A REPLACEMENT FOR 63Ni IN ION MOBILITY SOURCES?

PULSED CORONA DISCHARGE: 

A REPLACEMENT FOR 63 Ni IN ION MOBILITY SOURCES? 

C. Hill & P. Thomas 

DIAS, UMIST, PO BOX 88, Manchester, M60 1QD, UK. 

Introduction 

Although 63 Ni is a stable and long lived ionisation 

system that requires no power, and promotes the 

formation of a wide range of product ions in the 

positive and negative its continued use is 

problematic. This is due to the regulations 

surrounding its manufacture, use, and disposal. 

Further, for the same reasons 63 Ni is not an option 

for the development of new ion mobility 

applications in environmental, clinical and the 

food science sectors. Pulsed corona discharge 

(PCD) sources have been developed as 

alternatives to 63 Ni. To investigate the reactant 

ion chemistry associated with a PCD source a 

switchable high resolution ionisation ion mobility 

spectrometer (SHRIMS; Graseby Dynamics) was 

interfaced to a Model C50 Quadrupole Mass 

Spectrometer (Extrel). The preliminary findings of 

this research are presented here showing that 

63 

Ni reactant ion chemistries may be obtained 

from a pulsed corona discharge in both the 

positive and negative modes of operation. 

Instrumentation 

The PCD was housed in the source of the 

SHRIMS, see Figure 1. 4 kV was applied to the 

repeller plate and then divided down on the 

SHRIMS by a resistor network to provide the 

source and extractor voltages along with the 

potentials for the field defining electrodes in the 

reaction region. The reaction region was 8 cm 

and had an electric field gradient of 250 V cm-1. A 

Bradbury-Nielson ion gate connected the reaction 

region to the drift tube, which was also 8 cm long. 

The gating signal was derived from the time the 

corona pulse was initiated. The gate width, the 

time the gate remains open for, and the gate 

delay the time between the corona striking and 

the gate being opened were controlled from the 

PCD control unit. The electric field gradient in the 

drift tube was 250 V cm -1 . 

The Faraday detector was constructed from gold 

foil with a 50 mm pin hole at its centre to allow the 

passage of a small proportion of the ions into the 

mass spectrometer for analysis. A Viton o-ring 

was used to make a seal between the Faraday 

detector and the first differential pumping stage of 

the mass spectrometer. Biasing voltages were 

applied to the Faraday plate and the skimmer 

cone. The ion signal underwent a two-stage 

amplification, initially it was passed through the 

head amplifier before being further amplified in 

the IMS Control Unit. 

Data collection was carried out using a National 

instruments PCI-6024E data acquisition card 

fitted to a PC running a Labview virtual 

instrument. The signal obtained from the Faraday 

plate detector and, the total number of ion counts 

from the channeltron mass spectrometer detector 

were plotted against drift time. This system was 

used to study positive and negative mode ion 

chemistries produced by the pulsed corona 

discharge. 

Positive mode studies: 

In the positive mode it was found that the reactant 

ion peak chemistry of air was very similar to that 

seen for the 63 Ni. Further studies with ammonia 

doping were undertaken with dipropyleneglycolmonomethylether 

(DPM) to identify the 

product ions formed, and to investigate the effect 

variable gate delay had on their formation in air. 

Received for review July 20, 2002, Accepted August 15, 2002 

Copyright © 2003 by International Society for Ion Mobility Spectrometry

C. Hill & P.Thomas: „PULSED CORONA DISCHARGE ...”, IJIMS 6(2003)1,4-8, p. 5 

7 

10 

6 

9 

1 

2 

3 

4 

5 

A1 

8 

A2 

11 

12 

Mass spectrometer 

8a 

8b 

Shutter 

grid detail 

Figure 1: 

Schematic diagram of the switchable high-resolution ion mobility spectrometer 1. inlet for gas to be analysed, 2. repeller 

plate, 3. electrode assembly, 4. source region, 5. extractor plate, 6. field defining electrodes reaction region, 7. gas exhaust 

8. shutter gate, 10. drift gas inlet, 11. screen grid, 12 Faraday detector and pin hole orifice 

Ammonia and DPM permeation sources were 

used to to generate concentration levels of 2.39 

mg m -3 and 0.162 mg m -3 respectively in the inlet 

gasses to the instrument. A water permeation 

source was also used to maintain the 

concentration of water in the instrument at 75 mg 

m -3 . 

Figure 2 shows the mass spectrum of the product 

ions obtained from a DPM challenge to the pulsed 

corona discharge ionisation source. The reactant 

100 

80 

Intensity 

60 

40 

20 

0 

0 20 40 60 80 100 120 140 160 180 200 

Mass to charge ratio 

Figure 2: 

Mass spectrum taken for positive mode pulsed corona discharge-SHRIMS-MS for trace DPM in air with the gate open 



Number of counts per thousand scans 

500 

400 

300 

200 

100 

0 

9 11 13 15 17 19 21 

Gate delay/ms 

Figure 3: 

The effect of varying gate delay on the different ion populations produced by a DPM challenge: diamonds = m/z 54 peak, 

triangles = m/z 106 peak , squares = m/z 166 peak , and circles = m/z 314 peak 

ions are discernible along with monomer and 

dimer product ions. The peak at m/z 106 was 

isolated from the DPM response and is believed 

to be a contaminant present in the DMP, it has 

yet to be identified. 

