In the Laboratory
Development of a Multiple-Element Flame Emission Spectrometer Using CCD Detection
W
Caryn S. Seney* and Karen V. Sinclair Department of Chemistry, Mercer University, Macon, GA 31207; *
[email protected] Robin M. Bright,** Paul O. Momoh, and Amelia D. Bozeman Department of Chemistry, Fort Valley State University, Fort Valley, GA 31030; **
[email protected] Simultaneous multiple-element analysis of environmental samples can offer an alternative method to single-element methods such as atomic absorption spectroscopy (AAS). For example, with the full wavelength coverage of the chargecoupled device (CCD) detector coupled with an echelle spectrograph, the system allows for simultaneous multipleelement spectrometry to be performed (1). In addition, the measurement of the background and analyte signals at the same time, with proper data reduction, can result in the elimination of background flicker noise (1), thereby increasing the overall signal-to-noise. Moreover, the necessity to keep samples for many single-element determinations and lengthy analysis times enhances the probability of contamination or inconsistent results when compared to fresh samples (2). Thus, utilizing simultaneous multiple-element analysis provides faster analysis times, increased throughput, less sample used, and oftentimes less opportunity for contamination. Frequently these simultaneous multiple-element analyses are performed using an inductively coupled plasma– atomic emission spectrometer (ICP–AES) (2–6); however, since the equipment is expensive and there is increased complexity of operation, acquisition, and maintenance of instruments, purchase of these instruments for use in the undergraduate laboratory is often not feasible. Furthermore, the design of the instruments creates serious limitations to the educational value of the instrument, often resulting in “black box” arrangements in which the students are unable to observe the actual optical path of the instrument. As shown in a survey of laboratory directors, “black box” instrumentation is training students to be technicians, not scientists (7). Equally important is that an immense quantity of information can be gathered simultaneously by using ICP–AES; yet many instruments do not take advantage of this ability because they employ detection systems based on single wavelength detection such as photomultiplier tubes (PMTs) (8). Although current techniques using PMTs for detection are able to analyze a large array of elements with linear dynamic ranges covering six or more orders of magnitude (9), the limited multichannel abilities of multiple PMTs often reduce the instrument’s capabilities to observe multiple spectral lines for each element or to simultaneously observe the spectral area around the analytical lines of interest (9). PMTs limit the flexibility in line selection, are not easily reprogrammed to record a new set of analytical wavelengths, and cost progressively more as the number of lines analyzed increases because more PMTs are needed to monitor additional wavelengths (8). However, the development of the CCD detector coupled with an echelle spectrometer permits valid, simultaneous multiple-element spectrometry over a wide spectral range (1,
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10), and with the high resolution, sensitivity, and large dynamic range exhibited by CCDs a more promising approach to AES in which a CCD detector can measure a large number of wavelengths simultaneously is realized (9). CCD detectors generally offer high sensitivity because of their low noise levels and high quantum efficiency and are thus suitable for low-light-level imaging and spectroscopy (11). Also, the CCD bandwidth of 200–1100 nm allows for many types of spectroscopic analysis including multiple-element analysis using atomic emission (1, 9, 12, 13). Throughout the literature a number of AES laboratory experiments ranging from quantitative flame emission procedures (14–24) to the construction of inexpensive emission spectrometers for the undergraduate laboratory (25–28) have been described. Our instrument design utilizes the characteristics of the CCD detector, allowing students to align, collect, and manipulate data, thereby gaining a deeper understanding of the instrument, multiple-element emission analysis, and limitations imposed by either solution preparation or instrumental parameters; all of which can be studied within five, three-hour laboratory periods. The experiment described herein is not designed for immediate implementation by instructors, but is instead an adaptation of a common multiple-element ICP–AES experiment to a multiple-element flame AES experiment for smaller chemistry departments that may not have an ICP–AES instrument but may have a flame atomic absorption (AA) instrument that can be adapted by the purchase of a monochromator and CCD system for a reasonable price. Useful experiments that might be adapted include ones for AAS (14, 15, 20, 22), AES (16, 23), and ICP–AES (19, 21). Experimental
