1
The precision is better than that obtained for the estimation of the capacity of carboxylic acid resins ( 3 ) . The capacity of sulfonic acid resins could be estimated more rapidly by measuring the absorbance at 1008 cm-l relative to the absorbance of the resin a t 1450 cm-l. In this way, the necessity for accurate weighing and the use of an internal standard of potassium thiocyanate could be eliminated. However, the calibration curve (Figure 4) was no longer linear. For resins of the same cross-linking, the band a t 1127 cm-l, indicative of p-substitution, could be used to determine capacity. The absorbance was measured relative to that of the internal standard a t 2065 cm-l, and a linear relationship to capacity was found (7 = 0.996, P < 0.001). The standard deviation in capacity was estimated to be 0.2 mequiv g-l, and repeated determinations using a resin with 8%DVB gave a figure of 0.15 mequiv 8-l.
s i
u co
0 0
Capacity
CONCLUSIONS Both the capacity and cross-linking of sulfonic acid resins could be measured by IR spectrophotometry with a relative standard deviation of 5 pph under the best conditions. Pyrolysis-gas chromatography was more precise (relative standard deviation 2 pph) for the measurement of capacity in the optimum range, but was less precise (relative standard deviation 10-28 pph) for the estimation of crosslinking. I t was quicker than IR spectrophotometry, since no sample preparation was required, but frequent calibration was necessary, and the calibration curve for the determination of capacity was not linear. IR spectrophotometry required more time for the preparation of potassium bromide discs containing an internal standard, but linear calibration graphs were obtained which could be used repeatedly.
LITERATURE CITED (1) (2)
J. R. Parrish, Anal. Chem., 45, 1659 (1973). E. Blasius, H. Lohde, and H. Hausler, 2. Anal.
II
Chem., 264, 278, 290
(1973).
meqlg
Figure 4. Plot of ion-exchange capacity vs. ratio of absorbance at
1008 cm-' to absorbance at 1450 cm-' (3)
J. Klaban and V . Radl, Chem. Prum., 21, 445 (1971); Chem. Abstr., 76,
73106t (1972). (4) H. Kvedar, Kem. lnd., 19, 141 (1970); Chem. Abstr., 73, 99526t (1970). (5) D. Whittington and J. R. Millar, J. Appl. Chem. (London). 18, 122 (1968). (6) K. W. Pepper, J. Appl. Chem. (London), 1, 124 (1951). (7) J. R. Millar, J. Chem. SOC., 1960, 1311. (8)V. W. Meloche and G. E. Kalhus, J. lnorg. Nucl. Chem., 6, 104 (1958). (9) S. E. Wiberley, J. W. Sprague, and J. E. Campbell, Anal. Chem., 29, 210 (1957). (10) A. Strasheim and K. Buijs, Spectrochim. Acta, 17, 388 (1961). (11) J.Dechant,Z. Chem.,5, 114(1965). (12) L. H. Jones, J. Chem. Phys., 25, 1069 (1956); 28, 1234 (1958). (13) M.V. Belyi, G.I. Ermolenko,and I. Ya. Kushnirenko, Ukr. Fiz. Zh. (Russ. Ed.), 19, 489 (1974); Chem. Abstr., 81, 18844k (1974). (14) G. I. Ermoienko, I. Ya, Kushnirenko, and Kh. K. Maksimovich. 2h. Prikl. Spektrosk., 20, 460 (1974); Chem. Abstr., 81, 18673u (1974). (15) D. J. Gordon and D. Foss Smith, Spectrochlm. Acta, Part A, 30, 1953 (1974).
RECEIVEDfor review March 26, 1975. Accepted June 12, 1975.
Flameless Atomic Absorption Spectrometry Employing a Wire Loop Atomizer M. P. Newton' and D. G. Davis2 Department of Chemistry, University of New Orleans, New Orleans, La. 70 122
The use of a versatile flameless atomlzer for atomic absorption spectrometry, which employs an electrically heated tungsten alloy wire loop, Is reported In this study. Several sampllng methods were investigated Including electrolytic deposition. The relative standard deviation was usually between l-3.5%. The detection limits of 19 elements have been compared for the various sampling methods.
ers, however, are expensive, require water cooling, large power supplies, and are not always sufficiently versatile to eliminate special sample preparation. The purpose of this work was to develop an atomizer which would overcome these disadvantages. Sampling methods were also developed to minimize interferences and background absorption while allowing maximum sensitivity, reproducibility, and versatility.
