1150
Anal. Chem. 1987, 59, 1150-1157
Determination of the Rare-Earth Elements in Geological Materials by Inductively Coupled Plasma Mass Spectrometry Frederick E. Lichte,* Allen L. Meier, and James G . Crock
U S . Geological Survey, Branch of Geochemistry, Box 25046, M I S 928, Denver Federal Center, Denver, Colorado 80225
A method of analysis of geological materials for the determination of the rare-earth elements using the inductively coupled plasma mass spectrometric technique (ICP-MS) has been developed. Instrumental parameters and factors affecting analyticai results have been first studied and then optimized. Samples are analyzed directly foilowing an acid digestion, without the need for separation or preconcentration with limits of detection of 2-1 1 ng/g, precision of f2.5 % relative standard deviation, and accuracy comparable to inductively coupled plasma emission spectrometry and instrumental neutron activation analysis. A commercially available ICP-MS instrument is used with modifications to the sample introduction system, torch, and sampler orifice to reduce the effects of high salt content of sample solutions prepared from geologic materials. Cor'rectlonsfor isobaric interferences from oxide ions and other diatomic and triatomic ions are made mathematically. Special internal standard procedures are used to compensate for drift in metakmetal oxide ratios and sensitivity. Reference standard values are used to verify the accuracy and utility of the method.
Several methods of analysis exist for the determination of the rare-earth elements (REE) in geological materials (1-3). Currently, the two methods most widely used are instrumental neutron activation analysis (INAA) (4) and inductively coupled plasma atomic emission spectrometry (ICP-AES) (5, 6). Neutron activation analysis is particularly useful because of its high sensitivity. When coupled with radiochemical separation, this technique offers both high sensitivity and excellent accuracy. The primary drawbacks of the technique are the time and cost for each analysis. The ICP-AES method is more generally available than INAA but detailed studies involving REE must utilize an ion-exchange technique for separation and concentration of the REE from the matrix. This step reduces spectral interferences and offers significant preconcentration of the REE over straight digestion procedures. However, the ion-exchange step increases the time of the analysis and requires the use of high-purity acids and reagents to keep contamination to a minimum. The sensitivity is sufficient to determine most of the REE at or above chondritic abundance ( 5 ) . Inductively coupled plasma mass spectrometry (ICP-MS) is a relatively new analytical technique (6-8). Unique features of ICP-MS over ICP-AES when used for elemental determinations are increased sensitivity, simplicity of the background spectra, and the ability to measure specific isotopes. Limits of detection for most elements including the REE have been reported to be in the low nanogram per liter range (9). The background spectra consist primarily of singly charged ions, singly charged oxides of refractory elements, and doubly charged ions of some elements. Other ions such as MOH' and MC1+ also exist and appear in the background when the concentration of the metal is high in the sample. In the ICP-MS technique, ions generated by an inductively coupled plasma are directed t o a quadrupole mass spectrom-
eter through a differentially pumped region bounded by an outer sampler cone and an inner skimmer cone. The sampler cone has a 1.1-mm orifice and the skimmer cone has a 0.89-mm orifice. This arrangement allows the plasma to be operated a t atmospheric pressure, making most of the sample introduction systems previously developed for ICP-AES applicable. The most widely used sample introduction method uses nebulization of aqueous solutions and transport of the aerosol into the plasma. Even though the nebulization process is relatively inefficient, the matrix and analyte species are both transported to the mass spectrometer with equal efficiency. Solutions containing high concentrations of matrix salts can quickly deposit on the sampler and skimmer resulting in a gradual closing of these orifices. Accompanying the closing of the orifices is a loss in sensitivity and a change in the metal to metal oxide ion ratios. Additionally, samples containing high acid concentrations can significantly reduce the lifetime of the sampler cone. Thus, the design and material used for fabricating this orifice is of primary importance. This study examines the instrumental parameters of the ICP-MS and describes a procedure for the analysis of a variety of geological materials to determine all of the REE (La-Lu), with the exception of Pm, Sc, and Y. Promethium is a short-lived radioisotope no longer present in the earth's crust, and scandium and yttrium are not required for the interpretation of the REE pattern for most petrogenetic studies. A multiacid digestion is used for dissolution of the sample and the REE are determined directly from this solution. A commercially available ICP-MS instrument is used. Modifications have been made to the sample introduction system, torch, and sampler cone to reduce the effects of the high salt content of sample solutions prepared from geologic materials. These modifications and special procedures are designed to compensate for drift in sensitivity, drift in the ratio of metal to metal oxide ions, and corrections for interferences.
EXPERIMENTAL SECTION Sample Preparation. The dissolution of rocks has been described elsewhere (5). Briefly, 1.00 g of sample, which has been ground to pass through a 100-meshsieve, is digested in a Teflon beaker on a hot plate with appropriate amounts of aqueous HCl, HN03,HCL04,and HF. The digest is taken to dryness on a hot plate and heated to 150 "C to decompose and evaporate fluorides, chlorides, and perchlorates. Residual fluorides can precipitate the REE, and chlorides will form diatomic ionic species in the ICP that interfere with the determination of some of the REE. The residue is dissolved with 2 mL of nitric acid and 0.5 mL of hydrogen peroxide. One hundred micrograms of cadmium and thorium are added by using 100 pL of a loo0 kg/mL stock solution and the volume is brought to 200 mL with 3% "OB. Cadmium is used as an internal standard for correction of sensitivity changes and thorium is measured with its oxide ion to monitor and correct changes in metal to metal oxide ion ratios. Instrumentation. A Sciex Elan ICP-MS system was used for this study. Modifications of the sampler cone, torch, sample delivery system, and lens power supplies were made to improve performance of the instrument for the analysis of high salt content samples. Instrumental conditions used are given in Table I. The ions measured and interfering ions are listed in Table 11.
