Sample Preparation Techniques Used for Gas Chromatography

Robert GrobBy Robert L. Grob, Ph.D., Emeritus Professor of Analytical Chemistry, Villanova University

Most scientists when faced with the analysis of a sample give little or no attention to how the sample was obtained, i.e., the sampling process. This is not the topic of discussion for this editorial. Suffice it to say one should realize that the results of the analysis of a sample can only be as reliable as the sample is representative. One must remember; to obtain reliable analytical data three components are always involved: 1) the system (a representative sample), which consists of the analytes of interest and the matrix (the part not requiring analysis but which can interfere with the instrumentation), 2) a measuring instrument, and 3) the analyst or observer (a human being!). Well over 50% of the analysis time is spent on sample preparation and, since you have a human being as part of this system, sample preparation is the most error-prone and labor-intensive task in the analytical laboratory. For this discussion we will assume ALL samples are homogeneous and representative! If the final measurement of analyte concentration is by GC, one should strive to circumvent complete sample matrices; e.g., nonvolatile components and interfering analytes.

One criterion for an analyte is that it must have a vapor pressure of 0.1 Torr at operating conditions in gas chromatography (GC). Thus, it must be able to be vaporized in the system inlet. The sample can be a gas, liquid or in some cases a solid (thermally stable or capable of producing a definite pyrolysis pattern). Most sample preparation techniques for GC are based on variations of extraction theory whereby the analytical chemist may change solvent, temperature, pressure, phases or volumes. Thus, an understanding or comprehension of liquid-vapor, liquid-liquid and liquid-solid equilibria are assumed.

IMPORTANT NOTE: Before you attempt the gas chromatographic analysis of an unknown sample obtain as much information about the sample as possible. Randomly injecting an unknown sample into a gas chromatograph does not reveal complete analytical data about the sample!

Having three states of matter (gas, liquid & solid), we have a series of sample preparation techniques available.

Gas-Liquid or Gas-Solid Equilibria

The main sample preparation techniques which can be classified by these equilibria are: Static headspace technique: analytes of interest (volatiles) are equilibrated in a closed vial at a specified temperature & pressure. A gas-tight syringe is used to transfer the headspace sample into the gas chromatographic injection port. Dynamic headspace technique: analytes of interest are swept or purged onto an adsorbent and then thermally desorbed into the gas chromatograph. Solid-phase extraction (SPE): may be used to concentrate analytes from gaseous or liquid samples and often is used to clean up and concentrate liquid extracts. The adsorbed analytes can be eluted with a solvent or thermally desorbed. Solid-phase microextraction (SPME): may be used for both gaseous and liquid samples. A fused-silica polymer coated fiber (e.g., with polydimethylsiloxane) is exposed to the stirred sample. The shielded-fiber is then inserted into the injection port of the gas chromatograph. Distillations: these are predominately macro scale techniques and are rarely employed as sample preparation techniques for GC. Stir bar sorptive extraction (SBSE): this is a dynamic variation of SPME in which a spinning glass-covered magnetic bar (coated with a thick layer of polydimethylsiloxane) is used to sorb analytes of interest, which can be removed by thermal desorption in the gas chromatographic injection port.

Liquid-Liquid or Liquid-Solid Equilibria

The technique of liquid-liquid extraction has lost appeal to the analytical chemist because of (1) the time needed to reach equilibrium, and (2) the volume of solvent needed for quantitative recovery of analytes (the environmental restrictions on waste disposal of used solvents). Quantitative extraction of organic species from aqueous systems requires that the organic moiety (1) is non-polar, (2) does not dissociate in the aqueous phase, and (3) does not dimerize or polymerize in the organic phase. Thus, liquid-liquid extractions have limitations as sample preparation techniques for GC UNLESS the equilibrium between the two phases exhibits a large numerical partition coefficient. If this is the case, one may resort to micro liquid-liquid extractions, where the concentration factor is >1200 times that for the macro-technique [J. Chromatogr. 106,299 (1975) and J. Chromatogr. 177,135 (1979)]. Thus, classical liquid-liquid extractions have been replaced by modern & efficient techniques and are more prevalent in organic synthesis laboratories or for the separations of metal complexes, metal chelates, and/or ion-pairing reagents.

A number of classical liquid-solid equilibria techniques are available (e.g., ion exchange or Soxhlet extraction) but only Soxhlet extractions have application in sample preparation prior to GC. This technique is not commonly used because: (a) a large volume of solvent is needed for the sample extraction, (b) an evaporation step is required to concentrate the sample, (c) lack of thermal stability and volatility of some sample analytes, and (d) interference from contaminants in the extraction thimbles (requires a blank extraction prior to sample extraction).

