Environmental Laboratory 17 Table 1. HS-GC-FID operating conditions for GRO determination in water


TRACE 1310 GC Parameters Inlet Module and Mode: Split Ratio:


Septum Purge Mode, Flow (mL/min):


Carrier Gas, Carrier Mode, Pressure (kPa):


Oven Temperature Program Temperature 1 (°C): Hold Time (min): Temperature 2 (°C): Rate (°C/min):


Hold Time 2 (min):


FID Temperature (°C): Air Flow (mL/min): Flow (mL/min): Flow (mL/min):


H2 N2


Acquisition Rate (Hz):


SSL, split 20:1


Constant, 5 He, constant pressure, 150


50 1


220 15 5


300 350 35 40 25


TriPlus 500 HS Autosampler Parameters Incubation Temp. (°C): Incubation Time (min): Vial Shaking:


85 30


Fast Vial Pressurization Mode:


Vial Pressure (kPa) (Auxiliary Gas Nitrogen):


Vial Pressure Equilibration Time (min):


Loop Size (mL):


Pressure 200


1 1


Loop/Sample Path Temp. (°C): 105 Loop Filling Pressure (kPa):


150


Loop Equilibration Time (min): 1 Needle Purge Flow Level: Injection Mode:


5 Injection Time (min):


Standard 1


concentration of a substance that can be measured and reported with 99% confidence that the analyte concentration is greater than zero.2


According to the Wisconsin method3 for


GRO determination, the required limit of quantitation (LOQ) is 100 µg/L (ppb) or less for water samples and the method blank should not exceed a concentration of 50 µg/L (ppb). The method detection limit (MDL) was assessed analyzing n = 7 blank tap water samples (5 mL) and n = 7 tap water samples spiked at the concentration of 12.5 µg/L (ppb). MDL and LOQ were then calculated applying Equations 1 and 2, respectively.


The recovery was calculated using Equation 3 and was in the range 80% to 120%, with an average value of 105%. MDL, LOQ, and percent recovery results for the spiked samples are reported in Table 2. None of the investigated compounds could be detected in the tap water samples as shown in Figure 2.


(Equation 1) MDL = t(n-1,1-α=0.99)


Where:


Figure 2. Comparison between chromatograms obtained analyzing n = 7 real tap water samples (unspiked) and a tap water sample spiked at 12.5 µg/L (ppb) (red trace). None of the investigated gasoline organics could be detected in the unspiked tap water samples.


t = Student’s t-value appropriate for the single-tailed 99th percentile t statistic and a standard deviation estimate with n-1 degrees of freedom, for


n = 7 injections: t = 3.143 S = standard deviation of the replicate analysis


(Equation 2) * S


LOQ = 10 * S Where:


S = standard deviation of the replicate analysis


Table 2. Calculated MDL, LOQ, and % recovery for n = 7 tap water samples spiked at a concentration level of 12.5 µg/L (ppb)


Gasoline Range Organics


Methyl tert-butyl ether (MTBE) Benzene Toluene


Ethylbenzene


m-Xylene, p-Xylene o-Xylene


1,3,5-Trimethylbenzene 1,2,4-Trimethylbenzene Naphthalene Average


Spiked Conc. (µg/L)


12.5 12.5 12.5 12.5 12.5 12.5 12.5 12.5 12.5


Average Measured Conc. (µg/L, n = 7)


11.5 12.8 13.7 12.8 12.8 12.4 14.4 13.3 14.6 13.1


Calculated MDL (µg/L)


1.4 1.2 1.7 1.3 0.8 0.8 1.7 1.7 2.2 1.4


Calculated LOQ (µg/L)


4.4 3.9 5.5 4.0 2.7 2.6 5.5 5.3 7.1 4.6


Average Recovery (%, n = 7)


92


103 110 102 103 100 115 107 117 105


(Equation 3) Average %R = (Cave


Where: Cave


Csp


Figure 1. Calibration curves for GRO obtained by injecting seven concentration levels (6.25 to 10,000 µg/L). R2 relative standard deviations (% RSD) are shown. Each calibration level was prepared and analyzed in triplicate (n = 3).


as well as response factors


/Csp


) * 100%


= average concentration of the spiked samples = initial spike concentration


To assess the method accuracy (%) in tap water samples containing raw gasoline matrix, 30 µL of the sample stock solution (prepared as described in the sample preparation section) were diluted into two flasks previously filled with 30 mL of tap water and fortified with the standard solution at a concentration of 1000 µg/L (ppb) and 10,000 µg/L (ppb). A blank matrix solution was prepared by adding 30 µL of sample stock solution to 30 mL tap water. Then, 5 mL of each fortified solution and the blank matrix were transferred into 10 mL headspace vials (n = 5) and analyzed to assess the recovery. The average recoveries (%) for the spiked matrix samples were calculated using Equation 3 and confirmed to be within 80–120% of the spiked levels with an average value of 96.5% (Table 3). Chromeleon CDS matrix correction feature allowed for automated subtraction of the background leading to a precise quantitation of the spiked samples.


Precision System repeatability was assessed using n = 10 consecutive injections of tap water samples spiked at a concentration of 50 µg/L (ppb) and n = 10 tap water samples spiked with the 5% raw gasoline solution. Peak area %RSDs obtained for both


www.envirotech-online.com AET October / November 2019


MTBE


Benzene


Toluene


Ethylbenzene m-Xylene, p-Xylene o-Xylene


1,3,5-Trimethylbenzene 1,2,4-Trimethylbenzene


Naphthalene


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