Instrument Index


The Micro Residue On Ignition (ROI) System

The Micro Bulk Density / Tap Density System

The Microsieve System






The Micro Residue On Ignition (ROI) System


Micro Residue On Ignition (ROI) System
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DETAILS: The Prototype Micro Residue On Ignition (ROI) System [Micro Platinum Crucibles, Fuzzy-Logic Thermal Controller, Electric Bunsen Burner, and Vacuum Dessicator]

ABSTRACT:  Scaled-down equipment and methodology for performing Residue On Ignition (ROI) have been developed that requires only 1/10 the typical sample requirement, employs readily portable equipment, produces highly accurate temperature programming, and uses less than 500 watts of readily available 120 VAC power.  The components of the Micro ROI System are pictured above which supplies needs unmet by commercial analytical laboratories.

BACKGROUND:  In order to determine the amount of non-volatile inorganic material (filler) in organic materials such as plastics and elastomers, the entire material is first weighed into platinum crucibles, then charred with pure concentrated sulfuric acid. The sample and crucible are subsequently heated progressively and reproducibly to several hundred degrees to drive-off volatiles.  Finally, the cooled sulfated residue in the crucible is reweighed.  This procedure is termed Residue On Ignition (ROI) and the test is defined in the USP  (United States Pharmacopoeia)

COMMENTS:  Often the sample is less than the USP recommended 1-2 grams and commercial test companies will refuse to perform the test.  If several smaller samples are combined to meet the initial mass requirement, information on the amount of inorganic residue (and its variation) in a single sample is lost.  This was the case for stem gasket elastomers in Metered Dose Inhaler (MDI) valves. Therefore, the Micro ROI system was created and employed to gather the data for this typical example.

DISCUSSION:  The Micro ROI System follows a validated temperature program curve that can be monitored and plotted for compliance purposes.  Since a fuzzy-logic controller is used, variations in power line voltage and heater element response are eliminated.  Thus, the Micro ROI System delivers temperature-time curve reproducibility unmatched by larger typical muffle furnace heaters.  The Micro ROI System holds the final temperature at 100C for 30 minutes during which the residue sample-containing platinum micro-crucibles are transferred to the small desiccator provided. This operation reduces any ambient moisture uptake as the sample cools by the sulfate salts of magnesium, calcium, and other elements which are highly hydroscopic.  

Finally, the Micro ROI System has a dual-use in producing not only ROI data but lithium or sodium tetraborate sample fusions as well.  These fusions allow most refractory oxide materials and super-metal alloys to be uniformly dissolved in molten-salt at high temperatures (920C-740C) http://www.socachim.com/flux_selection_criteria.htm and subsequently analyzed reproducibly by other analytical methods such as X-Ray Fluorescence, Atomic Emission, Atomic Absorption, or SEM/EDX analyses.

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The Micro Bulk Density / Tap Density System



The Micro Bulk Density / Tap Density System
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DETAILS: The Prototype Micro Bulk Density / Tap Density System [Digital Tapper, Truncated Graduated Cylinders (TGCs), and Drop Height Measuring Device]

ABSTRACT:  An analysis of the USP (United States Pharmacopoeia) measurement methodology used in Bulk Density/Tapped Density determinations of pharmaceutical powders has led to improvements that increase precision, dramatically reduce the test volume of powder, and provide information about the dynamics of powder compaction.  Employing the Truncated Graduated Cylinder (TGC) device pictured above increases precision in the Bulk Density determination by approximately a factor of 10.  The collection of compaction data at multiple points and the generation of a curve provides greater statistical certainty of the end point value and allows an analysis of likely operator errors to aid in training and further reduction of those errors.    

HISTORICAL:  One of the frequently used tests of the physical properties of pharmaceutical powders is the Bulk Density/Tap Density determination as defined in the USP.  In that procedure, a 250-ml graduated cylinder is weighted and carefully filled to greater than 80% of capacity with the test powder.  The volume is read from the cylinder scale and the powder is weighed by difference.  The mass/volume is the Initial Bulk Density or simply BD.  The powder and cylinder are subsequently raised 14+-2mm by a cam-actuated device at about 300 cycles per minute and allowed to fall repetitively under the influence of gravity onto a circular anvil (typically nylon).  After 500 taps the compacted powder volume is again measured using the graduated cylinder scale and the Tapped Density or TD is calculated using the formula (mass/compacted volume).  Provision is made in the USP to use a 100 ml graduated cylinder to reduce the volume for scarce or expensive powders; however, many labs have reduced the scale still further to use a 10-ml graduated cylinder.  Mechanical holders for 10-ml volume cylinders are commercially available from the manufactures of BD/TD test instruments. 

CONCEPT & DEFINITIONS (TGC & DTD):  Since the initial value of volume is most difficult to read accurately due to the unevenness of the top of the uncompacted freshly-filled power bed, a unique mechanical modification was made to a 10-ml glass graduated cylinder which I have termed Truncated Graduated Cylinder (TGC) to improve measuring/reading precision. 

The number of taps is sequentially digitally set into the test machine and the volume read at each point and the tapped density calculated and plotted to generate a Dynamic Tap Density (DTD) curve.  The points selected for plotting are as follows:  0, 10, 25, 50, 100, 250, 500, 750, 1000, 1250, 1500, 1750 and 2000 and have the following approximately evenly weighted intervals 10, 15, 25, 50, 150, 250, 250, 250, 250, 250, 250, 250 which are sequentially set into the tapping instrument. 

COMMENTS:  Note that the points at 0, 500, 1250, and 2000 are points that correspond with the USP test.  Ideally the DTD curve should level-off to a constant density value at about 500 taps.  If there is considerable increasing slope to the curve, it suggests that the particles are being fractured during the >200g deceleration as the TGC strikes the anvil during each tap. This is observed when a sieved silica gel particle standard is run.

