Introduction

Natural rubber is used in a wide range of products from daily necessities such as shoe soles and hoses to industrial products such as tires. It is important to control off-flavor components in natural rubber, because the sap of natural rubber trees has a strong odor. In general, the SCAN/SIM measurement by HS-GC/MS is used for this analysis. However, it has possibility that the SIM measurement is affected by matrix components, because some natural rubbers contain many matrix components. Therefore, it is difficult to analyze off-flavor components contained in trace amounts by using the SIM measurement. On the other hand, the SRM measurement of GC/MS/MS use a combination of selected precursor ion and generated product ion from its selected precursor ion, so selectivity of detection ion and detection sensitivity will be improved. As a result, the influence of matrix components can be reduced, and it is possible to detect components contained in trace amounts. In this application, we report the analysis of off-flavor components in natural rubber by using HS-GC/MS/MS.

Measurement

The measurement was performed using a MS-62071STRAP trap-type HS and GC triple quadrupole mass spectrometer JMS-TQ4000GC UltraQuad™ TQ. Natural rubber (Sample A and B) was used as the measurement sample. Off-flavor components content of sample A is larger than that of Sample B. The amount of measurement sample was reduced from 200 mg to 50 mg, because the combination of chemical trap mode of JEOL's HS and SRM measurement enable more sensitive measurement. The measurement conditions is shown in Table 1, and the target components and their SRM transition are shown in Table 2.

Table 1 Measurement condition

HS condition
Sample Temp. 150°C
Sampling mode Trap mode
Heating time 20 min
Trap tube AQUATRAP1 (GL Sciences Inc.)
GC condition
Column VF-5MS(30 m length, 0.25 mm i.d., 0.25 μm film thickness)
Inlet Split/Splitless
Inlet Temp. 250°C
Flow 2 mL/min, Constant flow
Injection Mode Split (20 :1)
Oven Program 40°C (3 min) → 3°C/min → 100°C (1min) → 8°C/min → 250°C
MS condition
Ion Source Temp. 200°C
Interface Temp. 250°C
Ionization Mode EI+, 70 eV
Measurement Mode SCAN/SRM
Mass range m/z 10-500
Collision Gas N2, 10%

Table 2 SRM transition

Component R.T.(min) Quantitative ion Reference ion 1 Reference ion 2
Acetic acid 1.7 60->43 CE:5 60->45 CE:10 60->60 CE:5
Isovaleric aldehyde 2.1 44->43 CE:15 58->57 CE:10 58->58 CE:5
Propionic acid 2.6 74->55 CE:15 74->73 CE:15 57->57 CE:5
Isobutyric acid 3.6 73->55 CE:10 88->73 CE:15 73->73 CE:5
Toluene 3.7 91->65 CE:15 92->91 CE:15 92->92 CE:5
Butyric acid 4.5 60->42 CE:10 73->55 CE:10 60->60 CE:5
Isovaleric acid 6.2 60->42 CE:15 87->69 CE:10 60->60 CE:5
Valeric acid 7.9 60->42 CE:10 73->55 CE:10 60->60 CE:5
Skatole 28.6 130->77 CE:20 130->130 CE:10 131->130 CE:15
Result SCAN measurement result of sample A and B

The TICC and EIC of sample A are shown in Fig. 1. The chromatogram peaks of the target component were clearly observed by SCAN measurement, because sample A content a lot of off-flavor components. Similarly, the chromatogram peaks of sample B were confirmed at the same RT, although the peak intensity was low.

No. Component
1 Acetic acid
2 Isovaleric aldehyde
3 Propionic acid
4 Isobutyric acid
5 Toluene
6 Butyric acid
7 Isovaleric acid
8 Valeric acid
9 Skatole

Fig. 1 TICC and EIC of Sample A

NIST library search result of sample A and B

The M.F. value in the NIST library search result is shown in Table 3, and the mass spectra of peak No. 8 and peak No. 9 are shown in Fig. 2. Sample A has a high M.F. value of 820 to 938 due to the higher concentration of off-flavor components. On the other hand, sample B contain lower concentration of off-flavor components, and more than half of the NIST library search results showed low M.F. value or no result for the target component. As shown in Fig. 2, the mass spectra of peak No. 8 and peak No. 9 obtained from sample B were significantly different from those of sample A and the NIST library data. The chromatogram peaks observed from sample B were mainly composed of matrix components. Since single QMS measurements such as SCAN and SIM are expected to be greatly affected by matrix components, SRM measurement, which can suppress the effect of matrix components, might be a suitable for measurement method.

