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Prod331 > Stability of Formulations > Flashcards

Stability of Formulations Flashcards

1
Q

What should we investigate when thing of the stability of formulations?

A

The structure of the solid/liquid interface, the structure if the electrical double layer (for change stabilised suspensions), absorbtion of surfactants, coalesce/phase inversion/particle size/ distribution during storage (for emulsions)

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2
Q

What are the different emulsion instabilities?

A

Phase inversion, creaming, sefementation, Oswald ripening, coalescence, flocculation

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3
Q

How do you measure particle and droplet distribution

A

This is essential for Oswald ripening, flocculation and coalescence (in emulsions) of the formulation. Dilute the concentrated formulation with its own dispersion medium, (can be obtained by centrifugation of the formulation)

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4
Q

What does optical/light microscopy tell us?

A

Ali’s Favourite, information on size, shape, and aggregation of particle size

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5
Q

What are the limitations of light optical microscopy? Who was this discovered by?

A

Wavelength of visible light, can’t see anything after the smallert wavelength, any object that’s less than half the wavelength of the microscopes illumination source is not visible. This was discovered by Ernest Abbie

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6
Q

How can you improve the optical microscopes limitation?

A

Phase contrast, differential interference contrast, polarised light microscopy

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7
Q

Describe phase contrast

A

This utilises the difference between the diffracted waves from the main image and the direct light from the light source

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8
Q

Describe differential interference contrast (DIC)

A

Utalises a phase difference to improve contrast

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9
Q

Describe polarised light microscopy

A

Illuminating sample with linearly or ciclualrly polarised light in reflection or transmission mode.

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10
Q

Describe what dark field microscopes uses

A

The ultramicroscope uses dark field illumination to extend the useful range of optical microscopy to small particles not visible in a bright light illumination

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11
Q

How does dark field illumination form an image?

A

Dark field illumination utilises a hollow cone of light at a large angle of incidence. The image is formed by scattered light from the particles against a dark background.

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12
Q

Describe the visibility of particles using darkfeild microscopes compared to bright light illumination. What is the outcome of the images?

A

Particles about 10 times smaller than those visible by bright light illumination can be detected. However, the image obtained is abnormal and the particle size cannot be accurately measured. This is the reason electron microscopy has displaced ultramicroscope, except for dynamic studies by flow ultra-microscopy

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13
Q

What are advantages to dark field microscopy

A

Simple procedure, can be used on live transparent (would otherwise need to be stained or killed), images appear spectacular/visibly impressive, allows for the visualisation of objects below the resolution of the microscope

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14
Q

What are the disadvantages to dark field microscopy?

A

Sensitive to dirt, not suitable for all (refractive index) needs high intensity light

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15
Q

Describe phase contrast microscopy

A

A contrast enhancing technique that can be utilised to produced high-contrast images of transparent specimens, such as living cells, micro-organisms, thin tissue slices

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16
Q

Explain how phase contrast works

A

It utilises the differences between the diffracted waves from the main image and the direct light from the light source, the phase contrast technique employs an optical mechanism to translate minute variations in phase into corresponding changes in amplitude, which can be visualised as differences in image contrast

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17
Q

Describe differential interface contrast (DIC), who was this technique developed by?

A

Gives a better contrast than phase contrast. It ustlises a phase difference to improve contrast but the separation and recombination of a light beam into two beams is accomplished by prisms. (Uses polarised light, only see the refacted light in one direction) Developed by Georges Nomarski in 1952.

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18
Q

How can particle size be measured in optical microscopy?

A

Particle size or triplet sizing can be carried out bu using manual, semiautomatic or automatic image analysis technique

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19
Q

What is electron microscopy?

A

Electron microscopy utilises an electron beam to illuminate the sample. Due to the very short wavelength of electrons the resolving power on an electron microscope exceeds that of an optical microscope. The resolution depends of the accelerating voltage which determines the wavelength of the electron beam and magnifications as high as 200,000 can be reached with intense beams, but this could damage the sample

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20
Q

What is TEM?

A

Transmission electron microscopy

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21
Q

What is SEM?

A

Scanning electron microscopy

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22
Q

How do conventional transmission electron microscopes resemble optical microscope?

A

Image contrast (intensity variation) is produced by the variation in optical absorbtion from point to point on the specimen

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23
Q

What is the main advantage of electron microscopy?

A

High resolution, sufficient for resolving details separated by only a fraction of a nano-metre

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24
Q

What are the disadvantages of electron microscopy?

A

There is a risk or altering or damaging the sample that may introduce artefacts and possible aggregation of the particles during particle separation, can’t use live sample

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25
Q

What do you need to do if the particles in your sample do not conduct electricity, for electron microscopy?

A

Sample has to be coated with a conducting layer, such as gold, carbon or platinum to avoid negative charging by the electron beam

26
Q

What does TEM display?

A

The image of the specimen on a fluorescent screen, and the image can be recorded on a photographic plate or film.

