Ana&trca Chrmca Acta, 250 (1991) 85-94

Elsevrer Scznce Pubhshers B V , Amsterdam

85

Use of magnesium as a test element for inductively coupled plasma atomic emission spectrometry diagnostics

J M Mermet

Laboratolre des Scrences Analytrques, Bat 308, Unrverslte Claude Bernard-LJon I, 69622 VdIeurbanne Cedex (France)

(Received 23rd May 1991)

To optnmze the atonuzation and lomzatmn processes m an mductlvely coupled plasma used as a source m atonuc

ennsslon spectrometry, the Mg II 280 270-nm/Mg I 285 213-nm hue mtenslty ratio 1s used A theoretical ratio 1s calculated assunung a local thermodynanuc eqmhbrnun A review of previously pubhshed expenmental values of the ratlo IS Bven as a function of the parameters mfluencmg the energy transfer between the plasma and the mjected species In particular, the effects of the power, the tamer gas flow-rate, the 1 d of the torch mjector, the use of a

sheathmg gas and the presence of hydrogen are described Values of the ratio close lo the theoretical values are obtamed with the use of a high power (> 14 kw), a low tamer gas flow-rate (< 0 6 1 nun-‘) and a large 1 d of the

InJector (Z 2 mm) This optmuzatlon can also be apphed to the mmnmzation of Interference. effects due to the presence of sodmm

KqwordF Ermsslon spectrometry, Magnesmm, Plasmas

The mductlvely coupled plasma (ICP) 1s cur- rently wdely used as a ra&atlon source m atonuc ermsslon spectrometry (AES) or as an lomzatlon source m mass spectrometry (MS) for elemental analysis Its advantages have been largely de- scribed [l-3] determmatlon of most elements of the Penodlc Table, excellent lumts of detection, m particular m ICP-MS, and low levels of physlcal or chermcal mterferences because of a h& kmetlc temperature Actually, the plasma is part of an mstrument and it does not seem that it can be clamed that an ICP system 1s absolutely free from mterferences The mterferences can ongmate in the discharge Itself, because of mefflclent energy transfer from the plasma to the sample They can also occur as a result of changes m the sample mtroductlon system

Smce the mtroductlon of this source [4,5], Q- agnostics have been apphed to the ICP to obtam an understandmg of the basic processes that trans- form the mjected species mto photons or ions

These processes can be summarued as the atorm- zatlon step, 1 e , from particles to free atoms, and the excitation and lomzation steps The fust step 1s of the order of &seconds and the followmg steps are of the order of nanoseconds Except for a few elements for wbch excitation of the ion reqmres too high an energy, most elements etiblt spectra of neutral atoms and smgly lomzed atoms (atormc and lomc hnes)

One major cntenon for the understandmg of the plasma processes 1s the closeness to the local thermodynarmc equlhbnum (LTE) Plasmas are composed of species wth a very large &fference m mass electrons and heavy argon particles Follow- mg energy deposlhon through the h&-frequency field, the two species will etiblt fast relaxation processes towards two different Maxwelhan dlstn- butlons, 1 e , two different kmetic enerees wtth T, and T,, the kmetlc temperature of the electrons and the heavy particles, respectively [6] The sec- ond process where the two sub-systems would

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86 JM MERMET

tend to thermal eqmhbnum wth only a smgle temperature 1s usually very slow owmg to the poor efficiency of the energy transfer between the elec- trons and the heavy particles The ICP belongs to the category of thermal plasmas where there 1s a correlation between the behavlour of the electrons and the heavy particles, 1 e , T,, tends to T, Thermal plasmas should be close to being m LTE

Several cntena for the assumption of LTE have been described [6,7] for both optically thm and thuzk hydrogen-hke plasmas The electron number density, n _ must reach a nummum value so that the electron-atom superelastic colhslons pre- dommate over the radatlve processes of the lowest excited levels of the plasma gas With a thck plasma, the ermtted radiation from the resonance levels 1s reabsorbed, which consequently decreases the value of n, reqmred to obtam a LTE Both Gnem’s [7] and Drawm’s [6] cntena lead to the same value of ne wthm a factor of 3, I e , 1 5 X

