Symptoms Which Look Like "Drift" in an
Olis RSM 1000 Spectrophotometer and
What Can Be Causing Them

The stability of an Olis RSM 1000 spectrophotometer over time is excellent. As a dual beam instrument, the two detectors produce a flat baseline without drift. In tests performed in-house, we find that no discernible change over a 600 second period in which time 600 scans (each the average of 1,000) were collected. More exactly, an amplitude change of 0.000048 AU was seen. Stability of ±0.001 AU over 10, 20, and 30 minutes is routinely achieved.

If drift — changes in output not caused by the sample — is observed, a number of causes could be responsible: the lamp, the photomultiplier tubes, the ambient temperature, contamination of the observation area, and/or a photolyzing sample.

Lamp

If the lamp is not perfectly aligned, changes in the lamp as it heats up could cause physical movement of the arc, which would cause variations in the distribution of light between the sample and the reference beams. To correct, adjust the lamp using the knobs on the lamp housing back to achieve maximum light throughput in both channels. Illustrated instructions are available in the Olis RSM Hardware handbook, page 25.

Photomultiplier Tubes

To quote from the Hamamatsu technical pages "While operating a photomultiplier tube continuously over a long period, anode output current of the photomultiplier tube may vary slightly with time, although operating conditions have not changed. This instability of anode output current is called drift." Another characteristic of photomultiplier tubes is hysteresis, or failure to return exactly to a previous setting, even when nothing has changed. Quoting again from Hamamatsu, "Photomultiplier tubes exhibit a temporary instability in anode output current for several seconds to several tens of seconds after voltage and light are applied, i.e., sensitivity may overshoot or undershoot before reaching a stable value. This instability is called hysteresis and may be a problem in spectrophotometry and other applications." Whether or not a drift problem is due to the photomultiplier tubes can be determined by careful examination of the data from each PMT. One can examine the two signals which were collected to produce the absorbance in the Data Reduction Mode (select Single Beam). Can one discern that the signal from one PMT changed in a way similar to the drift observed in the absorbance signal?

Room Temperature

Wide swings in ambient temperature will be noticed by the components in any spectrophotometer.

Contamination

A stopped-flow which is leaking near the observation cell could cause 'drift' problems if the cell windows become contaminated with reagents seeping out onto the windows. It can be useful to remove the stopped-flow mixer from the spectrophotometer so that drift can be evaluated without the complication of the stopped-flow.

Sample

Finally, it is important to know that many samples are photolabile, and as they undergo photolysis, exhibit an increase or decrease in absorbance as they photolysis, which could be (mis)interpreted as 'drift.' Secondly, it is important to recall that diffusion can occur when a stopped-flow is left for many minutes between shots. It is advised that one treat the first shot after minutes of inactivity with care, as it might not be "clean."