- Email: [email protected]
Practical field-portable atomic absorption analyser
Journal o f Geochemical Exploration, 19 (1983) 689--704 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
PRACTICAL
FIELD-PORTABLE
ATOMIC ABSORPTION
689
ANALYSER
C.G. CASTLEDINE and J.C. ROBBINS
Scintrex Limited, 222 Snidercroft Road, Concord, Ont. L4K 1B5 (Canada) (Received August 5, 1982; accepted for publication March 14, 1983)
ABSTRACT Castledine, C.G. and Robbins, J.C., 1983. Practical field-portable atomic absorption analyser. In: G.R. Parslow (Editor), Geochemical Exploration 1982. J. Geochem. Explor., 19: 689--704. A new compact atomic absorption spectrophotometer has been developed. Designated the Scintrex AAZ-2, the system used flameless atomization and Zeeman modulation for maximum sensitivity and selectivity respectively. The most important factor of AA instrumentation is the method used for vapourizing the sample. The atomizer furnace in the AAZ-2 is a tungsten ribbon, rather than the conventional carbon furnace, to avoid the need for high electrical power input and consequent water cooling. The ribbon is Ushaped to fit between the pole pieces of a powerful electromagnet which provides the magnetic field required for Zeeman modulation of the absorption signal. Zeeman modulation refers to the spectral displacement of atomic absorption lines in the presence of a magnetic field. By pulsing the field, the emission from the source lamp alternately does, or does not, coincide with the analyte absorption. A synchronous detection system then allows the extraction of the modulated signal from any unmodulated absorption due to nonspecific absorbers such as particulate scattering or some molecular species. Zeeman modulation provides a method of baseline correction that is both more efficient and simpler than the commonly used deuterium arc correction and requires no moving components. The AAZ instrument has already been characterized for a number of metals, in particular, those of exploration interest such as Au, Ag, Cu, Pb, Ni, Co. Zinc has been determined but difficulties in avoiding high blank concentrations have been found. Detection limits are typically 1 ng/ml or better (e.g. Au -- 0.5 ng/ml, Ag -- 0.06 ng/ml). The metals usually requiring conversion to a gaseous hydride form before analyses have also been studied. Tin in the sub-ppm range has been determined after a simple extraction procedure. Work on arsenic, selenium, antimony and bismuth is planned. The entire machine weighs 26 kg and consumes 0.5 kW making it suitable for use in remote areas using portable power generating equipment. Argon at low pressure and flow rate is the only gas needed for operation. When used for analysis of samples in a base camp, close to the scene of a geochemical drilling program, the AAZ-2 can provide quick turnaround of the results and therefore on-the-spot guidance of these programs without loss of sensitivity or data quality. In addition, the possibility of contamination by high-level samples is somewhat less in a field camp than in a long established laboratory.
0375-6742[83[$03.00
© 1983 Elsevier Science Publishers B.V.
690 INTRODUCTION
Atomic absorption spectrometers Atomic absorption spectrometry (AAS) has been a significant factor in the rapid growth o f geochemical techniques, thanks to the relatively low cost and high sensitivities achieved in analysis. In general, however, atomic absorption spectrometers are quite large, are n o t intended for field use and consume significant quantities of electricity, combustion and support gases. In view of the success of geochemical methods, particularly in the reconnaissance phases of exploration programs, there is a definite need for compact, rugged equipment with the same sensitivity as its larger counterparts but which can be readily deployed in the field or in temporary laboratories and facilities. Although n o t all geochemists agree on the desirability of analysis literally in-field, there can be little d o u b t that under proper supervision and controlled analytical conditions, the benefits of determinations close in time and space to collection of the samples are well worth pursuing. At the reconnaissance level, large numbers of samples must be processed at minimum cost b u t with adequate accuracy. Ease of handling, analytical time, equipment reliability and simplicity of operation become significant factors in the design of an instrument. To carry o u t atomic absorption spectroscopy, the original liquid or solid sample must be converted to a gaseous state. Electrothermal atomization is a practical m e t h o d of sample vaporization for equipment in the field, although there are penalties attached to its use. Relative to flame atomization, the precision is probably lower, the dynamic range more limited and the analytical cycle time considerably longer. These disadvantages are outweighed by the greatly improved detection limits, small volumes of sample required and the lack of combustion gases. Considering the simplicity of the equipment, AAS results are remarkably specific. Chemical interferences exist in which the atomic absorption process may be undesirably suppressed or enhanced b y the reaction of the analyte with other components of the sample. It is only at extremely low levels, though, that d o u b t m a y exist of the actual identity of the absorption signal. Spectral interferences due to the presence of a neighbouring line of an unwanted element are known but are quite rare. Broadband absorption by molecular or particulate species is, however, more c o m m o n . A series of methods known as background correction techniques have been developed to reduce this problem. Zeeman atomic absorption spectroscopy (ZAAS), which uses an applied magnetic field to split and displace atomic spectral lines either in emission or absorption, represents an elegant method of correction. The variant described in this paper is especially convenient in that the modulation does n o t require any moving mechanical parts and the magnet itself is quite compact, allowing the design of practical and portable field equipment.
691
Zeeman atomic absorption Background correction refers generally to techniques in which absorption of the analytical spectral line is compared to that of the neighbouring wavelength, with the assumption that the interfering absorption is constant across the spectral interval considered. The principal difference between the various methods is the generation of the reference wavelength. Continuum sources, e.g. deuterium arcs, and hollow cathode lamps with a suitable emission wavelength have been used but there are problems of optical matching and mechanical alignment. The reference wavelength in ZAAS is generated by the spectral shift of parts of atomic absorption or emission lines in the presence of a magnetic field. At high resolution, spectral lines are seen to consist of, sometimes one, b u t more generally, several close-spaced components attributed to modification of energy levels b y hyperfine and isotope effects. The splitting of spectral lines in the presence of a magnetic field to form characteristic Zeeman patterns is determined b y the particular optical transition involved. The spect r o s c o p y that details the generation of the various Zeeman patterns is well d o c u m e n t e d (Koizumi and Yasuda, 1976; Brown, 1977) and will n o t be repeated here except to note that the magnetic field also introduces polarization effects. Emission or absorption processes taking place transverse to the Mj
1p1
,C
+1
+1
0
0
s~"
"~__
~
-1
0.-
285.2 nrn
ISo
Mj g
7r
-1
0+
0
. . . . . . . .
0
v*
Field-Free
In Magnetic Field
I
I
0.-
71"
0"4.
Fig. 1. Normal Zeeman effect in magnesium (from Brown, 1977).
692
magnetic field give rise to o components which can be resolved from those parallel to the magnetic field, the ~ components. The normal Zeeman effect is observed when the line splits into three components {Fig. 1), a ~ c o m p o n e n t in which the wavelength remains constant and two o (-+) which split symmetrically about the line center. The normal effect is observed when both terms involved in the transition are singlets or when the multiplicity does not change. The intensity of the two o components together equals the ~ component. All other transitions exhibit 'anomalous' Zeeman effects. The patterns (Fig. 2) are quite variable; in general, splitting is still symmetrical about the origin but the n components also shift although less than the o-+. There are m a n y ways of utilizing the shifted wavelengths or the polarization p h e n o m e n a produced by the Zeeman effect, depending on whether the field is applied to the light source or the absorbing atoms and whether it is continuous or pulsed. Liddell and Brodie (1980) list and compare seven variations. Continuous magnetic fields are easy to generate but require some Mj
Mjg
312 6/3 1/2 2/3 -1/2 -2/3 -3/2 -6/3
2P3~
D2
1/2 -1/2
2Pll 2
D1
7/-
7/"
O" Cr
o
11-
I' ? T O
° (7
112
1
2Sl12
Field-Free
1/3 -1/3
-1/2
1 -1
In Magnetic Field
iiir
iiii
D1
D2
(589.59 nm)
(589.00 nm)
4P"~
Fig. 2. A n o m a l o u s Z e e m a n e f f e c t in s o d i u m (from Brown, 1977).
