-
Written by Gianpaolo Rota, Application Specialist | July 9, 2024
Overcoming sample prep pain points in geochemical and mining applications
INTRODUCTION
A wide range of purity and regulatory requirements in the mining and geochemical
industries are in place to ensure safe and high-quality final products, and metals analysis
plays a key role in helping operations meet these standards. Similar to analytical testing
in most industries, sample preparation plays a major role in successfully characterizing
materials. Unfortunately, poor sample preparation and human error are the leading
two causes of mistakes in chemical analysis.1 Thus, it is imperative that labs focus on
procuring the right equipment and sample prep processes to alleviate these common
pain points in mining and geochemistry applications.
SRC TECHNOLOGY AND THE ULTRAWAVE 3
How It Works
Single Reaction Chamber (SRC) technology is an attractive alternative for sample digestion that uses a 1-L stainless steel reaction chamber into which multiple samples are placed, as opposed to individual reaction vessels that are processed in batches. With the ultraWAVE 3, which is based on the SRC design, samples are weighed and put into vials with loose-fitting caps and suspended from the machine. Then, the rack is lowered into a base load of acidified water that ensures all samples are at the same temperature and pressure conditions, enabling runs of mixed sample types and chemistries.
As seen in FIGURE 1, the reaction chamber is prepressurized at the beginning of runs, which raises the boiling points of all reagents and prevents cross-contamination. In a typical microwave program, the temperature is ramped up and then held for a period of time at a temperature that will ensure full dissolution of samples. At the end of the run, an external chiller cools the reaction chamber before it is depressurized; as a result, users do not have to manually open pressurized vessels containing acid fumes like they would with a rotor-based system. Finally, samples are removed from the depressurized chamber, the caps are removed, and the samples are diluted to final volume. The samples can then be run on an ICP-OES or ICP-MS.
Single Reaction Chamber (SRC) technology is an attractive alternative for sample digestion that uses a 1-L stainless steel reaction chamber into which multiple samples are placed, as opposed to individual reaction vessels that are processed in batches. With the ultraWAVE 3, which is based on the SRC design, samples are weighed and put into vials with loose-fitting caps and suspended from the machine. Then, the rack is lowered into a base load of acidified water that ensures all samples are at the same temperature and pressure conditions, enabling runs of mixed sample types and chemistries.
As seen in FIGURE 1, the reaction chamber is prepressurized at the beginning of runs, which raises the boiling points of all reagents and prevents cross-contamination. In a typical microwave program, the temperature is ramped up and then held for a period of time at a temperature that will ensure full dissolution of samples. At the end of the run, an external chiller cools the reaction chamber before it is depressurized; as a result, users do not have to manually open pressurized vessels containing acid fumes like they would with a rotor-based system. Finally, samples are removed from the depressurized chamber, the caps are removed, and the samples are diluted to final volume. The samples can then be run on an ICP-OES or ICP-MS.
Benefits of SRC Technology
Traditional rotor-based technology can complicate mixedsample analysis, i.e., analyzing samples of differing composition, because mixed samples lead to different temperature and pressure conditions in each vessel, resulting in inconsistent digestion quality. SRC technology, like the ultraWAVE 3, offers a way for users to confidently run mixed samples regardless of type, weight, and acid chemistry because they’re at the same temperature and pressure conditions.
The ultraWAVE 3 maximum operating temperature of 280 °C and pressure capability of 199 bar enable it to handle more difficult matrices and larger sample amounts than rotor-based systems can. Critical metallic components of the ultraWAVE 3 are made of Hastelloy, which is resistant to acids and thus makes this device compatible with any acid chemistry.
FIGURE 2 compares the operator time per sample for both a rotor-based system and the ultraWAVE 3. Vial capping for the ultraWAVE 3 takes ~2 seconds per sample; no vessel assembly is required, nor is vessel venting because the chamber will automatically vent reaction gases at the end of each run. Applying these figures to the preparation of 5000 samples, traditional rotor-based systems consume ~19 days of operator time, while ultraWAVE 3 takes ~10 days. Comparing the final operator time per sample—110 seconds versus 64 seconds— ultraWAVE 3 requires 47% less user effort, which improves turnaround time and lowers labor costs.
Traditional rotor-based technology can complicate mixedsample analysis, i.e., analyzing samples of differing composition, because mixed samples lead to different temperature and pressure conditions in each vessel, resulting in inconsistent digestion quality. SRC technology, like the ultraWAVE 3, offers a way for users to confidently run mixed samples regardless of type, weight, and acid chemistry because they’re at the same temperature and pressure conditions.
