Article pubs.acs.org/ac
Standard Dilution Analysis Willis B. Jones,† George L. Donati,*,† Clifton P. Calloway, Jr.,‡ and Bradley T. Jones† †
Department of Chemistry, Wake Forest University, Winston-Salem, North Carolina 21709, United States Department of Chemistry, Physics and Geology, Winthrop University, Rock Hill, South Carolina 29733, United States
‡
ABSTRACT: Standard dilution analysis (SDA) is a novel calibration method that may be applied to most instrumental techniques that will accept liquid samples and are capable of monitoring two wavelengths simultaneously. It combines the traditional methods of standard additions and internal standards. Therefore, it simultaneously corrects for matrix effects and for fluctuations due to changes in sample size, orientation, or instrumental parameters. SDA requires only 200 s per sample with inductively coupled plasma optical emission spectrometry (ICP OES). Neither the preparation of a series of standard solutions nor the construction of a universal calibration graph is required. The analysis is performed by combining two solutions in a single container: the first containing 50% sample and 50% standard mixture; the second containing 50% sample and 50% solvent. Data are collected in real time as the first solution is diluted by the second one. The results are used to prepare a plot of the analyte-to-internal standard signal ratio on the y-axis versus the inverse of the internal standard concentration on the x-axis. The analyte concentration in the sample is determined from the ratio of the slope and intercept of that plot. The method has been applied to the determination of FD&C dye Blue No. 1 in mouthwash by molecular absorption spectrometry and to the determination of eight metals in mouthwash, wine, cola, nitric acid, and water by ICP OES. Both the accuracy and precision for SDA are better than those observed for the external calibration, standard additions, and internal standard methods using ICP OES.
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trometry, where the optical path length is variable and depends upon the positioning of a sample droplet pressed between two plates.3 Other common applications involve flowing streams such as gas and liquid chromatography.4,5 The internal standard method will not correct for sample matrix effects, but it will correct for external perturbations as long as the internal standard species has exactly the same response to the perturbations as the analyte. The method is quicker than standard additions, especially if more than one sample is analyzed. One potential drawback, however, is that the effect of the sample matrix on analyte sensitivity must be exactly the same as its effect on the internal standard. Since the internal standard calibration curve (a plot of the analyte-to-internal standard signal ratio versus the concentration of analyte) is constructed using solutions prepared in pure solvent, the signal ratio for a fixed amount of analyte/internal standard must be the same in every sample matrix. This often is not the case. The combination of the standard additions and the internal standard calibration methods has been shown to improve the results for Cd detection in urine, a typically complex matrix, without sample digestion or any other matrix modification using inductively coupled plasma optical emission spectrometry (ICP OES).6 In this case, the calibration methods were combined in the traditional sense: various amounts of a Cd
tandard dilution analysis (SDA) is a novel calibration method that may be applied to most instrumental analysis techniques that will accept liquid samples and are capable of monitoring more than one wavelength simultaneously. SDA combines the traditional methods of standard additions and internal standards. The method of standard additions usually involves the preparation of a series of solutions, each containing an equal amount of sample but a different amount of added analyte. Since each solution contains the same amount of sample, any effect by the matrix on the analyte should be replicated and corrected. While the mathematics of the method could be complex depending upon the specific procedure employed,1 it typically boils down to the preparation of a calibration curve using the prepared solutions and then determining the concentration of analyte in the sample from the x-intercept of the line. The method is time-consuming and cumbersome, especially if more than one sample must be analyzed. On the other hand, it offers a powerful correction if the sample matrix significantly affects the analytical signal.2 Alternatively, if the analytical signal is affected by external factors such as light source fluctuations or changes in sample volume or position, the internal standard method is more appropriate. In this case, identical amounts of a judiciously chosen species (internal standard) is added to each analyte solution prepared for a calibration curve. The same amount of internal standard is also added to each sample, which must not contain that species naturally. Typical applications for the internal standard method include infrared absorption spec© XXXX American Chemical Society
