323 4 Determinative Techniques to Measure Organics and Inorganics They laughed when they heard Aston say, he would weigh tiny atoms one day. But he had the last laugh — with his mass spectrograph, he “weighed” them a different way. —Anonymous CHAPTER AT A GLANCE Column chromatographic determinative techniques for trace organics Introduction and historical 323 Differential migration 327 Principles of countercurrent distribution 330 Scope of chromatographic separations 335 Theoretical basis of column chromatography 339 Chromatographic resolution 351 Gas Chromatography 357 Gas pneumatics and inlets 359 Capillary columns 369 Programmed column temperature 386 Cryogenic techniques 390 Element selective GC detectors 392 Atomic Emission Detector 417 Gas Chromatography-Mass Spectrometry 423 Principles of the quadrupole 425 Principles of the ion-trap 433 Tuning a quadrupole 436 Principles of Time-of-flight 445 Interpretation 447 Tandem strategies and techniques 449 High Performance Liquid Chromatography 452 Mobile phase/stationary phase considerations 458 UV and fluorescence detectors 464 Principles of LC-MS interfaces 476 © 2006 by Taylor & Francis Group, LLC 324 Trace Environmental Quantitative Analysis, Second Edition Ion chromatographic determinative technique for trace inorganics Principles of cation-anion exchange 478 Principles of suppressed ion chromatography 481 Atomic spectroscopic determinative techniques for trace metals Introduction and historical 490 Choosing among techniques 493 Inductively-coupled plasma atomic emission 497 Principles 497 Inductively-coupled plasma-mass spectrometry 506 Principles 506 Metals speciation 510 Atomic Absorption Historical and Principles 512 Graphite Furnace 515 Other determinative techniques for trace organics and trace inorganics Infrared Absorption Spectroscopy 518 Oil and grease 521 Total organic carbon 525 Capillary Electrophoresis 528 Theoretical 530 Indirect photometric detection 537 References 540 To the observer who does not have the technical background in TEQA and walks into a contemporary environmental testing laboratory, a collection of black boxes (instruments) with cables connecting the black boxes to personal computers and other high-tech devices should be what makes the first impression. This observer will see people, some of whom wear white lab coats, running around, holding various glassware, such as vials, syringes, beakers, test tubes, or whatever else it is lab people handle when at work in the busy lab. Observers will, upon being invited to tour, see different departments within the corporate structure. Some department personnel process analytical data generated by these black boxes; some personnel prepare samples for introduction into the black boxes; and other personnel enter data into a Laboratory Information Management System (LIMS) that reads a bar code label on a given sample and tracks the status of that sample as various analytical methods and instruments are used to generate the data. Some instruments are noisy, some are silent, some incorporate robot-like arms, and some incorporate samples directly, whereas others require sample preparation; all contribute to the last and no less important step in TEQA: determination. Instruments, computers, and accessories all comprise what the Environmental Protection Agency (EPA) refers to as determi- © 2006 by Taylor & Francis Group, LLC native techniques, hence the title of this chapter. In Chapter 2, we discussed the important outcomes of using determinative techniques to perform TEQA. In Chapter 3, we discussed the means by which environmental samples and biological Determinative Techniques to Measure Organics and Inorganics 325 specimens are made suitable and appropriate for introduction to these instruments (i.e., the science of sample preparation for TEQA). This chapter on determinative techniques therefore completes the thorough discussion of TEQA. To the sufficiently educated observer, the contemporary environmental testing laboratory is a true testimonial to man’s ingenuity, a high-tech masterpiece. However, unlike a work of art, this observer is quick to discover that this artistic endeavor is a work in progress. This observer may see a robotic arm of an autosampler depositing 5 µL of