Two recent approaches for coupling capillary scale liquid chromatography and electron ionization mass spectrometry are reviewed and discussed.
· The first one, Cap-EI, is the latest evolution of the micro-scale particle beam interface, in which the nebulizer has been optimized to overcome the limitations of the former approach, in terms of sensitivity and linearity.
· The second one is a miniaturized interface for nano- and micro-HPLC, in which the interfacing process takes place into a suitably modified ion source. Because the eluate from the column is completely transferred into the ion source for ionization, superior sensitivity, linearity, and reproducibility are obtained2.
ii) Chemical ionization:
EI having disadvantage is that due to higher energy electron beam, there are so many daughter ions and so chances for the complex spectra. Because of less energy required for Chemical ionizationthan electron ionization yields less fragmentation, and usually a simpler spectrum. Chemical Ionization (CI) is especially useful technique when no molecular ion is observed in EI mass spectrum, and also in the case of confirming the mass to charge ratio of the molecular ion.
The chemical ionization process begins when a reagent gas such as methane, isobutane, or ammonia is ionized by electron impact. Sample ions are formed by the interaction of reagent gas ions and sample molecules. This phenomenon is called ion-molecule reactions and produces either the reagent gas ions or reagent gas neutrals. Some of the products of theseion-molecule reactions can react with the analyte molecules to produce analyte ions. Reagent gas molecules are present in the ratio of about 100:1 with respect to sample molecules. Positive ions and negative ions are formed in the CI process. Depending on the setup of the instrument only positive ions or only negative ions are recorded.
The main problem in this technique is choice of reagent gas. It depends on two factors.
· Proton affinity PA
· Energy transfer
NH3is the most used reagent gas in CI because of the low energy transfer of NH4+ compare to CH5+. With NH3 as reagent gas, usually MH+ and MNH4+ (17 mass units difference) are observed.
In a CI, ions are produced through the collision of the analyte with ions of a reagent gas that are present in the ion source. First step for this is to make ion of reagent gas. Electrons entering the source will preferentially ionize the reagent gas. The resultant collisions with other reagent gas molecules will create ionization plasma. Positive and negative ions of the analyte are formed by reactions with this plasma.
Positive ion mode:
GH+ + M MH+
+ G
Primary Ion Formation
CH4 + e- CH4+
+2e-
Secondary Reagent Ions
CH4 + CH4+
CH5+ + CH3
CH4 + CH3+
C2H5+ + H2
Product Ion Formation
M + CH5+
CH4 + [M + H] +(protonation)
AH + CH3+
CH4 + A+(H−
abstraction)
M + CH5+
[M + CH5] +(adduct formation)
A + CH4+ CH4+
A+ (charge exchange)
Chemical ionization reaction mass spectrometry (CIRMS) is applied for the first time to a range of organic gases with a variety of functional groups. This technique, recently developed, is an extension to proton transfer reaction-mass spectrometry (PTR-MS) using a greater variety of chemical ionization reagents in the ionization process. Clean sources of the reagents H3O+, NH4+, NO+ and O2+ have been obtained without any mass pre-selection. The reactions of these reagent ions with a range of test VOCs are found to be rapid, with the chemistry generally paralleling that observed previously with selected ion flow tube measurements, although with some important differences. CIRMS is shown to be a more versatile technique than conventional PTR-MS with the potential for rapid multi-reagent analysis of chemical mixtures3.
Application of chemical ionization method in estimating relative values of proton affinity, susceptibility to alkylation, and ionization energies of aromatic and heteroaromatic compounds, from that it is concluded that
· The proton affinity of furan, thiophene, and their alkyl derivatives is greater than that of the corresponding benzene derivatives.
· The alkylation of heteroaromatic compounds proceeds to a considerably greater degree than that of toluene; furan and its homologs are more prone to form [M+C4H9]+ cluster-ions than are the corresponding thiophene derivatives.
· The charge-transfer reactions that take place under conditions of chemical ionization in binary mixtures provide a means for obtaining information on the ionization energies of the components of the mixture4.
