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Materials analysis by laser-probe mass spectrometry
i33 InfemafionaI
Journ
Q Elscvicr Scicutic
(1976) 133444 Amsterdam - Printed iu The Netherlands
of .Mhss Spectrome~ry and Ion PIi~tsicS, 21
Publishing Company,
,MATERIALS ANALYSIS BY LASER-PROBE MASS SPECXROMETRY _. R_ A_ B!NGHAXI AEI Scientific Apparatus Limited, Burton Dock Roud, Chnston, Manchester (GE. Britah)
P_ t- SALTER De~rmettr
of Applied Pli>xies, Umkersity o_f HZ& Huti. Humkrside
(First received 29 August
1975; in fiud form 27 November
(Ct. Britain)
1975)
AB!xRAcT
A ruby laser has been used as a source of ion production for the analysis of
solid materi& by a double-focussing mass spectrometer_ The analytical qualities are discussedand cotiparison made with the conventional rf spark source. Results of analysis are given for bo’rhconducting and non-conducting materi& and quantitative aspects are c&idered_
INIRODUCTIOX
The use of the Iaser as an ion source in mass spectrometry has increased in popularity in recent years in step with improvements in Iaser technology. Much of the work on laser-solid interaction has been weU summa&d in Ready’s book
[I]_ Most work involving mass spectrometry has been of a specialised nature involving studies of laser-produced pIasma [243, or the examination of par&u&
materiaIs such as thin metalIic films on glass or single crystals to study their molecular properties [5-g]. Very little work to date has indicated the potential of the laser as a quantitative anatytical source- In several cases secondary ionizing tcchniques such as spark excitation or electron beam [lo, II] are used and the laser serves mereIy to evaporate the sample- However, Q-switched Iasers provide not onIy a means of evaporating a solid sample but also of producing a highly ionized pIume of material which expands from the sample surface- The Iaser provides not only the promise of a microanalytical technique (since the beam can be focussed to small areas of a few microns diameter), but. also offers the possibilities of both
134
b&k and surfa& anaIysisEIoy and co-workers at Centre #Etudes Nucleares de Grenoble have demonstrated the possibility of quantitative surface analysis using a ruby laser and a &&-focus&g mass spectrometer (see refs- X2-14)_ A major advantage of the laser source is that no sample preparation is required and the cohductivity of the
sample has little or no influence on the i&n production. This overcomes the need for sample mixing with a conducting material, such as graphite, as employed in
spark-source mass spectrometry- It avoids the possibility of sample contamination and also of dilution of the sample material, with resultant reduction in analytical sensitivity_ Furthermore very small samples which are found difficult to handle with the conventional spark-source technique can be examined. The neutrality of the lakerkam means &at no problems such as surface charge on tie sample can affect the analysis, as encountered in anaIytica1techniques which use a bombarding char@-particle beam as the ioni*ng source-
To investigate the capabilities of the Iaser source_ a pulsed ruby laser was attached to the ion source of an AEI MS7 (double focussing, Mattauch-Herzog
geometry) mass spxtromcter. The arrangement is shown in Fig. 1. The laser beam is r&&ted by a prism int.0a side port of the existing ion source. Figure 2 shows how the source of the MS7 is modified for laser analysis- A window repiaces the
conventional spark-source micromanipuhtor control and the laser beam is re-
Laser beam
fkcted throush
a lens system
onto the sample surface_ The angle of incidence of
the beam is ca_ 45 ‘_ The lens system is a long working distance Vickers 20 x microscope objective (focal tenth 8 mm)_ Both the lens and the sample can be controlled b,v micromanipulators from outside the vacuum and the focal point is arranged to lie on the optical axis of the mass spectrometer. No secondary ionizing
technique is used. The ion source is otherwise in the standard condition for sparksourcez analysis
except for substantiat screenin, 0 in the ion-expansion region to prevent excessive charged-particle flow to earth- Intense ion pulses are produced and these can draw large cllrrents from the accelerating voltage supply and also
create space charge problems within the mass spectrometer unless they are controlied. An optic4 viewing system is attached to the source so that the specimen surface can be examined_ A He-Ne laser is used to align the source optics with the ruby laser and also to pinpoint the area for examination. Pn this way one can select any de&d point on the specimen surface for analysis. The ruby laser parameters are given in Table 1. The laser output of ca. 5 mJ and pulse Iength 20 ns provides a peak power of 025 MW_ The use of a Q-switched hrser means that a single pulse interacts with the sample as compared with a series TABLE 1
Pt&lettgrh Emrg,F Cr,-srol
Q-SS&CI?CCf Wrm/engrh PuLrt~prrironram MO&
3-ons EIOmJ
Ruby Dye-vamdyl phthaIocyaninc 6943 A 10 5 per pulse (msx) Singte TEMoo
136 of pukes (spikes) in a normal p&c fascr- Tht output of o Q-switched Iascr is more rcproducibk and consequcntJy one would eqxct the variation in ion production from consccutivc fascr puJ_c~to be JCS_The output is single mode and gives Lhc followins power de&ties at the sample surfkcr
