Spectroscopic studies of microwave-excited plasma

309

SPECTROSCOPIC

STUDIES

OF

MICROWAVE-EXCITED

PLASMA

The plasma torch, which has been employed as an excltatlon source for cmls\lon and absorptron spectrophotometry, appears to have very dlfferent properties from chemical flame\, and so causes new problem\. One of the advantages of the plasma IS Its high temperature which serves to dlssoclatc molecule\ and cxcttc atoms of high excltatlon energies The high temperature. however, does not havca bencflclal effect on other cattons present. Elements with low excltatton potentials may be 40 Ionized that neutral atomic lines cannot be observed, and the degree of Ionization tends to be severely affected by catlon addttlvcs Accordrngly, the plasma torch method IS rather prone to interferences In thl\ study of the Interferences of catlons In plasma, the followmg factors wcrcconsldercd thedlstIIbutlonsoftheclectron tcmpcraturc. thedcgrccofionl;latlon and total number of atoms and ions, and the effects of cations (alkah and alkalme earth metals)on thesequantltlesandon theemIssIon lntensltlesfromdlffercnt elements. The supporting gases for the plasma were argon, nitrogen and carbon dloxlde, which, molecules, respectlvcly, were expected to produce being mono-, dl-, and trlatomlc different results.

The calculatton used was based on Roltzmann’s law which IS vahd provtdcd that the system observed IS In thermal equillbrlum The lntcnslty ratio of two atomic lines cmltted from a certain element can be expressed as follows 1/1’ = (c#/g’f’)(2/2)”

exp(E’-

E/kT)

(1)

where I IS the relative mtenslty. (1 the \tatlqtlcal weight of the upper state,/ the osctllator strength. R the wavelength. E the energy of the upper state. It the Boltzmann constant, and T the electron temperature (K) . the prrme mdrcates the other atomic line of the element

The intensity

ratlo of the atomic

line and the lonlc ltne 1s expressed

where A 1s the degree of ionlzatlon, Q IS the partition Indicate the atom and the ion, respectively.

function

as follows.

and subscripts

a and I

K

310 011rek~trrw number

Drstrihulron a4

The intcnstty follows I/& = (I-

whcrc

of titom~

ratto of the center

z)NQ,(-K)/(l

N IS the total number

KITAGAWA.

I-

TAKEUC~lI

cinti ran.\

and other

\ecttorts

tn the torch

can be written (3)

--AN,Q(7) of atoms

and tons and the subscrtpt

c means

the center

I‘XI’I;.RIMI,NTAI

A Httacht 300 UEIF Pl;tstnrt Scan’ wits fitted wtlh four matn part5 ;i source apply, a dt\ch;trge untt wtth a tnagnclron (Httacht 2MX9). a drtvtng untt for a pulse \canntng motor, a Czcrny-Turner type grating monochrotnator, and ILdctcctor untt wtth a HTV 106 photomulttpltcr tube whose scnstttvtty curve was used to correct the observed tntcnstty

In order to esttmatc the elcc(ron temperature and the degree of totltzattotl. the clctncnt sclcctcd was manganese whtch emtts not only strong atomtc lines of dtffercnt cnctgtcs but also comparable tonic lines. The prtnctpul ltncs of manganese ;IIC shown in Table 1.

I’RINCII’AL .. I.rtw Mn Mn Mn MI] Mn

I” I” I” II II”

L;MISSION ____-

LINES -_

01.

MANGANI-Sl: _- __

CVtrr’c~/crlc/l/l(wt1)

{l/-l’dttV

fJ/‘pcV olc’,q\’

403 30 279 259 257

0 32‘) 3 03 7 39 50 79

442 444 4 17 4x1

08 I1 48 37 61

(c V)

