ANALYTICAL METHODS--Discontinuation of the National Water Quality Laboratory determinations for "total" nitrite, "total" nitrite plus nitrate, "total" ammonia,and "total" orthophosphate (using the four-channel analyzer)




In Reply Refer To:                               December 2, 1992
Mail Stop 412


OFFICE OF WATER QUALITY TECHNICAL MEMORANDUM 93.04

Subject:  ANALYTICAL METHODS--Discontinuation of the National
          Water Quality Laboratory determinations for "total"
          nitrite, "total" nitrite plus nitrate, "total" ammonia,
          and "total" orthophosphate (using the four-channel
          analyzer)

                             SYNOPSIS
     The purpose of the memorandum is to inform Division personnel
that determinations of "total" nitrite, "total" nitrite plus
nitrate, "total" ammonia, and "total" orthophosphate in both the
standard and low-level concentration ranges are being removed from
the National Water Quality Laboratory (NWQL) catalog.  Effective
immediately, project personnel are instructed to discontinue
requests for the laboratory codes (LC) listed below:


Nutrient species and bottle designation   Laboratory codes:
                                         Routine   Low level
____________________________________________________________
Ammonia, Total.....................RC     LC0123    LC0836
Nitrite, Total.....................RC     LC0302    LC0840
Nitrite plus nitrate, Total........RC     LC0304    LC0839
Orthophosphate, Total..............RC     LC0297    LC0838
____________________________________________________________


These laboratory analytical codes will be deleted from the
laboratory catalog and the Schedules, Parameters, and Network
Files System on January 1, 1993.  Project personnel must check the
laboratory schedules they are using to determine if the laboratory
codes listed above are included.

     Deletion of these laboratory codes follows a technical
examination of the method used to produce data for nutrient
species on unfiltered and filtered samples, and from a statistical
evaluation of the data produced by NWQL during 1989.  The
conclusion reached from evaluations of the method and data is: no
valid basis exists for distinguishing between unfiltered and
filtered determinations of nutrient species using the four-channel
method.  Moreover, the continuous-flow analyzer used in the four-
channel method is unsuitable for unfiltered samples.

     Division personnel are advised of the following changes
related to deletion of the laboratory codes listed above:

1.  After January 1, 1993, no analysis will be run for nitrite,
nitrite plus nitrate, ammonia, or orthophosphate on RC (Raw,
Chilled) samples.  The only RC samples to be analyzed will be
those that request LC1686 for total phosphorus and (or) LC1688
for total Kjeldahl nitrogen (ammonia plus organic nitrogen).

2.  All samples submitted to the laboratory for determination of
nutrient species using the four-channel method must be
designated "FC" (Filtered, Chilled) and, therefore, must be
filtered in the field.

3.  Effective immediately, project personnel may not request
reruns on nutrient determinations using the four-channel method
based on a criterion that concentrations reported in the RC
(unfiltered) sample must equal or exceed those on the FC
(filtered) sample.
                              PROBLEM

     Determination of "total" (unfiltered) and "dissolved"
(filtered) nitrite, nitrite plus nitrate, ammonia, and
orthophosphate are requested routinely on identical or replicate
samples.  Concentrations of these nutrient species are determined
simultaneously using a four-channel, continuous flow, colorimetric
analyzer.  No digestion is performed on unfiltered (RC) samples
prior to analytical determinations; analytical procedures are
identical for "dissolved" and "total" determinations.  Therefore,
no analytical basis exists for the distinction between analyte
concentrations in the "dissolved" and "total" fractions.
Moreover, unfiltered samples with large concentrations of solids
can foul the four-channel analyzer on which these determinations
are performed.  Therefore, these samples may be laboratory
filtered at the analyst's discretion, although the results are
reported as a "total" concentration.  Records of laboratory
filtrations have not been kept.

     The problems inherent in using the four-channel method for
nutrient determinations on unfiltered samples have been summarized
by C.J. Patton of the Methods Research and Development Program,
NWQL:

1.  Colorimetric analysis of turbid solutions is contrary to
accepted analytical practice.  Particulates in reagent-treated
samples scatter light, resulting in increased absorbance that
is indistinguishable from absorbance caused by the colorimetric
reaction.  The consequence is to bias the analysis for
unfiltered samples with appreciable particulate material toward
higher concentrations than for filtered samples.

2.  Rigorous chemical digestion is required to measure "total"
nutrient concentrations in environmental samples.  Unfiltered
(RC) samples are not digested prior to analytical
determinations by the four-channel method.

