181 / 413
• 7 •
2008 vol. 2
Consistent chromatographic resolution of 3.5 or
greater for alachlor ESA and acetochlor ESA was
easily achieved as shown in Figure 1. Surrogate
recoveries, matrix spikes, minimum detection lim-
its, and internal standard recoveries produced
consistently acceptable results at low concentra-
tions and showed no interferences from the drink-
ing water matrix. The method reporting limits
(MRL) listed in Table I are based on seven replicate
fortified blanks prepared at the proposed MRL of
0.013µg/L in drinking water. An LCMRL of 0.012
to 0.014µg/L was established and validated with a
calculated detection limit of 0.004µg/L or less.
Precision and accuracy were demonstrated using
four replicate fortified blanks at 0.2µg/L; recovery
and RSD values easily met method requirements
(Table II). All analytes were detected in laboratory
blanks at
≤
1/3 MRL values demonstrating low
system background levels.
The optimized method developed here shows
superior sensitivity for the ESA and OA degradates
of chloroacetanilide herbicides alachlor, ace-
tochlor, and metolachlor, as well as reliable resolu-
tion between isomers alachlor ESA and acetochlor
ESA. This method is simplified by Restek's suite of
Method 535 products. All of the reference materi-
als, sample preparation products, and HPLC
columns needed are now available from a single
source, to facilitate successful Method 535 analysis.
References
1. C. Borton. EPA Method 535; Detection of Degradates of
Chloroacetanides and other Acetamide Herbicides in Water by
LC/MS/MS. Applied Biosystems, Foster City, CA, 2008.
LCMRL
Calculated Detection
Analyte
(µg/L)
Standard Deviation
Limit in Matrix (µg/L)
alachlor OA
0.013
0.28
0.003
acetochlor OA
0.014
0.27
0.003
alachlor ESA
0.013
0.18
0.002
metolachlor OA
0.013
0.21
0.003
acetochlor ESA
0.012
0.29
0.004
metolachlor ESA
0.012
0.18
0.002
Seven matrix spikes prepared at 0.013µg/L (proposed MRL).
Table I
Reliably achieve minimum detection limits of 0.004μg/L or less.
Analytes
Average Recovery (%)
%RSD
dimethachlor ESA
100.1
9.2
metolachlor OA
95.0
8.5
metolachlor ESA
94.8
8.9
alachlor OA
96.6
8.5
acetochlor OA
97.0
8.9
alachlor ESA
92.5
8.6
acetochlor ESA
94.3
8.0
Four lab fortified blanks spiked at 0.2µg/L.
Method requirements: average recovery ±30% of the true value, %RSD
≤
20%.
Table II
Outstanding accuracy and precision using Ultra C18 HPLC
columns.
Ultra C18 Columns (USP L1)
Excellent for a wide range of analyses
Physical Characteristics:
3µm Column, 2.1mm
cat. #
price
100mm
9174312
particle size: 3µm, spherical
pore size: 100Å
carbon load: 20%
endcap: fully endcapped
pH range: 2.5 to 7.5
temperature limit: 80°C
Compound
Solvent
Conc.
cat.#
(ea.)
price
acetochlor ESA sodium salt
M
100
33092
acetochlor OA
M
100
33094
alachlor ESA sodium salt
M
100
33096
alachlor OA
M
100
33099
metolachlor ESA sodium salt
M
100
33200
metolachlor OA
M
100
33201
M=methanol
Method 535 Individual Compounds
Volume is 1mL/ampul. Concentration is µg/mL.
butachlor ESA sodium salt
Method 535 Internal Standard
100µg/mL in methanol, 1mL/ampul
cat. # 33202
(ea.)
dimethachlor ESA sodium salt
Method 535 Surrogate Standard
100µg/mL in methanol, 1mL/ampul
cat. # 33203
(ea.)
CarboPrep™ SPE Cartridges
Nonporous graphitized carbon
SPE Cartridge
Tube Volume,
Bed Weight
qty.
cat#
price
CarboPrep 90
6mL, 500mg
30-pk.
26092
Resolution of all target ana-
lytes, including alachlor ESA
and acetochlor ESA isomers,
can easily be achieved.