concentration range for target analytes in this method

was evaluated between 0.5 and 20 g/L for a 40-mL sample.

e. Safety: See Section 1090. The toxicity or carcinogenicity of

each reagent used in this method has not been precisely defined.

Regard each chemical compound as a potential health hazard and

minimize exposure to these chemicals. Maintain in the laboratory a current awareness file of OSHA regulations regarding the

safe handling of the chemicals specified in this method. A

reference file of MSDSs also should be made available to all

personnel involved in the chemical analysis. Additional references to laboratory safety are available.4–6

The toxicity of the extraction solvent methyl t-butyl ether

(MtBE) has not been well defined. Susceptible individuals may

experience adverse affects upon skin contact or inhalation of

vapors. Therefore, use protective clothing and gloves, and use

MtBE only in a chemical fume hood or glove box. The same

precaution applies to pure standard materials.

Diazomethane is a toxic carcinogen that can explode under

certain conditions. Use the diazomethane generator behind a

safety shield in a well-ventilated fume hood. Never allow the

generator to be heated above 90°C and avoid all grinding surfaces, such as ground-glass joints, sleeve bearings, and glass

stirrers. To minimize safety hazards, the diazomethane generator

apparatus used in the esterification procedure produces micromolar amounts of diazomethane in solution. If the procedure is

followed carefully, no possibility for explosion exists.

Although hydrogen can be safely used as a carrier gas, the

potential for fire or explosion does exist if the gas system is

mishandled. If unsure of the safety guidelines for using hydrogen

as a carrier gas, seek advice from the instrument manufacturer.

2. Apparatus

a. Sample containers, amber glass bottles with a minimum

volume of 40 mL, fitted with PTFE-lined screw caps.

b. Extraction vials, 60-mL clear glass vials with PTFE-lined

screw caps.

c. Autosampler vials, 2.0-mL vials with screw or crimp cap

and a PTFE-faced seal.

d. Standard solution storage containers, 10- to 20-mL amber

glass vials with PTFE-lined screw caps.

e. Clear vials, 7-mL glass, disposable, with PTFE-lined screw

caps for extract drying and derivatization.

f. Pasteur pipets, glass, disposable.

g. Pipets, Class A, 2.0-mL and 4.0-mL glass, or adjustable

volume dispensers.

h. Volumetric flasks, Class A, suggested sizes 5 mL, 10 mL,

and 100 mL.

TABLE 6640:I. SINGLE-LABORATORY METHOD DETECTION LEVELS IN

REAGENT WATER

Compound*

Fortification

Level

g/L

Primary

Column MDL

g/L

Secondary

Column MDL

g/L

Dalapon 0.100 0.05 0.07

3,5-Dichlorobenzoic acid 0.050 0.21 0.05

Dicamba 0.050 0.03 0.04

Dichlorprop 0.100 0.43 0.12

2,4-D 0.100 0.06 0.07

Pentachlorophenol 0.010 0.01 0.08

Silvex 0.025 0.03 0.02

2,4,5-T 0.025 0.02 0.03

2,4-DB 0.100 0.25 0.18

Chloramben 0.050 0.06 0.08

Dinoseb 0.100 0.17 0.08

Bentazon 0.100 0.07 0.19

Dacthal 0.050 0.11 0.11

Quinclorac 0.050 0.08 0.11

Picloram 0.050 0.08 0.05

Acifluorfen 0.250 0.31 0.09

* To assure exact identification of analytes, CAS numbers are listed as follows:

Acifluorfen, 50594-66-6; Bentazon, 25057-89-0; Chloramben, 133-90-4; 2,4-D,

94-75-7; Dacthal, 1861-32-1; Dalapon, 75-99-0; 2,4-DB, 94-82-6; Dicamba,

1918-00-9; 3,5-Dichlorobenzoic acid, 51-36-5; Dichloroprop, 120-36-5; Dinoseb,

88-85-7; pentachlorophenol 87-86-5; Picloram, 1918-02-1; Quinclorac, 84087-

01-4; Silvex, 93-72-1; 2,4,5-T, 93-76-5.

ACIDIC HERBICIDE COMPOUNDS (6640)/Micro Liquid-Liquid Extraction GC Method

2
i. Microsyringes, various sizes.

j. Balance, analytical, capable of weighing to the nearest

0.0001 g.

k. Diazomethane generator: See Figure 6251:3 for a diagram

of an all-glass system custom-made for these validation studies.

Micromolar generators also are available from commercial

sources.* If a generator from a commercial source is used,

follow manufacturer’s instructions.

l. Gas chromatograph, capillary GC.† Modifications should

include a high-pressure (50 psi) split/splitless injector, fast temperature ramp oven (50°C/min) and a low-volume (150-L)

micro ECD detector. A data system capable of fast sampling (20

points/peak) also is required.

m. Primary GC column,‡ 180-m ID, 40 m long, 0.2-m film

thickness, fused silica capillary with chemically bonded (14%

cyanopropylphenymethylpolysiloxane), or equivalent bonded,

fused silica column.

n. Confirmation GC column,§ 180-m ID, 40 m long, 0.2-m

film thickness, fused silica capillary with chemically bonded (5%

phenylmethylpolysiloxane), or equivalent bonded, fused silica

column. Columns of other dimensions may be used as long as the

QC criteria of the method are met.

3. Reagents

Use reagent-grade or better chemicals in all analyses. For

reagents b through f, use high-purity HPLC-grade materials

demonstrated to be free from analytes and interferences. Unless

otherwise indicated, use reagents conforming to the specifications of the Committee on Analytical Reagents of the American

Chemical Society, where such specifications are available. Other

grades may be used, provided it is first determined that the

reagent is of sufficiently high purity to permit its use without

lessening the quality of the determination.

