Qianli
Xie
* and
Robert
Kerrich
Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK, Canada S7N 5E2. E-mail: xieq@duke.usask.ca; Tel: (306) 966-5691; Fax: (306) 966-8593
First published on 4th December 2001
Isotope ratios were measured from the low mass (25Mg/26Mg) to the high mass (203Tl/205Tl) region, using an ELAN 5000 and a Platform Hex-ICP-MS, to investigate mass bias effects on the two instruments. For the ELAN 5000, the percentage mass discrimination per atomic mass unit (MD%) varies from 5% for 25Mg/26Mg to 0.7% for 203Tl/205Tl, whereas for the Platform it ranges from 10% to 0.11%, respectively. The Platform has higher mass bias at low mass and lower mass bias at the high mass region compared to the ELAN 5000. The difference in mass bias between the two instruments is attributed to the different ion transferring lens system, i.e., electrostatic lens system in the ELAN 5000 and hexapole collision cell in the Platform. For the Platform Hex-ICP-MS, the addition and variation of collision/reaction gases (He and H2) does not introduce any additional mass bias. The precision and accuracy of isotope ratio measurement by the Platform Hex-ICP-MS were assessed using the NBS982 Pb isotope reference material and an elemental Sn standard spiked with 117Sn-enriched spike. The overall precision was found to be 0.1–0.6% relative standard deviation.
Several aspects of instrument performance negatively affect the precision and accuracy of isotope ratio measurement by ICP-MS: specifically poor sensitivity, signal stability, spectral interferences, and mass discrimination. One way to reduce or eliminate spectral interference is to employ a high resolution magnetic sector mass analyzer. However, a drawback of the high resolution mass analyzer is the inherent decrease in instrument sensitivity.2 Alternatively, a “chemical resolution” technique has been successfully applied to quadrupole ICP-MS. The technique utilizes a “collision” or “reaction” cell filled with various gases (e.g., He, H2, NH3 or Xe) to reduce or eliminate spectral interferences.7–12 In most current models of ICP-MS that utilize the “chemical resolution” technique, the collision/reaction cell also acts as ion focusing optics that homogenize ion energy and hence increase ion transmission efficiency.9,10,13
Mass bias is the deviation of measured isotope ratios by a mass spectrometer from the “true value”, due to mass discrimination that occurs in all mass spectrometers. In particular, several factors have been found to affect bias in ICP-MS, such as detector “dead time”, the electrostatic ion focusing lens and the “space charge” effect occurring at the interface region.1,14 In general, mass bias in TIMS is time dependent, whereas mass bias change with time in ICP-MS has been demonstrated to be much slower under given instrumental parameters such as nebulization gas flow rate, torch position, and rf power. Hence, mass bias in ICP-MS can be predicted and more accurate corrections can be applied.15 However, with the addition of a “collision cell”, some concerns have arisen as to whether this introduces unpredictable mass bias effecs.16,17
In this study, isotope ratios were measured at low and high mass regions, using a conventional ICP-MS and a hexapole “collision cell” ICP-MS, to compare the mass bias of the two. The effect of varying collision gases on the mass bias of the hexapole ICP-MS (Hex-ICP-MS) was also investigated. The precision and accuracy of isotope ratio measurement using a Hex-ICP-MS was assessed by the measurement of Pb isotope ratios in NBS-982 standard, and the Sn isotope composition of an elemental Sn standard with natural isotope abundance, but spiked with 117Sn-enriched isotope in various proportions.
