Anal. Chem. 1990,62,2370-2376
2,370
Figures of Merit for Furnace Atomization Plasma Emission Spectrometry R. E. Sturgeon,* S. N. Willie, V. Luong, a n d S. S. Berman Division
of
Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada K1A OR9
Analytical figures of merit for nine elements (vlz. Ag, Cd, Pb, Ni, Be, Fe, Bi, Cu, and P) are presented that serve to broadly characterize the furnace atomization plasma emission spectrometry technlque. Detection power lies In the low plcogram reglon; precislon of replicate signals obtained with injection of samples by hand ranges from 2.2 to 12.1% relative standard deviation and the linear range of CalIbr8tiOn spans 2-4 orders of magnitude. Relative sensitlvities of alternative atom and Ion lines are given for each element. Sufficient energy is avallabk to effklently detect analyte ion lines having excitation energies of up to 13 eV. Static and dynamic background spectra are presented which reveal broad-band contributions from, mdecular species such as CO, N,, NO, and OH as well as atom lines from the He support gas.
Although widely recognized as an atomizer for atomic absorption, the graphite furnace (GF) has also been successfully utilized as an emission source for thermal excitation of atomic vapor ( I , 2). Unfortunately, the relatively low temperatures attained preclude detection of elements having emission lines with excitation potentials much greater than 4.5 eV (2). This problem can be partially circumvented by relying on the high sample vaporization efficiency of the GF as a means of introducing analyte vapor into hotter excitation sources, such as the inductively coupled plasma (3). With these processes decoupled, such tandem devices provide a promising means of achieving relatively efficient yet independent optimization of both sample introduction/atomization and analyte excitation. A somewhat more advantageous arrangement is a combined source, wherein a single device permits retention of independent control over these two processes. Falk et al. ( 4 ) have earlier addressed the advantages of the low pressure FANES (furnace atomic nonthermal excitation spectrometry) system, which combines the attractive thermal vaporization attributes of the GF with nonequilibrium excitation in a hollow cathode discharge for high multielement detection power. Subsequently, Ballou et al. (5)described a low pressure hollow anode plasma source, also based on a GF (anode) but containing a coaxially centered graphite rod (cathode) extending the length of the tube. Although work with this hollow anode based FANES system appears equally promising ( 5 ) ,both sources suffer from the need to operate at low pressures (.
5z W
t-
z
I! I 0.0
I TIME, 8
I 252.6
I I
254.6
WAVELENGTH, nm Flgure 2. Signal transients and background spectra for phosphorus: (a) 2.5 ng, P I 253.6 nm; (b) 5 Mg, P I1 525.3 nm; (c) background wavelength scan: 50 W plasma, 0.082 nm SBW, furnace at 700 K.
quasi-continua from line wings, molecular bands from the discharge gas impurities or from reaction products between such impurities and sample constituents, spectral lines emitted by free atoms or ions of the discharge gas and its impurities, spectral lines emitted by free atoms or ions of the concomitants of the sample, and blackbody radiation from the atomizer tube wall. The majority of these sources is active in contributing to the background observed in the FAPES source. A wavelength scan covering the spectral interval 200-700 nm is presented in Figure 4. This scan was obtained at a furnace temperature of 700 K and a forward plasma power of 50 W. A highly structured background is evident in the region 200-500 nm, containing intense molecular bands arising from CO, NO, OH, and N2 as well as intense lines from He I. Bands arising from the fourth positive system of CO have not been labeled on Figure 4a in an effort to keep the congestion of information to a minimum. However, the following band heads are evident: (3,10),(4,11),(5,12),(3,11),(6,14),(7,15), (4,13),(8,16),and (7,16)at 204.6,206.8,209.0,212.8,219.7, 222.2,223.8, 224.7, 231.2 nm, respectively. The highly structured feature centered at 283 nm reflects both OH as well
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 21, NOVEMBER 1, 1990
I
50
1 Cadmium
P s
200
210
220
230
240
250
270
260
280
290
300
WAVELIENGTH, nm
0.0
1.o
0.5
1.6
Copper
0.0
1.o
2 .o
24
Iron
P
c1
5 .O
I 3
2'
gz
12 1
400
/
410
1
420
,
/
430
W
,
1
440
,
1
,
450
1
,
460
l
,
470
l
,
480
I , 490
500
WAVELENGTH, nm
I-
z 0.0
2 .o
1.o
3 .o
TIME, s Figure 3. Effect of forward plasma power on signal intensities: (-) 50 W, (---) 60 W, (--) 75 W; Cd, 250 pg; Cu, 500 pg; Fe, 5 ng.