Observe safety precautions; see 6640B.1e.

a. Reagent water: Use purified water that does not contain any

measurable quantities of any target analytes or interfering compounds greater than 1/3 the MRL for each compound of interest.

b. Methyl tert-butyl ether (MtBE).

c. Acetone.

d. Carbitol (diethylene glycol monoethyl ether).

e. Hexane:MtBE wash solvent, 90:10 (v/v), unpreserved.

f. Hexane.

g. Sodium sulfate, Na2SO4, pesticide-grade, granular, anhydrous. If interferences are observed (see 6640B.1c), it may be

necessary to heat the sodium sulfate in a shallow tray at 400°C

for up to 4 h to remove phthalates and other interfering organic

substances. Alternatively, extract it with methylene chloride in a

Soxhlet apparatus for 48 h. Store in a capped glass bottle rather

than a plastic container.

h. Acidified sodium sulfate: Acidify by slurrying 100 g of

muffled sodium sulfate with enough ethyl ether to just cover the

solid. Add 0.5 mL conc sulfuric acid dropwise while mixing

thoroughly. Remove ether under vacuum. Mix 1 g of resulting

solid with 5 mL reagent water and measure pH of the mixture.

The pH must be below 4. Store in a desiccator or at 100°C to

keep reagent dry.

i. Copper II sulfate pentahydrate, CuSO4  5H2O.

j. Sodium hydroxide solution, NaOH, 4N: Dissolve 16 g NaOH

pellets in reagent water and dilute to 100 mL.

k. Potassium hydroxide solution, KOH, 37% (w/v): Dissolve

37 g KOH pellets in reagent water and dilute to 100 mL.

l. Sodium sulfite, Na2SO3, for use as a dechlorinating agent.

m. N-methyl-N-nitroso-p-toluenesulfonamide(MNTS) solution: Prepare a solution containing 5 g MNTS in 50 mL of a

50:50 (v/v) mixture of MtBE and carbitol. This solution is stable

for 1 month or longer when stored at 4°C in an amber bottle with

a PTFE-lined screw cap.

n. Sulfuric acid, H2SO4, conc.

o. Silica gel, 35 to 60 mesh.

p. Hydrogen, 99.999% pure or better, GC carrier gas.

q. Nitrogen, 99.999% pure or better, ECD make-up gas.

r. Internal standard solutions: 4,4-Dibromooctafluorobiphenyl (99%) is used as an internal standard for the method. This

compound has been shown to be an effective internal standard

for the method analytes, but other compounds may be used if the

QC requirements are met. The solutions listed in ¶s 1) and 2)

below are stable for at least 30 d. Replace solution if ongoing QC

indicates a problem.

1) Internal standard stock solution, 2.0 mg/mL: Prepare an

internal standard stock solution by accurately weighing approximately 0.0200 g of neat 4,4-dibromooctafluorobiphenyl. Dissolve the neat material in MtBE and dilute to volume in a 10-mL

volumetric flask. Transfer solution to an amber glass vial with a

PTFE-lined screw cap and store at 0°C. The resulting concentration of the stock internal standard solution will be approximately 2.0 mg/mL.

2) Internal standard primary dilution standard, 2.5 g/mL:

Prepare an internal standard fortification solution at approximately 2.5 g/mL by adding 12.5 L of the stock standard to

10 mL MtBE. Transfer primary dilution to an amber glass vial

with a PTFE-lined screw cap and store at 0°C.

3) MtBE extraction solvent with internal standard, 2.5 g/mL:

The internal standard 4,4-dibromooctafluorobiphenyl is added to

the extraction solvent before analyte extraction to compensate for

any volumetric differences encountered during sample processing.

Make this solution just before extraction. The addition of 1 mL of

primary dilution standard (2.5 g/mL) to 99 mL MtBE results in a

final internal standard concentration of 25 ng/mL.

s. Surrogate (SUR) analyte standard solution: 2,4-dichlorophenylacetic acid (99%) is used as a surrogate compound to

evaluate extraction and derivatization procedures. This compound is an effective surrogate for the method analytes, but other

compounds may be used if QC requirements are met. The

solutions listed in 1) and 2) below are stable for at least 30 d.

Replace solution if ongoing QC indicates a problem.

1) Surrogate stock solution, 1.0 mg/mL: Prepare a surrogate

stock standard solution of 2,4-dichlorophenylacetic acid by

weighing approximately 0.0100 g of neat material. Dissolve the

neat material in acetone and dilute to volume in a 10-mL volumetric flask. Transfer solution to an amber glass vial with a

* Aldrich Catalog, No. Z10, 889-8, or equivalent.

† Hewlett Packard Model 6890, or equivalent.

‡ DB-1701, or equivalent.

§ DB-5, or equivalent. Diazald®, Aldrich Chemical Co., or equivalent.

ACIDIC HERBICIDE COMPOUNDS (6640)/Micro Liquid-Liquid Extraction GC Method

3
PTFE-lined screw cap and store at 0°C. The resulting concentration will be 1.0 mg/mL.

2) Surrogate primary dilution standard/sample fortification

solution, 100 g/mL: Prepare a primary dilution standard at

approximately 100 g/mL by the addition of 1 mL stock standard to 10 mL acetone. Transfer primary dilution to an amber

glass vial with a PTFE-lined screw cap and store at 0°C. The

addition of 10 L of primary dilution standard to the 40-mL

aqueous sample results in a surrogate concentration of 25 ng/mL.

t. Analyte standard solutions: Obtain analytes as neat or solid

free acid standards or as commercially prepared ampulized solutions from a reputable standard manufacturer. Do not use premethylated standards for preparing analyte standards. Prepare analyte stock and primary dilution standards as described below. The

solutions listed in ¶s 1) and 2) below are stable for at least 30 d.