Received: November 6, 2014 Accepted: January 19, 2015
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DOI: 10.1021/ac504152x Anal. Chem. XXXX, XXX, XXX−XXX
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mACA and SI = mICI), the ratio of the analyte to internal standard signals is described by
standard reference solution were added to identical amounts of urine sample, and a constant concentration of an internal standard was also added to each solution. While the results were improved by the combination of the methods, the solution preparation was tedious. SDA uses a similar approach, with the advantage of simplicity. Because of the gradual dilution obtained by mixing only two solutions, many standard calibration points are obtained in a single run. In practice, SDA is performed by continually monitoring two signals for a solution containing a constant amount of sample and varying amounts of a standard solution containing both analyte and internal standard. This may be accomplished in molecular absorption spectrometry by partially filling a cuvette with a solution containing 50% sample and 50% standard mixture. As data are collected for this solution, a second solution containing 50% sample and 50% solvent is slowly added. Analyte and internal standard signals are collected as the solutions mix in the cuvette. In this case, the relative amount of sample remains constant (50%) while the relative amount of standards added decreases (the standards are diluted). Similarly, elements may be determined by ICP OES if a solution containing 50% sample and 50% standard mixture is added to the typical sample reservoir. As the peristaltic pump delivers this sample to the ICP, a solution containing 50% sample and 50% solvent is added to the same reservoir. Data are collected as the two solutions are mixed. The relative amount of sample remains constant, while the standards are diluted. Gradient dilution techniques have been applied in flow systems in the past. A calibration procedure for flow injection flame atomic absorption spectrometry was reported in 1998.7 Later, a gradient ratio standard addition method was reported for the same technique.8 Some of these ideas were discussed among a set of univariate calibration techniques for flow injection analysis.9 The same research group has described a manifold for calibration in flow injection analysis,10 and also a technique for flow injection gradient titration.11 More recently, a gradient dilution method that overcomes matrix interferences in ICP OES has been described.12 This approach uses an HPLC pump to dilute the sample provided to the ICP while monitoring two emission lines continually. This allows an automated approach to finding the highest sample concentration where the matrix effects are minimized. SDA, on the other hand, requires no flow since the dilution is performed in a single container. To demonstrate the viability of the SDA method, six commercial mouthwash samples have been analyzed for the presence of FD&C dye Blue No. 1, using FD&C dye Yellow No. 6 as the internal standard. Subsequently, a suite of eight metals (Al, Cd, Co, Cr, Cu, Fe, Ni, and Pb) has been determined in mouthwash, wine, cola, nitric acid, and water by ICP OES using Y as the internal standard.
SA mC m (C std + CAsam) m C sam m C std = A A = A A = A A + A A SI mIC I mIC I mIC I mIC I (1)
Notice that this is the equation for a line if we plot y as (SA/ SI) versus x as (1/CI). The slope and intercept of this line are given by slope =
mA CAsam mI
intercept =
mA CAstd mIC I
(2)
The SDA method is performed by combining a sample containing an unknown amount of analyte, with a standard solution containing a fixed ratio of analyte to internal standard (CAstd/CI = constant). If the relative amount of sample in the solution remains constant during the mixing process, then the matrix effects are constant and (mA/mI) is constant. Therefore, the intercept is also constant. The concentration of analyte in the sample is contained in the value for the slope of the line and is given simply by CAsam =
slope C std × A intercept CI
(3)
The final term in eq 3 is the concentration ratio of analyte to internal standard in the prepared standard mixture, so the concentration of analyte in the sample is easily obtained from the calibration plot. Eq 1 demonstrates the types of interference that SDA may correct. Note that the concentration of both the analyte and the internal standard must be within the linear dynamic range of the traditional calibration curve method. Also, the signals measured for the analyte and internal standard (SA and SI) must not include contributions from any other species. Given these conditions, the SDA method will correct any matrix effects that affect the sensitivity of the analyte, any chemical interferences that might affect the sensitivity of the analyte, and any fluctuations caused by variations in the flows of liquids or gases, light source power, or sample position. The SDA method will not correct for spectral interference unless the magnitude of that interference could be measured beforehand and subtracted from the appropriate signals.
