Wavelength Range For IR Materials - Microns & CM-1

Sealed Liquid Cells - Characteristics, Filling and Use

SEALED LIQUID CELLS comprise two IR transmission windows separated by a lead spacer which is amalgamated to the windows with mercury. This construction creates a seal which is perfect for infrared spectroscopy as it is virtually impervious to solvents and does not contain anything that could create spurious absorbances. In most cases sealed cells are sold with a metal front plate containing 2 Luer fittings for filling and a back plate for mounting, such as ICL’s SL-3 and SL-4 sealed cells. In the case of ICL sealed cells, there is also an amalgam seal to the front plate of the cell which we lap optically flat to enhance the quality of seal. These cells are also available simply as sandwich cells which comprise just 2 windows (one drilled) and a mercury amalgamated spacer. The SL-2 sandwich cell contains 2 windows, 1 mercury amalgamated spacer and 1 lead gasket which is amalgamated to a front plate containing Luer fittings. Sandwich cells can be used to replace the optics in complete sealed cells such as the SL-3 and SL-4 or with demountable cell bodies such as ICL’s Precision Demountable Cell (0006-497) or SL-2 (0006-4153).

SEALED CELLS are available in a variety of precise pathlengths ranging from 0.015mm to 10mm. The pathlengths can be calibrated to 4 decimal places and matched from cell to cell for consistent results. The availability of precise pathlengths makes sealed cells the tool of choice for quantitative analysis of liquids. Sealed cells can also be reconditioned at modest cost, which makes them relatively inexpensive. ICL provides a cell reconditioning service.

ALTHOUGH THE MERCURY to lead amalgams used to seal these cells have ideal characteristics for infrared spectroscopy, the seal will break if too much pressure is applied to it. Since the volumes and pathlengths of these cells are small, the simple act of injecting the sample into the cell with a luer syringe can create enough pressure to rupture the seal, particularly for cells with pathlengths of less than .05mm. For flow cell applications where pressure is contemplated, try one of our high pressure flow cells. ICL’s high pressure sealed liquid flow cells are rated for constant and pulsed pressures up to 1000 psi.

THE PROPER METHOD for filling a sealed liquid cell is to create a low pressure area in the cavity of the cell using an empty luer syringe while using a second luer syringe to fill the cell. See Fig. 1. After the empty luer syringe is attached to the cell, the plunger is drawn back, thereby creating a low pressure area which will cause the sample to flow into the cell from the other luer syringe without the need to depress the plunger of the sample syringe. To remove the sample, use an empty syringe and simply pull the plunger out slowly or use a cell cleaning accessory to create a partial vacuum. See Fig. 2. It is preferable to both fill and empty the cell from the lower Luer fitting. ICL’s SL-3 and SL-4 cells are designed with this filling technique in mind. The placement of one of the luer fittings on top of the cell leaves more space between the 2 syringes making it easy to pull a partial vacuum with one syringe while filling the cell with the other syringe. See Fig. 1. The design also discourages filling the cell from the top which frequently results in the sample being spilled on the cell windows while it also facilitates simply turning the cell upside down and dumping the cell contents out of the top port without spilling the sample contents on the window.

Useful Formulae for Spectroscopy

CALCULATION OF MAXIMUM
ALLOWABLE PRESSURES ON
CIRCULAR WINDOW MATERIALS

Calculation of maximum pressure will depend upon a number of user selectable parameters. For instance, the window material, window size, flange size and a safety factor all may be varied depending upon application.

To calculate maximum allowable pressure, we must assume that the maximum stress in a uniform circular plate is given by the equation:

where k is a constant, the value for which depends upon whether or not the window is clamped — use 0.75 for clamped windows and 1.125 for unclamped (See Fig. 2). Smax is the maximum stress, D is the window diameter under pressure (ie, the portion of window not supported by the flange as shown in the schematic in Figure 2), P is the load (expressed in psi), and t is the thickness of the window material. In the formula solving for t, the window diameter D can be expressed in any unit of measure such as mm or inches. To avoid plastic deformation a safety factor must be introduced where SF is a safety factor and Fa is apparent elastic limit for the material itself. Where apparent elastic limit is not available, use yield stress. Allowing for a safety factor (SF), the equations for calculation of maximum allowable pressure and for minimum window thickness (t) where the operating pressure (P) is known are:

The apparent elastic limits of some IR optical materials are listed below:


CaF2
5300
KCl
330

BaF2
3900
KBr
160

KRS-5
3800
CsI
810

NaCl
350
MgF2
7200

ATR SPECTROSCOPY

The formulae set forth below are useful for ATR (MIR) spectroscopy.

Penetration Depth

The depth of penetration is defined by the formula:

where l is the wavelength of the infrared light, n1 is the refractive index of the ATR crystal, is the angle of incidence of the infrared beam at the boundary and n21 is the ratio of the refractive indices of the sample, ns and ATR crystal,

Since the evanescent wave decreases in intensity exponentially from the surface of the crystal, the penetration depth, dp, is defined as the distance at which the amplitude of this wave has decreased to (1/e) or 37% of its original value.

Effective Angle of Incidence

This is the angle of incidence of the infrared beam internally in the ATR crystal when a variable angle HATR such as the Varimax™ is used for analysis. When the scale angle, scale, is not equal to the crystal face angle, face, the effective angle,  is different than the scale angle due to refraction.


Number of Reflections

The number of reflections in the crystal gives a measure of the intensity of the resulting spectrum. This number is a function of the effective angle of incidence , and the length, I, and thickness, t, of the crystal.

Effective Pathlength

The effective pathlength, Peff, is defined as the product of the penetration depth dp, and the number of bounces, N, the IR beam makes within the crystal: