Patent Application
20140273248
At present, the Ca isotope technique is the only method of measuring changes in bone mineral balance (BMB). This technique was originally invented for measuring changes in BMB induced by microgravity and for clinical applications relating to osteoporosis.
Variations in natural Ca isotope composition are a highly sensitive, non-invasive biomarker of bone mineral balance (BMB), providing information on net changes in bone mass that cannot be derived from conventional biochemical markers of bone formation and resorption, on time scales far shorter than those accessible to X-ray techniques like DEXA. The Ca isotopic method has great promise as a prognostic and ultimately a diagnostic biomarker in detecting, monitoring and tailoring effective individual treatments for diseases that involve abnormal loss or gain in skeletal mass, including many cancers.
However, progress in developing the Ca isotope biomarker is impeded by the analytical techniques currently used for high-precision isotope ratio measurements on the most efficient system presently available, multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). These techniques were developed mainly for geological applications, which typically require relatively few analyses of inorganic samples. Such techniques have not been optimized for measuring the large number of samples required for application to clinical research and practice. This difficulty is compounded by the fact that high concentrations of organic material in biological samples (e.g., blood and urine) makes processing them much more difficult and time consuming than the processing of inorganic samples, like rocks.
Widespread application of the Ca isotope biomarker thus requires the development of sample preparation and analysis techniques suited to the type and number of samples that will be generated in clinical settings.
The embodiments described herein relate to the application of the Ca isotope method to the diagnosing and monitoring the progression of cancers that affect bone. Further embodiments relate to evaluating treatments for cancer that may have an affect on bone density.
These and other aspects of the invention will be apparent upon reference to the following detailed description and figures. To that end, any patent and other documents cited herein are hereby incorporated by reference in their entirety.
Among other embodiments, two innovations described herein include: validation of an automated system of sample preparation, and optimization of analytical techniques on state of the art MC-ICP-MS instrumentation. Successful implementation of these innovations will increase sample throughput. Thus optimized, a MC-ICP-MS will be fast and efficient enough to support both advanced clinical research and application of the Ca isotope biomarker to clinical practice.
Embodiments described herein relate to the idea that cancer originating in or metastasizing to bone may change the bone mineral balance (BMB) of the skeleton, causing either a net gain or a net loss in bone mass. In addition, treatments for cancer may also have the adverse side effect of causing bone loss. Also, treatments are sometimes aimed at slowing bone loss, but there is no good way to monitor effectiveness. For these reasons there is a clinical need for a method of measuring short-term changes in BMB of cancer patients.
Thus, in one aspect, embodiments of the invention relate to the application of the Ca isotope method to cancer detection and treatment modification. The Ca isotope method is fully described in two publications:
Morgan, J. L., Gordon, G. W., Arrua, R. C., Skulan, J. L., Anbar, A. D., and Bullen, T. D., 2011. High-precision measurement of variations in calcium isotope ratios in urine by multiple collector inductively coupled plasma mass spectrometry. Analytical Chemistry. 83:6956-62.
Morgan, J. L., Skulan, J. L., Gordon, G. W., Romaniello, S. J., Smith, S. M., and Anbar, A. D., 2012. Rapidly assessing changes in bone mineral balance using natural stable calcium isotopes. Proceedings of the National Academy of Sciences, 109:9989-9994, which are hereby incorporated by reference. The method described in these references was used to generate the data of
The applicability of the Ca isotope method to cancer stems from the fact that some cancers affect the skeleton in ways that alter bone mineral balance (BMB), producing detectable changes in the Ca isotope composition of blood, urine and other biological materials. See, for example,
Embodiments also relate to the clinical application of the Ca isotope method to cancers where the onset to skeletal involvement marks an important transition in the progress of the disease, the early detection of which may affect both treatment options and prognosis.
Specifically, these embodiments cover:
1. The detection of osteolytic bone lesions that often accompany the transition from monoclonal gammopathy of undetermined significance (MGUS) to multiple myeloma. Osteolytic bone lesions cause a negative shift in BMB that has been detected through Ca isotope analysis of blood.
2. The detection of the metastasis of breast and prostate cancer to bone. This metastasis may involve osteolytic or osteoblastic bone lesions (osteoblastic in prostate cancer, primarily osteolytic in breast cancer). The former leads to bone loss and a negative shift in BMB. The latter may lead to bone gain through hyperostosis, and a positive shift in BMB. Both changes can be detected through Ca isotope analysis of blood, urine or other tissues.
