Analytical Procedures

Recommended analytical procedures

All batches of zinc are thoroughly quality-controlled at Indiana University in multiple tests using VSMOW and SLAP waters, to ensure quantitative conversion of water to elemental hydrogen and reproducibility of stable isotope ratios.  For best results, we recommend adherence to our analytical guidelines (see below).

Please inquire with us if you are unsure which type of zinc is best suited for your application. Before ordering zinc you may request a small free sample for testing (international shipping charges may apply).

Ampules can be reliably marked by engraving. Ampules should be pre-baked at 500 ºC to burn off organic contaminants. We suggest to load 100 mg of zinc for reaction with every 2 mg of water. Avoid mixing of batches of Indiana Zinc because different batches may have different hydrogen blanks. The ampule containing zinc can be connected to a vacuum line with an Ultra-Torr® fitting. The apparatus is designed to admit water from a syringe. Other setups for admission of water are required for certain applications, such as work on fluid inclusions in minerals or work on water that is derived from combustion of organic hydrogen. 

The vacuum line connects via Ultra-Torr® fitting to the top left. From the right side, the syringe pierces through a septum (see also picture below). Another Ultra-Torr® connection at the bottom holds the 6 mm o.d. glass ampule containing zinc. This setup is specialized for the injection of liquid water with a syringe. Other arrangements are necessary if water is derived from fluid inclusions or from combusted organic matter.
We recommend to inject water at a ratio of 2 microliter of water (= 2 mg) for each 100 mg of Indiana Zinc.

(b) Pre-heating of Indiana Zinc under vacuum

Indiana Zinc contains some adsorbed atmospheric moisture and some chemisorbed hydrogen, probably in form of hydroxy-groups.

It is recommended to heat the loaded zinc under a good vacuum for at least 5 minutes at 350 ºC, to reduce the hydrogen blank. Heating with a torch is not recommended because open flames offer little temperature control. Overheating needs to be avoided because zinc would excessively sublimate and may lose its reactivity. Consistency in the technique is important to achieve good reproducibility. 

A small, lightweight heating block or heating sleeve (= coil of electrically insulated heating tape around a small copper tube with ca. 7 to 8 mm inner diameter; apply additional external thermal insulation for easy handling; an integrated thermocouple should permit monitoring of core temperature) can be built to slip over the bottom of the ampule, to enclose the end of the zinc-containing glass ampule. The sleeve is fed a constant voltage from a variac transformer to hold a core temperature of 350 ºC. After 5 minutes, a faint metallic mirror should have sublimated onto the interior glass surface just above the heated zone. Absence of this mirror indicates insufficient temperature and/or poor vacuum. A gas torch can be used to briefly flame and outgas the ampule above the heating sleeve.

(c) Loading of water

Most analytical applications provide enough water to use a syringe for injection through a septum. We suggest the use of a 10 microliter gastight syringe that injects 2 microliter of water for each 100 mg of Indiana Zinc. The septum interface should be located near the Ultra-Torr® fitting, with as little internal surface and as few stopcocks as possible.

Before injection, the zinc-containing part of the ampule must be under good vacuum and at the temperature of liquid nitrogen. Any leak in the system would admit incondensible nitrogen, oxygen, etc. and would provide a diffusion barrier that prevents water from quantitatively collecting at the bottom of the ampule. The complete freeze-down of injected water should occur within five minutes. Following (i) visual confirmation that no liquid phase is left in the syringe and (ii) monitoring of a sufficiently low pressure in the system, the ampule is flame-sealed off with a small glassblowing torch. Fully loaded and properly sealed ampules can be stored for long periods of time without danger of isotope fractionation. Water will not readily diffuse through glass walls. 

Every batch of water analyses should use the same batch of zinc and should include internal laboratory water isotope standards and/or VSMOW and SLAP for proper normalization of isotopic results relative to VSMOW (zero permil) and SLAP (-428 permil). A syringe can carry water from one injection to the next, thus causing a memory effect. According to a suggestion by Dr. Gilles St-Jean (ISOGEOCHEM list, 9 Dec 2003), a syringe can be thoroughly dried between injections as follows:

  • (a) If clean water is to be analyzed, use 3 rinses with acetone, then insert needle through the septum to expose the barrel for 60 seconds to vacuum, while pumping the plunger once every 20 seconds.
  • (b) For saline or urine samples, rinse the syringe 3 times with deionized water, followed by 3 rinses with acetone, and 60 seconds of vacuum drying while pumping the plunger once every 20 seconds.

(d) Generation of hydrogen

The reaction of water with Indiana Zinc occurs quantitatively at 500 ºC over 30 minutes. Temperature ramp-up time does not count. We recommend the following options:

  • (i) Spread the zinc over the entire length of a glass ampule and place the ampule horizontally into a 500 ºC oven. It is important to place a reliable thermocouple very close to where the zinc is located. Some ovens without forced convection have strong thermal stratification.  Improper heating conditions may result in incomplete reaction.
  • (ii) With the zinc at the bottom of the ampule, place the ampule vertically into a heating block that is at 500 ºC. The upper part of the ampule, which sticks out of the heating block, should be at about 300 ºC. Convection of water vapor and hydrogen gas will cause rapid conversion of water to elemental hydrogen.
Before any heating, glass ampules can be conveniently engraved using a diamond-coated engraving tool. During heating at 500 ºC for 30 minutes, Indiana Zinc reacts quantitatively with water. The zinc surface becomes oxidized and turns gray and dull.

(e) Mass-spectrometry

Following reaction to elemental hydrogen, the ampules can be scored with a glass knife and directly fitted to an interface at the mass-spectrometer in manual dual-inlet mode. Indiana Zinc does not produce much zinc oxide dust, but as a precaution a small safety screen should be placed at the inlet system to protect valve seats from particulates.

(f) Disposal of Indiana Zinc

Indiana Zinc cannot be regenerated after use. The 'recycling' via leaching in nitric acid will not yield satisfactory results. After use, Indiana Zinc should be disposed in the same way as required for regular zinc shavings.

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