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The major concern in shipboard microbiological study is whether microbes from the drilling fluid are introduced into the recovered core material during coring. Therefore, it is critical to verify whether recovered cores are contaminated. Perfluorocarbon tracer (PFT) can be used to quantify the amount of contamination due to drilling fluid. It is strongly recommended that this test be routinely conducted when coring for microbiological studies.
PFTs are chemically inert and can be detected with high sensitivity. The JRSO has two chemicals it uses as chemical tracers to monitor potential contamination of sediment and rock samples on the JOIDES Resolution.
Table 1: Physical and chemical properties of perfluorocarbon tracers
Property | PFMCH | PFMD |
CAS Number |
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Molecular Formula | C7F14 | C11F20 |
Molecular Weight (g/mol) | 350.05 | 512.09 |
Boiling Point °C | 76 | 160 |
Density (g/mL) | 1.788 | 1.972 |
Solubility in Water (mg/L) | ~2 | ~10 |
Solubility in Methanol (mg/L) | 104 |
|
Solubility in Hexane (mg/L) |
| 470,000 |
Vapor Pressure @ 25C (kPa) | 14.11 | 0.29 |
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A Note about the Relative Volatility of PFMCH and PFMD
At 70¿C70°C, 100% of the PFMCH can be expected to be in the volatile phase, whereas the Antoine-derived curve for PFMD predicts that the partial pressure of PFMD in the 20 mL vial would be approximately 3% of atmosphere (0.03 bar).
It is impossible to heat the vial to the boiling point of the PFMD, however, because it will boil the water in the samples and exceed the pressure capacity of the vial. Even temperatures close to 100¿C 100°C will liberate significant water vapor, which can be problematic for the gas chromatography conditions.
It is therefore recommended that the samples be heated at 85¿C85°C, at which temperature, the PFMD can be expected to have a partial pressure of approximately 6% of atmosphere (0.06 bar). Although the majority of the PFMD is not volatilized in the vial, it is consistently volatilized at a steady temperature in both the standard vials and the sample vials, so a consistent concentration of PFMD will be measured by the GC-µECD.
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- 20 mL headspace vials (HP 5182-0837) and metal caps with PFTEPTFE/rubber seals
- Note: the PFTE PTFE side should face the vial and sample; the rubber side faces outward when the lid is crimped onto the vial
- Manual vial crimper
- 10 mL, 1 mL, and 200 µL syringes
- 0.1–1.0 mL gas-tight syringes
- Oven gloves and metal tray
- GC septa: 11 mm diameter, usable up to 250°C or 400°C
- GC column: Agilent column (15 m x 0.250 mm x 5 µm)
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PFMD's ready solubility in hexane (47% w/v) makes serial dilutions of this tracer much more straightforward, but does require some caution on the user's part because the hexane will evaporate at the oven temperature in the incubation oven.
Warning! The user should be careful how much of the hexane-dissolved standard is injected into a headspace vial. At the incubation oven temperature, nearly 100% of the hexane will move into the gas phase. Adding 1 mL of hexane to a 20 mL headspace vial will create nearly 11 atmospheres of pressure at 70¿C70°C, which will likely shatter the vial!
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The GC2 system comprises an HP 6890 gas chromatograph (GC) with a micro-electron capture detector (µECD).
The GC inlet is operated in splitless mode. PFT gas samples obtained using the headspace extraction method may be injected manually after incubation for 30 minutes at 85 deg. C, or can be injected by the Gerstel autosampler (whose incubator oven should be set to 85¿C85°C). The injection port liner assembly is connected to a megabore column (Rt-Alumina BOND/KCl, 50 m, 0.53 mm ID, 10 µm thickness), and then to a µECD detector, which requires both carrier and makeup gases (nitrogen).
Ensure the syringe installed in the autosampler has the Teflon-tipped plunger (Figure 6).
Figure 6: Different syringes used be by the Gerstel Autosampler. The rubber plunger of the syringe shown on top causes significant sample carryover, likely due to tracer penetrating pore spaces within the rubber. It is best to use the syringe with the teflon-tipped plunger shown on bottom.
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- Line pressure: 50 psi.
- Supply tubing: copper equipped with 1/8 inch Swagelock Swagelok fitting.
- Flow rate is crucial to prevent damage to the 63Ni foil in the µECD. Do not raise the temperature of the µECD from room temperature without N2 flow!
- Both the µECD and column are sensitive to oxygen; therefore, an oxygen/moisture trap and oxygen indication trap are highly recommended for the nitrogen supply lines.
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- Rinse syringes with methanol and bake them in the oven at 70¿C 70°C for 12 hours to drive off any possible trace of PFT.
- Prepare dilutions of PFT as per the instructions above.
- In the Agilent Open Lab program, choose Method and Run Control.
- Choose the latest PFT method file and wait until the Ready message is lit.
- Again! Do not raise the detector temperature without sufficient nitrogen quality and flow!
- If injecting manually, incubate the standards in the 85¿C 85°C oven for 30 minutes beforehand, as if they were samples. The Gerstel oven should be set to the same temperature.
- Set up a Gerstel autosampler sequence for the calibration standards, or inject each level manually, beginning from the most dilute to the highest concentration. (The samples should be injected by the Gerstel or manually, the same as the standards.)
- Allow the gas chromatograph to return to the Ready state before injecting the next standard; repeat until all of the standards has been run (for both PFMCH and PFMD, if both PFT's PFTs are expected to be used).
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- Set the incubation oven to 85¿C 85°C and program the Gerstel to heat and agitate the vials for 30 minutes prior to injection.
- Set the Gerstel to withdraw and inject 0.25 mL of headspace from the heated vial.
- Wait for GC temperature program to cycle and GC to return to ready (~15 min).
- Repeat steps 1–3 for each sample to be analyzed
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Calibrating the instrument produces instrument response factors to absolute component concentrations. To prepare a calibration for quantitation of unknown samples, the retention time(s) for the peak(s) of interest and the amount of component injected must be known.
Calibration
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Curve
The graphical representation of the amount and response (peak area) for PFT from the calibration samples defines the calibration curve. Because the ECD is not linear across its range of detection, multiple calibration standards are run to calibrate for PFT. Various curve-fit calculations are available to determine optimum regression coefficient including linear, log, power, exponential, quadratic, and cubic.
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Chemical Hazards:
- Incompatible substances: oxiding oxidizing agents, strong acids, strong bases
- Emits toxic fumes (HF) under fire conditions
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PAL SAMPLER AND METHOD | PARAMETERS OF PAL CYCLE |
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