Anal. Chem. 1989, 67, 1857-1862 (11) Tan, S. H.; Horiick, G. Appl. Spectrosc. W86, 4 0 , 445-460. (12) Iyengar, G. V.; Kollmer, W. E.; Bowen, H. J. M. The Elemental Composithm of Human Tissues and B W Fluids, 1st 4.;Verlag Chemie: Weinheim, New York, 1978.
RECEIVED for review January 25,1989. Accepted May 15,1989. We thank the National Fund for Scientific Research (Belgium,
1857
W W O ) and the Interuniversity Institute for Nuclear Sciences ( 1 1 ~for ~ financial ) The plasmaQuad was acquired by a grant of the “Fund for Medical Scientific Research (FGWO)”. H. Vanhoe is indebted to the Institute for Scientific Research in Industry and Agriculture (IWONL) for a research fellowship. C. Vandecasteele is Research Director of the National Fund for Scientific Research (Belgium).
Determination of Trace Metals in Reference Water Standards by Inductively Coupled Plasma Mass Spectrometry with On-Line Preconcentration Diane Beauchemin*P1and S. S. Berman
Analytical Chemistry Section, Chemistry Division, National Research Council of Canada, Ottawa, Ontario K I A OR9, Canada
INTRODUCTION Although the use of inductively coupled plasma mass spectrometry (ICP-MS) is rapidly expanding because of the many features of this technique (summarized in four review articles (1-4)), its application to the analysis of saline waters remains limited. This is largely due to the low tolerance of the technique to dissolved solids with the highest recommended level being of 0.2%, if a solution is to be continuously nebulized without inducing undue instrumental drift caused by solid deposition on the orifice ( 3 , 5 ) . Another restriction comes from effects of concomitant elements that are nonspectroscopic interferences often resulting in a suppression of analyk signals (e.g., ref 6-8). Thus, the analysis of seawaters requires a preliminary treatment in order to reduce their salt content prior to analysis by ICP-MS. This can be accomplished by, for instance, preconcentration on silica-immobilized a technique that allows the 8-hydroxyquinoline (I-8-HOQ) (9), concentration of a number of trace metals while separating them from the univalent major ions and, to some extent, the divalent ions such as Ca and Mg. This technique was successfully applied to the analysis of the coastal seawater reference material CASS-1 (10) and the open ocean water ref-
erence material NASS-2 (11). It presents, however, the disadvantages of being time-consuming and of using large volumes of sample. Flow injection analysis (FIA) can be used to both speed up the preconcentration process and reduce sample consumption (e.g. ref 12-13). The first on-line application of FIA to the preconcentration of trace metals in seawater was realized by Olsen and co-workers (14). They used a miniature ion-exchange column of Chelex-100 resin to determine Pb, Cd, Cu, and Zn by flame atomic absorption spectrometry (FAAS). Hartenstein and co-workers (15, 16) used a similar setup to enhance the sensitivity of inductively coupled plasma atomic emission spectrometry (ICP-AES), improving the detection limits by over 20 times for Ba, Be, Cd, Co, Cu, Mn, Ni, and P b (15) compared to conventional continuous aspiration. A detailed study of on-line preconcentration systems in FAAS was performed by Fang and co-workers (17) who compared different types of resins (among them, Chelex-100 and I-8HOQ). Their observation was that, owing to the smaller exchange capacity of I-8-HOQ and the comparatively high stability of the magnesium complexes, the recoveries of most of the heavy metals from a seawater matrix were not acceptable with I-BHOQ, even if this material almost always had the highest concentration factor; good recoveries were however obtained with Chelex-100. Nonetheless, Malamas and co-workers (18),as well as Marshall and Mottola (19) successfully used I-8-HOQ for on-line preconcentration in FAAS. They found that the excellent resistance to swelling of silica with changes in solvent composition was an advantage of I-8-HOQ over polymer-based ion exchangers such as Chelex-100. This work will describe the implementation of the on-line preconcentration technique in ICP-MS, using a miniature column packed with I-8-HOQ (20);a preliminary assessment of the system with the analysis of the riverine water reference material SLRS-1, which has a low salt content and can be run continuously (as was done for its direct analysis in another work (21))without clogging the torch and/or the interface; and, finally, the application of this system to the analysis of open ocean water NASS-2, a reference material whose salt content precludes its direct analysis by ICP-MS.