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Fig. 1 Schematics of instrument configuration of the Elan 5000 (a) and Platform ICP-MS (b). |
a Instrument read-back. | |
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PE Elan 5000— | |
Rf forward power | 1100 W |
Cool gas flow rate | 10 L min−1 |
Intermediate gas flow rate | 0.8 L min−1 |
Nebulizer gas flow rate | 0.76 L min−1 |
Sample uptake rate | 1 ml min−1 |
Sampler/skimmer cones | Ni, 1.1 and 0.89 mm diameter, respectively |
Lens settings/Va | P: −69, B: +9.1, S2: −8.3, E1: +8.2 |
Acquisition | Peak hopping |
Dwell time | 30 ms peak |
Point peak | 1 |
Micromass platform— | |
Rf forward power | 1350 W |
Cool gas flow rate | 14 L min−1 |
Intermediate gas flow rate | 0.85 L min−1 |
Nebulizer gas flow rate | 0.76 L min−1 |
Sample uptake rate | 0.70 ml min−1 |
Sampler/skimmer cones | Ni, 1.1 and 0.7 mm diameter, respectively |
Extraction lens | −400 V |
Exit lens | 400 V |
Hexapole bias | −1.6 V |
Ion energy | 2 V |
Multiplier | 450 V |
He gas flow rate | Variable, optimum 5.0 ml min−1 |
H2 gas flow rate | Variable, optimum 2.8 ml min−1 |
Acquisition mode | Single ion measuring (peak hopping) |
Dwell time | 200 ms |
Point/peak | 1 |
fMD = Rtrue/Rmeasured | (1) |
MD(%) = (fMD−1) × 100/Δm | (2) |
Platformb | 25Mg/26Mg | 63Cu/65Cu | 86Sr/88Sr | 95Mo/96Mo | 107Ag/109Ag | 151Eu/153Eu | 184W/186W | 203Tl/205Tl |
---|---|---|---|---|---|---|---|---|
1 | 0.8218 | 2.0278 | 0.1147 | 0.9427 | 1.0476 | 0.9056 | 1.0790 | 0.4186 |
2 | 0.8210 | 2.0054 | 0.1148 | 0.9438 | 1.0424 | 0.9118 | 1.0689 | 0.4148 |
3 | 0.8234 | 1.9939 | 0.1143 | 0.9422 | 1.0385 | 0.9060 | 1.0721 | 0.4170 |
4 | 0.8318 | 2.0053 | 0.1162 | 0.9480 | 1.0450 | 0.9065 | 1.0671 | 0.4182 |
5 | 0.8228 | 2.0107 | 0.1149 | 0.9344 | 1.0472 | 0.9042 | 1.0776 | 0.4228 |
6 | 0.8132 | 2.0178 | 0.1142 | 0.9358 | 1.0359 | 0.9052 | 1.0734 | 0.4139 |
7 | 0.8213 | 2.0285 | 0.1154 | 0.9267 | 1.0384 | 0.9032 | 1.0816 | 0.4202 |
8 | 0.8209 | 2.0147 | 0.1144 | 0.9386 | 1.0452 | 0.9069 | 1.0734 | 0.4169 |
9 | 0.8168 | 2.0243 | 0.1140 | 0.9323 | 1.0457 | 0.9001 | 1.0682 | 0.4192 |
10 | 0.8232 | 2.0111 | 0.1145 | 0.9433 | 1.0488 | 0.9096 | 1.0696 | 0.4184 |
Average | 0.8216 | 2.0139 | 0.1147 | 0.9388 | 1.0435 | 0.9059 | 1.0731 | 0.4180 |
Standard | 0.0048 | 0.0110 | 0.0007 | 0.0065 | 0.0044 | 0.0032 | 0.0049 | 0.0026 |
RSD (%) | 0.58 | 0.55 | 0.58 | 0.69 | 0.43 | 0.36 | 0.46 | 0.61 |
f MD | 1.1055 | 1.1140 | 1.0406 | 1.0167 | 1.0315 | 1.0112 | 1.0043 | 1.0022 |
MD(%) | 10.58 | 5.71 | 2.03 | 1.67 | 1.58 | 0.56 | 0.22 | 0.11 |
Elan 5000 | 25Mg/26Mg | 63Cu/65Cu | 86Sr/88Sr | 95Mo/96Mo | 107Ag/109Ag | 151Eu/153Eu | 184W/186W | 203Tl/205Tl |
---|---|---|---|---|---|---|---|---|
a 10 ppb solution for Platform and 100 ppb for Elan 5000. b Collision gas settings: He = 5.0 ml min−1, H2 = 2.8 ml min−1. c From IUPAC (1997). | ||||||||
1 | 0.8669 | 2.0332 | 0.1139 | 0.9466 | 1.0139 | 0.8772 | 1.0640 | 0.4265 |
2 | 0.8412 | 2.0388 | 0.1211 | 0.9179 | 1.0449 | 0.8896 | 1.0256 | 0.3972 |
3 | 0.9357 | 1.9527 | 0.1155 | 0.9049 | 1.0361 | 0.8720 | 1.0696 | 0.4042 |
4 | 0.9342 | 2.1382 | 0.1137 | 0.9556 | 1.0524 | 0.8567 | 1.0466 | 0.3992 |
