20


Table 3. Average Relative Response Factors (AvRF) calculated for each compound, and %RSD calculated over the 13 calibration points, from 2.5 to 20,000 ng/mL. Compounds are grouped by reference ISTD.


Compound name


Naphthalene-d8 Naphthalene


2 - methyl Naphthalene 1 - methyl Naphthalene Acenaphthylene Acenaphthene


Acenaphthene-d10 Dibenzofuran


Figure 1. Chromatographic separation and peak shape for 19 investigated PAHs, 5 labelled ISTD, and 2 surrogate standards in a solvent standard at 0.1 ppm acquired in SIM. Tracks of the quantification ions are reported for each peak in different colours.


Calibration and linearity


A calibration curve ranging from 2.5 ng/mL to 20,000 ng/mL was prepared to assess linearity with the average relative response factor (AvRF) calibration fit type and the %RSD <15% criterion required in the EPA Method 8270E. Calculated %RSDs were <10% for all the compounds across the entire calibration range. The extended dynamic range of the XLXR detector allowed for using only one calibration curve rather than two different ones for the analysis of trace level and highly contaminated samples. Calculated AARF and %RSD results are reported in Table 3. Examples of calibration curves are shown in Figure 3.


Fluorene


Phenanthrene-d10 Phenanthrene


Anthracene Fluoranthene


Terphenyl-d14 Pyrene*


Benz[a]anthracene Chrysene-d12


Chrysene


Benzo[b]fluoranthene Benzo[k]fluoranthene Benzo[a]pyrene


Perylene-d12


Dibenzo[a,h]anthracene Indeno[1,2,3-cd]pyrene Benzo[g,h,i]perylene


ISTD


1.20 1.48 1.25


5.94 7.64 7.01


Surrogate


2.17 1.85


ISTD


1.48 1.63 1.55 1.70


5.64 4.85 5.12 6.06


8.78 6.84


ISTD


1.28 1.25 1.21


6.00 4.88 7.10


ISTD


1.99 1.65


7.00 9.05


Category ISTD


1.16 0.80 0.69 2.51 1.57


6.60 5.68 7.74 9.43 8.69


AvRFs RSD %


Figure 3. Examples of calibration curve for some of the 19 PAHs analysed (from left to right: naphthalene, dibenzofuran, phenanthrene, fluoranthene, benzo[b]fluoranthene and benzo[g,h,i]perylene), annotated with the Average Response Factor %RSD. Linearity spans over five orders of magnitudes, from 2.5 to 20,000 ng/mL.


Figure 2. Chromatographic separation of benzo[b]fluoranthene and benzo[k]fluoranthene. Peak resolution is calculated as the ratio between the height of the peak valley and the height of the peak apex of the smaller compound.


Sensitivity: Instrument detection limits (IDLs) and method detection limit (MDLs)


To determine the instrument detection limit (IDL) for each analyte, the response and its standard deviation were taken into account [8]. This approach gives much more reliable LOD values as it considers not only the signal intensity, but also the consistency of the response. IDLs were calculated by eight repeated injections of the 2.5 ng/mL calibration solution in DCM, using Equation 1 [8].


IDL = t(n–1,1–α=0.99) Where:


× mOC × %RSD (Equation 1)


t = Student t-value for one-tailed distribution at 99% confidence with 7 degrees of freedom (2.998)


mOC = mass of analyte on column


%RSD = %RSD of the response (peak area) over the 8 repeated injections. %RSD must be consistent (%RSD < 15%).


IDLs ranged from 0.3 to 0.97 pg on columns, as reported in Table 4.


The method detection limit (MDL) is defined as the minimum measured concentration of a substance that can be reported with 99% confidence that the measured concentration is distinguishable from method blank results [9]. For the calculation of MDL, blank extracts of water and soil were selected, pooled, and spiked at different levels (2.5 ng/mL and 5 ng/ mL) after the extraction; eight repeated injections were made, and the calculations were done applying the formula reported in Equation 2.


MDLs = t(n-1,1–α=0.99) Where:


× Ss (Equation 2)


t = Student t-value for one-tailed distribution at 99% confidence with 7 degrees of freedom (2.998) Ss


= sample standard deviation of the calculated concentration


MDLs ranged from 0.44 to 6.94 pg on column for water, and from 0.48 to 7.60 pg on column for soil.


LAB ASIA - APRIL 2022


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