Volume 53, Number 2, 1999 APPLIED SPECTROSCOPY 1910003-7028 / 99 / 5302-0191$2.00 / 0

q 1999 Society for Applied Spectroscopy

Direct Insertion of Trace Metals Preconcentrated onActivated Charcoal into an Inductively Coupled Plasma

CAMERON D. SKINNER, MURIELLE CAZAGOU, JOANNE BLAISE,and ERIC D. SALIN*Department of Chemistry, McGill University, Montreal, Quebec, Canada H3A 2K6

Trace metals have been concentrated onto activated charcoal byusing oxalic acid, iminodiacetic acid, and 8-hydroxyquinoline. The

charcoal was directly analyzed in an ICP by placing several parti-

cles in a direct sample insertion (DSI) cup, or fabricating a DSIprobe with an activated charcoal cap. Detection limits in the tens

to hundreds of parts per trillion were found for Pb, Cd, Zn, and

Cu. The recovery for C u was 2± 5% but may be improved by re-activation of the plasma-cleaned charcoal. C leaning the charcoal,

derived from wood, in the plasma prior to adsorption was necessary

to remove trace metals.

Index Headings: Direct sample insertion; Activated charcoal; Pre-concentration; Inductively coupled plasma; ICP.

INTRODUC TION

In our laboratory, we have studied sample introductionmethodologies that attempt to improve the ef® ciency ofanalyte introduction into the inductively coupled plasma.These methods have included direct sample insertion(DSI),1,2 alone and in combination with on-line and phys-ical sample preconcentration.3,4 Each of these methodshas been effective in increasing sensitivity and loweringdetection limits; when combined, they provide a syner-getic improvement.5 We have also reported the use ofhigh-ef® ciency (90%) pneumatic nebulization of sampledirectly into heated DSI cups.4 However, combining thesemethods is not without penalty, preparing the sample canbe expensive in terms of equipment, reagent, and time.Additionally, the concentrate must be dried for severalminutes in the DSI cup. Contamination from reagentsused to concentrate and elute the sample from precon-centration systems can also be problematic.

A better solution would seem to be a fusion of DSIand preconcentration, rather than an interface between thetwo techniques. To this end, we report on experiments inwhich metals are chelated in solution and adsorbed di-rectly onto activated charcoal that can be inserted intothe plasma. This approach has many potential advantag-es: samples may be treated in batches, since only theprobe needs to be retrieved, remote sampling is convien-ient, the potential for sample contamination is lessened,and the probes may also be reusable.

Activated carbon has been used as an adsorption sur-face for many metal preconcentration methods from avariety of sample types. Most of these methods rely onthe adsorption capacity of chelates onto the carbon butdo not analyze the carbon directly, except in the case ofneutron activation analysis (NAA)6,7 and X-ray ¯ uores-cence (XRF).8 Rare earth elements are unusual because

Received 8 August 1997; accepted 14 October 1998.* Author to whom correspondence should be sent.

they may be adsorbed directly from the solution phase asa hydroxide without the aid of complexation with an or-ganic molecule.9 Often mineral acids are used to digestthe activated carbon and release the metals. Although thisprocedure does provide preconcentration of the analytes,contamination from the reagents is signi® cant. In someinstances, organic solvents have been used to elute themetal complexes from activated carbon.10

Activated charcoal is similar to activated carbon, butis generally produced from charcoal derived from naturalsources such as sugar, wood, and coconut shells. Themain difference between the two is the ash left in thecharcoal. Activated charcoal is available commercially asbricks and loose particles of various sizes. The loose par-ticles can be collected and placed into a graphite DSIcup, while the brick type of activated charcoal can bemachined into DSI probes. Graphite that has been ex-posed to the plasma was also examined, because the sur-face develops pitting and appears to change in what maybe a process similar to the activation process discussedabove.