3.  The method was not designed for turbid samples, nor does the
equipment function optimally when samples with high particulate
concentrations are introduced.  The continuous-flow analyzer
used in the four-channel method has conduits with internal
diameters of 1 millimeter or less.  Particulates in the "RC"
samples can coat or partially block the mixing coils and flow
cells of the colorimetric continuous-flow analyzers.  This
causes analytical problems that range from an abrupt shift in
baseline absorbance to total malfunction.

                      HISTORICAL BACKGROUND

     The rationale that originated the analysis for "total"
concentrations of these four nutrient species in unfiltered
samples is not well documented.  The practice arose, in part,
from a confusion embedded within our terminology.  An unfiltered
sample yields, by definition, a "total" analysis (that is, an
analysis of the raw sample).  Therefore, the concentrations
analyzed in unfiltered nutrient-species samples designated "RC"
were reported as "total" so that (a) the determination on a
filtered sample reported as "dissolved" is distinct (without
regard to sample preparation and analytical method), and (b) raw
and filtered sample designations are consistent with those
specified for other inorganic constituents.  In addition, the
U.S. Environmental Protection Agency (USEPA) required field
collection of an unfiltered or "total" sample for these
nutrients, and U.S. Geological Survey (USGS) projects using USEPA
methods required an analogous procedure and laboratory schedule.
To the best of our knowledge, however, USEPA protocols require
filtration of particulate-laden samples in the laboratory prior
to analytical determinations.  In contrast, filtration of
particulate-laden samples at the NWQL is discretionary.

     The chronology of methods used by USGS laboratories for
determinations of total and dissolved nutrient species is
partially documented as internal reports and memorandums.  The
1979 edition of Techniques of Water-Resources Investigations
(TWRI), book 5, chapter A1 (Skougstad and others) first published
methods used at NWQL since 1973 to determine "total" nutrients by
the four-channel method.  Differences between these methods and
those listed in the subsequent TWRI edition (1989) and editions of
the NWQL Services Catalog are incompletely defined.  In each time
period, however, the only apparent difference in the method
between the "total" and the "dissolved" nutrient species was that
unfiltered samples were decanted before analysis.

     For example, the analytical methods for nitrite and for
nitrite plus nitrate determinations on unfiltered samples specify
that the determinations be performed on the supernatant portion of
the sample "by decanting a suitable portion from a well-settled
sample" (Skougstad and others, 1979).

     Moreover, Skougstad and others (1979) discussed the
interference of suspended material on the method for total
orthophosphate as follows:

     "...as phosphorus is easily adsorbed on sediment, the
     orthophosphate recovered from the supernatant solution
     above a suspended-sediment solution after some time
     has elapsed may be less than the orthophosphate that
     would have been determined in the filtrate from a
     sample filtered at the time of collection.  The amount
     recovered may also depend on the type of sediment
     (clay, sand, etc.)."

                  STATISTICAL EVALUATION OF DATA

     A statistical analysis was performed by C.J. Patton (Methods
Research and Development Program, NWQL) and Jeff Pritt (Quality
Management Group, NWQL) of all calendar year 1989 data to compare
paired values of "total" and "dissolved" samples using a sign
test.  The number of paired values involved in the analysis for
each species is listed in table 1:


   Table 1.--Number of paired values of the FC and RC
        nutrient samples statistically analyzed
______________________________________________________
   Nutrient species         Number of paired values:
                               FC and RC samples
______________________________________________________
   Ammonia                          4,228
   Nitrite                            643
   Nitrite plus nitrate             1,459
   Orthophosphate                   1,025
______________________________________________________


     The data were analyzed twice, using graphical inspection and
two statistics software packages.  Branch of Systems Analysis
staff carefully reviewed the methods applications and data
interpretation, and confirmed the conclusions presented here.  A
USGS Open-File Report is planned for publication during fiscal
year 1993 to provide a detailed description of the problem, the
data analyzed, the statistical methods used, and the conclusions
reached.

     Visual inspection of the plotted differences in data values
for paired analyses [unfiltered ("total") versus filtered
("dissolved")] indicated that the mean difference was about zero.
A sign test on the filtered-unfiltered pairs indicated that the
medians were not significantly different from each other at the
95-percent confidence level.  The conclusion is that the
analytical values reported for pairs of unfiltered and filtered
concentrations for the identified nutrient species are
statistically indistinguishable.

