137.Fundamentals of Analytical Chemistry(2) – Introduction(2).

9.Confidence Limit (CL)-

  •  The confidence limit is the level of confidence we have about a value being  statistically  significant.

Confidence Limit (CL)=  100 × (1 − α) (%) , where , α ⇒ standard Deviation.

  • Lesser the standard deviation, more is the confidence limit.This means that if the values of a data set are closer to the mean (less standard deviation), we can be more confident that our value lies in the expected range and is thus significant (more CL). If the value of CL is less it means that we are unsure about it being in the expected range of values.

10.Limit Of Detection (LOD) – 

  • This is the lowest limit below which the sample cannot be measured.
  • LOD is the minimum amount/concentration of a component that can be detected with a given degree of confidence.
  • It is the lowest quantity of a substance that can be distinguished from the absence of that substance (blank value) within a stated confidence limit(about 1% generally).

11.Limit Of Quantification  (LOQ)

  • It is the concentration at which quantitative results can be reported with high degree of confidence.
  • The quantification limit is the lowest amount of analyte in a sample which can be quantitatively determined with suitable precision and accuracy i.e we can measure it quantitatively.

To understand these concepts let us take an example .Let us assume that we are at the airport with lots of noise from the jets taking off.

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If a person next to you,
∗ speaks softly → Voice < LOD ∴ We cannot hear his voice.
∗ speaks a bit loudly → LOQ > Voice > LOD ∴We hear his voice but cant decipher what he said.
∗ speaks loudly → Voice > LOQ  ∴ We can clearly hear him.
Although , during the course of time, the noise from the jet could also get reduced ! Thus, the quantities LOD and LOQ change.So, the detection limits (i.e LOD and LOQ)  are dependent on both the signal intensity (voice ) and the noise (jet noise).

12.Limit of linearity(LOL)

  • This is the maximum value of concentration of a component upto which the instrument produces  a linear response.Beyond LOL, response becomes non-linear.

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13.Sensitivity

  • It is a measure of the ability of the method to discriminate between two small concentration differences in the analyte.
  • It is measured in terms of the slope of the calibration curve.

203


In analytical chemistry, the data required to perform experiments and the results obtained often are numerical values. Thus, it is very important that a system be developed that is universally accepted and followed for common understanding of scientists. In this post we will discuss some basic numerical entities which are commonly used in analytical chemistry.

SI units

We have already studied SI units in post 13. Some important SI units are –

Measurement

Unit

Symbol

Mass

kilogram

kg

Volume

liter L

Temperature

kelvin

K

Time

second s
Amount of substance mole

mol

current

ampere A
Length meter

m

These are some fundamental SI units.Other units are defined using these units.

Physical Quantity

Formula

SI unit

Area Length * Breadth m2
Volume Length*Breadth*Height m3
Density Mass per unit volume Kg m-3
Frequency Cycles/sec Hz

(Hz= s-1)

Pressure Force/area Pascal(Pa)

(Newton /metre2 ,Nm-2).

Concentration Molarity – moles/dm3 mol dm-3
Force Mass * acceleration Newton (N)

Kg m s-2

Velocity Distance/time m s-1
Heat Energy Joule(J)

Significant figures.

It is also imperative that we have knowledge of significant figures. This concept is a reflection of the measurement’s uncertainty. Significant figures are the digits in a measured quantity, including all digits known exactly and one last digit whose quantity is not certain. Refer post 19 and post 20 for details.


In the next post we shall continue discussing more concepts used often in analytical chemistry. Till then,

Be a perpetual student of life and keep learning….

Good day!

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