Solid Phase Micro-extraction (SPME)

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Contributors: Catherine H. Stephens, Alba Alvarez Martin
==Overview==
Technique: SPME
Formal name: Solid phase microextraction
Sample image of the data: Image of SPME fibers as well as the accessory used to expose the SPME fiber to the analyte/sample. 
(SPME doesn’t generate data on its own. It is a sampling technique (ie, a way to collect a sample). The analyte/sample is adsorbed by a coating applied the core of a SPME fiber (there are different coatings available that are sensitive to adsorbing different types of analyte). When you are done collecting your sample (can be as short as 20 minutes to collect), you insert the fiber into an analytical instrument (gas chromatograph, liquid chromatograph) to desorb the analyte from the fiber and the instrument generates the data.)
Schematic of a SPME fiber loaded into a fiber holder. (a) SPME Fiber holder. (b) Cross-section of SPME fiber assembly. Originally published by Supelco. From "SPME fiber for analysis of pesticide residues in fruits and vegetables: a review" 2012
 Image of SPME Arrow fibers, not loaded into a fiber holder. Image from CTC Analytics AG website
What this techniques measures (compound classes, physical structures, etc): 
* Volatile organic compounds (VOCs) and semi-volatile organic compounds (SVOCs) if you are sampling air/gas
* Solvents if you are sampling a liquid
Selection of SPME fiber coatings and the analytes they are designed to collect: 
* carboxen / polydimethylsiloxane (CAR/PDMS) - Gases and low molecular weight compounds (MW 30-225)
* divinylbenzene / carboxen / polydimethylsiloxane (DVB/CAR/PDMS) - Trace compound analysis (MW 40-275)
* polydimethylsiloxane (PDMS) - volatiles (MW 60-275)
* polyacrylate (PA) - polar semi-volatiles (MW 80-300)
* polydimethylsiloxane / divinylbenzene (PDMS/DVB) - Amines and polar compounds (liquid chromatography injection only)
* carbowax-polyethylene glycol (PEG) - Alcohols and polar compounds (MW 40-275)
Limitations of this technique (egs: doesn't detect metals, must have certain quantities present, etc): 
Not for inorganic materials, not for metals, not for very small molecules 
Can/how can this technique be made quantitative? SPME tends to be a qualitative sampling technique. There is research showing you can work on adding internal standards if you are sampling air or VOCs coming from a solid if you are sampling in an enclosed jar (ie, have a very controlled experiment – see references). If you are sampling air in a gallery space, for example, you cannot do quantitative analysis, only qualitative.
==Samples==
Phases it can be used to examine (gas, liquid, solid) SPME is best for analyzing gases and liquids. If you are doing an experiment in a closed jar, you can measure the gases/VOCs that come out of a solid.
Is this technique non-destructive? 
* If you are sampling chemicals in the air, yes
* If you are sampling the VOCs that come out of a solid sample, no
How invasive is this technique? 
* It can be noninvasive if you are analyzing gases/VOCs in air.
* It takes about 0.3 grams of material to do an experiment where you are sampling the VOCs coming out of a solid material
Minimum size of sample necessary to use this technique? ~ 10 mg
Sample preparation methods [ed note: feel free to add references like ASTM or ISO methods, including numbers and name]
* The AIC Materials Working Group has a written protocol for sampling VOCs that come out of solid samples. This technique requires the use of a gas chromatograph-mass spectrometer to analyze the data
* American Standard Test Method ASTM D6520 – 18 method: Standard Practice for the Solid Phase Micro Extraction (SPME) of Water and its Headspace for the Analysis of Volatile and Semi-Volatile Organic Compounds
==Applications==
Examples of how this technique is used in the field? To look for hazardous chemicals coming from construction materials. To look for chemicals in storage and displays spaces that are hazardous to art objects. 
Risks associated with using this technique? That you might have a chemical present in your space or in your sample that does not get absorbed by the fiber and therefore, when you analyze the data, you might think the chemical is not present. 
==Budgetary Considerations==
Approximate cost to purchase this instrument? A SPME fiber is about $600. It lasts for ~ 50 – 100 samples, but you should monitor the fiber to make sure the coating is still present. It can rub off or be dissolved off (if you do liquid sampling with the wrong solvent) and your sensitivity will drop. 
Annual cost to maintain this instrument? A SPME fiber is considered to be a consumable product. The expensive part is getting the fiber analyzed. A gas chromatograph or liquid chromatograph cost more than $20,000 USD. 
Sample analysis costs? Unknown.
==Case Studies==
* Conservation Applications of Solid-Phase Microextraction
About the Lecture: Solid-phase microextraction (SPME) is a simple, sensitive, and economical method for collecting and preparing samples for analysis by gas or liquid chromatography. The technique uses polymer-coated fibers to concentrate samples from gas or liquid phases without using solvents. The fiber is then injected into the chromatograph for analysis. The Research and Testing Lab at the National Archives and Records Administration has used SPME for the past several years in combination with gas chromatography/mass spectrometry. Plasticizers used with document lamination films are easily identified using SPME. Volatile organic pesticides previously used to treat Native American artifacts have also been studied. SPME fibers are currently being used to study acetic acid and other pollutant gases in Archives storage areas. A modified fiber is being tested for use as a very simple method for collecting time-weighted average readings of pollutant. 
* "Analysis of laminated documents using solid-phase microextraction" by Mark Ormsby JAIC 2005 Vol 44 (1) 13-26.
* The Art of Museum Conservation Using Solid Phase Microextraction-Gas Chromatography-Mass Spectrometry – interview with Katherine Curran 
==Additional Information==
*Complementary Techniques: DTD, headspace 
*Variations of this technique 
*Analyzing volatile organic compounds (VOCs) with SPME Technique for analyzing SPME samples used by the Material Working Group's Materials Testing and Standards group
==References==
* Wikipedia entry for SPME 
* Lukman Bola Abdulra’uf, Wasiu Adebayo Hammed, and Guan Huat Tan (2012). SPME fibers for the analysis of pesticide residues in fruits and vegetables: A review, Critical Reviews in Analytical Chemistry, 42, 152–161.
* Quantitative SPME method 
* Pawliszyn research group website (inventor of the SPME technique) 
* Katherine Curran, Mark Underhill, Lorraine T. Gibson, Matija Strlic (2016). The development of a SPME-GC/MS method for the analysis of VOC emissions from historic plastic and rubber materials. Microchemical Journal, 124, 909-918.
* Supelco SPME homepage
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