Instrumentation for simultaneous multielement atomic absorption spectrometry with graphite furnace atomization. Fresenius J Anal Chem (1996) 355:501â509.
Fresenius J Anal Chem (1996) 355 : 501–509
© Springer-Verlag 1996
LECTURE
James M. Harnly
Instrumentation for simultaneous multielement atomic absorption spectrometry with graphite furnace atomization
Received: 5 December 1995 / Revised: 5 February 1996 / Accepted: 9 February 1996
Abstract Graphite furnace-atomic absorption spectrometry (GF-AAS) is restricted to the determination of 4 to 6 elements simultaneously due to the limitations of hollow cathode lamps. However, a consideration of prototype continuum source instruments and recent advances in the fields of spectrometer and detector technology suggests that a multielement GF-AAS instrument, with the multielement versatility associated with atomic emission spectrometry, is possible. Such a multielement instrument would employ a continuum source and provide 1.) multielement determinations for 30 to 40 elements, 2.) wavelength and time integrated absorbance measurements which are independent of the source width, 3.) detection limits comparable to line source AAS with the potential for another order of magnitude improvement using atomization at elevated pressures, 4.) extended calibration ranges limited only by the memory of the atomizer, and 5.) high resolution inspection of the spectra surrounding the analytical wavelength. Such an instrument could provide figures of merit comparable to inductively coupled plasma-mass spectrometer with considerably less complexity.
Introduction Graphite furnace-atomic absorption spectrometry (GFAAS) is a powerful analytical technique that has found universal application for the determination of trace metals. Furnace atomization provides efficient volatilization/atomization and a relatively long residence time (compared to techniques using pneumatic nebulizers) which results in excellent sensitivities and detection limits. In addition, furnace atomization offers small sample size re-
J. M. Harnly US Department of Agriculture, Agricultural Research Service, Beltsville Human Nutrition Research Center, Food Composition Laboratory, Building 161, BARC-East, Beltsville, MD 20705, USA
quirements, low cost of operation, simple spectra, and in situ chemical and thermal pre-treatment of samples. Historically, the biggest disadvantage of GF-AAS has been the sample analysis rate: a result of the single element nature of AAS and the slow repetition rate (an atomization every 2 to 3 min) of furnace atomization. There has been continued interest in the development of a multielement GF-AAS instrument that would increase the analytical rate but retain the excellent sensitivities and detection limits of single element operation. The primary obstacle to multielement AAS has been the lack of a suitable multielement radiation source. Hollow cathode lamps (HCLs) and electrodeless discharge lamps (EDLs) are basically single element sources furnishing the electronic spectra of the cathode metal and the fill gas. Today, all commercially available AAS instruments employ a line source (LS-AAS). Multielement lamps are problematic and combining the radiation from a series of lamps leads to reduced intensities and poorer signal-to-noise ratios (SNRs). Lesser obstacles are the necessity for compromise atomization conditions and the lack of an extended calibration range. The most subtle obstacle has been the pervading attitude that multielement AAS can be achieved through “simple” modifications of existing single element technology. In the past, this philosophy has resulted in “limited element” LS-AAS instruments which failed to achieve either the analytical figures of merit of single element instruments or the desired versatility of a truly multielement instrument. The success of multielement AAS is dependent upon an integrated instrument design aimed specifically at multielement determinations. The most recent commercially available multielement instrument in the field of GF-AAS is capable of determining 6 elements simultaneously with state-of-the-art detection limits [1, 2]. This instrument employs a combination of 4 single and multielement EDLs and HCLs and uses the high optical transmission of an echelle spectrometer and the high quantum efficiency of a solid state photodiode detector to offset the intensity lost in combining the four beams. Although this instrument is a significant step
