静電気学会誌，33, 1 (2009) 38-42
J. Inst. Electrostat. Jpn.

Fundamental Processes of Corona Discharge－Surface Analysis of Traces Stained with Discharge on
Brass Plate in Negative Corona－
Kanako SEKIMOTO* and Mitsuo TAKAYAMA*,1(Received August 20, 2008; Accepted February 10, 2009)

The surface profile and chemical components of the trace stained with negative corona discharge on the brass
plate have been analyzed with surface profiler and laser desorption/ionization mass spectrometry, respectively.
The trace pattern was made up of several concentric circles and the pattern seemed to reflect the distribution of
inhomogeneous electric field strength between point-to-plane electrodes. The surface profile of the trace
showed a concave pattern like a crater. Abundant NOx- and their complexes with Cu were observed in thecenter region of the crater, while abundant carbon cluster ions Cn- (n=2-10) and NOx- related ions wereobserved in the rim region of the crater. The results obtained indicated that NOx- ions were mainly produced on
the field line arising from the needle tip apex with high electric field strength, while the periphery of the needle
tip with lower field strength resulted in the carbon cluster ions.
1. Introduction

discharge are not yet well understood. Primary ions such

Corona discharge has been used as an ionizer in a wideas N2+・, O2+・, O- and O2- produced in corona discharge

range of research and industrial fields such as

move along the electric field line between the electrodes.

environmental, analytical and atmospheric sciences, andSimultaneously, they alter more stable ion species called

possibly even commercial electric appliances. In mass

terminal ions or lose the charge through a number ofspectrometry, recent atmospheric pressure ionization

collisions with common air constituents and by-products(API) techniques, e.g., direct analysis in real timeof discharge because the mean free path of air (66.3 nm)

1)

is very short. Terminal ions generated via successive

2)

ion-molecule reactions have few reactive collisions(DART) , atmospheric-pressure solids analysis probe(ASAP) and charge assisted laser desorption/ionization
3)(CALDI) , contain corona discharge devices to supply

during the majority of their lifetimes and can exist stably

the charge for ionization. Unique application for thesein air. A series of ion-molecule reactions from primary toionization techniques is the direct detection of chemicals

terminal ions have been reported as ion evolutions inon surfaces without requiring sample preparation, such as

both positive and negative corona discharge under

wiping or solvent extraction. The API techniques haveatmospheric pressure conditions (Scheme 1)4-6). Negative

demonstrated success in sampling hundreds of chemicals,

ion evolution is so complex compared with positive ion,drugs in abuse, explosives and toxic industrial chemicals

because the sequence of ion-molecule reactions would beon various surfaces such as asphalt, human skin, surfaces

controlled by trace gases such as CO2 and NOx in air. Theof fruits and vegetables, and clothing. Therefore, the API

studies of corona discharge reported so far show thattechniques using corona discharge are extremely usefulnegative ion evolution is quite complex and that it isfor the rapid, noncontact analysis of substances on

difficult to regulate the formation of specific negative ion

1-3)

surfaces, liquids and gases.

species.

Despite substantial recent progress of the API

We have recently established an atmospheric pressuretechniques, the elementary processes involved in ion

corona discharge system coupled with a mass

formation occurring in atmospheric pressure corona

spectrometer that successfully leads to regular andreproducible generation of positive and negative core

Key Words: corona discharge, brass plate, negative corona,surface profile, chemical components* International Graduate School of Arts and Sciences,
Yokohama City University, 2-2 Seto, Kanazawa-Ku,
Yokohama 236-0027, Japan.
1takayama@yokohama-cu.ac.jp

ions X+ and Y- and their hydrated cluster ions X+(H2O)nand Y-(H2O)n6). The positive cluster ions H3O+(H2O)nwith core ion H3O+ were dominantly observed under
any conditions in positive corona discharge, in39Fundamental Processes of Corona Discharge（SEKIMOTO & TAKAYAMA）(a) positive ion evolution
N2

e-

N2+・

H2O

(a) Positive mode(n = 20)

N2H+

H3O+(H2O)n

H2O

H2O
O2

O2

e-

O2+・

H2O

H2O+・

H3O+

H2O

O2+・(H2O)(n = 85)