27
Q

What particle rage can TEM be used to examine?

A

0.001-5 micrometers

28
Q

What is freeze fracture?

A

A technique by where you freeze the sample and break the frozen ice, separating/pulling the layers of the sample apart. e.g. cell, splitting the extracellular layer from the cytoplasmic layer

29
Q

What does SEM display and how does it work?

A

Particle topography by scanning a very narrow focused beam across the particle surface. The electron beam is directed normally or obliquely at the surface. The backscattered or secondary electrons are detected in a raster pattern and displayed on a monitor screen. The image is provided by secondary electrons exhibits good three dimensional detail

30
Q

When was SEM first demostrated and then developed

A

1935 and 1938

31
Q

What do SEM sample emit if the specimen has sufficient energy to excite the atoms on the surface?

A

x-rays. The energy required for x-ray emissions is charecteriec of a given element and since the emission is related to the number of atoms present, quantitative determination is very important advantage of SEM analysis by energy dispersive x-ray analysis.

32
Q

What does CLSM stand for?

A

Confocal laser scanning microscopy

33
Q

What is CLSM useful for?

A

Identification of formulations

34
Q

How does CLSM work?

A

It uses a pinhole aperture or variable width slit to illuminate only the focal plane by the apex of a cone of laser light

35
Q

What image does CLSM produce?

A

Out-of-focus items are dark so do not detract from the contrast of the image,. As a result of extreme depth discrimination the resolution is considerably improved. These images are combines to make a clearer image and 3D images.

36
Q

What do you have to pay attention to when considering the size of your sample?

A

Scale bars

37
Q

What does SPM stand for?

A

Scanning probe microscopy

38
Q

When was SPM founded?

A

1981

39
Q

What can SPM measure?

A

Physical, chemical and electrical properties of the sample by scanning the particle surface with a tiny sensor of high resolution

40
Q

How does SPM work?

A

It does not measure the force directly. Deflection of a cantilever and the tiny stylus (the tip) which works as a probe. It generates a 3D image and allows calibrated measurement in three coordinates (x, y, z). SPM not only produces a highly magnified image, but provides valuable information on sample characteristics. SMP can be operated under ambient conditions and, with some limitations in liquid media.

41
Q

What is the deflection of the cantilever monitored by?

A

Tunnelling current, laser deflection beam from the back side of the cantilever, optical interferometry, laser output controlled by the cantilever in the laser cavity, change in capacitance

42
Q

What does STM stand for?

A

Scanning tunnelling microscopy

43
Q

What is STM used for?

A

STM is an instrument for imaging surfaces at the atomic level

44
Q

When was STM developed?

A

1981

45
Q

What does STM measure?

A

An electric current that flows through a thing insulating layer (vacuum or air) separating two conductive surfaces. The electrons are visualised through the tunnel through the dielectric and generate a current, I, that depends exponentially on the distances, s, between the tiny tip od the sensor and the electrically conductive surface of the sample. Electricity goes through the tip, machine will show the point where the electricity went through

46
Q

What does AFM stand for?

A

Atomic Force microscopy

47
Q

What does AFM allow?

A

Allows one to scan the distribution of a sample using a very small tip made of silicon nitride. This tip is attached to a cantilever that is characterised by its spring constant, resonance frequency and quality factor. The sample rests on a piezoceramic tube which can move the sample horizontally (x, y) and vertically (z)

48
Q

How is the displacement of the cantilever measured?

A

It is measured by the position of a laser beam reflected from the mirrored surface on the top side of the cantilever, detected by photodetector

49
Q

What two modes can AFM be operated in?

A

Contact or non contact mode

50
Q

What is contact mode for AFM

A

Close contact with the surface

51
Q

What is non contact mode for AFM

A

The tip hovers 5-10nm above the surface

52
Q

Describe the process of a single-molecule force spectroscopy

A

An AFM tip tethered to an antibody engages the sample which results in a force curve

53
Q

What are scattering techniques useful for?

A

Very useful methods in characterising formulations and they can give quantitative information

54
Q

What is the issue with scattering techniques?

A

Need to use sufficiently dilute samples to avoid interference such as multiple scattering.

55
Q

What can electromagnetic radiation be used

A

Light, x-ray, neutrons

56
Q

What are examples of light scattering techniques?

A

Time average, Turbidity measurements, Light diffraction technique, Dynamic (quasi-elastic) light scattering /photon correlation spectroscopy, Backscattering techniques

57
Q

Describe time average light scattering

A

This method of dispersion is sufficiently diluted to avoid multiple scattering is illuminated by collimated light beam (usually laser) and the time-average intensity of scattered light is measured a function of scattering an 0

58
Q

Describe turbidity measurement

A

Can be used to measure particles size, flocculation and particle sediment

59
Q

Describe light scattering

A

Is a method that measures the time dependent fluctuation of scattered intensity

60
Q

Describe light diffraction techniques

A

Rapid and non destructive technique to determine particle or droplet size distribution

Prod331 (6 decks)

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