*Author to whom correspondence should be sent. Present address: Department of Chemistry, Queen’s University, Kingston, ON K7L 3N6, Canada.
EXPERIMENTAL SECTION Instrumentation. The inductively coupled plasma mass spectrometer used for this work was the Perkin-Elmer SCIEX
A preliminary lmplementatlon of on-ilne preconcentration in inductlvely coupled plasma mass spectrometry (ICP-MS) improved the detection limlts of several elements by a factor of 2-7 compared to ICP-MS alone. The on-line preconcentration system was first assessed by using the method of standard additions to determlne Mn, Co, NI, Cu, Pb, and U in the certlfied riverine water SLRS-1 whose salt content was low enough to allow monltoring both the preconcentration and the elution processes. Results In good agreement with the certified values were obtained for all but Ni, because of a spectral Interference by CaO from coeluted Ca. The system was successfully applied to the determlnation of Mn, Mo, Cd, and U In the reference open ocean water NASS-2 by using an isotope dliutlon technlque and the method of standard additions.
0003-2700/89/0361-1857$01.50/0
0 1989 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 17, SEPTEMBER 1, 1989
Table 1. ICP-MS Operating Conditions torch rf power reflected power plasma gas flow auxiliary gas flow nebulizer gas flow
Plasma Conditions conventional ICP-AES 1.2 kU’ 55
\v
L/min 2.0 L, min 0.9 I., min 14
Mass Spectrometer Settings Bessel box stop -6.1 to -6.7 \.’ Bessel box barrel 3.1-3.5 V Einzel lenses 1 & 3 -15.1 \.‘ Einzel lens 2 -130 V Bessel box end lenses -8.6 \.‘ sampler orifice diameter 1.14 mm skimmer orifice diameter 0.89 mm interface pressure 0.8-1.2 Torr mass spectrometer pressure (2.5-5.5) X Torr P
=-
W ‘
1,
I
Figure 1. On-line preconcentration setup: C, carrier (deionized distilled water): B, buffer (0.1 M ammonium acetate, pH 5.0); P, peristaltic pump; S, sample injection loop; E, eluent injectkm loop; H, short column of Id-HOQ; 0, output to the nebulizer: W, to waste: D, deionized distilled water.
ELAN 250 (Thornhill, ON, Canada). Four modifications were made to the originally supplied instrument. A mass flow controller (Model 5850, Brooks Instrument Division, Emerson Electric, Hatfield, PA) was added to the nebulizer gas line, and a peristaltic pump (Minipuls 11, Gilson Medical Electronics Inc., Middleton, WI) was used to maintain the sample delivery to the nebulizer a t 1.1 mL/min. Also, a conventional ICP-AES torch was used instead of the approximately 15 mm longer one that was provided with the instrument. Finally, an x,y,z translation stage (22) was installed under the torch box to allow the precise and reproducible translation of the torch box in three dimensions. The operating conditions used throughout this work are summarized in Table I. The elutions were recorded in real time and stored on the hard disk with the “multiple elements” software provided with the instrument. With this software, up to 4 masses could be monitored from the same injection. The measurements were made using the multichannel mode by peak hopping rapidly from one mass to the other, staying only a short time (dwell time) of 20 ms a t each mass, until the total measurement time of 0.06 s was reached. Three measurements (of 20 ms each) were made per peak (one measurement being done at the central mass while two others were done at *0.05 u from the assumed peak center). A resolution of 0.8 u (peak width) a t 10% peak height was maintained throughout the study. Under these measuring conditions, a data point was generated only every second because of data processing (to plot the points in real time) and dead time. On-Line Preconcentration Setup. Three Teflon sample injection valves (Rheodyne, Inc., Cotati, CA) were installed between a peristaltic pump and the nebulizer, as shown in Figure 1, in a fashion similar to that in ref 19. One (“S”in Figure 1)was used for injecting the sample in a flow of carrier; another (“E” in Figure 1)was for the eluent; and the last one served as a bypass valve, to direct the output from the column either to the nebulizer of the ICP-MS or to waste (in which case, an alternative flow of deionized distilled water (DDW) could be fed to the nebulizer). The preconcentration procedure was the following. The sample was injected (using interchangeable loops with injection volumes from 100 WLto 10 mL) in a flow of carrier (DDW for most of the work) and neutralized by using a buffer of 0.1 M ammonium acetate at pH 5.0 (adjusted with HCl). It then passed through