5 | 0.8406 | 1.9774 | 0.1161 | 0.9431 | 1.0174 | 0.8959 | 1.0640 | 0.4224 |
6 | 0.8519 | 2.0064 | 0.1127 | 0.9366 | 1.0497 | 0.8797 | 1.0576 | 0.4103 |
7 | 0.8621 | 2.0363 | 0.1100 | 0.9210 | 0.9914 | 0.8733 | 1.0986 | 0.4105 |
8 | 0.8349 | 2.1430 | 0.1131 | 0.9266 | 1.0418 | 0.8875 | 1.0777 | 0.4048 |
9 | 0.8462 | 1.9467 | 0.1041 | 0.8725 | 1.0149 | 0.8961 | 1.0609 | 0.4226 |
10 | 0.8216 | 2.0532 | 0.1139 | 0.9430 | 1.0362 | 0.8789 | 1.0538 | 0.4303 |
Average | 0.8635 | 2.0326 | 0.1134 | 0.9268 | 1.0299 | 0.8807 | 1.0618 | 0.4128 |
Standard | 0.0398 | 0.0677 | 0.0043 | 0.0245 | 0.0196 | 0.0121 | 0.0192 | 0.0118 |
RSD | 4.61 | 3.33 | 3.83 | 2.64 | 1.90 | 1.38 | 1.80 | 2.87 |
f MD | 1.0518 | 1.1038 | 1.0528 | 1.0298 | 1.0452 | 1.0402 | 1.0150 | 1.0148 |
MD(%) | 5.19 | 5.20 | 2.65 | 2.99 | 2.26 | 2.01 | 0.75 | 0.74 |
Natural ratioc | 0.9083 | 2.2436 | 0.1194 | 0.9544 | 1.0764 | 0.9161 | 1.0777 | 0.4189 |
For the Platform ICP-MS, the relative standard deviation (RSD) of measured isotope ratios varies between 0.4 and 0.7%. In all isotope ratios fMD is >1, consistent with light isotopes being transmitted less efficiently in ICP-MS.19 The mass discrimination per atomic mass unit (MD%) varies from 10% for 25Mg/26Mg to 0.11% for 203Tl/205Tl, similar to previously determined MD% values by the same type of instrument.2
For the Elan 5000 the RSD of isotope ratio measurement is between 2 and 5%, significantly higher than that of the Platform, despite the elemental concentration in the analyte being higher for the Elan 5000 than for the Platform. The mass bias factor (fMD) is also >1, and the MD% varies from 5% for 25Mg/26Mg to 0.7% for 203Tl/205Tl, similar to those reported by Heumann et al.1 on an Elan 5000.
There is also a difference in MD(%) between the Elan 5000 and Platform (Table 2 and Fig. 2). Specifically, the Platform appears to have larger MD% at low mass, but smaller MD% at high mass compared with the Elan 5000. Heumann et al.1 also found differences in mass bias among three quadrupole ICP-MS instruments (Spectromass 2000, Elan 5000, and HP4500), and attributed the difference to the different instrument design.1
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Fig. 2 Comparison of mass bias per amu versus mass between the Elan 5000 and Platform ICP-MS. See text for details of measurement and instrument parameters. |
The Platform uses a “Daly-type photomultiplier” with a linearity up to 1 × 108 counts s−1. At count rates >1 × 108, a strong “after glow” will occur, and subsequent counts will not be registered, a phenomenon similar to the dead time in a continuous dynode electron multiplier. Fig. 3 shows the instrument response with varying Ba concentration measured on the Platform ICP-MS. The data indicates that the instrument response remains linear up to 2 × 106 counts s−1, corresponding to a Ba concentration of 10 ng g−1. The analyte signal on the Platform in this study was between 1 × 105 and 1 × 106. Thus, a detector “dead time” correction was considered unnecessary.10 Accordingly, detector dead time is unlikely to be the reason that the two instruments have different mass bias.