Ammonia desorption chemical ionization of ether linked phospholipids of the type 1-0-alkyl-2-0-acetyl-snglycero-3-phosphocholine (platelet-activating factors) and a series of analogues revealed a systematic fragmentation pattern that is characteristic for these compounds5.
iii) Negative Chemical Ionization:
Chemical ionization for gas phase analysis is either positive or negative. Almost all neutral analytes can form positive ions through the reactions described above.
However, many important compounds of environmental or biological interest can produce negative ions under the right conditions. For such compounds, negative ion mass spectrometry is more efficient, sensitive and selective than positive-ion mass spectrometry.
In order to see a response by negative chemical ionization, the analyte must be capable of producing a negative ion (stabilize a negative charge) for example by electron capture ionization. Because not all analytes can do this, using NCI provides a certain degree of selectivity that is not available with other, more universal ionization techniques (EI, PCI). NCI can be used for the analysis of compounds containing acidic groups or electronegative elements (especially halogens).
Negative ions can be produced by a number of processes. One of them is Resonance electron capture in which capture of an electron by a neutral molecule to produce a molecular anion. The electron energy is very low, and the specific energy required for electron capture which depends on the molecular structure of the analyte. Negative ionization may also occur by ion-molecule reactions, e.g., proton abstraction, which is dependent on the analyte and the NICI reagent gas used. Irrespective of the mechanism of ionization, NICI is characterized as a soft ionization technique, whereby NICI spectra exhibit prominent molecular anions, and therefore molecular weight information.
Electron attachment is an endothermic process, so the resulting molecular anion will have excess energy. Some molecular anions can accommodate the excess energy. Others may lose the electron or fall apart to produce fragment anions. But in most of cases molecular ions observed in negative ion chemical ionization mass spectra are usually M- or [M-H].
Negative ion mode:
[G-H]- + M ------> [M-H]- + G
In negative-ion chemical ionization, electron transfer from ionized reagent gas (e.g. NH2- may transfer an electron to a molecule having a greater electron affinity than NH2).
Also as a buffer gas methane is used to slow down the electrons in the electron beam until some of the electrons have just the right energy to be captured by the analyte molecules. The buffer gas can also help stabilize the energetic anions and reduce fragmentation. This is really a physical process and not a true "chemical ionization" process.
Because of the high electronegativity of halogen atoms, NCI is a common choice for their analysis. This includes many groups of compounds, such as PCBs, pesticides, and fire retardants. Most of these compounds are environmental contaminants, thus much of the NCI analysis that takes place is done under the auspices of environmental analysis.
NICI is used for the analysis of saturated triglycerides. Analyses of triglycerides in food products have typically been difficult for the chromatographer - triglycerides are among the least volatile of food components analyzed by gas chromatography. Triglycerides in natural products are a complex mix of saturated and mono- or poly-unsaturated fatty acid chains.
2. Field Desorption Ionization:
Field desorption (FD)/field ionization (FI) refers to an ion source for mass spectrometry first reported by Beckey in 19696.
These methods are based on electron tunneling from an emitter that is biased at a high electrical potential. The emitter is a filament on which fine crystalline 'whiskers' are grown. When a high potential is applied to the emitter, a very high electric field exists near the tips of the whiskers.
Figure 2: Schematic of field desorption ionization with emitter
There are two kinds of emitters used on JEOL mass spectrometers: carbon emitters and silicon emitters. Silicon emitters are robust, relatively inexpensive, and they can handle a higher current for field desorption. Carbon emitters are more expensive, but they can provide about an order ofmagnitude better sensitivity than silicon emitters.
Field desorption and ionization are soft ionization methods that tend to produce mass spectra with little or no fragment-ion content.
i) Field Desorption (FD)
In FD, the analyte is applied as a thin film directly to the emitter, or small crystals of solid materials are placed onto the emitter. Slow heating of the emitter then begins, by passing a high current through the emitter, which is maintained at a high potential (e.g. 5 kilovolts). As heating of the emitter continues, low-vapour-pressure materials get desorbed and ionized by alkali metal cation attachment.