The height-of-drop varied between two Tap Density instruments of different age from the same manufacturer.  A TD height test-fixture was designed and fabricated that would mount universally on the steel tops of the TD instruments using magnetic feet.  An picture of the TD height-measuring test-fixture is shown above.  This device permitted the measurement  and setting of the height-of-drop using stainless steel washers installed on the four top mounting screws just under the tops to 14mm within about 0.2mm for both the TD units to more than meet the USP requirement.  The maximum height range variation found initially was 9.5mm (out-of-specification) to 15.9mm (just-within-specification).  That is a 46% difference using a base of 14mm in height-off-drop between TD instruments from the same manufacturer in the same building at our location!  No wonder folks get different values for BD/TD determinations!!

Since the Dynamic Tap Density curves are used to aid in confidence and to detect likely operating errors, the number of taps needs to be determined accurately and reproducibility.  It was found that the number of taps set vs. the number executed varied for the two commercial units employed in our labs.  The older unit taps varied between 1 and 2 higher than that set due to the inertia of the cam and motor system.  A call to the manufacturer netted the response, “Since the smallest number of taps in most determinations was 500, the known 1 to 2 tap variation would be small.” [However, this would give considerable error in the 10-tap value for DTD]  In addition, the older unit would frequently stop with the holder and cylinder lifted from the anvil.  The motor and cam were replaced with a computer-controlled stepper-motor so that the height was 14-mm+-0.2 (see paragraph above) and the number of taps set were the same as the number executed.  [A site visit to the TD unit manufacturer revealed that the end users often unknowingly abused the older Tap Density instruments.  In one case, the nylon tip which rides on the metal cam surface either broke or wore away, but the user continued operation even though there was a grinding noise as evidenced by a step about 1 mm deep on the part of the metal cam surface in contact with the lifting pin on the sample holder]. 

In the case of the newer commercial TD unit, the number of taps executed was almost always 1 higher than the value set.  Thus, always undersetting the desired value by 1 was an acceptable solution for collecting accurate DTD data.

There is another variation which must be considered in BD/TD measurements, which is the mass of the holder and sample-containing cylinder, and the mass of the sample itself.  These were found to be reasonably similar within about 5% between the two instruments, but remember that the potential energy varies directly with the mass of the assembly/sample as it is converted to kinetic energy to de-acceleration force during each tap.

OPTIMIZATION OF TAP RATE:  Finally, the rate of tapping likely has an effect.  It is noticed that at the USP specified rate of 5 taps per second (300 cycles per minute nominal) that the surface of many powders fly into the air inside and outside the uncovered graduated cylinder particularly during the early portion of the tapping test.  In the TGC this is minimized by physically restricting the headspace with a parafilm cover.  The vertical acceleration of the powder has been increased in recent models by TD instrument manufacturers by lifting during only 180 degrees of the cycle.  This increases the probability of always stopping in the sample holder down-position.  In the case of the retrofitted computer-controlled stepper-motor TD instrument, the tapping speed can be set between wide limits—in the test case shown 150 taps per minute were selected (1/2 that suggested by the USP) and little powder was observed being thrown into the area above the compacted powder bed surface. 

The trade-off for DTD was a doubling of testing time due to a slower tapping rate and the additional readings and resetting of the TD machine between values.  This is often compensated for by only needing duplicate analyses instead of triplicate or more since the overlaying of two curves gives great confidence of good precision of the TD testing.  The reduction in the number of replications also conserves sample material.

TRAINING ADVANTAGE:  As mentioned in the abstract, the DTD plots can be used to help in training in the use of BD/TD instruments.  Data collected while training three different operators suggest that operator 1 filled the cylinder too quickly and compacted the bed more than the others (higher initial BD), and that operator 2 had left a void in the powder bed (abrupt change in the curve at one point), while operator 3 was closest to previous values collected on the same lactose monohydrate sample by others.

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The Microsieve System



The Microsieve System
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DETAILS: The Prototype Microsieve System [Sieve Stack, Timed Shaker, and Palmtop / Spreadsheet]

The Microsieve was designed and fabricated to address an unmet need in the Pharmaceutical Industry to determine accurately the mass (volume) Particle Size Distribution (PSD) of small quantities of powder in the 10-100 milligram range).  The Microsieve is a primary standard PSD system (NIST traceable to length and mass) and uniformly covers the size range from >600 microns to <38 microns. 

It is made from chemically-resistant passivated-316 stainless steel and is closed to prevent loss of potent fine-particle material which is frequently a problem with commercially avaliable 3-inch diameter screen sets. The Microsieve mass recoveries are typically around 99%.  The Microsieve is 1/10 the diameter of a standard 3-inch sieve and offers 1/100 the fine-particle hold-up which causes larger area sieves to be unusable for small quantities of powder. 

The Microsieve System includes a digitally-timed mechanical shaker which has been optimized to produce the correct acceleration to make a rapid analytical separation of particles of difficult shapes such as columnar or acicular (needle-like) morphology.   A palmtop computer with a validated spreadsheet calculation/plotting program is part of the Microsieve System as well.
 
The accuracy/reproducibility of the Microsieve System PDS measurement is typically less than 5% if the Microsieve powder samples are loaded from a modified rotary-riffler (not shown in the picture but available).  In typical use, each of the six Microsieve sample fractions is of convenient quantity to be examined and photomicrographed to document powder morphology.  This is particularly useful in addressing size-segregation problems found in pharmaceutical solid phase mixtures and formulations.

* An analytical balance with 0.1 mg resolution/accuracy is required to weigh the powder sieve fractions.  

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