ISSN:0306-1353    

DOI:10.1039/AA97101FX001

出版商:RSC    

年代:1971

数据来源: RSC

摘要:

1 Light Sources While a variety of light sources is available f o r absorption and fluorescence work t h e most important are the hollow cathode lamp and the vapour discharge lamp driven by d.c. low frequency a.c. or microwave power. The ideal light source for fluorescence should emit high intensity over t h e whole of t h e absorptiotl line-width of t h e atom. In atomic absorption t h e ideal source should b e highly stable and monochromatic emit- ting only a t t h e peak of t h e absorption line while intensity would not be a prime consideration. In practice most of t h e light sources used in atomic absorption have a narrower line width t h a n t h e absorption profile in the flame (445). Whatever type of light source is used it should be reliable and with long life.1.1 HOLLOW CATHODE LAMPS The present state of development of these lamps has been reviewed ( 1 9 5 659) and emphasis placed o n t h e improved stability and light o u t p u t achieved by careful selec- tion of filler gas pressure out-gassing and electrode design. Most manufacturers supply lamps for some 50 t o 6 0 elements. The range of multi-element lamps is much smaller a n d usually revolves around a combination of Fe Cu Mn Ni Co and Cr; Ca Mg and Na K may also b e satisfactorily combined. Falk and Lucht suggested ( 6 5 3 ) t h a t t h e essential process f o r excitation of atoms of t h e carrier gas i n t h e hollow cathode discharge is three b o d y re-combination with direct electron excitation while f o r other atoms it is direct electron excitation.A s t u d y of t h e line width from a B lamp indicated t h a t unusual emission occurred during t h e initial rapid movement of t h e sputtered particles away from t h e cathode surface (167). This led t o large Doppler widths and some wavelength shift. The effect depended markedly o n whether t h e lamp was filled with Ar or Ne. The line width of t h e Ca 422.673 nm line in a Ne-filled hollow cathode lamp varied from 0.00092 t o 0.001 5 4 nm ( 6 3 ) and corresponded t o t h e Doppler broadening expected f o r a cathode temperature o f 347 t o 429OK. The intensity of t h e o u t p u t of hollow cathode lamps can be increased b y pulsing t h e current through t h e lamp u p t o unusually high levels.Further studies of this mode have been carried o u t by Prugger e t a1 ( 7 3 ) and Kielkopf (309). The latter investigated t h e operating parameters of hollow cathode lamps operated with pulses between 3 0 0 and 1500 amps. The general behaviour of t h e lamps was similar f o r any cathode metal a n d geometry t h e intensity of emission increasing with filler gas pressure and being stable in t h e range 5 t o 50 torr. This extremely high current mode of operation has n o t been applied t o absorption o r fluorescence spectroscopy and may warrant further investigation if t h e light o u t p u t is reproducible.Prugger et a1 extended t h e medium current work of Dawson and Ellis* L'vovt and L o w e t b y scanning t h e line profiles of t h e emission from hollow cathode lamps using a Fabry-Perot interferometer. They confirmed t h a t t h e intensity of t h e emission pulse was u p to 200 times greater t h a n i n d.c. operation and that line broadening and self reversal were only observed with increased power dissipation. They found t h a t t h e corresponding t o t a l flux of radiation could b e increased u p t o 2 0 fold f o r t h e resonance lines of Al Ba and Ca. Human and * J. B. Dawson and D. J. Ellis Spec. Actu 1967,23A 565. t B. V. L'vov "Atomic Absorption Spectrochemical Analysis" Adam Hilger London (1970) p.5 8 ff. 5 R. M. Lowe Spec. Actu 1 9 6 9 2 4 B 191. 2 3 Part I Fundamentals and Instrumentation Butler (64 4 7 7 ) examined a n alternative method of increasing t h e intensity of light o u t p u t which employed t h e hollow cathode lamp as a n atomic vapour generator. This vapour was t h e n excited at 2450 MHz. The increase in resonance line intensity (for Cu Mg Ca and Ni) was between 5 and 1 0 fold b u t t h e stability o f t h e light o u t p u t was reduced b y a factor of 2.5. This technique would appear t o have no significant advantage despite considerable increase i n t h e complexity of t h e system. There have been some modest developments in t h e design of hollow cathode lamps.One of these incorporated a lens as t h e window of t h e t u b e (691). The advantage of placing a condenser lens close t o t h e hollow cathode is likely t o be marginal. Modific- ations of t h e high intensity hollow cathode lamp for use in atomic fluorescence have been described b y Lowe (74). The secondary discharge is confined t o t h e region of t h e negative glow of t h e hollow cathode discharge. Lamps have been made for Fe Ni Co Cr Ag Cu and Au and have a light o u t p u t several times greater than t h a t of t h e earlier high intensity lamps. In a survey of light sources attention was drawn (662) t o t h e heated cathode high intensity lamp (426 463).The o u t p u t of these lamps f o r Cu Fe Pb a n d Mg is a t best 10 times greater t h a n t h a t of a conventional hollow cathode lamp and t h e analytical sensitivity u p t o 20% greater. Undoubtedly t h e most expensive hollow cathode lamp available is t h e "Glomax" demountable model (Barnes Engineering Co. 3 0 Commerce Road Stamford Conn. 06902 USA). This is a continuously pumped lamp with water cooling o f t h e cathode mount t h e b o d y diameter is similar t o t h a t of conventional sealed hollow cathode lamps a n d t h e unit can be mounted in a n y atomic absorption instrument. The lamp can be operated in t h e pressure range 0.5 t o 4 0 t o r r and with currents as high as 1,000 mA t h e nominal power dissipation being u p t o 2 5 0 W.Gas flow rates are 2 t o 1 0 0 ml min-l. It seems probable t h a t t h e development of conventional hollow cathode lamps is reaching its limit. The problem of instability however remains and leads t o fluctuations of t h e order of 0.1% per second. In atomic fluorescence where t h e o u t p u t signal is directly proportional t o light source brightness this instability is n o t a serious problem b u t i n absorption it can be a serious limitation i n obtaining low detection limits. In addition t o all t h e major instrument companies suppliers of hollow cathode lamps include:- (1) Cathodeon Ltd Nuffield Road Cambridge England.(2) S. a n d J. Juniper a n d Co 7 Potter Street Harlow Essex England. See also t h e Analytical Chemistry 1971 -72 Laboratory Guide. 1.2 DISCHARGE TUBES The use of electrodeless discharge tubes (EDTs) in analytical spectroscopy has been reviewed (464 1034). Tubes are now available f o r approximately 3 0 elements. Multi- element tubes have been prepared f o r Li Na a n d K (464) and f o r Co and Ni (394). The former were used f o r atomic absorption and t h e latter f o r fluorescence. Consider- able effort is being directed a t improving t h e manufacture of tubes t o give more reproducible a n d stable light sources (109 419 695). A special s t u d y has been made of t h e manufacture of vacuum-jacketed tubes f o r U235 and U238 (462).Thompson ( 4 5 ) used a Ge EDT as a primary source for t h e low-sensitivity measurement of calcium b y absorption a t its 422.673 nm line. The Ge line was 0.016 nm from t h e Ca and there Part I Fundamentals and Instrumentation 4 was analytically useful overlap between the lines giving good linearity of calibration curves up t o 1 500 pg m1-l. To improve the stability of EDTs a special cavity has been designed (425) t o give smooth air flow over the tube for cooling purposes. The power supply for this lamp provided current stabilisation t o 0.5%. A simple circuit t o provide current stabilisation against changes in supply voltage has been described (823).A 5% fluctuation in line voltage produced only a 0.1% change in light output. A detailed study of factors affecting lamp performance has been carried out by Cook et a1 (399). They evaluated the performance of Hg Se and T1 EDTs in absorption and fluorescence analysis using four different types of %-wave resonant cavity and with varying conditions of lamp preparation modulation stability cooling cavity tuning and deterioration of cavity surfaces. Modulation of light sources is standard practice in absorption and fluore- scence spectroscopy. There are a number of advantages in effecting this modulation within the lamp.Several workers have described systems for modulating the output of EDTs (302 573 855 896). Frequencies as high as 150 KHz have been tried but Dagnall et a1 (855) found the optimum frequency t o be ca 20 KHz preferably using a square wave. Micro-wave excitation has also been used t o generate UV lines and continuum radi- ations. The spectra of low pressure rare gas (Ar Kr and Xe) lamps at a power input of 8 0 W was a continuum with superimposed lines (1 01 6). At high pressure (400 torr) and high power (1 kW) the spectrum of a Xe tube was a continuum with an intense band at 280-300 nm (18). A refillable low pressure xenon lamp has been described (752). At an input power of 30 watts the 147 nm line flux was 1.6 X 1013 photons s-l sr-l.Similar results were obtained (244) when gas mixtures including Ne Xe He N 0 were used. The 02concentration was 1013 atoms cm-3 and the flux of the 130 nm was 1014 photons s-l sr-l. The source temperature was 2700°K. The Xe arc with metallic halide additives and confined in narrow bore quartz tubes was investi- gated (1018) as a possible excitation source in fluorescent discharge tubes. The power dissipation ranged from 2 t o 30 watts per linear cm. A continuum emission was obtained if the vapour pressure of the metallic halide was high. The flux density of the 184.9 nm radiation in the low pressure Hg discharge has been investigated (433). The intensities nearly saturated at about 2 amps cm-2 which suggested that this discharge would be potentially useful as a radiation standard.The pressure range varied from t o 0.5 torr and the line width from 0.006 nm (2 mtorr) t o 0.1 nm (0.75 torr). An attempt was made t o increase the intensity of the Hg 253.7 nm radiation by decreasing self absorption (97). Introduction of a higher pressure of foreign gas (Ar) reduced self absorption but as the Ar reduced the electron energy fewer Hg atoms were excited and there was no gain in line intensity. Electrodeless discharge tubes may be obtained from (1 ) EM1 Electronics Ltd Hayes Middlesex England. (2) EDT Supplies Ltd 26 Grove Park Terrace London W 4 England. (3) Southern Spectral Sources Frimley Road Camberley Surrey England.(4) Ophthos Instrument Co 9600 Overlea Drive Rockville Md. 20850 USA. For use with conventional metal vapour discharge lamps a simple circuit has been described which allowed the discharge lamp t o be modulated in phase with a lock-in amplifier (264). The lamp was driven at half mains frequency and the polarity of the electrodes alternated in each cycle. This produced light and dark periods of equal duration. The output of a gas discharge lamp has been modulated at twice the mains 5 Part I Fundamentals and Instrumentation frequency by inserting an inductor in series with the lamp (693). A phase lag occurred and this had t o be corrected before the lamp could be used in a lock-in amplifier system.1.3 LASERS The laser is an attractive source for atomic fluorescence work as it has high intensity and narrow line widths. Unfortunately no means has yet been found of producing laser sources using the energy levels of the resonance transitions. Tunable lasers have been developed whose tuning ranges include the wavelength of some resonance lines but their emission band width is relatively broad and the freedom of tuning is limited. Capelle and Phillips (280) have developed dyelasers t o generate 5 X lob9 s pulses of l o 3 watts with a 0.3 nm band width over the wavelength region 414-642 nm. UV tunable lasers have been made using frequency doubling of the emission from a dye laser (248 649).The Rhodamine 6G dye-solution laser (245) was tuned from 571-615 nm (half power point) with a peak output of 0.41 J at 595 nm and a band width a t half maximum of 0.17 nm. Other workers (654) using Rhodamine and other dyes covered the range 340-1000 nm; the widths of tuning bands were from 20- 180 nm and the emission bandwidth was 4 X 1 O4 nm. One of the first analytical applications of laser excitation in atomic fluorescence (253) was the measurement of sodium vapour in the atmosphere (200 atoms cm-3 in a path length of 4 Km). Fluorescence was excited by a 0.2 J/pulse Rhodamine 6G/ ethanol dye laser at an altitude of 80 t o 120 Km. The detector was a photomultiplier attached t o a 45 cm diameter telescope and photon count rates of 0.03 counts per laser pulse were recorded.A theoretical study by Piepmeier (930) of the use of lasers in atomic fluorescence showed that a laser with sufficient power density and spectral band width could saturate the excited state of an element thereby almost eliminating the dependence of analyte fluorescence intensity upon the power density of the primary light source. Fraser and Winefordner (413 923) used a pulsed tunable dye- laser (10 kW peak power) pumped with an N2 laser t o excite atomic fluorescence in flames. Detection limits of 0.01 ppm t o 0.3 ppm have been obtained for elements such as Ca and Fe. An argon laser has been used to excite Cz molecular fluorescence (908).Peterson et a1 (229) found that introduction of a small amount of a weakly absorb- ing atom or molecule into a laser system could lead t o the extinction of the laser action owing t o atomic absorption. The effect was demonstrated by introducing Na and I into the resonant cavity of a dye-laser generating a continuum emission with a band width of 2 t o 10 nm. Detection limits for atoms and molecules were two orders of magnitude lower than by conventional methods. 1.4 MISCELLANEOUS LIGHT SOURCES There is a continuing need in absorption and fluorescence for a source suitable for multi-element analysis. Applications of Grimm glow discharge and "Glomax" lamps have been investigated (221 592 593) for this purpose and their operating parameters determined.They have a high intensity output but are less stable than hollow cathode lamps. However it appears that these sources may be more suitable for excitation in emission analysis (see section 2.3). An alternative approach t o the light source problem is the wall-stabilised metal vapour arc (301) of the modified Maecker type. When the spectrum of this arc was investigated using a Fabry-Perot interferometer it was found that the line widths were broader than those of the conventional hollow cathode lamp. Part I Fundamentals and Instrumentation 6 High resolution atomic absorption spectra of U were obtained b y combining flash photolysis and flash discharge (434).U14 vapour in a quartz cell was decomposed by a pulse from a Xe flash lamp and a discharge passed through t h e vapour. The emission spectrum f r o m this lamp was simpler than that from a Ne filled hollow cathode lamp. In atomic spectroscopy there has been little attempt t o produce absolute standards of intensity although t h e wider use of standard light sources would facilitate inter- laboratory comparison of emission sensitivities. A W ribbon lamp fitted with a sapphire window and operating at 39 amps has been suggested as a standard for t h e range 150-270 n m (240). The blackbody temperature was 2798OK.The count rate was 11 cps at 150 nm (corresponding t o an emission of 4.5 X l o 6 photons cm-2 s-l sr-l nm-l) and 3 X l o 4 cps at 270 nm (2.7 X 1013 photons cmh2 s-' s f ' nm-l). A ribbon W lamp operated at a known temperature and assumed emissivities has been used as a standard source f o r determining t h e relative efficiency of a monochromator and detector system (1 90). The developments in light sources which have taken place over t h e years have contributed a great deal t o t h e advancement of analytical atomic spectroscopy. Never- theless some improvement in stability a n increase in light output and some reduction in price would be welcome.

