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Rapid analysis with transversely heated graphite furnace atomic absorption spectroscopy
lm34-w7/93$300+ 00 Q 1993 Peqmm Press Ud
S~tm,&inm Acta, Vol 48B, NO 11, PP 1435-1443.1993 mntcdIII&at Bntain
TECHNICALNOTE
Rapid analysis with transversely heated graphite furnace atomic absorption spectroscopy (Received and accepted 16 May 1993)
1. IIVTR~DUCTION
MODERNgraphite furnace techniques, characterixed by the use of the stabilixed temperature platform furnace, STPF [l], have proven to be accurate but slow. Typical analyses require an analytical cycle time of 2-4 min per determination. While retaining the conditions that have made furnace atomic absorption spectroscopy (AAS) accurate, methods have been proposed to speed up the analyses [2] by avoiding the pyrolysis step and making the drying step much more rapid. These proposals have been used by others, both with solution samples [56] and with solids sampled as slurries [7]. Some of the earlier literature was reviewed previously [2]. The complete temperature program from most of these papers is about 30 s and the analytical cycle time is less than 1 min. A new graphite furnace instrument, which utilixes longitudinal Zeeman effect background correction [S], and a transversely heated furnace [9] was developed. It is intended to speed up the analytical throughput and provide conditions even closer to those theoretically required for furnace AAS, In this paper experiments were conducted to test these capabilities. A disadvantage of the o;6nventional longitudinally heated furnace is that the ends of the furnace tube are cold. With this furnace design FALKet al. [lOI had shown that it was preferable to start the atomixation step from room temperature to heat the greatest length of the furnace tube to atomixation temperature. From this theory, a cooldown step [ll] was inserted between the pyrolysis step and the atomixation step so that the atomixation step proceeded from room temperature. When the furnace is heated transversely, the full length of the furnace is heated rapidly and at the same time. There are no colder ends and the cooldown step is not required, saving some analytical time. A relatively small sample is taken, 10 p.l, and it is deposited on the platform heated to about 100°C. The rate of sample delivery onto the platform can be controlled so that all the sample is dry by the time the full 10 Pl is deposited. The major advantage of the fast furnace programs are that each firing can be completed in less than a minute compared to 2-4 min for conventional furnace methods. In this work, a representative group of analytes, Cd, Cu, Cr, Se, As, Tl, V and Pb, were determined in several reference materials.
2. EXPERIMENTAL 2.1. Instncmentation The analyses were carried out with the Perk&Elmer 4100ZL or 51OOZLinstruments. This included the fume extraction system and the recirculating water bath used to control the temperature of the furnace. On these instruments the platform can be preheated before sample deposition. It is also possible to reduce the rate at which the sample is injected onto the platform, For the measurements of As, Pb, Se and Tl, EDLs were used with a Perkin-Elmer EDL power supply system. 2.2. Reagent The standard solutions for each analyte were prepared by stepwise dilution from loo0 mg/l stock solution. The acids used were “Seastar” grade. 1435
1436
Technical note Table 1. Wavelength, atomization temperature
and characteristic mass Characteristic
Element
As cd Cr cu Pb Se Tl V
Wavelength (nm) 193.7 228.8 357.9 324.8 283.3 196.0 276.8 318.4
Atomization temperature (“Cl 2300 1600 2300 2ooo 1600 1900 1600 24w
Observed (P& 44
1.6 6 17 33 48 52 55
mass (pg) Reported
WI 42 1.3 7 17 30 45 53 42
2.3. Samples and sample preparation A variety of reference materials were used for this study: NIST SRM 2670 Normal and Elevated Urine, NIST SRM 1573 Tomato Leaves, NIST SRM 1575 Pine Needles, NIST SRM 1645 River Sediment, NIST SRM 1643 Trace Metals in Water, and BCR CRM 144 Domestic Sewage Sludge. Several Environmental Resources Association (ERA), Arvada, CO 80002, waste water samples were analyzed: ERA 2524 Waste Water and ERA 9937 Waste Water. The NIST SRM 2670 normal and elevated freeze-dried urine were reconstituted, as specified on the SRM data sheet, by dilution with 20 ml of water, shaking thoroughly. Aliquots of 0.20 g of NIST SRM 1573, 1575 and 1645, and BCR CRM 144 were weighed into Teflon containers. Each sample was mixed with 1.0 ml of cone I-INOs and 1.0 ml of cone HCl. The container was assembled in an acid digestion bomb (Parr Instrument Company, Part No. 4745). The bomb was heated in an oven at 150°C for 8 h. After cooling, the container was removed from the bomb. Two milliliters of cone I-IF and 1 ml of cone HC104 were added. (Note: care must be taken in the use of perchloric acid, using a hood approved for that purpose.) The solution was heated on a hot plate at 120°C to incipient dryness. One milliliter of cone HNOs was added. The solution was heated again to incipient dryness. Five milliliters of water was added to the container. The solution was heated at 90°C for 20 min. After cooling, the solution was diluted to 20 ml. Three separate sample solutions were made for each SRM. The blank solution was prepared in parallel. The ERA 2524 waste water was prepared by pipetting 5.0 ml of the concentrate solution into a one liter volumetric flask and diluting to the mark with water. The ERA 9937 waste water was prepared by pipetting 5.0 ml of the concentrate solution into a 500-ml volumetric flask, adding 2 ml cone HN03 and diluting to the mark with water. The above prepared sample solutions were diluted to provide an analyte concentration which would give an integrated absorbance between about 0.1-0.2 s, with an injection volume of 10 p,l. The calibration curve for each analyte was obtained using serial dilution to produce four standard solutions. The reagent blank was subtracted. Four replicates were measured for each standard and each sample solution. Each of the reference materials was analyzed for each of the analytes for which reference values were available. The confidence limits in the tables are + 1 standard deviation.
