Chromium Isotopes Detection in their Ores with Minimal Errors

The industrial production and use of chromium have grown considerably during the past fi ve decades. Abundances of the chromium isotopes in terrestrial samples are identical to 0.01%. Among the dominant species of chromium, the trivalent form widely occurs in nature in chromite ores which is extremely immobilized especially in water bodies. Samples were mixtures of separated chromium isotopes and the calibration was made with the same species as those used in the measurements. The method had simplifi ed the conversion of the ores to chromyl fl uoride since the element could be readily separated as lead chromate from the leaching of chromite-sodium peroxide fusions. Isotope assay of chromyl fl uoride under certain conditions was measured and the measurements of chromium isotopic anomalies ratios and isotope abundance of the chromite ores have been assessed. These provided suffi cient quantitative mass spectrometric data, which were analyzed to calculate the abundance and the mean atomic mass of the questioned isotopes. Based on the high mass spectroscopy stability and the correction factors, the results were of good precision (incl. negligible systematic errors normally associated to inter-laboratory discrepancies) and the Cr isotopes availability (52Cr > 53Cr > 50Cr > 54Cr) was in conjunction with other classical tools such as oxygen isotopes. This paper is important for paleoecological, environmental, archeological, forensic, and nuclear researchers. Research Article Chromium Isotopes Detection in their Ores with Minimal Errors Loai Aljerf1* and Nuha AlMasri2 1Department of Basic Sciences, Faculty of Dental Medicine, Damascus University, Mazzeh Highway, Almazzeh, Damascus, Syria 2Department of Chemistry, Faculty of Medicine, Syrian Private University, Damascus, Syria *Address for Correspondence: Loai Aljerf, Department of Basic Sciences, Faculty of Dental Medicine, Damascus University, Mazzeh Highway, Almazzeh, Damascus, Syria, Tel: +963-93 34 46 993; Email: envirochrom@hotmail.com; loai.aljerf@aol.com Submitted: 21 July 2018 Approved: 03 September 2018 Published: 04 September 2018 Copyright: 2018 Aljerf L, et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


Introduction
Chromium is one of the elements on which no systematic study of the isotopic abundances has been made with regard to the geological source of the materials [1,2]. All the results of previously reported works were obtained with chemical reagents taken from the usual stores of these materials. As is evident in table 1, there is no real agreement among any of the results.
The main supply of chromium chemicals available in many countries, is from the chromite deposits [7,8]. Thus, it appears that the variation of the results listed in table 1, with the exception of those of Nowak [3], may be due to fractionation brought about in the chemical processing of the ores or by natural geological processes; or, they may be due to unknown discriminations involved in the various instruments with which the measurements were made. Table 1: Previous determinations of the abundances of the chromium isotopes, in atoms percent.

Measurement
Ion source reagent observer 1 Gas discharge Cr(CO) 6 [3] 2 Thermionic Cr metal [4] 3 Electron impact CrCl 3 [5] 4 Electron impact CrCl 3 [6] Isotope 50 4.9 4.49 4.31±0.04 4.41±0.06 With this in mind, a study was made of the abundances of the Cr isotopes in samples of chromite and other chromium bearing minerals collected from well-characterized deposits. The present paper reports the results of the initial studies dealing with the primary mineral, chromite. Descriptions are given of the precautions taken to eliminate instrumental discriminations and the development of a technique by which highly precise measurements can be made with this and the other transition elements. A report of the studies on secondary chromium minerals will be given in a subsequent publication.

