Abstract:
The possibility of quantitation of 76 pesticides in products(apples) in 8 minutes with the help of quadrupole mass detector was demonstrated.
Introduction
Helium has been widely used as a carrier gas in GC-MS. However, the use of hydrogen as an alternative carrier gas has increased recently due to ever increasing cost of helium, eliminated need for plumbing and other infrastructure to deal with cylinders as well as some other significant advantages. The sensitivity of the GC-MS analysis using Hydrogen carrier gas is comparable to that for Helium, while the speed of chromatography can be much increased, reducing the time per analysis. Besides, Hydrogen can be produced by a hydrogen generator, which is much more convenient compared to heavy and bulky helium cylinders, yet there is no transportation cost involved any more.
Another benefit of Hydrogen use is preservation of the Electron Multiplier emission properties resulting in its longer lifetime and, thus, reducing cost of GC-MS ownership.
GC-MS detector components are optimized for both helium and hydrogen operation, and do not require any hardware change or adjustment by a user. Long-term tests have shown sufficient reliability, safety and reproducibility of Q-Tek GC-MS in operation with hydrogen as a carrier gas.
Experimental
Samples of apples were obtained from a supermarket and mashed into puree to simulate blank matrix. Calibration levels – including blank 0; 0,005; 0,01; 0,025; 0,05; 0,1 ug/ml were prepared by spiking a mixture of 80 pesticides into the apple puree matrix, which did not contain residual amounts of pesticides prior to. Then samples were extracted with Acetonitrile by QuEChERS method. The sensitivity of the single quadrupole MS detector operated in SIM mode is far enough to analyze samples without pre-concentration. Thus, after the matrix clean-up step to remove interfering components of the matrix in extracts, the samples were ready to inject into GC-MSD.
The initial GC method developed for Helium carrier gas was optimized for Hydrogen to obtain sufficient linear velocity of the carrier gas for best separation. Using hydrogen carrier gas, it is recommended to use 20-25m long capillary columns with ID in range of 0.1-0.2 mm. This would provide an optimal linear velocity range, high intensity and symmetry of the peaks, and significantly reduce the analysis time. The peaks become narrower, therefore chromatographic separation parameters do not suffer (fig 1). The GC-MSD method parameters are outlined in Table 1, Retention time and monitored ions for target pesticide compounds used for data acquisition are listed in Table 2.
Fig.1 3-fold reduction of GC separation time after switch from He to Hydrogen carrier gas.
Table. 1. Parameters of the instrumental method.
| Q-Tek GC-MS system configuration | |
| GC-MS system | Q-Tek GC-MS |
| inlet | Split/Splitless (liner: single taper with wool) |
| GC column | -1MS Ultra (25m x 0,20mm; 110um) |
| GC Method parameters | |
| Injected Sample volume | 1 uL |
| Inlet mode | Pulsed Splitless; 30 psi 0.75 min, |
| Inlet temperature | 250°C |
| Carrier gas flow mode | Constant Flow H2 |
| Oven program | 80°C (0.75 min);
40,0°C/min up to 150°C, 30,0°C/min up to 300°C hold 1,0 min; |
| Carrier gas flow and type | (Hydrogen) 1,5 mL/min |
| GC-MS transfer line temperature | 280°C |
| Detector settings | |
| Ion source | Inert, Electron impact (EI) |
| Ion source temperature | 230°C |
| Solvent delay | 2,0 min |
| Data Acquisition mode | SIM (see below details) |
| Dwell Time | Automatically calculated by iDwell® Time |
Table. 2. SIM table of the data acquisition method.
Fig.2 Extracted Ion Chromatograms of pesticide compounds at their concentration of 0.01 µg/ml.
Results and discussion
Reducing the column length from 30m to 25m and the inner diameter from 0.25 mm to 0.2 mm led to decrease of analysis time from 24 to 8 minutes, compared to previously used separation conditions for helium. The Q-Tek GC-MS mass-spectrometric detector tolerates flow rate of carrier gas (hydrogen) up to 2 ml / min. Figure 2 shows Extracted Ion Chromatograms of target pesticides at their concentrations of 0.01 µg/ml. 6-level calibrations for Chlorpyrifos, Propargite, Tau-fluvalinate, Deltametrine and O-Phenylphenol are shown in Fig 3 below . Table 3 shows the linearity of 6-level matrix calibrations (coefficient of determination R2) .
Table. 3. Experimental calibration linearity values.
Fig.3 6-level calibrations for Chlorpyrifos, Propargite, Tau-fluvalinate, Deltametrine and O-Phenylphenol.
Organic components co-extracted with the analytes during sample preparation cause the “matrix” effect. Apples, for example, contain antioxidants (fig.5) that interfere with detection of some target pesticides.
Fig.5 Matrix effect from antioxidants present in apples.
As can be seen from the chromatogram of O-Phenylphenol, the peak at 170 m/z is split due to the presence nearby of intense peak of 2,4-di-tert-butylphenol (fig.6). However, the calibration dependence for this analyte has a high linearity value R2 > 0.998 (Table. 3).
Fig 6. Matrix interference on O-Phenylphenol peak @ m/z=170.
Conclusion
The results confirm that the single-quadrupole GC-MSD instrument can successfully analyze more than 75 compounds in less than 8 minutes using hydrogen as a carrier gas. Despite the extremely short chromatographic separation time for rather high number of target analytes, it is possible to achieve sufficient separation and high sensitivity of the analytical method at LOQ level near 0.005 mg/kg. Thus, significant savings in both time as well as analysis cost can be achieved without compromising detection capability of the method. Long-term tests in the laboratory demonstrated good reproducibility of the method.
Q-Tek GC automatic leak detection system shuts off the carrier gas supply as soon as the hydrogen leak has been detected to ensure operation safety.