This article not subject to U.S. Copyright. Published 1987 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 8, APRIL 15, 1987 W a i e r outlet
Table I. Instrument Operating Conditions Plasma Settings power 1600 W gas flows (Ar) plasma 13 L/min 1.0 L/min nebulizer 0.25 L/min sheath 15 mm from load coil sampling distance 1.86 mL/min sample uptake rate
7 1
,
1151
\
/ ' Auxiliary
Sheath
Ion Lens Settings 5.5 v B lens P lens 0.2 v E-1lens -25 V S-2 lens OV
Water
SarnDler
Figure 2. Schematic cross section of the torch used for this study showing relationship of the sheath gas supply tube to the aerosol injection tube and locations of the argon gas supplies and water cooling jacket.
Measurement Parameters 200 ms dwell time" 5 s/mass integration time total integration time 135 s wash time 20 s "Each mass was integrated for 200 ms in sequential order of m / e and a total of 25 integration at each m / e were taken. This has now been changed to 20 ms dwell time and 4 s/mass integration time. Table 11. Isotopes Measured and Interfering Ions measured isotope Cd 114 Ba 135 La 139 Ce 140 Pr 141 Nd 146 Eu 151 Sm 152 Gd 157 Tb 159
measured isotope
interfering ions
Dy 163 Ho 165 Er 168 Tm 169 Yb174 Lu 175 Hf 178 Th 232 Tho 248
BaO+ BaO+, CeO+ Pro+, CeOH+ NdO+, CeOH+
interfering ions
1.1
1.3
1.5
1.7
POWER ( K W )
NdOH+, SmO+ SmO+ SmO+ EuO+, SmOH+ GdO+ TbO+, GdOH+
Figure 3. Relationship of power to intensity for Pr' (0)and of the Pr' normalized to Cd' (+) at sheath gas flow rate of 0.25 L/min. 12
,
I
300 250
7 0
0 200 C 2 -I
150
1.1 mm
H
ikp/ 2
1 crn
t
'
3.7 c m
'
-
I Figure 1. Cross section of the platinum sampler cone.
The sampler cone design is shown in Figure 1. The orifice material is fabricated from pure (99.99%) platinum with a thickness of 0.56 mm. A collar made by cutting out the original sampler is used to hold the new sampler in place. The vacuum seal is attained by pressing the knife edge of the original sampler on the outside of the platinum sampler. The sampler cone is drilled with a $57 drill bit (1.09 mm) and a coating of Thoz is deposited on the platinum by nebulizing a 1%solution of Th(N03)4-4Hz0into the plasma for approximately 1h. The Thoz coating on the orifice protects the platinum from deterioration from the hot plasma gases. The sample introduction system, which has been relocated outside the plasma box, incorporates a Babington type nebulizer and straight spray chamber. The torch used for this study, as shown in Figure 2, has an independent argon gas supply which sheaths the sample gas flow for the last centimeter of the sample injection tube of the torch. It replaces the auxiliary gas flow in normal operation. We call this flow the sheath gas. The primary use of the sheath gas flow is to offer adjustment of the sample injection velocity into the plasma without affecting the performance of the nebulizer or the rate of total sample delivery. The torch is water cooled to improve its liietime in the hot environment of the torch box and close proximity to the interface. Argon gas flow rates for the nebulizer and sheath gases are controlled with
,
0.1
I
I
0.2
I
I
0.3
I
I
I
0.4
I
0.5
SHEATH G A S FLOW RATE(L/MIN)
Figure 4. Relationship of sheath as flow rate to intensity for Pr' (0) and of the Pr' normalized to Cd (+) at 1600 W of power.
9
a Matheson Model 8209 mass flow controller.
RESULTS AND DISCUSSION Plasma Conditions. To realize the high sensitivity of the ICP-MS technique, sample introduction, plasma, and mass
t
spectrometer parameters must be optimized concurrently. The nebulizer used was a Babington type nebulizer which requires solution to be pumped by a peristaltic pump at between 0.5 and 5 mL/min delivery rate. The sensitivity for the REE increases proportionately with increased sample introduction rate. However, the best compromise performance for sensitivity, precision, and sample consumption was obtained with the nebulizer operated at 1.86 mL/min sample delivery and a gas flow rate of 1.0 L/min. Input power to the plasma and sample injection velocity were studied with respect to sensitivity and oxide levels for the REEs and cadmium. Cadmium was selected as a potential internal standard to correct for matrix effects. Figures 3 and 4 show the relationships of plasma input power and sheath gas flow rate with sensitivity for praseodymium. Highest
1152
ANALYTICAL CHEMISTRY, VOL. 59, NO. 8, APRIL 15, 1987 0.34
1
I
w
eX 6 0 b
2
5 4
w 0 a 3 W
O 2 1
0 P L A S M A POWER ( K W ) Flgure 5. Relationship of power to the ratio of the intensity for Pro'
to intensity for Pr' at sheath gas flow rate of 0.25 L/min.