502 Table 1 Commercially available AA instruments
A. No. of Elements B. Optical design C. Performance D. Background corr. method E. Radiation source F. Source combiner
Thermo Jarrell Asha (AA Scan 4)e
Hitachi Instrumentsb (Z9000)f
Leeman Labs, Inc. c (Analyte 5)g
Perkin-Elmer Corporationd (SIMAA 6000)h
4 Selectable Single beam Pseudo double beam Self-reversed source HCLs Stepped mirror
4 Selectable Single beam Double beam Inverse Zeeman (Trans. DC field) HCLs None
5 Fixed (As, Pb, Sb, Se & Tl) Double beam in space Double beam Self-reversed source HCLs Reverse polychromator
6 Selectable Single beam Double beam Inverse Zeeman (Long. AC field) HCLs 4 faceted mirror
Transverselyheated Notch filters
Transverselyheated Echelle polychromator
1 PMT 50
60 photodiodes 54
G. Furnace
End-heated
circle End-heated
H. Disperser
Stepped grating Czerny-Turner monochromator 1 PMT 5
Quad. CzernyTurner Mono chromator 4 PMTs 60
1.0 0.6j
1.0 0.85–0.95k
0.48i 0.6 j
0.48i 1.0
1.0
0.5l
0.8m
0.25n
0.014AsTopp
0.005AsTopq
0.022AsTopr
0.019AsTops
I. Detectors J. Absorbance comp. frequency (Hz) K. Sensitivity factors Furnace length Background Corr. L. Absorbance noise factors Normalized optical path transmission Luminosity (mm2sr)o a
Franklin, MA, USA Naperville, IL, USA c Lowell, MA, USA d Norwalk, CT, USA e References 4 and 5 f References 6 and 7 g Reference 3 h References 1 and 2 i Transversely-heated furnace reduces sensitivity by 52% j Self-reversed background correction reduces sensitivity by average of 40%
k Inverse Zeeman background correction with transverse dc field decreases sensitivity by 5 to 15% l Fixed polarizer decreases intensity by 50% m Notched filter has maximum transmission of 80% n Four faceted mirror of beam combiner decreases intensity by 75% o Luminosity = Φ 2 HCL = BAsAgTop/f , Reference 8 p A = 1600 mm2, f = 330 mm g q A = 1024 mm2, f = 450 mm g r A = 2704 mm2, f = 350 mm g sA = 4700 mm2, f = 501 mm g
forward, it is still possible to hypothesize an “ideal” multielement GF-AAS instrument. This instrument would provide simultaneous detection for 30 to 40 elements without sacrificing detection limits, accurate background correction, and extended calibration ranges (5 to 6 orders of magnitude of concentration) for each element. These capabilities imply a continuum source (HCLs could never be used for this many elements), high optical transmission and detection efficiency, efficient multielement atomization with low memory between atomizations, and high speed data acquisition and signal processing. This paper will consider the current field of commercially available multielement LS-AAS instruments, their most desirable features, and how they address the problems facing multielement determinations with a graphite furnace atomizer. In addition, the impact of prototype continuum source AAS (CS-AAS) instruments and developments in the field of atomic emission spectrometry
(AES) will also be considered. Finally, a multielement CS-AAS instrument for the future will be proposed using a hypothetical, solid state detector.
b
Commercial multielement AAS instruments Table 1 lists the characteristic of 4 multielement LS-AAS instruments that are commercially available today [1–7]. Other multielement instruments are being produced, but they are not manufactured on a routine basis and will not be considered here. The one common feature of all 4 instruments is the use of HCLs as the radiation source. Consequently, the maximum number of elements that can be determined simultaneously is 6. All other features vary significantly. Two of the latest instruments employ optical designs that vary significantly from that of conventional AAS instruments. The SIMAA 6000 employs an echelle
