by George M. Hood, Peter C. Allee, and Philip G. Pucel

The most expensive part of an environmental assessment is often laboratory services. Control of these costs while still collecting adequate data to assess a site is sometimes the difference between solvency and bankruptcy, especially for small companies. Fortunately, the use of effective field screening techniques can significantly reduce the quantity of samples going to the laboratory for analysis, thus controlling overall project costs.

Field screening can:
• Provide decision quality data to on-site personnel;
• Reduce project costs by minimizing the number of samples sent for laboratory analysis;
• Generate data quickly;
• Supply focus for future efforts by delineating clean vs. impacted areas;
• Increase understanding of site contaminants by analyzing fewer samples for more extensive analytical suites rather than analyzing more samples for fewer parameters, for the same amount of money; and
• Encourage more extensive sampling/field screening programs thus reducing the chance of missing "hot spots" as might occur with more expensive approaches.

At a minimum a field screening method should include:
• Speed (time is money!);
• Adequate sensitivity;
• Selectivity, if appropriate;
• Acceptable levels of relative accuracy and repeatability;
• Appropriateness for the contaminants expected to be encountered at the site;
• Ease of use by "non-chemists";
• Safety features (minimized exposure to dangerous materials such as halogenated solvents, toxic reagents, etc.);
• Equipment which is field rugged and simple to operate; and
• Minimum support services such as running water, heat, electricity, etc.

One generally accepted field screening method for many volatile organic compounds (VOCs) is headspace analysis with a hand-held photoionization detector (PID). Briefly, a sample is collected in an air-tight container and the sample is allowed to equilibrate with the air above it (called headspace). The headspace is then analyzed with the PID to obtain a relative numerical value. The procedure takes 5-10 minutes. Based on the PID data, samples are then selected for further, more precise, laboratory analysis. Within limitations, the method works well for many VOCs.

A field screening procedure of equal simplicity is needed for semi-volatile organic compounds (SVOCs). Chemical and Environmental Consultants, Inc. (CEC) has field evaluated a rapid, widely applicable method for field screening petroleum contaminated soils and wastes for SVOCs. The method is similar in application to U.S. EPA method 418.1 and allows for the field screening of soil and waste samples in about 20 minutes. The method uses fluorescence spectroscopy analysis of a solvent extract of the soil or waste sample.

Fluorescence spectroscopy is a recognized laboratory technique. It is widely used in the biochemical research laboratory for the detection of derivatized amino acids and peptides, chlorophyll, and drugs such as adriamycin and is a commonly available detector for High Pressure Liquid Chromatography (HPLC) methods.

As part of this technique, CEC developed three proprietary extraction systems to extract soil/waste samples in the field. This was necessary due to the limited effectiveness of some of the commonly used extracting solvents such as methanol (used in EPA method 4030 for example) when dealing with heavier hydrocarbon fractions such as diesel fuel, lubricating oils, etc. Preliminary evaluations conducted by CEC’s chemical staff, indicated the solubility of diesel fuel in methanol to be less than 3% while the solubility of 10-30 lubricating oil is less than 0.02% (data not shown).

Use of methanol to extract soil/waste containing these heavy hydrocarbon fractions would meet with marginal success. If the compounds of interest can not be extracted from the matrix due to solubility limitations, the extract analysis will not yield accurate results. While some consulting firms insist on using “EPA approved” methods, it should be remembered that these techniques are not universal and if better methodology exists for specific matricies or compounds, that methodology should be used. Each site and any suspected contaminants should be evaluated carefully so the most effective methodology can be selected if the best results are to be achieved.

CEC believes Lubsolv and Asphasolv offer the best currently available alternative for soil/waste samples impacted with midrange petroleum distillates and heavier petroleum residues such as tank bottom sludges, residuum, and asphaltic type materials. The advantage of these solvent systems is a more complete extraction of the hydrocarbon fraction from the soil or waste. The disadvantage is that EPA laboratory procedures use the less effective solvents called for in the EPA standard protocols that may result in only a fraction of the contaminants being extracted. This practice will result in lower laboratory findings - often the reporting of non-detect levels- when compared to the results obtained using more effective extracting systems.

Advantages. Fluorescence spectroscopy can use its insensitivity to aliphatic hydrocarbons to advantage in that these solvents are frequently better choices for extracting heavy petroleum mixtures from soil, waste and water than the more commonly used solvents such as carbon disulfide, freon, methanol, etc.. Heavier aliphatic and alicyclic hydrocarbons such as cyclohexane, iso-octane, petroleum ether, mineral spirits, etc. are good solvents for extracting intermediate range distillates. These solvents can be mixed with alcohols and other additives to generate solvent mixtures capable of solvating even asphaltic range wastes. Once dissolved, these extracts may find further used as samples for other field analytical techniques such as thin layer chromatography (TLC).

