Analysis of trace metals in the air The atomic absorption spectroscopic technique is used to measure these pollutants in a direct method and in a discret e fashion
which the metals captured by the cup are atomized by passing several hundred amps of current through the walls of the cup. An AAS measures the absorbance signal. Because this discrete-sample approach has seen much wider application than the continuous sampling approach, the rest of this paper will be devoted to describing various aspects of it.
Darryl D. Siemer Marquette University Milwaukee, Wis. 53233 Direct atomic absorption determination of trace metals in air has been accomplished in two fundamentally different ways. The first approach involves drawing air samples continuously first through incandescent carbon granules, and then through a longpath heated-quartz absorption tube mounted in an atomic absorption spectrometer (AAS). The hot carbon reacts with air to produce carbon monoxide, which serves to reduce metal species carried in the air stream to free metal atoms. An advantage of this technique is that it can be used to measure either the total metal concentration, or the “molecular” forms, by simply inserting a filter in the sample inlet line. The main feature of the method is that analytical numbers are gotten without delay. The immediate absorbance signal is a measure of the instantaneous concentration of a metal in the air. There are, however, three serious weaknesses inherent in the method: Only volatile metals can be run because of limited atomization temperature capability. It is necessary to bring the entire
Batch process instrument to the sample site-often not practical. Because no filter is used to concentrate the samples, the sensitivities available with the method are marginal for many metals at typical ambient concentration levels. The second approach involves using a graphite furnace atomizer into which entire air filters are placed and the metals on them subsequently atomized and determined. A “clean” porous graphite cup is first put into a filter adapter and the air sample is drawn through. The cup is then taken to the laboratory and placed between the rods of a carbon cup atomizer, after
0013-936X/78/0912-0539$01.00/0 0 1978 American Chemical Society
The discrete sampling approach There are several important advantages to the direct graphite furnace atomic absorption spectrometric (GFAAS) approach to air particulate anaiysis. First, the method is extremely sensitive. Air samples of from 20 cc to one liter in volume are sufficient to measure accurately most metals at concentrations apt to be considered hazardous in air. This is true both because all of the collected sample is simultaneously atomized and measured, and because the absence of extensive sample preparation permits very low reagent blanks to be easily and reproducibly achieved. Other desirable features include: simplicity of the entire procedure from sampling to the final determination wide applicability to most elements with only minor changes in analytical conditions (atomization program) Volume 12, Number 5, May 1978 539
no need for the AA instrument to be dedicated to this type of analysis-conversion to regular solution analysis requires only changing the atomizer cup from a porous graphite type to the conventional impervious graphite type relatively low cost-laboratories already possessing GFAAS equipment need only buy or make air sampling kits to use with it (a 100 cc plastic syringe is a suitable pump in many cases) very rapid sample turnaroundfrom seconds to a few minutes for sampling and about two minutes for the actual metal determination. This general analytical approach is best used when analytical numbers are needed quickly on limited sample volumes. It can be an invaluable tool for “sniffing out” pollutant sources in industrial environments quickly enough to permit operators to do something about the problem as it is occurring.
Filtering air samples There are two distinctly different types of filter media that can be used in the direct GFAAS approach. The first is conventional membrane; for example, a Millipore or Nucleopore material. Sampling is done by placing a disk of the membrane in a GFAAS atomizer cup that has a perforated bottom similar to a Buchner funnel. The cup is then placed in the filter adapter, and the air sample is drawn through it (see Method a). After sampling, the cup is brought to the laboratory and inserted into the atomizer. A drop of phosphoric or other high-boiling oxyacid is added to the cup. A careful pyrolysis or “ashing” temperature program is then run through to burn away the organic filter and normalize the sample chemistry. Finally, the temperature of the cup is stepped up to atomize the material to be analyzed, and the actual atomic absorption metal determination is completed. The second and more elegant approach to filtering is to use porous graphite-often a part of the atomizer itself-for the sample collection. Graphite filters can be cleaned in the atomizer before they are used for sampling. This allows much lower blanks (for many metals), and, consequently, better detection capabilities for small samples than are achievable with the membrane filters. Graphite filters are reuseable many times and do not have to be “ashed off’ prior to the actual analytic atomization. The consensus of opinion of various workers who have compared the two 540
Environmental Science & Technology
filter types is that graphite is generally more retentive. Among the primary reasons for this is that graphite filters are generally much thicker than the membrane filters, and that the considerable tortuosity of the channels through the graphite favors fine particle capture. A study of metal concentrations in graphite filters a t various depths below the surface has indicated that significant amounts of the metals are caught in the channels within the graphite and are not simply “screened out” on the surface. A final advantage is that gold-plated porous graphite is an effective filter for atomic mercury, as well as for the particulate-bound forms. This makes it possible to measure this element in both of its commonly found forms. Conventional ultraviolet (UV) mercury analyzers measure only the atomic form.