Characteristic positive ions produced are radical molecular ions and cation attached species such as [M-Na]+. The ion produced during desorption by the attachment of trace alkali metal ions present in the analyte.
Sample introduced by direct insertion probe. The sample is deposited onto the tip of the emitter by dipping the emitter into an analyte solution or by depositing the dissolved or suspended sample onto the emitter with a microsyringe.
As the drawback of this is, it is sensitive to alkali metal contamination and sample overloading and relatively slow analysis and as the emitter current is increased the sample must be thermally volatile to some extent to be desorbed.
The recently developed liquid injection FD ionization (LIFDI)7 technique "presents a major breakthrough for FD-MS of reactive analytes8" Transition metal complexes are neutral and due to their reactivity, do not undergo protonation or ion attachment. They benefit from both: the soft FD ionization and the safe and simple LIFDI transfer of air/moisture sensitive analyte solution. This transfer occurs from the Schlenk flask to the FD emitter in the ion source through a fused silica capillary without breaking the vacuum.
But main advantage is that samples undergo little or no fragmentation during FD ionization and give simplest mass spectra of complicated compounds.
This technique is suitable for high molecular mass and/or thermally labile substances such as polymers, peptides, carbohydrates and organic or inorganic salts. FD remains one of the only ionization techniques that can produce simple mass spectra with molecular information from hydrocarbons and other particular analytes. The most commonly encountered application of FD at the present time is the analysis of complex mixtures of hydrocarbons such as that found in petroleum fractions.
ii) Field Ionization (FI)
During Field Ionization, sample molecules become ionized by the ‘quantum tunneling’ of a valence electron as they pass close to the tips of emitter electrodes. The electrodes are essentially a large collection of carbon micro-needles, deposited on tungsten wire, surrounded by a very high electric field within the ion source.
Sample introduced by heated direct insertion probe gas inlet. Samples are introduced in the same way as for electron ionization (EI).
Here the sample must be thermally volatile.
In this technique spectra exhibit some fragmentation due to the heat used for sample volatilization though the spectra will remain simple.
This is use as a interface for the gas-liquid chromatography mass spectrometry. FI sensitivity studies of the molecular ion for a variety of carbonyl compounds with relative intensities by electron impact between 0.1 and 0.6% suggest that some type of carbonyl compounds significantly for sensitive to FI than others. The percentage of sample which permeates the membrane ranged between 30 and 72% for these carbonyl compounds9.
Field ionization of alkali metal Rydberg states: application as an ultraviolet radiation detector with absolute wavelength calibration10.
These both techniques are used for the analysis of ring-type fractions of coal-derived oils, obtained by liquid chromatography. These techniques produce only the molecular ion and its isotopic signals, free from fragment ions, of a molecule, consequently the exact molecular-weight-distribution profile of chromatographic fractions.11
3. Particle Bombardment:
In these methods, the sample is deposited on a target that is bombarded with atoms, neutrals, or ions. The most common approach for organic mass spectrometry is to dissolve the analyte in a liquid matrixwith low volatility and to use a relatively high current of bombarding particles (FAB or dynamic SIMS). Other methods use a relatively low current of bombarding particles and no liquid matrix (static SIMS). The latter methods are more commonly used for surface analysis than for organic mass spectrometry.
The primary particle beam is the bombarding particle beam, while the secondary ions are the ions produced from bombardment of the target.
i) Fast Atom Bombardment (FAB):
Fast atom bombardment (FAB) is an ionization technique used in mass spectrometry. 12
The analyte is dissolved in a liquid matrixsuch as glycerol, thioglycerol, m-nitrobenzyl alcohol, or diethanolamine and a small amount (about 1 microliter) is placed on a target. The target is bombarded with a fast atom beam (for example, 6 keV xenon atoms) that desorb molecular-like ions and fragments from the analyte. Cluster ions from the liquid matrix are also desorbed and produce a chemical background that varies with the matrix used.