Flgure 7. Measured percentage of metal oxlde to metal ions for most REE and thorium, wing 1600 W of power and 0.25 L/min sheath gas
flow rate.
,
0.45
0.12 +
1
0.40
0.1 1
0.35
L
5
i
+
f
O.1°
0
I
0.09
0.25 0.20 0.15
0.08 0.07
0.30
! 0.1
I
I
I
I
I
I
0.2 0.3 0.4 S H E A T H GAS FLOW RATE(L/MIN)
I
I
0.10
0.5
0.05
Figure 6. Relationship of sheath gas flow rate to the ratio of the intensity for Pro+ to intensity for Pr' at 1600 W of power. sensitivity occurs at low power and high sheath gas flow rate. In Figure 3, the sheath gas flow rate was set a t 0.25 L/min and the power was adjusted from 1100 to 1700 W. The flat response of the praseodymium/cadmium normalized curve from 1500 to 1700 W indicates that cadmium is an effective internal standard for REE within this power range. Therefore, the power was set at 1600 W and the sheath gas flow rate was adjusted from 0.1 to 0.5 L/min as shown in Figure 4. Again, the flat response of the normalized praseodymium curve indicates that cadmium is effective as an internal standard for variations in gas flow rate a t 1600 W of power. The data on reference standards presented in Table I11 demonstrate the effectiveness of cadmium as an internal standard to compensate for variable matrix. Oxides. Isobaric interferences of some metal oxide ions are quite high on selected REE; therefore, the response of these metal oxide ions was studied with respect to plasma conditions. Praseodymium oxide is of extreme importance in this study because of its overlap with gadolinium-157. For samples enriched in the light REE, a significant fraction of the signal intensity for gadolinium-157 can come from (praseodymium oxide)-157. Therefore, conditions that minimize the oxide ions should be chosen. Figures 5 and 6 show the relationship of power and sheath gas flow rate on praseodymium oxide to praseodymium ions. The graph in Figure 5 shows that as the power is increased, the ratio of P r o to P r ions decreases. The graph in Figure 6 shows that the oxide ion ratio of praseodymium increases with increasing sheath gas flow rate. Based on the information shown in Figures 3-6, compromise conditions of 1600 W of power and 0.25 L/min sheath gas flow rate were selected to achieve a balance between sensitivity
! 1.1
I
I
I
I
1.3 1.5 POWER (KW)
I
I 1.7
Flgure 8. Relationship of power to the ratlo of the intensity for Pro' to intensity for Pr' (0)and the ratio of the intensity for Tho' to the intensity for Th' (+) at sheath gas flow rate of 0.25 Llmln.
and oxide levels. The location of the initial radiation zone (IRZ)as described by Koirtyohann et al. (10) using 500 pg/mL of sodium solution was 6 mm from the sampler orifice. The load coil was located 15 mm from the sampler cone. Oxide Correction. As shown in the preceding section, the operating conditions of the plasma are set to minimize matrix effects and oxide ion formation. The remaining isobaric interference from the oxide ions, however, must be corrected mathematically. The oxide correction of P r o on gadolinium-157 requires that a correction be made that is highly accurate. Even after the oxide abundance is minimized, a 10% error in the oxide ratio correction can result in a 20% error in the gadolinium result. We found that over a 3-h period of running samples, the oxide ratios could drift by as much as 100%. This is probably due to a gradual closing of the sampler cone although there are several contributing factors that are not yet understood. The metal to metal oxide ion ratios of elements studied all responded similarly to plasma conditions. Therefore, an additional study to test this apparent relationship was made of the relative oxide response to instrumental and sample parameters. The measured levels of oxide occurrence for most REE and thorium are shown in Figure 7. Thorium and thorium oxide ions at m / e 232 and 248 have no isobaric interferences and can be measured accurately; therefore, the ThO+/Th+ response was compared to the rare earth metal oxide responses. The comparison is shown in Figures 8 and 9. As shown, the oxide ratio can change considerably, but within the boundaries about 1600 W of power and 0.25 L/min sheath gas flow rate the normalization if very good.
