MALDI Application: Visualizing fragmentation channels of polyethylene oxide with different end groups using the JMS-S3000 SpiralTOF™ with TOF–TOF option

  • Summary

MSTips No.279

Tandem mass spectrometry is a powerful tool for polymer characterization. It can obtain information about polymer end groups, repeating structures (linear, cyclic, or branched), and copolymerization. High-energy collision–induced dissociation (HE-CID) is a fragmentation method that is available only in tandem time-of-flight mass spectrometry (TOF–TOF). The informative fragmentation channels, which are difficult to observe with commonly used low-energy CID, are often observable in HE-CID spectra. In MSTips 270, we proposed a method to visualize this abundant structural information and enable intuitive analysis using the "Remainders of KM" (RKM) plot method. In this report, we applied the method to analyze polyethylene oxide (PEO) with different end groups.

Experiment

We dissolved polypropylene glycol HO (C2H4O)nH, polyethylene glycol monolauryl ether HO (C2H4O)nC12H25, and polyoxyethylene monocetyl ether HO (C2H4O)nC16H33 in methanol (10 mg/mL). We used α-cyano-4-hydroxycinnamic acid (α-CHCA; 10 mg/mL in methanol) and sodium trifluoroacetate (NaTFA; 10 mg/mL in methanol) as matrix and cationizing agent, respectively. The sample, α-CHCA, and NaTFA were mixed at 1:10:1 (v/v/v), spotted on a target plate, and air dried. Product-ion spectra were acquired in positive-ion mode using JMS-S3000 SpiralTOF™ with the TOF–TOF option. We analyzed the data using msRepeatFinder version 3.0.

Product ion spectra

All product-ion spectra showed strong sodium-adduct ions [M+Na]+. The HE-CID spectra were acquired by selecting [HO(C2H4O)28H+Na]+ (m/z 1273.7), [HO(C2H4O)nC12H25+Na]+ (m/z 1265.8) and [HO(C2H4O)nC16H33+Na]+(m/z 1277.9) as precursor ions (Fig. 1). The red arrows in each product-ion spectrum in Figure 1 represent the mass difference corresponding to 44u (C2H4O). Several fragmentation channels with 44u interval were observed in each product ion spectrum; however, it is time consuming to assign each series one by one. Also note that the differences between the three polymers cannot be understood intuitively.

Fig.1 Product ion spectra of three types of polyethylene oxides.

Figure 1. Product ion spectra of three types of polyethylene oxides.

In Figure 2 below, product ion spectra of three types of PEO are overlaid in an RKM plot (base unit EO). We excluded the precursor and sodium ions from the display region in order to clarify the fragmentation channels of the repeating structures. In this figure, the peak colors of [HO(C2H4O)nH+Na]+, [HO(C2H4O)nC12H25+Na]+, and [HO(C2H4O)nC16H33+Na]+ are overlaid as blue, red, and green, respectively. The peak series displayed in black denote fragment ions commonly observed in all three types of PEOs. The fragmentation peak series in the RKM plot of Figure 2 are divided into three characteristic parts, each of which is confirmed in Figures 3–5, respectively.

Fig.2 Overlaid RKM plot (base unit: propylene oxide C2H4O from the repeat unit list) of three types of polyethylene oxides.

Figure 2. Overlaid RKM plot (base unit: propylene oxide C2H4O from the repeat unit list) of three PEO types

First, we focused on the black series (Fig. 3), i.e., the series commonly observed in all product ion spectra. These two series were considered to be fragmentation channels that did not include different end-group structures. We confirmed them to be the fragmentation channels shown in Figure 3, which we observed as [M+Na]+.
Fig.3 Common peaks in RKM plot of three types of polyethylene oxides.

Figure 3. Common peaks in RKM plot of three types of polyethylene oxides.

Second, we focused on the series of red and green (Fig. 4). These four series were considered to be fragmentation channels, which included different end-group structures. We confirmed these as the fragment channels shown in Figure 4, which we observed as [M+Na]+. Third, we focused on the series with slope (Fig. 5), which we observed in the part near the precursor ion (right side of RKM plot). "Series with slope" are series without 44u intervals. They have 14u (CH2) intervals and are assigned to be the alkyl chain cleavage, which is a unique fragmentation channel in HE-CID.

Conclusion

It is easy to visualize the fragmentation channels of polymer series using an RKM plot. By overlapping product-ion spectra obtained from precursor ions with different end groups in an RKM plot, fragmentation channels common to all PEOs can be distinguished from fragmentation that is dependent on end-group composition. This is a proven method for helping to analyze different series of end groups observed in the mass spectra of polymers in real samples.

Fig.4 Peaks only observed in each types of polyethylene oxides.

Figure 4. Peaks that are dependent on end group composition.

Fig.5 Peaks originated from charge remote fragmentation of the alkyl chain in the end group.

Figure 5. Peaks originating from charge-remote fragmentation of the alkyl chain in the end group.

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