ISSN:0306-1353    

DOI:10.1039/AA9710100002

出版商:RSC    

年代:1971

数据来源: RSC

摘要:

Part I Fundamentals and Instrumentation 6 2 Excitation Sources and Atomising Systems This section contains reports of work concerned with atom formation and/or excit- ation relevant t o atomic emission absorption and fluorescence analysis. 2.1 ARCS AND SPARKS Although several alternative means of sample atomisation and excitation exist the large majority of practical analyses by emission spectroscopy use arc or spark excit- ation. These techniques are of course well-established but remain an area of consider- able research interest. The following pages reflect the diversity of the investigations recently reported and demonstrate how complex are the physics and chemistry of these sources and their interaction with analytical samples.Excitation processes in spark discharges (415) and in the d.c. arc plasma (424) have been studied. French workers (1066) have shown that all elements e.g. halogens N 0 Pb etc. in salts and oxides could be determined by means of a battery of low tension (1000 V) but powerful (1000 J in 200 ps) condensers. This source permitted work at atmospheric pressure in both the visible and ultraviolet spectral range. The conditions governing the arcing of samples in briquetted form using a Zeiss Q-24 prism spectrograph and ABR - 3 source have been examined (85). An inter- mittent a.c. arc combined with rotation of the sample ensured constant intensity of spectral lines provided that impulse duration impulse frequency and current were suitably chosen.The matrix effect with the a.c. arc was buffered by C powder even in the case of calcites fluorites and barytes which have in their matrix the spectro- chemically efficient cations Ca" and Ba* (642). An a.c. arc has been used (775) instead of the more usual spark excitation in a comparative study of intensity ratio and density ratio methods. Muntz (23) used a helium sheathed tubular electrode arc with ultrasonic nebulisation of sample solutions t o examine seven different bases (Ni Cu Al Mg C o Fe Ti) t o type the alloy from one set of standard solutions containing 7 Part I Fundamentals and Instrum en tation 18 elements.Arc excitation of fissile materials was compared with hollow cathode excitation as an alternative t o mass spectrometry (574). The trend in many industries is for fast analysis and spark sources of higher repetition rates have been developed. For various reasons the maximum repetition rate of these sources is about 400 sparks s-l. The source described by Schroeder and Strasheim (625) produced up t o 1000 sparks s-l and some analytical experiments indicated that analysis speed could be increased by a factor of 10-20 with no loss of reproducibility. Spark-off curves precision sensitivity self absorption and matrix effects were discussed in application t o the analysis of wear metals in lubricating oils (2 19 948)." M u l t i s ~ u r c e ~ ~ conditions which have been successful for many years in the steel industry are now provided at the high repetition rates of up t o 400 sparks s-l (420) and tables of discharge characteristics and precision data have been given (553 554 872 9 12). Factors influencing calibration drift were associated with the counter electrode (5 5 3). Modification of a commercial air interrupter high voltage spark source for produc- tion of high current discharges with wave forms ranging from fully oscillatory t o fully unidirectional was discussed (564 964 967). Line intensities in low energy neutral and first ion spectra were enhanced significantly without corresponding increase in background when the electrode was rotated above 1200 r.p.m.t o ensure reproducible spatial geometry. A simple device in which the cathode spot of a d.c. arc operating in an inert gas moved rapidly over the surface of a metal sample producing a fine aerosol was described (422 952). This aerosol can be transported in the inert gas up t o 20 m t o any suitable excitation device such as the stable low powered d.c. capillary arc chamber which has been designed for optical excitation of the aerosol (553 1006). Ignition difficulties which can occur when sparking highly alloyed steels were investi- gated with a special electronic controlled spark generator (624).Removal of the inter- element and metallurgical effects in the spectrographic analysis of high and low alloy steel was claimed f o r the low energy high power density micro spark method (804). The micro-spark method has also been used (1075) with a Cu-needle electrode t o examine the microhomogeneity of ingots prepared in moulds with directed cooling. A 21 amp d.c. plasma arc assembly designed t o exclude air was described for the determination of non-metals mixed with a suitable solvent. The wt. % of H 0 S and N was estimated from the diatomic spectral bands C - H 314.3 nm 0 - H 306.3 nm C - S 257.5 nm and C - N 388.3 nm. Results were given for N and S in crude oil (56 7 5 7).A study was made (626) of the distribution of particles in a d.c. arc of 8.5 A during the vaporisation of very small amounts of elements. The introduction of ammonium compounds into d.c. arc plasma was shown t o influence the apparent sensitivity of trace elements (84). Electrodes with double cavities were used to study the carrier effect of various salts and possible explanations of these effects were given. Morris and Worden (31) determined 24 trace elements in M o by a spectrographic method using a low current arc in Ar. The sample was packed in graphite and a small quantity of CO was produced when the oxide was reduced in the anode. This change in composition of the arc gas was essential for the excitation of trace impurities.The stability of a d.c. arc was improved by using a reduced pressure static argon atmosphere with a Ta sphere attached t o the. counter electrode. This led t o greater sensitivity and the technique was useful in trace analysis especially of Cr in blood hair and urine (257). A d.c. arc plasma burning in air at atmospheric pressure was used Part I Fundamentals and Instrumentation 8 (545) t o investigate the direct determination of trace elements in rare-earth matrices. The trace elements concentrated in the cathode region while the rare earth elements concentrated in the anode region. The cathode region was less dense and trace elements were determined independent of rare earth matrices.The electrodeposition of trace elements on mercury coated electrodes was used (584) t o determine Ag Bi Cd Co Cr Fe Ga Ge In Mo Ni Pb Sn T1 and Zn simultaneously by a high voltage spark discharge. This method of preconcentration avoided some causes of erratic results reduced contamination from reagents and gave satisfactory photographically recorded spectra for all elements except Pb. Pitz (238) measured the absolute intensity of the carbon arc in the region 190-250 nm by comparing it with the intensity of an Hg high pressure lamp and deuterium discharge lamps calibrated by means of synchrotron radiation. Once standardised the carbon arc was very reproducible.The spectrum was a continuum decreasing in intensity t o short wavelengths and with superimposed fine line structure. Spectral radiances of the order of l o 5 erg s-l cm-2 s f 1 nm-l were found. A special apparatus was designed (632) t o study the parameters affecting light intensity fluctua- tions in emission spectrometry. Conclusions were drawn about the use of hemi- spherical electrodes argon atmospheres and changes in polarity. The changes in the carbon arc under the influence of a stationary homogeneous magnetic field were studied by Leushacke and Nickel (365 366 55 1). When low fields of 1 0 G were applied the arc became mechanically unstable and started t o rotate twisting t o a helix with higher inductions.The increase in spectral line intensities was studied and was due basically t o an intensification of the cathode layer effect. The effects of added elements having low ionisation potential in a d.c. arc plasma burning in an inhomogeneous magnetic field were investigated by Georgijevic et a1 (550 552). The influence o n the electron density of the plasma was considerable with changes affecting behaviour and the ratio of spectral line intensity t o background. The addition of small concentrations could be advantageous for the determination of other elements with low ionisation potentials. The influence of lithium carbonate o n temper- ature distribution in an arc has also been investigated (629). To photograph the plasma produced by spark discharges Strasheim and Blum (623) used a rotating mirror ultra high speed camera with a minimum framing interval of 0.25 p s .The spectral radiation coming from different zones of the spark gap was recorded by means of a time resolving technique and the formation of cathode spots on different structures of sample surface was observed (623). Results for the radial temperature distribution in the free burning arc according t o Krinberg theory were compared with experimental determinations in Ar N and air (627 628). It was found that the convection transport of heat was not sufficient explanation for the radial temperature distribution found in the outer zones of the arc.The experimental and theoretical radial temperature distribution curves for arcs burn- ing in N Ar C 0 2 with added water vapour Li and I vapour were discussed (79 80 81 82 83 670). Dieudonne and Bril (1057) also studied the characteristics of atomic excitation in arcs in rare gas atmospheres. Energy levels higher than in air were reached favouring the appearance of high excitation potential lines. The photographic measurement of arc radii was described using an adjustable velocity? explosively driven double probe (697). The influence of pressures of up t o 40 atmospheres on a d.c. arc was reported and showed different characteristics for varying electrode materials (568). The background intensity was found t o be at least as good a reference as an internal 9 Part I Fundamentals and Instrumentation standard particularly because it was applicable for quite a wide spectral range in an examination of a gas stabilised 15 amp d.c.arc (566). Marinkovic and Vickers (33) described a stabilized d.c. arc for the production of atoms for AAS which should have great potential in emission spectroscopy. The arc was stable with a temperature similar t o that obtained in N2 0 - C2 H2 flames. Detec- tion limits for Al B Mg V and W were broadly comparable with those obtained using flames. A 3.4 m Jarrell-Ash Ebert spectrograph was used t o investigate high temperature reactions in three component systems (605).These reactions were interdependently connected with the evaporation of the elements and in the case of cathodic evapor- ation the burn-off of the material depended strongly on the atmosphere during burn- ing (604). Two papers discussed different techniques of measuring transport parameters in d.c. arc plasmas (608 962). The action of spectral carriers and their role in d.c. arcs was discussed and the influence of fluoride chloride and iodide carriers on the excitation of trace impurities in alumina was studied (76 77 78). Decker (69) showed that electrode temperatures in a d.c. arc were dependent on electrode dimensions and o n the electrical power in the arc system.With a fixed arc gap the cathode temperature was directly proportional t o the power and the log of the anode temperature was directly proportional t o the log of the power. As the electrode temperature appeared t o be independent of both the atmosphere used and the nature of the sample it should be possible t o vary the electrode temperature t o suit the type of sample being analysed. Curry and Cooley described (374) a compact gas jet operated at 12 amps d.c. with a reduction in background emission and a shorter burn time when compared with earlier designs by Helz or Margoshes. For the direct spectrographic determination of highly volatile fluorides such as B and Si present as impurities in UF4 it was necessary (603) t o suppress the U excitation during the d.c.arc exposure and t o improve sensitivity. With samples mixed with alumina the limit of detection did not meet specifications but using zinc oxide at a concentration of 35% B and Si spectra were obtained free from most interference from U lines enabling their determination with good sensitivity and reproducibility. A specially shaped electrode was used (564) t o collect the material evaporated from a given microsector of mineral by a laser beam. The electrode was then subject t o a separate excitation in certain cases a d.c. arc in a magnetic field or in an impulse discharge. When applying this laser microanalysis technique t o the detection of impurities in galena there was an increase above the expected intensity of a great number of spectral lines.The intensification factor for CuI 323.8 nm was 4.65 with the background unchanged and thus improved the signal t o noise ratio. Investigations were conducted into the nature of selective electrical erosion of different alloys in spark and arc sources used in spectrochemical analysis. In a d.c. arc the erosion corresponded directly t o the melting points in Ti - Ni Mg - Al and Mg - Zn binary alloys and volatilisation was mainly a thermal process (71 0). The decisive role of thermal conductivity o n the erosion mechanism was shown using an a.c. arc with Cu Ni Cu - Mn Cu - Zn and Cd - Zn alloys (702).The dependence - of electrode erosion on electrode composition in an arc discharge was much stronger for the anode than the cathode and it was indicated that the transport rate of elec- trode material into the discharge caused part of a third element effect described (703). Erosion by spark discharges was shown t o have both thermal and electromechanical Part I Fundamentals and Instrumen tation 10 properties and hard brittle materials with high melting points underwent stronger spark erosion than malleable materials with low melting points (709 710). A linear relationship was shown between t h e spectral intensities and t h e amount of erosion loss of Cu Mn Si and Ti by spark discharge from various Fe alloys and a n A1 alloy (339 3 44).2.2 PLASMAS The characteristics and applications of plasmas generated in an inert gas stream have been studied b y several groups. Of t h e more basic work t h e apparatus used b y Mullen et a1 (753) t o measure high temperature plasma reactions was interesting. In this t h e R F induction plasma produced by means of a 100 kW 1 MHz R F generator was passed through a reaction channel t o react with supersonically injected target gas molecules. The reaction products were then monitored b y quadrupole mass spcctro- metry. Kleinmann and Cajko (75) investigated spectrophysical characteristics (spatial distribution of radiation shape of spectral lines temperature of particles electron density continuum intensity) of a high frequency (50 MHz) low power (270 W ) discharge in Ar.The discharge was shown not t o b e in thermodynamic equilibrium. Isagawa and Niki (350) studied t h e distribution of spectral line intensity and continuum in t h e plasma j e t along t h e flame axis measuring t h e Fe 371.99 nm Mg 285.21 n m and Mg 280.27 nm lines. They concluded that line t o continuum ratio was maximised b y cooling the plasma b y means of a "cooling device" rather than by allowing it t o cool by spouting itself i n t o t h e atmosphere. Barnes and Schleicher (610) reported t h e beginning of a study aimed at developing a description of t h e physical and chemical interactions in an induction plasma discharge and their influence o n spectrochemical processes.Two other studies of plasma temperatures have been noted (705 740). Measuring t h e temperature i n a 55A DC plasma jet b y t h e two-line method (Zn 307.2 nm and Zn 307.6 nm) it was found (705) that at and above 1 6 mm the temperature was n o t influenced b y t h e effective ionisation potential of t h e samples. Comparisons have been made (740) between temperature measurements of argon induction plasmas and calculations based o n t h e Elenbaas-Heller equation. Agreement was good for the central portion of t h e plasma column while discrepancies nearer t h e walls were attributed t o over-estimation of gas temperatures when Saha equilibrium was assumed.A patent has been granted (242) covering a high frequency discharge plasma torch generator with a single A1 electrode. The electrode is watercooled and use of A1 at t h e t i p of t h e electrode was claimed t o preclude erosion even a t high temperatures. Analytical applications of plasmas have increased and commercial instruments using these as t h e excitation source are now available (see Section 6.2). Greenfield (774) has recently reviewed his considerable experience in this field. Plasma jets using graphite electrodes microwave torches capacitively- and inductively-coupled plasma torches and electrodeless inductively-coupled torches were described and t h e advantages of the latter type stressed.Power generators incorporating Hartley-type oscillators were stated t o remove mismatching effects caused b y introduction of aerosols into plasmas. Ultrasonic nebulisation was advocated for introduction of liquids and fluidised-bed devices f o r solids. This technique has been applied (1 1 1 ) t o t h e analysis of powders using a modified Radyne SC15 plasma generator. Fassel and co-workers have made significant contributions in particular by demon- strating t h e selectivity and sensitivity of this technique when used with suitable optical equipment. Recent reviews (224 9 3 1 ) reveal that they favour use of a n induction- 11 Part I Fundamentals and Instrumentation coupled plasma operated at 40-50 MHz a n d 2 kW making analytical measurements in t h e tail flame (T-3000" K) rather t h a n in t h e toroidal plasma (T-900O0K).While.it has been claimed (846 8 9 1 ) t h a t a n Ar plasma has been developed t o t h e stage a t which it is available as a routine analytical tool t h e experience of some workers has been less favourable. A 2 kW 6 0 MHz induction-coupled plasma for example showed rather irreproducible behaviour (454) which made operation difficult. Robin ( 4 4 4 ) reported results obtained with a STEL (France) 4-6 MHz generator which confirmed t h a t while plasmas have advantages in emission spectro- scopy t h e y are inferior to flames when used in t h e absorption mode chiefly because t h e high background emission tends t o saturate t h e detector.Demonstrations of t h e applicability of plasmas t o practical analyses have been provided b y Vecsernyes and Zaray (327) Heemstra (56) Gebhardt and Horn (609) and Kleinmann (1032). The last a u t h o r noted t h e severe matrix effects of elements with low ionisation energies in a high frequency low voltage plasma. Vecsernyes (61 1 ) has also discussed t h e analytical significance of nine operating parameters in t h e design and operation of a laboratory-built plasma. He commented particularly o n t h e experimental difficulties associated with direct analysis of powder samples.Finally t w o examples of t h e use of low-power microwave-generated plasmas should be noted. Fallgatter e t a1 ( 3 8 ) used a 1 0 0 W 2450 MHz generator t o produce a plasma with temperature -5000" K. The system however was n o t in thermodynamic equilibrium and also suffered from memory effects. Aldous et a1 (829) used a similar system equipped with an EM1 9 6 0 1 B photomultiplier and simple amplifier-detector system t o detect t h e transient emission signals produced from -0.1 pl of sample introduced b y evaporation from a Pt o r W loop heated b y electrical resistance. Both of these investigations were restricted t o volatile elements in aqueous solutions.It has recently been announced t h a t ARL are to produce a n emission spectrometer based upon a low-power microwave plasma which it is claimed will provide empirical formulae of eluent fractions from a gas chromatographic separation of organic mixtures. I t will cover C H N 0 P S and halogens. 