as the presence of other additional (unidentified) species. Beyond 500 nm, the plasma background is relatively structureless and, with the exception of some discharge gas lines, the few features evident in Figure 4e have not been identified. The molecular bands arise predominantly from impurities in the He gas and from ingress of ambient atmosphere into the furnace through the sample introduction port (11, 12). Dynamic background signals arising from the "atomization" of acidified DDW blanks are illustrated in Figures 1 and 2. Two cases can be discerned: a relatively flat background that is associated with the more volatile elements and a background that increases with time, eventually achieving a plateau. This latter feature is characteristic of the less volatile eIements that atomize at elevated temperature (Ni, Be, Fe, Cu) when blackbody continuum from the hot furnace wall reaches the detector. Such a continuum photon flux is orders of magnitude greater than that arising from the plasma continuum itself ( 4 ) . An efficient optical mask will significantly reduce this problem. Figures 2 and 5 illustrate the structure of the static background in the neighborhood of the analytical lines selected (f1.0nm). These wavelength scans were obtained with the furnace set at the appropriate char temperature used for each element (cf. Table 11). It is obvious that there is highly structured background in the vicinity of many of the analysis
500
510
530
520
540
550
560
570
580
590
E
WAVELENGTH, nm
OH
600
610
A'X'-Xw
620
630
640 650 660 WAVELENGTH, nm
1
Figure 4. Plasma background in the 200-700 nm region: 50 W plasma; 0.082 SBW; furnace at 700 K.
lines: NO bands near lines for Cd, Fe, and Be; CO bands near Ni and OH bands near Bi and Pb. Although the intensities of these structures are quantitatively presented in these figures, they refer to conditions prevailing at the char temper-
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 21, NOVEMBER 1, 1990
Table IV. Sensitivity"
Table 111. Absolute Detection Limits, pg element
A, nm
Ag
328.1 320.1 234.8 306.7 228.8 324.8 248.3 302.1 232.0 341.5 253.6 214.9 283.3
Be Bi Cd cu Fe Ni
P Pb a
FAPES" peak area 1.2
4.8
5.0 25 2.0 4.9 50
16 55 6.8 14 51
FANES peak 4.0
164
45 2 1.5 6.8 0.7
405 2.3
94
152
21
46
210 12
element
peak height, nA/ng
peak area, nA.s/ng
Ag Be Bi Cd cu Fe Ni
370 89 46 51 45 4.4 3.1 71 4.7
120 37 15 7.4 16 4.3 2.1 36 1.1
P Pb a
Data obtained under conditions given in Tables I and 11.
Table V. Precision and Linear Range"
LOD based on 3~7bcriterion where q is the estimate of the SD
precision, % peak area
of repetative measurements of the blank.
