Personal comments on the Picotrace system

After having used the Picotrace system for a few years, I have a few observations regarding the system. First, my primary interest is dissolving igneous and metamorphosed igneous rocks, ranging from basaltic to granitic compositions. Naturally, zircon dissolution is a primary concern.

• Our low-pressure HF dissolution procedure is at 120°C, and clearly failed to dissolve zircons over digestion times up to a month, and so was useful only for zircon-free mafic rocks.
• Our LiBO2 fusion method was quite successful, of course, but when we got our new ICP-MS instrument we banned this method because Li and B contamination would compromise our water analysis programs.
• We tried microwave digestion at temperatures up to 240°C, but with disappointing results. Part of the problem was that, with our (then) state-of-the-art system, digestions could be run at 240°C for only half an hour before cooling was required to protect plastic structural components. This means that the 24 or so hours needed for zircon dissolution would be difficult to achieve. 5 hour dissolution experiments, requiring 10 half hour runs were insufficient to dissolve zircons, similar to the findings of others. We also had numerous problems with vessel leaks, damage to the fabric reinforcing sleeves, and venting incidents during heating. We ascribe these problems principally to running the system at its pressure and temperature limits.

Hot plate

The Picotrace hot plate is entirely made of Teflon, coated with Teflon, or covered with Teflon. No metal parts are exposed. It seems very well designed and works exactly as described in the Picotrace literature. Our hot plate is located in a hood and run with the sash full open.

Hot plate controller

This is a programmable controller that monitors and controls the hot plate and sample temperatures. It can ramp temperature up over time, hold temperature for long periods, and turn the hot plate off after a set time. Operating the controller is not obvious, but the instructions are reasonably clear. After some practice, programming for normal runs is easy. After a power failure, however, the system woke up in the mode for changing the many system settings. It took a few minutes with the manual to figure out how to get it back to the normal run mode. There are a great many system settings and I would not look forward to having to reset them all.

Sample blocks and pressure plates

The two sample blocks are Teflon coated aluminum, each with 16 drilled, round-bottomed holes for the sample vessels. The sample vessels are machined, white Teflon, with the interiors "densified" by some process. To remove surface porosity, I suppose. The normal lids for dissolution fit over each vessel individually. The sealing surfaces are flat, and the seal is very effective. We have had only two samples leak over thousands of sample days of operation. Each lid is held in place by a Teflon-coated aluminum pressure disk, which is pressed down onto the vessel with a hand-tightened bolt on an overlying plate. Initially we had some relatively minor problems.

1. We had difficulties getting the sample blocks to reach their maximum operating temperature. See here for our solution.
2. The Teflon-coated aluminum block is supposed to be quite resistant to corrosion. We found corrosion starting after four 24-hour runs at 175°C using HF only. Its Teflon coating is very thin, like that of the pressure disks, and not at all like that of the hot plate. Corrosion involves the appearance of white crystalline salts on the surface of the black Teflon coating, and bubbling up of the Teflon coating. Detachment of the coating is supposed to occur after awhile, but 4 runs seems a bit soon. We clean the holes after each run with DI water and scrubbing with a wet cellulose sponge, and store the blocks in a dessicator to avoid continued corrosion. Before a run we always spray the vessel holes with Teflon spray. This seems to slow the corrosion rate tremendously. I suspect that the Teflon spray coating is laterally porous, which allows acid vapors coming through the sample vessel walls to diffuse away.
3. The Teflon-coated aluminum pressure disks each have a hole in the middle that is not coated. Corrosion in the holes, and at the tip of the pressure bolt that presses down over this hole, started immediately from acid vapors diffusing through the Teflon lids. When the pressure plates are removed after a run, white aluminum and brown stainless steel corrosion products fall out onto the sample vessel lids. After a few runs the Teflon coating on the disks on the side against the vessel started to bubble up as corrosion products formed under the coating. The Teflon coating is very thin, nothing like the thickness on the hot plate itself. The purpose of the hole in the pressure plates is unclear. Teflon spray (dry film lubricant) does not seem to help reduce corrosion in the holes. We made a set of 6 mm thick Teflon disks, put between the pressure plates and vessel lids. This extra thickness of Teflon seems to dramatically reduce corrosion of the pressure disks, though alignment of the pressure plates with the pressure bolts requires considerable care.
4. One pressure disk has stuck to the Teflon lid because I used the wrong Teflon spray, and part of its Teflon coating tore away when it was pried off. This gave me the idea that the wrong teflon spray can be used to stick the pressure disks and 6 mm Teflon spacers together. It worked, so now we just leave them stuck together all the time. The 6 mm Teflon spacers seem to have eliminated most corrosion (see immediately below).