503
polychromator and a monolithic detector containing 60 photodiodes [1, 2]. Approximately 15 of the diodes are allotted for secondary lines, lines approximately 2 orders of magnitude less sensitive than the primary lines. This allows for extension of the dynamic calibration range. The Analyte 5 [3] combines the radiation from 5 HCLs using a reverse polychromator configuration (i.e. the HCLs are located at the exit slit positions and the combined beams appear at the entrance slit). This approach is optically efficient and is very effective for a dedicated suite of elements. Both the SIMAA 6000 and the Analyte 5 use transversely heated furnaces which are more suitable for multielement determinations. The most critical figure of merit for the instruments listed in Table 1 is the signal-to-noise ratio (SNR). In principle, the SNR can be computed as the ratio of the analytical sensitivity of the furnace and the baseline absorbance noise. The furnace sensitivity (Item K) is dependent on the length of the furnace and the background correction method. The absorbance noise (Item L) is dependent on the product of the luminosity [8] (the radiation intensity reaching the detector) and the detector quantum efficiency. The luminosity is dependent on the line radiance for the HCL (B), the slit area (As), the grating area (Ag), the optical transmission factor (Top), and the focal length of the spectrometer (f). Table 1 assumes that the detector quantum efficiency and the HCL lamp currents and performance are comparable. Unfortunately, there is a lack of published data for the optical transmission factor, Top, for most instruments, and the optimal slit area is highly variable and is dependent on the element, the method of back-
ground correction, the furnace, and the optical design of the instrument. Consequently, the luminosity is reported only as a factor of As Top. A more practical basis for comparison of the SNRs is the detection limits cited by the manufacturers (Table 2). Perkin-Elmer reports detection limits for the SIMAA 6000 [11] which are comparable to their previous single element instrument, the 4100ZL [10]. The detection limits for the Analyte 5 from Leeman Labs [3] range from 2 to 8 times worse than those reported for the SIMAA 6000, although still well below the required EPA limits for water quality determinations. The detection limits for the Z9000 [14], from Hitachi Instruments, can be expected to be comparable to those for the Analyte 5. Like the Analyte 5, this instrument was developed for EPA water quality determinations. Thermo Jarrell Ash has openly acknowledged that the lower sensitivity of the self-reversed source background correction system leads to detection limits which are approximately a factor of 2 poorer than those without background correction. In addition, in the multielement mode the low absorbance computation frequency and the settling noise for the galvanometers positioning the beam combiner and the spectrometer grating result in detection limits that are at least 4 times worse than those found in the single element mode [15].
New technology In the author’s opinion, the most interesting new technological developments are the use of the transversely heated
Table 2 Detection limits Element
Perkin-Elmer Zeeman 5000a
Zeeman 4100ZLb
Leeman Labs Analyte 5d
SIMAA 6000c Single element
CS-AAS LPDA detectore
SCD detectorf
24. 1. –.
Hypotheticalh
Multielement
As Cd Co
20. 0.3 6.
10. 0.4 8.
6. 0.1 –.
8. 0.2 –.
18. –. –.
28. 0.4 4.
Cr Cu Mn
1. 1. 1.
2. 5. 2.
0.4 4. 0.6
1. 7. 1.
–. –. –.
–. 0.6 0.5
–. –. 0.3
0.1 2. 0.4
Mo Ni Pb
4. 10. 5.
4. 16. 3.
1. –. 4.
2. –. 8.
–. –. 9.
–. 11. 0.9
–. –. 0.7
0.2 10. 5.
Sb Se Sn
15. 30. 20.
8. 15. 10.
–. 9. –.
–. 22. –.
120. 60. –.
–. 50. 26.
12. 49. –.
–. 12. 1.
T1 V
10. 20.
6. 6.
9. 2.
12. 4.
12. –.
–. –.
2. –.
–. 1.
a
e
b
f
Reference [9] Reference [10] c Reference [11] d Reference [3]
Reference [12] Reference [13] h Computed as described in text
6. 0.08 1.
