Gerard Kwakkenbos

In-source decay of hyperbranched polyesteramides in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Hyperbranched polyesteramides (DA2), prepared from hexahydrophthalic anhydride (D) and diisopropanolamine (A) have been characterized, by use of matrix-assisted laser desorption/ ionization time-of-flight mass spectrometry (MALDI-TOF-MS), field desorption (FD)-MS, and electrospray ionization (ESI)-MS. MALDI of polyesteramides produces protonated molecules. The spectra show a complex chemical composition distribution and end-group distribution which are mainly composed of two series of homologous oligomers DnAn+1 − mzH2O and DnAn − mH2O, where m = 1–2. Signals from protonated molecules DnAn+1 and DnAn are almost absent in the MALDI spectrum, whereas these ions are responsible for the base peak of DnAn+1 − mH2O and DnAn − mH2O (m = 1–2) clusters in the ESI spectrum. The absence of −OH end-groups signals in the MALDI spectrum is due to a metastable decay of protonated DnAn+1 and DnAn ions in the ion source of the MALDI mass spectrometer prior to ion extraction. In-source decay results in the formation of protonated lower DnAn+1 − mH2O and DnAn − mH2O oligomers and their corresponding neutrals, leading to wrong conclusions concerning the relative end-group distribution as a function of the degree of polymerization and the chemical composition.

Selection of buffers and of an ion-pairing agent for thermospray liquid chromatographic—mass spectrometric analysis of ionic compounds

Abstract The applicability of ammonium formate, ammonium acetate and ammonium bicarbonate as volatile electrolytes for thermospray liquid chromatographic—mass spectrometric analysis of three decomposition products of α-aspartame is demonstrated. With these buffers, the pH range of the mobile phases for silica-based reversed-phase columns can be covered. With respect to the buffer capacity and ultraviolet transparency, a buffer salt concentration of 50 mM was used in the mobile phase. The retention times of the compounds studied in the volatile buffers were comapared with those obtained in sodium acetate and sodium phosphate buffers and were found to match quite well. For the reversed-phase ion-pair separation of α-aspartame and β-aspartame in combination with thermospray mass spectrometry, a method is described using trifluoroacetate as pairing ion. Liquid chromatographic—mass spectrometric analysis of l -aspartyl- l -phenylalanine shows that the highest signal-to-noise ratio is obtained in ammonium formate. For the compounds studied, no correlation could be found between the signal-to-noise ratio and source temperature, or between signal-to-noise ratio and vaporizer temperature.

Volatile ion-pairing agents for liquid chromatographic—thermospray mass spectrometric determination of amino acids and amino acid amides

Abstract The behaviour of volatile ion-pairing agents in terms of performance of the combined liquid chromatography—mass spectrometry is described. The applicability of heptafluorobutanoic acid, tridecafluoroheptanoic acid and nonadecafluorodecanoic acid as volatile ion-pairing agents is demonstrated by the analysis of mixtures of amino acids and the corresponding amino acid amides. Translation of a non-volatile to a volatile ion-pair system that is suitable for thermospray mass spectrometry is described. Using nonadecafluorodecanoic acid, the retention times of the compounds studied were comparable to those obtained with n -dodecylsulphonic acid. Further, with nonadecafluorodecanoic acid, the repeatability of the tests and the calibration graphs of the compounds investigated were good. The sensitivity of the mass spectrometer towards the compounds of interest was greatly improved by postcolumn addition of trifluoroacetic acid and ionization with gaseous ammonia. Using nonadecafluorodecanoic acid, the method was applied to the analysis of samples from bio-organic synthesis.

Thermospray liquid chromatography/mass spectrometry study of diastereomeric isoindole derivatives of amino acids and amino acid amides.

A thermospray liquid chromatography/mass spectrometry (TSP-LC/MS) method is described for determination of the enantiomeric excess of alpha-amino acids and alpha-amino acid amides as their o-phthalaldehyde/N-acetyl-L-cysteine (OPA/NAC) derivatives. The source temperature is an important factor in optimizing the sensitivity of the TSP-LC/MS analysis, whereas the repeller voltage is of minor importance. On-column mass spectra were acquired for the OPA/NAC derivatives of several alpha-amino acids and alpha-amino acid amides. For the main fragment ions, mass spectra fragmentation pathways are proposed. The applicability of the method is demonstrated by means of the enantiomeric excess determination of valine in a sample from an enzymatic hydrolysis experiment. Using single ion monitoring, the detection limit of D-valine in the presence of excess L-valine is 10 pmol. The present TSP-LC/MS method is useful for validating the results obtained from LC/UV or LC/fluorescence methods for the enantiomeric excess determination of alpha-amino acids and alpha-amino acid amides.

Low-molecular-weight model study of peroxide cross-linking of ethylene-propylene-diene rubber using gas chromatography and mass spectrometry II. Addition and combination reactions.