3. The detection of osteolytic or osteoblastic lesions resulting from the metastasis of other cancers to bone. In addition, embodiments cover the application of the Ca isotope method to monitoring changes in bone mineral balance in cancer patients treated with aromatase inhibitors, a known cause of osteoporosis.
A tracerless Ca isotope biomarker allows for the early diagnosis of cancer metastasis to bone, and for detection of osteoporosis caused by aromatase inhibitors. The biomarker is non-invasive and does not require X-rays. There currently are no other technologies that allow for rapid detection of changes in bone mineral balance. Because the technique involves no radiological or other risks, it may be used to continuously monitor patients for bone lesions indicating cancer metastasis or bone loss resulting from cancer treatment. Moreover, quantitatively, the onset of a bone lesion will lead to a larger, faster change than is typical in osteoporosis. In other words, the two would be distinguished by the expected pattern of change in Ca isotopes over time, with metastatic bone cancer being marked by a decrease in BMB over a relatively short time, and age-related osteoporosis causing a slow drop in BMB over a long time.
Natural calcium is a mixture of six isotopes (masses 40, 42, 43, 44, 46, and 48). Calcium isotope compositions conventionally are expressed as δ44Ca ‰, which is the fractional difference, in parts per thousand (‰) between the 44Ca/40Ca ratio of a sample and a laboratory reference material, so that a sample with a δ44Ca of 0‰ has the same isotopic composition as the reference material, while δ44Ca values above and below 0‰ are isotopically heavier and lighter than the reference material, respectively.
Although calcium serves a great many biological functions in the human body, the calcium flux into and out of the skeleton so great that associated fractionation largely controls the calcium isotope composition of soft tissues.
Bone selectively incorporates isotopically light calcium as it mineralizes. As a result δ44Ca of bone calcium is lower than that of soft tissue and of the average δ44Ca of diet. The calcium isotope “fractionation factor” during bone mineralization, which is the instantaneous isotopic difference between newly formed mineral and the soft tissue calcium from which it precipitates, has not been directly measured but is estimated at about −1.3‰.
Calcium isotopes are not fractionated by bone resorption, because osteoclasts dissolve bone mineral in bulk. Isotope fractionation is possible during mineral formation because calcium ions can freely move and compete for sites in growing crystals. However, once incorporated into bone mineral, Ca atoms are locked in a crystal lattice; whether or not they are dissolved depends only on their position in the crystal, and is not affected their mass. Hence calcium returned to soft tissue by bone resorption will have the same isotopic composition as skeletal calcium.
Because calcium isotopes are fractionated by about −1.3‰ during bone formation but are not fractionated during bone dissolution, the soft tissue calcium pool from which bone mineral precipitates is about 1.3‰ higher than bone calcium. In addition, mass balance dictates that when the mass of calcium in the skeleton is constant, the mass and isotope composition of calcium entering and leaving the body will be equal. Thus at ‘steady state’ (where rates of bone formation and resorption are equal) soft tissue and dietary δ44Ca will be equal. Increase in skeletal mass will cause positive excursions in soft tissue δ44Ca, while a decrease in mass will cause negative excursions. These excursions can be detected by mass spectrometric analysis of blood or urine.
The time required for soft tissue δ44Ca to respond to changes in bone mineral balance has not been determined. In the only study of calcium isotopes in humans that has been conducted to date, dramatic changes in urinary δ44Ca were detected in the first samples taken, four weeks after the start of bed rest. But given the short residence times of calcium in soft tissue compartments such changes probably occur in far less than four weeks. It is likely that changes in bone mineral balance will produce detectable shifts in soft tissue δ44Ca on a timescale of days or even hours.
The Ca isotope biomarker employs concepts and techniques developed for geological and environmental science applications that rarely or never have been applied to medicine. Widespread clinical application of the Ca isotope biomarker requires the development of sample preparation and analysis techniques suited to the type and number of samples that will be generated in clinical settings.