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Fig. 3 Count rate versus Ba concentration measured by the Platform, showing linear response of the detector up to 2 × 106 counts s−1. |
In all ICP-MS designs, there is a substantial column of positively charged particles behind sampler and skimmer cones; positively charged particles are mutually repelled. In this region, the light ions tend to disperse more than heavy ions, resulting in mass discrimination against light isotopes. This is termed “space charge” effect.23 More recently, Heumann et al.1 suggested that when a supersonic jet of charged particles passes through the ICP-MS interface region, light isotopes should preferably be pumped down and therefore more light isotopes leave the central beam compared with the heavier isotopes. They termed this the “nozzle separation” effect. “Nozzle separation” effect also introduces mass discrimination against light ions. However, because the “space charge” and “nozzle separation” produce mass bias in the same direction, it is difficult to distinguish the two. The “space charge” effect has been considered to have the strongest mass bias effect in ICP-MS, inducing a mass discrimination consistently against the light isotope.23
For an ICP-MS with an ion focusing lens system, such as the Elan 5000, the total bias is a combination of ion lens system and “space charge/nozzle separation” effects, and is dependent on ion mass and energy. In contrast, for the Platform ICP-MS equipped with a collision cell, the total bias is dominated by “space charge” and “nozzle separation” effect. Furthermore, the collision cell also homogenizes ion energy, reducing energy spread to <1 eV.24 Hence, the total bias in a Platform is dependent on mass, but independent of ion energy. This is, perhaps, the reason that the Elan 5000 has lower apparent mass bias at low mass, but higher apparent bias at high mass (Fig. 2), whereas the Platform appears to have mass bias that is correlated exponentially with mass (Fig. 2). Palacz recently also showed the different mass bias between a hexapole single focusing MC-ICP-MS and a double focusing MC-ICP-MS, and suggested that the ESA employed in a double focusing MC-ICP-MS may offset the bias–mass correlation.15 The exponential law has been considered a good approximation for mass bias occurring in an ICP-MS interface.25–29
Table 3 and Fig. 4 present the fMD and MD(%) of various isotope ratios measured under different gas settings. All other parameters remain the same (see Table 1). The fMD varies from 1.1 at low mass (25Mg/26Mg) to 1.002 at high mass (203Tl/204Tl). There is no significant difference in bias per amu under different gas settings, except for 25Mg/26Mg, where MD% changes from 10.5 under He = 5.0 ml min−1 and H2 = 2.8 ml min−1 to 7.7 under He = 5.0 ml min−1 and no H2 (Table 3, Fig. 4). These results suggest that change of hexapole gas settings does not significantly affect mass bias in the Platform. In contrast, changing lens setting in a conventional ICP-MS with an electrostatic ion focus lens system can induce mass bias against light or heavy isotopes depending on ion energy.14,19,20,22,23 It appears that in hexapole ICP-MS, mass bias occurs predominantly in the interface region induced by “space charge” and “nozzle separation” effects.