sample into the graphite tube of a graphite furnace atomic absorption spectrophotometer (GFAA). He may also peer into a monitor that reveals an electron- impact mass spectrum of a priority pollutant, semivolatile organic compound. He will become aware very quickly whether or not this particular instrument is running samples or is still running calibration standards in an attempt to meet the stringent requirements of EPA methods. If this person is interested in the progress made by a particular sample as it makes its way through the maze of methods, he can find this information by peering into the sample status section of the LIMS software. This chapter takes the reader from the uninformed observer described above to the educated observer who can envision the inner workings of a contemporary environmental testing laboratory. This is the chapter that deals with the determinative step, a term coined by the EPA. Beginning with the SW-846 series of methods, the sample prep portion was separated from the determinative portion. This separation enabled flexibility in conceptualizing the total method objectives of TEQA in the SW-846 series. This author believes that separating the sample prep from the deter- minative also makes sense in the organization of this book. Of the plethora of instrumental techniques, 3–11 GC, GC-MS (mass spectrometry), HPLC, AA, ICP-AES, and ICP-MS are the principal determinative techniques employed to achieve the objectives of TEQA as applied to both trace organics and trace inorganics analysis. The separation sciences have been coupled to the optical spectroscopic and mass spectrometric sciences to yield very powerful so-called hyphenated instruments. These six techniques are also sensitive enough to give analytical information to the client that is the most relevant to environmental site remediation. For example, one way to clean up a wastewater that is contaminated with polychlorinated volatile organics (ClVOCs) is to purge the wastewater to remove the contaminants, a process known as air stripping. It is important to know that the air-stripped wastewater has a concentration of ClVOCs that meets a regu- latory requirement. This requirement is usually at the level of low parts per billion. no place in the arsenal of analytical instruments pertinent to TEQA. Recall from instruments provide instrumental detection limits (IDLs), whereas method detection combination serves to significantly lower the overall detection limits and is one of the prime goals of TEQA. This chapter introduces those six determinative techniques referred to earlier and adds several others. We first discuss those fundamental principles, vital to the practice of both GC and HPLC, that facilitate a more meaningful understanding of column chromatographic separations that are particularly relevant to the quantitative determi- © 2006 by Taylor & Francis Group, LLC limits (MDLs) combine the sample prep step with the determinative step. This A determinative technique that can only measure as low as parts per hundred has Chapter 2 that techniques relating the acquisition of data directly from analytical 326 Trace Environmental Quantitative Analysis, Second Edition nation of trace organics. We then introduce the operational aspects of these instruments largely from a user perspective. A strong emphasis is placed on GC-MS, as this has become the dominant determinative technique for organics in TEQA. Ion chromato- graphic techniques as applied to trace inorganics are then introduced, and this topic provides an important link to the other major class of enviro-chemical/enviro-health chemical contaminants, trace metals, where atomic spectroscopy, as the principal determinative technique, dominates. A link between infrared absorption spectros- copy and TEQA is made through quantitative oil and grease and total organic carbon measurements. Finally, capillary electrophoresis is introduced and applied to the separation, detection, and quantification of trace inorganic anions in surface water via indirect photometric detection. 