Continuous Liquid-Sample Introduction for Bunsen Burner Atomic Emission Spectrometry Gregory D. Smith, Caryn L. Sanford, and Bradley T. Wake Forest University, Winston-Salem, NC 27109
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With the cost of commercial instruments far outrunning the budgets of many undergraduate universities, numerous educational institutionsare unable to ourchase the instrumentation necessary for advanced chemistry courses. Academic institutions incapable of affording laboratory equipment often rely on "dry 1abs"in order to give students exoerience with the scientific techniques being studied. ~ i e s dry e labs usually consist of a briefdiscussik on how the instruments work and a handout of data collected on equipment a t some larger university. The students must then analyze the data in order to understand the techniques, without any hands-on, practical experience. Cost is not the only obstacle that commercial instruments present. Such analytical equipment commonly suffers from the "black box" syndrome ( 1 ) .Instrument manufacturers tend to hide most of the working parts of the device behind computer terminals and sleek plastic facades. This form of instrument packaging is beneficial for the laboratory technician who must complete a large number of analvses without interactine extensivelv with the system. However, such a device is often inappropriate for the academic laboratom because the student needs a basic understanding ol'the principles in\wlvt!d in [he technique. Atomic Emlssion Soectromct~vI X S I is one ofthe must popular techniques carried out in undergraduate instrumental analysis laboratories. Approximately 43% of all schools offering such courses included a n AES experiment in 1991 (1).In most of these cases, either the airlacetylene flame or the inductively coupled plasma was used as the excitation source. A larger proportion of academic laboratories could offer the experiment if a simpler, less expensive excitation source could be used. !Chis paper describes a laboratory-constructed atomic emission spectrometer with modular instrumentation components and a simple
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'Author to whom correspondence should be addressed
Erlenmeyer Flask
Plastic Tubing .. .. .. .. .. ... ... ... ... ... ......
To Air Tuhine (TOs A p l e ) -
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Figure 2. Detailed schematic of the sample-introductionsystem Bunsen burner atomizer with continuous sample introduction. The Bunsen burner atomization system could also serve for classroom demonstrations of the emission spectra of group 1A and 2A elements. The device provides a brightly colored, stable flame as long a s solution is aspirated. The demonstration is much more striking than that observed with the typical wire-loop atomizer. Experimental Instrumentation Aschematic d i a a a m of the Bunsen burner atomic emission spectrometeris given in Figure 1and a detailed schematic of the sample introduction system is ziven in Fimre 2. The nebulize; was constructed using a-5.75-in. disposable Pasteur pipet (Corning part no. 7095B-5x1. Typical polyethylene nebulizer tubing (0.023 in. i.d. x 0.038 in. 0.d.) was used for sample introduction (PerkinElmer). The nebulizer was constructed by temporarily sealing both ends of the pipet with cork. The pipet was heated slowly a t a single point in the middle using a normal Bunsen burner flame. The expanding air pressure within the n i ~ eforced t a small onenine a t the hottest section of gl&; The nebulizer t u k n g was then threaded through the opening until the tip of the tubing was flush with the small end of the pipet. Epoxy was used to form a n airtieht seal around the tubing a t the opening created in the pipet Sample Introduction .Caution: The Erlenmeyer flask should he wrapped with tape (or constructed from plastic) to minimize the dangers from shattering should a flashback occur at the burner.
Bunsen Bumer
Chamber
Figure 1. Schematic diagram of the Bunsen burner atomic emission sDectrometer.
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Journal of Chemical Education
The larger end of the pipet was connected with heavywalled rubber tubing to a laboratory air jet. Because air pressure is applied to this tube, thinner-walled rubber tubing would tend to burst. Stronger polyethylene tubing also worked well. A liquid sample can be drawn through the nebulizer tube by opening the air jet and passing a bighvelocity stream of air past the end of the tube (Bernoulli effect). The rapidly moving air created a fine mist of sample solution a t the pipet tip. To collect the larger droplets
10
0
20 Time (min)
F~gt.rc4 Cnan recoroer trace of the re atwe emnsston s gnals co ccted fortwo potass Lm standard solut ons and lour u JIG ocor Sam. pies Determination of Sodium and Potassium in Beer Samples
Fgure 3 Photograph ofthe system durng the asplratlon of a sodum standard solution while passing the smaller droplets into the burner, a sirnple spray chamber was prepared using a l-L Erlenmeyer flask, a two-holed ruhher stopper, and a short length of 0.25411. plastic tubing (Fig. 2). The flask with stopper was held horizontally by a clamp. The nebulizer was inserted into one of the holes in the stopper and the plastic tubing was inserted into the other hole. The sample was sprayed into the flask. Large droplets had sufficient momentum to strike the sides of the flask. and were collected ;IS waste. Smaller droplets passed through the nlnstir tubing ;ind into the flame. The external end o r t h e piastic tubingwas placed near (1-5 in.) one of the air inlets a t the bottom of a normal Bunsen burner. For additional stability, this tuhe can he held with a second clamp assembly. Sample, oxidant, a n d fuel a r e mixed within the barrel of the burner, and the analyte is atomized and excited in the flame. Aspiration Rate a n d Droplet Size The sample mist was sufficiently fine that no condensation was visible on the outside of the burner after several hours of nspirat~on.'The. aspiration rate ind droplet i z r cnn he crudt:ly ;~diuitcdby using dilrerenr lrnmhs of n t h or by lizer tubing, by operating a t different air adjusting the penetration of the nebulizer and plastic tuhing into the spray chamber. In practice, these parameters were adjusted to give a reproducible signal, and then held constant afterwards. High air pressures are not recommended because they could dislodge the nebulizer assembly from the ruhher stopper. An air pressure regulator would he useful, but i t is not required. The spray chamber was drained after approximately 2 h of sampling. This was accomplished by removing the stopper and emptying the contents of the flask. A drain tube similar to that used with most AES systems could be used if the flask had a side arm. Sample-to-sample memory effects were eliminated by aspirating water for 1min after each sample run. Modular Units for Focus a n d Detection The remainder of the atomic emission spectrometer comprised modular units designed like those used in commercial instruments. The system was constructed using a l - m optical rail. A 75-mm focal-length lens (Melles Griot) was used to focus the flame emission onto the entrance slit of a medium resolution monochromator (0.35-m focal length, 1200 grooveslmm, 2 n d m m RLD, McPherson Model 270).
