GC-TOFMS Application: JMS-T2000GC AccuTOF™ GC-Alpha Sensitivity in nitrogen carrier gas ③ - CI ion source

  • Summary

MSTips No. 376

Introduction

Due to the global shortage of helium gas supply, the demand for alternative gas for GC-MS carrier gas is increasing. Nitrogen gas is the most suitable gas due to its availability and high safety, but it is known that the influence of nitrogen ions generated by the MS ion source causes a decrease in sensitivity. So we have checked the influences of nitrogen carrier gas on JMS-T2000 GC AccuTOF ™ GC-Alpha, and report on MS Tips No. 374-376. This report shows the results of the CI (Chemical Ionization) ion source.

Measurement

Table 1 shows the details of the measurement conditions in this experiment. In the positive ion CI (CI+) method, 1 μL of benzophenone 100 pg / μL was injected. In the negative CI (CI-) method, 1 μL of OFN (octafluoronaphthalene) 10 pg / μL were injected. Helium and nitrogen were used as carrier gases, and the S/N sensitivity and mass accuracy (error) of molecular ions were compared. The carrier gas flow rate was set to 1.0 mL / min in helium and 0.6 mL / min in nitrogen based on the optimum linear velocity of each carrier gas.

Table 1 Measurement condition

GC : 8890GC (Agilent Technologies, Inc.)
Injection volume 1 μL
Mode Splitless
Column DB-5MS UI
(Agilent Technologies, Inc.)
30m x 0.25mm, 0.25μm
Oven temperature 50°C (1min) - 40°C/min
-250°C (2min)
Carrier flow He : 1.0 mL/min
N2 : 0.6 mL/min
TOFMS : JMS-T2000GC AccuTOF™ GC-Alpha
Ion source CI ion source
Ionization ①CI+, ②CI-
CI reaction gas Methane
Ionization energy
(filament current)
200eV (300μA)
Mass Range m/z 100-500
Detector voltage 2500V

Results ① CI+ method

Figure 1 shows the extracted ion chromatograms (m/z 183.08 ± 0.02) of the measurement result of benzophenone in the CI+ method. The sensitivity was decreased to about 1/2 in nitrogen. Since nitrogen is difficult to ionize in the CI method, which is soft ionization, the decrease in sensitivity was suppressed.

Figure 1. EICs of benzophenone (CI+ method)

Figure 1. EICs of benzophenone (CI+ method)

Figure 2 shows the mass spectra of the benzophenone measurement result in the CI+ method. Protonated ions [M+H]+ (m/z 183.0804) were strongly observed, and their mass errors were as good as 1 mDa or less in both results.

Mass spectra of benzophenone (CI+ method)

Figure 2. Mass spectra of benzophenone (CI+ method)

Results ② CI- method

Figure 3 shows the extracted ion chromatograms (m/z 271.99 ± 0.02) of the OFN measurement result in the CI- method. The sensitivity was improved about twice in nitrogen. Since it was difficult to ionize reaction gas impurity in addition to nitrogen in CI- method, which is soft ionization, it is considered that the sensitivity was improved.

Figure 3. EICs of OFN (CI- method)

Figure 3. EICs of OFN (CI- method)

Figure 4 shows the mass spectra of the OFN measurement result in the CI- method. The mass errors of the molecular ions M-・(m/z 271.9878) were as good as 1 mDa or less in both results.

Figure 4. Mass spectra of hexadecane (FI method)

Figure 4. Mass spectra of OFN (CI- method)

Conclusion

The influences of nitrogen carriers on the CI ion source of JMS-T2000GC AccuTOF™ GC-Alpha were checked. In the CI+ method, the sensitivity was decreased to about 1/2. In the CI- method, the sensitivity was not decreased. The mass errors of the molecular ions were as good as 1 mDa or less in both CI+ method and CI- method.

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GC-TOFMS Application

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