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EXPERIMENTAL SECTION Visible Absorption Spectrometry. FD&C Blue No. 1 (B1) and FD&C Yellow No. 6 (Y6) crystalline dyes were obtained from Rainbow Colors, LLC (Windsor, CT). Caution is advised when handling the solid dyes to prevent accidental inhalation. Blue no. 1 also may be harmful to the aquatic environment. An aqueous solution was prepared containing a mixture of 6.03 μM B1 (analyte) and 37.6 μM Y6 (internal standard). Concentrations were recorded to three significant figures. A modular UV−vis apparatus consisting of a sample holder with integrated light source (tungsten coil lamp), a glass cuvette, fiber optic, and an USB4000 spectrometer (Ocean Optics, Dunedin, FL) was used to analyze the samples. Solution 1, containing 50% of the standard mixture and 50% sample, was placed in the glass cuvette (1.0 mL aliquot), and the signals at 635 nm (analyte) and 480 nm (internal standard) were recorded using a 1-s integration time. Then 2.0 mL of solution 2, containing 50% sample and 50% distilled water, were slowly added to the same cuvette using a micropipette with no manual
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MATHEMATICAL APPROACH The mathematical approach is straightforward. The analyte species (A) in the sample is combined with a standard mixture containing both the analyte (A) and an internal standard species (I). Analyte signal (SA) arising from both the sample (sam) and the standard (std) mixture is monitored at one wavelength, while the internal standard signal (SI, from the standard mixture only) is monitored at a different wavelength. Given that the signal in each case is equal to the calibration sensitivity (m) times the concentration of each species (SA = B
DOI: 10.1021/ac504152x Anal. Chem. XXXX, XXX, XXX−XXX
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RESULTS AND DISCUSSION Visible Absorption Spectrometry. FD&C Blue No. 1 and FD&C Yellow No. 6 are certified color additives used in many foods and consumer products. Since B1 is added as a colorant to relatively simple commercial preparations, it is easy to detect using the traditional technique of visible molecular absorption spectrometry. Because it is inexpensive and detectable at very low levels using a simple colorimeter, it has been employed in many educational laboratory experiments. Straightforward calibration techniques have been employed to determine B1 in candies,14 powdered drinks,15,16 and mouthwash.17 Of these, mouthwash preparations seem to have the most variable sample matrices, so these were chosen to evaluate the initial viability of the SDA method. SDA signal profiles for four replicates of sample C are shown in Figure 1. Closed points represent the absorbance of the
or mechanical mixing. The analysis time depended on how fast solution 2 was added to solution 1 which, in turn, determined the number of calibration points obtained (i.e., slower dilutions provided more calibration points). In this experiment, typical runs lasted 90 s, resulting in approximately 50 calibration points. Six commercial mouthwash samples (A−F), each containing different amounts of FD&C Blue No. 1 (based on the darkness of their blue color), and varying matrix concomitants were analyzed: Listerine Ultraclean (A), Scope (B), Crest (C), Listerine Fluoride (D), Act (E), and Breath Rx (F). For comparison, the same system, samples, standard solutions, and integration times were used to analyze the samples by the traditional methods of external calibration, standard additions, and internal standard. Atomic Emission Spectrometry. A mixture of metal ions was obtained in two solutions from Teledyne Leeman Laboratories (Hudson, NH). The first solution, used as the reference standard, consisted of Al, Cd, Cr, Co, Cu, Fe, Ni, Pb (analytes), and Y (internal standard) at 100 mg/L in 5% (v/v) HNO3. Yttrium was used in all SDA measurements because it has been shown to be an effective internal standard for a variety of analytes and sample types in ICP OES determinations.13 The second solution was used to spike several samples and check the efficiency of the SDA method for different matrixes. It was identical to the first one, with the exception of containing Sc instead of Y. The sample and reference standard solutions used in the SDA runs were prepared by diluting the original solutions 10-fold. For the SDA method, five samples were spiked with the multielement solution containing Al, Cd, Cr, Co, Cu, Fe, Ni, Pb, and Sc: deionized water; 40% HNO3 (v/v) (Fisher Scientific, TraceMetal grade−caution: use eye and hand protection when handling concentrated nitric acid); 50% (v/ v) Listerine mouthwash; regular Pepsi (Pepsico, Inc.); red wine (Twisted Wine Cellars, Manteca, CA). The reference standard used in each run was prepared by diluting the Y-containing stock solution 1:10 in deionized water. A Prodigy High Dispersion ICP OES (Teledyne Leeman Laboratories) was used in all SDA experiments. Emission signals were obtained by time-resolved analysis (TRA) at 1 s intervals and over the course of 5 min (for a total of 300 data points). The typical recommended instrumental operating conditions were employed. Four steps were required to apply the SDA method in ICP OES using a single peristaltic pump channel: (i) deionized water was introduced into the plasma for approximately 30 s and a baseline signal was obtained; (ii) a solution containing 50% sample and 50% standard (solution 1) was then introduced and the signal increased until a plateau was observed; (iii) after a stable maximum was achieved, the solution containing the blank (50% sample and 50% blank, solution 2) was added to the tube containing solution 1, which resulted in the gradual dilution of the reference standard; and (iv) after the signal reached a stable minimum due to the dilution of solution 1 by solution 2, deionized water was introduced once again to rinse the system and ready it for the next run. For comparison, the same system and stock solutions were used to carry out the traditional methods of external calibration, standard additions, and internal standardization for the analysis of each sample. For each of the traditional methods, five different calibration solutions were used to prepare each plot.