1 ox

” Ltne obscrvod

Other clcmcnt~ (chromtum. vanudtum, anttmony, boron. sclcntutn and phosphtnc) wet-c also cxatntncd bccausc they po\scsscd htgh cxcttatton potcnttals and/or bccausc Ihctr molecules had htgh dtssoctntton cncrgtes. The elements chosen as addtttvcs were alkalt and alkaltne cat th metals (Itthtum, sodtum, potasstum, magnesium. colctum. C -tnd stronttutn) whtch offered typtcal electrontc effects. Standard solutton of mangancsc was prepared by dtssolvtng the tnetal tn soluttons of alkali and alkaltnc earth hydrochlortc actd purtfted by dtsttllatton; metals were prepared by dtssolvtng their dried carbonates in hydrochlortc actd. and others by dtssolvtng thetr acids or salts tn water. RESULlS

AND

DISCUSSION

The atomtc Ar~trlChrt~r ACV~I.60

manganese ( 1972)

Itncs 403 OXnm and 280 11 ntn were used to estimate

the

M IC K OWAVE-EXCITED

311

PLASMAS

dlstrlbutlon of the electron temperature In the plasma torch. A IO-’ M manganese solution was nebullzed into the torch The emlwon was observed through slits (30 /tm wide and 1 mm high), so that the emitted hght was observed a\ an average rntcnslty over the tcctangle. Flow rates of gases were held constant at CCI 5 1 mm- ‘, and the pressure of nebullzltron was mamtalned constant (2 kg cm - 2, The mlcrowave power supply wa\ about 250 W for argon or nitrogen plasma Howcvcr. when carbon dloxlde was and the alummum electrode was introduced. the plasma could not bc mulntalncd. cxhaustcd by oxygen radlca14, rcsultlng aluminum oxldc dccrcascd the efficiency of the mlcrowavc radlatlon and made the plasma unstable Accordingly. a larger power supply ((XI 450 W) was required to maintam the plasma, and prolonged observations were rmpo\slblc Under the optimal condrtlons. the electron tcmpcrature was about 3800°K even 111the hottest region. Apparently. the torch furnished by carbon dloxrdc was the smallest. the torch with nitrogen was larger and that with argon the largc\t The electron temperature profiles observed rn the argon plasma torch and 111 the nltropcn torch iu-e shown 111Figs I and 2, re\pcctlvcly. The results show that the electron temperature was hlghcr in the argon plasma than m the nltrogcn one. which may bc due to the vibration of the nltrogcn molecule Part of the mdlated mlcrowavc energy IS finally dlrtrlbutcd among the cncrglc\ of vlbrutlon. rotntlon. translation .tnd transltlon which appcrtaln to vul~ous spcc~c\ in the plasma If thcrc I\ a thermal cqulllbrtum In a system of spec~cs 1. the cncrgy of the system c’, = (N,/Q,) Z$ o,, cxp (- Q,,/LYJ. When the whole \ystcm IS In equlllbrlum. 7; IS lndcpcnclent of 1 Tljc total energy mu\t be constant when the plasma IS regularly suppllcd wlth the rnlcrowavc power. hence the larger N,. the smaller 7;. Temperatures of other systems can bc affected by N, 21s far as energy transfct 1s concerned, cvcn

D~st&e

from

yhe

centra13ax~s(mm)

Distance

Fig

1 Prdilc

of clcctron

tcmpcrature

of manganese

atom5

In argon

Frg

2

of electron

tcmpcraturc

of manganese

atoms

m nltrogcn

Profile

from

plasma plasma

the

torch torch.

central

axis (mm)

K

312

KII-AGAWA,

-I- -1AKEUCHI

when pcrfcct

eclulllbrlum 15 not achlevcd throughout the \ystcm Thcrcforc, temperatures decrease when molcculcs po\szs\lng a high dcgrec of freedom are Introduced thecffect dccreascs for dlffcrcnt plasmas In the order argon > mtrogcn > Into p)usma

carbon

dloxldc

With regard to the gradlcnt plasma torch had a planar region sug,geQ\ discqir~librium

