ANALYSIS OF ALCOHOL TRACERS USED AS PARTITIONING TRACERS
1.0 PURPOSE
The purpose of this SOP is to ensure reliable and reproducible analytical results of alcohols in water samples for laboratory-based or on-site (field-based) GC-FID analyses, and to permit traceability of possible causes of error in analytical results.
2.0 SCOPE
This SOP describes the analytical procedures used by Clemson University (CU) for analysis of alcohols used as partitioning tracers, in both laboratory and field studies to estimate the amount and distribution of residual non-aqueous phase liquids (NAPLs) present in the saturated zone. This SOP may not be specifically applicable to the activities of other organizations.
This SOP was written by John Coates, Charles Wright, and Cindy Lee at Clemson University. The method described here was modified from a protocol provided by Gary Pope at the University of Texas-Austin and D.P. Dai, H.K. Kim, and P.S.C. Rao at the University of Florida.
The alcohols selected for use in the CU laboratory and field studies are methanol, and two of the following: 2,2-dimethyl-3-pentanol, 2-methyl-2-pentanol, 3-methyl-3-hexanol, 2-methyl-2-hexanol, or 6-methyl-2-heptanol.
The method involves gas chromatography (GC) techniques for estimation of concentrations of alcohol(s) in water samples; a flame-ionization detector (FID) is used to quantify the analyte concentrations in the sample. The method has been found to provide reliable and reproducible quantitation of alcohol tracers for concentrations> 1-2 mg/ml; this value is then considered to be the Method Detection Level (MDL). The standards calibration curve for FID response was found to be linear up to 1,000 mg/ml for the alcohols, and it may be linear even beyond this value, but was not tested. [In laboratory and field experiments, the output concentrations of the alcohol tracers will not exceed this upper limit.]
Samples selected for GC-FID analysis may be chosen on the basis of preliminary screening to determine approximate concentration ranges, and select appropriate GC parameters (e.g., sample injection volumes; concentration range for standard curves, etc.) However, strategies for sample screening themselves are outside the scope of this SOP.
Water samples from laboratory and field experiments may be sub-sampled into 2-ml GC vials for analysis.
3.0 RESPONSIBILITIES
All NAPL project staff, faculty, and students are responsible for knowing the procedures outlined below for analysis of alcohol tracers used in the Clemson test.
4.0 PROCEDURES
4.1 Sample Containers, Collection, Transportation and Storage
Sample Containers. Water samples collected in the field are contained in 4-ml glass sample vials (Fisher Catalog # 03-393G or equivalent) or 20 ml glass sample vials (Fisher # 03-339-14C) with open caps and Teflon-faced septa. Water samples collected in the laboratory are contained in GC vials (2 ml) with open caps and Teflon-faced septa (Baxter Catalog # C4901-230 and # C4901-238 or equivalent). The glass vials or the caps are not reused.
Sample Collection. Each sample vial is completely filled with aqueous samples, such that no head space of air exists, and capped. The vials are not opened until the time of sub-sampling or analysis.
Transportation and Storage. For field studies, the samples are stored in coolers containing "blue ice," and later stored in refrigerators in a trailer located on the site. Samples may be subjected to on-site GC analysis, and/or shipped back to CU laboratories; samples are packed in coolers and shipped via overnight air express (e.g., FedEx). The samples are stored in a refrigerator at 4C, until they are ready for GC analysis. After sub-sampling, the samples are returned to cold storage. For laboratory studies, the samples are stored in a refrigerator if the period prior to analysis is expected to exceed eight hours.
4.2 Sub-Sampling and Dilution
For laboratory analysis, 1 ml aliquot of sample is transferred from the 4-ml sample vials to the 2-ml GC vials using an Oxford P-7000 pipetter (Baxter Catalog # P5058-1L or equivalent) with disposable polypropylene tips. For field sample analysis, 200 ml aliquot of sample is transferred from 4-ml sample vials to 2-ml GC vials fitted with 250 ml glass micro-inserts.