FAB is a relatively soft ionization technique and produces primarily intact protonated molecules denoted as [M+H]+ and deprotonated molecules such as [M-H]-. The nature of its ionization products places it close to electrospray and MALDI13.
The first example of the practical application of this technique was the elucidation of the amino acid sequence of the oligopeptide efrapeptin D. This contained a variety of very unusual amino acid residues. The sequence was shown to be: N-acetyl-L-pip-AIB-L-pip-AIB-AIB-L-leu-beta-ala-gly-AIB-AIB-L-pip-AIB-gly-L-leu-L-iva-AIB-X. PIP = pipecolic acid, AIB = alpha-amino-isobutyric acid,leu = leucine, iva = isovaline, gly = glycine. This is a potent inhibitor of the mitochodrial ATPase activity.
Figure 3: Ionization Process by Fast Atom Bombardment
The detection by fast atom bombardment mass spectrometry (f.a.b.-m.s.) of high molecular mass (greater than 1000 Da) material in coal-derived products was investigated. The pentane insoluble (PI) fractions of liquefaction extracts from maceral concentrates of two UK coals (Linby and Cortonwood), prepared in a low-residence time (less than 10 s) flowing-solvent reactor and fractions of hydropyrolysis tar prepared in a hot-rod reactor at 500 °C were examined.F.a.b.-m.s. spectra of the Cortonwood liptinite concentrate liquefaction extract PI-fraction mounted in a thiodiethanol matrix indicated ions up to molecular masses of about 4000 Da. This appears to be the highest reported molecular mass identified by fast atom bombardment in a coal-derived material14.
The application of fast atom bombardment (FAB) mass spectrometry to the determination of lead isotope ratios. FABMS is applied to the direct analysis of lubricant additives, thermally labile or involatile organic compounds such as macromolecules and ionic dyestuffs, and inorganic compounds15.
ii) Secondary Ion Mass Spectrometry (SIMS):
Secondary ion mass spectrometry (SIMS) is a technique used in materials science and surface science to analyze the composition of solid surfaces and thin films by sputtering the surface of the specimen with a focused primary ion beam and collecting and analyzing ejected secondary ions. These secondary ions are measured with a mass spectrometer to determine the elemental, isotopic, or molecular composition of the surface. SIMS is the most sensitive surface analysis technique, being able to detect elements present in the parts per billion range.
In the early 1960s two SIMS instruments were developed independently. One was an American project, led by Liebel and Herzog, which was sponsored by NASA at GCA Corp, Massachusetts, for analyzing moon rocks16. The other at the University of Paris-Sud in Orsay by R. Castaing for the PhD thesis of G. Slodzian17. These first instruments were based on a magnetic double focusing sector field mass spectrometer and used argon for the primary beam ions. In the 1970s, K. Wittmack and C. Magee developed SIMS instruments equipped with quadrupole mass analyzers. 18,19
Figure4:Ionization Process by Secondary ion mass spectrometry
The SIMS technique provides a unique combination of extremely high sensitivity for all elements from Hydrogen to Uranium high lateral resolution imaging, and a very low background that allows high dynamic range. This technique is "destructive" by its nature. It can be applied to any type of material that can stay under vacuum.
SIMS is widely used for analysis of trace elements in solid materials, especially semiconductors and thin films. The SIMS ion source is one of only a few to produce ions from solid samples without prior vaporization. The SIMS primary ion beam can be focused to less than 1 μm in diameter. During SIMS analysis, the sample surface is slowly sputtered away. Continuous analysis while sputtering produces information as a function of depth, called a depth profile. When the sputtering rate is extremely slow, the entire analysis can be performed while consuming less than a tenth of an atomic monolayer. This slow sputtering mode is called static SIMS in contrast to dynamic SIMS used for depth profiles. Shallow sputtering minimizes the damage done to organic substances present on the sample surface. The resulting ion fragmentation patterns contain information useful for identifying molecular species. Only dynamic SIMS will be treated in this surface analysis computer aided instruction package because only dynamic SIMS yields quantitative information.
