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The field ionization properties of electrochemically produced Ni- and Co-emitters
Zntemoiionol Job of Mass Spectromety OJixsevierScientitc~~Company.~-
TEEE E-IEJLD IONIZATION PIuDmm PRODUCEDNi-AND&RlWJ.TERS
and Ion Phyaict. 25 (1977) 147-154 Printed iu The Netherlands
:A7 ‘-?_
OF --@ALLY
S.J. RRDDY, Institute
F-W. RijIJiGEN, A. MAAS and IXD. BECKETY of Physic& Chemktzy. Univemi ty of Bonn. Wegehtr.
12. D-6300
Bonn (W_
Germ-Y) (Reazioea
‘Ibe
2 February 1977)
emi&
on
~_ic$fiidioniration
properties of Ni- and Co-emittess
of the type recently
ixr_kodPcea in
and~dd~~tiofmnrespectrometryhavebeen~~ted. rscwem3 that these properks are pnnapally controlled by the bulk strucftue of
t.bemia-oneedtes.Aftex preparationthemi~deaarcinmlatorsor poorconductcws at room temperatrue since they are composed mainly of the oxide of aickel or cobalt mqxctk&_ The “activation” of these emit&m by a heat treatment lef&s to tbe formation of electrically conducting microneedles.
INTRODUCTION
In tbe past several publications on electrochemical methods for production of field anodes f&r field ionization (m/field desorption (FD) mass spectrometzy have appeared [l-3] _ Using these methods metals are deposited on a metallic carrier wire (most& a 10 m tungsten wire) to form dendritic microneedles. These microneedles produce tile high electric field strengths of several 10’ v cm-’ which are reqti for field ionization. The emical methods for production of field anodes is advantageofusingelectrocb the short time (a few minutes only necesary for groti) and tile possibility of using a number of different metals as field ion emittefs for surface studies. In the present paper we shall report on the emission properties of electrochemically produced Cc+ and N&emitters_ This study was originally started with the intention of exploring the possibilities of these emitters for surface studies_ The low evaporation field strengtJ~sof Co and Ni [4] suggested a quick and reproducible production of ads~rba&+W metal e However,inthecorweofthisstudgitkrrnedoutthatthemicroneedles inrefs.2and3arenotmetallic; ‘callyasdesuibed produced &.ectxM’ minantly oxidic structure with semiconducting properties theyhaveapredo ___ tr&anentreqGred Acharactenshcmd=Xtlo - nofthisstructureisthethexmal foi production Of &Id ionization cuuenti
I48
Ni- and Coemitters were produced according to the method described in refs. 2 and 3, In the c8se of S an aqueous 6 M Co(NO& solution at 85°C was used. An ek&ro4&emicalcellhavingacathodecon&tingofa 10 JHII tungsten wire of 3 mm length and-an anode of a platinum wire 0.5 mm in diameter was used. The distance between these two electrodes was about 3 mm_ For the formation of metal dendrites 25 pulses of 15 V amplitude and 13 ms duration were applied to the ekctrodes. In the case of nickel emitters only 5 pulses of 25 V and 1.5 ms were applied. The Ni(NO& solution was 4 M, and was held at a temperature of 90°C The emitters were “activated” by heating the wires in vacuum for about 15minata current of 40 mA (ca 600°C) to achieve a high ion emission_ For the mass spectrometric studies a single focusing SO” magnetic mass spcctromewwasused. Al -on et-n microscope (Philips EM 300) was used for the structure ckacidation of the microneedles, A heated sample holder enabled studies to be made up to 1000°C. The temperature was measured by a PtFUl/ E%thermocouple. RESULTS
AND DISCUSSION
Figuresland2show examples of Co- and Niemittcrs_ The ionization efficiencies of these emitters for FI of organic substances were about 1 order of magnitude lower than for carbon emitters- However, it has to be assumed that the shape and the density of the microneedles may not represent the optimum attainable for FI of organic substances_ Comparison of the Ni- and Co-emitters with carbon emittezs for applications in FD mass spectrometry is more difficult, since the properties of FD emitters are strongiy substancespecific. This was shown recently with the example of the FD of salts 163 _ Therefore the problem of the usefulness of the% emitters for FD of organic substances will not be disc& here. Tbe most important parameter for the pattern of FI mass spectza is the field strength- The intPnnity ratio of the fkagment ions C& to the
molecular ions GE& in the FI spectrum of n-heptane W the ‘bxcLium” fieldt3tmngm~?]-Thisinte~rati0liesbetwee!n 10-4and 2 - lo-’ tith n.icke!land cob&t emitters. The corresponding field sbngths are about l-2 - 1o’p v cm-’ -Theaveragefieldstrengthsaresomewhathigher
with carbon emitters than with the “metal” emitters, The inratio H20z/H~O* from the water background shows larger differenoes between ~nazbdnickelorco~temi~.Thisratioisno~much~ than 1 with the “metal” emitters. The carbon emitters usuatly gbow an intensity ratio HzOt/HSO*> 1, This intensity ratio is a merrsure of the contribution of emission areas of high field skeng&. to the total~cu#ssio~n [S]. In the case H,O’/If,O’ << 1 the ionization efc _ ncp of moJec+s kith Mgh
Pig.
