Micro and Trace Analysis by a Combination of Ion ... - ACS Publications

Micro and Trace Analysis by a Combination of Ion Exchange Resin-Loaded Papers and X-Ray Spectrography WILLIAM J. CAMPBELL, ERNEST F. SPANO, and THOMAS E. GREEN

Downloaded by UNIV OF NEBRASKA-LINCOLN on September 14, 2015 | http://pubs.acs.org Publication Date: July 1, 1966 | doi: 10.1021/ac60240a013

U. S. Department o f the interior, Bureau of Mines, College Park Metallurgy Research Center, College Park, Md.

b Ion exchange resin-loaded paper disks were found to be an excellent media for collecting microgram quantities of cations and anions from sohtions by a simple filtering process and for presenting the collected ions to a fluorescent X-ray spectrograph. The high X-ray sensitivity, expressed as counts per second per microgram, and low magnitude of matrix effects make the combination of ion exchange collection and X-ray spectrographic determination a preferred procedure for trace analysis. Chemical and X-ray characteristics of Reeve-Angel cation and anion exchange resin-loaded paper disks were investigated. Chemical characteristics include exchange capacity, effects of pH, salt concentration, and competing ions, and distribution of collected ions. X-ray characteristics include X-ray transmission coefficient as a function of wavelength, relationship of X-ray intensity to quantity, atomic number, and distribution of collected ions, reproducibility of intensity measurements, and limits of detection.

A

PPLICATIONS

OF

ION

EXCHANGE

resin-loaded papers in conjunction with X-ray spectrography were recently developed by the authors to provide analytical methods (9, 10) required for the Bureau of Mines high purity metals program. Because of the potential application of the same technique to other analytical problems, a more detailed investigation of the chemical and X-ray spectrographic characteristics of these ion exchange resin-loaded papers was undertaken. Although X-ray spectrography is generally limited to the determination of minor and major constituents, X-ray analysis can be extended to the trace range by preconcentrating the elements t o be determined. Several techniques, such as precipitation, electrodeposition, and ion exchange, have been employed for the collection of microgram quantities of elements and their presentation to the X-ray spectrograph. Of these preconcentration techniques, the use of ion exchange membranes and ion ex-

fie Vial Owith cut f f bollom

Screw far rolaling holder in speclograph

poper disk

Polyelhylene cop. w i t h 3 cm hole. sealed Io funnel

w i t h 3 c m hole cop

@ ePln,;",e'

i!i

SAUPLE HOLDER

FILTERING APPARATUS

Figure 1

Filtering apparatus and sample holder

change resin-loaded papers offers the best potential as a general analytical method (1-3). Luke (5) reported on the use of '/s-inCh diameter ion exchange membrane disks for the collection of cations and anions. The disks were used in conjunction with a focusing X-ray spectrograph to achieve high sensitivity for microgram and submicrogram quantities of the elements. However, these small membrane disks have a very limited exchange capacity, S O fig., and require several hours to reach equilibrium with the solution being analyzed. Cation and anion exchange resinloaded papers, available from ReeveAngel, have several advantages over the membranes used by Luke (5) and others (1). The ions are collected on the paper disks by a simple filtering technique. Ions, a t submicrogram to milligram concentration, can be collected in a few minutes because the filtering process produces intimate contact between the solution and the resin particle. Dispersion of the resin through the paper also produces a more effective surface area per unit weight of resin than is available with resin-loaded

membranes, thereby increasing the effective capacity of the disk. The exchange capacity of the 3.5-cm. diameter disk is approximately one hundred times that of the l/s-inch diameter disk used by Luke. Although there have been several publications on the application of ion exchange resin-loaded papers (6, 9, 10) a need existed for a critical evaluation of the chemical and X-ray spectrographic characteristics of these papers. These Characteristics are the subject of this report. Chemical parameters investigated include exchange capacity, effects of pH, salt concentration, and competing ions, and distribution of collected ions. X-ray characteristics include X-ray transmission coefficient as a function of wavelength, relationship of X-ray intensity to quantity, atomic number, and distribution of collected ions, reproducibility of intensity measurements, and limits of detection. The results of these investigations show that the combination of ion exchange collection and X-ray spectrographic determination provide a general analytical method for micro- and trace analysis. The microgram amounts collected may represent major and minor VOL. 38,

NO. 8, JULY 1966

987

constituents in micro samples or trace constituents in macro samples. For the latter it is usually necessary to make a preliminary separation of the trace constituents from the matrix material.


EXPERIMENTAL

Apparatus and Reagents. X-ray spectrographic instrumentation used in this investigation consisted of a Philips Electronics 60-kv. 50-ma. fullwave rectified power supply with an FA-60 tungsten target tube, an inverted three-position sample holder, 0.005- by 4-inch or 0.02- by 4-inch parallel-plate collimator, P E T (pentaerythritol) or LiF crystal, scintillation counter or flow-proportional counterboth detectors used with pulse amplitude discrimination. The sample holder and apparatus used for filtering solutions through the disks are shown in Figure 1. The Plexiglas holders had to be replaced periodically because of embrittlement resulting from exposure to high energy X-rays. Properties of Reeve-Angel cation (SA-2) and anion (SB-2) exchange resin-loaded papers were emphasized in these investigations. The weak acid (WA-2) and weak base (WB-2) papers were considered qualitatively but not investigated extensively. Calcium impurities in the SA-2 disks made it necessary to clean the disks before they were used for calcium collection and determination. Calcium was removed by placing five to ten disks, which had been soaked in water for a few minutes, in the filtering apparatus and filtering a 40-ml. portion of 6N hydrochloric acid through the disks five times. A second 40-ml. portion of 6N hydrochloric acid was then filtered through the disks and the disks were washed free of acid with water. The resin in the disks was converted to the sodium form by filtering a 40-ml. portion of saturated sodium chloride solution through the disks five to seven times. Treatment with fresh portions of sodium chloride solution was repeated until the pH of the sodium chloride solution did not change on filtering. The disks were washed free of sodium chloride with water and dried. SB-2 paper disks were converted from the chloride form to the hydroxide form by filtering several 40-ml. portions of 5% sodium hydroxide solution through the disks, washing the disks with distilled water, and then drying them. All chemicals used were reagent grade. Demineralized-distilled water, used throughout the procedure, was prepared by single stage distillation followed by passage through a demineralizer. Procedure. Standard solutions of cations were prepared by dissolving weighed portions of high purity metals, oxides, or salts in a minimum amount of acid and diluting to known volumes. Standard solutions of anions were prepared by dissolving weighed portions of the appropriate ammonium, sodium, or potassium salt in water and diluting to volume. Working solutions 988

