48 Water/Wastewater
aliquot and place it in a vial, along with a 50 µL aliquot of five deuterium labeled internal standards. The organic solvent content of the sample should not exceed 5%. Collect the samples in baked amber glass bottles and store at 4 °C until analyzed. Pass the water sample through a PFTE filter (0.2 µm) before addition of internal standards, in order to prevent plugging of the analytical column. The sample is then ready for direct injection into the LC/MS/MS system. Blanks should also be passed through the filter to check for interferences.
Results and Discussion Method 538
Table 2 shows the 10 organophosphate analytes included in EPA Method 538 along with the polynuclear aromatic heterocycle, quinoline. These 11 analytes represent important possible drinking water contaminants [1]. Five deuterated standards are also part of the method and are shown in Table 3. One advantage of the EPA method is that solid phase extraction (SPE) is no longer needed for sample preparation, which means that total analysis time is cut at least in half. In addition, suppression from the sample matrix is reduced because the matrix is not concentrated as may occur with SPE. Although concentration of the sample may enable lower detection limits, this advantage will be mitigated by suppression effects. In addition, the sensitivity of the instrument negates the need for concentration of the sample. The method is quite simple, requiring only the addition of the internal standard mixture to the water sample.
Analyte
Acephate Aldicarb
Aldicarb sulfoxide Dicrotophos
Diisopropyl methylphosphonate (DIMP) Fenamiphos sulfone Fenamiphos sulfoxide Methamidophos Oxydemeton-methyl Quinoline Thiofanox
Chemical Abstract Services Registry Number (CASRN)
30560-19-1 116-06-3 1646-87-3 141-66-2 1445-75-6
31972-44-8 31972-43-7 10265-92-6 301-12-2 91-22-5
39196-18-4
Table 2: Ten Organophosphate Pesticides and Quinoline are the 11 Compounds Measured in EPA Method 538 as Drinking Water Contaminants
Internal standards
Acephate-d6 Diisopropyl methylphosphonate-d14 (DIMP-d14) Metamidophos-d6 Oxydemeton-methyl-d6 Quinoline-d7
Table 3: The Five Deuterated Internal Standards Used in EPA Method 538 [1] Compound
Acephate Acephate Aldicarb Aldicarb
Aldicarb sulfoxide Aldicarb sulfoxide Dicrotophos Dicrotophos DIMP DIMP Fenamiphos sulfone Fenamiphos sulfone Fenamiphos sulfoxide Fenamiphos sulfoxide Methamidophos Methamidophos Oxydemeton-methyl Oxydemeton-methyl Quinoline Quinoline Thiofanox Thiofanox
Precursor ion
206 184 213 213 229 229 238 238 181 181 336 336 320 320 142 142 269 247 130 130 241 241
Product ion
165 143 116 89
166 109 193 112 139 97
308 266 292 233 125 94
191 169 103 77
184 57
Dwell
10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10
Fragmentor (V)
90 50 90 90 70 70 70 70 70 70
110 110 110 110 70 70
110 70
110 110 90 90
Table 4. Transitions, Fragmentation Energies, and Collision Energies Used for Each of the 11 Standards for EPA Method 538 Compound Limits of Detection and Linearity
The EPA Method 538 calls for one MRM transition per compound [1]. The adaptation of the method described includes a second transition in order to provide a confirmation ion for each detected compound. This change also conforms to standard analytical procedures that call for a second confirming transition for analysis by LC/MS/MS using triple quadrupole methods, as well as ion-ratio percentages. Table 4 shows the transitions for each of the 11 compounds, along with the fragmentation and collision energies. Table 5 shows the transition used for each of the deuterated labeled standards used for quantitation, as well as their fragmentation and collision energies.
The extracted ion chromatogram (EIC) for the 11 compounds of EPA Method 538 is shown in Figure 1, using a 10-minute rapid gradient with UHPLC (Table 1). The 11 compounds elute in approximately 6 minutes. The more polar compounds, such as methamidophos, acephate, and aldicarb sulfoxide, elute in the first minute of the chromatogram. The more hydrophobic compounds, such as diisopropyl methylphosphonate (DIMP), aldicarb, fenamiphos sulfoxide, and sulfone, along with thiofanox, elute at the end of the chromatographic run. Good peak shape, which improves sensitivity and increases the limit of detection, was accomplished with this gradient.
Acephate-d6 DIMP-d14 Methamidophos-d6 Oxydemeton-methyl-d6 Quinoline-d7
Transition
190 149 195 99 148 97 253 175 137 81
Fragmentation energy
50 70 70 70
110
Collision energy
0 5
10 10 35
Table 5: Transitions, Fragmentation Energies, and Collision Energies for each of the 5 Labeled Standards for EPA Method 538
Compound
Acephate Aldicarb
Aldicarb sulfoxide DIMP Dicrotophos Fenamiphos sulfone Fenamiphos sulfoxide Methamidophos Oxydemeton-methyl Quinoline Thiofanox
Fortified conc. (ng/L)a
500 5 5
10 10 5 5
50 5
10 5
LOD (ng/L)b
500 2 1
10 10 5 5
50 5
10 2
MRL (ng/L)c
1000 5 2
20 20 10 10
100 10 20 5
a. Spiking concentration used to determine LOD b. Limit of Detection (determined as three times signal-to-noise) c. Method Reporting Limit (determined as six times signal-to-noise with two transitions per compound taken into account)
Table 6: Limits of Detection for EPA Method 538 AET Annual Buyers’ Guide 2012
www.envirotech-online.com
Collision energy (V)
5 0 5
15 5
10 0 5 0 5
10 15 10 20 10 10 5
10 25 35 5
15 Polarity
Positive Positive Positive Positive Positive Positive Positive Positive Positive Positive Positive Positive Positive Positive Positive Positive Positive Positive Positive Positive Positive Positive
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