Anal. Chem. 1982, 5 4 , 2491-2495 (22) Warner, I . M.; Davldam. E. R.; Chrlstlan, (5. D. Anal. Chem. 1977, 49, 2155-2159. (23) Ho, C.-N.; Chrlstlan, G D.; Davldson. E. R. Anal. Chsm. 1980, 52, 1071-1079. (24) Shelly, D. C.; Fogarty, M. P.; Warner, I. M. HRC CC J . High ResoM. Chromafogr. Chromatogr. Commun. 1981, 4 , 616-626. (25) Cherkasov, A. S. Bull. Acad. Sci. USSR,Phys. Ser. (Engl. Trans/.) 1956, 20,436-439.
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(26) Cherknsov, A. S. Opt. Spekfrosk. 1959, 7 , 211-214.
RECEIVED for review June 16,1982. Accepted September 13, 1982. This work was supported in part by grants from the and the Office Department Of of Naval Research.
Inductively [Coupled Plasma Emission Spectrometry with Internal Standardization and Subtraction of Plasma Background FIuctuat ions8 Gary J. Schmldt" anid W. Slavln The Perkin-Elmer Corporation, Norwalk, Connecticut 06856
The Improvements resulting from the use of Internal standardlzation In lnductlvely coupled plasma emission spectrometry have been experlnientaily determlned. I n addlllon, we have also slmultaneousiy measured plasma background fluctuations and corrected analyte signals for these variatlons. These technlques have been applied to the determination of several elements In various matrlx solutions and some USGS water samples and have resulted In signlflcant Improvements In both measurement preclslon and accuracy. When determining Mn in a 5 % NaCl solutlon, precision was Improved 25-fold when using internal standardization. Correlation coefficients better than 0.99 were obtained for flve elements determined In the USG!; water samples.
Since its introduction ( 1 , 2), inductively coupled plasma emission (ICP) spectrornetry has grown rapidly and has become an extremely valuable technique for trace element determination. Its usefulness has extended into many areas of interest, including the analysis of geological, environmental, biomedical, and agricultural materials. A review of many ICP applications has been given (3). A number of reports have appeared in the literature describing the use of ICP for determining trace elements in water samples including wastewaters ( 4 , 5 ) , natural waters (6, 7), and seawater (7-10). Difficulties that confronted the analyst when developing these methods included choosing the proper analyte emission line to reduce interelemeint interference and to provide adequate sensitivity and choosing the proper background correction intervals to minimize continuum or wing-overlap spectral interference. The difficulties encountered in fulfilling these requirements have been minimized by the use of modern computer-controlled instrumentation. Of equal importance to the precision and accuracy of the methodology is the choice of appropriate standardization and calibration procedures. Samples consisting of relatively pure aqueous solutions may usually be standardized with simple aqueous standards. However, samples which contain complex or variable matrix constituents may lead to falsely reduced analyte concentrations due to the effect of matrix components on the various plasma processes. In these situations, careful matrix matching of stand'ard is required to ]provide acceptable results. Unfortunately, mmples are usually highly variable
and the matrix is largely unknown. Internal standardization has been used to compensate for these changes in analyte emission intensity as well as to improve measurement precision by correcting for various plasma noise sources. Since many of these noise sources affect different analytes similarly, adding an element as an internal standard permits the emission signal from the analyte to be corrected by monitoring the internal standard signal. This results in improvements in analytical precision and accuracy. By determination of the ratio of the emission intensities of the analyte element and the internal standard, much of the noise associated with the ICP measurement can be compensated for. Barnett et al. (11,12)have studied both theoretically and experimentally the various parameters associated with selecting acceptable analyte/internal standard element pairs. Watters and Norris (13) have discussed the instrumental factors which contribute to random error in emission measurements and have noted some of the advantages of internal standardization. Several workers have used internal standardization to improve the quality of the emission measurement. Feldman (14) found a 2- to %fold improvement in precision and improved analytical accuracy when using internal standardization. Uchida et al. (15) used Y as an internal standard and found a 2- to 20-fold improvement in precision for some elements when using a microsampling technique with 5-pL sample volumes. Skogerboe and Coleman ( 1 6 ) evaluated the use of a microwave-induced plasma for multielement emission analysis and used In as an internal standard. They found a significant improvement in analytical accuracy even when large amounts of sodium were present in the sample. Salin and Horlick (17) noted that the use of internal standardization either improved or degraded precision depending on the choice of analyte/internal standard emission line pairs. The correct choice of internal standards has received considerable attention, Barnett et al. (11,1.2)proposed guidelines for choosing internal standards based on consideration of the ionization energy, excitation energy, and partition function of the elements. Recently, Myers and Tracy (18)have shown that by the proper choice of ICP operating parameters a single internal standard element improved the analytical performance independently of whether an ion or neutral atom line was ehosea fci the analyte.