a short column (3.0 mm i.d. X 4.5 cm long) of silica-immobilized 8-hydroxyquinoline (ca. 80 mg dry weight, 37-75 pm particle size) which was described in detail in ref 20. After a suitable wash time (of a t least 1min), elution was accomplished (in slightly less than 1min) by injecting 1mL of eluent (2 M HC1/0.1 M “0,). The flow rate of the carrier line was equal to that of the buffer line of 1.1mL/min, which produced a flow rate of 2.2 mL/min through the column. This was the maximum flow rate that was allowed because of the back pressure produced in the system. Tygon tubing of 0.76 mm i.d. (Mandel Scientific Co., Ltd., Rockwood, ON, Canada) was used to feed the carrier and buffer, while Teflon tubing of 1 mm i.d. was used for the other flow lines. Reagents. All acids were purified by subboiling distillation in a quartz still (23). Purified ammonia was prepared by isothermal distillation of reagent grade stock. The enriched 62Ni, “Cu, @ ‘M ‘ o, W d , and mrPb isotopes used for the stable isotope dilution analysis were purchased from the Oak Ridge National Laboratory. The was the National Bureau of Standards SRM U-930. All the stable isotopes were dissolved as described previously (24) and their concentrations were checked by reverse spiking. The marine reference waters SLRS-1 and NASS-2 were acidified to pH 1.6 immediately after collection. The riverine water SLRS-1 was gathered in the St. Lawrence River at 2-3 m depth, several kilometers upstream from Quebec City (QuGbec, Canada) and about 30-40 km upriver from the saltwater mixing zone. The seawater NASS-2 was collected a t a depth of 1300 m, southeast of Bermuda. (Complete information on the procurement of these water reference materials and other marine reference materials can be obtained from S. Berman, Marine Analytical Chemistry Standards Program, Division of Chemistry, National Research Council of Canada, Ottawa, ON, Canada K1A OR6.) Analysis Procedure. Instrument Operating Conditions. Both the ion lens voltages of the instrument and the plasma operating conditions (Table I) were chosen while continuously aspirating a multielement standard solution through the bypass valve (while the effluent of the column is being routed to waste), so as to provide a compromise between high sensitivity and low oxide levels. Data Treatment. The raw count rates were transferred to a VAX-11 computer (Digital Equipment Corp., Maynard, MA) and processed by using programs written in FORTRAN. The count rates recorded for each elution peak were first smoothed with a seven-point Savitzky-Golay (25) moving window; background correction was performed by using points on both sides of each peak. The area and height of each peak were then measured. For the determination of the detection limit, the mean and standard deviation of several (typically eight) injections of the eluent were calculated. Isotope ratios were obtained by either ratioing the areas or the heights of the isotopes considered. Isotope Dilution Calculations. The analyte concentration in the waters was calculated by using the following formula: C=
- BsR) V(BR - A )
where C is the analyte concentration in the water sample (microgram per liter), M s is the mass of the stable isotope spike (nanogram), Vis the volume of water sample to which the isotopic spikes were added (milliliter), A is the natural abundance of the reference isotope, B is the natural abundance of the spike isotope, As is the abundance of the reference isotope in the spike, BS is the abundance of the spike isotope in the spike, K is the ratio of the natural and spike atomic weights, and R is the measured ratio (reference isotope/spike isotope) corrected for mass discrimination where needed (as explained later) measured after the addition of the spike. It should be noted that the ratio used in the isotope dilution calculations was obtained by first subtracting the intensities of the column blank from those of the reference and spike isotopes and by then ratioing the resulting blanksubtracted intensities. RESULTS AND DISCUSSION Preliminary Assessment of On-Line Preconcentration ICP-MS. The miniature column of I-8-HOQ used in this work was developed by Nakashima and co-workers (20) for off-line
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 17, SEPTEMBER 1, 1989 8" "
Table 11. Detection Limits (pg/L) for Mn, Co, Ni, and Cu under Different Preconcentration Conditions
Mn
co
Ni
cu
conditions
0.9 0.09 0.05 0.03
0.2 0.03 0.05 0.05
0.2 0.06 0.03 0.09
3 0.9 0.2 0.3
no buffer" bufferb
J
I, a
? i ,-. 8000+
a
buffer'
all bufferd
"Carrier = buffer = DDW at 1.1mL/min. bCarrier = DDW at 2 mL/min; buffer = 0.1 M ammonium acetate, pH 5.0, at 0.5 mL/ min. cCarrier = DDW at 1.1 mL/min; buffer = 0.1 M ammonium acetate, pH 5.0, at 1.1 mL/min. dCarrier = buffer at 2 mL/min; buffer = 0.1 M ammonium acetate, DH5.0, at 0.5 mL/min.