The ultraWAVE 3 maximum operating temperature of 280 °C and pressure capability of 199 bar enable it to handle more difficult matrices and larger sample amounts than rotor-based systems can. Critical metallic components of the ultraWAVE 3 are made of Hastelloy, which is resistant to acids and thus makes this device compatible with any acid chemistry.
FIGURE 2 compares the operator time per sample for both a rotor-based system and the ultraWAVE 3. Vial capping for the ultraWAVE 3 takes ~2 seconds per sample; no vessel assembly is required, nor is vessel venting because the chamber will automatically vent reaction gases at the end of each run. Applying these figures to the preparation of 5000 samples, traditional rotor-based systems consume ~19 days of operator time, while ultraWAVE 3 takes ~10 days. Comparing the final operator time per sample—110 seconds versus 64 seconds— ultraWAVE 3 requires 47% less user effort, which improves turnaround time and lowers labor costs.
CASE STUDY: SRC DIGESTIONS OF MOLYBDENUM CONCENTRATES
Molybdenum concentrates are difficult, time-consuming
samples to work with. Freeport-McMoRan works with up
to nine daily samples of these concentrates, requiring nine
different tests. In addition to oil content, reactivity can be
affected by whether the sample is a sulfide or oxide. To ensure
all samples can be run, a robust digestion is required, but
selection of an optimal system for this is subject to manpower,
funding, and lab space constraints.
At Freeport, a rotor-based microwave system was being used for molybdenum concentrate sample digestion. For the samples to be fully digested with this technology, a two-step digestion is required in which nitric acid is added and the temperature is set to 200 °C in the first step. For the second step, the samples are cooled before hydrochloric acid is added and the sample is digested at 180 °C. Although a successful digestion is achieved, the process costs a great deal of time and labor that isn’t compatible with the company’s allotted manpower and time constraints.
To digest the molybdenum sulfide and oxide together, there is a 15-minute reaction time and 15-minute ramp to 180 °C before being held there for 20 minutes.
This method works on elements of interest including Al, Ca, Cu, Fe, K, P, and Pb, and additionally saves on cost and manpower. Unfortunately, unlike the two-step process, there is carbon residue left over (that requires cleaning with methanol) and occasionally the vessel plug is hard to remove as it cools because the pressurization in the chamber creates a vacuum effect. So, this was not a completely satisfactory procedure, either.
Freeport evaluated the ultraWAVE 3 in the hopes of achieving a more robust digestion. However, for this technology to fit in the lab workflow, certain requirements had to be met. The system had to digest molybdenum sulfide and oxide together, maintain a run, keep the process (from acid addition to pouring) at around ~100 minutes, and match the accuracy of rotor-based digestion. If these requirements were met/ exceeded, then the lab could decrease acid consumption and sample handling.
Method Development
In preparation to work with Milestone application specialists, the lab needed to evaluate initial acid volumes to accommodate the shift in vessel size from 55 mL to 15 mL. For initial settings, the lab started with 4 mL of HCI, 3 mL of HNO3, and 1 mL of HBF4. To avoid carbon residue, the temperature of the ultraWAVE 3 was ramped to 260 °C in 20 minutes and was then held there for 15 minutes.
Runs performed on the initial settings resulted in the molybdenum sulfide samples having very low recovery of all the elements and the molybdenum oxide samples having an incomplete digestion and low recovery of some elements.
The top of FIGURE 3 shows the initial results that suggest variance between the measured values and true values of the concentrates. After working with the Milestone application specialists, the nitric acid level was increased for the sulfides and decreased for the oxides, which resulted in the desired values as shown in the bottom of FIGURE 3.
Next, the Freeport team needed to determine the temperature requirements. The team started with a 20-minute ramp to 260 °C and was held for 15 minutes, and ≤13 samples were analyzed. When a full rack was digested, some of the inner vials had incomplete digestions indicated by a white solid left in solution. In collaboration with the specialists, the lab switched to a higher temperature with a ramp that helps the system reach 280 °C within 18 minutes, which resulted in complete digestions in all vials and no statistical difference based on the location of the vials within the chamber. After the appropriate settings were determined, the team began testing to determine if the ultraWAVE 3 preparations were compatible with the lab’s requirements for accuracy and precision. To test this, molybdenum sulfide and oxide samples were analyzed with each method several times, and the results were compared with known values. For added assurance, the standard deviation of both methods was compared. Ultimately, both methods yielded acceptable recoveries and had similar standard deviations.
Results
Based on the original requirements that the Freeport team set for the ultraWAVE 3, the system performed as needed. It is capable of digesting molybdenum sulfide and oxide together, takes ~80 minutes from acid addition to pouring, and meets the precision of traditional rotor-based microwaves. Additionally, it requires less handling and thus less staff exposure to hazardous materials, as well as decreased acid consumption (57% less HCI was used per sulfide and 57% less HCI and 80% less HNO3 was used per oxide).