A sample dilution is prepared if response for any peak in the sample exceeds the calibration range. The dilution should allow the analyst to obtain the greatest response within the analytical range, or refer to screen results to determine if initial dilution is required. For on-site analyses, samples are diluted if screening analyses indicate that the alcohol tracer concentrations exceed 200 mg/ml.
4.3 Apparatus and Materials
Glassware. Disposable micro-pipettes (10, 100 ml; Fisher Catalog # 21-175B; 21-175F)
and Class A volumetric pipettes (1 or 2 ml) are required for sample dilution. Drummond
Dialamatac microdispensers (Fisher Catalog # 21-170-15A and # 21-170-15D or equiva-
lent) may be used with the disposable micropipettes. GC vials (2 ml) with Teflon-faced
caps (Baxter Catalog # C4901-230 and # C4901-238 or equivalent) are required for GC
analysis. National Scientific 250 ml glass micro-inserts (Baxter Catalog # C4901-80) will
be used during field analysis. Hamilton syringes (10 to 250 ml, Baxter Catalog # S9661-6 or equivalent) are required for addition of internal standard.
Gas Chromatograph System. An analytical GC system is required, complete with a temperature-programmable oven, and either an integrator or a PC-based data acquisition/analysis software. Also required are other accessories, including analytical columns and the gases required for GC-FID operation.
Two GC systems are used at CU; a Hewlett Packard (HP) 5890A equipped with a FID and a HP 6890 equipped with an autosampler and FID. The HP 5890A is interfaced with an HP 3396A integrator. The HP 6890 is interfaced with a IBM-compatible PC loaded with HP GC ChemStation (Rev A.03.03) software.
with a 3.0-mm film thickness (Baxter Catalog # C4577-486 or equivalent).
Reagents. Reagent water is defined here as the water in which an interferant is not observed at the MDL of the parameters of interest. Laboratory reagent water is distilled deionized tap water in the laboratory studies and reagent grade water (Baxter Catalog # C4351 or equivalent) in the field studies.
Standard Solutions. Analytical standard solutions are prepared from pure materials in the laboratory. Stock standard solutions, at approximately 1000 mg/ml of each alcohol, are prepared in reagent water and kept in 12 ml glass vials (Baxter Catalog # C4802-12 or equivalent) with Teflon-lined caps; minimal headspace ensures no volatile losses. These stock solutions are stored at 4C.
Old stock solutions are discarded and a fresh batch prepared every month. Any time a comparison with the check standards indicates a problem, a new batch of standards must be prepared.
calibration standards are prepared by diluting stock standard solutions in
reagent water.
4.4 Calibration
Calibration Standards for Laboratory Studies. Six calibration standards are prepared as follows; from a stock standard solution at a concentration of approximately 1,000 mg/ml, appropriate dilutions are prepared in reagent grade water using Class A volumetric pipettes or Hamilton glass syringes. Target concentration values for the calibration curve are 1, 10, 20, 100, and 200 mg/ml.
Since working calibration standards are prepared by dilution of a stock standard solution, precautions must be taken to ensure the accuracy of the stock mixture. Two stock solutions are prepared from primary weights and dilution using separate analytical balances. Target concentrations of the stock standard solutions are 1000 mg/ml. Dilution from each are used to prepare a single set of calibration standards. Using this technique, systematic errors may be detected and corrective actions taken to ensure the preparation of accurate working standards.
Internal Standard. Isopentanol is used as the internal standard for the analysis of the alcohol tracers. A stock solution of isopentanol is prepared at 8,000 mg/ml for laboratory analysis and at 1600 mg/ml for field analysis. Ten ml of the stock solution is added to each 1 ml (200 ml in the case of field analysis) aqueous sample before analysis.
4.5 Quality Control
GC injector septa must be changed every 30 to 40 injections or sooner if any related problems occur.