1. Ekc~emical3y
produced Xi-emitter, carrierz lOpmW-wins
ionizationpotentials is veqr small.
Consequently, the “metal” emitters are not suitable for ionization of molecules having an ionization energy of more than about 11 eV. This,however, is not essential for analytica\ applications. On the other hand, several thermal effects are important for the use of the ‘Lmetal” emitters in FI/FD mass spectrometry, It is well known that after production of the emitters no, or or& a very weak ion emission is observed at room temperature_ A bigb emission is achieved only after tbennal tread ment of the emitters. F’urther, not ordy the intensity but also the energy distribution of field ions depends on the heat treatment. Narrow peak shapes speckam, corresponding to a narrow energy distriiution of the inthemass ions. are usually only achieved after or during heating of the anode with the high field applied, In tbe case of N&emitters the opposite effect is also 0-k after heating to hi@ again a peak temperature s P6OO”C) broadening to lower w is found. The peak broadening can resuit in mass, This phenomenon was not severalpeaksbelongingtithesame obsenEdwiUIc33-emi~Figure3showsan example of +is effect which misinterpretationofspect+takenwithaair@efocusi4Z --WM_h
marnr~meter.Thepotentiatdeficitofthe~oris(relatmetotheanode potential)Swhidl ~ndstotbepealrsirifi,canrangeuptosomelOOV-
-
The temper&are dependence of the peak group is proof that these p&s (F’ig. 3b) are correlated _to the same ion. The energy deficit decreases w& increasing anode temperature and al3 peaks move steadily along the mass scale to the correct mass position (m/e = 100). Finally, with both wpes of emitters are around SOO”C, far below the melting points of Ni (1453°C) and Co (1495X), a rupture of the microneedles from the metaRic support is observed with both Wand Pt. The question of the causes of the structure of the electrochemically produced microneedles arises. Or@nally it was assumed that the initial inactivity of the emitters is caused by electrically isolating or semiconducting is surface layers (for example by oxide films). However, this assumption nsistent with the observation that the same thermal treatment for activaEz of the field anodes is also necessary after strong etching of the emitters pe. Influence of the by acids sufficient to be visible under the light mim metal wims (tung&en or platinum) on the thermal treatment was not observed. Consequently, the reason for the thermal effects can only be found in an irreversible change of the structure of the microneedles. This means that the microneedles of both Ni- and Co-emitters are electrically of the experiment nonconducting or only semiconducting at the beginniq and that the electrical conductivity which is necessary for ionization is only achieved by thermal treatment. The conductivity of the needles in the electrochemical cell is favoured by the high temperature of the electrolyte (ahout 90°C). The structure and the temperature-dependent structural changes of the -on electron f microneedles were observed with a high resoIution microscope having a heated specimen holder. With both types of emitter a transparent region on the tips of the microneedles was selected for the 100 kV electron beam. The temperaturedependent changes of the morphology -on and and structure of the microneedles could be observed in f with the aid of electron diffra&on. Figure 4 shows typical changes of the shape and structure of the micro. and the heating needles of cobalt emitters depending on the tempemture time. Figure 4(a) shows the tip of a microneedle. which is only weakly transparent at-room temperature. The corresponding difbction pattern (Fig. 4b) consists of diffuw rings and therefore points to a largely amorphous structure of the needles which are of cobalt oxide-with increasing temperature the rings hecome sharper corresponding to an increase of the . ‘on. Above 300°C a pronounced contraction of the degre& of crysM&&
needle volume was observab le. In the temperatwe range 4-OO-!500°C distinct
reflexesappeared~fromcrgatnnine~inthevicinityofthewireand later also from t&e tips of tie microneedles as indicated by dark field These distinct reflexes originated from cobalt oxide, and at ternaboveBboUt500QC~&ommetallicco~Figure4(c)~o~thetipof tbemicroneedleat6000CaPterabout40minh~tiine_Tfietipis-_comPared with pis- 4(a) - m~ch~andshowskgerstrongbrtransparent
images.