0003-2700~82/0354-2491$01.25/0 0 1982 American Chemlcal Society
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982
Mn
INTERFERENCE FILTER
1 FNS
Mn/Sc
BEAMSPLITTER
BACKGROUND PHOTODIODE
7-
LENS
’X
I
0
I5
30
TIME (SECONDS)
r---------q
rt-, PLASMA
CHROMATOR
Flgure 1. Schematic of the optical arrangement showing the ICP/5000 monochromator and the experimental photometrlc system.
In addition to the calculation of the analyte/internal standard ratio, we have also simultaneously subtracted the plasma background emission fluctuations from each emission reading prior to calculation of the i n t e r d standard ratio. In this work, this was achieved by using an experimental dual filter photometer one channel of which measured argon line emission in order to monitor background emission fluctuations. This background emission was subtracted from both the analyte and internal standard emission signals to provide the appropriate net emission signals, These were then used to determine the internal standard ratio which corrects for various plasma noise sources. A more complete discussion of these procedures has been given (18). In this paper we have determined experimentally the improvements in analytical accuracy and precision which were obtained when these procedures were implemented.
EXPERIMENTAL SECTION Instrumental Methods. A Perkin-Elmer Model ICP/5000 inductively coupled plasma emission spectrometer equipped with a Model 3600 data station and a PR-100 printer was used. A quartz torch and spray chamber and a glass concentric nebulizer were used for all experiments. A Model 56 dual-pen recorder was used to provide graphical representation of the spectra. A peristaltic pump was used to introduce sample solutions into the nebulizer. The nebulizer argon was obtained from a gas cylinder set at 80 psi. A needle valve was inserted between the nebulizer gas rotameter and the pressure gauge to permit direct adjustment of the nebulizer gas flow, rather than the gas pressure. This arrangement permitted the nebulizer flow ta remain highly stable even during fluctuations in the input pressure. The coolant (plasma) gas was obtained from a separate liquid argon source. For all experiments, Sc was used as the internal standard. Simultaneous measurement of the internal standard and the analyte element were made by using an experimental dual photometric system mounted on the rear of the torch unit. The optical arrangement i s shown schematicallyin Figure 1. This photometer incorporated a beam splitter which split the plasma emission into equal portions and directed it to each channel of the photometer. For these experiments, one channel of the photometer was set to measure Sc at the 424.7 nm emission line and the other channel was used to measure plasma background. Plasma background was measured by using a group of argon lines at 419.0-420.0 nm. This portion of the plasma background was chosen since it is removed from measurement wavelengths of interest. The fluctuations in plasma background at these wavelengths are highly correlated with those observed at other wavelengths.
45
i
0
I
I
2
TIME IMINUTES)
Flgure 2. Graphical representation of various emission signals from an aqueous solutlon containlng 1 mg/L Mn and 10 mglL Sc. Mn wavelength is 257.6 nm, photomultiplier tube voltage is 550 V, and slit width Is 0.02 nm.
The two simultaneously measured signals from the photometer were fed into an analog signal processor. The signal processor used these two signals (internal standard and background emission) to correct the analyte signal for fluctuations resulting from plasma background variations and variations associated with aerosol formation. Reagents. Individual stock solutions of the elements at a concentration of 1g/L were obtained from Alfa Products, Danvers, MA. Dilutions of these stock solutions were made with deionized water to provide appropriate working concentrations. All sample solutions were prepared in polyethylene screw-capped bottles previously rinsed with 50% nitric acid. Plasma Conditions. The plasma operating conditions were set to the following values except where noted in the text. The forward rf power was set at 1250 W and the reflected power was typically 53 W. The coolant (plasma) argon flow was 16 L/min and the nebulizer gas flow was 0.75 L/min. Analyte measurements were made at a viewing height of 15 mm above the top of the load coil. The peristaltic pump delivered the sample solution at a liquid flow rate of 1 mL/min. Wavelength and other spectrometer settings are noted in the appropriate figure legends. Signal Compensation Procedure. Analyte emission signals were processed by subtracting the plasma background emission (measured by one channel of the filter photometer) to provide the net analyte emission intensity. The net analyte intensity, I*‘, may be expressed as IA’ = IA - IB (1) where ZA is the total observed analyte emission intensity and I , is the background emission intensity measured at the group of argon lines at 419.0 to 420.0 nm. Likewise, the internal standard emission was processed similarly and expressed as I{ = I , - ZB where ZI is the total observed internal standard emission intensity plus background intensity measured by the experimental photometer. The internal standard corrected ratio, IC, was obtained by dividing the net internal standard emission into the net analyk emission (3)
Measurement of all three emission signals was made simultaneously. A detailed discussion of the various parameters affecting signal compensation has been given (18). The subtraction of the plasma background emission from the analyte and internal standard emission readings was done to remove the noise associated with the fluctuations in plasma background. Since all emission measurements are made simultaneously, these fluctuations may be effectively removed, resulting in improved correlation of the analyte and internal standard signals, as will be shown. This procedure should not be confused with subtraction of a “blank” emission signal during standardi-
ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982
2493
SIGNAL COMPENSATION
-O r"
