Example use of the pyrolysis-GC/FID method for one set of analytical methods

6.3.1.1 Collection of samples

Conduct sample collection according to standard methods (e.g. Alaska Methods for DRO and RRO). For each sample, collect approximately 500 g of soil. This will allow for 250 g of soil for the DRO/RRO analysis and 250 g for the DRBO/RRBO analysis. If both sets of analyses will be conducted in the same laboratory, send the full 500 g sample to the lab for splitting. If the DRO/RRO and DRBO/RRBO will be conducted in separate laboratories, split the samples in the field. Prepare, package, and ship samples according to standard methods.

6.3.1.2 Laboratory preparation of samples

Split the sample into two equal, representative portions if the sample was not split in the field. Perform DRO and RRO analyses of one portion, using standard methods (e.g. AK102 and AK103 for Alaska). Oven-dry the sample for DRBO/RRBO following American Standards and Testing Methods D 2974. Record the oven-dry mass. If DRO/RRO analysis is to be performed at the same laboratory, the oven-dried DRBO/RRBO sample may be used to calculate moisture content

for the DRO/RRO sample. Grind the dried sample with a mortar and pestle and shake through a #30 sieve. Visually inspect the material not passing the sieve to ensure that it contains only mineral matter. Repeat the grinding process as needed until all vegetable material is reduced to a fine powder. A ball mill may be used to grind tough fibrous organic matter. Weigh and set aside the coarse fraction. The fraction passing the sieve is referred to as the dry fines. Stir the dry fines, pour into a vial or jar of volume at least double that of the fines, and shake thoroughly to homogenize. Store the dry fines in a desiccator until use. Long-term storage of the fines is possible providing the samples remain dry.

Calculate the coarse adjustment factor (CAF) as follows:

CAF= (total oven − dry mass) ÷ [(total oven − dry mass)

− (mass of coarse fraction)] (6.1)

Conduct a loss on ignition test on a subsample of the dry fines as follows. Weigh an aluminum sample dish. Add approximately 10 g of dry fines and weigh again; subtract the weight of the dish to determine the original sample mass. For samples with high organic content and high moisture content, less than 10 g of dry fines may be available; in this case, it is acceptable to use as little as 1 g of dry fines for the LOI test. Place the aluminum dish and sample in a muffle furnace

at 550◦C for 1 hour. Remove, cool in a desiccator, and weigh again. Subtract

the weight of the dish to determine the combusted sample mass. Discard the combusted fines. Calculate loss on ignition (LOI) by:

LOI= [(original sample mass)

− (combusted sample mass)] ÷ (original sample mass) (6.2)

Calculate the target mass: Target mass= (2 mg) ÷ LOI. Place a plug of quartz

wool in the bottom of a 2 mm ID quartz sample tube for pyrolysis. Add dry

fines to the sample tube to achieve the target mass of fines, ± 10%, and weigh

on the microbalance. Use of a static ionizing unit, such as the Staticmaster

_®

model number 2U500, is recommended to minimize the effect of static on the microbalance. Add a small plug of quartz wool at the top of the tube to hold

the sample in place. Inject the sample with 5μl of a 0.8 mg ml−1 solution of

polyalphamethylstyrene dissolved in methylene chloride.

6.3.1.3 Sample Analysis

Carrier gas and flow rates

Pyrolysis results are sensitive to changes in the flow conditions through the pyrolysis-GC apparatus. Use of a fine pressure regulator in the line between the nitrogen tank and the pyrolysis-GC is recommended. Maintain the GC col-

flow at approximately 50 ml min−1, and GC purge vent flow at approximately

15 ml min−1. Adjust the inlet pressure of the carrier gas to obtain the desired

flow rate.

Pyrolyzer

In the development of this method, pyrolysis was performed on a CDS Analytical Pyroprobe 2000 with an autosampler AS2500. Maintain the pyrolysis

interface chamber at 280◦C. Before pyrolyzing the sample, allow the sample to

sit in the 280◦C interface for 10 minutes while the nitrogen carrier gas flow-

ing through it is purged to vent. At the end of the purge, switch the nitrogen gas flow online to the GC and begin pyrolysis. To pyrolyze, heat the sample at

5◦C min−1 from 280◦C to 700◦C and then hold at 700◦C for 9.9 seconds. Keep

the pyrolyzer online to the GC for two minutes following the start of pyrolysis, then discharge to vent and supply clean carrier gas to the GC for the remainder of the run.

Gas Chromatograph

The pyrolyzer was connected to an HP 5890 GC with a flame ionization

detector. Maintain the injection port at 270◦C and the detector at 300◦C. Use a

cross-linked methyl siloxane, 25 m× 0.2 mm × 0.33 μm film thickness capillary

column, such as the HP-1, and a FID. Run the GC in splitless mode for the first minute of each run, then return to split mode. Use an 80-min temperature

program: hold at 40◦C for 10 minutes, ramp up at 3◦C min−1to 85◦C, ramp up

at 5◦C min−1to 270◦C, and hold at 270◦C for 18 minutes.

6.3.1.4 Interpretation of data

Peak identification and integration

Run a blank of quartz wool injected with the surrogate, polyal- phamethylstyrene, to determine the time at which the surrogate elutes, approx- imately 20 minutes. Retention times vary slightly from sample to sample and also shorten over time as the column ages. Retention times shorten more over time for the surrogate than for the compounds of interest, and more for earlier- eluting compounds. Pyrograms of organic soils contain a distinctive series of double peaks (Figures 6.4 and 6.5). These double peaks are referred to with let- ters from ‘‘a” to ‘‘l.” The double peaks are further split into a first and second as, for example, e1, e2, f1, f2, etc., with 1, the first, and 2, the second peak in each pair, and retention times increasing with alphabetical order. The double peaks are usually much larger than nearby single peaks in the pyrogram. Lag times between the first and second peaks of each pair, and between the first peak of a pair and the first peak of the next pair, decrease with increasing retention time.