500
160 50 20
I@ IO9 IO’O JO”
In gwwral, pwious; work in this JicJdhas btvn carried cwt rt tihtivcl~ low ~&WY densities (ca. IO’ W cm-‘) [I_ 47. IS, J6]_ Most of the prcscnt work has bcltn nrricd out usins powx dcrsitics of 10’“-IO1’ W cm”‘_ At thrvc hi& fluxes material is rcmowd from Lhesample earl? in the laser puk and the laser radiation is then absorbed by inwrsc brcmsstrahlung [J7. IS], producing a hish!y ionized phsma which expands array from the sample surface. ScvcmJ mod& have been proposed for the laser intenction with solid Largxs [J9-?I]_ At low kr llwrcr (tO”-IO’ W cm-‘) rhc amount of matcriai remowd from a wmpkdepends stron& on the thermal conductivity of the sample [I ] and when tlnaJy&g impuritk at low lluxcs one might expect preferential removal of some ckmentr. At high JJu..esthis is not the case and undtr Lhc conditions used hcrc the Jwr is non-selective for a wide rage of elements as will be dcmonztratcd by the results_ The specimens used arc ffat but this is not a critical paramctcr. Irregular surfkcs can be c~xtrnkd but a reduction of ions through the mu= spxtromctcr occurs if the angle &cLween the mw-spcctrometcr axis and the spccimcn surface is large_ We have examined irreguiar rurfxes such as the fkxturcd surfaces of rock samples and JUW found &at Lhc ion production is very simihr LOthaf obtained from metal nmpk Fig,u-e 3 showz a typical crater obtained fern a singk laser shot on il mildsteel sample- ‘J%e central region is deep wvhcrczlsthe outer fringe is only sli$tJ_v crodcd- Over 95 7: of the maLeria1 rcmovcd is within the central r&on and the crater diameter is taken as the width of this central region_ Typical diameten on steel are 20 pm arid typical depths 5pm_ The diameter and depth obtained depends on the specimen material and power density used_ The diameter of the crater can be varied over the range 20-500 pm by defocussing the lens s_ystem-
In the prezent work the ions were detected on Ilford Q3 photoplates.
the pulse repetition been possibk;
Since
rate of the ruby lxxr is low eJectrica1 detection would not have
however,
the photopiatc
provides an ideal detector since aII masses
in the range 6-230 can be coJIecrcd simultaneously from one laser pulse- The use of the photoplate restricts sensitivity compared to electrical detection but gives an overall elemental survey of the analysis for different source panmeteis. Table 2 compares the ruby laser and rf spark sources_ it can be seen how simihr
rhe two sources are in ion production
and detection
under the conditions
used. The rf spark conditions are chosen for routine analysis (i.e. 25 kV spark rate 300 pulses per s). It is interesting Volta=_ pulse length 2OO~ts,pulse rep&ion to note that both sources remove the same amount of material per pulse and the tota number of ions reaching the detector is aIso the same_ This indicates that with a 1-r capable of pulsing at the same rate as the rf spark the same analytical spbped \vould k obtained
down to the equivalent
sensitivity
IeveI_
A sir&e faser puke produces a visual photoplate sensitivity of 1000 p-pm. atomic and removes cu. O:O&pz of material_ Increased numbers of pulses recorded on the same spectrum increase the element sensitivities proportionally so that, for example, 1000 pulses would be required for 1 p-p-m_ atomic sensitiviLy_ The present work has covered the ranse 1-1000 pulses and iondetection is found to be linear over this range_ Further work with a high repetition rate laser (such as Nd YAG) is required LO determine whether ion detection is still iinear over a large number of shots_ Figure 4 shows a secLion of specxra on a photoplate over the mass range 20-75 for two materials_ The upper spectra are obLained from successive laser shots (I-3-IO-3tHoO pukes respectively) on a specimen of slate and the lower set the same sxics on a galena crystd. The first comment Lo make is that the spectra are very simihr to those obtained with the rf spark The resolving power is similar. being ca_ uloo (50 % pz:tk-hei_& definition for the photoplaLe)_ There are few
fii
-?- PI~~topkuc speara ofslate and pkna
crysaL
139 molecular species but there are considerable numbers of muitiply-charged species of the same orders of intensity as found with thespark Thii can be attributed to the high power density conditions used for ionization- The main elements of slate are Si, Al, K and also some Mg and Fe. These are basically from the constituent minerals: Quartz (SiO?), feldspar (KAlSi,O,), kaoIinite (A14(Si40i,)(OH)J, muscovite (KA12(AISi30,0)(OH)z), biotite (K(hrfgFe)(A1Si3010)(OH)t) and some small crystals of iron pyrite (FeS). From the spectrum of the slate it can be seen that the highly voiatile eiement potassium is not excessively intense compared with the aluminium and silicon, as may be expected at lower flux densities 171. The lines of ‘6Fe and 32S arc erratic and this may be attributed to the vailation in the iron-pyrites crystal distribution in the slate. In the gaIena crystal the third ionization spties, Pb3+, can be seen around mass 69, and Zn’+ is also present In Figure 5 several different spectra have been recorded over the mass range 20-70. The top three spectra are l-3-10 shots on a mild-steel sample an& the Fe+ .
