GC-TOFMS Application: Examining detection sensitivity of EI/FI and PI ion sources using nitrogen as a GC carrier gas

  • Summary

MSTips No.299

General

The global helium shortage is a very serious issue that is affecting a variety of scientific research organizations that utilize this gas for their work. In particular, gas chromatography-mass spectrometry (GC-MS) is a widely used analytical technique that traditionally uses helium as the GC carrier gas. Consequently, it has become critical to identify other carrier gases for GC-MS in order to minimize the effects of the helium shortage. Previously, we changed the GC-MS carrier gas from helium (He) to nitrogen (N2) to determine how this affects sensitivity when using the dedicated EI ion source. The results showed that the detection sensitivity for N2 as the GC carrier gas was 1/18 of the level when compared to He.

In this work, we compare sensitivity levels when using the combination EI/FI and EI/PI ion sources when He and N2 are used as the GC carrier gas.

Experiment

Table 1 shows the measurement conditions. The GC column used for the analysis was a DB-5MS (30 m × 0.25 mm × 0.25 μm). He and N2 carrier gases were used for the measurements. Samples used were octafluoronaphthalene (OFN 100 pg / μL), hexadecane (10 ng / μL), and benzophenone (100 pg / μL). One μL of each was injected to evaluate the EI/FI ion source in EI+ and FI+ modes and the EI/PI ion source in the PI+ mode, respectively.

Table 1. Measurement conditions

Instrument JMS-T200GC
Injection mode Splitless
Injection Volume 1 μL
Column DB-5MS (Agilent Technologies), 30 m × 0.25 mm, 0.25 μm
Oven temp. 40°C (1min) - 30°C / min – 250°C (2min)
Carrier Flow He 1.0 mL / min, N2 0.55 mL / min
Ion Source temp. 250°C

Results

As a starting point, the EI/FI combination ion source was tested with both He and N2 to determine their affects on sensitivity. Figure 1 shows the extracted ion chromatogram for 100 pg of OFN (m/z 271.976±0.01) for each carrier gas when using EI+ mode for this combination EI/FI source. Interestingly, the OFN S/N ratio was nearly equal for both gases, which was different from the findings reported previously for the dedicated EI ion source in which the sensitivity level declined when the N2 carrier gas was used. These results suggest that the EI/FI ion source, with its open ion volume design, reduces space charging from nitrogen ions inside the ion source, which in turn prevents a decline in sensitivity.

Figure 1

Figure 1. EIC for 100 pg of OFN in EI+ mode with EI/FI ion source using He and N2 as the GC carrier gas

Figure 2 shows the extracted ion chromatogram for 10 ng of hexadecane (m/z 182.073±0.01) when He and N2 carrier gases were used in the FI+ mode for the EI/FI ion source. The S/N ratio of hexadecane was nearly equal for both gases and this observation is again likely related to the open design of the EI/FI ion source as well as to the fact that FI does not ionize N2.

Figure 2

Figure 2. EIC for 10 ng of hexadecane in FI+ mode with EI/FI ion source using He and N2 as the GC carrier gas

Finally, Figure 3 shows the extracted ion chromatogram for 100 pg of benzophenone (m/z 226.265±0.01) when He and N2 carrier gases were used in the PI+ mode for the EI/PI ion source. The S/N ratio for both carrier gases was nearly equal for this measurement as well. In this case, despite the EI/PI source having an enclosed ion volume design, PI+ mode does not ionize N2, thus minimizing space charging effects from the nitrogen ions that would result in poor sensitivity.

Figure 3

Figure 3. EIC for 100pg of benzophenone in PI+ mode with EI/PI ion source using He and N2 as the GC carrier gas

Summary

Sensitivity levels were examined for the EI/FI ion source in EI+ and FI+ modes and for the EI/PI ion source in PI+ mode when He and N2 were used as the GC carrier gas. For the open EI/FI ion source, there was little or no difference in sensitivity in the FI+ mode, where ionization efficiency of the carrier gas was low. There was also no difference in sensitivity in the EI+ mode, where charge accumulation/space charging could be a concern from nitrogen ion formation. With the closed design of the EI/PI ion source, there was also little or no difference in sensitivity when He and N2 were used as a GC carrier gas due to the fact that N2 ionization efficiency is low in PI+ mode. These results indicate that N2 can be used as an alternative GC carrier gas for the EI/FI and EI/PI combination sources with minimal loss in sensitivity for EI+, FI+, and PI+ modes.

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