Choice of Chelation Chemistries. Most column-basedpreconcentration methods rely on simple chelation li-gands bound to larger polymers. One of the most com-mon is Chelex-100, which employs iminodiacetic acidgroups as the chelating ligands on a polystyrene back-bone. In the past, small amounts of this material wereplaced in the DSI cup, but very careful ashing and in-sertion was required in order to avoid extinguishing theplasma. Even so, resin pyrolysis products still remainedand were responsible for changes in the excitation ef® -ciency of the plasma.5

The poor success with the resin indicates that immo-bilized chelating reagents and large chelating moleculesmay not be suitable for plasma analysis. For this reason,adsorption of small chelating molecules directly onto ac-tivated charcoal was investigated. Fortunately, there aremany good chelating reagents that may be suitable forthe task: 8-hydroxyquinoline,8,11 potassium ethyl xan-thate,12 ammonium pyrrolidinedithiocarbamate,13,14 dithi-zone, and others15,16 have been used with activated char-coal. Iminodiacetic acid (IDAC) and oxalic acid are alsowell known for their chelation properties and have beeninvestigated.17

The aim of this initial study is to obtain informationabout the viability of preconcentration of metal chelatesonto activated charcoal. As such, the scope of the inves-tigation is limited to a few chelation molecules and alimited test set of elements.

EXPERIMENTAL

Preparation of Probes and Charcoal. The method ofpreparation of standard DSI cups has been recently de-

192 Volume 53, Number 2, 1999

FIG. 1. Diagram of regular DSI cup as well as activated charcoal DSIprobe. See Table I for dimensions.

TABLE I. Dimensions of DSI probes.

Dimensions depictedin Fig. 1

Regular graphitecup (mm)

Activated charcoalcap probe (mm)

abcdWeight (mg)Stem thickness

5.566.50.25´´́

440.75

2.754.01.702.254.20.75

TABLE II. Suppliers and manufacturers.

Electrodes for DSI cupsElectrode type: S-8 high density

Bay CarbonBay City, U.S.A.

Loose activated charcoal (6-14 Mesh) andbrick activated charcoal

Fisher Scienti® cMontreal, Canada

8-Hydroxyquinoline BDH ChemicalsToronto, Canada

Oxalic acid and iminodiacetic acid FlukaNew York, U.S.A.

scribed, and the reader should refer to this publication fordetails on the techniques used to machine the cups.18 Fig-ure 1 shows the two types of probes used for this inves-tigation. The ® rst probe is termed a regular cup and istypical of the DSI cups that are used in our laboratory.This cup is normally used to hold a small volume ofliquid sample which, after drying, can then be insertedinto the plasma for atomic analysis atomic emission spec-trometry (AES) or mass spectrometry (MS). For this in-vestigation, the cup is used to hold several particles ofactivated charcoal.

The second probe in Fig. 1 is a variant of the standardcup. The stem was machined so that a cup of activatedcharcoal could be placed on the stem. A small cylinderof charcoal was cut from the block by using a piece of6.35 mm (1/4 in.) aluminum tubing mounted in a drillpress as a cutter. The tube has several teeth cut into theend, and each tooth was given a small set to provideclearance between the cylinder and the inside of the tubeduring cutting. The charcoal rod was then mounted ontoa lathe and machined to the required shape with regulartools. The dimensions of the two types of probes are giv-en in Table I.

The graphite cups and the activated charcoal werecleaned prior to use by inserting them into the plasma,(see Table II for sources of materials). About 60 s (at 1kW) was required to clean the graphite cups, but the ac-tivated charcoal required a more extensive cleaning. Asmentioned earlier, the activated charcoal retained manyminerals from the wood charcoal. The loose activatedcharcoal was cleaned by placing several particles into acleaned DSI cup and inserting it into the plasma (1.25kW for 2 min). The activated charcoal probes were alsocleaned at 1.25 kW for a period of 2 min. Activated char-coal adsorbs gases, so the material (both loose and thecap) had to be heated under the plasma to drive off theadsorbed gases; otherwise the plasma would extinguish.

Other researchers have used hydrochloric acid treat-ments to remove trace metals from activated carbon.6,19

A batch of charcoal particles was soaked in concentratedHCl overnight and the solution was decanted. This pro-cedure was repeated twice, and then the charcoal waswashed with Milli-Q water until the wash solutions wereat a pH of 7.