2.4. Instrumental conditions The wavelength and atomization temperature used for each element and the characteristic mass found are listed in Table 1. The sensitivities for As and Pb were degraded from that shown in Table 1 by about 30% when Pd and Mg were used as modifier. For the standards, the agreement of the characteristic mass with the manufacturer’s values (121 was good. The several furnace programs that were used are shown in Table 2. In every case, the analyses were standardized against analytical working curves prepared with simple aqueous standards in dilute nitric acid.
Technical note
1437
Table 2. Furnace programs A: Fast furnace program, no pyrolysis or cooldown steps Ramp Hold Temperature Step (s) (s) (“C) 1 2 3
120 250 *
4
2400
1 1 0 1
15 7 5 2
Total program time: 32 s. Injection temperture: 120°C; pipette speed: 100%. Injection volume: 10 ~1. * See Table 1 for atomizatton temperature.
B: Standard furnace program Temperature
(“C) Ramp
Step 1 2 3 4
As
Pb
(s)
Hold (s)
120 1000 2300 2400
120 850 1800 2400
1 10 0 1
60 20 5 2
Total program time: 99 s. Injection temperture: 20°C; pipette speed: 100%. Injection volume: 20 ul sample + 10 ~1 modifier. Modifier: 15 pg Pd + 10 ug Mg(NO,),.
C: Fast furnace program using a modifier Temperature (“C) Step 1
2 3
As
Pb
Ramp (s)
Hold (s)
1000 2300 2400
850 1800 2400
1 0 1
15 5 2
Total program time: 24 s. Injection temperature: 120°C; pipette speed: 60%. Injection volume: 7 ~1 sample + 3 4 modifier. Modifier: 5 pg Pd + 3 t.t.gMg(NO,),.
3. RESULTS 3.1. Removal of pyrolysis and cooldown steps The possibility of removing the pyrolysis and cooldown step prior to atomization and of using no modifer was investigated by analyxing the volatile elements, Cd, Pb and Tl, the elements of intermediate volatility, As, Cu and Se, as well as the refractory elements, Cr and V, in various sample matrices. These included urines, plant materials, sludge, sediment and waste waters. The furnace program used for these analyses is listed in section A of Table 2 with atomization temperatures from Table 1. The sample drying was done in two steps. The furnace program required 32 s, plus about 30 s for the autosampler to dispense the sample solution and for the furnace to cool to room temperature. The real cycle time for each measurement was about 1 min. The analytical results for each element, except As and Pb, in the various samples are listed in Tables 3-6. These data for the fast furnace technique, eliminating the pyrolysis and cooldown step, agree with the certified or reference values within the uncertainty limits.
1438
Techmcal note Table 3. Cd results Material NIST 1643b water NIST 2670 urine (elevated) ERA 2524 waste water ERA 9937 waste water BCR 144 sewage sludge NIST 1645 river sediment
Found
Certiiied
20 + 1 (I@) 0.089 2 0.002 (me/l) 55 + 1 (Pfl) 117 + 2 (lJ@) 3.61 2 0.14 (i&g) 10.1 2 0.5 (l&g)
20 2 1 0.088 f 0.083 50 (40-60) 106 (87-125) 3.41 f 0.25 10.2 -c 1.5
Table 4. Cr results Material NIST 1643b water NIST 2670 urine (normal) NIST 2670 urine (elevated) NIST 1573 tomato leaves NIST 1575 pine needles ERA 2524 waste water ERA 9937 waste water NIST 1645 river sediment
Found
Certified
18.8 f 0.1 (Pti) 0.013 + 0.001 (mgfl) 0.089 + 0.002 (mgfl) 4.1 + 0.1 (l@g) 2.8 + 0.1 (k@g) 215 f 2 (i&l) 248 + 4 (&I) 2.71 f 0.34 (“/)
18.9 f 0.4 (0.013) 0.085 + 0.006 4.5 2 0.5 2.6 ” 0.2 227 (181-272) 236 (193-278) 2.96 k 0.28
Table 5. Cu results Material NIST 1643b water NIST 2670 urine (normal) NIST 2670 urine (elevated) NIST 1573 tomato leaves NIST 1575 pine needles NIST 2524 waste water ERA 9937 waste water BCR 144 sewage sludge NIST 1645 river sediment
Found
Certified
21.4 + 0.6 (t&l) 0.14 + 0.01 (mgn) 0.35 f 0.01 (mgn) 9.3 2 0.4 (P&g) 2.5 2 0.1 (k&g) 217 2 3
22.1 f 0.4
19P! (A 696 114 (i&g) 108 f 9 Wg)
0.13 5 0.02 0.37 2 0.03 11 f 1 3.0 + 0.3 192 (154-230) 226 (185-267) 713 2 26 109 2 19
1439
Technical note Table 6. Se, Tl and V results Material
Analyte
Found
Certified
0.46 rt 0.05 (mgfl) 89 f 2
0.46 + 0.03
NIST urine ERA waste ERA waste
2670 (elevated) 2524 water 9937 water
Se
ERA waste ERA waste
2524 water 9937 water
Tl
(I@) 161 + 3 (I.%%
ERA 9937 waste water NIST 1645 river sediment
V
78
(63-94)
142 (106-168)
36 -e 1 (I@) 73 2 1 (I@)
35 (28-43)
259 + 13 (I@) 20.1 5 1.3
248 (203-293)
74 (55-87)
23.5 f 6.9
(I&&
3.2. Determination of As and Pb The analytical results for As and Pb in some of these materials is shown in Tables 7 and 8,
