!!! Chromatography Problem? Call
800-526-3687' for the solution!
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Chromatography Question? Call
800-526-3687' for the answer! We can help with applica tion advice, we can help with column selection, we can help with technical trou ble-shooting or with an accessory that will do the trick, or even a custom made column for your unique needs.
Table I. Elements, Analytical Wavelengths, and Sensitivities Obtained with Helium Glow Excitation Element
Wavelength (A)
ment of the electron microprobe (4). This elegant but expensive instrument can analyze picoliter samples for a number of elements at once. Our approach has been to make spe cial-purpose instruments suited to a single type of analysis. The areas that have concerned us have included os molality, cation analysis for sodium, potassium, etc., and other organic me tabolites. Salt and Water Osmotic forces are very important in the recovery of water from the fil tered tubule fluid. The kidney recov ers about 99% of the filtered water. Osmometry is a convenient way to measure the water activity, and freez ing point depression can be used if millidegree sensitivity can be achieved. Ramsay developed a microfreezing point depression apparatus in which samples within a capillary were observed with a microscope as the ice bath in which they were suspended was slowly warmed (5). The operator had to decide when the last bit of ice was about to melt and then read the temperature of the bath to the nearest
Chrompack, Inc. PO Box 6795 Bridgewater, NJ 08807 201 -722-8930 CIRCLE 32 ON READER SERVICE CARD
1 0.5 4 9 2 4 4 6 1 5 equal to twice
millidegree with a Beckmann ther mometer. Prager and Bowman devel oped a microfreezing point apparatus with Peltier elements to control the sample temperature (6). In their ap paratus, the samples were suspended in oil held in holes in a copper bar which formed the top link of a twostage cooler. A thermistor on the link was part of a servo loop that allowed the operator to hold the temperature at any point while inspecting each sample with a microscope. Figure 1 shows this arrangement. In this way, the smallest ice crystal could be held in equilibrium with the fluid portion, and the true melting point could be estimated with good precision. Also, standards could be put in adjacent holes so that simultaneous calibra tions could be carried out. This al lowed operators much greater control and measurement accuracy than did the older techniques. Sodium is the major osmotically ac tive cation, and potassium is impor tant because the ratio of the extracel lular concentration to the intracellular concentration determines the cell membrane potential. Changes in po-
nL Constriction Pipet
Cap Inlet Outlet Reference Thermistor
LiOH Granule
Hg Drop
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Sensitivity a (X 10~ 1 4 mol)
Ca 4227 0.02-2 Cd 3261 0.01-0.1 Cu 3724 0.01-0.1 Fe 2488 0.1-1 2537 0.01-0.1 Hg Κ 7699 0.1-1 Mg 2852 0.02-2 Na 5896 0.1-1 Pb 4058 0.01-0.1 Zn 2139 0.01-1 " Sensitivity is defined as the amount of element needed to give a signal increment the standard deviation of the blank.
If you're working with HPLC, GC, or Capillary GC (or want to know how to switch over to capillary GC, easily and economically) call 800-526-3687*. You'll reach Chrompack, special ists in chromatography
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Concentration range tested (mM)
CCI2F2
co2 Carrier
Carrier Gas 1 jiL/s H 2 S0 4
Sample Thermistor
Figure 2. Arrangement for the extraction and measurement of picomole amounts of C0 2 from nanoliter samples
392 A · ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984