162r-
Fig. and
-
.
--
_-
pattems of a tip of a “Co” micronedIe (a) rsraaionofthe~tter,(c)md(d)~tWO~C.
observed: the microneedles are inithllyalso amorphous or microcryg tdine, their diffraction patterns at room temperature however, do not oniy indicate the existence of NiO but aiso weak metaliic nickel. ‘Ibe degree of crystaihzation increases with rising temperature and distinct reflexes coming fIom NiO and Ni are observed. Morphological changes are at about 200°C cieariy obsemable. In some cases a contraction of the needle volume of more than 50% was observed. The dectron microscope observations explain the initial insulator properties of the microneedies because NiO and Co0 are electrically nonconducting. The increase of the conductivity of the needles during and after the thermal treatment is probably due to an increase of the density of the needle material and/or to the formation of metallic particles. The morphological changes which are &ready clearly observable at lower temperatures are presumably caused by inclusions of nickel and cobalt salts which have very low melting points (ca. 55°C) and decomposition temperatures (c-a. 130°C) whereas NiO melts at 1990°C. Co0 at 1935°C and C&O3 at 895°C [9]. The low degree of crystallisation and the heterogeneous structure of electrochemicaiiy produced microneedles is very probable the result of a fast needle growthwere
CONCLUSION The structure of the microneedles of the Ni- and Coemitters is mainly oxidic and becomes partially metallic after a thermal treatment. A broad and sometimes also structured energy distribution of the FI products results from this oxidic structure of the emitters. Thus the Ni- and Co-emitters are less suitable for mass spectrometers which do not have energy focusing and they are not suitable for the same reason for field ionization kinetic studies. ACKNOWLEDGThe authors would iike to thank Prof. Dr. M.M. Bursey, North Carolina and Dr. J. Heitbaum, Bonn, for valuable discussions, and the Deutsche Forschungsgemeinscbaft and the Fonda der Deutschen Chemischen Industrie for support of this work. S.J. Ready is grateful to the Deutsche Akademische Austauschdienst for a res?zxmzhscholarship.
and V.G. Golovatyi.Prib.Tekhn. Ehp.,16(1973) LV.Goldenfeld,RN.Bondaremko 166. ~.W~abrrrn~D~.~to~~~SImmonoandMdM.Burslcy.TPt.J.~Spectro=. 1cngw~.17(1975)208. Hintm.T.S.C~IpitksandK.b¶. hU&Btusey.C.E.Ekchkeiuer,M.C.~D-M. ‘lh~~J.PhyrE.9(1976)1&1 E.WJHiiIk md T.T. Ibopg. Fidd fop Microccopp, EZswier. New York, 1969.
s’ ED. Beckey and E-B, Schnltcn. Angew. Chem., Int, Ed. Eugl... 14 (1975) 403. 6 F-W_ RElIgem, tJ. Gieaa.. HAHeinenRlldSd.Re!ddy.rnLJ.MahSpe!chm-Ion pfim. 24 (1977) 235; F-W_ a U. Gikasmaun and Ed. Heinen. Z. Nahrforsc?~. T&IA. 31(1976)172B. MesaDettmik. 80 (1972) 147. 7 F.Spier,H.J.HkinenandHJLBeckey, 8 D-M_ ‘lhyOr, PmW.XEJ&pa and ZLD_ m. Surf- &iv, 40 (1973) 264_ 9 Handbook af Chemistry and Physics, The Chemical Ehbber Punching Co., 1974.