- - - -= l o
"\%b 00
20
40
-
S
1 b-
*\
4-I\td->,
,
'
,
10 0
80
60
12 0
time (min)
preconcentration of trace metals from seawater in a flowsystem prior to their determination by graphite furnace atomic absorption spectrometry (GFAAS).The main purpose of their work was to decrease the sample consumption. The procedure followed was essentially the same as the column method described in ref 9, but on a smaller scale. Briefly, the sample was pumped through the column at 2 mL/min. The column was washed with a small volume of DDW and the elution was then carried out by pumping 1.7 mL of 2 M HC1/0.1 M HN03 (at 1 mL/min) through the column. The pH of the sample was adjusted prior to its preconcentration (no buffer was used). In contrast, when the same miniature column was installed on-line (as shown in Figure l),it appeared that neutralization of the sample with a buffer was mandatory if reproducible elutions were to be obtained. This is illustrated in Table I1 where the effect of different proportions of bufferlcarrier on the detection limits of Mn, Co, Ni, and Cu is summarized. There was a dramatic improvement as soon as the ammonium acetate buffer was introduced, which leveled off when the proportion of buffer became equal or greater than that of the carrier (DDW). Since, according to the results in Table 11, it was not necessary to use an all buffered system (i.e. buffer as the carrier), a buffer flow rate of 1.1mL/min, equal to that of the carrier (DDW), was used thereafter. The blank signals (Le. injection of eluent without prior injection of samples), sensitivities, and detections limits obtained in these preconcentration conditions, using peak area and peak height, are compared in Table 111. They are based on 100-pL injections of an aqueous 10 pg/L multielement standard (with subsequent elutions with 1 mL of eluent). In general, the detection limits using peak height were better or similar to those using peak area. (It should be noted that similar results could be obtained by using 1-mL injections of an aqueous 1 pg/L multielement standard; the 100-pL injection loop was preferred for this preliminary characterization of the system, because of the shorter loading time required.) The most important reproducible blanks were observed for
Figure 2. Preconcentrationand elution processes of =Mn (-), %0 (.-.), BONi(---), and =Cu showing that with too short a wash time, two elutions are required for a complete recovery of the elements. Injections of 1 ng (as 100 pL of 10 pg/L) of each element were done at time 0.0 and that indicated by the "S" arrow. Injections of 1-mL are indicated by the other arrows. "a" eluents (2 M HCi/O. 1 M "0,) and "b" indicate respectivety analyte elutions and column blank elutions. (-e-),
L_-_.
3500 ;
3000 i
3
1
25004
\
,b
*r%
8
-\----
1000
-.-..... . .-. ..-..... ~
4
b
:d-l. 0
7 00
-
7
'
'
10
~
...
"1
,
20
30
L-_40
50
60
time (rnin)
Figure 3. Preconcentration and elution processes of (-), '''Cd (. s), '*'Sb (---), and *OSPb (-.-). An injection of 1 ng (as 100 pL of 10 pg/L) of each element was done at the time indicated by the "S" arrow. Injections of 1 mL of eluent (2 M HCVO.1 M "0,) were done at time 0.0 and that indicated by the other arrow. "a" and "b" indicate respectively analyte elution and column blank elution.
