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INTRODUCTION

Charge exchange CI was among the ionization types studied

decades ago during work by researchers investigating fundamental

characteristics of chemical ionization1. However, it was not

subsequently widely adopted for use in vacuum source mass

spectrometry. This is likely due, at least in part, to practical

difficulties associated with maintaining the proper reagent gas

density to promote efficient charge exchange in a vacuum sourced

mass spectrometer. Atmospheric pressure ionization sources on the

other hand have a very high gas capacity allowing optimum

conditions for charge exchange CI without straining the vacuum

system of the mass spectrometer2. The atmospheric pressure gas

chromatography (APGC) ionization source discussed here is able to

be operated in charge exchange mode using nitrogen as the

reagent gas. It is possible to employ other reagents such as CS2 or

benzene for charge exchange ionization however, the following

discussion will only address nitrogen mediated charge exchange. It

is also possible, through the choice of various reagents and

application of different operating conditions, to generate

protonated species or to perform hydrogen deuterium exchange

(HDX) experiments. These latter approaches have been the subject

of previous coverage3.

Despite having a sound understanding of the principles of charge

exchange, the relatively new implementation of APGC led to the

need to thoroughly investigate source conditions for optimized

performance. Since the source was designed to minimize the

number of critical operating parameters only a small number of

these needed to be investigated. Parameters most likely to effect

the ionization were determined to be the corona pin position and

current in addition to the cone gas and auxiliary gas flow rates.

Although it is certain that varying the gas composition would also

have an effect on ionization, these experiments were limited to the

use of nitrogen. The results of the systematic study of these

conditions were subsequently applied to the analysis of real world

samples.

Characteristics of Charge Exchange Chemical Ionization (CI) in an Atmospheric Pressure Gas Chromatography (APGC) Source and Applied Uses

Steven Lai1, Rhys Jones2, David Douce2 , Jody Dunstan2, Douglas Stevens3

1. Waters Corporation, Beverly, MA 01915 2. Waters Corporation, Manchester, UK M23 9LZ 3. Waters Corporation, Milford, MA 01757

METHODS

APGC analysis was performed on a variety of samples and standards. Samples for the determination of petroleum biomarkers

and dioxins were examined as well as a standard for US EPA Method 8270. Samples were analyzed on QTof (Xevo G2 QTofTM), a QTof ion

mobility hybrid (Synapt G2-STM) and tandem quadrupole (Xevo TQ-STM) instruments using the APGC source option operated in nitrogen mediated charge exchange mode which is also referred to as ‘dry

source’ mode.

References

1. Munson, B. Chemical ionization mass spectrometry, Analytical Chemistry, 43 (13), pp 28A– 43A DOI:

10.1021/ac60307a042, 1971

2. McEwen, C.; McKay R. A combination atmospheric pressure LC/MS:GC/MS ion source: Advantages of

dual AP-LC/MS:GC/MS instrumentation, JASMS, 16 (11), pp 1730 - 1738 DOI: 10.1016/

j.jasms.2005.07.005, 2005

3. Stevens, D.; Hsu, C.S. Atmospheric Pressure Gas Chromatography Mass Spectrometry (APGC/MS) for Novel Applications in Petroleum and Chemical Research, Oral presentation at 242nd American Chemical

Society National Meeting, Denver, CO 2011

4. Douce, D.; Abohlin, P.; Sapawi, A.A. The Identification of Source Environment from the Analysis of

Rock Oil Extracts Utilizing the High Selectivity of Triple Quadrupole Gas Chromatography/Mass

Spectrometry, Waters document #720000982, 2004

5. Hancock, P.; Dunstan, J.; Hall, K.; Harriman, G. Investigating Source, Age, Maturity, and Alteration

Characteristics of Oil Reservoirs Using APGC/MS/MS Analysis of Petroleum Biomarkers. Waters

document #720004645en, 2013

RESULTS AND DISCUSSION

During the qualification of the instrument a thorough investigation of

the APGC source conditions was performed on a StepWaveTM

equipped QTof ion mobility hybrid instrument (Synapt G2-STM).

Optimum corona pin position and corona pin current in addition to source gas flows were determined. Initial work investigated the corona pin location and its effect on sensitivity for a range of

compounds from the US EPA 8270 mixture.

CONCLUSIONS

Systematic investigation of optimum charge

exchange conditions for the APGC source led to the determination of optimum operating conditions and

provided insight into the interplay between parameters.

By combining charge exchange ionization with MSE acquisition mode, it was possible to acquire accurate

mass measured intact molecular ion and fragmentation data while also providing the ability to

compare fragmentation data, generated through CID of M+. molecular ions, to NIST EI library spectra.

Since the molecular ion formed using charge

exchange CI is the same form as molecular ions formed by EI (when present at all) then the product

ions generated upon CID are shown to be predominantly the same.

Petroleum biomarker analysis performed using EI

GC/MRM and atmospheric pressure charge exchange

CI GC/MRM was demonstrated to be able to use the same MRM transitions for the determination of cyclic

biomarkers.

Sensitive detection of TCDD was demonstrated using charge exchange with APGC operated in MRM mode

by applying the same transitions as used with EI GC/MRM.

Figure 3. Two primary mechanisms applied in APGC positive ion

mode (I) Charge Exchange and (II) Protonation. Note that the mo-lecular ion formed under charge exchange conditions, M+., is the

same form as is observed using EI if a molecular ion is observable in the EI spectrum at all.