TEEE E-IEJLD IONIZATION PIuDmm PRODUCEDNi-AND&RlWJ.TERS
and Ion Phyaict. 25 (1977) 147-154 Printed iu The Netherlands
:A7 ‘-?_
OF --@ALLY
S.J. RRDDY, Institute
F-W. RijIJiGEN, A. MAAS and IXD. BECKETY of Physic& Chemktzy. Univemi ty of Bonn. Wegehtr.
12. D-6300
Bonn (W_
Germ-Y) (Reazioea
‘Ibe
2 February 1977)
emi&
on
~_ic$fiidioniration
properties of Ni- and Co-emittess
of the type recently
ixr_kodPcea in
and~dd~~tiofmnrespectrometryhavebeen~~ted. rscwem3 that these properks are pnnapally controlled by the bulk strucftue of
t.bemia-oneedtes.Aftex preparationthemi~deaarcinmlatorsor poorconductcws at room temperatrue since they are composed mainly of the oxide of aickel or cobalt mqxctk&_ The “activation” of these emit&m by a heat treatment lef&s to tbe formation of electrically conducting microneedles.
INTRODUCTION
In tbe past several publications on electrochemical methods for production of field anodes f&r field ionization (m/field desorption (FD) mass spectrometzy have appeared [l-3] _ Using these methods metals are deposited on a metallic carrier wire (most& a 10 m tungsten wire) to form dendritic microneedles. These microneedles produce tile high electric field strengths of several 10’ v cm-’ which are reqti for field ionization. The emical methods for production of field anodes is advantageofusingelectrocb the short time (a few minutes only necesary for groti) and tile possibility of using a number of different metals as field ion emittefs for surface studies. In the present paper we shall report on the emission properties of electrochemically produced Cc+ and N&emitters_ This study was originally started with the intention of exploring the possibilities of these emitters for surface studies_ The low evaporation field strengtJ~sof Co and Ni [4] suggested a quick and reproducible production of ads~rba&+W metal e However,inthecorweofthisstudgitkrrnedoutthatthemicroneedles inrefs.2and3arenotmetallic; ‘callyasdesuibed produced &.ectxM’ minantly oxidic structure with semiconducting properties theyhaveapredo ___ tr&anentreqGred Acharactenshcmd=Xtlo - nofthisstructureisthethexmal foi production Of &Id ionization cuuenti
I48
Ni- and Coemitters were produced according to the method described in refs. 2 and 3, In the c8se of S an aqueous 6 M Co(NO& solution at 85°C was used. An ek&ro4&emicalcellhavingacathodecon&tingofa 10 JHII tungsten wire of 3 mm length and-an anode of a platinum wire 0.5 mm in diameter was used. The distance between these two electrodes was about 3 mm_ For the formation of metal dendrites 25 pulses of 15 V amplitude and 13 ms duration were applied to the ekctrodes. In the case of nickel emitters only 5 pulses of 25 V and 1.5 ms were applied. The Ni(NO& solution was 4 M, and was held at a temperature of 90°C The emitters were “activated” by heating the wires in vacuum for about 15minata current of 40 mA (ca 600°C) to achieve a high ion emission_ For the mass spectrometric studies a single focusing SO” magnetic mass spcctromewwasused. Al -on et-n microscope (Philips EM 300) was used for the structure ckacidation of the microneedles, A heated sample holder enabled studies to be made up to 1000°C. The temperature was measured by a PtFUl/ E%thermocouple. RESULTS
AND DISCUSSION
Figuresland2show examples of Co- and Niemittcrs_ The ionization efficiencies of these emitters for FI of organic substances were about 1 order of magnitude lower than for carbon emitters- However, it has to be assumed that the shape and the density of the microneedles may not represent the optimum attainable for FI of organic substances_ Comparison of the Ni- and Co-emitters with carbon emittezs for applications in FD mass spectrometry is more difficult, since the properties of FD emitters are strongiy substancespecific. This was shown recently with the example of the FD of salts 163 _ Therefore the problem of the usefulness of the% emitters for FD of organic substances will not be disc& here. Tbe most important parameter for the pattern of FI mass spectza is the field strength- The intPnnity ratio of the fkagment ions C& to the
molecular ions GE& in the FI spectrum of n-heptane W the ‘bxcLium” fieldt3tmngm~?]-Thisinte~rati0liesbetwee!n 10-4and 2 - lo-’ tith n.icke!land cob&t emitters. The corresponding field sbngths are about l-2 - 1o’p v cm-’ -Theaveragefieldstrengthsaresomewhathigher
with carbon emitters than with the “metal” emitters, The inratio H20z/H~O* from the water background shows larger differenoes between ~nazbdnickelorco~temi~.Thisratioisno~much~ than 1 with the “metal” emitters. The carbon emitters usuatly gbow an intensity ratio HzOt/HSO*> 1, This intensity ratio is a merrsure of the contribution of emission areas of high field skeng&. to the total~cu#ssio~n [S]. In the case H,O’/If,O’ << 1 the ionization efc _ ncp of moJec+s kith Mgh
Pig.
