Fast GC-MS/MS Analysis of Polyaromatic Hydrocarbons using Waters Quattro micro GC

This application note demonstrates fast GC-MS/MS analysis of polyaromatic hydrocarbons using Waters Quattro micro GC.

Fast GC

GC-MS analysis of polyaromatic hydrocarbons (PAHs) is recognized as one of the most sensitive methods for the analysis of these persistent organic pollutants (POPs). PAHs are easily resolved using standard GC columns without a requirement for derivatization. Most separations can be achieved in less than 30 minutes using columns such as 30 m 0.25 mm ID 0.25 μm df 5% phenyl polysiloxane type phases. The use of a narrow bore, thin film column allows an increase in chromatographic resolving power, coupled with a reduction in analysis time.

As the column ID is reduced, the column efficiency increases. For example, a column of 0.25 mm ID has 2,500 plates per meter, whereas a column of 0.1 mm ID has 6,700 plates per meter. The injection volume is critical to avoid overloading of the GC Column. Figure 1 shows that injecting half the volume of a standard onto the column can give greater intensity and better chromatography. The figure shows benz(a)anthracene and chrysene acquired under fast GC conditions, using 0.5 μL and 1 μL injection volume onto a 0.18 mm ID GC Column.

Fast GC-MS/MS

The column used for this analysis was a DB5-ms 20 m 0.18 mm ID 0.18 μm df Column with a constant He flow rate of 0.7 ml min^-1, installed in an Agilent 6890 GC oven, directly interfaced to a Waters Quattro micro GC Mass Spectrometer operated in EI+ mode. The GC oven was fitted with a fast GC oven insert, and was configured in the fast oven ramping mode. All injections were made in pulsed splitless mode with an injection volume of 0.5 μL, using a purge time of 0.4 minute, 0.4 min pulse (280 kPa). The GC temperature ramp employed was: - 50 °C/0.4 min, 100 °C min^-1 to 90 °C, 65 °C min^-1 to 190 °C, 50 °C min^-1 to 265 °C, 40 °C min^-1 to 310 °C, hold 4 minutes. The GC ramp and column combination employed results in peak widths of between 0.9 and 3 seconds, with a run time of less than 9 minutes. All critical pairs were suitably resolved under these conditions. Figure 2 shows the separation for phenanthrene and anthracene (top trace) and benzo(b)fluoranthene, benzo(k)fluoranthene and benzo[a]pyrene (bottom trace).

Figure 2 also shows the number of scans across each peak with a minimum of 8 data points per peak. This is possible due to the ability of the Quattro micro GC to acquire with dwell times and interchannel delay times of 10 ms. The top chromatogram shows 12 data points across a GC peak 1.2 seconds wide; while acquiring 3 MRM channels simultaneously. This exceeds the quantitative requirement for a minimum of 8 data points.

All US EPA 16 PAHs were separated and determined with the last eluting peak eluting at 8.21 minutes. Excellent linearity was observed over a concentration range from 0.25 pg to 250 pg on column, with all LODs (based upon peak to peak signal to noise of 3:1) being 0.25 pg or less. Table 1 presents the r² values, retention times and LODs for the US EPA 16 PAHs. The linear dynamic range is affected when acquiring fast GC because the loading capacity of the column is exceeded at lower concentrations due to the narrow peak widths. Comparing the LODs determined with those acquired under ‘normal’ chromatographic conditions, fast GC operation can be seen to give a significant sensitivity improvement. The calibration curves acquired were linear up to 250 pg, but above this concentration, the dynamic range of the column is exceeded when acquiring under these conditions.

The MRM transitions optimized for the PAHs are given in Table 1. All transitions were optimized with a collision cell gas pressure of 3e^-3 mbar (Argon). In some cases, parent to parent MRM transitions are quoted. In these cases, a third transition has been optimized, because parent to parent transitions may not always offer adequate selectivity in the presence of a matrix, especially the hydrocarbon matrix often encountered during PAH analysis.

The comparative sensitivity for injections of a spiked 0.1 μgL^-1 canal water extract is shown in Figures 3 and 4. Figure 3 shows a 1 μL injection of the sample extract acquired under standard chromatographic conditions. Figure 4 shows a 0.5 μL injection of the same extract acquired using the fast GC conditions described above. It can be seen that the sensitivity has more than doubled, while maintaining good chromatographic resolution.

This example of polyaromatic hydrocarbon analysis using fast GC chromatographic conditions clearly demonstrates the ability of the Quattro micro GC to acquire data using very short dwell times. All data was acquired using 10 ms dwell and inter-channel delay times when using fast chromatographic conditions, with the exception of the last eluting peaks, where longer dwell times could be utilized due to the broader chromatographic peaks. This is enabled by the ability of Waters MassLynx Software control to allow flexible dwell times within an acquisition function. This allows the user to maximize the time spent monitoring transitions when necessary. Fast GC conditions can be used to enhance the sensitivity of a method. However, this enhancement comes at the cost of reduced dynamic range due to column loading capacities.

The data presented here shows that it is possible to determine all US EPA 16 PAHs within a 9-minute analytical run time, without compromising chromatographic resolution.