0'9I 7 1
0.8 RELATIVE
EMISSION
I
__.
Mn/Sc
I
I
IO 15
0
I5 45
30
20 60
25 RUN 75 MINUTES
Figure 4. Effect of 5 % NaCl matrix on Mn emission intensity. 22Or
0
30
60
TIME (SECONDS) SIGNAL COMPENSATlON
Figure 3. Graphical representation of various emission signals from
____-----
a 5 % NaCl solution contalriing 1 mg/L Mn and 10 mg/L Sc. Instrumental conditions are as listed in Figure 2. RELATIVE EMISSION 'go
zation, this procedure still being utilized in this work. Figure 2A shows an example of the background-subtracted Mn signal and the background-subtracted Sc signal. These signals were obtained simultaneously. The sample was an aqueous solution containing 1 mg/L Mn and 10 mg/L &. The fluctuations in the signals were largely due to variationw in the aerosol formation and transport processes. Additional fluctuation due to variations in the plasma background emission has already been subtracted. There was a high degree of correlation between the two signals. Due to the highly correlated nature of these signals, resulted in division to provide the internal standard ratio, IC, removal of the signal fluctuations as shown in Figure 2B. As a result, a significant decrease in the noise associated with the emission measurement W~BSobtained. Figure 3 shows similar emission signals ffor Mn and Sc in a solution containing 5% NaCl. In this situation, very large signal fluctuations were observed due to the transient clogging of tho nebulizer tip by the high dissolved solids content of the sample. However, even though the efficiency of nebulization was considerably degraded under these conditions, the ratio of analyte element to internal standard was not affected. Therefore, as can be seen, these very large fluctuations were nearly completely removed using signal compensation. Note that the observed emission fluctuations were plotted to a different ordinate scale as compared to those shown in Figure 2.
RESULTS AND DISCUSSION A series of experiments were performed to evaluate the improvements in precision and accuracy obltained when using internal standardization with subtraction of plasma background fluctuations. Therie results were compared with results obtained when these procedures were not employed. Sample matrices consisted of aqueous solutions containing various amounts of salts or acids. An experiment was ruin to determine the effect of high concentrations of sodium chloride on the precision and accuracy of Mn determination. Mn was added at a concentration of 1mg/L to an aqueous solution containing 5% NaCl. Sc was used as the internal standard and added to provide a concentration of 50 mg/L. A single aqueous solution containing Mn and Sc a t the same concentrations as in the salt matrix was used for standardization. Figure 4 shows the results obtained when measuring the h4n emission intensity in this matrix. Each point on the graph represents the average of 10 replicate readings using a 1-s integration time. Each measurement was made by scanning across the emission peak t o determine peak intensity.
I60
NORMAL
0.1 0.2
0.4
NaCl
Figure 5. Effect of
0.6
0.6
1.0
PA)
low concentrations of NaCl on Mn emission in-
tensity.
As can be seen, data obtained without the use of internal standardization (normal) showed a significant decrease in the observed emission intensity due to the presence of the NaC1. In addition, there was considerable fluctuation in the average emission intensity between each reading. Therefore, both measurement precision and accuracy were degraded under these conditions. In this particular experiment, the average within-run precision (n = 10) was about 8% for the "normaln run and the average emission intensity was decreased about 30%, relative to the standard, due to the high concentration of NaCl. Comparative data obtained by using internal standardization (signal compensation) revealed very little change in emission intensity due to the presence of the sodium chloride over a period of about 75 min. Under these conditions, the nebulizer efficiency was very erratic due to the partial clogging of the nebulizer tip by the NaC1. This can clearly be seen from the data obtained when internal standardization was not utilized. Since the ratio of analyte element to internal standard did not change under these conditions, these variations in nebulizer efficiency were removed. The measurement precision was determined to be about 0.3% for this set of data, representing a %-fold improvement. Since the concentration of NaCl used in the above experiment was relatively high, an experiment was performed to determine the effect of smaller fluctuations in the NaCl content. These concentrations, SI%, are typical of those which might be found in some river water samples. Water samples containing 1 mg/L Mn and 10 mg/L Sc were prepared by adding NaCl to provide concentrations equal to O%, 0.01%, 0.05%, O.l%, 0.2%, 0.5%, and 1.0%. The peak heights of the Mn emission signals were measured and plotted against the concentration of NaCl. The results are shown in Figure 5. Standardization was by use of a single aqueous standard
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982
Table I
Table 111. USGS Water Samples
amt of Mn, mg/L 10
1 1 1
1 1 0.1 0.1 0.1
sample matrix
precision ( n = 10) internal standardization yes no
4% NaCl
0.29
3.6
H*0
10% H,PO, 10%HNO, 25% HNO, 50% HNO,
0.11 0.19 0.17 0.33 0.27
0.4 0.7 2.4 1.1 0.8
0.2% LiBO 0.5% LiBO, 1.0% LiBO,
0.41 0.56 0.58
1.0 0.8 2.5
element and sample no. 59 61 63 71 77
mg/L 1 1 1 1
sample matiix 10% H,PO, 10% HNO, 25% HNO, 50% HNO,
59 61 63 71 77
1