8-Hydroxyquinoline as a Chelator. 8-Hydroxyquin-oline (oxine) was the ® rst chelator investigated. There isample literature to indicate that oxine is an excellent che-lating reagent.20 Vanderborght and Van Grieken foundthat the adsorption of oxine onto activated charcoal wasindependent of pH in the 2 to 10 pH range.8

Oxine is only sparingly soluble in water (approx. 50mg/L) but is highly soluble in methanol. To circumventthe poor solubility in water, we investigated several ap-proaches. A methanolic solution (2.5 mL of 25 mg/mL)was added to 1 g of cleaned charcoal; an additional 8.2mL of methanol was added to facilitate mixing. The mix-ture was stirred for 15 min; then the ``doped’ ’ activatedcharcoal was removed and dried on a hot plate. To testthe ef® ciency of the doped charcoal, we added 65 mg ofthe doped and regular charcoal to 200 mL of water and200 mL of 30 ppb Cu solutions and left them for 24 h.The following day, the activated charcoal was removedfrom the solution and dried in a desiccator overnight. Ananalysis was performed by placing 10 to 20 mg of thecharcoal in a regular DSI cup and inserting it into a 1.25kW plasma. The grains were handled by using a vacuumline with an Eppendorf pipette tip on the end. Each par-ticle was dropped into the cup by pinching the vacuumline. The operating conditions are given in Table III. Thesolutions were also analyzed via ICP-MS (inductivelycoupled plasma mass spectrometry) after the charcoalwas removed.

The second method tested involved preparation of1500 ppm oxine in methanol and addition of 1 mL ofthis solution to some of the samples. Two separate setsof samples (30 mL each) were prepared: two blanks andtwo solutions containing 10 ppb of Pb, Cd, Zn, and Cu(one set with oxine and one without). The pH was notadjusted, and the activated charcoal portions of the DSIprobes were immersed in the solutions for 24 h.

The HCl acid-cleaned charcoal was also tested for its

APPLIED SPECTROSCOPY 193

TABLE III. Operating conditions for the plasma.

Drying stepPlasma forward power 50 W

Drying time 1 minAnalysis

Plasma forward powerRe¯ ected power

1.5 kW

, 5 WPlasma gasAuxiliary gas

14 L/min1.6 L/min

Insertion depthInsertion time

0 mm ATOLCa

15 sViewing height 15 mm ATOLCInsertion speed

Regular cupsActivated charcoal cups

240 mm/s6 mm/s

a Above top of load coil.

preconcentration capacity with 8-hydroxyquinoline. Twoblanks were prepared, one with 200 mL of Milli-Q waterand the second with 200 mL of 1 ppm oxine. Two 30ppb Cu standards were also prepared, the ® rst in Milli-Qwater the second in 1 ppm oxine. To each of these so-lutions approximately 80 mg of the acid-cleaned activat-ed charcoal was added and left overnight. The followingday, the solutions were analyzed as well as the charcoal.The pH levels of 3 and 10 were investigated.

In the previous sets of experiments, loose charcoal wasimportant in our investigation because of its relative easeof preparation and introduction into the sample solution.Unfortunately, the removal, handling, and analysis of thecharcoal after chelation and adsorption were problematic,as will be discussed later. For the remainder of the ex-periments, only activated charcoal probes and regulargraphite cups were considered.

Iminodiacetic Acid as a Chelator. Iminodiacetic acidis the chelating functional group that is used in Chelex-100, a popular stationary phase for metal preconcentra-tion. It forms strong complexes with most transition met-als but not with alkali and alkali earth metals. A 10 ppmsolution of iminodiacetic acid was prepared. Four solu-tions were prepared: a blank of 33 mL of Milli-Q water;30 mL of Milli-Q water plus 3 mL of the 10 ppm imi-nodiacetic acid; 30 mL of 30 ppb Pb, Cd, Zn, and Cuplus 3 mL of Milli-Q waters and 30 mL of 30 ppb Cuplus 3 mL of the iminodiacetic acid. The pH was adjustedto 8, and the heads of the activated probes were immersedin the solutions overnight, removed, dried inductively (byplacing them in the load coil and applying low power),and inserted into the plasma.

Oxalic Acid as a Chelator. Oxalic acid was investi-gated because it forms chelates with many metals. It alsohas the additional bene® t of a low sublimation tempera-ture (157 8 C) and readily decomposes. These propertiesfacilitate the removal of the organics from the cup andmay avoid the problem of organic residues disturbing theplasma.

The rate of adsorption onto the charcoal was deter-mined by placing the activated charcoal probes into 30mL of 30 ppb Cu solutions to which 3 mL of the oxalicacid solution had been added. The cups were left in thesolution for 0.5, 1, 2, and 24 h. The cups were then re-moved, dried, and analyzed; the operating conditions arelisted in Table III.