102* and 4 x 102* rne3, respectively for c = lo4 K

These n, values are seldom observed m an ICP, wluch seems to etiblt a more or less slgrufrcant departure from LTE Moreover, the LTE cntena have been estabhshed for plasmas unth no molecu- lar species present m the &scharge, which 1s not the case for analytical plasmas Molecular species ongmate, m particular, from &ssociation of water

An alternative to venfy the departure from LTE is the Qfference between the various temper- atures [8] measured m the plasmas, namely T,,, the excitation temperature, T,,, the kmetic tem- perature, T,o,,, the lomzatlon temperature, and T, In the ICP, T, IS m the range 8000-11000 K [9-121 and 1s lllgher than the kmetic temperature for whch the best determmatlon 1s obtamed by measunng the Doppler width [13,14] Usually, T,,

< Tkm i Tm < T, Possible departure from LTE has been a matter

of &scusslon [15-211 ms departure can be ex- plamed by the &fflculty m measurmg either the electron number density or the vanous tempera- tures mentioned above Another cntenon can be used and 1s based on the measurement of an lomc to atormc hne mtenslty ratio For a system m LTE, such a ratio 1s related to the electron number density through the Saha equation Assummg LTE,

a theoretical ratio can be calculated so that expen- mental ratios can be compared wth It The ad- vantage of this method 1s its amphaty

Not only do expenmental lomc to atonuc hne intensity ratios allow an evaluation of the depar- ture from LTE, but also they wdl reflect any meffzlency m the atormzatlon, excltatlon and lomzation processes It has been demonstrated [22,23] that most atonuc hnes are relatively msen- sltlve to changes m the operating parameters, m contrast to lomc hnes They have been called [22] hard and soft hnes, respectively Therefore, the lomc to atonuc hne mtenslty ratlo urlll allow the measurement of the departure from LTE and venflcahon of the optumzatlon of the operatmg parameters of the plasma Moreover, it ullll be possible to assess whether any change m the signal 1s due to vanations m the properties of the plasma In part~ular, it can be used m the &agnostlcs of some interference effects, m particular those due to easily lomzable elements which are often pres- ent m the matnx

SELECTION OF THE TEST ELEMENT

Using the Saha equation where T, 1s mvolved, and the hne mtenslty relation&p where T,,, 1s involved via the Boltzmann function [8], the lomc to atormc hne ratlo 1,/I, can be deduced

1,/I, = (4 83 x 102’/n,)(g,A,h,/g,A,h,)~3’2

X exp( - -%JkT, )

xexp[ - (E& - Q&&T,,]

where g is the statistical we@t and A the transl- tlon probabtity, X the wavelength, E,,, the excl- tatlon energy, E,,, the lomzatlon energy and k the Boltzmann constant For tis equation n, must be expressed m mw3, and when LTE 1s assumed, T,=T_=T

Among the possible elements, magnesmm has often been used [24-451 wth the hne selection Mg II 2&o 270 nm (or Mg II 279 553 nm) and Mg I 285 213 nm One advantage of dus selection 1s the closeness of the excitation energes of the atonuc and lomc hnes, 35 051 and 35 669 cm-‘, respec- tively ms makes the mfluence of the second