Replace solution if ongoing QC indicates a problem.

1) Analyte stock standard solution: Prepare separate stock standard solutions for each analyte of interest at a concentration of 1 to

5 mg/mL in acetone. Method analytes may be obtained as neat

materials or ampulized solutions (99% purity) from a number of

commercial suppliers. Store stock standard solutions at 0°C.

For analytes that are solids in their pure form, prepare stock

standard solutions by accurately weighing approximately 0.01 to

0.05 g pure material in a 10-mL volumetric flask. Dilute to

volume with acetone.

For analytes that are liquid in their pure form at room temperature, place approximately 9.8 mL acetone in a 10-mL volumetric flask. Let flask stand, unstoppered, for about 10 min to

allow solvent film to evaporate from the inner walls of flask, and

weigh to the nearest 0.1 mg. Use a 10-L syringe and immediately add 10.0 L standard material to the flask by keeping

syringe needle just above the surface of the acetone. Be sure that

the standard material falls dropwise directly into the acetone

without contacting inner wall of flask. Calculate concentration in

milligrams per milliliter from net gain in weight. Dilute to

volume, stopper, then mix by inverting flask several times.

2) Primary dilution standard (PDS): Prepare primary dilution

standard solution by combining and diluting stock standard solutions with acetone. Store primary dilution standard solution at

0°C. As a guideline to the analyst, the analyte concentrations, in

micrograms per milliliter, used in the primary dilution standard

solution during method development are listed as follows.

Acifluorfen 5.0

Bentazon 10

Chloramben 5.0

2,4-D 10

Dalapon 10

2,4-DB 10

Dacthal acid degradation 5.0

Dicamba 5.0

3,5-Dichlorobenzoic acid 5.0

Dichlorprop 10

Dinoseb 10

Pentachlorophenol 1.0

Picloram 5.0

2,4,5-T 2.5

2,4,5-TP (Silvex) 2.5

Quinclorac 5.0

u. Calibration standards: Prepare a five-point calibration

curve by fortifying reagent water with the primary dilution

standard. Let the standard reach room temperature, then add a

designated amount of each calibration standard in acetone into

separate, 40-mL portions of reagent water to produce a calibration curve ranging from below or at the MRL to approximately

10 to 20 times the lowest calibration level. Treat these aqueous

calibration standards like samples and therefore add all preservatives and other reagents. Extract by the procedure described

below. Store calibration standard solutions in acetone at 0°C.

4. Procedure

a. Sample preparation: Remove samples from storage and let

them equilibrate to room temperature. Place 40 mL water sample

in a pre-cleaned, 60-mL glass vial with a PTFE-lined screw cap,

using a graduated cylinder. Add 10 L room temperature surrogate standard (100 g/mL 2,4-dichlorophenylacetic acid in

acetone) to the aqueous sample.

b. Hydrolysis: Because many of the herbicides included in this

method are applied as a variety of esters and salts, hydrolyze

them to the parent acid before extraction. Include this step in

analysis of all extracted field samples, laboratory reagent blanks

(LRBs), laboratory-fortified matrices (LFMs) and calibration

standards. Failure to perform this step may result in data that are

biased low for some targets in field samples.7 Add 1 mL 4N

NaOH solution to each glass vial, then cap and shake for several

seconds. Check sample pH with pH paper or a pH meter. If

sample does not have a pH 12, adjust pH by adding more 4N

NaOH solution. Let sample remain at room temperature for 1 h,

shaking contents periodically.

c. Hexane:MtBE wash (optional): Use this wash step to quantify the Dacthal degradation products from the parent Dacthal.

This wash aids in sample cleanup and removes any Dacthal from

the sample that would interfere with the quantitation of the

Dacthal degradation products. After hydrolysis, add 5 mL 90:10

(v:v) hexane:MtBE and shake vigorously for 3 min. Let phases

separate for approximately 5 min, then remove and discard the

top hexane/MtBE layer. If differentiation between Dacthal and

its degradation products is not required, this step may be omitted.

d. Microextraction: Adjust pH to 1 by adding approximately

1 mL conc sulfuric acid. Cap, shake, and then check pH with a

pH meter or narrow-range pH paper. Add more sulfuric acid as

needed to properly adjust pH.

Quickly add approximately 2 g copper II sulfate pentahydrate

and shake until dissolved. This colors the aqueous phase blue

and allows the analyst to better distinguish between the aqueous

phase and the organic phase.

Quickly add approximately 16 g muffled sodium sulfate and

shake until almost all is dissolved. Sodium sulfate addition

increases the ionic strength of the aqueous phase, further driving

the chlorophenoxy acids into the organic phase, and also decreases MtBE solubility in the aqueous phase, allowing greater

volumetric recovery. Add this salt and the copper II sulfate

pentahydrate quickly so the heat generated from addition of the

acid will help dissolve the salts.

Add exactly 4.0 mL MtBE extraction solvent with internal

standard to each vial and shake vigorously for 3 min. Let phases

separate for approximately 5 min.

ACIDIC HERBICIDE COMPOUNDS (6640)/Micro Liquid-Liquid Extraction GC Method

4
e. Preparation of diazomethane: Assemble diazomethane generator (Figure 6251:3) in a hood. See 6640B.2k above. The

collection vessel is a 10- or 15-mL glass vial equipped with a

PTFE-lined screw cap and maintained at 0 to 5°C.