504 Table 3 SNRs for three Perkin-Elmer instrumentsa
1.5
1.5
1.3
1.3
1.1
1.1
0.9
0.9
0.7
0.7
0.5 500
1000
1500
2000
2500
0.5 3000 500
Zeeman 5000
1000
1500
2000
2500 3000
Fig. 1 The ratio of experimentally determined characteristic masses to those listed by the manufacturer as a function of the atomization temperature, with and without 5 µg of Pd and 3 µg of Mg(NO3)2, for g) Cd (324.7 nm), m) Pb (283.3 nm), p) Cu (324.7 nm), r) Cr (357.9 nm), and M) V (313.3 nm). The dashed line is the predicted decrease in sensitivity for diffusion limited loss (reproduced from Reference [17])
A. Luminosity (mm2 sr) B. Integration time (ms) C. Optical path transmission D. Total intensity (= A × B × C) (mm2 sr ms) E. Absorbance noise (∝1/EF D) F. Absorbance signal (furnace dependent) G. Signal-to-noise ratio (F/E) a
Fig. 2 The ratio of experimentally determined characteristic masses (from Fig. 1) to the square root of the peak widths, with and without 5 µg of Pd and 3 µg of Mg(NO3)2, for g) Cd, m) Pb, p) Cu, r) Cr, and M) V (reproduced from Reference [17])
furnace in LS-AAS and the development of echelle polychromators and solid state detectors in LS-AAS and AES. Transversely heated furnaces, i.e. the transversely heated graphite atomizer (THGA) and the integrated contact furnace (ICF), are critical for multielement AAS. Based on the design of Frech et al. [16], these furnaces offer spatial and temporal isothermality which allows atomization at lower temperatures and decreases memory between atomizations. For multielement AAS, the pyrolysis temperature is dictated by the most volatile element and the atomization temperature is dictated by the least volatile element. The biggest concern is the loss in sensitivity and detection limits of volatile elements atomized at temperatures which are much higher than those used in the single element mode as illustrated in Fig. 1 [17]. It can be seen that the loss in sensitivity is not as severe as predicted by
Zeeman 4100ZL
SIMAA 6000 Single element
Multielement
0.0024
0.0020
0.0078
0.0078
300
720
648
648
0.5
1.0
1.0
1.0
0.36
1.47
5.66
1.42
1.66
0.82
0.42
0.84
1.0
0.48
0.48
0.48
0.60
0.58
1.14
0.57
Reference [1, 2]
diffusion (dotted line). This better-than-predicted performance is the result of the analyte volatilizing before the final furnace temperature is reached and an increase in the absorption coefficient at higher temperatures [17]. The high sensitivities at lower temperatures are also accompanied by broader peaks and longer integration times. Figure 2 shows the SNRs computed by dividing the sensitivities in Fig. 1 by the square root of the peak widths (the variance of the integrated absorbance is proportional to the integration time for the shot noise limited case) [17]. The loss in sensitivity is offset by the shorter integration times for the sharper, better defined peaks at higher temperatures. Thus, the SNRs at higher temperatures are generally better than those at lower temperatures. Echelle spectrometers are of interest because they offer high resolution and luminosity and a compact, two dimensional spectra suitable for monolithic solid state detectors. In LS-AAS, the increased luminosity of the echelle (for the SIMAA 6000) has been used to offset the source intensity lost when combining HCLs for multielement operation. As shown in Table 3, the luminosity of the SIMAA 6000 is a factor of 4 greater than that of the Zeeman 4100ZL despite the fact that the optical transmission factor, Top, decreases by 25% [1]. In the multielement mode, this means that the calculated SNR of the SIMAA 6000 is comparable to that of the 4100ZL. In the single element mode, the SNR is a factor of 2 better. The predicted SNRs (Table 2, item G) are confirmed by the published detection limits in Table 2. This strategy of increasing the spectrometer luminosity had been used previously to counter the reduced sensitivity encountered when the THGA (Zeeman 4100ZL) was substituted for the end-heated atomizer (Zeeman 5000). The THGA has two-thirds the length and half the sensitivity (Table 3, item F) of the end-heated furnace em-
505 Table 4 Solid state detectors
Detector size (mm × mm) Configuration Number of sub-units Sub-unit detector size Pixel size (µm × µm) Read noise (e–) Read time
SIMAA 6000a SCDb
CIDc
30 × 60
13 × 19
12 × 14
imbedded diodes 60 diodes
imbedded CCD arrays 114 linear CCD arrays 1 × 20 to 1 × 80 pixels 12.5 × 80 or 12.5 × 170 15 ~2 ms/array
1 array
1 diode 1 000 × 2 000
1 000 ~4 ms/ 60 diodes QE (%) at 200 nm 70
60
512 × 512 pixels 4 × 14 pixels 23 ×27 225 ~5 ms/ 4 × 14 pixels 35 (with phosphor)
a
Reference [1, 2] Reference [18, 19] c Reference [20] b
ployed by the Zeeman 5000. The lack of a fixed polarizer (Table 3, item C) and longer integration times (Table 3, item B) resulted in greater intensity reaching the detector and reduced absorbance noise levels (Table 3, item E). Again, the predicted SNRs (Table 3, item G) are confirmed by the detection limits shown in Table 2. In both AAS and AES, the compact, two dimensional spectra of the echelle are essential to the use of monolithic solid state detectors. The solid state detector of the SIMAA 6000 is a first for LS-AAS but is relatively unsophisticated compared to the detectors being used in AES (Table 4). The segmented charge coupled device (SCD) of the Optima (Perkin-Elmer Corp., Norwalk, CT, USA) [18, 19] and the charge injection device (CID) of the Iris (Thermo Jarrell Ash Corp., Franklin, MA, USA) [20] represent dramatic advances in detector technology (Table 4). The SCD offers low read noise (~15 e–), fast read times, high quantum efficiency in the UV (60% at 200 nm), and selective (but fixed) wavelength coverage. The CID offers complete wavelength coverage and low read noise (~25 e–) with 100 non-destructive reads. Both of these solid state devices are potential detectors for multielement AAS. The current limitations are their size (Table 4), their cost, and the rapid read time (each array or sub-array must be read at a minimum of 50 Hz) demanded by the transient furnace signal. Optimistically, these limitations can be overcome.
