Anal. Chem. 1987, 59, 2191-2196
2191
Registry No. Lactose, 63-42-3.
laxis 2
LITERATURE CITED
1
FI i I
I
1204
nm
2180nm
Flgure 8. CFA on the near-Infrared spectra of milk: parametric curve (-), spectra of milk (M); P, samples with high protein content (67.7 and 67.0 g/L); F I , samples wlth high fat content (range 58.0-69.0 g/L); F2, samples with low fat content (range 19.0-24.5 g/L).
Spectral patterns that are characteristic of the principal constituents are obtained by morphological analyses, and the resemblances between spectra are visualized on the similarity maps of PCA. An assignment of the wavelengths can be realized by using CFA.
Williams, P. C. CerealChem. 1975,52,581. Norris, K. H.; Barnes, F. E.; Moore, J. E.; Shenk, J. S. J . Anim. Sci. 1978,4 3 , 8 8 9 . Barnes, F. F.; Marten, G. C. J . Anim. Sci. 1979,48, 1554. Frank, J. F.; Birth, G. J . Dairy Sci. 1981,65, 1110. Baer, R. J.; Frank, J. F.; Loewenstein, J. J. Assoc. Off. Anal. Chem. 1983,6 6 , 858. Goulden, J. D. S. J . Dairy Sci. 1958,25, 228. Martens, H.; Jensen, S. A. Proceedings of the 7th WorM Cereal and Bread Congress, Prague, Elsevier: Amsterdam, 1982; pp 607-647. Devaux, M. F.; Bertrand, D.; Martin, G. Cereal Chem. 1988,6 3 , 151. Cowe, I. A,; McNicoJ. J. W. Appl. Spectrosc. 1985, 39, 257. Robert, P.; Bertrand, D.; Demarquilly, C. Anim. Feed Sci. Techno/. 1988, 76, 215. Norme AFNOR V. 04.210, Association Franpaise de Normalisation 1975, Paris. Norme AFNOR V.04.211, Association Franpaise de Normalisation, 1971, Paris. Document 174, 1983; FWration Internationale de Laiterie, International Dairy Federation: Square Vergote 41, Bruxeiies. Foucart, T. Analyse Factorielle -Programmation sur microordinateurs; Masson: Paris, 1982. Ben-Gera, I.; Norris, K. H. I s r . J . Agric. Res. 1988, I B , 117. Norris, K. H.; Williams, P. C. Cereal Chem. 1984,67, 158. Mc Clure, W. F.; Hamid, A,; Giebnecht, F. G.; Weeks, W. W. Appl. Spectrosc. 1984,3 8 , 322. Robert, P.; Bertrand, D. Sci. Aliments 1985,5 , 501. Lefevre, J. Introduction aux ana/yses statistiques mu/tidimensionelles, 3rd ed.;Masson: Paris, 1983. Le Nouvei, J. Thesis, Universit6 Rennes I, 1981. Benzecri, J. P.; Benzecri, F. Pratique de I'analyse des donn6es; Dunod: Paris. 1980.
RECEIVED for review October 27,1986. Accepted May 11,1987.
Sample Analysis Using Plasma Source Mass Spectrometry with Electrothermal Sample Introduction C. J. Park,' Jon C. Van Loon,* Peter Arrowsmith: and J. B. French
Institute for Aerospace Studies, University of Toronto, 4925 Dufferin Street, Downsuiew, Ontario, Canada M3H 5T6
A detailed procedure is outlined for the analysis of practical samples by plasma mass spectrometry wlth an electrothermal vaporizer for sample Introduction. Results have been obtalned for the determlnatlon of As, Cu, Mn, Pb, Rb, V, Zn, and Ag in National Bureau of Standards (NBS) orchard leaves (SRM 1571) and oyster tissue (SRM 1566) and of Pb In a human blood sample used in an lnterlaboratory comparison study. Simultaneous multhnass analysls is demonstrated for Pb isotopes In NBS 981. Generally, the results agreed wlth certified values. Absolute detectlon llmlts at the plcogram level were obtalned that are 10- to 100-fold better than those reported for ICP atomic emlsslon spectrometry and 10-fold better than those obtalned by nebullzatlon wlth Inductlvely coupled plasma mass spectrometry. Relative standard deviations ranged from 2 % to 13%.
Sample introduction into inductively coupled plasmas remains a weak point for efficient utilization of these sources Present address: Geological Survey of Canada, Ottawa, Ontario,
Canada.