1. HOW DO YOU KNOW WHICH DETERMINATIVE TECHNIQUE TO USE? Which determinative technique to use is dictated by the physical and chemical nature serves as a useful guide. Let us consider how we would determine which instrumental technique to use for the following example. Ethylene glycol, 1,2-ethanediol (EG), and 1,2-dichloroethane (1,2-DCA) consist of molecules that contain a two-carbon backbone with either a hydroxyl- or chlorine-terminal functional group. The molec- ular structures for these are as follows: These two molecules look alike; so, could we use the same instrument and conditions to quantitate the presence of both of these compounds in an environmental sample? Nothing could be farther from the truth. Some relevant physical properties of both compounds are given in Table 4.1. The presence of two hydroxyl groups enables ethylene glycol to extensively hydrogen bond both intramolecularly (i.e., to itself) and intermolecularly (i.e., between molecules) when dissolved in polar sol- vents such as water and methanol. In stark contrast to this associated liquid, 1,2-DCA interacts intramolecularly through much weaker van der Waals forces and is inca- pable of interacting intermolecularly with polar solvents while being miscible in TABLE 4.1 Physico-Chemical Properties of Ethylene Glycol and 1,2-Dichloroethane Compound T (mp) (°° °° C) T (bp) (°° °° C) Soluble in HOCH 2 CH 2 OH −12.6 197.3 Polar solvents CICH 2 CH 2 CI −35.7 83.5 Nonpolar solvents OH HO C1 C1 © 2006 by Taylor & Francis Group, LLC of the analyte of interest. The organics protocol flowchart introduced in Chapter 1 Determinative Techniques to Measure Organics and Inorganics 327 nonpolar solvents such as chloroform and ether. The boiling point of EG is almost twice as large as that of 1,2-DCA. These significant differences in physical properties would also be reflected in octanol–water partition coefficients. 1,2-DCA can be efficiently partitioned into a nonpolar solvent or to the headspace, whereas any attempt to extract EG from an aqueous solution that contains dissolved EG is useless. Because both compounds are liquids at room temperature, they do exhibit sufficient vapor pressure to be said to be amenable to analysis by gas chromatography. How- ever, it may prove difficult to chromatograph them on the same column. The fundamental differences between a hydroxyl covalently bonded to carbon and a chlorine atom bonded to carbon become evident when one attempts to separate the two. We will continue to use the physical-chemical differences between EG and 1,2-DCA to develop the concept of a separation between the two compounds by differential migration through a hypothetical column and through a series of con- secutive stages known as the Craig distribution. 2. WHAT IS DIFFERENTIAL MIGRATION ANYWAY? Around 100 years ago, Mikhail Tswett, a Russian botanist, demonstrated for the first time that pigments extracted from plant leaves, when introduced into a packed column, whereby a nonpolar solvent is allowed to flow through calcium carbonate, initially separated into green and yellow rings. He called this separation phenomenon chromatography, derived from the Greek roots chroma (color) and graphein (to write). If additional solvent is allowed to pass through, these rings widen and separate more, and further separate into additional rings. In Tswett’s own words: 1 Like light rays in the spectrum, the different components of a pigment mixture, obeying a law, are resolved on the calcium carbonate column and then can be qualitatively and quantitatively determined. I call such a preparation a chromatogram and the corre- sponding method the chromatographic method. His work in establishing the technique of liquid–solid adsorption chromatogra- phy would languish for 30 years until resurrected by Edgar Lederer in Germany. A timeline titled Historica Chromatographica published recently and bench- marks key advances in all of chromatography and serves to recognize those that often go unnoticed; it is summarized in tabular format below: 2 Year Key Advances Pioneers 1990 Persuasive perfusion PerSeptive Biosystems, part of PerkinElmer, introduces perfusion chromatography, in which samples move both around and through the resin beads 1985 Superior supression Dionex researcher Pohl introduces micromembrane suppressors for use in ion chromatography at Pittcon 1981 Microcolumn SFC Novotny and Lee, pioneers in microcolumn liquid chromatography, introduce capillary