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Standard Operating Procedures

Silver Salt Preparation

Introduction

This document describes the procedure for cleaning and preparing aqueous samples (ice cores, snow pits, aerosols, etc.) for Δ17O analysis of nitrate and sulfate via silver salt pyrolysis. This version was most recently modified by Qianjie Chen and has had many previous contributors (Lei Geng, Ben Johnson, Eric Sofen, Joesph Erbland, Shelly Kunasek). Samples are cleaned by filtering and hydrogen peroxide to remove large material and organics (not necessary to use H2O2 if samples are not with high organics). They are then sent through a Dionex Ion Chromatograph to separate anion species. Samples are then run through a cation exchange resin loaded with silver ions to replace any cations in the sample with silver (sodium form is required before the conversion of silver salt if the samples are with high organics, see details below).  The resulting silver salt solution then undergoes a series of evaporation/re-dissolving steps to transfer into small quartz/metal capsules. Silver (quartz) capsules are used for nitrate and gold (quartz) for sulfate.  After samples are prepared they are run on a ThermoFinnigan MAT 253 for oxygen isotope analysis. See Lorax Protocol.docx for details. 

UNLESS OTHERWISE NOTED ALWAYS USE MILLIQ WATER!! 

Filtering

A rough filtering step is recommended, even if samples do not appear dirty.  Use the filtering setup on the eastern bench in the lab. The filtering device is a two piece glass cylinder that is held together by a metal clamp (filter holder) and fits into a vacuum Erlenmeyer flask. Always clean the glassware before use. 

Rinse a Whatman filter paper [Grade 5 CAT no. 1005-325  (2.5µm)] with MiliQ water and place in between the two glass pieces and clamp together. Fit the filtering device on a clean flask, and attach the black vacuum hose to the barbed fitting.  Fill glass with 10 mL of water, and turn pump on. Pump water through, turn vacuum off. Repeat several times (>3 times) to thoroughly clean the filter paper. 

Switch to a clean Erlenmeyer, and pour sample in the filtering column.  Turn vacuum on and filter to completion. For especially dirty samples, repeating the process is recommended. Make sure to use a new, clean filter paper for each sample! 

Removing Organics

This step attempts to remove organic material, which can combust with sample oxygen during pyrolisys, reducing sample yield. 

First, cations in samples need to be replaced with sodium, especially for samples with a pH < 7, because at solutions with very low pH (i.e., at the end of sample drying in the freezer dryer) sulfate will potentially exchange oxygen atoms with water, altering the oxygen isotopic composition of sulfate samples. This step is done by following the procedures described in "Lorax_OfflineResinProtocol", except charging the resin with Na2SO4 solution instead of Ag2SO4 solution (see "Lorax_OfflineResinProtocol" for details.

Second,  ensure that samples are 7 mL or less in volume. If they are more, use a hot plate to evaporate sample until it reaches this volume, or split sample in to several test tubes. Add sample to an 8 mL plastic test tube (in cabinet near sample freezer on shelf labeled “Small Test Tubes”). In the hood, pipette 1 mL of 30% hydrogen peroxide (H2O2).  

Place test tubes in centrifuge rack and into freeze dryer on east bench next to the drying oven. Make sure the test tube lids are off!! Select proper settings on freeze dryer: Method H2O and Temp 45°C. Typically samples take about 6-8 hours to evaporate to completion, so set timing for 8 hours and make sure that the heating is on for the duration of the run. 

Ensuring that all H2O2 is removed is paramount, as any remaining will continue to react and contaminate samples. You may repeat cleaning procedure for samples that require it. 

After all liquids are completely evaporated, redissolve sample by adding water, capping test tube, and placing in the sonicator for at least 30 minutes.  

Running through Yertle

The next step involves separating anions on the Ion Chromatography machine, named Yertle. Instructions for starting the machine and running samples are described here.  Samples will be loaded, ions (e.g., Cl-, NO3- and SO42-) in sample matrix will be separated and then the sulfate and/or nitrate fractions are collected by a fraction collector.  

Fractions collected from the IC are in acidic form (H+). Following the procedures in "Lorax_OfflineResinProtocol"  and converting the acidic forms to silver salts. One more step of "removing organics" is recommended before silver salt conversion, especially when low yields are observed and the interference of organics is suspected. 

Cleaning Capsules

All capsules need to be cleaned prior to use, and the same procedure can be used for all three materials (gold, silver, quartz).  Never directly handle capsules, always use tweezers. The following steps are recommended:

  1. Place capsules in a clean beaker and fill with hexane. Cover with foil and sonicate for 30 minutes. Pour out hexane to a waste bottle, shake the cups/capsules to remove liquid inside the cups/capsules. Rinse cups/capsules with clean water (18 MΩ) completely (inside and outside), a wash bottle is recommended to rinse the inner part of the capsules/cups; 
  2. Repeat step one with soapy water;
  3. Repeat step one with DI water for 3 times; 
  4. Repeat step one with 18 MΩ water for 3 times. Pour out water, shake out water inside the cups/capsules. Put all cups/capsules in a clean beaker, cover with foil and dry in the oven in ATG 325 for overnight at 60 C. After rinsing, bring the cups/capsules to IsoLab, bake out capsules in the muffle oven following the method of muffle oven operation. The T of muffle oven is set to 1000°C for several hours, any trace organics in cups/capsules are removed.  When cool, place in a clean container and store in desiccator.

Transferring samples to capsules

Once samples are converted in to silver form, carefully label and load into centrifuge disc and into the freeze dryer. Make sure that the disc is balanced (i.e. equally spaced test tubes).  Use the same set up as the hydrogen peroxide cleaning step (Method H2O, temp=45°C) and set the timing to run overnight (12-14 hours).  

After overnight freeze drying, you will need to re-dissolve your sample in a small amount of water, and transfer into one test tube if there are multiple tubes per sample.  Ag2SO4 is soluble at 0.8g/100mL (1µmol/38µL) and Ag2NO3 at 234g/100mL (1µmol/0.07µL). Add about twice the water you think you need, put a clean cap on the test tube, and sonicate (in 30 minute increments) until all material is dissolved. Combine samples that are in multiple tubes in one.  

Place quartz cups in a sample tray.  Using small pipette, transfer 110µL of sample at a time into capsules (the capacity of the quartz cups is ~115 µL). Weigh the sample tray, according the mass of the sample tray to make a "fake sample tray" with the same mass. Put two trays in the freezer dryer, following the setup (Method H2O and Temp 45°C), to dry  the samples for ~2 hrs. After samples are completely dried, keep adding aliquots until a designated amount of sulfate is added.   

After sample drying, always keep the samples in the same tray, cover with foil (to isolate from light), and store the samples in the freezer dryer in Isolab until isotope analysis. See method protocol in IsoLab for how to operate the freezer dryer there.