__ .
. -v
-..__
.. _
___.
._
_
l+_ 5_ Photoplace spectra of steel and individual rock minerals
and Fe’* spxies can be seen. The other spectra in Fig. 5 are recorded from different crystals selected from the broken surface of a piece of rock These crystals varied in angle to the laser bea= and mass-spectrometer optical axis whichaffected the number of ions extracted imo the mass spectrometer as mentionedpreviouslySpectra 4-6 were l-3-10 laser shots onacrystai of copp,or pyrites (CS) producing the copptr and sulphur isotopes, and also some iron which is in the crystal at a fairly hi& level- The remaining spectra are fromdifferent crystais, mainly calcites,
and of various numbers of laser shots. Again, the element calcium which is a major constituent of these minerals is not excessively intense_
140 SeveraI photoplates of steel samples were recorded and it was calculated that the visuaI sensitivity of a sin$e faser pufse for iron was 1000 p.p.m.. Using this value as a criterion for general photoplate sensitivity several standards of other materials have been analysed to obtain a measure of accuracy of the technique for bulk analysis, Table 3 shows the anaIysis obtained for cartrids brass standard TABLE 3 WWAJ_ A-SUYsIs OF -
z Cd Sll Sb Pb Bi
NBS
a3mearm.cion
Cakzdhed mwu7tumKibn
O-OOXl om55 0.037 O-013 am a003 a0055 O-016 0.012 O-OS OlKlO4
OAlOZ4 a0065 0.047 O-01I O-O@4 O-005 oaO7 0.0055 O-MIS O-019 O_OOO5
Gicrn
P Mn Fe Ni
BR%SSSLUW-
i 101 WIllI
THE hfi~/RURY
LASER (% W)
The concentrations have been calculated in % wt_ for comparison with the given concentrations- The spectra recorded were of 1-3-10-30-100 laser shots and the whole anaIysis took about 30 min to p&x-m, owins to the slow rep&ion rate of the Iaser_ The Iorgoest exposure of 100 pulses had a sensitivity of CL IO p-p-m_ atomic- It can bc seen that the anaIytica1 results are comparable with the given analyticat vaIue~ over tic range of efements certified by the suppliersTabk 4 shows the anaIysis obtained for .Johnson-Matthey aluminium alloy M3SlIOI.
imimtl
Giren cvKzceKlnurion
Cizhhed CORCCKIRUIiOKl
MS
122
Si -Ii cr
OX? O.OO7 O-O6
L69 0-10 O_OO~4 0.W
Mtl Fe
02 aIs
O-12 0.18
Ni CU
0.012
O_OlO
OLOIS
O-Q23
141
standard ACI. Again a visual estimation of 1000 p-p-m- per Iaser shot was assumed for calculation of the anaIytical vafues, which arc in rcasonabIe acmcnt with the given concentrations_ TabIe 5 gives the anaIytical vatues obtained for a steel standard NBS467_ The quoted relative-sensitivity factors have been determined from the ratio of caku-
TABLE
5
VISU.U AELILYSSOF NBS STEELSTAXDARD467
Ekaze~~ Girenconcearrarion e/,ut)
B
O.ooO2
0 Si
0.004
P S -l-i
V Cr hfn Ni CO
O-26 O-033 cko1 026 O&?l O-036 02s O-088
Zr
O-074 O-067 0.003 o-14 0-W
MO
O-021
CU GC As
Nb J%S
sn -r;? \v Pb
0.29 OSNM O-IO 0.23 O-20 OBOO6
Cakdaredcancentrariaa RelafiresemiiicityfacIor o.ooo49 0.00s
0.32 0.028 0.006 023 0.027 0-W 0_20 O-12 0.06 O-082 o-007 O-08 0.06 0.021 0.05 o-011 0.098 O-W5 O-068 OMb
2.45
2.0 1.25 0.84
O-6 O-8 O-67 1.23 o-7 1-M O-81 1.22 33 0.57 0.21 LO
0.17 38 0.98 0.28 0.34 67
0 Mean not including lead l-07_ b Contiixuion from prerious sample.
fated-to-given concentrations. These factors give a measure of the element sensitivity with respect to iron, the reference element, and show that whilst the mean is fairly ciose to unity, the variation is a factor of 3. This is very similar to the result obtained with the rf spark source_ To ilkstrate the capability of the Iaser sour-cc for non-conducting materiak a glass standard NBS611 has also been examined and the results are given in Table 6. The ion production was comparable with. that for the metal standards and, again assuming a sensitivity of 1000 p.p.m. per laser shot, gave analytical results which compare ‘very favourably with the @en concentrations, although the sensitivity appears to fall off at the Iower masses.
142
Hell?&!@
Ginn conce@mMn
Ci&uIoO!d connnlmlrbn
u 3-h
462 457
42.5 249
Pb AS Sr Rb 2n cu Ni CO Fk Mn F K B
426 25-l 515 416 433 440 458 390 45s 485 437 461 351
44s 231 377 430 457 343 316 zs3 260 216 257 557 141 _-
It can be seen that within the parameters used throughout these experiments the laser-mas+spectrometer combination provides the basis for a quantitative analyticA tool. One advantage for routine anaiysis is that opentor intervention is unnecessary for bulk analysis since thea&ment of the point of ion production with the mass spectrometer optics is not crncial and the Iaser can be left to run unattended- Furthermore it should be possible to insert several specimens in the source at one loading in a multi-specimen holder and so avoid tie pumpdown time between each specimen, This offers the possibilities of an autotiated or semiautomated system which cannot be contemplated with the rf spark owing to the more complex alignment ,of two ekctrodes. Speed of sample throughput could be greatiy increzzd by such an arran~mentThe reproducibility of the mass spectra obtained from single laser shots has been measured and gives a coefficient of variation of &-20x- Therefore, over a large number of laser shots the standard error could be limited to the pkotoplate emulsion variation of f4 x_ The analysis rcsuits reported here were determined visually, which gives grcatcr sensitivity, but are less accurate measurements than can be obtained by the use of the microdensitomcter. No element corrections were made for any analysis and it can be seen that the analytical values vary over onty a factor of 3, despite using a common sensitivity estimate for all materials_ These results are comparable with those obtained with the rf spark under the s&e conditions ACCUEX~ of .
