Two factors should be considered:
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.
For more information, SPME Fiber Selection.
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.
For reliable quantitative results from manual SPME extractions, we recommend these guidelines:
The sample volume must be consistent among samples. For direct immersion applications, completely fill the sample container. For headspace applications, fill the container between 1/2 and 3/4 full.
When extracting semivolatiles, use a consistent stirring rate. If the rate is inconsistent, precision drops, particularly for analytes with high distribution constants.
Keep the amount of organic content in the samples minimal (<1%) and consistent among all samples. Large amounts of organic solvent can greatly affect the distribution constants. Some solvents, such as methanol, act as co-solvents with the water, and can reduce the recovery of analytes extracted.
Use internal standards that are similar to your analytes. Internal standards help to offset extraction variables. Standard preparation is important. Prepare standards at 1-2mg/mL solvent, then spike 0.05-5uL of the standard into the water sample prior to the extraction.
Keep extraction time and temperature consistent among samples. The extraction time generally is 10 minutes for volatiles, 20-30 minutes for semivolatiles.
For more information about consistent results from SPME, refer to Bulletin 923.
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.
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.
In analyses of semivolatile and volatile analytes, headspace SPME offers several important advantages over headspace analyzers.
When you use a headspace analyzer to trap semivolatile analytes in water-based samples, you must heat the sample to 110 °C or higher. Much water will be transferred with the analytes. Often the analyte-containing volume is large and you must cryogenically refocus the analytes at the column inlet. Water that is transferred with the analytes can freeze in the line.
With headspace SPME, samples usually require warming only to 40-60 °C. The analytes will concentrate on the fiber with little water vapor. Then, because the fiber is desorbed rapidly in the injection port, the analytes usually desorb as a focused plug -- no cryogenics required. In addition, with headspace SPME sample volume is small, e.g., 3 mL in a 4 mL vial.
Even volatile organic compounds (VOCs) can be desorbed from an SPME fiber with sufficient speed to eliminate the need for cryogenics. Use a Carboxen™/polydimethylsiloxane fiber to retain extremely volatile analytes (gases at room temperature). Only ethane, methane, and some fixed gases are too small to be retained in the pores of Carboxen particles. Other SPME fibers are suitable for extracting semivolatiles by headspace SPME.
The decision to use headspace SPME or fiber immersion SPME is based almost entirely on the vapor pressure of the analytes. If there is significant vapor pressure, headspace SPME is advantageous; analyte recoveries are cleaner and faster than by immersion SPME, and the fiber will last longer. If you have a solid sample matrix, headspace SPME is mandatory.
For more information about SPME of volatile analytes, refer to Application Note 11 and Application Note 56. For more information about SPME of semivolatiles, refer to Application Note 6, Application Note 17, and Application Note 81.