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.
MeasurementThe 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 |
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
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
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 |
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)
ConclusionThe 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.
AcknowledgmentsWe would like to thank Mr. Unno and Ms. Yoshitani of Sumitomo Rubber Industries, Ltd. for providing samples for the preparation of this application note.
Plastics / Polymer
Chemistry
Related productsJMS-TQ4000GC Triple Quadrupole Mass Spectrometer
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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.