143 analysis can thus be obtained by comparison between calculated analytical values and those given by the suppliers. Improved accuracy could be obtained by using a microdensitometer and appIying element sensitivity corrections but wouId invoIve spectra containing larger numbers of laser shots to compensate for Ioss of measurement sensitivityIt has been shown that the laser-mass-spectrometer system is capable of providing bulk analysis, but by increasing the spot diameter a larger area can be sampled. Ilncreasin,= the beam diameter reduces the power density on the sample surfaec and so the depth of vaporisation is reduced_ This provides the possibility of sampling thin layers of the surface, a technique used by Eloy and Dumas [I21 and Eloy 1141, who were able to sample Iayers ca. 2000 A pzx Iaser shot. Each spectrum is a record of a single shot so that by taking a series of spectra of consecutive laser shots the distribution of elements can be measured through the surface IaJxrs_ Conversely by focussing the laser to a fine focus, spatial analysis can be obtained across the specimen surface. Line scans can be obtained by recording a series of spectra across the area of interest and the photoplate will record all elements above 1000 p.p_m_simultaneousIy for each single shot spectrumMore than one shot in each position will increase elemental sensitivity according to the number of shots taken. It should now be possible to use faster pulsing lasers such as Nd YAG for ion production_ These instruments are capable of pulse repetition rates of up to 25 KHz, although rzues as high as this would introduce specimen heating problemsIt should be quite feasible to employ a laser pulsing at %-IO0 Hz and this would give a much greater throughput of ions. Photoplate analysis speed would be much increased and coutd be similar to that already obtained by the rf spark. Again by using a higher repetition rate, electrical detection of the ion beam could be used and this would give greater sensitivity and precision of analysis_
The laser described is capable of being used as an ion source on a mass spectrometer for materials analysis_ Conducting and non-conducting materials can be analysed at a speed and accuracy similar to the rf spark technique but without the necessity of preliminary specimen preparation_ The simplicity makes it an ideal system for routine or semi-automatic anaIysis_ The technique is capabIe of bulk analysis, spatial analysis or thin-layer analysis without the probIem of a iarge range of relative-sensitivity factors found in other techniques providing simiiar typzzs of analysis. The laser is found to be equal in all respxts to the rf spark, with further advantages of its own, and is now fully capable of replacing the spark source while giving a more flexibie analytical system for materials analysis with a mass spectrometer,
144
This paper is published with the permission of Mt. C. R. Bates, Managing Dinxtor of AEI Scientific Apparatus Limited, Urmston, Manchester, One of the authors (P_LS_) wishes to acknowledge the receipt of a Science Research Council C&!3.E studentship in conjunction with A-W-R-E_, Aldermaston, during the periodinwhich this work was carrkd out,. He would also like to acknowledge the keen interest in the work shown by Professor S. A. Ramsden and Dr. G_ J_ Pert of the Department of Applied Physics, University of HulI_
i J_ IF_Rrsdr, fi_#rcLsof High Pmrer Lpav Ridiafkm, grcss Catato~ cud No_ x%37597)_ 2 R E Honig, Appl. P&s- Lm.. 3 (1963) S-
3 4 5 6 7 8
9 10 II
It
Audcmic,
New York (Library
of
Con-
i- L_ Dumas, Thesis, University of Grenoble. De 1970_ E Bernid G, J. F. Ready and F_ J. Akn, J_ Appl P@s.. 45 (1974) 2950. N- C- Fknncr and N. R DaIy, Rcr. &_i_ fksnwn_, 37 (1956) 106% N- C- Femer and N. R_ D&y. &her_ SC& 3 (19621) 259. E C_ Be&m, Thcsk, Pennsylvania State University. June !973_ F-J_ Vytola, k J. Pironc end R O_ Mumma_ Proceafings I&h hmtta~ Cunferencr A¶arsSpecrrvnwry and AI&d Topics, 1968, pp. 299-301. Y. P_ Zakhov and I_ PA. Procu, kc_ Ahtd_ NatA SSSR. 38 (1974)238_ HI- J_ won Dietze and H_ Z&n. &rpcrikncnreUe Tech& da Physik XX, fief- 5. 1972. p_ u)I_ S- O_ Ekbcsry. B. F_ Scribncr and hrl. Margoshcs. Appi- Upr., 6 (1967) SlJ- F_ EIoy and J_ L Dmnxi, Mezhod- Ph_m- ~mf_, 2 (1966) %l_
13 f_ F_ my, rUeth&_ Pliys- AxaL, 4 (1968) 161_ I4 J_ F_ Eloy. Afehtf_ P&s- And, 5 (1969) 15715 R H_ Scott, P_ F_ S_ Jackoa and .A_ Strashcim. Nuture (London), 232 (1971) 623_ 16 V- P_ -v ad I_ N_ Rotas. Prib. 7-d&_ &p-, 3 (1973) 16t 17 J. M_ Dstson and C_ R. obtrrnan, P&S- f&i& 5 (1962) 517. t 8 J_ M_ Damn, P_ Kaw and B. Green, P&-s_ E&c& 13. (1969) 575.