4.6 Instrument Procedures
Gas Chromatography Condition. Recommended operating conditions are as follows:
Oven temperature begins at 40C for 1 min, then is increased 30C/min to 200C with an injector temperature at 200C and detector temperature at 240C. The injection volume is 1 ml using spitless injection. The carrier gas initial flow is set at 1.5 ml/min. The linear velocity is 52 cm/s. Combined make up flow and column flow are set at 50 ml/min, the H2 is at 40 ml/min, and the air at 450 ml/min.
A typical chromatogram for a mixture of several alcohol tracers in aqueous samples analyzed using the HP 6890 GC system is attached here as Figure 1.
4.7 Sample Preparation
Sub-sampling. Samples are transferred from sample vials to GC vials and capped with open-top, Teflon-lined septa caps.
Dilution. Prepare a sample dilution if responses for any peaks in the sample exceed the calibration ranges. The dilution should allow the analyst to obtain the greatest response within the analytical range, or refer to screen results to determine if initial dilution is required. For on-site analyses, samples were diluted if screening analyses indicated that the alcohol tracer concentrations exceeded 200 mg/ml.
4.8 Sample Analysis
Analysis. The samples are allowed to reach ambient temperature prior to GC analysis. From the GC vials with sub-sampled (and diluted, if necessary) samples, 1 ml is withdrawn with a glass syringe and manually injected in the HP 5880 injector. For the HP 6890 GC system, the GC vials are loaded onto the autosampler and 1 ml injections are made.
Analyte Identification. Analyte identification is not based on absolute retention time. A relative retention time for each analyte of interest is established by comparison to a specific reference peak (e.g., the solvent peak, a major component peak, a spiked internal standard or reference). The analyte of interest must elute at the same relative retention time as the standard; small changes in the absolute retention times are expected due to variations in flow rate, column temperature or other operational parameters. The primary criterion is that the relative retention time of the sample component must be within ± 0.1 RRT units of the standard.
Analyte Quantitation. When an analyte has been identified, the concentration is determined by an internal standard calibration technique where
or
RRF is the relative retention factor, Area (A) is the area of the analyte peak, Mass (A) is the mass of the analyte, Mass (IS) is the mass of the internal standard, and Area (IS) is the area of the internal standard peak.
Interferences. Contamination by carryover can occur whenever high-level and low-level samples are sequentially analyzed.
analyses.
4.9 Safety
The main safety issue concerning the use of the GC at a field site relates to the compressed gases in use. The FID gases (hydrogen and air) form explosive mixtures. It is important to keep this in mind at all times and be aware of the hazard potential in the event of an undetected hydrogen leak. All gas connections must be properly tested at installation.
High-pressure compressed-gas cylinders must be secured to a firm mounting point, whether they are located internally or externally. Gas cylinders should preferably be located outside the trailer on a flat, level base, and the gas lines run inside through a duct or window opening. If the gases are located outside, then some form of weatherproofing for the gauges will be necessary. As a temporary measure, heavy-duty polyethylene bags, secured with tie-wraps, have been used successfully; this may not be very elegant but it is very effective for short-term use of the GC. A more permanent protective housing must be built if the GC is located at the trailer for an extended time period. If it is not possible to arrange external sitting easily, the gas cylinders should be secured to a wall inside the trailer. (The cylinders will be attached to the table used for the GC inside the trailer.)
The main operating drawback to locating the gas cylinders externally is that it is not easy to monitor the cylinder contents from the instruments; the gas which could be used up most quickly is air for the FID, particularly if two instruments are hooked up to the same supply and they are running continuously. It may be worth having a reserve cylinder of air, just in case.
Miscellaneous Safety Issues. It is good laboratory practice to make sure the flame is attended at all times. When it is necessary to change the injection liner on the GC, the detector gases should be shut off. The column must be connected to the detector before igniting the flame. The trailer should be kept well ventilated when using the GC. Refer to the Materials Safety Data Sheets (MSDS) for additional information on environmental toxicity data and for safety information, procedures, and regulations.