117

the tall

of the electron and

the rnlddlc 111the

temperature,

the arcumfcrcncc

a steep

preclpa,

which

region

Under the same condltrons as dcscrlbed above, the rclatlve Intensity of the JO~IC llnc 257 61 nm of manganc\e was mea\urcd When cqn (2) was used to compute the degree of lomzatlon, It was assumed that thermul equlllbrrum exlsted between the clectronlc systems of the atoms and Ions. r.e that the electron temperature of the atomic system, ‘I,, was equal to that of the lonlc system, -4 The ionic llncs of manganese wcrc hardly detcctablc when carbon dloxldc wa\ Introduced. The observed profiles In argon and nltrogcn plasmas arc shown In Figs 3 and 4, rcspectivcly. The dcgrec of IonlzatJon 111nItroget> plasma decllnctl (6(Y),, to IO”(,) as the electron tcnipcrnturc fell (3700°K to 2900°K) . m the argon plasma ii maximum occurred about 4 mm from the central ~XJS and 22 mm above theelectrodc. which mdlcates that at the root of the torch. argon lacking vlbratlonal or rotatIonal energy does not ylcld suffiaent energy Further cvldcncc was provldcd by other \ystcms In Figs 5 and 6 are shown the relatlvc lntensltles of strontium llncs (from SrCI,) and boron (from H,B03). respcctlvely The emlsslon mtcnslty of boron 117nltrogcn plasma was slmllar to that in argon plasma, probably. boric acid IS cn~lly dtssoclatcd In mtrogcn plasma so

Distance

from the

central

a%s(mm~

Ulstonce

Fig

3 ProTtlc ordcglcc

of ~omzal~on

of marqgncsc

.itoms

In argon

I-Q

4 Prolilc

of lomutton

of mangancsc

atoms

In nitrogen

~,utl

clrm

of dcgrec ntttr

60

(I

972)

plasnhr pl,lsm,l

from

the

central

axIs

MICROWAVE-EXCITED

Dlstonce

from

the

PLASMAS

central

ax6

Utstance

(mm1

I-lg 5 Ret.~t~vc lordi. (II) k(t)

Intcnsltj in drgorl

of \tronllutn I1nc4 (%(I) 46701 enc. (C) Sr(t) 111nitrogen one.

I-lg

Intcnvty

of boron

6 Rchtivc

lint

(249 77 nm)

nrn, \r(tt) (1~) \r(tt)

(A) Argon

from

the

407 77 rim) (A) Sr(tt) in nltrogcn one

phsni.l

larch,

(IS) nltropcn

central

axls(mm)

In .lrgon

ptdwid

one

dis\oclatlon IS the predominant rcactlon. whcreus strontium 1s easily dissociated In argon plasma and high temperature IS Important rn\ofar 21s lomiratlon does not predommatc. But higher tcmpcruturcs lncrea\e lonlzltlon, causing wcakcr :itomlc hnes and stronger lonlc lines that

Equatton (3) was used to estlmatc the relative dlstrlbutlon of the total number of atoms and Ions. In practice, however. as the torch varted in thickness along its length, a correction based on a cyllndrlcnl state was made The horizontal and vertical dlstrlbutlons determined In this way are shown In Fig 7. The results lndlcate that In the horizontal plane the verges of the torch have a hlghcr spcc~cs conccntratlon than the center, but that the maxlmum occurs 111 a vertical dIrectIon There 1s therefore a problem that the sample aerosol cannot be thoroughly mrxed with the plasma EJfect.5 of trlkali trmi c~lkalrrte etrrth tnettrls The emlsslon mtenstty from a system as follows* C, = (srvhn’~‘~/Q(T)m,)(g,

f,,,/i-2

m thermal exp (--%lCJ

equlllbrlum

can be represented (4)

where h 1s Planck’s constant, e the charge of electron, nt, the electron mass and N the number of atoms. Calculation’ of values of the function exp (- E,/kT)/Q(T) for various elements indicates that the emlsslon lntenslty increases as the electron

Height above the electrode

(vertrcal) (mm)

x

x

MICROWAVE-EXCITED

315

PLASMAS

M++Nu=M+Na+ If this \lmplc as follows

reaction

(6) can be asumcd,

the equlllbrwn

constant

can bc exproscd

K = KM?NJ+ MQhf + Qd ew MM-Emlsston

mtensttv

(vertlcol)

b

(arbltrarv

(7)

units)

