Monday, 18 February 2013

12 - The devil is in the detail - trace metal sampling and contamination

A lot of people especially such as those working in the pharmaceutical industry or hospitals, as well as people in their own homes like to keep things clean to avoid different kinds of contamination. In most cases a contamination problem can be cleaned up. For the trace metal samples that the chemistry team are collecting during this research cruise this is not the case and there are numerous potential contaminant sources that need to be avoided otherwise all the hard work and money that went into preparing for this cruise to get these samples could be wasted.

High concentrations of trace metals in the environment can be deadly to life but most life cannot thrive without small quantities of elements such as iron and zinc. This is why we study the chemistry of these metals in the ocean.

Iron is an essential nutrient for phytoplankton growth in the ocean. Phytoplankton are at the base of the food chain in ocean ecosystems but also photosynthesise and use up carbon dioxide in the ocean so the amount that these organisms can grow has important implications for the linked ocean-atmosphere system. Knowing how iron is supplied to the oceans and how this has changed in the past will help us to make better predictions about how our environment may change in the future.

Iron is only present in extremely small concentrations in the ocean due to its low solubility in oxic seawater. This means that in some regions of the ocean the lack of iron limits phytoplankton growth. In order to understand why some regions of the ocean are iron-limited, the potential sources of iron need to be sampled to examine how this element behaves in the environment.

The rear view of the ROV with
the niskin bottles mounted on the
port side. We use these to sample
water close to the vents.
One potential source of iron to the oceans is hydrothermal venting. Vents emit hot metal rich fluids to the ocean which mix with and become diluted by seawater to form a plume. As this mixing occurs, iron in the vent fluid forms iron oxide and sulphide minerals and the larger mineral particles fall to the ocean floor. During this process some of the iron forms smaller nano scale minerals or can become associated with organic matter, and it is these smaller particles that can be transported away from the vent by currents providing a source of iron to the ocean.

Due to the low concentrations of iron we are dealing with, samples are very easily contaminated. Certain precautions have to be taken to make sure we are measuring the actual concentration of iron in the water that was sampled.

The first step to prevent contamination of any samples begins weeks before any research cruise starts. The plastic bottles used to store samples may look clean and new, but they can contain particles which may spoil samples, and to remove absolutely everything the bottles need to be washed in baths of acid for weeks. Washing hundreds of plastic bottles in acid was my very first task upon starting my PhD last October.

The second of these precautions is the titanium frame the 10L niskin bottles are mounted on. A stainless steel frame would be the obvious choice when designing this type of equipment, however this would introduce the possibility for sample contamination.

Aly sampling the CTD
The RRS James Cook  has a clean lab on board and any processing of samples to be analysed for trace metals has to be done in this lab. As iron can be present as different particles of different sizes down to the nano-scale, sample processing involves taking several sub-samples from the niskin bottles and filtering those samples through different types of filters in different ways. I spend most of my time in the clean lab trying to keep track of all this but also trying to do it as quickly as possible as the longer samples are left in the niskin bottles the more likely that the samples chemistry will change as more particles may form in the niskin bottles over time. So not only am I trying to prevent my samples from outside sources of contamination but I'm also trying to minimise natural processes occuring in the samples that change their original composition. Unfortunately the clean lab is also up one deck, so the heavy 10L niskin bottles have to carted up and down the stairs before the sample can be removed.

Left to right: filtering from niskin bottles under pressure from nitrogen line., vacuum pump filtering, pre-rinsing of 0.02┬Ám filters for in-line filtering
I spend the vast majority of my time on the ship in the clean lab, which can be a bit strange at first as the lab is constantly moving around with the ship, there are no windows and the environment inside the lab is very different to that outside. So it can be a surprising but nice shock after hours of sample processing in the cold dry clean lab to come back outside into the real world and remember you’re in the middle of the Caribbean Sea!

By A.J. Mackenzie Lough


  1. At the same time it can be nice to be in the middle of the Caribbean and go into your cool, clean lab!
    I just found your blog and will be following.

  2. I really enjoyed reading this blogpost, keep on writing such interesting posts.
    sampling equipment