In the past several years, newer techniques have become available. Accelerated solvent extraction (ASE), sometimes referred to as pressurized liquid extraction (PLE) or pressurized fluid extraction (PFE), may be used for solid and semi-solid samples. Elevated temperatures and pressures used in these techniques cause hydrogen bonds and dipole interactions to be reduced, and surface wetting is increased. Water may be used as the solvent if it is below its critical point. Then, it is known as subcritical water extraction (SWE); which makes it similar to SFE.

A similar sample preparation technique is microwave-assisted extraction (MAE); sometimes referred to as microwave-assisted solvent extraction (MWE). The pressure generated is ca. a few hundred psi; however, the extraction container must be microwave transparent (e.g., PTFE or quartz). The solvent used may be microwave absorbing or non-microwave absorbing. In place of microwaves, ultrasonic vibrations may be used to assure good contact between sample and solvent. This is a fast technique but efficiency is not as high as with other techniques. Low concentrations of analytes in samples require multiple extractions. This sample preparation technique is referred to as ultrasonic extraction (USE).

A technique which became very popular during the 1980s is supercritical fluid extraction (SFE). Supercritical fluids (SFs) are dense gases above their critical temperature & pressure. Thus, SFs possess properties which resemble both liquids and gases. Analytes are more soluble in SFs when they are in their liquid state; thus, analyte melting points and solubility in the SF are important properties to consider. SFEs are fast and very efficient.

Another group of sample preparation techniques are solid-phase extraction (SPE), solid-phase microextraction (SPME) and stir bar sorptive extraction (SBSE). SPE is a technique (invented in the late 1970s) referring to a non-equilibrium exhaustive removal of analytes (semi-volatiles and non-volatiles) from a liquid sample by retention on a solid phase (sorbent) and then subsequent removal of selected analytes by solvent elution. Particulate matter in the sample can interfere with the analysis. Thus, particulate matter may sorb some analytes of interest and cause low analytical recoveries. NOTE: Remove particulates, by filtration, prior to SPE analysis! The efficient use of this technique requires optimization of the sorption and desorption processes.Another group of sample preparation techniques are solid-phase extraction (SPE), solid-phase microextraction (SPME) and stir bar sorptive extraction (SBSE). SPE is a technique (invented in the late 1970s) referring to a non-equilibrium exhaustive removal of analytes (semi-volatiles and non-volatiles) from a liquid sample by retention on a solid phase (sorbent) and then subsequent removal of selected analytes by solvent elution. Particulate matter in the sample can interfere with the analysis. Thus, particulate matter may sorb some analytes of interest and cause low analytical recoveries. NOTE: Remove particulates, by filtration, prior to SPE analysis! The efficient use of this technique requires optimization of the sorption and desorption processes.

An extension of SPE, known as solid-phase microextraction (SPME), came about in 1989; the technique utilizes a liquid or solid stationary phase on a fiber, tube, vessel walls, suspended solids, stirrer, or disk/membrane. The technique may be applied to volatiles, semi-volatiles, and non-volatiles and is an equilibrium technique.

An excellent treatment of sample preparation techniques may be found in Chapters 11 and 15 of Modern Practice of Gas Chromatography, R.L. Grob and E.F. Barry (Eds.), John Wiley & Sons, Hoboken, NJ, 4th ed., 2004.

Remembering Bob Grob

By Joe Konschnik

We all lost a friend and pioneer in the field of chromatography recently when Dr. Bob Grob passed away on October 22, 2006 at the age of seventy-nine. His friends and colleagues will remember Bob as a truly outstanding man who touched many lives. We acknowledge that Dr. Grob’s numerous accomplishments in the field of chromatography undoubtedly rank him among the most highly respected and gifted educators, authors, and analytical chemists.

On a more personal side, those who knew Bob Grob, whether his family, former graduate students, colleagues, or friends, would agree that Bob was an outstanding mentor and leader. Bob truly cared for people and was involved in their lives, encouraging them to work hard and strive for excellence in everything they did. Bob set high standards and expectations for himself and expected the same from those around him. His former graduate students have described him as being like a father to them during some of their most challenging years. Bob always kept his commitments and was a very unselfish giver of his time and resources, displaying his concern for people by placing their needs before his own. He gave of his time and resources to help others during times of crisis or difficulty.

Bob was extremely well organized, and lived every day by his lists, which guided him through the completion of those things he considered most important. He loved to laugh and enjoyed sharing jokes with others in various formats including emails. His hearty laugh was easily distinguished in a crowded room. Bob was a gatherer who preferred to be around people, and enjoyed spending time with others, whether at professional events or just at lunch with a friend. Bob was uniquely blessed, being able to enjoy a very full scientific career doing what he really loved, teaching, writing, and consulting, and was very humble regarding his accomplishments.

Dr. Grob will be missed greatly by the many of us who knew him. We are grateful for his life, friendship, leadership, and accomplishments. His legacy will live on for many years to come as we enjoy our memories of him and continue to benefit from his teaching and many publications. We offer our deepest sympathies to his wife, Marge, his family, and close friends.