Figure 8. EI GC/MS/MS data from vacuum sourced instrument

(Quattro micro GCTM). This is a source rock extract of a sample from Southeast Asia that has a marine source input and was analyzed in

20044. The abundance and distribution of C30 steranes support additional evidence in the data of a marine source input.

Figure 1.

Top: Xevo TQ-S tandem quadrupole MS system with LC and GC

capability (set up shown in the GC mode of operation).

Bottom left: APGC attachment leading into the source.

Bottom right: APGC connection close up photographs (source chamber (top) and heated exchange interface (bottom)).

LC GC

Figure 9. APGC GC/MS/MS data from tandem quadrupole (Xevo TQ-

STM). This is a North Sea oil analyzed in 2012. The abundance and distribution of C30 steranes support additional evidence in the data

of a marine source5. Note the improved separation of the final four C30 sterane isomers which aids in determination the state of degra-dation of the sample as well.

Figure 10. APGC MRM chromatograms generated on Xevo TQ-S for

primary and secondary transitions for 2,3,7,8-TCDD. The table below the chromatograms shows MRM transition used for EI MRM analysis for the same compound on a vacuum sourced GC/MS/MS instrument

(Quattro micro GCTM).

Figure 4. Response of charge exchange molecular ion with

varying corona pin position.

Figure 11. %RSD results for APGC MRM acquisitions (n=42) on

Xevo TQ-STM demonstrating 5%RSD average for the analysis of diox-ins and dioxin-like analytes.

Figure 2. Cut away view of the APGC ionization/reaction chamber showing orientation of components during GC/MS operation. Nitro-

gen gas flow path is indicated by blue arrows. Z and black arrow in-dicate the direction of adjustment for the corona pin position investi-gation. Yellow arrow indicates direction of cone gas flow. Auxiliary

flow is a second nitrogen gas inlet which adds gas directly to the source environment.

Figure 5. Heat map of selected analytes from a standard for US EPA

Method 8270 showing the optimum (green) corona current setting for relative response of the charge exchange form of the molecular

ion for each compound.

APGC MSE fragmentation spectrum

NIST 2011 EI spectrum

These observations show that reduced auxiliary gas flow maintains

charge exchange conditions, while an increase in auxiliary gas flow can introduce moisture and hence contribute to protonation effects

in the spectrum. Increased cone gas stabilizes the residence time of the compound in the corona region producing optimal charge exchange conditions and thereby increasing sensitivity.

These conditions were applied to the acquisition of a range of

standards to investigate collision induced dissociation (CID) fragmentation and its similarities with standard EI spectra. Through the use of the MSE acquisition mode concurrent fragmentation and

intact molecular ion information is produced. This provided evidence that precursor ions created by charge exchange produce fragment

ions similar to fragments in an EI spectrum. In Figure 7, the fragmentation spectrum from the MSE acquisition is

shown for a compound from the US EPA 8270 standard with the NIST 2011 library search result. While not all analytes will return

this level of correspondence between the two ionization types, it is common for charge exchange APGC spectra to show considerable

similarity with EI data. A later study of product ion and MRM data further supported the MSE results.

Direct application of MRM transitions, previously determined using EI/MS/MS, was investigated in APGC/MS/MS analysis of petroleum

biomarkers as shown in Figures 8 and 9.

Figure 4 shows that for a range of chemical components, the

optimum corona pin position for charge exchange was the same. Intensity response maximized at 1 to 1.5 mm from the center point

of the sample cone orifice showing the pin location was important. It can relate to capillary column position and is therefore required to be optimized experimentally.

Corona current was another variable investigated to determine

optimal instrument response. The following table shows the variation in charge exchange conditions (green color, as protonation increases the color changes from green, to yellow and then red) with the

highest signal intensity (normalized to 100). The majority of compounds produce the best charge exchange intensity at

approximately 5µA, with a slight preference for a higher corona current at early eluting compounds and a lower corona value for a late eluting component. A compromise value of 5µA would be

appropriate in this analysis.

Finally the gas flows within the source environment were investigated

simultaneously with the cone gas and the auxiliary gas altered independently, resulting in the 3D contour plots shown in Figure 6. In

this example, three sample groups (compounds with similar contour plots were grouped to illustrate an averaged response) show quite similar signal intensities with the maximum charge exchange

response to be shown at low auxiliary gas flow (50L/Hr), and an elevated cone gas flow of 200 to 225 L/Hr (light blue areas). In

addition these parameters show the best signal responses are also found at these gas flow rates.

Figure 6. Three dimensional contour plots showing relationship

between cone gas and auxiliary gas settings with corresponding normalized experimental data.

Z

N2+●

N2e-

2e-

2N2

N4+● M●+

MCorona Pin

M●+

M

(I) Charge Transfer

“Dry” source conditions

Favoured by relatively non-polar

compounds

N2+●

N4+●

H2O

H2O+●

H2O

H3O+●

+OH●

[M+H]+

M(II) Protonation

Modified source conditions eg. with H2O / MeOH

present

Favoured by relatively polar compounds

Corona Pin

(I) Charge Exchange

M+.

M+.

EI+ Data

APGC+ Data

The similarity in fragment ions from the EI and APGC/CID systems

was also used in the analysis of dioxins and furans, as demonstrated in Figures 10 and 11.

Figure 7. Comparison of high energy MSE data from APGC acquisi-

tion on Xevo G2 QTof (upper spectrum) to EI library spectrum for 4-bromophenyl phenyl ether from NIST 2011 (lower spectrum).