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Q: What SPME fiber should I use to extract a class of analytes?
Two factors should be considered:
- Polarity of the analyte
- Volatility and molecular size of the
analyte
Polar analytes are attracted to polar phases (e.g., polyacrylate and Carbowax
coatings). Fibers with a thicker film (100µm) are better for volatiles,
but also can be used for less volatile compounds (longer extraction times
are required). Porous fibers (with Carboxen or divinylbenzene coating) also can
retain small analytes, and are ideal for C2-C6 analytes Thinner film fibers
(7µm and 30µm polydimethylsiloxane) are better for larger molecules.
Q: How can I improve SPME recovery?
- Select the most appropriate fiber (see the previous question).
- Agitate or stir the sample.
- If you are performing headspace sampling, minimize the headspace volume
(<30%) Select the appropriate temperature (40-90°C). Analytes
can desorb from the fiber if the temperature is too high.
- Add 25% NaCl to the sample and adjust the pH (reduce the pH for acids, increase the pH for bases).
- Minimize the amount of organic solvent in the sample.
Q: Is SPME quantitative?
Yes. A calibration curve to determine linear range is required for each
analyte. Using internal standards with properties similar to those of the
analytes in the sample improves precision. Usee of Standard Additions is
best used for complex matrices. For gas chromatography/mass spectrometry
analysis, isotopes are ideal as internal standards. Consistent extraction
conditions and stirring or agitation is required.
Q: How can I extend SPME fiber life?
Most fiber damage is the result of needle bending. Bending usually occurs
while the fiber is piercing the vial septa. Adjust the black needle depth
gauge so that only 0.5-1cm of the needle is exposed. Pierce the vial septa,
then screw the needle into the vial by rotating the needle depth gauge.
Maintain the injection port temperature below the maximum recommended temperature
of the fiber. Conduct headspace sampling when ever possible to minimize
extraction or carry over of high molecular weight compounds that coat the
fiber.
Q: My background is too noisy. How can I reduce extraneous peaks?
- Proper conditioning of the fiber before using it. Recondition the
fiber for a longer time.
- Check the inlet liners. Most peak noise is caused by septa particles
in the liner.
- Change the vial septa.
- Reduce the inlet temperature by about 20°C. Use the lowest temperature
that produces sharp peaks and minimizes carryover.
Q: Can analytes be extracted from organic solvent using SPME?
We
do not recommend extraction from organic solvents. However, you may extract
analytes using the headspace above polar organic solvents, at the risk of
a reduced distribution constant, fiber damage from swelling, and a large
solvent peak on your chromatogram. |
