Product Review
Measuring nitrogen and phosphorus in the presence of hydrocarbons
The pesticide, petrochemical, tobacco, and pharmaceutical industries have more in common than you might think. What puts them all in one category is their need to quantify trace levels of nitrogen and/or phosphorus (N/P) compounds in complicated, often unknown, matrices. In such analyses, selectivity—the ability to measure N/P in the presence of hydrocarbons^ is key. The thermionic nitrogen/phosphorus detector (NPD) has found widespread use in GC for the detection of N/P compounds, from organophosphorus pesticides and triazine herbicides to drugs of abuse. Compared with a typical flame ionization detector (FID), an NPD is about 500 and 50 times more sensitive for P and N compounds, respectively. The linear response range for an NPD is generally on the order of 104 or 10^ much less than an FID (~107). But what's important is that the NPD response to N/P is 104-106 times larger than its response to carbon. An NPD is not specific to just N and P—other materials will respond, says Edward Kikta, of FMC Corporation's Agriculture Division, who frequently uses NPDs for
Thermionic NPDsfindwidespread use in gas chromatography.
technology and practical considerations for potential NPD buyers.
The design An NPD is structurally similar to an FID. Several NPDs use FID components, such as the base and amplifier. According to the manufacturers, such designs allow for easy switching between an FID and an NPD. Some gas chromatographs, however, can pesticide analyses. The response to P in par- accommodate an FID and an NPD on the same instrument. Many systems can even ticular, and N in some cases, is enhanced handle more than one detector operating with an NPD, he says, depending on the simultaneously with independent temperaplasma temperature and how the plasma is set up (flow rates of gases), as well as on the ture control. "Presumably you could have an electron capture detector and an NPD," number of C-N and C-P bonds in die molesays David Kurtz of Penn State University's cule coming dirough. "There are many facPesticide Research Laboratory As long as tors mat are not fully understood mat can alter relative response. Even though we have the detector does not destroy the sample and the chromatograph can accommoresponse differences in omer detectors, it is date it, it can be used along with an NPD exaggerated with an NPD." Are all NPDs alike? Analytical Chemistry An FID cannot be used concurrently with an NPD because an FID is destructive to surveyed several manufacturers in the GC the sample business tofindout. Specifications and features of selected commercially available The critical difference between an FID NPDs are provided in Table 1. This review, and an NPD is the magnitude of the H2 although not comprehensive, aims to proflow, says Paul Patterson, president of vide a representative overview of NPD DETector Engineering and Technology. Analytical Chemistry News & Features, September 1, 1998 5 9 9 A
Product Review Table 1 . Summary of selected thermionic nitrogen/phosphorus detectors for GC
a
Model
TRACE GC 2 0 0 0 Series
PET NPD
HP 6 8 9 0 NPP
Manufacturer
CE Instruments Strada Rivoltana 20090 Rodano Milan, Itlay 39 2 95059 341
DETector Engineering & Technology 486 N. Wiget Ln. Walnut Creek, CA 94598-2408 925-937-4203
Hewlett-Packard 2850 Centerville Rd. Wilmington, DE 19808 800-227-9770
URL
www.ceinstruments.com
None
www.hp.com/go/chem
Price (U.S. Dollars)
$2900
Up to $3600 (complete NPD); modular components also sold separately
$1950 (manual pneumatic control) 3)3050 (electronic pneumatic control)
Compatible with other GC systems
No
Yes
No
Bead material
Ceramic
TI\j-c. (black ceramic) TID-4 (white ceramic)
Ceramic
Detection limit Nitrogen Phosphorus
5 x 1 0 _ 1 4 gN/s 2 x 10~14 gP/s
TID-4: 7 x 10" 14 gN/s (acrylonitrile) TID-2: 7 x 10" 1 4 gP/s (malathion)