Ca isotopes and cancer. A preliminary study conducted with the Mayo Clinic, Scottsdale, δ44/42Ca analysis of blood from 19 patients showed that blood δ44/42Ca was significantly lower in patients with multiple myeloma than in patients with monoclonal gammopathy of undetermined significance (MGUS), an asymptomatic precursor to multiple myeloma (
We expect such results will be possible with other cancers as well because the Ca isotope technique is sensitive enough to detect changes in bone mineral balance likely to be caused by cancerous bone lesions in general. The mathematical model allows changes in soft tissue δ44/42Ca to be quantitatively related to BMB. The drop in urinary δ44/42Ca observed in both of the bed rest studies translates to a net bone loss rate of about 4±1.2%/year, which is consistent with bone loss rates calculated from DEXA measurements. Given current instrumental precision, the lower limit of bone loss detectable by the Ca isotope method is about 2%/year, about four times smaller than the rate of bone loss observed or inferred in multiple myeloma and metastatic breast cancer.
The potential impact on patient care is profound because the Ca isotope technique reveals changes in bone mineral balance at least ten times faster than DEXA, the only other method by which BMB currently can be directly measured. Bone loss is reflected by changes in urinary δ44/42Ca within ten days of the start of bed rest, and after an even shorter interval after the actual start of significant bed-rest induced bone loss, whereas 90 days or more would be required for a rate of bone loss of 4%/year to cause a decrease in bone loss detectable by DEXA.
The relationship between δ44/42Ca and BMB is well enough understood and documented to permit immediate clinical application of the method to situations where baseline measurements are available, and to merit clinical research with large sample suites to validate more widespread applications. The chief obstacle to such work is not theoretical, but technical. Ca isotope analysis of blood and urine currently demands complex multi-stage sample preparation performed by a skilled technician. While superficially similar work is regularly done in clinical laboratories, preparation of biological samples for isotopic analysis requires specialized facilities and expertise that are neither widely available in nor well suited to hospitals and clinics. In particular, the automation and streamlining of sample analysis described below should allow us to have sample turnaround times that will make Ca isotope analysis an important new tool for oncologists.
Isotopic analysis by any method benefits from or requires complete recovery and a relatively pure sample of the element of interest. Complete recovery is necessary because it is essential that the measured isotopic composition be the same as that of the starting material, which cannot be assumed if isotopes of the element are fractionated during sample processing and some of the element to be measured is lost during sample processing. Standard ion exchange chromatography induces isotopic variations larger than the natural variations we wish to measure.
Additionally, most isotope analytical techniques, including the multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) that we currently employ for Ca isotope analysis, require extremely high purity. Impurities complicate the mass bias effects during ion transmission into the mass spectrometer that are typically an order of magnitude larger than the natural mass fractionation. To this end, the sample solution has to be as similar as possible to the inorganic standard, since small variations in the relative transmission efficiency between samples and standards are easily on the scale of the natural variations of interest. Interferences, including doubly charged ions and polyatomic ions, are one complication, but matrix effects, including alkali metal concentrations, acid molarity and organics derived from the ion exchange resin, also can cause analytical artifacts that degrade the accuracy and precision of results.
The procedures to deal with these problems have been worked out. Quantitative recovery of high purity Ca is relatively straightforward when dealing with inorganic materials with high Ca concentration such as Ca carbonate or Ca phosphate. A sample is dissolved in acid and Ca isolated using ion exchange chromatography. Modifications or repetitions of this process may be necessary, but achieving required Ca purity usually is not difficult, although it may be time consuming. Purifying Ca from biological materials such as urine and blood is a much more daunting task.
Ca has a high affinity for organic materials (proteins, cell residue etc.—referred to here as “organics”) that are abundant in such samples. These organics must be degraded prior to any other purification step. Removal of organics is a laborious process generally requiring a combination of ashing, microwave digestion and multiple rounds of acid digestion. Sufficient digestion to measure Ca concentrations is insufficient for measuring isotope ratios precisely. Only after organics have been efficiently destroyed is a sample ready for ion chromatography. Two different ion exchange columns are necessary to separate different elements, and a secondary round of acid digestion is necessary to destroy organic residues from the pre-cleaned resins. Determination of quantitative recovery is achieved by measuring the Ca concentration of an aliquot of the sample both prior to and after sample purification.
However, currently all these steps in sample preparation are done manually by highly trained personnel, a process that takes multiple days and adds enormously to the cost and time required for Ca isotope analysis. It also produces unacceptable slow sample turnaround time for routine clinical application. As the time required to process samples is dictated by the chemistry of the samples and the purity requirements of the analytical techniques, it probably cannot be reduced substantially. However, automation of the sample preparation process would greatly increase sample throughput by eliminating the need for a lab technician, allowing 24-hour sample processing, and allowing the simultaneous parallel processing of many samples.