2 . 3 GLOW DISCHARGE LAMPS Although t h e most important application of glow discharge lamps in atomic spectro- scopy is as t h e light source i n atomic absorption o r fluorescence t h e y can be used as t h e excitation source f o r analytical emission spectroscopy. Their advantages in this mode include high signal-to-noise ratio relatively few inter-element interferences and t h e requirement of only a few mg of sample.The development of this application has been largely attributable t o Grimm". During t h e past year h e has described their application (421 595) particularly to t h e determination of t h e gases N H and 0 in metals such as Ni alloys. The sample was placed in a cavity diameter 2 0 mm and formed t h e cathode. The lamp contained 3 t o r r of Ar ran a t 0.5 to 1 kV and 1 5 0 mA and a 60 second exposure was used. T h o r n t o n ( 4 2 3 ) described t h e determination of trace metals i n Ni alloys placing 0.01 g of sample i n t h e hollow-cathode in a n He atmosphere. Further practical applications of t h e glow lamp have included t h e analysis of special steels ( 5 5 5 ) using a technique i n which t h e line and background emission intensities were measured alternately b y t h e .same photomultiplier b y means of an oscillating * W.Grimm Naturw. 1967,22 586. W. Grimm Spectrochim. Acta 1968,23B 443. Part I Fundamentals and Instrumentation 12 quartz disc; determination of non-metallic Contaminants 0 S P and halogens (8 14) and the determination of U235 in natural U (352). In the last analytical measurement was made at 424.4124 nm using the 424.4372 nm U238 line as standard - the Jarrell- Ash 3.4 m Ebert-type grating spectrograph having the dispersion necessary t o separate these lines completely.The U sample t o be analysed was deposited into the hollow- cathode cavity electrolytically and the discharge was carried by a flowing Ar stream. Other descriptions of the design and application of such lamps for emission analysis have been given by Johnson and Gram (865 866 949) Dogan et a1 (681) Ropert (596 1072) Boumans (592) Jager and Butler (216 594) Moal and Brossier (597) and Dieudonne and Bril (593) - in most instances using glow discharge sources manu- factured by R.S.V. GmbH. Three further papers are noteworthy. In the first (598) design parameters were optimised and the stability of the discharge was shown t o be critically dependent upon the distance between the electrodes.The use of this lamp f o r determination of P and also for the continuous analysis of solutions was con- sidered. A simple demountable water-cooled graphite hollow-cathode system for solution analysis was described by Prakash and Harrison (1 06). Finally a twin hollow- cathode system was described (599) which permitted excitation of sample and standard material under identical conditions. Hollow cathode discharges have also been used as sputtering devices for the non- flame atomic absorption analysis of alloys (54 169 7 11). Atomic fluorescence gave (169) limits of detection for Ni Cr and Cu in Fe base alloys of the order of 10 pg/g. Atomic absorption investigations showed that precision was poorer than that obtain- able with a flame (5- 15%) (54) and that analysis time was long - varying from 15-20 min (71 1) t o 3-4 min (54).From the summary above it will be seen that this is an area in which there is considerable activity. A more complete understanding of the cathodic sputtering processes is being built-up and the stability of hollow-cathode discharges - an internal standard need not be used - has been pointed out. It will be interesting t o note whether present research efforts result in the adoption of this technique for routine emission analysis in the near future. 2.4 LASERS Laser microprobe analysis is now an established technique in emission spectroscopy and various aspects of its application have been described (560).Webb and Webb (314) for example used a Jarrell-Ash Q-switched ruby laser with a Hilger medium quartz spectrograph for semi-quantitative analysis. Microprobe analysis has been applied t o studying minerals (564) and the welding seams of steels (563). It has been shown (62) that staining of a metal sample surface during analysis was prevented by prior application of a thin coating of collodion. The laser microprobe has been used (706) for atomic absorption analysis. Here the samples were held at 45O t o the laser beam and the optical axis of a spectrograph. The integrated absorption signal from high alloy steel samples was measured.Two studies of laser microprobe analysis in atmospheres other than air have appeared. With a Carl-Zeiss Jena LMA-1 laser Petrakiev and Dimitrov (5 62) concluded that in N2 O2 and Ar atmospheres spectral line intensities and signal-to-background ratios increased by up t o 1 5-fold resulting in significant improvements in reproduc- ibility and sensitivity. Treytl et a1 (406) however investigated the use of N 2 0 2 Ar Ne and vacuum and concluded that special atmospheres were of no benefit in general work. Line-to-background ratio showed no systematic changes with change of energy 13 Part I Fundamentals and Instrumentation between 1.2 3.6 and 8 mJ but was best in vacuum a t 1.2 mJ.This group also reported (255) that spectral emission of metallic elements viz.Ag and Mg was suppressed in organic matrices such as albumin and serum owing t o reduced sample vaporisation. Treytl et a1 have also (40) described a method of maximising the signal-to- background ratio from a Q-spoiled laser by photoelectric time differentiation of the emission spectra. With a similar aim Scott and Strasheim (559 971) used a high- repetition rate Nd laser with a time-resolved direct-reading detection system and achieved reproducibility which permitted use of the technique for quantitative analysis of metal samples.More fundamental studies have been made of the interaction of a laser beam with a sample. Tlalka (561 ) reviewed various theories of the mechanism of this interaction. Allen (741) probed a laser-produced plasma with an electron beam and obtained the target surface temperature as a function of time and the mass vaporisation rate. Finally Piepmeier et a1 (870 968 1008) have also studied the processes occurring when a Qswitched Nd laser atomised a metallic Cu sample. At atmospheric pressure the crater diameter (90 p m ) and sample consumption (35 ng) were relatively constant with variation in power from 10 t o 100 mJ. At 1 torr however both increased with increasing power and spectral changes also occurred.2.5 FLAMES 2.5.1 Flame Types The dominant position of premixed air/C2 H2 flames for atomic absorption analysis remains unaltered. These flames have gained acceptance as sources for atomic emission and on shielding with an inert gas have proved attractive for fluorescence applications. The current state of the art was clearly defined by Koirtyohann (447) in his plenary lecture in Paris. He pointed out that the flame/burner/nebuliser system is still the most troublesome item of today's atomic absorption instruments. We still use inefficient pneumatic nebulisers which require large sample volumes; high precision seems t o take place t o sensitivity in most instrument designs; working curves inexplic- ably change during a series of analyses; we have no coherent theory of chemical interference for many elements; a non-quenching interference-free flame is still required for atomic fluorescence; solid sample analysis is not part of current practice; and there are still many elements which are determined only poorly by atomic absorption - boron being an example.The premixed 0 2 / H 2 flame has received little attention as an analytical source. A recent study (453 926) showed that despite a high maximum temperature (2950°K) detection limits were worse by factors of 4 t o 10 than those obtained in NzO/C2H2 flames. Solute volatilisation interferences were serious particularly in fuel-rich flames and equilibrium calculations indicated that vapour-phase metal monoxide formation was extensive.The premixed N20/C2,H2 flame has now been generally accepted as useful for atomic emission*. A list of detection limits has been published (266 1010) for 68 elements and the use of gas chromatography integrators for evaluation of spectral scans has been reported (1042). The use of the N2 shielded N 2 0 / C 2 H 2 flame supported on a slot-burner gave u p t o 3 times improvement in detection limit over the unshielded flame (1042) for A1 396.2 Re 346.1 and Re 448.9 nm. * E. E. Pickett and S. R. Koirtyohann Spectvochirn Acta 1968,23B 235. Part I Fundamentals and Instrumen tation 14 Methyl acetylene appears t o be a less satisfactory alternative t o acetylene for use in premixed flames (664).Flames based on the use of liquid fuels (e.g. hexane acetone) and air (100 260 490) may have some application in the design of portable instruments for field use. Bailey and Rankin introduced fuel into a conventional burner chamber by means of a second nebuliser. Performance was similar t o that using gaseous alkane fuels. 2.5.2 Burners and Nebulisers The important parameters in burner design which affect flame flashback have been reviewed by a number of authors (101 618 849 902 1009). Using these criteria it is possible t o design burners which will not flash back under any conditions of gas flow.Such burners would have plate separations of 0.3 mm for N 2 0 / C 2 H 2 and 0.5 mm for air/C2H2 if a conventional slot design were adopted but would be of limited analy- tical use. Capillary burners have been designed (618) which are safe usable and have convenient configurations. The minimum total gas flow required t o maintain a stoichi- ometric flame on several commercial burners has been calculated (61 8 849 902 1009). One important observation was that the measured burning velocity of the stoichiometric N2 O/Cz H2 flame (61 8 ) + + C2 Kz 3 N 2 0 = 2CO H 2 0 3Nz C / O = 0.667 was found t o be 285 cm/sec as against the only other value of 160 cm/sec*.An improved N2 0 / C 2 H2 burner (304 820) of gold-plated copper construction featured water cooling and a slot width of 0.64 mm tapered t o 0.48 mm at the base. The burner warmed u p rapidly operating at a head temperature of 60°C. Increases in sensitivity of 2.0 t o 2.6 times were claimed over the Techtron AB 50 burner which it superficially resembled. Burners with independent sample aerosol introduction have been designed (478) for use with ultrasonic nebulisers. Separation of the nebulisation and combustion gas supplies resulted in constant nebuliser performance irrespective of flame variations. Ultrasonic nebulisers have not found widespread use in atomic spectroscopy of their poor stability.Owen (881) has experimented with various designs and reported (479) a direct impi\ngement device which gave reproducible performance comparable with that of a pneumatic nebuliser. The sample solution was pumped at 0.25 t o 3 ml/min across the vibrating face of a piezo-electric disc protected by a glass coating. Vibration at 1.4 MHz produced a fine aerosol with a uniform drop size and efficiencies of up to 50%. An improved spray chamber design by Nicolas (19) is now commercially available on Techtron instruments. The chamber has a constriction about one-third of the distance from the inlet. Fuel gas is introduced at the nebuliser end of the chamber at a low velocity point.The constriction aids mixing of fuel and oxidant gases removes large droplets and successfully damps out shock waves which are propagated in conventional spray chambers. The efficiency of nebuliser chambers can be increased by the use of infra-red heating as in some types of burner available from Beckman Instruments. Some recent experiences with heated chambers have been reported by Uny et a1 (70 132) who * W. G. Parker and H. G. Wolfhard Proc. 4 t h Int. Symp. Combustion 420 ( 1 9 5 2 ) andProc. 5th Int. Symp. Combustion 7 1 8 (1954). 15 Part I Fundamentals and Instrumentation claim a n increase i n sensitivity of 1 0 t o 30 times depending o n t h e element and b y Rawson (465) who has constructed a stainless steel unit having similar performance.While correction for background i n flame emission has been compensated b y repeti- tive optical scanning (641) a n older alternative approach is t h a t of modulated intro- duction of t h e sample. I n t h e past this has involved t h e use of periodic sample injection via a rotating t a p at a b o u t 50 Hz. Difficulties were usually encountered as t h e modulation technique also caused flame background fluctuations. In t h e most recent reports (35 639) t h e sample introduction was only partially modulated. A latex t u b e leading t o a t o t a l consumption burner which supported a premixed nitrous oxide/ acetylene flame was sandwiched between t h e plates of a piezoelectric bimorph.Modulation was produced b y compression of t h e tubing wall. A dual channel device was also described where sample and blank were aspirated i n t o t h e same burner through separate tubes and modulation was 1 80° out-of-phase thus maintaining a constant liquid flow-rat e. The noise spectra of premixed N2 0 / C 2 H2 flames supported o n a sprayer burner* were measured (638) over t h e frequency range lov3 t o 0.5 Hz. Below 0.1 Hz t h e noise was mainly inversely proportional t o t h e frequency (l/f noise). Between 0.1 Hz and 0.5 Hz "white" noise predominated. Noise arising from t h e nebuliser ( b o t h l/f and white) made a significant contribution t o t h e t o t a l noise of t h e analytical signal when a sample solution was aspirated i n t o t h e flame.The noise arising from Swan band emission ( 7 7 1 ) in a n air/C2H2 flame was shown t o cause interference o n several F e and Cr lines in AAS using a commercially available instrument. 2.5.3 Discrete Sampling Devices Several discrete sampling techniques f o r use with flames are currently available. Usually t h e sample solution is measured i n t o a boat o r coil and after evaporation to dryness inserted i n t o t h e flame. A Ta sampling boat similar t o t h a t developed b y Kahn et a l t has been used f o r atomic fluorescence (482) and improved by t h e use of electrical pre-heating (8 92).Pre-evaporation is quicker if prior extraction into an organic solvent is carried o u t . T h e Delves sampling cup$ is now available from Perkin Elmer and has been described recently in their literature (92 292). A boat of pyrolytic graphite was used (492) for t h e introduction of A1 solutions ( u p t o 1Opl) i n t o a N2 0 / C 2 H2 flame. The evaporation time was 3-5 sec although Cd was evaporated in less than 1 sec in an air/C2H2 flame. Several easily vaporised elements (e.g. Cd Zn Pb and Ag) may be determined b y placing a d r o p of solution o n a Pt loop which is t h e n inserted i n t o t h e f l a m e s . A similar device has been developed (685) for use with an air/C2H2 flame f o r use i n t h e determination of Zn in A1 and Cu alloys b y atomic absorption.Harris and Newton (1 0 4 5 ) have injected dithizone extracts containing Cu directly into t h e capillary of a conventional pneumatic nebuliser. As little as 30 pg Cu could b e detected i n 1 0 0 1.11 a n d it is possible t h a t under these conditions t h e nebuliser could have a higher efficiency t h a n with continuous operation. Best results were obtained with a n oscilloscope readout. The direct analysis of solid samples has commanded little attention f r o m flame spectroscopists until recently. Introduction of a powdered sample i n t o t h e unburned * V.G. Mossotti and M. Duggan Appl. Optics 1968 7 1325. t H. L. Kahn G. E. Peterson and J. E. Schallis At. Absorp. Newsl. 1968 7 35. $ H. T. Delves Analyst 1970 95 431. 0 G. M. Katz Anal. Biochem. 1968,26 381. Part I Fundamentals and Instrumen tation 16 gas mixture of a flame has been described (533) for Pd on a number of catalyst supports. Aspiration of a suspension of stannous oxide stannic oxide or stannic sul- phide has been used (261) for the determination of Sn. In both instances the atomic absorption response was dependent on the nature of the sample. Direct analysis of electrically conducting samples has been achieved (1007) by an apparatus in which the sample forms the cathode of a low current d.c.arc. The cathode spot moves t o give uniform sampling and the aerosol produced is carried in a stream of Ar t o a N2O/C2H2 flame. Coefficients of variation of 2.6% have been obtained for Cr Mn and Ni in steels by atomic emission. Other devices which could possibly be of use for solid sample nebulisation are described in the section dealing with non-flame atomisation. 