Question

(a) Explain how solid phase microextraction works. Why is cold trapping necessary during injection with this technique? Is all the analyte in an unknown extracted into the fiber in solid phase microextraction?

(b) Explain the differences between stir-bar sportive extraction and solid-phase microextraction. Which is more sensitive and why?

Short answer

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Step-by-step solution

 a) microextraction

A solid phase microextraction is a sample preparation method wherein the compound of interest is extracted from liquids, air, or even sludge without using a solvent. A fused-silica fiber coated with a $10-\mathrm{to}100-\mathrm{\mu m}$ -thick film of stationary phase similar to those used in gas chromatography is the most significant component. The stirring or heating. It is recommended first to determine by experiment the time at which the fiber will reach equilibrium with the analyte and use this time for extraction.

Then, fiber is retracted, and syringe is inserted into a gas chromatograph equipped with a 0.7-mminner-diameter injection port liner after sample collection. The fiber should be extended such that it reaches the hot injection liner, where analyte is thermally desorbed from the fiber in spitless mode for a fixed time. The function of the narrow port is to maintain the desorbed analyte in a narrow band. Cold trapping is necessary during injection to collect the desorbed analyte at the head of the column before chromatography.

In solid phase microextraction, not all analytes in an unknown are extracted into the fiber. The mass of analyte ( m in $\mathrm{\mu g}$ ) absorbed in the fiber is expressed in the equation below:

$\mathrm{m}=\frac{{\mathrm{KV}}_{\mathrm{f}}{\mathrm{c}}_{\mathrm{o}}{\mathrm{V}}_{\mathrm{s}}}{{\mathrm{KV}}_{\mathrm{f}}+{\mathrm{V}}_5}$

where:

- ${\mathrm{V}}_{\mathrm{f}}=$volume of film on the fiber

- $V_5=$volume of solution being extracted

- ${\mathrm{C}}_{0=}$initial concentration of analyte in the solution

- $K=$partition coefficient

stir-bar sportive extraction and solid-phase microextraction

Another method of sample extraction that is closely related to solid-phase microextraction is stir-bar sportive extraction. This technique is ~ 100 times more sensitive for trace analysis than solid phase microextraction. In this method, a magnetic stirring bar enclosed in a glass jacket is coated with a 0.5-to 1-mm-thick layer of sorbent such as poly (dimethyl siloxane), that is similar to the stationary phase in nonpolar gas chromatography columns. The stirring bar is then placed in an aqueous mixture and stirred for 0.5-4 hours to absorb hydrophobic analytes. The mass of analyte extracted is also expressed as the equation above, but the volume of sorbent $(V_f)$is increased from $~0.5\mathrm{\mu }$L to 25-125 $\mathrm{\mu }$L. Thus, 50 to 250 times more analyte is extracted with the stir bar method as compared to solid phase microextraction.

Final answer

a) A solid phase microextraction is a sample preparation method wherein the compound of interest is extracted from liquids, air, or even sludge without using a solvent.

b) Another method of sample extraction that is closely related to solid-phase microextraction is stir-bar sportive extraction