621
Anal. Chem. 1982, 54, 621-624 (26) DiGioia, A. J.; Lichter, R. L. J . Magn. Reson. 1977, 2 7 , 431. (27) Levy, G. C.; Gargioli, J. D. J . Magn. Reson. 1973, IO, 231 (28) Levy, G. C.; Dechter, J. J.; Kowalewski, J. J . Am. Chem. Soc. 1978, 100, 2308. (29) Sievers, R. E., Ed. "Nuclear Magnetlc Resonance Shift Reagents"; ACademiic Press: New York, 1973. (30) Lind, U. D.; Hoard, J. L. J . Am. Chem. SOC. 1965, 8 7 , 1611. (31) Elgavlsh, G. A,; Reuben, J. J . Am. Chem. Soc. 1076, 9 8 , 4755. (32) Elgavish, G. A,; Reuben, J. J . Am. Chem. Soc. 1077, 9 9 , 1762. (33) Farnell, L. F.; Randall, E. W.; White, A. I. J . Chem. Soc., Chem. Comniun. 1972, 1159. (34) Freeman, R.; Pachler, K. 0. R.; La Mar, G. N. J . Chem. Phys. 1971, 5 5 ,. 4586 -. - - -.
(35) Hawkes, G. E.; Herwig, K.; Roberts, J. D. J . Org. Chem. 1074, 39, 1017.
(36) Liebfrltz, D.; Roberts, J. D. J . Am. Chem. SOC.1973, 9 5 , 4996. (37) Levy, G. C.; Edlund, U. J . Am. Chem. SOC. 1075, 9 5 , 4996. (38) Vandegaer, J.; Chaberek, S.;Frost, A. E. J . Inorg. Nucl. Chern. 1959, I f , 210. (39) Harder, R.; Chaberek, S.J . Inorg. Nucl. Chem. 1959, 7f, 197.
RECEIVED for review July
17, 1981. Accepted December 7, 1981. The support of the National Science Foundation through Grant CHE 79-13022 is greatly appreciated. T.J.W. thanks John Frohliger and the Society of Analytical Chemists of Pittsburgh for a summer fellowship, awarded by the Analytical Division of the American Chemical Society.
Magnesium Nitrate as a Matrix Modifier in the Stabilized Temperature Platform Furnace Waiter SHavln," G. R. Carnrick, and D. C. Mannlng The Perkin-Elmer Corporation, Norwalk, Connecticut 06856
Magneslurn, as Mg(NO,),, has been added to solutions to be analyzed Ifor Mn and AI to permlt charrlng at hlgher tlemperature. The atomlzatlon Is delayed to a higher temperature whlch, with the platform technlque, permlts the tube to reach a stable temperature before atomlzatlon of Mn. We lbelleve that Mn, AI, and Mg are reduced to the oxlde In the solid phase, prior to vaporlzaiion of the oxide. The functlon of the Mg additlain Is to Imbed the analyte In a matrlx of Mg oxlde, delaying vaporlzetlon of the analyte untll the Mg oxide Is vaporlzed. With these condltlons, Interference effects on Mn and AI are greatly reduced. Prellmlnary data lndlcate tlhat thls matrix modlfler Is advantageous also for Cr, NI, and Co.
We recently described the determination of Mn in seawater (1). We found that there was less interference on Mn in the seawater than in an equivalent concentration of NaC1. We could char a seawater sample a t 1400 "C with little loss of Mn, while Mn in simple aqueous solutions was lost above 1200 "C. In studies of A1 (2), we found that adding a Mg salt, .MgC12, delayed the appearance of the Al peak in tlhe atomization step. Recently, Genc et al. (3) argued from activation energy measurements that Mn reached the atomic state after the Mn chloride, nitrate, or sulfate was converted to the oxide on the graphite surface. The oxide wa3 then vaporized and dissociated in the vapor state. Consistent with this model, they found that the appearance temperature for Mn was 11475 f 10 K indelpendently of whether the Mn was present as the chloride, nitrate, or sulfate. We speculated that the effect of the seawater on the Mn determination was due to Mg salts present in seawater. The alkaline earths were probably reduced to the oxide after hydrolysis, prior to vaporization. The Mn was trapped within the MgO crystals, delaying its volatilization. The bulk of the NaC1, on the other hand, was driven off a t char temperatures near 1000 "C. This suggested that we might be able to improve the analytical conditions for metals that are atomized from their oxide in the furnace by adding a massive amount of a metal Eialt that is converted to a relatively refractory oxide. The experimental data shown here support our speculation. 0003-270~0/82/0354-0621$0 1.25/0
Table I. Instrument Parameters dry
char
atom
clean
cool
2600 1 6 300
20 1 20 300
Manganese temp ("C) ramp (SI hold (s) int. gas flow (mL/min)
250 1
60 300
1400 10 70 300
2200a 0 8 0
Aluminum temp ("C) ramp hold int. gas flow (mL/min)
250
1700
1 60
1
300
45 300
2400 0 6 0
2600 1 6 300
20 1
20 300
a Some of these experiments used an atomization temperature of 2300 "C, which we believe provided identical results.
