TITLE: Production of The Aluminum Nitric-acid Keg (TANK) for mitigating explosions during the oxidation of black carbon into benzenepolycarboxylic acid (BPCA) molecular markers
AUTHORS: Madelyn Miller, Alex Collins, Riley Barton, Morgan Schaller, Sasha Wagner

[DESCRIPTION]
Black carbon is often measured using the benzenepolycarboxylic acid (BPCA) method, where condensed aromatics are oxidized into BPCA molecular markers for subsequent separation and quantification via HPLC. The process involves placing sample and excess nitric acid into glass ampules, flame sealing, and heating (often between 150-180 ºC for multiple hours). During oxidation, pressure from within the ampule can cause it to explode, resulting in sample loss and possible destruction of oxidation ovens. Another issue that arises from such explosions is the "domino-effect" where when one sample explodes, it also destroys surrounding samples, exacerbating the aforementioned consequences. Here we provide schematics and instructions for the production of The Aluminum Nitric-acid Keg, or TANK. This apparatus was designed to isolate individual samples to stop the domino-effect, decreasing sample loss and in turn limit released nitric acid fumes. It should be noted, however, that despite the singular intention of preventing cascading explosions, employment of TANK has resulted in a dramatic decrease in our lab's explosions overall, possibly due to more consistent heating of samples. 

For a step-by-step protocol describing the BPCA method, please see Barton and Wagner (2022) and the associated protocol here at protocols.io!


[BEFORE_START]
Determine the size of TANK right for you. To do this, measure the internal size of your oxidation oven and decide how many samples you can/want to oxidize at the same time. The schematics included in this protocol produce a TANK that is compatible with an HP 5890 Series II Gas Chromatograph Oven, measuring 6"(L) × 10"(W) × 7"(H) and  holding up to 28, 20 mL glass ampules at a time. The internal dimensions of your oxidation oven are likely the limiting factor in the number of ampule wells you can drill. When deciding the dimensions for your TANK, remember to leave room on the sides of the oven for the handles. While the handles we recommend do collapse, the significant weight of TANK makes it beneficial to leave enough room on the sides to insert the whole unit with the handles out. 

We recommend reading the protocol in its entirety before beginning production to familiarize yourself with the order of operations. 

[GUIDELINES]
Feel free to optimize TANK to your personal needs. These instructions can be used to manufacture TANK yourself or to assist the staff at your institution's machine shop. If you do use TANK for your black carbon, or other, analyses, please cite this protocol.

[STEPS]
SECTION: Gather materials and equipment
1. A more detailed list of the necessary materials and equipment, including recommendations and alternatives, can be found in the "Materials" section.

Materials
Aluminum Blocks 
         - 1 for Lid
         - 1 for Body
2 Alignment post rods
2 Folding chest handles and corresponding screws
2 Latches and corresponding screws
Cutting fluid
Optional: Placement guides (from the provided "Blueprint and Templates" PDF), printed to scale

Equipment
Drill bits meant for metal (at least 3/8", 7/16" and 1") 
Tap and die set 
Drill press
Vise
Wire brush
2 3/8" dowel centers 
Hydraulic press 
Center hole punch 
Belt sander
1" countersink drill bit 



SECTION: Adjust materials if necessary
2. Before beginning drilling, adjust any materials as necessary. 
For example, it is sometimes difficult to find  aluminum blocks in your required dimensions. Be sure your dimensions allow room on TANK's sides for the addition of handles and clasps. Cutting the aluminum block to size before drilling holes will save time, reduce weight, and decrease the likelihood of drilling holes in the incorrect locations. 
Additionally, alignment posts may need to be cut down to size.

 
SECTION: Drill holes for ampules
4. Once your lid and body are at the correct dimensions, layout the center points for your ampule wells, being sure that no center point is closer to the long edge than 1.125" and no closer to the short edge than 1.25" (to ensure room for latch and handle screws). In addition, each center point should be at least 1.25" from one another. This can be achieved with a simple ruler and marker, a marking gauge, or (if your dimensions are equivalent to ours) a to-scale printout of the provided guide. 

Create a starting point for the drill by making a divot at each center point using either a center-hole punch or a simple hammer and nail. 
 

SECTION: Adding alignments posts
14. Prepare the alignment posts by sanding down the corners on a belt sander to allow for easier insertion. 

Then, using a hydraulic press, set the posts into the lid. 

 


 
SECTION: Attach handles and latches
20. Secure the latches and handles to the sides of TANK with the appropriate screws. 

 

SECTION: Wash and dry TANK
21. The ampule wells and alignment-post holes will be filled with a mixture of aluminum shavings and lubricant. Prior to use, clear out the holes with compressed air to remove the shavings and scrub with water, a test tube brush, and a strong degreasing soap. Rinse well and allow to dry thoroughly. 