1. Ekc~emical3y
produced Xi-emitter, carrierz lOpmW-wins
ionizationpotentials is veqr small.
Consequently, the “metal” emitters are not suitable for ionization of molecules having an ionization energy of more than about 11 eV. This,however, is not essential for analytica\ applications. On the other hand, several thermal effects are important for the use of the ‘Lmetal” emitters in FI/FD mass spectrometry, It is well known that after production of the emitters no, or or& a very weak ion emission is observed at room temperature_ A bigb emission is achieved only after tbennal tread ment of the emitters. F’urther, not ordy the intensity but also the energy distribution of field ions depends on the heat treatment. Narrow peak shapes speckam, corresponding to a narrow energy distriiution of the inthemass ions. are usually only achieved after or during heating of the anode with the high field applied, In tbe case of N&emitters the opposite effect is also 0-k after heating to hi@ again a peak temperature s P6OO”C) broadening to lower w is found. The peak broadening can resuit in mass, This phenomenon was not severalpeaksbelongingtithesame obsenEdwiUIc33-emi~Figure3showsan example of +is effect which misinterpretationofspect+takenwithaair@efocusi4Z --WM_h
marnr~meter.Thepotentiatdeficitofthe~oris(relatmetotheanode potential)Swhidl ~ndstotbepealrsirifi,canrangeuptosomelOOV-
-
The temper&are dependence of the peak group is proof that these p&s (F’ig. 3b) are correlated _to the same ion. The energy deficit decreases w& increasing anode temperature and al3 peaks move steadily along the mass scale to the correct mass position (m/e = 100). Finally, with both wpes of emitters are around SOO”C, far below the melting points of Ni (1453°C) and Co (1495X), a rupture of the microneedles from the metaRic support is observed with both Wand Pt. The question of the causes of the structure of the electrochemically produced microneedles arises. Or@nally it was assumed that the initial inactivity of the emitters is caused by electrically isolating or semiconducting is surface layers (for example by oxide films). However, this assumption nsistent with the observation that the same thermal treatment for activaEz of the field anodes is also necessary after strong etching of the emitters pe. Influence of the by acids sufficient to be visible under the light mim metal wims (tung&en or platinum) on the thermal treatment was not observed. Consequently, the reason for the thermal effects can only be found in an irreversible change of the structure of the microneedles. This means that the microneedles of both Ni- and Co-emitters are electrically of the experiment nonconducting or only semiconducting at the beginniq and that the electrical conductivity which is necessary for ionization is only achieved by thermal treatment. The conductivity of the needles in the electrochemical cell is favoured by the high temperature of the electrolyte (ahout 90°C). The structure and the temperature-dependent structural changes of the -on electron f microneedles were observed with a high resoIution microscope having a heated specimen holder. With both types of emitter a transparent region on the tips of the microneedles was selected for the 100 kV electron beam. The temperaturedependent changes of the morphology -on and and structure of the microneedles could be observed in f with the aid of electron diffra&on. Figure 4 shows typical changes of the shape and structure of the micro. and the heating needles of cobalt emitters depending on the tempemture time. Figure 4(a) shows the tip of a microneedle. which is only weakly transparent at-room temperature. The corresponding difbction pattern (Fig. 4b) consists of diffuw rings and therefore points to a largely amorphous structure of the needles which are of cobalt oxide-with increasing temperature the rings hecome sharper corresponding to an increase of the . ‘on. Above 300°C a pronounced contraction of the degre& of crysM&&
needle volume was observab le. In the temperatwe range 4-OO-!500°C distinct
reflexesappeared~fromcrgatnnine~inthevicinityofthewireand later also from t&e tips of tie microneedles as indicated by dark field These distinct reflexes originated from cobalt oxide, and at ternaboveBboUt500QC~&ommetallicco~Figure4(c)~o~thetipof tbemicroneedleat6000CaPterabout40minh~tiine_Tfietipis-_comPared with pis- 4(a) - m~ch~andshowskgerstrongbrtransparent
images.