Furnaces Several different graphite furnace atomizer configurations have been successfully used for these analyses. Not all designs are equally suitable for this application, so each will be discussed separately. The only instrument for which an air sampling kit is commercially available is the Varian Techtron Model 63 or Model 90 carbon cup atomizer (see Method a). The “Microsampling” kit includes bottom-perforated graphite atomizer cups, a punch for membrane filter disks, some membrane filters, and a 200-cc spring-loaded syringe air Pump. The bulk of the more recently published work in the field has been done with this system. However, for some commonly determined elements, the filter “blanks” have proved to be too high, and porous graphite (RW-1 and VSP- 1, Ringsdorff Werke) cups were used with the sampler instead of the stock membrane filters. A homemade version of the Varian
Model 63-90 carbon tube or “miniMassman” furnace was used for this author’s earlier efforts in the field (see Method b). Use of the walls of a tube instead of the bottom of a cup for filtering the air sample gives a far larger filter surface area and allows much faster sampling with lower pressure drops. For example, filtration rates of about 1-2 liters per minute are achievable with the tubes as compared with about 50-100 cc per minute with the cups. It is also much easier to make tubular filters with reproducible dimensions than it is to make filters of other geometries. A possible disadvantage of the tubular filter is that it is not easily possible to pretreat the entire active filter surface with a reagent-for example, phosphoric acid-prior to atomization in order to normalize analyte chemistry. However, this step may not be necessary because the particles are actually caught somewhat within the surface of the graphite filter. This filter requires volatilized metal species to diffuse through a layer of hot graphite before they can enter the optical path. The prolonged contact with the hot graphite serves to reduce the various chemical forms to the free metal. It also renders the acid treatment less important than it is when metal salts are evaporated strictly from the surface of impervious graphite surfaces as is the case when membrane filters are used. Perhaps the best furnace design that has been used for this application is the Woodriff furnace. Basically, it consists of a long (275 mm) atomizer tube enclosed in a heavily thermally insulated steel can. Sample cups are placed onto a pedestal that is then inserted up through the bottom of the furnace, and seals a hole cut in the center of the atomizer tube (see Method c). The interior of the furnace is kept at a constant temperature high enough to
Commercially available graphite furnace atomizers Model 70/2000/21 00/2?00 HGA Series
Perkin-Elmer Corp. horualk, Conn.
Model 3 5 5 / 5 5 5 Flameless Atoniiiers
Instrumentation Laboratory Inc. Wilmington, Mass.
Model CRA 63/90 Serics
Varian Techtron, Palo ,Alto. Calif.
Model FLA 10/100 Series
Jarrel-Ash Div. of Fisher Scientific Waltham, Mass.
~ - - l l s a _ _ v _ _ ~
_F__r
-TI-^I---
_i--sl--
T o n - s t r e a m process Methods for direct atomic absorption analysis
--_
b
5
graphite
C
-Graphite pedestal d
Model 4551555 features removable sample planchet*
Microboat‘
atomize the sample of interest. The cup is rapidly heated by combined convective and radiant energy transfer from the cup seat and other internal furnace parts. Because the furnace is very large, volatilized sample atoms are contained within it for a time which is long, relative to the time that it takes for the atoms to leave the surface of the graphite sample cup and get into the optical path. This prevents differences in evaporation rates of the materials to be analyzed from strongly affecting the peak absorbance signal. The system is thus rendered more immune to matrix effects than are the short-tube, thermally pulsed furnaces.