693 further m e t h o d of separating the polarization components: the polarizing filters required are n o t very efficient in the ultraviolet. Modulated fields can be applied to the source {e.g. Robbins, 1972) or to the sample as in the present work. This last variant seems to be the most versatile since it can be used with all optical transitions and allows the use of standard AAS light sources (hollow cathode or electrodeless lamps). The effectiveness of Zeeman AAS (ZAAS) is measured b y the change in atomic absorption induced b y the magnetic field. Systems using modulated fields without polarization selection have a theoretical limit of 50% for normal Zeeman transitions since the unshifted ~ c o m p o n e n t is not rejected; for anomalous transitions efficiencies up to 100% can be achieved. Conversely, up to 100% modulation of normal transitions can be achieved b y systems using constant magnetic field and polarization modulation b u t the efficiency of anomalous transitions m a y be very much lower as a compromise magnetic field must be found that maximizes the difference between the ~ and e shifts. The intensity of the magnetic field required to achieve useable modulation can be estimated. Spectral lines are n o t infinitely narrow. The spectral shift must be sufficient to move a substantial fraction of the energy outside the envelope caused b y pressure and Doppler broadening. The presence of hyperfine or isotopic structure may also further broaden the apparent line width. As an order of magnitude estimate, line widths of elements from the middle of the periodic table at atomization temperatures of, say, 2300°K would be approximately 10--15 GHz. Spectral shifts can be roughly estimated at some 0.2 GHz/T to imply a minimum field requirement of 0,5--0.75 T. One tesla (T) equals 10 kilogauss. INSTRUMENTAL General A front view of the AAZ-2 is shown in Fig. 3. The instrument requires t w o support systems: p o w e r at 1101220 V +- 20%, 50]60 Hz and inert gas (usually argon) supplied to the instrument at low pressure. Consumptions are 500 W and 0.5 1/min, respectively. No other support {e.g. cooling water) is required. The instrument weighs a b o u t 26 kg and measures approximately 9 × 36 X 49 cm. Once the light source has been installed and various instrumental parameters dialed in, operation consists of deposition of the sample on the filament with a micropipette and activation of the temperature program. The analysis result is displayed at the completion of the temperature program. The temperature cycle m a y be programmed up to 4 min in duration b u t is generally 1 to 11/2 min.
694
Fig. 3. Front view of the AAZ-2~ Zeeman modulation In the AAZ-2 a modulated magnetic field is applied to the analyte. The inp u t p o w e r required to generate the pulsed magnetic field is proportional to at least the square of the pole gap distance and also increases with the modulation frequency. The modulation period (the reciprocal of the frequency} should be much shorter than the transient absorption pulse to allow adequate sampling. Electrothermal atomization can generate very brief absorption pulses lasting sometimes as short as 50--100 ms. In the Scintrex AAZ-2, a field o f 1.4 T at a frequency of 50 Hz is obtained in a 3-mm gap with 70 W input p o w e r in a magnet weighing some 6 kg. The modulation is generated by efficient power-recovery circuitry proriding essentially square wave rather than the more c o m m o n l y used sinusoidal waveforms. Square wave modulation maximizes the time in each cycle spent either in the field " o n " or the field " o f f " condition, rather than intermediate states during which data must be discarded. For the sake of optical throughput and simplicity of design, plane polarization characteristics are not measured in the AAZ-2. Filament design A cross-sectional view of the filament c o m p o n e n t s used in the AAZ-2 spectrometer is shown in Fig. 4. The U-shaped configuration represents a compromise between simple rod atomizers and the tube-type furnaces which
695
Field h.= Direction I ~
Fig. 4. Cross-section of the atomizer showing the tungsten strip between the magnetic poles. present difficulties in fabrication and the design of electrical connections. The vertical walls provide some containment of the atomic vapor and insulation from the relatively cool magnet poles. The transverse geometry allows the filament to expand w i t h o u t distortion at high temperature b u t maintains rigidity by virtue of small flanges formed in the sides of the filament. This arrangement also makes electrical connection of the filament easier and more reliable than t u b e designs. Metal foil filaments were chosen in preference to graphite furnaces largely because of the greater design flexibility in combining the requirements for sample containment with the need to minimize the volume of the magnetic field. Graphite tubes could fit in the 3-mm gap available b u t would have been very fragile. The higher mass and greater emissivity of graphite tubes relative to tungsten filaments would have also materially increased the p o w e r required with the problem of the subsequent removal of the heat. Tungsten has been used in preference to b o t h m o l y b d e n u m and tantalum which are both much easier to fabricate b u t oxidize and become brittle at high temperatures. Rhenium and rhenium/tungsten (West et al., 1979) foils have been used in a related furnace application b u t the solubility of rhenium in dilute acids limits its use for aqueous solutions. The filament is m o u n t e d using a simple alignment tool on a detachable carrier which provides mechanical support and electrical connection. The power to the filament is supplied b y a constant voltage drive with a variable d u t y cycle. Atomizing currents typically range up to 60 amps (rms) with applied voltages of some 2.5 to 3 V. The temperatures at the normal operating p o w e r levels during the atomization cycle have not y e t been accurately determined b u t are expected to be approximately 2500°K. The rise time is a b o u t 0.8 s, which yields a rate of temperature rise of 3000°K/s, which is considerably higher than conventional graphite furnaces (Sturgeon and Chakrabarti, 1978) b u t less than recent figures for capacitor discharge heating (Charkrabarti et al., 1980). A sheath of inert gas is formed around the filament b y argon or other gases flowing from a series of fine holes in the floor of the sample cavity. The gas flows at a rate of 0.5 l/min around the filament and discharges
696 through the sample inlet port. Provision is made to shut off the gas flow during the atomization periods for analyses in which stopped gas flow condi~ tions are found to be superior.
Optical system The optical l a y o u t is shown in Fig. 5. Light from the hollow cathode lamp is focussed just above the tungsten strip atomizer, then refocussed on the m o n o c h r o m a t o r entrance slit. The miniature m o n o c h r o m a t o r uses a concave grating (10 cm focal length) to give a resolution of a b o u t 2 nm with the 0.5mm slits usually used. The resolution, which is rather less than that conventionally used in AAS, appears adequate. Light leaving the exit slit is detected b y a side~)n photomultiplier. This is m o u n t e d in a steel housing to reduce any effects on sensitivity caused b y stray fields from the Zeeman magnet.
Monochromator TungstenStrip
HollowCathodeLamp
Qu-rtz Lens Electromagnet
I .... it
Quartz Lens '
PMT Frequency to C
s
t
Fig. 5. Optical layout of the AAZ-2.