supercritical fluid chromatography (SFC) © 2006 by Taylor & Francis Group, LLC 328 Trace Environmental Quantitative Analysis, Second Edition Year Key Advances Pioneers 1975 IC advent Small, Stevens, and Bauman develop ion chromatography combining a cation exchange column (separator) and strongly basic resin (stripper) to separate cations in dilute HCl 1974 Capillary zone electrophoresis (CZE) under glass Virtanen introduces commercial CZE in glass tubes, based largely on pioneering work by Hjerten 1966 I see HPLC Horvath and Lipsky develop high-pressure liquid chromatography (HPLC) at Yale University 1966 Sugar, sugar Green automates carbohydrate analysis, improving on the earlier efforts of Cohn and Khym, who used a borate- conjugated ion exchange column to separate mono- and disaccharides 1960 GC’s heart of glass Desty introduces the glass capillary column for GC, used in his analysis of crude petroleum; the technology was later commercialized by Hupe & Busch and Shimadzu 1958 Automating AA analysis Stein, Moore, and Spackman automate amino acid (AA) analysis using ion exchange and Edman degradation 1955 Going to market First gas chromatographs were introduced in the U.S. by Burrell Corp., PerkinElmer, and Podbielniak 1953 Exclusive science Wheaton and Bauman define ion exclusion chromatography, where one solution ion is excluded from entering the resin beads and passes in the void volume 1948 Reversing phases Boldingh develops reversed-phase chromatography when separating the higher fatty acids in methanol against a solid phase of liquid benzene supported on partially vulcanized Hevea rubber 1945 One small step to GC Prior describes gas–solid adsorption chromatography when separating O 2 and CO 2 on charcoal column 1941 Protein pieces Martin and Synge develop liquid–liquid partition chromatography when separating amino acids through ground silica gel 1938 Spotting the difference Izmailov and Shraiber develop drop chromatography on thin horizontal sheets, a precursor to thin-layer chromatography 1937 The road to white sands Taylor and Urey use ion exchange chromatography to separate lithium isotopes, work that eventually led to the separation of fissionable uranium for the Manhattan Project 1922 Clarifying butter Palmer, who is later recognized for popularizing chroma- tography’s use, separates carotenoids from butter fat 1913 Water world First U.S. use of zeolites in water softening based on earlier work in Germany by Gans 1906 Our Father … Tswett develops the concept of chromatography while attempting to purify chlorophylls from plant extracts; this discovery gained him the cognomen “Father of Chromatography” 1903 Food for thought Goppelsroeder develops theory of capillary analysis when using paper strips to examine alkaloids, dyes, milk, oils, and wine, improving on the earlier work of his mentor, Schoenbein © 2006 by Taylor & Francis Group, LLC Determinative Techniques to Measure Organics and Inorganics 329 If a mixture containing EG and 1,2-DCA is introduced into a column, it is possible to conceive of the notion that the molecules that make up each compound would migrate differentially through the packed bed or stationary phase. Let us assume that this hypothetical column tends to retain the more polar EG longer. This separation of EG from 1,2-DCA is shown as follows: We observe that the dispersion of the molecules as represented by σ 2 is found to be proportional to the distance migrated, z, according to where k, the constant of proportionality, depends on the system parameters and operating conditions. Because k is a ratio of the degree of spread to migration distance, k can be referred to as a plate height. The resolution, R s , between the separated peaks can be defined in terms of the distance between the apex of the peaks and the broadening of the peak according to where τ is defined as being equal to 4 (p. 109). 