Analytical Instrument Documents

It is not always easy to identify minor unknown components in complex LC/MS datasets. The new DART™ ion source screened for components that were not immediately recognized in LC/MS analysis of tea samples. LC/TOFMS datasets can contain high-resolution, exact-mass data for all ionized components of a complex mixture. Even with concurrent UV detection and chromatographic enhancement software, it is not always easy to identify all of the components that are present in the dataset. Furthermore, suppression effects may mask important information. Here, a new technique known as Direct Analysis in Real Time (DART™) was used to screen tea samples and provide elemental compositions for minor components that were “buried” in LC/MS data collected for tea analysis. DART is a powerful new ionization method that permits direct analysis of solid, liquid, or gas samples at atmospheric pressure and ground potential. DART has been applied to rapid in-situ analysis of a very wide range of materials ranging from drugs to explosives, foods, and beverages.

DART can be used to analyze polymers, cements, resins, and glues by increasing the gas temperature to 450-550° C to induce pyrolysis. This has been applied to a variety of polymers including Nylons, polypropylene and polyethylene, polyethylene terephthalate (PET), polyesters, poly(methyl methacrylate) (PMMA), polycarbonate, phenoxy resin, polystyrene, and cellulose. Examples are shown here for standard samples of Nylon, polystyrene, and cellulose.

A batch of contaminated welding wire received from a vendor by a customer was causing problems in a manufacturing process. Visual comparison of the clean and contaminated wire did not show any obvious differences, but the contamination was readily observed on backscatter electron images obtained with the JEOL IT300 scanning electron microscope.

The potential of the time-of-flight mass spectrometry (TOF-MS) to innovate the analysis of soft drinks is described using gas chromatography (GC) hyphenated to TOF-MS and a new type of ion source, direct analysis in real time (DART), coupled to high-resolution TOF-MS. Head-space solid-phase microextraction (SPME) was used to isolate/extract volatile compounds followed by GC–TOF-MS to identify tainted compound in contaminated soft drinks. Direct analysis in real time–time-of-flight mass spectrometry (DART–TOF-MS) was also used to obtain negative and positive ion profiles of different soft drinks to determine the presence of various compounds, including antimicrobial preservatives, artificial sweeteners, acidulants and saccharides, without any sample preparation and chromatographic separation.

GC/MS analysis can be carried out by connecting the GC output to the DART® ion source with a simple interface. Because the GC column is not introduced into vacuum, there are no restrictions on gas flow rates. No fragile electron filament is used. Conditions can be adjusted to produce chemical ionization (CI) mass spectra or mass spectra resembling electron ionization (EI) mass spectra.

Analyzing fiber samples has always been difficult by DART®. The problem has been that there is no easy way to hold the fiber in the gas stream without losing it into the vacuum system. A fiber can be secured in the DART gas stream with forceps or other means, but if the DART gas is too hot, the fiber can break off and be lost into the mass spectrometer vacuum system through the atmospheric pressure interface. A thermal desorption/pyrolysis stage (The Biochromato, Inc. “ionRocket™”) designed for use with DART produces highly reproducible thermal desorption profiles that show outgassing, additives, and high-quality pyrolysis DART mass spectra for materials. Because fiber samples placed in the disposable copper sample “pots” are not positioned directly in the DART gas stream, a single fiber can be analyzed without risk of loss into the vacuum system.

Exact masses have been used for decades to calculate elemental compositions for known and unknown molecules. The traditional approach calculates all possible combinations of user-specified atoms that fall within a given error tolerance of a measured mass. The number of possible combinations increases dramatically with increasing mass and as more atoms are included in the search set. In many cases, it is not possible to determine a unique composition based on mass alone. A common source of error in measuring isotopic abundances with scanning mass spectrometers is related to fluctuations in ion current during measurement. The AccuTOF family of mass spectrometers overcomes this problem by analyzing all of the isotopes formed at the same instant. Combined with a high-dynamic-range detector, this provides highly accurate isotopic abundances. It has been shown that accurately measured isotopic abundances can be combined with measured exact masses to dramatically reduce the number of possible elemental compositions for an unknown. It is often possible to deduce a unique elemental composition, facilitating the identification of unknown substances.

We examined different parts of a hot pepper to determine which part of the pepper contains the highest concentration of capsaicin. Different sections of the pepper were placed between the DART and the AccuTOF orifice. Little capsaicin was found in the fleshy part of the pepper; higher concentrations were found in the pepper seeds. The highest concentration of capsaicin was found in the membrane inside the pepper pod onto which the seeds are attached.

Organometallic compounds play an important role in chemistry, as recently recognized by the awarding of the 2005 Nobel Prize in Chemistry to Chauvin, Schrock and Grubbs. Characterization of organometallic compounds by mass spectrometry can sometimes be complicated by problems with solubility and reactivity. Electron ionization can be used for some voltatile organometallics. Fast atom bombardment (FAB) and electrospray ionization (ESI) are useful provided suitable solvents can be used. Field desorption (FD) is often effective, but FD emitters can be fragile and the analysis should be carried out by an experienced operator. DART (Direct Analysis in Real Time) complements these methods and provides an alternative; it is fast and does not require solvents. The sampling area is purged with an inert gas, reducing the likelihood of undesirable reactions. Further, AccuTOF-DART permits exact mass measurements without requiring the presence of a reference standard during the sample measurement. DART is extremely robust and does not require special operator training.

The AccuTOF™ equipped with Direct Analysis in Real Time (DART™) is capable of analyzing drugs in pills and capsules with no sample preparation. In most cases, the pill can simply be placed in front of the DART and the active ingredients can be detected within seconds. This application note shows a wide variety of pills that have been analyzed by using DART. The examples include prescription drugs, over-the-counter medicines, and illicit drugs that were confiscated by a law-enforcement agency.