Figure 1. Molecular absorbance measured for FD&C Blue No. 1 (closed circles) at 635 nm, and FD&C Yellow No. 6 (open circles) at 480 nm, during the standard dilution analysis of four replicates of commercial mouthwash sample C.
analyte (B1) measured at 635 nm; open points represent the absorbance of the internal standard (Y6) measured at 480 nm. Gaps between each run are the times required to replace the solution and rinse the cuvette. All replicates in this figure were used to produce a single SDA calibration plot, which is shown in Figure 2. The SDA plot has the inverse of the concentration of Y6 (mM−1) on the x-axis versus the ratio of the absorbance of B1 and Y6 on the y-axis. In all cases, the concentration of Y6 at any given time was calculated using the absorbance measured for Y6 at that time (Figure 1) and the maximum absorbance observed for Y6 prior to the onset of dilution. Therefore, mixing need not be complete prior to data collection, and data are collected “on-the-fly”. Notice that despite the complexity of the sample matrix, the trendline almost perfectly bisects each calibration point, even though all four replicates are included on the same graph. In fact, the correlation coefficient for the trendline was 0.9999, and R2 was 0.9998. Regression analysis performed by Microsoft Excel reported a standard error in the slope of only 0.10%. The error in the intercept was 0.16%. The four separate SDA runs gave B1 concentrations in mouthwash C with a relative standard deviation of only 0.19%. SDA calibration plots for all six mouthwash samples are shown in Figure 3. Clearly, SDA can provide results with a high degree of precision. C
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Figure 2. SDA plot of the four replicates for mouthwash C (Figure 1) plotted as a single run.
Table 1. Concentration and Precision Values for the Determination of FD&C Blue No. 1 in Six Mouthwash Samples Using Different Calibration Methodsa sample
EC
IS
SA
Average Amount of FD&C Blue No. 1 Found (μM) 0.74 0.79 0.33 1.1 1.4 2.0 3.2 4.1 3.8 7.2 7.3 7.2 6.2 7.1 7.6 10.7 13.3 10.2 Precision (% relative standard deviation) A 5.5 5.4 3.7 B 1.3 1.4 0.7 C 1.5 1.2 1.8 D 1.9 1.9 1.4 E 4.3 6.3 5.5 F 3.2 4.0 1.9 average 3.0 3.4 2.5 A B C D E F
Figure 3. SDA plots for six different mouthwash samples (A−F). Each plot includes multiple replicates. The scale is expanded to show the intercepts more clearly, so many data points fall beyond the limits of the axes (see Figure 2).
SDA 0.54 2.1 3.6 7.6 7.9 10.1 0.88 0.23 0.19 0.37 0.44 0.39 0.42
The traditional methods employed five replicates (n = 5), while SDA employed four replicates analyzed as a single run with 150 calibration points total (Figure 3). a
The concentration of B1 in each sample is contained in the slope of the SDA line. The amount of B1 in the sample is found by simply dividing the slope by the relative sensitivities determined from the intercept (eq 2), or by dividing the slope by the intercept and then multiplying it by the relative concentrations of B1 and Y6 in the standard mixture (eq 3). For comparison, the original solutions, blank, samples, and instrumentation used for the SDA determinations were also used to prepare traditional external calibration (EC), internal standard (IS), and standard additions (SA) curves. Results are shown in Table 1. Notice that, generally, %RSD values are higher for the EC and IS methods. This most likely happens because these methods are more severely affected by matrix effects, and the samples evaluated in these experiments present a significant variation in their composition (e.g., sample A has 21.6% v/v ethanol; sample C has no ethanol). Notice that these samples have different y-intercepts (Figure 3). This is because
the different sample matrices affect the calibration sensitivities for B1 and Y6 to different degrees. Not only does SDA correct for this effect but it also provides a visual indication of its magnitude (by comparing the intercepts). As expected, SA and SDA are less sensitive to matrix effects, and their results are more closely related. Therefore, the SDA accuracy should be as good as that measured with the standard additions method, with the advantage of the enhanced precision provided by the correction for fluctuations due to internal standardization. Results in Table 1 clearly show that the SDA method can be as effective as, or even more effective than, the traditional calibration methods. Considering SDA’s low RSDs, it may also contribute to higher powers of detection. In addition to accurate, precise, and sensitive determinations, the SDA method also allows for high sample throughput since only D
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Analytical Chemistry one standard solution needs to be prepared, and the sample is diluted only twice (once with blank and once with this standard solution). Atomic Emission Spectrometry. Figure 4 shows the emission signals obtained for Al in a mouthwash matrix, Fe in a
Figure 5. SDA plots for Al in mouthwash, Fe in nitric acid, and Pb in wine.