12 We will have more to say on this topic later. It also becomes evident that ∆z is proportional to the migration distance z and σ is proportional to the square root of the migration distance. Expressed mathematically, we have and These equations tell us that the distance between zone centers increases more rapidly than the zone widths. From the definition of R s , this suggests that resolution improves with migration distance. We will have more to say about resolution when we take up chromatography. Differences in the rates of analyte migration, however, Response to presence of organic compounds 1,2- DCA EG Time after injection EG and 1,2-DCA σ 2 = kz R z s = ∆ τσ ∆zz∝ σ∝ z © 2006 by Taylor & Francis Group, LLC 330 Trace Environmental Quantitative Analysis, Second Edition do not explain the fundamental basis for separating EG from 1,2-DCA. For this, we begin by discussing the principles that underline the Craig countercurrent extraction experiment. 3. WHAT CAUSES THE BANDS TO SEPARATE? We just saw that, experimentally, EG and 1,2-DCA differentially migrate through a stationary phase when introduced into a suitable mobile phase, and that chromatog- raphy arises when this mobile phase is allowed to pass through a chemically selective stationary phase. It is not sufficient to merely state that EG is retained longer than 1,2-DCA. It is more accurate to state that EG partitions to a greater extent into the stationary phase than does 1,2-DCA, largely based on “like dissolves like.” The stationary phase is more like EG than 1,2-DCA due to similar polarity. This is all well and good, yet these statements do not provide enough rationale to establish a extraction (LLE) and also considered successive or multiple LLE. What we did not discuss is what arises when we transfer this immiscible upper or top phase or layer to a second sep funnel (first Craig stage or n = 1; see below). Prior to this transfer, the second sep funnel will already contain a fresh lower phase. Equilibration is in contact with the lower phase in the first funnel. What happens if we then transfer the upper phase from this second sep funnel to a third sep funnel that already contains a fresh lower phase? This transfer of the upper phase in each sep funnel to the next stage, with subsequent refill of the original sep funnel with a fresh upper phase, can be continued so that a total of n stages and n + 1 sep funnels are used. It becomes very tedious to use sep funnels to conduct this so-called countercurrent extraction. A special glass apparatus developed by L.C. Craig in 1949 provides a means to perform this extraction much more conveniently. Twenty or more Craig tubes are connected in series in what is called a Craig machine. Once connected, up to 1000 tubes previously filled with a lower phase can participate in countercurrent extraction by a mere rotation of the tubes. The following is a schematic diagram of a single Craig tube of 2 mL undergoing rotation: b 8 cm 5 c m 9 cm d c e a ABC 4 cm © 2006 by Taylor & Francis Group, LLC allowed to occur in the second sep funnel, while the fresh upper phase is brought true physical-chemical basis for separation. In Chapter 3, we introduced liquid–liquid Determinative Techniques to Measure Organics and Inorganics 331 It is this rotational motion that removes the extracted organic phase while a fresh organic phase is introduced back into the tube. The phases separate in position A, and after settling, the tube is brought to position B. Then, all of the upper phase flows into decant tube d through c, as the lower phase is at a. When the tube is brought to position C, all of the upper phases in the decant tube are transferred through e into the next tube, and rocking is repeated for equilibration. The tubes are sealed together through the transfer tube, location e in the figure, to form a unit. These units are mounted in series and form a train having the desired number of stages (pp. 111–112). 12 The Craig countercurrent extraction enables one to envision the concept of discrete equilibria and helps one understand how differences in partition coefficients among solutes in a mixture can lead to separation of these solutes. Consider a cascade of n + 1 stages, each stage containing the same volume of the lower phase. We seek to explain the foreboding-looking Table 4.2. Let us also assume that the total amount of solute is initially introduced into stage 0 (i.e., the first Craig tube in the cascade). The solute is partitioned between the upper and lower phases, as we saw in Equation (3.16), represented here according to (4.1) (4.2) TABLE 4.2 Countercurrent Distribution of