EXPERIMENTAL SECTION The Perkin-Elmer Model 5000 atomic absorption spectrophotometer and HGA-500 furnace were used with the AS40 autosampler. The Data System 10 was used to record absorbance signal profiles. New pyrolytically coated graphite tubes (part no. B009-1504) and pyrolytic platforms (part no. 0290-2311) were used with 20-pL sample aliquots. The wavelength, slit width, and lamp conditions were taken from the Analytical Methods Book (4). The furnace programs are shown in Table I. Maximum power heating was used for the atomization step. The required char time varied depending on the matrix and the amount of dissolved solids. A shorter char time might be satisfactory for many applications. The purpose of the photodiode in maximum power heating is to indicate when the tube has reached a preset temperature. The maximum power available is applied to the furnace tube at the beginning of the atomization step until the photodiode viewing the emission of the tube indicates that the preset temperature has been reached. Then the voltage is reduced to a value which will maintain that temperature until the conclusion of the atomization step. Preliminary work indicated that the cutoff setting of the photodiode strongly influenced the shape of the thermal profile. The temperature conditions for the furnace were typically set (5) by programming a temperature on the keyboard which the pyrometer indicated provided the desired final 0 1982 Arnerlcan Chemical Society
622
ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982 1. 21
W
;:y r
SEAWATER
w-
MGCL2
A
sErmm*c
21w
3 2000
m
5I-
1.2T
1700
MG (N03) 2 1500
0
1
2
3
4
5
6
0
1
2
3
3
0
T I M E
Figure 1. Comparison of three thermal programs for the determination of Mn in several materials. In all cases the char temperature was 1200 "C and the final atomization temperature setting was 2300 "C. The temperature at which maxlmum power was stopped was set to 2000, 2300, and 2600 "C. The upper curve was 20 pL of seawater. The middle curve was the same amount of MgCI, that is in seawater and the lower curve the same amount of Mg, present as Mg(NO,),.
4
5
6
SEC.
SEC.
0
1
2
3
4
5
6
SEC.
Figure 2. Recorder traclngs of the thermal profiles of the graphite tube used in Figure 1. The char temperature was 1200 "C. 2200
c
steady-state temperature. The furnace was fired to this temperature for a few seconds and the photodiode potentiometer was adjusted so that the red indicator light just turned on. The Ircon recording pyrometer was used with a fast recorder to record temperature profiles as previously described (6). The temperature indicating system appeared to have a response time of about 0.1 s.
RESULTS AND DISCUSSION Using three sets of analytical conditions which will be described below, we compared the absorption signals obtained for Mn in a seawater sample, in a solution of 0.1% MgC12,and in a solution of 0.16% Mg(NO&. Approximately 0.4 ng of Mn was added to each solution. The position and shape of the absorbance profiles of Figure 1were quite similar for all three sample solutions. Since it seemed very likely that the MgClz and Mg(NO& would have hydrolyzed by the completion of the char step, it was really the Mg that produced the advantage and therefore any easily hydrolyzed Mg salt would do equally well. This indicated that it was the MgClz in the seawater which provided the beneficial properties in our seawater experience. However, since chloride is known to produce interference in the vapor phase, we chose to use Mg(NOJ2 as a matrix modifier. One of our observations in previous experiments with Mn in NaClZ, not shown here, was that the Mn absorbance signal included a strong shoulder which was not present with seawater or Mg(NO& We believed that the elimination of the shoulder resulted from a delay in the appearance of Mn in the presence of Mg, which permitted the furnace system to come more closely to constant temperature. The purpose of a matrix modifier in conjunction with the platform was to delay the atomization until the graphite tube approached a stable temperature. Thus it was important to determine the time dependence of the thermal profile of the tube, the conditions which control the profile, and the relationship in time between the thermal profile and the absorbance signal. Experiments were conducted to determine the time dependence of the thermal profile of the tube. Misadjustment of the setting of the photodiode potentiometer in either direction had a relatively large effect on the thermal profile. This was explored in Figure 2 using a nominal temperature setting of 2300 "C for the atomization step. The thermal profiles were compared when the maximum power cutoff was adjusted correctly and when it was deliberately adjusted to 300 "C less and 300 "C more than was proper. The Ircon pyrometer was used to determine the cutoff temperature for the diode. The three thermal profiles are shown in Figure 2. The three profiles shown were typical of the large number of firings made during this experimental study. The profiles obtained were very repeatable within a run, but it was difficult to get precisely the same profile each time we attempted to repeat previous settings.