respectively. The fast furnace results for As and Pb in urine were very low, indicating loss of analyte before it could be reduced to the atomic vapor. The urine was reanalyxed for As and Pb, and As was determined in the sediment solution using a standard STPF program (part B of Table 2) which included a matrix modifier and pyrolysis. The values found for these determinations are shown in Tables 7 (As) and 8 (Pb) and they are in satisfactory agreement with the expected vahres. Probably the presence of the modifier prevented loss of analyte at low tempertures. 3.3. Removal of the drying step An alternative fast furnace program was sought which might provide good results for the As and Pb determination while retaining use of the matrix modifier. The possibility of removing the drying step entirely was investigated by slowing deposition of the sample solution onto the preheated platform. Copper was chosen for this experiment. Ten microliters of a 50-&l Cu standard solution was injected onto a platform, which was already heated to 120°C. The injectiou rate was reduced to 60% compared with the program of Table 2, section A. To see if the sample was completely dry, this injection was followed by a firing at a much higher temperature where splattering would have occurred if the 10 pJ of solution had not been dry. The furnace program Table 7. As results Found Material
Fast furnace*
NIST 1643b water NIST 2670 urine (elevated) ERA 2524 waste water ERA 9937 waste water NIST 1645 river sediment
55 f 1 (I@) 0.024 + 0.018 (mgfl) 105 + 1
Standard furnace?
Certified (49)
0.53 + 0.03
0.48 + 0.10 81 (64-97)
(WLgfl) 128 f 1 (I@) 41 + 3 (I&)
* Conditions of Table 2, section A. t Conditions of Table 2, section B.
110 (82-130) 71 * 3
(66)
1440
Technical note Table 8. Pb results Found Material
Fast furnace’
Standard furnace?
Certified
24.0 + 0.8
NIST 1643b water NIST 2670 urine (elevated) ERA 2524 waste water NIST 9937 waste water BCR 144 sewage sludge NIST 1645 river sediment
(lW 0.032 f 0.005
24.1 f 0.7 0.108 f 0.008
0.109 f 0.004
(mgfl) 184 + 3
172 (138-206)
(I@) 215 + 4
239 (196-282)
(WY) 491 * 10
495 2 19
(t&g) 739 +: 24
714 f 28
(t&g)
* Conditions of Table 2, section A. t Conditions of Table 2, section B. Table 9. Furnace program for testing the infhrence of drying Temperature (“C)
Step 1 2 3
Ramp (s)
Hold (s)
1 0 1
25 5 2
* 2000 2400
* Variable lOO-1ooo”C. 10 t.Llof 50 p&/l cu. Platform at 120°C; pipette speed 60%.
shown in Table 9 was used for the measurement. The temperature in step 1 was varied from 100 to 1000% and the resulting data are shown in Table 10. Since the Cu integrated absorbance and the precision were independent of the initial temperature, the water must have been fully evaporated prior to the first step. The influence of the hold time in step 1 was also tested at the temperature of 800°C in Table 11. The results show that the sensitivity and precision did not change even at a hold time as short as 10 s. No attempt was made to use a hold time less than 10 s, since 1 or 2 s are necessary anyway for setting the instrumental baseline (BOC) before atomization. Table 10. Influence of step 1 temperature signal* Temperature (“C) loo 200 300 400 500 600 700 800 900 1000 * Hold time: 25 s.
Absorbance (s) 0.141 0.145 0.146 0.146 0.146 0.148 0.149 0.149 0.146 0.144
on Cu
RSD (%) 1.47 1.32 0.57 0.54 0.69 0.88 0.72 0 71 0.46 0.52
1441
Technical note Table 11. Influence of step 1 hold time on Cu signal* Hold time (s)
Absorbance (s)
RSD (%)
0.149 0 147 0.147 0.150
O.% 0 54 0.69 0.99
25 20 15 10
* Step 1 temperture: 800°C Table 12. As and Pb results with modifier Found
Material
Analyte As
NIST 2670 urme (elevated) NIST 1645 river sediment NIST 2670
Pb
No modifier
With modifier
Standard furnace
Certified
0.02 f 0.02 (mg/l) 41 f 3 (l&g) 0.03 + 0.005
0.51 f 0.06
0.53 2 0.03
0.48 * 0.1
72 + 5
71 + 3
(66)
1.103 + 0.008
0.108 + 0.008
0.109 f 0.004
urine (elevated)
(mgfl)
These experiments with Cu indicated that we could omit the drying step and retain a short high temperature pyrolysis step to remove some matrix prior to atomization. These possibilities were tested using the As and Pb determinations in the materials that had produced problems in the original fast program. The new fast furnace program for As and Pb are shown in section C of Table 2, using a lO-pl aliquot composed of 7 u.1 of sample and 3 ~1 of the modifier. The analytical results with this fast program are shown in Table 12, along with the data for those materials from the earlier tables. The results from this new fast program are in excellent agreement with the reference values. The signal profile of Pb in urine obtained with and without the pyrolysis step are compared in Fig. 1. The left side of Fig. 1 compares the background with and without pyrolysis and the modifier. The right side compares the analytical signal showing the improved recovery of analyte. The background was greatly reduced compared to the result without pyrolysis and the modifier.
