ICP Standards: Guide to Washout Considerations

Inductively Coupled Plasma (ICP) Mass Spectrometry employs an inductively coupled plasma to ionise a sample. It atomizes the sample and forms atomic and small polyatomic ions which can then be detected.

In a trace-level, elemental analysis method, emission spectra of a sample are learnt to recognise and quantify the elements present through use of characterised ICP standards and quantified by the intensity of the same lines.

Trace analysis is often thought of as an analyte concentration low enough to cause challenges. The challenges may be caused by the sample size, matrix, or washout issues.

What Can be Done about Washout?

Washout refers to sample solution being removed from the sample introduction system during analysis. Ensuring that the previous sample has been completely removed before introducing the next sample for analysis will eliminate carryover contamination from one sample to the next. This carryover contamination can cause incorrect results by either introducing interferences that should not be present or artificially adding elements of interest.

Some solutions contain elements that need Hydrofluoric Acid (HF) to maintain stability. Sample introduction systems are used in ICP and are often made of borosilicate glass. Introducing samples containing HF will cause Boron and Silicon to leach out of the glass sample introduction system parts and into the sample solution. ICP analysts will sometimes use a HF resistant system such as PFTE and PFA, however certain elements will still leach out of the coatings used in the plastic sample introduction parts. This carryover from HF resistant sample introduction system parts will increase as the concentration of HF increases.

When trying to remove washout issues with ICP standards, it is vital to ensure that all parts of the sample introduction system are clean. Some elements are more challenging to remove through standard cleaning processes and may require that parts are soaked in cleaning solution or even replaced more frequently. One specific part that can be easy to overlook is PVC Peristaltic Pump Tubing, see: https://www.youtube.com/watch?v=3mlBlBKzaew

Performing the correct rinse protocol is a critical part of avoiding washout issues. It can take some experimentation to understand how much time should be spent rinsing the equipment, and which rinse solution should be used, but it is of the utmost importance. Dilute acid or water may be sufficient to ensure the elimination of carryover contamination for relatively simple sample analysis. However, depending on the instrument method and the analytes/concentrations of interest the ICP analyst may choose to use more exotic rinse solutions.

ESSLAB advocate changing peristaltic tubing on a regular basis to obviate potential contamination issues and reducing cleaning/washout procedures. Aged pump tubing is prone to contamination with so called “sticky elements”; Good housekeeping and regular replacement result in runs of more samples free of “memory” interferences.

Summary