In the Laboratory
A Matrix Modification Experiment for Use in Electrothermal Atomic Absorption Spectrophotometry Michael N. Quigley* Department of Chemistry and Biochemistry, The Bayer School of Natural and Environmental Sciences, Duquesne University, Pittsburgh, PA 15282 Frederick Vernon Department of Chemistry and Applied Chemistry, University of Salford, Salford M5 4WT, UK A previously published article in this Journal described the determination of trace-metal ion concentrations in real or artificial seawater using coprecipitation or chelating ion-exchange to preconcentrate the ions prior to analysis (1). The use of either technique identifies the overall procedure as an indirect method of determination. Direct determination of trace-metal ions in seawater is also possible using the right combination of analytical instrumentation and chemistry. For electrothermal atomic absorption spectrophotometry, matrix modification is potentially superior to indirect analyses because contamination from added reagents is greatly reduced. In large part, the success of matrix modification depends upon the geometry of the electrothermal atomizer and the efficiency with which the volatilization products are removed. For example, contemporary graphite furnace atomizers usually include an “ends-to-center” method of purging the graphite cuvette with nitrogen or argon. This is in contrast to older versions that rely only on envelopment of the cuvette with the gas to prevent oxidation of the carbon. However the furnace is constructed, background correction is a still a requirement of the matrix modification technique.
Matrix modification Matrix modification in general relies on the addition of a reagent to a “test” solution in order to alter the constituents of the matrix during the dry and ash stages of the electrothermal process. With regard to seawater analysis, additives have been used in a number of ways: •
To increase the volatility of the matrix constituents— particularly NaCl—by conversion to other compounds. For example, a large excess of ammonium nitrate added to seawater converts sodium chloride to sodium nitrate and ammonium chloride in accordance with the equation
NaCl + NH4NO3 → NaNO3 + NH4Cl NaNO3 has a melting point of 307 °C (i.e., ca. 500 °C lower than NaCl), and NH4Cl sublimes at 335 °C (2). •
•
To decrease the volatility of the analyte by conversion to thermally stable compounds (e.g. with nitric acid or nitric acid/sulfuric acid mixtures). To increase the volatility of the analyte by organic compound addition (e.g. with ethylenediaminetetraacetic acid or citric acid).
ticle, as has advice on the preparation of an artificial seawater and on instrumental settings for atomic absorption spectrophotometers (1). Volumetric glassware should be acid-washed and then rinsed with deionized water before use. Students can gain confidence in their analytic approach by determining the known concentration of a typical trace transition metal ion in 0.5 M NaCl solution. Use analytical grade reagents to prepare (i) 1 L of 0.5 M NaCl solution; (ii) 100 mL of 1,000 mg/L Mn2+ stock standard solution; (iii) serial dilutions of the Mn2+ stock standard solution to cover the range 0–8 µg/L Mn2+; (iv) 50 mL of 50% (w/w) NH4NO3 solution; (v) 100 mL of a 0.5 M NaCl solution “spiked” with 2 µg/L of Mn2+. Use the instrumental conditions shown in Table 1 as a guide. Using an automatic pipet with disposable tips, inject the following solutions into the furnace: 10 µL of 0.5 M NaCl solution 10 µL of 50% NH4NO3 solution 10 µL, equal mixture of the above solutions
• • •
Monitor the time/absorbance profile with a strip chart recorder. Note the high degree of scatter induced by the presence of NaCl, and contrast this with the result when NH4NO3 is added. See Figure 1. Determine the concentration of Mn2+ in the spiked solution by first noting the absorbances of the following: 10 µL of Mn2+ spiked solution + 10 µL of deionized water + 10 µL of 50% NH4NO3 solution
•
Table 1. Guide to Instrumentatal Conditions Used for the Direct Determination of Mn2+ in Natural or Artificial Seawater Absolute mass range 2–100 pg Concentration rangea Wavelength
279.5 nm
Bandpass
0.5 nm
Lamp currentb
4.3 mA
Dry timec
20/25 sec
Dry tempc
75/100 °C
Ash
Experimental Procedure Details concerning the collection and treatment of natural seawater have been published in a previous ar-
timec
Dry tempc
980
20/20 sec 400/600 °C
Atomize timec
0/5 sec
Atomize tempc
2000/– °C
a
Based upon a 10-µL injection volume. Use a deuterium lamp set at 25 mA, or other suitable background correction technique. c All times and temperatures are in two stages. b
*Corresponding author. Address: Bradfield Hall, Cornell University, Ithaca, NY 14853.