Background 1.20
I
5 f e 8
With PyrOlySiS
S
C 0
3
Time (se@
D
0
3.00
Time (84~)
Fig. 1 The absorbance profile of Pb m urine comparing the results when no matrix modifier is used (a) and when the Pd + Mg matrix modifier is used (b) with the program of Table 2, section C.
1442
Technical note 4. DISCUSSION
These experiments showed that for many analytes and samples, the simple programs which omit the pyrolysis and the matrix modifier work very well. For some situations the pyrolysis step was still required even with Zeeman background correction with the new transversely heated furnace. However, a furnace program of less than 25 s using slow injection into a preheated furnace and a matrix modifier provided very similar results compared to the standard furnace analyses. More recent experiments indicate that the temperature to which the platform should be preheated might be reduced to 100°C. The conditions in this paper should provide a good starting point for optimizing these fast furnace methods. From the experience of these experiments, it is expected that, using preheated injection, the dry, pyrolysis and cooldown steps can be removed from the program if no interference is found. A program time of less than 15 s can be used for the sample analysis and, perhaps eventually, more than 100 samples can be analyzed per hour when the instrumentation will reduce the time to prepare for the next sample. Little additional time will be required in those situations where pyrolysis is found to be useful. This hypothesis wasn’t tested. It will be necessary to decide how to accommodate the BOC setting in the program.
5. SUMMARY A new graphite furnace instrument using a transversely heated furnace tube with a longitudinal Zeeman correction system has been specifically designed to provide more nearly stabilized temperature platform furnace (STPF) conditions than previous furnace systems. Because there are no cold ends on this furnace tube on which to condense analyte and matrix materials, the vapor phase interferences are expected to be smaller. Also, the cooldown step can be avoided, thus saving time. This instrument permits the delivery of sample into a furnace already heated. The delivery rate of the autosampler can be slowed. These opportunities make it feasible for the sample to be dry on the platform by the time the delivery is complete. Several elements of environmental significance were chosen for test: As, Pb, Se, Tl, Cd, Cu, Cr and V. In almost all of these situations, the analyte was fully recovered without using a matrix modifier or a pyrolysis step. However, As and Pb in urine and As in sediment required a modifier and pyrolysis step for accurate results. A new fast furnace protocol was developed to accommodate use of a matrix modifier and this new protocol was successful for Pb and As in these matrices. All the procedures required less than 1 min total cycle times and produced results in agreement with certified values. This is in contrast with conventional methods which require 2-3 min per firing. These results confirm that graphite furnace methods can be accomplished with a throughput greater than 60 determinations per hour, and eventually, it may be possible to increase this rate beyond 100 determinations per hour. Acknowledgement-We
thank JEANNESCHICKLI for her experimental help in some of these experiments.
The Perkin-Elmer Corporation 761 Main Avenue Nonvalk, CT 068.59-0237 U.S.A.
ZHANG LI GLEN CARNRICK
WALTER SLAVIN
Bonaire Technologies Box 1089 Ridgefield, CT 06877 U.S.A. REFERENCES
[l] W. Slavin and G Camrick, SpectrocIarm Acru 39B, 271 (1984). [2] W. Slavin, D. C. Manmng and G. R. Carnnck, Specrrochrm. Acra 44B, 1237 (1989). [3] Llan Liang, P. C. D’Haese, L. V. Lamberts, F. L Van de Vyver and M. E. De Broe, Anal. Chem. 63, 423 (1991). [4] Llan Liang, Specrrochrm. Acra 473, 239 (1992) [5] M. B. Knowles, J. Ad. At. Speclrom. 4, 257 (1989). [6] D. J. Halls, Tufanru 37, 555 (1990).
Technical note
1443
[7] D. Bradshaw and W. Slavin, Spec~ochim. Acta 44B, 1245 (1989). [8] M. T. C. de Loos-Vollebregt, L. de Galan and J. W. M. van Uffelen, Spectrochim. Actu 43B, 1147 (1988). [9] W. Frech, D. C. Baxter and B. Hiitsch, Anal. Chem. 58, 1973 (1986). [lo] H. Falk, A. Glisman, L. Bergann, G. Minkuntz, M. Schubert and J. Skole, Spectrochrm. Actu 4OB, 533 (1985). [ll] D. Manning and W. Slavin, Specrrochun. Acta 403, 461 (1985). [12] List of Characteristtc Masses for 4100 ZL, Bodenseewerk Perk&Elmer, F. R. G. (1991).