The COSIMA instrument onboard Rosetta will be the first instrument to determine the composition of cometary dust with secondary ion mass spectrometry in 201420.
Secondary ion mass spectrometry (SIMS) is useful for measuring Mg/Ca in both primary calcite and diagenetic minerals in planktonic foraminifera. The excellent spatial resolution (<10 μm) and small amount of material removed (<2 ng) makes it easy to avoid targets that include obvious embedding material and encrusting or infilling minerals such as secondary calcite and authigenic clays in diagenetically altered samples21.
An instrumental method based on the use of secondary ion mass spectrometry (SIMS) is presented for the identification of uranium particles, and the determination of their isotopic composition22.
4. Atmospheric Pressure Ionization:(Spray Methods):
In these methods, a solution containing the analyte is sprayed at atmospheric pressure into an interface to the vacuum of the mass spectrometer ion source. A combination of thermal and pneumatic means is used to desolvate the ions as they enter the ion source. Solution flow rates can range from less than a microliter per minute to several milliliters per minute. These methods are well-suited for flow-injection and LC/MS techniques.
i) Electrospray Ionization (ESI):
EI is the classical ionization method in mass spectrometry, which also refered as electron impact ionization. This is the oldest and best-characterized of all the ionization methods. In this technique neutral analytes in the gases phase are sprayed out from a fine needle, positioned with its end pushing into a cage formed by metal surround. There is a big voltage between the needle and the cage, so the little sprayed droplets undergoes in a strong electric field. The ions already floating around in the droplets repel each other. Eventually the repulsive forces between the ions overcome the surface tension of the droplet, and it explodes into many tiny droplets. Thus, this process can either produce a molecular ionwhich will have the same molecular weight and elemental composition of the starting analyte, or it can produce a fragment ionwhich corresponds to a smaller piece of the analyte molecule. Here energy for the fragmentation is provided up to 70eV. Decreasing the electron energy can reduce fragmentation, but it also reduces the number of ions formed.
Figure5:Ionization Process by Electrospray Ionization
Liquid chromatography–mass spectrometry (LC-MS): Electrospray ionization is the ion source of choice to couple liquid chromatography with mass spectrometry. The analysis can be performed by two way, i) Online, by feeding the liquid eluting from the LC column directly to an electrospray, ii) Offline, by collecting fractions to be later analyzed in a classical nanoelectrospray-mass spectrometry setup.
Noncovalent gas phase interactions: Electrospray ionization is also ideal in studying noncovalent gas phase interactions. The electrospray process is capable of transferring liquid-phase noncovalent complexes into the gas phase without disrupting the noncovalent interaction. This means that a cluster of two molecules can be studied in the gas phase by other mass spectrometry techniques.
An interesting example of this is studying the interactions between enzymes and drugs which are inhibitors of the enzyme. Because inhibitors generally work by noncovalently binding to its target enzyme with reasonable affinity the noncovalent complex can be studied in this way. Competition studies have been done in this way to screen for potential new drug candidates.
The structural characterization of natural soil organic matter: A critical component of environmental processes and the global carbon cycle. This technique represents a significant advance in the identification of compounds within humic substances23.
ii) Atmospheric Pressure Chemical Ionization (APCI):
Atmospheric pressure chemical ionization (APCI) is an ionization method used in mass spectrometry, which is a form of chemical ionization which takes place at atmospheric pressure24.
The significant difference is that APCI occurs at atmospheric pressure and has its primary applications in the areas of ionisation of low mass pharmaceutical compounds. APCI is not suitable for the analysis of thermally labile compounds. The general source set-up shares a strong resemblance to electrospray ionisation (ESI) and as such is most commonly used in conjunction with HPLC or other flow separation techniques. Where APCI differs to ESI, is in the way ionisation occurs. In ESI, ionisation is bought about through the potential difference between the spray needle and the cone along with rapid but gentle desolvation. In APCI, the analyte solution is introduced into a pneumatic nebulizer and desolvated in a heated quartz tube before interacting with the corona discharge creating ions.