2.5.4 Theoretical Studies An accurate measurement of flame temperature is essential in understanding the spectroscopic equilibrium and kinetic processes involving species contained in the flame. Reif et a1 (450 61 4 932) have emphasised the point that spectroscopic tenfper- ature measurements on conventional analytical flames may be in error by several hundreds of degrees from the average o r weighted average temperature.The uncertainty arises from the non-isothermal nature of all flames used for analysis and Abel inversion techniques must be used t o convert experimental lateral temperature profiles t o true radial profiles. Illustrative calculations were based on temperature and concentration profiles generated by equations similar t o those used by Sasaki". Two of the most easily applied spectroscopic temperature measurement techniques the two-line method and the slope method depend o n accurate transition probabilities for Fe lines and a proper choice of lines. Reif et a1 (451 615) determined relative transition probabilities for 4 3 Fe lines relative t o the Fe 373.487 nm line.The work was carried out in an isothermal region of a premixed NzO/CzHz flame using the reversal temperature of the Fe 371.994 nm line t o determine the flame temperature with an experimental error of +1O0K. Durie and Smith (172) found that chemilumine- scence increased the measured Na D line reversal temperatures of premixed propane/ O2 /N2 flames when small amounts of S O 2 NO or Ar were introduced. + + + ("S) Na H SO2 -+ (2P) Na HSOz + + (2 S ) Na H 4- NO + ( 2 P) Na HNO Flame temperature measurements based on an atomic fluorescence technique have been proposed (183 485).Direct-line fluorescence of T1 at 535.0 nm is excited using a continuum source at the resonance wavelength of 377.6 nm. Resonance fluorescence at 377.6 nm is then excited by 535.0 nm radiation from the thermally populated metastable P!42 level. The effects of flame temperature on the atomic fluorescence of Pb at the direct-line 405.8 nm and the resonance line of 283.3 nm have received comment from Bailey (1 12) and thermally-assisted fluorescence of In and Ga has been discussed by Omenetto and Rossi (25). Calculations of the degree of thermal ionisation of metal additives in flames assum- ing the Saha equilibrium have been tabulated for 48 elements at three flame temper- atures ( 3 78) using the assumptions of earlier authors?.Although the calculation was * Y. Sasaki Jap. J. Appl. Phys. 1966,S 439. t D. @. Manning and L. Capacho-Delgado Anal; Chim A d a 1966,36 312. 17 Part I Fundamentals and Instrumentation admittedly approximate it should serve as a useful guide t o practising analysts. Signi- ficant deviations have however been observed (61 7 939) between calculated and experimentally determined degrees of ionisation. The differences are known from earlier work t o arise from either slow recombination of ions or chemi-ionisation. Ionic absorption measurements (619) have been suggested as a possible but unlikely complement t o atomic absorption for analysis.The measurement of the free a t o m fraction (p) of metal additives in flames continues t o undergo developments in experimental technique. Ag has been shown t o have a /3 = 1 .O in H2 / 0 2 /Ar flames and can therefore be used as a reference element for a relative absorption technique (677 907) using a continuum source and iso- thermal flame. The relative measurement approach obviates the necessity for determin- ing nebuliser efficiency and spectral response of the detection system. Accurate values for oscillator strengths were required in common with many other methods for deter- mining 0. Willis (72) using an absolute technique noted that the value of p approached 1 .O for Na Cu Ag and Au in a premixed air/C2 H2 flame.Errors by earlier workers were shown t o result from neglect of hyperfine structure. A method for measuring p factors which was not dependent on a knowledge of oscillator strengths (307) used relative measurements of atomic and metal monoxide emission spectra in a series of flames having the same temperature but different fuel-to-oxidant ratios. Nitrogen was used t o reduce the temperature of the hotter flames t o that of the more fuel-rich flames studied. The method required that the metal and its monoxide be the only species present. It was applied t o the rare-earth elements. A detailed investigation (665 754) of the behaviour of Cr in H2/02/N2 flames showed that comparable concentrations of Cry CrO C r 0 2 and HCr03 were present In the gas phase in all flames studied (1 800-2700" K).Calculations of free atom fractions based on tabulated data have been used t o explain equilibrium atom-formation processes in N2 O / C 2 Hz flames ( 3 12,903). Com- puterised calculations ( 3 12) have enabled quantitative predictions t o be made on the suitability of unusual flames for atomic spectroscopy e.g. 02/C2H2 02/C2N2 N 2 0 / H 2 NO/C2H2 N20/C3H8. Although some of the tabulated data for metal compounds is of limited accuracy this approach can only serve to eliminate much of the speculation associated with this area of flame chemistry.While these calculations showed that the degree of metal atom formation could be adequately described by an equilibrium model it is difficult t o appreciate the contrary views of other workers (754) who have adopted a much less rigorous approach. Evidence for non-equilibrium processes in low pressure flames has been presented (448) and both chemiluminescence and chemiatomisation phenomena observed. Excitation of metal atoms in hydrogen flames occurs (390) in the recombination of H' and OH' radicals. In hydrocarbon flames alternative mechanisms can give excitation t o energies of up t o 9.0 eV. The role of particulate chromium oxide in radical recombination reactions has been studied (665 754) in H2 /02 /N2 flames.The Doppler half-width (AX*) of an atom in a flame is easily calculated if the flame temperature is known. The Lorentz (collisional) half-width (AXL ) cannot be calculated accurately because of uncertainties in collisional cross-section and molecular concentrations of colliding species. A convenient means of determining ALL is via the a-parameter a = (ln2)1/2(AXL/AX,) and this can be obtained from experimental curves of growth*. The growth curves may be based on emission or absorption measurements. * C. van Trigt T. Hollander and C. T. J. Alkemade J. Quant. Spectry. Rad. Transfer 1965 5 813. Part I Fundamentals and Instrumentation 18 A method based on atomic fluorescence measurements by Winefordner et a1 (678) using H2 / 0 2 /Ar flames gave results which were systematically greater than those measured by emission.The advantages of the fluorescence method were discussed. Experimental growth curves in atomic fluorescence using continuum or line source excitation were in good agreement with theoretical predictions (7 1 ) for Mg 285.2 nm in an isothermal flame of well-defined geometry. Practical analytical curves cannot be determined under such favourable conditions. Svoboda and Winefordner (48 9) have discussed departures from ideality and their effects on the shapes of atomic fluore- scence curves.Van Gelder (88) has made a similar study of atomic absorption curves and cites self-absorption in the light source imperfect light collimation through the flame hyperfine structure and especially measurement of non-resonance radiation by the detector as the main causes of premature curvature. Atomic line profiles and related topics were the subject of an excellent review by de Galan (445). Measure- ments of the atomic absorption coefficient of the sodium D1 line have been made (254) using magnetic scanning of a vapour cell at 100 t o 180°C. Atomic line profiles from light sources and flames have been measured experimentally (637) using a piezo- electric Fabry-Perot interferometer. The self-absorption caused by cooler outer regions of a flame used for atomic emission will affect the measured line profile.This has been studied in detail (616) using a two flame atomic absorption technique. One flame acting as the light source represented the inner isothermal portion of the theoretical flame while the second flame represented the cooler outer regions of the theoretical model where self-absorption occurs. The quantitative theory of flame equilibrium processes is now quite well developed and fair agreement with experiment can be obtained. In contrast the study of chemical interferences is predominantly a kinetic problem as well as a problem of identifying the involatile species involved. Classical approaches involving measurement of degrees of interference at varying burner heights with differing excesses of inter- ferent and with varying flame conditions continue t o be made for aqueous solutions (65 441 458 459 460 1053 1061 1064) and for oils (1021).A more direct approach t o the problem of chemical interference is that adopted by Hieftje (461 925 990) who has developed the single droplet technique introduced by himself and Malmstadt". Using this method droplets of a uniform diameter are introduced singly into a tall well-defined flame and the rates of desolvation volatilisation and dissoci- ation measured as a function of distance travelled. The determination of aerosol size distribution using spectral attenuation measurements and based on Mie scattering theory has been reported (1017).Page and Newman (640) used a Langmuir probe t o measure the volatilisation and diffusion of aerosol droplets of a 1 M KC1 solution. Each droplet evaporated t o give a small 'filament' of vapour about 3 p m in diameter separated from its neighbours by a mean of 500 y m or 170 diameters. 2.6 NON-FLAME CELLS 1971 saw a profusion of publications describing the use of non-flame atom cells for atomic absorption and fluorescence spectroscopy. The high absolute sensitivity obtain- able permits the analysis of very small samples and the low emissive backgrounds assist performance in atomic fluorescence. Furthermore the devices are usually small and may be totally enclosed t o allow analysis of radioactive samples or t o extend the wavelength range t o the vacuum ultra-violet.Kirkbright (436) published an excellent review with 65 references of non-flame devices and their historical development. * G. M. Hieftje and H. V. Malmstadt Anal. Chem 1968,40 1860. 19 Part I Fundamentals and I n s t r u m e n t a t i o n 2.6.1 Tube Furnaces and Filaments A number of instrument companies have made designs commercially available. The Perkin-Elmer HGA-70 is based o n t h e Massman t u b e furnace and features a program- mable power supply. It has been described i n several application reports (1 5 166 1 7 1 189 384 397 512 522 8 4 4 918).Varian-Techtron market a carbon rod device based o n t h e West carbon filament and also their so-called "mini-Massman" which is interchangeable with t h e carbon rod. Both are used with a programmable power supply and have been described with applications in a number of reports (1 13 1 14 116 175 209 2 5 9 412 4 7 1 650 8 4 1 851 8 7 6 8 8 6 945 1043 1044). Instru- mentation Laboratory Inc. have developed a n electrically heated Ta strip with a V- shaped indentation to receive t h e sample. The strip is operated in a n Ar atmosphere and has been described in detail (271 480 570 845 8 4 8 8 7 7 935 984). Most of t h e reports o n these commercially available devices were somewhat repetitive and journal publications usually covered t h e symposium material also.Other instrument com- panies known t o b e actively evaluating non-flame a t o m cells include Jarrell-Ash Rank Precision Industries Shandon Southern Instruments (469) and Carl Zeiss (571 ). In addition several workers have reported t h e use of either unspecified o r home-built devices based o n accepted designs of t u b e (Massman) furnaces (218 51 5 5 3 4 ) o r o f filament atomisers (393 41 7 470). A t u b e furnace f o r continuous operation (2 1 1 ) employed a n ultrasonic nebuliser t o introduce liquid samples. Solid samples were introduced using graphite cups (145 2 1 1). Simultaneous correction for molecular absorption and light scattering inter- ference was made.Direct measurements of atmospheric pollutants have been made using a heated cylindrical absorption tube packed with carbon rods through which t h e air was drawn continuously*. Lifetime of t h e rods might be expected t o be limited because of oxidation by t h e flowing air (834 9 8 9 ) . An inductively heated furnace incorporating a 6 kW Radyne power unit gave a maximum temperature of 21 70" K ( 4 8 ) ; a n improved version gave a maximum temper- ature of 2670°K (148). Soviet scientists (484 495 572 715 722 7 2 6 ) have developed a graphite cell which could be used f o r t h e determination of solid samples (about 2 0 mg mixed with an equal weight o f graphite powder).The crucible was clamped between graphite rods in a water cooled holder and heated b y a current < 4 0 0 A supplied from t h e secon- dary winding of a step-down transformer. A pulse of 5 sec duration gave a heating rate of 1800"K/sec and a final temperature of 2570°K. The device was used for b o t h atomic absorption and fluorescence analysis. A graphite cell f o r t h e measurement of vapour pressures and heats of vaporisation has been constructed b y Nickolaev e t a1 (525) and t h e use of King furnaces and heat pipe ovens described by Tomkins ( 4 7 3 ) t . A King furnace enclosed in a glove box has been used ( 5 3 9 ) f o r t h e atomic absorption determination of T e and other radioactive elements.Relevant t o t h e use of non-flame sampling was a paper b y Antonetti et a1 (5 1 8 ) who discussed t h e micromanipulative techniques necessary when volumes of 1-11 were t o be analysed. An automated system was constructed ( 9 0 1 ) for AAS and AFS where the instru- ment generated a d r o p of reproducible size placed it o n a Pt loop and vaporised t h e * H. P. Loftin et al Spectry. Lett 1970,3 161. t c.f. F. S. Tompkins and B. Ercoli App. Optics 1967 6 1299. Part I Fundamentals and Instrumentation 20 sample by passing a 6.3 V a.c. The measurement device was gated through the Pt loop and the total integrated signal converted t o digital output.About 150 analyses per hour could be achieved. 2.6.2 Cells for Mercury Determination Although flameless atomic absorption was used as long ago as 1939 by Woodson* who determined Hg in air the widely accepted cold-vapour absorption technique involving SnC12 reduction was introduced in 1964 by Poluektov et a1.F and developed by Hatch and OttS. Manning has published a thorough review of the topics§. Despite the number of publications very little new instrumentation has been described. This is no doubt because of the ease with which reliable determinations may be carried out at low levels using inexpensive modifications of commercial atomic absorption spectrometers. Nearly all manufacturers of atomic absorption instruments provide conversion kits for Hg determination.Furthermore heated devices such as tube furnaces and filaments have been used for this purpose. Several instruments specific- ally designed for Hg analysis are listed in Table A. Several reports (370 41 1 524) have described instrument adaptations where Hg is obtained by strongly heating the sample is adsorbed on Pt (524) or Au (370,411) and is subsequently liberated by heating and swept into an absorption cell. Automated digestion and measurement systems have been constructed (1 57 202 409 5 17) and the use of a Wickbold apparatus for sample preparation particularly with organic solutions has been described (504)q. Although Hg vapour has usually been detected by atomic absorption techniques the use of atomic fluorescence offers some additional advantages including elimination of spurious absorption signals by condensed species.In the system of Thompson and Wildy (764) there was no optical cell or recirculation system. Hg vapour escaped from the end of a 10 nm id glass tube at the optical centre of the fluorescence system. A d.c. glow discharge and atomic emission has also been used (408) for the detection of evolved Hg. An enclosed absorption cell has been described (28) which features a side-arm containing an Hg reservoir. An analytical calibration curve for the cell could be con- structed by placing the side-arm in a variety of cooling mixtures.The apparatus was used for the determination of Hg in lunar rocks (549). 2.7 OTHER EXCITATION AND ATOMISING SYSTEMS Two interesting papers have been noted which fall into none of the above categories. In the work described by Hirokawa (860) the analytical sample was excited by ion bombardment the ions being produced by a low voltage impulse discharge through N z . This indirect method was useful for analysing both metals and non-conducting materials such as oxides. In the latter instances mixing with a conducting material e.g. carbon was unnecessary. Analytical emission lines monitored included Ti I 319.19 nm Ti I1 309.19 nm C I1 250.91 nm Fe 1309.81 nm Si 1243.52 nm and B I 249.68 nm.Rousselet et a1 (468) described a system where the sample (volume * T. T. Woodson Rev. Sci Ins. 1939,10 308. N. S. Poluektov R. A. Vitkun and Y. V. Zelyukova Zh. Anal. Khim 1964,19 873. 5 W. R. Hatch and W. L. Ott,AnaZyt. Chem. 1968,40 2085. 8 D. C. Manning,At. Absorp. Newsl. 1970 9 97. 1 See ASTM Method D 2785-70 or Institute of Petroleum Method IP 243/69T for description of Wickbold apparatus. mg a filters testing.and Two liquids.gives operate gases 0.0002 .Mains on catalyst SnC1 feature to 0-0.2 1-1 an testing.through and discharged.battery ranges silver circulator.mg Batteries down atmosphere are operated scales with hot air two a for current use m-3.gases two in to air Mains mg determinations on for atmosphere for batteries on built-in by mainly rechargeable 0.0002-0.5 circulator.for /day.m-3 analysis are rechargeable direct used AAS air 0-0.7 Description with when be m-' mg Ni/Cd gas designed for injected Carried lb.liquids and off detector 6 mg and detector also 0.01 volt and Built-in m-3 turn to 12 sample Can to instrument furnace.liquid m-3 mg continuously.p1 0.0025 FLAMELESS Weight portable instrument instrument m3.portable is down system.for mg 50 BY use.beam mg 0-0.2 hours cut-out Drift 7% reduction hours a photometer.Hand-held Measures 0-1.0 8 Double Designed 0.002-5 operated.Laboratory reduction Hand-held ranges for safety Laboratory Typically in to mP3.MERCURY OF address DETERMINATION THE and Division) Lane U.S.A.Supplier's 11 Ltd England.England FOR Inc. Instruments Heath Heath Co.Division Division 15238 Research Crown AB 60153 (Coleman Ltd Bucks.The The Ins.Drive Pa Surrey Bromma Ill.Cheshire Industries Cheshire Mond Mond 11 Instruments House Berks.Elmer Elmer Dept Dept Alpha Incentive 11074 D D Ltd Ltd INSTRUMENTS & & Baccharach AMBAC 625 Pittsburgh U.S.A.Shandon-Southern Camberley IRD Development Box S-161 Sweden.Wallac Crown Theale Perkin Maywood Perkin Beaconsfield ICI R Runcorn ICI R Runcorn A TABLE MV-2 Sniffer Name Probe Vapour MAS-SO Meter Mercury Mercury W J HGM-2300 Coleman Total Mercury Part I Fu ndam en tals and Ins tru m en ta tion 22 10-100 nl) was deposited in a W cup and bombarded under vacuum by electrons accelerated by a 2 k V cm-l electric field.High voltage stability and vacuum quality were important criteria for successful operation.

