The Effect of Sample Concentration and Complexity on SPME Fiber Selection

By: Bob Shirey, Reporter US Vol 25.4

Selecting the Appropriate SPME Fiber for your Application

Bob Shirey

Previously, in the Reporter (Vol 24.3) we discussed how to select the appropriate fiber assembly, focusing on fiber core, needle gauge and assembly design. In the Reporter (Vol 25.3) we discussed the affect of analyte molecular weight and polarity on fiber selection. This article discusses the affect of sample concentration and complexity on fiber selection.

There are 2 mechanisms for the extraction of analytes into fiber coatings, absorption and adsorption. Absorbent coatings are primarily bonded gums that act somewhat like a sponge. The analytes partition in and out of the coating. The primary mechanism for retention of the analytes is the coating thickness and the size of the analyte. The coating type determines the affinity of an analyte for the phase. Usually there is little competition of analytes for the fiber coating unless one analyte is in a much greater concentration than the other analytes.

Adsorption type fibers more physically or chemically retain the analytes. Usually adsorption coatings contain particles that have pores (adsorbents). These particles are typically suspended in an adsorbent phase to bind the particles to the fiber. Analytes will typically migrate to the particles and either be retained by interaction with the surface such as pi-pi interactions, or they migrate into the pores where they are more tightly retained. The size of the pore determines if a given analyte will be retained. Because there are a limited number of pores per given fiber, there is a limited analyte capacity that the fiber can retain. If a given analyte has a higher affinity for a pore site, one analyte could possibly be displaced by another analyte. This is based upon Langmuir’s Isotherm for uniform pores.

By monitoring a group of analytes over a wide concentration range, one can determine the type of mechanism and the capacity of the fibers for the analytes. In this study we primarily focused on small polar and non-polar analytes.

Carboxen™ 1006 used in the Carboxen Polydimethyl siloxane (PDMS) fibers has tapered pores. Since the pores are not uniform, analytes with different size and shapes fit in different regions in the pores. Figure 1 shows the loglog plot of 7 analytes across a wide concentration range.

Figure 1. Plot of Analyte Response using Carboxen-PDMS Fiber

The results show that all of the analytes were extracted at 5 ppb. The polarity of the analytes increases in order from top to bottom. The responses for all of the analytes began to level off at 5 ppm, indicating that the maximum capacity for the analytes with this fiber coating. Between 10 ppm and 100 ppm the responses remained constant. Where lines crossover each other, this could be an indicator of displacement. Generally when displacement occurs the response for the displaced analyte will decrease and the analyte doing the displacement will increase. The decreased response for isopropanol may simply be a solubility issue as the concentration of the other analytes increased.

The Divinylbenzene (DVB) fibers have larger micropores and a more uniform mesopore that could lead to displacement. Figure 2 shows a plot of the extraction of the same analytes under the same conditions using a PDMS-DVB coated fiber.

Figure 2. Plot of Analyte Response using PDMS-DVB Coated Fiber

The results indicate that the response for the polar analytes is not detected until the concentration increases to 50-100 ppb while the non-polar analytes could be extracted at 5-10 ppb. The response begins to level off for some analytes between 10-50 ppm and there appears to be some displacement. The response for methylene chloride continues to climb while the slopes of the response lines for dioxane and acetone begin to decline.

A longer extraction time would show this effect more dramatically. Figure 3 shows the same analysis using the absorbent 100 μm PDMS coated fiber.

Figure 3. Plot of Analyte Response using 100 μm PDMS Fiber

The results show that the minimum detection limits are much higher for these smaller analytes, but the linearity is excellent up to 100 ppm. This was the highest concentration level evaluated. No displacement of the analytes was observed.

The following summary can be made based upon the results of this study and can be used as a guideline for selecting the appropriate SPME fiber.

  1. Adsorbent fibers are better for analytes at low concentration levels
  2. Adsorbent fibers have a limited capacity, so linear range for each analyte needs to be determined
  3. It is best to keep extraction times under 30 min for adsorbent fibers to reduce displacement
  4. Absorbent fibers are better for complex samples with varied concentration ranges
  5. DVB-Carboxen-PDMS fiber is good for complex samples at low concentration levels due to the 2 adsorbent beds.
  6. Absorbent fibers are better for screening samples at high concentration levels
  7. Absorbent fibers are a better option for dirty samples that may contain multiple unknown compounds.

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