Electronics system The electronics system supports the varied power requirements in the AAZ-2 including magnet and filament drives. It also processes the o u t p u t o f the photomultiplier providing a digital display of the integrated absorption measured. The drying, ashing and atomizing events applied to the 10-#l liquid sample deposited on the tungsten strip are sequentially controlled by the electronics. Controls are available for use by the operator to optimize the various steps. The drying step dries off the solvent. Ashing removes potential interferences which are more volatile than the element of interest and leaves the sample in a uniform, readily atomizable state. The signal processing is concerned with the demodulation of both the Zeeman signal and a second higher frequency modulation applied to the lamp. The lamp modulation is included t o reject emission o f light from the filament itself at high temperatures. The lamp is repetitively turned on and o f f at approximately 800 Hz. Photomultiplier currents during the " o f f " period are ascribed to emission from the filament and used to correct the signal during the subsequent " o n " period. A voltage-frequency converter produces a series of pulses with frequencies proportional to the photomultiplier output. The pulses are fed to
697 an up-down counter synchronized with both Zeeman and the lamp modulation frequencies. At the end of the atomization period, the counter has integrated the d o u b l y differenced signals as a value proportional to the absorption b y the atoms of interest. This value is displayed on a digital panel readout.
Computer interface To obtain video and hard-copy representations of transient absorption curves, a dedicated c o m p u t e r interface was developed. The system uses a custom interface to convert the analogue photomultiplier o u t p u t and timing information to usable levels for the Apple II computer. Signal conversion in the Apple II is provided b y a Mountain Computer A/D, D/A board. Transient signals m a y be viewed on the c o m p u t e r monitor or o u t p u t onto an X-Y recorder for permanent record keeping. While this system has proved useful for experimental development, the production version will use the already digitized signals generated in the AAZ-2. EXPERIMENTAL The efficiency of Zeeman splitting has been established for several elements. This efficiency is presented in this work as the " d e p t h of modulation" defined as the percentage reduction in absorbance during the magnetic field-on periods when compared with the unperturbed absorbance. Computer generated curves were produced for zinc at 214 nm and silver at 338 nm, with and w i t h o u t Zeeman modulation. A study of the influence of magnetic field strength on the silver transitions at 328 and 338 nm was also performed. Except where otherwise noted, standards were prepared in 0.1 N HNO3 using analytical grade chemicals. Analysis was performed using single element hollow cathode lamps and temperature programming as determined experimentaUy and published in an applications manual for the AAZ-2. Calibration curves were constructed from three or more standards in the working concentration range for the element of interest. A suite of rock samples has been analyzed for gold content. The samples were first roasted to destroy sulfides then leached in the cold with hydrobromic acid--bromine. After separating solids, the gold was extracted as a bromide into methyl iso-butyl ketone. After backwashing the organic extract with 0.1 N HBr the gold content of the organic layer was determined on the AAZ-2. Standards were prepared by extracting the appropriate a m o u n t of an aqueous standard from 3 N HBr into MIBK and backwashing the organic with 0.1 N HBr. Comparative analyses were supplied b y Cominco Ltd. (Toronto, Ontario, Canada) analysts w h o used fire assay followed by acid dissolution and AA analysis. Discrepancies in comparative measurements may be caused b y differences in standards, sample inhomogeneity,
698
extraction efficiency, etc. To avoid these difficulties, the performance of the AAZ alone was compared with another AA instrument. The same extracted solutions were analyzed for gold on both the AAZ-2 and a Perkin-Elmer 603 with an HGA 500 graphite furnace. A comparative study of silver determinations was conducted on a suite of ore pulps. The m e t h o d entailed a nitric acid--hydrogen peroxide leach with the heat of reaction of the reagents as the only warming of the digestion. Comparative analyses were again supplied by Cominco Ltd. using perchloricnitric-hydrofluoric acid digestion and flame AA for measurement. Detection limits have been determined for a number of other elements. These were determined with Zeeman background correction using 10 microlitre samples. The detection limit is defined as twice the standard deviation of the blank. This is not a very useful figure in realistic terms, but serves to indicate performance relative to other AA spectrometers. The e n h a n c e m e n t of signals obtained from tin standards was investigated. Two methods of enhancement were tried: inclusion of 3% hydrogen in argon atmosphere and extraction as an oxinate (8-hydroxyquinoline chelate) into butyl acetate. The Geostandards AN-G (Anorthosite), BE--N (Basalt) and MA-N (Granite) have been analyzed for a variety of trace elements. Digestion methods consisted of h o t acid attack for 1--2 hours followed by dilution and subsequent measurement on the AAZ-2. For copper and lead, nitric acid alone was used. For cadmium, cobalt and nickel a combination of nitric and hydrochloric acids was employed. RESULTS AND DISCUSSION
Figure 6 shows transient absorption curves for zinc at 214 nm and silver at 338 nm, with and w i t h o u t Zeeman modulation. The modulation gives the With Zeeman Modulation
No Zeeman b l o d u l a t i o n
L\r 7
,~-
/
--~-~ ~I~ ~--~~-
En
..
%
Ag 338 nm
Fig. 6. C o m p u t e r generated absorption curves s h o w i n g the effect of Z e e m a n m o d u l a t i o n .
699 curves a saw-toothed appearance with successive "field o f f " and "field o n " periods showing, respectively, the unperturbed absorption and the Zeeman split absorption. The depth of modulation for this zinc line is only a b o u t 35%. This is relatively inefficient when compared with the silver line which yields virtually 100% depth of modulation. The effect of increase in magnetic field strength on the t w o main lines of silver is shown in Fig. 7. Although both transitions are anomalous the splitting of the 338-nm line is more efficient. The unperturbed peak absorbance for the 328-nm line is a b o u t twice that of the 338-nm line b u t the latter is usually preferred for AAZ-2 analysis due to its better depth of modulation. Scatter diagrams for the t w o suites of gold analyses and the silver analyses are given in Figs. 8, 9 and 10. As expected, correlation is much better where all analytical and sampling biases are eliminated and the performance of the instrument alone is compared.
ir !ij!:, .....
f/i'
LT].
a)
_~Y
Fig. 7. Effect of increasing magnetic field strength on efficiency of Zeeman splitting for the silver transitions at 338 and 328 nm: a) 0.6 T; b) 0.9 T; c) 1.2 T; d) 1.5 T.
700
JO,O00~-
. . . . . . . . . . . . . . .
!
i
I i /
I000
~
-I ~
~ i
/
IJ
I00 .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
~ .
.
.
.
.
.
.
m -r oJ N <~ <~ x LU rr FZ
i0 ~
-
i
1.0
1.0 ng/g
Fire
I0 Ass0y - AA
I00
I000
IOltO0
Fig. 8. Comparative gold analyses - ore pulps. I000
•
/i
""
I00
t
r
N
X w r~ t-.--
.~- ...... - - - - .....
I0-
0 o9
LOI0
ng/ml
I0 IO0 I000 Perkin Elmer 6 0 3 with HGA 5 0 0 Graphite Furnonce
F i g . 9. C o m p a r i s o n
of instrumental
analyses for gold on the same extracted
solution.
701 IO00
0 z I
I00 @
0 O
(M N < I
X i,i rr" I.z
I0-
o9
1.0 1.0
I0 Ng/g
I00
I000
Flame AA (HCIO4-HNOs)
Fig. 10. C o m p a r a t i v e silver a n a l y s e s - - o r e p u l p s .