ISSN:0306-1353    

DOI:10.1039/AA9710100006

出版商:RSC    

年代:1971

数据来源: RSC

摘要:

Part I Fu ndam en tals and Ins tru m en ta tion 22 3 Optics 3.1 LIGHT GATHERING AND BEAM MANIPULATION In emission and fluorescence analysis significant gains in sensitivity can be achieved by effective collection of the light emitted by the source. The auxiliary optics used for flame and arc or spark methods differ because of the geometry of the light source the spark being small and compact the flame extended. Boumans (636) has pointed out that the use of a cylindrical lens t o focus an image of the source on the entrance slit of the spectrograph or monochromator is well known practice in arc emission spectro- graphy and could be of benefit in flame photometry. The advantages of this arrange- ment are uniform illumination of the entrance slit full illumination of the grating and selection of a narrow pencil of radiation from an optimum zone of the flame.Two papers (25 98) described the use of a reflecting elliptical A1 cylinder t o collect light from the flame. The flame was located a t the primary focus of the ellipse and the entrance slit of the monochromator at the secondary. Around the entrance slit was a spherical mirror with a central aperture t o allow light t o enter the monochromator. The focus of the spherical mirror was back into the flame. Using this system ten-fold gains in sensitivity were reported. This optical arrangement may be particularly useful in conjunction with low background atom reservoirs such as shielded flames or plasmas.Improved sensitivity in atomic absorption has been achieved by inserting lens masks (104). These limit the absorption path t o a precisely defined zone of the flame corres- ponding t o the highest atomic vapour density and improve sensitivity by 5 t o 50% for a number of elements. Although this approach leads t o loss of light in most cases the intensity of the light source is sufficient t o ensure that the precision of the measure- ment is not determined by shot noise. In atomic absorption and fluorescence polarisation of the light from the source has been used t o separate the signals from the measurement and reference channels (1 023) the reference channel being used t o measure non-specific background absorp- tion in a furnace system.Schull and Winefordner (29) used polarisation in an attempt t o reduce scattering effects in atomic fluorescence. There was no change in the ratio between the fluorescence and scattered light with and without polarisation. Further the use of a single polariser produced a much greater reduction in signal than would have been expected from the transmittance values of the polariser at that wavelength. New materials available for U.V. transmitting glass fibres have been briefly reviewed (226). 50% transmission at 254 nm through a bundle 1 m long was reported with usable transmission at 200 nm using shorter paths.The acceptance angle of such a fibre was +30° and the bending radius approximately 20 mm. The intensity of light entering an emission spectrometer can be controlled by a rotating cylindrical shutter (125) driven at speeds of up t o 5000 r.p.m. (with air) or up t o 3000 r.p.m. (with water) and consisting of two cylinders mutually adjustable t o give any desired fraction of light transmission. 23 Part I Fundamentals and Instrumentation 3.2 WAVELENGTH SELECTION 3.2.1 Dispersive Systems The present position of diffraction gratings has been reviewed by Loewen (20). The h > constrdction of a very high resolution Echelle grating system (- 450,000 at 6h 500 nm) with a dispersion of 0.0097 nm mm-' and aperture f/47 has been described (694).In a novel spectrograph designed b y Lantwaard and Van de Stadt (696) the incoming light was collimated by a series of grids which absorbed non-axial light the dispersing element was a Fresnel zone plate and the spectrum was formed on photo- graphic film placed parallel to and aside from the axis of the system. The dispersion of the system was 1 nm mm-' and the resolution about 5000. The viewing angle was 0.5". Wavelength accuracy varied from +2 nm in the violet t o +20 nm in the red. It is being increasingly appreciated that t o obtain the lowest detection limit precise measurement of the background signal adjacent t o the analytical line is important (447).Haisch et a1 (682) used an array of slits consisting of a number of parallel glass fibres for this purpose. Each fibre acted as a slit covering a spectral range of 0.005 nm. By this means background measurements could be made extremely close t o the analytical line and thus provided accurate background correction. A simpler system used a quartz plate t o scan a narrow waveband in the region of the analytical line. The system may be arranged t o give either two measurements one on the spectral line and one on the adjacent background ( 5 5 5 ) or a continuously variable signal (641). In the second case Rains and Menis achieved detection by means of a lock-in amplifier tuned t o twice the frequency of the vibrating quartz plate.An instrumental accessory with this facility has recently become available from Jarrell-Ash. By appropriate instru- mentation a derivative spectrogram may be obtained if both quartz plate scanning and main wavelength drive scanning are used together. Wide range wavelength scanning has been carried out using a stepping motor (241) t o drive the dispersing element; the scan rate was 480 nm s-l the spectral range 330-1 140 nm the dispersion 6.8 nm mm-l and the output of the photodetector fed into a 1024 channel signal averager. Up t o 128 scans could be made of the spectral source. Single channel scanning is an ineffic- ient way of gathering spectral information and substantial improvement (1 O2 - l o 3 times) can be effected by using Hadamard transform spectroscopy (HTS) (233 235).In HTS the intensity distribution in a dispersed spectrogram is temporally encoded the dispersed light is then recombined and focused into a single detector. The resul- tant signal contains information on both the spatial distribution and intensity in each point of the spectrum. The information is recovered by decoding the signals with the aid of a computer. The general principles of selective spatial or temporal modulation of light signals were considered by Harwitt (247).Devices based on these principles have greater information collection capacity but require elaborate data processing facilities. 3.2.2 Interferometers Lerner (234) discussed the limitations of dielectric interference filters when used in wide angle optical receivers. With omnidirectional radiation there was negligible improvement in the signal-to-noise ratio by using an interference filter placed between the light source and detector. Several papers have reported slight modifications which improve the performance of Fabry-Perot interferometers. These devices are of relatively large aperture and high Part I Fundamentals and Instrumentation 24 resolution hence they could be of value where continuous spectrum sources are being used for atomic absorption.There are however problems of stability and alignment with interferometers. Berney (279) described a development which allowed adjust- ment of the plate separation without disturbing the parallelism so that as long as t h e temperature remained constant no mis-alignment occurred. Several workers (23 1 698 739) have used piezo electric control t o give the fine wavelength adjustment of the interferometer plates and the drive for spectrum scanning. By incorporating a pair of quarter wave plates in a bi-refringent Fabry-Perot interferometer a narrow band pass with a large solid angle for light acceptance was achieved (228).A multiplex Fabry-Perot interferometer has been described by Hirschberg et a1 (230). Multi-plexing was achieved by rotating a multi-zone disc at 5 Hz in front of a similar disc held stationary the pair being centred in the fringe pattern of the Fabry-Perot. The trans- mitted radiation fell on the photomultiplier. The advantages of this system were (1 ) The practical upper limit t o the number of channels was set only b y the finesse of the interferometer; (2) Only one detector was needed; (3) The system was much faster than pressure-scanning and more stable than one using vibrating plates. The resolution was of the order of 20,000. A Michelson interferometer has been modified by Dohi and Suzuki (246).In this instrument one of the reflecting mirrors was replaced by a diffraction grating. The remaining mirror was vibrated t o vary the path difference and thus t o provide small wavelength scanning. The grating was rotated t o provide the spectral scan. This instrument provided data similar t o that of reference 641 but had a greater light gathering power and higher resolution. General improvement in detection limits is obtained by higher resolution and the incorporation of systems for background subtraction. The theory of derivative spectro- metry has been treated by Anderson and Hager (1025) and recent applications of rapid scanning interferometry by Griffiths (9 1 7).These techniques may achieve a significant improvement in performance but they are elaborate and require computer data processing. For these reasons they are not likely t o become widely used in analytical atomic spectroscopy. A high resolution Echelle grating monochromator has been shown t o give selectivities in emission flame photometry comparable with that obtain- able by atomic absorption (87 1 ). Fabry-Perot interferometers have been used t o study the profile of lines emitted by hollow cathode lamps (63 274) and the emission and absorption profiles of lines from flames (637). Combined monochromator-interferometer instruments achieved spectral bandwidths of 0.002 nm and can be used for atomic absorption measurements using a continuous source (150 W xenon arc lamp) (944 954).Winefordner et a1 (954) also examined the possibility of using a continuum source with a medium resolution mono- chromator and refractor plate t o effect narrow waveband scanning of the output of t h e system. Both systems gave detection limits of 0.01-0.1 yg ml-' for a number of elements.

ISSN:0306-1353    

DOI:10.1039/AA9710100022

出版商:RSC    

年代:1971

数据来源: RSC

摘要:

Part I Fundamentals and Instrumentation 24 4 Detector Systems The only development noted in the field of photographic recording was that of gamma compensating filters. The principle practical application and methods of preparation of such filters have been described by Nagy (807 808). 25 Part I Fundamentals and Instrumentation 4.1 PHOTO-ELECTRIC DEVICES Trassy and Robin ( 1 0 7 0 ) have discussed t h e principles and technical solutions in building a n electronic detection system for AAS with particular reference t o its use with a n h.f. plasma torch. 4.1.1 Photomultipliers Photomultipliers with a spectral response covering t h e range 180-600 nm are widely available with q u a n t u m efficiencies rising as high as 25%.Some tubes have a n extended red response stretching to 900 nm. The q u a n t u m efficiency o f "solar blind" photo- multipliers is 7% a t 2 5 0 nm and 0.02% a t 350 nm (243) while that of t h e RCA C31000E photomultiplier at 632.8 nm is 6.1% (237). Young and Schild (249) found t h e photo-electron collection efficiency of t h e EM1 9 5 5 8 t u b e t o be 75%. The dark current due t o thermal emission f r o m t h e photocathode tends t o be greater with cathodes of extended red response. T o minimise dark current t h e cathode area should be as small as possible compatible with receiving all t h e signal from t h e instrument.Owing t o inhomogeneities i n t h e photocathode (278) it should be fully illuminated during measurements. T o overcome t h e effect of cathode inhomogeneity d e Galan and Wagenaar (368) when studying line profiles introduced a field lens behind t h e exit slit t o project a n image o f t h e grating o n t o a larger area of the photomultiplier surface. Several a t t e m p t s have been made t o improve photocathode quantum efficiency b y improving t h e optical capture o f photons (282). In o n e arrangement (43 1 ) t h e p h o t e cathode was illuminated obliquely b y shining t h e incident light i n t o t h e side of t h e lens o n t h e cathode end of t h e photomultiplier this gave a q u a n t u m efficiency enhancement o f 50- 100%.A comparable improvement ( 4 3 0 ) was obtained b y passing the incident light through a quartz prism optically coupled t o t h e face of the photo- multiplier; t h e angle of incidence was such t h a t multiple reflections occurred within t h e envelope of t h e tube. As t h e photomultiplier is n o t a storage device t o measure low light levels some form of integrating electronics must b e incorporated i n t o t h e system. Image dissector tubes (image orthicon and vidicon tubes) store photo-images in t h e phosphor until released b y t h e scanning electron beam. The image of a spectrum can b e formed o n t h e photocathode and it can be used f o r multi-element analysis.It has been claimed ( 2 3 2 ) that t h e advantages o f this system are high q u a n t u m efficiency simplicity and cost when compared with a n array of photomultiplier tubes and true multiplex operation by storage of the information f r o m short pulses. The resolution of the system appeared t o be 1 6 line pairs rnm-l. The disadvantages of t h e system (428) included a small region of linearity a poor signal-to-noise ratio some lag and "sticking" of images in pulsed studies and t h e danger of burn-out at high light levels. The resolution was 1 nm and t h e storage time 1 sec. The S / N ratio varied from 1O:l to 3 0 l and t h e dynamic range was limited t o a factor of 5.The use of a commercial Russian television set (PTY-101) with a television camera tube (LI-17) for detection of emission spectra has been described (1091 1092). Qualitative and quantitative analyses were possible in t h e range 400-750 nm b u t detection limits were inferior t o those obtained by photoelectric detection. 4.1.2 Solid State Detectors The sensitivity of solid state detectors is still significantly less than that of photo- multiplier systems. Where light levels are n o t low however such devices have some Part I Fundamentals and Instrumentation 26 advantages and surface barrier diodes developed for nuclear counting have been evalu- ated as photodiodes for detection of radiation from 4 0 0 nni t o t h e near infra-red (87).Results indicated excellent performance in comparison with t h e more conventional diffused surface junction types of photodiodes. Bournans ( 5 6 9 ) has made a detailed study of t h e use of photodiodes and phototransistors as detection devices for multi- channel emission spectrometry. The detector consisted of a n array of equidistant photodiodes o r phototransistors each 3 mm long and 25 p m wide t h e interspace between t h e diodes being 5 pm. An array of 2 0 diodes was assembled. This system offers t h e possibility of t r u e multiplex operation with simultaneous subtraction of background signals f r o m t h e emission signal.Studies of t h e performance of t h e system are not yet complete. 4.2 SIGNAL PROCESSING 4.2.1 Photon Counting Photon counting as a means of measuring low light levels has been used for a t least 2 0 years. Only recently however has its use become widespread due principally to t h e newly available high-speed counting systems. Most photomultipliers may b e used as p h o t o n counters. However owing t o the limitations of t h e associated electronics non-linearities may arise d u e t o pulse pile up. The principal advantages of photon counting lie in t h e measurement of low light intensities and in t h e production of digital signals with consequent simple integration. The detection and measurement of very low light levels were discussed b y Flint (1).By careful choice of t h e photo- multiplier operating conditions and detection system light levels of about 1 0 photons s-l could be detected in t h e ultra-violet t o blue spectral region (more were required in t h e red). Unless extreme precautions were taken stray light reaching t h e photo- multiplier could influence t h e o u t p u t . Even when lock-in amplification was used Flint considered it pointless spending time and money t o reduce t h e dark current t o t h e pA range when stray light produced a photocurrent greater t h a n this. Many commercial spectrometers slit assemblies and photomultiplier housings were insufficiently light tight f o r critical work.In atomic spectroscopy it is n o t usual t o be measuring such low light intensities. Other applications of p h o t o n counting include t h e determination of t h e relative quantum efficiencies of monochromator and detector systems (96). There has been some dispute ( 2 5 1 2 5 2 ) o n t h e contribution of small noise pulses t o the pulse height spectrum obtained from a photomultiplier. Jones et a1 (250) showed t h a t to obtain t h e same relative variance f o r a signal t h e observation time for capacitative store should be 2.6 times greater t h a n t h a t required f o r p h o t o n counting with digital store. The application of p h o t o n counting t o spectroscopy has been examined b y several workers Aldous a n d Bailey (491) Dagnall e t a1 (193 376 487 and 1041) Dawson (446) a n d King and Williams (950).The latter compared t h e relative sensitivity of p h o t o n counting with photographic detection for t h e measurement of Pb and Ni b y emission spectroscopy. Photon counting has been compared with lock-in amplification detectors f o r measuring transient signals in atomic fluorescence using t h e carbon rod atomiser (491). The transient response o f p h o t o n counting and d.c. systems was com- pared f o r fluorescent and absorption measurements of Cd vaporised from a Pt wire in a flame (1041). The digital system gave a lower detection limit b y factors of between 2 and 5.A detailed examination (446) of t h e comparative performance of analog and digital systems f o r t h e measurement of flame emission absorption and fluorescence 27 Part I Fundamentals and Instrumentation signals using a n EM1 9 5 9 2 A photomultiplier showed t h a t b o t h analog and digital systems gave detection limits close t o t h e ultimate in emission and fluorescence set by t h e randomicity of photoelectron emission. In atomic absorption however t h e limit was set b y t h e instability of t h e light source. When using photon counting systems care must be taken t o ensure t h a t t h e response of t h e system is sufficiently fast t o avoid a n y possibility of pulse overlap as this will lead t o spuriously large pulses; if voltage discrimination is used this should n o t lead t o t h e loss of small signal pulses unless it is demonstrated that these arise from dynode emission.At t h e present time t h e highest convenient counting rate is of t h e order of l o 6 counts s-l. When a low background atomising system is used t h e photomultiplier dark signal may be compar- able with t h e emission from t h e a t o m reservoir. In this instance cooling of t h e photo- cathode can lead t o lower detection limits. When evaluating t h e performance of p h o t o n counting and other methods for t h e measurement of light intensity it is desirable t h a t some measure of t h e linearity of t h e response of t h e system be made.A scheme f o r t h e testing of t h e linearity of absorp- tion spectrophotometers has been outlined b y Hawes (276) and is based o n t h e use of Bouguer's Law and t h e superposition of optical fields. A theoretical analysis o f t h e linearity of the response of a photomultiplier (67) shows t h a t t h e deviation from linearity is less than 3% for all photomultipliers when t h e anode current is less t h a n 10% of t h e current in t h e final resistor of t h e dynode supply chain. 4.2.2 Fast Response Systems With t h e use of impulse atomisation i n a variety of spectroscopic techniques time resolution of t h e resultant signal can be used t o give improved analytical precision nd t o study atomisation and excitation processes.Photomultiplier signal currents have been gated i n t o integrating capacitors with t h e aid of a complementary switched pair of FETs a t t h e rate of 2 0 MHz (305 672). An RCA C70045 photomultiplier has been operated t o give a time resolution of 1 ns (692). Fast gating of t h e photomultiplier signal was used by Treytl et a1 (40 8 7 3 ) t o eliminate t h e continuum emission signal which occurred almost entirely during t h e initial 1 0 0 ns of t h e pulse generated when a Q-spoiled laser beam was used t o excite t h e emission of solid samples. 4.2.3 Modulation and Signal-to-Noise R a t i o It has been shown (771) in o n e instrument t h a t modulation of the light source in atomic absorption did n o t completely remove flame interference effects.If a strong band occurred a t t h e same wavelength as an analytical line a negative absorption reading resulted from a lean flame (i.e. emission was greater t h a n absorption) while in a rich flame positive absorption readings were obtained (i.e. absorption greater than emission). These effects may be attributed t o a frequency in t h e noise spectrum o f t h e flame emission passing through t h e electrical filters. The strong atomic emission from Ca in t h e nitrous oxide flame leads t o interferences in atomic absorption when light source modulation is effected b y a mechanical chopper interposed between the lamp and flame (39).In this system emission from t h e flame was reflected back from t h e hollow cathode lamp window and thus was modulated b y t h e chopper. The inter- ference could be avoided by tilting t h e window of t h e hollow cathode lamp and would n o t occur in systems using electronic source modulation. The effect of sine- and square-wave modulation of t h e light intensity o n t h e photon count distribution obtained for differing integration periods has been evaluated (227). Part I Fu ndam en tals and Ins tru men tu t io n 28 When t h e integration time is greaJer t h a n t h e modulation time t h e photocount distri- bution is Poissonian and 0 2 N = N.S / N ratio measurements have been carried o u t b y several workers (236 273 429 446). It is found that t h e S / N ratios for current measuring and p h o t o n counting systems are very similar and t h a t these d o n o t change significantly with change of photomultiplier supply voltage. A comparison between photomultipliers a n d photodiodes (273) shows t h a t in general photomultipliers have a better S/N a t photo-anodic currents less than 5 X lop9 amps b u t photodiodes are marginally better a t higher currents.