0.2–10 µg/L
Journal of Chemical Education • Vol. 73 No. 10 October 1996
In the Laboratory
Figure 2. Based upon the information given in the text example, the concentration of Mn2+ in solution is calculated using the x-axis intercept and the formula: concentration (µg/L) = absolute mass (g)/1×10{12 × injection volume (µL); i.e., {({29.8 × 10{12 g Mn2+)/1×10{12 × 10 µL. Figure 1. Temperature profile (a) and absorbance profiles of 10 µL of 0.5 M NaCl solution alone (b), and in the presence of 10 µL of a 50% aqueous solution of NH4NO3, as matrix modifier (c). See text for details.
•
10 µL of Mn2+ spiked solution + 10 µL of 6 µg/L Mn2+ solution + 10 µL of 50% NH4NO3 solution
•
10 µL of Mn2+ spiked solution + 10 µL of 8 µg/L Mn2+ solution + 10 mL of 50% NH4NO 3 solution
•
10 µL of Mn2+ spiked solution + 10 µL of 10 µg/L Mn2+ solution + 10 µL of 50% NH4NO3 solution
Use the data to plot a standard addition curve of absorbance (peak height or area) vs. absolute mass of Mn2+ added. Remember that absolute mass in grams is equal to 1 × 10{12 × µL (vol) × µg/L (conc). Extrapolate the curve so that it bisects the negative x-axis. See Figure 2. This bisection point corresponds to the absolute mass of Mn2+ in the 10 µL of solution. Alternatively (or in addition), use a linear regression program to calculate the y-axis (absorbance) intercept, multiply this value by two, and interpolate this to the best-fit curve in order to obtain the absolute mass of Mn2+ added (3). Once the absolute mass of Mn2+ present in the 10 µL of 0.5 M NaCl solution has been determined, calculate the concentration of Mn2+ in the 0.5-M NaCl solution using the formula above. For example, say that some solutions prepared and used according to the above description gave the following absorbance values: 0 pg Mn2+ added = 0.210 A; 60 pg Mn2+ added = 0.650 A; 80 pg Mn2+ added = 0.776 A. (A small error has been introduced for the purposes of illustration.) Using linear regression analysis, the following
values are obtained: correlation coefficient = 0.9996; slope = 7.13 × 109; y-axis intercept = 0.212. Multiplying the yaxis intercept value by 2 and interpolating this to the bestfit curve gives an absolute mass value of 2.98 × 10{11 g Mn2+ (i.e., 29.8 pg Mn2+). Using the formula above, the concentration of the spiked solution is calculated as 2.98 µg/L Mn2+. Since the true concentration of Mn2+ in the spiked solution is known, calculate the relative error using the formula %E = [(xi–xt )/xt] × 100, where xi is the concentration value obtained and xt is the true value—in this case, 2 µg/L Mn2+. Use the value for relative error to correct for the determination of Mn2+ in natural or artificial seawater.1 The mean concentration of all forms of manganese in natural seawater has been determined as 0.9 mg/L (4). Use a similar rationale for the determination of other trace metal ions in seawater. Notes 1. The instructor should “spike” the artificial seawater with a 2+ known concentration of Mn , in the same way that the 0.5 M NaCl solution is spiked.
Literature Cited 1. Quigley, M. N.; Vernon, F. J. Chem. Educ. 1996, 73, 671–675. 2. Lide, D. R. (Ed.). CRC Handbook of Chemistry and Physics, 73rd ed.; CRC: Boca Raton, FL, 1992. 3. Quigley, M. N. Chem 13 News 1992, 212, 4. 4. Kennish, M. J. (Ed.). Practical Handbook of Marine Science, 2nd ed.; CRC: Ann Arbor, MI, 1994.
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