Increased efficiency also could be realized by refining mass spectroscopic techniques. As previously explained, Ca isotope ratios are measured relative to a standard reference material, which maximizes precision and assures inter-lab reproducibility. In theory, a standard would need to be analyzed only once, and the results of all sample analysis compared to that single measure. In practice, because subtle changes in the operating conditions of MC-ICP-MS instruments affect beam stability and cause drift in peak locations, bracketing every sample with a standard before and after is standard practice. Frequency of standard analysis has a large effect on the speed of sample throughput, because time spent analyzing standards is time that cannot be spent analyzing samples. Newer-generation MC-ICP-MS instruments offer more stable operating conditions and better baseline measurements, permitting less frequent standard measurements. In particular, ions from the 40Ar or 40Ca ion beams can be deflected in older model instruments that lack baffles in the flight tube—resulting in variable and non-zero baselines.
Optimizing sample throughput with MC-ICP-MS can be achieved through improvements in sample processing, and by developing analytical techniques that take advantage of recent technical improvements incorporated into new MC-ICP-MS instruments.
Sample processing. We have validated an existing automated ion exchange system from Elemental Scientific, Inc (Prepfast) for sample purification of U isotopes (
This system can be adapted for Ca isotope purification, and modified so that all four steps in sample preparation (sample digestion, purification, recovery determination, and preparation for analysis) can be automated into the same system.
Sample digestion: Our initial sample types, serum and urine, are high in organic matter, including proteins that bind calcium. Our sample purification technique relies on ion exchange chemistry and protein-bound calcium is inefficiently recovered due to poor and variable distribution coefficients. This inefficient recovery can induce isotopic fractionations on the scale of the isotopic variation signals we want to look at. Hence, we need an efficient and highly effective technique for organic degradation. Based on preliminary experiments, high intensity short-wave UV radiation offers the most promise as an in-line system for eliminating organic contaminants prior to sample purification. Samples are loaded into centrifuge tubes in one rack of the Prepfast; while the previous sample was being purified, the following sample would be transferred by a valve in the Prepfast to a quartz tube and subjected to a 1400 W source of UV radiation, with a maximum at 240 nm. This requires an additional multi-port valve to the Prepfast to accommodate the additional digestion location.
Sample purification: Our existing ion exchange chromatography protocol requires several days of active participation by a trained technician, and permits only limited reuse of the ion exchange resin.
Our existing purification techniques utilizes a Biorad AG1X-12 ion exchange resin, followed by an Eichrom Sr-specific resin for separation of Ca and Sr. Our existing purification protocol can use the Biorad resin a maximum of five times before the resin no longer performs adequately. In part because of the required use of concentrated HBr to efficiently separate K from Ca. Ultrapure HBr is expensive, and releases bromine gas, a hazard to lab personnel. The Eichrom resin is not reused because small amounts of elemental carryover can cause significant problems. However, recent technological developments including development of the TODGA ion exchange resin shows significantly better distribution coefficients for Ca.
A significant advantage of the Prepfast system is it can be hooked up in-line to an ICP-MS. The online elution measurements for concentrations using a quadrupole ICP-MS dramatically reduces the development and validation time for a new ion exchange protocols (
Sample recovery determination: In order to ensure that analytical artifacts do not compromise our signal, we must recover over 90% of the Ca and reduce the Sr/Ca ratio to >0.0001, and increase the Ca/(Σalkali metal) to 2. These conditions must be confirmed for all samples, which requires that an aliquot of each purified sample be set aside for elemental concentration measurement. This requirement can be met using the Prepfast system, by loading and automatically diluting a sample aliquot both before and after chemistry. High precision syringe pumps can accurately and repeatedly dispense small volumes. In addition, up to five reagents can be dispensed into four locations. Frequent analysis of in-house standards and spot checks of chemical recovery could be sufficient to ensure quantitative recovery.
Sample preparation for analytical measurement. The Prepfast system can evaporate samples and reconstitute them in acid for MC-ICP-MS measurement. If trace organics from the ion exchange resin are an issue, a second short-wave UV system could be installed for post-chemistry digestion.
