Analytical chemistry (orchemical analysis) is the branch ofchemistry concerned with the development and application of methods to identify the chemical composition of materials and quantify the amounts of components inmixtures. It focuses on methods to identify unknown compounds, possibly in a mixture or solution, and quantify a compound's presence in terms ofamount of substance (in any phase),concentration (in aqueous orsolution phase), percentage by mass or number of moles in a mixture of compounds (or partial pressure in the case of gas phase).
Analytical chemistry has been important since the early days of chemistry, providing methods for determining which elements and chemicals are present in the object in question. During this period, significant contributions to analytical chemistry included the development of systematicelemental analysis byJustus von Liebig and systematized organic analysis based on the specific reactions of functional groups.[3]
Most of the major developments in analytical chemistry took place after 1900. During this period, instrumental analysis became progressively dominant in the field. In particular, many of the basic spectroscopic and spectrometric techniques were discovered in the early 20th century and refined in the late 20th century.[5]
Theseparation sciences follow a similar timeline of development and have also became increasingly transformed into high-performance instruments.[6] In the 1970s many of these techniques began to be used together as hybrid techniques to achieve a complete characterization of samples.
Starting in the 1970s, analytical chemistry became progressively more inclusive of biological questions (bioanalytical chemistry), whereas it had previously been largely focused on inorganic orsmall organic molecules. Lasers have been increasingly used as probes and even to initiate and influence a wide variety of reactions. The late 20th century also saw an expansion of the application of analytical chemistry from somewhat academic chemical questions toforensic,environmental,industrial andmedical questions, such as inhistology.[7]
Modern analytical chemistry is dominated by instrumental analysis. Many analytical chemists focus on a single type of instrument. Academics tend to either focus on new applications and discoveries or on new methods of analysis. The discovery of a chemical present in blood that increases the risk of cancer would be a discovery that an analytical chemist might be involved in. An effort to develop a new method might involve the use of atunable laser to increase the specificity and sensitivity of a spectrometric method. Many methods, once developed, are kept purposely static so that data can be compared over long periods of time. This is particularly true in industrialquality assurance (QA), forensic and environmental applications. Analytical chemistry plays an increasingly important role in the pharmaceutical industry where, aside from QA, it is used in the discovery of new drug candidates and in clinical applications where understanding the interactions between the drug and the patient are critical.[8]
The 21st century has been defined by the digitalization of analytical chemistry. The handling of large datasets ("big data") from instruments like Orbitrap mass spectrometers has made advanced data analysis, including machine learning, an essential skill.[9] This era also focuses strongly on sustainability, leading to the principles of Green Analytical Chemistry, which aim to minimize the environmental impact of chemical analyses.[10]
The presence ofcopper in this qualitative analysis is indicated by the bluish-green color of the flame
Although modern analytical chemistry is dominated by sophisticated instrumentation, the roots of analytical chemistry and some of the principles used in modern instruments are from traditional techniques, many of which are still used today. These techniques also tend to form the backbone of most undergraduate analytical chemistry educational labs.[citation needed]
Inorganic qualitative analysis generally refers to a systematic scheme to confirm the presence of certain aqueous ions or elements by performing a series of reactions that eliminate a range of possibilities and then confirm suspected ions with a confirming test. Sometimes small carbon-containing ions are included in such schemes. With modern instrumentation, these tests are rarely used but can be useful for educational purposes and in fieldwork or other situations where access to state-of-the-art instruments is not available or expedient.[citation needed]
Quantitative analysis is the measurement of the quantities of particular chemical constituents present in a substance. Quantities can be measured by mass (gravimetric analysis) or volume (volumetric analysis).[13]
The gravimetric analysis involves determining the amount of material present by weighing the sample before and/or after some transformation. A common example used in undergraduate education is the determination of the amount of water in a hydrate by heating the sample to remove the water such that the difference in weight is due to the loss of water.[14]