MAGNESIUM AS TF!ST ELEMENT IN ICP-AES 87

exponential, 0 91 for T = 10000 K and 0 88 for T = 7000 K, almost neghgble Moreover, the ac- curacy of gA values 1s acceptable [46] wth &4 = 5 32 x 10’ and 14 85 x 10’ s-l for Mg II 280 270 nm and Mg I 285 213 nm, respectively ms work wdl be concerned v&h the Mg II 280 270-nm hne mtenslty In order to ut&ze data from other pub- hcatlons concermng the Mg II 279 553-nm hne, a conversion factor was apphed The conversion to Mg II 280 270 nm 1s obtamed by dlvldmg the hne intensity by 2, as these two hnes have almost the same A values and E,,,, 35761 cm-’ for Mg II 279 553 nm, and differ only m the g values, 4 and 2 for Mg II 279 553 nm and Mg II 280 270 nm, respectively The closeness of wavelengths usually avoids a wavelength response correction

Another advantage of using the Mg II 280 270- nm line 1s its high sensltlvlty to parameter changes It has been shown [47] that hnes with an energy sum, 1 e , the sum of the lomzatlon energy and the excltatlon energy, m the range 11-13 eV are the most sensitive to any change m the energy transfer between the plasma and the mJected species tis 1s the case for the Mg II 280 270-m hne as Its energy sum is 12 1 eV

It 1s interesting that there 1s currently a good knowledge of the excitation mechamsms of Mg lomc hnes The hnes such as Mg II 279 078 nm or Mg II 293 651 nm have an energy sum above 16 eV (Fig l), 1 e , shghtly lugher than the lomzatlon energy of Ar (15 76 ev) It has been reported that charge-transfer excitation by argon Ions should be efficient m an ICP [48,49] Theoretical calcula-

3d ‘D 8 88 (16 51)

Cl594 45 2s I I I , ; 8 8 85 86 (16 (18 30) 51)

-1576

3p *P 4 43 (12 08)

4 42 (12 07)

Fig 1 Energy levels of Mg Ion arlth the excltatlon energy and the energy sum (lomzatlon energy and excttatlon energy) m eV

r” 2 a-

z” 2-

0 1 10 100

Electron number denslty (lOZa rnm3)

Fig 2 Tbeoretlcal calculation of the Mg II 280 270-nm/Mg I

285 213-nm hne intensity ratlo as a function of the electron

number density usmg the Saha and the Boltzmann equations

The upper curve 1s obtamed assummg the presence only of Ar, the lower curve IS obtamcd assummg an Ar-Mg tmxture

(Ar/Mg = 104)

tions [20] and recent experunents [50,51] have m&cated that hnes ongmatmg from the 3d ‘D and 4s ‘S levels (Fig 1) are populated by a charge-exchange process from the ground-state Mg atoms This is m contrast to the iomc hnes ongmatmg from the 3p ‘P level (e g , Mg II 280 270 nm), whch are populated by electron unpact from the ground-state Mg ions This means that the 3d ‘D and 4s ‘S levels are overpopulated with respect to LTE, and explams why a Boltzmann plot usmg Mg lomc hnes below and above 16 eV does not provide a hnear graph [48]

Calculatron of the Mg II 280 270-nm/Mg I 285 213-nm hne mtenslty wtzo

When the electron number density 1s de- termmed usmg a method mdependent of the LTE, e g , the Stark effect on H, [8], a correspondmg T, value can be deduced from the Saha equation, assummg a smgle temperature and argon as the predommant species Then, T, and n, values can be used m Eqn 1 together with the gA values previously gven for Mg Equation 1 becomes

1,/r, = (1 58 X 1021/n.)T3/2 exp( -88732/T)

(2) A theoretical 1,/l, ratio 1s deduced as a function of n, (Fig 2) Under the LTE assumption, the

88 JM MERhET

TABLE 1

Temperature (T), electron number density calculated wth Ar as the only species [n, (Ar)], electron number density assummg a bmary Ar-Mg nnxture wth Ar/Mg = lo4 [n, (Ar-Mg)], Mg II 280 270~nm/Mg I 285 213-nm hne mtenslty ratlo calculated from Eqn 2 [Z,/Z, (Eqn 2)], raho for Mg II/Mg I states calculated from the Saha equation (n,/n,) and correspondmg Mg II 280 270-nm/Mg I 285 213-nm hne mtemnty rabo calcu- lated from Eqn 4 [Z,/Z, (Eqn 4)]

1% /Ia (Eqn 2) Fir-Mg)

h/n, A/L (Eqn 4)

(m-?