2
0 T I M E
CSEC)
Figure 3. Data system absorbance profiles of 0.4 ng of Mn alone and in the presence of varying amounts of Mg(NO,),. The thermal profile of the graphite tube is drawn in from curves like Figure 2. The char
temperature was 1200 "C. Using the conditions in Figure 2, the several absorbance profiles shown in Figure 1were obtained. Thus the temperatures shown in Figure 1 are the temperatures of the tube a t which the maximum power is discontinued. In all cases, the final steady-state temperature was the same, 2300 "C. The very large difference between the absorbance profiles that were found was caused by the fact that atomization was occurring while the temperature was still changing. The rising portion of the absorbance signals for maximum power cutoff setting of 2300 and 2600 "C were similar since atomization had started while the temperature was still rising. The 2600 "C signals returned rapidly to base line because the vapor-phase temperature was several hundred degrees greater during the short time when the atomic cloud was present. In Figure 3, absorbance profiles are shown for Mn alone and in the presence of several concentrations of Mg(N03)> In order to observe the delay produced by the Mg(N03)2,it was necessary to use a lower char temperature that would retain Mn in the solution without Mg(N03)zthrough the char step; thus the char temperature was reduced to 1200 "C. The other conditions were as in Table I. The figure showed a delay of about 0.25 s in the presence of 3 Fg of Mg(NO&. Increasing concentrations of the Mg added still further delay. The Mn-only absorbance signal showed the characteristic shouldering or doubling which we observed for the platform analysis when the analyte peak was evolved before the temperature had reached its maximum. The thermal profile for these conditions was drawn on Figure 3. The shape of the absorbance profiles for two different atomization temperatures is compared more directly in Figure 4. The shapes for 2100 "C and 2300 "C were equally satisfactory. The thermal profiles were drawn from recorder tracings. The effect of a lower atomization temperature was to move the time when steady-state temperature was achieved earlier by about 0.07 s per 100 "C. Most of the area of the 2300 "C absorbance profile was within the steady-state thermal conditions. Still more of the 2100 "C profile was within these limits, but the profile returned more
ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982 623
'T
0,6r
2300°C T
W U Z
K 0 Ln 0
__
I
4
0-
T I M E
CSEC>
,Absorbanceprofiles for a solution oontalnlng 0.4 ing of Mn and 300 pg of Mg(N03), at two atomization temperatures. The thermal profiles of lthe graphlte tube are drawn in for 2100 and 2300 OC. The char temperature was 1400 OC.
't
\.
I
;1
100
L. 1wo
p9 Ca Cl2
3
Flgure 4.
$ 1
L-
00
c-----------t--------+--+---0
O0.12 I
Flgure 7. The interference of CaCI, on Mn absorbance. The sample contained 0.4 ng of Mn and 15 Mg of Mg(N03)2.For comparison, the curve Is shown uslng the wire technique ( 8 ) .
'T
,-
WiTHOUT MOOiFiER
-_- - _ _ _ _ _ _ _
0 0
4
T I M E CSEC> . 10 100 1000 + + pg NaCl
Interference of NaCl on Mn absorbance. The sample contained 0.4 ng of Mn and 15 pg of Mg(N03),. The samples without modifier were charred at 1100 OC and atomized at 2300 OC.
Figure 5. The
$1 Li
08
Absorbance profiles for 1 ng of AI alone and for 1 ng of The AI alone profile was obtained by charring at 1500 OC whlle the profile with AI plus Mg(NO& was obtained by charring at 1700 OC. Flgure 8.