Detectio n limits for a variety of elements are given in Table I. The inh e r e n t high sensitivity of the 214-nm line for zinc and its mobility have caused problems of c o n t a m i n a t i o n bot h in analysis o f samples and standards alike. Spurious c o n t a m i n a t i o n f r o m pipette tips, containers, etc. as well as m or e or less c o n s t a n t c o n t r i b u t o r s such as water and reagent impurities combine to f o r m a significant problem. T he d e t e c t i o n limit on this line could be improved if this high c o n t a m i n a t i o n were eliminated. In some cases the 308-nm line o f zinc can be used t h o u g h it is some three orders of magnitude less sensitive; a drawback which limits the usefulness of this approach. TABLE I Some detection limits with the AAZ-2 Element
Wavelength (nm)
Detection limit a (g)
Equivalent concentrationb (ng/ml)
Ag Au Cd Co Cu Ni Pb Zn Zn
338.3 242.8 228.8 240.7 324.8 232.0 283.3 213.9 307.6
6 5 5 1 4 4 8 2 4
0.06 0.5 0.05 1.0 0.4 4 0.8 0.2 400
X 10 -13 x 10 -12 × 10 -13 X 10-11 × I 0 -12 × 10-11 X 10 -12 X 10 -'2 × 10 -9
aMeasured with Zeeman modulation. b C o n c e n t r a t i o n in a 10-ul s a m p l e .
0.02 8.2 25.0 2.0 18.0
This work
0.031--0.18 10--56 8--27 0.62---13 9--70
R a n g e of values
Geostandards a
AN-G
-25 19 2 35
Recommended value 0.09 34 79 4.2 250
This work
a G e o s t a n d a r d s values f r o m G o v i n d a r a j u ( 1980).
Cadmium Cobalt Copper Lead Nickel
Element
C o m p a r a t i v e results for t h e G e o s t a n d a r d s (ug/g)
T A B L E II
0.089--0.21 43--75 43--109 3--17 160--327
Range of values
Geostandards
BE-N
-61 72 4 267
Recommended value 2.0 0.1 97 15 2.5
This work
1.8--2.2 0.35--38 76--187 10--86 2--92
R a n g e of values
Geostandards
MA-N
2 1 140 29 3
Recommended value
703
The sensitivity of the 214-nm line can be detuned to make backgrounds manageable b y substituting helium for argon as the inert-gas atmosphere. This has the effect of reducing the sensitivity b y a b o u t an order of magnitude. Atomization p o w e r must be increased to compensate for the increased heat loss p r o m o t e d by the much higher thermal conductivity of helium. Although this study has n o t been completed, the ability to lessen the sensitivity, especially for zinc, should prove very useful. Results obtained from the Geostandards are presented in Table II. Although the data show a low bias, values generally lie within the range of values reported for these samples (Govindaraju, 1980). Cadmium in AG-N is below any other reported data. AG-N and BE-N have no r e c o m m e n d e d cadmium values since the number of reported values was not sufficient to characterize them properly. Cobalt values were also generally lower than those reported. Discrepancies have been attributed to the inefficiency of the acid attack used in this study versus the total digestions or total analyses assumed to have been used by the other laboratories. Re-evaluation of these samples using total digestion is planned. Initial measurements on tin in aqueous solution using a pure argon atmosphere yielded a relatively p o o r sensitivity of several ppm. The formation of oxides during the final stages of drying suppresses the signal obtained, a p h e n o m e n o n which is well k n o w n to give p o o r sensitivity for flame AA analysis of tin. This already low signal completely disappears when ashing of a b o u t 500°C is introduced implying that only the volatile tin components which remain after drying give rise to the signal. Introduction of a small amount, in this study 3%, of hydrogen to the argon atmosphere immediately increased sensitivity some t w e n t y times. As ashing was successively increased, the signal was f o u n d to increase. The gain at an ashing temperature of 1000°C for 10 s is a further ten times increase in sensitivity over the measurements made w i t h o u t ashing. The net gain for the addition of both hydrogen and ashing was, therefore, some 200 times. An alternate m e t h o d of impeding oxide formation is to extract the tin into an organic solvent. Extraction into b u t y l acetate of the 8-hydroxyquinoline complex of tin (IV) was used. Direct analysis of this extract yielded an enhancement of a b o u t 200 times relative to the aqueous standard. Introduction of the partial hydrogen atmosphere and the ashing steps had no effect on the signals obtained. While it is assumed that the hydrogen atmosphere provides a reducing environment, it is n o t clear as y e t if the organic extract simply deprives the tin of water thereby impeding oxide formation or whether the decomposition of the chelate molecule may provide a localized reducing environment. CONCLUSIONS
The design and performance of a new atomic absorption analyzer suitable for field use has been described. The major innovation is the development of a miniaturized metal filament for sample vaporization which, in turn, per-
704
mits the use of a compact magnetic field for background correction via the Zeeman effect. The magnetic field is electrically modulated to avoid the need for mechanical oscillation of any optical components. The instrument has been thoroughly characterized for determination of gold down to 2 ng/g in various geological sample materials. A preliminary set of data (Cd, Co, Cu, Pb, Ni) on three standard geochemical reference standards has also been presented. Good agreement with the exception of Co with accepted values has been achieved. Development continues on the determination of low levels of As, Sb, Se and the more refractory elements such as Cr and V. ACKNOWLEDGEMENTS
The authors wish to thank W. Kostiak who performed many of the analyses presented here and J. Kinrade for development of the tin extraction procedure. They also acknowledge the significant contributions to earlier phases of the program by B. Gupta and B. Radziuk. The cooperation of B. Kato of Cominco Limited for supplying comparative results for gold and silver analyses is also greatly appreciated. Partial support through an IRAP grant "Development of Portable Atomic Absorption Spectrometers" and more recently, partial support through "Atomizer 1226" for development of the tungsten filament are gratefully acknowledged.
REFERENCES Brown, S.D., 1977. Zeeman effect-based background correction in Atomic Absorption Spectrometry. Anal. Chem., 49: 1269A--1281A. Chakrabarti, C.L., Hamed, H.A., Wan, C.C., Li, W.C., Bertels, P.C., Gregoire, D.C. and Lee, S., 1980. Capacitive discharge heating in Graphite Furnace Atomic Absorption Spectrometry. Anal. Chem., 52: 167--176. Govindaraju, K., 1980. Report (1980) on three GIT-IWG rock reference samples: Anorthosite from Greenland, AN-G;Basatt d'Essay-la-CSte, BE-N, Granite de Beauvoir, MA-N. Geostandards Newslett,, 4: 49--138. Koizumi, H. and Yasuda, K., 1976. A novel method for Atomic Absorption Spectroscopy based on the Analyte-Zeeman effect. Spectrochim. Acta, 31B: 523--525. Liddell, P.R. and Brodie, K.G., 1980. Application of a modulated magnetic field to a graphite furnace in Zeeman Effect Atomic Absorption Spectrometry. Anal. Chem., 52: 1256--1260. Robbins, J.C., 1972. Zeeman spectrometer for measurement of atmospheric mercury vapour. In: M.J. Jones (Editor), Geochemical Exploration 1972. Institute of Mining and Metallurgy 1973, London, pp. 315--323. Sturgeon, R.E. and Chakrabarti, C.L., 1978. Recent advances in electrothermal atomization in graphite furnace atomic absorption spectrometry. Prog. Analyt. Atom. Spectrosc., 1 : 5--199. West, M.H., Molina, J.F., Yuan, C.L., Davis, D.G. and Chauvin, J.V., 1979. Determination of metals in waters and organic materials by flameless atomic absorption spectrometry with a wire loop atomizer. Anal. Chem., 51 : 2370--2375.