Prototype instrumentation In addition to commercial developments, there have been new developments in prototype, research AAS instruments using a continuum source (CS-AAS) and solid state detectors (linear photodiode arrays and CCD arrays). CSAAS, with array detection, integrates absorbance as a function of wavelength and time, permitting more funda-
Fig. 3 Time integrated absorbance for LS-AAS and wavelength and time integrated absorbance for CS-AAS
mental absorbances [12], improved detection limits [12], extended calibration ranges [21], and inspection of the spectrum around the wavelengths of interest [13]. Absorbance measured with an HCL is limited to the width of the HCL emission line while absorbance measured by CS-AAS with array detection covers the whole profile (Fig. 3). As a result, absorbance for CS-AAS, unlike that of LS-AAS, is independent of the source width and is dependent only on the efficiency of the atomizer. Time integrated (and normalized) absorbance for LS-AAS has units of seconds. The wavelength and time (normalized by the spectral width of a pixel) integrated absorbance of CS-AAS has units of picometer-seconds. Whereas, the analytical sensitivity for LS-AAS is defined as characteristic mass, mo (the mass necessary to give an absorbance of 0.0044 s), the sensitivity for CS-AAS is defined as intrinsic mass, mi (the mass necessary to give an absorbance of 0.0044 pm-s). Sensitivities for LS-AAS and CS-AAS are shown in Table 5 for a number of instrumental configurations. Conversion of characteristic mass for LS-AAS to intrinsic mass is accomplished by normalizing the HCL absorbance by the width of the emission line and multiplying by a correction factor to account for the differences in bandwidths between the lamp line and the absorption profile [12]. The intrinsic masses for LS-AS and CS-AAS agree within a factor of 2. The characteristic masses for LS-AAS were determined in the single element mode using optimum atomization temperatures while the intrinsic masses for CS-AAS were all measured at 2770° K. Characteristic masses would be expected to increase (grow worse) as the atomization temperature exceeds the optimum suggested by the manufacturer. Such a trend is not obvious in Table 5. In general the primary source of error is the accuracy with which the width of the HCL and absorption profile are known. The detection limits for CS-AAS are comparable to those for LS-AAS as shown in Table 2. With a linear photodiode array as a detector, the limiting noise for CS-AAS is the read noise and the best SNRs and detection limits in Table 2 are achieved with a 500 µm entrance slit width
506 Table 5 Continuum source intrinsic massesa
a
All values determined with HGA-500 graphite furnace atomizer (Perkin-Elmer Corp.) b Reference [9] c Reference [12] d Reference [13]
Wavelength (nm)
HCL source
Continuum source instrument
mob
Spectraspan IIIb
mic
LPDA (25 µm)
LPDA (50 µm)
H 20b LPDA (50 µm)
Optimad SCD (25 µm) 10. 20. –.–
As Se Zn
193,7 196.0 213.9
17. 30. 0.45
10. 11. 0.23
7.6 11. 0.50
7.7 10. 0.50
–.– –.– 1.4
Pb Sn Cd
217.0 224.6 228.8
5.0 20. 0.35
2.1 11. 0.16
1.9 9.2 0.27
1.8 9.4 0.19
2.1 –.– 0.23
4.5 –.– 0.29
Ni Co Fe
232.0 240.7 248.3
13. 6.0 5.0
5.7 2.5 1.9
8.0 2.3 1.6
8.4 2.3 1.8
6.4 –.– –.–
–.– –.– –.–
Mn Pb Cu
279.5 283.3 324.7
2.0 11. 4.0
0.63 3.8 1.1
0.83 3.9 1.0
0.88 3.8 1.1
–.– –.– –.–
0.76 2.1 –.–
