C. M. ALBRIGHT, Jr. Engineering Research Laboratory, E. 1. du Pont de Nemours & Co., Inc., Wilmington 98, Del.
Basic Instrumental Schemes for Chemical Analysis This simplified approach to understanding and selecting process analysis instruments considers the fundamental interactions of matter and energy
Table I. Relationships between Various Kinds of Energy and Matter Can Be Established and Observed to Determine Chemical Com position
ADVANCES in chemical analysis have an important role in the growth of the chemical process industry, in both size and complexity. Instrumental analysis techniques have advanced rapidly during the past 10 years, but n o orderly system has become available for classifying them. Therefore, no systematic approach can be made to the selection of the best technique for a given requirement. For example, instruments are frequently classified on the basis of their construction-e.g., optical or electricalor on the basis of the kind of measurements they make--e.g., pH, gas chromatography, index of refraction, or infrared or ultraviolet absorption. Neither of these systems is adequate. Undoubtedly, this situation has been tenable only because of the wealth of personal experience of the leaders in this field. This discussion introduces a new perspective for viewing instrumental chemical analysis-i.e., from the standpoint of interactions between matter and energy. This folloivs directly from the theory in physical chemistry that every configuration of atoms in a molecule can be represented by a definite system of electron energy states which, in turn, reflect the physical state or relationship of groups of molecules. By establishing and observing interaction between a substance and some measurable form of energy, these energy states which are characteristic of the composition of a particular substance can be inferred. 484
Interaction with Electromagnetic Radiation
Amount and Wave Length of Emitted Radiation Thermally Excited Emission Emission spectroscopy Flame photometry Electromagnetically Excited Emission Fluorescence Raman spectrophotometry Induced radioactivity X-ray fluorescence Amount and Wave Length Distribution of Transmitted or Reflected Radiation X-ray absorption (gross or critical edge) or diffraction Ultraviolet, visible, infrared, or microwave spectrophotometry Colorimetry Light scattering Refractive index Nuclear quadrupole moment Interaction with Other Chemicals
Amount of Sample or Reactant Consumed Orsats Automatic titrators Amount of Reaction Product Impregnated paper tape devices Continuous reaction types (miniature chemical plant) Thermal Energy Liberation Combustion Other exothermic reaction
INDUSTRIAL A N D ENGINEERING CHEMISTRY
P R O C E S S CONTROL Classification
As shown in Table I, this probing energy can be clzssified into four groups: Electromagnetic radiation-quantity and quality of radiation diffracted, emitted, reflected, or transmitted Chemical affinity or reactivityamount of material reacted, product formed, heat liberated, or determination of equilibrium attained Electric and magnetic fields-current, voltage, or flux changes produced in energized circuits containing the sample Thermal and mechanical-amount of energy transmission or work done, determination of changes in state Interactions with Electromagnetic Radiation. Because electromagnetic radiation varies in energy with frequency-that of the highest frequency or shortest wave length having the highest energy-radiation frequencies can be selected to interact with different levels of electron energy and permit observation of different chemical charac-
teristics. For example, gamma rays interact with atomic nuclei, x-rays interact with inner shell electrons, visible and ultraviolet radiations interact with valence electrons and strong interatomic bonds, while infrared radiation and microwaves interact with weak interatomic bonds and molecular vibrations or rotation. Chemical Reactivity. The inherent ability of chemical reactions to recognize particular substances is dependent upon the short-range exchange of valence or chemical energy between the two parties to the reaction in such direction as to achieve maximum stability or equilibrium. The method is applicable to systems of known stoichiometric and thermodynamic behavior where singular reactions occur. Unfortunately, many otherwise useful reactions may be masked by the presence of unwanted reactive substances in the sample, so that in practice they can be applied only to systems of known composition limits.