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Fig. 4 Mass bias per amu (MD%) under three different hexapole gas settings for the Platform. The three settings are: (□) He = 5.0 ml min−1 and H2 = 2.8 ml min−1; (△) He = 5.0 ml min−1 and H2 = 0 ml min−1; and (○) He = 2.5 ml min−1 and H2 = 2.8 ml min−1. Other parameters remain the same (see Table 1 and text for details). |
25Mg/26Mg | 63Cu/65Cu | 86Sr/88Sr | 95Mo/96Mo | 107Ag/109Ag | 151Eu/153Eu | 184W/186W | 203Tl/205Tl | |
---|---|---|---|---|---|---|---|---|
a Measured with 10 ppb multi-element solution. | ||||||||
He = 5.0 ml min−1; H2 = 2.8 ml min−1 | ||||||||
f MD | 1.1055 | 1.1140 | 1.0406 | 1.0167 | 1.0315 | 1.0112 | 1.0043 | 1.0022 |
MD(%) | 10.55 | 5.70 | 2.03 | 1.67 | 1.58 | 0.56 | 0.22 | 0.11 |
He = 5.0 ml min−1; H2 = 0 ml min−1 | ||||||||
f MD | 1.0779 | 1.0748 | 1.0203 | 1.0156 | 1.0203 | 1.0111 | 1.0054 | 1.0017 |
MD(%) | 7.79 | 3.74 | 1.02 | 1.56 | 1.01 | 0.55 | 0.27 | 0.09 |
He = 5.0 ml min−1; H2 = 2.8 ml min−1 | ||||||||
f MD | 1.0748 | 1.1104 | 1.0204 | 1.0140 | 1.0168 | 1.0047 | 1.0039 | 1.0020 |
MD(%) | 7.48 | 5.52 | 1.02 | 1.40 | 0.84 | 0.24 | 0.19 | 0.10 |
Measurement # | S1 (6 ppb) | S2 (11 ppb) | S3 (16 ppb) | ||||||
---|---|---|---|---|---|---|---|---|---|
117 cpsb | 120 cps | 117Sn/120Snc | 117 cps | 120 cps | 117Sn/120Snc | 117 cpsb | 120 cps | 117Sn/120Snc | |
a Measured with: dwell time 0.2 s; inter-channel delay 0.02 s; 1 acquisition/measurement, total time/measurement 1 min. Other parameters as in Table 1. b Counts per second. c Bias corrected ratios. See text for details. | |||||||||
1 | 287997 | 295552 | 1.0014 | 363128 | 602349 | 0.6195 | 411103 | 863655 | 0.4892 |
2 | 282433 | 291080 | 0.9971 | 352519 | 587534 | 0.6166 | 403128 | 849517 | 0.4877 |
3 | 277396 | 287252 | 0.9924 | 351191 | 584561 | 0.6174 | 405763 | 851065 | 0.4900 |
4 | 279189 | 286293 | 1.0022 | 350850 | 583329 | 0.6181 | 410988 | 861023 | 0.4889 |
5 | 273643 | 280157 | 1.0038 | 353887 | 588990 | 0.6175 | 407849 | 856734 | 0.4889 |
Average | 280132 | 288067 | 0.9994 | 354315 | 589353 | 0.6178 | 407766 | 856399 | 0.4889 |
Standard deviation | 5423 | 5733 | 0.0046 | 5071 | 7609 | 0.0011 | 3430 | 6123 | 0.0008 |
Relative standard deviation (%) | 1.94 | 1.99 | 0.46 | 1.43 | 1.29 | 0.18 | 0.84 | 0.71 | 0.17 |
Spiked ratio | 1.0103 | 0.6236 | 0.4931 | ||||||
Deviation from true ratio | −0.0109 | −0.0058 | −0.0041 |
The precision of 117Sn/120Sn ratio based on 5 measurements ranges from 0.46% in S1 to 0.17% in S3. The accuracy, measured by the difference between the bias-corrected observed ratio and the spike-adjusted, true ratio, ranges from −0.0109 in S1 to −0.0041 in S3 (Table 4). The difference between bias corrected and spiked ratios suggests that there is still a residual error in bias correction. Such residual error most likely stems from the external mass bias correction, i.e., mass bias factors were measured on a different solution and applied to samples assuming they have similar mass bias. As mass bias in ICP-MS occurs predominantly in the interface region and is dependent on sample matrix, such external correction may not fully correct the bias effect. The experiments indicated that the instrument mass bias was stable over 4 h.