Table 3 NIST library search result

No. Component M.F.
Sample A Sample B
1 Acetic acid 932 892
2 Isovaleric aldehyde 932 933
3 Propionic acid 845 731
4 Isobutyric acid 868 725
5 Toluene 938 940
6 Butyric acid 822 618
7 Isovaleric acid 900 853
8 Valeric acid 859 -
9 Skatole 820 -

Fig. 2 The difference of mass spectra by influence of contaminant

Repeatability of peak area of each component in Sample B

The repeatability of peak area value obtained by SCAN measurement is shown in table 4, and the repeatability of peak area value obtained by SRM measurement is shown in table 5. The result of SCAN measurement showed overall high coefficients of variation especially Skatole and Butyric acid. This may be due to lower peak intensity or the influence of matrix components. On the other hand, the result of SRM measurement showed good repeatability and a coefficient of variation range from 0.1 to 8.6% due to the suppression of the influence of matrix components and improved sensitivity.

Table 4 Repeatability of peak area by SCAN

Component n = 1 n = 2 n = 3 Ave. STDEV C.V.(%)
Acetic acid (m/z 60) 26727058 23448580 30907431 27027690 3738502 13.8
Isovaleric aldehyde (m/z 58) 37894217 36797432 30177488 34956379 4174815 11.9
Propionic acid (m/z 74) 1118609 1169287 1238393 1175430 60128 5.1
Isobutyric acid (m/z 73) 441535 476884 494190 470870 26838 5.7
Toluene (m/z 91) 2066383 2268404 2248838 2194542 111419 5.1
Butyric acid (m/z 60) 540842 638348 465206 548132 86801 15.8
Isovaleric acid (m/z 60) 1623757 1962426 1671893 1752692 183223 10.5
Valeric acid (m/z 60) 505776 500742 414297 473605 51424 10.9
Skatole (m/z 130) 16467 10732 13107 13435 2882 21.4

Table 5 Repeatability of peak area by SRM

Component n = 1 n = 2 n = 3 Ave. STDEV C.V.(%)
Acetic acid (60→43) 48198138 46441666 46676467 47105424 953573 2.0
Isovaleric aldehyde (44→43) 31611384 32652805 33453197 32572462 923531 2.8
Propionic acid (74→55) 1620757 1524801 1530497 1558685 53831 3.5
Isobutyric acid(73→55) 3658550 3502348 3510171 3557023 88012 2.5
Toluene (91→65) 8652579 8161044 8422329 8411984 245931 2.9
Butyric acid (60→42) 2326212 2132262 2092492 2183655 125049 5.7
Isovaleric acid (60→42) 4355087 4349861 4359477 4354808 4814 0.1
Valeric acid (60→42) 2327284 2243168 2132341 2234264 97776 4.4
Skatole (130→77) 50718 43135 44845 46233 3977 8.6
Confirmation of Valeric acid and Skatole in sample B by SRM measurement

The SRM chromatogram peaks of Valeric acid and Skatole are shown in Fig. 3(a) and(b). Regarding Valeric acid and Skatole that the mass spectra of the target component could not be obtained by SCAN measurement, the observed peaks can be confirmed as the target component based on the multiple monitor ions and the relationship between precursor ion and product ion.

Fig. 3 SRM chromatogram peaks of valeric acid (a) and skatole (b)

Conclusion

The SCAN/SRM measurement of GC/MS/MS is an effective measurement method for the off-flavor analysis in materials with many matrix components. In particular, the SRM measurement use a combination of selected precursor ion and generated product ion from its selected precursor ion, so selectivity of detection ion and detection sensitivity will be improved. As a result, the influence of matrix components can be reduced, and it is possible to detect components contained in trace amounts. In addition, HS-GC/MS/MS measurement combined with JEOL's HS enables higher sensitivity analysis and reduces the amount of sample for measurement.

Acknowledgments

We would like to thank Mr. Unno and Ms. Yoshitani of Sumitomo Rubber Industries, Ltd. for providing samples for the preparation of this application note.

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Jeol Ltd. published this content on 09 April 2024 and is solely responsible for the information contained therein. Distributed by Public, unedited and unaltered, on 09 April 2024 07:23:02 UTC.