Septum Bleeding in Flame Ionization Programmed Temperature, Gas Liquid Chromatography SIR: I n high sensitivity, programmed temperature gas liquid chromatography (PTGLC) extraneous ghost peaks were encountered that have not been described in the literature. These ghost peaks are eluted during blank (no sample or solvent injection) programmed temperature runs as well as during regular sample analyses and are extremely persistent. Repeated column purges change the intensity of, but do not ah-ays eliminate, these peaks. I n fact, after many blank runs, the intensities may sometimes increase. Because low injection block temperatures (100" to 200' C.) were being used to avoid thermal decomposition of the unstable samples being analyzed, these artifacts mere a t first attributed to sample condensation and/or decomposition near or on the septum or in the sample splitter. This deposit was thought to bleed off gradually and concentrate on the cool column between runs and then appear during a regular temperature programmed run as spurious peaks. Ilowever, when cleaning and flushing the injection block with solvents and changing spptums proved fruitless, the silicone gum rubber (SGR) spptum itself was suspected of bleeding. This mas confirmed b y a series of esperiments described belonr. This elution of contaminants would be noticed neither in isothermal GLC, because the continuous septum bleeding mould not be distinguished from column
Table I.
bleeding with most detectors, nor in PTGLC using the conventional thermal conductivity detector because of the low concentrations involved. However, when PTGLC is combined with the sensitive (flame) ionization detector, septum bleeding seriously affects the validity of qualitative and quantitative analyses. The problem of septum bleeding can be rectified quite simply by conditioning the SGR septums in a bomb continually purged with nitrogen a t 300' C. for 90 hours. These conditioned septums still seal effectively after being punctured and are used routinely in this laboratory. They are generally replaced after I to 2 days of use, but have been used for as long as 5 days a t an injector temperature of 200' C. and for short periods a t 500' C. EXPERIMENTAL
Apparatus. F & &I Model 609 programmed temperature gas chromatograph with flame ionization detector. T h e splitter port on t h e injection block was modified b y replacing t h e S G R septum-splitter needle arrangement with a stainless steel Hoke needle valve. A second injection block consisting of a Swagelok "T" fitting (lj4-inch 0.d. tube) was installed in the column oven and attached directly t o the carrier gas supply line via a 12 x l/*-inch 0.d. stainless steel tube preheater. The third arm of the "T" rvas sealed either by a SGR septum or by a brass Smagelok plug. This
Study of Septum Bleed in Isothermal Injection Block"
Equilibrium base line height with Max. eak column heigit at 225' C. ( X 10-'2 (X amp.) amp.) 10.1 ... 11.2 ... 26.0 ... 615.0 ...
KO. of Injection port Injection blo2k peaks septum material temperature, C. eluted None 1. Lead gasket 20 None 2. Fresh SGR septum, Brand -4 20 10 3. Fresh SGR septum, Brand A 200 4. Fresh SGR septum, Brand A 200 (for 12 hr. with 12 column a t room temp. before regular column programmed temp. run) 5. SGR septum, Brand A (aged 215 None ... 10 1 for 90 hr. at 300" C. under nitrogen atmosphere) a Stock F and ? V I 609 injection block except that splitter port septum is replaced with a stainless steel Hoke needle valve. Column temperature programmed from 50' t o 225" C. a t 9" C./min.
1840 *
ANALYTICAL CHEMISTRY
Table 11. Typical Retention Times and Temperatues of "Ghost Peaks""
Retention
Peak
Retention time, min. from injection
G
13.6
172.3
H I J K L
14 0
15.0 15.5 16.0 17.1
temp., C.
176 0 185 0
189 5 104 0 203 5
a Peak retention times and intensities are variable, being dependent on the past history of septum os. column temperature. Stock F and b l Injection Block ntth modifications used here (see Experimental). Injection block temperature, 200" C.; initial column temperature, 50" C.; program rate, 9" C./min.