162r-
Fig. and
-
.
--
_-
pattems of a tip of a “Co” micronedIe (a) rsraaionofthe~tter,(c)md(d)~tWO~C.
observed: the microneedles are inithllyalso amorphous or microcryg tdine, their diffraction patterns at room temperature however, do not oniy indicate the existence of NiO but aiso weak metaliic nickel. ‘Ibe degree of crystaihzation increases with rising temperature and distinct reflexes coming fIom NiO and Ni are observed. Morphological changes are at about 200°C cieariy obsemable. In some cases a contraction of the needle volume of more than 50% was observed. The dectron microscope observations explain the initial insulator properties of the microneedies because NiO and Co0 are electrically nonconducting. The increase of the conductivity of the needles during and after the thermal treatment is probably due to an increase of the density of the needle material and/or to the formation of metallic particles. The morphological changes which are &ready clearly observable at lower temperatures are presumably caused by inclusions of nickel and cobalt salts which have very low melting points (ca. 55°C) and decomposition temperatures (c-a. 130°C) whereas NiO melts at 1990°C. Co0 at 1935°C and C&O3 at 895°C [9]. The low degree of crystallisation and the heterogeneous structure of electrochemicaiiy produced microneedles is very probable the result of a fast needle growthwere
CONCLUSION The structure of the microneedles of the Ni- and Coemitters is mainly oxidic and becomes partially metallic after a thermal treatment. A broad and sometimes also structured energy distribution of the FI products results from this oxidic structure of the emitters. Thus the Ni- and Co-emitters are less suitable for mass spectrometers which do not have energy focusing and they are not suitable for the same reason for field ionization kinetic studies. ACKNOWLEDGThe authors would iike to thank Prof. Dr. M.M. Bursey, North Carolina and Dr. J. Heitbaum, Bonn, for valuable discussions, and the Deutsche Forschungsgemeinscbaft and the Fonda der Deutschen Chemischen Industrie for support of this work. S.J. Ready is grateful to the Deutsche Akademische Austauschdienst for a res?zxmzhscholarship.
and V.G. Golovatyi.Prib.Tekhn. Ehp.,16(1973) LV.Goldenfeld,RN.Bondaremko 166. ~.W~abrrrn~D~.~to~~~SImmonoandMdM.Burslcy.TPt.J.~Spectro=. 1cngw~.17(1975)208. Hintm.T.S.C~IpitksandK.b¶. hU&Btusey.C.E.Ekchkeiuer,M.C.~D-M. ‘lh~~J.PhyrE.9(1976)1&1 E.WJHiiIk md T.T. Ibopg. Fidd fop Microccopp, EZswier. New York, 1969.
s’ ED. Beckey and E-B, Schnltcn. Angew. Chem., Int, Ed. Eugl... 14 (1975) 403. 6 F-W_ RElIgem, tJ. Gieaa.. HAHeinenRlldSd.Re!ddy.rnLJ.MahSpe!chm-Ion pfim. 24 (1977) 235; F-W_ a U. Gikasmaun and Ed. Heinen. Z. Nahrforsc?~. T&IA. 31(1976)172B. MesaDettmik. 80 (1972) 147. 7 F.Spier,H.J.HkinenandHJLBeckey, 8 D-M_ ‘lhyOr, PmW.XEJ&pa and ZLD_ m. Surf- &iv, 40 (1973) 264_ 9 Handbook af Chemistry and Physics, The Chemical Ehbber Punching Co., 1974.