Strategy:
In a previous publication [23] a preparation of chromyl luoride (CrO 2 F 2 ) by vacuum distillation from a solid-solid mixture of chromium trioxide (CrO 3 ), and cobalt (iii) luoride (CoF 3 ), was described. The high vapor pressure of chromyl luoride and its convenient preparation made this compound ideal as a means of introducing the element into the mass spectrometer (MS) as a gas, which affords the experimenter the advantage of extremely stable sample conditions. To avoid any possibility of introducing errors due to the possible isotopic effects on the fragmentation of this compound, CrO 2 F 2 + ions were chosen as the ion currents on which to base each abundance determination [24]. The anisotopic nature of luorine simpli ied the handling of the MS data.
Analytical protocols: Since considerable chemical manipulation is required to convert chromite ores (Fe, Mg)Cr 2 O 4 to CrO 3 , a simpler procedure for preparing CrO 2 F 2 was sought. In the course of this search, experiments revealed that the CrO 3 used in the above method could be replaced by lead chromate (PbCrO 4 ). This greatly simpli ied the conversion of the ores to chromyl luoride, since the element could be readily separated as lead chromate from the leaching of chromite (Cr 2 O 4 2-)-sodium peroxide (Na 2 O 2 ) fusions. To accomplish this, the ores were inely pulverized, fused with an excess of sodium peroxide, and leached with distilled water. The leaching was neutralized with glacial acetic acid, iltered to remove the silica, and then solid lead acetate was added to precipitate the chromium as lead chromate. Excess reagents were used throughout to ensure complete separation of the chromium, thus avoiding any possible isotopic fractionation effects during chemical treatment. The lead chromate was iltered, washed, air dried, and then ignited at from 400 to 500°C [25,26] to remove any residual organic matter as well as to dry the material. The ignited lead chromate was immediately transferred into a dry box through the route revealed in igure 1, where it was stored until used.
For conversion to chromyl luoride the lead chromate was inely ground and intimately mixed with an excess of CoF 3 . The mixture was placed in a copper reaction tube. Bronze turnings were placed above the charge to prevent blow-over of the reactants during subsequent evacuation of the reaction tube.
The reaction tube was removed from the dry box and attached to the remainder of the sample preparation apparatus. The assembled sample preparation apparatus consisted of a 0.25 in. copper reaction tube which was about 6 in. long, a 0.25 in. copper U-tube, and a packless, bellows-type valve (Hoke M482). The components were joined by means of standard lare ittings, and the assembled apparatus was attached to the inlet system of the mass spectrometer through a stainless steel itting (Hoke S24). An aluminum gasket was used to make the latter connection vacuum tight.

MS prepreparation method:
A furnace was placed about the reaction tube and the temperature was raised to 375°C. This initial heat served to outgas the reactants and reaction tube without any loss of chromyl luoride since the luorination reaction does not proceed at this temperature. The outgassing was accompanied by an increase of pressure which was monitored by the inlet vacuum system Pirani gauge. When the pressure had returned to near normal, a dry-ice-trichloroethylene (C 2 HCl 3 ) slush was placed about the U-tube to trap the chromyl luoride, and the furnace temperature was increased to 550°C. Again the pressure in the inlet vacuum system increased due to oxygen produced during the luorination of the lead chromate. When the pressure had returned to normal, the reaction was considered complete. The furnace was removed, and the chromyl luoride was then treated like any other condensable gas. Samples of this compound could be stored in the copper container for several days without any apparent deterioration.

MS optimization method:
The mass spectrometer employed in the study was a 180° instrument of 5 in. radius (Consolidated Electrodynamics Corporation, Model 21-220, modi ied in our laboratory). A 200 μA current of 70 V electrons ionized the chromyl luoride at an analyzer pressure of less than 5 x 10 -7 mm Hg, indicated by an ionization gauge placed within 12 in. of the ion source. The vacuum system was arranged to give differential pumping. Preliminary experiments to determine the optimum instrumental operating conditions were performed using a reference sample of chromyl luoride prepared from lead chromate, which had been prepared from reagent-grade chromic bromide (Br 3 Cr). This chromyl luoride was assayed over the complete mass range of the instrument and found to be 99+ percent pure with the principal impurity being HF.

MS optimization outcomes
Preliminary experiments showed that the sensitivity of the MS for the CrO 2 F 2 + ions was constant in the ion accelerating voltage range of from 1600 to 1700 V. This meant that the isotopic CrO 2 F 2 + ion currents could be voltage scanned in that range, which was more convenient than magnetic scanning. It was found that the most reproducible results were achieved when the CrO 2 F 2 + ions were manually scanned to obtain the desired mass spectrograms. Ten spectrograms were made by scanning the mass region 120-124 in alternate directions. Consecutive spectrograms were averaged, which resulted in nine sets of data for each assay of any particular chromyl luoride sample.

Oxygen isotopes intervention and their correction
The oxygen isotopes could not be ignored in any isotope determination using CrO 2 F 2 + ions, since ion currents at masses 121, 125 and 126 were observed and their magnitudes agreed with those calculated for ions containing 17 O and 18 O.
Since direct measurement of the oxygen isotopes in CrO 2 F 2 was not possible, an indirect measurement was made. Given that the production of chromyl luoride involved the luorination of PbCrO 4 by means of CoF 3 , some means of examining directly the abundance of the oxygen isotopes in this chromate was sought. The most direct procedure developed involved the reaction of lead chromate with potassium luobromite, KBrF 4 [26,27].
The liberated molecular oxygen was introduced directly into a dual collector, 60°, 6 in. radius mass spectrometer and the 34 O 2 / 32 O 2 and 33 O 2 / 32 O 2 ratios measured. Atmospheric oxygen, for which Nier [4] and Young et al. [28] gave an absolute ratio for 34 O 2 / 32 O 2 of 0.00409, was used as a comparison standard in order to place all measurements on an absolute basis.
All of the PbCrO 4 samples prepared from the ores listed in table 2 and the distilled water were examined. The results of the KBrF 4 luorinations along with that of CoF 3 are summarized in table 3.
From these results, it appeared that the oxygen in the PbCrO 4 was derived primarily from the distilled water used to leach the sodium peroxide-chromite fusions and to wash the precipitated lead chromate. To ensure that, fractionation of the oxygen isotopes did not occur during the luorinations. One experiment was performed in which four different samples of O 2 were collected at various times during the reaction. Assay of this gas gave identical results with those already cited.
An exchange experiment designed to con irm the nature of the exchange between CrO 4 2and H 2 O 8 was performed. Vacuum-dried sodium chromate was dissolved in water enriched in 18 O to 5 atom percent. As soon as complete solution was achieved, the water was distilled off and crystalline Na 2 CrO 4 recovered.
After being dried under vacuum at 70°C for 7 hr, this material was luorinated with KBrF 4 . (In this case the luorination proceeded smoothly at 200°C.) Measurement of  [29] and of Mills [30] whose measurements of the oxygen isotopes were based on the density of the water employed. On the basis of these results, it was concluded that correction of the observed CrO 2 F 2 + ion currents for the oxygen isotopes had to be based upon the observed abundances in the distilled water. Accordingly, corrections of the observed ion currents were made on the basis of 16 O/ 17 O = 2740, and 16 O/ 18 O = 508.9 for the oxygen isotopes in the distilled water and the assumption that the oxygen in chromyl luoride was a random collection of oxygen atoms.
Thus, the calculated abundances of the 16 Where M is the observed ion current, A is the relative abundance, and the superscript is the isotopic mass number. The factors 0.00073 and 0.00393 are the ratios 16 These were used in all the subsequent treatment of the data.

Experimental pressure effect
Since there was small probability of obtaining equal ion currents for all eighteen samples of chromyl luoride, the reference sample of chromyl luoride was assayed at one-half, at normal, and at twice the normal operating pressure to determine whether or not the assays were pressure dependent. The results of this experiment are shown in table 4. They indicate that no pressure effect on the assays occurred and that the electrometer tube input resistor was truly ohmic in its behavior.

Comparative inter-laboratory study
Comparison of the assays of the chromyl luoride prepared from the eighteen chromite ores depended on the stability of the instrument during the period required to assay the materials and the absence of any fractionation effects during the production of the CrO 2 F 2 and its subsequent volatilization into the inlet system of the  5 in columns A, B and C, respectively. These results indicated that comparison of the assays of the eighteen ore samples could be made with no concern about day-to-day instrumental drift or chemical fractionation effects.

Analytical challenges
Another potential error involved the exact nature of gas low in the mass spectrometer. The sample inlet system of the instrument was equipped with a viscous leak which consisted of a 5 in. length of copper capillary tubing of 0.005 in. internal diameter terminated by an adjustable constriction. Differential pumping in the ion source indicated the probability that mass discrimination due to effusive low was present. However, it has been shown that the nature of the correction factor that should be applied to isotope measurements for this and other instrumental discriminations is dependent upon the detailed physical conditions existing in a particular mass spectrometer [31,32]. It is therefore most reliable to determine these correction factors by calibration of the instrument by means of mixtures of separated isotopes.
Mixtures of separated chromium isotopes were irst considered for this calibration. Thus the calibration would have been made with the same species as those used in the measurements. Attempts to use CrO 2 F 2 prepared from mixtures of these materials were fruitless due to an inability to assay the separated isotopic materials satisfactorily with the instrument at hand. This was principally the result of an exchange between the chromium of the chromyl luoride and the chromium of the Nichrome V (9-21% Cr, 2.5% Mn (max), 1.0% Fe (max), 0.75-1.6% Si, 0.15% C (max), balance Ni) used in the construction of the ion source. This exchange resulted in assay errors whose magnitude depended on the difference of the sample composition from that of normal chromium.