Analytical improvements. Analytical efficiency can be improved by reducing the number of replicate measurements of samples and standards required to obtain good sample data. Such reductions can be achieved by improvements in sequence design and baseline measurements that are possible when new MS-ICP-MS instrumentation is combined with the improvements in sample preparation previously discussed.
Improved sequence design. Current techniques use the classic standard-sample-standard measurement technique during MC-ICP-MS analysis. Each sample requires a 60-second uptake time and 60-second wash time. Each analysis requires nearly nine minutes, and each sample is analyzed between three and fifteen times to get acceptable precision. Every five samples, one to two additional secondary standards are analyzed. In addition, each day a series of standards of varying concentration are analyzed, to evaluate the acceptable range of standard-sample concentration matching required for data of adequate quality. This means that only about 40% of the analyses are of actual samples, and nearly 30% of the analysis time is spent either waiting for sample uptake or washout to occur. Hence, less than 30% of the instrument time is spent analyzing samples.
Modifications of the instrumental sequence files may be performed to significantly improve the proportion of time in sample analysis, without sacrificing data quality. The first modification is to eliminate the washout time by using the uptake time of the following sample to rinse out the previous sample, which could produce a 15% improvement in sample throughput. The second modification is to evaluate running two or even three samples bracketed by standards, rather than just one. If the instrument is sufficiently stable, this may still produce acceptable data while potentially doubling sample throughput. Having consistent sample types and residual matrix will be essential to minimize variability in the instrumental mass bias. Hence, this improvement will feed off the improvements in the chemical preparation for consistent sample purity.
The specific sequence of steps we propose for automated preparation of Ca samples in a four-rack PrepFast instrument are as follows:
Baseline measurements. Our existing instrument, a Thermo Neptune MC-ICPMS, has been a reliable platform for developing and validating our measurement techniques. However, newer models of this instrument have substantial improvements that would make the measurements easier, more precise and far faster.
Better sensitivity is achieved through the addition of a high-performance scroll pump in place of the rotary pump on our existing model. This scroll pump has better vacuum performance in the front end, enabling better transmission of analyte ions and less dispersion by residual gas pressure.
Because we use a flow rate of 15 L/minute of argon gas to create and maintain the plasma that serves as our ion source, the ion beam at mass ˜40 (40Ar and 40Ca) is many orders of magnitude larger than our sample. In a straight ion flight tube, this large ion beam can be deflected and reflected, entering the Faraday cups along with the intended sample ion beams. The intensity of the deflected ion beam varies across our nine-detector array, so that even very small differences in the physical position of the detectors cause large differences in intensity of the deflected ion beam reaching each cup (
We anticipate that far fewer replicates will be required to produce our current precision, given an instrument with installed baffles. For other metal mass-dependent isotope systems, typical samples are measured in triplicate with a precision of 1 pptt or better. Achieving this efficiency with Ca would more than double our sample throughput.
Several modifications to the Prepfast ion exchange chromatography system are required to implement our improvements to the Ca isotope method for clinical samples. These include: 1) addition of another multi-port valve to incorporate the quartz tube needed for sample digestion 2) integration of short-wave UV radiation source for sample digestion 3) incorporation of a heater block in the eluent rack to allow evaporation of samples to dryness and 4) modifications to the system's software to allow computer control of these processes.
Specific steps that will be taken to implement each innovation are given in the table below.
Hydroxylapatite precipitation experiments. There are a number of published techniques for precipitating pure (stoichiometric) hydroxylapatite. Many of these techniques use concentrated reagents and relatively high temperature (>80° C.) to quickly and efficiently produce hydroxylapatite. Other techniques are intended to mimic conditions of bone mineralization, and produce hydroxylapatite at physiological temperature and pH. In the former experiments hydroxylapatite generally precipitates directly from solution; the initial precipitate from physiological solutions generally is a poorly crystalline material that matures to hydroxylapatite over a period of hours to days. Measurements of Ca isotope fractionation during both types of hydroxylapatite precipitation will reveal the sensitivity of fractionation to the physical conditions of formation. High (ca 80-90° C.) temperature methods that produce hydroxylapatite after little aging will be used to explore the dependence of Ca isotope fractionation on temperature, pH and precipitation rate; precipitation under CO2 free but otherwise physiological conditions will be used to measure Ca isotope fractionation between Ca2+ and the initial poorly crystalline precipitate, and changes in the Ca isotope composition of this precipitate as it matures into stoichiometric hydroxylapatite.