19 A- Caruso and R Gntton, Pt’mu PhJ-s-, 10 (196S) S67_ 10 H. Pudl. Z_ Nzrwfkrck. 25 (1970) 1SO721 C_ fiuquigmn and F_ Flow, P&-s. F7uidr, 13 f 1970) 3S6.
Journ
Q Elscvicr Scicutic
(1976) 133444 Amsterdam - Printed iu The Netherlands
of .Mhss Spectrome~ry and Ion PIi~tsicS, 21
Publishing Company,
,MATERIALS ANALYSIS BY LASER-PROBE MASS SPECXROMETRY _. R_ A_ B!NGHAXI AEI Scientific Apparatus Limited, Burton Dock Roud, Chnston, Manchester (GE. Britah)
P_ t- SALTER De~rmettr
of Applied Pli>xies, Umkersity o_f HZ& Huti. Humkrside
(First received 29 August
1975; in fiud form 27 November
(Ct. Britain)
1975)
AB!xRAcT
A ruby laser has been used as a source of ion production for the analysis of
solid materi& by a double-focussing mass spectrometer_ The analytical qualities are discussedand cotiparison made with the conventional rf spark source. Results of analysis are given for bo’rhconducting and non-conducting materi& and quantitative aspects are c&idered_
INIRODUCTIOX
The use of the Iaser as an ion source in mass spectrometry has increased in popularity in recent years in step with improvements in Iaser technology. Much of the work on laser-solid interaction has been weU summa&d in Ready’s book
[I]_ Most work involving mass spectrometry has been of a specialised nature involving studies of laser-produced pIasma [243, or the examination of par&u&
materiaIs such as thin metalIic films on glass or single crystals to study their molecular properties [5-g]. Very little work to date has indicated the potential of the laser as a quantitative anatytical source- In several cases secondary ionizing tcchniques such as spark excitation or electron beam [lo, II] are used and the laser serves mereIy to evaporate the sample- However, Q-switched Iasers provide not onIy a means of evaporating a solid sample but also of producing a highly ionized pIume of material which expands from the sample surface- The Iaser provides not only the promise of a microanalytical technique (since the beam can be focussed to small areas of a few microns diameter), but. also offers the possibilities of both
134
b&k and surfa& anaIysisEIoy and co-workers at Centre #Etudes Nucleares de Grenoble have demonstrated the possibility of quantitative surface analysis using a ruby laser and a &&-focus&g mass spectrometer (see refs- X2-14)_ A major advantage of the laser source is that no sample preparation is required and the cohductivity of the
sample has little or no influence on the i&n production. This overcomes the need for sample mixing with a conducting material, such as graphite, as employed in
spark-source mass spectrometry- It avoids the possibility of sample contamination and also of dilution of the sample material, with resultant reduction in analytical sensitivity_ Furthermore very small samples which are found difficult to handle with the conventional spark-source technique can be examined. The neutrality of the lakerkam means &at no problems such as surface charge on tie sample can affect the analysis, as encountered in anaIytica1techniques which use a bombarding char@-particle beam as the ioni*ng source-
To investigate the capabilities of the Iaser source_ a pulsed ruby laser was attached to the ion source of an AEI MS7 (double focussing, Mattauch-Herzog
geometry) mass spxtromcter. The arrangement is shown in Fig. 1. The laser beam is r&&ted by a prism int.0a side port of the existing ion source. Figure 2 shows how the source of the MS7 is modified for laser analysis- A window repiaces the
conventional spark-source micromanipuhtor control and the laser beam is re-
Laser beam
fkcted throush
a lens system
onto the sample surface_ The angle of incidence of
the beam is ca_ 45 ‘_ The lens system is a long working distance Vickers 20 x microscope objective (focal tenth 8 mm)_ Both the lens and the sample can be controlled b,v micromanipulators from outside the vacuum and the focal point is arranged to lie on the optical axis of the mass spectrometer. No secondary ionizing
technique is used. The ion source is otherwise in the standard condition for sparksourcez analysis
except for substantiat screenin, 0 in the ion-expansion region to prevent excessive charged-particle flow to earth- Intense ion pulses are produced and these can draw large cllrrents from the accelerating voltage supply and also
create space charge problems within the mass spectrometer unless they are controlied. An optic4 viewing system is attached to the source so that the specimen surface can be examined_ A He-Ne laser is used to align the source optics with the ruby laser and also to pinpoint the area for examination. Pn this way one can select any de&d point on the specimen surface for analysis. The ruby laser parameters are given in Table 1. The laser output of ca. 5 mJ and pulse Iength 20 ns provides a peak power of 025 MW_ The use of a Q-switched hrser means that a single pulse interacts with the sample as compared with a series TABLE 1
Pt&lettgrh Emrg,F Cr,-srol
Q-SS&CI?CCf Wrm/engrh PuLrt~prrironram MO&
3-ons EIOmJ
Ruby Dye-vamdyl phthaIocyaninc 6943 A 10 5 per pulse (msx) Singte TEMoo
136 of pukes (spikes) in a normal p&c fascr- Tht output of o Q-switched Iascr is more rcproducibk and consequcntJy one would eqxct the variation in ion production from consccutivc fascr puJ_c~to be JCS_The output is single mode and gives Lhc followins power de&ties at the sample surfkcr