Atomic absorption has become one of the most widely used analytical techniques. The weak link in atomic absorption is the flame atomizer ( I ) . Flameless atomizers have been developed which eliminate the disadvantages of the flame atomizers (2-8). Some of these flameless atomiz-
Reagents. Reagent grade chemicals and doubly distilled deion-
EXPERIMENTAL
Present address, Merck Co., Inc., Rahway, N.J. Author to whom correspondence should be addressed.
ized water were used in all studies. Solutions were stored in polyethylene or polypropylene bottles. Dilute solutions were remade before each set of experiments. These solutions were made by adding five or more microliters of a concentrated stock solution, with a disposable plastic pipet to 100 ml of the doubly distilled deionized water. Apparatus. All work reported here was performed on a Varian
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
2003
Encloture
w To A n o d e
Figure 1. Heating and plating apparatus
Figure 3. Wire atomizer equipped for electroplating
(A) Heating-plating switch, (6)Push button onloff, (C) Drying-atomizing swltch, (D) 6-Voh step down transformer, (E) Atomization Variac, (F) Drying Variac, (G) Atomizer head, and (H) Manual poiarograph
onto the wire by soaking the wire in the sample solution, electrodeposition, or with a 5-p1 pipet. For the last method of sampling, the wire was mounted pointing up as shown in Figure 3, but was mounted pointing down for the other sampling procedures. The stainless steel electrical contacts which hold the wire were insulated from the body of the atomizer with asbestos paper. The atomizer has the advantages of being simple, inexpensive, and requiring only low power. The use of a tungsten-rhenium wire for a flameless atomizer was previously reported (IO). This material was preferable to pure tungsten because it is more ductile and resistant to oxidation. A sliding enclosure as shown in Figure 3 was used to increase sensitivity by holding the atoms in the optical path of the spectrophotometer for longer times. The enclosure also helped to keep the wire from becoming oxidized. A loop of 2-mm 0.d. gave good sensitivity and precision. Although the exact size of the loop was not important, the same loop should be used for both the sample and standards. The greatest sensitivity and reproducibility were achieved when the wire loop was positioned in the center of the radiation beam a t its focal point. When the wire was submerged in the solution for sampling, no loop was formed (Figure 3) so that very little water would be retained on the wire after rinsing. Temperatures. Studies were performed t o determine the relationship between drying, atomization, and cleaning temperatures and the absorbance signal. A high drying temperature could cause loss of sample due to rapid boiling or splattering. Extremely low drying temperatures would result in unnecessarily long drying times. The exact temperature during the drying step did not appear to affect the absorbance signal as long as the sample was dry and no sample metal was vaporized from the wire. The accurate reproduction of the atomization temperature was critical. At low temperatures, the sample came off the wire slowly so a broad low peak resulted; while, a t very high temperatures, the vapor left so quickly that the recorder could not fully respond to the fleeting signal. I t was found in the latter part of this project that linearly raising the atomization temperature (LATP), after drying, from room temperature to one considerably hotter than required to volatilize the sample was desirable when interferences were present. The cleaning temperature had to be high enough to completely volatilize any residual material off the wire but not so high as to oxidize or distort the shape of the wire. Sheathing Gases. Argon and nitrogen were used as sheathing gases and were passed over the wire loop to prevent oxygen from reaching the wire. If oxygen reached the wire, smoke was produced which caused blockage of radiation and resulted in excessive background absorption signals, in addition to destroying the wire. A flow rate of 2-3 l./min was satisfactory for either nitro-
I
Scale ExDPnd X 10
No Scale E i p o n d
,
20
0
Time
(Seconds)
Figure 2. Typical peak profiles
Techtron AA5 single beam atomic absorption spectrophotometer. A Varian A25 recorder with a 0.5-second full scale response was used for most of these studies. All timed steps were accomplished with the aid of a Standard S-10 electric timer. A Metrohm E-405 one-speed stirrer was employed as needed. Gas flow rates were measured with Gilmont flow meters. Slit widths, wavelengths,and lamp currents suggested by Varian Techtron were used throughout this work (9). The burner assembly was replaced by flameless atomizing devices designed and constructed in these laboratories. All of the atomizers developed utilized an electrically heated wire. The wire used was a IO-mil 97% tungsten-3% rhenium alloy wire (General Electric tungsten 3D 218-CS process). The apparatus that supplied the current to heat the wire and allowed the electrodeposition of certain elements is shown in Figure 1. An electric motor was sometimes used to drive a Variac at 4 rpm so that the temperature of the wire could be increased reproducibly with time. This will be referred to as linear atomization temperature programming (LATP).A Sargent polarograph was used to electroplate metals on the wire from a 10-ml plastic beaker. Approaches to mounting the wire will be discussed later. Data Treatment. A typical peak profile is shown in Figure 2. The peak heights were proportional to the analyte concentration. Several determinations of each sample were necessary to be confident that the peak heights observed were good representations of signals that should be obtained from that particular sample. If the precision during an experiment was satisfactory,about k3% or less relative average deviation, three readings were usually taken for each example.