-
Sn, P b (see Figure 4),and especially Cu. (The same observation was made for Fe and Zn; in their case however, the blank was much higher and quite irreproducible so that no reliable results could be obtained.) Measurements of isotope ratios during the elution of column blanks revealed that these elements were released by the column (they were probably preconcentrated from the reagents used). Typical elutions of 1ng of each of several elements, as well as column blanks, are illustrated during the first 5.5 min of
Table 111. Figures of Merit with On-Line Preconcentration ICP-MS (Carrier = DDW at 1.1 mL/min; Buffer = 0.1 M Ammonium Acetate, pH 5.0, at 1.1 mL/min) peak height
element Mn
peak area blank," counts sensitivity, counts/(pg/L) 181 zi 24' 46 f 15 37 f 5 296 f 16 36 f 13 23 f 3 150 f 5 151 f 6
1557 i 56 927 f 39 480 f 110 217 i58 120 f 41 282 f 13 208 f 21 357 f 48 226 f 15
DL,bNg/L
blank, counts/s
sensitivity, (counts/s)/(rg/L)
296 f 22 6500 f 130 co 0.05 123 f 73 4020 f 310 Ni 93 f 26 1660 f 410 0.03 cu 555 f 43 0.2 578 f 57 Mo 97 i 14 247 f 72 0.3 43 f 2 Cd 1042 f 78 0.03 618 f 27 Sn 262 f 11 0.07 Pb 1400 f 180 266 i.14 0.05 U 2 f 2 19 f 6 0.03 790 f 100 "Signal observed for the column blank (1.4-2.0-min interval). bDetection limit (based on 3u) which could be reproduced over month. cPrecisionexpressed as the standard deviation (n = 3-5). 0.05
DL, rg/L 0.01
0.05 0.05 0.2 0.2 0.006 0.05 0.03 0.02 at least a
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 17, SEPTEMBER 1, 1989
Table IV a. Comparison of Some Typical Detection Limits (rg/L) Observed with Direct Aspiration vs On-Line Preconcentration on-line
ratios
continuous preconcn continuous/ continuous/ element aspiration height area height area Mn co
cq -Tr 00
i
4
.
--
%__-
4
+ I _ _ * * -
13
20
30
40
50
cu
- ~_-_-
4
+
60
Cd Pb
4 70
0.04 0.04 0.07 0.03 0.07
80
90
'CO
' 3
time (mtn)
Figure 4. Peaks observed with the injection of six consecutive 1-mL aliquots of eluent following the injection of 1 ng (as 100 pL of 10 pgIL) of each of 88M0 (-), '"Cd (. -.), and *08Pb (---). The first injection of the eluent was carried out at time 0.0 (about 1 rnin following the sample injection).
Figure 2, and in Figure 3. Figure 2 shows results for Mn, Co, Ni, and Cu. It is interesting to see (looking at the 0.0-5.5-min interval) that Mn, Co, and Ni came out at essentially the same time (1min after injection of eluent), whereas Cu eluted about 30 s later. This behavior is in good agreement with the stability constants of 8-HOQ complexes (26), with Cu2+forming the strongest one. The 5.5-12-min interval illustrates the importance of a preelution wash with this on-line preconcentration system. When too short a time (in this instance, 30 s) was allowed after injection of the sample and before elution, disproportioned elution peaks were obtained and more than one injection of eluent was required for a complete recovery of analyte. However, when the wash time was increased to a t least 1 min, reproducible peaks (such as those obtained in the 0.0-5.5-min interval) were seen. This wash time, which is longer than that required to simply traverse the column, probably accounts for diffusional processes as well as actually washing the column. (During the analysis of seawater, if the wash time was too short (i.e. less than 1 min), a bright yellow Na emission was seen in the plasma as soon as the bypass valve was switched to monitor the elution.) Figure 3 illustrates the cases of Mo, Cd, Sb, and Pb. S b is obviously not retained (this was true at both pH 5.0 and 8.0). The elution pattern of Mo was different from that of the other elements considered in this work. Like Cu, Mo eluted later, but several injections of eluent were required (Figure 4) in order to recover Mo completely. I t should be noted that symmetrical peaks were observed only for unretained elements (Sb) or molecular species, such as 40Ar35C1 at m / z 75, and 35C1160Ha t m / z 52, resulting from the coelution of chlorine-containing compounds. These peaks were wide and symmetrical, in contrast to the elution peaks which were, in general, tall, narrow, and tailing toward the end of the elution. A similar behavior was observed in FAAS by Olsen and co-workers (14) who reported that nonselective absorption of light by Na from the matrix gave a low and wide peak during the preconcentration period, whereas a high and narrow peak was obtained during elution of the analyte. The detection limits and sensitivities obtained for Mn, Co, Cu, Cd, and P b by direct aspiration are compared to those obtained with on-line preconcentration in parts a and b of Table IV, respectively. Standard solutions in the range 1-100 pg/L were used for direct aspiration while a maximum of 100 pL of 10 pg/L was used for the on-line preconcentration. Except for Cu whose column blank is the limiting factor, peak area detection limits were similar to those obtained by continuous nebulization (Table IVa), which was to be expected since the same measurement period was used in both cases. The results obtained by using peak height showed a different