Present address: IBM Almaden Research Center, San Jose, CA
95120-6099.
of atoms and ions for optical emission and for mass spectrometry. Electrothermal devices for vaporization of samples into plasmas used in optical emission spectrometry have improved absolute detection limits compared to conventional nebulization (I, 2). Surprisingly, these improvements have been obtained despite relatively poor transport efficiencies (generally less than 20% 1. Most electrothermal vaporization (ETV) systems have been modifications of devices used in electrothermal atomic absorption spectrometry. Thus relatively little attention has been given to use of basic principles of flow dyamics in these designs. In this report an electrothermal vaporization device is described that has been designed for optimum gas flow. It has been applied to liquid sample introduction into an inductively coupled plasma source mass spectrometer.
EXPERIMENTAL SECTION Equipment. Design and optimum operation of the ETV were described previously (3). The filament consists of 0.05-mm-thick Re metal ribbon with a sample capacity of 5 fiL, which can be resistively heated to a maximum operating temperature of 1800 O C by using a crude homemade power supply, constructed from a 1-kW transformer and laboratory variac. Maximum output capability is 2.5 V and 30 A. The filament is encased in a glass dome, which is sealed to a metallic base by an O-ring. Argon, introduced tangentially in the base, carries the vaporized sample,
0003-2700/87/0359-2191$OlSO/O0 1987 American Chemical Society
2192
ANALYTICAL CHEMISTRY, VOL. 59, NO. 17, SEPTEMBER 1, 1987
through 50-cm-long by 0.5-cm-diameter Tygon tubing, into the inductively coupled plasma (ICP). The volume of the glass dome above the filament is 5 mL. This allows condensation of the aerosol into microparticulates of particle size 0.5-1.0 pm (measured for Zn) before contact with the glass dome walls can occur. Transport efficiency has been found to be between 83% and 85% for the elements studied. The 2-kW ICP generator (Henry Radio, Model 2500) must be operated and adjusted manually. A cross-flow nebulizer was used to introduce sample solution during initial optimization of ion optics and mass Fpectrometry (MS) parameters for each element. Plasma operating conditions were as follows: outer flow 14 L/min, intermediate flow 0.7 L/min, carrier argon flow 2 L/min. The forward and reflected powers were lo00 and near 0 W, respectively. The electrothermal vaporizer was operated as follows: dry cycle 100 "C for 20 s, ashing cycle 400-800 "C for up to 60 s, and vaporization cycle 1000-1800 "C for 5 s. The actual temperature chosen for the ashing and vaporization cycles depended on the element being determined and on the sample type. A quadrupole mass spectrometer, described initially by Douglas and French ( 4 ) ,was employed for the bulk of the work. Multimass capability of the ETV was briefly investigated by using a commercial ELAN 250. Ion optics in the mass spectrometer system consisted first of a ring-shaped lens biased to about -300 V to focus only positive ions. A Bessel box with entrance (-40 V) and exit (-40 V) apertures was employed next. The center photon stop was operated at +9 V. Radio frequency (rf)only rods at the entrace and exit to the quadrupole rod set were employed to minimize fringing fields. The above-stated voltages are approximate and must be optimized for each element separately. A plasma interface to the ion optical system consists of an approximately 0.4-mm-diameter sample orifice in a conical Ni structure mounted on a water-cooled copper flange. Immediately behind the sampler and separated by a region (12 X the diameter of the orifice), pumped mechanically to about 1 T, is a 1-mmdiameter skimmer orifice. An x-y recorder (Varian, Model 9176B) was employed for display of spectra. Reagents. All metal salts, solvents, and acids used were at least reagent grade quality. Atomic absorption standard solutions (Fisher Scientific),of metals (As, Cu, Mn, Pb, Rb, V, Zn, and Ag) at 1 mg/mL were used. Calibrating solutions (1ng/mL t o 10 hg/mL) were prepared by dilution of the above standards. The single element calibration solutions contained 1% nitric acid upon final dilution. National Bureau of Standards Standard Reference Materials were used for verification studies [orchard leaves (SRM 1571), oyster tissue (SRM 1566), and Pb isotope standards (SRM 981 and SRM 983)]. Proposed Procedure. After the system was pumped down to 1 torr in the interface region and 2 X lo-' torr in the mass spectrometer (approximately 2.5 from a cold start), all electronic systems were turned on the the plasma was ignited. A 1 pg/mL solution of the element to be determined was aspirated and the ion optic component voltages were varied to obtain a maximum analyte signal. The instrument was operated until analyte signal drift stabilized, usually a minimum of 1 h (drift should be less than 5%/h). While the ICP-MS system was operating, the nebulizer was replaced with the ETV device. Orchard leaves (SRM 1571) and oyster tissue (SRM 1566) samples were dissolved in closed Teflon digestion vessels with appropriate mixtures of HNO,, HC104, and HF acids (5). The acid content of the solutions after dilution to volume was 1% "0,.