RESULTS AND DISCUSSION Optimization of Apparatus. T h e Atomizer. Various atomizer designs were tried, leading to the development of the atomizer shown in Figure 3. A major advantage of this atomizer is that the sample solution may be introduced either directly by applying aliquots or by submerging the wire into the sample solution. Samples were introduced 2004
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
Table I. Effect of Hydrogen Gas on Sensitivity M‘ith H2 Sample
Sampling method
S o H2 (abs)
(W
1 PPm Ag 1 ppm Au 0.1 ppm Cd 1 ppm C r 1 ppm C r 0.1 ppm Cu 1 ppm F e 1 PPm Mg 1 ppm P b 10 ppm Se 100 ppm Sn 1 ppm Z n
10-sec soak 10-sec soak 5 pl 5 $1 20-sec soak
0.26 0.10 0.60
0.39 0.30 0.61 0.32 0.20 0.10 0.50 0.17 0.50 0.15 0.37 0.79
5 ,A1 5 Ill
10-sec soak 5 Ill 5 111 5 jl.1
10-see soak
0.06 0.00 0.01 0.05
0.01 0.50 0.03
0.02 0.79
gen or argon. Hydrogen was also introduced into the sheathing gas stream in the latter studies. A flow rate of 0.4 l./min was used. Hydrogen further protects the wire from oxidation especially at extremely high temperatures. This greatly extended a loop lifetime in those circumstances which required high temperatures. A more detailed discussion on the effect of varying parameters such as loop size and position, drying and cleaning temperatures, sheathing gases and flow rates, slitwidths and enclosure dimensions has been reported (11). Sampling Methods. Aliquot Sampling. The first sampling technique routinely used involved applying a 5-p1 sample onto the wire loop with an “Autopete” having disposable polyethylene tips. Sampling was easy and reproducible. The sample was applied to the wire loop and dried a t low temperature. The enclosure was then moved into place and the sample atomized at moderate temperature while the signal was being recorded. The wire was cleaned at a high temperature after each sample. The sensitivity of some elements could be greatly increased by passing hydrogen gas over the wire in addition to the sheathing gas. The hydrogen provided a reducing atmosphere which increased the atomization efficiency of some of the elements. Examples showing the effect of hydrogen on the sensitivity of some elements are given in Table I. The precision for the aliquot sampling method compares quite satisfactorily with other flameless methods. One can commonly expect 1-3.5% relative standard deviation. Spontaneous Preconcentration Sampling. While investigating the influence of the magnitude of the applied potential in an electrodeposition preconcentration procedure, an interesting phenomenon was discovered. Metal ions would preconcentrate onto the tungsten-alloy wire when no potential was applied. This spontaneous preconcentration, which gave greater sensitivity than the aliquot sampling method in most cases, was employed as a sampling technique (10). In addition, by use of this method, it was often possible to separate that analyte from interferences present in the sample. The sampling procedure was to slide the enclosure away from the wire loop and raise the stirrer, thus immersing the wire loop in the sample solution for a measured amount of time. Next the stirrer was lowered, the rinsing mini-beaker positioned under the wire, and the wire immersed in the rinse water for about two seconds. The duration of the rinse was not important. The wire was then dried, the sample atomized, etc. as in the aliquot sampling method. This procedure took about one minute in addition to the soaking time in the sample solution which was 10-20 seconds in most of our studies, although much longer times were used if extreme sensitivity was desired.
