Headspace sampling, analytical method

Where is the initial analyte concentration in the liquid phase, C( the concentration of analyte in the gas phase, K the gas-liquid partition coefficient for the analyte at the analysis temperature, V, the volume of liquid phase, and V, the volume of gas phase (318-321,324,325). From equation (8.3) it can be seen that the concentration of the analyte in the headspace above a liquid in equilibrium with a vapor phase will depend on the volume ratio of the geis and liquid phases and the compound-specific partition coefficient which, in turn, is matrix dependent. The sensitivity 1 of the headspace sampling method can be increased in some instances adjusting the pH, salting out or raising the... 

HS-GC methods have equally been used for chromatographic analysis of residual volatile substances in PS [219]. In particular, various methods have been described for the determination of styrene monomer in PS by solution headspace analysis [204,220]. Residual styrene monomer in PS granules can be determined in about 100 min in DMF solution using n-butylbenzene as an internal standard for this monomer solid headspace sampling is considerably less suitable as over 20 h are required to reach equilibrium [204]. Shanks [221] has determined residual styrene and butadiene in polymers with an analytical sensitivity of 0.05 to 5 ppm by SHS analysis of polymer solutions. The method development for determination of residual styrene monomer in PS samples and of residual solvent (toluene) in a printed laminated plastic film by HS-GC was illustrated [207], Less volatile monomers such as styrene (b.p. 145 °C) and 2-ethylhexyl acrylate (b.p. 214 °C) may not be determined using headspace techniques with the same sensitivities realised for more volatile monomers. Steichen [216] has reported a 600-fold increase in headspace sensitivity for the analysis of residual 2-ethylhexyl acrylate by adding water to the solution in dimethylacetamide. 

Hcxanc can be determined in biological fluids and tissues and breath using a variety of analytical methods. Representative methods are summarized in Table 6-1. Most methods utilize gas chromatographic (GC) techniques for determination of -hexane. The three methods used for preparation of biological fluids and tissues for analysis are solvent extraction, direct aqueous injection, and headspace extraction. Breath samples are usually collected on adsorbent traps or in sampling bags or canisters prior to analysis by GC. 

To imitate water samples, trip blanks are prepared in volatile organic analysis (VOA) vials with septum caps lined with polytetrafluoroethylene (PTFE). The vials are filled without headspace with analyte-free water. For soil sampled according to the requirements of EPA Method 5035, field blanks may be vials with PTFE-lined septum caps, containing aliquots of methanol or analyte-free water. 

A method was described for the accurate and precise determination of monomethyl-Hg by species sped Pc isotope dilution calibration using SPME in combination with GC separation and ICP-MS detection [46], Samples were digested with methanolic KOH, derivatized in aqueous solution with sodium tetra-propylborate and headspace sampled with a polydimethylsiloxane SPME fused silica Pber. The analyte was then directly transferred from the Pber to the head of the GC column for desorption. 

Solid Phase MicroExtraction (SPME) is a solvent-free sample preparation method based on the adsorption of analytes directly from an aqueous sample onto a coated fused-silica fiber. Headspace SPME was used in combination with gas chromatography-mass spectrometry/ selective ion monitoring (GC/MS-SIM) to analyze for TCA in wine. 

SPME is a patented sample preparation method for GC applications (32-36). The solvent-free technique was developed in 1989 by Janusz Pawliszyn (http. /Avww.science.uwaterloo.ca/ -janusz/spme.html) at the University of Waterloo in Ontario, Canada, and a manual device made by Supelco, Inc. has been available since 1993. In 1996, Varian Associates, Inc., constructed the first SPME autosampler. SPME involves exposing a fused silica fiber that has been coated with a non-volatile polymer to a sample or its headspace. The absorbed analytes are thermally desorbed in the injector of a gas chromatograph for separation and quantification. The fiber is mounted in a syringe-like holder which protects the fiber during storage and I netration of septa on the sample vial and in the GC injector. This device is operated like an ordinary GC syringe for sampling and injection. The extraction principle can be described as an equilibrium process in which the analyte partitions between the fiber and the aqueous phase. 

Therefore, as noted in Chapter 2, most analytical methods involve some sample preparation step. Most often, such a step involves the transfer of the target analytes from the solid to a liquid phase. By exception, some techniques such as headspace or pervaporation involve the physical removal of volatile analytes from the solid sample. 