To analyze sample and standard solutions the glass dome was removed from the ETV base and a 2-pL aliquot of sample or standard solution was pipetted onto the Re filament. [The ETV bypass intermediate Ar flow (2 L/min) is used during this operation.] The glass dome was replaced and clamped tightly onto the ETV base by using the toggle clamps. The ETV Ar flow of 2 L/min was diverted by means of the three-way stopcock to a vent. The sample solution was treated at about 100 "C until the drop of liquid was completely evaporated (visual observation). The standard or sample was charred, if necessary (e.g., necessary for blood sample) at 400-800 "C for 30-60 s. (This temperature range is suitable for the determination of, Cu, Mn, Pb, Rb. V,
Zn, and Ag. If As is to be determined, 2 p L of 1%nickel nitrate solution is added before charring the sample or standard solution and the drying step is repeated. With the three-way stopcock, the ETV Ar flow (at 2 L/min) is switched to the plasma. The bypass Ar flow was stopped and the sample was vaporized at up to 1800 "C. The transient signal was recorded. Signal integration must be used in computing elemental concentration in orchard leaves and oyster tissue solutions. Calibration is performed by the method of standard additions, using 2-pL aliquots. RESULTS AND DISCUSSION Spectral overlap in ICP-MS generally is less severe than optical spectral interferences. In ICP-MS the most serious interference problem appears to be ionization suppression (6, 7). Ionization Suppression and Vaporization Interferences. All elements, present in significant amounts in a sample, can potentially cause ionization suppression, but those with ionization potentials below 8 eV are of greatest concern (6). When sufficient matrix substances are present to significantly increase the electron density in the ICP or significantly reduce the argon reagent ion levels, they can affect the analyte ionization and hence the signal. Since the typical electron density or total ambient ion density in the axial channel of the plasma is 1015 ~ m - when ~ , the number density of easily ionizable matrix atoms in the axial channel approaches this value, the degree of ionization of the analyte atoms is lower than that in the absence of the matrix elements. The amount of an easily ionized element when introduced into the plasma by an ETV that will cause this effect has been found to be approximately 1 pg (Na). For interference studies, an ETV was designed with two identical filaments installed side by side (Figure 1). This was done to separate the plasma loading interference from other matrix effects of the ETV-ICP-MS combination. Thus separate aliquots of matrix and analyte could be vaporized simultaneously. One nanogram of analyte elements (75As,l14Cd, and 63Cu) and 1 pg of matrix elements (Na, Cr, Ni, Ca, and Se) were vaporized in various combinations from the two adjacent filaments. The time-intensity mass spectra for 7 5 A ~l14Cd, , and 63Cu for separated and combined vaporization are given in Figure 1, parts a, b, and c, respectively. The first profile of each pair arises from a vaporization of the matrix and analyte elements from the same filament, while the second profile results from separate but simultaneous vaporization of the matrix and analyte elements from two adjacent f i i e n t s . The difference between the traces in each pair is related to the vaporization interference caused by matrix elements. The difference between the analyte-only-peak (shown as the first peak in Figure 1)and the second trace of each pair corresponds to a plasma loading interference signal. The areas of the two peaks from vaporizations on the same and separate filaments are generally similar (*lo%) for each analyte and concomitant combination except for 75Aswith Ca concomitant and l14Cd with Se concomitant. This indicates that little analyte loss occurs during desorption from the filament. For As with Ca concomitant, the first trace (the same filament) gives less peak area than the second trace, while for Cd with Se concomitant, the first trace shows larger peak area than the second trace. This difference between the first and second traces might be due to interelement compound formation on the filament, thereby affecting transport efficiency of the analyte. The enhancement of l14Cd signal by Se concomitant on the same filament agrees with the observation by Millard et al. (8). For 63Cu, 1 pg of Se causes neither vaporization nor plasma loading interference, owing
ANAL.YTICAL CHEMISTRY, VOL. 59, NO. 17, SEPTEMBER 1. 1987
I
a
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0
b
i
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2000 A
2193
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Flgure 2. Signal characteristics of "Se vaporized from a Re filament at different filament temperatures.