1153
ANALYTICAL CHEMISTRY, VOL. 59, NO. 8, APRIL 15, 1987 Table 111. Comparison of Rare E a r t h Elemental Concentrations in Standard Reference M a t e r i a l s
BCR-1 Basalt" element
this study
ref valueb
La Ce Pr Nd Sm Eu Gd Tb
26.7 54.4 7.0 28.9 6.8 1.99 7.1 1.08
26.6 53.8 7.29 29.7 6.7 1.98 6.9 1.0
ref valuec
element
this study
ref valueb
DY
6.59 1.29 3.74 0.53 3.55 0.54
6.721 1.4 3.8 0.57 3.7 0.524
25.3 54.0
Ho Er Tm Yb Lu
28.6 6.7 1.95 6.63
ref valueC 6.3 3.61 3.39 0.54
SO-2 Soil"
element
this study
ref valueb
element
this study
ref valueb
La Ce Pr Nd Sm Eu Gd
46.0 115.0 14.2 59.1 12.6 3.42 11.0
46.0 111.0 13.4 57.0 12.2 3.43 10.3
Tb DY Ho Er Tm Yb Lu
1.63 9.03 1.66 4.30 0.57 3.47 0.47
1.3 8.6 1.71 4.1 0.54 3.39 0.47
GSP-1Granodioritea element
this study
ref valueb
element
this study
ref valueb
La Ce Pr Nd Sm Eu Gd
180.0 423.0 56.9 206.0 26.6 2.18 12.4
188.0 432.0 56.0 206.0 27.4 2.16 12.4
Tb DY
1.28 5.95 0.95 2.21 0.26 1.41 0.20
0.4 5.8 1.00 1.7 0.22 1.4 0.16
Ho
Er Tm Yb Lu
NBS 1632a Coal" element
this study
ref valueb
element
this study
ref valueb
La Ce Pr Nd Sm Eu Gd
14.7 30.6 3.74 13.4 2.62 0.53 2.01
14.5 28.5 3.3 13.0 2.6 0.49 2.4
Tb DY Ho Er Tm Yb Lu
0.31 1.84 0.36 1.01 0.15 0.98 0.13
0.3 2.1 0.38 0.91 0.4 0.90 0.15
"All values are pg/g from the average value of two replicate sample preparations. NBS 1632a coal was ashed at 525 "C. The values given are calculated from the ash content. bThe ICP-AES method of Crock and Lichte (5). 'The isotope dilution average presented by Hansen et
al. f31. 0.34
0.1s
I
c 1.5
\
0.15
10.5
0.08 "
I
I
0.1
0.2
0.3
I
I
I
0.4
I
0.5
SHEATH GAS FLOW RATE(LIMIN)
Flgure 9. Relationship of sheath gas flow rate to the ratio of the intensity for Pro+ to intensity for Pr' (0)and the ratio of the intenslty for Tho+ to the intensity for Th+ (+), using 1800 W of power.
Figure 10 shows the effect of both matrix salts and power on the oxide ratios of praseodymium and thorium. The matrix consists of sodium, iron, and aluminum a t 500, 200, and 500
I 1.1
I
1
I
1.3
1.5
1
1.7
POWER(KW)
Flgure 10. Relationship of power on the oxide ratios both with and without matrix salts. The curves are for PrO+/Pr+ without matrix ( O ) , PrO+/Pr+ with matrix (+), the ratio of PrO+/Pr* to ThO+/Th+ without matrix (o), and the ratio of PrO+/Pr+ to ThO+/Th+ with matrix (A).
pg/mL, respectively. The blank solution contains only praseodymium, cadmium, and thorium at 0.5 pg/mL. The thorium oxide response as shown is similar to the response of praseodymium oxide for both power and matrix content.
1154
ANALYTICAL CHEMISTRY, VOL. 59, NO. 8, APRIL 15, 1987
Application of this relationship should allow a means to compensate for changes in the REE oxide ratio by measuring the ThO+/Th+ ratio on each sample. Therefore, thorium is added to each sample and standard a t 0.5 pg/mL. The ThO+/Th+ ratio is measured concurrently with the REE oxide ratios and the observed percent change in the ThO+/Th+ ratio is used to correct for changes in the other metal oxide ratios due to matrix or drift. While synthetic standards can be run before and after each sample to determine current oxide correction factors, the application of the thorium correction eliminates this need. In short, even though we do not fully understand the processes involved in forming and dissociating diatomic ions, the metal oxide ratios of the elements studied for this method are proportional, and thorium oxide ratio can be used to correct changes in the ratio. This direct relationship is valid for as much as a 50% change in the oxide ratio. Larger changes can be corrected but require use of second-order equations. Since the cause of the drift is apparently a gradual closing of the sampler orifice, it is easier to analyze small groups of samples to keep the drift under 50% than to make larger corrections. Fifty samples can be analyzed within these constraints by using a single linear calibration curve for the calculation of the concentrations. Since this study was completed, a change in the data collection software now allows the data to be collected with a 20-ms dwell time and the total integration time has been reduced to 4 s per mass. The reduced integration time allows a longer wash time without changing the analysis time and it is now possible to analyze more samples without exceeding a 50% change in oxide ratios or cleaning the sampler or skimmer orifices. Lenses. The photon stop consists of a set of three interdependent lenses: a plate lens which forms an entrance and exit orifice to the photon stop system; a barrel (B) lens that is a cylinder lens within the plate lenses; and a stop (S-2) lens that is a disk of approximately equal diameter to those of the two orifices of the plate lens. The applied voltage on these lenses is normally optimized for maximum sensitivity. Indeed, a study of the applied voltage on the plate lens showed that the sensitivity for ions is affected by its potential. However, as higher negative voltages are applied to this lens the intensity of the background signal increases and the background becomes increasingly sensitive to the salt content of the sample. The plate lens was therefore optimized for minimal background by using a standard with high salt content. Instead of setting this lens at a negative potential as suggested in the manufacturer’s manual, using background as the criteria, we found that a slightly positive potential on this lens maintained a low background even with high salt content samples and high power applied to the plasma. In order to reach the positive voltage necessary to avoid what appears to be secondary emission of photons from ion collisions, the power supply for the plate lens was modified to a range of f 5 V and was set to +0.2 V. Additionally, in order to reduce the number of lenses involved in the optimization, the S-2lens was grounded and the B lens optimized for maximum sensitivity. Figure 11 shows the relative effect of the voltage on the B lens of the photon stop on sensitivity for the mass range of the REE. The B lens is in fact the most mass-sensitive lens in the instrument. A setting of 70, or +5.5 V, was chosen as a compromise setting because it gives approximately equal sensitivity across the mass range of the REE without significant loss of sensitivity. The S-2 lens was then retested and only slight improvements in sensitivity could be gained with this lens over its grounded state. Orifice Design. The sampler cone as supplied by the manufacture was redesigned to minimize two separate prob-
W
0
z
70
604
501A -
40
c
v)
z
W
I+-/
10 135
I
I
145
I
I
I
-
155 MASS UNITS
I
165
I
I
175
Flgure 11. Effect of the voltage on the B lens of the photon stop on intensity normalized to isotopic abundance for the mass range of the REE. The curves are for B lens = 40 ( O ) ,B lens = 50 (+), for B lens = 60 (o),B lens = 70 (A),B lens = 80 (X), and B lens = 90 (V).