0.01 0.05 0.2 0.006 0.03
0.05
4 0.8 0.4 5 2
0.05 0.2 0.03 0.05
0.8 0.8 0.4 1.0 1.4
b. Comparison of Some Typical Sensitivities ((counta/s)/(gg/L)) Observed with Direct Aspiration vs On-Line Preconcentration
element Mn co
cu Cd
Pb
continuous aspiration
on-line preconcn height
ratio height/ continuous
3701 3149 1436 783 145
6500 4020 578 1042 1400
1.8 1.3 0.4 1.3 1.9
behavior. The detection limits for Cu and Co were the same as with peak area, but those for P b and especially Mn and Cd were improved with on-line preconcentration (even with a preconcentration factor of 1). These improvements can be explained (at least partly) by considering the elution process. Although 1 mL of eluent is injected, the analyte is mostly eluted near the front of this volume. As can be seen in Figures 2-4, 1 ng of analyte came out over about 1 min but most of it was already out in the first 30 s. This generally resulted in an enhancement of the signal compared to that of 1ng/mL aspirated directly, as can be seen from the change in peak height sensitivities reported in Table IVb. However, on comparison of the changes in sensitivities (Table IVb) to the corresponding changes in detection limits (Table IVa), it appears that they are similar for Co, Cu, and P b whereas the improvement in detection limit is much greater than that in sensitivity for Mn and Cd. This means that the increased sensitivity is cancelled by increased noise (resulting from the shorter measurement time used for peak heights) in the case of Co, Cu, and Pb, whereas there would seem to actually be a reduction in noise for Mn and Cd. Although the difference between the results of Co and Mn can partly be explained by the lower recovery of Co experienced with I-8-HOQ (9),future work will aim at accounting for the above observations (which were reproducible over several months). Analysis of SLRS-1. The first application of the on-line preconcentration system was made to the certified riverine water SLRS-1. The method of standard additions was used (additions were made to an aliquot of SLRS-1, prior to injection, in order to double the concentrations of the analytes). A 1-fold preconcentration was performed for Mn, Ni, and Cu (i.e. 1-mL aliquots of the unspiked and spiked SLRS-1 were injected) while a 10-fold preconcentration was done for Co, Pb, and U. (In each case, the three elements stated were monitored from the same injection.) The results, computed by using both peak height and peak area, are summarized in Table V. A good agreement with the certified values was achieved by either peak area or peak height for Mn and Co, whereas peak area gave the best results for P b and U, and peak height gave the most accurate and precise result for Cu. Overall, it seems that within each pair of results (area vs height), the result of highest precision is also the most accurate. (These observations were also true for seawater.) However, high results (by both peak height and peak area) were obtained for Ni. It should be noted that, although the
ANALYTICAL CHEMISTRY, VOL. 61, NO. 17, SEPTEMBER 1, 1989
Table V. Concentration (pg/L) of Some Trace Metals in the Riverine Water SLRS-1As Determined by the Method of Standard Additions with On-Line Preconcentration ICP-MS,”with Peak Area and Peak Height
element
peak area
peak height
8000
certified valueb
5
0
Mn Co Ni Cu Pb
U
1.79 f 0.54c 0.047 f 0.011 1.41 f 0.40 4.0 f 1.5 0.099 f 0.018 0.295 f 0.012
1.81 f 0.55 0.044 f 0.014 1.26 f 0.38 3.60 f 0.88 0.082 f 0.020 0.243 f 0.048
1861
1
60001 ,
1.77 f 0.23 0.043 f 0.010 1.07 f 0.06 3.58 f 0.30 0.106 f 0.011 0.28 0.03
I \
i
, ,‘i C
*
“The preconcentration factor was 1for Mn, Ni, and Cu, and 10 for Co, Pb, and U. bThe uncertainties for the accepted values are 95% tolerance limits-not standard deviations. Standard deviation (n = 3).