m

ANALYTICAL CHEMISTRY

containing the cations or anions at concentrations of 2 to 100 pg. per ml. were prepared by aliquoting and diluting the standard solutions. Cation test solutions were prepared by diluting aliquots of the cation working solutions to the desired volume and adjusting to the desired pH by addition of sulfuric, nitric, or hydrochloric acid. Anion test solutions were prepared by diluting aliquots of the anion working solutions to known volumes. No p H adjustments were made. The following procedure was used to collect the cations or anions on the appropriate ion exchange resin-loaded paper disk. The disk, which had been soaked in water for a few minutes, was inserted in the filtering apparatus shown in Figure 1, and the test solution was filtered through the disk seven times. The disk was then removed, afresh disk inserted in the filtering apparatus, and the filtrations were repeated. The second disk was used to check for completeness of the exchange. The disks were dried slowly under a heat lamp to prevent curling. Disks were protected from contamination by wrapping them in filter paper before storing in paper envelopes. The elements collected on each disk were determined by means of the X-ray spectrograph. Intensities of the characteristic line of each element being determined were measured on both sides of each disk, the intensities were then averaged a n d corrected for background. Background counting rates were determined a t the same Bragg angle as the characteristic line, using pre-cleaned disks as the X-ray scatterer. CHEMICAL CHARACTERISTICS OF CATION EXCHANGE RESIN-LOADED PAPERS

Two Reeve-Angel cation exchange resin-loaded papers, designated as SA-2 and WA-2, were considered for detailed evaluation. Both papers consist of an ion exchange resin dispersed in alpha cellulose. When dry, they are rigid enough to remain flat in a plastic sample holder. The following description is given by the manufacturer. SA-2 Amberlite resin Resin type Approximate -~ weight per cent resin Resin form Approximate exchange capacity, meq./gram Functional pH range

WA-2

IR-120

IRC-50

45-50

45-50

1.9-2.0

4.6-5.0

1-14

5-14

Strong acid Weak acid Na+

H'

The ability of the SA-2 paper to function in the lower pH range is an advantage, particularly when working with metallic ions that are subject to hydrolysis. Initial tests of both the SA-2 and WA-2 paper indicated that the SA-2 paper was more efficient for determining trace quantities of the more common cations and had better

physical strength. SA-2 paper was therefore used in the present investigation on general properties of cation exchange resin-loaded papers. The greater exchange capacity and the stronger affinity of the WA-2 paper for specific ions could make WA-2 paper preferable for use in special applications. Chemical analysis by the Bureau of Mines of the untreated SA-2 papers gives the following composition (in weight per cent) : hydrogen-5.7, carbon-41.9, sulfur-5.8, sodium4.2 (calculated on the basis of one sodium atom per sulfur atom), and oxygen-42.4 by difference. The manufacturer's stated capacity of 1.9 to 2.0 meq. per gram of paper agrees with the calculated capacity of 1.8 meq. per gram based on a sodium content of 4.2 weight per cent. The 3.5-cm. diameter disks, approximately 0.03 cm. in thickness, ranged in weight from 0.11 to 0.14 gram, with two boxes containing disks weighing approximately 0.09 gram. In general, the disks of a given lot did not vary more than 0.005 gram. Except when the ultimate accuracy is required, variation of +0.01 gram from a nominal weight of 0.125 gram is acceptable. Based on a weight of 0.125 gram, the exchange capacity is approximately 0.23 meq. per disk. The usable exchange capacity is approximately 0.20 nieq. per disk, allowing a correction for the outer edge of the disk which is held in the filtering apparatus and thus not available for exchange. This calculated capacity, which was experimentally confirmed, corresponds to several thousand micrograms of exchanged ione.g. approximately 4,000 pg. of Fe+3 or 14,000 fig. of Ba+2. Effect of pH. The recommended filtration procedure was applied to a series of 40-ml. solutions containing 100 pg. each of Al+3, Ca+2, Cr+3, Mg+2, and Mn+2, 50 pg. each of C U + ~ , Fe+3, and Zn+2and 40 pg. of C O + and ~ Nif2 a t various pH levels. The per cent of each ion collected on the first disk was calculated by multiplying the X-ray intensity of the first disk by 100 and dividing by the sum of the X-ray intensities of the first and second disks. All intensities were corrected for background. KO correction for nonlinearity in the intensity to microgram relationship was required for these low concentrations. On the basis of the results given in Table I, pH 2 was selected as the optimum pH for collection of cations, Number of Filtrations Required. I n order to achieve maximum exchange, the filtration process must be repeated until equilibrium between the disk and the solution is established. The quantity of the ion collected during a single pass through the paper depends on the quantity of the ion in the solu-


tion, the number of unfilled exchange sites in the disk, and the affinity of the ion for the resin in the disk. Mechanical factors such as retention of the solution on the disk, on the walls of the container from which it is filtered, and on the stem of the filtering apparatus will also have an influence on the number of filtrations required. In order to establish a standard number of filtrations to be used in each test, solutions containing 500 pg. of Ba+2 were diluted to 40 ml., adjusted to pH 2, and filtered through disks the number of times indicated in Table 11. The depleted filtrates were then filtered seven times through a second disk to determine the amount of Ba+2 remaining in the solution. Similar tests were performed on 40-ml. solutions containing Cs+, C O + ~ and Cd+2. Only about 60% of the Cs+ ion was collected after equilibrium was reached. These results indicate that equilibrium is reached after five to seven filtrations. Seven filtrations were used in all subsequent tests unless specified otherwise. P e r Cent Exchange. The recommended procedure was applied to 40nil. solutions containing individual cations or groups of cations a t concentrations of 100 to 1000 pg. per 40 ml. All solutions were adjusted to pH 2, and then filtered through two disks sequentially. Except where noted, the cations were in HC1 solution. The per cent of each ion collected by the first disk is given in Table 111. Effect of Filtration Rate. If the filtration rate is not controlled, approximately 18 seconds is required for a 40-ml. solution to pass through a disk held in the filtering apparatus shown in Figure 1. In order to determine whether a slower filtering rate would increase the per cent retention of an ion of low affinity, 40-ml. solutions containing 500 pg. of Ag+ were adjusted to pH 2 and filtered seven times a t different flow rates. The rate of flow was controlled by a screw clamp on a short piece of rubber tubing attached to the stem of the filtering apparatus. Each filtrate was then passed seven times through a second disk a t the normal flow rate. In each of five tests, in which the average filtering time was 18, 29, 48, 110, and 535 seconds, respectively, between 80 and 82y0 of the silver was collected on the first disk. Thus it is concluded that the filtering rate does not have any significant effect on the quantity of a cation retained. Slower filtering rates did, however, result in a greater concentration of the silver on the side of the paper which had faced upward during the filtration thus affecting the observed X-ray intensities when long wavelength X-ray lines are used. The effect of distribution on observed intensity is discussed later in this paper.