PRACTICAL
FIELD-PORTABLE
ATOMIC ABSORPTION
689
ANALYSER
C.G. CASTLEDINE and J.C. ROBBINS
Scintrex Limited, 222 Snidercroft Road, Concord, Ont. L4K 1B5 (Canada) (Received August 5, 1982; accepted for publication March 14, 1983)
ABSTRACT Castledine, C.G. and Robbins, J.C., 1983. Practical field-portable atomic absorption analyser. In: G.R. Parslow (Editor), Geochemical Exploration 1982. J. Geochem. Explor., 19: 689--704. A new compact atomic absorption spectrophotometer has been developed. Designated the Scintrex AAZ-2, the system used flameless atomization and Zeeman modulation for maximum sensitivity and selectivity respectively. The most important factor of AA instrumentation is the method used for vapourizing the sample. The atomizer furnace in the AAZ-2 is a tungsten ribbon, rather than the conventional carbon furnace, to avoid the need for high electrical power input and consequent water cooling. The ribbon is Ushaped to fit between the pole pieces of a powerful electromagnet which provides the magnetic field required for Zeeman modulation of the absorption signal. Zeeman modulation refers to the spectral displacement of atomic absorption lines in the presence of a magnetic field. By pulsing the field, the emission from the source lamp alternately does, or does not, coincide with the analyte absorption. A synchronous detection system then allows the extraction of the modulated signal from any unmodulated absorption due to nonspecific absorbers such as particulate scattering or some molecular species. Zeeman modulation provides a method of baseline correction that is both more efficient and simpler than the commonly used deuterium arc correction and requires no moving components. The AAZ instrument has already been characterized for a number of metals, in particular, those of exploration interest such as Au, Ag, Cu, Pb, Ni, Co. Zinc has been determined but difficulties in avoiding high blank concentrations have been found. Detection limits are typically 1 ng/ml or better (e.g. Au -- 0.5 ng/ml, Ag -- 0.06 ng/ml). The metals usually requiring conversion to a gaseous hydride form before analyses have also been studied. Tin in the sub-ppm range has been determined after a simple extraction procedure. Work on arsenic, selenium, antimony and bismuth is planned. The entire machine weighs 26 kg and consumes 0.5 kW making it suitable for use in remote areas using portable power generating equipment. Argon at low pressure and flow rate is the only gas needed for operation. When used for analysis of samples in a base camp, close to the scene of a geochemical drilling program, the AAZ-2 can provide quick turnaround of the results and therefore on-the-spot guidance of these programs without loss of sensitivity or data quality. In addition, the possibility of contamination by high-level samples is somewhat less in a field camp than in a long established laboratory.
0375-6742[83[$03.00
© 1983 Elsevier Science Publishers B.V.
690 INTRODUCTION
Atomic absorption spectrometers Atomic absorption spectrometry (AAS) has been a significant factor in the rapid growth o f geochemical techniques, thanks to the relatively low cost and high sensitivities achieved in analysis. In general, however, atomic absorption spectrometers are quite large, are n o t intended for field use and consume significant quantities of electricity, combustion and support gases. In view of the success of geochemical methods, particularly in the reconnaissance phases of exploration programs, there is a definite need for compact, rugged equipment with the same sensitivity as its larger counterparts but which can be readily deployed in the field or in temporary laboratories and facilities. Although n o t all geochemists agree on the desirability of analysis literally in-field, there can be little d o u b t that under proper supervision and controlled analytical conditions, the benefits of determinations close in time and space to collection of the samples are well worth pursuing. At the reconnaissance level, large numbers of samples must be processed at minimum cost b u t with adequate accuracy. Ease of handling, analytical time, equipment reliability and simplicity of operation become significant factors in the design of an instrument. To carry o u t atomic absorption spectroscopy, the original liquid or solid sample must be converted to a gaseous state. Electrothermal atomization is a practical m e t h o d of sample vaporization for equipment in the field, although there are penalties attached to its use. Relative to flame atomization, the precision is probably lower, the dynamic range more limited and the analytical cycle time considerably longer. These disadvantages are outweighed by the greatly improved detection limits, small volumes of sample required and the lack of combustion gases. Considering the simplicity of the equipment, AAS results are remarkably specific. Chemical interferences exist in which the atomic absorption process may be undesirably suppressed or enhanced b y the reaction of the analyte with other components of the sample. It is only at extremely low levels, though, that d o u b t m a y exist of the actual identity of the absorption signal. Spectral interferences due to the presence of a neighbouring line of an unwanted element are known but are quite rare. Broadband absorption by molecular or particulate species is, however, more c o m m o n . A series of methods known as background correction techniques have been developed to reduce this problem. Zeeman atomic absorption spectroscopy (ZAAS), which uses an applied magnetic field to split and displace atomic spectral lines either in emission or absorption, represents an elegant method of correction. The variant described in this paper is especially convenient in that the modulation does n o t require any moving mechanical parts and the magnet itself is quite compact, allowing the design of practical and portable field equipment.
691
Zeeman atomic absorption Background correction refers generally to techniques in which absorption of the analytical spectral line is compared to that of the neighbouring wavelength, with the assumption that the interfering absorption is constant across the spectral interval considered. The principal difference between the various methods is the generation of the reference wavelength. Continuum sources, e.g. deuterium arcs, and hollow cathode lamps with a suitable emission wavelength have been used but there are problems of optical matching and mechanical alignment. The reference wavelength in ZAAS is generated by the spectral shift of parts of atomic absorption or emission lines in the presence of a magnetic field. At high resolution, spectral lines are seen to consist of, sometimes one, b u t more generally, several close-spaced components attributed to modification of energy levels b y hyperfine and isotope effects. The splitting of spectral lines in the presence of a magnetic field to form characteristic Zeeman patterns is determined b y the particular optical transition involved. The spect r o s c o p y that details the generation of the various Zeeman patterns is well d o c u m e n t e d (Koizumi and Yasuda, 1976; Brown, 1977) and will n o t be repeated here except to note that the magnetic field also introduces polarization effects. Emission or absorption processes taking place transverse to the Mj
1p1
,C
+1
+1
0
0
s~"
"~__
~
-1
0.-
285.2 nrn
ISo
Mj g
7r
-1
0+
0
. . . . . . . .
0
v*
Field-Free
In Magnetic Field
I
I
0.-
71"
0"4.
Fig. 1. Normal Zeeman effect in magnesium (from Brown, 1977).
692
magnetic field give rise to o components which can be resolved from those parallel to the magnetic field, the ~ components. The normal Zeeman effect is observed when the line splits into three components {Fig. 1), a ~ c o m p o n e n t in which the wavelength remains constant and two o (-+) which split symmetrically about the line center. The normal effect is observed when both terms involved in the transition are singlets or when the multiplicity does not change. The intensity of the two o components together equals the ~ component. All other transitions exhibit 'anomalous' Zeeman effects. The patterns (Fig. 2) are quite variable; in general, splitting is still symmetrical about the origin but the n components also shift although less than the o-+. There are m a n y ways of utilizing the shifted wavelengths or the polarization p h e n o m e n a produced by the Zeeman effect, depending on whether the field is applied to the light source or the absorbing atoms and whether it is continuous or pulsed. Liddell and Brodie (1980) list and compare seven variations. Continuous magnetic fields are easy to generate but require some Mj
Mjg
312 6/3 1/2 2/3 -1/2 -2/3 -3/2 -6/3
2P3~
D2
1/2 -1/2
2Pll 2
D1
7/-
7/"
O" Cr
o
11-
I' ? T O
° (7
112
1
2Sl12
Field-Free
1/3 -1/3
-1/2
1 -1
In Magnetic Field
iiir
iiii
D1
D2
(589.59 nm)
(589.00 nm)
4P"~
Fig. 2. A n o m a l o u s Z e e m a n e f f e c t in s o d i u m (from Brown, 1977).