time (rnin)
Figure 5. Preconcentrationand elution processes of %n (-), @ i“ and %u (---)from riverine water SLRS-1. Injections of 4 X 250 HL (1 mL) SLRS-1 were done at time 0.0 and those indicated by the “S” arrows. Injections at 1-mL eluent (2 M HCVO.1 M “0,) are indicated by the other arrows. “a” indicates the peaks observed during the preconcentration period, “b” the elution, and “c” a column blank. (.e.),
most abundant isotope was monitored for all the other elements, @“i was used to avoid the isobaric interference of @Fe on %Ni. One plausible explanation for the higher Ni result would be an isobaric interference from %al60, which may result from a coelution of Ca. It was noted during the analysis of SLRS-1 that some Ca went through the column during the preconcentration. This is illustrated in Figure 5 where a 1-fold preconcentration of SLRS-1 was accomplished by injecting 1mL of SLRS-1 (as four 250-1L injections) followed by elution with 1 mL of eluent and an additional injection of eluent to check if elution was complete (Le. if the column blank was observed). Four symmetrical peaks appeared during the preconcentration which were not due to Ni and may be attributed to 44Ca160because they remained identical during the injection of SLRS-1 spiked with Ni, whereas the taller asymmetrical peak observed during the elution increased. However, although some of the Ca was obviously not retained by the column, there was probably some Ca that was retained as well. This assertion is supported by the fact that the alkaline earths are complexed by I-8-HOQ (for instance, 8HOQ can be used for the determination of Mg (26))but to a smaller extent than the transition metals. This smaller retention can become significant when high concentrations of alkaline earths are present with traces of metals. For instance, Fang and co-workers ( I 7) reported unacceptable recoveries of most of the heavy metals from a seawater matrix because of the high concentrations of alkaline earths, in particular magnesium which had a concentration of 1300 mg/L. Analysis of NASS-2. The on-line preconcentration system was really put to the test by performing the analysis of the open ocean reference water NASS-2. This time, isotope dilution was more specifically considered, the method of standard additions being mostly reserved for monoisotopic elements. The isotopic spikes were chosen so as to obtain isotopic ratios close to unity. During each preconcentration of NASS-2, the effluent was directed to waste to prevent the
high salt content from reaching the nebulizer and the interface. The effluent was only monitored during elutions (performed after a wash period of a t least 1min). The natural isotopic ratios of standard solutions of Ni, Cu, Mo, and Cd were first measured (by on-line preconcentration) to assess the extent of mass discrimination. The ratios were compared to IUPAC values (27) and found to all be in good agreement. Therefore, no correction for mass discrimination was made. In the case of Pb, a solution of NBS 981 was used. The mPb/208pb ratio of the NBS standard solution came out higher than expected for no apparent reason. Also, no reliable result could be obtained for P b by isotope dilution analysis, for any of the reference waters considered in this work, whereas good results were obtained in previous works ( I I , 2 I ) ,using the original column preconcentration with 1-8-HOQ. The results obtained by isotope dilution analysis are summarized in Table VI, along with the results obtained by standard additions, those obtained in a previous work (11) by isotope dilution analysis with off-line preconcentration, and the certified values. The isotope dilution determination with on-line preconcentration of Mo, Cd, and U yielded results in good agreement with the certified values, as did the method of standard additions for monoisotopic Mn. However, isotope dilution with on-line preconcentration gave results too high for Ni and Cu, although the method of standard additions (using 63Cu) gave a Cu result in closer agreement with the certified value. On comparison of the on-line preconcentration results with those obtained with the off-line method, it appears that isotope dilution yields good results for Ni and Cu but not Mo when used off-line. Since the major difference between the on-line and off-line preconcentration techniques is the neutralization with the ammonium acetate buffer (which is not used with the off-line method), it would appear that neutralization of the sample with a buffer improved the