Add a sufficient amount of MtBE (approximately 7 mL) to

first tube to cover the impinger. Add 10 mL MtBE to collection

vial. Set nitrogen flow at 5 to 10 mL/min. Add 4 mL MNTS

solution (6640B.3m) and 3 mL 37% KOH solution to the second

impinger. Connect tubing as shown in Figure 6251:3 and let

nitrogen flow purge diazomethane from reaction vessel into the

collection vial for 30 min. Cap vial when collection is complete

and maintain at 0 to 5°C. When stored at 0 to 5°C, this diazomethane solution may be used over a period of 72 h.

Using a Pasteur pipet, transfer sample extract (upper MtBE

layer) to a 7-mL glass vial. Add 0.6 g acidified sodium sulfate

and shake. This step dries the MtBE extract.

Using a Pasteur pipet, transfer extract to a second, 7-mL glass

vial. Add 250 L diazomethane solution to each vial. The

contents of the vial should remain slightly yellow, indicating an

excess of diazomethane. More diazomethane may be added if

necessary. Let the esterification reaction proceed for 30 min.

Remove any unreacted diazomethane by adding 0.1 g silica

gel. Effervescence (evolution of nitrogen) is an indication that

excess diazomethane was present. Let extracts stand for 0.5 h.

Transfer extract to an autosampler vial. A duplicate vial may

be filled with excess extract.

Analyze sample extracts as soon as possible. The sample extract

may be stored up to 21 d if kept at 0°C or less. Keep extracts away

from light in amber glass vials with PTFE-lined caps.

f. Gas chromatography: Use apparatus specified in 6640B.2l,

m, n, and hydrogen carrier gas, 6640B.3p. CAUTION: Observe

safety guidelines when using hydrogen as a carrier gas. If

further safety information is required, seek advice from the

instrument manufacturer regarding hydrogen use.

Pay strict attention to established column installation guidelines for proper cutting and placement of capillary columns

within the instrument. If a loss of response is noted for an

analyte, trimming approximately 1 m from the head of the

column may restore response. A guard column is recommended

if conditions in the laboratory necessitate frequent column trimming.

Chromatographic conditions and analyte retention times for

the primary column are given in Table 6640:II A chromatogram

from this column is provided in Figure 6640:1. Chromatographic

conditions and analyte retention times for the secondary column

are given in Table 6640:III. A chromatogram from this column

is provided in Figure 6640:2.

g. Calibration: Establish GC operating parameters equivalent

to the suggested specifications in Table 6640:II. Calibrate GC

system using the internal standard (IS) technique. Other columns

or conditions may be used if equivalent or better performance

can be demonstrated. Prepare a set of at least five calibration

standards as described in 6640B.3u. The lowest concentration of

calibration standard must be at or below the MRL; preferably use

a minimum of four calibration standards between the MRL and

the upper quantitative range.

Use GC data system software to generate a linear regression or

quadratic calibration curve using the internal standard. This

curve may be generated by plotting Ax/Ais vs. Qx/Qis, where Ax

and Ais are integrated peak areas of the analyte and internal

standard, respectively, and Qx and Qis are quantity of analyte and

internal standard injected in concentration units, respectively.

TABLE 6640:II. CHROMATOGRAPHIC CONDITIONS AND AVERAGE RETENTION TIME DATA FOR PRIMARY COLUMN

Peak Number

(Figure 6640:1) Compound

Average RT*

min

RSD

%

1 Dalapon 7.06 0.017

2 3,5 Dichlorobenzoic acid 11.32 0.002

3 2,4-Dichlorophenylacetic acid (SUR) 11.80 0.002

4 Dicamba 11.91 0.002

5 Dichlorprop 12.21 0.002

6 4,4-Dibromooctafluorobiphenyl (IS) 12.25 0.002

7 2,4-D 12.37 0.002

8 Pentachlorophenol 12.49 0.002

9 Silvex 12.70 0.002

10 2,4,5-T 12.90 0.002

11 2,4-DB 13.10 0.003

12 Chloramben 13.15 0.003

13 Dinoseb 13.44 0.003

14 Bentazon 13.60 0.003

15 Dacthal 13.68 0.002

16 Quinclorac 13.85 0.004

17 Picloram 14.02 0.003

18 Acifluorfen 15.20 0.004

* Average of seven runs.

Primary column: DB-1701, 40 m  0.180 mm ID, 0.20-mm film thickness, injector temperature 200°C, liner 2 mm straight Siltek deactivated, injection volume 1 L

of 200 ng/mL (highest level component) standard, splitless injection. Hold to 1 min then purge @ 50 mL/min, detector temperature 360°C, detector make-up gas nitrogen

at 20 mL/min. Temperature program: 45°C initial, program at 5°C/min to 80°C, then 50°C/min to 220°C, then 20°C/min to 280°C and hold for 3 min. Data collection via

HP GC Chemstation at a rate of 20 Hz.

Carrier gas: Hydrogen (UHP).

Detector make-up gas: Nitrogen (UHP).

ACIDIC HERBICIDE COMPOUNDS (6640)/Micro Liquid-Liquid Extraction GC Method

5
Figure 6640:1. Chromatogram of chlorphenoxy herbicides on a primary column. Peak identifications and chromatographic conditions are given in Table

6640:II. Peak has been attenuated for illustration.