     As a result, in July 1991, NWQL ceased to apply the internal
quality-assurance criterion requiring a rerun on nutrient samples
for which the "dissolved" exceeded the "total" concentration for
sample pairs analyzed using the four-channel method.  Instead, the
NWQL has been applying a two-sided test to RC and FC sample pairs
that requires respective concentrations for the pair to be
equivalent within analytical variance.

               INSTRUCTIONS TO DIVISION PERSONNEL

1.  Effective immediately, project personnel are instructed to
discontinue requests for the laboratory analytical codes shown
on table 2.  These laboratory codes will be discontinued on
January 1, 1993.  Project personnel must check the laboratory
schedules they are using to determine whether the laboratory
codes listed on table 2 are included.


Table 2.--Laboratory codes to be deleted from the NWQL catalog
________________________________________________________________
Nutrient species and bottle designation    Laboratory codes:
                                          Routine   Low level
________________________________________________________________
Ammonia, Total.....................RC     LC0123     LC0836
Nitrite, Total.....................RC     LC0302     LC0840
Nitrite plus nitrate, Total........RC     LC0304     LC0839
Orthophosphate, Total..............RC     LC0297     LC0838
________________________________________________________________


2.  After January 1, 1993, the only RC samples to be analyzed will
be those that request LC1686 for total phosphorus and (or)
LC1688 for total Kjeldahl nitrogen (ammonia plus organic
nitrogen).  RC samples will not be run on the four-channel
analyzer.  Therefore, no determination is available from NWQL
for "total" concentrations on the following individual species:
ammonia, nitrite, nitrite plus nitrate, and orthophosphate.

3.  All samples submitted to the laboratory for determination of
nutrient species on the four-channel analyzer (ammonia,
nitrite, nitrite plus nitrate, and orthophosphate) must be
filtered in the field and the sample bottle labeled with an
"FC" designation.

4.  Effective immediately, project personnel are asked not to
request reruns on nutrient determinations using the four-
channel method based on a criterion that the "total" (RC, or
unfiltered) concentration must equal or exceed the "dissolved"
(FC, or filtered) concentration reported for the sample pair.

5.  Measurement of unfiltered or filtered concentrations of
phosphorus and Kjedahl nitrogen can be obtained by requesting
the appropriate laboratory codes listed in table 3, for which
rigorous procedures for complete sample digestion are applied
using the modified Jirka method that was implemented by NQWL in
1992 (see Office of Water Quality Technical Memorandum 92.10).


   Table 3.--Laboratory codes to be used for determination of
   phosphorus and nitrogen concentrations by complete digestion
               of the unfiltered and filtered sample
_________________________________________________________________
       Nutrient determination           Sample       Laboratory
                                     designation        codes
_________________________________________________________________
Phosphorus
                 unfiltered,digested     RC            LC1686
                 filtered,digested       FC            LC1685

Nitrogen, Kjeldahl
 (ammonia plus organic nitrogen)
                unfiltered,digested      RC            LC1688
                filtered,digested        FC            LC1687
_________________________________________________________________


     One 125-mL "FC" bottle of filtered sample is sufficient sample
volume for determination of:  nitrogen and phosphorus species on the
four-channel analyzer, filtered phosphorus (LC1685), and filtered
Kjeldahl nitrogen (LC1687).  A separate 125-mL "RC" bottle of
unfiltered sample is required for total phosphorus (LC1686) and
total Kjeldahl nitrogen (LC1688) determinations.

     Division personnel are advised that it is invalid to make the
following interpretations with respect to comparison of unfiltered
and filtered four-channel nutrient data:  (1) to attribute
environmental significance to differences or similarities between
concentrations for the "total" and "dissolved" determinations; (2)
to suggest concentrations measured in the unfiltered sample
represent the total concentration present in the sample for the
nutrient species analyzed; and (3) to implement a quality-assurance
strategy based on the premise that the concentration measured in
the unfiltered sample must exceed or equal the concentration
measured in the filtered sample.

                          REFERENCE CITED

Skougstad, M.W., Fishman, M.J., Friedman, L.C., Erdmann, D.E.,
   Duncan, S.S., 1979, Methods for Determination of Inorganic
   Substances in Water and Fluvial Sediments, U.S. Geological
   Survey Techniques of Water-Resources Investigations, book 5,
   chapter Al, 545 p.




                                    David A. Rickert
                                    Chief, Office of Water Quality

This memorandum does not supersede any Office of Water Quality
Technical Memorandum, but refers to Office of Water Quality
Technical Memorandum 92.10.

Key Words:  Laboratory methods, nutrient species, nitrite,
            nitrate, ammonia, orthophosphate, four-channel method.

Distribution:  A, B, S, FO, PO