I nte r act ion with Other Che micaIs
(continued) Equilibrium Solution Potential (Oxidation-Reduction) Redox potentiometry
h:tal-metal
ion equilibria
Reaction to Electrical and Magnetic Fields Electrical Effects Caused by Ionization in Sample Mass spectrometry, gaseous conduction (ionization conferred by bom bardment) Polarography, amperometry, and coulometry Electrical conductivity, dielectric constant, oscillometry Electrical Effects Caused by Magnetic Properties of Sample Nuclear magnetic resonance Electron paramagnetic resonance Miscellaneous Physical Effects Caused by Electric or Magnetic Properties of Sample Paramagnetic displacement (oxygen analysis) Electrolytic deposition I
Interaction with Thermal or Mechanical Energy Effects Observed Chiefly by Measurement of Temperature T herma I conductivity Melting or freezing and boiling or condensation points Other Measurements Affected by Temperature or Thermodynamic Equilibrium Vapor pressure (gas-liquid or gas-solid) Thermal expansion Miscellaneous Mechanical Effects Viscosity-mechanical drag or motion attenuation Sound velocity Density
Interaction with Electric or Magnetic Fields. Important and unique electrical, magnetic, or other measurable physical effects, chiefly dependent upon charge-carrying abilities, may result from either inherent ionization or polarization properties of specific constituents in a sample or from such properties that may be conferred to certain constituents in the sample upon its insertion within an electric or magnetic circuit. Interaction with Thermal or Mechanical Energy. These are true interactions between matter and energy, although of a relatively gross nature compared to the first three groups. For example, the distinguishing ability of some gas molecules to become highly excited in vibration or rotation enables them to conduct relatively large amounts of thermal energy away from heated bodies with which they collide. Thermal conductivity analyzers operate on this principle and are especially suited to the determination of a gas such as hydrogen, which has a high thermal conductivity, occurring in nitrogen which has a low thermal conductivity. Thermal conductivity is predominantly used for detecting the amounts of the constituent molecules in a sample separated in a chromatograph column which are sequentially presented to the detector as a series of two-component mixtures, the carrier gas being the second component. Another example of gross energy transfer is the measurement of viscosity, wherein work is done on a substance with such devices as oscillating or rotating disks or by dropping a weight through it, thus affording an insight regarding the intermolecular forces which must be overcome and providing for measurement of such variables as concentration or degree of polymerization. Identification and Measurement of Constituents Analysis can be defined as the separation or resolution of a sample into its component parts so that they are individually distinguishable. In a system for the classification of chemical analysis instruments, the mechanisms which determine what is in the sample are as important as the energy measurements that determine how much of a given constituent is in the sample. Table I1 outlines the physical techniques employed in many common analysis instruments for these purposes. A number of these instruments are nonspecific with regard to the identity of the constituents whose presence is measured quantitatively. Good practice in the selection of instrumental analysis schemes demands the early consideraVOL. 52, NO. 6
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Table 11. Inherent Physical Techniques Are Employed to Determine Identity and Quantity in Common Analysis Instruments Instrument
Quantity
Identity
Chemical reaction analyzers, Choice of specific chemi- Magnitude of physical change or amount of titrimeters, etc. cal reaction reactant Colorimeters Amount of light absorded or Not determined reflected Coulometry, polarography Choice of applied poten- Amount of current flowing tial Dielectric constant, electrical Not determined Measure frequency change conductivity or amount of current flowing Emission spectroscopy Wave length of emitted Amount of emitted light light Gas chromatography Relative solubility in Thermal conductivity vs. selected solvent calibration infrared absorption Wave-length distribution Amount of radiation transmitted Mass spectroscopy Mass distribution (vz. cali- Amount of ion current bration for molecules) Nuclear magnetic resonance Measure rad iofreq uency Amount of radiofrequency energy received in dea t which nuclei precess tector coil Magnitude of light beam Refractive index Not determined deflection Solution potential Not determined Measure electrical potential Thermal conductivity Proportional to cooling Not determined effect on heated wire Ultraviolet absorption Wave-length distribution Amount of light transmitted X-ray absorption Wave-length distribution Amount of radiation transmitted