(the largest possible on the echelle used). With a CCD detector, photon shot noise is limiting and the SNR is independent of the spectral bandwidth. The detection limits shown in Table 2 were obtained with an entrance slit width of 25 µm. Thus, state-of-the-art detection limits can be obtained while retaining the high resolution of the echelle. CS-AAS permits extension of the dynamic calibration ranges because intensity measurements can be made in the wings of the absorption profile. Two approaches [21] have been used in computing absorbances: wavelength selected absorbance (WSA) for the high resolution, shot noise limited case (Fig. 4 A) and wavelength integrated absorbance (WIA) for the large entrance slit, read noise limited case (Fig. 4 B). Figure 4 A illustrates how a trio of WSAs are computed for the pixel pairs, 0 and 1, –3 and 4, and –14 and 15 using pixels –20 and 21 as the reference
Fig. 4 A, B Modeled data showing intensity as a function of pixel number for two analyte concentrations using A a 25 µm entrance slit and B a 500 µm entrance slit. In A wavelength selected absorbances (As) are computed using the extreme shaded pixels (–20 and 21) in the wings as the reference intensity and each of the three pairs of shaded pixels (0 and 1, –3 and 4, and –14 and 15) as the analytical intensities. In B wavelength integrated absorbance (Aλ) is computed using the average of the shaded pixels (–35 to –20 and 21 to 36) in the wings as the reference intensity and summing the absorbances for the pixles covering the absorption profile (–15 to 15)
Fig. 5 Modeled calibration curve (m) and relative concentration error plot (M) for wavelength integrated absorbance (Aλ); data obtained as shown in Fig. 4 B. Concentration has been normalized by intrinsic mass so that a concentration of 1 ng ml–1 has an absorbance of 0.0044 pm-s. SNR at lowest point of the modeled calibration curve is 3.0 (reproduced from Reference [21])
507
Fig. 6 A, B Modeled calibration curves (A) and relative concentration error plots (B) for wavelength selected absorbances (As) computed using (M) pixels 0 and 1, (m) pixels –3 and 4, and (R) pixels –14 and 15 as the analytical intensities (Fig. 4 A). Concentration has been normalized by intrinsic mass so that a concentration of 1 ng ml–1 has an absorbance of 0.0044 pm-s. SNR at the lowest point of the calibration curve is 3.0 (reproduced from Reference [21])
tration for both WIA and WSA in Figs. 5 and 6. In both figures the concentration has been normalized by the intrinsic mass [21]. WIA produces a single curve (Fig. 5) which provides calibration over 6 orders of magnitude of concentration. Absoprtion in the wings of the absorption profile allows calibration long after the absoprtion at the line center has reached a maximum. A trio of WSAs produce a trio of curves which cover a similar wavelength range (Fig. 6). The calibration ranges in Figs. 5 and 6 could be further extendend by moving the reference and sample pixels further from the line center. Of course, this increases the possibility of spectral line overlap interferences. With CSAAS, however, spectral interferences can readily be observed and avoided. This is illustrated in Fig. 7 for the atomization of a 2 ng Se standard with and without 1 µg of Pd as a matrix modifier [13]. This data was acquired with the Optima spectrometer (a CCD detector) in the high resolution (3 pm/pixel) mode. Data from the National Institute for Standards and Technology list the weakly absorbing Pd line at 196.026 nm and the Se line at 196.011 nm. The potential interference of the Pd is easily recognized and can be eliminated by proper selection of the pixels for the signal (pixels 13–15) and reference (pixel < 8 and pixel > 16) intensities. Finally, CS-AAS permits enhancement of the analytical sensitivities and detection limits by atomization at elevated gas pressures [22]. At pressures greater than 1 atmosphere, the analyte diffusion