6500 101~10~~ 96 166 x 1020 34 2 5 96 7ooO 283~10’~ 102 3 28X1020 50 0 8 80 7500 6 90X10Zo 10 8 715x1020 57 2 10 16 8000 1 51X102’ 114 151 x 102’ 60 1 10 75 8500 301x102’ 120 3 01 x 102’ 62 2 1120 9000 557~10~’ 126 5 57x102’ 62 7 11 36 9500 970x10= 133 970x102’ 62 9 1145

lo000 160x1022 138 160~10~~ 62 3 1140

value of the ratio vanes between 10 and 13 over the range of n, normally observed m an ICP

A more sophisticated approach 1s to calculate the Saha equation assummg a bmary Ar-Mg nux- ture [52] still under LTE It 1s possible to deduce the n,/n, ratio (ion and atom number densities, respectively) and to obtam the Z,/Z, ratto, consld- ermg that the Z,/Z, ratio 1s constant and equal to 0 55

Z/Z, = (~,/~,)(g,A,X~g,A=X,)(z,/z,)

Xexp[ -(j%-Z%)/kT] (3)

and

Z/Z, = 0 2( n,/n,) exp( - 889/T) (4)

For an Ar/Mg ratio of 104, the mfluence of Mg on n, 1s only appreciable below 7000 K (Table 1) The maxunurn value obtamed for n, is near to 1020 m- 3 The n/n, ratio values and the correspondmg Z/Z, values are reported m Table 1 [40] The Z/Z, ratio 1s also shown as a function of n, m Fzg 2 For n, values between 5 X 10” and 5 X 1021 rnm3, tis ratio 1s shghtly below that obtamed assummg only the presence of Ar An Ar/Mg ratio of lo4 corresponds to a high con- centration of Mg, dependmg on whether the total argon flow-rate 1s considered or only the argon located m the central channel The actual values

of the ratio would be between the two curves gven m Fig 2

INFLUENCE OF ENERGY TRANSFER

Although the argon ICP etiblts a high kmetic temperature, the energy transfer from the sur- roundmg plasma to the central channel where the sample 1s located can be mefflclent Ths transfer of energy wdl depend on some operating parame- ters of the ICP such as the power, the tamer gas flow-rate, the inner diameter (1 d ) of the injector, the amount of aerosol, the use of a sheathmg gas and the presence of molecular gases In part~ular, the velocity of the aerosol at the exit of the m~ec-

tor wdl depend on both the earner gas flow-rate and the 1 d of the mjector

Role of power Using a 27-MHz ICP, an injector 1 d of 15 mm

and a aer gas flow-rate of 0 9 1 mu-*, Caugh- hn and Blades [34] obtamed results for power m the range l-2 kW The vanatlon of the electron number density was between 7 x 10zo and 4 x 102’ rnp3 It can be seen m Fig 3 that the Mg II/Mg I ratio Increased mth mcreasmg power, but re- mamed below the values calculated under LTE Figure 3 also shows results obtamed wth a sun&u 27-MHz ICP, but with an mJector 1 d of 2 mm [4O] and a tamer gas flow-rate of 1 1 nun-’ For

“1 11 12 13 14 15 16 17 18 18 2

Power (kW)

Ag 3 Effect of power on the Mg II 280270~nm/Mg I 285 213-nm hne mtenslty ratlo for two mjector mner dmms ters A 27-MHz generator was used for both expenments (W) 15mmtd [34],(.)2mmId [4O]