Since there is a true thermal equilibrium between the gas within the optical path and the walls of the furnace, it is possible to use non-resonance lines for reproducible atomic absorption (AA) analysis. This expedient permits tailoring the sensitivity of the metal determination to meet varying analytical needs by simply changing the furnace temperature. At this time the most serious practical problems of using this furnace are: The furnace is too bulky to fit into most commercially available atomic absorption spectrometers. Its maximum temperature for continuous operation is marginal for
determining some refractory elements. Most importantly, it is not commercially available. The Instrumentation Laboratory Model 455-555 square tube atomizer, using rectangular “microboat” sample planchets, is readily adapted to direct air particulate analysis. This has been done by placing tiny porous graphite filter disks (cut from 6.5 mm diameter spectroscopic graphite rods) into the microboats which are, in turn, inserted into a slot cut in the bottom of the atomizer tube (see Method d). If microboats made of porous graphite and a suitable adapter to hold them while sampling were readily available, this writer would consider this furnace design to be the best of the commercially available models for air analysis. The reason for this statement is that it is not necessary to disturb the actual atomizer tube to insert a sample when this furnace is used. The popular, round-tube Massman furnace design, as exemplified by the Perkin-Elmer Model 2200 or the Jarrel-Ash model FLA 100, has not yet been used for direct air analysis. The relatively large tubes used in this design are easily removed and replaced (a necessary criterion) and could easily be made of porous graphite. If porous tubes were used in adapters fashioned to permit only the central centimeter or so of the tube to serve as a filter, this furnace design could be predicted to give excellent service for this application. However, unless the sample particulates are somehow restricted to the center of the tube, the very large longitudinal thermal gradient inherent in this furnace configuration would give broad, poorly defined, atomic absorption signals. Other likely problems would include difficulty in cleaning contaminated tubes and in selection of suitable atomization programs to separate analyte signals from bulk matrix nonatomic absorption signals. Latest designs are better Most of the latest commercially available furnace designs are significantly better than their forebears for this, as well as for the more conventional solution analysis applications. These improvements are largely in the capability of the newer power supplies to very rapidly “step” to an effective atomization temperature. They then Volume
IL,Number 5, May 1978
541
Cases in which direct AA has been reported hold that temperature until all of the material to be analyzed has diffused from the optical path. Doing this significantly improves the sensitivity and, more importantly, renders the analysis less prone to matrix interferences. The improved atom diffusion control also makes signal integration a fundamentally more effective procedure. Both because of the unknown and often variable composition of the bulk of particulate matter in an air sample, and because of the limited amount of sample preparation inherent in this technique, it is desirable to take advantage of every instrumental “crutch”available, in order to ensure accuracy. This means that signal integration, simultaneous background correction, and carefully selected atomization temperature programs should all be used as a matter of course, unless extensive experience in running analyses on a given sample type indicates that one or more of these expedients may be eliminated. Table 1 describes the work of some of the major contributors to the field,
measurement of beryllium levels in a room in which a gas mantle lantern has been lit-extremely high levels were found near the lantern u hen a fresh mantle was lighted for the first time; afterwards the beryllium release-rate declined rapidly determination of cadmium and lead in consecutive puffs from the same cigarette demonstration of the fact that a single Christmas candle having a metal wire-wick stiffener could pollute a rather large room with lead particulates to concentration levels usually found in the middle of city traffic investigation of mercury levels at various points and times in a combination office and laboratory building measurement of lead within a moving automobile at various points as i t covered a typical “commuter run” determination of several metals in a welding shop
and gives the approximate dates when the work was done. It is apparent that little recent work has been done in the U.S. Possible reasons for this may include the following reasons: Laws and regulations usually specify average, not instantaneous or very short-term, exposure limits to airborne metals. There has been no significant federal funding available for work in the area for several years. With only one exception, the air sampling equipment must be constructed by the analyst interested in trying out the method. Many analysts are not aware of the technique and it may need further literature exposure to “catch on”.
Some limitations A serious limitation of the direct GFAAS method is the limited useful analytical range inherent in atomic absorption spectrometry. The analyst must choose his sampling volume to give eventual analyte absorbance signals within a range of about an order
TABLE 1
Significant contributions to direct atomic absorption air analysis literature Prlnclpal Investigators and dates of work
Robinson, Loftin
1970-1974 Lech, Siemer, Woodriff
1972-1974
Siemer, Lech, Woodriff
1972-1974
Siemer, Koteel, Wei
Atomizer type
Metals
Hg, Pb, Cd
Rf heated quartz tubes
none
Mn, Pb, Hg
Woodriff furnace
porous graphite cup
ambient air in semi-urban area Hg done with gold-plated filters- welding shop
Pb, Hg, Cr, Be Cd, Mn Zn
homemade Varian CRA 63 design “Min i-Massman’ ’
porous graphite tube
artificial environments outside air, welding shop, room with a propane mantle lamp (Be), cigarette smoke
Pb
I.L. 455 and tungsten filament atomizer
porous graphite disk
urban air- compared direct AA to electrochemical AA method
Pb, Cd
Varian 63
millipore disk in cup
air in car parks, moving car, urban area, cigarette smoke
millipore disks or nucleopore disks or porous graphite cup
air in both urban and nonurban areas. Used cups as “fallout” gauges in some cases
1973-1974 Nolier, Bloom
1974present
~
542
Sample types
ambient air in urbanized area
1975-1976
Matousek, Brodie
Filter
CUP
Pb, Cd, Cu, Ni Zn
~
Environmental Science & Technology
Varian 63 cup
I
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
I
of magnitude if he is to get an accurate quantitative analysis. Another problem is that furnace atomic absorption cannot be utilized for simultaneous multielement analyses with state-of-the-art equipment (with one exception-a two-channel instrument). These constraints dictate that the person making the analysis know both what he wants to measure and roughly how much of it is apt to be found . . . unless a simple “more than” or “less than” answer will serve the purpose of the analysis. . . before the sample is filtered. However, for many of the problems facing industrial or governmental hygienists, the advantages of direct GFAAs far outweigh the limitations. It is expected that many more application will be seen in the near future.