~ p + ‘Concentration (ppm)
Figure 4. Calibration curve for electroplating preconcentration Sampling IO-second plating in sea water, 3.0 volts
To achieve maximum reproducibility, the same area of the wire should be submerged in the sample for each run. This could be accomplished mechanically by using a “stopper” on the stirrer supporting bar. I t has been reported that the free atom population falls off exponentially as a function of the distance of the vaporizing sample from the optical beam (12-14). Thus the effect of a slight variation in the length of wire submerged would be insignificant compared to the total signal if the variation was on a portion of the wire which was a reasonable distance away from the optical path. This was confirmed by the reproducibility of this technique. Thus it was just as effective to raise the solution a few mm above the area where the source shone on the wire. The mechanism of the preconcentration phenomenon was investigated and it was concluded to be similar to ion exchange processes. The evidence that led to this conclusion was discussed elsewhere (10). The relative standard deviation for the spontaneous preconcentration sampling technique was 1-4%. Calibration plots obtained with the aliquot and soaking methods and absorbance vs. time plots have been shown elsewhere (10, 15, 16). These calibration curves become nonlinear a t higher concentration because the recorder could not keep up with the fast signal. Electrolytic Reduction Sampling. Electrolytic reduction of metal ions onto the wire loop was investigated as an alternative sampling method. The loop was utilized as a cathode and a platinum wire as the anode (Figure 3). Electroplating added an additional step to the soaking procedure when using this apparatus. This consisted of throwing switch A in Figure 1 which applies a previously selected potential to the wire loop before it was submerged. After the sample had been deposited and the wire rinsed, the circuit which heated the wire was connected and the analysis carried out as with the simple soaking procedure. The chosen potential was applied by means of the Sargent polarograph. For selective plating, controlled potential electrolysis can be utilized. One advantage of reducing the metal ions onto the wire was that it sometimes led to an increase in sensitivity. It was also possible to decrease interference due to ion exchange when the exchangeable metal ions were reduced to neutral atoms on the wire. The presence of various electro-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
2005
Table 11.Examples of Exchange Interference Being Lessened by Electrodeposition \Vith
KO interference ,Metal
0.1 ppm Cd 1 ppm Cu
Interference
(1)
interference
a
(1)
(2)
0.00 0.07 0.00 0.00
0.18 0.63 0.23 0.21
(2)
Sea water
0.31 0.30 0.30 0.70 0.05 ppm Mg 10-3.U ZrCl, 0.36 0.30 0.05 ppm Mg 10-3.~1 A1(N03)3 0.36 0.30 a (1)No potential applied. (2) Potential applied. 1.11 H N 0 3
lytes in to M concentrations did not significantly affect the amount of metal going on the wire nor the apparent current efficiency, and thus electrolyte was not added to samples. Figure 4 shows a typical calibration plot in a matrix which necessitates electrolytic reduction. In some complex matrices, accurate analysis would have been impossible for some metals if a potential were not applied. Sea water, urine, and some concentrated salt solutions in which the element of interest is in trace quantity are examples of this type of matrix. Table I1 shows how electroplating gave some relief from interferences. The precision of the electrodeposition technique was about the same as with the soaking method. Detection Limits. The detection limits using the various sampling techniques are compared to the flame method in Table 111. These detection limits could be lowered by employing a multiple loop and longer submersion times for the preconcentration sampling techniques or by applying multiple samples to the wire before atomization in aliquot sampling. Another comparison of the sensitivity of the wire loop atomizer to the flame method is the number of grams that give a signal twice that of the noise level. This is the absolute detection limit (Table IV). Minimization of Background Absorption a n d I n t e r ferences. Background absorption and interferences are caused by material in the sample other than the analyte. At
least three possibilities exist. First is the fact that solid materials in the light path may scatter light from the hollow cathode resulting in excessive background. It is also possible that condensing vapors may collect atoms of the analyte preventing absorption. Finally, extraneous materials may affect the rate a t which atoms are generated from the atomizer. Greatest accuracy can be achieved if the background and interference are eliminated rather than attempting background corrections. This is true because the background and interferences may cause excessive irreproducibility as well as enhancing or lowering the signal. In addition, corrections for nonlinear calibration plots may be troublesome. When using the wire loop atomizer, "interference" tables, such as those often reported by many workers, are valid only if the conditions are exactly reproduced. Such tables can indicate, however, whether or not interferences are likely, but cannot be easily used to make corrections. In addition, many real samples may have several interfering materials whose effects may not be additive, and it is not often that all interfering materials are known. Temperature Control. Drying the sample completely before atomization separates the solvent from the analyte, thus eliminating a large portion of the potential background. If something in the sample gives rise to a background signal and can be ashed or is volatile relative to the element to be determined, the wire may be heated to a temperature which will drive this substance off while the less volatile anal@ remains on the wire loop. One must exercise care to avoid analyte loss when ashing the sample. The volatility of the sample matrix can affect the rate at which the analyte leaves the wire and, thus, the concentration of neutral ground state atoms vs. time relationship. One way to minimize any volatility effect is to increase the atomization temperature with time which assures that the temperature during atomization is never extremely high or low for any given sample. The example given in Figure 5 indicates at the low temperature setting, 16, the cadmium is vaporized slowly when the less volatile K2Cr207 is present producing a broad low peak. A moderate temperature, 20, will give some relief to this problem. The high temperature
Table 111. Comparison of Detection Limits (ppm) Obtained with the Wire Loop Atomizer, the Flame Method, and the Graphite Tube Atomizer Flame AA
Metal
Ag As Au Be Cd co Cr cu Fe Hg
a
3 x 10'~ 2.5 x 10'' 1 x 10'2
2x 6 x 6 x 6.6 x lo-' 3 x 10-3 5
2 x 10'1
La
1
Mg
3 3 x 10-3
Mn Ni Pb
8 x 2 x 10-2
Sb
4 x 10'2
Se
1
Sn V Zn
3 x 10-2 1 x 10" 2x
5 - u l aliquot
2 2 1.3 2.5
x x x x 4 x
200-sec electrolytic reduction
10-4 10-2
2.4 x l o e 6 1.3 x lo-'
4-3
4.4 10-5 5 x 10-1 6 x 4 x 10-~
10-3 10-5
3 10-3 2.5 x lo-' 4 x 10-i 1.3 x
1.3 x 2.5 10-5 5 x 10-4
5 x 10'4"'
7 x 10-2
...