500
160 50 20
I@ IO9 IO’O JO”
In gwwral, pwious; work in this JicJdhas btvn carried cwt rt tihtivcl~ low ~&WY densities (ca. IO’ W cm-‘) [I_ 47. IS, J6]_ Most of the prcscnt work has bcltn nrricd out usins powx dcrsitics of 10’“-IO1’ W cm”‘_ At thrvc hi& fluxes material is rcmowd from Lhesample earl? in the laser puk and the laser radiation is then absorbed by inwrsc brcmsstrahlung [J7. IS], producing a hish!y ionized phsma which expands array from the sample surface. ScvcmJ mod& have been proposed for the laser intenction with solid Largxs [J9-?I]_ At low kr llwrcr (tO”-IO’ W cm-‘) rhc amount of matcriai remowd from a wmpkdepends stron& on the thermal conductivity of the sample [I ] and when tlnaJy&g impuritk at low lluxcs one might expect preferential removal of some ckmentr. At high JJu..esthis is not the case and undtr Lhc conditions used hcrc the Jwr is non-selective for a wide rage of elements as will be dcmonztratcd by the results_ The specimens used arc ffat but this is not a critical paramctcr. Irregular surfkcs can be c~xtrnkd but a reduction of ions through the mu= spxtromctcr occurs if the angle &cLween the mw-spcctrometcr axis and the spccimcn surface is large_ We have examined irreguiar rurfxes such as the fkxturcd surfaces of rock samples and JUW found &at Lhc ion production is very simihr LOthaf obtained from metal nmpk Fig,u-e 3 showz a typical crater obtained fern a singk laser shot on il mildsteel sample- ‘J%e central region is deep wvhcrczlsthe outer fringe is only sli$tJ_v crodcd- Over 95 7: of the maLeria1 rcmovcd is within the central r&on and the crater diameter is taken as the width of this central region_ Typical diameten on steel are 20 pm arid typical depths 5pm_ The diameter and depth obtained depends on the specimen material and power density used_ The diameter of the crater can be varied over the range 20-500 pm by defocussing the lens s_ystem-
In the prezent work the ions were detected on Ilford Q3 photoplates.
the pulse repetition been possibk;
Since
rate of the ruby lxxr is low eJectrica1 detection would not have
however,
the photopiatc
provides an ideal detector since aII masses
in the range 6-230 can be coJIecrcd simultaneously from one laser pulse- The use of the photoplate restricts sensitivity compared to electrical detection but gives an overall elemental survey of the analysis for different source panmeteis. Table 2 compares the ruby laser and rf spark sources_ it can be seen how simihr
rhe two sources are in ion production
and detection
under the conditions
used. The rf spark conditions are chosen for routine analysis (i.e. 25 kV spark rate 300 pulses per s). It is interesting Volta=_ pulse length 2OO~ts,pulse rep&ion to note that both sources remove the same amount of material per pulse and the tota number of ions reaching the detector is aIso the same_ This indicates that with a 1-r capable of pulsing at the same rate as the rf spark the same analytical spbped \vould k obtained
down to the equivalent
sensitivity
IeveI_
A sir&e faser puke produces a visual photoplate sensitivity of 1000 p-pm. atomic and removes cu. O:O&pz of material_ Increased numbers of pulses recorded on the same spectrum increase the element sensitivities proportionally so that, for example, 1000 pulses would be required for 1 p-p-m_ atomic sensitiviLy_ The present work has covered the ranse 1-1000 pulses and iondetection is found to be linear over this range_ Further work with a high repetition rate laser (such as Nd YAG) is required LO determine whether ion detection is still iinear over a large number of shots_ Figure 4 shows a secLion of specxra on a photoplate over the mass range 20-75 for two materials_ The upper spectra are obLained from successive laser shots (I-3-IO-3tHoO pukes respectively) on a specimen of slate and the lower set the same sxics on a galena crystd. The first comment Lo make is that the spectra are very simihr to those obtained with the rf spark The resolving power is similar. being ca_ uloo (50 % pz:tk-hei_& definition for the photoplaLe)_ There are few
fii
-?- PI~~topkuc speara ofslate and pkna
crysaL
139 molecular species but there are considerable numbers of muitiply-charged species of the same orders of intensity as found with thespark Thii can be attributed to the high power density conditions used for ionization- The main elements of slate are Si, Al, K and also some Mg and Fe. These are basically from the constituent minerals: Quartz (SiO?), feldspar (KAlSi,O,), kaoIinite (A14(Si40i,)(OH)J, muscovite (KA12(AISi30,0)(OH)z), biotite (K(hrfgFe)(A1Si3010)(OH)t) and some small crystals of iron pyrite (FeS). From the spectrum of the slate it can be seen that the highly voiatile eiement potassium is not excessively intense compared with the aluminium and silicon, as may be expected at lower flux densities 171. The lines of ‘6Fe and 32S arc erratic and this may be attributed to the vailation in the iron-pyrites crystal distribution in the slate. In the gaIena crystal the third ionization spties, Pb3+, can be seen around mass 69, and Zn’+ is also present In Figure 5 several different spectra have been recorded over the mass range 20-70. The top three spectra are l-3-10 shots on a mild-steel sample an& the Fe+ .