Because water is transparent in the range <200-700 nm, it can be used as a solvent or as part of a solvent system. In some instances, aqueous extracts or water samples can be evaluated directly without the need for extensive sample preparation.

The low cost ($20 per sample) associated with field fluorescence analysis encourages the screening of more sampling locations thus minimizing the potential to miss "hot spots" which may be overlooked with more expensive approaches. With the ability for one person to screen 30-40 samples or more in an 8 hour day, this technique can provide abundant real time data points for on-site personnel charged with decision making.

While a number of techniques lend themselves to field screening (TLC, immunoassay, gas chromatography), not all are rapid, sensitive, and/or capable of quantitation. Some techniques require the operator to visually compare sample extracts to a series of standards. While this is adequate for Yes/No situations, engineers and regulators frequently feel more at ease with instrumentally derived numbers, which the fluorometer provides. This reduction in subjective interpretation also reduces the chance for errors by field technicians with minimal experience.

Since this method is a field screening technique, the intent is to detect as many compounds as possible. Hence, there are no "interferences" as such. The only conceivable interferences would be inorganic or naturally occurring compounds that fluoresce in the region of the spectrum where the organic contaminants also fluoresce. This would lead to a false positive result. Naturally occurring compounds can be corrected for by background subtraction.

Limitations. One possible limitation to fluorescence spectroscopy lies in the fact that aliphatic hydrocarbons don't fluoresce in the region <200-700 nm. This renders the technique useless for these compounds alone. However, most petroleum derived mixtures such as gasoline, kerosine, diesel fuels, heating oils, lube oils, asphalt, etc., contain compounds that do fluoresce strongly in this region of the electromagnetic spectrum. In fact, this technique is more sensitive for mixtures such as diesel fuel than is the older and accepted 418.1 Infrared method. This in turn provides lower detection limits for diesel fuel using this method than those achieved with 418.1.

Like the hand-held PID, which is used widely to screen for VOCs, the purpose of this technique is to provide a relative indication of the degree of impact in a series of samples. The numerical results expressed on the readout of the fluorometer are entirely relative to the standard used and to the instrument response to the individual components in that particular matrix.

Method. This method provides for the extraction of soil or waste samples and the semi-quantitative analysis of the extracts for SVOCs. The procedure consists of weighing a sample of soil or waste (placed in a tared disposable container) and extracting the sample with one of the proprietary solvent systems. An aliquot of the extract is introduced into the fluorometer and the fluorescence of the sample is determined. The fluorescence intensity of the sample is then used to calculate the concentration of total fluorescent compounds (TFCs) in the investigative sample. Quantitative results are usually achieved in an average of 20 minutes or less.

A stock standard is prepared at a known concentration. Serial dilution of this standard is used to prepare working standards at concentrations appropriate to the project requirements. Generally, working standards are prepared at concentrations of 100ppm, 10ppm, and 1ppm. Standards such as #2 diesel fuel or #30 weight lubricating oil were chosen because they are readily available even in small towns near remote sites. If available, a pure sample of the suspected contaminant should be used for the preparation of the stock standard and working standards. However, this option is not always available.

The operating conditions for the fluorometer will depend on the types of compounds sought and should be maximized for the matrix being evaluated. For example, if phenol is being determined, a 270 nm excitation wavelength can be used with emission monitoring at 330 nm.

The fluorometer is calibrated using a three or four point external standard technique. The unit is zeroed using a blank consisting of the appropriate extraction solution. Low, intermediate, and high standards are then analyzed under the same conditions as the investigative samples. The instrument readings and settings are then recorded in the notebook. The initial calibration is used to determine the concentration of total fluorescent compounds (TFCs) in the investigative samples as well as serving as the basis for continuing calibration assessments.

The initial calibration curve is reverified, at a minimum, at the beginning of each day. Standard curve verification is performed using the mid-point calibration standard. The verification check must be within +/-10% of the previous day's value. If not, corrective action must be taken. A new calibration curve must be established if corrective action can not reverify the initial calibration curve. All samples analyzed since the previous in calibration run must be reanalyzed. Frequent verifications are therefore recommended (after each 10-20 samples.)

Sample preparation and analysis

1) Break up any large chunks of soil or waste and mix to ensure a reasonably homogenous sample.

2) Place 5-25 grams (+/- 0.1 gm) of waste or soil in a tared vial and record the sample weight.

3) Add 25.0 ml. of extracting solution, cap the container, and sonicate the sample for 1 cycle (about 5 minutes).

4) Allow the sample container to stand briefly to allow any solids, such as sand, to settle.

5) Carefully remove an aliquot of the extract using a 2 or 5 ml syringe, taking care not to remove any of the precipitated solids.

6) Inject the sample into the instrument and obtain the fluorescence reading. If the reading exceeds the value for the high standard, prepare a dilution and work with the dilution. Record the reading(s) in the lab notebook and record any dilutions.