Internal correction is also commonly used in isotope ratio measurement by mass spectrometry. For example, in Nd isotope ratio measurement, mass bias is determined using 146Nd/144Nd and applied to other isotope ratios (e.g., 143Nd/144Nd) in the same sample. In cases where there is no pair of stable isotope (e.g., Pb) or all isotopes are to be measured (Ca, Fe, Cu, etc.), a different element close in mass can be used. For example, Tl has been used for Pb isotopes,22 and Zn for Cu isotopes.29Table 5 presents Pb isotope measurement of the standard NBS982 by hexapole ICP-MS, using Tl for internal mass bias correction. An NBS982 solution with 20 ppb Pb was spiked with 10 ppb high purity Tl solution (Inorganic Ventures, Inc.). Mass bias factor was determined using measured 203Tl/205Tl and the IUPAC value (0.4189), and applied to Pb isotope ratios using an exponential correction law.25
Measurement # | 206Pb/204Pb | 207Pb/204Pb | 208Pb/204Pb | 207Pb/206Pb | 208Pb/206Pb | 208Pb/207Pb |
---|---|---|---|---|---|---|
a Ratios were corrected for mass bias using the 203Tl/205Tl ratio. See text for details. | ||||||
1 | 36.8380 | 17.0503 | 35.9616 | 0.4628 | 0.9910 | 2.1411 |
2 | 36.3725 | 16.8636 | 35.6922 | 0.4636 | 0.9953 | 2.1468 |
3 | 36.4566 | 17.0327 | 35.9056 | 0.4672 | 1.0013 | 2.1432 |
4 | 36.5009 | 17.0850 | 35.7566 | 0.4681 | 0.9978 | 2.1317 |
5 | 36.6340 | 17.0631 | 35.9459 | 0.4658 | 0.9953 | 2.1369 |
6 | 36.7084 | 17.0923 | 36.1047 | 0.4656 | 0.9995 | 2.1465 |
7 | 36.5952 | 17.2067 | 36.1416 | 0.4702 | 1.0018 | 2.1306 |
8 | 36.9915 | 17.2687 | 36.4923 | 0.4668 | 1.0027 | 2.1479 |
9 | 36.6806 | 17.1249 | 36.2489 | 0.4669 | 1.0023 | 2.1470 |
10 | 36.4334 | 17.0139 | 35.7641 | 0.4670 | 0.9956 | 2.1319 |
11 | 35.8323 | 16.8769 | 36.0798 | 0.4710 | 1.0069 | 2.1378 |
12 | 36.2122 | 16.9513 | 36.1913 | 0.4681 | 0.9994 | 2.1350 |
13 | 36.1443 | 16.9717 | 35.8753 | 0.4696 | 0.9926 | 2.1138 |
14 | 36.1170 | 16.9091 | 36.1563 | 0.4682 | 1.0011 | 2.1383 |
15 | 36.2262 | 16.9605 | 36.5097 | 0.4682 | 1.0078 | 2.1526 |
16 | 35.9458 | 16.7921 | 36.1034 | 0.4672 | 1.0044 | 2.1500 |
17 | 36.3160 | 17.0106 | 36.4107 | 0.4684 | 1.0026 | 2.1405 |
18 | 36.3110 | 16.9584 | 36.4819 | 0.4670 | 1.0047 | 2.1513 |
19 | 35.9790 | 16.9331 | 36.5892 | 0.4706 | 1.0170 | 2.1608 |
20 | 36.2698 | 16.9670 | 36.1680 | 0.4678 | 0.9972 | 2.1317 |
21 | 36.0520 | 16.8923 | 36.0346 | 0.4686 | 0.9977 | 2.1332 |
Average | 36.3627 | 17.0011 | 36.1245 | 0.4676 | 1.0007 | 2.1404 |
Standard deviation | 0.3053 | 0.1147 | 0.2611 | 0.0020 | 0.0058 | 0.0102 |
Relative standard deviation (%) | 0.84 | 0.67 | 0.72 | 0.43 | 0.58 | 0.48 |
Certified values | 36.7385 | 17.1595 | 36.7443 | 0.4671 | 1.0002 | 2.1413 |
Deviation from true ratios | −0.3757 | −0.1583 | −0.6199 | 0.0005 | 0.0005 | −0.0009 |
The precision of bias corrected Pb isotope ratios ranges from 0.43% to 0.84% (Table 5). The RSD% for ratios involving 204Pb is higher than other isotope ratios. This is due to: (1) lower abundance in 204Pb; and (2) background correction of Hg contribution on mass 204. The deviation from the certified value ranges from 0.0005 (0.1%) for 207Pb/206Pb to −0.6199 (1%) for 208Pb/204Pb. The accuracy is also poorer for ratios involving 204Pb. A precision of 0.1% RSD or less has been achieved on Pb isotope ratio measurement of NBS981 with 100 ng g−1 Pb on a Platform.10 Mason et al. obtained a precision of 0.03–0.05% in 34S/32S measurement of 10–50 mg L−1 solutions using a Platform with He, H2 and Xe as collision gases.11
This journal is © The Royal Society of Chemistry 2002 |