MAGNESIUM AS TEST ELEMENT IN ICP-AES

the h@est power of 18 kW a value w&m the LTE range was obtamed

It should be noted that there is no direct rela- tion&p between the electron number density, as two different Mg II/Mg I ratios can be obtamed at the same density With the expenments men- tioned above, at 175 kW, Caughhn and Blades [34] measured an electron number density of 3 4 X 102’ mV3, which is not very different from the value of 2 8 X 1021 mm3 3 obtamed by Mermet [40] at 1 8 kW However, the Mg II/Mg I ratios were 6 15 and 12 6, respectively Thus absence of a relatlonshlp has also been desctlbed by Oleslk and Den [43)

The effect of the pressure has been also de- scribed [14] It can be compared urlth a change m the power A decrease m the pressure corresponds to a reduction m the Mg II/Mg I hne ratio An ICP mstalled far above the sea level wrll e&bit poorer atonuzatmn and lomzatlon capabhtles

Role of resrdence time Although the role of the forward power 1s nn-

portant, it has already been seen that the mfluence of the 1 d of the mjector has to be considered In fact, the residence tune 1s related to both the tamer gas flow-rate and the mJector 1 d At ob- servation heights above the load coti, It has been reported that the velocity of the large droplets still exlstmg m the plasma 1s almost independent of the canner gas flow-rate [53] Apparently, an accel- eratlon stage occurs between the tip of the injector and the normal analytical zone dunng the atonu- zatlon process A lower speed of the cold gas at the exit of the mjector corresponds to a longer atonuzation process

Figure 4 shows the effect of the tamer gas flow-rate on the Mg II/Mg I hne ratio for three inner hameters of the m~ector, 1 e , 1 5, 2 and 3 mm [54] The generator was operated at 40 MHz It 1s mterestmg that the most commonly used injector 1 d has been 15 mm for years With this mqector, the hqhest ratio was obtamed at 0 6 1 mm-‘, where the efflclency of a pneumatic nebu- hzer is poor A value of 15 was reported wth an qector of 14 mm 1 d , but the tamer gas flow-rate wasonly041mm-‘[43],andat071mn-‘the ratio was less than 5 In this work a raho of 10

89

0 06 066 0, 075 08 086 09 095 1

Carrier gas flow-rate (I /mm)

Fig 4 Effect of carrier gas flow-rate on the Mg II 280 270- nm/Mg I 285 213-nm lme intensity ratI for three mJeCtOr

mner dmmeters A 40-MHz generator was used (A) 15 mm ~d,(~)2mm~d,(.)3mm~d

was obtamed at 0 75 1 mn-’ with an 1 d of 3 mm Figure 5 shows the results obtamed wth a 40-MHz generator prototype [55] The nebuhzer was work- mg at 0 95 1 mm-l The effect of the observation height on the Mg II/Mg I ratio for three different mJectors of 1 8, 2 1 and 3 mm 1 d IS shown m Fig 5 Even with such a tamer gas flow-rate, ratios near 10 can be reached mth the 2 1 and 3 mm 1 d inJectors

Nowak et al [17] also observed that closeness to LTE occurs at low earner gas flow-rates (< 0 5 1 mm-‘) and h@ power

13 5 7 9 11 13 15 17 19 21 23 25 27 29 3,

Observation height ALC (mm)

Fig 5 Effect of observation he@ above the load co11 (ALC) on the Mg II 280 270-nm/Mg I 285 213-run lme mtenslty ratio for three injector mner diameters A 40-MHz prototype was used (r)lSmm~d,(m)21mm~d,(.)3mm~d

90

Role of amount of aerosol The amount of aerosol vanes with the tamer

gas flow-rate, and so must also be taken mto account It 1s possible to study tlus vmation by decreasmg the speed of the penstaltlc pump which feeds the pneumatic nebuhzer Bates and Oleslk [45] used a Memhard concentnc nebuhzer at 0 8 1 mu-l The flow-rate was vaned between 0 4 and 2 0 ml mm-‘, wluch corresponded to an amount of aerosol between 5 2 and 8 3 mg mm - ’ The Mg II line decreased as the amount of aerosol m- creased and the Mg I hne remamed constant up to an amount of 6 8 mg mm-’ and then also de- creased Therefore, m this expenment, where the mjector 1 d was 15 mm, the ratio was dependent on the amount of aerosol However, a smular expenment was cmed out urlth an 1 d of 3 mm, and a reduction of 50% m the amount of aerosol did not produce a change m the Mg II/Mg I ratio [56], which remamed at 14 for a tamer gas flow- rate of 0 7 1 mu-’ It seems that the mfluence of the amount of aerosol IS also hnked to the rest- dence tune