Na Concentration (mgIl2 fl 02)
K Concentration (rngil2fl 02)
Busch
2.4 k0.5
92f7
Guinness Extra Stout
3.5 k0.6
180+23
Kiliian's
8.0 i 1.2
98f 12
Milwaukee's Best Lite
3.8 i0.7
50f7
Sample
The portion of the flame just above (3-5 mm) the bright blue cone was imaged onto the slit. The slit width was adjusted to 30 bm, giving a spectral handpass of 0.06 nm. The emission was detected using a photomultiplier t u h e (Hamamatsu R955) and photometer (McPherson). Data was collected on a standard strip chart recorder (Fischer Recordall Series 5000). Any similar equipment could he used, and the monochromator could be replaced with suitable bandpass filters. A voltmeter could replace the chart recorder. Sample Analysis
To increase student interest in the experiment, a real sample was chosen for analysis. Four commercial beers (Guinness Extra Stout, George Killian's, Busch, Milwaukee's Best Lite) were analyzed for potassium (766.5 nm) and sodium (589.0 nm). The potassium determination requires a red-sensitive photomultiplier tube. Beer samples were degassed by rapidly stirring a portion that had been allowed to reach room temperature. For the determination of sodium, the degassed beer samples were aspirated directly. For the determination of potassium, a 2-mL portion of degassed beer was diluted to 100 mL using distilled water before aspiration. Stock solutions (1000 m g L ) were prepared for both elements as previously described (2).A series of standard solutions spanning the range from 1 to 40 @mL was prepared for both elements by serial dilution of the stock solution with distilledldeionized water. Calibration curves were prepared and analytical figures of merit were calculated for the system. Results and Discussion Figure 3 is a photograph of the Bunsen burner system taken during the aspiration of a sodium standard solution. Calibration curves were prepared for both sodium and poVolume 72 Number 5 May 1995
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tassium. The observed limit of detection (LOD) for both metals was 0.2 pg1mL. The LOD was calculated using the IUPAC definition (3). The calibration curves were linear over the range 0.2 to 30 pg1mL. Typical data collected for two standard solutions and four beer samples are shown in Figure 4. The observed sodium and potassium content of the beer samples is given in the table. The reported concentration is given in mg per 12 fl oz (mglserving).The concentrations are reported along with 95% confidence intervals. I n each case, three different bottles of the beer were analyzed, each in triplicate. The reported confidence interval is therefore based on nine measurements. For the analysis of a single bottle, the standard deviation was found to be approximately one-half that observed for the three bottles. The ex~ e r i m e nwas t reueated after a two-month veriod with new bottles of the s a k e beer samples, new standard solutions, r Bunsen burner. The results did and a new a s ~ i r a t o and not vary signkcantly from the original findings. Conclusion
The Bunsen burner AES system is sensitive enough to analyze real samples containing sodium and potassium. In
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Journal of Chemical Education
addition to these elements, brightly colored flames were observed during the aspiration of barium, calcium, cesium, copper, lithium, manganese, and strontium. Quantitative determination of these elements could be carried out in appropriate samples. If the wavelength-selection device and detector are not available, the continuous liquid sample introduction into the Bunsen burner works beautifully for simde inoreanic cation flame tests. The continuous emission is easier to distinguish than the typical wire-loop test. The nebulizer and spray chamber are sufficiently inexpensive that many of these devices could be fabricated for a large general chemistry laboratory section.
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Acknowledgment
This work was supported by Grant No. DUE-9251294 of the NSF Instrumentation and Laboratory Improvement Program (Leadership Projects in Laboratory Development). Literature Cited 1. Jones, B.T.J. Chsm. Edue. 1992,69,A266A269. 2. Smith, B. W.:Parsons.M. L.J. Chrm Edue. 197S,50,619481. 3. Long,G.L.; Winefordner,J. V A n a l . Chsm. 1983.55,712A-724A.