*.. 1.3 x 10-5 8x 8x 1x 8x 5x 2.5 x 1x 3x
lom5 lo-'
10-2
IO-* 10-2 10-5
2 x 10-6 6x 6x 1x 1x 1x 2x 1.6 x 4 x
Copper wire used.
2006
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12. OCTOBER 1975
lo-' lo"
10-d 10-2 10-1 10-3 10-1
10'7
200-sec spontaneous preconcentration
1.5 x 5x 3 5x
110-~ lo-'
10-4 10-1
7 x 10-6 5 x 10-5 2 x 10-5 8 x lom5 5 x 10-4 5 x 1o-Ja
...
2 x 10-6 1.3 x 8 x lo-' 1x 2.5 x 1.5 x 2 x 1x 6 x
10-4 lo-' lo-* 10-3
10-1
5 - u l gra hite tube atomizer $lode1 HGA 2100 Perkin-Elmer
1 x 10-4 2 x 10-3 2 x 10-3
6 x lom4 6 x 8x 2x 1 x 10-3 4 x 10-4
...
... 2 x 10-4
2 x 10-2 1 x 10-4 4x 1 x 10-2 2 x 10-2 1 x 10-1
2 x 10-5
Ag As Au Be Cd co Cr cu Fe Hg Mg Ni Pb Sb Se Sn
3 x 10-9 2.5 x 1 x 10-8 2 x 6 > lom1' 6 x lo-' 6.6 x lo-* 3 x 10-9 5 10-9 2 x 10-7 3 x 10-'0 3x 8 x lom9 2 x 10-8 4 x 10-8 1 x 10-6 3 x 10"a
V
1 x 10-7
Zn
2 x 10-9
Mn
a
Flanea
This work, wire loop
1 x 10-12 1 x 10-10 7.5 x 1.2 x lo-" 2 x 1043 1.5 x lo-"
1.2 x 2x 6.5 x 3.5 x 6.5 x 4x 4x 5x 4x 2.5 x 1.2 x 5x 1.5 x
10-12 10-'2 lo-" 10-l' 10-l~ 10-l2 10-l2 10'10
10-10 10-10
10-1' 10-l~
r o d a atomizer
ABS.
2 x 10-l~ 1 x 10-10 1 x 10-1' 9 x 10-l~ 1 x 10-13 6x 5 x 10-12 7 x 10-l2 3 x 10-12 1 x 10-'0 6x 5 x 10-l~ 1 x 10-l1 5 x 10-12 3 x lo-" 1 x 10-10 6 x lo-" 1 x 10-10
A TIME
TIME
1
,5
8x
'Ramping Variac
ABsL I
.I 0
Hydrogen l a m p
il'ith
Voriac 2 0
,5t
M Ith H2
sample
Interferent
hoH2
H2
XoH2
10 ppb Ag 1 ppm Au 1 ppm Pb 2.5 ppm Cd
10-3.1ZNi lO-'Jf Ni 10-2dI Ni Acid albumin digest 10-3dI C r
0.09 0.39
0.05
0.05
0.02 0.43 0.14 0.02
0.28 0.13 0.03
b.00 0.00 0.00 0.00
0.24
0.39
0.16
0.00
0.5 ppm P b
.I
0
Table V. Elimination of Background Absorption by the Use of Hydrogen Gas (No Ashing)
0.18
+2
.Of DDm . . Cd t IO-' K2Cr207
Varioc 16
,4t
5- U l
+2
Cd
t
.4
Varian Techtron.