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Flgure 1. (a) Effect of 1 pg of various elements on the signal of 1 ng of 75Aswhen vaporization of analyte and matrix is from the same filament (first peak) and two separate filaments (second peak). (b) Effect of 1 pg of various matrix elements on the slgnal of 1 ng of 'I4Cd when vaporization Is from the same fbment and separate filaments. (c) Effect of 1 pg of various matrix elements on the signal of 1 ng of %u when vaporization is from the same filament and 2 separate filaments.
probably to the high volatility and low degree of ionization of Se (about 1/27 times that of Na) in the ICP. The absolute amount of a matrix element rather than the molar ratio of the matrix to the analyte element appears to determine the level of both vaporization and plasma loading interferences. This would be expected because the more matrix element on the filment, the lower the rate at which the analyte leaves the filament surface and the higher the matrix atom concentration in the ICP. To investigate the above possibility, analyte signals from 20 pg of analyte (75As, l14Cd,63Cu)with 20 ng of concomitant (Na, Cr, Ca, Se) were compared with those from 200 pg of analyte with 200 ng of matrix. For the selenium matrix no appreciable difference in the matrix effects was observed between the two groups of samples (20 pg of analyte with 20 ng of concomitant and 200 pg of analyte with 200 ng of concomitant). However, for
the other concomitant elements studied (Na, Cr, Ca) the matrix effects with 200 ng of concomitant at the same molar ratio were 3-5 times more severe than those with 20 ng of concomitant. This indicates that in most instances the absolute amount of the matrix elements rather than the molar ratio of the matrix to the analyte will likely be important in determining the level of matrix effects. From these results it is clear than in the presence of microgram levels of matrix both vaporization and plasma loading interferences are possibilities. Microgram levels of matrix elements will often be present with analyte in aliquots of practical samples. Compound Formation. Compound formation on the filament is indicated by the appearance of multipeaked signal tracings. A similar phenomenon has been noted by others, particularly in furnace atomic absorption (9). The multiple peak could be caused by the presence or formation of different analyte compounds on the filament. Sometimes in graphite furnace atomic absorption spectrometry the compound formed could be identified from the analyte appearance temperature. In practice, low heating rates aid in studying this phenomenon because they exaggerate the time intensity profile. Conversely, very high heating rates tend to obviate this effect. Compound formation involving the analyte and the Re filament and the analyte and sample matrix constituents was detected. Of the elements studied, only Se and Ca were found to form compounds with the Re filament. Double peaks of 7sSe generated during the vaporizations of 2 ng of Se a t three different temperatures (heating rates) are shown in Figure 2. Considering that Se is a relatively volatile element, the first peaks probably are due to vaporization of 78Sewhile the second peaks may result from vaporization of a selenium-rhenium compound. A single peak occurs at temperatures above 1900 "C. When 200 ng of arsenic was added to 2 ng of '%e on a filament, a single peak appeared even at a lower final temperature (1500 "C). The formation of a selenium-arsenic compound may occur which prevents selenium-rhenium compound formation. The 75As time intensity profile in Figure 3 indicates arsenic-selenium compound formation on the filament. The peaks were observed when 20 ng of Se (matrix) was added to 0.2 ng of As (analyte). The second peak appears exactly at the same time (temperature) as the 75Assingle peak. The first peak appears to be generated by the vaporization of a volatile arsenic compound in the presence of selenium the melting point of arsenic selenide is 360 O C compared to that of arsenic a t 817 "C. The peaks can be eliminated by increasing the heating rate (use of higher final temperature, 2100 "C for Se and 1500 "C for As).
2104
ANALYTICAL CHEMISTRY, VOL. 59, NO. 17, SEPTEMBER 1, 1987 (01
FILAMENT TEMPERATURE
0%
Time (sec)
Figwe 3. Signal characteristics of 75As vaporized from a Re filament in the absence and presence of Se.
Time (sec)
Figure 4. Effect of nitric acid concentration on "Cu signal for 2 ng of Cu (ftlament temperature 900 "C).