7) Determine the concentration of TFCs and record this result in the lab notebook and on the report form.

Calculations. A mathematical approach must be used in those cases where the weight of sample and the weight of extracting solutions are different. The following equation is used:

Conc. of SVOCs = f x Sample Reading x Vol. of Extract/Sample Weight x Dilution Factor
where f = Ó Concentration of Standards / Ó Readings of Standards.

As an example, assume a 1 mg/L standard gave an instrument reading of 9 units, a 10 mg/L standard gave a reading of 100 units, and 100 mg/L standard gave an instrument reading of 1010 units. Also assume 21.555 grams (0.021555 Kg) of soil was extracted with 25 ml (0.025L) of solvent and the extract had to be diluted 1:5 to give an instrument reading 612 units. The calculation would be accomplished as follows:

f = 1+10+100 / 9+100+1010 = 0.0992
Concentration of SVOCs = 0.0992 x 612 x (0.025 / 0.021555) x 5
= 704 mg / Kg

Case study. As part of a solid waste management unit (SWMU) investigation at a refinery in the U.S., CEC evaluated 26 potential disposal sites ranging in size from a few square feet to several acres. CEC proposed the use of onsite headspace/PID field screening for VOCs and extraction/fluorescence field screening for SVOCs. The results generated by the field screening techniques would be used to select the most impacted samples from each SWMU for additional laboratory characterization.

VOCs. Samples were field screened for VOCs using a Thermo Environmental Instruments (TEI) 580B photoionization detector. Each sample was collected in a Ziplock bag and the headspace was analyzed for VOCs. The concentrations of VOCs determined by field screening ranged from 0.0 ppm to 2000 ppm ("over range" reading). Since it is difficult to conveniently dilute a VOC sample in the field (as can be done with the SVOC procedure), all samples with VOC concentrations greater than the "over range" reading were simply recorded as ">2000 ppm." Samples were selected for laboratory analysis based on the field screening results. Quality Assurance/Quality Control samples were also analyzed in the field and in the laboratory.

Six hundred and ninety nine (699) investigative samples were field screened for VOCs using a TEI Model 580B photoionization detector (PID). The samples were collected in individual Ziplock bags and allowed to equilibrate with the headspace air for about 10 minutes. The Ziplock was opened slightly, the probe of the PID was inserted, and a reading was obtained. For duplicate (QA/QC) samples, the bag was immediately resealed and the sample allowed to reequilibrate with the headspace air. Approximately ½ hour later, a second reading was obtained using the same protocol.

A total of 55 samples were sent to the laboratory for analysis by EPA method 8260 and Skinner List VOC characterization. Only 9 of the 85 targeted analytes were reported in the 55 samples submitted. The names and range of concentrations for these compounds are shown in Table 1.

Table 1.
Range of VOC concentrations reported in laboratory analyzed SWMU samples

Compound	Number of samples reported in	Concentration range
Acetone	12	0.050 - 2.03 mg/ Kg
Acrolein	1	0.45 mg/ Kg
Benzene	23	0.019 - 13.9 mg/ Kg
Chloroform	3	0.006 - 0.026 mg/ Kg
Ethyl Benzene	26	0.005 - 35.8 mg/ Kg
Methyl ethyl ketone	6	0.015 - 0.65 mg/ Kg
4-Methyl-2-pentanone	1	0.62 mg/ Kg
Toluene	24	0.002 - 9.82 mg/Kg
Xylenes	33	0.005 - 61.5 mg/Kg

Field quality assurance was provided by calibration of the PID to a factory prepared 100ppm isobutylene standard on a daily basis and by verifying the instrument response on a random basis throughout the day. Field quality control was provided by analyzing duplicate samples. The results obtained for the 64 duplicate analyses are presented in Table 2. Samples whose mean concentration was below 50ppm (designated with an asterisk in Table 2) generally had large %RSDs with the average being >100%. If these low values are removed from consideration, the %RSD averages 29%

Table 2.
Results and statistics for VOC duplicate analysis

Sample ID	First reading	Second reading	Mean	Std. Dev.	%RSd
1	44.0	63.0	53.5	13.4	25.1
2*	3.0	37.0	20.0	24.0	120
3	129	86.0	108	30.4	28.3
4	106	82.0	94.0	17.0	18.1
5	60.0	75.0	67.5	10.6	15.7
6	511	155	33	252	75.6
7	367	188	278	127	45.6
8	213	46.0	130	118	91.2
9	179	50.0	115	91.2	79.7
10	312	229	270	58.7	21.7
11	142	53.0	97.5	62.9	64.5
12	314	145	230	119.5	52.1
13	165	129	147	25.5	17.3
14	194	196	195	1.41	0.7
15	181	90.0	136	64.3	47.5
16	288	171	230	82.7	36.0
17	213	103	158	77.8	49.2