lems: clogging and short lifetime. The original sampler cone is fabricated from nickel which oxidizes in the hot plasma environment. Its lifetime is especially shortened by using the high power settings and high acid content samples required for dissolution of the REE in geological materials. The modified orifice is made from pure platinum and coated with thorium oxide. The thorium oxide coating is very refractory but not permanent. The diameter of the orifice can be decreased by depositing more thorium or matrix salt onto it or increased by removing the deposit through ablation. Ablation is a normal process which occurs when solutions containing little or no salt are nebulized. Thus, even though the salt from the samples can clog the orifice by increasing this coating, it can also be removed during the wash cycle of the autosampler. At 1600 W, an equilibrium of deposition/ablation occurs when the sampling and wash times are equal. The platinum is thus protected from deterioration and has now been in use for over 1 year without noticeable wear. The ablation process does not result in the formation of ions from the sampler material. Thus, neither platinum nor thorium ions are observed in the background spectra. The original sampler cone is shaped to give a laminar expansion of gases under supersonic flows within its cone. Salt from samples deposit on this cone and can affect this flow pattern. Deposits also build up just inside the orifice changing the effective thickness at the orifice. The modified orifice as shown in Figure 1 is nearly blunt. Visual examination of the inside of the sampler cone indicates that indeed the orifice itself is coated with a glassy slag and that salts are deposited as a powder in a 2-mm-wide band approximately 2 mm from the orifice. This contrasts with the original orifice where the deposit forms directly on the inside of the cone and gradually decreases with distance. We estimate that this design change has decreased the clogging problem by 10-fold and the effect of clogging is now equal between the skimmer orifice and sampler orifice. The effect of the distance between the sampler and skimmer was tested by varying the depth of the sampler cone. New samplers were fabricated to alter the depth so that the distance between sampler and skimmer was half, equal, and double that of the original design. All three designs appeared to give comparable results in terms of sensitivity and oxide levels. Therefore, the depth of the modified sampler chosen for the rest of this work was equal to the original design. This blunt design appears to have reduced the drift in sensitivity and oxide ratios resulting in a much more stable and reliable system. Dilution Factors. The ICP-MS technique is noted for its high sensitivity and large dynamic range. For the instrumental
ANALYTICAL CHEMISTRY, VOL. 59,
conditions used in this study, the linear range was approximately 0.01-5000 pg/L. The dilution factor of the sample needs to be adjusted to bring the concentrations of the elements of interest within this linear range. Several other factors must also be considered in choosing the proper dilution for analysis of acid solutions of geologic materials. Sample solutions that have minimal dilution should offer lower limits of detection than those with higher dilution factors. However, more concentrated solutions increase the rate of orifice clogging and the percent suppression of the signal by matrix elements. From these considerations, the analysis of dilute solutions is favored. However, increased contamination, cost of reagents, and the integration time required to achieve comparable limits of detection are also significant considerations. A compromise to address these considerations is necessary. In this study, a dilution factor of 200 was chosen, i.e., 1g of sample dissolved and diluted to a final volume of 200 mL. For most silicates, this dilution results in a total dissolved salt concentration of approximately 0.25%. Internal Standard. Cadmium was chosen as the internal standard because of its low normal abundance in igneous rocks. Its effectiveness as an internal standard was shown in the power and sheath gas studies presented in Figures 4 and 5. A change in sensitivity by a factor of 5 results in less than a 10% change in the REE/Cd ratio. Interferences. As shown in the Experimental Section, there are a number of isobaric interferences from diatomic and triatomic ions. The interference from isobaric metal oxides and hydroxide ions must be subtracted from the elemental ion intensity. The natural abundance of the REE favors the light REE and the even atomic numbered elements are more abundant than odd numbered members. Therefore, polyatomic species of the light REE and hydroxide, l8O, or chloride from the matrix solution contribute significant interferences on the heavier less abundant REE and must also be included in the calculation. Samples diluted with 3% hydrochloric acid required corrections for isobaric overlap from ions of LaC1+ and BaC1+ on ytterbium and CeCP on lutetium that were nearly 50% of the total signal. For this reason, dissolution of the sample