Metallic l a m p
DDm
.5
Carbon Metal
,';I1 .01
Table IV. Comparison of Absolute Detection Limits (g) For t h e Wire Loop a n d Flame Atomizers (1 ml)
TIME
needed to eliminate this broading due to the presence of the K&r207 will create a new problem in the absence of other less volatile substances because of the slow response of the recorder to the signal produced. This works especially well with the more volatile elements. That linear atomization temperature programming (LATP) can reduce volatility problems is shown in Figure 5. Hydrogen Gas. I t has been observed that hydrogen gas added along with the sheathing gas eliminates or reduces background absorption in many circumstances as well as increasing sensitivity. A flow rate of 0.4 l./min of hydrogen in addition to 2 l./niin of the sheathing gas is passed over the wire while the sample is atomized. An example of how the use of hydrogen can eliminate background absorption is shown in Table V. When the continuum hydrogen lamp is used in place of the metallic lamp, the background is measured. The use of hydrogen gas also greatly reduced or completely eliminated chemical interferences in which the foreign matter affected the efficiency of ground state atom production. The hydrogen gas provided a reducing atmosphere that reduced any oxygen present and kept the analyte from being oxidized by other substances. Preconcentration. If the above methods were not adequate for relief of background absorption or interference effects for some samples, the aliquot sampling method was replaced by a soaking or plating procedure. In this way, a separation of the analyte from the substance giving rise to
Figure 5. Use of ramping atomization temperature to minimize error due to different volatillty rates
the prob1e.m was often accomplished. This situation was exemplified in the analysis of sea water. The salt concentration was so high that, even after a tenfold dilution, light scattering background prohibited the analysis of any element present. When a sample of cadmium was electroplated onto the wire loop and rinsed, no background signal was observed since the salt matrix was washed off by the rinse water. Reduction of interference effects could also be accomplished by separating the analyte from all other materials in the sample by preconcentration sampling. Table VI shows the. effectiveness of hydrogen gas, LATP, and preconcentration sampling toward minimization of interference effects. An important new kind of interference problem sometimes occurred in spontaneous preconcentration sampling when analyte ions were complexed or replaced from the wire by other ions. Electrolytically reducing the ions, once they preconcentrated on the wire, often offered an easy way to overcome this problem. It should be noted that electroplating was desirable only when there were significant interference problems. The reason was that the electrodeposition step had a greater tendency in some cases to also preconcentrate the interferent when compared to spontaneous preconcentration. This produced greater chemical and physical interference. When the wire was coated with an interferent upon electrolysis, normally there was a decrease in the amount of analyte preconcentrated on the wire. The magnitude of the applied potential played another important role in electrodeposition. This involved the interrelationship between the p H around the wire loop and the applied potential on interference effects.
ANAL.YTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
2007
Table VI. Comparison of Interferences for Lead Analysis When Employing Various Conditions and Sampling Methods for the Wire Loop Atomizer
so4-2
and salt excess
Interfer ence A1 Ba Ca Cd Cr
cu Fe K La Li Mg
Na 4"
Ni
ABS. x 100'
x 1000a
x ZOOb
x 400'
l L 4 d 12 r
x 4003
04
110 +30 +50 +25 +35 +50 0 110 +50
-30 +25 0 -3 5 +30 +35 0 A10 +50
+70
+60
+15 +15 +45 +10 +35 -3 0 -100 -3 0 0
0 +30 +50 0 +20
-10 -40
-55 0 0 -25 -25 -10 0 0 -10 0
-15 0 0
0 0 0 0
-15 -10 0 0 0 0 0 0 0 0 0 0 0 0 0 0
3
0
For example, a t a pH of about 3 or higher, lead ions went on the wire mainly by the ion exchange mechanism. Once the metal was on the wire, it could be reduced depending on the applied potential. If the potential was great enough, the water near the loop was reduced, thus raising the pH. The high pH generated around the wire caused some metals, especially trivalent ones, to precipitate as hydroxides. Any precipitate would tend to prevent other cations from preconcentrating on the wire by collection of these cations in or on the precipitate formed. At pH 2 or below, there was less of a tendency for metal ions to permanently preconcentrate spontaneously onto the wire because of hydrogen ion competition. If an appropriate potential was applied to the loop, the analyte ions were reduced on the wire as they preconcentrated; thus they were not exchanged off the wire. If trivalent cations were also present, they would not normally interfere because the analyte ions were reduced to neutral atoms and the pH around the wire loop would not get alkaline enough to cause precipitation of the trivalent metals. Trivalent cations gave less interference at high pH for spontaneous preconcentration because these metals were then complexed as hydroxides and therefore did not tend to compete for sites as they did a t lower pH. An extra advantage of selective plating at a low p H was that a fraction of the preconcentrated interfering species which was not reduced would have been exchanged off the wire by hydro-
0
5
.'