The corona discharge replaces the electron filament in CI - the atmospheric pressure would quickly "burn out" any filaments - and produces primary N2°+ and N4°+ by electron ionisation. These primary ions collide with the vaporized solvent molecules to form secondary reactant gas ions - e.g. H3O+ and (H2O)nH+ (see fig. 2).
Figure6:Ionization Process by Atmospheric Pressure Chemical Ionization
These reactant gas ions then undergo repeated collisions with the analyte resulting in the formation of analyte ions. The high frequency of collisions results in a high ionisation efficiency and thermalisation of the analyte ions. This results in spectra of predominantly molecular species and adducts ions with very little fragmentation. Once the ions are formed, they enter the pumping and focussing stage in much the same as the other atmospheric pressure ionisation sources.
Assuming nitrogen is the sheath and nebulizer gas with atmospheric water vapour present in the source, then the type of primary and secondary reactions that occur in the corona discharge (plasma) region during APCI are as follows:
The most abundant secondary cluster ion is (H2O)2H+ along with significant amounts (H2O)3H+ and H3O+. The reactions listed above are ways to account for the formation of these ions during the plasma stage.
The protonated analyte ions are then formed by gas-phase ion-molecule reactions of these charger cluster ions with the analyte molecules. This results in the abundant formation of [M+H]+ ions.
Atmospheric pressure chemical ionization (APCI) mass spectrometry (MS) has proven to be a very valuable technique for analysis of lipids from a variety of classes. This instrumental method readily produces useful ions with gentle fragmentation from large neutral molecules such as triacylglycerols and carotenoids, which are often difficult to analyze using other techniques25.
This technique was used in the identification of triacylglycerol molecular species in lymph samples from rats given either a structured lipid or safflower oil. The structured lipid was MLM-type (M, medium-chain fatty acid; L, long-chain fatty acid) and manufactured from caprylic acid (8:0) and the oil26.
Determination of morphine and its 3- and 6-glucuronides, codeine, codeine-glucuronide and 6-monoacetylmorphine in body fluids can be done by liquid chromatography atmospheric pressure chemical ionization mass spectrometry27.
Used to the differentiation of stereoisomeric C19-norditerpenoid alkaloids28.
Most explosives are susceptible to impurities in the solvent and form adducts ions instead of molecular-type ions. These explosives can be identified by this technique29.
5. Laser Desorption
In this case there is use of laser beam for the ionization of the atoms or molecules. Therefore technique is known as laser ionization. And the ionization is occurs due to desorption of laser beam, so also known as laser desorption. One of the mass ionization technique is based on this is Matrix-Assisted Laser Desorption Ionization (MALDI).
Laser desorption methods use a pulsed laser to desorb species from a target surface. So, need to use a mass analyzer like Time-of-Flight (TOF) or Fourier Transform Ion Cyclotron Resonance (FTICR). Magnetic sector mass spectrometer equipped with an array detector is also used for the detection of ions produced by MALDI.
Major application of this technique is in quantitative analysis of benzene which is a major industrial air pollutant.30
i) Matrix-assisted laser desorption/ionization (MALDI):
Matrix-assisted laser desorption/ionization (MALDI) is a soft ionization technique used in mass spectrometry, allowing the analysis of biomolecules (biopolymers such as proteins, peptides and sugars) and large organic molecules (such as polymers, dendrimers and other macromolecules), which tend to be fragile and fragment when ionized by more conventional ionization methods.
The ionization is triggered by a laser beam (normally a nitrogen laser). A matrix is used to protect the biomolecules from being destroyed by direct laser beam and to facilitate vaporization and ionization.
The matrix consists of crystallized molecules like 3,5-dimethoxy-4-hydroxycinnamic acid (sinapinic acid), α-cyano-4-hydroxycinnamic acid (alpha-cyano or alpha-matrix) and 2,5-dihydroxybenzoic acid (DHB).31
Analyte ions are considered to be produced from a protonation or deprotonation process involving an analyte molecule colliding with a matrix ion in the gas phase. With the cluster ionization model, charged particles are desorbed with a strong photoabsorption by matrix molecules. Analyte ions are subsequently produced by desolvation of matrix from cluster ions. 32
There are main three steps involved in the mechanism of MALDI:
(i) Formation of a 'Solid Solution': It is essential for the matrix to be in access thus leading to the analyte molecules being completely isolated from each other. This eases the formation of the homogenous 'solid solution' required to produce a stable desorption of the analyte.