H2O

H3O+(OH・)

H2O(n = 54)

H3O+(H2O)

H2O(n = 29)(b) Negative mode(n = 50)(b) negative ion evolution
H2O
higher E
e-

CO2
CO2

O2-

NO2

HNO3

lower Ee-(n = 74)

HNO3-

NO2

O-

O2
lower E
e-

HO・

HO-

HO-(H2O)n

NO2-

CO3-

(c) Negative modeNO3-(H2O)n

NO2

CO4-

NO2
NO

(n = 9)NO3-

Scheme 1 Sequential progress of (a) positive and (b)negative ion evolutions in corona discharge underatmospheric pressure conditions. Higher and lower E
represent the kinetic energy of electrons emitted fromthe needle tip.
agreement with other previous studies4,5). The mass(n = 29)

Fig. 1 Corona discharge mass spectra of ambient air.
(a) Water cluster ions H3O+(H2O)n observed inpositive mode, (b) HO-(H2O)n observed in negativemode at low corona voltage, and (c) NO3-(H2O)nobserved in negative mode at high corona voltage.

spectrum of the positive cluster ions H3O+(H2O)nstrength on the formation of negative ions would(n=3-88) is shown in Fig. 1a. The abscissa of the masscontribute towards the understanding of the ion

spectrum represents mass-to-charge ratio (m/z) of ionsevolutions occurring in corona discharge.

and the vertical axis indicates relative ion abundance. In

Here we attempt to trace the negative ions produced in

the case of negative corona, it was found that various

negative corona discharge on the plate electrode and to-different negative core ions Y were generated according

analyze the traces stained with discharge on the plateto the needle voltage and that the needle voltage could besurface, by using microscope, surface profiler and laser

6)

related to the lifetimes of ions . The lowest corona

desorption/ionization (LDI) mass spectrometry.

voltage resulted in the dominant formation of hydroxidecore ion HO- with a lifetime of 10-3 s, whereas other core

2. Experimental

ions such as NOx- and COx- with longer lifetimes (> 1 s)

2.1

were produced at higher needle voltage. Fig. 1b and c

The corona discharge device used here consisted ofshow the mass spectra consisting of the dominantpoint-to-plane electrodes under ambient air with relative-

Atmospheric pressure corona dischargecluster ions HO (H2O)n (n=3-88) andNO3 (H2O)n (n=0-40), respectively. It is reasonable tohumidity of 50 % and 24℃. The corona needle as a pointconsider that the phenomena regarding negative corona

Tokyo, Japan), made of stainless steel with a diameter ofdischarge would be understood from the standpoint of

200 μm and 20 mm in length. The needle tip with glossy

inhomogeneous electric field strength on the needle tip,

surface was ca. 1 μm in the radius of curvature and the

because the electric field strength affects the production

shape of the tip surface was adequately approximated inof primary ions and discharge by-products such as

the form of hyperboloids of revolution (Fig. 2a). The

neutral NOx which are significantly involved in the ion

needle was located perpendicular to the brass plate (0.1

evolution to form typical negative core ions HO , NO2-,
NO3-, HNO3-, CO3- and CO4- as shown in Scheme 1b.

mm thick) with a gap of 3 mm (Fig. 2b). Discharge

Therefore, further study of the influence of the field

respectively.

negative-

electrode used was an insect pin with headless (Shiga,

voltage and irradiation time were -4.0 kV and 15 min,40

第 33 巻静電気学会誌(a)(b)

第 1 号（2009）- 4 kV
5 mm

Needle
10 m

×1000

3 mm

A
Brass plateA

Fig. 2 (a) The optical micrograph of the needle tip
used and (b) a configuration of the point-to-planeelectrodes.
2.2

Microscope, surface profiler and laser
desorption/ionization mass spectrometry

Optical micrographs and surface profile of the tracestained with discharge on the plate were obtained by

Fig. 3 The optical micrograph of the trace stainedwith discharge on the brass plate.

using a BX51 optical microscope (OLYMPUS, Tokyo,yfield line

Japan) and an Alpha-Step IQ surface profileru  const(needle) equipotential linev  const(KLA-Tencor, San Jose, CA, USA), respectively. LDI
mass spectra were acquired on an AXIMA-CFR
instrument (Shimadzu Corp, Kyoto, Japan). A nitrogenlaser (337 nm) was used to irradiate and ionize thedcomponents of the trace stained with discharge on theplate. The laser beam profile on the target was 200 μm in
diameter. The mass spectra were obtained withnegative-ion reflectron mode. The acceleration potential
was set to 20 kV using a gridless-type electrode. The ion
source and analyzer were maintained at 10-5 Pa.