 

SECTION: Use TANK for oxidation of black carbon
22. Gently place sealed ampules into the ampule wells, being careful not to drop them. 

 


SECTION: Drill holes for ampules
5. Ensure there is adequate clamping force applied to the block on the drill press platform. We used a 6" cross slide vise mounted to a wood board, which was in turn mounted to the platform, all with nuts and bolts. 

 

SECTION: Drill holes for ampules
8. Next, each ampule well must be drilled to its final size using a 1" drill bit and a drill speed of 250-400 rpm. Since you will no longer have the divots as a clear indication of center, be careful when positioning the block for the 1" hole. 
Employ the same methods recommended in Step 6 for efficient cutting.

 

 
  
SECTION: Drill holes for ampules
9. When all the ampule wells are finished, chamfer the edges to eliminate sharp corners. This can be done with either a large countersink drill bit in the drill press (or even a handheld drill), or a small chamfering bit in a handheld router. 

 

SECTION: Adjust materials if necessary
3. To remove any sharp edges and corners, knock them down using a belt sander, file, or chamfering bit in a hand router.
SECTION: Use TANK for oxidation of black carbon
23. Place the lid on and secure it to the body with the draw latches. 
SECTION: Use TANK for oxidation of black carbon
24. Transfer TANK to your oxidation oven and run oxidation protocol.

 

SECTION: Use TANK for oxidation of black carbon
25. Once oxidation is complete, allow samples and TANK to return to room temperature. Then remove TANK from the oven, set aside the lid, and remove the ampules. If the ampules are still warm, or you cannot get a good grip on them, use a large pair of tweezers to grab them. Alternatively, a piece of soft tubing can be slid over the top of an ampule until snug to aid in removal.

In case of an explosion, remove all intact samples. Before cleaning, neutralize any nitric acid. To clean, tip TANK over to remove any glass, then rinse with water and dry.  

 

SECTION: Drill holes for ampules
6. Once your block is aligned and secured, apply some cutting fluid to the divot and drill a smaller pilot hole at each divot to a depth of 5.5". We recommend a 3/8 - 1/2" bit, and a drill speed of 700-1000 rpm. This is an important step to reduce the sheer volume of material being removed by the 1" bit. 

 

SECTION: Drill holes for ampules
7. When finished with the pilot holes, clear them of cuttings with compressed air.

 

SECTION: Adding alignments posts
10. On the lid, either measure in 1/2" from each side on two opposite corners or employ a to-scale printout of the provided guide and make starting divots using a center hole punch or a simple hammer and nail. 
SECTION: Adding alignments posts
11. Using the cross slide vise and the drill press, drill two holes in the lid using a 3/8" drill bit to a depth of 3/4". 
SECTION: Adding alignments posts
12. Before setting the alignment post: To position the corresponding holes in the body for the alignment posts to fit into, place the dowel centers in the post holes in the lid, line the lid up with the body so the centers are touching the top of the body and the lid and body's sides are flush with each other, and gently hammer on the lid above the dowel centers to make corresponding divots in the body. 

 

SECTION: Adding alignments posts
13. Using the cross slide vise and the drill press, drill two holes in the body using a drill bit between 7/16" and 1/2" (to allow wiggle room for the posts) to a depth of 1.5". 
SECTION: Adding alignments posts
15. Place the lid on the body to ensure the alignment posts fit in easily. If not, redrilling the holes in the body slightly larger should alleviate the problem. 

 

SECTION: Attach handles and latches
16. 
 

With the lid on, place the parts of the latch on each side of TANK, ensuring there is some tension on the latch so the parts aren't too close to each other (the hook should attach to the lid and the loop to the body). Mark the screw holes with a marker. Alternatively, if you are using the recommended latches, you can use the included printout guide, but the guide should be employed as one piece with the lid sitting on the body to ensure the hook and latch are in line with one another. Then make a divot with a center hole punch or hammer and nail. 
SECTION: Attach handles and latches
17. Drill holes at each marking using an appropriately sized drill bit (for the recommended No. 6 screws, use a #36 bit) to a depth of 1/2".

Then, tap the holes (for the recommended No. 6 screws, use a 6-32 tap) using some cutting fluid. 

 

SECTION: Attach handles and latches
18. Place the handles on TANK's sides, ensuring the hinge of the handle sits at least halfway up (with the lid on) to prevent balance issues. Mark the screw holes with a marker. Alternatively, if you are using the recommended handles and dimensions, you can use the included printout guide. Then make a divot with a center hole punch or hammer and nail. 
SECTION: Attach handles and latches
19. Drill holes at each marking using an appropriately sized drill bit (for the recommended No. 10 screws, use a #21 bit) to a depth of 1/2".

Then, tap the holes (for the recommended No. 10 screws, use a 10-32 tap) using some cutting fluid.