The bottom line is that the sample blocks and the pressure plates will not last forever, and will probably start to deteriorate at once, at least if HF is used. Always use Teflon spray in the sample block holes, and I recommend 6 mm Teflon disk spacers between the aluminum pressure disks and the vessel lids, as described above. It may be that the sample blocks can be recoated with Teflon. There are many shops that can do this around the U.S.

Teflon-coated aluminum pressure disk (black) shown with the 6 mm additional Teflon disk we always use to reduce corrosion of the pressure disk. The hole in the pressure disk has to be cleaned out periodically to remove dust-producing corrosion products. The corrosion products come from both the aluminum disk (white powder) and from the tip of the pressure bolt (dark brown powder).

Same as above, upside down, to show that there is no fancy machining. Aligning the two disks between the sample vessels below and the pressure bolts above can be a pain. Fortunately, after a run or two the two disks tend to stick together, especially if you use the sticky Teflon spray. We just leave them stuck together.

Drying cover

The sub-boiling drying cover is made of a machined Teflon slab that covers all 16 sample vessels on a block. Each slab is connected to two 0.2 μm filters at one end and an aspirator-neutralization system at the other. The cover works well though it is very important to be patient. I became impatient one day and heated the samples too fast. The samples boiled and I ended up with boiling splatters all over the insides of the vessels and the drying cover. Some vessels dry faster than others.

Two pairs of 2-liter plastic sparging bottles with tubing are supplied, to be used to neutralize the acid vapors that are driven off the samples through the drying cover. The bottles are supposed to contain NaOH solution to neutralize the acids. The bottles each have a Teflon top plug and supplied tubing. The PVC tubing supplied is large diameter and stiff, making it difficult to handle and difficult to keep the bottles upright.

We built a plastic box that holds the two bottles upright despite the tubing pulling in different directions.

The aspiration rate through the bottles supplied with the digestion system bottles was difficult to control. Either vapors were not drawn through the bottles at all, because of leaks around the PVC tubing where it passes through the bottle top plugs, or the bottles collapsed under the vacuum. There was little middle ground. The Picotrace-supplied acid neutralization system is inadequate in this regard. I strongly recommend making your own neutralization system out of non-collapsible bottles. I recommend Nalgene 2126 series plastic vacuum bottles, such as 2126-2000. If you do this, the Picotrace plugs won't fit. You will have to drill and perhaps tap the caps that come with the Nalgene bottles. We drilled and tapped our bottles as shown below.

Outside view of our drilled and tapped Nalgene bottle top. Note that we used the Picotrace-supplied fitting for the large-diameter Teflon tubing (left) that comes from the sample vessel drying cover, but used our own 6 mm OD tubing and fitting for the tube that goes to the aspirator (right).

Inside view of our drilled and tapped Nalgene bottle top. Our threads were quite tight so we didn't bother using Teflon tape to seal the threads.

Neutralizing solution

The sparging bottles are supposed to capture and neutralize acid evaporated from the samples, preventing the acids from contaminating the vacuum system, which is possibly aspirator water going down the drain. Picotrace recommends NaOH for neutralization, which will certainly work. I cleverly decided to use Na2CO3 instead, which is cheaper and less corrosive to skin. This was a mistake. It seemed to work fine for dozens of runs. One day I saw a big drop of acid run into the bottle from the samples and there was an eruption of foam that shot back up the tubing into the samples, ruining the batch. It just goes to show that you can watch something a hundred times and miss the one time that something important happens. Luckily, our batch of Na2CO3 had a very odd trace element composition (very high La but very little of the other REE, high Cr, high Co, low Ni, etc.) that permitted us to determine which samples of all previous runs were contaminated. Fortunately, only the immediately preceding batch was bad. Two batches ruined; live and learn. Now we just use tap water to collect and dilute the evaporated acid, and dispose of the acid water as we do our other dilute acid wastes.

We had another problem with the Na2CO3 neutralizer during HF evaporation, that may also be a problem with NaOH as the neutralizer. NaF crystallized around the end of the tube that brought evaporated acid drops into the first sparging bottle. This NaF crust gradually slowed air flow, and sometimes prevented overnight evaporation of the acid. Yet another reason to use just tap water.