Effxzency of energy transfer The h&frequency field 1s coupled with the

electrons located at the penphery of the plasma, owmg to the skm effect Then, the energy 1s trans- nutted to the centre of the discharge through colhslons Change m frequency has been reported to mo&fy the energy transfer [35] and a vanatlon m the Mg II/Mg I line ratio resulted between 5 and 100 MHz [39] Usually, the ratio decreases with increase m frequency and it 1s dfficult to obtam ratio values larger than 6 at 100 MHz

Another way to mo&fy the couplmg efflclency 1s to use a lammar sheathmg gas added at the exit of the spray chamber Ongmally designed [57] to avoid any deposmon on the mner wall of the injector, it has been shown [47] that the use of a sheathmg gas reduces the energy transfer between the surroundmg plasma and the injected speaes An example 1s gven m Fig 6, where a sheathmg gas flow-rate m the range 0 06-O 42 was added to a earner gas flow-rate of 0 75 1 mm-’ The genera- tor was operating at 40 MHz and the injector 1 d was 3 mm [58] Although the earner gas flow-rate and therefore the amount of aerosol were the

JM MERMET

005 01 015 02 025 03 035 04 045

Sheathmg gas flow-rate (I /mm)

Fig 6 Effect of sheathmg gas flow-rate on the Mg II 280 270- nm/Mg I 285 213-nm lme mtenslty ratio for a tamer gas

flow-rate of 0 75 1 mm-’ A 40-MHz generator was used The

upper curve corresponds to the ad&tlon of 002 1 mu-’ of

hydrogen

same, the vanatlon of the ratio was by a factor up to 4 One possible way to overcome ths problem 1s to add a molecular gas to argon The thermal conductlvlty of argon 1s ca 0 1 W m-l K-’ m the temperature range 3000-7000 K [59] Molecular gases such as mtrogen and hydrogen etiblt a h&er thermal conductlvlty In particular, hydro- gen etiblts a value of 15 W m-l K-’ at 3800 K because of Its dtssoclatlon Hydrogen should be preferred to mtrogen, as hydrogen already exists m the plasma and enuts a few hnes whereas mtrogen produces molecular bands It has been reported [60] that addlhon of 0 05 1 mm-’ of hydrogen produced an enhancement of up to a factor of 10 for lomc hnes with an excltakon energy m the range 6-8 eV At 4 4 eV, whch 1s the excitation energy of the Mg II 280-nm lme, the enhancement factor 1s only 2 Atonuc hnes are almost msensltlve to hydrogen An enhancement factor of 2 1s only obtamed for atormc lmes wth an excitation energy greater than 5 eV, which exceeds that of the Mg I 280 273-nm hne There- fore, use of hydrogen leads to a shght lmprove- ment m the Mg II/Mg I ratio (Fig 6)

When hydrogen 1s not present, e g , m the ab- sence of water, It 1s known [31,59,61-641 that mjectlon of dry aerosols corresponds to tempera- tures lower than those observed for the conven- tional mjection of wet aerosols It has been re- ported [43] that the Mg II/Mg I hne ratio be-

MAGNESIUM AS TEST ELEMENT IN ICP-AES

comes mdependent of the tamer gas flow-rate between 0 4 and 10 1 mm-’ for a dry aerosol A ratio value of 10, wbch 1s less than the maxnnum ratlo of 15, was obtamed at 0 4 1 nun-l for a wet aerosol