10
15
0
1
1
5
IO
1
15
A i ' Concentration b p b )
0
+10
0,
5
'I Ag Concentration (ppbi
-20 0 0 0 0 0 0 0 0 0 0
Zn 0 NaAc -50 -5 0 NaBr 0 0 -100 NaCl -60 0 -15 NaC10, -15 0 -90 NaCN 0 -95 0 -10 Na2C0, -7 0 -95 0 -15 0 Na F 0 +10 0 0 -10 NaHCO, -100 -100 -10 0 0 NaI -100 -100 -80 0 -15 0 NaN03 +15 430 0 0 -20 Na2S04 0 -85 0 -10 -90 -60 -15 0 Na2S203 -6 0 Judy Chauvin Thesis ( I I ) , 2-p1 aliquot, no HZgas, no LATP. b5-p1 aliquot, 1.0 ppm Pb, 10-3M int. with Hz gas with LATP. c Spontaneous preconcentration (20-second submersion time) 0.5 ppm Pb, lO-3M int. with Hz gas with LATP. Electrolytic reduction at 3.0 volts for 20 seconds with HZgas and LATP, 0.5 ppm Pb, 10-3 interferents.
2008
c104.1
20
Sampling method
Flgure 6. Standard addition plots employing aliquot sampling 5 pi, ~ o - ~interferences M
gen ions, thus further lowering the chemical interference upon atomization. The interrelationship of pH, potential, and the amount of interference for anions seemed to be less critical and straightforward than for cations. Compensation of Interferences by Use of the Stand a r d Addition Method. In some cases, when analyzing complex samples, the methods to eliminate interferences, proposed above were inadequate. Thus the signal derived from the sample could not be accurately applied to a calibration curve obtained using simple aqueous standard solutions. To obtain accurate results, any interferences present had to be compensated for correctly. Matching the standards to the samples was often impractical or impossible when the composition of the sample matrix was very complex, not known, or when many different types of samples were being analyzed. The problems mentioned above were usually solved easily by the standard addition method. The best results were obtained when the standard addition technique was used in conjunction with the approach likely to be the most effective in eliminating interferences, because interference effects could produce relatively poor precision. It can be seen from Figure 6, and that despite the widely differing degrees of interference present, relatively accurate analysis was possible. Practical Applications. When analyzing "real" samples, it is important to choose the best sampling method and conditions for the particular matrix under investigation. The examples cited below demonstrate this and the versatility of the wire loop atomizer when combined with the various sampling methods. Since the volatility effect is so important, LATP should be used for all determinations. No special sample preparation was required for any of the analyses discussed in this section, although the oyster samples did necessitate simple digestion. Drinking waters from seven bottling companies and several taps and drinking fountains were analyzed as examples of samples with minimum interferences and high sensitivity requirements. Aliquot sampling may be sufficient for a limited number of elements; however, spontaneous preconcentration employing relatively long soaking times was needed in some circumstances. A few brands of commercial drinking water had significant amounts of dissolved minerals in them, thus hydrogen gas and the standard addition
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
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Table VII. Electrodepositiond Detection Limits (ppm) in Sea Water hletal
Ag AU
Cd
co Cr
cu Mn Ni
Sb Zn a
D e t e c t i o n limit
4 13 x 4 x 2 x :I x '1 X x :. x ij x 8 x
10-~ 10-6 1cV.J 10-4 10'6
10"' 10-7
Amount normally present in sea .watei
ABS.
3 x 10-4 4 x io-6+ x 1 0 - ~
Cd" CQncenlrOllOn Added (ppbl
3 x 1 0 - ~x4 5 x 10-5-7 x 4 x 10-'-2.5 x l o m 3 1 x 10-3-2.5 x lo-' 7 x 10-4 1 x 10'4-2.5 x 2 x 10-4 7 ~ . 1 0 - ~ - x2 io-'
C o n c r n l r o l i o n Ippbl
Cd"
Voltage = 3.0 volts, 300-second submersion time.
