Over the last 10 years, several solvent-free microextraction techniques for gas chromatography (GC) and mass spectrometry (MS) have been developed. Two of these techniques, solid-phase microextraction (SPME) and stir-bar sorptive extraction (SBSE), are available commercially for the analysis of volatile compounds, such as flavors in foods and beverages, and toxic organic compounds in environmental applications. Other techniques, such as open tubular trapping, inside needle capillary adsorption trap (1), in-tube SPME, capillary microextraction, needle trap, and headspace solid-phase dynamic extraction (2), were also developed for different applications. The basic principle for all of these techniques is essentially the same. Volatile and semivolatile compounds are adsorbed on a sorbent coating, often packed on the interior surface of a capillary column or stainless steel needle. After the sample is concentrated on the coating, the compounds are desorbed thermally in the heated injection port of a gas chromatograph and are analyzed by GC–MS.

Recently, an automated dynamic headspace extraction technique called in-tube extraction was developed by CTC Analytics (Zwingen, Switzerland) for its Combi PAL autosampler and used with an ion-trap gas chromatography–mass spectrometry (GC–MS) system. In this device, analytes are adsorbed on a sorbent trap contained inside of a side-hole needle attached to a gas-tight syringe. Loading takes place by continuously passing the headspace above a sample across a sorbent trap inside the needle. This dynamic headspace extraction results in added sensitivity over conventional static headspace techniques. The analytes are then desorbed off the needle trap into a GC injection port and analyzed by ion-trap MS. Until recently, very little work had been done to determine optimized extraction procedures for in-tube extraction methods (3,4).

Presently, volatile organic compounds (VOCs) in drinking water typically are measured using one of two methods: purge-and-trap GC–MS or headspace GC–MS. Purge-and-trap GC–MS is a very sensitive technique, but it is subject to carryover and contamination due to long sample pathways in the concentrator. Also, compounds sensitive to active sites will degrade rapidly on dirty traps, poor connections, and cold spots. Headspace analysis eliminates several of these problems, but its sensitivity is not as good, particularly for less volatile compounds. In-tube extraction and thermal desorption is highly sensitive and is a good alternative to these conventional methods for the analysis of trace levels of organic chemicals.

This study developed an analytical method using in-tube extraction and an ion-trap mass spectrometer for VOCs and compared results with those obtained using a standard purge-and-trap reference method, U.S. EPA Method 524.2. The EPA method has specific quality control parameters for reproducibility, accuracy, and method detection limit (MDL), providing an excellent standard point of reference for the evaluation of the in-tube extraction technique.

Materials and Reagents

Figure 2: Extracted ion chromatogram of vinyl chloride at 0.1 ppb using the in-tube extraction technique.
 
A total of 62 VOCs along with a volatile gases stock solution at 2000 mg/L and three internal standards at 2000 mg/L were purchased from Ultra Scientific (North Kingstown, Rhode Island). All solutions were in purge-and-trap-grade methanol. These stock solutions were further diluted in methanol to prepare substocks at 2, 20, and 200 mg/L and then spiked into 100-mL volumetric flasks to make 0.1-, 0.2-, 0.5-, 1-, 2-, 5-, 10-, 20-, and 50-μg/L standard solutions in reagent water. Internal standards were added to give a final concentration of 5 μg/L. A 12.5-mL volume of each standard solution was transferred into a 20-mL screw cap headspace vial with a PTFE-coated silicone septum.

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