Getting good and consistent results for trace element analysis using ICP-MS can be a daily challenge. For a start, contamination lurks everywhere, ready to trip you up. Next, there is the problem of sample-to-sample washout/memory effects randomly popping up to frustrate you. Then there’s the issue of interferences, which, of course, are at their worst for those elements that are most important to you. On top of these challenges, there’s also the headache of sample and calibration solution preparation, which is not only labour intensive but also fraught with the risk of error as well as contamination.
In this first part of a three part series, I’ll explore how you can identify and solve the problems caused by contamination in your laboratory.
Typical Troublesome Elements and Their Sources
Elemental contamination is a widespread issue, which encompasses contamination from sample preparation and sample-to-sample washout/memory effects (the latter appearing as cross contamination between samples). The usual suspects that cause the most trouble for trace level analysis include Al, Cr, Cu, Fe, Mn, Ni, Pb and Zn, but others, such as Hg, Mo and Sb, can cause random errors if they are at high levels in one sample, then low in the next.
During sample preparation – which includes everything from sample digestion and dilution, to blank and calibration solution preparation – the biggest sources of contamination are labware and reagents. Pipette tips, volumetric flasks and their caps, sample tubes and their lids, de-ionised water supplies and reagents such as nitric acid are all potential sources of contamination. The next biggest trace element contamination source is you, the analyst. Hair, skin cells and dust from clothing are all significant sources of trace elements.
Solving Labware, Reagent and Laboratory Personnel Contamination Problems
The largest component in any liquid sample for trace element analysis using ICP-MS is most often water, so the purity of your de-ionised water supply is critical. Ideally, the water you use should have a resistivity of 18.2 MΩ cm to ensure that elemental contamination is at a minimum. The next most significant reagents are the acids used to stabilise elements in solution. Nitric acid is most commonly used, followed by hydrochloric acid which is required for certain elements as discussed below. For routine applications, ultra-high purity acids are not required – it is sufficient to use trace element grade purity reagents such as Fisher Chemical’s TraceMetal® or PrimarPlus® grade acids.
The next significant problem is surface contamination of sample bottles, volumetric flasks and sample tubes and their stoppers/lids. To ensure that this is eliminated, these items should be rinsed with or soaked in dilute (0.5% (v/v)) nitric acid, then washed with de-ionised water before use. As coloured flask stoppers often contain high levels of metals (e.g. Cd in red stoppers), using only colourless stoppers is advisable. Sample tubes with blue or orange lids are generally not a problem, but it is recommended to check that they are not a source of contamination. The simplest way to do this is to rinse one of the lids with deionised water, soak it in dilute nitric acid overnight then run an ICP-MS scan of the acid to identify if anything has leached from the lid. Alternatively, sourcing pre-washed labware can be a good idea. Glassware generally causes more problems than plastic ware, so use plastic where possible, and recycle this as much as possible, of course. Pipette tips are trickier to clean, so test tips from different suppliers to check if they are clean enough to use straight from the box, although you can also pre-clean them by pipetting dilute nitric acid and dispensing it to waste first, before using the tip for your sample preparation.
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To avoid contaminating samples with hair, skin or dust from yourself, wearing powder free gloves and lab coats and tying back long hair are strongly recommended. Wearing protective eyewear is of course essential too. Coughing and sneezing close to your samples also risks transferring elemental contamination into your samples, so take care to avoid this as far as possible.
Addressing Washout / Memory Effects
With sample-to-sample washout/memory effects, the usual troublemakers are ‘sticky’ elements such as Hg, Mo and Sb, as already mentioned. These elements take longer to washout between samples as they adhere more strongly than other elements to the surface of the sample probe and tubing along which the samples pass on their way into the ICP-MS. The simplest solution to this problem is to increase the wash time between samples, but this can lead to much longer analysis times. A mixed rinse solution containing both dilute nitric and hydrochloric acids (generally at 2% (v/v) concentration) is more effective at washing out sticky elements than just nitric acid alone. In the case of Hg, it is also advisable to ensure HCl (typically at 0.5% to 2% (v/v)) is present in all blanks, calibration solutions and samples, or alternatively, to add Au at 200 ppb to all solutions to be analysed to stabilise Hg in solution (as Hg2+) and so prevent it causing washout problems. If it’s practical for you to do so, including Au (again at 200 ppb) in your autosampler rinse solution further helps to keep the Hg background to a minimum throughout your analysis.
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