Analytical Methods for Urine and Blood. Specific biomarkers of lewisite exposure are currently based on a very limited number of in vitro experiments (Jakubowski et al., 1993 Wooten et al., 2002) and animal studies (Logan et al., 1999 Fidder et al., 2000). Wooten et al. (2002) developed a solid-phase microextraction (SPME) headspace sampling method for urine samples followed by GC-MS analysis. It is the most sensitive method reported to date with a lower limit of detection of 7.4 pg/mL. Animal experiments have been limited in number and in their scope. In one study of four animals, guinea pigs were given a subcutaneous dose of lewisite (0.5 mg/kg). Urine samples were analyzed for CVAA using both GC-MS and GC coupled with an atomic emission spectrometer set for elemental arsenic (Logan et al., 1999). The excretion profile indicated a very rapid elimination of CVAA in the urine. The mean concentrations detected were 3.5 pg/mL, 250 ng/mL, and 50 ng/mL for the 0-8, 8-16, and 16-24 h samples, respectively. Trace level concentrations... 

There is as yet no perfect extraction method for all types of samples. A minimum of a liquid extraction and a headspace sampling should lead to a fairly well rounded flavor or fragrance profile and analysis. The more techniques used to study a particular subject, the better the quality of the analytical results. However with experience the choice of extraction techniques will ensure more detailed analyses from fewer extracts. 

The direct headspace (HS) technique has been used to determine VOCs in water samples. This method overcomes comphcations associated with the sample matrix and can be applied to a wide range of concentrations. HS requires little sample preparation. Salt is usually added to improve the partitioning into the gas phase and the sample is heated to temperatures of about 50-60 °C, which enhances the volatilisation of the analyte, increasing the efficiency of the extraction process and consequently the sensitivity. Typically, the HS is directly sampled with a p,L-lock valve-gastight syringe and injected into the GC. This method has been used for the determination of the isotope composition of MTBE, ETBE and TAME, reporting detection limits of 3-6 mgL for 5 C and 8-20 mgL for the 3 H [37-40]. 

A third sample preparation method is purge and trap, which aims to extract as close to all of the analyte as possible from the solid or liquid sample and is a deviation from headspace sampling. It works by bubbling a purge gas such as helium through the heated sample vial. The gas carries analyte up into an adsorption tube packed with selective stationary phase. After all the analyte has been trapped in the tube, the gas flow is reversed through the tube to remove any residual solvents. The tube is then directed to the injector port and, heated to desorb the analytes, which are then cold-trapped onto the head of the GC column. From there, the concentrated sample is heated for GC separation. 

Several papers investigated the use of SPME for VFA analysis in wastewater and in air. Briefly, a fiber is exposed to the sample headspace or inserted directly into the sample. Analytes adsorb onto the fiber and are subsequently desorbed at high temperatures in the GC injection port. SPME is a solvent-free technique which introduces less potential contaminants into the GC compared to direct injections. SPME is also rapid since no further sample preparation steps are required. It may be used for routine analysis provided that the specific autosampler required for this method is available and that the optimized method conditions are suitable for autosampler application. Further information on principles and other applications of this technique can be found elsewhere. " " Parameters which have been optimized for VFA analysis are fiber coating, fiber exposure time, sample temperature, sample pH, sample agitation, potential salt addition, and desorption parameters. Surrogate standards employed for VFA analysis were 2-ethylbutyric acids for GC/FID or GC/MS and C-labeled organic acids for GC/MS. The method was optimized using standards in deionized water and only a few wastewater samples were analyzed as examples. 

There are many differences between samphng from the liquid phase (direct SPME) and from the headspace (HS-SPME). The factors afifechng direct SPME and HS-SPME, and the conditions that lead to the optimum performance of the analytical method, are different due to the nature of each process. In direct SPME the mass transfer rate of analytes is limited by the diffusion in the liquid phase, while in HS-SPME the limiting rate is the transport of analytes from the sample to the headspace. Because diffusion in the liquid phase is much slower than in the headspace and transport of analytes from the hquid to the vapor phase can be accelerated by proper conditions, the time taken to reach equilibrium by HS-SPME is shorter than in direct SPME. A comparative study showed, how for the optimal conditions of each method, the time taken to reach equilibrium in HS-SPME was shorter than for direct SPME (see Table 14.3). Limits of detection were also slightly better for HS-SPME than for direct SPME. 

Analysis of environmental samples is similar to that of biological samples. The most common methods of analyses are GC coupled to MS, BCD, a Hall s electrolytic conductivity detector (HECD), or a flame-ionization detector (FID). Preconcentration of samples is usually done by sorption on a solid sorbent for air and by the purge-and-trap method for liquid and solid matrices. Alternatively, headspace above liquid and solid samples may be analyzed without preconcentration. Details of commonly used analytical methods for several types of environmental samples are presented in Table 6-2. 

---