Analytical Instrument Documents

DART mass spectrometry commonly uses helium as the DART gas. With the looming helium shortage, other gases are being evaluated for DART.

Some of the characteristic compounds that are responsible for the bright colors of autumn leaves are readily detected by using various ambient ionization methods with the AccuTOF-DART mass spectrometer system.

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.

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.

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.

DART provides very simple mass spectra that are free of multiple charging and alkali metal cation adducts such as [M+Na]+  and [M+K]. This facilitates identification of target compounds in mixtures and simplifies assignment of elemental compositions for unknowns.