with nitric acid is preferred to dissolution with hydrochloric acid. This dissolution is more time-consuming, but the correction for the chloride ions is eliminated and the precision of measurement in improved for Yb and Lu. Isotopic Choice. For the ICP-MS experiment, individual isotopes of the elements are generally used to quantify the concentration of the element. The isotopes used in this method are listed in Table 11. The REE in geological materials are generally at low concentrations; therefore, in most cases, the most abundant isotope was measured. In some cases less abundant isotopes were selected because isobaric interferences on the primary isotope required large corrections. Barium oxide overlaps europium and samarium. At low concentrations of barium, the interference of the oxide is negligible; however, in samples where the barium is present at high concentrations the degree of overlap can be significant. Therefore, the less abundant barium-135 isotope (5% abundance) can be measured for the oxide correction. The hafnium-173 overlap on ytterbium requires that hafnium be measured, although the interference by hafnium is quite small. No correction was made for the mass dependence of the Einzel lenses and photon stop. As mentioned above, gadolinium, because of an isobaric interference from praseodymium oxide, is quite difficult to determine accurately. Both gadolinium-157 and -158 were investigated to test the utility of each. Gadolinium-157 has a praseodymium oxide interference whereas both NdO and CeO overlap gadolinium-158 requiring two corrections instead of one. The magnitude of correction is approximately equal
NO.8, APRIL 15, 1987
1155
for most samples. Gadolinium-157 gave better results and was chosen as the primary isotope for quantifying gadolinium. Gadolinium-158 is also measured, but only as a secondary measurement. Background. The background intensity is independent of mass and was between 3 and 4 countsfs as measured at the uranium-234 mass. For the 47 samples measured for this study, the standard deviation was 0.9 counts/s. This deviation of the background was used to determine the limits of detection. Standardization. The initial standardization of the method was done by using matrix matched standards made from the individual rare earth element oxides. From this standardization, the values for Pikes Peak granite were determined and this standard was then used to calibrate the instrument for subsequent runs. A plot of the sensitivity of each isotope (normalized for abundance) vs. mass was used to check the validity of the standardization. The data points should fit a smooth curve. As shown in Figure 11, the relative sensitivities of the REE depend greatly on the applied voltage on the barrel lens. The oxide interferences are determined from standards containing 0.5 pg/mL of the interfering element and thorium. It is not necessary to matrix match the standards for the oxide corrections, but is is recommended. The calculation method requires dividing all sensitivities by the internal standard and subtracting oxide isobaric interferences. Calculations. Isobaric interferences from hafnium were subtracted by using Sciex software in the quantitative analysis program. All other data were calculated by using a commercial spreadsheet software on a personal computer. The following Equations are used to calculate the results:
La = (ILa - I U ) / I C d x 20.5
(1)
Ce = (Ice - I u ) / I C d x 23.1
(2)
Pr = (Ip, - I U ) / I C d x 20.5
(3)
- I")/Icd
(4)
Nd = Sm = ((Ism - IU) -
(INd
x 118.6
RBaO FThO/RThO) - (Ice x R c e O X FThO/RThO))/ICd X 80.4 (5)
(IBe
Eu = ((IEu
Gd =
- IU)
-(IBa
((IGd -
RBaO
Iu)- (IPr x
RPrO
x
FThO) - (Ice
FTho/RTho))/Icd
Tb =
( ( I T b - Iu)- ( I N d
RNdO
RCeOH
Dy = ((ID, - I U ) - (Ism (UHo
- IU)
-
RSmO
(Ism
RCeOH x x 143.4 (7)
FThO/RThO)
- (Ice
23*1 (8)
FThO/RThO))/ICd
RNdOH
Ho =
44.7 (6)
FThO/TThO))/ICd
FThO/RThO)
- (INd 95.6 (9)
FThO/RThO))/ICd
X 24.4
X RSmO X FThO))/ICd
(10) Er = ((IEr
Tm =
- IU)
((ITm
-
(Ism
RSmO
- I U ) - (IEu
(ISmOH
REuO
RSmOH
91.4 (11)
FThO/RThO))/ICd
FThO/RThO)
FThO/RThO))/ICd
-
24*8 (12)
Yb = ((IYb - I U ) - (IGd
Lu =
((ILu - I U ) - ( I T b RGdoH
RGdO
FThO/RThO))/ICd
78.9 (13)
FThO/RThO) - (IGd &bo FTho/RThO))/ICd x 25.8 (14)
x
where I, is the count rate of the element x, Iu is the background as measured at mass 234, F T h o is the ratio of T h o to T h as determined for that sample, R T h o is the ratio of T h o
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 8, APRIL 15, 1987
Table IV. Pikes Peak Granite, Comparison of Mean and Standard Deviation of Proposed ICP-MS Method and an ICP-AES Method; All Values Are in p g / g element
La Ce Pr Nd Sm Eu Gd Tb DY Ho Er Tm Yb Lu
ICP-MS meanb SD
ICP-AES" meanb SD
148.0 284.0 34.3 119.0 22.7 1.85 17.5 3.04 18.5 3.67 10.6 1.65 10.4 1.37
149.0 286.0 33.5 127.0 24.5 2.03 21.1 3.0 18.4 3.75 10.7 1.41 11.0 1.53
3.0 5.0 0.6 1.6 0.5 0.08 0.9 0.09 0.4 0.08 0.2 0.03 0.2 0.04
2.0 2.0 0.9 1.o
0.4 0.04 0.1 0.0 0.2 0.05 0.2 0.03 0.2 0.02
"ICP-AES method of Crock and Lichte ( 5 ) . *Mean was calculated from the determinations of three separate splits, each prepared three times.
