The Dart Ionization Principles Biology Essay

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Mass spectrometry is very valuable areas in analytical instrumentation. The use of mass spectrometry is well established in pharmaceutical and organic chemistry, as well in forensic science, materials science and environmental research. But presently available ion source put extreme limitations on the speed and convenience of sample analysis by mass spectrometry. In 2005, the new ion source reported which overcome these limitations. This new technique referred to as Direct Analysis in Real Time (DART). The ionization of low -molecular mass mixtures present on the surface of solid or liquids in a gas stream without sample preparation or chromatographic separation of components of the sample is a unique characteristic of (DART) Mass spectrometry compared to traditional mass spectrometry. DART-MS allows one to carry out a fast non-contact analysis of various samples, there for hundreds of chemicals and its agent has been successfully tested using DART and its analytical applications. {1, 2}

The DART ionization principles:-

DART is based on the atmospheric pressure interaction of long-lived electronic excited state atoms or vibronic excite state molecules with the sample and atmospheric gases. {1}

Where the

1) Gas inlet

2) Needle electrode

3) Disk electrodes

4) Gas heater

5) Grid electrode

6) Insulator cap

7) Gas outlet

Figure 1 Schematic (a) and detailed view (b) of a DART source {2}

A schematic diagram of the DART ion source is shown in Fig.1. A gas flows through a discharge chamber where electrons, ions and metastable molecules or atoms are produced. Then, highest charged particles removed as gas passes through perforated lenses or grids and only the neutral gas particles as well as metastable particles remain and then this remain gas particles is heated to provide thermal desorption of analytic molecule. Then perforated lens and grid at the exit of DART provides some very useful function such as it performances as a source of electrons by surface penning ionization, it prevents ion-electron recombination as well ion-ion recombination, it also act as an electrode to stimulate ion drift toward the orifice of the mass spectrometer's atmospheric pressure interface. In DART-MS experiments, there are quite a few ionization mechanisms are possible which depends on the reaction gas and polarity. Most experiments carried out with hilum gas but some of them carried out with nitrogen or neon such as penning ionization which is the simplest process, involving transfer of the energy of a long-lived excited-sate metastable molecule or atom(N*) to the analyte molecule (M) which have lower ionization potential than the internal energy of (N*). This reaction result in production of a radical molecular cation M+- and an electron (e−). {1, 2}

N* + M → M+- + N + e−. {1, 2}

Figure2 Gas-phase reaction as stage of the DART ionization Mechanism {4}

The positive ion spectra showed signals of M+ and (M+H)+.

The negative ion spectra showed signals of (M+H) −.

Applications:-

DART-MS has many applications in both Qualitative analysis and Quantitative analysis which been used to analyse wide range of analytes. The main advantage of using DART is that material can be analysed directly on surface such as TLC plate, glass, and etc. without using wipes or solvent removal. {2}

Qualitative analysis:-

In Qualitative analysis, analysis of pharmaceutical tablets is the fastest. In this scenario, only the specific signals of the active pharmaceutical ingredients (API) shown frequently by the DART mass spectra. That because the matrix is non-volatile. For example, DART mass spectra of Mexidol, in which API is ethylmethylhydroxypyridine. The protonated API particles (M+H)+ contains quite strong signal in the DART spectrum at 138m/z which provide instant confirmation of the presence of the desired component in the tablet. Therefore the active ingredient in Mexidol can be rapidly detected. {2, 3}

Figure 3 DART mass spectrum of Mexidol tablet. {2}

The DART mass spectrometry shows great result in all cases. Using the DART application is very easy to obtain data about the presence or absence of an active ingredient. The dimer ion (2M+H)+ also observed which is useful for additional conformation of the active ingredients.{3}

Quantitative analysis:-

Combination of DART-MS with different kind of thin-layer chromatography (TLC) is of mainly very useful for analysis of complex multicomponent mixtures. TLC with DART-MS detection is projected to screen and subsequently identify drug compounds. DART conformation of TLC analysis takes less than 10 mints to complete and it is sensitive and selective in compare to GS-MS.{2, 5} It has been reported successful application of DART-MS in the natural products analysis field. Three known herbal drugs (Namely- Angelicae gigantis radix, Evodiae fructus and Schisandrae fructus were extracted using TLC/DART-MS. As shown in experiment procedure, to analyse TLC plate using DART-MS, the developed TLC plate was air dried and observed under UV light to localize the band matching to each marker standard compound and the plate was cut to around 0.5cm. Then the band of marker compound in medicine were directly positioned under the helium gas stream of DART ion source and as the band of marker compound pass through the DART gas stream, the protonated particle was appeared on the mass spectrum. {6}

Figure 3 TLC condition, detection methods and marker compound for each herbal medicine. {6}

Figure 4 TLC plate was visualized under UV light of 365 nm. {6}

Coupling of TLC with DART-MS can be used as real time reaction monitoring device. Some compounds which have molecular mass above 6000 were detected using DART source (at 500-c). So TLC/DART-MS also appropriate for the identification and separation of demethoxy-curcumin, curcumin and bisdemethoxycurcumin in turmeric extracts.{2}

There are many other DART-MS application available for analyse wide range of organic compounds, drugs, explosive etc. Some more examples are shown below.

In fig 5 where DART used to analyse drug containing anti-malarial dihydroartemisinin and no active ingredients. {1}

In fig 6 where DART was used for detection of drug and metabolites in raw, blood, saliva, urine. {1}

In fig 7 where DART was used for the detection of explosive such as TNT, HMX and etc. {1}

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