ATOMIC FORCE MICROSCOPY

Atomic force microscopy (AFM) or scanning force microscopy (SFM) is a very high-resolution type of scanning probe microscopy, with demonstrated resolution on the order of fractions of a nanometer, AFM provides a 3D profile of the surface on a nanoscale, by measuring forces between a sharp probe (<10 nm) and surface at very short distance (0.2-10 nm probe-sample separation). The probe is supported on a flexible cantilever. The AFM tip “gently” touches the surface and records the small force between the probe and the surface.

The AFM measures the forces acting between a fine tip and a sample. The tip is attached to the free end of a cantilever, The cantilever is typically silicon or silicon nitride with a tip radius of curvature on the order of nanometres and it is brought very close to a surface. Attractive or repulsive forces resulting from interactions between the tip and the surface will cause a positive or negative bending of the cantilever. The bending is detected by means of a laser beam, which is reflected from the back side of the cantilever. The figure  shows the basic concept of AFM :

What types of forces are measured?

Plot of force as a function of probe-sample separation

The dominant interactions at short probe-sample distances in the AFM are Van der Waals interactions. However long-range interactions (i.e. capillary, electrostatic, magnetic) are significant further away from the surface. These are important in other SPM methods of analysis. During contact with the sample, the probe predominately experiences repulsive Van der Waals forces (contact mode). This leads to the tip deflection described previously. As the tip moves further away from the surface attractive Van der Waals forces are dominant (non-contact mode).

Modes of operation

CONTACT MODE

In the So-called contact-AFM mode, the tip makes soft “physical contact” with the surface of the sample. The deflection of the cantilever Dx is proportional to the force acting on the tip, via Hook’s law, F=-k. x, where k is the spring constant of the cantilever. In contact-mode the tip either scans at a constant small height above the surface or under the conditions of a constant force. In the constant height mode the height of the tip is fixed, whereas in the constant-force mode the deflection of the cantilever is fixed and the motion of the scanner in z-direction is recorded. By using contact-mode AFM, even “atomic resolution” images are obtained. For contact mode AFM imaging, it is necessary to have a cantilever which is soft enough to be deflected by very small forces and has a high enough resonant frequency to not be susceptible to vibration instabilities. Silicon Nitride tips are

used for contact mode. In these tips, there are 4 cantilever with different geometries attached to each substrate, resulting in 4 different spring constants.

Probe with four different cantilevers with different spring constants (N/m

Advantages:

• High scan speeds
• Atomic resolution” is possible
• Easier scanning of rough samples with extreme changes in vertical topography

Disadvantages:

• Lateral forces can distort the image.
• Capillary forces from a fluid layer can cause large forces normal to the tip sample interaction
• Combination of these forces reduces spatial resolution and can cause damage to soft samples.

INTERMITTENT MODE (TAPPING)

The imaging is similar to contact. However, in this mode the cantilever is oscillated at its resonant frequency, Figure 4. The probe lightly “taps” on the sample surface during scanning, contacting the surface at the bottom of its swing. By maintaining a constant oscillation amplitude a constant tip-sample interaction is  maintained and an image of the surface is obtained.

Advantages:

• Higher lateral resolution (1 nm to 5 nm).
• Lower forces and less damage to soft samples in air.
• Almost no lateral forces.

Disadvantage:

• Slower scan speed than in contact mode.

  

  NON CONTACT MODE

(Attractive VdW) The probe does not contact the sample surface, but oscillates above the adsorbed fluid layer on the surface during scanning. (Note: all samples unless in a controlled UHV or environmental chamber have some liquid adsorbed on the surface). Using a feedback loop to monitor changes in the amplitude due to attractive VdW forces the surface topography can be measured.

Advantage:

• VERY low force exerted on the sample(10^-12 N), extended probe lifetime.

Disadvantages:

• Lower lateral resolution, limited by tip-sample separation.
• Contaminant layer on surface can interfere with oscillation; usually need ultra-high vacuum (UHV) to have best imaging.
• Slower scan speed to avoid contact with fluid layer.
• Usually only applicable in extremely hydrophobic samples with a minimal fluid layer.

Applications :

The number of applications for AFM has exploded since it was invented in 1986 and Nowadays this technique is involved in many fields of Nanoscience and nanotechnology. The remarkable feature of STM and AFM instruments is their ability to examine samples not only in an ultrahigh vacuum but also on ambient conditions or even in liquids. AFM can image the non-conducting surfaces, and therefore it is very suitable for biological systems.