~~

Per Cent of Cations Collected on SA-2 Paper Disk as a Function of pH

Table 1.

Per cent collected Cu Fe Mg ... 88 98 .,. 1.5 ... 97 >99 .., 2.0 98" >99 >99 >99b 3.0 ... >99 97 ... 5 . 0 ______________________________________ precipitate formed Vacuum X-ray spectrographic determination. b By atomic absorption spectrophotometric determination of Mg in the filtrate. pH 1.0

Ca 94 >99 >99 >99

A1

Cr 96 >99 98 95

Co 86 98 >99 97

0

Table II.

Variation of Cation Exchanged with Number of Filtrations

No. of filtrations

Per cent collected on first disk c o +2 Cd +a 71 66 ._ 97 91 99 98 >99 >99 >99

Ba+a 68 97 >99 >99 >99

1

3 5

7 20

Table 111.

a

0

100

200

300

400

500

SOLUTION VOLUME, m l

Figure 2. Effect of solution volume on per cent cation retained

Effect of Solution Volume. In previous applications of the combined ion exchange paper-X-ray spectrographic technique (.9, IO), 40 ml. was arbitrarily selected as a convenient working volume. Preconcentration procedures based on ion exchange column separation or solvent extraction were designed to collect the ions being determined in 50-ml. beakers. Smaller volumes can be used. However, transfers of solutions, p H adjustments, and other operations become awkward. Since the per cent of an ion exchanged varies with volume, this relationship was investigated. The equilibrium distribution coefficient, Kd, for exchange of an ion, B, between a solution and an ion exchange resin (7) is given by the equation

where b, is the quantity of ion B collected by the resin, b, is the quantity of ion B remaining in solution, w is the exvolume of the solution-usually pressed in weight, and g is the weight of

62 62 62 59

Per Cent of Cations Collected on SA-2 Paper

Per cent exchanged 62 83 98

Cation

cs

c s+

+

Ag+

c [Alf3, Cr+3, Ti+'] Ba+2, Bi+3 aCd+2, aLa+3, Pb+l, Srlz, U O Z + >o~ Y + ~ >99 Fe+', [Ca+2, Co+2, Cu Mn+2, Ni+2, Zn+z( >99 a In nitric acid. In sulfuric acid. e Ions collected simultaneously from one solution.

resin. If the total quantity of B in the system is br, then b, - b, can be substituted for b,. Equation 1 can then be rearranged in the form

where b, bt

-

per cent exchanged 100

=-

This equation states that for ions of large Kd, change in volume will have little effect on the per cent of the ion exchanged. For ions of low Kd, change in per cent exchanged with change in volume will be significant. To measure the effect of changes in volume on per cent exchanged, solutions containing 500 fig. of Ag+ or 100 pg. of Cs+ (both have a low Kd) were diluted to various volumes, adjusted to pH 2, and filtered seven times through one disk. Solutions containing 500 fig, of Sr+* (high Kd) were treated in the same way. In these tests, the per cent of an ion collected on each disk was calculated as 100 times the X-ray intensity from the disk divided by the sum of the intensities of a series of disks on which the ion had VOL. 38, NO.

a,

JULY 1966

989

loo,

Table IV.

,

-9

Effect of Alkali Salt Concentration on the Retention of Strontium

Milliequivalents of alkali ion in 40 ml.

Li + >99. >99. 99. 98. 96. 95.

0.0 0.5 1.o

1.5 2.0 2.5

Per cent of strontium retained on first disk Na + K+ >99. >99. 98. 99. 97. 96. 94. 94. 91. 90. 89. 87.

NH4+ >99. 98. 96. 94. 92. 89. 20

Table V. Amount of Ca+2, Sr+21 and Ba+2 Collected on Successive Disks

Milliequivalent collected 2nd 3rd 4th Disk Disk Disk Disk 0.036 0.056 0.080 0.028 Ca Sr 0.054 0.066 0.066 0.014 Ba 0.102 0.070 0.028 0.002 Total 0.192 0.192 0.174 0.044


1st

Table VI.

Anion Br -

I103-

504-2

vas-

Cr20,-2 Cr04-2 Mn04hf004-2 Fe( CN)a-' PtCle-2 HP04-'

Collection of Anions by SB-2 Paper

Resin in C1- form 97 88

>99

71

>99 98 >99 >99 >99 >99 >99 99 76

Resin in OH- form >99 98 >99 90 >99 >99 >99 >99 >99 >99 >99 >99 99

been collected from a small volume. The results given in Figure 2 illustrate that there is a strong volume dependence for ions of low Kd; for ions of high Kd the per cent exchange is essentially independent of volume. The curves in Figure 2 are theoretical curves, based on Equation 2, normalized through the experimental value for a volume of 50 ml. Effect of Alkali Salt Concentration. All cations in the solution compete for the available exchange sites in the disk. Therefore, the presence of a high concentration of an alkali salt would be expected to have an adverse effect on the collection of desired cations. The extent of this competition depends on the concentration of the alkali salt and on the Kd values of both the alkali ion and the cation being determined. The effect of increasing concentrations of alkali and ammonium salts on the recovery of strontium, a high Kd ion, was determined by applying the recommended filtration procedure to solutions containing 1000 pg. of strontium and up to 2.5 meq. of Li+, Na+, K+, or NH4+ ions. All these solutions were diluted to 40 ml. and adjusted to pH 2 before filtra990


tion. The results are given in Table IV. These data show that when the combined ion exchange-X-ray spectrographic procedure is used to determine small quantities of cations after their preliminary separation from a matrix material, the alkali salts introduced during the preliminary separation should be kept to a minimum. Also excess acids should be removed by evaporation, rather than neutralized by bases. If the introduction of alkali salts is unavoidable, lithium reagents should be used instead of sodium, potassium, or ammonium reagents (9). Combined Effect of Solution Volume and Alkali Salt Concentration. As shown in Figure 2, dilution of the sample has a greater effect on cations of low Kd value than on cations of high Kd value. Dilution of the sample should therefore decrease the adverse effect of alkali salts on the retention of cations of high Kd. This hypothesis was tested by applying the recommended procedure, but with volume as a variable, to solutions containing 1000 pg. of strontium and 2.5 meq. of sodium chloride. A similar series of tests was performed on a series of solutions containing 500 pg. each of barium, strontium, and cobalt and 5.0 meq. of sodium chloride. The results of these tests, presented graphically in Figure 3 show that, as predicted, dilution of the sample decreases the adverse effect of an alkali salt. Thus the analyst can reduce the competitive effect of alkali salts by using Li+ and working with large volumes. Selectivity among Competing Cations. The affinity of a cation exchange resin for cations of the same valence increases with increase in atomic number of the cations. The ion exchange resin-loaded disks should therefore be expected to show a selectivity among members of a series such as Ca+2, S+, Ba+2. To test this selectivity, a solution containing 0.2 meq. each of Caf2, Sr+2, and Ba+2 was diluted to 40 ml., adjusted to pH 2 and filtered seven times through each of a series of disks until the three cations were completely collected. The total concentration of the three cations was three times the exchange capacity of an individual disk. The quantity of each cation collected on each disk is given in Table V.