The ef® ciency of the oxalic acid preconcentration on

activated charcoal probes was determined by comparingthe signals from 30 ppb metals solution with and withoutoxalic acid. The solution was prepared by immersing theactivated charcoal of the probe in a solution containing3 mL of 9.5 ppm oxalic acid solution and 30 mL of 30ppb Pb, Cd, Zn, and Cu solution. A blank was preparedby adding 3 mL of water to 30 mL of the metals solution.The pH was adjusted to 9 and the cups were left in thesolutions for 2 h with stirring.

Comparison of Graphite Cups and Activated Char-coal Probes. The adsorption capabilities of the activatedcharcoal probes relative to the regular graphite cups werefound by placing the two types of probes in the followingsolutions: 33 mL of distilled water, 33 mL of 10 ppb Cusolution, and 30 mL of 10 ppb Cu with 3 mL of 10 ppmoxalic acid solution. The cups were left in the solutionsfor 2 h.

RESULTS AND DISC USSION

Cleaning the Charcoal. Many researchers have usedhydrochloric acid to clean activated carbon prior to tracemetal determinations. When charcoal cleaned in thismanner was inserted into the plasma, large Cu signalswere observed indicating that acid washing of charcoalis insuf® cient to remove metals that would vaporize inthe plasma. Perhaps these metals are removed from thesmaller particles of activated carbon that are usually usedfor adsorption. Figure 2 illustrates the extent of the Cucontamination on acid-washed activated charcoal. Threetraces are shown: the solid lines are from three successiveinsertions of the acid-washed charcoal into the plasma at1.25 kW. For comparison, the dashed trace is from plas-ma-cleaned activated charcoal that has adsorbed approx-imately 16 ng of Cu. There is no Cu signal observedwhen the plasma-cleaned charcoal is inserted into theplasma. An approximate equivalent concentration of 1.3ppm of Cu is left in the charcoal after the acid cleaningwhen the data from the Cu adsorbed onto the plasma-cleaned charcoal are used for calibration and linearity isassumed. The large signal from the acid-washed charcoalin comparison with the signal that was observed for atypical preconcentration precludes the direct use of theacid-washed charcoal. The plasma-cleaned charcoal wascompared with the acid-cleaned charcoal for its precon-centration capacity with the use of 30 ppb Cu standardsand oxine. The plasma-cleaned charcoal adsorbed aboutone third the amount of the acid-cleaned charcoal.

Experiments with 8-Hydroxyquinoline. Particles ofthe undoped activated charcoal were placed into a regularDSI cup and inserted into the plasma without incident,but the doped charcoal samples were more problematic.When the cup was inserted, the plasma contracted, andthere was a large green carbon emission as the re¯ ectedpower brie¯ y rose to about 125 W. During this time, thebackground changed rapidly. Figure 3 shows the back-ground signals vs. time at the four wavelengths that wereinstalled on the spectrometer at the time of the experi-ments. The ® rst four traces show that the change is dueto a broadband shift in the background that is not seenwith the undoped cups (® fth trace). Large changes in theplasma conditions are undesirable because they may af-fect the excitation conditions and the reproducibility of

194 Volume 53, Number 2, 1999

FIG. 2. Cleaning of activated charcoal. The solid traces are from acid-washed activated charcoal. The dashed trace is the signal observed from 16ng of Cu preconcentrated onto the same mass of charcoal.

FIG. 3. Background emissions from doped activated charcoal particles. The activated charcoal was inserted into the plasma at 1.7 s. The signalsare from the wavelength that is used for background correction of the analyte emission. The top four traces are from doped charcoal (oxine); thebottom trace is from charcoal that was not doped.

the method. Additionally, large background changes ne-cessitate the use of high-speed background correction toaccurately determine the signal intensity. The high or-ganic load on the doped charcoal probe is the probablecause for the large shift in the background. The problemsobserved with the doped charcoal in the plasma are sim-ilar to those observed by Rattray and Salin and are con-sistent with a high organic load.5

The loose charcoal was doped to avoid dif® culties withthe poor solubility of the oxine and to simplify the ana-lytical procedure. Additionally, having the chelatingagent directly bound to the probe is very appealing sinceit eliminates the problem of chelate precipitation in thesample solution. Loading the charcoal with oxine wasthought to be necessary so that the Cu would be able to® nd sites on the charcoal where binding with the oxinewould be possible as well as to maximize the speed ofchelation. However, with all the problems that have been

observed with this method, it seems that doping the char-coal does not provide any advantage.

