Nd Chemistry Overview
The purpose of this document is a quick primer in Nd chemistry, to briefly describe why certain stages are done and to warn about the potential pitfalls and mistakes that can be made along the way. Nd chemistry is broken into three different sets of columns, utilizing two different types of resin and several different reagents.
Stage 1 Columns- AGW50x-4 Resin, HCl
The purpose of the first stage of Nd column chemistry is to remove all Fe from the samples. Sample collects can be prepped for the next round of chemistry using a hard dry down, as the loading solution (2M HNO3) is strong enough to pick up the collection into solution. This stage is only done with samples that contain >5% FeOt. Fe can act as an interference on the REE Spec column.
Stage 2 Columns- REE SPEC Resin
The purpose of the second stage of Nd column chemistry is to preferentially extract REE from all other elemental chemistry. Fe creates an interference on this column, which is why we run Stage 1 Columns first. With these columns, the difficulty comes from making sure the resin bed remains within the downstem portion of the column, and it isn’t disturbed at any point. When resin is lifted out of the resin bed, it can settle on the sides of the column, where it has the ability to bind to REE cations and cause them to not be eluted during collection. This lowers REE yield. The dry down for the collection from this column is a soft dry down, more information on preparing the loading solution for Stage 3 chemistry can be found in the next section.
Information about the REE Spec Resin and its use can be found in:
Horwitz, E.P., Chiarizia, R., Dietz, M.L., Diamond, H., Nelson, D., 1993, Separation and preconcentration of actinides from acidic media by extraction chronmatography: Analytica Chimica Acta, v. 281, p. 361-372.
Stage 3 Columns- AGW50x-4 Resin, MLA
The purpose of the final stage of Nd column chemistry is to preferentially collect all Nd from your sample, and separately collect Sm if needed. This round of columns is the crux of these procedures, as well as the most time consuming. It is incredibly important that the sample loaded onto the column is dissolved in 0.68 N HCl; this is difficult because this reagent is not strong enough to necessarily re-dissolve a hard dry down. The addition of two drops of H3PO4 before drying down the Stage 2 collection helps significantly with this. The vaporization point of H3PO4 is significantly higher than the dry down temp for this stage, so it forces the material to stay in solution within the H3PO4 that remains during dry down. Due to the low volume of H3PO4 added at this point, it does not cause an interference on the column. The setup of these columns can cause a bit of difficulty, the frit is not permanently in the column like in other columns, it is manually added and can get stuck. The key is to just drop the frit into the reservoir of the column, and then spray it downward with H2Omq. If it gets stuck in the column, invert the column and use the Nd-3 cleaning bottle (tube at the end of H20mq bottle) to spray water from the bottom of the column upwards. This should dislodge the frit.
The 3rd stage of Nd chemistry is done using a reagent called MLA, also known as α-HIBA. Unlike the previous two columns, where a changing to a specific reagent effectively eluted the desired chemistry, elution of Nd (and Sm when it applies) is based on the rate at which MLA transports Nd through the resin. Due to similar characteristics between different REE’s, the same reagent is used throughout. Similar mass elements such as Pr, Eu and Sm all cause isobaric and oxide interferences on the mass spectrometer, so having a pure Nd sample is important to limit error within the analysis. The differential rate of transport for REE’s down the column is dependent on the concentration and pH of MLA. It is very important that the length of the resin bed is the same as used in the calibration, and that the columns are accurately calibrated for the elution of desired REE’s (Fig. 1). The calibration can vary based on the sample matrix, so sample prep of individual mineral separates may vary . There are two steps in the checklist that suggest removing resin to get the resin bed to the correct length. Be careful on this, as the resin will contract during the resin cleaning step, and can cause your resin bed to become too short. Make sure to leave slightly extra resin in each column prior to the small water resin previous to sample loading. The goal is to have the resin bed reach the bend in the column (Fig.2).
Figure 1- Current Nd-3 Column Elution curve (5/21/15). The horizontal axis is volume of MLA through the column, the vertical axis is the percent eluted of each REE, showing the differential elution of individual elements throughout the column procedure.
Figure 2- The goal height of the resin bed. The resin bed should stop right at the curved beginning of the reservoir of the column.
When loading sample into Stage 3 Columns, it is important to gently drip the load directly onto the top of the resin bed, not onto the sides of the columns. The sample should be loaded slowly drop by drop, with the intention of having all of the sample loaded into the resin bed prior to adding any additional MLA to the column. Make sure to load the sample gently though, it is important the resin bed is not disturbed during loading. The way the sample is loaded affects the rate REE’s travel down the resin bed, so it is important to match this as closely as possible. Mistakes here can affect the Nd & Sm yield of the column, and also allow collection of undesired REE’s.
Following Stage 3 chemistry, Nd is in solution with MLA. MLA causes organic interferences on the mass spectrometer, and limits the ability for the machine to effectively ionize Nd. The addition of concentrated HNO3 and 6M HCl for two hours sealed is to dissolve any MLA that’s left after dry down, and then evaporate it prior to filament loading. If thin, bladed crystals appear in the beaker during dry down after being fluxed with HNO3-HCl, MLA was not completely removed from your sample, and it needs to be fluxed again. It is not uncommon for some sample to need to be fluxed multiple times prior to the complete removal of MLA.
Information about the AGW50x-4 Procedure and original method can be found in:
Boyet, M., Carlson, R.W., 2005, 142Nd evidence for early (>4.35 Ga) global differentiation of the silicate earth: Sceince, v. 3099, 576-581.