I 0

100

200 XO SOLUTION VOLUME, m l

400

800

Figure 3. Effect of solution volume and sodium concentration on per cent cation retained

The ratioof Ba+2:Sr+2:Ca+2 (2.8:1.5:1) collected on the first disk is in qualitative agreement with the ratios of their Kd values reported by Strelow (12). These results indicate that the disks show only a limited selectivity among cations of different affinity when the recommended filtration procedure is followed. CHEMICAL CHARACTERISTICS OF A N I O N EXCHANGE RESIN LOADED PAPERS

Initial tests were performed using two Reeve-Angel anion exchange resinloaded papers designated as SB-2 and WB-2. The following descriptions of the papers mere supplied by the manufacturer. SB-2 Amberlite resin in the paper Resin type Approximate per cent resin in the paper Resin form Approximate exchange capacity meq./gram Functional pH range

WB-2

IRA-400 IR-4B Strong base Weak base 45-50 C1-

45-50 OH -

1.5-1.6

4.0-4.2

0-12

0-9

The manufacturer of the Amberlite resins states that the most effective performance of the IR-4B resin is obtained a t a pH below 5.5. Because of its greater pH range, better physical strength when wet, and more efficient collection of most anions, the SB-2 paper was selected for all tests on anion collection discussed in this report. The 1.5 to 1.6 meq. per gram exchange capacity stated by the manufacturer corresponds to an exchange capacity of 0.17 to 0.19 meq. per 0.11 to 0.12 gram disk. This calculated value agrees with the experimentally determined value of 0.15 meq. per disk if an allowance is made for the loss of exchange capacity for that portion of the disk held in the filtering apparatus.

io,ooo

OF ION EXCHANGE LX-RAY CHARACTERISTICS RESIN-LOADED PAPERS

I


I

3

I 5

7

DISK NUMBER

Figure 4. Distribution of consecutive disks

S f i 2 on

The order of selectivity of IRA-400 anion exchange resin is reported to be citrate > sulfate > oxalate > iodide > nitrate > chromate > bromide > thiocyanate > chloride > formate > hydroxide > acetate (7). Anions ahead of chloride in this series will replace chloride in the resin. Also the hydroxide ion is more readily replaced than the chloride ion, therefore it is sometimes advantageous to convert the resin to the hydroxide form. The collection of each of 13 anions on both the chloride and hydroxide forms of SB-2 paper was investigated by filtering 40-ml. solutions, each containing 500 pg. of the anion to be determined, through a disk seven times and then seven times through a second disk. When hydrolyzable cations such as Al+3 and Fe+3 were present, the pH was adjusted to approximately 3.5 with acetic acid. The per cent of each anion was calculated as 100 times the average X-ray intensity measured on the first disk divided by the sum of the X-ray intensities measured on the first and second disks. The results are given in Table VI. The greater exchange of bromate, iodate, and phosphate anion with the resin in the hydroxide form as compared t o the chloride form is attributed to the position of these anions in order of selectivity of the IRA-400 resin. The Kd values for these anions are significantly higher for replaceable hydroxide ions as compared to replaceable chloride ions. The effects of solution volume, competing ions, and other variables were not investigated for the SB-2 paper. By analogy these effects are assumed to be similar to the effects of corresponding variables on the collection of cations as described for the SA-2 paper. Applications of SB-2 paper were described by Minns (6).

I n the chemical section of this report it was established that microgram quantities of various ions can be quantitatively collected by ion exchange resinloaded papers. The next step in developing analytical applications is to establish methods for relating measured X-ray intensity to the quantity of each ion collected by the paper. Parameters that affect the intensity-concentration relationship are the distribution of ions through the paper, absorption or enhancement of spectral lines by other ions-matrix effect, reproducibility of the procedure including the statistical counting error, and sensitivity of response in counts per second per microgram. Each of these parameters requires critical consideration. I o n Distribution and I t s Effect on X-Ray Intensity. Previous investigations ( 1 , .2) showed that exchanged ions were uniformly distributed on both sides of ion exchange membranes provided the membranes were completely immersed in a strongly agitated exchange media. When strong agitation was employed, both sides of the membrane gave equal X-ray intensities; however, the distribution of ions through the membranes was not investigated. The filtration procedure used with ion exchange resin-loaded papers results in a non-uniform distribution of exchanged ions with the highest concentration toward the top surface of the paper. Immersion of the papers into the solution which is being analyzed, coupled with strong agitation, was considered but rejected as it was found that the paper deteriorated rapidly upon agitation. Also the filtration procedure requires only a few minutes, compared to several hours for the immersionagitation procedure. Ion exchange resin-loaded paper is a physical mixture of alpha cellulose and ion exchange resin. There is approximately 50 per cent of each by weight. Optical microscopy and electron probe microanalyzer scans show that the resin particles range from approximately 10 to 40 microns in diameter with an average size of 25 to 30 microns. The distribution of the exchanged ions was investigated by electron probe microanalyzer examination of SA-2 papers through which various amounts of Fe+3had been filtered. The iron distribution was measured by two microprobe techniques-translation of the sample while maintaining the electron beam stationary and electron beam scanning over areas approximately 60 microns square. Both techniques show that the iron distribution is uniform throughout the resin particles. Intuitively one would expect to find a concentration gradient through the resin particles. However, Span and Ribaric ( 8 ) stated