The charcoal particles used in this series of experi-ments had a large range of sizes, which raised concernsabout signal normalization and mass biasing. Mass biascan occur when the analytical signals are normalized byusing the mass of the charcoal, but the physical processof adsorption is surface-area dependent. As an example,consider the case of a sphere. The area is 4 p r 2, whereasthe mass is 4/3 r p r3 (r is the radius and r is the density).It can be seen that any signal that is normalized with themass will still retain a dependence on the radius whichvaries with the mass. Activated charcoal is unlike asphere in that it exhibits an extended surface so that thesurface area is proportional to the mass of the particle.As a veri® cation of this assumption, each particle of adoped charcoal sample (12 particles) was analyzed, andsignal vs. mass was plotted. There was a proportional

APPLIED SPECTROSCOPY 195

TABLE IV. Test of the effectiveness of the three chelators on ac-tivated charcoal probes. The peak area signals have been scaled tothe largest signal. Exposure time was 2 h.

Chelator Pb Cd Zn Cu

OxineIDACOxalic acid

10.780.45

10.080.05

10.120.09

10.750.97

FIG. 4. Adsorption vs. time for Cu/oxalic acid complex on activatedcharcoal probes. (The individual measurements are denoted with 1 andthe average with M).

TABLE V. Detection limits in ppb for the three chelators tested.

Chelator Pb Cd Zn Cu

OxineIDACOxalic acidLiquid neubulization

0.50.50.4

10

0.040.30.30.3

0.030.10.20.7

0.050.090.070.7

relationship between the signal magnitude and the mass.All signals for the loose activated charcoal experimentshave been normalized by using the mass of the particlesin the cup ( ø 10±15 mg).

Both the loose activated charcoal and the sample so-lutions were analyzed after the charcoal (doped and reg-ular) had been left in contact with the Cu solution for 24h. The solutions were analyzed by ICP-MS with conven-tional liquid nebulization. There was no Cu detectable inthe Milli-Q water used to prepare all of the solutions.The water that was in contact with the regular activatedcharcoal had no detectable Cu, but the water in contactwith the doped charcoal contained 1.8 ppb. The 30 ppbCu solutions were found to have 5.6 ppb and 25.0 ppbfor the regular and doped charcoal, respectively. The highconcentration of the solution from the doped charcoalwas surprising and was veri® ed via the technique of stan-dard additions.

These results are contrary to what was expected, sincethe adsorption of Cu directly from the solution by theactivated charcoal should be less than that of the dopedcharcoal. It may be that the doping is responsible for theloss of sorption capacity of the charcoal. When largequantities of 8-hydroxyquinoline are used, the entire sur-face of the charcoal may become coated and prevent theadsorption of additional molecules, whereas the undopedcharcoal has a ``clean’ ’ surface onto which the Cu mayadsorb. Cu is known to adsorb directly onto activatedcharcoal.21 The detection limit for Cu, with the use of thedoped charcoal, was found to be 1 ppb by using threetimes the noise of the background divided by the sensi-tivity obtained with the chelator.

When the oxine was used with the DSI probes that hadan activated charcoal cap, improvements in the recoverywere observed for three of the four metals studied. ForCu, Zn, and Cd the increase in peak area was 4.5, 82,and 94, respectively. The only element that did not showany signi® cant improvement was Pb (1.2 times larger).The addition of oxine to the sample solution also im-proves the reproducibly considerably; the average %RSD (relative standard deviation) went from 110 to 18%(N 5 3). Signi® cant reductions in the noise were alsoobserved for the oxine blank compared to the water blank(71 to 25%). The high variability in the absence of oxineis probably due to the differences in the surface characterof the activated charcoal on the probes. The noise is re-duced in the presence of oxine, because the metal com-plexes are capable of adsorbing onto a wider range ofsites on the activated charcoal than the metal alone.

Experiments with IDAC . The use of iminodiaceticacid as a chelating compound on the loose charcoal forCu alone did not increase the signal signi® cantly. WhenIDAC was tested with the activated charcoal probes, thesignals were signi® cantly lower than those obtained with

oxine (Table IV). However, the presence of the IDAC didreduce the noise in the case of the activated charcoalprobes. The noise went from 20 to 9.4% (N 5 2).

Experiments with Oxalic Acid. Oxalic acid was test-ed only with the activated charcoal probes. Table IVshows that it was the least effective chelator studied.

