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To get a better understanding of the practical capabilities of this new technology for the separation and analysis of semi- volatile organic compounds, a comparison was made between conventional LTM column technology and the planar LTM column described here for a standard of 18 polycyclic aromatic hydrocarbons (PAHs) with a boiling point range of 200-570°C. This comparison is exemplified in Figure 4.
Figure 5: Schematic of the toroidal ion trap The following observations were made.
• Peak shapes are poorer due to lower efficiency and resolution reduced in the conventional column – peaks were wider with a noisier background
• The retention times of the peaks in the conventional LTM column were substantially longer
• Peak intensities in the flat low thermal mass column were significantly higher than those in conventional one
• Conventional column technology could not analyse components 16, 17 and 18 in the PAH mix, they did not elute because of their very low volatility (boiling points 524-550°C)
It is likely that the poor peak shapes were caused by the cooler sites existing on the conventional column, which slowed down the movement of the analytes, decreased their partition coefficient and mass transfer, but increased the longitudinal diffusion band in the mobile phase. As a result the peaks become broadened and took a longer time to elute from the column. In fact, it can be seen that some high boiling point compounds got so broad that they cannot be seen as a normal Gaussian peak shape and as a result, were not detected.
The Mass Spectrometer
The instrument’s mass spectrometer uses a novel toroidal ion trap, which is well-suited for miniaturisation compared to other types of mass spectrometers, such as conventional cylindrical ion traps or linear quadrupoles. The benefits of using smaller ion traps is that they can operate at higher pressure so vacuum requirements are less stringent, allowing for smaller pumps which reduces both size and weight. The toroidal ion trap geometry translates into larger trapping volumes despite its miniaturised size, resulting in high ion counts and increased sensitivity, low noise levels and good spectral quality This means the instrument can operate off battery power for longer periods than any other field portable GC/ MS. A schematic of the instrument’s toroidal ion trap is shown in Figure 5.
Figure 6: The toroidal ion trap mass spectrometer provides multiple scans across a narrow chromatographic peak
Sample Preparation Module
The capability and flexibility of this portable GC-MS technology can be further improved using a compact, battery-operated, rugged sampling accessory (SPS-3, PerkinElmer Inc, Shelton, CT) for use in the field [7]. The choice of sampling modules includes:
• Heated headspace (HS) for volatiles in solid and liquid samples
• Purge and trap (P&T) for volatiles in liquid (aqueous) samples
The ion trap mass analyser is heated to 150-200°C depending on the application and operates under vacuum, which results in the electrodes staying clean for long periods of time. This reduces the need for frequent maintenance, while increasing mass spectral quality and reproducibility. Performing at an elevated temperature also leads to long-term MS resolution stability, providing <0.5 m/z mass resolution (FWHM) over the 41-500 Daltons (da) mass range. Most chromatographic peaks are ≥ 1 sec wide, ensuring that a multiple suite of compounds can be fully resolved and analysed in 1 min or less. The scan rate of the mass spectrometer is 10 -15 scans per sec, which provides multiple data points across the narrow chromatographic peaks resulting in excellent mass spectral quality. Typical repeatability at this scan rate is ~ 10% RSD for ppm-ppb levels using the SPME sampling method and lower at higher concentrations. The principle of quantifying with multiple data points across the chromatographic peak is shown in Figure 6.
• Thermal Desorption (TD) for volatiles using a conventional TD tube
• Internal standard addition module
These modules require the use of the needle trap (NT) to transfer the analytes to the GC/MS. In addition to the SPS-3 sampling module, the needle trap can be used independently to sample gases without the sampling module, while solid phase micro- extraction (can be utilised for gas and liquid samples and a coiled wire filament (CWF) can be used for semi volatiles dissolved in solvent samples. However, for this study, a SPME system was the sampling device of choice.
Experimental
The experimental conditions for the rapid screening of VOCs/SVOCs in cocoa beans and chocolate products are described below.
Sample Preparation
Cocoa beans, cocoa butter, and finished chocolate product were provided by Theo Chocolate (Seattle, WA). Each sample was placed in a 20mL headspace vial and capped. A CUSTODION™ SPME syringe with a 65 µm Polydimethylsiloxane/ Divinylbenzene (PDMS/DVB) fibre was used for extraction by exposing the fibre directly into the headspace above each sample for 10 min at 60°C, as shown in Figure 7. This sampling technique improved sample off- gassing and analyte collection of VOCs and SVOCs in the sample.
Figure 7: Schematic of the SPME syringe sampling the headspace vial
Analytical Conditions
Following each sample preparation, the SPME syringe was inserted into the injection port of the portable GC-MS where the target analytes were desorbed into a low thermal mass injector (270°C) coupled to a capillary GC column (MXT-5, 5 m x 0.1 mm, 0.4 µm - Restek, State College, PA). After an initial 10 second hold at 50°C, the GC temperature was increased at 2°C/sec to 280°C. The capillary GC was coupled to a toroidal trap MS detector having a mass range of 41-500 m/z and a scan rate of 10-15 scans/sec. The full chromatographic separation conditions are shown in Table 1, while the mass spectrometer parameters are shown in Table 2.
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