Possible applications of AFM are :

• Substrate roughness analysis.
• Step formation in thin film epitaxial deposition.
• Pinholes formation or other defects in oxides growth.
• Grain size analysis.
• Phase mode is very sensitive to variations in material properties, including surface stiffness, elasticity and adhesion.
• Comparing the tip-samples forces curves for materials to study the ratio of Young´s Modulus (graphite as a reference for measure of the indentation).
• Obtaining information of what is happening under indentation at very small loads .
• By In situ AFM analysis with changes in temperature we can study changes in the structure.

References :

1.  Basic Theory Atomic Force Microscopy (AFM) by  Robert A. Wilson and Heather A. Bullen,* Department of Chemistry, Northern Kentucky University, Highland Heights

2.  Principles of atomic force microscopy (AFM) written by Arantxa Vilalta-Clemente , Aristotle University, Thessaloniki, Greece and Kathrin Gloystein, Aristotle University, Thessaloniki, Greece

3.  http://en.wikipedia.org/wiki/Atomic_force_microscopy

HIGH PERFORMANCE LIQUID CHROMATOGRAPHY

High-performance liquid chromatography (sometimes referred to as high-pressure liquid chromatography), HPLC, is a chromatographic technique used to separate a mixture of compounds in analytical chemistry and biochemistry with the purpose of identifying, quantifying or purifying the individual components of the mixture.

Basic block diagram

HPLC is accomplished by injection of a small amount of liquid sample into a moving stream of liquid (called the mobile phase) that passes through a column packed with particles of stationary phase. Separation of a mixture into its components depends on different degrees of retention of each component in the column. The extent to which a component is retained in the column is determined by its partitioning between the liquid mobile phase and the stationary phase. In HPLC this partitioning is affected by the relative solute/stationary phase and solute/mobile phase interactions. Thus, unlike GC, changes in mobile phase composition can have an enormous impact on your separation. Since the compounds have different mobilities, they exit the column at different times; i.e., they have different retention times, tR. The retention time is the time between injection and detection. There are numerous detectors which can be used in liquid chromatography. It is a device that senses the presence of components different from the liquid mobile phase and converts that information to an electrical signal. For qualitative identification one must rely on matching retention times of known compounds with the retention times of components in the unknown mixture.Quantitative analysis is often accomplished with HPLC. An automatic injector providing reproducible injection volumes is extremely beneficial, and are standard on modern commercial systems.

HPLC is just one type of liquid chromatography, meaning the mobile phase is a liquid. In this lab you will use what is called reversed phase HPLC. Reversed phase HPLC is the most common type of HPLC. What reversed phase means is that the mobile phase is relatively polar, and the stationary phase is relatively non-polar. Thus non-polar compounds will be more retained (i.e. Have longer retention times) than a polar compound. In normal phase HPLC, the mobile phase is relatively non-polar and the stationary phase is relatively polar. Other more general types of HPLC include partition, adsorption, ion-exchange, size-exclusion, and thin-layer chromatography.

Application :

• Quantitative/qualitative analyses of amino acids, nucleic acids, proteins in physiological samples.
• Measuring levels of active drugs, synthetic by-products, degradation products in pharmaceuticals.
• Measuring levels of hazardous compounds such as pesticides and insecticides.
• Monitoring environmental samples.
• Purifying compounds from mixtures.

GLOW DISCHARGE OPTICAL EMISSION SPECTROMETRY

GDOES made its first appearance in 1968 and was designed primarily for bulk spectrochemical analysis of various metals and their alloys. Since its introduction, this method has been steadily developed and has excelled in the areas of surface and coating analysis as well. Compared with conventional excitation techniques, the striking feature of Glow Discharge Technology is the ability to discern defined surface layers of the material being examined and analyze their chemical composition. In the field of metal analysis GDOES is ideal for concentration profile analysis and surface analysis. All kinds of surface treatment processes as well as surface coating processes can be monitored by analyzing the surface and near-surface areas the treated material. Coating thickness and chemical composition can be accurately measured using the technique of depth profile analysis. GDOES is the preferred method of analysis for materials that were previously impossible to analyze by traditional methods, and it is one of the fastest methods available.


A stream of argon ions mill material from the sample surface. The sputtered material is then excited in a low pressure plasma discharge and resulting light emission is used to characterize and quantify the sample's composition. Glow Discharge offers an improved excitation source for fast, economical, accurate, and reliable sample turnaround. This source ultimately removes material from the sample surface which reduces the effects of metallurgical and chemical history inherent in all samples.
.