to T h as determined for the oxide isobaric interferant standard, and RM is the ratio of the isobaric MO to M as determined from the single element standard. The sensitivity and oxide factors are determined each day and checked for drift at the beginning, middle, and end of each 40 samples. Results of Reference Materials. The analytical results of several reference materials are presented in Table I11 and are compared with other values reported in the literature to demonstrate the accuracy of the method. These materials were chosen to represent a wide range of geological materials and concentrations of REE. The data presented were collected in a single run consisting of 47 samples and standards and calculated from calibration equations by using the cadmium and thorium correction factors as described above. Each standard was prepared in duplicate and analyzed in random order. The average of the duplicate value is reported. The values appear to compare quite favorably between the two methods with average differences under 5 % . To determine the within run variation of the method, five replicates of Pikes Peak granite, an in-house standard, were run with the other reference materials. Digests of this standard were randomly interspersed between the 20th and 47th position in the analytical run and the standard deviations of the individual results were calculated. The data presented in Table IV demonstrate the precision of the method and compare favorably with the ICP-AES method. The average relative standard deviation for all REE and 2.5% with a range of 1.7-5.1%. Gadolinium showed the poorest precision at 5.1%. Limits of Detection. The limits of detection as defined by 3x the standard deviation of the background are presented in Table V and are compared with the ICP-AES method, which uses a cation-exchange preconcentration step. The detection limits in the solutions analyzed would be between 10 and 55 pg/mL depending on the element. If the cation exchange method were applied to the ICP-MS technique, the detection limits would be lower by a factor of 40. A direct comparison of the two methods indicates the ICP-MS technique is more sensitive than ICP-AES by factors ranging between 40 and 4000 depending on the element. Chondritic Plots. The occurrence of REE in rocks and minerals is an important indicator of the petrogenesis of the host rock. When the concentrations of the REE are normalized to their chondritic abundance, odd-even atomic number abundance affects are eliminated and a simple graph
Table V. Limits of Detection for ICP-MS and ICP-AESn element
ICP-MSb
ICP-AES'
La Ce Pr Nd Sm Eu Gd
0.002 0.002 0.002 0.009 0.006 0.003 0.011 0.002 0.007 0.002 0.007 0.002 0.006 0.002
0.022 0.15 0.32 0.11 0.15 0.0045 0.09 0.29 0.125 0.018 0.04 0.026 0.005 0.009
Tb DY Ho Er Tm
Yb Lu
a Detection limit is defined as 3 times the standard deviation of the background for the 40 samples analyzed in this study. All values are in pg/g. gram of sample dissolved in 200 mL. 'One gram of sample dissolved in 5 mL following ion exchange separation.
0.20:
I
I
I
La Ce Pr Nd
I
I
I
I
I
,
I
I
I
I
Sm Eu Gd Tb Dy Ho Er Tm Y b Lu
Figure 12. Logarithm of the ratio of sample concentration to chondrite abundance plotted against REE in increasing atomic number order for BIR-1 (+), and NBS-688 (0). three standards: GSP-1 (0).
can show the enhancement of the light or heavy REE with respect to chondritic abundance. Additionally, europium and cerium can occur in the +2 and +4 oxidation states, respectively, where the other REE are in the +3 state. Europium is relatively easily reduced, and is of particular interest because its ionic radius is quite different for each oxidation state and its concentration with respect to its adjacent members, samarium and gadolinium, is useful in assessing the redox conditions during its placement into the host rock. The chondritic plots of three standards are presented in Figure 12. The logarithm of the ratio of the concentration of the element to chondritic concentration (5)is plotted against the atomic number of the REE. These plots in conjunction with the data presented offer an excellent means to assess the quality of the REE data obtained by using this procedure. The concentrations for many of the elements in BIR-1 are in fact below the limit of detection for the ICP-AES technique even with an ion-exchange preconcentration step. The method presented here allows for the determination of all of the naturally occurring REE in a single procedure that simplifies the sample preparation of the ICP-AES method and yet offers lower limits of detection. Registry No. La, 7439-91-0; Ce, 7440-45-1; Pr, 7440-10-0; Nd, 7440-00-8; Sm, 7440-19-9; Eu, 7440-53-1; Gd, 7440-54-2; T b , 7440-27-9; Dy, 7429-91-6; Ho, 7440-60-0; Er, 7440-52-0; Tm, 7440-30-4; Yb, 7440-64-4; Lu, 7439-94-3.
LITERATURE CITED ( 1 ) Topp, N. E. The Chemistry of Rare-Earth Elements; Elsevier: New York, 1965.
