Blank sample

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In the area of ​​qualitative chemical analysis, a blank sample is a procedure that is intended to ensure the functionality and usability of the selected detection method. There are two different approaches:

If the blank sample is positive, the substance to be detected is added to the analysis mixture. The detection reaction must therefore occur. If the detection reaction (generally a precipitate or a color reaction) does not occur in this case, the test is unreliable because either the reagents are outdated or because the mixture to be analyzed contains components that inhibit the detection reaction.

In the case of the negative blank test, only the reagents are subjected to the detection procedure without adding the substance to be examined. In this case, the reaction must be negative, that is, the typical reaction must not take place, since the corresponding substance is not contained. If the reaction does occur, the reagents are contaminated and useless for this detection or there is a systematic procedural error.

Summary:

  • The positive blank test ensures that the substance being searched for will be found if it is present.
  • The negative blank test ensures that the searched substance will not be found if it is not present.
  • The double blank sample, i.e. both the positive and the negative together, ensure the reliability of the method used for the present case.

In quantitative analytical chemistry , a blank sample is understood to be a sample that is presumably free from the substance to be determined (the analyte ). Theoretically, such a sample should give the analysis value zero. In the case of relative methods ( instrumental analysis ) the measured value zero is theoretically to be expected (e.g. no peak visible). In practice, however, a measurement signal can be caused either by unavoidable contamination of the reagents used with the analyte or by interfering influences ( matrix effects , interference ). The value obtained in this way is called the blank value . Blank values ​​limit the detection capability of an analytical method. The detection and quantification limit can be determined using a blank sample using the blank value and its statistical fluctuation.

literature

J. Strähle, E. Schweda: Jander · Blasius - Introduction to the inorganic-chemical practical course. 14th edition. Hirzel, Stuttgart 1995, ISBN 978-3-77-760672-9 .