[kamiyu]

Hi there, although I can make a nice series of absorption spectra to determine the molar absorptivity, I would like to ask you for help for the following tecqniues problems (I would like to pursue most advanced tecqniues):

1. Is it desirable to warm up the machine (spectrophotometer) for 20 minutes before taking the measurement??

2. Has anyone used the pipette ball? I know how to use it but I found the liquid inside keeps slowly sucking towards the bulb even though I stop pressing the "sucking region", which is very annoying...

3. I found the baseline of my machine will shift down a very little bit (from -0.007 to -0.015) as I continue using the machine. Any explanation??

4. My solvent for measurement is DCM, you know, which is very volatile. Do you have any suggestion about how to minimize the error caused due to solvent evaporation??

[Arkcon]

Hmmm... technique questions, kinda all over the place, but still, we're glad to help people learn.  So, OK, point by point:

Hi there, although I can make a nice series of absorption spectra to determine the molar absorptivity, I would like to ask you for help for the following tecqniues problems (I would like to pursue most advanced tecqniues):

1. Is it desirable to warm up the machine (spectrophotometer) for 20 minutes before taking the measurement??

Warmup can be important, especially for older systems.  But maybe not as critical.  If you really care, do prepare  test solution, and check it at various time points.  Try to see how the instrument behaves.  Then you can file a report, and have it on hand.  So you'll have the answer for someone auditing your process.

2. Has anyone used the pipette ball? I know how to use it but I found the liquid inside keeps slowly sucking towards the bulb even though I stop pressing the "sucking region", which is very annoying...

Some people are experts at using those, but I'm not.  At any rate, there should be a valve that stops the draw when released.  I mean for the black bulb in this image: The red one would still draw, you break the seal in that case.  Kinda wondering about the problems you're having, 'tho.  Maybe you just need new bulbs, they do wear out over the years. 

3. I found the baseline of my machine will shift down a very little bit (from -0.007 to -0.015) as I continue using the machine. Any explanation??

First off, is that significant for your application?  Instruments do drift, it just happens.  You'll have to explain if you're working at that low a level.

4. My solvent for measurement is DCM, you know, which is very volatile. Do you have any suggestion about how to minimize the error caused due to solvent evaporation??

Again, what how much error are you looking to avoid.  Consider: you've prepared a 0.10 N solution and you get a reading of 0.5000 AU.  The solution evaporates to a concentration of, oh I'll be crazy and say, 0.11.  What absorbance would you get?  You can calculate that using Beer's law.  How much volume would you have to lose to get such a concentration, anyway?  Measure the volume of a cuvette, I can only guess the path length, you have to figure out height of solution.  If you're using DCM, that may be why you're having pipette problems.  You may get more accuracy making solutions by weight.

[Corribus]

Just adding slightly to what Arkcon wrote:

1. Is it desirable to warm up the machine (spectrophotometer) for 20 minutes before taking the measurement??

I think it's a great rule of thumb. Lamp intensity can fluctuate quite a bit in the first 5-10 minutes after turning it on. This will be most problematic in very concentrated solutions (which you shouldn't be using anyway) and very dilute solutions.  In addition, instrument validation procedures will often fail within the first 5-10 minutes of turning on. If you're not doing the warmup, this may also explain your baseline shift (point 3).

2. Has anyone used the pipette ball? I know how to use it but I found the liquid inside keeps slowly sucking towards the bulb even though I stop pressing the "sucking region", which is very annoying...

For volatile solvents, I recommend either gravimetric determination of concentration or a good quality volumetric flask.

3. I found the baseline of my machine will shift down a very little bit (from -0.007 to -0.015) as I continue using the machine. Any explanation??

See above for warm-up. Fluctuating temperature can also cause this, which is another reason to wait after you turn the lamp on. The lamp produces heat, which raises the temperature by a small margin in the sample chamber. Waiting 20-30 minutes after turning on the lamp can allow thermal equilibration of the instrument.

As Arkcon already said, though, some drift is often unavoidable. It will depend on the degree of the shift and the time it takes for the shift to happen. The shift you mention over 8 hours operating time will be far less significant than the same shift over 5-minutes. A lot will depend on your particular needs and error tolerance. If it becomes a big problem, the lamp may be approaching the end of its life, or there may be another problem, in which case you might need a service call. Most modern instruments have an internal validation program in their software and drift is often included in the diagnostic. There will be threshold value for drift under which the software considers normal. You can look to see what yours is and run the appropriate diagnostic. If the validation fails, call a service technician to troubleshoot. If it passes, then you may just have to live with the drift because this is considered "normal" by the manufacturer.

4. My solvent for measurement is DCM, you know, which is very volatile. Do you have any suggestion about how to minimize the error caused due to solvent evaporation??

Use a sealed cuvette and evaporation won't be an issue.

[voidSetup]

My 2 cents,

When using those pipette bulbs, pull the liquid above the desired level, and quickly take it off and cover the pipette with your thumb. Then slowly allow some to drain by lifting your thumb to get it to the right level.

For the drift, some folks I work with have said that cleaning the windows in the cuvette compartment can help. The lamp can definitely be an issue too.

[kriggy]

Whats the amount of solvent you are pippeting? I used mechanical pippetes like this one

but they are expensive. You can also use siringe and needle if you want, I think it might be better to not use the bulb but it depends on volumes you need to measure.