(ii) Matrix Excitation: The laser beam is focussed onto the surface of the matrix-analyte solid solution. The clusters ejected from the surface consist of analyte molecules surrounded by matrix and salt ions. The matrix molecules evaporate away from the clusters to leave the free analyte in the gas-phase.
(iii) Analyte Ionisation: The photo-excited matrix molecules are stabilised through proton transfer to the analyte. Cation attachment to the analyte is also encouraged during this process. It is in this way that the characteristic [M+X]+ (X= H, Na, K etc.) analyte ions are formed. These ionisation reactions take place in the desorbed matrix-analyte cloud just above the surface. The ions are then extracted into the mass spectrometer for analysis.
For the complete procedure there is just 2 µl of total sample volume is required and loaded onto MALDI sample plate in volatile solvents.
Figure7:Ionization Process by Matrix-assisted laser desorption/ionization
The determination of RNA sequences using base- specific enzymatic cleavagesis a well established method. MALDI-MS of the generated fragments is presented asan efficient technique for a direct read out of the nucleotide sequence. 33
In polymer chemistry MALDI can be used to determine the molar mass distribution.
6. Inductively Coupled Plasma (ICP):
Inductively coupled plasma (ICP) is a type of plasma source in which the energy is supplied by electrical currents which are produced by electromagnetic induction.
Figure 8: Ionization Process by Inductively Coupled Plasma
Inductively Coupled Plasma Spectroscopy technique is also called "wet" sampling method because here samples are introduced in liquid form for analysis. In the case of mass spectrometry, when ions are produced by inductively coupled plasma it is known as ICP-MS (Inductively coupled plasma mass spectrometry). This is a technique which is a type of mass spectrometry that is highly sensitive and capable of the determination of a range of metals and several non-metals at concentrations below one part in 1012. ICP-MS is also capable of monitoring isotopic speciation for the ions of choice.
Plasma is a gas that contains a sufficient concentration of ions and electrons to make the gas electrically conductive. The plasmas used in spectrochemical analysis are essentially electrically neutral, with each positive charge on an ion balanced by a free electron. In these plasmas the positive ions are almost all singly-charged and there are few negative ions, so there are nearly equal amounts of ions and electrons in each unit volume of plasma.
ICP burner head consists of three turns of radio-frequency (RF) induction coil wrapped around the upper opening of a quartz chamber. High-purity argon gas is fed through the plasma gas inlet. A spark from a coil to ionize the Ar gas. The Ar+ ions are immediately accelerated by the powerful RF field that oscillates about the load coil at a frequency of 27MHz. The accelerated ions transfer energy to the entire gas by collisions between atoms. Once the process has begun, the ions absorb enough energy from the electric field to maintain the plasma at temperature of 6000-10000K. It is so hot that the quartz burner must be protected by argon coolant gas flowing around the outer edge.
After injecting the sample, the plasma's extreme temperature causes the sample to separate into individual atoms. Next, the plasma ionizes these atoms (M → M+ + e-) so that they can be detected by the mass spectrometer.
The high sensitivity of inductively coupled plasma mass spectrometry (ICP-MS) has resulted in an increased popularity of this technique for the analysis of metal-based anticancer drugs. In addition to the quantitative analysis of the metal of interest in a sample, ICP-MS can be used as an ultrasensitive metal selective detector in combination with speciation techniques such as liquid chromatography34.
The application of inductively coupled plasma mass spectrometry (ICP-MS) to multielement analysis in fingernail and toenail as biological indices for metal exposure is presented. The ICP-MS measurements were performed using a Thermo Elemental X7CCT series35.