ature. It can be experimentally seen that the intensities of the NO bands decrease dramatically as the temperature of the furnace increases while there is a corresponding increase in the intensities of the CO bands. This simply reflects the increasingly reducing atmosphere within the furnace as the temperature rises (11, 12). Cleanup of the He gas and enclosure of the furnace in a more efficient housing to eliminate ingress of ambient atmosphere, such as that described by Ballou et al. (5),should significantly reduce molecular background and permit access to elements having sensitive emission lines in the 305-320 nm (Al, Mo, V) and 350-360 nm (Cr, U) regions. It is evident that use of a high-resolution spectrometer and efficient transfer optics coupled with rapid dynamic background correction should eliminate many potential spectroscopic problems that may be encountered with the present system. Figures of Merit. Table I11 compares absolute limit of detection (LOD) for the FANES system with those obtained in this study by FAPES. The former data are compiled from the work of Littlejohn (13). The LOD for the FAPES system is based on the 3fJb criterion where fJb is the estimate of the standard deviation of repetitive measurements of the background or blank signal. No information is given in ref 13 as to the manner in which the FANES LOD is calculated. In general, detection power lies in the picogram range and compares favorably with those data reported for the FANES system. Room for significant improvement in these data is expected with use of more efficient transfer optics, dynamic background correction, a high-resolution spectrometer, chemical modifiers, and an isothermal furnace such as that described by Frech et al. (14). Of the several lines for each element that were investigated, those reported in Table I11 resulted in the best detection limits. Peak height quantitation was found to result in LODs superior to those obtained with signal integration, particularly for the more volatile elements possessing small signal halfwidths. It is noteworthy that nonmetals such as P can be detected with the FANES and FAPES systems. In the present case, the LOD reported for P is superior to the best values obtained from either GFAAS, i.e., 3000 pg (15),or FANES, Le., 150 pg (16). Table IV summarizes the sensitivities of the elements in terms of absolute response per nanogram for both peak height (nanoamperes) and area (nanoamperes seconds); Table V gives the precision of replicate measurements and approximate linear range of calibration curves. Precision is largely determined by the repeatability of injection of samples into the furnace by hand. Evidently, a precision as good as 2% relative
Ag Be Bi Cd cu Fe Ni
P Pb
9.4 2.2 6.5 7.5 2.3 5.8 2.5 12.1 7.7
6.0 2.2 7.7 3.5 2.5 3.7 4.0 12.5 3.5
a Precision expressed as % RSD for n 100-fold estimated LOD.
log (linear range) peak area 3.9 3.3 3.3 4.4 2.9 4.4 2.0 1.7 2.4
3.3 2.8 3.0 4.5 2.1 4.3 1.6 1.7 2.7
> 8 a t concentrations 10-
Table VI. Plasma Gas Lines line, nm He He He He He He He
1667.8 1587.6 1501.6 1492.2 1471.3 1447.1 1388.9
excitation energy, eV
re1 sens
23.07 23.06 23.08 23.72 23.58 23.72 23.00
1.0 0.70 0.64 0.07 0.06 0.11 0.57
standard deviation can be achieved for this operation since this was attained for Be and Ni. Relatively poor precision is seen with volatile elements, possibly reflecting variable preatomization losses of molecular species (particularly P) and any instability in the plasma which occurs during this early period of rapid heating of the furnace and expulsion of the internal gas. It is expected that these data could be improved with the use of chemical modifiers and/or atomization from a platform which would serve to shift the signals to higher temperatures. The linear ranges of calibration curves are shorter than expected for an emission source and typically span only 3 orders of magnitude, not significantly greater than those for GFAAS. It is possible that self-absorption may limit the linear range in this system because of the extremely large temperature gradient that exists between the center and ends of the furnace (17).This shortcoming could be eliminated with the use of an isothermal integrated contact cuvette type of furnace (14). Alternatively, a higher powered plasma of expanded volume may extend the linear range, but this aspect was not investigated. Analyte Excitation. Analyte excitation and ionization likely occur via electron impact. As noted earlier, no analyte emission was detected during sample atomization if the plasma was not on, thermal excitation being negligible. A number of intense He atom and ion lines were observed in a 50-W plasma. Their relative sensitivities are summarized in Table VI along with their corresponding excitation energies. Strong
12
12
Nickel
Cadmium
P >-’
6 -
6 v)
z W I-
z
228.8 I
--I
12
I
1
I
1
I
Beryllium
1 ;
248.3
J
I I
I
233.7
235.7
12
247.5
249.3
36
Lead
2 >:
12
12
Copper
2
Silver
$ v)
6 -
z W
c
z
524.7
excitation of He should be indicative of strong analyte excitation. As the forward power to the plasma was increased from 50 to 75 W, the intensities of the He 1587.6- and 447.1-nm lines increased %fold. No study of the effect of internal He gas flow rate on the intensity of plasma gas lines was made. Although not possible to undertake with the present setup, it is expected that plasma gas line intensities will be markedly affected by the ambient pressure (18)and future investigations will be aimed at elucidating this. Table VI1 presents a summary of the relative sensitivities of several alternative emission lines of each anal*. Also given are the relative sensitivities of these lines observed in an Ar inductively coupled plasma (ICP) (19). It is evident that sufficient energy is available in the He plasma to ionize and excite populations of a number of analytes, including nonmetals. In most cases, the alternative atom line is comparable in relative intensity to that observed in the ICP. However,
large discrepancies are evident with ion line intensities, leading to the obvious conclusion that the degree of ionization of analytes is significantly lower in the FAPES source. The excitation temperature of the FAPES source has been measured (Boltzmann plot with both He I and Fe I thermometric species) to be 3200 f 200 K (20). The data in Table VI1 also point to the utility of utilizing the major compilations of wavelength tables that are available, such as Boumans’ (19),to select appropriate analyte lines for measurement and take advantage of the wealth of information presented in this literature concerning potential spectroscopic interferences.