Figure 6.4. Chromatogram showing the double peak series with peaks labeled.

Figure 6.5. Expanded view of the double peak series used for calculation of biogenic

Peaks a1 and a2 elute approximately 18.5 minutes after the surrogate. Peaks b1 and b2 are the next pair after a; c1 and c2 are after b, etc. If the pyrogram does not contain the distinctive double-peak pattern, DRBO and RRBO are non- detectable (ND) by the pyrolysis-GC/FID method.

Identify peak pairs d through j. The peaks of interest for the Alaska standard methods were d1, e2, f2, g2, h2, i1, i2, and j1. These were selected by trial and error from the suite of double-peak compounds. Visually examine a close- up view of the integrated chromatogram to ensure that the peaks of interest are integrated properly. Adjust integration parameters if necessary. For each peak of interest in the sample, calculate the numerical value of the associated

biogenic indicator as, e.g., d1 = (area of peak d1 in pyrogram) ÷ [(mass of

sample pyrolyzed)× (CAF)]. For each sample, calculate the weighted area sums xD

and xR:

xD = 0.0200d1 + 0.0864g2 − 0.1086h2 − 0.0265i1 + 0.0527j1 (6.3)

xR = 0.0200d1 − 0.0287e2 + 0.0156f2 + 0.0605g2 − 0.0648i2 (6.4)

Equations 6.3 and 6.4 were derived from a statistical analysis of the pyrogram with respect to biogenic DRO and RRO. A complete accounting of the statis- tics and origin of the equations can be found in Garland (1999). The unresolved fraction of the chromatogram includes peaks associated with non-lipophilic com- pounds and is important to the analysis.

Calibration

Three uncontaminated soils of known DRBO and RRBO are used as stan- dards to create a calibration curve. Standards are run according to the same con- ditions as the sample. The DRBO calibration curve is created by plotting known

DRBO of the standards on the vertical axis and calculated xD on the horizontal

axis. The y = mx + b form of the equation of the best-fit line is the calibration

curve, with xD = x and DRBO = y. Similarly, the known values of RRBO and

calculated xRare used to create the RRBO calibration curve. DRBO and RRBO of

unknown samples are found by inserting the calculated values of xDand xRfor

the sample into the equations of the calibration curves. It is important to note that the calibration equations actually convert the pyrolysis-GC/FID data to the relevant standard method. In this case, the result is the equivalent of the Alaska Methods for DRO and RRO. Full detail relative to the development of the DRBO and RRBO calibration method is presented in Garland (1999).

6.3.1.5 Summary of the pyrolysis-GC/FID method

The pyrolysis-GC/FID method could serve as one tool for quantifying biogenic interferences in samples. At this time, there are very few laboratories that have pyrolysis interfaces for GCs. In order for this method to become widely

available, commercial laboratories would need to invest in the technology. This would likely require regulatory agency certification of the method. Pyrolysis- GC/FID is not without its limitations. In particular, pyrolysis can be subject to matrix effects. The study described herein attempted to overcome matrix effects by analyzing large numbers of samples of differing origin and mineral content. Any one sample, however, particularly those with unusual matrices (e.g. marine sediments) could have unexpected matrix effects leading to inaccurate quantification of DRBO or RRBO. In the course of the method development, however, no samples were discovered for which inaccurate DRBO or RRBO was quantified.

It is important to note that this method is quite detailed and requires sig- nificant development. It is unlikely that this would be a routine method for biogenic interference where inexpensive, and easy methods, such as silica gel cleanup or background sampling are practical. In particularly difficult cases, it may be worth developing and applying the pyrolysis-GC/FID method.

6.4 Guidelines and recommendations

It is important to remember that a solvent extracts a different portion of the organic matter in the contaminant and in the organic soil. In cold region soils where the organic content can be over 50% of the soil mass, large errors in contaminant concentration can result. A very non-polar solvent such as hexane will extract the least amount of NOM but also sacrifices the semi-polar frac- tion of petroleum products. Therefore, more refined products, such as gasoline and diesel fuel, are better suited to very non-polar solvents. The more complex, asphaltene-containing oils require a slightly more polar solvent to capture all of the petroleum. In addition, weathering generally makes an oil more polar as microbes add oxygen to some of the molecules, or the non-polar light fraction evaporates.

Whole soil techniques such as the pyrolysis-GC/FID method can eliminate some of the error inherent in the soil extraction process. Without this method, use of a background (if available) and solvent cleanup will improve one’s ability to identify biogenic interference. The limitations to background and cleanup methods are well documented, however. It is critical to understand the limita- tions of any petroleum quantification method prior to use.

6.5 Future research

The need persists to better understand the role of NOM in contami- nant analyses. As contaminants weather they are chemically transformed and indeed become more like NOM. Future research must address the transition

from when a contaminant is a contaminant to when it is part of the natural environment.

Future research must continue to address the interaction between NOM and contaminants, and the impact of that interaction on contaminant extraction and analysis. Since the chemical and physical properties of NOM are unique to a given environment, it is important to characterize NOM in a way that allows one to predict contaminant behavior in any NOM matrix.

Treatability studies: microcosms,