0v  0 (plate)x

Fig. 4 The coordinate system in x-y plane
representing electric field distribution between aplane and a perpendicular needle by a gap of d.

3. Results and discussion

electric field between point-to-plane electrodes by a gap

3.1

Traces stained with discharge on the brassof d m and the electric field strength on the needle tip

plate in negative corona

surface E(u) Vm-1 are given as follows:

Figure 3 shows an optical micrograph of the tracestained on the brass plate. The stained trace patternconsisting of several concentric circles with the point A
was observed. The center A was the intersection pointwith the needle axis (Fig. 2b). The stained trace patternobtained here seems to reflect the electric field
distribution between point-to-plane electrodes and theinhomogeneous field strength on the needle tip surface.
If it is assumed that the needle tip used here is shapedin the form of hyperboloids of revolution around y axis(y > 0) and the brass plate is located at the x-z plane (y =7)0), according to the method of Eyring et al.
coordinatexv, u , y v, u ofx v , u  









1/ 2
1/ 2
d
d
u 1  v 2 , y v , u  v u2 1v0v02
v  1  v0E u    0  22
d  u  v 0  1 

1/ 2



0
1

1  v0 1  v0 2log
1  v0A set of hyperboloids for constant v ( 0  v  1 ) andan orthogonal set of half ellipses for constant u(    u   ) represent equipotential and field lines,
respectively. The variables  0 and v0 represent thepotential difference between the electrodes and aparameter accounting for the characteristic of a hyperbola

thecorresponding to the contour of the cross-sectional needle

two-dimensional

tip as an equipotential surface, respectively. In the case of41

Fundamental Processes of Corona Discharge（SEKIMOTO & TAKAYAMA）

50 m

9.0

(a)

8.5

10 m

log E

8.0

62

100

46

7.5

187

50

250

7.0
0

6.5
-0.010-0.005+0.0050x [m]Fig. 5 The calculated electric field strength on
the needle tip surface as a function of x axis on
the plate.

50

100

150

200

(b)

20

250

187

0

0

0

-10

-8

-6

-4

-2

0

+2

+4

+6

+8

m/z

300

62

50

10

250

46

30
thickness [nm]

100

50 m

+0.010

50

100

150

200

250

m/z

300

50 m

+10

(c)

x [mm]

Fig. 6 Uneven surface profile of the trace depositedon the plate surface. Vertical axis represents the
thickness of the deposits on the plate.

10 m
*

100

62
46

the experimental conditions here v0 is 0.99955. The
coordinate system in x-y plane and the calculated field

50

0strength distribution on the needle tip as a function of x

*

187

*
50

100

150

* * **

200

250

300

*

m/z

50 m

axis on the plate are shown in Fig. 4 and Fig. 5,(d)respectively. Three-dimensional coordinate can beobtainedbyrevolution

about

the

y

axis

of

two-dimensional coordinate. It should be noted that the

*
299

100

terminal positions of field lines arising from eachposition u on the needle tip correspond to the pattern ofconcentric circles on the plate. It can be deduced,
therefore, that the trace stained on the plate here is
produced by the interaction between surface materials on
the plate and terminal negative ions generated via ion
evolution occurring on each field line. That is, thedeposits or products in the center region (point A in Fig.
3) are originating from the negative ions produced on the

*
118

50

*
207
*
* 243
223

*
90
0

50

100

150

200

250

*
331
300

m/z

Fig. 7 The optical micrographs and LDI mass spectra of
the traces stained with (a) the highest field strength at x=0mm, (b) relatively high field strength at x=+3.0 mm, (c) low
field strength at x=+7.0 mm and (d) without discharge.
Asterisk indicates the ions originated from the brass plate.
Those ion peaks could not be identified.

