Negative Atmospheric Pressure Chemical Ionization Research Articles

Negative atmospheric pressure chemical ionisation low-energy collision activation mass spectrometry for the characterisation of flavonoids in extracts of fresh herbs

The flavonoid composition of commonly eaten fresh herbs such as dill, oregano and parsley was analysed by combined LC, MS and low-energy collision induced dissociation (CID) MS–MS. Negative atmospheric pressure chemical ionisation (APCI) MS and MS–MS were used to provide molecular mass information and product-ion spectra of the glycosyl compounds. The most prominent fragment was found to arise from the aglycone ion, which provides molecular mass information about the glycosyl substituent and the aglycone. Product-ion spectra of the aglycone verified the identity by comparison with product-ion spectra of authentic standards. Methoxylated flavonoids provide characteristic fragmentation, i.e., loss of ·CH 3, which add to the usefulness of the method for identifying unknown flavonoids. Negative-mode APCI-MS is thus demonstrated to be a good alternative to the commonly employed positive mode operation.

Rapid screening procedures for the hydrolysis products of chemical warfare agents using positive and negative ion liquid chromatography–mass spectrometry with atmospheric pressure chemical ionisation

Qualitative screening procedures have been developed for the rapid detection and identification of the hydrolysis products of chemical warfare agents in aqueous samples and extracts, using liquid chromatography–mass spectrometry with positive and negative atmospheric pressure chemical ionisation (APCI). Previously reported screening procedures, which used positive APCI or electrospray ionisation (ESI), were modified by using LC conditions that allowed acquisition of positive and negative ion mass spectra. APCI was generally found to be more robust than ESI, probably due to variable adduct ion formation with ESI, depending on the condition of the sample and the system. Negative APCI provided selective detection of acidic analytes and allowed facile differentiation of alkyl alkylphosphonic acids from isomeric dialkyl alkylphosphonates. The combination of positive and negative APCI, using a C 18 column and water–methanol mobile phase modified with ammonium formate, provides a rapid screening procedure for chemical warfare agent degradation products, with limits of detectability in the range 10–100 ng/ml. In the case of proficiency test samples, where analyte concentrations are in the range 1–10 ppm, introduction of the sample by infusion may provide an even faster preliminary screening procedure.

Effect of high-performance liquid chromatography mobile phase components on sensitivity in negative atmospheric pressure chemical ionization liquid chromatography-mass spectrometry

We have investigated the effect of several common buffers (10-mM formic acid, 10-mM ammonium acetate, and 100-mM ammonium acetate) on the ionization of a series of model compounds that are amenable to negative atmospheric pressure chemical ionization to determine the extent of ionization quenching that can occur. In addition, we have compared the sensitivity of these standard mobile phases to a mobile phase that does not contain an acidic buffer component, but rather a base (N-methylmorpholine). The results showed that, as expected, the sensitivity for the test analytes was greatest in the mobile phase that lacked acidic components. In general, ionization of analytes that contained a single, more weakly acidic functional group was inhibited to a greater degree by more strongly acidic buffer components. In some cases, ionization was quenched completely by acidic buffer components, Ionization of compounds that were more strongly acidic was quite good in all mobile phases tested. Differences in the ionization efficiencies of the analytes in each mobile phase were correlated with the gas-phase reagent ions present. As a point of reference, each of the analytes also was analyzed in the positive ion mode and the signal intensities were compared to those obtained in the negative ion mode. In addition, the utility of mobile phases that contained N-methylmorpholine for chromatographic separations was demonstrated.

Applications of a versatile technique for trace analysis: atmospheric pressure negative chemical ionization.

The ability to use ambient air as a carrier and reagent gas in an atmospheric pressure chemical ionization source allows instantaneous air analysis to be combined with hypersensitivity toward a wide variety of compounds. The TAGA (Trace Atmospheric Gas Analyser) is an instrument which is designed to use both positive and negative atmospheric pressure chemical ionization (APCI) for trace gas analysis; this paper describes several applications of negative APCI which demonstrates that the technique is not limited to environmental monitoring. Examples are described which suggest that the TAGA can be used for the detection of illicit drugs and explosives, and for the analysis of breath or skin emissions, as well as for air pollution measurements. The applications are not restricted by the use of ambient air as a reagent gas; addition to the air carrier of various gases allows specific reagent ions such as Cl- or Br- to be generated. Furthermore, in certain situations pure gas carriers can be used to provide even more flexibility in the ion chemistry, with a short term absorber-desorber system used to transfer the sample from the ambient air into the ion source region. The potential uses for APCI are expanding continuously as the understanding of the complex ion-molecule chemistry grows. This paper underlines the complementary relation between the development of new negative chemical ionization (NCI) techniques and practical applications using the TAGA system.