I

3

Dlstan?ce

Distance

FIB

from the kntrol

from

the

central

ax~s:hor~zontol)(mrd

---k---

Ulstance

I

d

0 rrom

tne

central

5 . axtsCmm1

axts(mm)

8 Effect of potassium (a) on OH cmlssmn (306 4 nm), (b) on H, he. (c) on argon line (394 8 tlm) vcrtlcal dlstrlbutlon (A ,Ind I>) No pot,lwum added, (B :mcl E) (0 0) Hortzontal dwrlbutmn. (O---O) 15 pmol K ml-’ added. (C and F) 30 pmol K ml-’ added

K

KITAGAWA,

T

‘IAKEUCfff

(a>

0

4,0000~

Concentration of add tt Ives (pmol/ml)

t 0 Concentrath

of addlth%s

(pmol/ml)

L

I

Concentratlo? of add&-: (pmol/mll

f+lg 9 hfrccts of ;Ilk,dl .~nd .tlk,dmc c,lrth mchls (%I)on tlw clcctron tcmpcr,~turc of mang,mc\c atom\. (b) .Ind (c) on 01-1 cnllbvon, and (d) on contmuum (at 280.0 nm) (A) Llthlum acfdcd. (U) sodium ‘Iddcd. (C) ~OL~~SILIIII .~ddcd,(D) wlaum added,(C) \tronttum ,~ddcd. (I-) m,lyncslum ad&d

where V, and V,,, are the Ionization potentials of M and Na, respectrvely. I< 1s larger at lower temperatures or at larger values of (V, - V,,) With regard to ionized elements, additton of a cation depresses ionizatron and increases the total number of atoms, so that atomic lines may bc strengthened m rntensrty. A decrease rn the electron temperature contributes more conspicuously to the emrssron mtensrty of elements wrth a high excrtation energy. The shaft of the maxrmtrm peak may depend on the excrtatton potential. Thus enhancement effect for atomrc lines was observed3 m nitrous oxideacetylene flames where ionic lines were found to be reduced, whereas they generally increased in the pl,Isma torch. The degree of ionization of manganese can be seen ,4r1~11 Chm

AL?(I, 60

(1072)

of

ConcentFatlon

sodwm

(pmol/ml)

OC I 0 Concentrot&

I 0

1

Concentr&n

t

of

sod:Zm

i 4

El 1

1

1

of sod~~~m’&mol/ms;)

I

~pr*o&~

big

IO Effcctsof\odwm on cmlswn lntcnsltyof vGirmu\ clc~ncnt~ (‘I) Sr(l), 460 7 nm , (b) Sr(JJ), 407 8 nm, (c) Cr(l). 425 4 nm. (d) V(J). 437 9 nni (c) H(1). 240 t( nm (f)Sh(J) . 231 I tin7 (A) Argon pl.lwn;t torch. (B) mtrogcn torch

I

I

0 Concentration

5

L

1c

of addltwes

pmol/ml Fig

11 Ltthlum

Elfects

added,

of

alkah and alkalme earth metals on the degree of tontwtron of manganese atom5 (B) sodium added, (C) potawum added, (D) calcrum added, (El strontium xtdcd

(A)

318

K

Incrca\lng from t-~g 1 I Thc\e results lndlcatc and ions arc incrcad by cutlon ilddltlvc4

Ki,\lJ

KITACIAWA.

that the conccntratlon\

I-

‘1-AKE.UCtll

of both atom\

M I.

LII\AMMI

NI A\\lJN
Munganlo\ung wurdc 111 c~ncn mlkrowcllcncrrcgtcn Plasmabrcnnct (2450 dcsscn Profd untct aucht wurdc. drc ElektroncnM 1 17 und C’LI 200 W) gcspruht, tcmpcratur. dcr lonrsatlon~gracl sow~c dlc Gcsamtzahl da htome lmd Ioncn wurdcn crmlttdt Dcr Elnllus\ van Alkali- und l~rd~~lk~~l~~~~~t~~ll~n hangt von den Iomsattonspotcntralcn sowohl dcs Mangun\ als such da /,ugcfuhrtcn Mctalls ab