TABLE 6640:III. CHROMATOGRAPHIC CONDITIONS AND AVERAGE RETENTION TIME DATA FOR CONFIRMATION COLUMN

Peak Number

(Figure 6640:2) Compound

Average RT*

min

RSD

%

1 Dalapon 5.04 0.057

2 3,5-Dichlorobenzoic acid 10.33 0.007

3 2,4-Dichlorophenylacetic acid (SUR) 11.31 0.004

4 Dicamba 11.46 0.006

5 Dichlorprop 12.19 0.006

6 4,4-Dibromooctafluorobiphenyl (IS) 12.71 0.004

7 2,4-D 12.34 0.005

8 Pentachlorophenol 13.16 0.006

9 Silvex 13.29 0.004

10 2,4,5-T 13.48 0.005

11 2,4-DB 13.94 0.005

12 Chloramben 13.34 0.004

13 Dinoseb 14.01 0.004

14 Bentazon 14.28 0.006

15 Dacthal 15.00 0.004

16 Quinclorac 14.88 0.004

17 Picloram 14.59 0.004

18 Acifluorfen 16.38 0.003

* Average of seven runs.

Confirmation column: DB-5, 40 m  0.180 mm ID, 0.20-mm film thickness, injector temperature 200°C, liner 2 mm straight Siltek® deactivated,

injection volume 1 L of 100 ng/mL (highest level component) standard, splitless injection. Hold to 1 min then purge @ 50 mL/min, detector

temperature 360°C, detector make-up gas nitrogen at 20 mL/min. Temperature program: 45°C initial, program at 4°C/min to 80°C, then program at

30°C/min to 280°C and hold for 2 min. Data collection via HP GC Chemstation at a rate of 20 Hz.

Carrier gas: Hydrogen (UHP).

Detector make-up gas: Nitrogen (UHP).

ACIDIC HERBICIDE COMPOUNDS (6640)/Micro Liquid-Liquid Extraction GC Method

6
5. Quality Control

The quality control practices considered to be an integral part

of each method are summarized in Table 6020:I.

Quality control (QC) requirements include the initial demonstration of capability (IDC), the determination of the method

detection level (MDL), and subsequent analysis in each analysis

batch of a laboratory reagent blank (LRB), continuing calibration

check (CCC) standards, a laboratory-fortified sample matrix

(LFM), and either a laboratory-fortified sample matrix duplicate

(LFMD) or a field duplicate (FD) sample. This section details the

specific requirements for each QC parameter. The criteria discussed below are considered the minimum acceptable QC criteria. Laboratories are encouraged to institute additional QC

practices to meet their specific needs.

Process all QC samples through all steps of the procedure

(6640B.4a–f), including hydrolysis and methylation. Add sample preservatives as described in 6640A.3 before extracting and

analyzing the QC samples.

a. Initial demonstration of capability IDC: Requirements for

this demonstration are described below.

1) Initial demonstration of low system background—Before

any samples are analyzed, demonstrate that a LRB is reasonably

free of contamination and that the criteria in ¶ c below are met.

Process all QC samples through all steps of the method.

2) Initial demonstration of precision—Prepare, extract, and

analyze four to seven replicate LFBs fortified at 5 g/L, or near

the mid-range of the initial calibration curve. Process all QC

samples through all steps of the analysis procedure. The relative

standard deviation (RSD) of the results of the replicate analyses

should be less than 20%.

3) Initial demonstration of accuracy—Using the same set of

replicate data generated for ¶ 2) above, calculate average recovery. The average recovery of the replicate values should be

within 20% of the true value.

4) Method detection level (MDL)—Prepare, extract, and analyze

at least seven replicate LFBs at a concentration estimated to be near

the MDL, over a period of at least 3 d (both extraction and analysis

should be conducted over at least 3 d) using the analysis procedure

of 6640B.4a–f. Process all QC samples through all steps of the

method. Select a fortification level with a signal of two to five times

the noise level and at or below the laboratory’s MRL. The appropriate concentration will depend on the sensitivity of the GC/ECD

system. Add sample preservatives as described in 6640A.3 to these

samples. Calculate the MDL as follows:

MDLSt(n1, 10.99)

where:

t(n  1, 1    0.99)  Student’s t value for the 99% confidence level

with n1 degrees of freedom,

n  number of replicates, and

S  standard deviation of replicate analyses.

Figure 6640:2. Chromatogram of the chlorphenoxy herbicides on confirmation column. Peak identifications and chromatographic conditions are given in

Table 6640:III.

ACIDIC HERBICIDE COMPOUNDS (6640)/Micro Liquid-Liquid Extraction GC Method

7
NOTE: Calculated MDLs need only be less than 1/3 of the

laboratory’s MRL to be considered acceptable. Do not subtract

blank values when performing MDL calculations. The MDL is a

statistical determination of precision only.1 If the MDL replicates are fortified at a low enough concentration, it is likely that

they will not meet precision and accuracy criteria, and may result

in a calculated MDL that is higher than the fortified concentration.

b. Minimum reporting level (MRL): The MRL is the threshold

concentration of an analyte that a laboratory can expect to

quantitate accurately in an unknown sample. The MRL should

not be established at an analyte concentration that is less than

either three times the MDL or a concentration yielding a signalto-noise (S/N) ratio of less than five. Depending upon the study’s

data quality objectives, it may be set at a higher concentration.

Although the lowest calibration standard should be at or below

the MRL, the MRL cannot be established at a concentration

lower than the lowest calibration standard.

c. Laboratory reagent blank (LRB): An LRB is required with

each extraction batch of samples to determine the background

system contamination. If the LRB produces a peak within the

retention-time window of any analyte that would prevent the

determination of that analyte, determine the source of contamination, and eliminate the interference before processing samples.