ISSN:0306-1353    

DOI:10.1039/AA9710100024

出版商:RSC    

年代:1971

数据来源: RSC

摘要:

Part I Fu ndam en tals and Ins tru m e n tu t io n 28 5 Data Processing 5.1 EMISSION SPECTROSCOPY Paksy ( 8 0 6 ) has used t h e double standard evaluation method for t h e spectrographic analysis of steels and a new method f o r handling data f r o m t h e capillary arc source has also been demonstrated (897). Evaluation of photographic plates has been automated b y means of a magnetic tape programmable densitometer (1 87). A "Steinheil" spectrocomparator-densitometer allowed qualitative and quantitative analysis t o be performed o n spectral films o r plates irrespective of t h e spectral dispersion together with t h e evaluation of back- ground and line densities.The measurement programme manually set f o r t h e first spectrum was stored o n a magnetic tape recorder and automatically applied to t h e subsequent spectra. An automatic densitometer was constructed b y a group handling a daily average of 2 3 0 emission and mass spectra plates (602). With t h e high scanning speed of 1 c m s-l a 25 cm spectrum measured a t 5 g m intervals yielded information f o r 50,000 measuring points. Out of this abundance of data all that was relevant t o t h e lines needed f o r qualitative and quantitative analysis was extracted b y a n analogue t o digital converter and a P9205 computer (16K 16 bit words) coupled t o t h e measur- ing instrument. The computer programme was given f o r this data reduction (600) which was performed simultaneously with t h e reading because of t h e reading speed and limited computer memory.Another computer orientated rnicrodensitometer has been described (405) and flexible (762) off line (601) uses of data processing included. A device including lines of known densities was used t o hold film (679) so t h a t semi-quantitative analysis was possible. Lavaud (1063) has used a time-sharing computer f o r densitometric data treatment in photographic emission spectroscopy. A description of t h e use of a computer for operating a direct reading spectrometer (186) was given with regard t o convenience and speed and in terms of increased precision. A Polyvac 1.5 m E l 0 0 0 (661 773 1 0 8 2 ) with a computer replacing t h e usual electronic console has been described.Fixed calibration curves were used f o r all elements with rotational and translational corrections f o r matrix effects. The use of basic calibration curves with a computer system t o adapt spectrometric analysis t o t h e demands of production units in a steelworks has been discussed (631). Present and f u t u r e improvements in performance of a laboratory in a BOF steel plant ( 6 3 3 ) were also discussed. The experimental application of a digital computer gave common curves for t h e analysis o f low alloy steel high alloy steel Fe glass A1 alloy and slags (630). The general curve could be approximated b y a n exponential function which indicated in accordance with t h e experimental results t h a t this method had been founded o n a general natural law.The analytical requirements (634) and t h e hardware and organis- ation of software ( 6 3 5 ) were given f o r a computerised emission spectrometry system. 29 Part I Fu n d a m en tals and Ins tru m e n tat ion Computer interpretation with t h e aid of disc storage of t h e output of a Jarrell-Ash 1.5 m Atomcounter was used for wear metals in oil (960) and for trace levels of fourteen elements in blood serum (24). Flexible computer techniques suitable for a n R & D environment (963) a small computer application (188) and a ratio output system (839) have also been described.Margoshes (68) described three instruments incorpor- ating mini computers one of which used a television camera tube as a detector. A Hewlett-Packard computer was used as a control and computation unit with an ARL 29000 spectrometer using t h e Analytica AB (Sweden) Tape machine (750). A theoretical analysis was made of t h e errors which arise due t o averaging spectral line intensities in t h e presence of electron temperature fluctuations in continuous plasma sources (432). Photographic spectrograms of Gd (I) and Gd (11) were measured in t h e 246.8-875.2 n m range using an automatic comparator and a CDC - 3200 computer (306). Wavelengths based o n a Th wavelength calibration of the spectro- grams were obtained for about 18,000 spectral lines.Systematic errors in the wave- lengths due t o shifts in t h e spectrograms of t h e Gd spectrum relative t o the T h spectrum could be greatly reduced by means of a correction deduced from discreg ancies between wavelengths obtained from different grating orders. The accuracy of t h e final wavelengths was tested using t h e combination principle and found t o be +2 X nm. 5.2 ABSORPTION SPECTROSCOPY Computers have been employed in solving many problems of a theoretical nature (3 12 378 414 450 614 6 1 6 9 0 3 9 3 2 ) b u t these calculations are outside t h e scope of this reljort.Computer calculation of analytical results will b e of benefit for large workload requirements and there is also some justification if marginal economies can be achieved using spare capacity o n a n existing computer". The current literature refers t o only a few systems covering a wide range of degree of sophistication. At one end of t h e scale t h e direct concentration readout with or without curve straightening may be adequate. Off-line data processing may b e carried o u t for example using a BASIC program (966) which computes a least squares fit t o a poly- nomial calculates concentrations in convenient units and prints out a final analytical report. A curve fitting routine for use with limited memory capacities was described (379) f o r a Wang 3 6 0 desk calculator.Here t h e calculator needed a card reading capacity of 160 logic steps and 6- 10 memory locations. Varian Techtron have published some reports of computer systems employing their model 3 4 Data Acquisition Unit (1 18 476 920 925 964). The unit accepts data from up t o four spectrometers and converts t h e data o n t o punched paper tape for processing. The use of a large computer (IBM 360 48K memory 3 discs) t o monitor many various instruments was described by Laporte (475).

ISSN:0306-1353    

DOI:10.1039/AA9710100028

出版商:RSC    

年代:1971

数据来源: RSC

摘要:

Part I: Fu ndam en tals and Ins tru m en tat ion 29 6 Complete Instruments In this section an attempt will be made to review the currently available range of commercial instruments as well as significant laboratory-built equipment. Unfortun- ately, the coverage of commercial instrumentation is more uneven than the compilers would have wished. The main causes of this have been threefold, namely: variation in the response from manufacturers t o requests for information on recently introduced instruments, problems of establishing the availability of equipment throughout the * J. Lacy, J. M. Skinner and C. Woodward, 3rd SACConference, Durham, (1971), Paper €315. 30 Part I: Fundamentals and Instrumentation world and the basic difficulty of becoming aware of all models from all manufacturers.Regrettably, therefore, this listing will inevitably be incomplete. Information from readers on any conspicuous omissions will be of great assistance in compilation of the second volume of this report and consequently will be welcomed by the editors. Despite these limitations it is felt that the information below is of considerable potential value t o readers and it is hoped that they will find it useful. 6.1 ARClSPARK EMISSION SPECTROMETERS Photographic emission spectroscopy, a long-established technique, is now used mainly in conjunction with other analytical methods for miscellaneous inorganic analysis. During the past decade, the more important instrumental developments have been in multi-channel direct-reading spectrometers frequently with associated computer facil- ities.These are, however, necessarily rather expensive because of the electronic requirements for detection and data-processing. Recent developments of less costly, sequential, integrating spectrometers using a single detector are therefore to be welcomed by users whose financial resources are not unlimited. The ARL 33,000 (185, 663, 1076, 1077) and Carl Zeiss Jena DSA 240 are examples of this type. These appear, along with other currently available commercial arclspark emission spectro- meters, in Table B. Dudeney (9 58) has discussed the difficulties encountered by instrument manufac- turers attempting to produce many identical spectrometers. His experience was based on Baird Atomic’s supply of forty instruments to an inflexible specification for the U.S. government. 6.2 FLAME EMISSION Commercial instrumental developments in flame emission spectroscopy are in two main areas: (a) the automation of simple, often filter, photometers to permit easy handling of large numbers of samples, (b) introduction of flame- or plasma-based instruments operating at a temperature high enough to excite most elements, with a monochromator capable of resolving the resulting emission spectrum sufficiently to permit simultaneous or sequential multi-element analysis. Examples of both of these classes are given below. Several manufacturers produce rather similar flame emission spectrophotometers designed basically for use in clinical laboratories where rapid handling of large numbers of samples is essential.Such instruments frequently have two measuring channels, normally used for Na and K, with Li measured as an internal standard. Depending upon the model selected, emission readings may be integrated and read out on a meter scale or displayed digitally. The full sample-handling capacity is realised if an auto- matic sample changer is used together with a printer unit. These accessories permit unattended operation for periods of an hour or more and provide almost fully auto- mated systems at modest cost. Instruments of this type are supplied by, amongst others, Baird-Atomic Inc. (model KY-3), Instrumentation Laboratory Inc. (models 143 and 343), Shandon-Southern Instruments Ltd.(model 1740), Evans Electro- selenium Ltd. (models 170, 227 and 450) and Beckman. At least two manufacturers (Spectrametrics Inc. and Hitachi) produce emission instruments in which the excitation source is a plasma. To a degree the performance of TABLE B COMMERCIALLY AVAILABLE EMISSION SPECTROMETERS Identification Supplier Description V-70 Vacuum instrument serving as production control polychro- mator or research tool. Wavelength scanning facility, Angstrom-West Division Angstrom Inc 600 Fig Avenue, Monrovia Calif. 91016, U.S.A. Quantometer 3 1,000 1 m vacuum grating, 170-407 nm, up to 80 elements, 60 simultaneously. Quantometer 33,000 Applied Research Laboratories SA, En Vallaire CH- 1024 Ecublens Lausanne Switzerland Integrating sequential analysis using programmable cards.Gratings range (1) 256-610 nm, ( 2 ) 170-407 nm. Also suitable for flame emission. Quantovac 80 Similar to Quantometer 3 1,000 but with new electronic console. Quantometer 80 1 m grating, 190-610 nm, up to 80 elements, 6 0 simultaneously. Applied Research Laboratories Ltd., Wingate Road Luton, Bedfordshire England Quantovac 29,500 1 m grating, 170-235 nm, 2160 lines/mm. Blazed for 200 nm, up to 1 8 elements. No details available Baird-Atomic Inc Bedford Mass. 01730, U.S.A. See Applied Research Laboratories Bausch and Lomb PGS-2 Q- 24 VEB Carl Zeiss Jena German Democratic Republic DSA- 24 0 2 m plane grating spectrograph, 200-280 nm. Prism spectrograph, 200-580 nm covered in one photograph.Quartz prism scanning spectrometer, 210-550 nm. Dispersion 0.78 am/mm a t 250 nm. Automatic recorder with tape punch and reader. C.Z. Scientific Instruments Ltd Zeiss England House 93/97 New Cavendish Street London W1A 2AR, England See Rank Precision Industries. Hilger and Watts TABLE B COMMERCIALLY AVAILABLE EMISSION SPECTROMETERS - continued ~~~ ,- Description Identification Supplier 66-000 Compact Atom-counter 65-100 Vacuum Atom-counter Jarrell-Ash/Fisher 590 Lincoln Street Waltham Mass. 02154, U.S.A. Direct reading, 190-800 nm, up to 60 elements. Vacuum attachment (66-070) permits C and S determination. Direct reading. Up to 45 elements including C, S and P in steel. V A Howe & Co. Ltd 88 Peterborough Road London S.W.6, England 70-310 70-314 66-750 78-090 3.4 m Ebert spectrograph with photographic or direct reading detection.Range 200-300 nm, dispersion 0.5 nm/mm. Relatively inexpensive direct reader, up to 30 elements. 1.5 m Wadsworth spectrograph. Choice of gratings to cover 420-960 nm or 210-480 nm. Resolution of 0.01 nm, aperture €/24. No information received. Jobin Yvon 26 Rue Berthollet 94 Arcueil (Seine) Paris, France PV 8300 (Philips) Vacuum direct reader featuring new measuring console (PV 8700). Up to 80 elements. Gratings (1) 1200 lines/mm, 170-430 nm; (2) 1440 lines/mm, 177-407 nm. MBLE 80 Rue des Deux Gares Brussels, Belgium ESA 1 Stigmatic plane grating 2160 lines/mm. Sequential analysis.ESA 3 Optica S.A.S. 20139 Milano Via Gargano 21 Italy Czerny-Turner plane grating, 21 60 lines/mm. Vacuum. Dispersion 0.36 nm/mm. Used as polychromator (9 elements) 177-217 nm or monochromator 160-500 nm. Polyvac E600 Direct reading vacuum instrument. 2 X 66" fluorite prisms 177-310 nm. Up to 25 elements. Various options in data presentation. E800 Rank Precision Industries Ltd Analytical Division Hilger and Watts 31 Camden Road London N.W.l, England Direct reading air-path instrument for production control. Up to 35 elements. Polyvac E 1000 Computer-controlled direct reading vacuum instrument. Dual gratings give 6 systems in range 159.6-864.3 nm. Dual spark chamber system available. Up to 60 elements. R.P.I.Inc. 260 North Route Westnyack N.Y. 10994, U.S.A. €398 Medium Quartz E742 Large Quartz 0.6 m prism, 200-1000 nm, available with direct reading attachment (E54 9). Well-known, “classical” 1.1 m prism spectrograph. 191 -800 nm. a.c. spark or d.c. arc excitation. E l I I I1 18 1 .5 m Czerny-Turner grating spectrographs, 1200 lines/mm. 200-1200 nm, aperture f/20. 3.5 m spectrometer. No details available. RSV GmbH, 8031 Hechendorf/Pilsensee West Germany Analymat 1.5 m spectrometer, choices of gratings and readout units. Glow discharge excitation. Automatic sample changing. No details available. GV-200 Shimadzu-Seisakusho Ltd. 14-5 Uchikanda 1-chome Chiy oda-ku Tokyo 101, Japan Spectraspan 101 Spectraspan 201 Spectraspan 401 Compact spectrometers using Echelle grating with prism order separation. Dispersion 0.04 nm/mm at 200 nm.Incorporate argon plasma jet excitation source for liquids and gases, DC arc for solids. Spectrametrics Inc. 2nd Avenue Burlington Mass. 01803, U.S.A. Techmation Ltd. 58 Edgware Way Edgware Middlesex HA8 8JP, England 1802 1500 SP Spex Industries Inc P. 0. Box 198 Metuchen N.J. 08840, U.S.A. 1 m Czerny-Turner spectrograph/spectrometer. Grating/camera provides 9 in. (200 nm) of spectrum in one exposure. f/8.4. Versatile 0.75 m Czerny-Turner vacuum instrument. Optimum range 110-250 nm, aperture f/6.8, dispersion 1.1 nm/mm. 1500 DP Versatile 0.82 m Czerny-Turner vacuum instrument. Choice of gratings, aperture f/7.4. Dispersion 0.5 nm/mm.Glen Creston The Red House 31 The Broadway Stanmore, Middlesex England 34 Part I: Fundamentals and Instrumentation these is intermediate between that of a flame emission spectrophotometer and of a direct-reading arc/spark spectrometer. Spectrametrics have two instruments giving single element (model 101 ) or simultaneous ten-element (model 20 1) analyses, the sample being nebulised into an Ar plasma with excitation temperature approximately 10,OOOo K. Optical dispersion is by means of an Echelle grating crossed with a prism which yields a dispersion of approximately 0.04 nm/mm at 200 nm. The Hitachi model 300 similarly uses an Ar plasma torch, excitation temperature -7000° K, with nebulisation of the sample, via a heating chamber, into the plasma flame.Multi- element analyses are obtained by use of the scanning monochromator. These instru- ments are of the type needed if the capabilities of emission spectroscopy as applied t o liquid samples are t o be realised, i.e. sample introduction is simple, excitation is efficient and optical dispersion is adequate t o permit simultaneous or sequential multi-element analysis. It should be pointed out that, in addition t o those instruments described above, most standard atomic absorption spectrophotometers also have flame emission capabil- ities. Other interesting work on complete flame emission instruments has included Mavrodineanu's description (443, 887, 929) of a flame spectrometer designed and built at the National Bureau of Standards, Washington.This consisted of a 1.0 m spectrometer with multi-channel phototube housings, signal integration and print-out facilities and a punch-tape arrangement for computer calculations. The instrument could be used as a monochromator, spectrograph or multi-channel spectrometer. in the last mode, simultaneous determination of eight elements by emission or six by emission and two by absorption was possible. Several groups have described complete systems for carrying out automated analyses by flame emission. Bissett et a1 (894) discussed practical problems such as baseline correction, time compensation for handling delays, minimisation of carbon build-up and burner memory effects, in the continuous determination of Ca in brine.i n a typical biological application, a combination of an IL 143 flame photometer, IL 144 dilutor and Technicon Sampler I1 has been applied (208) t o the automated flame photometric determination of serum Na and K. Two applications have appeared of flame photometric detection following gas chromatographic separation both of which actually rely on detection of emission of molecular species. A Tracor inc. flame photometric detector has been linked with a Hewlett-Packard model 5750 gas chromatograph for detection of P and S in pesticide residues in foodstuffs (10). Secondly, a modified Beckman DU with the photocells replaced by an EM1 9601B photomultiplier and a capillary burner supporting an N2 /H2 diffusion flame has been designed for detection of relatively volatile fluorides (1 38).6.3 SPECIALISED EMISSION INSTRUMENTS Refinements to direct-reading spectrometers t o provide more rapid analyses, e.g. high- repetition source units, are only short-term improvements on the present situation. The real need in industries using spectrometers for process control is for analysis in situ, e.g. of steel while molten. Bojic and Barbier (557) described preliminary labor- atory work with this aim which yielded some encouraging results but was, as the authors pointed out, still far removed from being applicable to a practical situation. Three other interesting instruments are note worthy. Part I: Fundamentals and Instrumentation 35 A small portable polychromator has been developed (27) to monitor O2 and H2 products in the W arc/inert gas welding process.The H 656.28 nm and 0 777.19 nm lines were monitored with the Ar 852.14 nm line as internal standard. To allow for variable arc position, the focusing assembly was mounted on the torch head and a fibre optic bundle transmitted the light t o the spectrometer. An oscillating prism in the entrance beam produced an a.c. signal above the d.c. background and the read-out was sensitive only t o signal above background. Gram et a1 (904) described an emission-resonance dedicated instrument for Hg detection in pollution monitoring which used a demountable hollow-cathode lamp, resonance detection and a phase-sensitive subtraction type lock-in amplifier. Mainka and Baeckmann (574) compared emission spectroscopy using a 3.5 m spectrometer with mass spectrometry for isotope analysis in fissile materials. Emission lines must be narrow and clearly high resolving power is of paramount importance.The authors were unable t o determine Pu241 using arc excitation because the line half-widths (S eff = 0.0035 nm) were too large t o permit adequate separation between lines from different isotopes. 6.4 ATOMIC ABSORPTION/FLUORESCENCE SPECTROMETERS Descriptions of commercial instruments in the scientific literature have included: the Jarrell-Ash Model 800 dual double-beam spectrometer (660), the Perkin-Elmer single- beam Models 103 and 107 (93) and double-beam Model 306 (94). Thomerson (194) showed that the performance of the Pye-Unicam SP90 was improved significantly by the use of a nebuliser impact bead and aperture stops.A testing routine for a new atomic absorption instrument has been presented by Perkin-Elmer workers (889). A single-beam atomic absorption spectrometer (850) has been built which can compen- sate for variations in source intensity and non-specific absorption. Source radiation was passed through the atom reservoir and detected using a repetitively scanning monochromator with both a.c. and d.c. amplifiers. The ratio of the two signals was related t o concentrations. A spectrometer which measures the sum of atomic fluore- scence and emission signals has been constructed (122) for the determination of wear metals in lubricating oils. Few advantages of this approach over conventional atomic absorption could be claimed.A digital atomic fluorescence spectrometer, designed (1 20) for the determination of Se gave a limit of detection of 0.1 mg 1-l Se. The possibilities of on-line process control using atomic absorption have been explored (874, 922, 942), and a list of required specifications compiled. The main problems are those associated with safety, stability and automatic control. Automatic feed back control of the industrial process has also been discussed. Table C summarises currently available commercial atomic absorption spectro- photometers. 6.5 NON-DISPERSIVE INSTRUMENTS Atomic fluorescence spectroscopy has found little application in practical analysis t o date and has few advantages over atomic absorption.However, AFS easily lends itself t o non-dispersive analysis because of the selectivity of excitation obtainable with a modulated light source. In this way only the fluorescence of the element of interest is excited; the whole of the fluorescence spectrum being measured instead of only one wavelength. As there are few positional limitations a number of light sources may be arranged round the flame and can be used for simultaneous multi-element analysis if a suitable gating system is used. TABLE C COMMERCIALLY AVAILABLE INSTRUMENTS FOR ATOMIC ABSORPTION SPECTROSCOPY Identification See OCLI Instruments Model 440 Model 444 Model 44 8 Model 485 Model 495 Supplier Aztec Instruments Beckman Instruments Fullerton, Calif.92634, U.S.A. Bausch and Lomb 142 Linden Ave., Rochester, N.Y. 14625, U.S.A. ARL Ltd Wingate Road Luton, Bedfordshire Carl Zeiss 7082 Oberkochen Wiirttemburg, W. Germany Cary Instruments Evans Electroselenium Ltd Halstead, Essex, England Description Single beam, single- or triple-pass optics, grating 1200 lines/mm blazed at 250 nm, resolution to 0.2 nm, automatic filter selection, three lamp turret. Double or single beam, single- or triple-pass optics, grating 1200 lines/mm, blazed at 250 nm, resolution to 0.2 nm, automatic filter selection, three lamp turret. Single beam, single-pass optics, grating 1200 lines/mm, blazed at 250 nm, resolution to 0.2 nm, automatic filter selection, three lamp turret. Double or single beam, single- and triple-pass optics, grating 1200 lines/mm, blazed at 250 nm, resolution on 0.2 nm, automatic filter selection, 50 X scale expansion, meter display.As model 485, 100 X scale expansion, digital display. Range 190-800 nm with wavelength drive, 10 X scale expansion, direct concentration read-out. Double grating monochromator, 3-speed wavelength drive, automatic stray light and 2nd order filter selection. Use also as emission or UV solution spectrophotometer. FA-1 and FA-2 attachments for PMQ I1 spectrophotometer. Variety of single or double grating of prism monochromators available. Total comsumption or laminar flow burners. Converts to UV solution spectrophotometer. Single beam, 0.25 m modified Czerny-Turner monochromator, grating 576 lineslmm, dispersion 7 nm/mm, non-linear dial read-out, single lamp turret, inexpensive.Spectronic 760-AA Spectronic AC2-20 PMQ I1 See Varian-Techtron. EEL 140 EEL 240 Single beam, 0.25 m modified Czerny-Turner monochromator, grating 576 lineslmm, dispersion 6.6 nm/mm, f/lO aperture, 4 lamp turret, integration, meter read-out. See Jarrell-AshiFisher Fisher Scientific Co. Modular System Heath/S chlumberger Benton Harbor, Mich., 49022, U.S.A. Modular AA, flame emission, UV spectrophotometer with double or single beam optics and various amplifier/detector types. Model 5960A Simultaneous, multielement AAS for 6 elements, no longer produced. Hewlett Packard Palo Alto, Calif. 94303, U.S.A. See Rank Precision Industries. Hilger and Watts Hitachi 208 Single beam, Czerny-Turner monochromator, grating 1440 lines/mm, dispersion 1.8 nm/mm, 3 lamp turret, 20 X scale expansion, meter read-out directly in concentration, wave- length drive.Hitachi Ltd Nissei Sangyo Co. Ltd. 15-1 2 Nishi-Shimbashi ZChome, Minato-Ku, Tokyo, Japan. IL 153/353 Instrumentation Laboratory 11 3 Hartwell Ave., Lexington, Mass. 02173, U.S.A. Double beam, dual-channel, 0.35 m Ebert monochromator, grating 1200 lines/mm, dispersion 2.5 nm/mm, aperture f/9, six lamp turret, autocalibrate, autozero, integration, wave- length drive, digital display. IL 253 P K Morgan Ltd., 10 Manor Road Chatham, Kent. Single channel, double-beam, digital read-out, wavelength drive. Full specifications not available.Dial Atom I1 Single beam, 0.25 m Czerny-Turner monochromator, grating 11 80 lines/mm, dispersion 3.3 nm/mm, aperture f/7.5, two lamp turret, 10 X scale expansion, meter read-out. Jarrell-Ash/Fisher 71 1 Forbes Ave., Pittsburgh, Pa. 15219, U.S.A. Atomsorb Single beam, 0.25 m Ebert monochromator, grating 11 80 lines/mm, aperture f/3.6, up to 6 lamp turret, 20 X scale expansion, meter read-out. V A Howe & Co. Ltd 88 Peterborough Road London S.W.6. Model 82-500 (Maximum Versatility) Single beam, 0.5 m Ebert monochromator, grating 1180 lines/mm, various options available on gratings, slits and wave- length drive, multi-pass optics, six lamp turret, 20 X scale expansion. Dual double beam, predecessor of Model 810.Model 82-800 TABLE C COMMERCIALLY AVAILABLE INSTRUMENTS FOR ATOMIC ABSORPTION SPECTROSCOPY - continued Identification ~ ~~~~~ Model 82-810 Model 82-600 Supplier V A Howe & Co. Ltd. 88 Peterborough Road London S.W.6. Jobin Yvon 26 Rue Berthollet 94 Arcueil (Seine), France OCLI Instruments 132 Water St., South Norwalk, Conn. 06854, U.S.A. Optica Via Gargano 21-20139 Milan, Italy Perkin-Elmer Corp. Norwalk, Conn. 06854, U.S.A. Description Double beam, dual channel, two 0.4 m Ebert monochromators, gratings 1180 lines/mm, dispersion 2.1 nm/mm, 5 speed wave- length scan, integration, autozero, digital display. Simultaneous multielement (12 elements), 0.75 m spectrograph, concave grating 1180 lines/mm, meter read-out with magnetic needle lock.No information received. Previously sold by Aztec Instruments. Single beam, 0.5 m Czerny-Turner monochromator, grating 1200 lines/mm, dispersion 1.6 nm/mm, 3 or 6 lamp turret, purgeable mono- chromator, rotating optical bench. Single beam, 0.35 m Ebert monochromator, automatic filter insertion, pre-focused water-cooled hollow cathode lamps, integration, digital display. Single beam, 0.27 m Littrow monochromator, grating 1800 lines/mm, dispersion 1.6 nm/mm, single lamp turret, autozero, 50 X scale expansion, integration, meter display. As Model 103 with digital display. Single beam, 0.4 m Czerny-Turner monochromator, UV grating 2880 lines/mm, dispersion 1.0 nm/mm, VIS grating 1800 fines/mm, dispersion 1.6 nm/mm, changeover 420 nm, automatic filter insertion, single lamp turret, autozero, auto- concentration, curve corrector, 40 X scale expansion, gas flow interlocks, D, background corrector, digital display.Double beam, monochromator specification as Model 300, single lamp turret, % absorption meter display. As Model 303, absorbance meter display, concentration display on counter, Model 403 or 107 burner option. Atomic Analyser Mark 2 Model 6000 Model 103 Model 107 Model 300 Model 303 Model 305 Model 306 As Model 305, digital display, autozero, auto-concentration, curve correction, Model 403 burner. Model 403 As Model 306, automated flame ignition/extinguishing and safety monitoring. See Pye Unicam. Philips SP 90 Series 2 F'ye Unicam Ltd York S t , Cambridge England SP 1900 Single beam, Littrow monochromator, 30" rear aluminised silica prism, dispersion 3 nm/mm at 200 nm, 6 nm/mm at 250 nm and 32 nm/mm at 400 nm, three turret accessory, 10 X scale expansion, meter display.Single or double beam, Ebert monochromator, grating 1800 lines/mm, dispersion 2.2 nm/mm, 20 X scale expansion, 10 X scale contraction, integration, 6 lamp turret, digital display. As SP 1900 but with single lamp turret. SP 1950 Atomspek Mark 2 Single beam, 60" silica prism monochromator, dispersion 1.7 nm/mm at 200 nm, 44.6 nm/mm at 500 nm, 6 lamp turret, meter display, scale expansion. Rank Precision Industries Hilger and Watts 31 Camden Rd., London N.W.l, England A-3000 Single beam, 0.25 m Czerny-Turner monochromator, grating 700 line/mm, dispersion 6.0 nm/mm, four lamp turret, 10 X scale expansion, meter display.Shandon Southern Instruments Ltd., Frimley Road, Camberley Surrey, England AA-600 Single beam, 0.35 m Czerny-Turner monochromator, two lamp turret, 10 x scale expansion, meter display, wavelength drive. Shimadzu-Seisakusho Ltd 14-5 Uchikanda 1-chome Chiy oda-Ku Tokyo 101, Japan MAF-1 Single beam, simultaneous analysis for up to 4 elements, 0.5 m Ebert monochromator, grating 1200 lines/mm, dispersion 1.66 nm/mm, aperture fi8.5, four-pass optics, wavelength drive. MAF-2 As MAF-1 but with additional reference channel including independent monochromation/detection. TABLE C COMMERCIALLY AVAILABLE INSTRUMENTS FOR ATOMIC ABSORPTION SPECTROSCOPY - continued Description Identification Supplier Spectraspan 101 Spectraspan 201 Spectraspan 401 Spectrametrics Inc.2nd Ave., Burlington Mass. 01803, U.S.A. Compact spectrometers using Echelle grating with prism order separation. Dispersion 0.04 nm/mm at 200 nm. Although intended primarily as emission instruments they can be used for AA with a Xe continuum source and photoelectric or photographic detection. AFS -6 Technicon Industrial Systems Tarrytown, N.Y. 10591, U.S.A. Multielement spectrometer for simultaneous analysis of 6 elements by non-dispersive atomic fluorescence/emission using filters. Direct concentration read-out on printer. Hollow cathode or electrodeless discharge lamps.Not an AA instrument but included for completeness. AA-5 Single beam, 0.5 m Ebert monochromator, grating 638 lines/ mm, dispersion 3.3 nm/mm, aperture f/lO, four lamp turret, 10 X scale expansion, autozero, meter display. Varim-Techtron 679 Springvale Road N. Springvale, Vic., Australia 3 17 1 AA-120 Single beam, 0.25 m Ebert monochromator, grating 1276 lines/mm, dispersion 3.3 nm/mm, aperture f/10, four lamp turret, 10 X scale expansion, 2 speed wavelength drive, autozero, meter display. Varian Associates Ltd Russell House, Molesey Road Wal ton-on-Thames Surrey, England Model 1000 Single beam, 0.25 m Czerny-Turner monochromator, grating 1276 lines/mm, aperture f/8, four lamp turret, 10 X scale expansion, autozero, meter display.0 P Part I: Fundamentals and Instrumen tation 41 Work on a non-dispersive instrument for atomic fluorescence was carried out as far back as 1967". However, the problems presented by emission from high concentra- tions of Nay for example, precluded formal publication of this work. Although the emission problem could not be solved by modulation, the additional use of solar-blind photomultipliers? , responding t o ultraviolet but not visible radiation, overcame the difficulty. The wide optical acceptance angles of monochromatorless systems can improve the atomic fluorescence limits of detection over those obtainable with disper- sive instruments, and the short path-lengths involved may improve performance in the far ultraviolet. Work at the C.S.I.R.O. in Australia (121, 168, 673, 674) has been directed at evaluating the various components of non-disp ersive instrument at io n : high-int ensit y hollow cathode, conventional hollow cathode and vapour discharge lamps with several premixed flames, both separated and unseparated. The best limits of detection obtained (673) were frequently factors of ten better than those obtained using a monochromator instrument. The separated air/C2H2 flame was found to be best for general purposes (674), and N2 separated N2 O/C2 H2 flame gave adequate detection limits for some elements and could be improved further by noble gas shielding. The N2 O/Hz flame was generally unimpressive. Other monochromatorless systems have been reported by Elser and Winefordner (37) who used electrodeless discharge tubes and H2 /Ar/entrained air flames supported on a total-consumption burner for Cd, Hg and Zn. Marshall and Smith (506) deter- mined Zn and Fe in air/H2 flames also using electrodeless discharge tubes for excit- ation. Simultaneous, multi-element analysis using non-dispersive atomic fluorescence has been commercially developed in the Technicon AFS-6 instrument (882). This six channel instrument is based on the design of Mitchell and Johanssonz, and employs pulsed hollow-cathode lamps to stimulate the atomic fluorescence. The fluorescence emission is focused on to a photomultiplier after passing through an interference filter. The filters for each element are stepped into the optical path in sequence with the pulsing of the hollow-cathode lamps. A few applications have been described (1 5 1 , 483, 1019) and more may be expected from users whose high workloads justify the cost of such an instrument.