a Given Solute in a Craig Apparatus Stage Distribution01 234 Introduce solute and equilibrate p/q Total 1 1 First transfer 0/qp/0 (q + p) 1 Equilibrate p(q)/q(p) p(p)/q(p) Second transfer 0/q(q) p(q)/q(p) p(p)/0 Total q 2 2pq p 2 (q + p) 2 Equilibration p(qq)/q(qq) p(2pq)/q(2pq) p(pp) q(p 2 ) Third transfer 0/q 3 pq 2 /2 q 2 p 2p 2 q/qp 2 p 3 /0 Total q 3 3pq 2 3p 2 qp 3 (q + p) 3 Equilibrate p(q 3 )/q(q 3 ) p(3pq 2 )/q(3pq 2 ) p(3p 2 q)/q(3p 2 q) p(p 3 )/q(p 3 ) Fourth transfer 0/q 4 pq 3 /3q 3 p 3p 2 q 2 /3p 2 q 2 3p 3 q/p 3 qp 4 /0 Total q 4 4q 3 p 6p 2 q 2 4p 3 qp 4 (q + p) 4 p VD VD = +1 q VD = + 1 1 © 2006 by Taylor & Francis Group, LLC 332 Trace Environmental Quantitative Analysis, Second Edition and q is the fraction of total solute that partitions into the lower phase. Also, by definition, the following must be true: V is the ratio of the upper phase volume to the lower phase volume and is usually equal to 1 because both volumes in the Craig tubes are usually equal. D is the distribution ratio and equals K D , the molecular partition coefficient in the absence solutes to stage 0, each solute will have its own value for D, and hence a unique value for p and q. For example, for a mixture containing four solutes, we would realize a fraction p for the first solute, a fraction p' for the second solute, and so forth. In a similar manner, we would also realize a fraction q for the first solute, a fraction q' for the second solute, and so forth. We seek now to show how the p and q values of Table 4.2 were obtained. We also wish to show how to apply the information contained in Table 4.2. We then extrapolate from the limited number of Craig tubes in Table 4.2 to a much larger number of tubes and see what effect this increase in the number of Craig tubes has on the degree of resolution, R s . We start with a realization that once a mixture of solutes, such as our pair, EG and 1,2-DCA, is introduced into the first Craig tube, an initial equilibration occurs and this is shown at stage 0. Again, if p and q represent the fraction of EG in each phase, then p' and q' represent the fraction of 1,2-DCA in each phase. The first transfer involves moving the upper phase that contains a fraction p of the total amount of solute to stage 1. A volume of upper phase equal to that in the lower phase is now added to stage 0, and a fraction p of the total amount of solute in stage 0, p, is partitioned into the upper layer while a fraction q of the total, q, is partitioned into the lower phase. A fraction p of the total p remains in the upper layer in stage 1, and a fraction q of the total p is partitioned into the lower phase. The remainder of Table 4.2 is built by partition of a fraction p of the total after each transfer and equilibration to the upper layer and by partition of a fraction q of the total into the lower layer. The last column in Table 4.2 demonstrates that if each row labeled “total” is added, this sum is the expansion of a binomial distribution, (q + p) r , where r is the number of transfer. The fraction of solute in each nth stage after the rth transfer and corresponding equilibration can then be found using (4.3) This fraction represents the sum of the fractions in the upper and lower phases for that stage. For example, suppose we wish to predict the fraction of EG and the fraction of 1,2-DCA in stage 3 after four transfers. Let us assume that the upper phase is the less polar phase. Let us also assume that the distribution ratio for 1,2-DCA into the less polar upper phase is favored and that D 1.2−DCA = 4. Let us also pq+=1 f r nr n pq nrn = − − ! !( )! © 2006 by Taylor & Francis Group, LLC In Table 4.2, p is the fraction of total solute that partitions into the upper phase of secondary equilibria, as discussed in Chapter 3. If we introduce a mixture of [...]... for vs and v in Equation (4. 13) yields tR = t0 φm (4. 14) The mobile-phase volumetric flow rate, F, expressed in units of cubic centimeters per minute or milliliters per minute, is usually fixed and unchanging in chromatographic systems Thus, because tR = VR /F and t0 = V0 /F, Equation (4. 14) can be rewritten in terms of retention volumes: VR = © 2006 by Taylor & Francis Group, LLC V0 φm 342 Trace Environmental. .. 