The monomer species [(DPM)NH 4] + (m/z 166) and 

[(DPM)(N 2)NH 4] + (m/z 194) were accompanied by 

dimer [(DPM) 2.NH 4] + (m/z 314) and 

[(DPM) 2(N 2)NH 4] + (m/z 342) cluster ion peaks. 

In addition a peak at m/z 164 and associated 

dimer peaks at m/z 312 and m/z 310 thought to 

form through clustering with the m/z 166 species 

were observed. The mass selected ion mobility 

responses for m/z 164 and 166 were seen to give 

two clearly distinct peaks with drift times of 19.72 

Table 1: 

Table showing the main ions/clusters identified in the mass spectrum shown in Figure 5 

O 2 

- 

Ion/cluster identity 

m/z 

32 

Ion/cluster identity 

- 

HCO 3 

- 

NO 2 46 

- 

NO 3 62 

[(H 2O).O 2] - 50 [(H 2O)(CO 2). O 2] - 

94 

[(N 2).O 2] - and CO 3 60 [(NO 3) 2] - 

124 

ms and 19.96 ms respectively suggesting that 

they were formed in the reaction region and 

traversed the drift region as two separate entities. 

Similar behaviour was observed with the m/z 312 

and 310 peaks. 

The effect that varying the gate delay had on the 

maximum peak intensity was investigated and the 

resultant graph is shown in Figure 3. The 

relationships between monomer product ions (m/z 

166) and the reactant ions (m/z 54) and the 

formation of dimer product ions (m/z 314) from the 

monomer product ions is evident. Further, the 

concept of gate delay to impart additional 

selectivity into the process is demonstrated in this 

figure. 

m/z 

61 

Negative mode studies: 

Figure 4 is the mass 

spectrum obtained in the 

negative mode with a 

water concentration of ca 

75 mg m -3 and with the 

gate open. The main 

ions/clusters are outlined 

in Table 1. 



60 

50 

40 

Intensity 

30 

20 

10 

0 

0 20 40 60 80 100 120 140 160 180 200 

Mass to charge ratio 

Figure 4: 

Mass spectrum taken for negative mode PCD-SHRIMS-MS for air with gate open 

Figure 5 shows the selected mass ion mobility 

spectra for the individual ions/clusters identified in 

Table 1 at a gate delay of 11.5 ms. As would be 

expected O 2 

- 

(m/z 32) and ([H 2O].O 2) - m/z 50 have 

the same drift times. NO 3 

- 

(m/z 62) was clearly 

resolved from NO 2 

- 

(m/z 46). Two peaks were 

observed with an m/z value of 60. One coincided 

with the response for HCO 3 

- 

(m/z 61) and was 

consequently assigned to HCO 3- . The largest 

peak observed for m/z 60 had a higher mobility 

than HCO 3- , and was assigned to [(N 2)O 2] - . 

The plot shown in Figure 6 shows the effect of 

gate delay on the peak ion intensity for tuned ion 

Peak identifier 

Figure 5: 

Tuned ion spectra for negative mode PCDIMS-MS in air for a 11.5 ms gate delay 



Peak number of counts per 1000 scans 

400 

300 

200 

100 

0 

9 10 11 12 13 14 15 

Gate delay/ms 

Figure 6. 

The effect of varying gate delay on different ions representing different cluster formations: circles = m/z 60; diamonds = m/z 

50; triangles = m/z 61; and, squares = m/z 94 

responses of selected ions in the air reactant ion 

peak in negative mode. The m/z 60 species are 

interesting for there are two distinct maxima 

supporting the approach of assigning two species 

to this m/z value. Further, although the [(N 2)O 2] - 

peak did not exhibit the highest mobility, its 

maximum intensity was observed at a significantly 

shorter gate delay than the other species in the 

reactant ion peak suggesting that [(N 2)O 2] - was 

produced close to the ion source, where as the 

other ions were the products of reactions 

occurring in the reaction region. 

Summary 

In the positive mode PCD activity mirrors quite 

closely 63 Ni reactant ion chemistry. In the negative 

mode however the issue is more complex. Under 

certain conditions it is possible to produce 

reactant ion chemistry similar to 63Ni, however, 

use of PCD appears to introduce a degree of 

added complexity dependent upon the source 

conditions, the electrode d.c. and corona 

voltages. These relationships are being further 

investigated and will be the subject of future 

presentations and papers 

Acknowledgements 

Research on this project was funded by dstl at 

Porton Down. Graseby Dynamics also provided a 

substantial amount of help and guidance.

A REPLACEMENT FOR 63Ni IN ION MOBILITY SOURCES?

Create successful ePaper yourself

Delete template?

Save as template?