second injection block is not independently heated but assumes the oven temperature and is designated as a programmed temperature injection block. The carrier gas connections in the instrument were arranged so that either injection block alone, or both in series, could be attached to the chromatographic column. The series arrangement m s not used t o obtain the data reported in this communication. Basic sensitivities of electrometer according to the manufacturers is 4 X amp. for full-scale recorder deflection. GLC Column. ',',-inch 0.d. X 5 ft. atainless steel tubing packed nith 230to 320-mesh glass beads coated with 0.57, (.-./.;.) Dow Corning FS-1265 fluorinated silicone fluid. SGR Septums. Type A. obtained from U c r o t e k Instruments, Inc., Baton Rouge, La. Type 13, obtained from Kilkins Inbtrument and Research, Inc., Walnut Creek, Calif. SEPTUMCOSDITIOSISG. Xging the septum in the conventional injection block of the gas chromatograph n-as not found to be effective. Instead, the septums are heated for 90 hours a t 300' C. in a stainless steel cannister swept out by a continuous flom- of nitrogen. After the conditioning. a brownish residue should be wiped off the face of the septum with a lint-free cellulose tissue. OPERATISGCOXDITIOSS. Injection block and column temperatures and program rates as stated in Tables I, 11. and 111; detector block tempera-
ture, 300' C.; helium flow rate, 50 ml./min. ; hydrogen flow rate, 35 ml./min.; air flow rate, 450 ml./min. DISCUSSION
Tables I and I1 describe the intensity and number of peaks eluted from fresh and aged septums us. a lead gasket as a reference standard. The common procedure of letting the carrier gas flow through a hot injection block into a cool column overnight (Run KO. 4, Table I ) , when using ordinary S G R septums at a moderate temperature of 200" C., results in the collection of large amounts of contaminants on the column. During the short cooling period between runs (from 225' C. to room temperature and immediately starting another programmed run), the collection and elution of contaminants from the septum are sufficient to invalidate the nest run (Run KO.3, Table I). R u n KO. 5, 'Table I, shows that conditioned septums perform as satisfactorily at 200" C. as does the reference material, lead. The bleeding rates of SGR septums
Table 111. Study of Septum Bleed in Programmed Temperature Injection Block"
Injection port septum material Brass plug Fresh SGR septum, type A Type A (washed in acetone and aged for 0.5 hr. in N2 at 300" C.) Tvne B (washed in acetone nnrl aged for 0.5 hr. in Nz ttt300" C.) Type A (aged for 7 2 hr. at 300" C. in N2 atm.) Type A (aged for 90 hr. a t 300" C. in ?rT2 atm.) i r
~
Equilibrium base line height with column, injection block and septum at 225' C. ( X 10-12 amp. )
8.0 50.4
20.0
\
23.6
15.6 12.0
a Temperature of injection block, septum, and column programmed simultaneously from 25" t o 225' C.
us column substrate (Dom Corning FS-1265) are compared in Table 111, b y using the programmed injection block to keep the septum and the column a t the same temperature. The basic column substrate bleed rate is determined with a brass plug in place of the SGR septum in the programmed injector. Dow Corning FS-1265 bleeds relatively heavily a t 225' c'. (more than General Electric S E 30 and JE 51 silicone gum rubbers, or Dom Corning 550 and 710 silicone fluids) yet the (fresh) septum bleeding a t 225' C. is six times the rate of colunin Needing (Table 111). Thus, n-hen n-orking w t h low load, high temperature wlxtrates, septum bleeding does makc, R major contribution to total base line height in PTGLC, and this licight can I F jignificantly reduced through the use of preconditioned septums (Table 111). RICHARD H. KOLLOFF Agricultural Chemicals Division Monsanto Chemical Co. St. Louis 66, Mo. RECEIVED for review September 6, 1962. Accepted September 34, 1962.
Identification of Organic Compounds by Differential Thermal Dynamic Analysis SIR: One of the traditional analytical methods for identifying organic compounds determines the melting points of crystalline derivatives prepared from the sample and a standard reagent ( 5 ) . This procedure is often tedious, and somet'imes misleading because of the complications of side products. Recently we attempted to replace this multistep process by a one-step thermal dynamic technique using a differential thermal analysis apparatus. The sample !vas hcnted ~vitha specific reagent a t a programmed rate in a selected atmosphere. The thermogram showed the derivative-forming reaction, the physical transitions of the sample or the reagent in excess, and the physical transitions of the intermediates and products in a single run. Only a few milligrams of sample and reagent were required. I'reliniinary results are very promising. This new approach could lead to a new scheme of organic analysis.
function of the sample temperature. The principles and general applications can be found in excellent reviews (3, 4, 6). The apparatus used consisted of four major parts: cell assembly, temperature programmer, amplifier, and recorder. The last three parts and thermocouple junctions were similar t o those described by Vassallo and Harden (8). For crude experiments the temperature programmer could be replaced by an ordinary powerstat.
EXPERIMENTAL
Differential thermal analysis measures the thermal effects occurring in the sample by continuously recording the temperature difference between the sample and a reference material as a
Figure 1. Differential thermal analysis cell assembly
A schematic diagrani of the cell assembly is shown in Figure 1. This simple design gives high sensit'ivit'y and resolution, good control of atmosphere, quick cooling, fast change of thermocouples, and no cell cleanup. In aluminum block, 2 (0.75 X 1.5 inches), held in a llarinite seat, 3, is used as the heat sink. The heat source is a 0.23 X 1 inch, 30-rratt cartridge heater, 3 (Hot K a t t Co., Danvers, Mass.), which can be operabed up to 500" C. n-itliout difficulty. Ordinary melting point capillaries, 6 (1.5 t o 2.0 X 25 i i i m . ; KIllAX 34505), containing the reaction mixture and roasted glass beads (100- t o 140-1nesh, Potters I ~ ~ o s Carlstadt, ., K. J.j, respectively, are placed in two holes (0.070 x 1 inch) symmetrical in position t o t h e heater. The %-gage, glass-insulated Chroniel-Alumel thermocouples are directly inserted into the reaction mixture 2nd t'he reference material to measure the sample temperature and the temperature differential. A similar thermocouple, 5 , is placed close to the cwtridge h ~ a t e rt o control the temperature programming. The whole assembly is enclosed b y a borosilicate glass bell jar, 4 (3.125 x 3.375 inches), and sealed b y a neoprene O-ring, e, on the bottom aluminum plate, 1, which is supported by a three-leg clamp, 12. Electrical connections of the thermocouples and the heater arc made through VOL. 34, NO. 13, DECEMBER 1 9 6 2
1841
