Application
Whether you are performing analytical, preparative, or microbore HPLC analyses, the appearance of minor impurities with the major component is a common problem. Since most compounds separated via chromatographic techniques are similar in nature, these compounds may have wavelength absorbance maxima that are also similar. Additionally, many of these compounds have relatively broad spectral absorbance peaks. Nearby wavelengths
produce almost identical absorbances for most compounds.

More meaningful information can be obtained from the mixture by analyzing the sample at two distinctly different, but relevant, wavelengths. Prime examples of this application are the analysis of proteins, peptides, and other analytes where dual wavelength analysis is commonly carried out at 214nm for the aliphatic amino acid moieties and 280nm for the aromatic moieties. Other common analyses include Ninhydrin Amino Acid analysis at 440 and
570nm.

It is the 214nm requirement which presents a problem for both the user and the instrument company developing a suitable instrument for dual wavelength detection. The most desirable source of energy for 214nm absorbance has historically been the Zinc lamp which has a strong energy emission at 214nm. However, the relative instability of the lamp, and just as important, the cost of the lamp and its power supply, create a problem for a user
needing that wavelength.

The user often selected a variable wavelength detector instead of the fixed 214 detector since its price/performance is superior. In spectrophotometry, dual wavelength operation has been used for many years in peak purity analysis. The development of the variable wavelength HPLC detector as a dual wavelength instrument was a natural progression.

Most of the users, particularly with applications in biotech preparative HPLC, rarely employ the full capability of the dual wavelength detector they purchased even though they paid a premium price for the instrument.

The DDW-10 provides simultaneous absorbance information from two selected wavelengths. This information is available at the rear of the instrument from two separate analog signals. To simplify the system we chose not to burden the cost with internal firmware which can easily be provided by many of the competent chromatography software programs in todays marketplace.

With a hardware-only system such as DDW-10, data is processed in real time. This means the scale factor is determined from testing and identical experimental conditions are reproduced in the succeeding runs. Isn’t this exactly what is done in preparative chromatography and quality control?

Principles of Operation
The double beam optical system consists of continuum spectral energy source (Deuterium for UV operation; Tungsten for Visible operation), beam shaping optics, dual pathlength flowcell (standard D-Star sample/reference), broad spectrum beam splitter, two wavelength selection filters, and two dual channel UV/Vis silicon photodiodes. The measurement electronics are composed of two high performance log convertors and two signal processors. The front panel display provides continuous readout of the absorbance measurement from both channels. Autozero for both channels is also found on the front panel. Absorbance range selection for each channel is also conveniently located on the front of the instrument. Remote selection of absorbance ranges and autozero is available.

SPECIFICATIONS
Measurement	Simultaneous absorbance at two fixed wavelengths (214nm and 280nm standard).
Wavelength	Changeable 12.5mm diameter interference filters; 10nm bandwidth; optics and source lamps are capable of 195-360nm & 360-1000nm.
Lamps	Prefocused D2 and W; user replaceable.
Flowcell	2mm pathlength, 4µL preparative; 7mm pathlength, 10µL analytical (accessory)
Noise	>1% peak to peak at 0.005 absorbance full scale at 254nm (±2.5 x l0-5 AU)
Drift	<10% of full scale at 0.005 absorbance full scale (5x10-4 AU/Hr).
Linearity	Better than 2%.
Response time	0.5 sec. to 63% of final value (step change rise time)
Recorder ranges	0.005, 0.01, 0.02, 0.05, 0.10, 0.20, 0.50, and 1.00 absorbance full scale on a 10mV recorder.
Outputs/Channel	10mV recorder; 1.0V/AU integrator, remote control ea. ch.
Front display(s)	Recorder range indicator LEDs (8 each channel), absorbance display each channel (3.5 digit panel meters).
Size	9.4" (W) x 7.1" (H) x 14.3" (D), 22lb, 10kg
Power	115/230VAC, 50/60 Hz