The adsorption rate of the metal chelate was deter-mined by placing the probes in 30 ppb Cu solutions fordifferent lengths of time. Figure 4 shows that the adsorp-tion of the oxalic acid complex proceeds logarithmically.Long adsorption times are not uncommon for complexesfrom solution.22

A calibration curve was determined with oxalic acidin the concentration range of 0 to 30 ppb of Cu. Thecalibration curve is log linear over the concentrationrange studied.

The comparison between the activated charcoal probesand the graphite cups showed that the Cu signal was ap-proximately 25 times larger from the activated charcoalthan the signal from the graphite cup, even though themass of the cup is about 10 times greater. This result isprobably due to the smaller surface area of the graphite.

Detection Limits. The detection limits with the use ofthe activated charcoal caps were estimated by takingthree times the standard deviation of the background sig-nal divided by the sensitivity obtained from the 30 ppbsolution with the chelating agent. This approach assumesthat the calibration curve is linear to 30 ppb and that thenoise near the detection limit is primarily due to the back-ground. From the oxalic acid calibration curve, we knowthat the calibration curve is actually logarithmic, so thatthe detection limits are a conservative estimate. The de-tection limits are reported in Table V.

Aging of the Activated Charcoal Probes. During thecourse of the experiments with the activated charcoalprobes, it appeared that the observed signal depended on

196 Volume 53, Number 2, 1999

the number of times that the probe was inserted into theplasma. This observation was investigated by labelingnew probes and following the signal that was obtainedover the course of several preconcentration experiments.This experiment took place over the course of severaldays. The signals were normalized to the signal obtainedfrom a sample of a reference solution deposited in agraphite cup. No signi® cant change was observed withPb and Cu from the ® rst insertion to the fourth. Therewas a signi® cant decrease in the signal that was observedfrom Zn and Cd. The Zn signal decreased by over anorder of magnitude, and the Cd signal decreased by near-ly a factor of ® ve. The noise was similar for all the probesand did not change signi® cantly throughout the experi-ment. The charcoal portions of the probes were examinedwith both optical and electron microscopes. The only per-ceptible difference in the gross structure between the rawcharcoal and the probes that had been in the plasma sev-eral times was the appearance of cracks in the charcoal.The small tubes (artifacts of the vessels from the wood)did not appear to change in size or character. Small piecesof material, presumably pyrolysis products from the ox-ine, accumulated on the walls of the tubes. These parti-cles were around 1±5 m m in diameter. On the basis ofthis evidence, we concluded that the change in signal asa function of age in the plasma is likely due to changesin the micropore volume and the fraction of the total areathat the micropores occupy within the charcoal. Changesof this type have been observed with charcoal subjectedto elevated temperatures in inert atmospheres.23

Recovery. The recovery of Cu with the use of theactivated charcoal caps is incomplete. The recovery wasestimated by comparing the signal from aqueous standarddeposited on the activated charcoal cap to the signal ob-tained from the 30 ppb sample solution after 2 h of ex-posure. We found that the recovery was approximately2±3% of the total Cu in the solution. If all the chelatingcompounds have rates of adsorption similar to those ofoxalic acid, then the maximum amount will be nearly 5%after 24 h of exposure. This result is in sharp contrast tothe 80 to 100% recovery found with activated carbon/charcoal directly and in combination with chelatingagents.6,8,9,13,24±26 In most of these studies, the amount ofactivated carbon is much larger (50 to 500 mg), but thelarger amount of activated carbon is probably not re-sponsible for the increased recovery. The ratio of themass of the metal to activated carbon in those studiesranged from 0.72 to 12 m g/mg; in this study the ratio was0.72 m g/mg. The poor recovery is probably due to thedeactivation of the charcoal after exposure to the plasmaduring the cleaning and analysis procedures. It may bepossible to increase the recovery by reactivating the char-

coal after cleaning. The charcoal can, in theory, be re-activated by heating the charcoal in the presence of airor steam and carbon dioxide.

CONCLUSION

Plasma cleaning of activated charcoal is effective inremoving trace metal contaminants left in the material,but exposure to the plasma appears to adversely affectthe adsorption capacity of the charcoal. The activatedcharcoal probes appear to be more promising than theloose charcoal because they are easier to manipulate andcan be machined reproducibly, so that normalization ofthe signals to the mass of charcoal is not necessary. Metalchelates are adsorbed onto the charcoal, but the recoveryis poor. Doping the charcoal prior to exposure to the sam-ple solution is not advantageous. Of the chelating re-agents studied, 8-hydroxyquinoline appears to be themost promising.

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