Determination of C1-C4 alkanes by ion mobility spectrometry

Abstract

Flight instrumentation for the analyses of extraterrestrial environments must often perform under severely restricted conditions. Often, the detection and identification of a multitude of chemical species is required to fulfill the scientific objectives of the mission. It is therefore important that the analytical instrumentation have universal response. The gas chromatograph-ion mobility spectrometer (GC-IMS) has the potential to provide the sample separation, identification and sensitivity necessary for a successful analysis. However, the IMS has poor sensitivity for the C1-C4 alkanes. The abundance of these molecules at various extraterrestrial sites is often of great importance to exobiologists. This study focuses on optimizing IMS sample ionization mechanisms for the C1-C4 alkanes and other hydrocarbons of interest to exobiology.

Atmospheric pressure chemical ionization of alkanes, alkenes, and cycloalkanes

“…Until recently, the low proton affinities of alkanes were considered responsible for poor response in IMS [1], suggesting comparatively inefficient proton transfers from IMS reactant ions such as (H20)nH+ (n = 2 to 4). Trace detection limits and high quality mobility spectra have been obtained for alkanes when the values for n in (H20)nH+ have been reduced to ""1 with low moisture in helium [2] and high temperatures in air [3].…”

Capabilities and limitations of ion mobility spectrometry for field screening applications

“…These vapors completely eliminated the response of the IMS, producing a stable baseline, as shown in the lower tracing of this figure. When low levels of NH 3 were added to the spectrometer, a strong response for NH 3 was observed, demonstrating that IMS can be used as a sensitive and selective detection method for gas-phase NH 3 .…”

“…Hydrocarbon responses can be achieved in IMS when water is eliminated from the ionization region and the drift gas of the IMS. 3 Because trace amounts of water are so difficult to eliminate from the spectrometer, it is common practice to add water at the 10-ppm level to ensure a reproducible response in the IMS. When IMS is used to measure samples in air, humidity can be a problem, because the variable concentration of water in the air sample can change the response characteristics of the detector.…”

Mercury-induced fragmentation of n-decane and n-undecane in positive mode ion mobility spectrometry

“…Using soft ionization techniques (in most commercial devices this is a radioactive Ni-63 or H-3 source, other techniques involve electrospray or corona discharge) the ionization of the analyte normally leads to formation of singly charged analyte ions without fragmentation. Lowering this value to 100 ppbv (in dry helium, using a Ni-63 source) could increase the detection rate drastically 7 . proton bound dimers) form 1 .…”