The dicumyl-peroxide-initiated addition and combination reactions of mixtures of alkanes (n-octane, n-decane) and alkenes [5,6-dihydrodicyclopentadiene (DCPDH), 5-ethylidene-2-norbornane (ENBH) and 5-vinylidene-2-norbornane (VNBH)] were studied to mimic the peroxide cross-linking reactions of terpolymerised ethylene, propylene and a diene monomer (EPDM). The reaction products of the mixtures were separated by both gas chromatography (GC) and comprehensive two-dimensional gas chromatography (GCxGC). The separated compounds were identified from their mass spectra and their GC and GCxGC elution pattern. Quantification of the various alkyl/alkyl, alkyl/allyl and allyl/allyl combination products shows that allylic-radicals comprise approximately 60% of the substrate radicals formed. The total concentration of the products formed by combination is found to be independent of the concentration and the type of alkene. The total concentration of the products formed by addition to the alkene increases with increasing concentration of alkene. In addition, the total concentration of the formed addition products depends strongly on the type of the alkene used, viz. VNBH>ENBH approximately DCPDH, which is a consequence of differences in steric hindrance of the unsaturation. The peroxide curing efficiency, defined as the number of moles of cross-linked products formed per mol of peroxide, is 173% using 9% (w/w) 5-vinylidene-2-norbornane (VNBH). This indicates that the addition reaction is recurrent. All these findings are consistent with experimental studies on peroxide curing of EPDM rubber. In addition, the present results provide more-detailed structural information, increasing the understanding of the mechanism of peroxide curing of EPDM. The described approach to use low-molecular-weight model compounds followed by GC-mass spectrometry (MS) and GCxGC-MS analysis is proven to be a very powerful tool to study the cross-linking of EPDM.

Chemical ionization with gaseous ammonia for normal-phase liquid chromatographic—thermospray mass spectrometric applications

Abstract Addition of gaseous ammonia to a thermospray ion source, followed by filament-on ionization, provides chemical ionization reagent ions. The abundance of reagent ions is sufficient to obtain spectra which contain predominantly [M + NH 4 ] + ions. The advantage of this method of ionization lies in normal-phase LC—MS applications, where “traditional” ionization methods such as volatile buffer, filament-on or discharge-on ionization yields either no ionization or too much fragmentation, making identification very difficult or impossible. After modification of a Finnigan TSQ 70 ion source, the source pressure and effluent flow were optimized. The formation of [M + NH 4 ] + ions is dependent on the voltage applied to the repeller electrode. Low voltages yield spectra with predominantly [M + NH 4 ] + ions, whereas high voltages yield spectra showing mostly fragmentation and a small percentage of chemical ionization. The formation of the [M + NH 4 ] + ion of cyclohexanone is linear over two decades ranging from 0.2 to 20 ng of the compound injected into a normal-phase column. Higher concentrations give rise to a deviation of the linear relationship. Quantitative results are obtained, with a repeatability of 8% (R.S.D.).

Red-oils in ethylene plants: formation mechanisms, structure and emulsifying properties

In most ethylene plants, the caustic sections suffer from red-oils formation which results from the polyaldol condensation of some oxygenate species in basic media. These red-oils lead to solid material deposition which is generically referred to as fouling and which can cause severe energy losses or operational issues. This specific red-oil fouling can be successfully mitigated by regularly washing the caustic scrubber with a hydrocarbon washing stream which acts as a solvent. However, such washing streams can be further rerouted to the quench water system, increasing the risk of emulsion formation due to its amphiphilic components, i.e., red-oils, compromising the safe operation of the whole plant. The complexity of red-oil structures renders its analysis and characterization challenging. Consequently, there has been no study showing the effect of the type of hydrocarbon wash on the red-oils structure and its potential emulsifying properties. This study describes an in-house developed analytical technique based on UPLC-High resolution QTOF which allows characterizing hydrocarbon streams with complex mixtures of oxygenated species. This method has been successfully applied to monitor red-oil structures during a plant trial where Pyrolysis Gasoline and Toluene-Xylene hydrocarbon washes were successively applied. The data revealed that reactive compounds from the Pyrolysis Gasoline react with the polyaldol species, likely through a Diels–Alder mechanism, thus increasing the diene adduct concentration in the red-oil. It was further found that toluene-xylene washes hindered the emulsifying properties of the red-oils more effectively than Pyrolysis Gasoline washes, in that they could best inhibit the formation of polyaldols adducts.

Direct formation of an organonitrogen compound from a molybdenum nitrido species

The molybdenum nitrido complex 15NMo[N(R)Ar]3 (where R = C(CD3)2CH3, Ar = 3,5-C6H3Me2) reacted with the anhydride of trifluoroacetic acid at room temperature to afford the correspondent organonitrogen compound in almost quantitative yield without the necessity of using additional reagents to achieve the C-N coupling.