coefficient decreases and the residence time and integrated absorbance increase. Increasing the pressure does not enhance the analytical signal for LS-AAS since the absorption profile is broadened and shifted away from the HCL line. For LS-AAS, the increased residence time is offset by the reduction in sensitivity. With CS-AAS, the wavelength shift and broadened profile are easily accomodated by the selection of the appropriate pixels. Table 6 shows the effect of atomization at 6 atmospheres of Ar on the detection limits for 4 elements. The improved detection limits at 6 atmospheres reflect the enhancement for atomization at higher pressure and the use of an end-capped graphite furnace with the dosing hole plugged [22]. With an open, transversely-heated furnace, loss of the analyte by convection becomes increasingly dominant as the pressure increases and the diffusion rate
Fig. 7 Intensity as a function of pixel number for the atomization of 2 ng of Se with and without 1 µg of Pd as a matrix modifier. The wavelength profiles were obtained at (P) the analyte peak maximum (with respect to time) or 1.5 s after the peak maximum and (p) 1 s prior to the start of the atomization (reproduced from Reference [13])
Table 6 Detection limits (pg) for CS-AAS at elevated pressures with an end-capped THGA and a plugged dosing hole
intensities. Figure 4 B illustrates how WIA is computed by summing the absorbances for pixels –15 through 16. The average of pixels –20 through –35 and 21 through 36 are used as the reference intensity. Modeled wavelength and time integrated absorbances and the predicted relative concentration errors are plotted as a function of concen-
Cd Cr Mn Pb
228.8 357.9 279.5 283.3
aReference
[22] [10]
Element
bReference
Wavelength (nm)
4100ZLb
0.3 2. 2. 3.
Continuum-Source-LPDAa Tube open 1 Atm.
End-capped 6 Atm.
0.5 0.8 0.8 1.
0.05 0.1 0.06 0.09
508 Table 7 Hypothetical multielement CS-AAS Source: Atomizer:
Polychromator: Detector:
Fig. 8 Fe absorbance (solid line) as a function of time for a mixed standard of inorganic Fe and hematin (Fe in hemoglobin as a chloride, Fe (Hb)). The cup of the two-step furnace was heated to 300° C for 15 s and then to 2000° C for 1 s (dashed line). The furnace was heated to 2600° C (dashed line) (reproduced from Reference [25])
decreases. In this case, the integrated absorbance does not increase linearly with increasing pressure. With an endcapped furnace with a plugged dosing hole, the integrated analyte absorbance increases in proportion to increasing pressure. Preliminary results also indicate that halide interferences were significantly reduced as a result of the longer residence time [22]. Another prototype device which has considerable potential for multielement AAS is the two-step furnace [23]. With optimized temperature programming of the upper furnace and the cup, it is possible to atomize each element at temperatures very close to the optimum [24]. Placement of this two-step furnace in a sealed chamber permits the both the furnace and cup to be cleaned up by heating them both at pressures of 100 mTorr. The vacuum can also be used to dry the sample and to encourage selective volatilization of different analyte species. Figure 8 shows the absorbance versus time profile for Fe obtained with a mixed standard of iron chloride and hematin (hemoglobin as a chloride) [25]. The upper furnace was heated to a temperature of 2600° C. The first peak Fe (hematin) was obtained upon heating the cup to 300° C and the second peak Fe (inorg) was obtained upon heating the cup to 2000° C. Thus, Fe in hemoglobin can be selectively differentiated from the inorganic Fe.