S~tm,&inm Acta, Vol 48B, NO 11, PP 1435-1443.1993 mntcdIII&at Bntain
TECHNICALNOTE
Rapid analysis with transversely heated graphite furnace atomic absorption spectroscopy (Received and accepted 16 May 1993)
1. IIVTR~DUCTION
MODERNgraphite furnace techniques, characterixed by the use of the stabilixed temperature platform furnace, STPF [l], have proven to be accurate but slow. Typical analyses require an analytical cycle time of 2-4 min per determination. While retaining the conditions that have made furnace atomic absorption spectroscopy (AAS) accurate, methods have been proposed to speed up the analyses [2] by avoiding the pyrolysis step and making the drying step much more rapid. These proposals have been used by others, both with solution samples [56] and with solids sampled as slurries [7]. Some of the earlier literature was reviewed previously [2]. The complete temperature program from most of these papers is about 30 s and the analytical cycle time is less than 1 min. A new graphite furnace instrument, which utilixes longitudinal Zeeman effect background correction [S], and a transversely heated furnace [9] was developed. It is intended to speed up the analytical throughput and provide conditions even closer to those theoretically required for furnace AAS, In this paper experiments were conducted to test these capabilities. A disadvantage of the o;6nventional longitudinally heated furnace is that the ends of the furnace tube are cold. With this furnace design FALKet al. [lOI had shown that it was preferable to start the atomixation step from room temperature to heat the greatest length of the furnace tube to atomixation temperature. From this theory, a cooldown step [ll] was inserted between the pyrolysis step and the atomixation step so that the atomixation step proceeded from room temperature. When the furnace is heated transversely, the full length of the furnace is heated rapidly and at the same time. There are no colder ends and the cooldown step is not required, saving some analytical time. A relatively small sample is taken, 10 p.l, and it is deposited on the platform heated to about 100°C. The rate of sample delivery onto the platform can be controlled so that all the sample is dry by the time the full 10 Pl is deposited. The major advantage of the fast furnace programs are that each firing can be completed in less than a minute compared to 2-4 min for conventional furnace methods. In this work, a representative group of analytes, Cd, Cu, Cr, Se, As, Tl, V and Pb, were determined in several reference materials.
2. EXPERIMENTAL 2.1. Instncmentation The analyses were carried out with the Perk&Elmer 4100ZL or 51OOZLinstruments. This included the fume extraction system and the recirculating water bath used to control the temperature of the furnace. On these instruments the platform can be preheated before sample deposition. It is also possible to reduce the rate at which the sample is injected onto the platform, For the measurements of As, Pb, Se and Tl, EDLs were used with a Perkin-Elmer EDL power supply system. 2.2. Reagent The standard solutions for each analyte were prepared by stepwise dilution from loo0 mg/l stock solution. The acids used were “Seastar” grade. 1435
1436
Technical note Table 1. Wavelength, atomization temperature
and characteristic mass Characteristic
Element
As cd Cr cu Pb Se Tl V
Wavelength (nm) 193.7 228.8 357.9 324.8 283.3 196.0 276.8 318.4
Atomization temperature (“Cl 2300 1600 2300 2ooo 1600 1900 1600 24w
Observed (P& 44
1.6 6 17 33 48 52 55
mass (pg) Reported
WI 42 1.3 7 17 30 45 53 42
2.3. Samples and sample preparation A variety of reference materials were used for this study: NIST SRM 2670 Normal and Elevated Urine, NIST SRM 1573 Tomato Leaves, NIST SRM 1575 Pine Needles, NIST SRM 1645 River Sediment, NIST SRM 1643 Trace Metals in Water, and BCR CRM 144 Domestic Sewage Sludge. Several Environmental Resources Association (ERA), Arvada, CO 80002, waste water samples were analyzed: ERA 2524 Waste Water and ERA 9937 Waste Water. The NIST SRM 2670 normal and elevated freeze-dried urine were reconstituted, as specified on the SRM data sheet, by dilution with 20 ml of water, shaking thoroughly. Aliquots of 0.20 g of NIST SRM 1573, 1575 and 1645, and BCR CRM 144 were weighed into Teflon containers. Each sample was mixed with 1.0 ml of cone I-INOs and 1.0 ml of cone HCl. The container was assembled in an acid digestion bomb (Parr Instrument Company, Part No. 4745). The bomb was heated in an oven at 150°C for 8 h. After cooling, the container was removed from the bomb. Two milliliters of cone I-IF and 1 ml of cone HC104 were added. (Note: care must be taken in the use of perchloric acid, using a hood approved for that purpose.) The solution was heated on a hot plate at 120°C to incipient dryness. One milliliter of cone HNOs was added. The solution was heated again to incipient dryness. Five milliliters of water was added to the container. The solution was heated at 90°C for 20 min. After cooling, the solution was diluted to 20 ml. Three separate sample solutions were made for each SRM. The blank solution was prepared in parallel. The ERA 2524 waste water was prepared by pipetting 5.0 ml of the concentrate solution into a one liter volumetric flask and diluting to the mark with water. The ERA 9937 waste water was prepared by pipetting 5.0 ml of the concentrate solution into a 500-ml volumetric flask, adding 2 ml cone HN03 and diluting to the mark with water. The above prepared sample solutions were diluted to provide an analyte concentration which would give an integrated absorbance between about 0.1-0.2 s, with an injection volume of 10 p,l. The calibration curve for each analyte was obtained using serial dilution to produce four standard solutions. The reagent blank was subtracted. Four replicates were measured for each standard and each sample solution. Each of the reference materials was analyzed for each of the analytes for which reference values were available. The confidence limits in the tables are + 1 standard deviation.