Carbonate and Mg substitution experiments. Bone mineral is not pure hydroxylapatite, but typically contains substitutions, the most common of which are percent-level substitutions of Mg2+ for Ca2+ and CO32− for PO43−. Possible effects of such substitution on Ca isotope composition of hydroxylapatite will be investigated by precipitating Mg and CO3-rich under physiological conditions. If Ca isotope fractionation is found to be substitution-dependent, follow-up experiments will be conducted to quantify this dependence and constrain its effect on the Ca isotope composition of natural bone mineral. The degree of Mg and CO3 substitution in hydroxylapatite is easily manipulated by varying [Mg2+] in the precipitating solution and PCO2 in the air above the solution.
Hydroxylapatite-collagen experiments. Natural bone mineral is precipitated on a protein (primarily collagen) matrix, which accounts for up to 40% of the mass of mature bone. It is generally believed that collagen is laid down first, and is subsequently mineralized with hydroxylapatite. The presence of collagen could affect Ca isotope fractionation by, for example, affecting the diffusion of Ca2+ to crystallization sites within the collagen matrix. It is possible to replicate this process in vitro, by abiotically precipitating hydroxylapatite on collagen sponge obtained by demineralizing bone. It also is possible to co-precipitate collagen and hydroxylapatite from the same solution. Both of these experimental approaches will be used to determine whether, and how, the presence of collagen affects the Ca isotope composition of co-existing bone mineral. In addition, because it most closely resembles natural bone mineral, hydroxylapatite precipitated with collagen is used in the isotope exchange experiments described next.
Isotope exchange experiments. The possibility that equilibrium isotope fractionation affects the Ca isotope composition of hydroxylapatite is investigated in experiments that most closely replicate in vivo conditions—high Mg, high CO3 hydroxyapatite produced in the presence of collagen under physiological conditions. Equilibrium isotope fractionation between a mineral and the solution from which it precipitates requires isotopic exchange between solution and mineral. The rate of isotopic exchange can be measured using a 48Ca tracer, in parallel experiments in which solution containing an isotopically labeled solution is incubated with an unlabeled precipitate, and an unlabeled solution is incubated with a labeled precipitate. 48Ca will be used as a tracer because it is abundant enough to be available in relatively pure form (unlike 43Ca and 46Ca), yet scarce enough that very high enrichments (ca. 1000%) can with much less material than would be required for more abundant isotopes
The rate of isotopic equilibration is determined by measuring the change over time in tracer concentration in solution and precipitate in both experiments. Isotopic exchange rates will be measured during two phases of the hydroxylapatite precipitation experiments: during the maturation of hydroxylapatite from the initial poorly crystalline precipitate, and between precipitating solution and mature hydroxylapatite.
Experimental design and data. Table 1 summarizes the experimental suite. This plan may be modified based on emerging results. For example, if Ca isotope fractionation during the precipitation and maturation of pure hydroxylapatite is found to be the same as in the precipitation of highly CO3 and Mg substituted hydroxylapatite, there will be no need for a more detailed investigation of the effects of CO3 and Mg substitution.
In order to constrain mass balance, precipitation experiments will be done using batch, rather than continuous processes. Constant temperature and pH will be maintained. To the extent possible, precipitation reactions will be phosphate-limited so that only a relatively small fraction of available Ca2+ precipitates, which will minimize isotope distillation effects. Ca isotope compositions of starting solutions, ending solutions and precipitates will be measured. Mineralogy of precipitates will be determined using a combination of XRD, SEM and TEM. Except for the long-term exchange experiments, experiments in which initial precipitates are aged will be conducted in duplicate, permitting separate measurements of initial precipitates and precipitates after aging. Precipitates will be separated from solution by centrifuge, washed several times, and dried prior to isotopic, mineralogical and crystallographic analysis. Experiments will be scaled to produce a minimum of 100 ug of hydroxylapatite precipitate, enough material to meet all analytical demands. Ca isotope fractionation factors will be computed from the difference in δ44/42Ca between initial solution and bulk precipitate, using a Rayleigh distillation model as necessary to correct for distillation effects.