The 44Catime intensity profiles for three final filament temperatures were obtained for 5 ng of %a in the presence of 500 ng of arsenic. At a low heating rate, two peaks appear, indicating a Qossible interelement compound formation on the filament. However, a single peak was observed a t higher heating rates. Of the elements studied, Ca was unusual becuase no appreciable signal was observed at any filament temperatures except in the presence of an excess (any amount greater than the Ca level) of As. The compounds formed have not been iqentified for either the As or Ca. Anion Ihterference. Sample solutions should contain about 1% acid to ensure stability of trace elements in solution. Thus anion(s) associated with acid(s) are present at relatively high concentration. Acid effects have been demonstrated for conventional ICP-MS systems (i.e., those which involve aspiration Qf the sample solution into the ICP) (6). Signal suppression, particularly from P-, S-, and Cl-containing anions, can be severe. Sample introduction by ETV has a potential advantage when solutions with these anions are to be analyzed. With the ETV a matrix modifier such as NH4N03can be added and the sample can be ashed prior to introduction. This will allow removal of some of the acid anion. Because of the adverse effeds of phosphorus-containing anions on the ICP-MS system (IO), no investigation of these anions was attempted. The effect of different anions on copper signal (for 2 ng of ' T u ) was studied. When the molar ratio of W u to anion was 1:1, no adverse effects on the 63Cu signal were noted for chloride, nitrate, sulfate, or acetate. For '%u solutions with 3% and 6% acid the time-intensity profile was extended and the maximum reduced. Typical effects for nitric acid (filament temperature 900 "C) are shown in Figure 4. The 2-@Laliquot of solution was dried at 120 OC, but no ashing step was employed. The peak height and peak area both decreased with acid content. These changes might be due to two factors: (1) The anion lowers the desorption rate or compound volatility. (2) The anion causes an increase in Re loss from the filament,
900'C
3%
3%
3%
3%
HNO,
HF
HCL
H25Cd
(b) FILAMENT TEMPERATURE 1250'C
0%
3% HNC,
3% 3%
3%
HF
H2SOd
HCL
Figure 5. Effect of acids on "Cu signal for 2 ng of Cu at 900 OC and 1250 O C filament temperatures.
which results in plasma loading. The effects on the @Cusignal of other acid anions (3%)are given in Figure 5 for two filament temperatures (900and 1250 "C). At 900 "C, the effect of nitric acid and hydrofluoric acid is peak broadening, but the integrated signal is not reduced. At 1250 OC, the broadening is less. This temperature effect suggests that the peak broadening is due to the increased desorption energy or change in volatility by some chemical mechanism. However, with 3 % hydrochloric acid and sulfuric acid the peak is broadened and its magnitude reduced. Sulfuric acid causes peak broadening with significant reduction in the integrated signal. This effect becomes worse at a higher filament temperature (1250 "C). Hydrochloric acid does not cause the peak broadening at 900 "C but a significant reduction in the integrated signal occurs. At a higher filament temperature (1250 "C), the 63Cu peak has an enlarged tail. This enlargement of the tail, even at the higher filament temperature, may be due to a chemical compound effect. The effect of 3% acid solutions on the signal for 2 ng of 52Cr at a filament temperature of 1500 "C was studied. Similar anion effects were noted except for hydrochloric acid. In the presence of C1 a sharper 52Crpeak than the other peaks including that from 0% acid concentration appeared, indicating that chromium chloride is more volatile than the other Cr compounds. However, for both 63Cuand 52Cr,the integrated signal of the C1 compound is smaller than that of nitrate or fluoride compound. The integrated peak area of sulfate media is much lower than those of the other compounds for both 63Cuand 62Cr. Sulfate is reported to behave similarly in the graphite atomizer (IO). However, the mechanism of signal loss for both sulfate and chloride is not now known. The results obtained for 63Cu and 52Crsuggest that the anions affect the analyte signal significantly and as high a filament temperature as feasible should be used to reduce the peak broadening. In the analytical use of the ETV, an ashing step is