Reduce background contamination to an acceptable level before

proceeding. Background from method analytes or contaminants

that interfere with the measurement of method analytes should

be below 1/3 of the MRL. If target analytes are detected in the

LRB at concentrations equal to, or greater than, this level, then

all data for the problem analyte(s) may be considered invalid for

all samples in the extraction batch.

d. Continuing calibration check (CCC): A CCC is a standard,

prepared in the same extraction batch as the samples of interest,

that contains all compounds of interest and is extracted in the

same manner as the procedural standards used to prepare the

initial calibration curve. Prepare an appropriate number of CCCs

with each extraction batch. The CCC verifies the initial calibration at the beginning and end of each group of analyses, and after

every tenth sample. The beginning CCC for each analysis batch

should be at or below the MRL in order to verify instrument

sensitivity before any analyses. Subsequent CCCs should alternate between a medium and high concentration. Calculate the

concentration of each analyte and surrogate in the check standard. The calculated amount for each analyte for medium and high

level CCCs should be within 30% of the true value. The

calculated amount for the lowest calibration level for each analyte should be within 50% of the true value. If these conditions

do not exist, consider all data for the problem analyte invalid,

and take remedial action, which may require recalibration. Reanalyze any field sample extracts analyzed since the last acceptable calibration verification after adequate calibration has been

restored, with the following exception. If the continuing calibration fails because the calculated concentration is greater than

130% (150% for the low-level CCC) for a particular target

compound, and field sample extracts show no detection for that

target compound, non-detects may be reported without re-analysis. Prepare calibration checks, with the samples being analyzed, at the beginning of each day that samples are analyzed,

after every ten samples, and at the end of any group of sample

analyses.

e. Laboratory fortified blank (LFB): Because this method

utilizes procedural calibration standards, which are fortified

reagent waters extracted with each analytical batch, there is

no difference between the LFB and the continuing calibration

check standard. Consequently, the analysis of an LFB is not

required; however the acronym LFB is used for clarity in the

IDC.

f. Internal standard (IS): Monitor the peak area of each internal standard in all injections during each analysis day. The IS

response (as indicated by peak area) for any chromatographic

run should not deviate by more than 50% from the average

area measured during the initial calibration for that IS. A poor

injection could cause the IS area to exceed these criteria. Inject

a second portion of the suspect extract to determine whether the

failure is due to poor injection or instrument response drift. If the

reinjected portion produces an acceptable internal standard response, report the results. If the internal standard area for the

reinjected extract deviates more than 50% from the initial calibration average, check the CCC standards that ran before and

after the sample. If the CCC fails the criteria of ¶ d above,

recalibrate according to 6640B.4g. If the CCC is acceptable,

repeat sample extraction provided that the sample is still within

holding time. Otherwise, report results obtained from the reinjected extract, but indicate as suspect.

g. Surrogate recovery: The surrogate standard is fortified into

the aqueous portion of all samples, LRBs, and LFMs and

LFMDs before extraction. It is also added to the calibration

curve and CCC standards. The surrogate is a means of assessing

method performance from extraction to final chromatographic

measurement. When surrogate recovery from a sample, blank, or

CCC is less than 70% or greater than 130%, check calculations

to locate possible errors, standard solutions for degradation,

contamination, and instrument performance. If those steps do not

reveal the cause of the problem, re-analyze extract. If extract

re-analysis meets the surrogate recovery criterion, report data

only for re-analyzed extract. If extract re-analysis fails the 70 to

130% recovery criterion, check calibration by injecting the last

calibration standard that passed. If the calibration standard fails

the criteria of ¶ d above, recalibrate according to 6640B.4g. If

calibration standard is acceptable, repeat sample extraction provided that the sample is still within the holding time. If sample

re-extract also fails the recovery criterion, report all data for that

sample as a quantitative estimate because of surrogate recovery.

h. Laboratory-fortified sample matrix and duplicate (LFM and

LFMD): Include LFMs and LFMDs in each extraction batch to

determine whether the sample matrix adversely affects method

accuracy. If the occurrence of target analytes in the samples is

infrequent, or if historical trends are unavailable, include a

second LFM or LFMD from a duplicate field sample to assess

method precision. Extraction batches that contain LFMDs will

not require the analysis of a FD. If a variety of different sample

matrices are analyzed regularly, for example, drinking water

from groundwater and surface water sources, establish method

performance for each. Over time, document LFM data for all

routine sample sources for the laboratory. Within each extraction

batch, fortify a minimum of one field sample as an LFM for

every 20 samples extracted. Prepare the LFM by adding to a

sample an appropriate amount of analyte as given in ¶ 3t2).

Select a fortifying concentration that is at least twice the matrix

background concentration, if known. Use historical data and

ACIDIC HERBICIDE COMPOUNDS (6640)/Micro Liquid-Liquid Extraction GC Method

8
rotate through the designated concentrations when selecting a

fortifying concentration.

Calculate the percent recovery (R) for each analyte by using

the equation:

R  A  B	

C

 100

where:

A  measured concentration in fortified sample,

B  measured concentration in unfortified sample, and

C  fortification concentration.

Analyte recoveries may exhibit matrix bias. For samples fortified at or above their native concentration, recoveries should

range between 70 and 130%, except for low-level fortification

near or at the MRL, where 50 to 150% recoveries may be

acceptable. If the accuracy of any analyte falls outside the

designated range, and the laboratory performance for that analyte

is shown to be in control in the LFB, the recovery may be judged

to be matrix-biased. Label result for that analyte in the unfortified sample as suspect because of matrix effects.