2.4. Instrumental conditions The wavelength and atomization temperature used for each element and the characteristic mass found are listed in Table 1. The sensitivities for As and Pb were degraded from that shown in Table 1 by about 30% when Pd and Mg were used as modifier. For the standards, the agreement of the characteristic mass with the manufacturer’s values (121 was good. The several furnace programs that were used are shown in Table 2. In every case, the analyses were standardized against analytical working curves prepared with simple aqueous standards in dilute nitric acid.
Technical note
1437
Table 2. Furnace programs A: Fast furnace program, no pyrolysis or cooldown steps Ramp Hold Temperature Step (s) (s) (“C) 1 2 3
120 250 *
4
2400
1 1 0 1
15 7 5 2
Total program time: 32 s. Injection temperture: 120°C; pipette speed: 100%. Injection volume: 10 ~1. * See Table 1 for atomizatton temperature.
B: Standard furnace program Temperature
(“C) Ramp
Step 1 2 3 4
As
Pb
(s)
Hold (s)
120 1000 2300 2400
120 850 1800 2400
1 10 0 1
60 20 5 2
Total program time: 99 s. Injection temperture: 20°C; pipette speed: 100%. Injection volume: 20 ul sample + 10 ~1 modifier. Modifier: 15 pg Pd + 10 ug Mg(NO,),.
C: Fast furnace program using a modifier Temperature (“C) Step 1
2 3
As
Pb
Ramp (s)
Hold (s)
1000 2300 2400
850 1800 2400
1 0 1
15 5 2
Total program time: 24 s. Injection temperature: 120°C; pipette speed: 60%. Injection volume: 7 ~1 sample + 3 4 modifier. Modifier: 5 pg Pd + 3 t.t.gMg(NO,),.
3. RESULTS 3.1. Removal of pyrolysis and cooldown steps The possibility of removing the pyrolysis and cooldown step prior to atomization and of using no modifer was investigated by analyxing the volatile elements, Cd, Pb and Tl, the elements of intermediate volatility, As, Cu and Se, as well as the refractory elements, Cr and V, in various sample matrices. These included urines, plant materials, sludge, sediment and waste waters. The furnace program used for these analyses is listed in section A of Table 2 with atomization temperatures from Table 1. The sample drying was done in two steps. The furnace program required 32 s, plus about 30 s for the autosampler to dispense the sample solution and for the furnace to cool to room temperature. The real cycle time for each measurement was about 1 min. The analytical results for each element, except As and Pb, in the various samples are listed in Tables 3-6. These data for the fast furnace technique, eliminating the pyrolysis and cooldown step, agree with the certified or reference values within the uncertainty limits.
1438
Techmcal note Table 3. Cd results Material NIST 1643b water NIST 2670 urine (elevated) ERA 2524 waste water ERA 9937 waste water BCR 144 sewage sludge NIST 1645 river sediment
Found
Certiiied
20 + 1 (I@) 0.089 2 0.002 (me/l) 55 + 1 (Pfl) 117 + 2 (lJ@) 3.61 2 0.14 (i&g) 10.1 2 0.5 (l&g)
20 2 1 0.088 f 0.083 50 (40-60) 106 (87-125) 3.41 f 0.25 10.2 -c 1.5
Table 4. Cr results Material NIST 1643b water NIST 2670 urine (normal) NIST 2670 urine (elevated) NIST 1573 tomato leaves NIST 1575 pine needles ERA 2524 waste water ERA 9937 waste water NIST 1645 river sediment
Found
Certified
18.8 f 0.1 (Pti) 0.013 + 0.001 (mgfl) 0.089 + 0.002 (mgfl) 4.1 + 0.1 (l@g) 2.8 + 0.1 (k@g) 215 f 2 (i&l) 248 + 4 (&I) 2.71 f 0.34 (“/)
18.9 f 0.4 (0.013) 0.085 + 0.006 4.5 2 0.5 2.6 ” 0.2 227 (181-272) 236 (193-278) 2.96 k 0.28
Table 5. Cu results Material NIST 1643b water NIST 2670 urine (normal) NIST 2670 urine (elevated) NIST 1573 tomato leaves NIST 1575 pine needles NIST 2524 waste water ERA 9937 waste water BCR 144 sewage sludge NIST 1645 river sediment
Found
Certified
21.4 + 0.6 (t&l) 0.14 + 0.01 (mgn) 0.35 f 0.01 (mgn) 9.3 2 0.4 (P&g) 2.5 2 0.1 (k&g) 217 2 3
22.1 f 0.4
19P! (A 696 114 (i&g) 108 f 9 Wg)
0.13 5 0.02 0.37 2 0.03 11 f 1 3.0 + 0.3 192 (154-230) 226 (185-267) 713 2 26 109 2 19
1439
Technical note Table 6. Se, Tl and V results Material
Analyte
Found
Certified
0.46 rt 0.05 (mgfl) 89 f 2
0.46 + 0.03
NIST urine ERA waste ERA waste
2670 (elevated) 2524 water 9937 water
Se
ERA waste ERA waste
2524 water 9937 water
Tl
(I@) 161 + 3 (I.%%
ERA 9937 waste water NIST 1645 river sediment
V
78
(63-94)
142 (106-168)
36 -e 1 (I@) 73 2 1 (I@)
35 (28-43)
259 + 13 (I@) 20.1 5 1.3
248 (203-293)
74 (55-87)
23.5 f 6.9
(I&&
3.2. Determination of As and Pb The analytical results for As and Pb in some of these materials is shown in Tables 7 and 8,