times. Since the concentration of all eight analytes spiked in the samples was the same (10 mg/L), the results were combined for each matrix, and the different calibration methods were compared. As one would expect, the SDA results were similar to the ones from the standard additions method, and both were superior to the other calibration strategies (Table 2). The main
Figure 4. Emission signals obtained for Al in a mouthwash matrix, Fe in a HNO3 matrix, and Pb in a wine matrix during an SDA run.
HNO3 matrix, and Pb in a wine matrix during an SDA run using ICP OES. As discussed previously, in the first 30 s of each run, deionized water is introduced into the ICP to establish a blank baseline. Solution 1 is then introduced into the plasma until a stable maximum is achieved. At 120 s into the run, an aliquot of solution 2 is added to solution 1, which results in the gradual decrease of the emission intensities, giving rise to the “SDA region” of Figure 4. Note the 15 s delay between the solution mixing and the beginning of the SDA region as the solution travels through the tubing, to the nebulizer, and into the plasma. At 210 s, water is again introduced into the ICP to prevent memory effects. This cleaning step continues until the 300 s mark, when the system is ready for the next run. Similar to the molecular absorption determinations described previously, the ratio between analyte and internal standard signal intensities is plotted against the inverse of the internal standard concentration. Again, the concentration of internal standard at any given time is calculated by comparing the emission signal of the internal standard in the SDA region (Figure 4) with the signal prior to the onset of dilution. Figure 5 shows the SDA plots created from data in the “SDA region” of Figure 4 and using Y as the internal standard. As observed in Figure 4, the SDA region stretches over approximately 30 s. Because a data point is collected every second, a total of 30 standard calibration points is obtained. Some of these points are not shown in Figure 5 for clarity, but the trendline is constructed from all of the points. Because the standard reference solution used in all experiments contains 10 mg/L of all elements, the ratio between analyte and Y concentrations in the standard is equal to 1. Thus, the concentrations of analytes in the samples are calculated simply by dividing the calibration curve slopes by their intercepts (eq 3). The SDA results were compared to the standard additions, external calibration, and internal standard methods. To prevent any bias during data collection, the Time Resolved Analysis feature was used in all experiments with the same integration
Table 2. Limits of Detection and Concentrations of Eight Metals Found Using Different Calibration Methods EC
IS
SA
SDA
Limit of Detection, mg/L Al, 396.15 nm 0.2 0.6 0.1 0.08 Cd, 214.44 nm 0.1 0.4 0.3 0.03 Co, 228.62 nm 0.2 0.6 0.3 0.01 Cr, 267.72 nm 0.1 0.4 0.3 0.02 Cu, 324.75 nm 0.1 0.5 0.3 0.02 Fe, 259.94 nm 0.2 0.3 0.3 0.08 Ni, 221.65 nm 0.3 0.2 0.3 0.03 Pb, 220.35 nm 0.3 0.3 0.3 0.08 Average Concentration of Eight Metals Found, mg/L (10 mg/L of each metal added) water 10.0 10.9 10.1 10.2 nitric acid 9.1 11.6 8.9 9.8 mouthwash 13.4 12.6 11.2 10.3 cola 10.0 11.6 9.5 9.5 wine 14.6 13.4 11.2 10.5 average 11.4 12.0 10.2 10.1
n 5 5 5 5 5 5 5 5
24 24 24 24 24 120
advantages of the SDA method when compared to standard additions are its simplicity, its superior sample throughput, and the possibility of correcting for variations in physical parameters associated with the sample and its environment during the analysis. The limits of detection (LOD) were calculated by analyzing five blank samples and multiplying the standard deviation of the results by 3. In all cases, the blank was the matrix without the analytes added. The results for SDA were slightly better than those obtained by the other calibration methods (Table 2). This fact may be related to SDA characteristics such as the large number of calibration points used for the blank (30 points for SDA compared to 5 points for SA, for example) and minimal sample manipulation, which may result in lower standard deviations. In addition, blank measurements to determine LOD E
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method may randomly result in artificially high or low values, the average amount found is similar to that for SDA. However, the average percent error is significantly worse for SA (Table 3). In addition, SDA provides improvements in precision, calculated as percent relative standard deviation, when compared to all three calibration methods. For this proof of concept work, we have chosen two different techniques that are widely used and that can suffer from severe matrix effects. ICP-based methods are especially sensitive to such effects,18,19 and we have intentionally chosen complex matrices, with diverse compositions, to demonstrate the capabilities of SDA in comparison to the traditional calibration methods.