693 further m e t h o d of separating the polarization components: the polarizing filters required are n o t very efficient in the ultraviolet. Modulated fields can be applied to the source {e.g. Robbins, 1972) or to the sample as in the present work. This last variant seems to be the most versatile since it can be used with all optical transitions and allows the use of standard AAS light sources (hollow cathode or electrodeless lamps). The effectiveness of Zeeman AAS (ZAAS) is measured b y the change in atomic absorption induced b y the magnetic field. Systems using modulated fields without polarization selection have a theoretical limit of 50% for normal Zeeman transitions since the unshifted ~ c o m p o n e n t is not rejected; for anomalous transitions efficiencies up to 100% can be achieved. Conversely, up to 100% modulation of normal transitions can be achieved b y systems using constant magnetic field and polarization modulation b u t the efficiency of anomalous transitions m a y be very much lower as a compromise magnetic field must be found that maximizes the difference between the ~ and e shifts. The intensity of the magnetic field required to achieve useable modulation can be estimated. Spectral lines are n o t infinitely narrow. The spectral shift must be sufficient to move a substantial fraction of the energy outside the envelope caused b y pressure and Doppler broadening. The presence of hyperfine or isotopic structure may also further broaden the apparent line width. As an order of magnitude estimate, line widths of elements from the middle of the periodic table at atomization temperatures of, say, 2300°K would be approximately 10--15 GHz. Spectral shifts can be roughly estimated at some 0.2 GHz/T to imply a minimum field requirement of 0,5--0.75 T. One tesla (T) equals 10 kilogauss. INSTRUMENTAL General A front view of the AAZ-2 is shown in Fig. 3. The instrument requires t w o support systems: p o w e r at 1101220 V +- 20%, 50]60 Hz and inert gas (usually argon) supplied to the instrument at low pressure. Consumptions are 500 W and 0.5 1/min, respectively. No other support {e.g. cooling water) is required. The instrument weighs a b o u t 26 kg and measures approximately 9 × 36 X 49 cm. Once the light source has been installed and various instrumental parameters dialed in, operation consists of deposition of the sample on the filament with a micropipette and activation of the temperature program. The analysis result is displayed at the completion of the temperature program. The temperature cycle m a y be programmed up to 4 min in duration b u t is generally 1 to 11/2 min.
694
Fig. 3. Front view of the AAZ-2~ Zeeman modulation In the AAZ-2 a modulated magnetic field is applied to the analyte. The inp u t p o w e r required to generate the pulsed magnetic field is proportional to at least the square of the pole gap distance and also increases with the modulation frequency. The modulation period (the reciprocal of the frequency} should be much shorter than the transient absorption pulse to allow adequate sampling. Electrothermal atomization can generate very brief absorption pulses lasting sometimes as short as 50--100 ms. In the Scintrex AAZ-2, a field o f 1.4 T at a frequency of 50 Hz is obtained in a 3-mm gap with 70 W input p o w e r in a magnet weighing some 6 kg. The modulation is generated by efficient power-recovery circuitry proriding essentially square wave rather than the more c o m m o n l y used sinusoidal waveforms. Square wave modulation maximizes the time in each cycle spent either in the field " o n " or the field " o f f " condition, rather than intermediate states during which data must be discarded. For the sake of optical throughput and simplicity of design, plane polarization characteristics are not measured in the AAZ-2. Filament design A cross-sectional view of the filament c o m p o n e n t s used in the AAZ-2 spectrometer is shown in Fig. 4. The U-shaped configuration represents a compromise between simple rod atomizers and the tube-type furnaces which
695
Field h.= Direction I ~
Fig. 4. Cross-section of the atomizer showing the tungsten strip between the magnetic poles. present difficulties in fabrication and the design of electrical connections. The vertical walls provide some containment of the atomic vapor and insulation from the relatively cool magnet poles. The transverse geometry allows the filament to expand w i t h o u t distortion at high temperature b u t maintains rigidity by virtue of small flanges formed in the sides of the filament. This arrangement also makes electrical connection of the filament easier and more reliable than t u b e designs. Metal foil filaments were chosen in preference to graphite furnaces largely because of the greater design flexibility in combining the requirements for sample containment with the need to minimize the volume of the magnetic field. Graphite tubes could fit in the 3-mm gap available b u t would have been very fragile. The higher mass and greater emissivity of graphite tubes relative to tungsten filaments would have also materially increased the p o w e r required with the problem of the subsequent removal of the heat. Tungsten has been used in preference to b o t h m o l y b d e n u m and tantalum which are both much easier to fabricate b u t oxidize and become brittle at high temperatures. Rhenium and rhenium/tungsten (West et al., 1979) foils have been used in a related furnace application b u t the solubility of rhenium in dilute acids limits its use for aqueous solutions. The filament is m o u n t e d using a simple alignment tool on a detachable carrier which provides mechanical support and electrical connection. The power to the filament is supplied b y a constant voltage drive with a variable d u t y cycle. Atomizing currents typically range up to 60 amps (rms) with applied voltages of some 2.5 to 3 V. The temperatures at the normal operating p o w e r levels during the atomization cycle have not y e t been accurately determined b u t are expected to be approximately 2500°K. The rise time is a b o u t 0.8 s, which yields a rate of temperature rise of 3000°K/s, which is considerably higher than conventional graphite furnaces (Sturgeon and Chakrabarti, 1978) b u t less than recent figures for capacitor discharge heating (Charkrabarti et al., 1980). A sheath of inert gas is formed around the filament b y argon or other gases flowing from a series of fine holes in the floor of the sample cavity. The gas flows at a rate of 0.5 l/min around the filament and discharges
696 through the sample inlet port. Provision is made to shut off the gas flow during the atomization periods for analyses in which stopped gas flow condi~ tions are found to be superior.
Optical system The optical l a y o u t is shown in Fig. 5. Light from the hollow cathode lamp is focussed just above the tungsten strip atomizer, then refocussed on the m o n o c h r o m a t o r entrance slit. The miniature m o n o c h r o m a t o r uses a concave grating (10 cm focal length) to give a resolution of a b o u t 2 nm with the 0.5mm slits usually used. The resolution, which is rather less than that conventionally used in AAS, appears adequate. Light leaving the exit slit is detected b y a side~)n photomultiplier. This is m o u n t e d in a steel housing to reduce any effects on sensitivity caused b y stray fields from the Zeeman magnet.
Monochromator TungstenStrip
HollowCathodeLamp
Qu-rtz Lens Electromagnet
I .... it
Quartz Lens '
PMT Frequency to C
s
t
Fig. 5. Optical layout of the AAZ-2.