350 300 m, p-xylene 250 Benzene Toluene 200 100 6 .44 1.95 150 3.39 EthylBz o-xylene 6. 14 0. 84 7.67 50 0 1.0 2.0 3.0 4. 0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 FIGURE 4. 8 Separation and detection of BTEX components using static headspace capillary GC with FID © 2006 by Taylor & Francis Group, LLC 12.0 Trace Environmental Quantitative Analysis, Second Edition 40 0 ... Equation (4. 32) ′ k′ 0.01 0.05 0.10 0.15 0.5 1.0 3.0 5.0 10 50 ′ ′ [(k′ + 1)/k′]2 α Nrequired (α = 1.05) α Nrequired (α = 1.10) 10,201 44 1 121 58.8 9 4 1.8 1 .4 1.1 1.05 162,000,000 7,000,000 1,900,000 930,000 143 ,000 63,500 28,600 22,200 17,500 16,700 44 ,000,000 1,900,000 527,000 256,000 39,000 17,500 7800 6100 48 00 46 00 © 2006 by Taylor & Francis Group, LLC Determinative Techniques to Measure Organics and. .. Table 4. 2) shows this result to be identical to that shown in the table Next, we proceed to evaluate p and q for each of the two solutes Using Equations (4. 1) and (4. 2), we find the following: p (1, 2 − DCA) = 0.9, p(EG) = 0.0909 q (1,2-DCA) = 0.1, q (EG) = 0.909 Upon substituting these values for p and q for each of the two solutes in the mixture into Equation (4. 3), we obtain: f132 ,4 DCA = ( 4) (0.5)3... in a 22-mL headspace vial and sealed The temperature of the vial was brought to 65°C and kept at that temperature for approximately ½ h The headspace was sampled using a gas-tight syringe and injected into an Autosystem® GC, manufactured by PerkinElmer Corporation The © 2006 by Taylor & Francis Group, LLC 358 650 Separation of btex components 600 TP31001B.RAW HS-C-GC-FID 550 500 45 0 350 300 m, p-xylene... Organics and Inorganics 333 assume that EG prefers the more polar lower phase and has a distribution ratio that favors the lower phase and that DEG = 0.1 We also assume that the volume of upper phase equals that of the lower phase (i.e., Vupper = Vlower) and the volume ratio is therefore 1 Substituting into Equation (4. 3) without considering values for p and q yields f 3 ,4 = 4! p 3q = 4q 3 q 3!( 4 − 3)!... mobile phase and © 2006 by Taylor & Francis Group, LLC 344 Trace Environmental Quantitative Analysis, Second Edition stationary phase requires a length of column, and this length can be defined as H A column would then have a length L and a number of these equilibrations denoted by N, the number of theoretical plates Hence, we define the HETP, abbreviated H for brevity here, as follows: H= L N (4. 19) The... and the linear mobile-phase velocity u according to H = A+ B + CSu + CM u u (4. 25) This is a more contemporary van Deemter equation, and this equation takes on different contributions to the terms A, B, CS, and CM, depending on which form of chromatography is employed We will introduce specific parameters that comprise both C terms when we discuss GC and HPLC Equation (4. 25) is plotted in Figure 4. 4... ethyl acetate dissolved in n-hexane and chromatographed on a Height equivalent to a theoretical plate (H) H HS and HM HL A Linear velocity in cm/sec FIGURE 4. 4 Plate height vs linear carrier gas velocity showing the distinct contributions to plate height © 2006 by Taylor & Francis Group, LLC Determinative Techniques to Measure Organics and Inorganics 349 normal-phase, silica-based HPLC column The plot... Francis Group, LLC 352 Trace Environmental Quantitative Analysis, Second Edition the base is equal to four times the standard deviation of the Gaussian peak profile (i.e., tw = 4 t) We also assume that the variance of both peaks is equal, so that σ1 = σ 2 = σ τ Upon substituting this into Equation (4. 27), we get τ τ RS = 2 1 2 1 2 (tR − tR ) tR − tR = 8σ τ 4 τ (4. 28) Equation 4. 28 shows that the larger . Spectrometry 42 3 Principles of the quadrupole 42 5 Principles of the ion-trap 43 3 Tuning a quadrupole 43 6 Principles of Time-of-flight 44 5 Interpretation 44 7 Tandem strategies and techniques 44 9 High. Chromatography 45 2 Mobile phase/stationary phase considerations 45 8 UV and fluorescence detectors 46 4 Principles of LC-MS interfaces 47 6 © 2006 by Taylor & Francis Group, LLC 3 24 Trace Environmental Quantitative. 0/q 4 pq 3 /3q 3 p 3p 2 q 2 /3p 2 q 2 3p 3 q/p 3 qp 4 /0 Total q 4 4q 3 p 6p 2 q 2 4p 3 qp 4 (q + p) 4 p VD VD = +1 q VD = + 1 1 © 2006 by Taylor & Francis Group, LLC 332 Trace Environmental