1,000

0

Seven fillrotions

0 One filtration

I

2

3

4

5

D I S K NUMBER

Figure 5. Distribution of stacked disks

Ag+

on

that ‘(therate of ion exchange is mainly controlled by the diffusion in the particle if the concentration of the surrounding solution is higher than O . l M J by diffusion through the boundary film if the concentration is lower than O.OOlM, and by both processes otherwise.” With only a few exceptions all the applications considered by our laboratory are for concentrations below 0.00lM. Therefore diffusion through the boundary film is the rate controlling step. The observed uniform distribution of iron through the resin particle supports this conclusion. The distribution of the collected ions through the paper was investigated as follows. A solution containing 1500 pg. of Sr+2 was filtered once through a series of single precleaned SA-2 disks until the Sr+2 was completely exchanged. Seven disks were required. The intensity of the SrKa line was measured to minimize the effect of X-ray absorption in the disks, and all intensities were corrected for background. The quantity of Sr+* collected on successive disks was found to decrease exponentially as shown by the semilog plot of intensity per disk us. disk number in Figure 4. Similar results were obtained with Zn+2and Ag+. The next set of experiments was designed to determine if the same distribution would be obtained by filtration through a series of stacked disks, as opposed to filtration through individual disks sequentially. A solution of Cdf2 was filtered once through a series of five disks of SA-2 paper pressed tightly together in the filtering apparatus. The VOL. 38, NO. 8, JULY 1966

991

X-rays of interest. Therefore the transmission characteristics must be established before intensities can be used to evaluate distribution. Calculated and experimentally determined X-ray transmittances for SA-2 paper are shown in Figure 6. The theoretical curve was calculated using literature values for y, and the composiP


Figure 6. X-ray transmittances of SA-2 and SB-2 ion exchange papers

intensity of the CdKa line was measured on each disk and corrected for background. As expected, a single filtration through the stacked disks gave essentially the same exponential distribution found for filtration through individual disks. Similar results were found using a solution of Ag+. However, when the Cd+2 and Ag+ solutions were recycled seven times through the stacked disks, a marked change was observed in the Ag+ distribution as shown in Figure 5. No change was observed in the C d f 2 distribution. Thus Ag+ undergoes redistribution, whereas Cdf2 is strongly held by the resin particle in which first contact is made. The difference between the behavior of Ag+ and Cd+2is due to the lower Kd of Ag+ compared to that of Cd+2. For our proposed analytical applications, the distribution of the collected ions through each disk is of interest. If the distribution through each disk is exponential, there will be a large difference between the intensities measured on the two sides of the disk, particularly when long wavelength X-rays are used. The variation in intensity from side to side is a function of both ion distribution and transmission of the disk for the

Table VII. Theoretical Effect of Ion Distribution on X-Ray Intensity

Spectral I t / I b for line and exponential ‘Average I. wavelength distributiona Average I , SnKa 0.9A 1.04 1.00 ZnKa 1.4A 1.26 1 .oo CoKa 1.9A 1.60 1.01 VKa 2.5A 2.9 1.07 SnLa 3.6A 12. 1.33 MoLa5. 4A 155, 1.46 a Calculated ratio of intensities from top and bottom of the disk, assuming exponential distribution. * Ratio of calculated average intensity for exponential distribution to calculated average intensity for uniform distribution. 992


tion of the SA-2 paper as determined by chemical analyses. The agreement between calculated and experimentally determined I/Zo values is excellent except for the 4 to 6-A region. In the 4 to 6-A region most of the transmitted X-rays pass through holes in the paper rather than through the resin-cellulose mixture. The effect of these small holes or voids in the paper is negligible in the short wave-length region because of the high transmission of the paper for these high energy X-rays. Experimental values of I / I , for SB-2 papers are also shown in Figure 6. The dashed line represents the trend of the data; theoretical values of I/I0were not calculated for the SB-2 paper. Experimentally determined values of I / I , for WA-2 and WB-2 papers were similar to those found for SA-2 and SB-2, respectively. The poor $ransmission of low energy X-rays through ion exchange papers provides a means of evaluating the distribution of ions through the paper. A computer program was written in Fortran to calculate intensity ratios from the top and bottom of the paper. These calculations assume an exponential concentration distribution through the disk. To facilitate the calculations each disk was divided into twelve equally spaced intervals in which the distribution was considered to be uniform. The results of the theoretical calculations are summarized in Table VII. These calculations show that there will be a very large difference in intensity between sides of the paper if the distribution is exponential and long wavelength X-rays are used. However, the intensity ratios that the authors observed experimentally and those they calculated are markedly different. The experimentally determined intensity ratios were much closer to unity than was predicted for an exponential distribution through the paper. For example, the intensity ratio for CdLa using a series of stacked disks averaged about 1.8 to 1.9 for either one filtration or seven filtrations. If the Cd+2distribution through each disk was exponential, then the intensity ratio should have been approximately 100. The intensity ratio of AgLa from various disks used for a single filtration was about 1.1; this value is about 2 orders of magnitude lower than predicted for an exponential distribution. Thus the authors were presented with

conflicting data: Comparison of intensity ratios on individual disks indicate that the distribution is essentially uniform through the disk, whereas the studies on filtration through consecutive disks and closely stacked sets of disks indicate an exponential distribution (Figures 4 and 5). This conflict can be resolved on the basis of the size of the resin particles with respect to the thickness of the disk and the distribution of the resin particles through the disks. The particles have an average diameter of approximately one tenth the thickness of the disk and are homogeneously distributed in a disk containing an equal weight of cellulose and a considerable volume of empty space. Each ion therefore contacts only a few resin particles per filtration. If the average contact was one particle per filtration, the ratio of X-ray intensities measured on the two sides would be unity. However, as shown by the microprobe examinations, the average number of such contacts was between two and four per filtration. Therefore the observed small deviation from unity would be expected. As the number of particles increases-by increasing the number of disks-a more exponential distribution is observed. In the analytical procedure used by the authors the intensity of the analytical line is measured on both sides of the paper, then averaged. The second column in Table VI1 lists the ratio of averaged intensities for exponential vs. uniform distribution. For wavelengths less than 2A the average intensity for the two types of distribution varies less than 1%. In the longer wavelength region a measurable change in averaged intensity as a function of distribution is predicted. Therefore variations in distribution between standard and unknown are a possible source of analytical error in the long wavelength region. However, the magnitude of this error should be very small because standards and unknowns are prepared by similar techniques. A more uniform distribution of exchanged ions is produced by either increasing the number of competitive ions, such as H+, or by increasing the volume while keeping the quantity of competitive ions constant. For example, the effect of variable H + concentration in the intensity ratio of elements ranging from atomic number 20 to 30 is summarized in Table VIII. For X-rays of wavelength shorter than 2.4, at a pH of approximately 1, the intensity ratios of 1.00 to 1.01 were achieved for a series of divalent ions. At a pH of 3.0 the intensity ratios for these ions varied from 1.08 to 1.18. However, a t a pH of 3 the ions are more quantitatively retained on the disk than at a pH of 1. In summary, the following conclusions were reached from the study of ion