5
I I I I I I I I I
I
I I I I I I I I I I I I
Additional reading Robinson, J. W., Direct Determination of Metals in Air, EPA-650/2-73-011, August 1973, EPA Technology Series, EPA Office of Research and Development, Washington, DC (describes continuous sampling approach). Bloom, H., Noller, B . N., Application of Trace Analysis Techniques to the Study of Atmospheric Metal Particulates, paper presented at Clean Air Society of Australia and New Zealand Symposium on Analytical Techniques for the Determination of Air Pollutants, Melbourne University, May 23-27, 1977 (summarizes work done with carbon cup atomizer). Siemer, D., Woodriff, R., Direct AA Determination of Metallic Pollutants in Air With a Carbon Rod Atomizer, Spectrochim. Acta, 29B,269 (1974) (summary of work done with carbon tube atomizer). Lech, J., Ph.D. Thesis, Montana State University (1 974) (description of work done using Woodriff furnace atomizer).
i I I I I I I
0Yes, I need the reliable, competitively priced pollution monitoring resultsyour Analytical Chemistry Lab now offers. I understand you are now offering the air/water/process sample monitoring services of your Rockwell Analytical Chemistry Laboratory to commercial customers. I would like to hear more about a laboratory whose rigorous quality control program was formulated in collaboration with the EPA. I understand it can provide the kind of valid data I need to: (a) determine my operation’s costeffectiveness; (b) meet federal, state and local regulatory requirements; and (c) meet legal challenges. I am definitely interested in your fast turnaround and the fact that, through automation, you can offer your services at competitive prices. Please call me or send me a brochure with complete details.
0
0
Company Name Address City Phone number
State-
Zip ES-5
Air Monitoring Center, Atomics International Division, Rockwell International, 2421 West HiUcrest Drive, Newbury Park, CA 91320. Phone: (805) 498-6771.
Rockwell International where science gets down to business
I
1111111111111D111111lllll CIRCLE 25 ON READER SERVICE CARD
...................................... ... .. .. .. :.Synthetic :.Pyrethroids .
.... .. .. .. .. .. .. .. : .. . ... : .. ..................................... I
toxic organochlorine, organophosphate, a n d carbamate compounds.
Chemists, toxicologists, entomologists, a n d environmental specialists concerned w i t h pesticide development a n d production will w a n t this useful collection for its critical, up-to-date coverage of all aspects of synthetic pyrethroids including future applications against agriculture a n d forest insect pests.
CONTENTS
Darryl I).Siemer is an assistant professor of chemistry at Marquette University. Hir research interests include straightforwarc; applications of atomic spectroscopy tc analytical problems-often by uncon. ventional approaches.
ACS S y m p o s i u m Series No. 4 2 Michael Elliott, Editor Rothamsted Experimental Station A symposium sponsored by t h e Division of Pesticide Chemistry of the A m e r i c a n Chemical Society. T h e twenty-one contributions to this significant n e w v o l u m e are devoted to synthetic pyrethroids a n e w class of insecticides currently t h e subject of intense interest throughout t h e world since they promise to replace the more
Synthetic Pyrethroids Preferred Conformations Derived Pyrethroid Insecticides Most Potent Pyrethroids Insecticidally Active Synthetic Pyrethroid Esters Pyrethroid Like Esters of Cycloalkane Methanols Insecticidal Activities Neurophysiological Studies Central vs. Peripheral Action in the Nervous System Permethrin Synthesis Dichloromethyl Pentadienes Synthesis of the Acid Moiety Photochemical Reactions Permethrin Degradation Enzyme Substrate Specificity in Pyrethroid Metabolism Stereospecificity in Pyrethroid Metabolism Permethrin Metabolism Permethrin Metabolites Residue Methodology Gas Chromatographic Determination of Residues 229 pages (1977) clothbound $15.75 LC 77-1810 ISBN 0-8412-0368-7
SISiAmerican Chemical Society 1155 16th St., N.W.TWash.,D.C. 20036
Volume 12, Number 5, May 1978
543