A Glow Discharge Optical Emission Spectrometer (GD-OES) is built of a glow discharge source and one or more optical spectrometers, including detectors, either Photomultiplier tubes or solid state detectors, usually CCD's. A schematic layout is given to the above. The spectrometer displayed here using a concave grating in the Rowland circle or Paschen-Runge configuration and photomultiplier tubes for the light detection.

The use of solid state detectors, CCD's and photo diode array's have become a common alternative to Photomultiplier tubes. These detectors allow the acquisition of the entire spectrum, or at least a large portion of it, but are usually slower than Photomultiplier tubes and therefore not suitable for very short acquisition times used in thin film analysis.

The principle of operation is fairly easy to understand. In a glow discharge, cathodic sputtering is used to remove material layer by layer from the sample surface. The atoms removed from the migrate into the plasma where they are excited through collisions with electrons or metastable carrier gas atoms. The characteristic spectrum emitted by this excited atom is measured by the spectrometer.

GDOES can be used in many industries such as :

• Automotive industry and its suppliers.
• Metalworking industry.
• Iron and steel industry.
• Aerospace industry.
• Electronics industry.
• Glass and ceramics industry.
• Surface technology.
• Galvanizing industry.
• Photovoltaic industry.
• Scientific institutes.

Advantages of GDOES

• Limited matrix effect.
• Linear working curves.
• Minimal spectral interferences.
• Excellent precision.
• Analysis of difficult materials (as-cast iron, low melting point alloys).
• Automatic cleaning between samples.
• Low reference material and gas consumption.

FOURIER-TRANSFORM NEAR IR GRAIN ANALYZER

Near-infrared reflectance (NIR) spectroscopy is widely used for the quantitative determination of quality attributes such as moisture, protein, fat, and kernel hardness in agriculture and food products . It is an approved method for quantitative measurement of wheat protein and moisture content. NIR instruments for grain and grain product measurement have predominantly used grating monochromators to obtain spectral information, which is measured with a single or diode array detector. NIR instruments use various hardware configurations to obtain spectral information, and Fourier-transform near-infrared reflectance (FT-NIR) instruments are only one method to do this. FT-NIR hardware is generally more complex but advances in electronics, methods, and manufacturing have significantly improved the detector sensitivities, resolution, and immunity from vibration effects .

FT-NIR records the intensity of the entire spectrum as a function of the optical path differences (OPD) between two NIR beams in an interferometer. The two beams are created by splitting the measurement beam, i.e., the beam that is transmitted through or reflected from the specimen. One split beam travels over a different optical path length, via a moving mirror, and is recombined with the second beam to create an interference signal. The total interference signal results from the mirror traveling through a range of wavelengths and is transformed to spectral components via a fast Fourier transform.  Gave a detailed
explanation of FT-NIR and cited its advantages over conventional grating NIR spectroscopy as 1) higher signalto-noise ratios, 2) extremely high resolutions, and 3) fast and accurate frequency determinations. The first advantage is realized because there are fewer optical components to attenuate radiation which leads to greater power reaching the detector and a better signal-to-noise ratio. The second advantage is that resolution at discrete wavelengths is much better, and thus elements which interact within very narrow
bands can be detected. The third advantage states that spectral collection is much faster than with a dispersive instrument, since all wavelengths are measured simultaneously. While this last advantage is somewhat true, it can also be misleading since simultaneous measurement occurs only at discrete mirror positions. The total interference signal must be measured at discrete points during the mirror’s travel, which requires a finite time period. After the entire time-domain interference signal is measured, it is then
transformed to the frequency (spectral) domain via a fast Fourier transform (FFT) to obtain discrete wavelength information. When using FT-NIR at the shorter NIR wavelengths, compared to mid-infrared, the
interference pattern is more influenced by misalignment and may cancel any added advantage FT-NIR may have over grating NIR instruments. Thus small perturbations in mirror positioning or misalignment are sources of error and as a result, lasers are commonly incorporated to provide a position reference. To achieve a position reference, monochromatic laser output is directed through the interferometer onto its own detector. The precision of this alignment is not generally specified by the manufacturer. Other literature indicated that the advantages of   FT-NIR/IR are in the mid-infrared region, not the NIR region, because of the higher signal/noise (S/N) ratio using detector noise- limited instrumentation. NIR instruments do not use
detector-noise limited instrumentation. Biological materials absorbs over broad regions in the NIR, not at discrete wavelengths. Thus, the advantage of FT-NIR being able to detect absorbance in very narrow bands may not be a significant benefit. 

FT-NIR has been applied to study various attributes such as fat, protein, cholesterol, lactose, and internal quality in products of agriculture and food industries.

