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31


Table 1. Linearity, MDL (in terms of minimum mass detected on the tube for TO-15, and concentration in air for Method 325), precision and recovery for TD-SIFT-MS analysis of BTX. Precision and recovery data are at ca. twice the TO-17 MDL.


Target Compound


Benzene 0.9997 Toluene Xylene


Linearity (R2 ) Method Detection Limit


0.9999 0.9997


TO-17 / ng on tube 0.23 0.28 0.37


of up to 20 TDTs per hour per SIFT-MS instrument can be achieved using this fully integrated system - approximately an order of magnitude higher than the throughput of thermal desorption (TD)-GC/MS.


Experimental 1. SIFT-MS


SIFT-MS [7,8] is a real-time analytical technique for direct, comprehensive gas analysis to ultra- trace levels [9]. Data obtained by SIFT-MS instruments compare well with the accepted chromatographic method for volatile organic compound (VOC) analysis [10].


O2


SIFT-MS uses soft, precisely controlled chemical ionisation coupled with mass spectrometric detection to rapidly quantify VOCs to low part-per-trillion concentrations by volume (pptV). Eight chemical ionisation agents (reagent ions) are now available in commercial SIFT-MS instruments: H3 -, and NO3


O+ +, O- , O2 -, OH- , NO2 - [6]. These


reagent ions react with VOCs and inorganic gases in well controlled ion-molecule reactions, but they do not react with the major components of air (N2


, O2


Method 325 / pptV 2.1 5.1 7.9


Precision (at twice MDL) / %


13.8 9.5


13.0


Recovery (at twice MDL) / %


91 66 98


2. Automated thermal desorption-SIFT-MS


Autosampler integration enables the rapid, direct gas analysis provided by SIFT-MS to be applied to high-throughput TDT analysis. In contrast to chromatographic techniques that require cryo-focusing of thermally desorbed volatiles to provide optimal chromatographic resolution, the continuous, direct analysis provided by SIFT-MS means that volatiles are measured as they are desorbed.


, NO+ ,


Data presented here were obtained using an integrated Gerstel Multipurpose Sampler (MPS) (Gerstel, Mülheim an der Ruhr, Germany; www.gerstel.com). Addition of the Gerstel TD 3.5+ thermal desorption unit and the Gerstel CIS4 transfer line options facilitates simplifi ed, rapid analysis of thermally desorbed volatile compounds. Additionally, Gerstel hardware is controlled using Gerstel’s powerful Maestro software, which allows for optimal scheduling of sample transport, temperature ramp and cooling steps. This ensures that the highest sample throughput is achieved for the conditions required for a given method.


, and Ar). This


enables SIFT-MS to analyse air at trace and ultra-trace levels without pre-concentration.


Rapid switching between the eight reagent ions provides very high selectivity. The key benefi t of the additional ions is not primarily in the number of reagent ions, but in the multiple reaction mechanisms that provide additional independent measurements of each compound, delivering unparalleled selectivity and detection of an extremely broad range of compounds in real time.


In this paper, a Voice200ultra SIFT-MS instrument (Syft Technologies, Christchurch, New Zealand; www.syft.com) was utilised. Parameters for detection of benzene, toluene, and xylene isomers (BTX) using the positively charged reagent ions (H3


O+ , NO+ , and O2 + )


of SIFT-MS are from Ref. 11. Styrene and 1,3-butadiene parameters are from Ref. 12.


The fi nal concentrations were determined from the total integrated mass of each analyte (in environmental samples, xylene and ethylbenzene are reported as a combined total) released during desorption, and the tube loading parameters.


3. Standards and spiking of TDTs


Validation work follows procedures given in United States Environmental Protection Agency (US EPA) Method TO-17R [1]. Stock solutions of concentration 0.3 µL mL-1 were prepared by dissolving 30 µL each of benzene, toluene, and o-xylene in 100 mL methanol. These samples were stored in a sealed volumetric fl ask from which working solutions were prepared at fi ve concentrations (0.00015, 0.00150, 0.00750, 0.01500, 0.03750 µL mL-1


), by diluting the stock solution with methanol.


Unless otherwise noted, Tenax® TA tubes were spiked manually with 4 µL working solution of benzene, toluene and o-xylene at the working concentrations. At each level, three repeats were prepared to demonstrate precision. Blanks were prepared by spiking 4 µL of MeOH onto Tenax® tubes. Solvent purging was conducted by fl owing nitrogen at 100 standard cubic centimetres per minute (sccm) for 15 minutes after liquid spiking.


For the VIAQ study described later, a custom gas standard was procured that contained ca. 1 ppmV of each of the eight Chinese- regulated compounds in nitrogen.


4. Thermal desorption parameters


Thermal desorption conditions for the TD- SIFT-MS analysis of TDTs for each study were as follows. In all cases, nitrogen was used as the gas for tube desorption and the transfer line temperature was 200°C.


For validation of TD-SIFT-MS, the nitrogen desorption gas was at a constant pressure of 60 kPa and desorption fl ow of 19.9 sccm in splitless mode. The sample fl ow into the SIFT- MS instrument was 16.9 sccm. The initial TD temperature (held for 30 seconds) was 60°C for Tenax® TA (Method TO-17R) and 80°C for


Figure 2. Replicate TD-SIFT-MS analysis of tubes loaded from a gas standard containing BTEX + styrene.


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