At Freeport, a rotor-based microwave system was being used for molybdenum concentrate sample digestion. For the samples to be fully digested with this technology, a two-step digestion is required in which nitric acid is added and the temperature is set to 200 °C in the first step. For the second step, the samples are cooled before hydrochloric acid is added and the sample is digested at 180 °C. Although a successful digestion is achieved, the process costs a great deal of time and labor that isn’t compatible with the company’s allotted manpower and time constraints.
To digest the molybdenum sulfide and oxide together, there is a 15-minute reaction time and 15-minute ramp to 180 °C before being held there for 20 minutes.
This method works on elements of interest including Al, Ca, Cu, Fe, K, P, and Pb, and additionally saves on cost and manpower. Unfortunately, unlike the two-step process, there is carbon residue left over (that requires cleaning with methanol) and occasionally the vessel plug is hard to remove as it cools because the pressurization in the chamber creates a vacuum effect. So, this was not a completely satisfactory procedure, either.
Freeport evaluated the ultraWAVE 3 in the hopes of achieving a more robust digestion. However, for this technology to fit in the lab workflow, certain requirements had to be met. The system had to digest molybdenum sulfide and oxide together, maintain a run, keep the process (from acid addition to pouring) at around ~100 minutes, and match the accuracy of rotor-based digestion. If these requirements were met/ exceeded, then the lab could decrease acid consumption and sample handling.
Method Development
In preparation to work with Milestone application specialists, the lab needed to evaluate initial acid volumes to accommodate the shift in vessel size from 55 mL to 15 mL. For initial settings, the lab started with 4 mL of HCI, 3 mL of HNO3, and 1 mL of HBF4. To avoid carbon residue, the temperature of the ultraWAVE 3 was ramped to 260 °C in 20 minutes and was then held there for 15 minutes.
Runs performed on the initial settings resulted in the molybdenum sulfide samples having very low recovery of all the elements and the molybdenum oxide samples having an incomplete digestion and low recovery of some elements.
The top of FIGURE 3 shows the initial results that suggest variance between the measured values and true values of the concentrates. After working with the Milestone application specialists, the nitric acid level was increased for the sulfides and decreased for the oxides, which resulted in the desired values as shown in the bottom of FIGURE 3.
Next, the Freeport team needed to determine the temperature requirements. The team started with a 20-minute ramp to 260 °C and was held for 15 minutes, and ≤13 samples were analyzed. When a full rack was digested, some of the inner vials had incomplete digestions indicated by a white solid left in solution. In collaboration with the specialists, the lab switched to a higher temperature with a ramp that helps the system reach 280 °C within 18 minutes, which resulted in complete digestions in all vials and no statistical difference based on the location of the vials within the chamber. After the appropriate settings were determined, the team began testing to determine if the ultraWAVE 3 preparations were compatible with the lab’s requirements for accuracy and precision. To test this, molybdenum sulfide and oxide samples were analyzed with each method several times, and the results were compared with known values. For added assurance, the standard deviation of both methods was compared. Ultimately, both methods yielded acceptable recoveries and had similar standard deviations.
Results
Based on the original requirements that the Freeport team set for the ultraWAVE 3, the system performed as needed. It is capable of digesting molybdenum sulfide and oxide together, takes ~80 minutes from acid addition to pouring, and meets the precision of traditional rotor-based microwaves. Additionally, it requires less handling and thus less staff exposure to hazardous materials, as well as decreased acid consumption (57% less HCI was used per sulfide and 57% less HCI and 80% less HNO3 was used per oxide).
ultraWAVE 3
The new ultraWAVE 3 is the latest generation of SRC technology that further elevates the value of this technology for elemental analysis in terms of performance, time, workflow, and cost of ownership.
Download the Executive Summary
Microwave-assisted solvent extraction has demonstrated to be a well-established sample preparation technique that offers a reliable and efficient approach to their extraction.
Milestone™ Srl
Via Fatebenefratelli 1/5 - 24010 Sorisole (BG) - Italy
Phone: +39 035.573857 | Fax: +39 035.575498
Via Fatebenefratelli 1/5 - 24010 Sorisole (BG) - Italy
Phone: +39 035.573857 | Fax: +39 035.575498
Milestone™ Srl
Via Fatebenefratelli 1/5 - 24010 Sorisole (BG) - Italy
Phone: +39 035.573857 | Fax: +39 035.575498
Via Fatebenefratelli 1/5 - 24010 Sorisole (BG) - Italy
Phone: +39 035.573857 | Fax: +39 035.575498