Table VI. Concentration (pg/L) of Some Trace Metals in the Open Ocean Water NASS-2 As Determined by Isotope Dilution and the Method of Standard Additions with On-Line Preconcentration ICP-MS”
element Mn Ni cu Mo
isotope dilution
standard addition
previous workb
certified valueC
0.268 f 0.005 0.116 f 0.011 8.90 f 0.64 0.027 f 0.001 2.92 f 0.04
0.022 f 0.007 0.257 f 0.027 0.109 f 0.011 11.5 f 1.9 0.029 f 0.004 3.00 f 0.15
0.022 f 0.007d 0.332 f 0.001 0.190 f 0.020 12.1 f 0.9 0.033 f 0.001 2.91 f 0.17
0.095 f 0.011
Cd U 3.06 f 0.56 “The preconcentration factor was 1 for Mo and U and 10 for Mn, Ni, Cu, and Cd. *See ref 11. Determined by the isotope dilution technique with a 50-fold off-line preconcentration. The uncertainties for the accepted values are 95% tolerance limits-not standard deviations. dPrecision expressed as the standard deviation (n = 3-12).
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loading of Mo, whereas it was a source of interferences for the determination of Ni and Cu. I t is interesting to note that different sample injection volumes (i.e. preconcentration factors) can be used to speed up the analysis. For instance, only 1 mL was needed for Mo and U, while 10 mL was required for the lower concentrations (Mn, Ni, Cu, and Cd). CONCLUSIONS The implementation of on-line preconcentration in ICP-MS can both speed up the sample pretreatment of seawater and decrease the sample consumption drastically. With the original procedure (9),the preconcentration of NASS-2 required 500-mL aliquots (11)whereas only 10-mL aliquots were needed by on-line preconcentration. Furthermore, a preconcentration by a factor of 50 was found necessary with the original procedure in order to get reliable values for Cd, whereas a factor of 10 was quite sufficient with on-line preconcentration because of the additional enhancement in sensitivity (see Table IV)which results when elutions are followed in real time (most of the analyte coming out in the first half of the eluent reaching the column). Finally, at least 8 h (for the preconcentration as well as the actual analysis by ICP-MS) was required with the original procedure for the analysis of three samples of NASS-2 and three column blanks; only 45 min was needed with on-line preconcentration. In terms of “concentration efficiency” (CE), defined as the product of the enrichment (or preconcentration) factor (EF) and the sampling frequency in number of samples analyzed per minute (28), this converts to a CE value of 0.31 EF/min with the original procedure and of 0.67 EF/min with on-line preconcentration. This preliminary on-line preconcentration setup thus improved the concentration efficiency by a factor of 2. However, it was not applicable to all the elements normally determined with the original procedure (for instance, huge column blanks precluded the determination of Fe and Zn). Future work will deal with a more thorough application of this system to the analysis of saline waters as well as the optimization of several parameters of the setup in order to improve even more the concentration efficiency. These parameters will include the column dimensions, sampling frequency (which could probably be doubled if the buffer line was eliminated and 0.05 M ammonium acetate was used as the carrier, a t a rate of 2.2 mL/min), pH of the buffer, column blanks, etc. ACKNOWLEDGMENT The authors are grateful to J. W. McLaren for his helpful comments during the preparation of the manuscript.
Registry No. Mn, 7439-96-5; Co, 7440-484; Ni, 7440-02-0; Cu, 7440-50-8; Mo, 7439-98-7; Cd, 7440-43-9; Sn, 7440-31-5; Pb, 7439-92-1;U, 7440-61-1;water, 7732-18-5;8-hydroxyquinoline, 148-24-3.
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RECEIVED for review July 1,1988. Resubmitted February 27, 1989. Accepted June 2,1989. This is NRCC Publication No. 30431.
