HILL RESEARCH LABIsoprene Analysis by Ion Mobility Spectrometry-Mass Spectrometry

INTRODUCTION

RESULTS

Isoprene is one of the most important naturally emitted volatile organic compounds (VOC) in the atmosphere. It accounts for more than a third of globally annual VOC emissions. It plays a significant role in atmospheric photochemistry through facilitating the production of pollutants such as ground level ozone and secondary organic aerosols.Detecting isoprene in ambient air requires continuous measurement with sub ppbv detection limit, a large linear dynamic range, and a high selectivity. For micrometeorlogical flux measurements atop forest canopies, fast instrument response of 10 analysis per second is necessary.This study aims to demonstrate that IMS alone can be used for field flux measurements to rapidly detect and separate isoprene from monoterpenes.

METHODS

Figure 6. Mass-selected ion mobility spectra of isobaric isoprene dimer, -pinene and myrcene at 200ºC (Rp 75)

and 150ºC (Rp 85)

Head space sample vapor was directed into the ion mobility spectrometer (IMS) by a flow of nitrogen gas and ionized by secondary electrospray ionization (SESI).The samples analyzed were isoprene, myrcene, -pinene, R-(+)-limonene, and 3-carene. The structures of these compounds are shown on the right. The ion mobility spectrometry-mass spectrometry (IMS-MS) system consisted of an electrospray ionization (ESI) source, an IMS coupled to a 150 QC ABB Extrel quadrupole mass spectrometer (QMS), or an Ionwerks orthogonal time-of-flight mass spectrometer (TOF-MS).

Experimental data demonstrated the ability of IMS to separate isoprene and monoterpenes.The monomeric isoprene (K0 = 2.35 cm2 V-1 s-1 ) was observed for the first time by IMS (Figure 2).Dimer, trimer, and tetramer ions of isoprene were also detected. Pentamer ions were observed at higher sample concentrations (Figure 3).The mass-mobility trend line of isoprene ions is shown in Figure 4.The mobility time of the four monoterpenes analyzed were very similar (Figure 5). Separation between monoterpenes would require ultra-high resolution-IMS.As the IMS temperature decreased, the resolving power (Rp) of IMS increased, allowing for the separation of the isobaric isoprene dimer and monoterpenes (Figure 6).

Figure 4. 2D-IM-MS plot of isoprene ions, shown with

mobility drift times and mass-to-charge ratios

Figure 2. Mass-selected ion mobility spectra of isoprene at

low concentration

Figure 3. Mass-selected ion mobility spectra of isoprene at

high concentration

Isoprene 68Da

Myrcene136 Da

-Pinene136 Da

R-Limonene

136 Da

Figure 5. Ion mobility spectra of isobaric

monoterpenes at 200ºC

3-Carene136 Da

CONCLUSION

Inte

nsity

181614121086Drift Time (ms)

69 m/z

137 m/z

205 m/z

237 m/z

50403020100Drift Time (ms)

Inte

nsity

205 m/z

137 m/z

273 m/z

341 m/z

Inte

nsity

161412108Drift Time (ms)

Isoprene Dimer-PineneMyrcene

mixture(b) 150ºC

Inte

nsity

14121086Drift Time (ms)

Isoprene Dimer-Pinene (Intensity x5)Myrcene (Intensity x2)

(a) 200ºC

Inte

nsity

16128Drift Time (ms)

16128

(a)Myrcene

(b)-Pinene

137 m/z

137 m/z

273 m/z

273 m/z

273 m/z

137 m/z

137 m/z

273 m/z

(c)3-Carene

(d)Limonene

Department of Chemistry Hill Research Group

Ion Mobility Spectrometry

50 75 100 125 150 175 200 225 250

14500

13000

11500

10000

8500

7000

5500

m/z

Dri

ft T

ime

(mic

ro-s

econ

d)

Monomer

Dimer

Trimer

Figure 1. Ion mobility spectrometer coupled to a time-of-flight mass spectrometer

ABOUT THE PROJECT

A stand-alone IMS has the capability of separating isoprene from monoterpenes, with fast signal response that is suitable for micro-meteorlogical flux measurements.

This is a multidisciplinary project in collaboration with Dr. Hal Westberg and Jack Chen, from the Department of Civil and Environmental Engineering.The project involves measuring and modeling biogenic Volatile Organic Compounds (BVOC) emissions from vegetation. The goal is to understand the role of BVOC in regional atmospheric chemistry and terrestrial carbon exchange related to the global carbon cycle and climate change.

Camping trip with collaborators