method were required for accurate results. Measurable amounts of Cd, Cr. Cu, Pb, and Zn were found in one or more of the drinking waters. For sea water analysis, the electroplating method was utilized. Light scattering from salt particles prohibited the use of aliquot sampling. The extremely high sodium and magnesium content in sea water made it desirable that a reduction potential be applied to the wire loop during preconcentration to prevent exchange of the analyte from the wire. After the sample metal was electroplated on the loop, the loop was rinsed in doubly distilled deionized water for a few seconds before switching to the heating circuit. No background was observed due to salt when this procedure was followed. Table VI1 lists the detection of ten elements in a sea water matrix. The p H should be lowered for some elements to obtain increased sensitivity. Beverages such as fruit juice, soda, and beer were analyzed employing preconcentration sampling. When utilizing aliquots of sample, problems were encountered due t o the large amounts of dissolved materials present in these mixtures. No sample preparation was necessary except for dilution in some cases or, when a sample was too alkaline, 0.1 ml of concentrated " 0 3 was added to the 10-ml sample before electroplating. Hydrogen gas and the standard addition method were employed for all analyses. The most interesting results for beverages concerned 850 ppm tin and 25 ppm iron being found in canned grapefruit juice. Digested biological [samples were analyzed by aliquot sampling utilizing ashing and hydrogen gas to eliminate background absorption and to minimize interference effects. Whole kidney digest and cell fractions such as nuclei, mitochondria, microsomal suspensions, and solubles were analyzed for cadmium. The digests from control animals as well as from the cadmium-fed animals were within the detectable range of this method. Since this matrix is so complex, the standard a'ddition method was employed. All kidney digests were similar since they all went through the same digestion procedure. It was, therefore, not necessary to construct a standard addition plot for each sample. A standard addition curve was constructed for one sample and this one curve used as a standard calibration plot. This was accomplished by adjusting the concentration axis to make the intercept of the curve the zero point while retaining the original concentration scale, as shown in Figure 7 . When determining trace amounts of contaminants in
0
2
4
6
8
10 I2 14 16
C d 2 Concenlrnlion (ppbl
Figure 7. Example of the use of one standard addition plot for many similar samples 5-pl Samples of mouse kidney digest. (A) Original standard addition plot, (B) Adjusted concentration axis, (c) Fit other samples on curve
chemicals, the contaminants being determined are present in much lower concentration than the bulk chemical. T o overcome this situation on the wire loop, selective electrodeposition should be utilized if possible. If the bulk chemical is reduced more easily than the trace metal being determined, spontaneous preconcentration may work if the exchange interference is small enough to allow the analysis. In many circumstances, it was also possible to employ aliquot sampling if the major component was not present in too large a concentration. Trace analysis results for cadmium, silver, magnesium, and lead were obtained in various salts employing aliquot and preconcentration sampling. LITERATURE CITED (1) A. Walsh, Anal. Chem., 46, 698A (1974). (2) B. V. L'vov and A. D. Khartsysov, J. Anal. Chem., USSR, 25, 1565 (1970). (3) R. Woodriff, B. R. Culver, D. Shrader, and A. B. luper, Anal. Chem., 45, 230 (1973). (4) J. W. Robinson, R. Garcia, 0. Hindman, and P. Slevin, Anal Chlm. Acta, 6Q,203 (1974). (5) D. Clark, R. M. Dagnall, and T. S. West, Anal. Chim. Acta, 63, 11 (1973). (6) M. T. Glenn, J. Savory, S. A. Fein. R. D. Reeves, C. J. Molnar, and J. D. Wlnefordner, Anal. Chem., 45, 203 (1973). (7) C. W. Fuller and J. Whitehead, Anal. Chim. Acta, 68, 407 (1974). (8) A. Rattonetti. Anal. Chem., 48, 739 (1974). (9) Instruction Manual for Varian Techtron Model AA5, Appendix I, Palo Alto, Calif. (1971). (10) M. P. Newton, J. V. Chauvin, and D. Q. Davis, Anal. Left.,6, 89 (1973). (1 1) J. V. Chauvin, M.S. Thesis, University of New Orleans (1973). (12) J. Aggett and T. S. West, Anal. Chlm. Acta, 55, 349 (1971). (13) R. G. Anderson, H. N. Johnson, and T. S. West, Anal. Chim. Acta, 57, 281 (1971). (14) D. Alger, R. G. Anderson, I. S. Maines, and T. S. West, Anal. Chim. Acta, 57, 271 (1971). (15)J. V. Chauvin, M. P.Newton, and D. 0. Davis, Anal. Chim. Acta, 65, 291 (1973). (16) M. P. Newton, Ph.D. Dissertation, University of New Orleans (1974).
RECEIVEDfor review May 3,1975. Accepted June 30,1975.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
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