Instrument A schematic of the instrument is shown in Figure 1. The emission source is a 50-mm × 0.4-mm slot burner head that was extracted from a nonfunctional flame AAS controlled by the gas box from the same nonfunctional AAS. Should such equipment not be available, the alternative would be to make your own using a Bunsen burner as described by Smith et al. (28). Emission signals from the burner head are collected by a 3-in. focal length, 1-in. diameter biconvex fused silica lens (purchased from Edmund Scientific for $89 per lens), attenuated in order not to saturate the detector by an iris stripped from a camera, and then focused using a second 3-in. focal length, 1-in. diameter biconvex fused silica lens onto the entrance aperture of a CM110 1兾8-m monochromator (purchased from CVI Laser Corporation for approximately
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In the Laboratory
$2300). The monochromator contains two gratings from which the user may choose either 1200 grooves兾mm or 300 grooves兾mm. There are also slit widths available for the CM110 monochromator ranging from 0.125 mm to 2.4 mm. For our experiments, the monochromator was set to the 1200-grooves兾mm grating and a 0.125-mm slit width. Attached to the back of the monochromator is a ST-6 CCD detector, purchased from CVI Laser Corporation for $4000, which includes the camera head with a fine focus assembly, the communication interface box, and cabling. Kestrel Spec data acquisition software (purchased from CVI for $1500) collects the data from the CCD detector and then displays it on the computer. To make the instrument modular and open for the students to trace the light path requires purchasing lens holders, post holders, base plates, a breadboard, and optical bench hardware (which includes 2 Allen wrenches, 12 Allen-head screws, 12 thumb screws, 3 captive thumb screws, 12 flat head Phillips screws, and 12 Delrin washers, all from Edmund Scientific). Buying such components allows further versatility in the laboratory. Switching light sources and changing geometry of the instruments’ hardware allows for multiple types of analyses because CCD detectors can record signals from 200–800 nm allowing for many types of spectroscopic analysis, including Raman (29), fluorescence (30, 31), UV–vis (32), atomic absorption (33), and atomic emission.
relays interface box
camera head and CCD
iris
flame
lens
lens
monochromator
NO2
acetylene power gas control box
Figure 1. Schematic of atomic emission spectrometer.
nickel
Standard Preparation
Hazards Solutions containing nitric acid are used. Nitric acid is a strong oxidizer and may irritate the skin. It is suggested that students wear gloves during the preparation of solutions. Nitrous-oxide兾acetylene mixture is explosive, and ignition of the flame for the atomic emission experiments needs to be done with extreme caution as a backflash can occur. The flame assembly needs to be securely attached to a lab bench or heavy table because if a backflash occurs, the flame assembly may move. Goggles must be worn at all times during solution preparation and use of the instrumentation.
Signal
lead iron cobalt 250
275
300
325
350
375
400
Wavelength / nm Figure 2. Individual element spectra of Mg, Ni, Pb, Fe, and Co: 0.1-s exposure, 1 accumulation (number of exposures co-added into spectrum curves), 0.125-mm slit, 1200 grooves/mm. Each spectrum is vertically offset. manganese 403.1 nm
25
20
15
10
copper 327.4 nm
copper 324.8 nm
5
chromium 425.4 nm
chromium
0
Results and Discussion
300
Typical spectra for Cu, Mn, and Cr, after averaging the spectra and subtracting background, are shown in Figure 3. Comparison of Figures 3 and 2 shows that the averaging process produces less noise in the spectrum. Analyte peak position was determined and the intensity at each peak was then
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magnesium
Signal / 104
Each study involved the analysis of ten solutions of varying concentrations, prepared by dilution of an atomic absorption standard solution for each of the elements determined. The elements were grouped in the following way: (i) Na, Ca, and K; (ii) Cu, Cr, and Mn; (iii) Co and Fe; (iv) Mg and Ni and (v) Pb. The elemental groupings were designated such that Na, Ca, and K were analyzed together because all three require a lower emission temperature for analysis. Although Co, Fe, Mg, Ni, and Pb are all within the same spectral region, they were analyzed in separate groups owing to spectral overlap among these elements, as seen in Figure 2.
