Linearity/Breakthrough GC


Column: J & W GS-Al/KCl 50 m x 0.53mm with 5m 0.18 mm fused silica on MS end acting as a restrictor


Pressure: 18 psi Mode: Constant pressure


Oven program: 60°C (4 min) then 8°C/min to 150°C (0 min) Total run time: 15.25 min


Figure 2 illustrates system linearity for increasing volumes of 15 ppb level gas standards, with R2


volumes, for example, up to 25ml and 150ml respectively for CF4 and N2O.


values


of 0.99 for C2F6 and SF6. CF4 and N2O also had R2 values of 0.99 up to their respective breakthrough


samples and impacts the estimated minimum detection limit.


System Reproducibility


System reproducibility was determined at three different sample volumes: 25, 150 and 1000ml. Six samples of a 70 ppt standard dilution were taken at


Caused by large nitrogen/oxygen peak


C2F6 CF4 MS


Quad temperature: 150°C Source temperature: 230°C Full scan range: 10 – 300 amu


SIM ions (dwell time): 30 (100), 69 (100), 119 (20), 127 (100)


Sample Preparation


A four component 1 ppm custom gas standard in nitrogen was used to fill a 10L gas sampling bag. Successive dilutions of the bag contents with nitrogen were used to reach lower concentrations.


Conclusion Results


The excellent peak shape of a 25ml 100 ppb standard sample can be seen in Figure 1.


C2F6 CF4


Figure 3b. Chromatogram showing extracted SIM ion 127 for 0.1 ppb SF6 in real air. Sample volume was 25ml; 1L sample used for detection limit data.


C2F6


Thermal desorption offers an effective alternative to traditional solvent extraction techniques, providing superior sensitivity and lower detection limits. These features make the technique ideal for the analysis of CFC and HCFC air toxics in canisters and when combined with conventional GC/MS TD offers on- or off-line cryogen-free analysis of ultravolatile


greenhouse gases such as CF4 (bp: - 128°C), at levels below 50 ppt in air. The higher breakthrough volumes


of other ultravolatile compounds tested (C2F6 and SF6) facilitate even lower detection limits in these cases –


namely <<1 ppt for SF6 and C2F6. The estimated lower detection limit for N2O is 200 ppt. This is due to high background levels of the selected ion in real air samples. Though unavailable as a standard, the


SF6


reduced volatility of CF3Cl relative to CF4 would enable limits of detection significantly below 5 ppt for this compound, using the analytical system and conditions described.


N2O


The excellent detection limits and method performance data shown here demonstrate that cryogen-free on- and off-line monitoring of ultra- volatile greenhouse gases in air can be achieved at the lowest concentrations of interest.


Figure 1. Extracted ions 69 (black), 119 (blue), 172 (green) and 30 (red) from a full scan analysis of 25 ml of a 100 ppb gas standard.


Figure 3c. Chromatogram showing extracted SIM ion


30 for 400 ppb N2O in real air. Sample volume was 25ml; 150ml sample used for detection limit data.


Minimum Detection Limits The minimum detection limits were determined with a


25 ml sample volume for CF4 and 1L for C2F6 and SF6. Due to problems with absorption and carryover of N2O in the gas sampling bag, no reliable data could be obtained at low concentrations for this compound.


Figure 2. Linearity plots for CF4, C2F6, N2O and SF6 using a 15 ppb standard concentration, full scan


extracted ion.


Detection limits for N2O were therefore determined by extrapolating S:N data at higher concentrations. The MS was run in selected ion monitoring (SIM) mode to give the greatest sensitivity.


Table 2. Sampling and detection limit data, determined in SIM mode for standards in nitrogen (std) and real air samples (air). *Detection limit extrapolated from higher concentration data.


Figure 3 shows example data at low levels in real air. Table 2 gives the sample volume, sampling flow, lowest measured detection limits, root mean square (RMS), signal:noise (S:N) and estimated minimum detection limit (with S:N >3:1) for standards in nitrogen (Std) and real air samples (Air). The high S:N values even at low concentrations are due to the very


low noise levels of the SIM ions, the exception is N2O where the 30 ion background is high in real air


References and associated explanatory notes


1. Directive 2003/87/EC: EU Directive establishing European Union Emission Trading System (EU ETS). This is the largest multinational, emissions trading scheme in the world covering CO2 and methane. As of January 2008, the European Commission proposed a number of changes to the scheme, including the inclusion of other specified greenhouse gases, such as nitrous oxide and


perfluorocarbons.These changes are likely to come into effect from January 2013 onwards.


2. CARBON POLLUTION REDUCTION SCHEME: ‘Australia's low pollution future’ White Paper, 15 December 2008.


3. 3rd Indicative Occupational Exposure Limit Value (IOELV) Directive Update, European Commission, 2007


4. EN ISO 16017: Air quality – Sampling and analysis of volatile organic compounds in ambient air, indoor air and workplace air by sorbent tube/thermal desorption/capillary gas chromatography. Part 1: Pumped sampling.


5. ASTM D6196-03: Standard practice for selection of sorbents, sampling and thermal desorption analysis procedures for volatile organic compounds in air.


Figure 3a. Chromatogram showing extracted SIM ion


69 for 0.3 ppb CF4 and 0.1 ppb C2F6 in real air. Sample volume was 25 ml; 1L sample used for C2F6 detection limit data.


Table 3. Compound % RSD values at 3 different volumes; SIM mode; 70 ppt diluted standard. *determined at 15 ppb.


each volume. N2O % RSD values were obtained from 150ml of a higher concentration standard because of the sorption and carryover effects observed with lower level standards using gas sampling bags. Table 3 shows the % RSD (n=6) for the different volumes.


Values are not reported for CF4 at 150 and 1000ml as these volumes are above its breakthrough volume.


Applications were performed using the stated analytical conditions. Operation under different conditions, or with incompatible sample matrices, may impact the performance shown.


Spectroscopy Focus


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