in SDA begin with a relatively large analyte signal from the standard. This might help minimize small signal fluctuations from the background, which would be more prominent in the absence of an analytical signal. Furthermore, if the measurement is shot-noise-limited, as might be the case for an emission measurement using an internal standard, a larger initial signal should result in a lower LOD. On the other hand, the SDA method is time-limited (a single point per second is collected in ICP OES, for example). The traditional methods might employ longer integration times to improve detection limits. Table 3 displays the results of all four calibration methods separated by element. Individual entries in the table are an
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Table 3. Concentration of Eight Metals, Accuracy, and Precision Found Using Different Calibration Methods EC
IS
SA
SDA
Average Concentration Found, mg/L (10 mg/L added) Al 12.1 12.6 9.5 9.5 Cd 11.7 12.2 11.0 10.5 Co 11.8 12.3 10.2 10.0 Cr 11.0 11.6 10.3 10.2 Cu 11.6 12.3 9.5 9.7 Fe 11.2 11.8 10.5 10.3 Ni 11.1 11.7 10.4 10.2 Pb 11.0 11.6 10.1 9.9 average 11.4 12.0 10.2 10.1 Accuracy (average percent error) Al 24.8 26.1 4.8 5.0 Cd 20.8 22.2 16.0 7.3 Co 19.6 23.3 11.5 4.2 Cr 15.5 16.4 10.4 4.3 Cu 16.4 23.4 7.8 3.8 Fe 20.1 17.6 10.8 4.5 Ni 18.7 17.5 12.4 4.9 Pb 18.9 15.8 12.0 4.0 average 19.3 20.3 10.7 4.7 Precision (percent relative standard deviation) Al 30.5 17.4 2.9 3.5 Cd 24.4 10.9 18.8 7.7 Co 21.0 8.0 13.9 5.1 Cr 18.7 5.9 12.7 4.5 Cu 15.6 9.7 7.6 3.3 Fe 25.6 12.7 13.5 5.0 Ni 22.6 9.4 14.9 5.8 Pb 22.4 9.3 13.8 4.9 average 19.8 9.3 13.3 5.8
CONCLUSION Standard dilution analysis is simple and effective, providing superior accuracy, precision, and sample throughput when compared to other traditional calibration methods. It offers the best of both worlds: it simultaneously corrects for matrix effects due to the sample concomitants (as with standard additions) and for fluctuations due to changes in sample size, orientation, or instrumental parameters (as with internal standard). SDA requires no instrument modifications or special strategies for widely used techniques such as molecular absorption spectrometry and ICP OES. It can be applied to most instrumental analysis techniques that will accept liquid samples and are capable of simultaneous multianalyte determinations. The novel approach proposed by SDA is rapid, requiring less than 3 min to analyze each sample by mixing two solutions in a single container. That container may be the cuvette in a molecular absorption experiment or the sample tube in an ICP OES experiment. Since the same amount of sample is always present during the measurement, the relative effect of the matrix on analytes and internal standard is not a concern. This makes the selection of a suitable internal standard trivial. As shown in this proof of concept work, the SDA method may be successfully used in molecular and atomic spectrometry techniques.
n 15 15 15 15 15 15 15 15 120 15 15 15 15 15 15 15 15 120
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15 15 15 15 15 15 15 15 120
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel: +1 336 758-4815. Fax: +1 336 758-4656. Notes
The authors declare no competing financial interest.
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average of 15 SDA runs: five sample matrixes, n = 3. As observed in Table 2, SDA and standard additions present similar results. Purely from a recovery standpoint, SDA provides results of a slightly better accuracy than those of standard additions and significantly better than those obtained with the other methods. However, when average percent errors are compared, SDA accuracies are superior to the standard addition (SA) values by a factor of 2 and to the other methods by a factor of 4. This may be explained by the number of data points used to construct the plots for each method. The SDA plots are prepared using 30 data points, one collected each second during the SDA region of the dilution. The SA method employs a calibration curve prepared from only five solutions. Therefore, the best fit is superior with the SDA method and the accuracy is better. Since the quality of the best fit in the SA
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