AI with 50 pg of Mg(NO,),.
.rn/s WORK
pg Mg Clz
CHAR TEMPERATURE,OC
The interference of MgCIz on Mn absorbance. The sample contained 0.4 ng of Mn and 15 pg of Mg(NO,),. For comparlson, the similar curve Is shown from ref 7 and 8.
added.
slowly to the base line. Since the results appeared satiefactory at both temperatures we recommend 2201) " C for most work, although the atomization temperature is not critical, The choice of char temperature is critical since at too high a temperature Mn was lost and too low a temperature potential interferents remained. The char temperature was chosen to be about 100 OC less than that temperature a t which a losri of Mn signal was detectable. The effect of increasing concentrations of NaCl on IMn absorbance was compared in Figure 5 between the present method and a method without matrix modifier. The improvement with the new procedure is considerable. The interference from MgClz is shown in Figure 6. For comparison, our previous work (7) is shown using slightly different conditions. The added Mg in the present method shoulld have no effect a t high concentrations of MgCl2, since all the Mg is
presumably reduced to MgO. However, the higher charring temperature of this method might have been expected to help. The data show that it did not help. The interference data using the wire technique (8) show little interference up to very high concentrations of MgClZ. The interference on Mn from CaClz is shown in Figure 7. The present technique produced results that were similar to our previous results using the wire technique (8). The addition of Mg(NO& delays the appearance of the A1 absorbance signal by about 0.5-1 s. Figure 8 shows two absorbance profiles using 1ng of Al. One sample also had 50 pg of Mg(NOJ2. With the added Mg(NO&, the charring temperature was increased to 1700 "C, thus possibly removing more of the inorganic matrix. This also reduced the temperature difference between the char and atomization steps and, therefore, the time required to reach a stable temperature.
Figure 6.
Figure 9.
Char study for 2 ng of AI, alone, and with 50 pg of MgNQB
824
Anal. Chem. 1982, 5 4 , 624-629
Figure 9 shows the char study for Al, comparing A1 alone with Al in a matrix of 50 Mg of Mg(N03),. In the presence of Mg(N03),,
Al is not lost at 1800 OC while, without Mg(NO& Al is lost above 1600 "C. We have seen some variability in the temperature of the inflection point, and we therefore have standardized on a char temperature for A1 of 1700 OC with Mg(N03)?. The conditions described for the determination of Mn appear to be very rugged with a broad optimum range. We have selected a char temperature of 1400 OC, but a change of 100 OC in either direction should have no effect on the results. An atomization temperature for Mn of 2100-2400 OC appeared satisfactory and we have selected 2200 "C. Large changes in the amount of Mg added do not seem to have much effect on the analysis. We have selected a concentration of about 0.2% or 50 Mg in the aliquot used. The only disadvantage that we can see to concentrations as high as 1.5% is the difficulty of obtaining Mg(N03)zfree of contamination. The choice of 1700 "C as the char temperature for Al when the Mg(N03)2matrix modifier was used minimized the difference between char and atomization temperature. The appearance temperature of A1 was higher than Mn and therefore a higher atomization temperature, 2400 "C, appeared to be satisfactory. The use of Mg(N0J2 is particularly advantageous since it is a well-established aid in the dry ashing of organic materials (9). It is frequently added to samples to be ashed since the resultant Mg oxide acts as a stable diluent of the inorganic materials left as the organic compounds are destroyed. This is probably an important function in the graphite furnace charring step as well. The appearance temperature of Mn was shown by Maessen et al. (10) to be delayed by AI(NO3)3. In fact, they showed that the presence of the reduced an interference on Mn from BaC1, when integrated absorbance was used for quantitation. No argon flow was used during atomization in all of these experiments because the results were more precise and generally more sensitive than with gas flow. It was very important to use a cleanout step at the end of each analysis so that material from a previous analysis did not influence the subsequent analyses. Problems were found when the cleanout temperature was too low or the time too short. We do not understand why MgClz and CaClz at high concentration have a larger influence on Mn than does NaC1. Certainly
the interference that is observed is a vapor-phase molecular binding of Mn by C1. Perhaps some of the C1 is trapped within the M,O, crystals and is released in the atomization step. However, the freedom of the wire technique from MgC1, interference suggests that the C1 is condensed in the cooler portion of the tube and is revolatilized during the atomization cycle. Perhaps this becomes progressively more serious as the matrix is changed from NaCl to MgClz to CaCl, because the chlorides are progressively more stable in the same progression (11). We wed the method proposed here to determine Mn in stream waters using standards prepared in the matrix modifier. Recoveries were always near 100% and the results appeared to be accurate. Without the matrix modifier, the results were more variable. We have used the Mg(N03)2matrix modifier for the A1 determination also (2) and we were able to free the determination of interference from CuC1,. We have preliminary experience indicating that Mg(NO& is a useful matrix modifier for Ni, Co, and Cr, permitting higher char temperatures without loss of the analyte.