LASER BASED LIQUID PARTICLE COUNTER

A particle counter is an instrument that detects and counts particles. By its very nature a particle counter is a single particle counter, meaning it detects and counts particles one at a time. The nature of particle counting is based upon either light scattering, light obscuration, or direct imaging. A high energy light source is used to illuminate the particle as it passes through the detection chamber. The particle passes through the light source (typically a laser or halogen light) and if light scattering is used, then the redirected light is detected by a photo detector. If direct imaging is used, a halogen light illuminates particles from the back within a cell while a high definition, high magnification camera records passing particles. Recorded video is then analyzed by computer software to measure particle attributes. If light blocking (obscuration) is used the loss of light is detected. The amplitude of the light scattered or light blocked is measured and the particle is counted and tabulated into standardized counting bins.

  The laser beam passes through the walls of a glass container or a flow-thru cell

Particle counter uses as its basic light source a laser diode (650nm wavelength). The beam from this laser is spatially filtered and focused by a lens assembly to form a small and well-defined illuminated volume within the liquid being inspected. A scanning mechanism provides a circular displacement of this illuminated volume at a constant rate of speed. As the illuminated volume moves across a particle suspended in the liquid, some light from the beam will be scattered. This is known as Fraunhofer diffraction. Most of this scattered light is in the near-forward direction and is collected by the optical system of the photodetector assembly. The flash of light striking the photodetector will cause an electrical pulse in the preamplifier connected to the photodetector. The amplitude and width of this pulse are a function of the size of the particles, The analog signals generated by the light pulses are routed to a computer and digitized.

Applications :

• Quality control of hydraulic fluids and oils.
• De-ionized water and acid testing for semiconductor manufacturing.
• Vial and ampule inspection for pharmaceuticals.
• Silt and sediment sizing.
• Oceanographic particles.
• Sizing for corrosive chemicals and solvents.
• Cell counting where physical force would damage particles.
• Particle agglomeration studies.
• Water treatment plants.
• Filter efficiency control.
• Powdered solids manufacturing.

FOURIER TRANSFORM-INFRARED SPECTROSCOPY

Fourier Transform-Infrared Spectroscopy (FTIR) is an analytical technique used to identify organic (and in some cases inorganic) materials. This technique measures the absorption of infrared radiation by the sample material versus wavelength. The infrared absorption bands identify molecular components and structures.
When a material is irradiated with infrared radiation, absorbed IR radiation usually excites molecules into a higher vibrational state. The wavelength of light absorbed by a particular molecule is a function of the energy difference between the at-rest and excited vibrational states. The wavelengths that are absorbed by the sample are characteristic of its molecular structure . 

FTIR analysis became the main used technique when specific analytical topics have to be addressed, mainly when non-destructive analysis is needed. In this respect, according to our opinion, challenging analytical issues are raised in two important cases; the first one is that when historic (archaeological) or artistic materials have to be analyzed while the second issue came from the analysis of highly- specific biomaterials

The FTIR spectrometer uses an interferometer to modulate the wavelength from a broadband infrared source. A detector measures the intensity of transmitting or reflected light as a function of its wavelength. The signal obtained from the detector is an interferogram, which must be analyzed by a computer using Fourier transforms to obtain a single-beam infrared spectrum. The FTIR spectra are usually presented as plots of intensity versus wavenumber (in cm-1). Wavenumber is the reciprocal of the wavelength. The intensity can be plotted as the percentage of light transmittance or absorbance at each wavenumber


TYPICAL APPLICATIONS

• Identification of foreign materials
- Particulates
- Fibers
- Residues
• Identification of bulk material compounds
• Identification of constituents in multilayered materials
• Quantitation of silicone, esters, etc., as contamination of various materials

INTRODUCTION TO SPECTROSCOPY

Spectroscopy was originally the study of the interaction between radiation and matter as a function of wavelength (λ). In fact, historically , spectroscopy referred to the use of visible light dispersed according to its wavelength, e.g. By a prism. Later the concept was expanded greatly to comprise any measurement of a quantity as a function of either wavelength or frequency. Thus it also can refer to a response to an alternating field or the varying frequency (ν). Spectroscopy originated through the study of visible light dispersed according to its wavelength. Later the concept was expanded greatly to comprise any interaction with radiative energy as a function of its wavelength or frequency. A further extension of the scope of the definition added energy (E) as a variable, once the very close relationship E = hν for photons was realized (h is the Planck constant). A plot of the response as a function of wavelength—or more commonly frequency—is referred to as a spectrum.