•
350
400
450
Wavelength / nm Figure 3. Spectrum of 35 ppm copper, 70 ppm chromium, and 25 ppm manganese: 0.1-s exposure, 1 accumulation (number of exposures co-added into spectrum curves), 0.125-mm slit, 1200 grooves/mm.
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Table 1. Comparison of Detection Limits
Signal / 105
20
Cu, 327.4 nm Mn Cr Cu, 324.8 nm
15
Analyte
Detected Wavelength/ nm
Exp Detection Limit/(mg/L)
Reported Detection Limita/(mg/L)
10
5
0 0
10
20
30
40
50
60
Concentration / (mg / L) Figure 4. Calibration curve for Mn concentration ranges from 0.1 to 25 ppm, Cu concentration ranges from 1 to 35 ppm, and Cr concentration ranges from 1 to 70 ppm.
0.0001
0.05
0.00001
K
766.5
0.3
0.00001
Cu
324.8
5
0.03
327.4
6
0.3
425.4
5
0.001
Mn
403.1
Co
340.5
0.6
0.001
10
---
350.2
9
---
352.7
10
372.0
10
373.6
4
-- ---
0.2 0.01
381.8
20
Mg
285.2
30
Ni
340.6
7
---
345.2
5
---
352.5
5
363.6
40
368.3
30
Pb
0.001
0.02 --0.0002
a
where the standard deviations of the blank, σblank, are measured at the λmax for each metal and the slope of the line is measured from the calibration curve. The experimental detection limits are compared with those published in the CRC Handbook of Basic Tables for Chemical Analysis (34) (Table 1). As noted, our detection limits are on average three orders of magnitude greater than the detection limits given for a nitrous-oxide兾acetylene gas mixture. These results are expected owing to the higher background noise often exhibited by thermoelectrically air-cooled, temperature-sensitive CCD detector as compared to water-cooled or liquid nitrogen-cooled CCD detectors. By switching to either of these CCD cameras, detection limits should be lowered to within the reported range. Unfortunately, those detectors are also typically more expensive and may not be plausible for the undergraduate department to purchase and are often cumbersome and expensive to maintain. Conclusion The spectrometer was built and characterized through the analysis of the following environmentally significant elements: Ca, K, Na, Cu, Mn, Pb, Cr, Fe, Ni, Mg, and Co. Although detection limits were comparably higher than those published in the CRC (34), the instrument does afford reasonable multiple-element analysis. A detector producing less background noise, such as a nitrogen-cooled or water-cooled CCD, would lower the limit of detection below that of the thermoelectrically air-cooled CCD as used here. Another possibility to increase the signal with respect to background noise would be to increase integration time. However, increasing
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0.8
589.0
Fe
plotted versus concentration (Figure 4). Some analyte concentrations were omitted because they were either outside the linear range or below the detection limit. Graphs similar to Figures 3 and 4 were generated for each element set. The experimental detection limits, LOD, are calculated by
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Crb
70
3σblank LOD = Slope
Ca Na
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Note that the reference states that “the detection limit in real samples will be one or two orders of magnitude higher, or worse, than those stated here.” (34, pp 371–372). b The data from the second chromium peak could not be used to calculate the detection limit; the resolution of the peaks decreased as the concentration increased resulting in nonlinear data.
integration time also increases the time to collect and analyze the data. Because resolution of the spectra is limited by the instrumentation, moving to a different spectrometer with narrower slit widths would improve resolution with a tradeoff in signal. The intrinsic resolution of our instrument as indicated by the instrument manual is 0.2 nm using the 1200-mm兾groove grating; however, the data acquisition software does not do justice to the resolution of the instrument. The effective bandwidth of our instrument based on inspecting sodium peaks typically range from 2.3 to 2.7 nm, calcium peaks with effective bandwidths at approximately 1.9 nm, and potassium peaks with effective bandwidths at approximately 1.6 nm. Our research demonstrates the viability of developing a relatively inexpensive multiple-element emission spectrometer that utilizes a fairly new method of detection, while also allowing students the opportunity to receive a practical understanding of AES by letting them not only put the instrument together from its varied parts, but also giving them the opportunity to view the light path of the signal throughout the instrument. W
Supplemental Material
Instructions for the students and notes for the instructor are available in this issue of JCE Online.
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In the Laboratory
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