LITERATURE CITED (1) Carnrlck, 0. R.; Slavln, W.; Manning, D. C. Anal. Chem. 1981, 5 3 ,
1886-1872. (2) Manning, D. C.; Slavln, W.;Carnrick, G. R. Spectrochh. Acta, in press. (3) Genc. 0.; Akman, S.; Ozdural, Ates, S.; Balkis, T. Specfrochlm.Acta, Part 6 1981, 388,183-168. (4) "Analytical Methods for Furnace Atomic Absorption Spectrometry"; Perkln-Elmer Corp.: Norwalk, CT, Part No. 8010-0108, 1980. (5) "Instructions, HGA-500 Graphite Furnace"; Perkln-Elmer Corp.: Norwalk, CT, Part No. 993-9449. (6) Slavln, W.; Myers, S. A,; Manning, D. C. Anal. Chlm. Acta 1980, 117, 267-273. (7) Slavln, W.; Manning, D. C. Specfrochlm. Acta, Part 6 1980, 358, 701-714. (8) Manning, D. C.; Slavln, W. Anal. Chim. Acta 1980, 118, 301-306. (9) Gorsuch, T. T. "The Destructlon of Organic Matter"; Pergamon Press: New York. 1970 (10) Maessen, F . J: M. J.; Balke, J.; Masse, R. Spectrochim. Acta, Part 6 1978. 338. 311-324. (11) Robinson, P. L.; Smith, H. C.; Brlscoe, H. V. A. J . Chem. SOC.1926, 45. 836.
RECEIVED for review October 13, 1981.
Accepted December
11, 1981.
Depression of Calcium, Strontium, and Barium Signals by Phosphine in Atomic Spectrometry Gary L. Long and Charles B. Boss" Deparfment of Chemistry, North Carolina State University, Ralelgh, North Carolina 27650
The presence of phosphine, PH,, in the fuel severely depresses Ca, Sr, and Ba atomlc emission and absorption slgnals from alr/acetylene flames. The depresslon of the analyte signal Is belleved to occur from a slowlng of the free atom supply rate brought about by a reactlon between the combustlon products of PH, and the condensed phase analyte In the flame. The use of releaslng agents such as La3+ and EDTA Increases the sensltlvlty In the measurement of Ca, Sr, and Ba In PH, contamlnated flames but does not totally allevlate the depresslon. For hlgher sensltlvlty flame atomic spectrometric determlnatlons uslng alr/C,H, flames, purified or scrubbed C,H, should be used and La3+ Included In the samples and standards.
The determination of group 2A elements by flame atomic spectrometry is impeded by the presence of phosphorus 0003-2700/82/0354-0624$01.25/0
containing species in the analyte matrix. The most commonly known example is the Ca2+/P043-interference ( I ) where PO4* from acid or the salt combines with Ca2+ in the condensed phase to form refractory compounds of Ca. These slowly vaporizing species of Ca decrease the concentration of free vapor phase Ca atoms in the spectrometer's light beam, thus, causing a less intense analyte signal to occur. This paper reports on the PH3 depression of Ca, Sr, and Ba atomic spectrometric signals. Since PH3 is not very soluble in water, it is unlikely t h a t PH3 is absorbed into the sample aerosol to create a classical phosphorus depression. Unlike interferences, PH, is a contaminant group 2A element/PO:in the CzHz fuel and is not commonly present in the analyte matrix. However, a significant decrease in the analyte signal occurs when PH3 is present in the fuel gas. Most CzHz is made from CaCz and, frequently, CaCz is contaminated by Ca3P2. The Ca3Pzimpurity produces P H 3 through a reaction similar to the one that produces C2H2. 0 1962 American Chemical Society