__ .
. -v
-..__
.. _
___.
._
_
l+_ 5_ Photoplace spectra of steel and individual rock minerals
and Fe’* spxies can be seen. The other spectra in Fig. 5 are recorded from different crystals selected from the broken surface of a piece of rock These crystals varied in angle to the laser bea= and mass-spectrometer optical axis whichaffected the number of ions extracted imo the mass spectrometer as mentionedpreviouslySpectra 4-6 were l-3-10 laser shots onacrystai of copp,or pyrites (CS) producing the copptr and sulphur isotopes, and also some iron which is in the crystal at a fairly hi& level- The remaining spectra are fromdifferent crystais, mainly calcites,
and of various numbers of laser shots. Again, the element calcium which is a major constituent of these minerals is not excessively intense_
140 SeveraI photoplates of steel samples were recorded and it was calculated that the visuaI sensitivity of a sin$e faser pufse for iron was 1000 p.p.m.. Using this value as a criterion for general photoplate sensitivity several standards of other materials have been analysed to obtain a measure of accuracy of the technique for bulk analysis, Table 3 shows the anaIysis obtained for cartrids brass standard TABLE 3 WWAJ_ A-SUYsIs OF -
z Cd Sll Sb Pb Bi
NBS
a3mearm.cion
Cakzdhed mwu7tumKibn
O-OOXl om55 0.037 O-013 am a003 a0055 O-016 0.012 O-OS OlKlO4
OAlOZ4 a0065 0.047 O-01I O-O@4 O-005 oaO7 0.0055 O-MIS O-019 O_OOO5
Gicrn
P Mn Fe Ni
BR%SSSLUW-
i 101 WIllI
THE hfi~/RURY
LASER (% W)
The concentrations have been calculated in % wt_ for comparison with the given concentrations- The spectra recorded were of 1-3-10-30-100 laser shots and the whole anaIysis took about 30 min to p&x-m, owins to the slow rep&ion rate of the Iaser_ The Iorgoest exposure of 100 pulses had a sensitivity of CL IO p-p-m_ atomic- It can bc seen that the anaIytica1 results are comparable with the given analyticat vaIue~ over tic range of efements certified by the suppliersTabk 4 shows the anaIysis obtained for .Johnson-Matthey aluminium alloy M3SlIOI.
imimtl
Giren cvKzceKlnurion
Cizhhed CORCCKIRUIiOKl
MS
122
Si -Ii cr
OX? O.OO7 O-O6
L69 0-10 O_OO~4 0.W
Mtl Fe
02 aIs
O-12 0.18
Ni CU
0.012
O_OlO
OLOIS
O-Q23
141
standard ACI. Again a visual estimation of 1000 p-p-m- per Iaser shot was assumed for calculation of the anaIytical vafues, which arc in rcasonabIe acmcnt with the given concentrations_ TabIe 5 gives the anaIytical vatues obtained for a steel standard NBS467_ The quoted relative-sensitivity factors have been determined from the ratio of caku-
TABLE
5
VISU.U AELILYSSOF NBS STEELSTAXDARD467
Ekaze~~ Girenconcearrarion e/,ut)
B
O.ooO2
0 Si
0.004
P S -l-i
V Cr hfn Ni CO
O-26 O-033 cko1 026 O&?l O-036 02s O-088
Zr
O-074 O-067 0.003 o-14 0-W
MO
O-021
CU GC As
Nb J%S
sn -r;? \v Pb
0.29 OSNM O-IO 0.23 O-20 OBOO6
Cakdaredcancentrariaa RelafiresemiiicityfacIor o.ooo49 0.00s
0.32 0.028 0.006 023 0.027 0-W 0_20 O-12 0.06 O-082 o-007 O-08 0.06 0.021 0.05 o-011 0.098 O-W5 O-068 OMb
2.45
2.0 1.25 0.84
O-6 O-8 O-67 1.23 o-7 1-M O-81 1.22 33 0.57 0.21 LO
0.17 38 0.98 0.28 0.34 67
0 Mean not including lead l-07_ b Contiixuion from prerious sample.
fated-to-given concentrations. These factors give a measure of the element sensitivity with respect to iron, the reference element, and show that whilst the mean is fairly ciose to unity, the variation is a factor of 3. This is very similar to the result obtained with the rf spark source_ To ilkstrate the capability of the Iaser sour-cc for non-conducting materiak a glass standard NBS611 has also been examined and the results are given in Table 6. The ion production was comparable with. that for the metal standards and, again assuming a sensitivity of 1000 p.p.m. per laser shot, gave analytical results which compare ‘very favourably with the @en concentrations, although the sensitivity appears to fall off at the Iower masses.