Electronics system The electronics system supports the varied power requirements in the AAZ-2 including magnet and filament drives. It also processes the o u t p u t o f the photomultiplier providing a digital display of the integrated absorption measured. The drying, ashing and atomizing events applied to the 10-#l liquid sample deposited on the tungsten strip are sequentially controlled by the electronics. Controls are available for use by the operator to optimize the various steps. The drying step dries off the solvent. Ashing removes potential interferences which are more volatile than the element of interest and leaves the sample in a uniform, readily atomizable state. The signal processing is concerned with the demodulation of both the Zeeman signal and a second higher frequency modulation applied to the lamp. The lamp modulation is included t o reject emission o f light from the filament itself at high temperatures. The lamp is repetitively turned on and o f f at approximately 800 Hz. Photomultiplier currents during the " o f f " period are ascribed to emission from the filament and used to correct the signal during the subsequent " o n " period. A voltage-frequency converter produces a series of pulses with frequencies proportional to the photomultiplier output. The pulses are fed to
697 an up-down counter synchronized with both Zeeman and the lamp modulation frequencies. At the end of the atomization period, the counter has integrated the d o u b l y differenced signals as a value proportional to the absorption b y the atoms of interest. This value is displayed on a digital panel readout.
Computer interface To obtain video and hard-copy representations of transient absorption curves, a dedicated c o m p u t e r interface was developed. The system uses a custom interface to convert the analogue photomultiplier o u t p u t and timing information to usable levels for the Apple II computer. Signal conversion in the Apple II is provided b y a Mountain Computer A/D, D/A board. Transient signals m a y be viewed on the c o m p u t e r monitor or o u t p u t onto an X-Y recorder for permanent record keeping. While this system has proved useful for experimental development, the production version will use the already digitized signals generated in the AAZ-2. EXPERIMENTAL The efficiency of Zeeman splitting has been established for several elements. This efficiency is presented in this work as the " d e p t h of modulation" defined as the percentage reduction in absorbance during the magnetic field-on periods when compared with the unperturbed absorbance. Computer generated curves were produced for zinc at 214 nm and silver at 338 nm, with and w i t h o u t Zeeman modulation. A study of the influence of magnetic field strength on the silver transitions at 328 and 338 nm was also performed. Except where otherwise noted, standards were prepared in 0.1 N HNO3 using analytical grade chemicals. Analysis was performed using single element hollow cathode lamps and temperature programming as determined experimentaUy and published in an applications manual for the AAZ-2. Calibration curves were constructed from three or more standards in the working concentration range for the element of interest. A suite of rock samples has been analyzed for gold content. The samples were first roasted to destroy sulfides then leached in the cold with hydrobromic acid--bromine. After separating solids, the gold was extracted as a bromide into methyl iso-butyl ketone. After backwashing the organic extract with 0.1 N HBr the gold content of the organic layer was determined on the AAZ-2. Standards were prepared by extracting the appropriate a m o u n t of an aqueous standard from 3 N HBr into MIBK and backwashing the organic with 0.1 N HBr. Comparative analyses were supplied b y Cominco Ltd. (Toronto, Ontario, Canada) analysts w h o used fire assay followed by acid dissolution and AA analysis. Discrepancies in comparative measurements may be caused b y differences in standards, sample inhomogeneity,
698
extraction efficiency, etc. To avoid these difficulties, the performance of the AAZ alone was compared with another AA instrument. The same extracted solutions were analyzed for gold on both the AAZ-2 and a Perkin-Elmer 603 with an HGA 500 graphite furnace. A comparative study of silver determinations was conducted on a suite of ore pulps. The m e t h o d entailed a nitric acid--hydrogen peroxide leach with the heat of reaction of the reagents as the only warming of the digestion. Comparative analyses were again supplied by Cominco Ltd. using perchloricnitric-hydrofluoric acid digestion and flame AA for measurement. Detection limits have been determined for a number of other elements. These were determined with Zeeman background correction using 10 microlitre samples. The detection limit is defined as twice the standard deviation of the blank. This is not a very useful figure in realistic terms, but serves to indicate performance relative to other AA spectrometers. The e n h a n c e m e n t of signals obtained from tin standards was investigated. Two methods of enhancement were tried: inclusion of 3% hydrogen in argon atmosphere and extraction as an oxinate (8-hydroxyquinoline chelate) into butyl acetate. The Geostandards AN-G (Anorthosite), BE--N (Basalt) and MA-N (Granite) have been analyzed for a variety of trace elements. Digestion methods consisted of h o t acid attack for 1--2 hours followed by dilution and subsequent measurement on the AAZ-2. For copper and lead, nitric acid alone was used. For cadmium, cobalt and nickel a combination of nitric and hydrochloric acids was employed. RESULTS AND DISCUSSION
Figure 6 shows transient absorption curves for zinc at 214 nm and silver at 338 nm, with and w i t h o u t Zeeman modulation. The modulation gives the With Zeeman Modulation
No Zeeman b l o d u l a t i o n
L\r 7
,~-
/
--~-~ ~I~ ~--~~-
En
..
%
Ag 338 nm
Fig. 6. C o m p u t e r generated absorption curves s h o w i n g the effect of Z e e m a n m o d u l a t i o n .
699 curves a saw-toothed appearance with successive "field o f f " and "field o n " periods showing, respectively, the unperturbed absorption and the Zeeman split absorption. The depth of modulation for this zinc line is only a b o u t 35%. This is relatively inefficient when compared with the silver line which yields virtually 100% depth of modulation. The effect of increase in magnetic field strength on the t w o main lines of silver is shown in Fig. 7. Although both transitions are anomalous the splitting of the 338-nm line is more efficient. The unperturbed peak absorbance for the 328-nm line is a b o u t twice that of the 338-nm line b u t the latter is usually preferred for AAZ-2 analysis due to its better depth of modulation. Scatter diagrams for the t w o suites of gold analyses and the silver analyses are given in Figs. 8, 9 and 10. As expected, correlation is much better where all analytical and sampling biases are eliminated and the performance of the instrument alone is compared.
ir !ij!:, .....
f/i'
LT].
a)
_~Y
Fig. 7. Effect of increasing magnetic field strength on efficiency of Zeeman splitting for the silver transitions at 338 and 328 nm: a) 0.6 T; b) 0.9 T; c) 1.2 T; d) 1.5 T.
700
JO,O00~-
. . . . . . . . . . . . . . .
!
i
I i /
I000
~
-I ~
~ i
/
IJ
I00 .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
~ .
.
.
.
.
.
.
m -r oJ N <~ <~ x LU rr FZ
i0 ~
-
i
1.0
1.0 ng/g
Fire
I0 Ass0y - AA
I00
I000
IOltO0
Fig. 8. Comparative gold analyses - ore pulps. I000
•
/i
""
I00
t
r
N
X w r~ t-.--
.~- ...... - - - - .....
I0-
0 o9
LOI0
ng/ml
I0 IO0 I000 Perkin Elmer 6 0 3 with HGA 5 0 0 Graphite Furnonce
F i g . 9. C o m p a r i s o n
of instrumental
analyses for gold on the same extracted
solution.
701 IO00
0 z I
I00 @
0 O
(M N < I
X i,i rr" I.z
I0-
o9
1.0 1.0
I0 Ng/g
I00
I000
Flame AA (HCIO4-HNOs)
Fig. 10. C o m p a r a t i v e silver a n a l y s e s - - o r e p u l p s .