respectively. The fast furnace results for As and Pb in urine were very low, indicating loss of analyte before it could be reduced to the atomic vapor. The urine was reanalyxed for As and Pb, and As was determined in the sediment solution using a standard STPF program (part B of Table 2) which included a matrix modifier and pyrolysis. The values found for these determinations are shown in Tables 7 (As) and 8 (Pb) and they are in satisfactory agreement with the expected vahres. Probably the presence of the modifier prevented loss of analyte at low tempertures. 3.3. Removal of the drying step An alternative fast furnace program was sought which might provide good results for the As and Pb determination while retaining use of the matrix modifier. The possibility of removing the drying step entirely was investigated by slowing deposition of the sample solution onto the preheated platform. Copper was chosen for this experiment. Ten microliters of a 50-&l Cu standard solution was injected onto a platform, which was already heated to 120°C. The injectiou rate was reduced to 60% compared with the program of Table 2, section A. To see if the sample was completely dry, this injection was followed by a firing at a much higher temperature where splattering would have occurred if the 10 pJ of solution had not been dry. The furnace program Table 7. As results Found Material
Fast furnace*
NIST 1643b water NIST 2670 urine (elevated) ERA 2524 waste water ERA 9937 waste water NIST 1645 river sediment
55 f 1 (I@) 0.024 + 0.018 (mgfl) 105 + 1
Standard furnace?
Certified (49)
0.53 + 0.03
0.48 + 0.10 81 (64-97)
(WLgfl) 128 f 1 (I@) 41 + 3 (I&)
* Conditions of Table 2, section A. t Conditions of Table 2, section B.
110 (82-130) 71 * 3
(66)
1440
Technical note Table 8. Pb results Found Material
Fast furnace’
Standard furnace?
Certified
24.0 + 0.8
NIST 1643b water NIST 2670 urine (elevated) ERA 2524 waste water NIST 9937 waste water BCR 144 sewage sludge NIST 1645 river sediment
(lW 0.032 f 0.005
24.1 f 0.7 0.108 f 0.008
0.109 f 0.004
(mgfl) 184 + 3
172 (138-206)
(I@) 215 + 4
239 (196-282)
(WY) 491 * 10
495 2 19
(t&g) 739 +: 24
714 f 28
(t&g)
* Conditions of Table 2, section A. t Conditions of Table 2, section B. Table 9. Furnace program for testing the infhrence of drying Temperature (“C)
Step 1 2 3
Ramp (s)
Hold (s)
1 0 1
25 5 2
* 2000 2400
* Variable lOO-1ooo”C. 10 t.Llof 50 p&/l cu. Platform at 120°C; pipette speed 60%.
shown in Table 9 was used for the measurement. The temperature in step 1 was varied from 100 to 1000% and the resulting data are shown in Table 10. Since the Cu integrated absorbance and the precision were independent of the initial temperature, the water must have been fully evaporated prior to the first step. The influence of the hold time in step 1 was also tested at the temperature of 800°C in Table 11. The results show that the sensitivity and precision did not change even at a hold time as short as 10 s. No attempt was made to use a hold time less than 10 s, since 1 or 2 s are necessary anyway for setting the instrumental baseline (BOC) before atomization. Table 10. Influence of step 1 temperature signal* Temperature (“C) loo 200 300 400 500 600 700 800 900 1000 * Hold time: 25 s.
Absorbance (s) 0.141 0.145 0.146 0.146 0.146 0.148 0.149 0.149 0.146 0.144
on Cu
RSD (%) 1.47 1.32 0.57 0.54 0.69 0.88 0.72 0 71 0.46 0.52
1441
Technical note Table 11. Influence of step 1 hold time on Cu signal* Hold time (s)
Absorbance (s)
RSD (%)
0.149 0 147 0.147 0.150
O.% 0 54 0.69 0.99
25 20 15 10
* Step 1 temperture: 800°C Table 12. As and Pb results with modifier Found
Material
Analyte As
NIST 2670 urme (elevated) NIST 1645 river sediment NIST 2670
Pb
No modifier
With modifier
Standard furnace
Certified
0.02 f 0.02 (mg/l) 41 f 3 (l&g) 0.03 + 0.005
0.51 f 0.06
0.53 2 0.03
0.48 * 0.1
72 + 5
71 + 3
(66)
1.103 + 0.008
0.108 + 0.008
0.109 f 0.004
urine (elevated)
(mgfl)
These experiments with Cu indicated that we could omit the drying step and retain a short high temperature pyrolysis step to remove some matrix prior to atomization. These possibilities were tested using the As and Pb determinations in the materials that had produced problems in the original fast program. The new fast furnace program for As and Pb are shown in section C of Table 2, using a lO-pl aliquot composed of 7 u.1 of sample and 3 ~1 of the modifier. The analytical results with this fast program are shown in Table 12, along with the data for those materials from the earlier tables. The results from this new fast program are in excellent agreement with the reference values. The signal profile of Pb in urine obtained with and without the pyrolysis step are compared in Fig. 1. The left side of Fig. 1 compares the background with and without pyrolysis and the modifier. The right side compares the analytical signal showing the improved recovery of analyte. The background was greatly reduced compared to the result without pyrolysis and the modifier.
