Homework #2

Use the following TOX raw data to answer the following questions:

		Integrated Peak (ng)
Sample	Volume (mL)	1st column	2nd column
distilled water	50	605	555
200 ug/L MCAA	50	2405	1205
100 ug/L MCAA	50	1490	890
Tap Water	50	3492	1955
Tap Water	50	4059	1538
distilled water	50	510	660
groundwater	100	1288	799
groundwater	100	1892	865
groundwater	100	1484	990
groundwater	100	1553	921
groundwater	100	1399	1020
groundwater	100	1686	952
groundwater	100	1335	1100
distilled water	50	495	600

What is the TOX concentration of the tap water sample?
Are the two GAC cartridges adequately capturing the organic halogen compounds?
What is the % recovery of the MCAA standards?
What is the method detection limit? (refer to section 1030 in Standard Methods for this)

Solution

What is the TOX concentration of the tap water sample?

For this you must first calculate an average blank value from the distilled water replicates.

Blank Avg =

570.8

Then you subtract this blank from the individual column values to get the net organic halide in ng (as Cl). Finally you add the two net values and divide by the sample volume to get the TOX concentration in ug/L. See table below for these calculations:

		Integrated Peak (ng)			Net OX (ng)				TOX	MCAA	% recovery
Sample	Volume (mL)	1st column	2nd column	Sum	1st	2nd	sum	% first	conc (ug/L)	conc (ug/L)
distilled water	50	605	555	1160	34	-16	18
200 ug/L MCAA	50	2405	1205	3610	1834	634	2468	74%	49.4	200	65.8%
100 ug/L MCAA	50	1490	890	2380	919	319	1238	74%	24.8	100	66.0%
Tap Water	50	3492	1955	5447	2921	1384	4305	68%	86.1
Tap Water	50	4059	1538	5597	3488	967	4455	78%	89.1
distilled water	50	510	660	1170	-61	89	28
groundwater	100	1288	799	2087	717	228	945	76%	9.5
groundwater	100	1892	865	2757	1321	294	1615	82%	16.2
groundwater	100	1484	990	2474	913	419	1332	69%	13.3
groundwater	100	1553	921	2474	982	350	1332	74%	13.3
groundwater	100	1399	1020	2419	828	449	1277	65%	12.8
groundwater	100	1686	952	2638	1115	381	1496	75%	15.0
groundwater	100	1335	1100	2435	764	529	1293	59%	12.9
distilled water	50	495	600	1095	-76	29	-47
				Tap Water Mean =					87.6
				Groundwater Standard Deviation =					2.1	ug/L

						MDL = 3.14*SD =			6.6	ug/L

The values for the two tap water samples are 86.1 and 89.1 ug/L. So the average would be 87.6 ug/L

Are the two GAC cartridges adequately capturing the organic halogen compounds?

Here you compare the net values from the first column with those from the second column. This may be done as a %age of the first compared to the total. Results of this calculation are shown in the table above under the column “% first”. These values average about 85%, and the lowest is for the first tap water sample at 70%. This is good recovery as only 30% of 30% (or 9%) would be escaping the second column, even in this worst sample.

What is the % recovery of the MCAA standards?

Here you need to recognize that MCAA is only 37.5% chlorine by weight. Therefore you expect to get 37.5 ug/L and 75 ug/L for the two standards. The actual values fall well below, and they average 65.9 % of the expected value (see table above).

What is the method detection limit? (refer to section 1030 in Standard Methods for this)

Based on standard methods you need 7 replicates of a low level sample. The groundwater sample serves our purpose here. First you calculate the standard deviation of those 7 samples. This turns out to be 2.1 ug/L. Then you multiply by the appropriate t-statistic (i.e., 3.14) which gives us an MDL of 6.6 ug/L (see above).

Assigned: 24 Oct 2005

Homework #2

Solution

Due: 31 Oct 2005