Inductively coupled plasma dynamic reaction cell mass spectrometry (ICP-DRC-MS) was characterised for the detection of the six naturally occurring selenium isotopes. The potentially interfering argon dimers at the selenium masses m/z 74, 76, 78 and 80 were reduced in intensity by approximately five orders of magnitude by using methane as reactive cell gas in the DRC. By using 3% v/v methanol in water for carbon-enhanced ionisation of selenium, the sensitivity of 80Se was 104 counts s–1 per ng ml–1 of selenium, and the estimated limit of detection was 6 pg ml–1. The precision of the isotope ratios was close to the theoretical values for selenium concentrations at 1 and 10 ng ml–1. 36
Inductively coupled plasma mass spectrometry (ICP-MS) and laser ablation ICP-MS (LA-ICP-MS) have been applied as the most important inorganic mass spectrometric techniques having multielemental capability for the characterization of solid samples in materials science. ICP-MS is used for the sensitive determination of trace and ultratrace elements in digested solutions of solid samples or of process chemicals (ultrapure water, acids and organic solutions) for the semiconductor industry with detection limits down to sub-picogram per liter levels. Whereas ICP-MS on solid samples (e.g. high-purity ceramics) sometimes requires time-consuming sample preparation for its application in materials science, and the risk of contamination is a serious drawback, a fast, direct determination of trace elements in solid materials without any sample preparation by LA-ICP-MS is possible. The detection limits for the direct analysis of solid samples by LA-ICP-MS have been determined for many elements down to the nanogram per gram range37.
7. High pressure ionization:
High pressure ionization methods have been found to be particularly useful for drug identification since they often produce the base peak of the spectrum in the molecular ion region, thus serving to identify the molecular weight of the compound. However, some compounds, notably heroin and morphine, fragment easily when ionized with the high pressure reagent gases used so far38.
A high pressure ionization gauge is described with a novel electrode system of rugged construction. The electrodes consist of two parallel electron collector plates and two parallel ion collector plates mounted at right angles to each other with a filament running axially down the centre. The filament consists of a platinum-10% rhodium wire with an yttrium oxide coating allowing the gauge to operate in reactive gases such as atmospheric air up to a pressure of 5 torr. The evaluation of the gauge characteristics have enabled the optimum operating conditions to be achieved. The gauge sensitivity is equal to 0.11 torr-1 for air and constant within the pressure range 1 torr to 50 μtorr providing the electron accelerating potential is not greater than 42 V. The gauge can be usefully employed in the pressure range required for applications such as cathodic sputtering, vacuum melting, gas discharge studies, and as a secondary calibration standard39.
High-pressure ammonia chemical ionization mass spectrometry (CI-MS) and chemical ionization mass spectrometry/mass spectrometry (CI-MS/MS) methods for porphyrin analysis is done. Adjustment of ammonia CI parameters, in particular the ion source temperature, are used to alter the appearance of the mass spectra and, therefore, their information content. At high ion source temperatures (>523 K), protonation predominates and little fragmentation is observed. The daughter ion MS/MS spectra of the protonated porphyrins is shown to provide information regarding the nature of the peripheral macrocycle substituents. Operating at low ion source temperatures (typically 423 K) promotes a surface-assisted reduction and decomposition of the porphyrins macrocycle resulting in a relatively complex spectrum that consists of mono-, di-, and tripyrrolic fragment ions. From the masses and pattern of the pyrrolic fragments in the mass spectrum, the pyrrole sequence of the porphyrin can, in many cases, be determined. This technique can be used to sequence only individual porphyrin isomers, and the spectra are often difficult to reproduce and/or complicated by the presence of structurally uninformative peaks. A superior pyrrole sequencing method is developed recently, which is based on CI-MS/MS. In this method, the daughter ion spectrum of the reduction product, (M+7H)+ (i.e., the protonated porphyrinogen), produced in situ is used to pyrrole sequence the porphyrins40.
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Received on 28.03.2010 Modified on 15.04.2009
Accepted on 22.05.2010 © AJRC All right reserved
Asian J. Research Chem. 3(3): July- Sept. 2010; Page 518-527