Titration involves the gradual addition of a measurable reactant to an exact volume of a solution being analyzed until some equivalence point is reached. Titration is a family of techniques used to determine the concentration of an analyte.[15] Titrating accurately to either the half-equivalence point or the endpoint of a titration allows the chemist to determine the amount of moles used, which can then be used to determine a concentration or composition of the titrant. Most familiar to those who have taken chemistry during secondary education is the acid-base titration involving a color-changing indicator, such asphenolphthalein. There are many other types of titrations, for example, potentiometric titrations or precipitation titrations. Chemists might also create titration curves in order by systematically testing the pH every drop in order to understand different properties of the titrant.[15]
Electroanalytical methods measure thepotential (volts) and/orcurrent (amps) in anelectrochemical cell containing the analyte.[17] These methods can be categorized according to which aspects of the cell are controlled and which are measured. The four main categories arepotentiometry (the difference in electrode potentials is measured),coulometry (the transferred charge is measured over time),amperometry (the cell's current is measured over time), andvoltammetry (the cell's current is measured while actively altering the cell's potential).[18][19][20]
Chromatography can be used to determine the presence of substances in a sample, as different components in a mixture have different tendencies to adsorb onto the stationary phase or dissolve in the mobile phase. Thus, different components of the mixture move at different speeds. Different components of a mixture can therefore be identified by their respectiveRƒ values, which is the ratio between the migration distance of the substance and the migration distance of the solvent front during chromatography.
In combination with the instrumental methods, chromatography can be used in the quantitative determination of substances. There are different types of chromatography that differ from the media they use to separate the analyte and the sample. Inthin-layer chromatography, the analyte mixture moves up and separates along the coated sheet under the volatile mobile phase. Ingas chromatography, the gas phase separates the volatile analytes. A common method of chromatography using liquid as a mobile phase ishigh-performance liquid chromatography.[24]
Hyphenated separation techniques refer to a combination of two (or more) techniques to detect and separate chemicals from solutions. Most often the other technique is some form ofchromatography. Hyphenated techniques are widely used inchemistry andbiochemistry. Aslash is sometimes used instead ofhyphen, especially if the name of one of the methods contains a hyphen itself.[29]
The visualization of single molecules, single cells, biological tissues, andnanomaterials is an important and attractive approach in analytical science. Also, hybridization with other traditional analytical tools is revolutionizing analytical science.Microscopy can be categorized into three different fields:optical microscopy,electron microscopy, andscanning probe microscopy. Recently, this field is rapidly progressing because of the rapid development of the computer and camera industries.[31]
Devices that integrate (multiple) laboratory functions on a single chip of only millimeters to a few square centimeters in size and that are capable of handling extremely small fluid volumes down to less than picoliters.[32]
The vast amount of data produced by modern analytical instruments has made computational data analysis an integral part of the field. The field of chemometrics uses statistical and mathematical methods to design optimal experimental procedures and to extract meaningful information from chemical data.[33]
Key areas include:
Multivariate calibration: Used to develop models that correlate instrument responses (e.g., spectra) to analyte concentrations, essential in techniques like near-infrared spectroscopy.[34]
Pattern recognition: Employed to classify samples based on their analytical profile, with applications in food authenticity and medical diagnostics.
Machine learning and artificial intelligence: These techniques are increasingly used for predictive modeling, optimizing analytical methods, and automating data interpretation.[35]
Error can be defined as numerical difference between observed value and true value.[36] The experimental error can be divided into two types, systematic error and random error. Systematic error results from a flaw in equipment or the design of an experiment while random error results from uncontrolled or uncontrollable variables in the experiment.[37]
In error the true value and observed value in chemical analysis can be related with each other by the equation
where
is the absolute error.
is the true value.
is the observed value.
An error of a measurement is an inverse measure of accurate measurement, i.e. smaller the error greater the accuracy of the measurement.