Hydroxylapatite for long term exchange experiments will be co-precipitated in three batches, each large enough to produce 1000-1500 mg of precipitate. One batch will be spike with 48Ca to an enrichment of about 1000%, the other two batches will be unspiked. 20 aliquots of ca 50 ug of spiked hydroxyapatite will be mixed with 20 aliquots of unspiked solution from one of the two unspiked batches and vice versa. 20 aliquots of mixed solution and hydroxylapatite from the second unspiked batch also will be taken. All aliquots will be placed in sealed containers and continuously agitated at a constant temperature between 70 and 80° C. (high enough to significantly accelerate isotopic exchange without affecting the stability of hydroxylapatite). Aliquots from each of the three experiments (spiked hydroxylapatite+unspiked solution, unspiked hydroxylapatite+spiked solution, unspiked hydroxylapatite+unspiked solution) will be harvested in pairs over the course of a year (or more). The spiked experiments will measure the rate of isotopic equilibration, if it can be measured on the timescale of the experiments. The unspiked experiment will provide data needed to calculate the Ca2+-hydroxylapatite equilibrium fractionation factor. In addition to mineralogical analyses, crystal size and morphology will be monitored during long term precipitation, to gain information on the mechanism of any observed equilibration, and how the rate of equilibration relates to crystal size.
Analytical methods. In order to prevent interferences from degrading data quality, hydroxylapatite samples are acid digested followed by ion exchange purification. To ensure accurate results, quantitative recovery during column chemistry and efficient matrix removal, is verified by quadrupole ICP-MS measurement of aliquots before and after chemistry.
Isotope abundances will measured by multiple-collector (MC) ICP-MS (ThermoScientific Neptune). Samples are introduced into a 1200-W plasma with a desolvating sample introduction system (Apex, CPI International) in medium-mass resolution. 30 measurement cycles are collected for each analysis; ratios of the ion beams 44Ca/42Ca,44Ca/43Ca,48Ca/42Ca, and 48Ca/43Ca are calculated and averaged into a single measurement. Standard-sample-standard bracketing is used to correct for instrumental mass bias. Secondary standards, purified through chemistry and measured by a different lab with thermal ionization mass spectrometry (TIMS), are run every few samples to ensure measurement reproducibility and accuracy over time. The bracketing standards used for all samples and external standards are the in-house standard, National Institute of Standards and Technology Ca ICP solution (NIST lot no. X-10-39Ca).
The abundance ratio of δ44/42Ca can be measured using <25 μg of Ca with a typical precision of ±0.2 (±2 SI)) ‰ compared to the standard.
All samples will be characterized with SEM to determine crystal size, size distribution and morphology. Secondary electron imaging on the FEI XL30 environmental FESEM will provide spatial resolution of a few nanaometers for detailed analysis of crystal morphology and sample homogeneity. EDX will be used to assist in phase identification and test for chemical heterogeneity. FESEM will also be used to image collagens and their structural relationship to hydroxylapatite. FIB-SEM will be employed to image mineral-collagen aggregates in 3D by slicing into aggregates and imaging the sliced surfaces. This technique can be used for 3D tomography, if needed and for imaging of the apatite-collagen interface. TEM will be used to further characterize the structure of the hydroxylapatites in the run products, employing one of several analytical TEM instruments in the LE-CSSS. TEM imaging combined with selected-area electron diffraction or nanodiffraction will be used to determine crystallinity, crystallographic orientations and fabrics in physiological samples.
HRTEM imaging is particularly useful for characterizing the maturation process from amorphous to crystalline material and the crystal defects that may result from precipitation or maturation. Sample preparation for TEM will depend on the size of the particles synthesized. If the particles are 10s of nanometers in size, the samples can be imaged directly in TEM after dispersal onto holey carbon support grids. If the crystals are larger than this, they will have to be thinned for analysis. The simplest method is crushing followed by dispersal on a grid so that thin particle fragments can be images. Larger grain-sizes and apatite-collagen aggregates will be sectioned using the FIB lift-out techniques. This sections provide ideal samples for TEM imaging, diffraction and EDX microanalysis. FIB section orientation can be selected to investigate specific crystal orientations or fabric directions.
The claims are not intended to be limited to the embodiments and examples described herein.
This application claims the benefit of U.S. provisional Application No. 61/784,033 filed on Mar. 14, 2013, which is incorporated by reference herein in its entirety.
This invention was made with government support under NNX08AQ36G awarded by National Aeronautical & Space Administration (NASA). The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 61784033 | Mar 2013 | US |
