YSI 2700 SELECT, User Manual

Page: 52 / 165

4-3
In order to configure your instrument to measure a particular chemistry analyte, you need to:
» Approximate the analyte concentration or range of concentrations to be measured.
»
Decide if you must dilute your sample, and, if required, determine an appropriate
dilution factor and diluent.
» Decide what calibration value(s) is appropriate for the range of concentrations under
study.
» If possible, account for any interferences to your reading. For example, if you want to
measure starch, free dextrose must be considered. Methods to do these corrections are
described below.
Once you make the above determinations, you can decide whether one of the standard setups
described below will be appropriate or whether you will need to customize your setup. In either
case, the Measurement Parameter section of the instrument Setup Menu lists four submenus in
which you must confirm or assign appropriate parameters. These are sample size, calibration
method, black probe parameters and white probe parameters. If you have a single channel
unit, white probe parameters do not appear in the menu choices.
You may want to access the print setup command in your 2700 SELECT menu. From Run or
Standby mode, press [MENU], then select [5] for PrntSetup. From Main Menu display, press
[MENU], select [2] for Setup, then select [5] for PrntSetup. In either case, the instrument
printer will print a complete list of system parameters.
See Appendix C for more detail on accessing “PrntSetup” and also to review the sample print-
outs of a single channel unit set for dextrose and a dual channel unit set for simultaneous
dextrose and L-lactate analyses. The parameters you see in these printouts are default values.
For many applications, the default settings will be appropriate for you.
Now enter the Measurement Parameter menu (press [2] for MeasParameter). Within this menu
level you will confirm or change the parameter settings that define the chemistry or chemistries
of interest.
Sample Size (1-SampleSize). The default setting is 25
µ
L. The range of choices is 5 to 65
µ
L.
You may enter any integer value. Remember, however, this is a nominal volume. The instrument
does not depend on an accurate absolute value, but rather reproducible aspirations. This allows
the calibrator probe signal to be stored in memory and provide a reference value used to
internally calculate sample readings.
For each chemistry described, a sample size (usually 25
µ
L) will be recommended. Some of the
reasons for changing that sample size are listed below.
» The calibrator probe signal plateau current might be less than 10 nA. Refer to
Appendix D. You may want to increase the sample size to increase the probe signal.
» Probe plateau current is high enough, but you would like to improve the linear range.
For example, you might decrease sample size from 25
µ
L to 12
µ
L. Calibration current
might drop from 20 nA to 10 nA, but accuracy and precision at the upper end of your
concentration range should improve.
» You may want to measure low concentrations of a particular analyte. For example, you
might prepare a 500 mg/L dextrose calibrator for a particular application. If the
dextrose calibrator produces probe signal plateau currents significantly below 10 nA,
you could increase the sample size from 25 to 50
µ
L (or more) to bring the plateau
current into a reasonable range.