Overcoming solutions
For samples whose isotopic abundances were normal, errors from this effect were computed to be negligible. The dif iculty might have been circumvented by constructing a new source of some other material or gold plating the existing source. Both of these possibilities were considered and the latter was seriously contemplated. This will be done for the extension of the work to secondary chromium materials where the preliminary experiments indicate differences in the isotopic constitution.

Suggested corrective measures:
A compromise calibration for the present phase of the work was accomplished with a mixture of separated nitrogen isotopes which was carefully prepared and assayed according to the method described by Junk and Svec [33] to produce absolute abundance values. This mixture was then assayed with the mass spectrometer employed throughout the chromium tests under conditions in which the range of ion acceleration voltage was as nearly identical as possible with those under which the chromium assays were made. The value obtained here for the nitrogen isotope mixtures was 42.29 atom percent 15 N compared to 42.18 obtained by Junk and Svec [33]. However, the indicated agreement to one part in 423 was obtained only when the data were corrected for fractionation due to effusive gas low from the ion source. A correction factor equal to the square root of the inverse ratio of the masses involved had to be applied. Because the precision obtained in this calibration approached that obtained with the chromium measurements, it was safe to assume that all of the mass discrimination due to gas low could be considered to be the result of effusion of the sample material from the ion source.
After these preliminary experiments, the samples of chromyl luoride prepared from the eighteen chromite ores were assayed along with the reference sample during a 2 week period. The observed data for the ores were irst corrected for gas low discrimination and then for the oxygen isotope effect. The results are listed in tables 6,7. Table 6 gives the individual assays of the eighteen samples and the average assay. Table 7 gives the standard deviation associated with each individual assay and the average of the eighteen individual standard deviations.
Comparison of the results listed in tables 6,7, shows that the standard deviation from the average assay of the eighteen ores was nearly identical to the average standard deviation associated with each individual assay. It must therefore be concluded that there is no variation in the isotopic composition of chromium with respect to the source of chromite ore. In order to examine the statistics of the determinations further, every individual datum from each of the assays was grouped into a composite of 170 items for each isotope abundance.
The standard deviation of the composite data (Table 8) is larger but is more reliable than that of the average assay of the eighteen ores, because in the calculation of the latter, the standard deviation for each individual assay was ignored.
Despite the good precision of the data above, it was necessary to consider whether or not the values obtained represented the absolute abundances of the chromium isotopes and were not merely relative values. The results of the previously mentioned experiments, which were designed to measure and correct for discrimination effects due to gas low, voltage scanning, variation in gas pressure, non-ohmic electrometer input resistors, impure sample gases, and instrumental drift, led the writers to assume that the resulting measurements were absolute.

Conclusion
Using the available literature, chromite ores gathered from various deposits throughout the world were assayed for the abundances of the chromium isotopes. However, no differences in the relative abundances were observed.
Upon application of criteria to determine the absoluteness of the measured abundances of the isotopes of chromium, the only discrimination observed during the measurements was that due to effusive gas low out of the ionizing region of the ion source. The magnitude of this discrimination was determined from measurements of a mixture of separated nitrogen isotopes and corrections were made for it. On this basis, the abundances of the chromium isotopes reported in this communication can be considered absolute. Since chemical chromium has a common source, it is conservative to accept these absolute abundance values for the isotopes in reagent chromium. At the ±3σ level (99.7% con idence level), the recommended values in atoms percent are: Cr 50 = 4.352 ± 0.024 Cr 52 = 83.764 ± 0.036 Cr 53 = 9.509 ± 0.027 Cr 54 = 2.375 ± 0.018 Table 7: Standard deviation associated with each abundance determination of the chromite ores shown in Table 6 (Standard deviation x 1000).