Encyclopedia of Environmental Science and Engineering, Volume I and II

548 INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS

gain may refer to voltage, current or power ampli-

fication and its input and output impedances.

• Noise refers to random signals, usually continu-
  ous, that restricts the lower detection limit and
  accuracy of the signal. Noise arises from elec-
  tronic components and environmental sources and
  cannot, at times, be completely eliminated.


• The ratio of the amplitude of the signal to that of
  the noise is called the signal-to-noise, S/N, ratio.
  This ratio gives the ability to distinguish between
  signals and noise, that is a measurement of the
  quality of an instrument. One cannot usually dis-
  tinguish the signal from the noise when the ratio
  is less than about 2 or 3.


• Resolution or resolving power is the capabil-
  ity of displaying two signals differing slightly
  in value. The resolving power, R, of a mono-
  chrometer concerns absorption of emission spec-
  tral signals,

R  λ / d λ (3)

is the wavelength under consideration and d λ is the differ-

ence of wavelength between the two signals. In mass spec-

trometry resolution refers to the separation of two mass

peaks Ms and Ms  dMs, where dMs is the difference in

masses so that

R  Ms / dMs. (4)

For resolution for chromatographic methods see Part Two

Section III,B,4, a.

• Response time refers to the time needed for a pen
  of a potentiometric recorder to travel the total ver-
  tical distance on the Y axis.


• Sensitivity, S, describes the ratio of the change in
  the response or output signal, dI 0 of the instrument
  to a small change in the concentration or amount
  of the analyte, dC. The ratio is given as follows:

S  dI 0 / dC. (5)

• Linear dynamic range, LDR, describes the
  mathematical relationship between amount or
  concentration of the analyte and the response of
  the instrument. An increase in the analyte quan-
  tity results in a linear increase in response. The
  size of the range of quantities accommodated
  by the instrument response is the key factor for
  this parameter. For example in voltammetry
  the LDR is 10^ ^8 to 10^ ^3 M (molar), five orders
  of magnitude, in (ultraviolet) uv–visible spec-
  trophotometry, about 10 to 100, and for a GC
  with a FID (flame ionization detector) the LDR
  extends from 10^ ^1 to 10^7 ng (nanograms, 10^ ^9
  grams) or eight orders of magnitude. Obviously

the sensitivity remains constant in contrast to a

non-linear dynamic relationship.

• The reagent blank or blank in a spectroscopic
  determination is the signal obtained by the solution
  of the reagents without any analyte. The sample
  matrix is important to include, if known, in the
  blank. In many instances the effect of the matrix is
  determined indirectly.


• Accuracy defines, mathematically, the absolute
  error, e a , inherent in the method when comparing
  the analytical result, x i , with the true value, x t , of
  the analyte content of the sample.

e a  ( x i  x t ). (6)

Preparing a standard sample containing an accurately known

concentration of the analyte is required. This is not a simple

task, because homogeneity of any mixture is difficult to

obtain and ascertain.

• The precision of a method is concerned with the
  repeatability of the analytical results for a number
  of analyses on the same sample. There are several
  ways of expressing precision; standard deviation
  is a very effective and meaningful measure. The
  standard deviation, sd, for small sets of data is
  given as follows:

sd xiax No

i

N





().

,

2

1

12

∑^1

⎡

⎣

⎢

⎤

⎦

⎥

(7)

Here x i is the experimental value, x a , the average of the exper-

imental values, and No, the number of values. The standard

deviation is a measure of the average uncertainty of all the

measurements in the data set, x i , that is x 1 ,... , x N. 18,19

Types of Instruments

Analytical instruments can be classified according to cat-

egories based on various physical phenomena. The general

categories used in this article are spectroscopy, electrochem-

ical analysis, radiochemical analysis, chromatography, and

automated analysis. Table 4 illustrates these categories.

Spectroscopy

Introduction Spectroscopic instruments include optical and

other types of instruments. The optical instruments analyze

electromagnetic radiation, emr, while other spectroscopic

instruments deal with sound, mixtures of ions, electrons, and

other forms of energy. Other optical methods utilize instru-

ments that make refractometric and polarimetric measure-

ments. Refractometric measurements will be discussed in

the section on liquid chromatography.

Spectroscopy, classically, is that area of science where

the electromagnetic radiation, emr, emitted from or absorbed

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