Background 1.20
I
5 f e 8
With PyrOlySiS
S
C 0
3
Time (se@
D
0
3.00
Time (84~)
Fig. 1 The absorbance profile of Pb m urine comparing the results when no matrix modifier is used (a) and when the Pd + Mg matrix modifier is used (b) with the program of Table 2, section C.
1442
Technical note 4. DISCUSSION
These experiments showed that for many analytes and samples, the simple programs which omit the pyrolysis and the matrix modifier work very well. For some situations the pyrolysis step was still required even with Zeeman background correction with the new transversely heated furnace. However, a furnace program of less than 25 s using slow injection into a preheated furnace and a matrix modifier provided very similar results compared to the standard furnace analyses. More recent experiments indicate that the temperature to which the platform should be preheated might be reduced to 100°C. The conditions in this paper should provide a good starting point for optimizing these fast furnace methods. From the experience of these experiments, it is expected that, using preheated injection, the dry, pyrolysis and cooldown steps can be removed from the program if no interference is found. A program time of less than 15 s can be used for the sample analysis and, perhaps eventually, more than 100 samples can be analyzed per hour when the instrumentation will reduce the time to prepare for the next sample. Little additional time will be required in those situations where pyrolysis is found to be useful. This hypothesis wasn’t tested. It will be necessary to decide how to accommodate the BOC setting in the program.
5. SUMMARY A new graphite furnace instrument using a transversely heated furnace tube with a longitudinal Zeeman correction system has been specifically designed to provide more nearly stabilized temperature platform furnace (STPF) conditions than previous furnace systems. Because there are no cold ends on this furnace tube on which to condense analyte and matrix materials, the vapor phase interferences are expected to be smaller. Also, the cooldown step can be avoided, thus saving time. This instrument permits the delivery of sample into a furnace already heated. The delivery rate of the autosampler can be slowed. These opportunities make it feasible for the sample to be dry on the platform by the time the delivery is complete. Several elements of environmental significance were chosen for test: As, Pb, Se, Tl, Cd, Cu, Cr and V. In almost all of these situations, the analyte was fully recovered without using a matrix modifier or a pyrolysis step. However, As and Pb in urine and As in sediment required a modifier and pyrolysis step for accurate results. A new fast furnace protocol was developed to accommodate use of a matrix modifier and this new protocol was successful for Pb and As in these matrices. All the procedures required less than 1 min total cycle times and produced results in agreement with certified values. This is in contrast with conventional methods which require 2-3 min per firing. These results confirm that graphite furnace methods can be accomplished with a throughput greater than 60 determinations per hour, and eventually, it may be possible to increase this rate beyond 100 determinations per hour. Acknowledgement-We
thank JEANNESCHICKLI for her experimental help in some of these experiments.
The Perkin-Elmer Corporation 761 Main Avenue Nonvalk, CT 068.59-0237 U.S.A.
ZHANG LI GLEN CARNRICK
WALTER SLAVIN
Bonaire Technologies Box 1089 Ridgefield, CT 06877 U.S.A. REFERENCES
[l] W. Slavin and G Camrick, SpectrocIarm Acru 39B, 271 (1984). [2] W. Slavin, D. C. Manmng and G. R. Carnnck, Specrrochrm. Acra 44B, 1237 (1989). [3] Llan Liang, P. C. D’Haese, L. V. Lamberts, F. L Van de Vyver and M. E. De Broe, Anal. Chem. 63, 423 (1991). [4] Llan Liang, Specrrochrm. Acra 473, 239 (1992) [5] M. B. Knowles, J. Ad. At. Speclrom. 4, 257 (1989). [6] D. J. Halls, Tufanru 37, 555 (1990).
Technical note
1443
[7] D. Bradshaw and W. Slavin, Spec~ochim. Acta 44B, 1245 (1989). [8] M. T. C. de Loos-Vollebregt, L. de Galan and J. W. M. van Uffelen, Spectrochim. Actu 43B, 1147 (1988). [9] W. Frech, D. C. Baxter and B. Hiitsch, Anal. Chem. 58, 1973 (1986). [lo] H. Falk, A. Glisman, L. Bergann, G. Minkuntz, M. Schubert and J. Skole, Spectrochrm. Actu 4OB, 533 (1985). [ll] D. Manning and W. Slavin, Specrrochun. Acta 403, 461 (1985). [12] List of Characteristtc Masses for 4100 ZL, Bodenseewerk Perk&Elmer, F. R. G. (1991).