distribution and its effect on intensity. Exchanged ions are distributed uniformly through each resin particle, which are , in turn, homogeneously distributed through the paper on a macro scale but heterogeneously on a micro scale. Because of the small number of resin particles through the cross-section of the paper, the observed intensity ratios for a single disk correspond more closely to a uniform distribution than to the exponential distribution which might be expected. Intensity ratios between sides of a disk approach unity as the X-ray wavelength decreases, with increasing concentration of competing ions, with increasing flow rate, and for ions of lower Kd value. From the analytical viewpoint, the linear averaging of intensities is practical since standards and unknown are prepared in the same manner. Intensity to Concentration Relationship. Quantitative analysis is achieved by establishing the sensitivity for the element being determined (counts per second per microgram) plus the direction and magnitude of changes in sensitivity as a function of the number and kind of ions collected (interelement effect). This discussion of interelement effects will be limited to S.4-2 paper; however, the principles are equally applicable to other papers and membranes. Ion exchange resin-loaded papers are in the same class as thin films, that is, the samples are not “infinitely thick” relative to the characteristic X-ray lines being measured. Gunn (4)investigated the relationship of intensity to microgram quantities of various elements deposited on Mylar. He tibserved that the relationship was linear for amounts up to several hundred micrograms in the wavelength range 1 to 3A and for amounts varying from 2000 to 20,000 pg. for X-rays less than lb. As the amount of each element deposited increases, the response becomes increasingly nonlinear. With ion exchange papers the linear region extends over a greater microgram range because of two factors. The weight fraction of sodium in the ion exchange resin-loaded paper is only 0.042. Secondly, since sodium is being replaced, the change in absorption characteristics is a function of the differ-

Table VIII.

CaKa 1.25 1.48 2.04 2.34

PH S 1 1.5 2.0 3.0

Effect of pH on Intensity Ratio Ratio of intensities measured on top and bottom of disks CrKa MnKa FeKa CoKa NiKa CuKa 1.01 1.17 1.00 1.01 1.00 1.23 1.14 1.04 1.07 1.04 1.16 1.09 1.17 1.16 1.10 1.07 1.29 1.21 1.23 1.10 1.18 1.16 1.32 1.26

papers. Luke (6) discussed this problem qualitatively for ion exchange membranes and presented experimental data which showed the effect of Ba+2 additions on the spectral intensity of elements ranging from scandium to zirconium. Small changes in sensitivity, 2 to 5%, were observed in the short wavelength region,

P BA2- , =;

(;),.

0.21

[(p)P Qi

BA-2

+

-

z

(’)

P Mi

-

of sodium. Thus if the

When

P

!! of sodium is equal t o the cc- of a monoP

P

valent ion replacing sodium, the intensity to concentration relationship will be constant. One of the principal objectives of these investigations was to develop a quantitative expression for evaluating interelement effects in ion exchange

Q&] (7)

P

ing ion and

1.05

1.08

+ (:)2.042

ence between the !!values of the replacI.(

ZnKa 1.01 1.03

(!)

is substituted in Equation

P EA-2’

The mass absorption coefficient of an unreacted SA-2 paper for radiation of any wavelength is equal t o the sum of the mass absorption coefficient of each element of the paper times its weight fraction.

( EP )

= HA-2

(F) P

0.419 C

+ ( eP )X 0.057+

4, p in Equation 4 must be replaced by the new density p’. But this p’ is cancelled by the p’ in the denominator of Equation 7. Thus regardless of the number and kinds of ions exchanged, the p term in Equation 4 is equal to the density of the original SA-2 disk, 0.48 gram/ om3. Therefore the following working equation is derived VOL. 30, NO.

a, JULY

1966

993

7,

1

I

1

I

0.1

10

IO0

1,000

36

X

Figure 7. Relationship of spectral response (C/S/pg.) X-ray absorption parameter X

kv, IO ma

10


A

Fe 50

2'

I

100

1,000

ELEMENT, pg

I = Figure 8. sensitivity

(:),

Values of

and

(i), in this equa-

tion can becalculatedfiom Equation 7. Evaluation of K is not required as ratios between intensities are calculated. Equation 8 is of the form

1- e-O.Ol8ZX

0.68X ' a relationship which is essentially constant for small values of X as shown in Figure 7. Thus in the short wavelength region where

(5)

relative changes in

values are small, large

(5)

have only minor

effects on I and interelement effects are essentially negligible. For example, a change in X from 1 to 5 results in a very small change in I . In the longer wavelength region, where

(9

is larger, both

\r I

the number and kind of elements collected on the disk will have a greater effect on I . For values of X equal to 50, the change in I is approximately proportional to the change in X. The ion exchange X-ray spectrographic procedure is designed primarily for determining small quantities of collected cations. Therefore Q in Equation 7 will normally be small, and the change in

(')

P SA-2'

will also be small.

Consequently the fluorescent intensity will be-nearly constant for small values of Q. This constancy of sensitivity was experimentally verified by the authors over the past several years. Typical values of I for 0 to 1000 pg. of CoiZ, N P 2 , and Fe+3 are shown in Figure 8. There is no measurable change in the sensitivity for any of these ions over the range of 0 to 200 pg. In the 200 to 1000 pg. range there is a 5 to 10% de994

- -

YI

to the


crease in sensitivity. This change in sensitivity represents the combined effect of increased X-ray absorption, incomplete exchange (98% at 1000 pg.), and any nonlinearity in the proportional counter-pulse amplitude discrimination circuits. To evaluate interelement effects, Equation 8 was written in Fortran for processing on an IBM 7094 computer. This program was designed to predict deviations in response caused by changes in concentration of either the element being determined or other elements collected simultaneously by the paper. No corrections were applied to Equation 8 for enhancement of X-ray intensities resulting from absorption of higher energy X-rays generated in the sample. Enhancement is a t least an order of magnitude less than absorption effects. Also the enhancing elements are present in low concentration. The two types of matrix effects considered in the computer studies were: Variation in concentration of the element being determined, and variation in concentration of other elements while maintaining the element being determined a t a fixed concentation. Values for

- were taken from various sources, P

principally the compilation of ff data by P

Stainer ( 1 2 ) and the unpublished tables prepared by Kurt Heinrich of the National Bureau of Standards. The wavelength for

(E)l

was selected arbi-

. .

trarily. A strong characteristic line of the target element was selected if the line was close to but on the high energy side of the critical absorption edge of the element being determined. For example, WLa radiation was used as h1 for