Anal. Chem. 1987, 59, 1157-1164 (2) Banks, C. V.; Klingman, D. W. Anal. Chim. Acta 1956, 15, 356-363. (3) Hansen, G. N. I n Accuracy in Trace Analysis: Sampling, Sample Hendling, Analysls; LaFleur, P. D., Ed.; National Bureau of Standards: pp 937-949. Washington, DC, 1976; Spec. Publ. No. 422, VoI. II., (4) Haskin, L. A.; Wiideman, T. R.; Haskin, M. A. J . Radioanal. Chem. 1966, 1 , 337-348. (5) Crock, J. G.; Lichte, F. E. Anal. Chem. 1982, 5 4 , 1329-1332. ( 6 ) Houk, R. s.; Fassel, V. A,: Flesch, 6. D.; Svec, H. J.; Gray, A. L.; Taylor, C. E. Anal. Chem. 1980, 52, 2283. (7) Date, A. R.; Gray, A. L. Analyst (London) 1983, 108, 159. (8) Douglas, D. J.: Houk, R. S. f r o g . Anal. At. Spectrosc. 1985, 8 , 1-18.
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(9) Date, A. R.; Gray, A. L. Spectrochim. Acta, Part B 1985, 406, 115-122. (10) Koirtyohann, S. R.; Jones, J. S.; Yates, D. A. Anal. Chem. 1980, 52, 1965.
RECEIVED for review August 4,1986. Accepted December 19, 1986. The use of brand or manufacturer’s names is for descriptive purposes only and does not constitute endorsement by the U.S. Geological Survey.
System Peaks in Liquid Chromatography: Their Relation to the Adsorption Isotherm Shulamit Levin and Eli Grushka* Department of Inorganic a n d Analytical Chemistry, The Hebrew University, Jerusalem, Israel
System peaks appear In a llquld chromatographic system when the moblle phase contalns more than one component. The study shows that system peaks are Intimately related to the adsorptlon of mobile phase components on the stationary phase surface and that they can be utlilred In the calculation of their adsorptlon Isotherm. The system peaks can be used as monltors of the chromatographlc events In whlch moblle phase components partlclpate. The chromatographic system studled here conslsted of a reversed-phase column and an aqueous moblle phase contalnlng acetate buffer, cupric acetate, and heptanesulfonate. The dependence of the system peaks on the concentratlon of the moblle phase components, elther as solutes or In the moblle phase, helped In the ldentlflcatlon of the specles responsible for these peaks. The solvatlon layer on the stationary phase can be ascertained and examined. I n addition, the areas of the system peaks were used to obtaln the capacity factor and adsorption lsotherm of copper Ions. The capacity factors were determined without having to measure the void volume of the column.
A full description of the origin and formation of the system peaks was given in a previous publication (1). However, a brief review is in order. A state of equilibrium in the chromatographic column is achieved when all the components in the mobile phase are distributed between the stationary and the mobile phases according to their partition coefficients. At this stage the composition of the mobile phase entering the column is identical with that leaving the column. The arrival at the head of the column, after an injection, of a sample different in any sense from the mobile phase, causes a perturbation of the equilibrium a t that position. The system strives to reestablish the equilibrium by a relaxation process, which results in the appearance of the system peaks in the chromatogram. these peaks are in addition to those due to the solutes. The relaxation process is explained in detail in ref 1. The requirements from a liquid chromatographic system that will lead to the observation of system peaks were also discussed before (I). In general, system peaks appear whenever the mobile phase contains more than one component, providing that the detector used is sensitive to these components. Typical examples of systems that show such peaks can be found in references dealing with ion exchange chro-
matography using mobile phases containing UV-absorbing reagents (2-12) or other additives (13-17). Many other systems (18-33) exhibiting system peaks have been described in the literature. System peaks were discussed by many workers, generally on an individual basis and for particular chromatographic systems. On the whole, however, the idea that system peaks are related to the fundamentals of the chromatographic process has not received the appropriate recognition. The present work gives further evidence as to the importance of this phenomenon. Relation of the System Peaks to Adsorption Isotherms. When a pure solvent (water in the present case) is injected, the sample that arrives at the column head is relatively vacant with regard to the mobile phase additives. This results in reextraction of the mobile phase components from the stationary phase surface into the injected volume. Each of the extracted components migrates through the column with a characteristic velocity dictated by the partition coefficient of the component, and it appears as a peak in the chromatogram. The peaks can be either in the negative or positive direction, relative to the base line of the detector, depending on the response of the particular component vis-a-vis that of the bulk mobile phase. Quantitation of each peak by calibration curves of the suitable components, as described in ref 1,allows the evaluation of the amounts of the desorbed components in the vacant sample zone. These amounts are related to the quantities adsorbed on the stationary phase prior to the injection, which in turn, is indicative of the adsorption isotherms. Several investigators (22,25,34-37) have already related the system peaks to the species that are present in the extraction layer on the stationary phase. It was also understood that the capacity ratios of these species may be used to evaluate the amounts adsorbed on the stationary phase surface. However the main difficulty with such determination was in the calculation of the true capacity factors, since the measurement of the void volume in a multicomponent mobile phase system is subject to errors, as well as to some operative definitions (34,35,38,39).The present work presents a simple way to circumvent this problem, using two reasonable assumptions. One assumption states that the change in the injection volume, during the flow from the injection valve to the column head, is negligible. Thus, all the events that occur at the head of the column immediately after the injection can be related to this volume. The second assumption states that one of the mobile phase components acts as a probe which can quantify the amounts adsorbed of all or most of the mobile
0003-2700/87/0359-1157$01.50/0 0 1987 American Chemical Society