normally employed to destroy the organic constituents in the sample before vaporization. Ashing should also eliminate some of the anion and thus could enhance the analyte signals. For 3% sulfuric acid, about 100% increase in the peak height was observed when the sample solution was ashed for 1 min at 400 "C. Little enhancement in the analyte signal was recorded for ashing studies with other acid solutions. Thus the sample ashing at an appropriate temperature possibly eliminates some sulfates, and ashing is effective for improving the signals of the sample solutions high in sulfuric acid. Possible interactions of the acids with the rhenium filament were examined. The rhenium filament may react with the acid solutions, causing an increase in the evaporation of rhenium. The 187Resignals obtained when 3% acid solutions were used with vaporization at 900 OC final temperature, obtained by monitoring lE7Reduring the vaporzation step, are
ANALYTICAL CHEMISTRY, VOL. 59, NO. 17, SEPTEMBER 1, 1987 ~~
Table I. Trace Elements in NBS Orchard Leaves
element this work, (pg/g) As
NBS certified data pg/g
detection limit pg/g
10 f 2 12 f 1 300 f 20 91 f 4 82 f 6 45 f 3 25 f 3
0.32 0.07 6.7 0.1 0.25 1.1 0.2
10.5 f 1 12 f 1 360 f 40 106 f 5 85 f 2 54 f 2 25 2
cu Fe Mn Na Pb Zn
*
Table 11. Trace Elements in NBS Oyster Tissue
element
this work, pg/g
NBS certified data, pg/g
detection limit, pg/g
9.2 f 0.6 64 f 2.1 19.3 f 1.1 2.6 f 0.2 3.8 f 0.5 2.9 f 0.4 860 f 50 3.6 f 0.3
13.4 i 1.9 63 f 3.5 17.5 f 1.2 0.48 4.04 4.45 f 6.09 (2.8) 852 f 14 0.89 f 0.09
0.06 0.4 0.2 0.04 0.21 0.52 0.32
As
cu Mn Pb
Rb V Zn Ak!
1.0
*
Table 111. NBS SRM 981 Lead Isotopes relative abundance
background CNTS = 0 runno.
counts/s
% 204
90 206
% 207
% 208
1 2 3 4 5 6
49 8920 41 7720 45 7820 46 8150 45 6130 46 7320
1.60 1.56 1.47 1.54 1.61 1.58
23.90 23.12 24.12 24.10 24.03 23.95
22.34 22.74 21.25 23.40 22.22 23.18
52.14 52.56 52.15 50.94 52.12 51.28
mean SD (a - 1) RSD absolute error certified
46 1010
1.56
23.87 0.37 1.57 0.26 24.14
22.52 0.77 3.43 0.44 22.08
52.03 0.81 1.56 0.31 52.35
0.05 5.68
3.37 0.13 1.42
given in Figure 6. Nitric, sulfuric, and hydrochloric acids apparently interact with the filament to increase 18’Re vaporization. Hydrofluoric acid has no appreciable effect. Sample Analyses, National Bureau of Standards orchard leaves (SRM 1571) and oyster tissue (SRM 1566) were analyzed by the method of standard additions. The results are given in Tables I and 11together with detection limit data (3u) obtained in solutions of orchard leaves and oyster tissue, respectively. In most cases the values obtained were in sat-
isfactory agreement with the NBS Certified Values. The poorest agreements were obtained for P b and Ag in oyster tissue. In an attempt to improve the results for Ag and P b in oyster tissue, a 5 times larger sample weight was taken, and the dissolved sample diluted to the same final volume. No better results were obtained, perhaps due to plasma loading problems at these higher solution salt contents. A blood sample, supplied by the Ontario Ministry of Labour, Occupational Health Laboratory, was diluted 10 times with distilled deionized water, and the P b content was determined. The zo8Pbisotope was used. The result obtained was 0.50 pg/mL (i0.03).This was in satisfactory agreement with the 0.53 pg/mL value calculated from results of an interlaboratory comparison study. Two 2osPbpeaks were obtained in the time intensity profile for the blood sample. This was though to be due to the presence of two P b species, one of which was likely a lead chloride (the sample is very high in C1-). If a higher heating rate were employed, a single peak could be obtained. However a reduced peak area resulted, likely due to plasma loading from increased rhenium evaporation. Integration over the double peak was employed in the blood P b determinations. Multimass Analysis. Multimass detection is possible with a commercial ICP-MS system. For fast sequential multimass detection, the dynamic count rate range is reduced. At short dwell times (DT < 100 ms) the maximum signal count is limited by the dynamic count rate range of the ion multiplier detector (lo5 SI); e.g., the dynamic range is 300 for DT = 1 ms. An additional factor arises from reduced duty cycle: although duty cycle per mass is proportional to dwell time, it varies inversely with the number of masses and is always