Effect of quantity of ion exchanged on

excitation of iron. When such lines were not available, a value of h that was 0.2h less than the critical absorption edge was used. The magnitude of the matrix effect is related t o the wavelength of the analytiical X-ray line and the number and type of elements collected (see Figure 9). The elements listed on the bar graphs represent the cations added; the elements and wavelengths listed along the abscissa represent the element being determined. Intensities for a disk containing several hundred micrograms of the element being determined and for a disk containing the same quantity of this element and either 0.01 or 0.10 milliequivalent of the added cation were calculated using Equation 8. Changes in intensity were determined experinientally for the wavelengths and concentrations shown in Figure 9. These experimental values agreed qualitatively with those calculated from Equation 8. For example, addition of 0.1 meq. of lead to papers containing Sr(SrKa), C o ( C o K a ) , and Sn(SnLa) resulted in intensity decreases of 20, 35 and SO'%, as compared to predicted values of 13, 31, and 48%. Comparison of other calculated and experimental values demonstrate the same qualitative agreement. There are several important features of Figure 9 and Equation 8. First, even very large additions of low atomic number elements have little effect on line intensity in the short and medium X-ray region. Second, matrix effects can produce a significaut analytical error when long wavelength analytical lines are used, or when a large amount of a high atomic number element is added. For example, the error resulting from addition of lead is an order of magnitude greater for the SnLa lines as compared

3 CATION ADDED

E:

20

0.01 maq

I

0.10 meq

i


Figure 9. Effect of calcium, lead, strontium, and zinc on SnKa, SrKcu, CoKcu, and SnLa response

to SnKa. The p / p values of SA-2 paper for SnKa and SnLa differ by a t least an order of magnitude. Referring back to Figure 7 , for SnKa, X = 1.25, which is in the linear region, whereas for SnLa, X = 300, which is a region where the intensity is strongly dependent on the value of X. Although the analyst should be aware of the potential analytical error, most of the applications by the authors have been for trace analysis where the matrix effects are negligible. When interelement effects in ion exchange papers become significant, the analyst can resort to conventional matrix correction procedures such as comparison standards, internal standards, and addition techniques. An alternate procedure which merits evaluation would be to compare the transmission coefficient of the standard and unknown disks for the wavelengths of interest. This transmission-correction method is limited to the 1 to 3A X-ray region; in this region there is a strong but measurable dependence of the transmission coefficient on the number and type of exchanged cations. X-rays longer than 3-4A are not efficiently transmitted by SA-2 paper, thus small changes in transmission would be very difficult to measure accurately. For X-rays shorter than lA, interelement effects are insignificant, and corrections are not required. Reproducibility and Limit of Detection. Ion exchange membranes and ion exchange resin-loaded papers provide reproducible media for collecting and presenting microgram quantities for X-ray determination. Previous studies by the authors with ion exchange membranes showed that deviations in analysis could be reduced to 1 2 pg. a t the 200-pg. level. Similar results have been obtained for the ion exchange resin-loaded papers. A series of seven disks was prepared by filtering individual solutions having 100 pg. of Fe+3 through each disk. The seven papers gave sensitivities for FeKa ranging from 12.80 to 13.08 counts per second per microgram with an average sensitivity of 12.90 counts per second per

microgram. The standard deviation was 0.11 count per second per microgram which represents, in terms of concentration, ~ 1 % of the amount present. Other examples of the reproducibility achieved by the fi1tration-Xray method are shown in Figure 8. Note that sensitivity values do not change measurably over the 0 to 200-pg. range. Without excellent reproducibility the sensitivity values would be subject to much greater scatter. An accepted definition for limit of detection in X-ray spectrography is “that quantity of an element which

Table IX.

results in a line intensity above background equal to three times the square root of the background, using 10-minute counting times for each measurement” (1). When the elements are to be preconcentrated prior to determination, it is advantageous to express limits of detection in micrograms rather than in per cent of the element present in the sample prior to chemical preconcentration. Therefore the authors express all sensitivities and limits of detection in terms of micrograms rather than per cent, When the total quantity of ions collected is small, background corrections

Limits of Detection for Various Cations

Line intensity above background, Limit of detection, pg. C/S/rg. 1.6 1.2 ,023 5.0 3.5 0.12 2.9 0.61 3.0 0.14 2.3 0.77 6.2 0.17 18.2 0.05 9.0 0.22 3.2 0.15 17.4 0.11 13.9 0.09 5.6 0.12

Spectral Background, line count,s per Instrument measured second conditions KLY C 268 KLY A 0.87 Ba B 12.6 La1 Bi 214 C LLYl Kff CaC B 12.0 212 Cd C Kff Cea B 49.2 La1 coc 47.5 KLY C B Crc KLY 260 cs C 16.1 La1 cuc Kff C 256 B Fee KLY 94.9 La” B 28.8 La1 Mnc B 27.7 11.1 0.06 Kai Nic KLY C 116 23.9 0.06 Pb C 213 3.2 La1 0.55 Sr Kff C 148 0.28 5.3 Ti * B Kff 6.3 4.0 0.08 224 C UOt 0.72 2.5 Lffl Y O Kff C 1208 0.22 19.0 Znc KLY C 131 0.07 19.8 0 In nitric acid. b In sulfuric acid. c Ions collected simultaneously from one solution. Instrument conditions-A-Tungsten target, PET crystal, He path, 0.02- by 4.&inch collimator, flow proportional counter B-Tungsten target, LiF crystal, He path, 0.005- by 4.0-inch collimator, flow proportional counter C-Tungsten target, LiF crystal, air path, 0.005- by 4.0-inch collimator, scintillation counter Metal ion

Table X.

Salt analyzed

Limits of Detection for Various Anions

Spectral line measured BrKa ILffl SKa VKLY CrKu MnKa MoKa

Line intensity Background, above Instrument counts per background, Limit of conditions second C/S/pga. detection, pg.“ C 234. 5.7 0.33 B 9.4 5.5 0.068 7. .. -3 A 0.61 0.54 B 9.5 0.044 8.5 B 280. 10.9 0.19 B 33.7 13.2 0.054 257. C 4.8 0.41 B 170. 18.2 0.088 C 705. 0.76 4.3 A 1.2 1.12 0.12 of detection are in terms of micrograms of the element

FeKa PtLff, PKa a Line intensities and limits measured. Instrument conditions: A-Tungsten target, PET crystal, He path, 0.02- by 4.0-inch collimator, flow prooortional counter B-yungsten target, LiF crystal, He path, 0.005- by 4.0-inch collimator, flow proportional counter C-Tungsten target, LiF crystal, air path, 0.005- by 4.0-inch collimator, scintillation counter