Influence of energy transfer on the role of sodwm Although the mfluence of easily lomzable ele-

ments (EIE) such as Na, K, Ll and Cs, has been urldely stud&, there 1s no clear evidence of the= role m the change m the slgnal of conconutant elements If it were a.n iomzation mterference, both a reduction m the lomc hne intensity and an mcrease m the atormc hne mtenslty would be observed In fact, enhancement and depressive effects were reported on both types of hnes, de- pending on the power, the earner gas flow-rate and the observation height Savage and J%eftJe [32] reported an enhancement of 40% on the Mg I 28%nm hne and only 10% on the Mg II 279~nm hne by addmg Na for a earner gas flow-rate of 0 86 1 nun-’ Sumlar results [32] were found urlth Cs (42% and 23X, respectively) These results therefore corresponded to a decrease m the Mg II/ Mg I line ratlo They also noted that the amount of mterferences decreased as the power was mcreased and the earner gas flow-rate was decreased

The mfluence of the power and the tamer gas flow-rate can be illustrated usmg a 27-MHz gener- ator In Fig 7 a Perkm-Elmer cross-flow nebuhzer and m Fig 8 a Cetac ultrasomc nebuhzer mth desolvation was used, each unth an mjector 1 d of 2 mm Results are summarized for two sets of con&tlons rmd-power (1 kW) and a h& earner gas-flow rate (1 1 mm-‘) and high power (1 6 kW) and low carnef gas flow-rate [0 6 1 nur-l (cross- flow nebuhzer) or 0 5 1 nun-’ (ultrasomc nebu- hzer)] As described above, the latter con&tions will provide the most efflclent energy transfer It 1s evident from both Figs 7 and 8 that the addltlon of 10 g 1-l of Na has a ngmflcant mfluence on the Mg II/Mg I ratio at nud-power and lugh tamer gas flow-rate, m contrast to the efficient energy- transfer con&tlons, where the ratio 1s not changed In the latter mstance, a snrular decrease was ob- served on both hnes, vrespectlve of the observa- tlon height

Observation height ALC (mm)

Fig 7 Effect of observation be&t above the load cod and the presence of Na on the Mg II 280 270-nm/Mg I 285 213~nm hne mtenslty ratlo for two sets of operatmg condmons A 27-MHz generator and a Perkm-Elmer cross-flow pneumatic nebulmzr were used (0) 16 kW and 0 6 1 mm-‘, no Na, (m) 16 kW and 0 6 1 nun-‘, addlhon of 9 g 1-l Na, (A) 1 kW and 1lmm-‘,noNa,(r)1kWand1lmu~‘,add1tionof9gl~’ Na

To confnm the large mfluence of the energy- transfer efficiency on the role of Na, an expen- ment using a sheathmg gas was made It has been mentioned above that the use of a sheathmg gas degrades the energy-transfer Usmg the same con-

14

I

-2 3 4 5 a 7 a s 10 11 12 13 14 16 1s

Observation height ALC (mm)

Fig 8 Effect of observation he&t above the load ~011 and the presence of Na on the Mg II 280 270-nm/Mg I 285 213-nm lme mtenslty ratlo for two sets of operating oon&tmns A 27-MHz generator and a Cetac ultrasomc nebubzer w~tb a desolvahon were used (0) 1 6 kW and 0 5 1 mm-‘, no Na, (m) 16 kW and 0 5 1 nun-’ adchtlon of 9 g 1-l Na, (A) 1 kW and 11 mm-‘, no Na, (r) I kW and 1 1 nun-‘, addlhon of 9 g I-’ Na

JM tdFXh%ET

01 I 1 3 s 7 @ 11 13 15 17

Observabon height ALC (mm)