A hypothetical multielement CS-AAS instrument It is intriguing to speculate about the analytical capabilities that could be achieved for AAS by combining the technologies just discussed. This hypothetical instrument (Table 7) would consist of a continuum source, an echelle
Xe arc lamp Double furnace Upper – Side-heated (28 mm long) Lower – Carbon cup Vacuum/pressure cell High resolution echelle optimized for throughput at 200 nm Segmented CCD (30 mm × 60 mm) ~60 imbedded CCDs Arrays with 1 × 40 to 1 × 80 pixels Pixel 12.5 µm × 200 µm 15 e– read noise 60% QE at 200 nm Interrogation of each array at 50 Hz
similar to that employed by the SIMAA 6000, a monolithic detector similar in design to the SCD of the Optima (only larger), and a two-step furnace (with a transversely heated upper furnace) in a sealed environment such that the gas pressure could be increased to 6 atm or decreased to 10–4 atm. (~100 mTorr). The most problematic aspect of this instrument is the detector. First, the detector would have to be 30 mm by 60 mm, which is considerably larger than either the SCD or the CID (Table 4). Second, the use of segmented CCDs would be preferred since (at this time) they would offer the minimum noise for a single read, the fastest read time, and produce a shot noise limited instrument. Finally, it must be assumed that this detector could be produced in a cost effective manner. The predicted detection limits for the hypothetical instrument and those for the SIMAA 6000 are shown in Table 2. These values are based on several assumptions. First, the analyte sensitivities of the two instruments are comparable (Table 5). Elimination of the longitudinal magnet from the THGA would permit a considerably longer furnace to be used, but, for these calculations, that was not done. Second, the baseline absorbance noise is determined by assuming that photon shot noise is limiting, using previously determined ratios of the radiant power of a 300 W xenon arc lamp and a series of HCLs and EDLs [26], using appropriate equations for the radiant power of the HCL and the CS [9], and using the appropriate equation for the baseline absorbance noise [21]. Third, an entrance slit 25 µm wide and 1000 µm high is used and each pixel is 25 µm wide and range in height from 200 µm (in the visible region) to 1000 µm (in the far UV) since the cross dispersion of the SIMAA 6000 varies as a function of wavelength. Fourth, a pre-monochromator is used [27] which produces full height (1000 µm) and full intensity images in the UV and reduced height (200 µm) and intensity in the visible. And finally, wavelength selected absorbances are computed based on 2 reference and 2 sample pixels. The predicted detection limits for the hypothetical CSAAS listed in Table 2 are comparable to those for the SIMAA 6000 in the single element mode and approximately a factor of 2 better than those for the multielement mode. As stated earlier, these values assume no increase
509 Table 8 Comparison of techniques Detection limits (µg/L) (lowest 80%) Range of elements (routinely analyzed) Multielement capability Calibration range (orders of magnitude) Interferences Type Level Mass tolerance (wt%) Resolution Cost ($K)
in the length of the furnace or any increased intensity for the continuum source. LS-AAS with furnace atomization has been much maligned because of its low analytical rate. The hypothesized CS-AAS instrument compares very well with other techniques as shown in Table 8. The listed detection limits for inductively coupled plasma-mass spectrometry (ICP-MS) are 10 times lower than the hypothetical CSAAS, but these values are for pure solutions and for the single element mode. Complex matrices and multielement determinations will cause deterioration in these values. The primary disadvantage of CS-AAS is the inability to determine the non-metals. CS-AAS probably has a slight advantage over ICP-MS with respect to interferences and a significant advantage with respect to mass tolerance. Use of electrothermal vaporization (ETV) improves the mass tolerance and detection limits (in the single element mode) of ICP-MS, but further increases the complexity and cost of the instrument. In addition, multielement determinations with ETV-ICP-MS require reduced integration time for each mass unit and result in a decrease in the detection limits.
Conclusions Multielement AAS will ultimately be successful because it will offer figures of merit which are comparable or superior to those of available today for single element AAS. With the rapid advances in solid state electronics, a much larger, reasonably priced monolithic detector consisting of multiple linear CCD arrays seems inevitable. A CS-AAS instrument based on such a detector would provide analytical capabilities comparable to ICP-MS with considerable less complexity.
ICP-AES
ICP-MS
GF-AAS
0.1–30.
0.0001–0.001
0.002–0.1
70
75
45
Excellent 4–6
Fair 4–6
Excellent 4–6
Spectral Moderate 1–2 3 pm 60–200
Spectral Moderate 0.5 1 amu 150–250
Chemical Moderate > 100 3 pm 70–100
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