uv-vis detector question

[piracetam]

so I just got an Applied Biosystems 785A similar to this one
http://www.peaktekhplc.com/images/14940.jpg , but when I opened the hinged door, I noticed it has a capillary flow cell. Will I necessarily need a capillary pump? I got a Varian Vista 2510 hplc pump similar to this one http://www.pemed.com/lab/spectchr/varian2510_81.jpg , hoping this would work with the detector, but now I'm not so sure. this is all pretty new to me

[sassman]

He he, piracetam, I just ate a couple of grams of piracetam too. Now I can get my work done. Hey what part of central TX are you from? I am originally from New Braunfels myself. Now living in Indiana:( I miss Texas!!!

I can't answer your question, but I do know that it won't hurt much to hook it up and give it a try.

[Mark Tracy]

Capillary flow cells are easier to clog, may have lower pressure rating, and definitely have worse S/N ratio & linearity than standard cells. However, you probably can still operate at 1 mL/min. The advantage is that if you want to use smaller i.d. columns you won't need to invest in a new flow cell. You only need a smaller pump if you plan on doing gradients with microbore or capillary columns.

[piracetam]

thank you mr. tracy. you answered my initial question, and another one in the back of my mind... the reason for the wild baseline fluctuations. the abs. fluctuates to +/- 0.008 AU, after autozero following warmup.

sassman, I'm in San Marcos; love this hill country. be sure to also take lecithin (with phosphatidyl choline) if you're going to take piracetam frequently. it has a reputation for depleting ACh.


from what I've read recently, a flow splitter could be connected to line out on the pump, to reduce the flow to <100 µL typically handled by cap. flow cells.

so, to connect the cap flowcell inlet tubing to a microbore column, I suppose these would work? http://www.vici.com/vfit/cef_mic.php
I think I'll probably need something like an upchurch microtight adapter (1/16" to capillary) and tubing sleeves

[Mark Tracy]

The baseline problem could be a couple of things. One is a bubble or particle in the optical path. Another is that the pump is not stable and the detector is picking up RI or thermal effects. Try testing the detector with an empty flow cell.

The Upchurch fittings work well to connect quartz capillaries. Valco reducing fittings are good for connecting 1/32" steel capillaries to standard 1/16" hardware. The Valco part you linked to is a column end fitting used for manufacturing microbore columns; don't go changing the column ends, use adapters instead.

You can make a splitter from a Tee and two pieces of capillary tubing. To keep things consistent, put the splitter in the column oven with your column.

[piracetam]

the flow cell is empty.. sorry for not mentioning that.

I may just end up selling the detector, and replacing it with an HP 1050 series
with a standard flow cell. the 785A looks like it's set up for protein analysis, however I'm looking to separate and analyze organic compounds such as amines/amides and phenols.
I'm a novice putting together a hobbyist system, by no means state-of-the-art. I'm on a budget.

[Mark Tracy]

If the noise is 0.008AU with an empty cell, this is not acceptable. At minimum you need a new lamp ($$!). You probably will be better off with the HP1050 and a standard cell. Besides, replacement parts will be easier to find.

[piracetam]

that's kinda what I figured. thanks for the advice, it clears up a lot of would-be hassle.

[piracetam]

upon further inspection (30 min after warmup), I found the deviation to be +/- .001 AU, and the lamp meter shows about 620 out of 2000 hours (this detector is ridiculously sensitive). apparently, this is an LC Packings (Dionex) U-Z view capillary, based on the sticker designation "UZ-AB-234" under the flow cell. The varian 2510 pumps down to .01 ml/min. with these things in mind, should I just work around this? (microtee/adapters....remember, I plan to analyze primary/secondary amines/amides) after some consideration, I can't afford the rather pricey HP detectors (btw... how good is this thing, the ABI/PE 785A, being an "obsolete" detector?)

[Mark Tracy]

Using a standard HPLC pump at the bottom of its flow range is usually a bad idea. The cam cycle gets so long that gradients become impossible to control, and it takes forever for the flow rate to stabilize. Fortunately, you probably don't need to use a capillary column at 10µL/min. Use a 2.1 mm column at 0.2 mL/min and the flow cell will be OK and the pump much better behaved.

The "obsolete" detector probably still works within original specification. Unfortunately, the original spec did not use a third-party capillary flow cell, so you have no easy way to ascertain its performance. You are lucky to have a nearly-new lamp. I would say that as long as you don't try for ultimate low-level performance you should be OK.