Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory-Determination of ammonium plus organic nitrogen by a Kjeldahl digestion method and an automated photometric finish that includes digest cleanup by gas diffusion

The National Water Quality Laboratory
(NWQL) determined ammonium plus organic
nitrogen (Kjeldahl nitrogen) by using semiautomated,
block digester methods for filtered
and whole-water samples from 1986 until
October 1, 1991. During that time,
phosphorus was determined by a persulfate
digestion method. In 1991, projected
increases in demand for both tests by the U.S.
Geological Survey?s National Water-Quality
Assessment Program led the NWQL to
develop and validate methods for determining
both analytes in a common digest.
This report describes a rapid and
accurate method to determine Kjeldahl
nitrogen. The batch, high-temperature (block
digester), Hg (II)-catalyzed digestion step
used in the new methods I-2515-91/4515-91 is
similar to U.S. Geological Survey methods
I-2552-85/4552-85 and U.S. Environmental
Protection Agency method 351.2 except that
sample and reagent volumes are halved.
Prepared digests are desolvated at 220 degrees
Celsius (oC) and digested at 370oC in separate
block digesters set at these temperatures,
rather than in a single, temperatureprogrammed
block digester. This approach
permits 40 calibrants, reference materials, and
samples to be digested and resolvated in about
an hour. Ammonium ions originally present
in samples, along with those released during
the digestion step, are determined photometrically
by an automated, salicylatehypochlorite
Berthelot reaction procedure at a
rate of 90 tests per hour. About 100
microliters of digest are required per
determination. The upper concentration level
is 10 milligrams per liter (mg/L) with a
method detection level of 0.05 mg/L.
Repeatability for a sample containing about
4.1 mg/L of Kjeldahl nitrogen in a high
suspended-solids matrix is 3.1 percent.
Between-day precision for the same sample is
4.8 percent.
A gas diffusion cell in the air-segmented
continuous flow analyzer eliminates
particulates and ions that otherwise would
interfere in the photometric finish. A singlechannel
analyzer can process the resolvated
digests from two pairs of block digesters each
hour. Statistical analysis of paired data for
about 1,500 samples determined by U.S.
Geological Survey methods I-2552-85/4552-
85 and I-2515-91/4515-91 during method
validation revealed a median concentration
difference between the former and the latter
methods of about 0.1 mg-N/L. This result was
expected because digestion blank concentrations
(nearly equal to 0.1 mg/L) were not
subtracted from concentrations reported by
methods I-2552-85/4552-85. A 10-year
record of National Water Quality Laboratory
Kjeldahl nitrogen blind blank concentration
data also supports a step-change decrease in
Kjeldahl nitrogen concentrations of about 0.1
mg/L after methods I-2552-85/4552-85 were
replaced by methods I-2515-91/4515-91 on
October 1, 1991. Somewhat larger
concentration differences between the two methods were observed for a subset of about
350 samples with nitrate plus nitrite
concentrations greater than 1 mg-N/L.