Errors can be expressed relatively. Given the relative error():
The percent error can also be calculated:
If we want to use these values in a function, we may also want to calculate the error of the function. Let be a function with variables. Therefore, thepropagation of uncertainty must be calculated in order to know the error in:
A general method for analysis of concentration involves the creation of acalibration curve. This allows for the determination of the amount of a chemical in a material by comparing the results of an unknown sample to those of a series of known standards. If the concentration of an element or compound in a sample exceeds the detection range of the technique, it can simply be diluted in a pure solvent. If the amount in the sample is below an instrument's range of measurement, the method of addition can be used. In this method, a known quantity of the element or compound under study is added, and the difference between the concentration added and the concentration observed is the amount actually in the sample.[38]
Sometimes aninternal standard is added at a known concentration directly to an analytical sample to aid in quantitation. The amount of analyte present is then determined relative to the internal standard as a calibrant. An ideal internal standard is an isotopically enriched analyte which gives rise to the method ofisotope dilution.[39]
The method ofstandard addition is used in instrumental analysis to determine the concentration of a substance (analyte) in an unknown sample by comparison to a set of samples of known concentration, similar to using acalibration curve. Standard addition can be applied to most analytical techniques and is used instead of acalibration curve to solve thematrix effect problem.[40]
One of the most important components of analytical chemistry is maximizing the desired signal while minimizing the associatednoise.[41] The analytical figure of merit is known as thesignal-to-noise ratio (S/N or SNR).
Noise can arise from environmental factors as well as from fundamental physical processes.
Thermal noise results from the motion of charge carriers (usually electrons) in an electrical circuit generated by their thermal motion. Thermal noise iswhite noise meaning that the powerspectral density is constant throughout thefrequency spectrum.
Shot noise is a type ofelectronic noise that occurs when the finite number of particles (such aselectrons in an electronic circuit orphotons in an optical device) is small enough to give rise to statistical fluctuations in a signal.
Shot noise is aPoisson process, and the charge carriers that make up the current follow aPoisson distribution. The root mean square current fluctuation is given by[41]
wheree is theelementary charge andI is the average current. Shot noise is white noise.
Flicker noise is electronic noise with a 1/ƒ frequency spectrum; asf increases, the noise decreases. Flicker noise arises from a variety of sources, such as impurities in a conductive channel, generation, andrecombination noise in atransistor due to base current, and so on. This noise can be avoided bymodulation of the signal at a higher frequency, for example, through the use of alock-in amplifier.[42][43]
Analytical chemistry has applications across science and industry. It is fundamental toforensic science (e.g., DNA fingerprinting and toxicology),bioanalysis (e.g., measuring drug concentrations in pharmacokinetic studies),clinical analysis (e.g., blood glucose monitoring and COVID-19 PCR testing),environmental monitoring (e.g., testing for pollutants in water and air), andmaterials science (e.g., quality control of semiconductors and nanomaterials).[46]
Great effort is being put into shrinking the analysis techniques tochip size. Although few examples of such systems compete with traditional analysis techniques, potential advantages include size/portability, speed, and cost. Micrototal analysis system (μTAS) orlab-on-a-chip.Microscale chemistry reduces the amount of chemicals used.[47]
Many developments improve the analysis of biological systems. Examples of rapidly expanding fields in this area aregenomics,DNA sequencing and related research ingenetic fingerprinting andDNA microarray;proteomics, the analysis of protein concentrations and modifications, especially in response to various stressors, at various developmental stages, or in various parts of the body;metabolomics, which deals with metabolites;transcriptomics, including mRNA and associated fields;lipidomics - lipids and its related fields; peptidomics - peptides and its related fields; and metallomics, dealing with metal concentrations and especially with their binding to proteins and other molecules.[48]
Analytical chemistry has played a critical role in the understanding of basic science to a variety of practical applications, such as biomedical applications,environmental monitoring, quality control of industrial manufacturing, forensic science, and so on.[49]
The recent developments in computer automation and information technologies have extended analytical chemistry into several new biological fields. For example, automated DNA sequencing machines were the basis for completing human genome projects, leading to the birth ofgenomics. Protein identification and peptide sequencing by mass spectrometry opened a new field ofproteomics. In addition to automating specific processes, there is effort to automate larger sections of lab testing, such as in companies likeEmerald Cloud Lab and Transcriptic.[50]
Analytical chemistry has been an indispensable area in the development ofnanotechnology. Surface characterization instruments,electron microscopes and scanning probe microscopes enable scientists to visualize atomic structures with chemical characterizations.[51]
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