A blank look at… blanks

Draft: Published 22nd January 2019 by Andy Connelly.

Introduction

There are many definitions of a “blank”.  The official definitions often say something like: a sample that includes everything that will be present in the samples of interest except the analyte of interest. However, this is not what the word blank is always used to refer to. This is partly because this kind of ideal blank is not always available but also because other types of blanks are also important. In fact, while in an ideal world your blank would not contain any of the analyte of interested, in reality it will and how much is there is really important for your experiments.

There are three key reasons why blanks are important:

  1. Identifying contamination: Blanks allow you to identify if any of the analyte you are interested in that has been introduced from your sample collection and processing, from your experimental process, or from your analytical process. In other words, have trace sources of contamination been artificially introduced.
  2. Interferences: Running blanks will identify signals that arise from components other than the analyte you are interested in. This prevents missassigning a result to the analyse of interest when it may be produced by accidental measurement of a different analyte or an instrument issue.
  3. Statistical analysis: Blanks allow you to calculate some important statistical values such as the Limit of Detection of your experimental and analytical methods. They are sometimes also used as part of your calibration curve.

DISCLAIMER: I am not an expert on statistics. The content of this blog is what I have discovered through my efforts to understand the subject. I have done my best to make the information here in as accurate as possible. If you spot any errors or admissions, or have any comments, please let me know

Different types of blank

There are many different types of blanks that can be used depending what is suitable for your experiment. Each will identify contamination from / give information about a different part of your process.

Here I have split the different types of blanks into 4 different general headings with more specific versions of those general types of blanks below.

Field (or sampling) blank

Field blanks are generally used to identify contamination from the sample collection process. They are often DI water or a similar “clean” material that is handled, transported, and analyzed in the same manner as the samples collected the same day. Different stages of the sample collection process can be separated out:

  • Trip blank: blanks are carried to and from the field along with the other sample containers from the site. They may indicate contamination from the sample containers, cross-contamination during shipment, storage, or laboratory contamination.
  • Transfer blanks: are prepared in the field while splitting or transferring samples. They may indicate contamination from sampling equipment, the surroundings, sample containers, or cross-contamination during shipment.

Process (experimental or method) blank

These are used to identify contamination introduced from the processing or producing samples including reagents. They are often DI water or a similar “clean” material that has been through all of your experimental processes.

  • Matrix (or sample) blank: Normally a solution/material which is made up with the same composition as your sample matrix e.g. 10% HCl.
  • Reagent Blank: A blank specifically to test the analyte contribution from the reagents.

Instrument (or analytical) blank

A blank which is as clean as possible. This is run through the system to meant to test the instrument/analytical method’s for false signals and to get an ideal of the baseline created by the instrument. It also allows to you see if there is any carry over from previous samples. Sometimes referred to as a dummy analysis.

Spiked Blank

A blank sample (usually a process or matrix blank) that has been spiked with a known amount of analyte as part of a calibration or validation procedure (e.g. spike recovery).

Conclusion

Blanks are one of the most important parts in planning an experiment. There is nothing worse than collecting lots of data only to find out your method or the instrument is introducing contamination that makes the results useless. Blanks introduced at each step of the process will go along way to ensuring that this does not happen.

Further reading

M.F.Vitha, P.W. Carr, and G.A. Mabbott, Appropriate Use of Blanks, Standards, and Controls in Chemical Measurements, Journal of Chemical Education, Vol. 82 No. 6 June 2005.

 

 

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