field line arising from the needle tip apex with the highest
field strength (u=0 in Fig. 4), while those in the outer

stained or deposited on the plate, the surface of the trace

regions are attributed to negative ions moved along field

was analyzed with surface profiler, microscope and LDIlines arising from lower field strength ( u  0 in Fig. 4).

mass spectrometry. Fig. 6 shows the surface profile of the

3.2

Analysis of the trace stained on the plate

trace deposited on the plate. The optical micrographs andIn order to obtain a detailed information on the

LDI mass spectra for the traces obtained by the highest

relationship between the inhomogeneous electric field

field strength at x=0 mm, relatively high field strength

strength on the needle tip surface and the resulting trace(x=+3.0 mm) and low field strength (x=+7.0 mm) on the

42

静電気学会誌

第 33 巻

Table 1 Negative ion species observed in the traces
stained with discharge on the plate and correspondingm/z values
Ion speciesm/z

Cn- (n=2-10)
NO2NO3NO3-(NO3)Cu
NO3-(NO3)Cu2

12n
46
62
187
250

第 1 号（2009）

with increasing the electric field strength on the needletip6).
4. Conclusions
Negative ions produced in negative corona discharge
were deposited on the brass plate as an electrode. The
surface profile and chemical components of the tracestained with the discharge on the plate were analyzed
with surface profiler and laser desorption/ionization mass

needle tip and for brass plate as a blank are shown in Fig.

spectrometry, respectively. The trace pattern was made7. Taking into account the laser beam profile on the target

up of several concentric circles that seemed to reflect thewith a diameter of 200 μm, each LDI mass spectrum

distribution of inhomogeneous electric field strength

obtained here seems to be consisting of the components

between point-to-plane electrodes. The surface profile oftraced on the whole area of the plate shown in thethe trace showed a concave pattern like a crater. The

micrograph. The deposit at the center region attributed tochemical components of the trace in the center of theparticularly high field strength at =0～±0.5 mm was

crater were abundant NOx- and the complexes of NOx-thinner than that at outer region originated from lower
field strength at x=±0.5～±6.0 mm, as shown in Fig. 6.
The chemical components of deposit at the center region
were identified as NOx- and complexes of NOx- with Cu
as summarized in Table 1. The negative ion species NOxare well-known as terminal stable negative ions in air andwere dominantly observed at higher needle voltage6). The
detection of complexes of NOx- with Cu and thewith Cu, while those in the rim of the crater wereabundant carbon cluster ions Cn- (n=2-10), NOx- andNOx- complexes with Cu. In order to examine thedetailed relationships between the field strength andresulting negative ions, the field strength and its
distribution on the needle tip were calculated. Calculated
field strength and the experimental results here indicatedthat the abundant NOx- ions were produced on the fieldobservation of cracks on the center region at x = 0 mm

line arising from the apex of the needle tip with high(see the micrograph of Fig. 7a) suggested that the surface

electric field strength, while lower field strength in thewas coated with the products resulted from theperiphery of the needle tip resulted in the depositinteractions between abundant NOx- and surface
materials such as Cu. The mass spectrum obtained overconsisting of the carbon clusters. The HO- ion and any
complexes of HO- were not observed in the any traces.

the range at x=±0.5 ～ ±5.0 mm showed the peaks
corresponding to NOx- and carbon cluster ions Cn(n=2-10, ▽ in Fig. 7b). The abundances of NOx- andtheir complex ions decreased with increasing the distancefrom the center at x = 0 mm. Simultaneously, carbon cluster
ions and other ion peaks originated from brass plate
relatively increased as shown in Fig. 7c. The increase indistance from the center means the decreasing of the fieldstrength on the needle tip.
The results obtained here indicated that abundant NOxspecies were produced on the field line originated fromthe needle tip apex with high field strength, while in thelower field strength regions the products deposited on theplate were carbon cluster ions Cn- as the terminal
negative ions. These facts were in agreement with theprevious report that the generation of NOx- was promoted

This work was supported by Research Fellowships of theJapan Society for the Promotion of Science for Young Scientists(No.20･10498) and National Institute for Materials Science
Nanotechnology Support Network (project ID:AC20035).

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