CONCLUSIONS This study has demonstrated the analytical characteristics of the FAPES technique. Detection power is comparable to or better than that available with FANES and operation is
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 21, NOVEMBER 1, 1990
Table VII. Analyte Responseo relative sensitivity
FAPES
ICPb
1.0
0.48 0.05
line, nm
excitation energy, e V
B e 1234.8 B e 1249.5 B e I1 313.0
5.28 7.68 13.28 3.78 3.66 17.5 5.54 5.28 14.56
0.03 0.95 1.0 0.30 0.002 1.0 0.30 0.05
4.04 5.55 >7.29 5.41 7.37 14.77 3.82 3.78 13.23 4.40 5.67 14.67 4.99 3.32 4.11 13.07 7.21 7.18 7.18 13.38
1.0 0.05 6X 1.0 0.02 2 x 10-4 1.0 0.40 2 x 10-3 1.0 0.10 5 x 10-3 1.0 0.41 0.27 0.04 1.0 1 x 10-3 0.05 I x 10-5
A g I 328.1 A g 1338.3 A g I1 243.8
Ni I 232.0 Ni I 234.5 Ni I1 221.6 Bi 1306.8 Bi 1223.1 Bi I1 190.3 Cd I 228.1 Cd 1361.1 Cd 11 214.4 Cu 1324.8 Cu 1327.4 Cu I1 224.7
Pb 1283.3 Pb 1217.0 Pb I1 220.4
I 248.3 1372.0 1302.1 I1 238.2 P I 253.6 P 1255.3 P 1213.6 P I1 525.3 Fe Fe Fe Fe
1.o
1.0 0.49 0.025 0.64 0.29 1.0 1.0 0.93 0.06 0.88 0.04 1.0 1.0 0.51 0.39 0.51 0.38 1.0
LITERATURE CITED
0.28 1.0
Atomization and measurement conditions as given in Tables and 11. b F r o m r e f 19.
FAPES offers great latitude for use in both metal and nonmetal analyses and there are substantial data to suggest that the technique has considerable potential for future development. A number of improvements can be relatively easily implemented with this system that should have a significant impact on improving performance. These include analyte atomization from a platform and use of chemical modifiers, use of an isothermal integrated contact cuvette furnace, more efficient transfer optics, and imaging coupled to a high resolution spectrometer and dynamic background correaction (e.g., wavelength modulation). An improvement of lC-100-fold in LOD can be expected. Future work must also consider the areas of plasma characterization and the influence of matrix concomitants (interferences) on analyte response. Work is presently in progress to address these issues.
I
unencumbered by the need to establish a low-pressure discharge. Since furnace programming is similar, the cycle time per analysis is comparable to conventional GFAAS. Graphite tube lifetime is identical to GFAAS methodology and that of the center electrode is in excess of 200 cycles. Although the background spectrum exhibits many molecular bands, most of these can be eliminated or severely attenuated by working with a more enclosed furnace. The rapid signal transients that are developed require a fast response (