If an LFMD is analyzed instead of a FD, calculate relative

percent difference (RPD) for values in duplicate LFMs (LFM

and LFMD) by using the equation:

RPD  LFM  LFMD

LFM  LFMD	/2

 100

RPDs for duplicate LFMs should fall in the range of 30% for

samples fortified at or above their native concentration. Greater

variability may be observed when LFMs are spiked near the

MRL. At the MRL, RPDs should fall in the range of 50% for

samples fortified at or above their native concentration. If the

accuracy of any analyte falls outside the designated range and

the laboratory performance for that analyte is shown to be in

control in the LFB, the recovery may be judged to be matrixbiased. Label result for that analyte in the unfortified sample as

suspect because of matrix effects.

i. Field duplicates (FD1 and FD2): Within each extraction

batch, include a minimum of one field duplicate (FD) or LFMD

(¶ 5h above). FDs check the precision associated with sample

collection, preservation, storage, and laboratory procedures. If

target analytes are not routinely observed in field samples, analyze a LFMD to substitute for this requirement. Extraction

batches that contain LFMDs may not require the analysis of a

FD. Calculate the relative percent difference (RPD) for duplicate

measurements (FD1 and FD2) using the equation:

RPD  FD1  FD2

FD1  FD2	/2

 100

RPDs for duplicates should be in the range of 30%. Greater

variability may be observed when analyte concentrations are

near the MRL. At the MRL, RPDs should fall in the range of

50%. If the accuracy of any analyte falls outside the designated

range, and the laboratory performance for that analyte is shown

to be in control in the LFB, the recovery may be judged to be

matrix-biased. Label result for that analyte in the unfortified

sample as suspect because of matrix effects.

j. Quality control (QC) sample: Analyze a QC sample each

time that new primary dilution standards, ¶ 3t2), are prepared.

Ideally, obtain QC sample from second vendor. If a second

vendor is not available, use a different lot of the standard.

Although use of pre-methylated standards is prohibited for

preparing analyte standard solutions, pre-methylated standards may be used to prepare the QC sample. The QC sample

may be injected as a calibration standard or fortified into

reagent water and analyzed as a LFB. If the QC sample is

analyzed as a continuing calibration, acceptance criteria are

the same as for the CCC. If the QC sample is analyzed as a

LFB, acceptance criteria are the same as for a LFB. If measured analyte concentrations are not of acceptable accuracy,

check entire analytical procedure to locate and correct problem source.

6. Precision and Bias

Precision and bias data for this method are presented in Table

6640:IV. The effect of sample holding time on recovery from a

chlorinated surface water is shown in Table 6640:V and the

effect of extract holding time in Table 6640:VI.

7. References

1. GIAM, C.S., H.S. CHAN & G.S. NEF. 1975. Sensitive method for

determination of phthalate ester plasticizers in open ocean biota

samples. Anal. Chem. 47:2225.

2. GIAM, C.S. & H.S. CHAN. 1976. Control of blanks in the analysis of

phthalates in air and ocean biota samples. U.S. National Bur. Standards, Special Publ. 442, p. 701.

3. GLASER, J.A., D.L. FOERST, G.D. MCKEE, S.A. QUAVE & W.L. BUDDE.

1981. Trace analyses for wastewaters. Environ. Sci. Technol. 15: 1426.

4. OCCUPATIONAL SAFETY AND HEALTH ADMINISTRATION. 1976. OSHA

Safety and Health Standards, General Industry. 29 CRF 1910; OSHA

2206 (rev.), Occupational Safety & Health Admin., Washington,

D.C.

5. U.S. PUBLIC HEALTH SERVICE. 1977. Carcinogens—Working with

Carcinogens. Publ. No. 77-206, U.S. Dep. Health, Education, and

Welfare, Public Health Serv., Center for Disease Control, National

Inst. Occupational Safety & Health, Atlanta, Ga.

6. COMMITTEE ON CHEMICAL SAFETY, AMERICAN CHEMICAL SOCIETY. 1979.

Safety in Academic Chemistry Laboratories, 3rd ed. American

Chemical Soc., Washington, D.C.

7. BASSETT, M.V., B.V. PEPICH & D.M. MUNCH. 2000. The role played

by basic hydrolysis in the validity of acid herbicide data — Recommendations for future preparation of herbicide performance evaluation standards. Environ. Sci. Technol. 34:1117.