Spectrometry is the spectroscopic technique used to assess the concentration or amount of a given species. In those cases, the instrument that performs such measurements is a spectrometer or spectrograph.
Spectroscopy/spectrometry is often used in physical and analytical chemistry for the identification of substances through the spectrum emitted from or absorbed by them. Spectroscopy/spectrometry is also heavily used in astronomy and remote sensing. Most large telescopes have spectrometers, which are used either to measure the chemical composition and physical properties of astronomical objects or to measure their velocities from the Doppler shift of their spectral lines.

OPTICAL EMISSION SPECTROSCOPY

Optical Emission Spectrometry (OES) is a fast, accurate and reliable method for quantitative analysis of  metals to about 1–3% relative precision.Optical emission spectroscopy methods are among the most useful and flexible means of performing elemental analysis of Steel ,Copper , Aluminium,Magnesium, Lead etc and its alloys. Accurate analysis of metals and alloys requires that :

• The melt or bulk sample is homogeneous;
• Samples are representative of the melt or bulk sample  and
• The metallurgical structure of the sample is similar to that of the standards used. The sample requirements are met by chill-cast disk samples collected as described in ASTM E 716-94 (or equivalents AS2612-83 or JISH 1305-1976). For the point-to-plane method of spectrochemical analysis  , samples are machined on a lathe to give a smooth, clean surface just prior to analysis.
The sample is mounted in the spectrometer and a spark is generated between the sample and a tungsten electrode. The material is volatilized from the surface of the sample resulting in the emission of light. This light is diffracted into its component wavelengths inside the spectrometer. Fixed detectors simultaneously measure light intensity at wavelengths characteristic of each element. OES requires calibration of the spectrometer
with certified chill-cast standards, which must have the same matrix as the alloys to be tested. Therefore
quantitative OES can only be performed on chillcast samples collected from the melt according to
standard methods. Other techniques, such as Induction Coupled Plasma (ICP), can be used for
quantitative analysis of alloys in other forms, for example billet slices, ingots, extrusions and forgings.

OES PRINCIPLE


The OES system consists of four main parts :
• Sample stand.
• Spark generator.
• Optics assembly.
• Data Acquisition Electronics assembly.

The clean sample is mounted in the stand and a spark is generated between the sample and a tungsten electrode. An example of this setup is illustrated in Figure 1. High purity argon is used as a discharge atmosphere to prevent any interaction between the atmosphere and the sample surface.

The material is violated from the surface of the sample resulting in the emission of light. When atoms are excited by an external energy source, some electrons move into higher energy levels. As these electrons drop back to their original levels, the atoms emit light (photons) of characteristic wavelength. The spectrometer collects the light emitted from the sample and splits it into its component wavelengths using a diffraction grating. Fixed detectors (photo multiplier tubes in above figure) simultaneously measure light intensity at wavelengths characteristic of each element. This is illustrated in Figure above. The intensity of the signals depends on the number of photons produced per unit time. The spectrometer is programmed for fixed sample types, elements and concentration ranges and only give results within the limits of calibration .

WHAT IS ANALYTICAL INSTRUMENT ?

An Analytical Instrument can be any type of equipment, apparatus or device as is specifically designed, constructed and often, through trial and error, ingeniously refined to apply utmost efficiency in the utilization of well proven physical principle, relationship or technology to facilitate or enable the pursuit, acquisition, transduction and storage of repeatable, verifiable data, usually consisting of sets numerical measurements made upon otherwise unknown, unproven quantities, properties, phenomena, materials, forces or etc., preferably as those characterized over time by an increasing degree of accuracy and precision and, typically, those initially derived as isolated or dependent variable results from, or empirical observations made during, the course of such experimental procedures as are firmly based upon the scientific method and long accepted tenants of experimental design.

Scientific instruments are part of laboratory equipment, but are considered more sophisticated and more specialized than other measuring instruments as scales, meter sticks, chronometers,thermometers or even power or waveform generators. They are increasingly based upon the integration of computers to improve and simplify control, enhance and extend instrumental functions, conditions, parameter adjustments and data sampling, collection, resolution, analysis (both during and post-process), storage and retrieval. Individual instruments can also be connected as a local area network (LAN) and can be further integrated as part of a laboratory information management system (LIMS), that in addition to having Internet access to databases of such physical properties as compound spectra libraries through the World Wide Web for results comparisons and advanced data analysis as well as the obvious usefulness of now ubiquitous email for rapid exchange of informational text and images, consultation and peer review. Recently, the development has utilized open source principles borrowed from the software industry to rapidly improve low-cost open-source hardware for scientific measurements

Source: Wikipedia