Radical cation formation in characterization of novel C3-symmetric disks and their precursors by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

False results caused by solvent impurity in tetrahydrofuran for MALDI TOF MS analysis of amines

Tetrahydrofuran (THF) is one of the most frequently used solvents in the MALDI TOF MS analysis of synthetic compounds. However, it should be used with caution because a trace amount of 4-hydroxybutanal (HBA) might be generated and accumulated in THF during storage. Since only a tiny amount of analytes is required in MALDI MS measurements, a trace amount of HBA might have a significant effect on the MS results. It was found that HBA will quickly react with primary and secondary amino compounds, leading to false results about the sample composition with an extra series of ions with additional mass of 70 Da in between. The formation of HBA can be inhibited by butylated hydroxytoluene (BHT) antioxidant. Therefore, when THF is required as the solvent for sample preparation, it is strongly recommended to use a BHT-stabilized one, at least for the analysis of compounds with amino groups.

A pitfall of using 2-[(2E)-3-(4-tert-butylphenyl)-2-methylprop-2-enylidene]malononitrile as a matrix in MALDI TOF MS: chemical adduction of matrix to analyte amino groups

2-[(2E)-3-(4-tert-Butylphenyl)-2-methylprop-2-enylidene]malononitrile (DCTB) has been considered as an excellent matrix for matrix-assisted laser desorption/ionization (MALDI) of many types of synthetic compounds. However, it might provide troublesome results for compounds containing aliphatic primary or secondary amino groups. For these compounds, strong extra ion peaks with a mass difference of 184.1 Da were usually observed, which might falsely indicate the presence of some unknown impurities that were not detected by other matrices. On the basis of the possible mechanisms proposed, these extra ions are the products of nucleophilic reactions between analyte amino groups and DCTB molecules or radical cations. In these reactions, an amino group replaces the dicyanomethylene group of DCTB forming a matrix adduct via a -C=N-bond. An aliphatic primary amine could react easily with DCTB and the reaction could start once they are mixed in a MALDI solution. For an aliphatic secondary amine, on the other hand, the reaction most likely occurs in the gas phase. Protonation of amino groups by adding acid seems to be a useful way to stop DCTB adduction for compounds with one single amino group, but not for compounds with multiple amino groups. Unlike aliphatic primary or secondary amines, aliphatic tertiary amines and aromatic amines do not yield DCTB adducts. This is because tertiary amines do not have the required transferrable H-(N) atom to form an extra -C=N-bond, while aromatic amines are not sufficiently nucleophilic to attack DCTB. In view of the possible matrix adduction, care should be taken in MALDI time-of-flight mass spectrometry (TOF MS) when DCTB is used as the matrix for compounds containing amino group(s).

Matrix-enhanced secondary ion mass spectrometry (ME SIMS) using room temperature ionic liquid matrices

Room temperature ionic liquids (ILs) have many applications including as matrices in MALDI. We wished to investigate the efficacy of ILs as matrices in time-of-flight secondary ion mass spectrometry and in mass spectrometric imaging (MS imaging). Two ILs derived from alpha-cyano-4-hydroxycinnamic acid (CHCA) were synthesized and tested using phospholipids, cholesterol, and peptides. The molecular ion intensities of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE), cholesterol, and bradykinin were greatly increased using IL matrices. Further, detection limits were also improved: for DPPC and DPPE detection, limits were at least 2 orders of magnitude better using IL matrices. However, these IL matrices were not effective for the enhancement of angiotensin I ions. The data also indicate that IL matrices are suitable for imaging MS. The IL matrices did not cause changes to the sample surface via matrix crystallization or other processes; no "hot spots" were observed in the mass spectra. As a demonstration, an onionskin membrane was imaged. In the matrix-enhanced MS images, ions characteristic of proteins and other biomolecules were observed which could not otherwise be observed. Clearly ionic liquids deserve further investigation in SIMS and MS imaging.

Titanium dioxide anatase as matrix for matrix-assisted laser desorption/ionization analysis of small molecules

The use of inorganic species as assisting materials in matrix-assisted laser desorption/ionization (MALDI) analysis is an alternative approach to avoid interfering matrix ions in the low-mass region of the mass spectra. Reports of the application of inorganic species as matrices in MALDI analysis of small molecules are, however, scarce. Nevertheless, titanium dioxide (TiO(2)) powder has been reported to be a promising matrix medium. In this study we further explore the use of TiO(2) as a matrix for the MALDI analysis of low molecular weight compounds. We present results showing that nanosized TiO(2) anatase and TiO(2) rutile perform better as MALDI matrices than a commercial TiO(2) anatase/rutile mixture. Moreover, when using nanosized TiO(2) anatase as a matrix, high-quality mass spectra can be obtained with strong analyte signals and weak or non-existing matrix interference ions. Furthermore, our results show that the phase type plays an important role in the application of TiO(2) as a MALDI matrix.

Dehydrogenation of tertiary amines in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

In the matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI TOF MS) analysis of various compounds synthesized in our laboratory, strong [M - H]+ ion peaks were often observed for the molecules with tertiary amino groups. In this work, the MALDI TOF MS behavior of two groups of compounds that incorporate tertiary amino moieties was investigated. One group is bisurea dimethylanilines (BUDMAs) prepared for the study of molecular recognition in thermoplastic elastomers, and the other group is the poly(propylene imine) diaminobutane dendrimers. The results clearly demonstrate the appearance of the [M - H]+ ions. In order to understand the possible mechanisms for the generation of these ions, a series of model compounds, ranging from primary to tertiary amines, were investigated. Unlike the tertiary amines, no [M - H]+ ion peaks were recorded for the primary amines, and only barely detectable ones, if any, for some secondary amines. It appears that the tertiary amino groups play an important role in the formation of these ions. In addition to MALDI TOF MS analysis, these samples were also applied to electrospray ionization (ESI) MS where no [M - H]+ ions were observed. The results indicate that the generation of [M - H]+ ion is due to the unique MALDI conditions and is likely to be formed via dehydrogenation of a protonated tertiary amine resulting in an N=C double bond. The absence of [M - H]+ ion peaks for the primary and secondary amines is probably because upon their formation these ions could easily transfer one proton to the corresponding amines in the MALDI gas-phase plume, yielding neutral imines that cannot be detected by MS.

Double cation adduction in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry of electron deficient anthraquinone derivatives

Six anthraquinone derivatives were analyzed using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI TOF MS). Clear (pseudo) molecular ions were observed for all the compounds. Interestingly, for some derivatives, strong ions with double cation adduction were also recorded in the positive mode. It is remarkable that all these ions are singly charged. In this work, possible mechanisms for the double cation adduction were investigated and discussed. It appears that the double cation adduction was due to the electron deficient nature of the derivatives, and formed by taking up two singly charged cations and one electron. Substituents on the anthraquinone ring were found to have a significant effect on the double cation adduction. In contrast, no considerable influence of the acidity of MALDI matrix/solution was observed, even on the double proton adduction. Furthermore, it was demonstrated that double cation adduction might occur in the MALDI gas-phase plume. In addition to the anthraquinones, three more electron deficient compounds of different types, i.e. a perylene bisimide derivative (PB), 3,7-decanoylamino-4,8-dihydrobenzo[1,2-b:4,5-b']dithiophene-4,8-dione (TQ) and 6,6-phenyl C61-butyric acid methyl ester (PCBM), were also analyzed with MALDI TOF MS. The results indicate that the 'abnormal' double cation adduction might be a 'normal' phenomenon in the MALDI TOF MS analysis of many electron deficient compounds.