Fig 9 Effect of observation height above the load cm1 the presence of Na and the sheathmg gas flow-rate on the Mg II 280 270-nm/Mg I 285 213-nm lme mtenslty ratio for two sets

of operatmg condmons A 40-MHz generator and a Memhard

type C concentnc n~buhzer were used (0) Sheathmg gas flow-rate 0 06 1 mu-‘, no Na, (4) sheathmg gas flow-rate 0 06

I nun-‘, ad&uon of 9 g I-’ Na, (A) sheathmg gas flow-rate 0 42 1 mm-‘, no Na, (v) sheathmg gas flow-rate 0 42 I nun-‘, addltton of 9 g I-’ Na

&tlons as m Fig 6 [58], the effect of Na on the Mg II/Mg I ratio was studied for a sheathmg gas flow-rate of 0 06 and 0 42 1 mm-’ at &fferent observation heq&ts In Fig 9, it can be seen that Na had almost no effect for a sheathmg gas flow- rate of 006 1 mm-‘, the Mg II/Mg I ratio was above 10 For observation heights above 10 mm, the curves wth and w&out ad&tlon of Na were sumlar This 1s m contrast to the behavlour ob- served for a sheathmg gas flow-rate of 0 42 1 mm -’ where the curves with and without addition of Na differed slgmflcantly An mcrease of a factor of 2 m the ratio was obtained above an observation height of 10 mm Usmg a 5 ~-MHZ generator, a power of 6 kW, an injector 1 d of 2 mm and a tamer gas flow-rate of 15 1 mm-’ [26], it was also found that the ratio was mdependent of the Na concentration up to 1 g 1-l and equal to 15 The relatively high tamer gas flow-rate was compensated for by the htgh power and the large qector 1 d

The ICP 1s a source where it 1s possible to combme atonuzatlon of the sample and excitation

and lomzation of the free atoms The h& kmetic temperature etiblted by the plasma, usually above 5000 K, should pernut complete atomua- tlon of any type of sample to be observed As the atonuzation processes involved are of the same order of time as the residence trme of the sample, 1 e , a few rmlhseconds, complete atonuzation and efficient lomzatlon will be obtamed only if careful optnmzatlon of the ICP operating parameters is done The most nnportant parameters are the car- ner gas flow-rate and the 1 d of the injector The velocity of the cold gas at the exit of the mJector 1s dependent on both the tamer gas flow-rate and the 1 d of the mJector, whereas the amount of aerosol depends on the tamer gas flow-rate A reduction m the power also reduces the energy- transfer As for the optmuzatlon of the atonnza- tlon and lomzatlon, mterference effects such as those produced by the ad&tion of easdy lomzable elements, can be numnuzed with the same set of parameters A snnple way to venfy this optnmza- tion is to use an ionic to atormc hne mtensity ratio of an element It was seen that Mg 1s smtable for this purpose Values of the Mg II 280 270-nm/Mg I 285 213-nm line mtensity ratio above 10 corre- sponded to an optlrmzatlon of the ICP and to mmmuzation of mterference effects of easdy lomzable elements

The current trend to reduce the tamer gas flow-rate and to mcrease the 1 d of the qector is 111 good agreement with the efflclency of energy- transfer It seems that the use of an ultrasomc nebuhzer is Justified not only by the improvement m the hnuts of detection but also by the posslbll- lty of havmg an efficient nebuhzer, even at tamer gas flow-rates below 0 6 1 nun- ’ There is another trend, which 1s to reduce the power of the genera- tor This IS hnked to the reduction 111 the plasma gas consumption Another benefit 1s the decrease 111 the Size and cost of the generator However, it has been observed that too large a reduction m the power corresponded to a value of the Mg II/Mg I ratio of less than 10

To obtam efficient atonuzation and lomzation and also mmunum interference effects, the use of a cmer gas flow-rate below 0 6 1 mm-’ 1s recom- mended, together wfh an injector 1 d of at least 2 mm and a power higher than 14 kW Addition of

MAGNESIUM AS TEST ELEMENT IN ICP-AES 93

molecular gases such as hydrogen can be helpful If these condlhons are not used

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