Detectio n limits for a variety of elements are given in Table I. The inh e r e n t high sensitivity of the 214-nm line for zinc and its mobility have caused problems of c o n t a m i n a t i o n bot h in analysis o f samples and standards alike. Spurious c o n t a m i n a t i o n f r o m pipette tips, containers, etc. as well as m or e or less c o n s t a n t c o n t r i b u t o r s such as water and reagent impurities combine to f o r m a significant problem. T he d e t e c t i o n limit on this line could be improved if this high c o n t a m i n a t i o n were eliminated. In some cases the 308-nm line o f zinc can be used t h o u g h it is some three orders of magnitude less sensitive; a drawback which limits the usefulness of this approach. TABLE I Some detection limits with the AAZ-2 Element
Wavelength (nm)
Detection limit a (g)
Equivalent concentrationb (ng/ml)
Ag Au Cd Co Cu Ni Pb Zn Zn
338.3 242.8 228.8 240.7 324.8 232.0 283.3 213.9 307.6
6 5 5 1 4 4 8 2 4
0.06 0.5 0.05 1.0 0.4 4 0.8 0.2 400
X 10 -13 x 10 -12 × 10 -13 X 10-11 × I 0 -12 × 10-11 X 10 -12 X 10 -'2 × 10 -9
aMeasured with Zeeman modulation. b C o n c e n t r a t i o n in a 10-ul s a m p l e .
0.02 8.2 25.0 2.0 18.0
This work
0.031--0.18 10--56 8--27 0.62---13 9--70
R a n g e of values
Geostandards a
AN-G
-25 19 2 35
Recommended value 0.09 34 79 4.2 250
This work
a G e o s t a n d a r d s values f r o m G o v i n d a r a j u ( 1980).
Cadmium Cobalt Copper Lead Nickel
Element
C o m p a r a t i v e results for t h e G e o s t a n d a r d s (ug/g)
T A B L E II
0.089--0.21 43--75 43--109 3--17 160--327
Range of values
Geostandards
BE-N
-61 72 4 267
Recommended value 2.0 0.1 97 15 2.5
This work
1.8--2.2 0.35--38 76--187 10--86 2--92
R a n g e of values
Geostandards
MA-N
2 1 140 29 3
Recommended value
703
The sensitivity of the 214-nm line can be detuned to make backgrounds manageable b y substituting helium for argon as the inert-gas atmosphere. This has the effect of reducing the sensitivity b y a b o u t an order of magnitude. Atomization p o w e r must be increased to compensate for the increased heat loss p r o m o t e d by the much higher thermal conductivity of helium. Although this study has n o t been completed, the ability to lessen the sensitivity, especially for zinc, should prove very useful. Results obtained from the Geostandards are presented in Table II. Although the data show a low bias, values generally lie within the range of values reported for these samples (Govindaraju, 1980). Cadmium in AG-N is below any other reported data. AG-N and BE-N have no r e c o m m e n d e d cadmium values since the number of reported values was not sufficient to characterize them properly. Cobalt values were also generally lower than those reported. Discrepancies have been attributed to the inefficiency of the acid attack used in this study versus the total digestions or total analyses assumed to have been used by the other laboratories. Re-evaluation of these samples using total digestion is planned. Initial measurements on tin in aqueous solution using a pure argon atmosphere yielded a relatively p o o r sensitivity of several ppm. The formation of oxides during the final stages of drying suppresses the signal obtained, a p h e n o m e n o n which is well k n o w n to give p o o r sensitivity for flame AA analysis of tin. This already low signal completely disappears when ashing of a b o u t 500°C is introduced implying that only the volatile tin components which remain after drying give rise to the signal. Introduction of a small amount, in this study 3%, of hydrogen to the argon atmosphere immediately increased sensitivity some t w e n t y times. As ashing was successively increased, the signal was f o u n d to increase. The gain at an ashing temperature of 1000°C for 10 s is a further ten times increase in sensitivity over the measurements made w i t h o u t ashing. The net gain for the addition of both hydrogen and ashing was, therefore, some 200 times. An alternate m e t h o d of impeding oxide formation is to extract the tin into an organic solvent. Extraction into b u t y l acetate of the 8-hydroxyquinoline complex of tin (IV) was used. Direct analysis of this extract yielded an enhancement of a b o u t 200 times relative to the aqueous standard. Introduction of the partial hydrogen atmosphere and the ashing steps had no effect on the signals obtained. While it is assumed that the hydrogen atmosphere provides a reducing environment, it is n o t clear as y e t if the organic extract simply deprives the tin of water thereby impeding oxide formation or whether the decomposition of the chelate molecule may provide a localized reducing environment. CONCLUSIONS
The design and performance of a new atomic absorption analyzer suitable for field use has been described. The major innovation is the development of a miniaturized metal filament for sample vaporization which, in turn, per-
704
mits the use of a compact magnetic field for background correction via the Zeeman effect. The magnetic field is electrically modulated to avoid the need for mechanical oscillation of any optical components. The instrument has been thoroughly characterized for determination of gold down to 2 ng/g in various geological sample materials. A preliminary set of data (Cd, Co, Cu, Pb, Ni) on three standard geochemical reference standards has also been presented. Good agreement with the exception of Co with accepted values has been achieved. Development continues on the determination of low levels of As, Sb, Se and the more refractory elements such as Cr and V. ACKNOWLEDGEMENTS
The authors wish to thank W. Kostiak who performed many of the analyses presented here and J. Kinrade for development of the tin extraction procedure. They also acknowledge the significant contributions to earlier phases of the program by B. Gupta and B. Radziuk. The cooperation of B. Kato of Cominco Limited for supplying comparative results for gold and silver analyses is also greatly appreciated. Partial support through an IRAP grant "Development of Portable Atomic Absorption Spectrometers" and more recently, partial support through "Atomizer 1226" for development of the tungsten filament are gratefully acknowledged.
REFERENCES Brown, S.D., 1977. Zeeman effect-based background correction in Atomic Absorption Spectrometry. Anal. Chem., 49: 1269A--1281A. Chakrabarti, C.L., Hamed, H.A., Wan, C.C., Li, W.C., Bertels, P.C., Gregoire, D.C. and Lee, S., 1980. Capacitive discharge heating in Graphite Furnace Atomic Absorption Spectrometry. Anal. Chem., 52: 167--176. Govindaraju, K., 1980. Report (1980) on three GIT-IWG rock reference samples: Anorthosite from Greenland, AN-G;Basatt d'Essay-la-CSte, BE-N, Granite de Beauvoir, MA-N. Geostandards Newslett,, 4: 49--138. Koizumi, H. and Yasuda, K., 1976. A novel method for Atomic Absorption Spectroscopy based on the Analyte-Zeeman effect. Spectrochim. Acta, 31B: 523--525. Liddell, P.R. and Brodie, K.G., 1980. Application of a modulated magnetic field to a graphite furnace in Zeeman Effect Atomic Absorption Spectrometry. Anal. Chem., 52: 1256--1260. Robbins, J.C., 1972. Zeeman spectrometer for measurement of atmospheric mercury vapour. In: M.J. Jones (Editor), Geochemical Exploration 1972. Institute of Mining and Metallurgy 1973, London, pp. 315--323. Sturgeon, R.E. and Chakrabarti, C.L., 1978. Recent advances in electrothermal atomization in graphite furnace atomic absorption spectrometry. Prog. Analyt. Atom. Spectrosc., 1 : 5--199. West, M.H., Molina, J.F., Yuan, C.L., Davis, D.G. and Chauvin, J.V., 1979. Determination of metals in waters and organic materials by flameless atomic absorption spectrometry with a wire loop atomizer. Anal. Chem., 51 : 2370--2375.