VOL. 38, NO. 8, JULY 1966

995


are obtained from either precleaned disks or disks prepared by carrying a reagent blank through the procedure. When large quantities of ions are collected and the signal-to-noise ratio is low, background corrections should be applied by the usual techniques in fluorescent X-ray spectrography; that is, measurement of the scattered intensity on both sides of the analytical line. Scattered intensity in the short wavelength region was found to be 20% lower than for a disk containing 0.1 meq. of lead for a precleaned disk. The X-ray spectrograph used in these investigations was shared with other projects, so that components such as the X-ray tube, collimator, crystal, and detector were changed frequently. Thus the detection limits listed in Tables I X and X should be considered only as typical values. The elements of greatest sensitivity have characteristic Kcr and La spectral lines in the 1.0 to 2.5A region. Using standard X-ray spectrographic procedure, sensitivities of 0.050.10 pg. are achieved for these elements. Decreased sensitivity in the shorter wavelength region results from low absorption of the exciting radiation, whereas in the longer wavelength region the X-rays are strongly absorbed in the sample. In the long wavelength region, improvement in sensitivity by a t least one to two orders of magnitude is feasible by using a windowless or ultra-thin window X-ray tube having as the target an element that is a strong emitter of long wavelength radiation ( I S ) . Overall sensitivities can be increased by a t least a factor of 2 by decreasing the size of the ion exchange resin loaded paper. When using a 3.5-cm. disk most of the paper does not efficiently view the incident radiation, and the collimator does not view all of the paper. Luke (5)reported sensitivities of 0.01 pg. using ‘/pinch diameter ion exchange membranes in conjunction with curved crystal X-ray optics. However, the exchange capacity of the small membranes he used were 1 to 2 orders lower than that of the resinloaded paper used by the authors. In most trace analytical applications, the limit of detection and the analytical accuracy for low concentrations are strongly dependent on the variable amount of the determined element introduced by the reagents or contained in the disks as an impurity. For example, the following reagent blanks were required for a procedure for determining trace elements in tungsten: Ca-20 pg., Co-O.2 pg., Cu-7.2 pg., Mn-2.1 pg. , and Xi-2.5 pg. (9). Generally the residual amount of an ion remaining in a precleaned disk was significantly less than that added by reagents required for dissolution and separation steps. Therefore, as with all trace element determinations, the ion exchange-x-ray 996


Table XI.

Element Ni co Cr Fe Mn

Determination of Major Components in Alloys NBS sample 349 NBS sample 167

Stat.ed, %

Found, %

Stated, yo

Found, %

57.15 13.95 19.50

57.7 13.8 19.9

20.65 42.90 20.00 2.13 1.64

20.2 40.9 21.0 1.85 1.68

.*.

.43

...

spectrographic procedure requires critical evaluation of the various sources of impurities. In most examples studied by the authors the present X-ray sensitivities are more than adequate to reach the practical detection limits imposed by reagent impurities. APPLICATIONS

The technique of using ion exchange resin-loaded papers in X-ray spectrography was originally developed to fill a need for a method of determining trace quantities of metallic impurities after their separation from high purity tungsten (9, 10). The same technique has been used to determine metallic impurities separated from high purity molybdenum., It was also used in our laboratory to‘ determine trace metals in rat kidneys for another government agency. In solutions prepared from Zgram samples of kidney tissue, Ca, Cd, Co, Cu, Fe, Mn, Nil and Zn were determined a t levels of 1 to 90 pg. Ca and Zn were also determined on these samples by atomic absorption. Average differences between the results obtained for Ca and Zn by the two analytical methods were 4 and 6% of the amount present. Although the ion exchange-X-ray spectrographic technique was developed for determining trace components, it can be used to determine major components in situations where only small samples are available or where only small samples can be taken from objects of artistic or historical value. It can also be used to give a rapid analysis of complex alloys for which chemical methods use prohibitively long or complex procedures. A number of these complex alloys were analyzed for another research group a t this research center. Similar materials of known composition were analyzed simultaneously to indicate the accuracy of our procedure. The results obtained for the known alloys are given in Table XI. Individual standard disks were prepared for each cation to be determined. The good agreement between stated and found values in Table XI supports the authors’ statement that the measured intensity of the analytical line is essentially independent of the number and kinds of other ions collected for concen-

.51

tration less than 0.1 to 0.2 meq. If greater accuracy is required standard disks can be prepared that contain all of the ions being determined. Another proposed application of the ion exchange-X-ray technique is the analysis of water samples taken in the field, as in hydrogeochemical prospecting or the study of stream pollution. Only a small amount of equipment would be required in the field for the filtering operation. The air-dried disks could be returned by mail to a central laboratory for the X-ray spectrographic determination. ACKNOWLEDGMENT

The computer programs used in these investigations were prepared by James D. Brown, Research Physical Chemist. Electron probe microanalyzer data for the diffusion studies were provided by Philip Burkhalter, Research Physicist. Their advice and assistance is greatly appreciated. LITERATURE CITED

(1) Campbell, W. J., Am. SOC.Testing

Materials, Philadelphia, Spec. Tech. Pub. 349, pp. 48-69 (1963). (2) Campbell, W. J., Thatcher, J. W., U. S. Bur. Mines. Reot. Invest. 5966 I

-

(1962). (3) Grubb, W. T., Zemany, P. D., Nature 176, 221 (1955). (4) Gunn, E. L., ANAL.CHEM.33, 921 (1961). (5) Luke, C. L., Zbid., 36,318 (1964). (6) Minns, R. E., “Limitations of De-

tection in Spectrochemical Analysis,”

p. 45, Hilger and Watts, Ltd., London, 1964. (7) Samuelson, O., “Ion Exchangers in

Analytical Chemistry,” Wiley, New York, 1953. (8) Span, J., Ribaric, AI., J. Chem. Phys.

41, 2347 (1964). (9) Spano, E. F., Green, T. E., Campbell, W. J., U. S. Bur. Mines, Rept. Invest. 6308 (1963). (10) Zbid., 6565 (1964). (11) Stainer, H. M., U. S. Bur. Mines Inf. Circular 8166 (1963). 32, (12) Strelow, F. W. E., ANAL.CHEM.. 1185 (1960). (13) Thatcher, J: W., Campbell, W. J., U. S. Bur. Mines Rept. Invest. 6689 (1965).

RECEIVED for review January 17, 1966. Accepted May 9, 1966. The e uipment used is named for purposes of ijentification only and does not imply endorsement by the Bureau of Mines.

Micro and Trace Analysis by a Combination of Ion ... - ACS Publications

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