HP Spec: Time-based acquisition

Linear light sensors (photo diodes and CCD’s) have several advantages over single-channel detectors. Among those, the ability to acquire the entire spectrum at once is the major distinction. Coupled with high sensitivity, this allows to follow complex spectral changes in real time by acquiring multiple spectra one after another, turning spectroscopy from static into dynamic tool.

Notes

Chemstation includes a provision for time-based measurements when it is in Advanced mode. In this mode, Spectrophotometer Parameters dialog accessible from Instrument menu provides several options necessary to enable time-based measurement. 

Standard configuration, however, does not provide any real-time visual feedback of reaction in progress. Such “blind” measurement is a significant limitation, especially in long experiments.  Fortunately, this limitation was corrected using macros accessible from Denis→Monitor menu.

Both the standard Instrument->Setup spectrophotometer and Denis→Monitor→Set parameters dialogs control the same set of system parameters. Therefore, changing setting in either one dialog will reflect in another. Most parameters used in time-based measurements are exposed in Denis→Monitor→Set parameters dialog and are described first. Less frequently used parameters can be controlled from Instrument->Setup spectrophotometer dialog and are described below. Simple, single measurements can be well controlled from standard dialog.

Before & After

After the time based-measurements, acquired data are delivered to the Samples folder as individual spectra. It is highly recommended to:

  • Before measurement clear Samples folder of samples by saving them or by moving to other folders.
  • After measurement convert data into a matrix using Make Matrix macro to preserve time calibration, avoid lost spectra and to un-clutter folders. 

Setting time-based parameters

Recommended dialog for setting time-based acquisition and monitoring parameters can be accessed from Denis→Monitor→Set parameters menu. Hoover mouse or click on areas for more information.

initial absorption scale total acquisition time time between starts of successive acquisitions Time before first acquisition single accumulation change in duration from one cycle to the next time before cycle time begins to change acquisition parameters positions to monitor changes initial absorption scale not significant initial wavelength scale not significant selects wchin spectra are dispalyed maximal number of spectra to display

Cycle parameters

With the exception of Integration time, cycle parameters exposed in Denis→Monitor→Set parameters dialog are specific to time-based measurement:

Parameter Range Use
Run time >0 Total duration of experiment; acquisition will continue until this tine elapses
Cycle time Minimal cycle or longer How often a new acquisition is started
Delay time Typ. 0 Delay before the first acquisition is started
Integration 0.1-30 sec Signal integration time, cycle is often longer
Cycle 0 or 2-5% Change in cycle time from one acquisition to the next
Hold time 0 Delay time before change in cycle time begins to take effect.

Time traces display

Traces monitor panel specifies how time traces should be displayed if Traces window is shown.

Monitor at fields allows to specify up to six characteristic wavelengths. Traces of absorption changes at these positions will be displayed in Traces window.  

Y-scale specifies the initial intensity scale of the plot. It is possible to zoom in and autoscale during measurement, but there are no way to zoom out except by specifying larger range here. This is due to disabling of all user input during measurement.  If you want to see absorption changes in perspective to other data (such as earlier run) set up relative scale here.

Scale provides a choice of available scale changes, can be left as is.

Spectra overlay display

Spectra monitor panel specifies parameters of Spectral Overlay window if it is shown. 

X-scale specifies initial wavelength range to display. This parameter controls only visual feedback, and not the actual spectral range being collected. Since there is no speed benefit of limiting the wavelength range, it is recommended to always acquire the entire range from 190 to 1100 nm. If absolutely necessary, recorded wavelength range can be limited in standard spectrophotometers parameters dialog.

Y-scale is analogous to that for Traces monitor panel.

Monitor drop-down list specifies how many spectra should be displayed.

  • All spectra – everything acquired will be overlayed. Beware of busy overlays in long measurements.
  • Number of spectra – the maximal number of spectra specified below will be displayed.  If total number of measurements is larger than maximal number, selected spectra will be spread uniformly over current elapsed time skipping proportionally larger number of spectra as time gets longer. 
  • Last spectrum – only a single spectrum is displayed and updated with each measurement.

Scale provides a choice of available scale changes, can be left as is.

Setting real-time views

Once cycle and monitoring parameters have been set up, selected views must be shown. Available options appear at the bottom of  Denis→Monitor menu.

Spectral overlay window or time traces window can be shown. View all option brings up both monitor windows. 

Resetting views for next run

At the completing of the time-based measurement, Spectral overlay and Time traces monitor windows contain results of the previous run. These results are not cleared automatically at the beginning of the next measurement. This bug does not affect data that are being collected and saves, just the visual feedback.
Time traces and Spectral feedback windows are flushed separately. 

  • To flush Time traces monitor window clear all entries in wavelength list. Close and re-open Set parameters dialog. Input wavelengths again.
  • If Spectra Overlay window is set up to display the last spectrum, it will continue to do so dring the next measurement. In other cases, set Monitor option to Last Spectrum, then close and re-open Set parameters dialog. Restore Monitor option. 

Additional parameters in standard dialog

Most of parameters available in Denis→Monitor→Set parameters dialog are available through Denis→Monitor→Set parameters. Several option, however, are available only through this dialog.

Hoover mouse or click on areas for more information.

Wavelength range duration of single accumulation measure standard deviation - makes cycles longer measure and correct for stray light - makes cycle longer single acuisition options lamps control - there are better options cycle parameters standard entry for cycle parameters Probe light shutter control estimates for current settings standard control over time-based parameters

Parameter Typical Use
Wavelength 190-1100 / 1 Controls wavelength range being saved in to spectrum. Limiting range does not increase the rate of acquisition.
Std. deviation on Off Measures standard deviation in absorption intensity at each point. Rarely used
Stray light correction On or off Measured second spectrum for each measurement with source light closed. Significantly increases cycle time. May be replaced by a black drop cloth over  sample.
Lamps On Redundant option, can be controlled elsewhere
Shutter open On or Off When on forces sample light to be open all the time, which shortens cycle time. However, beware of exposing sample to UV light in long measurements.
Number of measurements Info Estimates number of spectra at current settings
Min. cycle time Info Calculates the shortest cycling possible at current parameters. Run time can be set to shorter time, but will be ineffective.

Blanks for time-based measurements

Sample blanks, or references, are measured using mostly same parameters as samples. For a single spectrum, blank is measured using single acquisition. Although time-based measurement involve multiple acquisitions, they too require only a single blank measurement before acquisition start. Chemstation, however, uses the Run time parameter not only for sample measurements, but for blank measurements as well. As the result, if you attempt to measure blank with Run time set to anything other than 0, the result will be uncertain, most likely with multiple spectra measured and discarded.

Therefore, before measuring any blank spectra for time-based measurements, open either of parameter dialogs and set Run time to 0. Restore this parameter after blank has been measured.

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