ACIDIC HERBICIDE COMPOUNDS (6640)/Micro Liquid-Liquid Extraction GC Method

9
TABLE 6640:IV. METHOD PRECISION AND BIAS IN SELECTED MATRICES

Low-Level Fortification Mid-Level Fortification

Matrix Compound

Fortification

Concentration

g/L

Mean

Recovery

%

RSD*

% S/N†

Fortification

Concentration

g/L

Mean

Recovery

%

RSD*

%

Reagent water Dalapon 1.0 108 2.2 578 10 107 2.3

3,5-Dichlorobenzoic acid 0.50 117 16 9 5.0 96 2.3

Dicamba 0.50 97 3.7 35 5.0 102 1.5

Dichlorprop 1.0 95 5.7 19 10 107 0.6

2,4-D 1.0 98 5.2 23 10 106 1.4

Pentachlorophenol 0.10 76 7.4 21 1.0 103 1.4

Silvex 0.25 84 20 9 2.5 107 4.4

2,4,5-T 0.25 96 7.3 16 2.5 105 1.3

2,4-DB 1.0 97 8.4 6 10 93 1.6

Chloramben 0.50 90 9.0 113 5.0 105 2.9

Dinoseb 1.0 103 6.7 8 10 119 4.6

Bentazon 1.0 100 2.1 41 10 98 0.7

Dacthal 0.50 92 6.5 100 5.0 100 1.9

Quinclorac 0.10 106 9.3 20 1.0 101 2.9

Picloram 1.0 92 2.6 23 10 99 3.1

Acifluorfen 0.50 77 5.9 80 5.0 107 4.5

2,4-Dichlorophenyl-acetic

acid (SUR) 25 107 8.0 0 25 112 1.3

Chlorinated surface water Dalapon 1.0 104 2.1 — 10 100 2.5

3,5-Dichlorobenzoic acid 0.50 110 3.0 — 5.0 104 2.6

Dicamba 0.50 93 4.4 — 5.0 102 2.9

Dichlorprop 1.0 93 11 — 10 101 1.2

2,4-D 1.0 97 5.2 — 10 101 1.2

Pentachlorophenol 0.10 86 3.2 — 1.0 100 1.0

Silvex 0.25 103 4.3 — 2.5 101 1.0

2,4,5-T 0.25 100 3.5 — 2.5 100 1.6

2,4-DB 1.0 104 5.9 — 10 101 2.3

Chloramben 0.50 93 2.0 — 5.0 102 2.6

Dinoseb 1.0 99 1.2 — 10 101 1.6

Bentazon 1.0 91 2.2 — 10 98 1.8

Dacthal 0.50 92 4.6 — 5.0 99 2.0

Quinclorac 0.10 96 15 — 1.0 105 2.5

Picloram 1.0 102 2.4 — 10 107 3.2

Acifluorfen 0.50 98 4.1 — 5.0 92 2.4

2,4-Dichlorophenyl-acetic

acid (SUR) 25 107 1.5 — 25 106 1.4

Chlorinated ground water Dalapon 1.0 105 1.5 — 10 97 5.5

3,5-Dichlorobenzoic acid 0.50 104 6.0 — 5.0 95 2.1

Dicamba 0.50 100 2.2 — 5.0 98 1.6

Dichlorprop 1.0 109 3.5 — 10 79 0.7

2,4-D 1.0 108 10 — 10 99 1.6

Pentachlorophenol 0.10 103 17 — 1.0 100 4.2

Silvex 0.25 96 3.0 — 2.5 97 1.6

2,4,5-T 0.25 91 3.4 — 2.5 97 1.5

2,4-DB 1.0 94 3.1 — 10 92 1.1

Chloramben 0.50 88 2.7 — 5.0 102 2.6

Dinoseb 1.0 100 4.0 — 10 105 2.6

Bentazon 1.0 101 2.8 — 10 101 1.8

Dacthal 0.50 62‡ 9.9 — 5.0 99 1.2

Quinclorac 0.10 107 5.7 — 1.0 94 1.5

Picloram 1.0 97 2.8 — 10 103 2.5

Acifluorfen 0.50 92 4.2 — 5.0 84 2.5

2,4-Dichlorophenyl acetic

acid (SUR) 25 95 3.1 — 25 106 1.1

* n  7

† Signal-to-noise ratios were calculated for each target compound peak by dividing peak height for each compound by peak-to-peak noise, which was determined for each

component from method blank over a period of time equal to full peak width in target analyte’s retention time window.

‡ Dacthal was present in the chlorinated surface water at about 1/5th fortification level. This was taken into account in the determination of the mean recovery.

ACIDIC HERBICIDE COMPOUNDS (6640)/Micro Liquid-Liquid Extraction GC Method

6-10
TABLE 6640:V. EFFECT OF SAMPLE HOLDING TIME ON RECOVERY FOR

SAMPLES FROM A CHLORINATED SURFACE WATER FORTIFIED WITH METHOD

ANALYTES*

Recovery

%

Compound Day 0 Day 2 Day 7 Day 14

Dalapon 91 90 91 108

3,5-Dichlorobenzoic acid 98 97 85 91

Dicamba 92 92 96 99

Dichlorprop 89 88 91 95

2,4-D 88 85 87 92

Pentachlorophenol 92 85 86 93

Silvex 91 88 86 94

2,4,5-T 89 89 85 91

2,4-DB 87 84 86 93

Chloramben 86 86 88 100

Dinoseb 105 95 99 111

Bentazon 91 87 95 99

Dacthal 92 91 86 94

Quinclorac 90 80 84 91

Picloram 94 85 85 99

Acifluorfen 109 96 103 123

* Sample storage stability is expressed as a percent recovery value calculated as

described in 6640B.5h.

TABLE 6640:VI. EFFECT OF EXTRACT HOLDING TIME ON RECOVERY FOR

SAMPLES FROM A CHLORINATED SURFACE WATER FORTIFIED WITH METHOD

ANALYTES*†

Recovery

%

Compound

Initial

Injection

Day 21

Reinjection

Dalapon 100 89

3,5-Dichlorobenzoic acid 104 113

Dicamba 102 104

Dichlorprop 100 101

2,4-D 101 102

Pentachlorophenol 100 102

Silvex 101 103

2,4,5-T 99 102

2,4-DB 101 102

Chloramben 102 105

Dinoseb 99 81

Bentazon 98 100

Dacthal 99 103

Quinclorac 105 127

Picloram 107 98

Acifluorfen 92 104

2,4-Dichlorophenylacetic acid 106 105

* Sample storage stability is expressed as a percent recovery value calculated as

described in 6640B.5h.

† All samples fortified at the same level used to collect the mid-level precision and

bias.

ACIDIC HERBICIDE COMPOUNDS (6640)/Micro Liquid-Liquid Extraction GC Method

6-11