ISSN:0306-1353    

DOI:10.1039/AA9710100029

出版商:RSC    

年代:1971

数据来源: RSC

摘要:

Part I: Fundamentals and Instrumen tation 7 Ancillary Equipment 41 It is intended that this section will include material, a knowledge of which could make the analyst’s job easier. This clearly includes a wide range of items - from sample handling devices t o standardised nomenclature and documentation. Despite the existence of commendable publications such as “Laboratory Equipment Digest”, the lack of easy access t o the wealth of information and products available is perhaps one of the major limitations t o the full exploitation of sophisticated analytical techniques to everyday problems. An attempt will be made t o overcome this deficiency. 7.1 STANDARDS One of the major problems confronting the user of a direct-reading emission spectro- meter is that of calibration.This requires a collection of standards covering the range * J. D. Winefordner and R. Smith in “Analytical Flame Spectroscopy”, Macmillan (London), R. Mavrodineanu, editor, p. 607. t P. L. Larkins, R. M. Lowe, J. V. Sullivan and A. Walsh, Spectrochim Acta, 1969,24B, 187. $ D. G. Mitchell and A. Johansson, Spectrochim Acta, 1970,25B, 175. 42 Part I : Fundamentals and Instrumentation of materials t o be analysed and the appropriate element concentrations therein. Such standards are available from a number of sources but building up a set can be both troublesome and expensive if a variety of materials is t o be analysed. Swinburn (558) described a technique for production of homogeneous cast steel standards which collaborative analysis showed to be suitable for calibration of direct- reading spectrometers. A mould for casting small A1 standards has also been described (9 16) which was used t o prepare additional standards t o those commercially available.Suitable standards are also needed for use during analysis of metals in organic liquids such as oils. Metal caprates have been recommended (590) as alternatives to cyclohexyl butyrates or naphthenates as standards for use in the emission spectro- graphic determination of wear metals in oils. Applications of the “Conostan” range of metallo-organic standards have also been described (898). Coulter (756, 840) has reviewed methods of production and the application of graphite and carbon in emission spectroscopy, concluding that “there is no general agreement as t o the best type of electrode material”.Evaluation techniques of various grades of graphite were described by Leistner and Dugas (589, 955) with particular reference to the analysis of used lubricating oils. They showed statistically that the physical property parameters of the graphite used in the direct-burn, rotating disc technique affected the reproducibility and accuracy of the method. 7.2 DOCUMENTATION New books on various aspects of analytical atomic spectroscopy are listed elsewhere. De Gregorio and Savastano (143) have prepared an atlas of the Fe arc and spark spectra from 220.6-465.6 nm using an R.S.V. 3.5 m Ebert grating spectrograph. This produced a 20-fold enlargement of the first order spectrum of a 600 grooves/mm grating.Three sensitive lines of each of 54 elements were marked on the Fe spectra and their spectral values collected in two tables. As atomic absorption becomes an established analytical method, there is an increas- ing need for standardisation of terms and of analytical methods (1059). Two Australian standards have been published (1 19) covering terms used in AAS and the chemical analysis of materials by AAS. In America, methods have been published by the AOAC (1 3) for the determination of Zn by AAS, Hg by flameless AAS and Cu and Ni in tea by AAS. Finally, Heyden and Son Ltd. have recently introduced a Series of Audio Visual Teaching Programmes on Atomic Absorption Spectroscopy (Series 3000), written by J.W. Robinson. The series comprise programmes on: - principles; instrumental requirements; optimising instrument conditions; sample preparation - calibration and calculation; and applications and special sampling techniques. 7.3 OTHER ANCILLARY EQUIPMENT Some items of hardware, which will be of general value, have been reported this year. A heating rack for decomposition of biological samples (772) was designed for digests at 100” C for 30-45 mins, followed by wet-ashing to dryness at 2 10” C. Simultaneous multi-element analysis of N, P, K, Ca (6) was achieved by a combination of colori- metric (N, P), atomic absorption (Ca) and flame emission (K) methods. A new auto- matic sample changer from Perkin-Elmer (17) handles up to 200 samples and a modification of the PE303 (41) enables the protective safety door of the Model 403 to be fitted.Decomposition bombs of ca 100 ml volume for use with hydrofluoric acid at up t o 160°C and 6 atms pressures have been used for the analysis of silicate materials Part I: Fundamentals and Instrumentation 43 (300). These bombs are made of steel and are lined with polytetrafluoroethylene. They are obtainable from: (a) Uni-Seal Decomposition Vessels, P.O. Box 9463, Haifa, Israel. (b) Perkin-Elmer AB, Goteborg, Sweden. (c) Parr Instrument Co., 21 Fifty Third St., Moline, Ill. 61265, U.S.A. (d) Bel-Art Products, Pequannock, N.J. 07440, U.S.A. (e) S. and J. Juniper, 7 Potter St., Harlow, Essex, England. Attachments for conversion of commercial atomic absorption spectrometers to either a solution fluorimeter", or a solution spectrophotometerj- have been published.It is probably not widely realised that atomic absorption spectrometers with hollow cathode light sources permit greater scale expansion than most commercial solution spectrophotometers. A number of novel sampling devices has been described. Only those of instrumental interest will be discussed herein. The methodology section of this report should be consulted for information on sampling techniques specific t o particular areas of applic- ation, e.g. pollution analysis, etc. Workers in the A.R.L. company have described (1006) a device whereby fine particles are produced from the surface of a solid metal sample by a d.c. arc, trans- mitted along tubing several meters long and swept into an analytical d.c.arc. Raiteri (1 78) has also designed a sampler avoiding the need to transport heavy materials. This permitted transfer of -10 mg of a large metallurgical product into the electrodes for subsequent spectrographic analysis. Schuessler (1 26) described a micro-sampling tool, consisting of a glass capillary probe attached by flexible tubing to a millipore filter holder. Under gentle vacuum, fine particles required for analysis could be detached and deposited on the filter prior to treatment, e.g. mixing with graphite, for analysis. The device was used, for example, to collect small foreign bodies from printed-circuit boards. Two sample-rotating devices have been described. The first (1 79) was mounted on the argon stand of an ARL Quantovac 31000 and utilised the selective volatilisation of lead during the determination of lead in stainless steel. The second (1013) was a platform suitable for supporting flat metal sample discs for spark analysis. The rotation speed was such that a fresh sample surface was presented t o each spark discharge and successive spark craters traced out a spiral so that no area was re- sampled. Two other papers are noteworthy. Fagan (869) has pointed out that an impervious graphite (Spectro FXI), originally intended for use in spectroscopic electrodes, had several other applications e.g. manufacture of crucibles, evaporating dishes and metal plate sampling electrodes. Finally, it has been pointed out (809) that the time occupied by photographic spectrography could be significantly reduced by use of a quick-developer (Agfa 36) and fixative (Agfa 304) for processing of plates. There was no loss of accuracy associated with this method. * R. Smith and A. E. Lawson, Spectrovision, 1971,26, 11. M. A. Hildon, Analyt. Chem., 1971,43,973.

ISSN:0306-1353    

DOI:10.1039/AA9710100041

出版商:RSC    

年代:1971

数据来源: RSC

摘要:

PART I I METHODOLOGY Introduction In Part 11, Methodology is interpreted as relating to all aspects of the application to chemical analysis of the techniques and instrumentation of atomic absorption, emission and fluorescence spectroscopy. The subject matter is treated under two principal headings: 1. General Techniques, where the preparation of the sample (section 1.1) and the treatment and significance of experimental data (section 1.2), as factors affecting the overall convenience, speed, accuracy, precision and sensitivity of analysis, are considered. 2. Applications, a survey of specific methods reported in various fields of materials analysis; each of these sections (2.1 to 2.8) contains a summary of the analytical data in tabular form. In sub-dividing these principal headings, as shown in the List of Contents, it proved difficult to achieve a rigid classification. In general, if a publication is relevant t o more than one section, the view has been taken that it is better to err in the direction of duplication of comment. Section 2.8 contains some entries which could not be readily placed elsewhere. 45

ISSN:0306-1353    

DOI:10.1039/AA9710100044

出版商:RSC    

年代:1971

数据来源: RSC

摘要:

EXPLANATION OF THE TABLES Each of the Applications sections, 2.1 t o 2.8 inclusive, is accompanied by a table which summarises the principal analytical features of the references from which the corresponding section is compiled. These Applications Tables form a convenient source of information for analysts interested in particular elements, matrices, sample treatments or atomisation systems. In many cases, sufficient detail is given for the analytical procedure t o be followed; absence of such detail usually means that the information was not directly available to the compiler of the table and hence the original reference should be consulted in such cases. The column headings of the tables and the various abbreviations employed are explained below. ELEMENT Xnm MATRIX CONCENTRATION TECH.The elements determined are listed in the alphabetical order of their chemical symbols, except that, for space economy, multi-element applications (5 elements or more) are given at the end of each table. The wavelength, in nanometres, at which the analysis was performed. An indication, necessarily brief, is given of the material analysed. The concentration range of the element in the matrix or original sample, expressed as % or pg/g for solids and mg/l for liquids. The atomic spectroscopy technique is indicated by either A (absorption), E (emission) or F (Fluorescence). 45 Part 11: Methodology ANALYTE The analyte is defined as the sample as it is presented to the analytical instrument, and its form is indicated by either S (solid), L (liquid) or G (gas or vapour). “d.1.” = detection limit in the analyte. SAMPLE TREATMENT A brief indication is given of the sample pre-treatment, if ATOMIS ATION REF. 46 any, required to produce the analyte. Abbreviations, where used, follow the Society for Analytical Chemistry list as given in “Analytical Abstracts.” The atomisation process is indicated by the abbreviations: a (arc), s (spark), f (flame) and p (plasma), usually with some additional descriptive detail. The number refers t o the main Reference List, which gives the title of the paper and the name of the author(s), with address.

ISSN:0306-1353    

DOI:10.1039/AA9710100045

出版商:RSC    

年代:1971

数据来源: RSC