tion of nonspecific techniques (such as electrical or thermal conductivity, color, and refractive index) because they are nearly always simpler and less expensive than general-purpose specific instruments (mass spectrometers, emission spectrometers, infrared and ultraviolet absorption spectrophotometers) which can be set up to determine almost any appropriate element or compound. The combination of nonspecific instruments with sample separation or
preselection techniques drawn from the standard methods of analytical chemistry provides excellent results for many industrial analysis problems. For example, using thermal conductivity, carbon dioxide can be determined in oxygen with reasonable accuracy to as little as 270 full-scale if the sample is uniform in temperature and no other gases are present. Should small amounts of other gases such as hydrogen be present, carbon dioxide specificity can be maintained by
using differential thermal conductivity measurement, first on the entire sample and then on the residual sample following selective removal of carbon dioxide on a fused caustic absorber. I n the event that water vapor or chlorine should also be present, they would react with the caustic and interfere with the determination. These can be removed prior to the analysis by absorbing the chlorine on antimony metal and the water on calcium sulfate, once again providing a specific carbon dioxide determination. However, this leads to an analysis train having two thermal conductivity cells and three chemical absorption steps. Gas chromatography owes much of its recent success to its inherent capacity to side-step such complexity in separating and resolving the components of a mixture. This two-step combination of techniques separates the components of a gas or vaporizable liquid sample in time so that a sequential series of simple two-component mixtures can be measured by thermal conductivity or some other nonspecific detection device. Abridged versions of the generalpurpose specific techniques are frequently applicable to certain analyses. For example, infrared or ultraviolet analysis can be performed for single components by measuring the ratio of radiation absorbed at two separate wave lengths, only one of which is responsive to the desired component. Similar abridged techniques are applicable to mass spectrometry and x-ray absorption.
General Applicability
Table 111. Guide to Selection of Common Instrumental Analysis Techniq ues Physical State of Sample Primary Applicability
Gas or Vapor
Liquid
Elemental or atomic constituents
Mass spectroscopy
Molecular constit. uents
Mass spectroscopy
Emission spectroscopy (70 elements) X-ray absorption (critical edge-elements Nuclear magnetic resonance (only nuclei with spin) Nuclear magnetic resonance
Nonspecific gross effects
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Gas chromatog raphy (limited only by solvent) Infrared absorption (all heteroatomic molecules) Chemical reaction analyzers, titrimeters, etc. Ultraviolet a bsorption Thermal conductivity
Solution potential, POlarography, coulometry, etc. Infrared absorption
Suspension, Slurry, or Solid Emission spectroscopy X-ray absorption heavier than Cu)
Nuclear m a g netic resonance Colorimetry
Chemical reaction analyzers, titrimeters, etc. Ultraviolet absorption Dielectric constant, Dielectric conelectrical conducstant tivity, etc. Specific gravity Density Refractive index
INDUSTRIAL AND ENGINEERING CHEMISTRY
Table I11 shows, in summary form, the applicability of a number of common instrumental techniques to samples in the gas, liquid, and solid form. Moreover, it relates the specificity of these techniques to the chemical complexity of the constituents. Many of the techniques suitable for gas or vapor samples can be applied to liquid samples upon quantitative vaporization. Very thin solid samples and finely-dispersed slurries or suspensions can be analyzed with techniques that are primarily applicable to liquid samples. RECEIVED for review September 15, 1959 ACCEPTEDFebruary 29, 1960 Division of Industrial and Engineering Chemistry, 136th Meeting, ACS, Atlantic City, N. J., September 1959. [Editors’ Note: The example describing thermal conductivity analysis for carbon dioxide and the material appearing in Table I are adapted from contributions by the present author appearing in “Process Instruments and Controls Handbook,” edited by D. M. Considine (McGrawHill, New York, 1957). The reader who has further interest in the subject may wish to refer to this handbook. 1