142
Hell?&!@
Ginn conce@mMn
Ci&uIoO!d connnlmlrbn
u 3-h
462 457
42.5 249
Pb AS Sr Rb 2n cu Ni CO Fk Mn F K B
426 25-l 515 416 433 440 458 390 45s 485 437 461 351
44s 231 377 430 457 343 316 zs3 260 216 257 557 141 _-
It can be seen that within the parameters used throughout these experiments the laser-mas+spectrometer combination provides the basis for a quantitative analyticA tool. One advantage for routine anaiysis is that opentor intervention is unnecessary for bulk analysis since thea&ment of the point of ion production with the mass spectrometer optics is not crncial and the Iaser can be left to run unattended- Furthermore it should be possible to insert several specimens in the source at one loading in a multi-specimen holder and so avoid tie pumpdown time between each specimen, This offers the possibilities of an autotiated or semiautomated system which cannot be contemplated with the rf spark owing to the more complex alignment ,of two ekctrodes. Speed of sample throughput could be greatiy increzzd by such an arran~mentThe reproducibility of the mass spectra obtained from single laser shots has been measured and gives a coefficient of variation of &-20x- Therefore, over a large number of laser shots the standard error could be limited to the pkotoplate emulsion variation of f4 x_ The analysis rcsuits reported here were determined visually, which gives grcatcr sensitivity, but are less accurate measurements than can be obtained by the use of the microdensitomcter. No element corrections were made for any analysis and it can be seen that the analytical values vary over onty a factor of 3, despite using a common sensitivity estimate for all materials_ These results are comparable with those obtained with the rf spark under the s&e conditions ACCUEX~ of .
143 analysis can thus be obtained by comparison between calculated analytical values and those given by the suppliers. Improved accuracy could be obtained by using a microdensitometer and appIying element sensitivity corrections but wouId invoIve spectra containing larger numbers of laser shots to compensate for Ioss of measurement sensitivityIt has been shown that the laser-mass-spectrometer system is capable of providing bulk analysis, but by increasing the spot diameter a larger area can be sampled. Ilncreasin,= the beam diameter reduces the power density on the sample surfaec and so the depth of vaporisation is reduced_ This provides the possibility of sampling thin layers of the surface, a technique used by Eloy and Dumas [I21 and Eloy 1141, who were able to sample Iayers ca. 2000 A pzx Iaser shot. Each spectrum is a record of a single shot so that by taking a series of spectra of consecutive laser shots the distribution of elements can be measured through the surface IaJxrs_ Conversely by focussing the laser to a fine focus, spatial analysis can be obtained across the specimen surface. Line scans can be obtained by recording a series of spectra across the area of interest and the photoplate will record all elements above 1000 p.p_m_simultaneousIy for each single shot spectrumMore than one shot in each position will increase elemental sensitivity according to the number of shots taken. It should now be possible to use faster pulsing lasers such as Nd YAG for ion production_ These instruments are capable of pulse repetition rates of up to 25 KHz, although rzues as high as this would introduce specimen heating problemsIt should be quite feasible to employ a laser pulsing at %-IO0 Hz and this would give a much greater throughput of ions. Photoplate analysis speed would be much increased and coutd be similar to that already obtained by the rf spark. Again by using a higher repetition rate, electrical detection of the ion beam could be used and this would give greater sensitivity and precision of analysis_
The laser described is capable of being used as an ion source on a mass spectrometer for materials analysis_ Conducting and non-conducting materials can be analysed at a speed and accuracy similar to the rf spark technique but without the necessity of preliminary specimen preparation_ The simplicity makes it an ideal system for routine or semi-automatic anaIysis_ The technique is capabIe of bulk analysis, spatial analysis or thin-layer analysis without the probIem of a iarge range of relative-sensitivity factors found in other techniques providing simiiar typzzs of analysis. The laser is found to be equal in all respxts to the rf spark, with further advantages of its own, and is now fully capable of replacing the spark source while giving a more flexibie analytical system for materials analysis with a mass spectrometer,
144
This paper is published with the permission of Mt. C. R. Bates, Managing Dinxtor of AEI Scientific Apparatus Limited, Urmston, Manchester, One of the authors (P_LS_) wishes to acknowledge the receipt of a Science Research Council C&!3.E studentship in conjunction with A-W-R-E_, Aldermaston, during the periodinwhich this work was carrkd out,. He would also like to acknowledge the keen interest in the work shown by Professor S. A. Ramsden and Dr. G_ J_ Pert of the Department of Applied Physics, University of HulI_
i J_ IF_Rrsdr, fi_#rcLsof High Pmrer Lpav Ridiafkm, grcss Catato~ cud No_ x%37597)_ 2 R E Honig, Appl. P&s- Lm.. 3 (1963) S-
3 4 5 6 7 8
9 10 II
It
Audcmic,
New York (Library
of
Con-
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13 f_ F_ my, rUeth&_ Pliys- AxaL, 4 (1968) 161_ I4 J_ F_ Eloy. Afehtf_ P&s- And, 5 (1969) 15715 R H_ Scott, P_ F_ S_ Jackoa and .A_ Strashcim. Nuture (London), 232 (1971) 623_ 16 V- P_ -v ad I_ N_ Rotas. Prib. 7-d&_ &p-, 3 (1973) 16t 17 J. M_ Dstson and C_ R. obtrrnan, P&S- f&i& 5 (1962) 517. t 8 J_ M_ Damn, P_ Kaw and B. Green, P&-s_ E&c& 13. (1969) 575.
19 A- Caruso and R Gntton, Pt’mu PhJ-s-, 10 (196S) S67_ 10 H. Pudl. Z_ Nzrwfkrck. 25 (1970) 1SO721 C_ fiuquigmn and F_ Flow, P&-s. F7uidr, 13 f 1970) 3S6.